Patent Publication Number: US-2022236417-A1

Title: Lidar System with Multi-Junction Light Source

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 63/142,095, filed 27 Jan. 2021, which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure generally relates to lidar systems. 
     BACKGROUND 
     Light detection and ranging (lidar) is a technology that can be used to measure distances to remote targets. Typically, a lidar system includes a light source and an optical receiver. The light source can include, for example, a laser which emits light having a particular operating wavelength. The operating wavelength of a lidar system may lie, for example, in the infrared, visible, or ultraviolet portions of the electromagnetic spectrum. The light source emits light toward a target which scatters the light, and some of the scattered light is received back at the receiver. The system determines the distance to the target based on one or more characteristics associated with the received light. For example, the lidar system may determine the distance to the target based on the time of flight for a pulse of light emitted by the light source to travel to the target and back to the lidar system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example light detection and ranging (lidar) system. 
         FIG. 2  illustrates an example scan pattern produced by a lidar system. 
         FIG. 3  illustrates an example lidar system with an example rotating polygon mirror. 
         FIG. 4  illustrates an example light-source field of view (FOV L ) and receiver field of view (FOV R ) for a lidar system. 
         FIG. 5  illustrates an example unidirectional scan pattern that includes multiple pixels and multiple scan lines. 
         FIG. 6  illustrates an example lidar system with a light source that emits pulses of light and local-oscillator (LO) light. 
         FIG. 7  illustrates an example receiver and an example voltage signal corresponding to a received pulse of light. 
         FIG. 8  illustrates an example light source that includes a seed laser diode and a semiconductor optical amplifier (SOA). 
         FIG. 9  illustrates an example light source that includes a semiconductor optical amplifier (SOA) with a tapered optical waveguide. 
         FIG. 10  illustrates an example light source with an optical splitter that splits output light from a seed laser diode to produce seed light and local-oscillator (LO) light. 
         FIG. 11  illustrates an example light source with a photonic integrated circuit (PIC) that includes an optical-waveguide splitter. 
         FIG. 12  illustrates an example light source that includes a seed laser diode and a local-oscillator (LO) laser diode. 
         FIG. 13  illustrates an example light source that includes a seed laser, a semiconductor optical amplifier (SOA), and a fiber-optic amplifier. 
         FIG. 14  illustrates an example fiber-optic amplifier. 
         FIG. 15  illustrates example graphs of seed current (I 1 ), LO light, seed light, pulsed SOA current (I 2 ), and emitted optical pulses. 
         FIG. 16  illustrates example graphs of seed light, an emitted optical pulse, a received optical pulse, LO light, and detector photocurrent. 
         FIG. 17  illustrates an example voltage signal that results from the coherent mixing of LO light and a received pulse of light. 
         FIG. 18  illustrates an example receiver that includes a combiner and two detectors. 
         FIG. 19  illustrates an example receiver that includes an integrated-optic combiner and two detectors. 
         FIG. 20  illustrates an example receiver that includes a 90-degree optical hybrid and four detectors. 
         FIG. 21  illustrates an example receiver that includes two polarization beam-splitters. 
         FIGS. 22-25  each illustrates an example light source that includes a seed laser, a semiconductor optical amplifier (SOA), and one or more optical modulators. 
         FIG. 26  illustrates an example voltage signal that results from the coherent mixing of LO light and a received pulse of light, where the LO light and the received pulse of light have a frequency difference of Δf. 
         FIG. 27  illustrates example graphs of seed current (I 1 ), seed light, an emitted optical pulse, a received optical pulse, and LO light. 
         FIG. 28  illustrates example time-domain and frequency-domain graphs of LO light and two emitted pulses of light. 
         FIG. 29  illustrates an example voltage signal that results from the coherent mixing of LO light and a received pulse of light. 
         FIG. 30  illustrates two example voltage signals that result from the coherent mixing of LO light with two different received pulses of light. 
         FIG. 31  illustrates an example light source and receiver integrated into a photonic integrated circuit (PIC). 
         FIG. 32  illustrates an example single-junction seed laser diode. 
         FIG. 33  illustrates an example multi junction seed laser diode with two laser junctions. 
         FIG. 34  illustrates an example multi junction seed laser diode with three laser junctions. 
         FIG. 35  illustrates an example single-junction semiconductor optical amplifier (SOA). 
         FIG. 36  illustrates an example multi junction SOA with two SOA junctions. 
         FIG. 37  illustrates an example multi junction SOA with three SOA junctions. 
         FIG. 38  illustrates an example multi junction light source with a multi junction seed laser diode and a multi junction SOA. 
         FIG. 39  illustrates an example multi junction light source with a single junction seed laser diode and a multi junction SOA. 
         FIG. 40  illustrates an example computer system. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
       FIG. 1  illustrates an example light detection and ranging (lidar) system  100 . In particular embodiments, a lidar system  100  may be referred to as a laser ranging system, a laser radar system, a LIDAR system, a lidar sensor, or a laser detection and ranging (LADAR or ladar) system. In particular embodiments, a lidar system  100  may include a light source  110 , mirror  115 , scanner  120 , receiver  140 , or controller  150 . The light source  110  may include, for example, a laser which emits light having a particular operating wavelength in the infrared, visible, or ultraviolet portions of the electromagnetic spectrum. As an example, light source  110  may include a laser with one or more operating wavelengths between approximately 900 nanometers (nm) and 2000 nm. The light source  110  emits an output beam of light  125  which may be continuous wave (CW), pulsed, or modulated in any suitable manner for a given application. The output beam of light  125  is directed downrange toward a remote target  130 . As an example, the remote target  130  may be located a distance D of approximately 1 m to 1 km from the lidar system  100 . 
     Once the output beam  125  reaches the downrange target  130 , the target may scatter or reflect at least a portion of light from the output beam  125 , and some of the scattered or reflected light may return toward the lidar system  100 . In the example of  FIG. 1 , the scattered or reflected light is represented by input beam  135 , which passes through scanner  120  and is reflected by mirror  115  and directed to receiver  140 . In particular embodiments, a relatively small fraction of the light from output beam  125  may return to the lidar system  100  as input beam  135 . As an example, the ratio of input beam  135  average power, peak power, or pulse energy to output beam  125  average power, peak power, or pulse energy may be approximately 10 −1 , 10 −2 , 10 −3 , 10 −4 , 10 −5 , 10 −6 , 10 −7 , 10 −8 , 10 −9 , 10 −10 , 10 −11 , or 10 −12 . As another example, if a pulse of output beam  125  has a pulse energy of 1 microjoule (μJ), then the pulse energy of a corresponding pulse of input beam  135  may have a pulse energy of approximately 10 nanojoules (nJ), 1 nJ, 100 picojoules (pJ), 10 pJ, 1 pJ, 100 femtojoules (fJ), 10 fJ, 1 fJ, 100 attojoules (aJ), 10 aJ, 1 aJ, or 0.1 aJ. 
     In particular embodiments, output beam  125  may include or may be referred to as an optical signal, output optical signal, emitted optical signal, output light, emitted pulse of light, laser beam, light beam, optical beam, emitted beam, emitted light, or beam. In particular embodiments, input beam  135  may include or may be referred to as a received optical signal, received pulse of light, input pulse of light, input optical signal, return beam, received beam, return light, received light, input light, scattered light, or reflected light. As used herein, scattered light may refer to light that is scattered or reflected by a target  130 . As an example, an input beam  135  may include: light from the output beam  125  that is scattered by target  130 ; light from the output beam  125  that is reflected by target  130 ; or a combination of scattered and reflected light from target  130 . 
     In particular embodiments, receiver  140  may receive or detect photons from input beam  135  and produce one or more representative signals. For example, the receiver  140  may produce an output electrical signal  145  that is representative of the input beam  135 , and the electrical signal  145  may be sent to controller  150 . In particular embodiments, receiver  140  or controller  150  may include a processor, computing system (e.g., an ASIC or FPGA), or other suitable circuitry. A controller  150  may be configured to analyze one or more characteristics of the electrical signal  145  from the receiver  140  to determine one or more characteristics of the target  130 , such as its distance downrange from the lidar system  100 . This may be done, for example, by analyzing a time of flight or a frequency or phase of a transmitted beam of light  125  or a received beam of light  135 . If lidar system  100  measures a time of flight of ΔT (e.g., ΔT represents a round-trip time of flight for an emitted pulse of light to travel from the lidar system  100  to the target  130  and back to the lidar system  100 ), then the distance D from the target  130  to the lidar system  100  may be expressed as D=c·ΔT/2, where c is the speed of light (approximately 3.0×10 8  m/s). As an example, if a time of flight is measured to be ΔT=300 ns, then the distance from the target  130  to the lidar system  100  may be determined to be approximately D=45.0 m. As another example, if a time of flight is measured to be ΔT=1.33 μs, then the distance from the target  130  to the lidar system  100  may be determined to be approximately D=199.5 m. In particular embodiments, a distance D from lidar system  100  to a target  130  may be referred to as a distance, depth, or range of target  130 . As used herein, the speed of light c refers to the speed of light in any suitable medium, such as for example in air, water, or vacuum. As an example, the speed of light in vacuum is approximately 2.9979×10 8  m/s, and the speed of light in air (which has a refractive index of approximately 1.0003) is approximately 2.9970×10 8  m/s. 
     In particular embodiments, light source  110  may include a pulsed or CW laser. As an example, light source  110  may be a pulsed laser configured to produce or emit pulses of light with a pulse duration or pulse width of approximately 10 picoseconds (ps) to 100 nanoseconds (ns). The pulses may have a pulse duration (Δτ) of approximately 100 ps, 200 ps, 400 ps, 1 ns, 2 ns, 5 ns, 10 ns, 20 ns, 50 ns, 100 ns, or any other suitable pulse duration. As another example, light source  110  may be a pulsed laser that produces pulses with a pulse duration of approximately 1-5 ns. As another example, light source  110  may be a pulsed laser that produces pulses at a pulse repetition frequency of approximately 80 kHz to 10 MHz or a pulse period (e.g., a time between consecutive pulses) of approximately 100 ns to 12.5 μs. In particular embodiments, light source  110  may have a substantially constant pulse repetition frequency, or light source  110  may have a variable or adjustable pulse repetition frequency. As an example, light source  110  may be a pulsed laser that produces pulses at a substantially constant pulse repetition frequency of approximately 640 kHz (e.g., 640,000 pulses per second), corresponding to a pulse period of approximately 1.56 μs. As another example, light source  110  may have a pulse repetition frequency (which may be referred to as a repetition rate) that can be varied from approximately 200 kHz to 3 MHz. As used herein, a pulse of light may be referred to as an optical pulse, a light pulse, or a pulse. 
     In particular embodiments, light source  110  may include a pulsed or CW laser that produces a free-space output beam  125  having any suitable average optical power. As an example, output beam  125  may have an average power of approximately 1 milliwatt (mW), 10 mW, 100 mW, 1 watt (W), 10 W, or any other suitable average power. In particular embodiments, output beam  125  may include optical pulses with any suitable pulse energy or peak optical power. As an example, output beam  125  may include pulses with a pulse energy of approximately 0.01 μJ, 0.1 μJ, 0.5 μJ, 1 μJ, 2 μJ, 10 μJ, 100 μJ, 1 mJ, or any other suitable pulse energy. As another example, output beam  125  may include pulses with a peak power of approximately 10 W, 100 W, 1 kW, 5 kW, 10 kW, or any other suitable peak power. The peak power (P peak ) of a pulse of light can be related to the pulse energy (E) by the expression E=P peak ·Δt, where Δt is the duration of the pulse, and the duration of a pulse may be defined as the full width at half maximum duration of the pulse. For example, an optical pulse with a duration of 1 ns and a pulse energy of 1 μJ has a peak power of approximately 1 kW. The average power (P av ) of an output beam  125  can be related to the pulse repetition frequency (PRF) and pulse energy by the expression P av =PRF·E. For example, if the pulse repetition frequency is 500 kHz, then the average power of an output beam  125  with 1-μJ pulses is approximately 0.5 W. 
     In particular embodiments, light source  110  may include a laser diode, such as for example, a Fabry-Perot laser diode, a quantum well laser, a distributed Bragg reflector (DBR) laser, a distributed feedback (DFB) laser, a vertical-cavity surface-emitting laser (VCSEL), a quantum dot laser diode, a grating-coupled surface-emitting laser (GCSEL), a slab-coupled optical waveguide laser (SCOWL), a single-transverse-mode laser diode, a multi-mode broad area laser diode, a laser-diode bar, a laser-diode stack, or a tapered-stripe laser diode. As an example, light source  110  may include an aluminum-gallium-arsenide (AlGaAs) laser diode, an indium-gallium-arsenide (InGaAs) laser diode, an indium-gallium-arsenide-phosphide (InGaAsP) laser diode, or a laser diode that includes any suitable combination of aluminum (Al), indium (In), gallium (Ga), arsenic (As), phosphorous (P), or any other suitable material. In particular embodiments, light source  110  may include a pulsed or CW laser diode with a peak emission wavelength between 1200 nm and 1600 nm. As an example, light source  110  may include a current-modulated InGaAsP DFB laser diode that produces optical pulses at a wavelength of approximately 1550 nm. As another example, light source  110  may include a laser diode that emits light at a wavelength between 1500 nm and 1510 nm. 
     In particular embodiments, light source  110  may include a pulsed or CW laser diode followed by one or more optical-amplification stages. For example, a seed laser diode may produce a seed optical signal, and an optical amplifier may amplify the seed optical signal to produce an amplified optical signal that is emitted by the light source  110 . In particular embodiments, an optical amplifier may include a fiber-optic amplifier or a semiconductor optical amplifier (SOA). For example, a pulsed laser diode may produce relatively low-power optical seed pulses which are amplified by a fiber-optic amplifier. As another example, a light source  110  may include a fiber-laser module that includes a current-modulated laser diode with an operating wavelength of approximately 1550 nm followed by a single-stage or a multi-stage erbium-doped fiber amplifier (EDFA) or erbium-ytterbium-doped fiber amplifier (EYDFA) that amplifies the seed pulses from the laser diode. As another example, light source  110  may include a continuous-wave (CW) or quasi-CW laser diode followed by an external optical modulator (e.g., an electro-optic amplitude modulator). The optical modulator may modulate the CW light from the laser diode to produce optical pulses which are sent to a fiber-optic amplifier or SOA. As another example, light source  110  may include a pulsed or CW seed laser diode followed by a semiconductor optical amplifier (SOA). The SOA may include an active optical waveguide configured to receive light from the seed laser diode and amplify the light as it propagates through the waveguide. The optical gain of the SOA may be provided by pulsed or direct-current (DC) electrical current supplied to the SOA. The SOA may be integrated on the same chip as the seed laser diode, or the SOA may be a separate device with an anti-reflection coating on its input facet or output facet. As another example, light source  110  may include a seed laser diode followed by a SOA, which in turn is followed by a fiber-optic amplifier. For example, the seed laser diode may produce relatively low-power seed pulses which are amplified by the SOA, and the fiber-optic amplifier may further amplify the optical pulses. 
     In particular embodiments, light source  110  may include a direct-emitter laser diode. A direct-emitter laser diode (which may be referred to as a direct emitter) may include a laser diode which produces light that is not subsequently amplified by an optical amplifier. A light source  110  that includes a direct-emitter laser diode may not include an optical amplifier, and the output light produced by a direct emitter may not be amplified after it is emitted by the laser diode. The light produced by a direct-emitter laser diode (e.g., optical pulses, CW light, or frequency-modulated light) may be emitted directly as a free-space output beam  125  without being amplified. A direct-emitter laser diode may be driven by an electrical power source that supplies current pulses to the laser diode, and each current pulse may result in the emission of an output optical pulse. 
     In particular embodiments, light source  110  may include a diode-pumped solid-state (DPSS) laser. A DPSS laser (which may be referred to as a solid-state laser) may refer to a laser that includes a solid-state, glass, ceramic, or crystal-based gain medium that is pumped by one or more pump laser diodes. The gain medium may include a host material that is doped with rare-earth ions (e.g., neodymium, erbium, ytterbium, or praseodymium). For example, a gain medium may include a yttrium aluminum garnet (YAG) crystal that is doped with neodymium (Nd) ions, and the gain medium may be referred to as a Nd:YAG crystal. A DPSS laser with a Nd:YAG gain medium may produce light at a wavelength between approximately 1300 nm and approximately 1400 nm, and the Nd:YAG gain medium may be pumped by one or more pump laser diodes with an operating wavelength between approximately 730 nm and approximately 900 nm. A DPSS laser may be a passively Q-switched laser that includes a saturable absorber (e.g., a vanadium-doped crystal that acts as a saturable absorber). Alternatively, a DPSS laser may be an actively Q-switched laser that includes an active Q-switch (e.g., an acousto-optic modulator or an electro-optic modulator). A passively or actively Q-switched DPSS laser may produce output optical pulses that form an output beam  125  of a lidar system  100 . 
     In particular embodiments, an output beam of light  125  emitted by light source  110  may be a collimated optical beam having any suitable beam divergence, such as for example, a full-angle beam divergence of approximately 0.5 to 10 milliradians (mrad). A divergence of output beam  125  may refer to an angular measure of an increase in beam size (e.g., a beam radius or beam diameter) as output beam  125  travels away from light source  110  or lidar system  100 . In particular embodiments, output beam  125  may have a substantially circular cross section with a beam divergence characterized by a single divergence value. As an example, an output beam  125  with a circular cross section and a full-angle beam divergence of 2 mrad may have a beam diameter or spot size of approximately 20 cm at a distance of 100 m from lidar system  100 . In particular embodiments, output beam  125  may have a substantially elliptical cross section characterized by two divergence values. As an example, output beam  125  may have a fast axis and a slow axis, where the fast-axis divergence is greater than the slow-axis divergence. As another example, output beam  125  may be an elliptical beam with a fast-axis divergence of 4 mrad and a slow-axis divergence of 2 mrad. 
     In particular embodiments, an output beam of light  125  emitted by light source  110  may be unpolarized or randomly polarized, may have no specific or fixed polarization (e.g., the polarization may vary with time), or may have a particular polarization (e.g., output beam  125  may be linearly polarized, elliptically polarized, or circularly polarized). As an example, light source  110  may produce light with no specific polarization or may produce light that is linearly polarized. 
     In particular embodiments, lidar system  100  may include one or more optical components configured to reflect, focus, filter, shape, modify, steer, or direct light within the lidar system  100  or light produced or received by the lidar system  100  (e.g., output beam  125  or input beam  135 ). As an example, lidar system  100  may include one or more lenses, mirrors, filters (e.g., band-pass or interference filters), beam splitters, optical splitters, beam combiners, couplers, polarizers, polarizing beam splitters, wave plates (e.g., half-wave or quarter-wave plates), diffractive elements, holographic elements, isolators, detectors, or collimators. The optical components in a lidar system  100  may be free-space optical components, fiber-coupled optical components, or a combination of free-space and fiber-coupled optical components. 
     In particular embodiments, lidar system  100  may include a telescope, one or more lenses, or one or more mirrors configured to expand, focus, or collimate the output beam  125  or the input beam  135  to a desired beam diameter or divergence. As an example, the lidar system  100  may include one or more lenses to focus the input beam  135  onto a photodetector of receiver  140 . As another example, the lidar system  100  may include one or more flat mirrors or curved mirrors (e.g., concave, convex, or parabolic mirrors) to steer or focus the output beam  125  or the input beam  135 . For example, the lidar system  100  may include an off-axis parabolic mirror to focus the input beam  135  onto a photodetector of receiver  140 . As illustrated in  FIG. 1 , the lidar system  100  may include mirror  115  (which may be a metallic or dielectric mirror), and mirror  115  may be configured so that light beam  125  passes through the mirror  115  or passes along an edge or side of the mirror  115  and input beam  135  is reflected toward the receiver  140 . As an example, mirror  115  (which may be referred to as an overlap mirror, superposition mirror, or beam-combiner mirror) may include a hole, slot, or aperture which output light beam  125  passes through. As another example, rather than passing through the mirror  115 , the output beam  125  may be directed to pass alongside the mirror  115  with a gap (e.g., a gap of width approximately 0.1 mm, 0.5 mm, 1 mm, 2 mm, 5 mm, or 10 mm) between the output beam  125  and an edge of the mirror  115 . 
     In particular embodiments, mirror  115  may provide for output beam  125  and input beam  135  to be substantially coaxial so that the two beams travel along approximately the same optical path (albeit in opposite directions). The input and output beams being substantially coaxial may refer to the beams being at least partially overlapped or sharing a common propagation axis so that input beam  135  and output beam  125  travel along substantially the same optical path (albeit in opposite directions). As an example, output beam  125  and input beam  135  may be parallel to each other to within less than 10 mrad, 5 mrad, 2 mrad, 1 mrad, 0.5 mrad, or 0.1 mrad. As output beam  125  is scanned across a field of regard, the input beam  135  may follow along with the output beam  125  so that the coaxial relationship between the two beams is maintained. 
     In particular embodiments, lidar system  100  may include a scanner  120  configured to scan an output beam  125  across a field of regard of the lidar system  100 . As an example, scanner  120  may include one or more scanning mirrors configured to pivot, rotate, oscillate, or move in an angular manner about one or more rotation axes. The output beam  125  may be reflected by a scanning mirror, and as the scanning mirror pivots or rotates, the reflected output beam  125  may be scanned in a corresponding angular manner. As an example, a scanning mirror may be configured to periodically pivot back and forth over a 30-degree range, which results in the output beam  125  scanning back and forth across a 60-degree range (e.g., a 0-degree rotation by a scanning mirror results in a 20-degree angular scan of output beam  125 ). 
     In particular embodiments, a scanning mirror (which may be referred to as a scan mirror) may be attached to or mechanically driven by a scanner actuator or mechanism which pivots or rotates the mirror over a particular angular range (e.g., over a 5° angular range, 30° angular range, 60° angular range, 120° angular range, 360° angular range, or any other suitable angular range). A scanner actuator or mechanism configured to pivot or rotate a mirror may include a galvanometer scanner, a resonant scanner, a piezoelectric actuator, a voice coil motor, an electric motor (e.g., a DC motor, a brushless DC motor, a synchronous electric motor, or a stepper motor), a microelectromechanical systems (MEMS) device, or any other suitable actuator or mechanism. As an example, a scanner  120  may include a scanning mirror attached to a galvanometer scanner configured to pivot back and forth over a 1° to 30° angular range. As another example, a scanner  120  may include a scanning mirror that is attached to or is part of a MEMS device configured to scan over a 1° to 30° angular range. As another example, a scanner  120  may include a polygon mirror configured to rotate continuously in the same direction (e.g., rather than pivoting back and forth, the polygon mirror continuously rotates 360 degrees in a clockwise or counterclockwise direction). The polygon mirror may be coupled or attached to a synchronous motor configured to rotate the polygon mirror at a substantially fixed rotational frequency (e.g., a rotational frequency of approximately 1 Hz, 10 Hz, 50 Hz, 100 Hz, 500 Hz, or 1,000 Hz). 
     In particular embodiments, scanner  120  may be configured to scan the output beam  125  (which may include at least a portion of the light emitted by light source  110 ) across a field of regard of the lidar system  100 . A field of regard (FOR) of a lidar system  100  may refer to an area, region, or angular range over which the lidar system  100  may be configured to scan or capture distance information. As an example, a lidar system  100  with an output beam  125  with a 30-degree scanning range may be referred to as having a 30-degree angular field of regard. As another example, a lidar system  100  with a scanning mirror that rotates over a 30-degree range may produce an output beam  125  that scans across a 60-degree range (e.g., a 60-degree FOR). In particular embodiments, lidar system  100  may have a FOR of approximately 10°, 20°, 40°, 60°, 120°, 360°, or any other suitable FOR. 
     In particular embodiments, scanner  120  may be configured to scan the output beam  125  horizontally and vertically, and lidar system  100  may have a particular FOR along the horizontal direction and another particular FOR along the vertical direction. As an example, lidar system  100  may have a horizontal FOR of 10° to 120° and a vertical FOR of 2° to 45°. In particular embodiments, scanner  120  may include a first scan mirror and a second scan mirror, where the first scan mirror directs the output beam  125  toward the second scan mirror, and the second scan mirror directs the output beam  125  downrange from the lidar system  100 . As an example, the first scan mirror may scan the output beam  125  along a first direction, and the second scan mirror may scan the output beam  125  along a second direction that is different from the first direction (e.g., the first and second directions may be approximately orthogonal to one another, or the second direction may be oriented at any suitable nonzero angle with respect to the first direction). As another example, the first scan mirror may scan the output beam  125  along a substantially horizontal direction, and the second scan mirror may scan the output beam  125  along a substantially vertical direction (or vice versa). As another example, the first and second scan mirrors may each be driven by galvanometer scanners. As another example, the first or second scan mirror may include a polygon mirror driven by an electric motor. In particular embodiments, scanner  120  may be referred to as a beam scanner, optical scanner, or laser scanner. 
     In particular embodiments, one or more scanning mirrors may be communicatively coupled to controller  150  which may control the scanning mirror(s) so as to guide the output beam  125  in a desired direction downrange or along a desired scan pattern. In particular embodiments, a scan pattern may refer to a pattern or path along which the output beam  125  is directed. As an example, scanner  120  may include two scanning mirrors configured to scan the output beam  125  across a 60° horizontal FOR and a 20° vertical FOR. The two scanner mirrors may be controlled to follow a scan path that substantially covers the 60°×20° FOR. As an example, the scan path may result in a point cloud with pixels that substantially cover the 60°×20° FOR. The pixels may be approximately evenly distributed across the 60°×20° FOR. Alternatively, the pixels may have a particular nonuniform distribution (e.g., the pixels may be distributed across all or a portion of the 60°×20° FOR, and the pixels may have a higher density in one or more particular regions of the 60°×20° FOR). 
     In particular embodiments, a lidar system  100  may include a scanner  120  with a solid-state scanning device. A solid-state scanning device may refer to a scanner  120  that scans an output beam  125  without the use of moving parts (e.g., without the use of a mechanical scanner, such as a mirror that rotates or pivots). For example, a solid-state scanner  120  may include one or more of the following: an optical phased array scanning device; a liquid-crystal scanning device; or a liquid lens scanning device. A solid-state scanner  120  may be an electrically addressable device that scans an output beam  125  along one axis (e.g., horizontally) or along two axes (e.g., horizontally and vertically). In particular embodiments, a scanner  120  may include a solid-state scanner and a mechanical scanner. For example, a scanner  120  may include an optical phased array scanner configured to scan an output beam  125  in one direction and a galvanometer scanner that scans the output beam  125  in an orthogonal direction. The optical phased array scanner may scan the output beam relatively rapidly in a horizontal direction across the field of regard (e.g., at a scan rate of 50 to 1,000 scan lines per second), and the galvanometer may pivot a mirror at a rate of 1-30 Hz to scan the output beam  125  vertically. 
     In particular embodiments, a lidar system  100  may include a light source  110  configured to emit pulses of light and a scanner  120  configured to scan at least a portion of the emitted pulses of light across a field of regard of the lidar system  100 . One or more of the emitted pulses of light may be scattered by a target  130  located downrange from the lidar system  100 , and a receiver  140  may detect at least a portion of the pulses of light scattered by the target  130 . A receiver  140  may be referred to as a photoreceiver, optical receiver, optical sensor, detector, photodetector, or optical detector. In particular embodiments, lidar system  100  may include a receiver  140  that receives or detects at least a portion of input beam  135  and produces an electrical signal that corresponds to input beam  135 . As an example, if input beam  135  includes an optical pulse, then receiver  140  may produce an electrical current or voltage pulse that corresponds to the optical pulse detected by receiver  140 . As another example, receiver  140  may include one or more avalanche photodiodes (APDs) or one or more single-photon avalanche diodes (SPADs). As another example, receiver  140  may include one or more PN photodiodes (e.g., a photodiode structure formed by a p-type semiconductor and a n-type semiconductor, where the PN acronym refers to the structure having p-doped and n-doped regions) or one or more PIN photodiodes (e.g., a photodiode structure formed by an undoped intrinsic semiconductor region located between p-type and n-type regions, where the PIN acronym refers to the structure having p-doped, intrinsic, and n-doped regions). An APD, SPAD, PN photodiode, or PIN photodiode may each be referred to as a detector, photodetector, or photodiode. A detector may have an active region or an avalanche-multiplication region that includes silicon, germanium, InGaAs, InAsSb (indium arsenide antimonide), AlAsSb (aluminum arsenide antimonide), or AlInAsSb (aluminum indium arsenide antimonide). The active region may refer to an area over which a detector may receive or detect input light. An active region may have any suitable size or diameter, such as for example, a diameter of approximately 10 μm, 25 μm, 50 μm, 80 μm, 100 μm, 200 μm, 500 μm, 1 mm, 2 mm, or 5 mm. 
     In particular embodiments, receiver  140  may include electronic circuitry that performs signal amplification, sampling, filtering, signal conditioning, analog-to-digital conversion, time-to-digital conversion, pulse detection, threshold detection, rising-edge detection, or falling-edge detection. As an example, receiver  140  may include a transimpedance amplifier that converts a received photocurrent (e.g., a current produced by an APD in response to a received optical signal) into a voltage signal. The voltage signal may be sent to pulse-detection circuitry that produces an analog or digital output signal  145  that corresponds to one or more optical characteristics (e.g., rising edge, falling edge, amplitude, duration, or energy) of a received optical pulse. As an example, the pulse-detection circuitry may perform a time-to-digital conversion to produce a digital output signal  145 . The electrical output signal  145  may be sent to controller  150  for processing or analysis (e.g., to determine a time-of-flight value corresponding to a received optical pulse). 
     In particular embodiments, a controller  150  (which may include or may be referred to as a processor, an FPGA, an ASIC, a computer, or a computing system) may be located within a lidar system  100  or outside of a lidar system  100 . Alternatively, one or more parts of a controller  150  may be located within a lidar system  100 , and one or more other parts of a controller  150  may be located outside a lidar system  100 . In particular embodiments, one or more parts of a controller  150  may be located within a receiver  140  of a lidar system  100 , and one or more other parts of a controller  150  may be located in other parts of the lidar system  100 . For example, a receiver  140  may include an FPGA or ASIC configured to process an output electrical signal from the receiver  140 , and the processed signal may be sent to a computing system located elsewhere within the lidar system  100  or outside the lidar system  100 . In particular embodiments, a controller  150  may include any suitable arrangement or combination of logic circuitry, analog circuitry, or digital circuitry. 
     In particular embodiments, controller  150  may be electrically coupled or communicatively coupled to light source  110 , scanner  120 , or receiver  140 . As an example, controller  150  may receive electrical trigger pulses or edges from light source  110 , where each pulse or edge corresponds to the emission of an optical pulse by light source  110 . As another example, controller  150  may provide instructions, a control signal, or a trigger signal to light source  110  indicating when light source  110  should produce optical pulses. Controller  150  may send an electrical trigger signal that includes electrical pulses, where each electrical pulse results in the emission of an optical pulse by light source  110 . In particular embodiments, the frequency, period, duration, pulse energy, peak power, average power, or wavelength of the optical pulses produced by light source  110  may be adjusted based on instructions, a control signal, or trigger pulses provided by controller  150 . In particular embodiments, controller  150  may be coupled to light source  110  and receiver  140 , and controller  150  may determine a time-of-flight value for an optical pulse based on timing information associated with when the pulse was emitted by light source  110  and when a portion of the pulse (e.g., input beam  135 ) was detected or received by receiver  140 . In particular embodiments, controller  150  may include circuitry that performs signal amplification, sampling, filtering, signal conditioning, analog-to-digital conversion, time-to-digital conversion, pulse detection, threshold detection, rising-edge detection, or falling-edge detection. 
     In particular embodiments, lidar system  100  may include one or more processors (e.g., a controller  150 ) configured to determine a distance D from the lidar system  100  to a target  130  based at least in part on a round-trip time of flight for an emitted pulse of light to travel from the lidar system  100  to the target  130  and back to the lidar system  100 . The target  130  may be at least partially contained within a field of regard of the lidar system  100  and located a distance D from the lidar system  100  that is less than or equal to an operating range (R OP ) of the lidar system  100 . In particular embodiments, an operating range (which may be referred to as an operating distance) of a lidar system  100  may refer to a distance over which the lidar system  100  is configured to sense or identify targets  130  located within a field of regard of the lidar system  100 . The operating range of lidar system  100  may be any suitable distance, such as for example, 25 m, 50 m, 100 m, 200 m, 250 m, 500 m, or 1 km. As an example, a lidar system  100  with a 200-m operating range may be configured to sense or identify various targets  130  located up to 200 m away from the lidar system  100 . The operating range R OP  of a lidar system  100  may be related to the time τ between the emission of successive optical signals by the expression R OP =c·τ/2. For a lidar system  100  with a 200-m operating range (R OP =200 m), the time τ between successive pulses (which may be referred to as a pulse period, a pulse repetition interval (PRI), or a time period between pulses) is approximately 2·R OP /c≅1.33 μs. The pulse period τ may also correspond to the time of flight for a pulse to travel to and from a target  130  located a distance R OP  from the lidar system  100 . Additionally, the pulse period τ may be related to the pulse repetition frequency (PRF) by the expression τ=1/PRF. For example, a pulse period of 1.33 μs corresponds to a PRF of approximately 752 kHz. 
     In particular embodiments, a lidar system  100  may be used to determine the distance to one or more downrange targets  130 . By scanning the lidar system  100  across a field of regard, the system may be used to map the distance to a number of points within the field of regard. Each of these depth-mapped points may be referred to as a pixel or a voxel. A collection of pixels captured in succession (which may be referred to as a depth map, a point cloud, or a frame) may be rendered as an image or may be analyzed to identify or detect objects or to determine a shape or distance of objects within the FOR. As an example, a point cloud may cover a field of regard that extends 60° horizontally and 15° vertically, and the point cloud may include a frame of 100-2000 pixels in the horizontal direction by 4-400 pixels in the vertical direction. 
     In particular embodiments, lidar system  100  may be configured to repeatedly capture or generate point clouds of a field of regard at any suitable frame rate between approximately 0.1 frames per second (FPS) and approximately 1,000 FPS. As an example, lidar system  100  may generate point clouds at a frame rate of approximately 0.1 FPS, 0.5 FPS, 1 FPS, 2 FPS, 5 FPS, 10 FPS, 20 FPS, 100 FPS, 500 FPS, or 1,000 FPS. As another example, lidar system  100  may be configured to produce optical pulses at a rate of 5×10 5  pulses/second (e.g., the system may determine 500,000 pixel distances per second) and scan a frame of 1000×50 pixels (e.g., 50,000 pixels/frame), which corresponds to a point-cloud frame rate of 10 frames per second (e.g., 10 point clouds per second). In particular embodiments, a point-cloud frame rate may be substantially fixed, or a point-cloud frame rate may be dynamically adjustable. As an example, a lidar system  100  may capture one or more point clouds at a particular frame rate (e.g., 1 Hz) and then switch to capture one or more point clouds at a different frame rate (e.g., 10 Hz). A slower frame rate (e.g., 1 Hz) may be used to capture one or more high-resolution point clouds, and a faster frame rate (e.g., 10 Hz) may be used to rapidly capture multiple lower-resolution point clouds. 
     In particular embodiments, a lidar system  100  may be configured to sense, identify, or determine distances to one or more targets  130  within a field of regard. As an example, a lidar system  100  may determine a distance to a target  130 , where all or part of the target  130  is contained within a field of regard of the lidar system  100 . All or part of a target  130  being contained within a FOR of the lidar system  100  may refer to the FOR overlapping, encompassing, or enclosing at least a portion of the target  130 . In particular embodiments, target  130  may include all or part of an object that is moving or stationary relative to lidar system  100 . As an example, target  130  may include all or a portion of a person, vehicle, motorcycle, truck, train, bicycle, wheelchair, pedestrian, animal, road sign, traffic light, lane marking, road-surface marking, parking space, pylon, guard rail, traffic barrier, pothole, railroad crossing, obstacle in or near a road, curb, stopped vehicle on or beside a road, utility pole, house, building, trash can, mailbox, tree, any other suitable object, or any suitable combination of all or part of two or more objects. In particular embodiments, a target may be referred to as an object. 
     In particular embodiments, light source  110 , scanner  120 , and receiver  140  may be packaged together within a single housing, where a housing may refer to a box, case, or enclosure that holds or contains all or part of a lidar system  100 . As an example, a lidar-system enclosure may contain a light source  110 , mirror  115 , scanner  120 , and receiver  140  of a lidar system  100 . Additionally, the lidar-system enclosure may include a controller  150 . The lidar-system enclosure may also include one or more electrical connections for conveying electrical power or electrical signals to or from the enclosure. In particular embodiments, one or more components of a lidar system  100  may be located remotely from a lidar-system enclosure. As an example, all or part of light source  110  may be located remotely from a lidar-system enclosure, and pulses of light produced by the light source  110  may be conveyed to the enclosure via optical fiber. As another example, all or part of a controller  150  may be located remotely from a lidar-system enclosure. 
     In particular embodiments, light source  110  may include an eye-safe laser, or lidar system  100  may be classified as an eye-safe laser system or laser product. An eye-safe laser, laser system, or laser product may refer to a system that includes a laser with an emission wavelength, average power, peak power, peak intensity, pulse energy, beam size, beam divergence, exposure time, or scanned output beam such that emitted light from the system presents little or no possibility of causing damage to a person&#39;s eyes. As an example, light source  110  or lidar system  100  may be classified as a Class 1 laser product (as specified by the 60825-1:2014 standard of the International Electrotechnical Commission (IEC)) or a Class I laser product (as specified by Title 21, Section 1040.10 of the United States Code of Federal Regulations (CFR)) that is safe under all conditions of normal use. In particular embodiments, lidar system  100  may be an eye-safe laser product (e.g., with a Class 1 or Class I classification) configured to operate at any suitable wavelength between approximately 900 nm and approximately 2100 nm. As an example, lidar system  100  may include a laser with an operating wavelength between approximately 1200 nm and approximately 1400 nm or between approximately 1400 nm and approximately 1600 nm, and the laser or the lidar system  100  may be operated in an eye-safe manner. As another example, lidar system  100  may be an eye-safe laser product that includes a scanned laser with an operating wavelength between approximately 900 nm and approximately 1700 nm. As another example, lidar system  100  may be a Class 1 or Class I laser product that includes a laser diode, fiber laser, or solid-state laser with an operating wavelength between approximately 1200 nm and approximately 1600 nm. As another example, lidar system  100  may have an operating wavelength between approximately 1500 nm and approximately 1510 nm. 
     In particular embodiments, one or more lidar systems  100  may be integrated into a vehicle. As an example, multiple lidar systems  100  may be integrated into a car to provide a complete 360-degree horizontal FOR around the car. As another example, 2-10 lidar systems  100 , each system having a 45-degree to 180-degree horizontal FOR, may be combined together to form a sensing system that provides a point cloud covering a 360-degree horizontal FOR. The lidar systems  100  may be oriented so that adjacent FORs have an amount of spatial or angular overlap to allow data from the multiple lidar systems  100  to be combined or stitched together to form a single or continuous 360-degree point cloud. As an example, the FOR of each lidar system  100  may have approximately 1-30 degrees of overlap with an adjacent FOR. In particular embodiments, a vehicle may refer to a mobile machine configured to transport people or cargo. For example, a vehicle may include, may take the form of, or may be referred to as a car, automobile, motor vehicle, truck, bus, van, trailer, off-road vehicle, farm vehicle, lawn mower, construction equipment, forklift, robot, golf cart, motorhome, taxi, motorcycle, scooter, bicycle, skateboard, train, snowmobile, watercraft (e.g., a ship or boat), aircraft (e.g., a fixed-wing aircraft, helicopter, or dirigible), unmanned aerial vehicle (e.g., drone), or spacecraft. In particular embodiments, a vehicle may include an internal combustion engine or an electric motor that provides propulsion for the vehicle. 
     In particular embodiments, one or more lidar systems  100  may be included in a vehicle as part of an advanced driver assistance system (ADAS) to assist a driver of the vehicle in operating the vehicle. For example, a lidar system  100  may be part of an ADAS that provides information (e.g., about the surrounding environment) or feedback to a driver (e.g., to alert the driver to potential problems or hazards) or that automatically takes control of part of a vehicle (e.g., a braking system or a steering system) to avoid collisions or accidents. A lidar system  100  may be part of a vehicle ADAS that provides adaptive cruise control, automated braking, automated parking, collision avoidance, alerts the driver to hazards or other vehicles, maintains the vehicle in the correct lane, or provides a warning if an object or another vehicle is in a blind spot. 
     In particular embodiments, one or more lidar systems  100  may be integrated into a vehicle as part of an autonomous-vehicle driving system. As an example, a lidar system  100  may provide information about the surrounding environment to a driving system of an autonomous vehicle. An autonomous-vehicle driving system may be configured to guide the autonomous vehicle through an environment surrounding the vehicle and toward a destination. An autonomous-vehicle driving system may include one or more computing systems that receive information from a lidar system  100  about the surrounding environment, analyze the received information, and provide control signals to the vehicle&#39;s driving systems (e.g., brakes, accelerator, steering mechanism, lights, or turn signals). As an example, a lidar system  100  integrated into an autonomous vehicle may provide an autonomous-vehicle driving system with a point cloud every 0.1 seconds (e.g., the point cloud has a 10 Hz update rate, representing 10 frames per second). The autonomous-vehicle driving system may analyze the received point clouds to sense or identify targets  130  and their respective locations, distances, or speeds, and the autonomous-vehicle driving system may update control signals based on this information. As an example, if lidar system  100  detects a vehicle ahead that is slowing down or stopping, the autonomous-vehicle driving system may send instructions to release the accelerator and apply the brakes. 
     In particular embodiments, an autonomous vehicle may be referred to as an autonomous car, driverless car, self-driving car, robotic car, or unmanned vehicle. In particular embodiments, an autonomous vehicle may refer to a vehicle configured to sense its environment and navigate or drive with little or no human input. As an example, an autonomous vehicle may be configured to drive to any suitable location and control or perform all safety-critical functions (e.g., driving, steering, braking, parking) for the entire trip, with the driver not expected to control the vehicle at any time. As another example, an autonomous vehicle may allow a driver to safely turn their attention away from driving tasks in particular environments (e.g., on freeways), or an autonomous vehicle may provide control of a vehicle in all but a few environments, requiring little or no input or attention from the driver. 
     In particular embodiments, an autonomous vehicle may be configured to drive with a driver present in the vehicle, or an autonomous vehicle may be configured to operate the vehicle with no driver present. As an example, an autonomous vehicle may include a driver&#39;s seat with associated controls (e.g., steering wheel, accelerator pedal, and brake pedal), and the vehicle may be configured to drive with no one seated in the driver&#39;s seat or with little or no input from a person seated in the driver&#39;s seat. As another example, an autonomous vehicle may not include any driver&#39;s seat or associated driver&#39;s controls, and the vehicle may perform substantially all driving functions (e.g., driving, steering, braking, parking, and navigating) without human input. As another example, an autonomous vehicle may be configured to operate without a driver (e.g., the vehicle may be configured to transport human passengers or cargo without a driver present in the vehicle). As another example, an autonomous vehicle may be configured to operate without any human passengers (e.g., the vehicle may be configured for transportation of cargo without having any human passengers onboard the vehicle). 
     In particular embodiments, an optical signal (which may be referred to as a light signal, a light waveform, an optical waveform, an output beam, an emitted optical signal, or emitted light) may include pulses of light, CW light, amplitude-modulated light, frequency-modulated (FM) light, or any suitable combination thereof. Although this disclosure describes or illustrates example embodiments of lidar systems  100  or light sources  110  that produce optical signals that include pulses of light, the embodiments described or illustrated herein may also be applied, where appropriate, to other types of optical signals, including continuous-wave (CW) light, amplitude-modulated optical signals, or frequency-modulated optical signals. For example, a lidar system  100  as described or illustrated herein may be a pulsed lidar system and may include a light source  110  that produces pulses of light. Alternatively, a lidar system  100  may be configured to operate as a frequency-modulated continuous-wave (FMCW) lidar system and may include a light source  110  that produces CW light or a frequency-modulated optical signal. 
     In particular embodiments, a lidar system  100  may be a FMCW lidar system where the emitted light from the light source  110  (e.g., output beam  125  in  FIG. 1  or  FIG. 3 ) includes frequency-modulated light. A pulsed lidar system is a type of lidar system  100  in which the light source  110  emits pulses of light, and the distance to a remote target  130  is determined based on the round-trip time-of-flight for a pulse of light to travel to the target  130  and back. Another type of lidar system  100  is a frequency-modulated lidar system, which may be referred to as a frequency-modulated continuous-wave (FMCW) lidar system. A FMCW lidar system uses frequency-modulated light to determine the distance to a remote target  130  based on a frequency of received light (which includes emitted light scattered by the remote target) relative to a frequency of local-oscillator (LO) light. A round-trip time for the emitted light to travel to a target  130  and back to the lidar system may correspond to a frequency difference between the received scattered light and the LO light. A larger frequency difference may correspond to a longer round-trip time and a greater distance to the target  130 . The frequency difference between the received scattered light and the LO light may be referred to as a beat frequency. 
     For example, for a linearly chirped light source (e.g., a frequency modulation that produces a linear change in frequency with time), the larger the frequency difference between the LO light and the received light, the farther away the target  130  is located. The frequency difference may be determined by mixing the received light with the LO light (e.g., by coupling the two beams onto a detector so that they are coherently mixed or combined together, or by mixing analog electric signals corresponding to the received light and the emitted light) to produce a beat signal and determining the beat frequency of the beat signal. For example, an electrical signal from an APD may be analyzed using a fast Fourier transform (FFT) technique to determine the frequency difference between the emitted light and the received light. If a linear frequency modulation m (e.g., in units of Hz/s) is applied to a CW laser, then the round-trip time ΔT may be related to the frequency difference between the received scattered light and the emitted light ΔF by the expression ΔT=ΔF/m. Additionally, the distance D from the target  130  to the lidar system  100  may be expressed as D=c·ΔF/(2m), where c is the speed of light. For example, for a light source  110  with a linear frequency modulation of 10 12  Hz/s (or, 1 MHz/μs), if a frequency difference (between the received scattered light and the emitted light) of 330 kHz is measured, then the distance to the target is approximately 50 meters (which corresponds to a round-trip time of approximately 330 ns). As another example, a frequency difference of 1.33 MHz corresponds to a target located approximately 200 meters away. 
     A light source  110  for a FMCW lidar system may include (i) a direct-emitter laser diode, (ii) a seed laser diode followed by a SOA, (iii) a seed laser diode followed by a fiber-optic amplifier, or (iv) a seed laser diode followed by a SOA and then a fiber-optic amplifier. A seed laser diode or a direct-emitter laser diode may be operated in a CW manner (e.g., by driving the laser diode with a substantially constant DC current), and a frequency modulation may be provided by an external modulator (e.g., an electro-optic phase modulator may apply a frequency modulation to seed-laser light). Alternatively, a frequency modulation may be produced by applying a current modulation to a seed laser diode or a direct-emitter laser diode. The current modulation (which may be provided along with a DC bias current) may produce a corresponding refractive-index modulation in the laser diode, which results in a frequency modulation of the light emitted by the laser diode. The current-modulation component (and the corresponding frequency modulation) may have any suitable frequency or shape (e.g., piecewise linear, sinusoidal, triangle-wave, or sawtooth). For example, the current-modulation component (and the resulting frequency modulation of the emitted light) may increase or decrease monotonically over a particular time interval. As another example, the current-modulation component may include a triangle or sawtooth wave with an electrical current that increases or decreases linearly over a particular time interval, and the light emitted by the laser diode may include a corresponding frequency modulation in which the optical frequency increases or decreases approximately linearly over the particular time interval. For example, a light source  110  that emits light with a linear frequency change of 200 MHz over a 2-μs time interval may be referred to as having a frequency modulation m of 10 14  Hz/s (or, 100 MHz/μs). 
       FIG. 2  illustrates an example scan pattern  200  produced by a lidar system  100 . A scanner  120  of the lidar system  100  may scan the output beam  125  (which may include multiple emitted optical signals) along a scan pattern  200  that is contained within a FOR of the lidar system  100 . A scan pattern  200  (which may be referred to as an optical scan pattern, optical scan path, scan path, or scan) may represent a path or course followed by output beam  125  as it is scanned across all or part of a FOR. Each traversal of a scan pattern  200  may correspond to the capture of a single frame or a single point cloud. In particular embodiments, a lidar system  100  may be configured to scan output optical beam  125  along one or more particular scan patterns  200 . In particular embodiments, a scan pattern  200  may scan across any suitable field of regard (FOR) having any suitable horizontal FOR (FOR H ) and any suitable vertical FOR (FOR V ). For example, a scan pattern  200  may have a field of regard represented by angular dimensions (e.g., FOR H  FOR V ) 40°×30°, 90°×40°, or 60°×15°. As another example, a scan pattern  200  may have a FOR H  greater than or equal to 10°, 25°, 30°, 40°, 60°, 90°, or 120°. As another example, a scan pattern  200  may have a FOR V  greater than or equal to 2°, 5°, 10°, 15°, 20°, 30°, or 45°. 
     In the example of  FIG. 2 , reference line  220  represents a center of the field of regard of scan pattern  200 . In particular embodiments, reference line  220  may have any suitable orientation, such as for example, a horizontal angle of 0° (e.g., reference line  220  may be oriented straight ahead) and a vertical angle of 0° (e.g., reference line  220  may have an inclination of 0°), or reference line  220  may have a nonzero horizontal angle or a nonzero inclination (e.g., a vertical angle of +10° or −10°). In  FIG. 2 , if the scan pattern  200  has a 60°×15° field of regard, then scan pattern  200  covers a ±30° horizontal range with respect to reference line  220  and a ±7.5° vertical range with respect to reference line  220 . Additionally, optical beam  125  in  FIG. 2  has an orientation of approximately −15° horizontal and +3° vertical with respect to reference line  220 . Optical beam  125  may be referred to as having an azimuth of −15° and an altitude of +3° relative to reference line  220 . In particular embodiments, an azimuth (which may be referred to as an azimuth angle) may represent a horizontal angle with respect to reference line  220 , and an altitude (which may be referred to as an altitude angle, elevation, or elevation angle) may represent a vertical angle with respect to reference line  220 . 
     In particular embodiments, a scan pattern  200  may include multiple pixels  210 , and each pixel  210  may be associated with one or more laser pulses or one or more distance measurements. Additionally, a scan pattern  200  may include multiple scan lines  230 , where each scan line represents one scan across at least part of a field of regard, and each scan line  230  may include multiple pixels  210 . In  FIG. 2 , scan line  230  includes five pixels  210  and corresponds to an approximately horizontal scan across the FOR from right to left, as viewed from the lidar system  100 . In particular embodiments, a cycle of scan pattern  200  may include a total of P x ×P y  pixels  210  (e.g., a two-dimensional distribution of P x  by P y  pixels). As an example, scan pattern  200  may include a distribution with dimensions of approximately 100-2,000 pixels  210  along a horizontal direction and approximately 4-400 pixels  210  along a vertical direction. As another example, scan pattern  200  may include a distribution of 1,000 pixels  210  along the horizontal direction by 64 pixels  210  along the vertical direction (e.g., the frame size is 1000×64 pixels) for a total of 64,000 pixels per cycle of scan pattern  200 . In particular embodiments, the number of pixels  210  along a horizontal direction may be referred to as a horizontal resolution of scan pattern  200 , and the number of pixels  210  along a vertical direction may be referred to as a vertical resolution. As an example, scan pattern  200  may have a horizontal resolution of greater than or equal to 100 pixels  210  and a vertical resolution of greater than or equal to 4 pixels  210 . As another example, scan pattern  200  may have a horizontal resolution of 100-2,000 pixels  210  and a vertical resolution of 4-400 pixels  210 . 
     In particular embodiments, a pixel  210  may refer to a data element that includes (i) distance information (e.g., a distance from a lidar system  100  to a target  130  from which an associated pulse of light was scattered) or (ii) an elevation angle and an azimuth angle associated with the pixel (e.g., the elevation and azimuth angles along which the associated pulse of light was emitted). Each pixel  210  may be associated with a distance (e.g., a distance to a portion of a target  130  from which an associated laser pulse was scattered) or one or more angular values. As an example, a pixel  210  may be associated with a distance value and two angular values (e.g., an azimuth and altitude) that represent the angular location of the pixel  210  with respect to the lidar system  100 . A distance to a portion of target  130  may be determined based at least in part on a time-of-flight measurement for a corresponding pulse. An angular value (e.g., an azimuth or altitude) may correspond to an angle (e.g., relative to reference line  220 ) of output beam  125  (e.g., when a corresponding pulse is emitted from lidar system  100 ) or an angle of input beam  135  (e.g., when an input signal is received by lidar system  100 ). In particular embodiments, an angular value may be determined based at least in part on a position of a component of scanner  120 . As an example, an azimuth or altitude value associated with a pixel  210  may be determined from an angular position of one or more corresponding scanning mirrors of scanner  120 . 
       FIG. 3  illustrates an example lidar system  100  with an example rotating polygon mirror  301 . In particular embodiments, a scanner  120  may include a polygon mirror  301  configured to scan output beam  125  along a first direction and a scanning mirror  302  configured to scan output beam  125  along a second direction different from the first direction (e.g., the first and second directions may be approximately orthogonal to one another, or the second direction may be oriented at any suitable nonzero angle with respect to the first direction). In the example of  FIG. 3 , scanner  120  includes two scanning mirrors: (1) a polygon mirror  301  that rotates along the Θ x  direction and (2) a scanning mirror  302  that oscillates back and forth along the Θ y  direction. The output beam  125  from light source  110 , which passes alongside mirror  115 , is reflected by reflecting surface  320  of scan mirror  302  and is then reflected by a reflecting surface (e.g., surface  320 A,  320 B,  320 C, or  320 D) of polygon mirror  301 . Scattered light from a target  130  returns to the lidar system  100  as input beam  135 . The input beam  135  reflects from polygon mirror  301 , scan mirror  302 , and mirror  115 , which directs input beam  135  through focusing lens  330  and to the detector  340  of receiver  140 . The detector  340  may be a PN photodiode, a PIN photodiode, an APD, a SPAD, or any other suitable detector. A reflecting surface  320  (which may be referred to as a reflective surface) may include a reflective metallic coating (e.g., gold, silver, or aluminum) or a reflective dielectric coating, and the reflecting surface  320  may have any suitable reflectivity R at an operating wavelength of the light source  110  (e.g., R greater than or equal to 70%, 80%, 90%, 95%, 98%, or 99%). 
     In particular embodiments, a polygon mirror  301  may be configured to rotate along a Θ x  or Θ y  direction and scan output beam  125  along a substantially horizontal or vertical direction, respectively. A rotation along a Θ x  direction may refer to a rotational motion of mirror  301  that results in output beam  125  scanning along a substantially horizontal direction. Similarly, a rotation along a Θ y  direction may refer to a rotational motion that results in output beam  125  scanning along a substantially vertical direction. In  FIG. 3 , mirror  301  is a polygon mirror that rotates along the Θ x  direction and scans output beam  125  along a substantially horizontal direction, and mirror  302  pivots along the Θ y  direction and scans output beam  125  along a substantially vertical direction. In particular embodiments, a polygon mirror  301  may be configured to scan output beam  125  along any suitable direction. As an example, a polygon mirror  301  may scan output beam  125  at any suitable angle with respect to a horizontal or vertical direction, such as for example, at an angle of approximately 0°, 10°, 20°, 30°, 45°, 60°, 70°, 80°, or 90° with respect to a horizontal or vertical direction. 
     In particular embodiments, a polygon mirror  301  may refer to a multi-sided object having reflective surfaces  320  on two or more of its sides or faces. As an example, a polygon mirror may include any suitable number of reflective faces (e.g., 2, 3, 4, 5, 6, 7, 8, or 10 faces), where each face includes a reflective surface  320 . A polygon mirror  301  may have a cross-sectional shape of any suitable polygon, such as for example, a triangle (with three reflecting surfaces  320 ), square (with four reflecting surfaces  320 ), pentagon (with five reflecting surfaces  320 ), hexagon (with six reflecting surfaces  320 ), heptagon (with seven reflecting surfaces  320 ), or octagon (with eight reflecting surfaces  320 ). In  FIG. 3 , the polygon mirror  301  has a substantially square cross-sectional shape and four reflecting surfaces ( 320 A,  320 B,  320 C, and  320 D). The polygon mirror  301  in  FIG. 3  may be referred to as a square mirror, a cube mirror, or a four-sided polygon mirror. In  FIG. 3 , the polygon mirror  301  may have a shape similar to a cube, cuboid, or rectangular prism. Additionally, the polygon mirror  301  may have a total of six sides, where four of the sides include faces with reflective surfaces ( 320 A,  320 B,  320 C, and  320 D). 
     In particular embodiments, a polygon mirror  301  may be continuously rotated in a clockwise or counter-clockwise rotation direction about a rotation axis of the polygon mirror  301 . The rotation axis may correspond to a line that is perpendicular to the plane of rotation of the polygon mirror  301  and that passes through the center of mass of the polygon mirror  301 . In  FIG. 3 , the polygon mirror  301  rotates in the plane of the drawing, and the rotation axis of the polygon mirror  301  is perpendicular to the plane of the drawing. An electric motor may be configured to rotate a polygon mirror  301  at a substantially fixed frequency (e.g., a rotational frequency of approximately 1 Hz (or 1 revolution per second), 10 Hz, 50 Hz, 100 Hz, 500 Hz, or 1,000 Hz). As an example, a polygon mirror  301  may be mechanically coupled to an electric motor (e.g., a synchronous electric motor) which is configured to spin the polygon mirror  301  at a rotational speed of approximately 160 Hz (or, 9600 revolutions per minute (RPM)). 
     In particular embodiments, output beam  125  may be reflected sequentially from the reflective surfaces  320 A,  320 B,  320 C, and  320 D as the polygon mirror  301  is rotated. This results in the output beam  125  being scanned along a particular scan axis (e.g., a horizontal or vertical scan axis) to produce a sequence of scan lines, where each scan line corresponds to a reflection of the output beam  125  from one of the reflective surfaces of the polygon mirror  301 . In  FIG. 3 , the output beam  125  reflects off of reflective surface  320 A to produce one scan line. Then, as the polygon mirror  301  rotates, the output beam  125  reflects off of reflective surfaces  320 B,  320 C, and  320 D to produce a second, third, and fourth respective scan line. In particular embodiments, a lidar system  100  may be configured so that the output beam  125  is first reflected from polygon mirror  301  and then from scan mirror  302  (or vice versa). As an example, an output beam  125  from light source  110  may first be directed to polygon mirror  301 , where it is reflected by a reflective surface of the polygon mirror  301 , and then the output beam  125  may be directed to scan mirror  302 , where it is reflected by reflective surface  320  of the scan mirror  302 . In the example of  FIG. 3 , the output beam  125  is reflected from the polygon mirror  301  and the scan mirror  302  in the reverse order. In  FIG. 3 , the output beam  125  from light source  110  is first directed to the scan mirror  302 , where it is reflected by reflective surface  320 , and then the output beam  125  is directed to the polygon mirror  301 , where it is reflected by reflective surface  320 A. 
       FIG. 4  illustrates an example light-source field of view (FOV L ) and receiver field of view (FOV R ) for a lidar system  100 . A light source  110  of lidar system  100  may emit pulses of light as the FOV L  and FOV R  are scanned by scanner  120  across a field of regard (FOR). In particular embodiments, a light-source field of view may refer to an angular cone illuminated by the light source  110  at a particular instant of time. Similarly, a receiver field of view may refer to an angular cone over which the receiver  140  may receive or detect light at a particular instant of time, and any light outside the receiver field of view may not be received or detected. As an example, as the light-source field of view is scanned across a field of regard, a portion of a pulse of light emitted by the light source  110  may be sent downrange from lidar system  100 , and the pulse of light may be sent in the direction that the FOV L  is pointing at the time the pulse is emitted. The pulse of light may scatter off a target  130 , and the receiver  140  may receive and detect a portion of the scattered light that is directed along or contained within the FOV R . 
     In particular embodiments, scanner  120  may be configured to scan both a light-source field of view and a receiver field of view across a field of regard of the lidar system  100 . Multiple pulses of light may be emitted and detected as the scanner  120  scans the FOV L  and FOV R  across the field of regard of the lidar system  100  while tracing out a scan pattern  200 . In particular embodiments, the light-source field of view and the receiver field of view may be scanned synchronously with respect to one another, so that as the FOV L  is scanned across a scan pattern  200 , the FOV R  follows substantially the same path at the same scanning speed. Additionally, the FOV L  and FOV R  may maintain the same relative position to one another as they are scanned across the field of regard. As an example, the FOV L  may be substantially overlapped with or centered inside the FOV R  (as illustrated in  FIG. 4 ), and this relative positioning between FOV L  and FOV R  may be maintained throughout a scan. As another example, the FOV R  may lag behind the FOV L  by a particular, fixed amount throughout a scan (e.g., the FOV R  may be offset from the FOV L  in a direction opposite the scan direction). 
     In particular embodiments, the FOV L  may have an angular size or extent Θ L  that is substantially the same as or that corresponds to the divergence of the output beam  125 , and the FOV R  may have an angular size or extent Θ R  that corresponds to an angle over which the receiver  140  may receive and detect light. In particular embodiments, the receiver field of view may be any suitable size relative to the light-source field of view. As an example, the receiver field of view may be smaller than, substantially the same size as, or larger than the angular extent of the light-source field of view. In particular embodiments, the light-source field of view may have an angular extent of less than or equal to 50 milliradians, and the receiver field of view may have an angular extent of less than or equal to 50 milliradians. The FOV L  may have any suitable angular extent Θ L , such as for example, approximately 0.1 mrad, 0.2 mrad, 0.5 mrad, 1 mrad, 1.5 mrad, 2 mrad, 3 mrad, 5 mrad, 10 mrad, 20 mrad, 40 mrad, or 50 mrad. Similarly, the FOV R  may have any suitable angular extent OR, such as for example, approximately 0.1 mrad, 0.2 mrad, 0.5 mrad, 1 mrad, 1.5 mrad, 2 mrad, 3 mrad, 5 mrad, 10 mrad, 20 mrad, 40 mrad, or 50 mrad. In particular embodiments, the light-source field of view and the receiver field of view may have approximately equal angular extents. As an example, Θ L  and Θ R  may both be approximately equal to 1 mrad, 2 mrad, or 4 mrad. In particular embodiments, the receiver field of view may be larger than the light-source field of view, or the light-source field of view may be larger than the receiver field of view. As an example, Θ L  may be approximately equal to 3 mrad, and Θ R  may be approximately equal to 4 mrad. As another example, Θ R  may be approximately L times larger than Θ L , where L is any suitable factor, such as for example, 1.1, 1.2, 1.5, 2, 3, 5, or 10. 
     In particular embodiments, a pixel  210  may represent or may correspond to a light-source field of view or a receiver field of view. As the output beam  125  propagates from the light source  110 , the diameter of the output beam  125  (as well as the size of the corresponding pixel  210 ) may increase according to the beam divergence Θ L . As an example, if the output beam  125  has a Θ L  of 2 mrad, then at a distance of 100 m from the lidar system  100 , the output beam  125  may have a size or diameter of approximately 20 cm, and a corresponding pixel  210  may also have a corresponding size or diameter of approximately 20 cm. At a distance of 200 m from the lidar system  100 , the output beam  125  and the corresponding pixel  210  may each have a diameter of approximately 40 cm. 
       FIG. 5  illustrates an example unidirectional scan pattern  200  that includes multiple pixels  210  and multiple scan lines  230 . In particular embodiments, scan pattern  200  may include any suitable number of scan lines  230  (e.g., approximately 1, 2, 5, 10, 20, 50, 100, 500, or 1,000 scan lines), and each scan line  230  of a scan pattern  200  may include any suitable number of pixels  210  (e.g., 1, 2, 5, 10, 20, 50, 100, 200, 500, 1,000, 2,000, or 5,000 pixels). The scan pattern  200  illustrated in  FIG. 5  includes eight scan lines  230 , and each scan line  230  includes approximately 16 pixels  210 . In particular embodiments, a scan pattern  200  where the scan lines  230  are scanned in two directions (e.g., alternately scanning from right to left and then from left to right) may be referred to as a bidirectional scan pattern  200 , and a scan pattern  200  where the scan lines  230  are scanned in the same direction may be referred to as a unidirectional scan pattern  200 . The scan pattern  200  in  FIG. 2  may be referred to as a bidirectional scan pattern, and the scan pattern  200  in  FIG. 5  may be referred to as a unidirectional scan pattern  200  where each scan line  230  travels across the FOR in substantially the same direction (e.g., approximately from left to right as viewed from the lidar system  100 ). In particular embodiments, scan lines  230  of a unidirectional scan pattern  200  may be directed across a FOR in any suitable direction, such as for example, from left to right, from right to left, from top to bottom, from bottom to top, or at any suitable angle (e.g., at a 0°, 5°, 10°, 30°, or 45° angle) with respect to a horizontal or vertical axis. In particular embodiments, each scan line  230  in a unidirectional scan pattern  200  may be a separate line that is not directly connected to a previous or subsequent scan line  230 . 
     In particular embodiments, a unidirectional scan pattern  200  may be produced by a scanner  120  that includes a polygon mirror (e.g., polygon mirror  301  of  FIG. 3 ), where each scan line  230  is associated with a particular reflective surface  320  of the polygon mirror. As an example, reflective surface  320 A of polygon mirror  301  in  FIG. 3  may produce scan line  230 A in  FIG. 5 . Similarly, as the polygon mirror  301  rotates, reflective surfaces  320 B,  320 C, and  320 D may successively produce scan lines  230 B,  230 C, and  230 D, respectively. Additionally, for a subsequent revolution of the polygon mirror  301 , the scan lines  230 A′,  230 B′,  230 C′, and  230 D′ may be successively produced by reflections of the output beam  125  from reflective surfaces  320 A,  320 B,  320 C, and  320 D, respectively. In particular embodiments, N successive scan lines  230  of a unidirectional scan pattern  200  may correspond to one full revolution of a N-sided polygon mirror. As an example, the four scan lines  230 A,  230 B,  230 C, and  230 D in  FIG. 5  may correspond to one full revolution of the four-sided polygon mirror  301  in  FIG. 3 . Additionally, a subsequent revolution of the polygon mirror  3   o   01  may produce the next four scan lines  230 A′,  230 B′,  230 C′, and  230 D′ in  FIG. 5 . 
       FIG. 6  illustrates an example lidar system  100  with a light source  110  that emits pulses of light  400  and local-oscillator (LO) light  430 . The lidar system  100  in  FIG. 6  includes a light source  110 , a scanner  120 , a receiver  140 , and a controller  150 . The receiver  140  includes a detector  340 , an amplifier  350 , a pulse-detection circuit  365 , and a frequency-detection circuit  600 . The lidar system  100  illustrated in  FIG. 6  may be referred to as a coherent pulsed lidar system in which the light source  110  emits LO light  430  and pulses of light  400 , where each emitted pulse of light  400  is coherent with a corresponding portion of the LO light  430 . Additionally, the receiver  140  in a coherent pulsed lidar system may be configured to detect the LO light  430  and a received pulse of light  410 , where the LO light  430  and the received pulse of light  410  (which includes scattered light from one of the emitted pulses of light  400 ) are coherently mixed together at the receiver  140 . The LO light  430  may be referred to as a local-oscillator optical signal or a LO optical signal. 
     In particular embodiments, a coherent pulsed lidar system  100  may include a light source  110  configured to emit pulses of light  400  and LO light  430 . The emitted pulses of light  400  may be part of an output beam  125  that is scanned by a scanner  120  across a field of regard of the lidar system  100 , and the LO light  430  may be sent to a receiver  140  of the lidar system  100 . The light source  110  may include a seed laser that produces seed light and the LO light  430 . Additionally, the light source  110  may include an optical amplifier that amplifies the seed light to produce the emitted pulses of light  400 . For example, the optical amplifier may be a pulsed optical amplifier that amplifies temporal portions of the seed light to produce the emitted pulses of light  400 , where each amplified temporal portion of the seed light corresponds to one of the emitted pulses of light  400 . The pulses of light  400  emitted by the light source  110  may have one or more of the following optical characteristics: a wavelength between 900 nm and 1700 nm; a pulse energy between 0.01 μJ and 100 μJ; a pulse repetition frequency between 80 kHz and 10 MHz; and a pulse duration between 0.1 ns and 20 ns. For example, the light source  110  may emit pulses of light  400  with a wavelength of approximately 1550 nm, a pulse energy of approximately 0.5 μJ, a pulse repetition frequency of approximately 750 kHz, and a pulse duration of approximately 5 ns. As another example, the light source  110  may emit pulses of light with a wavelength from approximately 1500 nm to approximately 1510 nm. 
     In particular embodiments, a coherent pulsed lidar system  100  may include a scanner  120  configured to scan an output beam  125  across a field of regard of the lidar system  100 . The scanner  120  may receive the output beam  125  (which includes the emitted pulses of light  400 ) from the light source  110 , and the scanner  120  may include one or more scanning mirrors configured to scan the output beam  125 . In addition to scanning the output beam  125 , the scanner may also scan a FOV of the detector  340  across the field of regard so that the output beam  125  and the detector FOV are scanned synchronously at the same scanning speed or with the same relative position to one another. Alternatively, the lidar system  100  may be configured so that only the output beam  125  is scanned, and the detector has a static FOV that is not scanned. In this case, the input beam  135  (which includes received pulses of light  410 ) may bypass the scanner  120  and be directed to the receiver  140  without passing through the scanner  120 . 
     In particular embodiments, a coherent pulsed lidar system  100  may include an optical combiner  420  configured to optically combine LO light  430  with a received pulse of light  410 . Optically combining LO light  430  with a received pulse of light  410  (which is part of the input beam  135 ) may include spatially overlapping the LO light  430  with the input beam  135  to produce a combined beam  422 . The combined beam  422  may include light from the LO light  430  and the input beam  135  combined together so that the two beams propagate coaxially along the same path. For example, the combiner  420  in  FIG. 6  may be a free-space optical beam-splitter that reflects at least part of the LO light  430  and transmits at least part of the input beam  135  so that the LO light  430  and the input beam  135  are spatially overlapped and propagate coaxially to the detector  340 . As another example, the combiner  420  in  FIG. 6  may be a mirror that reflects the LO light  430  and directs it to the detector  340 , where it is combined with the input beam  135 . As another example, a combiner  420  may include an optical-waveguide component or a fiber-optic component that spatially overlaps the LO light  430  and the input beam  135  so that the LO light  430  and the input beam  135  propagate together in a waveguide or in a core of an optical fiber. 
     In particular embodiments, a coherent pulsed lidar system  100  may include a receiver  140  that detects LO light  430  and received pulses of light  410 . A received pulse of light  410  may include light from one of the emitted pulses of light  400  that is scattered by a target  130  located a distance from the lidar system  100 . The receiver  140  may include one or more detectors  340 , and the LO light  430  and a received pulse of light  410  may be coherently mixed together at one or more of the detectors  340 . One or more of the detectors  340  may produce photocurrent signals that correspond to the coherent mixing of the LO light  430  and the received pulse of light  410 . The lidar system  100  in  FIG. 6  includes a receiver  140  with one detector  340  that receives the LO light  430  and the pulse of light  410 , which are coherently mixed together at the detector  340 . In response to the coherent mixing of the received LO light  430  and pulse of light  410 , the detector  340  produces a photocurrent signal i that is amplified by an electronic amplifier  350 . 
     In particular embodiments, a receiver  140  may include a pulse-detection circuit  365  that determines a time-of-arrival for a received pulse of light  410 . The time-of-arrival for a received pulse of light  410  may correspond to a time associated with a rising edge, falling edge, peak, or temporal center of the received pulse of light  410 . The time-of-arrival may be determined based at least in part on a photocurrent signal i produced by a detector  340  of the receiver  140 . For example, a photocurrent signal i may include a pulse of current corresponding to the received pulse of light  410 , and the electronic amplifier  350  may produce a voltage signal  360  with a voltage pulse that corresponds to the pulse of current. The pulse-detection circuit  365  may determine the time-of-arrival for the received pulse of light  410  based on a characteristic of the voltage pulse (e.g., based on a time associated with a rising edge, falling edge, peak, or temporal center of the voltage pulse). For example, the pulse-detection circuit  365  may receive an electronic trigger signal (e.g., from the light source  110  or the controller  150 ) when a pulse of light  400  is emitted, and the pulse-detection circuit  365  may determine the time-of-arrival for the received pulse of light  410  based on a time associated with an edge, peak, or temporal center of the voltage signal  360 . The time-of-arrival may be determined based on a difference between a time when the pulse  400  is emitted and a time when the received pulse  410  is detected. 
     In particular embodiments, a coherent pulsed lidar system  100  may include a processor (e.g., controller  150 ) that determines the distance to a target  130  based at least in part on a time-of-arrival for a received pulse of light  410 . The time-of-arrival for the received pulse of light  410  may correspond to a round-trip time (ΔT) for at least a portion of an emitted pulse of light  400  to travel to the target  130  and back to the lidar system  100 , where the portion of the emitted pulse of light  400  that travels back to the target  130  corresponds to the received pulse of light  410 . The distance D to the target  130  may be determined from the expression D=c·ΔT/2. For example, if the pulse-detection circuit  365  determines that the time ΔT between emission of optical pulse  400  and receipt of optical pulse  410  is 1 μs, then the controller  150  may determine that the distance to the target  130  is approximately 150 m. In particular embodiments, a round-trip time may be determined by a receiver  140 , by a controller  150 , or by a receiver  140  and controller  150  together. For example, a receiver  140  may determine a round-trip time by subtracting a time when a pulse  400  is emitted from a time when a received pulse  410  is detected. As another example, a receiver  140  may determine a time when a pulse  400  is emitted and a time when a received pulse  410  is detected. These values may be sent to a controller  150 , and the controller  150  may determine a round-trip time by subtracting the time when the pulse  400  is emitted from the time when the received pulse  410  is detected. 
     In particular embodiments, a controller  150  of a lidar system  100  may be coupled to one or more components of the lidar system  100  via one or more data links  425 . Each link  425  in  FIG. 6  represents a data link that couples the controller  150  to another component of the lidar system  100  (e.g., light source  110 , scanner  120 , receiver  140 , pulse-detection circuit  365 , or frequency-detection circuit  600 ). Each data link  425  may include one or more electrical links, one or more wireless links, or one or more optical links, and the data links  425  may be used to send data, signals, or commands to or from the controller  150 . For example, the controller  150  may send a command via a link  425  to the light source  110  instructing the light source  110  to emit a pulse of light  400 . As another example, the pulse-detection circuit  365  may send a signal via a link  425  to the controller with information about a received pulse of light  410  (e.g., a time-of-arrival for the received pulse of light  410 ). Additionally, the controller  150  may be coupled via a link (not illustrated in  FIG. 6 ) to a processor of an autonomous-vehicle driving system. The autonomous-vehicle processor may receive point-cloud data from the controller  150  and may make driving decisions based on the received point-cloud data. 
       FIG. 7  illustrates an example receiver  140  and an example voltage signal  360  corresponding to a received pulse of light  410 . A light source  110  of a lidar system  100  may emit a pulse of light  400 , and a receiver  140  may be configured to detect a combined beam  422 . The combined beam  422  in  FIG. 7  includes LO light  430  and input light  135 , where the input light  135  includes one or more received pulses of light  410 . In particular embodiments, a receiver  140  of a lidar system  100  may include one or more detectors  340 , one or more amplifiers  350 , one or more pulse-detection circuits  365 , or one or more frequency-detection circuits  600 . A pulse-detection circuit  365  may include one or more comparators  370  or one or more time-to-digital converters (TDCs)  380 . A frequency-detection circuit  600  may include one or more electronic filters  610  or one or more electronic amplitude detectors  620 . 
     The receiver  140  illustrated in  FIG. 7  includes a detector  340  configured to receive a combined beam  422  and produce a photocurrent i that corresponds to the coherent mixing of the LO light  430  a received pulse of light  410  (which is part of the input light  135 ). The photocurrent i produced by the detector  340  may be referred to as a photocurrent signal or an electrical-current signal. The detector  340  may include an APD, PN photodiode, or PIN photodiode. For example, the detector  340  may include a silicon APD or PIN photodiode configured to detect light at an 800-1100 nm operating wavelength of a lidar system  100 , or the detector  340  may include an InGaAs APD or PIN photodiode configured to detect light at a 1200-1600 nm operating wavelength. In  FIG. 7 , the detector  340  is coupled to an electronic amplifier  350  configured to receive the photocurrent i and produce a voltage signal  360  that corresponds to the received photocurrent. For example, the detector  340  may be an APD that produces a pulse of photocurrent in response to coherent mixing of LO light  430  and a received pulse of light  410 , and the voltage signal  360  may be an analog voltage pulse that corresponds to the pulse of photocurrent. The amplifier  350  may include a transimpedance amplifier configured to receive the photocurrent i and amplify the photocurrent to produce a voltage signal that corresponds to the photocurrent signal. Additionally, the amplifier  350  may include a voltage amplifier that amplifies the voltage signal or an electronic filter (e.g., a low-pass or high-pass filter) that filters the photocurrent or the voltage signal. 
     In  FIG. 7 , the voltage signal  360  produced by the amplifier  350  is coupled to a pulse-detection circuit  365  and a frequency-detection circuit  600 . The pulse-detection circuit includes N comparators (comparators  370 - 1 ,  370 - 2 , . . . ,  370 -N), and each comparator is supplied with a particular threshold or reference voltage (V T1 , V T2 , . . . , V TN ). For example, receiver  140  may include N=10 comparators, and the threshold voltages may be set to 10 values between 0 volts and 1 volt (e.g., V T1 =0.1 V, V T2 =0.2 V, and V T10 =1.0 V). A comparator may produce an electrical-edge signal (e.g., a rising or falling electrical edge) when the voltage signal  360  rises above or falls below a particular threshold voltage. For example, comparator  370 - 2  may produce a rising edge when the voltage signal  360  rises above the threshold voltage V T2 . Additionally or alternatively, comparator  370 - 2  may produce a falling edge when the voltage signal  360  falls below the threshold voltage V T2 . 
     The pulse-detection circuit  365  in  FIG. 7  includes N time-to-digital converters (TDCs  380 - 1 ,  380 - 2 , . . . ,  380 -N), and each comparator is coupled to one of the TDCs. Each comparator-TDC pair in  FIG. 7  (e.g., comparator  370 - 1  and TDC  380 - 1 ) may be referred to as a threshold detector. A comparator may provide an electrical-edge signal to a corresponding TDC, and the TDC may act as a timer that produces an electrical output signal (e.g., a digital signal, a digital word, or a digital value) that represents a time when the edge signal is received from the comparator. For example, if the voltage signal  360  rises above the threshold voltage V T1 , then the comparator  370 - 1  may produce a rising-edge signal that is supplied to the input of TDC  380 - 1 , and the TDC  380 - 1  may produce a digital time value corresponding to a time when the edge signal was received by TDC  380 - 1 . The digital time value may be referenced to the time when a pulse of light is emitted, and the digital time value may correspond to or may be used to determine a round-trip time for the pulse of light to travel to a target  130  and back to the lidar system  100 . Additionally, if the voltage signal  360  subsequently falls below the threshold voltage V T1 , then the comparator  370 - 1  may produce a falling-edge signal that is supplied to the input of TDC  380 - 1 , and the TDC  380 - 1  may produce a digital time value corresponding to a time when the edge signal was received by TDC  380 - 1 . 
     In particular embodiments, a pulse-detection output signal may be an electrical signal that corresponds to a received pulse of light  410 . For example, the pulse-detection output signal in  FIG. 7  may be a digital signal that corresponds to the analog voltage signal  360 , which in turn corresponds to the photocurrent signal i, which in turn corresponds to a received pulse of light  410 . If an input light signal  135  includes a received pulse of light  410 , the pulse-detection circuit  365  may receive a voltage signal  360  (corresponding to the photocurrent i) and produce a pulse-detection output signal that corresponds to the received pulse of light  410 . The pulse-detection output signal may include one or more digital time values from each of the TDCs  380  that received one or more edge signals from a comparator  370 , and the digital time values may represent the analog voltage signal  360 . The pulse-detection output signal may be sent to a controller  150 , and a time-of-arrival for the received pulse of light  410  may be determined based at least in part on the one or more time values produced by the TDCs. For example, the time-of-arrival may be determined from a time associated with the peak (e.g., V peak ) of the voltage signal  360  or from a temporal center of the voltage signal  360 . Alternatively, the time-of-arrival may be determined from a time associated with a rising edge of the voltage signal  360 . The pulse-detection output signal in  FIG. 7  may correspond to the electrical output signal  145  in  FIG. 1 . 
     In particular embodiments, a pulse-detection output signal may include one or more digital values that correspond to a time interval between (1) a time when a pulse of light  400  is emitted and (2) a time when a received pulse of light  410  is detected by a receiver  140 . The pulse-detection output signal in  FIG. 7  may include digital values from each of the TDCs that receive an edge signal from a comparator, and each digital value may represent a time interval between the emission of an optical pulse  400  by a light source  110  and the receipt of an edge signal from a comparator. For example, a light source  110  may emit a pulse of light  400  that is scattered by a target  130 , and a receiver  140  may receive a portion of the scattered pulse of light as an input pulse of light  410 . When the light source emits the pulse of light  400 , a count value of the TDCs may be reset to zero counts. Alternatively, the TDCs in receiver  140  may accumulate counts continuously over two or more pulse periods (e.g., for 10, 100, 1,000, 10,000, or 100,000 pulse periods), and when a pulse of light  400  is emitted, the current TDC count may be stored in memory. After the pulse of light  400  is emitted, the TDCs may accumulate counts that correspond to elapsed time (e.g., the TDCs may count in terms of clock cycles or some fraction of clock cycles). 
     In  FIG. 7 , when TDC  380 - 1  receives an edge signal from comparator  370 - 1 , the TDC  380 - 1  may produce a digital signal that represents the time interval between emission of the pulse of light  400  and receipt of the edge signal. For example, the digital signal may include a digital value that corresponds to the number of clock cycles that elapsed between emission of the pulse of light  400  and receipt of the edge signal. Alternatively, if the TDC  380 - 1  accumulates counts over multiple pulse periods, then the digital signal may include a digital value that corresponds to the TDC count at the time of receipt of the edge signal. The pulse-detection output signal may include digital values corresponding to one or more times when a pulse of light  400  was emitted and one or more times when a TDC received an edge signal. A pulse-detection output signal from a pulse-detection circuit  365  may correspond to a received pulse of light  410  and may include digital values from each of the TDCs that receive an edge signal from a comparator. The pulse-detection output signal may be sent to a controller  150 , and the controller may determine the distance to the target  130  based at least in part on the pulse-detection output signal. Additionally or alternatively, the controller  150  may determine an optical characteristic of a received pulse of light  410  based at least in part on the pulse-detection output signal received from the TDCs of a pulse-detection circuit  365 . 
     In particular embodiments, a receiver  140  of a lidar system  100  may include one or more analog-to-digital converters (ADCs). As an example, instead of including multiple comparators and TDCs, a receiver  140  may include an ADC that receives a voltage signal  360  from amplifier  350  and produces a digital representation of the voltage signal  360 . Although this disclosure describes or illustrates example receivers  140  that include one or more comparators  370  and one or more TDCs  380 , a receiver  140  may additionally or alternatively include one or more ADCs. As an example, in  FIG. 7 , instead of the N comparators  370  and N TDCs  380 , the receiver  140  may include an ADC configured to receive the voltage signal  360  and produce a digital output signal that includes digitized values that correspond to the voltage signal  360 . 
     The example voltage signal  360  illustrated in  FIG. 7  corresponds to a received pulse of light  410 . The voltage signal  360  may be an analog signal produced by an electronic amplifier  350  and may correspond to a pulse of light detected by the receiver  140  in  FIG. 7 . The voltage levels on the y-axis correspond to the threshold voltages V T1 , V T2 , . . . , V TN  of the respective comparators  370 - 1 ,  370 - 2 , . . . ,  370 -N. The time values t 1 , t 2 , t 3 , . . . , t N-1  correspond to times when the voltage signal  360  exceeds the corresponding threshold voltages, and the time values t′ 1 , t′ 2 , t′ 3 , . . . , t N-1  correspond to times when the voltage signal  360  falls below the corresponding threshold voltages. For example, at time t 1  when the voltage signal  360  exceeds the threshold voltage V T1 , comparator  370 - 1  may produce an edge signal, and TDC  380 - 1  may output a digital value corresponding to the time t 1 . Additionally, the TDC  380 - 1  may output a digital value corresponding to the time t′ 1  when the voltage signal  360  falls below the threshold voltage V T1 . Alternatively, the receiver  140  may include an additional TDC (not illustrated in  FIG. 7 ) configured to produce a digital value corresponding to time t′ 1  when the voltage signal  360  falls below the threshold voltage V T1 . The pulse-detection output signal from pulse-detection circuit  365  may include one or more digital values that correspond to one or more of the time values t 1 , t 2 , t 3 , . . . , t N-1  and t′ 1 , t′ 2 , t′ 3 , . . . , t′ N-1 . Additionally, the pulse-detection output signal may also include one or more values corresponding to the threshold voltages associated with the time values. Since the voltage signal  360  in  FIG. 7  does not exceed the threshold voltage V TN , the corresponding comparator  370 -N may not produce an edge signal. As a result, TDC  380 -N may not produce a time value, or TDC  380 -N may produce a signal indicating that no edge signal was received. 
     In particular embodiments, a pulse-detection output signal produced by a pulse-detection circuit  365  of a receiver  140  may correspond to or may be used to determine an optical characteristic of a received pulse of light  410  detected by the receiver  140 . An optical characteristic of a received pulse of light  410  may correspond to a peak optical intensity, a peak optical power, an average optical power, an optical energy, a shape or amplitude, a temporal duration, or a temporal center of the received pulse of light  410 . For example, a pulse of light  410  detected by receiver  140  may have one or more of the following optical characteristics: a peak optical power between 1 nanowatt and 10 watts; a pulse energy between 1 attojoule and 10 nanojoules; and a pulse duration between 0.1 ns and 50 ns. In particular embodiments, an optical characteristic of a received pulse of light  410  may be determined from a pulse-detection output signal provided by one or more TDCs  380  of a pulse-detection circuit  365  (e.g., as illustrated in  FIG. 7 ), or an optical characteristic may be determined from a pulse-detection output signal provided by one or more ADCs of a pulse-detection circuit  365 . 
     In particular embodiments, a peak optical power or peak optical intensity of a received pulse of light  410  may be determined from one or more values of a pulse-detection output signal provided by a receiver  140 . As an example, a controller  150  may determine the peak optical power of a received pulse of light  410  based on a peak voltage (V peak ) of the voltage signal  360 . The controller  150  may use a formula or lookup table that correlates a peak voltage of the voltage signal  360  with a value for the peak optical power. In the example of  FIG. 7 , the peak optical power of a pulse of light  410  may be determined from the threshold voltage V T(N-1) , which is approximately equal to the peak voltage V peak  of the voltage signal  360  (e.g., the threshold voltage V T(N-1)  may be associated with a pulse of light  410  having a peak optical power of 10 mW). As another example, a controller  150  may apply a curve-fit or interpolation operation to the values of a pulse-detection output signal to determine the peak voltage of the voltage signal  360 , and this peak voltage may be used to determine the corresponding peak optical power of a received pulse of light  410 . 
     In particular embodiments, an energy of a received pulse of light  410  may be determined from one or more values of a pulse-detection output signal. For example, a controller  150  may perform a summation of digital values that correspond to a voltage signal  360  to determine an area under the voltage-signal curve, and the area under the voltage-signal curve may be correlated with a pulse energy of a received pulse of light  410 . As an example, the approximate area under the voltage-signal curve in  FIG. 7  may be determined by subdividing the curve into M subsections (where M is approximately the number of time values included in the pulse-detection output signal) and adding up the areas of each of the subsections (e.g., using a numerical integration technique such as a Riemann sum, trapezoidal rule, or Simpson&#39;s rule). For example, the approximate area A under the voltage-signal curve  360  in  FIG. 7  may be determined from a Riemann sum using the expression A=Σ k=1   M V Tk ×Δt k , where V Tk  is a threshold voltage associated with the time value t k , and Δt k  is a width of the subsection associated with time value t k . In the example of  FIG. 7 , the voltage signal  360  may correspond to a received pulse of light  410  with a pulse energy of 1 picojoule. 
     In particular embodiments, a duration of a received pulse of light  410  may be determined from a duration or width of a corresponding voltage signal  360 . For example, the difference between two time values of a pulse-detection output signal may be used to determine a duration of a received pulse of light  410 . In the example of  FIG. 7 , the duration of the pulse of light  410  corresponding to voltage signal  360  may be determined from the difference (t′ 3 −t 3 ), which may correspond to a received pulse of light  410  with a pulse duration of 4 nanoseconds. As another example, a controller  150  may apply a curve-fit or interpolation operation to the values of the pulse-detection output signal, and the duration of the pulse of light  410  may be determined based on the curve-fit or interpolation. One or more of the approaches for determining an optical characteristic of a received pulse of light  410  as described herein may be implemented using a receiver  140  that includes multiple comparators  370  and TDCs  380  (as illustrated in  FIG. 7 ) or using a receiver  140  that includes one or more ADCs. 
     In  FIG. 7 , the voltage signal  360  produced by amplifier  350  is coupled to a frequency-detection circuit  600  as well as a pulse-detection circuit  365 . The pulse-detection circuit  365  may provide a pulse-detection output signal that is used to determine time-domain information for a received pulse of light  410  (e.g., a time-of-arrival, duration, or energy of the received pulse of light  410 ), and the frequency-detection circuit  600  may provide frequency-domain information for the received pulse of light  410 . For example, the frequency-detection output signal of the frequency-detection circuit  600  may include amplitude information for particular frequency components of the received pulse of light  410 . The frequency-detection output signal may include the amplitude of one or more frequency components of a received pulse of light  410 , and this amplitude information may be sent to a controller  150  for further processing. For example, the controller  150  may determine, based at least in part on the amplitude information, whether a received pulse of light is a valid received pulse of light  410  or an interfering pulse of light. 
     In particular embodiments, a frequency-detection circuit  600  may include multiple parallel frequency-measurement channels, and each frequency-measurement channel may include a filter  610  and a corresponding amplitude detector  620 . In  FIG. 7 , the frequency-detection circuit  600  includes M electronic filters (filters  610 - 1 ,  610 - 2 , . . . ,  610 -M), where each filter is associated with a particular frequency component (frequencies f a , f b , . . . , f M ). Each filter  610  in  FIG. 7  may include an electronic band-pass filter having a particular pass-band center frequency and width. For example, filter  610 - 2  may be a band-pass filter with a center frequency f b  of 1 GHz and a pass-band width of 20 MHz. Each filter  610  may include a passive filter implemented with one or more passive electronic components (e.g., one or more resistors, inductors, or capacitors). Alternatively, each filter  610  may include an active filter that includes one or more active electronic components (e.g., one or more transistors or op-amps) along with one or more passive components. 
     In addition to the M electronic filters  610 , the frequency-detection circuit  600  in  FIG. 7  also includes M electronic amplitude detectors (amplitude detectors  620 - 1 ,  620 - 2 , . . . ,  620 -M). An amplitude detector  620  may be configured to provide an output signal that corresponds to an amplitude (e.g., a peak value, a size, or an energy) of an electrical signal received from a filter  610 . For example, filter  610 -M may receive voltage signal  360  and provide to amplitude detector  620 -M the portion of the voltage signal  360  having a frequency component at or near the frequency f M . The amplitude detector  620 -M may produce a digital or analog output signal that corresponds to the amplitude, peak value, size, or energy of the signal associated with the frequency component f M . Each amplitude detector  620  may include a sample-and-hold circuit, a peak-detector circuit, an integrator circuit, or an ADC. For example, amplitude detector  620 -M may include a sample-and-hold circuit and an ADC. The sample-and-hold circuit may produce an analog voltage corresponding to the amplitude of a signal received from filter  610 -M, and the ADC may produce a digital signal that represents the analog voltage. 
     A frequency-detection circuit  600  may include 1, 2, 4, 8, 10, 20, or any other suitable number of filters  610  and amplitude detectors  620 , and each filter may have a center frequency between approximately 200 MHz and approximately 20 GHz. Additionally, each filter  610  may include a band-pass filter having a pass-band with a frequency width of approximately 1 MHz, 10 MHz, 20 MHz, 50 MHz, 100 MHz, 200 MHz, or any other suitable frequency width. For example, a frequency-detection circuit  600  may include four band-pass filters  610  with center frequencies of approximately 1.0 GHz, 1.1 GHz, 1.2 GHz, and 1.3 GHz, and each filter may have a pass-band with a frequency width of approximately 20 MHz. A 1.0-GHz filter with a 20-MHz pass-band may pass or transmit frequency components from approximately 0.99 GHz to approximately 1.01 GHz and may attenuate frequency components outside of that frequency range. 
     In particular embodiments, a light source  110  of a lidar system  100  may impart a particular spectral signature to an emitted pulse of light  400 . A spectral signature (which may be referred to as a frequency signature, frequency tag, or frequency change) may correspond to the presence or absence of particular frequency components that are imparted to an emitted pulse of light  400 . Additionally or alternatively, a spectral signature may include an amplitude modulation, frequency modulation, or frequency change applied to an emitted pulse of light  400 . For example, a spectral signature may include an amplitude or frequency modulation at a particular frequency (e.g., 1 GHz) that is applied to an emitted pulse of light  400 . As another example, a spectral signature may include an amplitude or frequency modulation at two or more particular frequencies (e.g., 1.6 GHz and 2.0 GHz) that is applied to an emitted pulse of light  400 . A received pulse of light  410  may include the same spectral signature that was applied to an associated emitted pulse of light  400 , and the photocurrent signal i (as well as the corresponding voltage signal  360 ) may include one or more frequency components that correspond to the spectral signature. A frequency-detection circuit  600  may determine, based on the voltage signal  360  (which corresponds to the photocurrent signal i), one or more amplitudes of the one or more frequency components. In the example of  FIG. 7 , the frequency-detection circuit  600  may include M band-pass filters  610  and M amplitude detectors  620 . Each band-pass filter  610  may have a center frequency corresponding to one of the frequency components (from f a  to f M ), and each amplitude detector  620  may produce a signal corresponding to the amplitude of one of the respective frequency components. The frequency-detection output signal produced by the frequency-detection circuit  600  may include M digital values corresponding to the amplitudes of the M frequency components. 
     In particular embodiments, a controller  150  may determine, based on the amplitudes of one or more frequency components associated with a received pulse of light  410 , whether the received pulse of light  410  is associated with a particular emitted pulse of light  400 . If one or more frequency components of a received pulse of light  410  match a spectral signature of a particular emitted pulse of light  400 , then the controller  150  may determine that the received pulse of light  410  is associated with the particular emitted pulse of light  400  (e.g., the received pulse of light  410  includes scattered light from the emitted pulse of light  400 ). Otherwise, if the frequency components do not match, then the controller  150  may determine that the received pulse of light  410  is not associated with the particular emitted pulse of light  400 . For example, the received pulse of light  410  may be associated with a different pulse of light  400  emitted by the light source  110  of the lidar system  100 , or the received pulse of light  410  may be associated with an interfering optical signal emitted by a different light source external to the lidar system  100 . As another example, a particular pulse of light  400  emitted by the light source  110  may include a spectral signature with an amplitude modulation at a particular frequency (e.g., 2 GHz), and a frequency-detection circuit  600  may include a filter  610  and amplitude detector  620  that determine the amplitude of a 2-GHz frequency component for a received pulse of light  410 . If the amplitude of the 2-GHz frequency component is greater than a particular threshold value (or within a range of two particular threshold values), then the controller  150  may determine that the received pulse of light  410  is associated with and includes light from the particular emitted pulse of light  400 . Otherwise, if the amplitude of the 2-GHz frequency component is less than the particular threshold value, then the controller  150  may determine that the received pulse of light  410  is not associated with and does not include light from the particular emitted pulse of light  400 . Additionally or alternatively, if the amplitude of a different frequency component (e.g., a 1.8-GHz frequency component) that is not part of a particular spectral signature is greater than a particular threshold value, then the controller may determine that the received pulse of light  400  is not associated with the emitted pulse of light  400  having that particular spectral signature. 
     In particular embodiments, the amplitudes of the one or more frequency components associated with a received pulse of light  410  may be scaled by a scaling factor. This scaling of the frequency-component amplitudes may be used to compensate for a decrease in the energy, power, or intensity of a received pulse of light  410  as a function of distance of the target  130  from the lidar system  100 . A controller  150  may receive, from a frequency-detection circuit  600 , digital values corresponding to the amplitudes of one or more frequency components of a received pulse of light  410 . Prior to comparing the frequency-component values to threshold values to determine whether the received pulse of light  410  is valid, the frequency-component values may be divided by a scaling factor that corresponds to an optical characteristic of the received pulse of light  410  (e.g., the energy, peak power, or peak intensity of the received pulse of light  410 ). Alternatively, the frequency-component amplitudes may be multiplied by a scaling factor that corresponds to D or D 2 , where D is a distance to the target  130  from which the corresponding emitted pulse of light was scattered. 
     In particular embodiments, a light source  110  may emit pulses of light  400  where each emitted pulse of light  400  has a particular spectral signature of one or more different spectral signatures. The spectral signatures may be used to determine whether a received pulse of light is a valid received pulse of light  410  that is associated with an emitted pulse of light  400 . A valid received pulse of light  410  may refer to a received pulse of light  410  that includes scattered light from a pulse of light  400  that was emitted by the light source  110 . For example, a light source  110  may emit pulses of light  400  that each include the same spectral signature. If a received pulse of light matches that same spectral signature, then the received pulse of light may be determined to be a valid received pulse of light  410  that is associated with an emitted pulse of light  400 . As another example, a light source  110  may emit pulses of light  400  that each include one spectral signature of two or more different spectral signatures. If a received pulse of light matches one of the spectral signatures, then the received pulse of light may be determined to be a valid received pulse of light  410  that is associated with an emitted pulse of light  400 . 
     In particular embodiments, a received pulse of light may be determined to match a particular spectral signature if the received pulse of light includes each of the one or more frequency components associated with the particular spectral signature. Additionally, a received pulse of light may be determined to match the particular spectral signature if the received pulse of light does not include any frequency components that are not associated with the particular spectral signature. Similarly, a received pulse of light may be determined to not match a spectral signature if (i) the received pulse of light does not include all of the one or more frequency components associated with the spectral signature or (ii) the received pulse of light includes one or more frequency components not associated with the spectral signature. Determining whether a received pulse of light  410  includes a particular frequency component may include determining the amplitude of the particular frequency component (e.g., based on a signal from an amplitude detector  620 ). If the amplitude of the particular frequency component is greater than a particular threshold value (or between a minimum threshold value and a maximum threshold value), then a controller  150  may determine that a received pulse of light  410  includes the particular frequency component. Additionally or alternatively, if the amplitude of the particular frequency component is less than the particular threshold value, then the controller  150  may determine that the received pulse of light  410  does not include the particular frequency component. 
     In particular embodiments, a light source  110  may emit pulses of light  400  where each emitted pulse of light  400  has a particular spectral signature of two or more different spectral signatures, and the spectral signatures may be used to associate a received pulse of light  410  with a particular emitted pulse of light  400 . For example, a light source  110  may emit pulses of light  400  with spectral signatures that alternate (e.g., sequentially or in a pseudo-random manner) between two, three, four, or any other suitable number of different spectral signatures. One spectral signature may include an amplitude modulation at 1.5 GHz, and another spectral signature may include an amplitude modulation at 1.7 GHz. A frequency-detection circuit  600  may include two filters and amplitude detectors that determine the amplitudes of the frequency components at 1.5 GHz and 1.7 GHz. Based on the amplitudes of the 1.5-GHz and 1.7-GHz frequency components of a received pulse of light  410 , the controller  150  may determine whether the received pulse of light  410  is associated with an emitted pulse of light  400  having a 1.5-GHz spectral signature or a 1.7-GHz spectral signature. If a light source  110  emits a first pulse with a 1.5-GHz modulation and a second pulse with a 1.7-GHz modulation, then a controller  150  may determine that a received pulse of light  410  with a 1.5-GHz frequency component is associated with the first emitted pulse. Emitting pulses of light  400  that have different spectral signatures may allow a frequency-detection circuit  600  and controller  150  to prevent problems with ambiguity as to which emitted pulse a received pulse is associated with. A received pulse of light  410  may be unambiguously associated with an emitted pulse of light  400  based on the frequency components of the received pulse of light  410  matching the spectral signature of the emitted pulse of light  400 . 
     In particular embodiments, a light source  110  may emit pulses of light  400  where each emitted pulse of light  400  has a particular spectral signature of one or more different spectral signatures, and the spectral signatures may be used to determine whether a received pulse of light is a valid received pulse of light  410  or an interfering optical signal. An interfering optical signal may refer to an optical signal that is sent by a light source external to the lidar system  100 . For example, another lidar system may emit a pulse of light that is detected by the receiver  140 , and the received pulse of light may be determined to be an interfering optical signal since it does not match the spectral signatures of the emitted pulses of light  400  from the light source  110 . A controller  150  may distinguish valid pulses from interfering pulses by comparing the frequency components for a received pulse of light with the expected frequency components associated with the spectral signatures imparted to emitted pulses of light  400 . If the frequency components of a received pulse of light do not match any of the one or more different spectral signatures imparted to the emitted pulses of light  400 , then the controller  150  may determine that the received pulse of light is invalid and is not associated with any of the emitted pulses of light  400 . For example, the received pulse of light may be an interfering pulse of light sent from a light source external to the lidar system  100 , and the interfering pulse of light may be discarded or ignored since it is not associated with any of the emitted pulses of light  400 . 
       FIG. 8  illustrates an example light source  110  that includes a seed laser diode  450  and a semiconductor optical amplifier (SOA)  460 . In particular embodiments, a light source  110  of a lidar system  100  may include (i) a seed laser  450  that produces seed light  440  and LO light  430  and (ii) a pulsed optical amplifier  460  that amplifies the seed light  440  to produce emitted pulses of light  400 . In the example of  FIG. 8 , the seed laser is a seed laser diode  450  that produces seed light  440  and LO light  430 . The seed laser diode  450  may include a Fabry-Perot laser diode, a quantum well laser, a DBR laser, a DFB laser, a VCSEL, a quantum dot laser diode, or any other suitable type of laser diode. In  FIG. 8 , the pulsed optical amplifier is a semiconductor optical amplifier (SOA)  460  that emits a pulse of light  400  that is part of the output beam  125 . A SOA  460  may include a semiconductor optical waveguide that receives the seed light  440  from the seed laser diode  450  and amplifies the seed light  440  as it propagates through the waveguide to produce an emitted pulse of light  400 . A SOA  460  may have an optical power gain of 20 decibels (dB), 25 dB, 30 dB, 35 dB, 40 dB, 45 dB, or any other suitable optical power gain. For example, a SOA  460  may have a gain of 40 dB, and a temporal portion of seed light  440  with an energy of 20 pJ may be amplified by the SOA  460  to produce a pulse of light  400  with an energy of approximately 0.2 μJ. A light source  110  that includes a seed laser diode  450  that supplies seed light  440  that is amplified by a SOA  460  may be referred to as a master-oscillator power-amplifier laser (MOPA laser) or a MOPA light source. The seed laser diode  450  may be referred to as a master oscillator, and the SOA  460  may be referred to as a power amplifier. 
     In particular embodiments, a light source  110  may include an electronic driver  480  that (i) supplies electrical current to a seed laser  450  and (ii) supplies electrical current to a SOA  460 . In  FIG. 8 , the electronic driver  480  supplies seed current I 1  to the seed laser diode  450  to produce the seed light  440  and the LO light  430 . The seed current I 1  supplied to the seed laser diode  450  may be a substantially constant DC electrical current so that the seed light  440  and the LO light  430  each include continuous-wave (CW) light or light having a substantially constant optical power. For example, the seed current I 1  may include a DC current of approximately 1 mA, 10 mA, 100 mA, 200 mA, 500 mA, or any other suitable DC electrical current. Additionally or alternatively, the seed current I 1  may include a pulse of electrical current so that the seed light  440  includes seed pulses of light that are amplified by the SOA  460 . The seed laser  450  may be pulsed with a pulse of current having a duration that is long enough so that the wavelength of the seed-laser light emitted by the seed laser  450  (e.g., seed light  440  and LO light  430 ) stabilizes or reaches a substantially constant value at some time during the pulse. For example, the duration of the current pulse may be between 50 ns and 2 μs, and the SOA  460  may be configured to amplify a 5-ns temporal portion of the seed light  440  to produce the emitted pulse of light  400 . The temporal portion of the seed light  440  that is selected for amplification may be located in time near the middle or end of the electrical current pulse to allow sufficient time for the wavelength of the seed-laser light to stabilize. 
     In  FIG. 8 , the electronic driver  480  supplies SOA current I 2  to the SOA  460 , and the SOA current I 2  provides optical gain to temporal portions of the seed light  440  that propagate through the waveguide of the SOA  460 . The SOA current I 2  may include pulses of electrical current, where each pulse of current causes the SOA  460  to amplify one temporal portion of the seed light  440  to produce an emitted pulse of light  400 . The SOA current I 2  may have a duration of approximately 0.5 ns, 1 ns, 2 ns, 5 ns, 10 ns, 20 ns, 50 ns, 100 ns, or any other suitable duration. The SOA current I 2  may have a peak amplitude of approximately 1 A, 2 A, 5 A, 10 A, 20 A, 50 A, 100 A, 200 A, 500 A, or any other suitable peak current. For example, the SOA current I 2  supplied to the SOA  460  may include a series of current pulses having a duration of approximately 5-10 ns and a peak current of approximately 100 A. The series of current pulses may result in the emission of a corresponding series of pulses of light  400 , and each emitted pulse of light  400  may have a duration that is less than or equal to the duration of the corresponding electrical current pulse. For example, an electronic driver  480  may supply 5-ns duration current pulses to the SOA  460  at a repetition frequency of 700 kHz. This may result in emitted pulses of light  400  that have a duration of approximately 4 ns and a pulse repetition frequency of 700 kHz. 
     A pulsed optical amplifier may refer to an optical amplifier that is operated in a pulsed mode so that the output beam  125  emitted by the optical amplifier includes pulses of light  400 . For example, a pulsed optical amplifier may include a SOA  460  that is operated in a pulsed mode by supplying the SOA  460  with pulses of current. The seed light  440  may include CW light or light having a substantially constant optical power, and each pulse of current supplied to the SOA  460  may amplify a temporal portion of seed light to produce an emitted pulse of light  400 . As another example, a pulsed optical amplifier may include an optical amplifier along with an optical modulator. The optical modulator may be an acousto-optic modulator (AOM) or an electro-optic modulator (EOM) operated in a pulsed mode so that the modulator selectively transmits pulses of light. The SOA  460  may also be operated in a pulsed mode in synch with the optical modulator to amplify the temporal portions of the seed light, or the SOA  460  may be supplied with substantially DC current to operate as a CW optical amplifier. The optical modulator may be located between the seed laser diode  450  and the SOA  460 , and the optical modulator may be operated in a pulsed mode to transmit temporal portions of the seed light  440  which are then amplified by the SOA  460 . Alternatively, the optical modulator may be located after the SOA  460 , and the optical modulator may be operated in a pulsed mode to transmit the emitted pulses of light  400 . 
     The seed laser diode  450  illustrated in  FIG. 8  includes a front face  452  and a back face  451 . The seed light  440  is emitted from the front face  452  and directed to the input end  461  of the SOA  460 . The LO light  430  is emitted from the back face  451  and directed to the receiver  140  of the lidar system  100 . The seed light  440  or the LO light  430  may be emitted as a free-space beam, and a light source  110  may include one or more lenses (not illustrated in  FIG. 10 ) that (i) collimate the LO light  430  emitted from the back face  451 , (ii) collimate the seed light  440  emitted from the front face  452 , or (iii) focus the seed light  440  into the SOA  460 . 
     In particular embodiments a front face  452  or a back face  451  may include a discrete facet formed by a semiconductor-air interface (e.g., a surface formed by cleaving or polishing a semiconductor structure to form the seed laser diode  450 ). Additionally, the front face  452  or the back face  451  may include a dielectric coating that provides a reflectivity (at the seed-laser operating wavelength) of between approximately 50% and approximately 99.9%. For example, the back face  451  may have a reflectivity of 90% to 99.9% at a wavelength of the LO light  430 . The average power of the LO light  430  emitted from the back face  451  may depend at least in part on the reflectivity of the back face  451 , and a value for the reflectivity of the back face  451  may be selected to provide a particular average power of the LO light  430 . For example, the back face  451  may be configured to have a reflectivity between 90% and 99%, and the seed laser diode  450  may emit LO light  430  having an average optical power of 10 μW to 1 mW. In some conventional laser diodes, the reflectivity of the back face may be designed to be relatively high or as close to 100% as possible in order to minimize the amount of light produced from the back face or to maximize the amount of light produced from the front face. In the seed laser diode  450  of  FIG. 8 , the reflectivity of the back face  451  may be reduced to a lower value compared to a conventional laser diode so that a particular power of LO light  430  is emitted from the back face  451 . As an example, a conventional laser diode may have a back face with a reflectivity of greater than 98%, and a seed laser diode  450  may have a back face with a reflectivity between 90% and 98%. 
     In particular embodiments, the wavelength of the seed light  440  and the wavelength of the LO light  430  may be approximately equal. For example, a seed laser diode  450  may have a seed-laser operating wavelength of approximately 1508 nm, and the seed light  440  and the LO light  430  may each have the same wavelength of approximately 1508 nm. As another example, the wavelength of the seed light  440  and the wavelength of the LO light  430  may be equal to within some percentage (e.g., to within approximately 0.1%, 0.01%, or 0.001%) or to within some wavelength range (e.g., to within approximately 0.1 nm, 0.01 nm, or 0.001 nm). If the wavelengths are within 0.01% of 1508 nm, then the wavelengths of the seed light  440  and the LO light  430  may each be in the range from 1507.85 nm to 1508.15 nm). 
       FIG. 9  illustrates an example light source  110  that includes a semiconductor optical amplifier (SOA)  460  with a tapered optical waveguide  463 . In particular embodiments, a SOA  460  may include an input end  461 , an output end  462 , and an optical waveguide  463  extending from the input end  461  to the output end  462 . The input end  461  may receive the seed light  440  from the seed laser diode  450 . The waveguide  463  may amplify a temporal portion of the seed light  440  as the temporal portion propagates along the waveguide  463  from the input end  461  to the output end  462 . The amplified temporal portion may be emitted from the output end  462  as an emitted pulse of light  400 . The emitted pulse of light  400  may be part of the output beam  125 , and the light source  110  may include a lens  490  configured to collect and collimate emitted pulses of light  400  from the output end  462  to produce a collimated output beam  125 . The seed laser diode  450  in  FIG. 9  may have a diode length of approximately 100 μm, 200 μm, 500 μm, 1 mm, or any other suitable length. The SOA  460  may have an amplifier length of approximately 1 mm, 2 mm, 3 mm, 5 mm, 10 mm, 20 mm, or any other suitable length. For example, the seed laser diode  450  may have a diode length of approximately 300 μm, and the SOA  460  may have an amplifier length of approximately 4 mm. 
     In particular embodiments, a waveguide  463  may include a semiconductor optical waveguide formed at least in part by the semiconductor material of the SOA  460 , and the waveguide  463  may confine light along transverse directions while the light propagates through the SOA  460 . In particular embodiments, a waveguide  463  may have a substantially fixed width or a waveguide  463  may have a tapered width. For example, a waveguide  463  may have a substantially fixed width of approximately 5 μm, 10 μm, 20 μm, 50 μm, 100 μm, 200 μm, 500 μm, or any other suitable width. In  FIG. 9 , the SOA  460  has a tapered waveguide  463  with a width that increases from the input end  461  to the output end  462 . For example, the width of the tapered waveguide  463  at the input end  461  may be approximately equal to the width of the waveguide of the seed laser diode  450  (e.g., the input end  461  may have a width of approximately 1 μm, 2 μm, 5 μm, 10 μm, or 50 μm). At the output end  462  of the SOA  460 , the tapered waveguide  463  may have a width of approximately 50 μm, 100 μm, 200 μm, 500 μm, 1 mm, or any other suitable width. As another example, the width of the tapered waveguide  463  may increase linearly from a width of approximately 20 μm at the input end  461  to a width of approximately 250 μm at the output end  462 . 
     In particular embodiments, the input end  461  or the output end  462  of a SOA  460  may be a discrete facet formed by a semiconductor-air interface. Additionally, the input end  461  or the output end  462  may include a dielectric coating (e.g., an anti-reflection coating to reduce the reflectivity of the input end  461  or the output end  462 ). An anti-reflection (AR) coating may have a reflectivity at the seed-laser operating wavelength of less than 5%, 2%, 0.5%, 0.1%, or any other suitable reflectivity value. In  FIG. 8 , the input end  461  may have an AR coating that reduces the amount of seed light  440  reflected by the input end  461 . In  FIG. 8  or  FIG. 9 , the output end  462  may have an AR coating that reduces the amount of amplified seed light reflected by the output end  462 . An AR coating applied to the input end  461  or the output end  462  may also prevent the SOA  460  from acting as a laser by emitting coherent light when no seed light  440  is present. 
     In particular embodiments, a light source  110  may include a seed laser diode  450  and a SOA  460  that are integrated together and disposed on or in a single chip or substrate. For example, a seed laser diode  450  and a SOA  460  may each be fabricated separately and then attached to the same substrate (e.g., using epoxy or solder). The substrate may be electrically or thermally conductive, and the substrate may have a coefficient of thermal expansion (CTE) that is approximately equal to the CTE of the seed laser  450  and the SOA  460 . As another example, the seed laser diode  450  and the SOA  460  may be fabricated together on the same substrate (e.g., using semiconductor-fabrication processes, such as for example, lithography, deposition, and etching). The seed laser diode  450  and the SOA  460  may each include InGaAs or InGaAsP semiconductor structures, and the substrate may include indium phosphide (InP). The InP substrate may be n-doped or p-doped so that it is electrically conductive, and a portion of the InP substrate may act as an anode or cathode for both the seed laser diode  450  and the SOA  460 . The substrate may be thermally coupled to (i) a heat sink that dissipates heat produced by the seed laser diode  450  or the SOA  460  or (ii) a temperature-control device (e.g., a thermoelectric cooler) that stabilizes the temperature of the seed laser diode  450  or the SOA  460  to a particular temperature setpoint or to within a particular temperature range. In the example of  FIG. 8 , the seed laser  450  and the SOA  460  may be separate devices that are not disposed on a single substrate, and the seed light  440  may be a free-space beam. Alternatively, in the example of  FIG. 8 , the seed laser  450  and the SOA  460  may be separate devices that are disposed together on a single substrate. In the example of  FIG. 9 , the seed laser  450  and the SOA  460  may be integrated together and disposed on or in a single chip or substrate. 
     In  FIG. 9 , rather than having a discrete facet formed by a semiconductor-air interface, the front face  452  of the seed laser diode  450  and the input end  461  of the SOA  460  may be coupled together without a semiconductor-air interface. For example, the seed laser diode  450  may be directly connected to the SOA  460  so that the seed light  440  is directly coupled from the seed laser diode  450  into the waveguide  463  of the SOA  460 . The front face  452  may be butt-coupled or affixed (e.g., using an optically transparent adhesive) to the input end  461 , or the seed laser diode  450  and the SOA  460  may be fabricated together so that there is no separate front face  452  or input end  461  (e.g., the front face  452  and the input end  461  may be merged together to form a single interface between the seed laser diode  450  and the SOA  460 ). Alternatively, the seed laser diode  450  may be coupled to the SOA  460  via a passive optical waveguide that transmits the seed light  440  from the front face  452  of the seed laser diode  450  to the input end  461  of the SOA  460 . 
     In particular embodiments, during a period of time between two successive temporal portions of seed light  440 , a SOA  460  may be configured to optically absorb most of the seed light  440  propagating in the SOA  460 . The seed light  440  from the seed laser diode  450  may be coupled into the waveguide  463  of the SOA  460 . Depending on the amount of SOA current I 2  supplied to the SOA  460 , the seed light  440  may be optically amplified or optically absorbed while propagating along the waveguide  463 . If the SOA current I 2  exceeds a threshold gain value (e.g., 100 mA) that overcomes the optical loss of the SOA  460 , then the seed light  440  may be optically amplified by stimulated emission of photons. Otherwise, if the SOA current I 2  is less than the threshold gain value, then the seed light  440  may be optically absorbed. The process of optical absorption of the seed light  440  may include photons of the seed light  440  being absorbed by electrons located in the semiconductor structure of the SOA  460 . 
     In particular embodiments, the SOA current I 2  may include pulses of current separated by a period of time that corresponds to the pulse period τ of the light source  110 , and each pulse of current may result in the emission of a pulse of light  400 . For example, if the SOA current I 2  includes 20-A current pulses with a 10-ns duration, then for each current pulse, a corresponding 10-ns temporal portion of the seed light  440  may be amplified, resulting in the emission of a pulse of light  400 . During the time periods τ between successive pulses of current, the SOA current I 2  may be set to approximately zero or to some other value below the threshold gain value, and the seed light  440  present in the SOA  460  during those time periods may be optically absorbed. The optical absorption of the SOA  460  when the SOA current I 2  is zero may be greater than or equal to approximately 10 decibels (dB), 15 dB, 20 dB, 25 dB, or 30 dB. For example, if the optical absorption is greater than or equal to 20 dB, then less than or equal to 1% of the seed light  440  that is coupled into the input end  461  of the waveguide  463  may be emitted from the output end  462  as unwanted leakage light. Having most of the seed light  440  absorbed in the SOA  460  may prevent unwanted seed light  440  (e.g., seed light  440  located between successive pulses of light  400 ) from leaking out of the SOA  460  and propagating through the rest of the lidar system  100 . Additionally, optically absorbing the unwanted seed light  440  may allow the seed laser  450  to be operated with a substantially constant current I 1  or a substantially constant output power so that the wavelengths of the seed light  440  and LO light  430  are stable and substantially constant. 
     In particular embodiments, a SOA  460  may include an anode and a cathode that convey SOA current I 2  from an electronic driver  480  to or from the SOA  460 . For example, the anode of the SOA  460  may include or may be electrically coupled to a conductive electrode material (e.g., gold) deposited onto the top surface of the SOA  460 , and the cathode may include or may be electrically coupled to a substrate located on the opposite side of the SOA  460 . Alternatively, the anode of the SOA  460  may include or may be electrically coupled to the substrate of the SOA  460 , and the cathode may include or may be electrically coupled to the electrode on the top surface of the SOA  460 . An anode may correspond to the p-doped side of a semiconductor p-n junction, and a cathode may correspond to the n-doped side. The anode and cathode may be electrically coupled to the electronic driver  480 , and the driver  480  may supply a positive SOA current I 2  that flows from the driver  480  into the anode, through the SOA  460 , out of the cathode, and back to the driver  480 . When considering the electrical current as being made up of a flow of electrons, then the electrons may be viewed as flowing in the opposite direction (e.g., from the driver  480  into the cathode, through the SOA  460 , and out of the anode and back to the driver  480 ). 
     In particular embodiments, an electronic driver  480  may electrically couple the SOA anode to the SOA cathode during a period of time between two successive pulses of current. For example, for most or all of the time period τ between two successive pulses of current, the electronic driver  480  may electrically couple the anode and cathode of the SOA  460 . Electrically coupling the anode and cathode may include electrically shorting the anode directly to the cathode or coupling the anode and cathode through a particular electrical resistance (e.g., approximately 1 Ω, 10Ω, or 100Ω). Alternatively, electrically coupling the anode and the cathode may include applying a reverse-bias voltage (e.g., approximately −1 V, −5 V, or −10 V) to the anode and cathode, where the reverse-bias voltage has a polarity that is opposite the forward-bias polarity associated with the applied pulses of current. By electrically coupling the anode to the cathode, the optical absorption of the SOA may be increased. For example, the optical absorption of the SOA  460  when the anode and cathode are electrically coupled may be increased (compared to the anode and cathode not being electrically coupled) by approximately 3 dB, 5 dB, 10 dB, 15 dB, or 20 dB. The optical absorption of the SOA  460  when the anode and cathode are electrically coupled may be greater than or equal to approximately 20 dB, 25 dB, 30 dB, 35 dB, or 40 dB. For example, the optical absorption of a SOA  460  when the SOA current I 2  is zero and the anode and cathode are not electrically coupled may be 20 dB. When the anode and cathode are electrically shorted together, the optical absorption may increase by 10 dB to 30 dB. If the optical absorption of the SOA  460  is greater than or equal to 30 dB, then less than or equal to 0.1% of the seed light  440  that is coupled into the input end  461  of the waveguide  463  may be emitted from the output end  462  as unwanted leakage light. 
       FIG. 10  illustrates an example light source  110  with an optical splitter  470  that splits output light  472  from a seed laser diode  450  to produce seed light  440  and local-oscillator (LO) light  430 . In particular embodiments, a light source  110  may include (i) a seed laser diode  450  with a front face  452  from which seed-laser output light  472  is emitted and (ii) an optical splitter  470  that splits the output light  472  to produce seed light  440  and LO light  430 . In  FIG. 10 , the output light  472  emitted by the seed laser diode  450  is a free-space optical beam, and the optical splitter  470  is a free-space optical beam-splitter that produces the free-space beams: seed light  440  and LO light  430 . In the examples of  FIGS. 8 and 9 , light emitted from the back face  451  of the seed laser diode  450  is used to produce the LO light  430 . In contrast, in the example of  FIG. 10 , both the seed light  440  and the LO light  430  are produced from the output light  472  emitted from the front face  452  of the seed laser diode  450 . The seed light  440  is transmitted through the splitter  470  and directed to the SOA  460 , and the LO light  430  is reflected by the splitter  470  and directed to the receiver  140  of the lidar system  100 . A light source  110  may include one or more lenses (not illustrated in  FIG. 10 ) that collimate the seed-laser output light  472  or focus the seed light  440  into the waveguide  463  of the SOA  460 . 
     The optical splitter  470  in  FIG. 10  is a free-space optical splitter that receives the seed-laser output light  472  as a free-space optical beam and produces two free-space beams: seed light  440  and LO light  430 . In  FIG. 10 , the free-space optical beam-splitter  470  reflects a first portion of the incident seed-laser output light  472  to produce the LO light  430  and transmits a second portion of the output light  472  to produce the seed light  440 . Alternatively, the beam-splitter  470  may be arranged to reflect a portion of the output light  472  to produce the seed light  440  and transmit a portion of the output light  472  to produce the LO light  430 . The free-space beam-splitter  470  in  FIG. 10  may have a reflectivity of less than or equal to 1%, 2%, 5%, 10%, 20%, 50%, or any other suitable reflectivity value. For example, the splitter  470  may reflect 10% or less of the incident seed-laser output light  472  to produce the LO light  430 , and the remaining 90% or more of the output light  472  may be transmitted through the splitter  470  to produce the seed light  440 . As another example, if the output light  472  has an average power of 25 mW and the splitter  470  reflects approximately 4% of the output light  472 , then the LO light  430  may have an average power of approximately 1 mW, and the seed light  440  may have an average power of approximately 24 mW. As used herein, a splitter  470  may refer to a free-space optical splitter, a fiber-optic splitter, or an optical-waveguide splitter. Additionally, an optical-waveguide splitter may be referred to as an integrated-optic splitter. 
     In particular embodiments, a light source  110  may include a fiber-optic splitter  470  that splits the seed-laser output light  472  to produce seed light  440  and LO light  430 . Instead of using a free-space optical splitter  470  (as illustrated in  FIG. 10 ), a light source  110  may use a fiber-optic splitter  470 . The fiber-optic splitter  470  may include one input optical fiber and two or more output optical fibers, and light that is coupled into the input optical fiber may be split between the output optical fibers. The output light  472  may be coupled from the front face  452  of the seed laser diode  450  into the input optical fiber of the fiber-optic splitter  470 , and the fiber-optic splitter  470  may split the output light  472  into the seed light  440  and the LO light  430 . The output light  472  may be coupled into the input optical fiber using one or more lenses, or the output light  472  may be directly coupled into the input optical fiber (e.g., the input optical fiber may be butt-coupled to the front face  452  of the seed laser diode  450 ). The seed light  440  may be directed to the SOA  460  by a first output fiber, and the LO light  430  may be directed to a receiver  140  by a second output fiber. The seed light  440  may be coupled from the first output fiber into the waveguide  463  of the SOA  460  by one or more lenses, or the seed light  440  may be directly coupled into waveguide  463  (e.g., the first output fiber may be butt-coupled to the input end  461  of the SOA  460 ). A fiber-optic splitter  470  may split off less than or equal to 1%, 2%, 5%, 10%, 20%, 50%, or any other suitable amount of the output light  472  to produce the LO light  430 , and the remaining light may form the seed light  440 . For example, a fiber-optic splitter  470  may split off 10% or less of the output light  472  to produce the LO light  430 , which is directed to one output fiber. The remaining 90% or more of the output light  472  may be directed to the other output fiber as the seed light  440 . 
       FIG. 11  illustrates an example light source  110  with a photonic integrated circuit (PIC)  455  that includes an optical-waveguide splitter  470 . In particular embodiments, a light source  110  may include an optical splitter  470  and a PIC  455 , where the optical splitter  470  is an optical-waveguide splitter of the PIC. A PIC  455  (which may be referred to as a planar lightwave circuit (PLC), an integrated-optic device, an integrated optoelectronic device, or a silicon optical bench) may include one or more optical waveguides or one or more optical-waveguide devices (e.g., optical-waveguide splitter  470 ) integrated together into a single device. A PIC  455  may include or may be fabricated from a substrate that includes silicon, InP, glass (e.g., silica), a polymer, an electro-optic material (e.g., lithium niobate (LiNbO 3 ) or lithium tantalate (LiTaO 3 )), or any suitable combination thereof. One or more optical waveguides may be formed on or in a PIC substrate using micro-fabrication techniques, such as for example, lithography, deposition, or etching. For example, an optical waveguide may be formed on a glass or silicon substrate by depositing and selectively etching material to form a ridge or channel waveguide on the substrate. As another example, an optical waveguide may be formed by implanting or diffusing a material into a substrate (e.g., by diffusing titanium into a LiNbO 3  substrate) to form a region in the substrate having a higher refractive index than the surrounding substrate material. 
     In particular embodiments, an optical-waveguide splitter  470  may include an input port and two or more output ports. In  FIG. 11 , the seed-laser output light  472  from the seed laser diode  450  is coupled into the input optical waveguide (input port) of the waveguide splitter  470 , and the waveguide splitter  470  splits the output light  472  between two output waveguides, output port 1 and output port 2. The seed-laser output light  472  may be coupled from the front face  452  of the seed laser diode  450  to the input port of the splitter  470  using one or more lenses, or the seed laser diode  450  may be butt-coupled to the input port so that the output light  472  is directly coupled into the input port. The seed light  440  is formed by the portion of output light  472  that is sent by the splitter  470  to output port 1, and the LO light  430  is formed by the portion of output light  472  that is sent by the splitter  470  to output port 2. The waveguide splitter  470  directs the seed light  440  to output port 1, which is coupled to waveguide  463  of the SOA  460 . Additionally, the waveguide splitter  470  directs the LO light  430  to output port 2, which sends the LO light  430  to a receiver  140 . An optical-waveguide splitter  470  may split off less than or equal to 1%, 2%, 5%, 10%, 20%, 50%, or any other suitable amount of the output light  472  to produce the LO light  430 , and the remaining light may form the seed light  440 . For example, the optical-waveguide splitter  470  may send 10% or less of the output light  472  to output port 2 to produce the LO light  430 , and the remaining 90% or more of the output light  472  may be sent to output port 1 to produce the seed light  440 . 
     In particular embodiments, a light source  110  may include one or more discrete optical devices combined with a PIC  455 . The discrete optical devices (which may include a seed laser diode  450 , a SOA  460 , one or more lenses, or one or more optical fibers) may be configured to couple light into the PIC  455  or to receive light emitted from the PIC  455 . In the example of  FIG. 11 , the light source  110  includes a PIC  455 , a seed laser diode  450 , and a SOA  460 . The seed laser diode  450  and the SOA  460  may each be attached or bonded to the PIC  455 , or the seed laser diode  450 , the SOA  460 , and the PIC  455  may be attached to a common substrate. For example, the front face  452  of the seed laser diode  450  may be bonded to the input port of the PIC  455  so that the output light  472  is directly coupled into the input port. As another example, the input end  461  of the SOA  460  may be bonded to the output port 1 of the PIC  455  so that the seed light  440  is directly coupled into the waveguide  463  of the SOA  460 . As another example, the light source  110  may include a lens (not illustrated in  FIG. 11 ) attached to or positioned near output port 2, and the lens may collect and collimate the LO light  430 . As another example, the light source  110  may include an optical fiber (not illustrated in  FIG. 11 ) attached to or positioned near output port 2, and the LO light  430  may be coupled into the optical fiber, which directs the LO light  430  to a receiver  140 . 
       FIG. 12  illustrates an example light source  110  that includes a seed laser diode  450   a  and a local-oscillator (LO) laser diode  450   b . In particular embodiments, a seed laser of a light source  110  may include a seed laser diode  450   a  that produces seed light  440  and a LO laser diode  450   b  that produces LO light  430 . Instead of having one laser diode that produces both the seed light  440  and the LO light  430  (e.g., as illustrated in  FIGS. 8-11 ), a light source  110  may include two laser diodes, one to produce the seed light  440  and the other to produce the LO light  430 . A light source  110  with two laser diodes may not include an optical splitter  470 . Rather, the seed light  440  emitted by the seed laser diode  450   a  may be coupled to a SOA  460 , and the LO light  430  emitted by the LO laser diode  450   b  may be sent to a receiver  140 . For example, the seed laser diode  450   a  may be butt-coupled to the input end  461  of the SOA  460 , and the LO light  430  from the LO laser diode  450   b  may be coupled into an optical fiber, which may direct the LO light  430  to a receiver  140 . 
     In particular embodiments, a seed laser diode  450   a  and a LO laser diode  450   b  may be operated so that the seed light  440  and the LO light  430  have a particular frequency offset. For example, the seed light  440  and the LO light  430  may have an optical frequency offset of approximately 0 Hz, 1 kHz, 1 MHz, 100 MHz, 1 GHz, 2 GHz, 5 GHz, 10 GHz, 20 GHz, or any other suitable frequency offset. An optical frequency f (which may be referred to as a frequency or a carrier frequency) and a wavelength λ may be related by the expression λ·f=c. For example, seed light  440  with a wavelength of 1550 nm corresponds to seed light  440  with an optical frequency of approximately 193.4 THz. In some cases herein, the terms wavelength and frequency may be used interchangeably when referring to an optical property of light. For example, LO light  430  having a substantially constant optical frequency may be equivalent to the LO light  430  having a substantially constant wavelength. As another example, LO light  430  having approximately the same wavelength as seed light  440  may also be referred to as the LO light  430  having approximately the same frequency as the seed light  440 . As another example, LO light  430  having a particular wavelength offset from seed light  440  may also be referred to as the LO light  430  having a particular frequency offset from the seed light  440 . An optical frequency offset (Δf) and a wavelength offset (Δλ) may be related by the expression Δf/f=−Δλ/λ. For example, for seed light  440  with a 1550-nm wavelength, LO light  430  that has a +10-GHz frequency offset from the seed light  440  corresponds to LO light  430  with a wavelength offset of approximately −0.08-nm from the 1550-nm wavelength of the seed light  440  (e.g., a wavelength for the LO light  430  of approximately 1549.92 nm). 
     In particular embodiments, a seed laser diode  450   a  or a LO laser diode  450   b  may be frequency locked so that they emit light having a substantially fixed wavelength or so that there is a substantially fixed frequency offset between the seed light  440  and the LO light  430 . Frequency locking a laser diode may include locking the wavelength of the light emitted by the laser diode to a stable frequency reference using, for example, an external optical cavity, an atomic optical absorption line, or light injected into the laser diode. For example, the seed laser diode  450   a  may be frequency locked (e.g., using an external optical cavity), and some of the light from the seed laser diode  450   a  may be injected into the LO laser diode  450   b  to frequency lock the LO laser diode  450  to approximately the same wavelength as the seed laser diode  450   a . As another example, the seed laser diode  450   a  and the LO laser diode  450   b  may each be separately frequency locked so that the two laser diodes have a particular frequency offset (e.g., a frequency offset of approximately 2 GHz). 
       FIG. 13  illustrates an example light source  110  that includes a seed laser  450 , a semiconductor optical amplifier (SOA)  460 , and a fiber-optic amplifier  500 . In particular embodiments, in addition to a seed laser  450  and a pulsed optical amplifier  460 , a light source  110  may also include a fiber-optic amplifier  500  that amplifies pulses of light  400   a  produced by the pulsed optical amplifier  460 . In  FIG. 13 , the SOA  460  may amplify temporal portions of seed light  440  from the seed laser  450  to produce pulses of light  400   a , and the fiber-optic amplifier  500  may amplify the pulses of light  400   a  from the SOA  460  to produce amplified pulses of light  400   b . The amplified pulses of light  400   b  may be part of a free-space output beam  125  that is sent to a scanner  120  and scanned across a field of regard of a lidar system  100 . 
     A SOA  460  and a fiber-optic amplifier  500  may each have an optical power gain of 10 dB, 15 dB, 20 dB, 25 dB, 30 dB, 35 dB, 40 dB, or any other suitable optical power gain. In the example of  FIG. 13 , the SOA  460  may have a gain of 30 dB, and the fiber-optic amplifier  500  may have a gain of 20 dB, which corresponds to an overall gain of 50 dB. A temporal portion of seed light  440  with an energy of 5 pJ may be amplified by the SOA  460  (with a gain of 30 dB) to produce a pulse of light  400   a  with an energy of approximately 5 nJ. The fiber-optic amplifier  500  may amplify the 5-nJ pulse of light  400   a  by 20 dB to produce an output pulse of light  400   b  with an energy of approximately 0.5 μJ. The seed laser  450  in  FIG. 13  produces seed light  440  and LO light  430 . The seed light  440  may be emitted from a front face  452  of a seed laser diode  450 , and the LO light  430  may be emitted from a back face  451  of the seed laser diode  450 . Alternatively, the light source  110  may include a splitter  470  that splits seed-laser output light  472  to produce the seed light  440  and the LO light  430 . 
       FIG. 14  illustrates an example fiber-optic amplifier  500 . In particular embodiments, a light source  110  of a lidar system  100  may include a fiber-optic amplifier  500  that amplifies pulses of light  400   a  produced by a SOA  460  to produce an output beam  125  with amplified pulses of light  400   b . A fiber-optic amplifier  500  may be terminated by a lens (e.g., output collimator  570 ) that produces a collimated free-space output beam  125  which may be directed to a scanner  120 . In particular embodiments, a fiber-optic amplifier  500  may include one or more pump lasers  510 , one or more pump WDMs  520 , one or more optical gain fibers  501 , one or more optical isolators  530 , one or more optical splitters  470 , one or more detectors  550 , one or more optical filters  560 , or one or more output collimators  570 . 
     A fiber-optic amplifier  500  may include an optical gain fiber  501  that is optically pumped (e.g., provided with energy) by one or more pump lasers  510 . The optically pumped gain fiber  501  may provide optical gain to each input pulse of light  400   a  while propagating through the gain fiber  501 . The pump-laser light may travel through the gain fiber  501  in the same direction (co-propagating) as the pulse of light  400   a  or in the opposite direction (counter-propagating). The fiber-optic amplifier  500  in  FIG. 14  includes one co-propagating pump laser  510  on the input side of the amplifier  500  and one counter-propagating pump laser  510  on the output side. A pump laser  510  may produce light at any suitable wavelength to provide optical excitation to the gain material of gain fiber  501  (e.g., a wavelength of approximately 808 nm, 810 nm, 915 m, 940 nm, 960 nm, 976 nm, or 980 nm). A pump laser  510  may be operated as a CW light source and may produce any suitable amount of average optical pump power, such as for example, approximately 1 W, 2 W, 5 W, 10 W, or 20 W of pump power. The pump-laser light from a pump laser  510  may be coupled into gain fiber  501  via a pump wavelength-division multiplexer (WDM)  520 . A pump WDM  520  may be used to combine or separate pump light and the pulses of light  400   a  that are amplified by the gain fiber  501 . 
     The fiber-optic core of a gain fiber  501  may be doped with a gain material that absorbs pump-laser light and provides optical gain to pulses of light  400   a  as they propagate along the gain fiber  501 . The gain material may include rare-earth ions, such as for example, erbium (Er 3+ ), ytterbium (Yb 3+ ), neodymium (Nd 3+ ), praseodymium (Pr 3+ ), holmium (Ho 3+ ), thulium (Tm 3+ ), dysprosium (Dy 3+ ), or any other suitable rare-earth element, or any suitable combination thereof. For example, the gain fiber  501  may include a core doped with erbium ions or with a combination of erbium and ytterbium ions. The rare-earth dopants absorb pump-laser light and are “pumped” or promoted into excited states that provide amplification to the pulses of light  400   a  through stimulated emission of photons. The rare-earth ions in excited states may also emit photons through spontaneous emission, resulting in the production of amplified spontaneous emission (ASE) light by the gain fiber  501 . 
     A gain fiber  501  may include a single-clad or multi-clad optical fiber with a core diameter of approximately 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 12 μm, 20 μm, 25 μm, or any other suitable core diameter. A single-clad gain fiber  501  may include a core surrounded by a cladding material, and the pump light and the pulses of light  400   a  may both propagate substantially within the core of the gain fiber  501 . A multi-clad gain fiber  501  may include a core, an inner cladding surrounding the core, and one or more additional cladding layers surrounding the inner cladding. The pulses of light  400   a  may propagate substantially within the core, while the pump light may propagate substantially within the inner cladding and the core. The length of gain fiber  501  in an amplifier  500  may be approximately 0.5 m, 1 m, 2 m, 4 m, 6 m, 10 m, 20 m, or any other suitable gain-fiber length. 
     A fiber-optic amplifier  500  may include one or more optical filters  560  located at the input or output side of the amplifier  500 . An optical filter  560  (which may include an absorptive filter, dichroic filter, long-pass filter, short-pass filter, band-pass filter, notch filter, Bragg grating, or fiber Bragg grating) may transmit light over a particular optical pass-band and substantially block light outside of the pass-band. The optical filter  560  in  FIG. 14  is located at the output side of the amplifier  500  and may reduce the amount of ASE from the gain fiber  501  that accompanies the output pulses of light  400   b . For example, the filter  560  may transmit light at the wavelength of the pulses of light  400   a  (e.g., 1550 nm) and may attenuate light at wavelengths away from a 5-nm pass-band centered at 1550 nm. 
     A fiber-optic amplifier  500  may include one or more optical isolators  530 . An isolator  530  may reduce or attenuate backward-propagating light, which may destabilize or cause damage to a seed laser diode  450 , SOA  460 , pump laser  510 , or gain fiber  501 . The isolators  530  in  FIG. 14  may allow light to pass in the direction of the arrow drawn in the isolator and block light propagating in the reverse direction. Backward-propagating light may arise from ASE light from gain fiber  501 , counter-propagating pump light from a pump laser  510 , or optical reflections from one or more optical interfaces of a fiber-optic amplifier  500 . An optical isolator  530  may prevent the destabilization or damage associated with backward-propagating light by blocking most of the backward-propagating light (e.g., by attenuating backward-propagating light by greater than or equal to 5 dB, 10 dB, 20 dB, 30 dB, 40 dB, 50 dB, or any other suitable attenuation value). 
     A fiber-optic amplifier  500  may include one or more optical splitters  470  and one or more detectors  550 . A splitter  470  may split off a portion of light (e.g., approximately 0.1%, 0.5%, 1%, 2%, or 5% of light received by the splitter  470 ) and direct the split off portion to a detector  550 . In  FIG. 14 , each splitter  470  may split off and send approximately 1% of each pulse of light ( 400   a  or  400   b ) to a detector  550 . Each of the splitters  470  in  FIG. 14  may be a fiber-optic splitter. One or more detectors  550  may be used to monitor the performance or health of a fiber-optic amplifier  500 . If an electrical signal from a detector  550  drops below a particular threshold level, then a controller  150  may determine that there is a problem with the amplifier  500  (e.g., there may be insufficient optical power in the input pulses of light  400   a  or a pump laser  510  may be failing). In response to determining that there is a problem with the amplifier  500 , the controller  150  may shut down or disable the amplifier  500 , shut down or disable the lidar system  100 , or send a notification that the lidar system  100  is in need of service or repair. 
     In particular embodiments, a fiber-optic amplifier  500  may include an input optical fiber configured to receive input pulses of light  400   a  from a SOA  460 . The input optical fiber may be part of or may be coupled or spliced to one of the components of the fiber-optic amplifier  500 . For example, pulses of light  400   a  may be coupled into an optical fiber which is spliced to an input optical fiber of the isolator  530  located at the input to the amplifier  500 . As another example, the pulses of light  400   a  from a SOA  460  may be part of a free-space beam that is coupled into an input optical fiber of fiber-optical amplifier  500  using one or more lenses. As another example, an input optical fiber of fiber-optic amplifier  500  may be positioned at or near the output end  462  of a SOA  460  so that the pulses of light  400   a  are directly coupled from the SOA  460  into the input optical fiber. 
     In particular embodiments, the optical components of a fiber-optic amplifier  500  may be free-space components, fiber-coupled components, or a combination of free-space and fiber-coupled components. As an example, each optical component in  FIG. 14  may be a free-space optical component or a fiber-coupled optical component. As another example, the input pulses of light  400   a  may be part of a free-space optical beam, and the isolator  530 , splitter  470 , and pump WDM  520  located on the input side of the amplifier  500  may each be free-space optical components. Additionally, the light from the pump laser  510  on the input side may be a free-space beam that is combined with the input pulses of light  400   a  by the pump WDM  520  on the input side, and the combined pump-seed light may form a free-space beam that is coupled into the gain fiber  501  via one or more lenses. 
       FIG. 15  illustrates example graphs of seed current (I 1 ), LO light  430 , seed light  440 , pulsed SOA current (I 2 ), and emitted optical pulses  400 . Each of the parameters (I 1 , LO light  430 , seed light  440 , I 2 , and emitted optical pulses  400 ) in  FIG. 15  is plotted versus time. The graph of seed current I 1  corresponds to a substantially constant DC electrical current that is supplied to a seed laser diode  450 . Based on the DC electrical current I 1 , the LO light  430  and seed light  440  produced by the seed laser diode  450  may each include CW light or light having a substantially constant optical power, as represented by the graphs of LO light  430  and seed light  440  in  FIG. 15 . For example, the LO light  430  may have a substantially constant average optical power of approximately 1 μW, 10 μW, 100 μW, 1 mW, 10 mW, 20 mW, 50 mW, or any other suitable average optical power. As another example, the seed light  440  may have a substantially constant average optical power of approximately 1 mW, 10 mW, 20 mW, 50 mW, 100 mW, 200 mW or any other suitable average optical power. As another example, the LO light  430  may have a substantially constant optical power of approximately 10 μW, and the seed light  440  may have a substantially constant optical power of approximately 100 mW. The LO light  430  or the seed light  440  having a substantially constant optical power may correspond to the optical power being substantially constant over particular time interval (e.g., a time interval greater than or equal to the pulse period τ, the coherence time T c , or the time interval t b −t a ). For example, the power of the LO light  430  may vary by less than ±1% over a time interval greater than or equal to the pulse period τ. 
     In particular embodiments, CW light may refer to light having a substantially fixed or stable optical frequency or wavelength over a particular time interval (e.g., over pulse period τ, over coherence time T c , or over the time interval t b −t a ). Light with a substantially fixed or stable optical frequency may refer to light having a variation in optical frequency over a particular time interval of less than or equal to ±0.1%, ±0.01%, ±0.001%, ±0.0001%, ±0.00001%, ±0.000001%, or any other suitable variation. For example, if LO light  430  with a 1550-nm wavelength (which corresponds to an optical frequency of approximately 193.4 THz) has a frequency variation of less than or equal to ±0.000001% over a particular time interval, then the frequency of the LO light  430  may vary by less than or equal to approximately ±1.94 MHz over the time interval. 
     In particular embodiments, the average optical power for LO light  430  may be set to a particular value based at least in part on a saturation value of a receiver  140 . For example, a seed laser  450  may be configured to emit LO light  430  having an average optical power that is less than a saturation value of a receiver  140  (e.g., less than a saturation value of a detector  340  or an amplifier  350  of the receiver  140 ). If a receiver  140  receives an input optical signal (e.g., combined beam  422 ) that exceeds an optical-power saturation value of the detector  340 , then the detector  340  may saturate or produce a photocurrent i that is different from or distorted with respect to the input optical signal. A detector  340  may saturate with an input optical power of approximately 0.1 mW, 0.5 mW, 1 mW, 5 mW, 10 mW, 20 mW, or 100 mW. If an amplifier  350  of a receiver  140  receives an input photocurrent i that exceeds an electrical-current saturation value, then the amplifier  350  may saturate or produce a voltage signal  360  that is different from or distorted with respect to the photocurrent signal i. To prevent saturation of the detector  340  or amplifier  350 , the optical power of the input beam  135  or of the LO light  430  may be selected to be below a saturation power of the receiver  140 . For example, a detector  340  may saturate with an input optical power of 10 mW, and to prevent the detector  340  from saturating, the optical power of a combined beam  422  may be limited to less than 10 mW. In particular embodiments, a limit may be applied to the average power of the LO light  430  to prevent saturation. For example, a detector  340  may saturate with an average optical power of 1 mW, and to prevent the detector  340  from saturating, the average optical power of LO light  430  that is sent to the detector  340  may be configured to be less than 1 mW. As another example, the average optical power of the LO light  430  may be set to a value between 1 μW and 100 μW to prevent saturation effects in a detector  340 . 
     In particular embodiments, the average optical power of LO light  430  may be configured by adjusting or setting (i) an amount of seed current I 1  supplied to a seed laser diode  450 , (ii) a reflectivity of the back face  451  of the seed laser diode  450 , (iii) a reflectivity of a free-space splitter  470 , or (iv) an amount of light split off by a fiber-optic or optical-waveguide splitter  470 . In the example of  FIG. 8  or  FIG. 9 , the seed current I 1  and the reflectivity of the back face  451  of the seed laser diode  450  may be configured so that the average optical power of the LO light  430  is set to a particular value (e.g., a value between 10 μW and 100 μW). In the example of  FIG. 10 , the seed current I 1  and the reflectivity of the splitter  470  may be configured so that the average optical power of the LO light  430  is set to a particular value (e.g., a value below 10 mW). In the example of  FIG. 11 , the seed current supplied to the seed laser diode  450  and the amount of light split off to output port 2 by the optical-waveguide splitter  470  may be configured so that the average optical power of the LO light  430  is set to a particular value (e.g., a value below 1 mW). 
     In  FIG. 15 , the hatched regions  441  of the seed light  440  correspond to temporal portions of the seed light  440  that are amplified by a SOA  460 . The SOA current I 2  includes pulses of electrical current, and each pulse of current may cause the SOA  460  to amplify a corresponding temporal portion  441  of the seed light  440  to produce an emitted pulse of light  400 . A temporal portion  441  of seed light  440  may refer to a portion of the seed light  440  located in a particular interval of time over which a pulse of current I 2  is applied to a SOA  460 . For example, the portion of seed light  440  located in the time interval between times t a  and t b  in  FIG. 15  corresponds to one temporal portion  441  of the seed light  440 . The corresponding pulse of SOA current between the times t a  and t b  results in the amplification of the temporal portion  441  and the emission of a pulse of light  400 . The duration of a temporal portion  441  (e.g., as represented by t b −t a ) or the duration of a SOA current pulse may be approximately 0.5 ns, 1 ns, 2 ns, 5 ns, 10 ns, 20 ns, 50 ns, 100 ns, or any other suitable duration. 
     Each emitted pulse of light  400  in  FIG. 15  may include a temporal portion  441  of seed light  440  that is amplified by a SOA  460 , and during the time period between successive pulses of SOA current I 2 , the seed light  440  may be substantially absorbed by the SOA  460 . The emitted pulses of light  400  are part of an output beam  125  and have a pulse duration of ΔT and a pulse period of τ. For example, the emitted pulses of light  400  may have a pulse period of approximately 100 ns, 200 ns, 500 ns, 1 μs, 2 μs, 5 μs, 10 μs, or any other suitable pulse period. As another example, the emitted pulses of light  400  may have a pulse duration of 1-10 ns and a pulse period of 0.5-2.0 μs. In particular embodiments, when a current pulse is applied to a SOA  460 , there may be a time delay until the optical gain of the SOA  460  builds up to exceed the optical loss of the SOA  460 . As a result, the pulse duration Δτ of an emitted pulse of light  400  may be less than or equal to the duration of a corresponding pulse of SOA current I 2 . For example, a SOA current pulse with a duration of 8 ns may produce an emitted pulse of light  400  with a duration of 6 ns. In the example of  FIG. 15 , the emitted pulses of light  400  may have a duration of approximately 5 ns, and the SOA current pulses may have a duration (e.g., as represented by t b −t a ) of approximately 5 ns to 10 ns. 
       FIG. 16  illustrates example graphs of seed light  440 , an emitted optical pulse  400 , a received optical pulse  410 , LO light  430 , and detector photocurrent i. Each of the parameters (seed light  440 , emitted optical pulse  400 , received optical pulse  410 , LO light  430 , and photocurrent i) in  FIG. 15  is plotted versus time. The seed light  440  may include CW light or light having a substantially constant optical power, and the temporal portion  441  of the seed light  440  may be amplified by a SOA  460  to produce the emitted pulse of light  400 . The emitted pulse of light  400  is part of output beam  125 , and the received pulse of light  410  is part of input beam  135 . The received pulse of light  410 , which is received a time interval ΔT after the pulse of light  400  is emitted, may include light from the emitted optical pulse  400  that is scattered by a target  130 . The distance D from the lidar system  100  to the target  130  may be determined from the expression D=c·ΔT/2. 
     In particular embodiments, a received pulse of light  410  and LO light  430  may be combined and coherently mixed together at one or more detectors  340  of a receiver  140 . Each detector  340  may produce a photocurrent signal i that corresponds to coherent mixing of the received pulse of light  410  and the LO light  430 . In  FIG. 16 , the received pulse of light  410  is coherently mixed with a temporal portion  431  of the LO light  430  to produce a corresponding pulse of detector photocurrent i. A temporal portion  431  of LO light  430  may refer to a portion of the LO light  430  that is coincident with a received pulse of light  410 . In  FIG. 16 , temporal portion  431  and the received pulse of light  410  are each located in the time interval between times t c  and t d . The coherent mixing of the pulse of light  410  and the temporal portion  431  may occur at a detector  340  of the receiver  140 , and the detector  340  may produce a pulse of detector photocurrent i in response to the coherent mixing. Coherent mixing of two optical signals (e.g., a received pulse of light  410  and LO light  430 ) may be referred to as optical mixing, mixing, optical interfering, coherent combining, coherent detection, homodyne detection, or heterodyne detection. 
     In particular embodiments, coherent mixing may occur when two optical signals that are coherent with one another are optically combined and then detected by a detector  340 . If two optical signals can be coherently mixed together, the two optical signals may be referred to as being coherent with one another. Two optical signals being coherent with one another may include two optical signals (i) that have approximately the same optical frequency, (ii) that have a particular optical frequency offset (Δf), or (iii) that each have a substantially fixed or stable optical frequency over a particular period of time. For example, seed light  440  and LO light  430  in  FIG. 16  may be coherent with one another since they may have approximately the same optical frequency or each of their frequencies may be substantially fixed over a time period approximately equal to coherence time K. As another example, the emitted pulse of light  400  and the temporal portion  431  of LO light  430  in  FIG. 16  may be coherent with one another. And since the received pulse of light  410  may include a portion of the emitted pulse of light  400 , the received pulse of light  410  and the temporal portion  431  may also be coherent with one another. 
     In particular embodiments, if two optical signals each have a stable frequency over a particular period of time, then the two optical signals may be (i) optically combined together and (ii) coherently mixed at a detector  340 . Optically combining two optical signals (e.g., an input beam  135  and LO light  430 ) may refer to combining two optical signals so that their respective electric fields are summed together. Optically combining two optical signals may include overlapping the two optical signals (e.g., with an optical combiner  420 ) so that they are substantially coaxial and travel together in the same direction and along approximately the same optical path. Additionally, optically combining two optical signals may include overlapping the two optical signals so that at least a portion of their respective polarizations have the same orientation. Once the two optical signals are optically combined, they may be coherently mixed at a detector  340 , and the detector  340  may produce a photocurrent signal i corresponding to the summed electrical fields of the two optical signals. 
     In particular embodiments, a portion of seed light  440  may be coherent with a portion of LO light  430 . For example, LO light  430  and seed light  440  may be coherent with one another over a time period approximately equal to the coherence time K. In each of  FIGS. 8-11 , the LO light  430  and the seed light  440  may be coherent with one another since the two optical signals are derived from the same seed laser diode  450 . In  FIG. 12 , the LO light  430  and the seed light  440  may be coherent with one another since the two optical signals may have a particular frequency offset. In  FIG. 16 , the temporal portion  441  of the seed light  440  may be coherent with the temporal portion  431  of the LO light  430 . Additionally, the temporal portion  441  may be coherent with any portion of the LO light  430  extending over at least the time interval ΔT or T c  (e.g., from approximately time t a  to at least time t d ). The coherence time T c  may correspond to a time over which light emitted by a seed laser diode  450  is coherent (e.g., the emitted light may have a substantially fixed or stable frequency over a time interval of T c ). The coherence length L c  is the distance over which the light from a seed laser diode  450  is coherent, and the coherence time and coherence length may be related by the expression L c =c·T c . For example, a seed laser diode  450  may have a coherence length of approximately 500 m, which corresponds to a coherence time of approximately 1.67 μs. The seed light  440  and LO light  430  emitted by a seed laser diode  450  may have a coherence length of approximately 1 m, 10 m, 50 m, 100 m, 300 m, 500 m, 1 km, or any other suitable coherence length. Similarly, the seed light  440  and LO light  430  may have a coherence time of approximately 3 ns, 30 ns, 150 ns, 300 ns, 1 μs, 1.5 μs, 3 μs, or any other suitable coherence time. 
     In particular embodiments, each emitted pulse of light  400  may be coherent with a corresponding portion of LO light  430 . In  FIG. 16 , the corresponding portion of the LO light  430  may include any portion of the LO light  430  (including temporal portion  431 ) extending from approximately time t a  to at least time t d , and the emitted pulse of light  400  may be coherent with any portion of the LO light  430  from time t a  to time t d . In  FIG. 15 , each emitted pulse of light  400  may be coherent with the LO light  430  over a time period from when the pulse of light  400  is emitted until at least a time τ (the pulse period) after the pulse is emitted. Similarly, in each of  FIGS. 8-11 , the emitted pulse of light  400  may be coherent with the LO light  430  for at least a time τ after the pulse  400  is emitted. In  FIG. 13 , the fiber-optic amplifier  500  may preserve the coherence of the pulse of light  400   a , and the emitted pulse of light  400   b  may be coherent with the LO light  430  for at least a time τ after the pulse  400   b  is emitted. 
     In particular embodiments, each emitted pulse of light  400  may include a temporal portion  441  of the seed light  440  that is amplified by a SOA  460 , and the amplification process may be a coherent amplification process that preserves the coherence of the temporal portion  441 . Since the temporal portion  441  may be coherent with a corresponding portion of the LO light  430 , the emitted pulse of light  400  may also be coherent with the same portion of the LO light  430 . An emitted pulse of light  400  being coherent with a corresponding portion of LO light  430  may correspond to temporal portion  441  being coherent with the corresponding portion of the LO light  430 . In the example of  FIG. 16 , the temporal portion  441  may be coherent with the LO light  430  over at least the time interval ΔT or T c  (e.g., from approximately time t a  to at least time t d ). Since the emitted pulse of light  400  may be coherent with the temporal portion  441 , the emitted pulse of light  400  may also be coherent with any portion of the LO light  430  (including the temporal portion  431 ) from approximately time t a  until at least time t d . An emitted pulse of light  400  being coherent with any portion of LO light  430  in the time period from time t a  until at least time t d  indicates that the emitted pulse of light  400  may be coherently mixed with any portion of the LO light  430  (including the temporal portion  431 ) over this same time period. The received pulse of light  410  includes light from the emitted pulse of light  400  (e.g., light from the emitted pulse of light  400  that is scattered by a target  130 ), and so the received pulse of light  410  may be coherent with the emitted pulse of light  400 . Based on this, the received pulse of light  410  may also be coherently mixed with any portion of the LO light  430  over the t a  to t d  time period. 
     In particular embodiments, an emitted pulse of light  400  being coherent with a corresponding portion of LO light  430  may correspond to the LO light  430  having a coherence length greater than or equal to 2×R OP , where R OP  is an operating range of the lidar system  100 . The coherence length L c  being greater than or equal to 2×R OP  corresponds to the coherence time T c  being greater than or equal to 2×R OP /c. Since the quantity 2×R OP /c may be approximately equal to the pulse period τ, the coherence length L c  being greater than or equal to 2×R OP  may correspond to the coherence time T c  being greater than or equal the pulse period τ. The LO light  430  and the seed light  440  may be coherent with one another over the coherence time T c , which corresponds to the temporal portion  441  in  FIG. 16  being coherent with the LO light  430  over the coherence time T c . Similarly, the emitted pulse of light  400 , which includes the temporal portion  441  amplified by the SOA  460 , may be coherent with the LO light  430  over the coherence time T c . If the coherence length of the LO light  430  is greater than or equal to 2×R OP  (or, if T c  is greater than or equal to τ), then an emitted pulse of light  400  may be coherent with any portion of the LO light  430  (including the temporal portion  431 ) from a time when the pulse of light  400  is emitted until at least a time τ after the pulse is emitted. This indicates that a received pulse of light  410  (which includes light from the emitted pulse of light  400  scattered from a target  130 ) may be coherently mixed with the LO light  430  as long as the distance D to the target  130  is within the operating range of the lidar system  100  (e.g., D≤R OP ). 
     In particular embodiments, each emitted pulse of light  400  may be coherent with a corresponding portion of LO light  430 , and the corresponding portion of the LO light  430  may include temporal portion  431  of the LO light  430 . The temporal portion  431  represents the portion of the LO light  430  that is detected by a receiver  140  at the time when the received pulse of light  410  is detected by the receiver  140 . In  FIG. 16 , the temporal portion  431  is coincident with the received pulse of light  410 , and both optical signals are located between times t c  and t d . Since the received pulse of light  410  includes scattered light from the emitted pulse of light  400 , the received pulse of light  410  may be coherent with the temporal portion  431  of the LO light  430 . The received pulse of light  410  and the temporal portion  431  may be coherently mixed together at a detector  340  of the receiver, and the coherent mixing may result in a pulse of detector photocurrent i, as illustrated in  FIG. 16 . 
     In particular embodiments, a received pulse of light  410  may be coherent with a temporal portion  431  of LO light  430 . In  FIG. 16 , the received pulse of light  410  and the temporal portion  431 , which are coherently mixed together, are coherent with one another. In particular embodiments, the coherent mixing of a received pulse of light  410  and a temporal portion  431  may not require that the coherence time T c  associated with seed light  440  or LO light  430  be greater than or equal to the pulse period τ. For example, the received pulse of light  410  and the temporal portion  431  may be coherently mixed even if the coherence time is less than ΔT or less than the pulse period τ. Coherent mixing may occur if the coherence time T c  associated with the seed light  440  or the LO light  430  is greater than or equal to the duration of the received pulse of light  410  or the duration of the temporal portion  431 . If a received pulse of light  410  and a temporal portion  431  each has a substantially fixed frequency over at least the duration of the temporal portion  431 , then the received pulse of light  410  and the temporal portion  431  may be coherently mixed together. As long as the received pulse of light  410  and the temporal portion  431  each has an optical frequency that is substantially stable over the duration of the pulse of light  410  or over the duration of the temporal portion  431 , then the two optical signals may be coherently mixed together. In the example of  FIG. 16 , the received pulse of light  410  and the temporal portion  431  may be coherent over the duration of the temporal portion  431  (e.g., the coherence time T c  may be greater than or equal to t d −t c ), and their electric fields may be coherently combined (e.g., summed together) and coherently mixed together. 
       FIG. 17  illustrates an example voltage signal  360  that results from coherent mixing of LO light  430  and a received pulse of light  410 . The LO light  430  and the received pulse of light  410  are each represented by a frequency-domain graph that illustrates the relative optical power versus optical frequency. The LO light  430  has a center optical frequency of f 0  and a relatively narrow spectral linewidth of Δν 1 . The pulse of light  410  has the same center frequency f 0  and a broader spectral linewidth of Δν 2 . The coherent mixing of the LO light  430  and the pulse of light  410  at a detector  340  may result in a pulse of photocurrent i which is amplified by an amplifier  350  that produces the voltage signal  360 . The upper voltage-signal graph illustrates the voltage signal  360  in the time domain and includes a pulse of voltage with a duration of Δτ′. The duration Δτ′ of the voltage pulse may be greater than the duration Δτ of the corresponding emitted pulse of light  400 . For example, the duration of an emitted pulse of light  410  may increase while propagating to and from a target  130  or due to pulse-broadening effects of scattering from the target  130 . Additionally or alternatively, the finite temporal response of a detector  340  or amplifier  350  may result in a voltage pulse with a longer duration than the duration of a corresponding emitted pulse of light  400  or received pulse of light  410 . The lower voltage-signal graph in  FIG. 17  is a frequency-domain graph of the voltage signal  360  that indicates that the voltage signal  360  has an electrical bandwidth of Δν. 
     A spectral linewidth of an optical signal (e.g., seed light  440 , LO light  430 , or pulse of light  410 ) may be referred to as a linewidth, optical linewidth, bandwidth, or optical bandwidth. The spectral linewidth or electrical bandwidth may refer to an approximate width of a spectrum as measured at the half-power points of the spectrum (which may be referred to as the 3-dB points). A spectral linewidth or an electrical bandwidth may be specified over a particular time period, such as for example, over a period of time approximately equal to a pulse duration (e.g., Δτ or t b −t a ), a temporal-portion duration (e.g., t d −t c ), a pulse period τ, a coherence time T c , or any other suitable period of time. A spectral linewidth or an electrical bandwidth may be specified over a time period of approximately 1 μs, 10 μs, 100 μs, 1 ms, 10 ms, 100 ms, 1 s, 10 s, 100 s, or any other suitable time period. For example, the LO light  430  may have a spectral linewidth Δν 1  of 4 MHz when measured over a 100-ms time interval. A spectral linewidth for an optical signal may be related to a variation in optical frequency of the optical signal. For example, LO light  430  having a spectral linewidth Δν 1  of 4 MHz may correspond to LO light  430  having a frequency variation of approximately ±2 MHz over a 100-ms time interval. 
     In particular embodiments, the seed light  440  or the LO light  430  may have a spectral linewidth Δν 1  of less than approximately 50 MHz, 10 MHz, 5 MHz, 3 MHz, 1 MHz, 0.5 MHz, 100 kHz, or any other suitable spectral-linewidth value. In the example of  FIG. 17 , the LO light  430  in  FIG. 17  may have a spectral linewidth Δν 1  of approximately 3 MHz, and the corresponding seed light (not illustrated in  FIG. 17 ) may have approximately the same spectral linewidth. When a temporal portion  441  of the seed light  440  is amplified to produce an emitted pulse of light  400 , the spectral linewidth of the emitted pulse of light  400  may have a broadened linewidth Δν 2  that is greater than Δν 1 . For example, an emitted pulse of light  400  and a corresponding received pulse of light  410  may each have spectral linewidth Δν 2  of approximately 10 MHz, 50 MHz, 100 MHz, 200 MHz, 300 MHz, 500 MHz, 1 GHz, 10 GHz, or any other suitable linewidth. As another example, the LO light  430  in  FIG. 17  may have a spectral linewidth Δν 1  of 5 MHz, and the received pulse of light  410  in  FIG. 17  may have a spectral linewidth Δν 2  of 100 MHz. As another example, the received pulse of light  410  in  FIG. 17  may have a duration ΔT of approximately 3-6 ns and a spectral linewidth Δν 2  of approximately 75-150 MHz. 
     In particular embodiments, an electrical bandwidth Av of a voltage signal  360  may be approximately equal to a numeric combination of the linewidths of the corresponding LO light  430  and received pulse of light  410 . The electrical bandwidth Av may be greater than both of the linewidths Δν 1  and Δν 2 . For example, the electrical bandwidth Av may be approximately equal to the sum of the linewidths of the LO light  430  and the received pulse of light  410  (e.g., Δν≅Δν 1 +Δν 2 ). As another example, the electrical bandwidth Av may be approximately equal to √{square root over (Δν 1   2 +Δν 2   2 )}. In  FIG. 17 , the LO light  430  may have a spectral linewidth Δν 1  of approximately 3 MHz, and the received pulse of light  410  may have a spectral linewidth Δν 2  of approximately 150 MHz. The electrical bandwidth Av of the voltage signal  360  may be approximately equal to the sum of the two linewidths, or 153 MHz. 
     In particular embodiments, a photocurrent signal i produced by a detector  340  in response to the coherent mixing of LO light  430  and a received pulse of light  410  may be expressed as i(t)=k|ε Rx (t)+ε LO (t)| 2 , where k is a constant (e.g., k may account for the responsivity of the detector  340  as well as other constant parameters or conversion factors). For clarity, the constant k or other constants (e.g., conversion constants or factors of 2 or 4) may be excluded from expressions herein related to the photocurrent i. In the expression for i(t), ε Rx  (t) is the electric field of the received pulse of light  410 , and ε LO (t) is the electric field of the LO light  430 . The electric field of the received pulse of light  410  may be expressed as E RX  cos [ω Rx t+ϕ Rx (t)], where E Rx  is the amplitude of the electric field of the received pulse of light  410 , which may be expressed as E Rx (t), since the electric field amplitude may vary with time. Similarly, the electric field of the LO light  430  may be expressed as E LO  cos [ω LO t+ϕ LO (t)], where E LO  is the amplitude of the electric field of the LO light  430 , which may also be expressed as E LO (t). The frequency ω Rx  represents the optical frequency of the electric field of the received pulse of light  410 , and ω LO  represents the optical frequency of the electric field of the LO light  430 . A frequency represented by ω is a radial frequency (with units radians/s) and is related to the optical frequency f (with units cycles/s) by the expression ω=2πf. Each of the frequencies ω Rx  and ω LO , which may be expressed as ω Rx (t) or ω LO (t), may vary with time or may be substantially constant with time. The parameter ϕ Rx (t) represents a phase of the electric field of the received pulse of light  410 , and ϕ LO (t) represents a phase of the electric field of the LO light  430 . Each of the phases ϕ Rx (t) and ϕ LO (t), which may be expressed as ϕ Rx  and ϕ LO , may vary with time or may be substantially constant with time. 
     The above expression for the photocurrent signal i may be expanded and written as i(t)=E Rx   2 +E LO   2 +2E Rx E LO  cos[(ω RX −ω LO )t+ϕ Rx (t)−ϕ LO (t)]. In this expanded expression for the photocurrent signal i(t), the first term E Rx   2  corresponds to the power of the received pulse of light  410 , and the second term E LO   2  corresponds to the power of the LO light  430 . If the received pulse of light  410  is a Gaussian pulse with a pulse width of Δτ, the first term may be expressed as E Rx   2 (t)=P Rx  exp [−(2√{square root over (ln 2)}t/Δτ) 2 ], where P Rx  is the peak power of the received pulse of light  410 . If the LO light  430  has a substantially constant optical power, the second term may be expressed as E LO   2 =P LO , where P LO  is the average power of the LO light  430 . In particular embodiments, a photocurrent signal i corresponding to the coherent mixing of LO light  430  and a received pulse of light  410  may include a coherent-mixing term. The third term in the above expression, 2E Rx E LO  cos[(ω Rx −ω LO )t+ϕ Rx (t)−ϕ LO (t)], may be referred to as a coherent-mixing term. If the received pulse of light  410  and the LO light  430  have approximately the same optical frequency, then ω Rx  is approximately equal to ω LO , and the coherent-mixing term may be expressed as 2E Rx E LO  cos [ϕ Rx (t)−ϕ LO (t)]. The coherent-mixing term represents coherent mixing between the electric fields of the received pulse of light  410  and the LO light  430 . The coherent-mixing term is proportional to (i) E Rx , the amplitude of the electric field of the received pulse of light  410  and (ii) E Lo , the amplitude of the electric field of the LO light  430 . The amplitude of the electric field of the received pulse of light  410  may be time dependent (e.g., corresponding to a Gaussian or other pulse shape), and the E LO  term may be substantially constant, corresponding to an optical power of LO light  430  that is substantially constant. 
     A coherent pulsed lidar system  100  as described herein may have a higher sensitivity than a conventional non-coherent pulsed lidar system. For example, compared to a conventional non-coherent pulsed lidar system, a coherent pulsed lidar system may be able to detect targets  130  that are farther away or that have lower reflectivity. In a conventional non-coherent pulsed lidar system, a received pulse of light may be directly detected by a detector, without LO light and without coherent mixing. The photocurrent signal produced in a conventional non-coherent pulsed lidar system may correspond to the E Rx   2  term discussed above, which represents the power of a received pulse of light. The size of the E Rx   2  term may be determined primarily by the distance to the target  130  and the reflectivity of the target  130 , and aside from boosting the energy of the emitted pulses of light  400 , increasing the size of the E Rx   2  term may not be practical or feasible. In a coherent pulsed lidar system  100  as discussed herein, the detected signal includes a coherent-mixing term, which is proportional to the product of E Rx  and E LO , and the improved sensitivity of a coherent pulsed lidar system  100  may result from the coherent-mixing term. While it may not be practical or feasible to increase the amplitude of E Rx  for far-away or low-reflectivity targets  130 , the amplitude of the E LO  term may be increased by increasing the power of the LO light  430 . The power of the LO light  430  can be set to a level that results in an effective boosting of the size of the coherent-mixing term, which results in an increased sensitivity of the lidar system  100 . In the case of a conventional non-coherent pulsed lidar system, the signal of interest depends on E Rx   2 , the power of the received pulse of light. In a coherent pulsed lidar system  100 , the signal of interest, which depends on the product of E Rx  and E LO , may be increased by increasing the power of the LO light  430 . The LO light  430  acts to effectively boost the coherent-mixing term, which may result in an improved sensitivity of the lidar system  100 . 
       FIG. 18  illustrates an example receiver  140  that includes a combiner  420  and two detectors ( 340   a ,  340   b ). In particular embodiments, a receiver  140  of a lidar system  100  may include an optical combiner  420  that (i) combines LO light  430  with a received pulse of light  410  (which is part of an input beam  135 ) and (ii) directs a first portion  422   a  of the combined light to a first output and directs a second portion  422   b  of the combined light to a second output. For example, combiner  420  may be a 50-50 free-space optical beam-splitter that reflects approximately 50% of incident light and transmits approximately 50% of incident light. In  FIG. 18 , the combined beam  422   a  is directed to detector  340   a  and includes a transmitted portion of LO light  430  and a reflected portion of the received pulse of light  410  (e.g., approximately 50% of the incident LO light  430  and approximately 50% of the received pulse of light  410 ). Similarly, the combined beam  422   b  is directed to detector  340   b  and includes a reflected portion of LO light  430  and a transmitted portion of the received pulse of light  410 . 
     In particular embodiments, a receiver  140  of a lidar system  100  may include one or more detectors  340  configured to produce one or more respective photocurrent signals i corresponding to coherent mixing of LO light  430  and a received pulse of light  410 . The receiver  140  in  FIG. 18  includes two detectors  340   a  and  340   b , and each detector produces a respective photocurrent signal i a  and i b . The portions of LO light  430  and received pulse of light  410  that make up the combined beam  422   a  may be coherently mixed at detector  340   a  to produce the photocurrent signal i a . Similarly, the portions of LO light  430  and received pulse of light  410  that make up the combined beam  422   b  may be coherently mixed at detector  340   b  to produce the photocurrent signal i b . 
     In particular embodiments, each of the detectors  340   a  and  340   b  may produce a photocurrent signal, and the two detectors  340   a  and  340   b  may be configured so that their respective photocurrents i a  and i b  are subtracted. For example, the anode of detector  340   a  may be electrically connected to the cathode of detector  340   b , and the subtracted photocurrent signal i a −i b  from the anode-cathode connection may be sent to amplifier  350 . The subtracted photocurrent signal may be expressed as i a (t)−i b  (t)=2E Rx E LO  cos [(ω Rx −ω LO )t+ϕ Rx (t)−ϕ LO (t)], which corresponds to the coherent-mixing term discussed above. The subtracted photocurrent signal does not include the terms E Rx   2  and E LO   2 . By subtracting the two photocurrents, the common-mode terms E Rx   2  and E LO   2  (as well as common-mode noise) that appear in each of the photocurrent signals i a  and i b  are removed, leaving the coherent-mixing term, which is the quantity of interest. Since subtracting may remove common-mode noise, the subtracted photocurrent signal may have a reduced noise compared to each of the photocurrent signals i a  and i b  alone. If the frequencies ω Rx  and ω LO  are approximately equal, then the coherent-mixing term may be expressed as 2E Rx E LO  cos [ϕ Rx  (t)−ϕ LO (t)]. 
       FIG. 19  illustrates an example receiver  140  that includes an integrated-optic combiner  420  and two detectors ( 340   a ,  340   b ). The integrated-optic combiner  420  in  FIG. 19  may function similar to the free-space optical combiner  420  in  FIG. 18 , but the integrated-optic combiner  420  may include optical waveguides that direct, combine, or split light (rather than having the light propagate as free-space beams). The integrated-optic combiner  420  may be part of a PIC that includes two input ports and two output ports. In  FIG. 19 , one input port receives the input beam  135  (which includes a received pulse of light  410 ), and the other input port receives the LO light  430 . The combiner  420  combines the input beam  135  with the LO light  430  and directs combined beam  422   a  to one output port and combined beam  422   b  to the other output port. The combined beam  422   a  is directed to detector  340   a  and includes portions of the LO light  430  and the received pulse of light  410  (e.g., approximately 50% of the LO light  430  and approximately 50% of the received pulse of light  410 ). The combined beam  422   b  is directed to detector  340   b  and includes the other portions of the LO light  430  and the received pulse of light  410 . In  FIG. 19  (as in  FIG. 18 ), the photocurrents from each of the detectors  340   a  and  340   b  are subtracted to produce a subtracted photocurrent signal i a −i b  that may be sent to an amplifier. The subtracted photocurrent signal in  FIG. 19  (as in  FIG. 18 ) may be expressed as i a (t)−i b  (t)=2E Rx E LO  cos [(ω Rx −ω LO )t+ϕ Rx (t)−ϕ LO (t)]. 
     In particular embodiments, a receiver  140  may include one or more lenses. For example, the receiver  140  in  FIG. 18  may include one or more lenses (not illustrated in  FIG. 18 ) that focus the combined beam  422   a  onto the detector  340   a  or that focus the combined beam  422   b  onto the detector  340   b . As another example, the receiver  140  in  FIG. 19  may include one or more lenses (not illustrated in  FIG. 19 ) that focus the input beam  135  or the LO light  430  into an optical waveguide of the combiner  420 . As another example, the receiver  140  in  FIG. 19  may include one or more lenses (not illustrated in  FIG. 19 ) that focus the combined beam  422   a  as a free-space optical beam onto the detector  340   a  or that focus the combined beam  422   b  as a free-space optical beam onto the detector  340   b . Alternatively, each of the detectors  340   a  and  340   b  in  FIG. 19  may be butt-coupled or affixed to an output port of the combiner  420  without an intervening lens. For example, detectors  340   a  and  340   b  may each be positioned close to an output port of the combiner  420  to directly receive the respective combined beams  422   a  and  422   b . In  FIG. 19 , rather than being free-space optical beams, the combined beams  422   a  and  422   b  may primarily be confined beams that propagate through a waveguide of the combiner  420  and are directly coupled, with a minimum of free-space propagation (e.g., less than 1 mm of free-space propagation), onto the detectors  340   a  and  340   b.    
       FIG. 20  illustrates an example receiver  140  that includes a 90-degree optical hybrid  428  and four detectors ( 340   a ,  340   b ,  340   c ,  340   d ). A 90-degree optical hybrid  428  is an optical-combiner component that may include two input ports and four output ports. Input light received at each of the two input ports is combined and split between each of the four output ports. In particular embodiments, a receiver  140  may include a 90-degree optical hybrid  428  that combines LO light  430  and an input beam  135  (which includes a received pulse of light  410 ) and produces four combined beams ( 422   a ,  422   b ,  422   c ,  422   d ). Each of the combined beams may include a portion of the LO light  430  and a portion of the received pulse of light  410 , and each of the combined beams may be directed to one of the four detectors of the receiver  140 . In  FIG. 20 , each of the four detectors may produce a photocurrent signal that corresponds to the coherent mixing of a portion of LO light  430  with a portion of the received pulse of light  410 . 
     In particular embodiments, a 90-degree optical hybrid  428  may be configured so that the combined beams directed to each of the output ports may have approximately the same optical power or energy. For example, the 90-degree optical hybrid  428  in  FIG. 20  may split the input beam  135  into four approximately equal portions and direct each of the input-beam portions to one of the detectors. Similarly, the LO light  430  may be split into four approximately equal portions directed to each of the four detectors. In the example of  FIG. 20 , the combined beam  422   a , which is directed to detector  340   a , may include approximately one-quarter of the power of the LO light  430  and approximately one-quarter of the energy of the received pulse of light  410 . Similarly, each of the other combined beams ( 422   b ,  422   c ,  422   d ) in  FIG. 20  may also include approximately one-quarter of the LO light  430  and approximately one-quarter of the received pulse of light  410 . 
     In particular embodiments, a 90-degree optical hybrid  428  may be implemented as an integrated-optic device. The 90-degree optical hybrid  428  in  FIG. 20  is an integrated-optic device that includes two integrated-optic splitters ( 470   a ,  470   b ) and two integrated-optic combiners ( 420   a ,  420   b ). Splitter  470   a  may split the received pulse of light  410  into two parts having substantially equal pulse energy, a first part directed to combiner  420   a  and a second part directed to combiner  420   b . Similarly, splitter  470   b  may split the LO light  430  into two parts having substantially equal power, a first part directed to combiner  420   a  and a second part directed to combiner  420   b . Each optical combiner may combine a part of the received pulse of light  410  with a part of the LO light  430 , and the combined parts may be split into a first combined beam (e.g., combined beam  422   a ) and a second combined beam (e.g., combined beam  422   b ). The combined beam  422   a  is directed to detector  340   a  and includes portions of the LO light  430  and the received pulse of light  410  (e.g., approximately 25% of the LO light  430  and approximately 25% of the received pulse of light  410 ). The combined beam  422   b  is directed to detector  340   b  and may include approximately 25% of the LO light  430  and approximately 25% of the received pulse of light  410 . 
     In particular embodiments, a 90-degree optical hybrid  428  may be implemented as a free-space optical device. For example, a free-space 90-degree optical hybrid  428  may include a beam-splitter cube that receives input beam  135  and LO light  430  as free-space beams and produces four free-space combined beams ( 422   a ,  422   b ,  422   c ,  422   d ). In particular embodiments, a 90-degree optical hybrid  428  may be implemented as a fiber-optic device. For example, a free-space 90-degree optical hybrid  428  may be contained in a package with two input optical fibers that direct the input beam  135  and LO light  430  into the package and four output optical fibers that receive the four respective combined beams and direct them to four respective detectors. 
     In particular embodiments, a 90-degree optical hybrid  428  may include a phase shifter  429  that imparts a 90-degree phase change (Δϕ) to a part of a received pulse of light  410  or to a part of the LO light  430 . For example, a splitter  470   a  may split the received pulse of light  410  into two parts, and a phase shifter  429  may impart a 90-degree phase change to one part of the pulse of light  410  with respect to the other part. As another example, a splitter  470   b  may split the LO light  430  into two parts, and a phase shifter  429  may impart a 90-degree phase change to one part of the LO light  430  with respect to the other part. In  FIG. 20 , splitter  470   b  splits the LO light  430  into two parts, and the phase shifter  429  imparts a 90-degree phase change to the part of LO light  430  directed to combiner  420   b . The other part of LO light  430  directed to combiner  420   a  does not pass through the phase shifter  429  and does not receive a phase shift from the phase shifter  429 . A 90-degree phase change may also be expressed in radians as a π/2 phase change. A phase change may be referred to as a phase shift. 
     In particular embodiments, a phase shifter  429  may be implemented as a part of an integrated-optic 90-degree optical hybrid  428 . For example a phase shifter  429  may be implemented as a portion of optical waveguide that only one part of the LO light  430  propagates through. The portion of optical waveguide may be temperature controlled to adjust the refractive index of the waveguide portion and produce a relative phase delay of approximately 90 degrees between the two parts of LO light  430 . Additionally or alternatively, the 90-degree optical hybrid  428  as a whole may be temperature controlled to set and maintain a 90-degree phase delay. As another example, a phase shifter  429  may be implemented by applying an external electric field to a portion of optical waveguide to change the refractive index of the waveguide portion and produce a 90-degree phase delay. In particular embodiments, a phase shifter  429  may be implemented as a part of a free-space or fiber-coupled 90-degree optical hybrid  428 . For example the input and output beams in a free-space 90-degree optical hybrid  428  may be reflected by or transmitted through the optical surfaces of the optical hybrid  428  so that a relative phase shift of 90 degrees is imparted to one part of LO light  430  with respect to the other part of LO light  430 . 
     In  FIG. 20 , the photocurrents from detectors  340   a  and  340   b  are subtracted to produce the subtracted photocurrent signal i a (t)−i b  (t)=E Rx E LO  cos [ω Rx −ω LO )t+ϕ Rx (t)−ϕ LO (t)]. If ω Rx  and ω LO  are approximately equal, then the subtracted photocurrent signal i a −i b  may be expressed as E Rx E LO  cos [ϕ Rx (t)−ϕ LO (t)]. Similarly, the photocurrents from detectors  340   c  and  340   d  are subtracted to produce the photocurrent signal i c (t)−i d  (t)=E Rx E LO  sin [(ω Rx −ω LO )t+ϕ Rx (t)−ϕ LO  (t)], which may be expressed as E Rx E LO  sin [ϕ Rx (t)−ϕ LO (t)] if the two frequencies are approximately equal. Each of the subtracted photocurrent signals represents a coherent-mixing term corresponding to the coherent mixing of a portion of the received pulse of light  410  and a portion of the LO light  430 . The two subtracted photocurrent signals are similar, except i a −i b  includes a cosine function, while i c −i d  includes a sine function. This difference between the two subtracted photocurrent signals arises from the 90-degree phase shift provided by the phase shifter  429 . Because a 90-degree phase shift is imparted to the LO light  430  directed to the combiner  420   b , the subtracted photocurrent signal i c −i d  includes a sine function (which has a 90-degree phase offset with respect to a cosine function). 
     The phase term ϕ Rx −ϕ LO  in the above subtracted photocurrent expressions represents the relative phase offset between the received pulse of light  410  and the LO light  430 . If the phase term ϕ Rx −ϕ LO  is approximately equal to 90° (modulo 2π), then the subtracted photocurrent signal i a −i b  may be approximately zero, and the subtracted photocurrent signal i c −i d  may be approximately E Rx E LO . Conversely, if the phase term φ Rx −ϕ LO  is approximately equal to 0° (modulo 2η), then the subtracted photocurrent signal i a −i b  may be approximately E Rx E LO , and the subtracted photocurrent signal i c −i d  may be approximately zero. Thus, both subtracted photocurrent signals vary based on the relative phase ϕ Rx −ϕ LO  between the received pulse of light  410  and the LO light  430 . The relative phase ϕ Rx −ϕ LO , which corresponds to the difference in optical path length between the input beam  135  and the LO light  430 , may vary by greater than or equal to π/8, π/4, π/2, π, or 2π over a particular time interval (e.g., due at least in part to relatively small changes in the optical path length caused by temperature change or small path-length changes). This variation in the relative phase may result in a significant time-dependent variation in each of the subtracted photocurrent signals. 
     The variation in the subtracted photocurrent signals may be addressed by processing or combining signals associated with the two subtracted photocurrent signals to produce an output electrical signal that is independent of the relative phase difference. For example, electrical signals associated with the two subtracted signals may be squared and then added together (e.g., a receiver  140  or controller  150  may produce an output electrical signal corresponding to (i a −i b ) 2 +(i c −i d ) 2 ). This squaring-and-summing operation results in an output electrical signal that is proportional to E Rx   2 E LO   2  (or, equivalently, P Rx P LO , which is the product of the power of the received pulse of light  410  and the power of the LO light  430 ) but does not depend on the relative phase difference ϕ Rx −ϕ LO . In this way, an output electrical signal may be obtained that is proportional to the power of the received pulse of light  410  and the power of the LO light  430  but is not sensitive to the relative phase difference ϕ Rx −ϕ LO . In a conventional non-coherent pulsed lidar system, the output signal may depend primarily on the power of a received pulse of light. Since the output electrical signal in a coherent pulsed lidar system  100  may depend on both P Rx  and P LO , the sensitivity of the lidar system  100  may be improved (with respect to a conventional non-coherent pulsed lidar system) by selecting a suitable power for the LO light  430 . 
       FIG. 21  illustrates an example receiver  140  that includes two polarization beam-splitters  650 . In particular embodiments, a receiver  140  may include a LO-light polarization splitter  650  that splits LO light  430  into two orthogonal polarization components (e.g., horizontal and vertical). Additionally, the receiver  140  may include an input-beam polarization splitter  650  that splits an input beam  135  (which includes a received pulse of light  410 ) into the same two orthogonal polarization components. In  FIG. 21 , the LO-light polarization beam-splitter (PBS)  650  splits the LO light  430  into a horizontally polarized LO-light beam  430 H and a vertically polarized LO-light beam  430 V. Similarly, the input-beam PBS  650  splits the input beam  135  into a horizontally polarized input beam  135 H and a vertically polarized input beam  135 V. The horizontally polarized beams are directed to a horizontal-polarization receiver, and the vertically polarized beams are directed to a vertical-polarization receiver. The receiver  140  illustrated in  FIG. 21  may be referred to as a polarization-insensitive receiver since the receiver  140  may be configured to detect received pulses of light  410  regardless of the polarization of the received pulses of light  410 . 
     In particular embodiments, a polarization-insensitive receiver  140  as illustrated in  FIG. 21  may be implemented with free-space components, fiber-optic components, integrated-optic components, or any suitable combination thereof. For example, the two PBSs  650  may be free-space polarization beam-splitting cubes, and the input beam  135  and the LO light  430  may be free-space optical beams. As another example, the two PBSs  650  may be fiber-optic components, and the input beam  135  and the LO light  430  may be conveyed to the PBSs  650  via optical fiber (e.g., single-mode optical fiber or polarization-maintaining optical fiber). Additionally, the horizontally and vertically polarized beams may be conveyed to the respective H-polarization and V-polarization receivers via polarization-maintaining optical fiber. 
     In particular embodiments, a receiver  140  may include a horizontal-polarization receiver and a vertical-polarization receiver. The H-polarization receiver may combine a horizontally polarized LO-light beam  430 H and a horizontally polarized input beam  135 H and produce one or more photocurrent signals corresponding to coherent mixing of the two horizontally polarized beams. Similarly, the V-polarization receiver may combine the vertically polarized LO-light beam  430 V and the vertically polarized input beam  135 V and produce one or more photocurrent signals corresponding to coherent mixing of the two vertically polarized beams. Each of the H-polarization and V-polarization receivers may include (i) an optical combiner  420  and two detectors  340  (e.g., as illustrated in  FIG. 18 or 19 ) or (ii) a 90-degree optical hybrid  428  and four detectors  340  (e.g., as illustrated in  FIG. 20 ). The H-polarization and V-polarization receivers may each preserve the polarization of the respective horizontally and vertically polarized beams. For example, the H-polarization and V-polarization receivers may each include polarization-maintaining optical fiber that maintains the polarization of the beams. Additionally or alternatively, the H-polarization and V-polarization receivers may each include a PIC with optical waveguides configured to maintain the polarization of the beams. 
     The polarization of an input beam  135  may vary with time or may not be controllable by a lidar system  100 . For example, the polarization of received pulses of light  410  may vary depending at least in part on (i) the optical properties of a target  130  from which pulses of light  400  are scattered or (ii) atmospheric conditions encountered by pulses of light  400  while propagating to the target  130  and back to the lidar system  100 . However, since the LO light  430  is produced and contained within the lidar system  100 , the polarization of the LO light  430  may be set to a particular polarization state. For example, the polarization of the LO light  430  sent to the LO-light PBS  650  may be configured so that the LO-light beam  430 H and  430 V produced by the PBS  650  have approximately the same power. The LO light  430  produced by a seed laser  450  may be linearly polarized, and a half-wave plate may be used to rotate the polarization of the LO light  430  so that it is oriented at approximately 45 degrees with respect to the LO-light PBS  650 . The LO-light PBS  650  may split the 45-degree polarized LO light  430  into horizontal and vertical components having approximately the same power. By providing a portion of the LO light  430  to both the H-polarization receiver and the V-polarization receiver, the receiver  140  in  FIG. 21  may produce a valid, non-zero output electrical signal regardless of the polarization of the received pulse of light  410 . 
     Coherent mixing of LO light  430  and a received pulse of light  410  may require that the electric fields of the LO light  430  and the received pulse of light  410  are oriented in approximately the same direction. For example, if LO light  430  and input beam  135  are both vertically polarized, then the two beams may be optically combined together and coherently mixed at a detector  340 . However, if the two beams are orthogonally polarized (e.g., LO light  430  is vertically polarized and input beam  135  is horizontally polarized), then the two beams may not be coherently mixed, since their electric fields are not oriented in the same direction. Orthogonally polarized beams that are incident on a detector  340  may not be coherently mixed, resulting in little to no output signal from a receiver  140 . To mitigate problems with polarization-related signal variation, a lidar system  100  may include (i) a polarization-insensitive receiver  140  (e.g., as illustrated in  FIG. 21 ) or (ii) an optical polarization element to ensure that at least a portion of the LO light  430  and input beam  135  have the same polarization. 
     A polarization-insensitive receiver  140  as illustrated in  FIG. 21  may ensure that the receiver  140  produces a valid, non-zero output electrical signal in response to a received pulse of light  410 , regardless of the polarization of the received pulse of light  410 . For example, the output electrical signals from the H-polarization and V-polarization receivers may be added together, resulting in a combined output signal that is insensitive to the polarization of the received pulse of light  410 . If a received pulse of light  410  is horizontally polarized, then the H-polarization receiver may generate a non-zero output signal and the V-polarization receiver may generate little to no output signal. Similarly, if a received pulse of light  410  is vertically polarized, then the H-polarization receiver may generate little to no output signal and the V-polarization receiver may generate a non-zero output signal. If a received pulse of light  410  has a polarization that includes a vertical component and a horizontal component, then each of the H-polarization and V-polarization receivers may generate a non-zero output signal corresponding to the respective polarization component. By adding together the signals from the H-polarization and V-polarization receivers, a valid, non-zero output electrical signal may be produced by the receiver  140  regardless of the polarization of the received pulse of light  410 . 
     In particular embodiments, a lidar system  100  may include an optical polarization element that alters the polarization of an emitted pulse of light  400 , LO light  430 , or a received pulse of light  410 . The optical polarization element may allow the LO light  430  and the received pulse of light  410  to be coherently mixed. For example, an optical polarization element may alter the polarization of the LO light  430  so that, regardless of the polarization of a received pulse of light  410 , the LO light  430  and the received pulse of light  410  may be coherently mixed together. The optical polarization element may ensure that at least a portion of the received pulse of light  410  and the LO light  430  have polarizations that are oriented in the same direction. An optical polarization element may include one or more quarter-wave plates, one or more half-wave plates, one or more optical polarizers, one or more optical depolarizers, or any suitable combination thereof. For example, an optical polarization element may include a quarter-wave plate that converts the polarization of an emitted pulse of light  400  or a received pulse of light  410  to a substantially circular or elliptical polarization. An optical polarization element may include a free-space optical component, a fiber-optic component, an integrated-optic component, or any suitable combination thereof. 
     In particular embodiments, an optical polarization element may be included in a receiver  140  as an alternative to configuring a receiver to be a polarization-insensitive receiver. For example, rather than producing horizontally polarized beams and vertically polarized beams and having two receiver channels (e.g., H-polarization receiver and V-polarization receiver), a receiver  140  may include an optical polarization element that ensures that at least a portion of the LO light  430  and the received pulse of light  410  may be coherently mixed together. An optical polarization element may be included in each of the receivers  140  illustrated in  FIG. 18, 19 , or  20  to allow the receiver to coherently mix the LO light  430  and a received pulse of light  410  regardless of the polarization of the received pulse of light  410 . 
     In particular embodiments, an optical polarization element (e.g., a quarter-wave plate) may convert the polarization of the LO light  430  into circularly polarized light. For example, the LO light  430  produced by a seed laser  450  may be linearly polarized, and a quarter-wave plate may convert the linearly polarized LO light  430  into circularly polarized light. The circularly polarized LO light  430  may include both vertical and horizontal polarization components. So, regardless of the polarization of a received pulse of light  410 , at least a portion of the circularly polarized LO light  430  may be coherently mixed with the received pulse of light  410 . In the receiver  140  illustrated in  FIG. 18 or 19 , the LO light  430  may be sent through a quarter-wave plate prior to passing through the combiner  420 . 
     In particular embodiments, an optical polarization element may depolarize a polarization of the LO light  430 . For example, the LO light  430  produced by a seed laser  450  may be linearly polarized, and an optical depolarizer may convert the linearly polarized LO light  430  into depolarized light having a polarization that is substantially random or scrambled. The depolarized LO light  430  may include two or more different polarizations so that, regardless of the polarization of a received pulse of light  410 , at least a portion of the depolarized LO light  430  may be coherently mixed with the received pulse of light  410 . An optical depolarizer may include a Cornu depolarizer, a Lyot depolarizer, a wedge depolarizer, or any other suitable depolarizer element. In the receiver  140  illustrated in  FIG. 20 , the LO light  430  may be sent through a quarter-wave plate or a depolarizer prior to passing through the splitter  470   b  of the 90-degree optical hybrid  428 . 
       FIGS. 22-25  each illustrates an example light source  110  that includes a seed laser  450 , a semiconductor optical amplifier (SOA)  460 , and one or more optical modulators  495 . In particular embodiments, a light source  110  may include a phase or amplitude modulator  495  configured to change a frequency, phase, or amplitude of seed light  440 , LO light  430 , or emitted pulse of light  400 . An optical phase or amplitude modulator  495  may include an electro-optic modulator (EOM), an acousto-optic modulator (AOM), an electro-absorption modulator, a liquid-crystal modulator, or any other suitable type of optical phase or amplitude modulator. For example, an optical modulator  495  may include an electro-optic phase modulator or an AOM that changes the frequency or phase of seed light  440  or LO light  430 . As another example, an optical modulator  495  may include an electro-optic amplitude modulator, an electro-absorption modulator, or a liquid-crystal modulator that changes the amplitude of the seed light  440  or LO light  430 . An optical modulator  495  may be a free-space modulator, a fiber-optic modulator (e.g., with fiber-optic input or output ports), or an integrated-optic modulator (e.g., a waveguide-based modulator integrated into a PIC). 
     In particular embodiments, an optical modulator  495  may be included in a seed laser diode  450  or a SOA  460 . For example, a seed laser diode  450  may include a waveguide section to which an external electrical current or electric field may be applied to change the carrier density or refractive index of the waveguide section, resulting in a change in the frequency or phase of seed light  440  or LO light  430 . As another example, the frequency, phase, or amplitude of seed light  440  or LO light  430  may be changed by changing or modulating the seed current I 1  or the SOA current I 2 . In this case, the seed laser diode  450  or SOA  460  may not include a separate or discrete modulator, but rather, a modulation function may be distributed within the seed laser diode  450  or SOA  460 . For example, the optical frequency of the seed light  440  or LO light  430  may be changed by changing the seed current I 1 . Changing the seed current I 1  may cause a refractive-index change in the seed laser diode  450 , which may result in a change in the optical frequency of light produced by the seed laser diode  450 . 
     In  FIG. 22 , the light source  110  includes a modulator  495  located between the seed laser  450  and the optical splitter  470 . The seed-laser output light  472  passes through the modulator  495  and is then split by the splitter  470  to produce the seed light  440  and LO light  430 . The modulator  495  in  FIG. 22  may be configured to change a frequency, phase, or amplitude of the seed-laser output light  472 . For example, the modulator  495  may be a phase modulator that applies a time-varying phase shift to the seed-laser output light  472 , which may result in a frequency change of the seed-laser output light  472 . The modulator  495  may be driven in synch with the emitted pulses of light  400  so that the emitted pulses of light  400  and the LO light  430  each have a different frequency change imparted by the modulator  495 . 
     In  FIG. 23 , the light source  110  includes a modulator  495  located between the seed laser  450  and the SOA  460 . The modulator  495  in  FIG. 23  may be configured to change a frequency, phase, or amplitude of the seed light  440 . For example, since the LO light  430  does not pass through the modulator  495 , the modulator  495  may change the optical frequency of the seed light  440  so that it is different from the optical frequency of the LO light  430 . In  FIG. 24 , the light source  110  includes a modulator  495  located in the path of the LO light  430 . The modulator  495  in  FIG. 23  may be configured to change a frequency, phase, or amplitude of the LO light  430 . For example, since the seed light  440  does not pass through the modulator  495 , the modulator  495  may change the optical frequency of the LO light  430  so that it is different from the optical frequency of the seed light  440 . In  FIG. 23 or 24 , the seed light  440  and LO light  430  may be produced by an optical splitter  470  that splits seed-laser output light  472  to produce the seed light  440  and the LO light  430 . Alternatively, in  FIG. 23 or 24 , the seed light  440  may be emitted from a front face  452  of a seed laser diode, and the LO light  430  may be emitted from the back face  451  of the seed laser diode. 
     In  FIG. 25 , the light source  110  includes three optical modulators  495   a ,  495   b , and  495   c . In particular embodiments, a light source  110  may include one, two, three, or any other suitable number of modulators  495 . Each of the modulators  495   a ,  495   b , and  495   c  may be configured to change a frequency, phase, or amplitude of the seed-laser output light  472 , seed light  440 , or LO light  430 . For example, modulator  495   b  may be an amplitude modulator that modulates the amplitude of the seed light  440  before passing through the SOA  460 . As another example, modulator  495   b  may be a phase modulator that changes the frequency of the seed light  440 . As another example, modulator  495   c  may be a phase modulator that changes the frequency of the LO light  430 . 
       FIG. 26  illustrates an example voltage signal  360  that results from the coherent mixing of LO light  430  and a received pulse of light  410 , where the LO light  430  and the received pulse of light  410  have a frequency difference of Δf. The LO light  430  has a center optical frequency of f 0  and a relatively narrow spectral linewidth of Δν 1 . The received pulse of light  410  has a center frequency f 1  and a broader spectral linewidth of Ave, and the frequency of the pulse of light  410  is shifted by Δf with respect to the frequency of the LO light  430  so that f 1 =f 0 +Δf. For example, the seed light  440  may be sent through a phase modulator  495  that shifts the optical frequency of the seed light by Δf. Alternatively, the optical frequency of the seed light  440  may be changed by changing the seed current I 1  supplied to a seed laser diode  450 . The SOA  460 , which amplifies a temporal portion  441  of the seed light  440 , may substantially maintain the optical frequency of the seed light  440 . As a result, the emitted pulse of light  400  or the corresponding received pulse of light  410  may also have approximately the same optical frequency offset of Δf with respect to the LO light  430 . 
     The coherent mixing of the LO light  430  and the pulse of light  410  at a detector  340  may result in a pulse of photocurrent i which is amplified by an amplifier  350  that produces the voltage signal  360  illustrated in  FIG. 26 . The upper voltage-signal graph illustrates the voltage signal  360  in the time domain and includes a pulse of voltage with a duration of Δτ′. The voltage pulse (which corresponds to the pulse of photocurrent i) exhibits periodic pulsations, each pulsation separated by a time interval 1/Δf. The lower voltage-signal graph is a frequency-domain graph of the voltage signal  360  that indicates that the voltage signal  360  is centered at a frequency of Δf and has an electrical bandwidth of Δν. The voltage signal  360  being centered at the frequency Δf indicates that the voltage signal  360  has a frequency component at approximately Δf, which corresponds to the periodic time-domain pulsations with time interval 1/Δf. The frequency component Δf in the voltage signal  360  arises from the frequency offset of Δf between the received pulse of light  410  and the LO light  430 . The coherent mixing of LO light  430  and a received pulse of light  410  may result in a photocurrent signal i with a coherent mixing term that may be expressed as E RX E LO  cos [2π·Δf·t+ϕ Rx −ϕ LO ]. Here, since the optical frequencies of the LO light  430  and the received pulse of light  410  are different, the coherent mixing term varies periodically with a frequency of Δf. This variation in the coherent mixing term corresponds to the periodic pulsations and the frequency component of Δf in the voltage signal  360  in  FIG. 26 . The graphs in  FIG. 26  are similar to those in  FIG. 17 , with the difference being that in  FIG. 26 , the LO light  430  and the received pulse of light  410  have a frequency difference of Δf (which gives rise to the periodic pulsations in the voltage signal  360 ), while in  FIG. 17 , there is no frequency difference (e.g., Δf is approximately zero, and there are no periodic pulsations in the voltage signal  360 ). 
     In particular embodiments, an optical frequency change of Δf applied to seed light  440  may correspond to a spectral signature imparted to an emitted pulse of light  400 . For example, a receiver  140  may include a frequency-detection circuit  600  (e.g., as illustrated in  FIG. 7 ) that determines the amplitude of the frequency component Δf in the voltage signal  360 . The frequency-detection circuit  600  may include a band-pass filter  610  with a center frequency of Δf, and a corresponding amplitude detector  620  may determine an amplitude of the Δf frequency component. The frequency-detection circuit  600  may be used to determine (i) whether a received pulse of light  410  is valid and is associated with a pulse of light  400  emitted by the light source  110  or (ii) whether a received pulse of light is not valid and is associated with an interfering optical signal. 
     In particular embodiments, an optical frequency change applied to seed light  440  or LO light  430  may be selected so that the frequency change Δf is greater than 1/Δτ (where Δτ is the duration of emitted pulse of light  400 ) or greater than 1/Δτ′ (where Δτ′ is the duration of a voltage pulse corresponding to a received pulse of light  410 ). For example, the frequency change Δf may be approximately equal to 2/Δτ, 4/Δτ, 10/Δτ, 20/Δτ, or any other suitable factor of 1/Δτ. As another example, an emitted pulse of light  400  with a duration Δτ of 5 ns may have a frequency change Δf of greater than 200 MHz. As another example, a light source  110  that emits 5-ns pulses of light  400  may be configured so that the emitted pulses of light have a 1-GHz frequency offset with respect to the LO light  430 . Having Δf greater than 1/Δτ may ensure that voltage signal  360  includes a sufficient number of pulsations that are distinct from the overall pulse envelope of the voltage signal  360 . In the example of  FIG. 26 , Δf is approximately equal to 3/Δτ, and the voltage signal  360  includes approximately seven pulsations superimposed on the pulse envelope. This difference between Δf and 1/Δτ may allow the frequency component Δf in the voltage signal  360  to be determined (e.g., by a frequency-detection circuit  600 ) distinctly from a frequency component associated with the overall pulse envelope of the voltage signal  360 . 
       FIG. 27  illustrates example graphs of seed current (I 1 ), seed light  440 , an emitted optical pulse  400 , a received optical pulse  410 , and LO light  430 . The graphs in  FIG. 27  each illustrate a particular quantity plotted versus time, including the temporal behavior of both the optical power and the optical frequency of the seed light  440  and the LO light  430 . In particular embodiments, a light source  110  may change an optical frequency of seed-laser output light  472 , seed light  440 , LO light  430 , or emitted pulses of light  400  by changing the seed current I 1  supplied to a seed laser diode  450  or by changing the SOA current I 2  supplied to a SOA  460 . Rather than incorporating a discrete optical modulator  495  into a light source  110 , a light source  110  may impart optical frequency changes based on the electrical current supplied to the seed laser diode  450  or the SOA  460 . For example, the light source  110  illustrated in  FIG. 6, 8, 9, 10, 11, 12 , or  13  may not include a modulator  495  and may impart an optical frequency change based on the electrical current supplied to the seed laser diode  450  or the SOA  460 . Changing the electrical current supplied to a seed laser diode  450  or a SOA  460  may cause a corresponding change in the optical frequency of the light emitted by the seed laser diode  450  or the SOA  460  (e.g., the change in optical frequency may result from a change in refractive index, carrier density, or temperature associated with the change in electrical current). For example, an electronic driver  480  may supply a seed laser diode  450  with a time-varying seed current I 1  that results in a frequency offset of Δf between a received pulse of light  410  and a corresponding temporal portion  431  of LO light  430 . As another example, a pulse of electrical current I 2  (e.g., as illustrated in  FIG. 15 ) supplied to a SOA  460  may cause the SOA  460  to produce an emitted pulse of light  400  that has a frequency offset of Δf with respect to a corresponding temporal portion  431  of LO light  430 . In this case, the seed current I 1  may be kept constant, and the frequency offset of the emitted pulse of light  400  may result from nonlinear optical effects within the SOA  460 . 
     In particular embodiments, a seed current I 1  may be alternated between K+1 different current values (where K equals 1, 2, 3, 4, or any other suitable positive integer) so that (i) each temporal portion  441  (and each corresponding emitted pulse of light  400 ) has a particular optical frequency of K different frequencies and (ii) each corresponding temporal portion  431  of the LO light  430  has one particular optical frequency that is different from each of the other K frequencies. In the example of  FIG. 27 , the seed current I 1  supplied to a seed laser diode  450  alternates between the two values i 0  and i 1 . The difference i 0 −i 1  between the two seed-current values may be approximately 1 mA, 2 mA, 5 mA, 10 mA, 20 mA, or any other suitable difference in seed current. For example, an electronic driver  480  may supply seed currents of approximately i 0 =102 mA and i 1 =100 mA, corresponding to a seed-current difference of 2 mA. The seed laser diode  450  produces seed light  440  and LO light  430 , and the optical power of the seed light  440  and the LO light  430  may exhibit changes when the seed current I 1  is changed. For example, when the seed current I 1  is reduced from i 0  to i 1 , the optical power of the seed light  440  or the LO light  430  may be reduced by less than approximately 1 mW, 5 mW, or 10 mW. Additionally, when the seed current I 1  is changed between the values i 0  and i 1 , the optical frequency of the seed light  440  and the LO light  430  may change by Δf between the respective values f 0  and f 1 . The frequency change Δf caused by a change in seed current I 1  may be any suitable frequency change between approximately 10 MHz and approximately 50 GHz, such as for example, a frequency change of 100 MHz, 500 MHz, 1 GHz, 2 GHz, or 5 GHz. 
     In particular embodiments, an electronic driver  480  may (i) supply electrical current i 1  to a seed laser diode  450  during a time interval when a pulse of light  400  is emitted by a light source  110  and (ii) supply a different electrical current i 0  to the seed laser diode  450  for a period of time after the pulse of light  400  is emitted and prior to the emission of a subsequent pulse of light  400 . Switching the electrical current from i 1  to i 0  may result in a change of the frequency of the LO light  430  by Δf, where the frequency change is with respect to: (i) the frequency of the seed light  440  or LO light  430  during the time interval when the pulse of light  400  is emitted and (ii) the frequency of the emitted pulse of light  400 . A photocurrent signal produced by coherent mixing of a received pulse of light  410  with the LO light  430  may include a frequency component at a frequency of approximately Δf. In the example of  FIG. 27 , the seed current I 1  is alternated in time between two current values (i 0  and i 1 ) so that (i) the temporal portion  441  of the seed light  440  has a frequency f 1  and (ii) the LO light  430  (including the temporal portion  431 ) during a period of time after the pulse of light  400  is emitted has a frequency of f 0 , where f 1 =f 0 +Δf. The emitted optical pulse  400  and the received optical pulse  410  may each have optical frequencies of approximately f 1 , corresponding to the frequency of the temporal portion  441 . The received optical pulse  410  may be coherently mixed with the temporal portion  431  of the LO light  430  (which may have a frequency of f 0 ) between the times t c  and t d  to produce a photocurrent signal having a frequency component at a frequency of approximately Δf. 
     In particular embodiments, seed current I 1  and SOA current I 2  maybe synched together so that (i) the seed current I 1  is set to a first value when a pulse of SOA current is supplied to the SOA  460  and (ii) the seed current I 1  is set to a second value during the time periods between successive pulses of SOA current. In  FIG. 27 , when a pulse of light  400  is emitted (between times t a  and t b ), the seed current I 1  is set to the value i 1 , and during the time periods between successive pulses of light  400 , the seed current I 1  is set to the value i 0 . The seed current I 1  may be set to the value i 0  for a period of time less than or equal to the pulse period τ, which corresponds to the time between successive pulses of light  400 . For example, the seed current I 1  may be set to i 0  from time t b  until at least time t d . At or before a time when a subsequent pulse of light  400  (not illustrated in  FIG. 27 ) is emitted, the seed current I 1  may be switched back to the value i 1 , which changes the frequency of the seed light  440  and LO light  430  back to f 1 . After that subsequent pulse of light  400  is emitted, the seed current I 1  may again be set to the value i 0 , which changes the frequency of the LO light  430  by Δf to f 0 . 
     In particular embodiments, an electronic driver  480  may supply seed current I 1  to a seed laser diode  450  where the seed current I 1  includes: (i) a substantially constant electrical current (e.g., a DC current) and (ii) a modulated electrical current. The modulated electrical current may include any suitable waveform, such as for example, a sinusoidal, square, pulsed, sawtooth, or triangle waveform. The constant-current portion of the seed current I 1  may include a DC current of approximately 50 mA, 100 mA, 200 mA, 500 mA, or any other suitable DC electrical current, and the modulated portion of the seed current I 1  may be smaller, with an amplitude of less than or equal to 1 mA, 5 mA, 10 mA, or 20 mA. The modulated portion of the electrical current may produce a corresponding frequency or amplitude modulation in the seed light  440  or the LO light  430 . For example, the modulated electrical current may be applied to the seed laser diode  450  when a pulse of light  400  is emitted so that the emitted pulse of light  400  includes a corresponding frequency or amplitude modulation. The modulated electrical current may not be applied during the time period between successive pulses of light  400 , and so, during this time the LO light  430  may not include a corresponding frequency or amplitude modulation. When a received pulse of light  410  is coherently mixed with the LO light  430 , the photocurrent signal may have a characteristic frequency component corresponding to the frequency or amplitude modulation applied to the emitted pulse of light  400 . For example, the characteristic frequency component may be detected or measured by a frequency-detection circuit  600  to determine whether a received pulse of light is a valid received pulse of light. 
     In particular embodiments, a light source  110  may be configured to impart a frequency change to an emitted pulse of light  400  based on (i) seed current I 1  supplied to a seed laser diode  450  or (ii) SOA current I 2  supplied to a SOA  460 . For example, in addition to or instead of imparting a frequency change to an emitted pulse of light  400  based on the seed current I 1 , a light source  110  may impart a frequency change to an emitted pulse of light based on the SOA current I 2  supplied to a SOA  460 . In particular embodiments, an electronic driver  480  may supply SOA current I 2  to a SOA  460 , where the SOA current is configured to impart a frequency change to an emitted pulse of light  400 . For example, the SOA current I 2  may include pulses of current, where each pulse of current results in the SOA  460  ( i ) amplifying a temporal portion  441  of seed light  440  to produce an emitted pulse of light  400  and (ii) imparting a frequency change to the emitted pulse of light  400 . A frequency change may be imparted to a temporal portion  441  while propagating through the SOA  460 , resulting in an emitted pulse of light  400  that has a frequency offset with respect to LO light  430 . The frequency change may result from a nonlinear optical effect in the SOA waveguide  463  or from a change in refractive index, carrier density, or temperature associated with a pulse of SOA current I 2 . For example, a pulse of SOA current may include a modulation (e.g., a linear or sinusoidal current variation added to the current pulse) that causes a refractive-index variation in the SOA waveguide  463 , which in turn results in a frequency change imparted to the emitted pulse of light  400 . A frequency change of Δf imparted to an emitted pulse of light  400  by a SOA  460  may result in a photocurrent signal (e.g., produced by coherent mixing of a received pulse of light  410  with LO light  430 ) with a frequency component at a frequency of approximately Δf. 
     In particular embodiments, a light source  110  may include an optical modulator  495  or an electronic driver  480  that imparts different frequency changes Δf k  to different temporal portions  441  of seed light  440 . An optical modulator  495  or an electronic driver  480  may apply a repeating series or a pseudo-random series of a particular number (e.g., 2, 3, 4, or any other suitable number) of different frequency changes to different respective temporal portions  441  of seed light  440 . For example, the optical modulator  495  in  FIG. 23  may change the optical frequency of a first temporal portion  441  of seed light  440  by Δf 1 , and the optical modulator  495  may change the optical frequency of a second temporal portion  441  of the seed light  440  by a different frequency-change value Δf 2 . The frequency changes applied to the temporal portions  441  may result in corresponding frequency changes to the emitted pulses of light  400  and the received pulses of light  410 . As another example, the electronic driver  480  in  FIG. 9  may supply three different values of seed current I 1  to the seed laser diode  450 . One value of the seed current may be applied to the seed laser diode  450  after a pulse of light  400  is emitted and prior to the emission of a subsequent pulse of light  400 . This value of seed current sets the optical frequency of the temporal portion  431  of the LO light  430 . The other two values of the seed current may be used to change the optical frequency of a first temporal portion  441  by Δf 1  (relative to the frequency of the temporal portion  431 ) and the optical frequency of a second temporal portion by Δf 2 . 
     In particular embodiments, different frequency changes may correspond to different spectral signatures that may be used to associate a received pulse of light  410  with a particular emitted pulse of light  400 . For example, a first received pulse of light  410  with a frequency change of Δf 1  may result in a photocurrent signal i having a frequency component at a frequency of approximately Δd 1 . A received pulse of light  410  that results in a frequency component at approximately Δf 1  may be associated with an emitted pulse of light  400  having a corresponding Δf 1  frequency change (e.g., the received pulse of light  410  may include light from the emitted pulse of light  400  that is scattered by a target  130 ). Similarly, a second received pulse of light  410  with a frequency change of Δf 2  may result in a photocurrent signal i having a frequency component at a frequency of approximately Δf 2 . A received pulse of light  410  that results in a frequency component at approximately Δf 2  may be associated with an emitted pulse of light  400  having a corresponding Δf 2  frequency change. An optical modulator  495  or an electronic driver  480  may alternate between the Δf 1  and Δf 2  frequency changes so that successive emitted pulses of light  400  have different frequency changes. The alternating frequency changes may allow a received pulse of light  410  to be unambiguously associated with an emitted pulse of light  400  based on the different frequency components associated with different received pulses of light  410 . 
     In particular embodiments, a frequency change imparted to an emitted pulse of light  400  may be referred to as a spectral signature and may be used to (i) determine whether a received pulse of light is a valid received pulse of light  410 , (ii) associate a received pulse of light  410  with an emitted pulse of light  400 , or (iii) determine whether a received pulse of light is an interfering optical signal. For example, a light source  110  may impart a spectral signature of one or more different spectral signatures to seed light  440  or to an amplified temporal portion  441  of the seed light  440  so that each emitted pulse of light  400  includes one of the spectral signatures. Each spectral signature may include a particular frequency change that may be imparted (i) using a modulator  495  (e.g., an electro-optic phase modulator or an acousto-optic modulator), (ii) based on the seed current I 1  supplied to a seed laser diode  450 , or (iii) based on the SOA current I 2  supplied to a SOA  460 . For example, a light source  110  may impart the same frequency change Δf to each emitted pulse of light  400  based on supplying two different values of seed current I 1  to the seed laser diode  450 . If coherent mixing of a received pulse of light  410  with LO light  430  produces a frequency component at approximately the same frequency Δf, then the received pulse of light  410  may be determined to be a valid received pulse of light. If coherent mixing of a received pulse of light with LO light  430  does not produce a frequency component at Δf (or the amplitude of the frequency component at Δf is below a particular threshold value), then the received pulse of light may be ignored or may be determined to be an interfering optical signal. As another example, a light source  110  may impart one of K different frequency changes to each emitted pulse of light  400  (where K equals 1, 2, 3, 4, or any other suitable positive integer). The frequency changes may be imparted in a repeating sequential manner or in a pseudo-random manner. If coherent mixing of a received pulse of light  410  with LO light  430  produces a frequency component at one of the K frequencies Δf k , then the received pulse of light  410  may be determined to be associated with a particular emitted pulse of light  400  having the frequency change Δf k . If coherent mixing of a received pulse of light with LO light  430  does not produce a frequency component corresponding to one of the imparted frequency changes (or the amplitude of the frequency components are below a particular threshold value), then the received pulse of light may be ignored or may be determined to be an interfering optical signal. 
       FIG. 28  illustrates example time-domain and frequency-domain graphs of LO light  430  and two emitted pulses of light  400   a  and  400   b . The time-domain graph of the LO light  430  indicates that the optical power of the LO light  430  is substantially constant. The frequency-domain graph of the LO light  430  indicates that the LO light  430  has a center optical frequency of f 0  and a relatively narrow spectral linewidth of Δν 1 . The pulse of light  400   a  represents an emitted pulse of light with pulse duration Δτ 2 , optical frequency f 1 , and spectral linewidth Δν 2 . The pulse of light  400   b  represents an emitted pulse of light with pulse duration Δτ 3 , optical frequency f 1 , and spectral linewidth Δν 3 . The pulses of light  400   a  and  400   b  each have an optical frequency f 1  that is shifted with respect to the LO light (e.g., f 1 =f 0 +Δf). For example, the frequency of the pulse of light  400   a  or  400   b  may be shifted by a phase modulator  495  or by an electronic driver  480  that changes the seed current I 1  supplied to a seed laser diode  450 . Compared to pulse of light  400   a , the pulse of light  400   b  has an additional modulation applied to it. For example, in addition to changing the seed current I 1  to shift the frequency of the pulse of light  400   b , an amplitude modulation (e.g., a linear or sinusoidal modulation) may be added to the seed current I 1  that results in additional variation that is imparted to the pulse of light  400   b . The additional modulation may result in a wider spectral linewidth so that Δν 3  is greater than Δν 2 . Additionally or alternatively, the additional modulation may result in an amplitude variation added to the pulse of light  400   b  in the time domain or in the frequency domain. The additional modulation added to the pulse of light  400   b  may be used as a spectral signature so that a corresponding received pulse of light  410   r  may be associated with the emitted pulse of light  400   b . A light source may apply two or more different modulations to different respective emitted pulses of light  400  so that a received pulse of light  410  may be unambiguously associated with a particular emitted pulse of light  400 . 
       FIG. 29  illustrates an example voltage signal  360  that results from the coherent mixing of LO light  430  and a received pulse of light  410   r . The received pulse of light  410   r  corresponds to the emitted pulse of light  400   b  in  FIG. 28  (e.g., the received pulse of light  410   r  may include light from the emitted pulse of light  400   b  that is scattered by a target  130 ). The voltage signal  360  is graphed in the frequency domain and exhibits variations in amplitude. These amplitude variations may result from the modulation added to the pulse of light  400   r  and may be used as a spectral signature. The frequency-domain graph of the voltage signal  360  includes peaks at the frequencies f a , f b , and f c . A receiver  140  may include a frequency-detection circuit  600  with three electronic band-pass filters  610  having three respective center frequencies f a , f b , and f c . Based on the amplitudes of these three frequency components, a receiver  140  or controller  150  may determine whether a received pulse of light  410   r  is associated with a particular emitted pulse of light  400   b . For example, if the amplitudes of the three frequency components match a spectral signature for a particular emitted pulse of light  400   b , then the received pulse of light  410   r  may be determined to include scattered light from that emitted pulse of light  400   b.    
       FIG. 30  illustrates two example voltage signals ( 360   a ,  360   b ) that result from the coherent mixing of LO light  430  with two different received pulses of light ( 410   a ,  410   b ). The LO light  430  and the received pulses of light  410   a  and  410   b  are each represented by a time-domain graph and a frequency-domain graph. The time-domain graph of the LO light  430  indicates that the LO light  430  has a substantially constant optical power. The frequency-domain graph indicates that the LO light  430  has a center optical frequency of f 0  and a relatively narrow spectral linewidth of Δν 1 . For example, the optical frequency f 0  may be approximately 199.2 THz (corresponding to a wavelength of approximately 1505 nm), and the spectral linewidth Δν 1  may be approximately 2 MHz. The received pulse of light  410   a  has a pulse duration of Δτ a  and a spectral linewidth of Δν a . The received pulse of light  410   b  has a pulse duration of Δτ b  (where Δτ b  is greater than Δτ a ) and a spectral linewidth of Δν b  (where Δν b  is less than Δν a ). As an example, the pulse of light  410   a  may have a 3-ns pulse duration and a 500-MHz spectral linewidth, and the pulse of light  410   b  may have a 6-ns pulse duration and a 250-MHz spectral linewidth. The coherent mixing of the LO light  430  and the pulse of light  410   a  at a detector  340  may result in a pulse of photocurrent i which is amplified by an amplifier  350  that produces the voltage signal  360   a . Similarly, the coherent mixing of the LO light  430  and the pulse of light  410   b  at a detector  340  may result in a pulse of photocurrent i which is amplified by an amplifier  350  that produces the voltage signal  360   b.    
     A pulse duration (Δτ) and spectral linewidth (Δν) of a pulse of light may have an inverse relationship where the product Δτ·Δν (which may be referred to as a time-bandwidth product) is equal to a constant value. For example, a pulse of light with a Gaussian temporal shape may have a time-bandwidth product equal to a constant value that is greater than or equal to 0.441. If a Gaussian pulse has a time-bandwidth product that is approximately equal to 0.441, then the pulse may be referred to as a transform-limited pulse. For a transform-limited Gaussian pulse, the pulse duration (Δτ) and spectral linewidth (Δν) may be related by the expression Δτ·Δν=0.441. The inverse relationship between pulse duration and spectral linewidth indicates that a shorter-duration pulse has a larger spectral linewidth (and vice versa). For example, in  FIG. 30 , pulse of light  410   a  has a shorter duration and a larger spectral linewidth than pulse of light  410   b . This inverse relationship between pulse duration and spectral linewidth results from the Fourier-transform relationship between time-domain and frequency-domain representations of a pulse. In the example of  FIG. 30 , the received pulse of light  410   a  may be a transform-limited Gaussian pulse with a pulse duration Δτ a  of 2 ns and a spectral linewidth Δν a  of approximately 220 MHz. Similarly, the received pulse of light  410   b  may be a transform-limited Gaussian pulse with a pulse duration Δτ b  of 4 ns and a spectral linewidth Δν b  of approximately 110 MHz. If a Gaussian pulse of light has a time-bandwidth product that is greater than 0.441, then the pulse of light may be referred to as a non-transform-limited pulse of light. For example, if the pulses of light in  FIG. 30  are non-transform-limited with a time-bandwidth product of 1, then the received pulse of light  410   a  may have a pulse duration Δτ a  of 2 ns and a spectral linewidth Δν a  of approximately 500 MHz. Similarly, the received pulse of light  410   b  may have a pulse duration Δτ b  of 4 ns and a spectral linewidth Δν b  of approximately 250 MHz. 
     When LO light  430  and a received pulse of light  410  are coherently mixed, a voltage signal  360  may be produced, and the voltage signal may include a voltage pulse having a particular frequency-domain representation. In  FIG. 30 , the graph of voltage signal  360   a  is a frequency-domain representation of the voltage signal that results from the coherent mixing of LO light  430  and received pulse of light  410   a . The graph of voltage signal  360   b  is a frequency-domain representation of the voltage signal that results from the coherent mixing of LO light  430  and received pulse of light  410   b . The voltage signal  360   a  includes frequency components that depend on a numeric combination of the linewidths of the LO light  430  and pulse of light  410   a . Similarly, the voltage signal  360   b  includes frequency components that depend on the linewidths of the LO light  430  and pulse of light  410   b . The voltage signal  360   a  has frequency components that extend over a wider frequency range than voltage signal  360   b , which corresponds to the spectral linewidth Δν a  of pulse of light  410   a  being larger than the spectral linewidth Δν b  of pulse of light  410   b.    
     In particular embodiments, an electronic driver  480  may supply pulses of current to a SOA  460 , and each pulse of current may cause the SOA  460  to (i) amplify a temporal portion  441  of seed light  440  to produce an emitted pulse of light  400  and (ii) impart a spectral signature to the temporal portion  441  so that the emitted pulse of light  400  includes the spectral signature. A spectral signature may be imparted by amplifying a temporal portion  441  of seed light  440  to produce an emitted pulse of light  400  having a particular spectral linewidth. The spectral signature may correspond to one or more of the frequency components associated with the spectral linewidth of the emitted pulse of light  400 . The seed light  440  may have a relatively narrow linewidth (e.g., which may be approximately equal to Δν 1  in  FIG. 30 ), and amplifying a temporal portion  441  of seed light  440  may result in the linewidth being broadened according to the inverse relationship between pulse duration (Δτ) and spectral linewidth (Δν). For example, amplifying a temporal portion  441  may produce a pulse of duration Δτ a  (e.g., as illustrated in  FIG. 30 ) from seed light  440 , which results in the spectral linewidth being broadened from Δν 1  to Δν a . 
     In particular embodiments, an electronic driver  480  may be configured to supply pulses of current to a SOA  460 , where each pulse of current imparts to each corresponding emitted pulse of light  400  a spectral signature of one or more different spectral signatures. For example, an electronic driver  480  may supply electrical current pulses having one or more different durations, and each current-pulse duration may result in an emitted pulse of light  400  having a particular pulse duration and a corresponding particular spectral linewidth. As another example, an electronic driver  480  may alternate between supplying two different pulses of current, where one pulse of current results in an emitted pulse of light  400  (e.g., associated with received pulse of light  410   a  in  FIG. 30 ) having a particular pulse duration and spectral linewidth, and the other pulse of current results in an emitted pulse of light  400  (e.g., associated with received pulse of light  410   b ) having a longer pulse duration and a narrower spectral linewidth. A particular spectral signature being imparted to a temporal portion  441  or to an emitted pulse of light  400  may result from a corresponding rise time, fall time, pulse duration, or pulse shape of a pulse of current supplied to the SOA  460 . For example, applying a pulse of current having a particular duration may result in an emitted pulse of light  400  that has a particular spectral linewidth corresponding to the duration of the current pulse. Shorter-duration current pulses supplied to the SOA  460  may result in emitted pulses of light  400  having shorter pulse durations and broader spectral linewidths. In  FIG. 30 , the pulse of light  410   a  may be associated with an emitted pulse of light produced by applying a 5-ns current pulse to a SOA  460 , and the pulse of light  410   b  may be associated with an emitted pulse of light produced by applying a 9-ns current pulse to the SOA  460 . As another example, applying a pulse of current having a particular rise time may result in an emitted pulse of light  400  having a particular spectral linewidth corresponding to the rise time of the current pulse. Current pulses with shorter-duration rise times may result in emitted pulses of light  400  having broader spectral linewidths. 
     In particular embodiments, a spectral signature of a pulse of light may be associated with a pulse characteristic (e.g., a rise time, a fall time, a pulse duration, or a pulse shape) of the pulse of light. For example, an emitted pulse of light  400  having a particular pulse duration or rise time may correspond to a particular spectral signature. Emitted pulses of light  400  or received pulses of light  410  having shorter pulse durations or shorter rise times may be associated with broader spectral linewidths. In  FIG. 30 , the shorter pulse duration Δτ a  of the received pulse of light  410   a  is associated with the broader spectral linewidth Δν a , and the longer pulse duration Δτ b  of the received pulse of light  410   b  is associated with the narrower spectral linewidth Δν b . 
     In particular embodiments, a spectral signature of an emitted pulse of light  400  or a received pulse of light  410  may correspond to one or more frequency components of the pulse of light. In  FIG. 30 , the frequency components of received pulse of light  410   a  that are located outside the spectral linewidth of the LO light  430  may correspond to the spectral signature of the received pulse of light  410   a . These frequency components may correspond to new frequency components outside of the Δν 1  linewidth that are imparted to a temporal portion  441  when an emitted pulse of light  400  is produced. For example, the spectral signature of the received pulse of light  410   a  may correspond to one or more frequency components located approximately in the range from f 0 −Δν a  to f 0 −Δν 1  and approximately in the range from f 0 +Δν 1  to f 0 +Δν a . Similarly, the spectral signature of the received pulse of light  410   b  may correspond to the frequency components located approximately in the range from f 0 −Δν b  to f 0 −Δν 1  and approximately in the range from f 0 +Δν 1  to f 0 +Δν b . 
     In particular embodiments, a spectral signature may correspond to the presence or absence of one or more particular frequency components in a received pulse of light  410 . A receiver  140  may include a frequency-detection circuit  600  configured to determine the amplitude of one or more frequency components of a received pulse of light  410 . Based on the amplitudes of the one or more frequency components, a receiver  140  or a controller  150  may determine whether a received pulse of light  410  ( i ) matches the spectral signature of an emitted pulse of light  400 , (ii) is a valid received pulse of light  410 , or (iii) is an interfering pulse of light. For example, a frequency-detection circuit  600  may include one or more band-pass filters  610  at frequencies that correspond to frequency components associated with one or more spectral signatures. If one or more particular frequency components each has an amplitude above or below a particular threshold value or within a particular range of values, then a receiver  140  or controller  150  may determine that a received pulse of light  410  is a valid received pulse of light that is associated with an emitted pulse of light  400 . For example, based on voltage signal  360   a  in  FIG. 30 , if the amplitude of frequency component f y  of a received pulse of light is above a particular threshold value, then a receiver  140  or controller  150  may determine that the received pulse of light is a valid received pulse of light that matches the spectral signature associated with pulse  410   a.    
     In particular embodiments, a light source  110  may emit pulses of light  400  with pulse durations and spectral linewidths that alternate between two or more different pulse durations and spectral linewidths (e.g., the pulse durations and linewidths of pulses  410   a  and  410   b  illustrated in  FIG. 30 ). Based on the example voltage signals  360   a  and  360   b  illustrated in  FIG. 30 , a frequency-detection circuit  600  may include two band-pass filters  610  having respective center frequencies of f x  and f y . As an example, the frequency-detection circuit  600  may determine the amplitude of the frequency component f y , and based at least in part on that amplitude, a receiver  140  or controller  150  may determine whether a received pulse of light matches the spectral signature associated with pulse  410   a  or pulse  410   b . If the amplitude of frequency component f y  of a received pulse of light  410  exceeds a particular threshold value, then a receiver  140  or controller  150  may determine that the received pulse of light  410  is associated with an emitted pulse of light  400  having the spectral signature associated with pulse  410   a . As another example, the frequency-detection circuit  600  may determine the amplitudes of the two frequency components f x  and f y , and based at least in part on those amplitudes, a receiver  140  or controller  150  may determine whether a received pulse of light matches the spectral signature associated with pulse  410   a  or pulse  410   b . If the amplitudes of frequency components f x  and f y  are each above or below a particular threshold value or within a particular range of values, then a receiver  140  or controller  150  may determine whether a received pulse of light  410  matches the spectral signature of pulse  410   a  or  410   b . Additionally or alternatively, if the ratio of the amplitudes of the two frequency components f x  and f y  is above or below a particular threshold value, then a receiver  140  or controller  150  may determine whether a received pulse of light  410  matches the spectral signature of pulse  410   a  or  410   b . For example, a receiver  140  or controller  150  may determine the ratio A(f y )/A(f x ), where A(f y ) is the amplitude of frequency component f y , and A(f x ) is the amplitude of frequency component f x . If the ratio is greater than a particular threshold value (e.g., if A(f y )/A(f x ) is greater than 0.25), then the corresponding received pulse of light  410  may be determined to be associated with an emitted pulse of light  400  having the spectral signature associated with pulse  410   a . Similarly, if the ratio is less than a particular threshold value, then the corresponding received pulse of light  410  may be determined to match the spectral signature of pulse  410   b.    
       FIG. 31  illustrates an example light source  110  and receiver  140  integrated into a photonic integrated circuit (PIC)  455  that is part of a coherent pulsed lidar system  100 . In particular embodiments, a coherent pulsed lidar system  100  may include a light source  110 , a receiver  140 , and a processor or controller  150 , and at least part of the light source  110  or at least part of the receiver  140  may be disposed on or in a PIC  455 . In the example of  FIG. 31 , both the light source  110  and the receiver  140  are disposed on or in the PIC  455 . As another example, the receiver  140  may be disposed on or in the PIC  455 , and the light source may be packaged separately from the PIC  455 . The light source  110  may emit (i) LO light  430  and (ii) an output beam  125  that includes pulses of light  400 , where each emitted pulse of light  400  is coherent with a corresponding portion of the LO light  430 . The receiver  140  may include one or more detectors  340  that detect the LO light  430  and a received pulse of light  410 , where the LO light  430  and the received pulse of light  410  are coherently mixed together at the receiver  140 . The received pulse of light  410  may include light from one of the emitted pulses of light  400  scattered by a target  130  located a distance D from the lidar system  100 , and the processor or controller  150  may determine the distance to the target  130  based on a time of arrival for the received pulse of light  410 . All or part of the processor or controller  150  may be attached to, electrically coupled to, or located near the PIC  455 . 
     In the example of  FIG. 31 , the light source  110  emits an output beam  125  that includes a pulse of light  400 , and the receiver  140  detects an input beam  135  that includes a received pulse of light  410  that may include light from the emitted pulse of light  400  scattered by a target  130 . In particular embodiments, a PIC  455  that is part of a lidar system  100  may include one or more seed laser diodes  450 , one or more waveguides  479 , one or more optical isolators  530 , one or more splitters  470 , one or more SOAs  460 , one or more lenses  490 , one or more polarization elements  465 , one or more combiners  420 , or one or more detectors  340 . The PIC  455  in  FIG. 31  includes the following optical components: seed laser diode  450 , optical isolator  530 , splitter  470 , SOA  460 , output lens  490   a , polarization element  465 , input lens  490   b , combiner  420 , and detectors  340   a  and  340   b . Additionally, the PIC  455  includes optical waveguides  479  that convey light from one optical component to another. The waveguides  479  may be passive optical waveguides formed in a PIC substrate material that includes silicon, InP, glass, polymer, or lithium niobate. The amplifier  350  or the pulse-detection circuit  365  may be attached to, electrically coupled to, or located near the PIC  455 . One or more optical components of the light source  110  or receiver  140  may be fabricated separately and then integrated with the PIC  455 . For example, the seed laser diode  450 , isolator  530 , SOA  460 , lenses  490   a  and  490   b , or detectors  340   a  and  340   b  may be fabricated separately and then integrated into the PIC  455 . An optical component may be integrated into the PIC  455  by attaching or connecting the optical component to the PIC  455  or to a substrate to which the PIC  455  is also attached. For example, an optical component may be attached to a PIC  445  using epoxy or solder. 
     In particular embodiments, a PIC  455  may include one or more optical waveguides  479  that direct seed light  440  to a SOA  460  and direct LO light  430  to a receiver  140 . For example, a light source  110  may include a PIC  455  with an optical waveguide  479  that receives seed light  440  from a seed laser diode  450  and directs the seed light  440  to a SOA  460 . As another example, an optical waveguide  479  may receive seed-laser output light  472  from a seed laser diode  450  and direct a portion of the seed-laser output light  472  (which corresponds to the seed light  440 ) to a SOA  460 . In  FIG. 31 , an optical waveguide  479  of the PIC  455  receives the seed-laser output light  472  from the front face  452  of the seed laser diode  450  and directs the output light  472  through the isolator  530  and then to the input port of the splitter  470 . The splitter  470  splits the seed-laser output light  472  to produce the seed light  440  and the LO light  430 . One optical waveguide  479  directs the seed light  440  from output port 1 of the splitter  470  to the SOA  460 , and another optical waveguide  479  directs the LO light  430  from output port 2 of the splitter  470  to the combiner  420  of the receiver  140 . 
     In particular embodiment, a PIC  455  may include one or more optical waveguides  479 , one or more optical splitters  470 , or one or more optical combiners  420 . The one or more waveguides  479 , splitters  470 , or combiners  420  may be configured to convey, split, or combine the seed-laser output light  472 , seed light  440 , LO light  430 , emitted pulses of light  400 , or received pulses of light  410 . In  FIG. 31 , the optical splitter  470  is an optical-waveguide splitter  470  that splits the seed-laser output light  472  to produce the seed light  440  and the LO light  430 . The integrated-optic optical combiner  420  in  FIG. 31  (which is similar to the combiner  420  illustrated in  FIG. 19 ) combines the input beam  135 , which includes the received pulse of light  410 , with the LO light  430  and directs combined beam  422   a  to detector  340   a  and combined beam  422   b  to detector  340   b.    
     In particular embodiments, a PIC  455  may include one or more lenses  490  configured to collimate light emitted from the PIC  455  or focus light into the PIC  455 . A lens  490  may be attached to, connected to, or integrated with the PIC  455 . For example, a lens  490  may be fabricated separately and then attached to the PIC  455  (or to a substrate to which the PIC  455  is attached) using epoxy or solder. The output lens  490   a  in  FIG. 31  may collimate the emitted pulses of light  400  from the SOA  460  to produce a collimated output beam  125 . The output beam  125  may be scanned across a field of regard by a scanner  120  (not illustrated in  FIG. 31 ). Light from an emitted pulse of light  400  may be scattered by a target  130 , and a portion of the scattered light may be directed to the receiver  140  as a received pulse of light  410 . The input lens  490   b  in  FIG. 31  may focus the received pulse of light  410  into a waveguide  479  of the PIC  455 , which directs the received pulse of light  410  to the combiner  420 . The combiner  420  combines the received pulse of light  410  with the LO light  430  and directs the combined beams  422   a  and  422   b  to the respective detectors  340   a  and  340   b . The LO light  430  and the received pulse of light  410  are coherently mixed together at the detectors  340   a  and  340   b , and the detectors  340   a  and  340   b  produce a subtracted photocurrent signal i a −i b , which is directed to the amplifier  350 . 
     A receiver  140  of a lidar system  100  that includes a PIC  455  may include 1, 2, 4, 8, or any other suitable number of detectors  340 . For example, a receiver  140  may include a single detector  340  that detects the LO light  430  and the input beam  135 . In the example of  FIG. 31 , the receiver  140  includes one integrated-optic combiner  420  and two detectors  340   a  and  340   b . The integrated-optic combiner  420  combines the LO light and the received pulse of light  410  and produces two combined beams  422   a  and  422   b . Detector  340   a  detects the combined beam  422   a  (which includes a first portion of the combined LO light  430  and received pulse of light  410 ), and detector  340   b  detects the combined beam  422   b  (which includes a second portion of the combined LO light  430  and received pulse of light  410 ). As another example, a receiver  140  may include two integrated-optic combiners  420  and four detectors  340  (e.g., one combiner  420  and two detectors  340  may combine and detect a first polarization component, and the other combiner  420  and two detectors  340  may combine and detect a second polarization component orthogonal to the first polarization component). As another example, a receiver  140  may include an integrated-optic 90-degree optical hybrid  428  and four detectors  340  (e.g., as illustrated in  FIG. 20  and described herein). As another example, a receiver  140  may include two integrated-optic 90-degree optical hybrids  428  and eight detectors  340  (e.g., one 90-degree optical hybrid  428  and four detectors  340  may combine and detect a first polarization component, and the other 90-degree optical hybrid  428  and four detectors  340  may combine and detect a second polarization component orthogonal to the first polarization component). 
     In  FIG. 31 , the light source  110  includes a seed laser diode  450  that emits seed-laser output light  472  that is split to produce seed light  440  and LO light  430 . The SOA  460 , which has a tapered waveguide  463  (e.g., a width of the SOA waveguide  463  increases from the input end  461  to the output end  462 ), amplifies the seed light  440  to produce the output beam  125 . For example, the SOA  460  may amplify temporal portions of the seed light  440  to produce an output beam  125  that includes emitted pulses of light  400 , where each amplified temporal portion of the seed light  440  corresponds to one of the emitted pulses of light  400 . The light source  110  may include an electronic driver  480  (not illustrated in  FIG. 31 ) that (i) supplies a modulated or substantially constant electrical current to the seed laser diode  450  and (ii) supplies pulses of current to the SOA  460 . The electronic driver  480  may impart frequency changes to seed light  440 , emitted pulses of light  400 , or LO light  430  based on the seed current I 1  supplied to the seed laser diode  450  or based on the SOA current I 2  supplied to the SOA  460 . The light source  110  may also include a fiber-optic amplifier  500  (not illustrated in  FIG. 31 ) that further amplifies light produced by the SOA  460 . The fiber-optic amplifier  500 , which may be similar to that illustrated in  FIGS. 13-14  and described herein, may receive an optical signal from the SOA  460  and further amplify the optical signal to produce an output beam  125 . For example, the SOA  460  may amplify portions of seed light  440  to produce pulses of light, and the fiber-optic amplifier  500  may further amplify the pulses of light to produce an output beam  125  that includes emitted pulses of light  400 . 
     In particular embodiments, a lidar system  100  that includes a PIC  455  may include a light source  110  with an optical isolator  530 . In  FIG. 31 , the light source  110  includes a seed laser diode  450 , an optical isolator  530 , and a SOA  460 , where the optical isolator  530  is located between the seed laser diode  450  and the SOA  460 . The optical isolator  530  may be an integrated-optic isolator, a fiber-optic isolator, or a free-space isolator. The isolator  530  in  FIG. 31  may include a Faraday-type isolator or a filter-type isolator and may be configured to (i) transmit seed light  440  to the SOA  460  and (ii) reduce an amount of light that propagates from the SOA  460  toward the seed laser diode  450 . 
     In particular embodiments, a coherent pulsed lidar system  100  that includes a PIC  455  may include an optical polarization element  465 . For example, the optical polarization element  465  in  FIG. 31  may alter the polarization of the LO light  430  so that the LO light  430  and the received pulse of light  410  may be coherently mixed. The polarization element  465  may ensure that at least a portion of the received pulse of light  410  and the LO light  430  have polarizations that are oriented in the same direction. The polarization element  465  may include one or more quarter-wave plates, one or more half-wave plates, one or more optical polarizers, one or more optical depolarizers, or any suitable combination thereof. For example, the polarization element  465  may include a quarter-wave plate that converts linearly polarized LO light  430  produced by the seed laser diode  450  into circular or elliptically polarized light. In the example of  FIG. 31 , the polarization element  465  may be an integrated-optic component. 
       FIG. 32  illustrates an example single junction seed laser diode  450 . The seed laser diode  450  includes one laser junction  700 , which may be referred to as a junction or a p-n junction. The laser junction  700  includes p-doped and n-doped cladding regions ( 712 ,  720 ), p-doped and n-doped waveguide regions ( 714 ,  718 ), and an active region  716 . The p-doped and n-doped regions of the laser junction  700  form a p-n junction that is forward biased when the seed current I 1  is supplied to the laser diode  450 . The active region  716  may be an undoped intrinsic region located between the p-doped and n-doped regions, or the active region  716  may be p-doped or n-doped. The seed current I 1  flows through the laser diode  450  and may produce optical gain in the active region  716  (which may be referred to as a light-producing region or a light-amplifying region). For example, a pulse of seed current may produce a pulse of light that propagates within the seed laser diode  450  back and forth between the back face  451  and the front face  452 . The direction of light propagation within the seed laser diode  450  may be referred to as a longitudinal propagation direction and may be approximately parallel to the direction of the seed light  440  produced by the seed laser diode. As the pulse of light propagates between the back and front faces, the pulse of light may experience optical gain in the active region  716  through stimulated emission of photons, and a portion of the amplified pulse of light may exit from the front face  452  to produce a pulse of seed light. 
     In  FIG. 32 , the light propagating between the faces and within the seed laser diode  450  may be confined laterally within an optical waveguide that includes the p-doped waveguide  714  and the n-doped waveguide  718 . The p-doped and n-doped waveguides may have a higher refractive index than the surrounding p-doped cladding  712  and n-doped cladding  720 . This refractive index difference between the waveguide and cladding regions may cause light to be confined within the optical waveguide and to propagate within the waveguide along the longitudinal direction. The laser mode  730  in  FIG. 32  represents an approximate transverse shape or optical intensity of the light propagating back and forth within the optical waveguide. The optical intensity of the laser mode  730  may be highest within the active region  716  where the optical gain is highest, and the intensity may decrease away from the active region. In other embodiments, a laser junction  700  may not include a separate or distinct optical waveguide. Instead, an optical waveguide may be provided by an active region  716  which may have a higher refractive index than surrounding layers. In this embodiment, the active region may act as both an optical waveguide that guides light within the laser diode as well as a gain region that amplifies the light. 
     In particular embodiments, a seed laser diode  450  that is part of a multi junction light source  110  may be a Fabry-Perot laser diode, a quantum well laser, a distributed Bragg reflector (DBR) laser, a distributed feedback (DFB) laser, a VCSEL, a quantum dot laser diode, or any other suitable type of laser diode. For example, the seed laser diode  450  of a multi junction light source  110  may be a DFB laser that includes a grating  740 . The grating  740  may be located within any suitable region or layer of a DFB laser (e.g., within a cladding or waveguide region). The seed laser diode  450  in  FIG. 32  is a DFB laser that includes a grating  740  located within the p-doped cladding  712 . The grating  740  is oriented parallel to the layers of the seed laser diode  450  and along the longitudinal propagation direction of light within the DFB laser. A grating  740  (which may be referred to as a Bragg grating or an optical grating) may have a refractive index that varies along a longitudinal axis of the seed laser diode  450 . For example, the refractive index of the grating  740  may vary periodically with distance along the longitudinal axis. A grating  740  may provide a distributed reflection of light within a particular wavelength range, and the grating  740  may provide for seed light  440  that has improved wavelength stability. For example, the seed light  440  produced by a DFB laser may have a smaller spectral linewidth or a reduced variation of wavelength over time or temperature with respect to another type of laser diode, such as for example, a Fabry-Perot laser diode. As another example, a Fabry-Perot laser diode may have a temperature-dependent wavelength variation of greater than 0.3 nm/° C., and a DFB laser may have a temperature-dependent wavelength variation of less than 0.1 nm/° C. 
     The seed laser diode  450  in  FIG. 32  includes an anode  711  and a cathode  723  that conduct the seed current I 1  into and out of the laser junction  700 . The anode  711  and cathode  723  may each include a layer of electrically conductive metal deposited onto the respective p-doped and n-doped contacts. Additionally or alternatively, the anode  711  and cathode  723  may each include a region of p-doped or n-doped semiconductor material that provides electrical conductivity to the seed current I 1  flowing through the laser diode  450 . For example, contact  710  may include a heavily p-doped region that acts as part of the anode  711 , and contact  722  may include a heavily n-doped region that acts as part of the cathode  723 . 
       FIG. 33  illustrates an example multi junction seed laser diode  450  with two laser junctions  700   a  and  700   b . In particular embodiments, a seed laser diode  450  of a multi junction light source  110  may be a multi junction seed laser diode that includes two or more laser junctions  700 . A seed laser diode that includes two or more laser junctions  700  may be referred to as a multi-junction seed laser diode. The multi junction seed laser diode  450  in  FIG. 33  includes two laser junctions  700   a  and  700   b , where each laser junction may be similar to the laser junction  700  illustrated in  FIG. 32 . For example, in addition to an active region  716 , each laser junction in  FIG. 33  may also include a p-doped cladding region  712 , a n-doped cladding region  720 , a p-doped waveguide region  714 , or a n-doped waveguide region  718 . Each laser junction in  FIG. 33  may include a semiconductor p-n junction that is forward biased when the seed current I 1  is supplied to the laser diode  450 . The seed current I 1  may flow through both junctions  700   a  and  700   b  and may produce optical gain in the respective active regions  716   a  and  716   b . The seed light  440  produced by the multi junction seed laser diode  450  includes each of the two seed-light portions  440   a  and  440   b . Junction  700   a  produces seed light  440   a , and junction  700   b  produces seed light  440   b . Seed light  440   a  and  440   b  may each be referred to as a seed-optical-signal portion, a seed-light portion, or a portion of seed light. 
     In particular embodiments, a multi junction seed laser diode  450  may include one or more tunnel junctions  750 , where one of the tunnel junctions is located between each pair of adjacent laser junctions  700 . The multi junction seed laser diode  450  in  FIG. 33  includes one tunnel junction  750  located between the pair of adjacent laser junctions  700   a  and  700   b . A tunnel junction  750  may provide electrical separation or isolation between adjacent laser junctions  700  which allows seed current I 1  to flow through the seed laser diode  450 . When seed current I 1  is provided to a multi junction seed laser diode  450 , a tunnel junction  750  may include a p-n junction that is reverse biased. Instead of blocking the flow of seed current, the tunnel junction  750  may be configured to provide good electrical conductivity when reverse biased so that the seed current is not blocked. Without a tunnel junction between two adjacent laser junctions  700 , the adjacent laser junctions may form a reverse-biased p-n junction that prevents the flow of seed current. For example, without tunnel junction  750  in  FIG. 33 , the n-doped lower portion of laser junction  700   a  and the p-doped upper portion of laser junction  700   b  may form a reverse-biased p-n junction that blocks the flow of seed current through the laser diode  450 . 
       FIG. 34  illustrates an example multi junction seed laser diode  450  with three laser junctions  700   a ,  700   b , and  700   c . Each laser junction in  FIG. 34  may be similar to the laser junction  700  illustrated in  FIG. 32 . For example, in addition to an active region  716 , each laser junction in  FIG. 34  may also include a p-doped cladding region  712 , a n-doped cladding region  720 , a p-doped waveguide region  714 , or a n-doped waveguide region  718 . Each laser junction in  FIG. 34  may include a semiconductor p-n junction, and seed current I 1  may flow through the laser junctions  700   a ,  700   b , and  700   c  and may produce optical gain in the respective active regions  716   a ,  716   b , and  716   c . Each laser junction  700  of a multi junction seed laser diode  450  may produce a corresponding seed-light portion. The seed light  440  produced by the multi junction seed laser diode  450  in  FIG. 34  includes seed-light portion  440   a  produced by junction  700   a , seed-light portion  440   b  produced by junction  700   b , and seed-light portion  440   c  produced by junction  700   c.    
     A multi junction seed laser diode  450  may include N laser junctions  700 , where N is an integer greater than or equal to 2. The multi junction seed laser diode  450  may produce seed light  440  that includes N seed-light portions, each seed-light portion produced by one of the N laser junctions. The multi junction seed laser diode  450  in  FIG. 34  includes three laser junctions ( 700   a ,  700   b ,  700   c ), and the seed laser diode produces seed light  440  that includes three respective seed-light portions ( 440   a ,  440   b ,  440   c ). 
     In  FIG. 34 , each of the tunnel junctions  750   a  and  750   b  is located between a pair of adjacent laser junctions and may provide electrical separation or isolation between the pair of adjacent junctions. Tunnel junction  750   a  is located between the pair of adjacent laser junctions  700   a  and  700   b , and tunnel junction  750   b  is located between the pair of adjacent laser junctions  700   b  and  700   c . In particular embodiments, a multi junction seed laser diode  450  may include N laser junctions  700  and N−1 tunnel junctions  750 , where Nis an integer greater than or equal to 2. Each tunnel junction  750  may be located between one pair of adjacent laser junctions  700 . In  FIG. 33 , the parameter N has a value of 2, and the multi junction seed laser diode  450  includes two laser junctions ( 700   a ,  700   b ) and one tunnel junction  750  located between the two laser junctions. In  FIG. 34 , the parameter N has a value of 3, and the multi junction seed laser diode  450  includes three laser junctions ( 700   a ,  700   b ,  700   c ) and two tunnel junctions ( 750   a ,  750   b ). 
     In particular embodiments, a multi junction seed laser diode  450  may include a grating  740   b  located within or near one of the laser junctions  700  of the seed laser diode. A grating may be located (i) between a p-doped contact  710  and an active region  716  of a laser junction  700  adjacent to the contact, (ii) between a n-doped contact  722  and an active region  716  of a laser junction  700  adjacent to the contact, or (iii) between any two adjacent active regions  716 . Laser junction  700   b  in  FIG. 34  includes a grating  740   b  located below the active region  716   b  and above the tunnel junction  750   b . The grating  740   b  in  FIG. 34  may be similar to the grating  740  in  FIG. 32 , and laser junction  700   b  may operate similar to a DFB laser. The grating  740   b  may stabilize the wavelength of the seed-light portion  440   b  produced by the laser junction  700   b . For example, without a grating present, the seed-light portion  400   b  produced by the laser junction  700   b  may have a temperature-dependent wavelength variation of 0.4 nm/° C., and with the grating  740   b , the temperature-dependent wavelength variation may be 0.08 nm/° C. Having the wavelength of the seed-light portion  440   b  stabilized may also cause the other seed-light portions  440   a  and  440   c  to become similarly stabilized. For example, the laser modes in adjacent junctions may be partially overlapped, and the wavelength-stabilized laser mode of the light propagating within laser junction  700   b  may cause the adjacent laser modes to become similarly wavelength stabilized. As a result, the three seed-light portions  440   a ,  440   b , and  440   c  may have approximately the same wavelength (e.g., the wavelengths of the three seed-light portions may be within 0.1 nm of one another). In particular embodiments, a multi junction seed laser diode  450  may include two or more gratings  740   b . Each grating  740   b  may be located within or near one of the laser junctions  700  and may be configured to provide wavelength stabilization for the corresponding seed-light portion produced by the laser junction. 
       FIG. 35  illustrates an example single-junction semiconductor optical amplifier (SOA)  460 . The SOA  460  includes one SOA junction  800 , which may be similar to the laser junction  700  in  FIG. 32  (e.g., the SOA junction  800  includes p-doped and n-doped cladding regions ( 812 ,  820 ), p-doped and n-doped waveguide regions ( 814 ,  818 ), and an active region  816 ). The p-doped and n-doped regions of the SOA junction  800  form a p-n junction that is forward biased when the SOA current I 2  is supplied to the SOA  460 . The active region  816  may be an undoped intrinsic region located between the p-doped and n-doped regions, or the active region  816  may be p-doped or n-doped. The SOA current I 2  flows through the SOA  406  and may produce optical gain in the active region  816 . For example, the SOA  460  may receive seed light  440  that includes a pulse of light, and the pulse of light may be amplified as it propagates through the SOA from the input end  461  to the output end  462 . The amplified pulse of light may then be emitted from the output end  462  as part of the output beam  125 . The p-doped waveguide  814  and n-doped waveguide  818  may form an optical waveguide that confines the seed light  440  as it propagates through the SOA  460  and is amplified within the active region  816 . The p-doped and n-doped waveguides may have a higher refractive index than the surrounding p-doped cladding  812  and n-doped cladding  820 , which may provide optical confinement within the optical waveguide. In other embodiments, a SOA junction  800  may not include a separate or distinct optical waveguide. Instead, an optical waveguide may be provided by an active region  816  which may have a higher refractive index than surrounding layers. In this embodiment, the active region  816  may act as both an optical waveguide that guides light as well as a gain region that amplifies the light. 
     The SOA  460  in  FIG. 35  includes an anode  811  and a cathode  823  that conduct the SOA current I 2  into and out of the SOA junction  800 . The anode  811  and cathode  823  (which may be similar to the anode  711  and cathode  723  in  FIG. 32 ) may each include a layer of electrically conductive metal deposited onto the respective p-doped and n-doped contacts. Additionally or alternatively, the anode  811  and cathode  823  may each include a region of p-doped or n-doped semiconductor material that provides electrical conductivity to the SOA current I 2  flowing through the SOA  460 . For example, contact  810  may include a heavily p-doped region that acts as part of the anode  811 , and contact  822  may include a heavily n-doped region that acts as part of the cathode  823 . 
       FIG. 36  illustrates an example multi junction SOA  460  with two SOA junctions  800   a  and  800   b . In particular embodiments, a SOA  460  of a multi junction light source  110  may be a multi junction SOA that includes two or more SOA junctions  800 . A SOA  460  that includes two or more SOA junctions  800  may be referred to as a multi junction SOA. The multi junction SOA  460  in  FIG. 36  includes the two SOA junctions  800   a  and  800   b , where each SOA junction may be similar to the SOA junction in  FIG. 35 . For example, in addition to an active region  816 , each SOA junction in  FIG. 36  may also include a p-doped cladding region  812 , a n-doped cladding region  820 , a p-doped waveguide region  814 , or a n-doped waveguide region  818 . Each SOA junction in  FIG. 36  may include a semiconductor p-n junction that is forward biased when the SOA current is supplied to the multi junction SOA  460 . The SOA current I 2  may flow through both junctions  800   a  and  800   b  and may produce optical gain in the respective active regions  816   a  and  816   b . The output beam  125  produced by the multi junction SOA  460  includes each of the two output-beam portions  125   a  and  125   b . SOA junction  800   a  amplifies seed-light portion  440   a  to produce output-beam portion  125   a , and SOA junction  800   b  amplifies seed-light portion  440   b  to produce output-beam portion  125   b . Output-beam portions  125   a  and  125   b  may each be referred to as an output-light portion, a portion of output beam  125 , or an amplified seed-optical-signal portion. 
     In particular embodiments, a multi junction SOA  460  may include one or more tunnel junctions  850 , where one of the tunnel junctions is located between each pair of adjacent SOA junctions  800 . The multi junction SOA in  FIG. 36  includes one tunnel junction  850  located between the pair of adjacent SOA junctions  800   a  and  800   b . The tunnel junction  850  in  FIG. 36  may be similar to the tunnel junction  750  in  FIG. 33  and may provide electrical separation or isolation between the adjacent SOA junctions  800   a  and  800   b  (e.g., so that the SOA current I 2  is able to flow through the multi junction SOA  460 ). The tunnel junction  850  may include a p-n junction that is reverse biased when SOA current I 2  flows through the multi junction SOA  460 . Instead of blocking the flow of the SOA current, the tunnel junction  850  may be configured to provide good electrical conductivity when reverse biased so that the SOA current is not prevented from flowing through the SOA  460 . 
     In  FIG. 36 , the output beam  125  (which includes output-beam portions  125   a  and  125   b ) is emitted from the output end  462  of the multi junction SOA  460 . In particular embodiment, the output end  462  of a multi junction SOA  460  may include an anti-reflection (AR) coating that reduces the reflectivity of the output end at the wavelength of the output beam  125 . Additionally, the input end  461  of the multi junction SOA  460 , which receives the seed-light portions  440   a  and  440   b , may include an AR coating that reduces the reflectivity of the input end at the wavelength of the seed light. An AR coating may provide a reflectivity of less than 5%, 2%, 0.5%, or 0.1% for the input end  461  or output end  462 . An AR coating applied to the input end  461  or the output end  462  may reduce the amount of light reflected back towards the seed laser diode  450  that supplies the seed light  440  to the SOA  460 . 
       FIG. 37  illustrates an example multi junction SOA  460  with three SOA junctions  800   a ,  800   b , and  800   c . Each SOA junction in  FIG. 37  may be similar to the SOA junction  800  illustrated in  FIG. 35 . For example, in addition to an active region  816 , each SOA junction in  FIG. 37  may also include a p-doped cladding region  812 , a n-doped cladding region  820 , a p-doped waveguide region  814 , or a n-doped waveguide region  818 . Each SOA junction in  FIG. 37  may include a semiconductor p-n junction that is forward biased when the SOA current I 2  is supplied to the multi junction SOA  460 . The SOA current I 2  may flow through the SOA junctions  800   a ,  800   b , and  800   c  and may produce optical gain in the respective active regions  816   a ,  816   b , and  816   c . The output beam  125  produced by the multi junction SOA  460  in  FIG. 37  includes output-beam portion  125   a  produced by SOA junction  800   a , output-beam portion  125   b  produced by SOA junction  800   b , and output-beam portion  125   c  produced by SOA junction  800   c.    
     A multi junction SOA  460  may include M SOA junctions  800 , where M is an integer greater than or equal to 2. The multi junction SOA  460  may (i) receive seed light  440  that includes M seed-light portions and (ii) produce an output beam  125  that includes M output-beam portions. Each SOA junction  800  of the multi junction SOA  460  may (i) receive one of the seed-light portions and (ii) amplify the received seed-light portion to produce a corresponding output-beam portion (which may be referred to as an amplified seed-optical-signal portion). The optical amplification of the received seed-light portion may occur primarily within the active region  816  of the SOA junction  800 . In  FIG. 36 , the multi junction SOA  460 , which includes two SOA junctions ( 800   a ,  800   b ), receives two seed-light portions ( 440   a ,  440   b ) and amplifies the seed-light portions to produce an output beam  125  that includes two corresponding output-beam portions ( 125   a ,  125   b ). The multi junction SOA  460  in  FIG. 37  includes three SOA junctions ( 800   a ,  800   b ,  800   c ), and each SOA junction amplifies one of the seed-light portions ( 440   a ,  440   b ,  440   c ) to produce an output beam  125  that includes three corresponding output-beam portions ( 125   a ,  125   b ,  125   c ). 
     In  FIG. 37 , each of the tunnel junctions  850   a  and  850   b  is located between a pair of adjacent SOA junctions and may provide electrical separation or isolation between the pair of adjacent SOA junctions. Tunnel junction  850   a  is located between the pair of adjacent SOA junctions  800   a  and  800   b , and tunnel junction  850   b  is located between the pair of adjacent SOA junctions  800   b  and  800   c . In particular embodiments, a multi junction SOA  460  may include M SOA junctions  800  and M−1 tunnel junctions  850 , where M is an integer greater than or equal to 2. Each tunnel junction  850  may be located between one pair of adjacent SOA junctions  800 . In  FIG. 36 , the parameter M has a value of 2, and the multi junction SOA  460  includes two SOA junctions ( 800   a ,  800   b ) and one tunnel junction  850  located between the two adjacent SOA junctions. In  FIG. 37 , the parameter M has a value of 3, and the multi junction SOA  460  includes three SOA junctions ( 800   a ,  800   b ,  800   c ) and two tunnel junctions ( 850   a ,  850   b ). 
     In particular embodiments, a multi junction SOA  460  may include one or more tapered optical waveguides  463 . Each tapered optical waveguide  463  may extend from the input end  461  to the output end  462  of the multi junction SOA, and a width of the tapered optical waveguide may increase from the input end to the output end. The single junction SOA  460  in  FIG. 35  may include one tapered optical waveguide similar to the tapered optical waveguide  463  illustrated in  FIG. 9  and described herein. For example, in  FIG. 35 , the optical waveguide (which includes the p-doped and n-doped waveguide regions  814  and  818 ) may have a tapered shape along a transverse direction, corresponding to the tapered optical waveguide  463  in  FIG. 9 . One or more of the other regions of the SOA  460  may also have a tapered shape. For example, the active region  816  may have a tapered shape matching the tapered shape of the p-doped and n-doped waveguide regions, and the p-doped and n-doped cladding regions may have a tapered shape. Additionally, the anode  811  or cathode  823  may have a corresponding tapered shape. In the examples of  FIGS. 36-37 , each SOA junction  800  may include a corresponding tapered optical waveguide  463 . For example, the multi junction SOA  460  in  FIG. 36  may include two tapered optical waveguides  463 , each waveguide similar to the tapered optical waveguide  463  illustrated in  FIG. 9  and described herein. In  FIG. 37 , each of the three SOA junctions ( 800   a ,  800   b ,  800   c ) in  FIG. 37  may include a tapered optical waveguide similar to the tapered optical waveguide  463  illustrated in  FIG. 9  and described herein. The optical waveguide of each SOA junction may have a tapered shape along a transverse direction. Additionally, each SOA junction may also include one or more other regions with a tapered shape (e.g., the active region or the cladding regions), and the anode  811  or cathode  823  of the multi junction SOA  460  may have a tapered shape. Each tapered optical waveguide of a multi junction SOA  460  may provide optical confinement to an associated seed-light portion as it propagates through and is amplified by the SOA  460 . For example, SOA junction  800   a  in  FIG. 37  may include a tapered optical waveguide that guides and confines the seed-light portion  440   a  while the light propagates through the SOA junction  800   a.    
       FIG. 38  illustrates an example multi junction light source  110  with a multi-junction seed laser diode  450  and a multi junction SOA  460 . In particular embodiments, a multi-junction light source  110  may include (i) a multi junction seed laser diode  450  with N laser junctions  700  and (ii) a multi junction SOA  460  with N SOA junctions  800 , where Nis an integer greater than or equal to 2. The parameter N may have a value of 2, 3, 4, 5, 10, or any other suitable value. In  FIG. 38 , the parameter N has a value of 3, which corresponds to the seed laser diode  450  having three laser junctions, and the SOA  460  having three SOA junctions. The seed laser diode  450  in  FIG. 38  is a multi junction seed laser diode  450  with three laser junctions ( 700   a ,  700   b ,  700   c ), and the seed light  440  produced by the multi junction seed laser diode  450  includes the three corresponding seed-light portions  440   a ,  440   b , and  440   c . The multi junction SOA  460  has three SOA junctions ( 800   a ,  800   b ,  800   c ), each SOA junction configured to optically amplify one of the seed-light portions to produce a corresponding output-beam portion. Laser junction  700   a  produces seed-light portion  440   a  which is amplified by SOA junction  800   a  to produce output-beam portion  125   a . Similarly, laser junction  700   b  produces seed-light portion  440   b  which is amplified by SOA junction  800   b  to produce output-beam portion  125   b , and laser junction  700   c  produces seed-light portion  440   c  which is amplified by SOA junction  800   c  to produce output-beam portion  125   c . The multi junction seed laser diode  450  in  FIG. 38  may be similar to the seed laser diode  450  in  FIG. 34 , and the multi junction SOA  460  in  FIG. 38  may be similar to the SOA  460  in  FIG. 37 . The multi junction seed laser diode  450  in  FIG. 38  may include a grating  740  (similar to that illustrated in  FIG. 34 ) that provides wavelength stabilization to the seed laser diode  450 . Alternatively, the multi junction seed laser diode  450  in  FIG. 38  may not include a wavelength-stabilization grating. The multi junction light source  110  in  FIG. 38  may include an optical combiner  920  or an output lens  490 , as illustrated in  FIG. 39  and described herein. 
     In particular embodiments, the seed light  440  from a multi junction seed laser diode  450  may be coupled to a multi junction SOA  460  by free-space coupling. For example, the multi junction light source  110  in  FIG. 38  may include one or more lenses located between the multi junction seed laser diode  450  and the multi junction SOA  460 . The lenses may collect the seed light  440  and focus each seed-light portion into a corresponding SOA junction of the SOA (e.g., one or more lenses may collect the seed-light portion  440   a  and focus it into a waveguide of SOA junction  800   a ). One lens assembly may be used to collect the seed light  440  and focus each of the seed-light portions, or a separate lens assembly may be used to collect and focus each seed-light portion separately. 
     In particular embodiments, the seed light  440  from a multi junction seed laser diode  450  may be coupled to a multi junction SOA  460  by optical fiber. Each seed-light portion may be coupled into an optical fiber that conveys the light to a corresponding SOA junction. The light source  110  in  FIG. 38  may include three optical fibers, one for each of the seed-light portions. For example, one optical fiber may collect the seed-light portion  440   a  from laser junction  700   a  and convey the light to the corresponding SOA junction  800   a . The seed-light portion  440   a  may be coupled into or out of the optical fiber by a lens, or the seed-light portion  440   a  may be coupled into or out of the optical fiber by butt-coupling an end face of the optical fiber to the corresponding laser junction or SOA junction. 
     In particular embodiments, the seed light  440  from a multi junction seed laser diode  450  may be coupled to a multi junction SOA  460  by a photonic integrated circuit (PIC). For example, the multi junction light source  110  in  FIG. 38  may include a PIC with three input ports (to collect each of the three seed-light portions) and three output ports (to deliver the seed-light portions to the corresponding SOA junctions). The seed-light portions may be coupled into or out of the PIC by one or more lenses or by butt-coupling the PIC to the front face  452  of the seed laser diode  450  or to the input end  461  of the SOA  460 . 
     In particular embodiments, the seed light  440  from a multi junction seed laser diode  450  may be directly coupled to a multi junction SOA  460 . For example, the front face  452  of the seed laser diode  450  may be directly coupled or connected to the input end  461  of the SOA  460 . Each seed-light portion may be directly coupled from a laser junction to a SOA junction without propagating through free space or an intervening optical element. In  FIG. 38 , there may be no gap between the seed laser diode  450  and the SOA  460 , and seed-light portion  440   a  may be directly coupled from laser junction  700   a  to SOA junction  800   a . Similarly, seed-light portions  440   b  and  440   c  may be directly coupled from their laser junctions into the respective SOA junctions  800   b  and  800   c . The seed laser diode  450  and the SOA  460  may be fabricated together so that they are integrated together and directly connected to one another, or the seed laser diode  450  and the SOA  460  may be fabricated separately and then affixed together (e.g., front face  452  may be attached with adhesive or epoxy to input end  461 ). 
     In particular embodiments, the seed light  440  from a multi junction seed laser diode  450  may be coupled to a multi junction SOA  460  by passive optical waveguides. For example, for a multi junction light source  110  with a N-junction seed laser diode  450  and a N-junction SOA  460 , the light source may include N passive optical waveguides. The passive waveguides may be located between the front face  452  of the seed laser diode  450  and the input end  461  of the SOA  460 , and each waveguide may convey a seed-light portion from a laser junction  700  to a corresponding SOA junction  800 . The light source  110  in  FIG. 38  may include three passive optical waveguides. For example, one passive optical waveguide may receive the seed-light portion  440   a  from laser junction  700   a  and convey the seed-light portion to SOA junction  800   a . The seed laser diode  450 , the SOA  460 , and the passive optical waveguides may be fabricated together onto a common substrate, and the passive waveguides may be made from similar semiconductor material. For example, the seed laser diode  450  and the SOA  460  may each include an InP, InGaAs or InGaAsP semiconductor structure grown on an InP substrate, and the passive optical waveguides may include an InP, InGaAs, or InGaAsP semiconductor structure. 
       FIG. 39  illustrates an example multi junction light source  110  with a single-junction seed laser diode  450  and a multi junction SOA  460 . The single-junction seed laser diode  450  produces a single beam of seed light  440 , which is split by the optical coupler  860  into three seed-light portions ( 440   a ,  440   b ,  440   c ). Additionally, the optical coupler  860  couples each seed-light portion into a corresponding SOA junction  800  of the multi junction SOA  460 . The SOA junctions ( 800   a ,  800   b ,  800   c ) each amplify a corresponding seed-light portion to produce an output-beam portion, and the optical combiner  920  combines the three output-beam portions ( 125   a ,  125   b ,  125   c ) to produce the output beam  125 . The single-junction seed laser diode  450  in  FIG. 39  may be similar to the seed laser diode  450  in  FIG. 32 , and the multi junction SOA  460  in  FIG. 39  may be similar to the SOA  460  in  FIG. 37 . The seed laser diode  450  in  FIG. 39  may include a grating  740  (e.g., similar to that illustrated in  FIG. 32 ) that provides wavelength stabilization to the seed light  440  produced by the seed laser diode  450  (e.g., the seed laser diode  450  may be a DFB laser). Alternatively, the seed laser diode  450  in  FIG. 39  may not include a wavelength-stabilization grating (e.g., the seed laser diode  450  may be a Fabry-Perot laser diode). 
     In particular embodiments, a multi junction light source  110  may include a single junction seed laser diode  450  and a multi junction SOA  460  with N SOA junctions  800 , where N is an integer greater than or equal to 2. The multi junction light source  110  may also include an optical coupler  860  located between the seed laser diode  450  and the SOA  460 . The optical coupler  860  may (i) split the seed light  440  produced by the seed laser diode  450  into N seed-light portions and (ii) couple each seed-light portion into a SOA junction of the multi junction SOA  460 . In  FIG. 39 , the parameter N has a value of 3. The coupler  860  splits the seed light  440  into three seed-light portions ( 440   a ,  440   b ,  440   c ), and the SOA  460  includes three respective SOA junctions ( 800   a ,  800   b ,  800   c ) into which the coupler  860  directs the seed-light portions. The coupler  860  may split the seed light  440  equally between the seed-light portions so that the seed-light portions have approximately equal power or energy. 
     In particular embodiments, an optical coupler  860  may include a diffractive optical element (DOE), a fiber-optic splitter, a free-space optical splitter, or a PIC-based splitter configured to split seed light  440  into N seed-light portions (where N is an integer greater than or equal to 2, and N is equal to the number of SOA junctions in the SOA  460 ). For example, the coupler  860  may include a DOE, such as a reflective diffraction grating, a transmissive diffraction grating, or a holographic element. The DOE may receive the seed light  440  as a free-space beam, or the DOE may receive the seed light  440  directly from the seed laser diode  450  (e.g., the DOE may be affixed to the front face  452  of the seed laser diode  450 ). The DOE may split the seed light  440  into N free-space seed-light portions that are angularly separated from one another. The coupler  860  may also include one or more lenses that collimate the seed light  440  or that couple the seed-light portions into the respective SOA junctions of the multi junction SOA  460 . As another example, the coupler  860  may include a 1×N fiber-optic splitter that splits the seed light  440  into N seed-light portions. The seed light  440  may be coupled into an input optical fiber of the fiber-optic splitter by a lens or by butt-coupling the input optical fiber to the front face  452  of the seed laser diode  450 . Additionally, the fiber-optic splitter may include N output optical fibers that couple each of the seed-light portions into one of the N SOA junctions (e.g., using one or more lenses, or by butt-coupling the output optical fibers to the input end  461  of the SOA  460 ). As another example, the coupler  860  may include a PIC with a 1×N optical-waveguide splitter that splits the seed light  440  into N seed-light portions. The PIC (which may be similar to the PIC  455  illustrated in  FIG. 11  and described herein) may have one input port that receives the seed light  440  and N output ports that direct the N seed-light portions to the N respective SOA junctions. Each output port of the PIC may couple one of the seed-light portions into one of the SOA junctions. The seed light  440  may be coupled into the input port by one or more lenses or by butt-coupling the input port of the PIC to the front face  452  of the seed laser diode  450 . Similarly, the seed-light portions may be coupled into the SOA junctions of the SOA  460  by one or more lenses or by butt-coupling the N output ports of the PIC to the input end  461  of the SOA  460 . As an example, the coupler  860  in  FIG. 39  may include a PIC with a 1×3 optical-waveguide splitter, and the coupler  860  may be butt-coupled (e.g., affixed with epoxy or adhesive) to both the front face  452  and the input end  461 , so that there is no air gap between the seed laser diode  450  and the coupler  860  and no air gap between the coupler  860  and the SOA  460 . A splitter (e.g., fiber-optic splitter, free-space optical splitter, or PIC-based splitter) of the optical coupler  860  in  FIG. 39  may be similar to the optical splitter  470  illustrated in  FIG. 10, 11, 14, 20, 22, 25 , or  31  and described herein, where the splitter of the coupler  860  has N output ports. 
     In particular embodiments, an optical coupler  860  may include one or more lenses that collimate seed light  440  or that focus each seed-light portion into a corresponding SOA junction. For example, a coupler  860  may include one lens assembly that focuses the seed-light portions into the corresponding SOA junctions. Alternatively, the coupler  860  may include N lens assemblies, where each lens assembly focuses one of the seed-light portions into a corresponding SOA junction. As another example, a coupler  860  that includes a free-space component (e.g., a free-space optical splitter or DOE) to split the seed light  440  may also include one or more input lenses to collimate the seed light  440  produced by the seed laser diode  450 , where the input lenses are located between the seed laser diode  450  and the free-space splitting component. Additionally or alternatively, the free-space coupler  860  may include one or more output lenses that couple the seed-light portions into the SOA junctions, where the output lenses are located between the free-space splitting component and the SOA  460 . 
     In particular embodiments, an optical coupler  860  may include an optical isolator. The optical isolator may (i) transmit seed light  440  to the SOA  460  and (ii) reduce the amount of light (e.g., back-reflected seed light, or amplified spontaneous emission light produced by the SOA) that propagates from the SOA  460  to the seed laser diode  450 . For example, an optical coupler  860  may include an optical isolator followed by a DOE, fiber-optic splitter, free-space optical splitter, or PIC-based splitter. The optical isolator may be an integrated-optic isolator, a fiber-optic isolator, or a free-space isolator. The optical isolator may be similar to the isolator  530  illustrated in  FIG. 14 or 31  and described herein. 
     In particular embodiments, a multi junction light source  110  may include an optical combiner  920 . The multi junction light source  110  may include a multi junction SOA  460  with N SOA junctions that produce N output-beam portions (where Nis an integer greater than or equal to 2). The optical combiner  920  may include a N×1 component that (i) receives the N output-beam portions from the SOA junctions and (ii) combines the N output-beam portions to produce an output beam  125 . For example, each of the N output-beam portions may include a pulse of light, and the N pulses of light may be combined spatially and temporally to produce an emitted pulse of light  400  that is part of the output beam  125 . The emitted pulse of light  400  may have a pulse energy that is somewhat less than or approximately equal to the sum of energies of the N pulses of light. For example, if each of the N pulses of light has an approximate energy of E, the emitted pulse of light  400  may have an energy between 0.8×N×E and N×E. The emitted pulse of light  400  having an energy of less than N×E may be caused by optical losses in the combiner  920 . The optical combiner  920  may include a free-space optical component, a fiber-optic component, or a PIC. For example, the optical combiner  920  may be a free-space component that includes a free-space combiner or a DOE (e.g., a diffraction grating) that combines the output-beam portions into a single output beam  125 . As another example, the optical combiner  920  may include a N×1 fiber-optic combiner. The combiner  920  in  FIG. 39  combines the three output-beam portions ( 125   a ,  125   b , and  125   c ) into a single output beam  125 . The output-beam portions may be combined so that they are substantially spatially overlapped with one another (e.g., output-beam portions  125   a  and  125   b  may be more than 50% overlapped along a direction orthogonal to the direction of propagation). Additionally, the output-beam portions may be combined so that they propagate along substantially the same beam-propagation direction. 
     The optical combiner  420  illustrated in  FIG. 18, 19, 20 , or  31  and described herein may be similar in some respects to the optical combiner  920  illustrated in  FIG. 39 . Both combiner  420  and combiner  920  may be free-space, fiber-optic, or integrated-optic components. However, while combiner  420  may be configured to receive multiple inputs and multiple outputs (e.g., combiner  420  in  FIG. 19  receives an input beam  135  and LO light  430  and produces two combined output beams  422   a  and  422   b ), the combiner  920  is a N×1 component configured to receive multiple input beams and produce a single output beam  125 . For example, in  FIG. 39 , the combiner  920  is a 3×1 component that receives three output-beam portions  125   a ,  125   b , and  125   c  and combines the three beams to produce the output beam  125 . 
     In particular embodiments, a multi junction light source  110  may include an output lens  490  that produces a collimated output beam  125 . The output lens  490  may receive N output-light portions produced by a multi junction SOA  460  and may collimate each of the output-light portions to produce a collimated output beam  125 . The output lens  490  may include one or more lenses of any suitable type (e.g., spherical lens, aspheric lens, or cylindrical lens). For example, the output lens  490  may include a cylindrical lens for fast-axis collimation and another lens for slow-axis collimation. In some embodiments, a multi junction light source  110  may not include an optical combiner, and an output lens  490  may directly receive and collimate the output-light portions to produce an output beam  125 . Alternatively, a multi junction light source  110  may include a lens  490  and an optical combiner  920 . The multi junction light source in  FIG. 39  includes an optical combiner  920  located between the SOA  460  and the lens  490 . The combiner  920  first combines the output-beam portions to produce a combined beam, and the lens  490  may then collimate the combined beam (which includes each of the output-beam portions) to produce the collimated output beam  125 . Alternatively, the locations of the lens  490  and combiner  920  may be interchanged with respect to  FIG. 39  so that the lens  490  is located between the SOA  460  and the combiner  920 . In this case, the lens  490  may first collimate each of the output-light portions, and the combiner  920  may then combine the collimated output-light portions to produce a collimated output beam  125 . 
     In particular embodiments, in addition to a seed laser diode  450  and a multi-junction SOA  460 , a multi junction light source  110  may include a fiber-optic amplifier  500 . The fiber-optic amplifier  500  may receive the output beam  125  from the multi junction SOA  460  and further amplify the output beam. The output beam  125  produced by a multi junction SOA  460  may be a free-space beam that is coupled into an input optical fiber of the fiber-optic amplifier  500  with one or more lenses. Alternatively, an input face of the input optical fiber may be butt-coupled to the output end  462  of the SOA  460  to directly couple the light from the SOA into the input optical fiber. A fiber-optic amplifier that is part of a multi junction light source  110  may be similar to the fiber-optic amplifier  500  illustrated in  FIG. 13 or 14  and described herein. 
     In particular embodiments, a multi junction light source  110  may include an electronic driver  480 . An electronic driver that is part of a multi junction light source  110  may be similar to the electronic driver  480  illustrated in  FIG. 8 or 9  and described herein. The electronic driver  480  may supply (i) seed current I 1  to the seed laser diode  450  to produce seed light  440  and (ii) SOA current I 2  to the multi junction SOA  460  to amplify the seed light. The multi junction light source  110  may be a pulsed light source that produces pulses of light  400 . In order to produce pulses of light  400 , the seed current I 1  may be substantially constant or may include pulses of electrical current, and the SOA current I 2  may include pulses of electrical current. For example, the seed current I 1  may include a substantially constant electrical current that causes the seed laser diode  450  to produce seed light  440  having a substantially constant optical power. Each pulse of electrical current supplied to the SOA  460  may cause the SOA to amplify a temporal portion of the seed light  440  to produce an emitted pulse of light  400 . The temporal portion of seed light may be split into N seed-light temporal portions, and each SOA junction may amplify one of the seed-light temporal portions to produce N pulses of light which are then combined to produce a single emitted pulse of light  400 . As another example, the seed current I 1  may include pulses of electrical current that cause the seed laser diode  450  to produce seed pulses of light. Each pulse of electrical current supplied to the SOA  460  may cause the SOA to amplify one of the seed pulses of light to produce an emitted pulse of light  400 . Each seed pulse of light may be split into N seed-light portions, and each SOA junction may amplify one of the seed-light portions to produce N amplified pulses of light which are then combined to produce a single emitted pulse of light  400 . The pulses of seed current I 1  and the pulses of SOA current I 2  may be supplied synchronously so that the two sets of pulses have approximately the same pulse frequency or are supplied at approximately the same times. 
     In particular embodiments, a lidar system  100  may include a multi junction light source  110  that includes (i) a seed laser diode  450  that produces seed light  440  and (ii) a multi-junction SOA  460  that amplifies the seed light  440  to produce an output beam  125 . The seed light  440  (which may be referred to as a seed optical signal) may include CW light or seed pulses of light (e.g., for use in a pulsed light system) or may include frequency-modulated light (e.g., for use in a FMCW lidar system). The output beam  125  (which may be referred to as an emitted optical signal) may include pulses of light (e.g., for use in a pulsed lidar system) or frequency-modulated light (e.g., for use in a FMCW lidar system). The multi junction light source  110  in  FIG. 38 or 39  may be part of a lidar system. 
     One or more of the lidar systems  100  described herein or illustrated in  FIG. 1-4, 6 , or  31  may include a multi junction light source  110 . In addition to the multi junction light sources  110  illustrated in  FIGS. 38 and 39 , one or more of the light sources  110  described herein or illustrated in  FIG. 1, 3, 6, 8-13, 22-25 , or  31  may be a multi junction light source. A multi-junction light source  110  may include (i) a seed laser diode  450  configured to produce seed light  440  (which may be referred to as a seed optical signal) and (ii) a multi junction SOA  460  configured to amplify the seed light to produce an emitted optical signal. The optical signal emitted by a multi junction light source  110  may correspond to an output beam  125  as described herein or as illustrated in  FIG. 1-4, 6, 8-11, 13-16, 22-25, 27, 31 , or  36 - 39 . The emitted optical signal may include pulses of light  400  or frequency-modulated light. Herein, a multi junction light source may be referred to as a light source. Additionally, a multi junction seed laser diode may be referred to as a seed laser diode, and a multi junction SOA may be referred to as a SOA. 
     In particular embodiments, a lidar system  100  may include a receiver  140  and a controller  150 . The receiver (which may be similar to the receiver  140  in  FIG. 6 or 7 ) may detect a portion of an emitted optical signal scattered by a target  130 , and the controller  150  may determine the distance from the lidar system to the target based on the round-trip time for the portion of the scattered optical signal to travel to the target and back to the lidar system. For example, the emitted optical signal may include a pulse of light  400  emitted by a multi junction light source  110 , and the portion of the scattered optical signal, which may be referred to as a received pulse of light  410 , may include a portion of the emitted pulse of light  400  scattered by the target. 
     In particular embodiments, a multi junction light source  110  may be part of a non-coherent pulsed lidar system. For example, a lidar system  100  that includes a multi junction light source  110  may be a pulsed lidar system where the emitted optical signal includes pulses of light  400 . The pulses of light  400  may have one or more of the following optical characteristics: a wavelength between 900 nm and 2000 nm; a pulse energy between 0.01 μJ and 100 μJ; a pulse repetition frequency between 80 kHz and 10 MHz; and a pulse duration between 1 ns and 100 ns. The multi junction light source  110  of a pulsed lidar system  100  may not produce LO light, and the lidar system may be referred to as a non-coherent pulsed lidar system (e.g., to distinguish it from a coherent pulsed lidar system). For example, the seed laser diode  450  in  FIG. 32, 33 , or  34  may be part of a non-coherent pulsed lidar system in which the seed laser diode  450  produces seed light  440  but does not produce LO light. As another example, the lidar system  100  in  FIG. 1 or 3  may be configured as a non-coherent pulsed lidar system in which the light source  110  is a multi-junction light source that emits an output beam  125  that includes pulses of light  400 . As another example, the lidar system  100  illustrated in  FIG. 6  may be configured as a non-coherent pulsed lidar system with (i) a multi junction light source  110  that emits pulses of light  400  (but does not produce LO light  430 ) and (ii) a detector  340  configured to detect received pulses of light  410  without combining or coherent mixing the received pulses of light with LO light. 
     In particular embodiments, a receiver  140  of a non-coherent pulsed lidar system  100  may include: one or more detectors  340 , one or more electronic amplifiers  350 , and a pulse-detection circuit  365 . Additionally, the receiver  140  may include a frequency-detection circuit  600 . The receiver  140  may detect an input light signal  135  that includes a received pulse of light  410  and may produce a pulse-detection output signal corresponding to the received pulse of light. The receiver  140  of a non-coherent pulsed lidar system  100  may be configured to detect a received pulse of light  410 , and LO light  430  may not be produced by the lidar system or detected by the receiver. The receiver  140  of a non-coherent pulsed lidar system  100  may be similar to the receiver  140  illustrated in  FIG. 6 or 7 , except no LO light may be provided to the receiver. The receiver  140  of a non-coherent pulsed lidar system  100  may be configured to directly detect a received pulse of light  410  without coherent mixing of the received pulse of light  410  with LO light. Each detector  340  of the receiver  140  may produce a pulse of electrical current corresponding to the received pulse of light  410 , and an electronic amplifier  350  may produce a voltage signal  360  with a voltage pulse that corresponds to the pulse of current. The pulse-detection circuit  365  may include one or more comparators  370  and one or more TDCs  380 , as described herein and illustrated in  FIG. 7 . 
     In particular embodiments, a multi junction light source  110  may be part of a coherent pulsed lidar system. One or more of the coherent pulsed lidar systems  100  described herein may include a multi junction light source  110 . For example, the light source  110  of the coherent pulsed lidar system  100  illustrated in  FIG. 6  may be a multi junction light source. A lidar system  100  that includes a multi junction light source  110  may be a coherent pulsed lidar system in which the emitted optical signal includes pulses of light  400 , and the light source  110  is further configured to produce LO light  430 . For example, the seed laser diode  450  in  FIG. 32, 33 , or  34  may be part of a coherent pulsed lidar system. In addition to producing seed light  440 , the seed laser diode  450  in  FIG. 32, 33 or 34  may also produce LO light (not illustrated in  FIGS. 32-34 ), and the LO light may be coherently mixed with a received pulse of light  410  at a receiver  140  of the lidar system. As another example, the seed laser diode  450  illustrated in  FIG. 8, 9, 10, 11 , or  13  may correspond to one of the seed laser diodes  450  in  FIGS. 32-34 . As another example, the multi junction light source  110  illustrated in  FIG. 38 or 39  may be part of a coherent pulsed lidar system. The seed laser diode  450  in  FIG. 38 or 39  may produce LO light (not illustrated in  FIGS. 38-39 ) in addition to the seed light  440 . The three seed-light portions  440   a ,  440   b , and  440   c  may be coherent with one another as well as with a portion of the LO light. Each of the three output-beam portions ( 125   a ,  125   b ,  125   c ) may include a pulse of light emitted from the three respective SOA junctions ( 800   a ,  800   b ,  800   c ). The three pulses of light, which may be coherent with one another, may be combined into a single emitted pulse of light  400  that is part of the output beam  125 . While propagating through the SOA  460 , adjacent portions of the laser modes of the three seed-light portions may be partially overlapped. This overlapping may provide coherent coupling between the adjacent seed-light portions which may ensure that the three pulses of light are coherent with one another. Additionally, the emitted pulse of light  400  (formed by combining the three pulses of light together) may inherent this coherence so that the emitted pulse of light  400  is coherent with a portion of the LO light. 
     In particular embodiments, a multi junction light source  110  may be part of a FMCW lidar system. A lidar system  100  that includes a multi junction light source  110  may be a FMCW lidar system in which the emitted optical signal includes a frequency-modulated (FM) output light signal, and the light source is further configured to emit a FM local-oscillator optical signal that is coherent with the FM output light. For example, the seed laser diode  450  in  FIG. 32, 33 , or  34  may be part of a FMCW lidar system in which the seed laser diode  450  produces FM seed light  440  as well as FM local-oscillator light (not illustrated in  FIGS. 32-34 ). The FM local-oscillator light may be mixed with a received optical signal (which includes a portion of the FM output light scattered by a remote target) to produce a beat signal. A receiver  140  or controller  150  of the FMCW lidar system may determine a frequency of the beat signal, and the distance to the target may be determined based on the frequency of the beat signal. As another example, the multi-junction light source  110  illustrated in  FIG. 38 or 39  may be part of a FMCW lidar system. The seed laser diode in  FIG. 38 or 39  may produce FM local-oscillator light (not illustrated in  FIGS. 38-39 ) in addition to frequency-modulated seed light  440 . Additionally, each SOA junction ( 800   a ,  800   b , and  800   c ) may amplify a portion of the FM seed light  440  to produce an output beam  125  that includes amplified FM seed light from each of the SOA junctions. 
     In particular embodiments, a multi junction light source  110  that includes a seed laser diode  450  and a multi junction SOA  460  may be configured as a three-terminal device. A three-terminal light source  110  may include (i) a common cathode and separate, electrically isolated anodes or (ii) a common anode and separate, electrically isolated cathodes. A seed laser diode  450  and a SOA  460  may each have a cathode and an anode, and a common-cathode configuration may refer to the cathodes of the seed laser diode  450  and the SOA  460  being electrically connected together into a single electrical terminal or contact that is connected to an electronic driver  480 . The seed laser anode  711  and the SOA anode  811  may be electrically isolated from one another. Alternatively, a light source  110  may be configured as a three-terminal common-anode device with a seed laser cathode  723 , a SOA cathode  823 , and a common anode. The common-anode configuration may refer to the anodes of the seed laser diode  450  and the SOA  460  being electrically connected together to form the common anode, while the cathodes of the seed laser diode  450  and the SOA  460  are electrically isolated. 
     Two terminals (e.g., two anodes or two cathodes) being electrically isolated may refer to the two terminals having greater than a particular value of electrical resistance between them (e.g., the resistance between two electrically isolated anodes may be greater than 1 kΩ, 10 kΩ, 100 kΩ, or 1 MΩ). Two terminals (e.g., two anodes or two cathodes) being electrically connected may refer to the two terminals having less than a particular value of electrical resistance between them (e.g., the resistance between two electrically connected cathodes may be less than 1 kΩ, 100 Ω, 10Ω, or 1Ω). A common-anode or common-cathode configuration may be provided by combining or electrically connecting the respective anodes or cathodes through a substrate. For example, a seed laser diode  450  and a SOA  460  may be fabricated separately and then affixed to an electrically conductive substrate so that their anodes or cathodes are electrically connected. As another example, a substrate may include an electrically conductive semiconductor material on which a seed laser diode  450  and SOA  460  are grown. The seed laser diode  450  and the SOA  460  may each include an InGaAs or InGaAsP semiconductor structure grown on an InP substrate. The InP substrate may be n-doped so that it is electrically conductive, and the cathodes of the seed laser diode  450  and the SOA  460  may each be electrically connected to the InP substrate so that the InP substrate acts as a common cathode. Alternatively, the InP substrate may be p-doped, and the anodes of the seed laser diode  450  and the SOA  460  may each be electrically connected to the InP substrate, which acts as a common anode. 
     One or more of the multi junction light sources  110  described herein may be configured as a three-terminal device (with a common cathode or a common anode). For example, the light source  110  in  FIG. 38  may be configured as a three-terminal common-cathode device having separate electrical connections between an electronic driver  480  and each of these three electrical terminals or contacts: (i) seed laser anode  711 , (ii) SOA anode  811 , and (iii) a common cathode that includes the seed laser cathode  723  and the SOA cathode  823  electrically connected together. In a three-terminal common-cathode device, the seed laser anode  711  and the SOA anode  811  may be electrically isolated from one another, and an electronic driver  480  may drive the seed laser diode  450  and the SOA  460  by supplying separate electrical signals to the seed laser anode and the SOA anode. The common cathode may electrically connect the n-doped contacts  722  and  822  and may act as a common return path for currents from the seed laser diode  450  and the SOA  460  to combine and return to the electronic driver  480 . 
     In particular embodiments, a multi junction light source  110  that includes a seed laser diode  450  and a multi junction SOA  460  may be configured as a four-terminal device. In a four-terminal light source  110 , the seed laser anode  711  and the SOA anode  811  may be electrically isolated from one another, and instead of having a common cathode, the seed laser cathode  723  and the SOA cathode  823  may be electrically isolated from one another. For example, the light source  110  in each of  FIGS. 38 and 39  may be configured as a four-terminal device with two electrically isolated anodes (seed laser anode  711  and SOA anode  811 ) and two electrically isolated cathodes (seed laser cathode  723  and SOA cathode  823 ). A four-terminal light source  110  may have separate electrical connections between an electronic driver  480  and each of these four electrical terminals or contacts: (i) seed laser anode  711 , (ii) seed laser cathode  723 , (iii) SOA anode  811 , and (iv) SOA cathode  823 . 
     One or more of the multi junction light sources  110  described herein may be configured as a four-terminal device. For example, in each of  FIGS. 38 and 39 , the p-doped contacts  710  and  810  may be electrically isolated (corresponding to electrically isolated anodes), and the n-doped contacts  722  and  822  may be electrically isolated (corresponding to electrically isolated cathodes). A light source  110  that is configured as a four-terminal device may have electrically isolated anodes and electrically isolated cathodes, and an electronic driver  480  may drive the anode  711  and cathode  723  of the seed laser diode  450  separately or independently from the anode  811  and cathode  823  of the SOA  460 . As compared to a three-terminal light source  110 , a light source configured as a four-terminal device may provide improved electrical isolation between the seed laser diode  450  and the SOA  460 . For example, in a four-terminal light source  110 , applying a pulse of current to the SOA  460  may result in a reduced amount of unwanted cross-talk current that is coupled to the seed laser diode  450 . 
       FIG. 40  illustrates an example computer system  4000 . In particular embodiments, one or more computer systems  4000  may perform one or more steps of one or more methods described or illustrated herein. In particular embodiments, one or more computer systems  4000  may provide functionality described or illustrated herein. In particular embodiments, software running on one or more computer systems  4000  may perform one or more steps of one or more methods described or illustrated herein or may provide functionality described or illustrated herein. Particular embodiments may include one or more portions of one or more computer systems  4000 . In particular embodiments, a computer system may be referred to as a processor, a controller, a computing device, a computing system, a computer, a general-purpose computer, or a data-processing apparatus. Herein, reference to a computer system may encompass one or more computer systems, where appropriate. 
     Computer system  4000  may take any suitable physical form. As an example, computer system  4000  may be an embedded computer system, a system-on-chip (SOC), a single-board computer system (SBC), a desktop computer system, a laptop or notebook computer system, a mainframe, a mesh of computer systems, a server, a tablet computer system, or any suitable combination of two or more of these. As another example, all or part of computer system  4000  may be combined with, coupled to, or integrated into a variety of devices, including, but not limited to, a camera, camcorder, personal digital assistant (PDA), mobile telephone, smartphone, electronic reading device (e.g., an e-reader), game console, smart watch, clock, calculator, television monitor, flat-panel display, computer monitor, vehicle display (e.g., odometer display or dashboard display), vehicle navigation system, lidar system, ADAS, autonomous vehicle, autonomous-vehicle driving system, cockpit control, camera view display (e.g., display of a rear-view camera in a vehicle), eyewear, or head-mounted display. Where appropriate, computer system  4000  may include one or more computer systems  4000 ; be unitary or distributed; span multiple locations; span multiple machines; span multiple data centers; or reside in a cloud, which may include one or more cloud components in one or more networks. Where appropriate, one or more computer systems  4000  may perform without substantial spatial or temporal limitation one or more steps of one or more methods described or illustrated herein. As an example, one or more computer systems  4000  may perform in real time or in batch mode one or more steps of one or more methods described or illustrated herein. One or more computer systems  4000  may perform at different times or at different locations one or more steps of one or more methods described or illustrated herein, where appropriate. 
     As illustrated in the example of  FIG. 40 , computer system  4000  may include a processor  4010 , memory  4020 , storage  4030 , an input/output (I/O) interface  4040 , a communication interface  4050 , or a bus  4060 . Computer system  4000  may include any suitable number of any suitable components in any suitable arrangement. 
     In particular embodiments, processor  4010  may include hardware for executing instructions, such as those making up a computer program. As an example, to execute instructions, processor  4010  may retrieve (or fetch) the instructions from an internal register, an internal cache, memory  4020 , or storage  4030 ; decode and execute them; and then write one or more results to an internal register, an internal cache, memory  4020 , or storage  4030 . In particular embodiments, processor  4010  may include one or more internal caches for data, instructions, or addresses. Processor  4010  may include any suitable number of any suitable internal caches, where appropriate. As an example, processor  4010  may include one or more instruction caches, one or more data caches, or one or more translation lookaside buffers (TLBs). Instructions in the instruction caches may be copies of instructions in memory  4020  or storage  4030 , and the instruction caches may speed up retrieval of those instructions by processor  4010 . Data in the data caches may be copies of data in memory  4020  or storage  4030  for instructions executing at processor  4010  to operate on; the results of previous instructions executed at processor  4010  for access by subsequent instructions executing at processor  4010  or for writing to memory  4020  or storage  4030 ; or other suitable data. The data caches may speed up read or write operations by processor  4010 . The TLBs may speed up virtual-address translation for processor  4010 . In particular embodiments, processor  4010  may include one or more internal registers for data, instructions, or addresses. Processor  4010  may include any suitable number of any suitable internal registers, where appropriate. Where appropriate, processor  4010  may include one or more arithmetic logic units (ALUs); may be a multi-core processor; or may include one or more processors  4010 . 
     In particular embodiments, memory  4020  may include main memory for storing instructions for processor  4010  to execute or data for processor  4010  to operate on. As an example, computer system  4000  may load instructions from storage  4030  or another source (such as, for example, another computer system  4000 ) to memory  4020 . Processor  4010  may then load the instructions from memory  4020  to an internal register or internal cache. To execute the instructions, processor  4010  may retrieve the instructions from the internal register or internal cache and decode them. During or after execution of the instructions, processor  4010  may write one or more results (which may be intermediate or final results) to the internal register or internal cache. Processor  4010  may then write one or more of those results to memory  4020 . One or more memory buses (which may each include an address bus and a data bus) may couple processor  4010  to memory  4020 . Bus  4060  may include one or more memory buses. In particular embodiments, one or more memory management units (MMUs) may reside between processor  4010  and memory  4020  and facilitate accesses to memory  4020  requested by processor  4010 . In particular embodiments, memory  4020  may include random access memory (RAM). This RAM may be volatile memory, where appropriate. Where appropriate, this RAM may be dynamic RAM (DRAM) or static RAM (SRAM). Memory  4020  may include one or more memories  4020 , where appropriate. 
     In particular embodiments, storage  4030  may include mass storage for data or instructions. As an example, storage  4030  may include a hard disk drive (HDD), a floppy disk drive, flash memory, an optical disc, a magneto-optical disc, magnetic tape, or a Universal Serial Bus (USB) drive or a combination of two or more of these. Storage  4030  may include removable or non-removable (or fixed) media, where appropriate. Storage  4030  may be internal or external to computer system  4000 , where appropriate. In particular embodiments, storage  4030  may be non-volatile, solid-state memory. In particular embodiments, storage  4030  may include read-only memory (ROM). Where appropriate, this ROM may be mask ROM (MROM), programmable ROM (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM), flash memory, or a combination of two or more of these. Storage  4030  may include one or more storage control units facilitating communication between processor  4010  and storage  4030 , where appropriate. Where appropriate, storage  4030  may include one or more storages  4030 . 
     In particular embodiments, I/O interface  4040  may include hardware, software, or both, providing one or more interfaces for communication between computer system  4000  and one or more I/O devices. Computer system  4000  may include one or more of these I/O devices, where appropriate. One or more of these I/O devices may enable communication between a person and computer system  4000 . As an example, an I/O device may include a keyboard, keypad, microphone, monitor, mouse, printer, scanner, speaker, camera, stylus, tablet, touch screen, trackball, another suitable I/O device, or any suitable combination of two or more of these. An I/O device may include one or more sensors. Where appropriate, I/O interface  4040  may include one or more device or software drivers enabling processor  4010  to drive one or more of these I/O devices. I/O interface  4040  may include one or more I/O interfaces  4040 , where appropriate. 
     In particular embodiments, communication interface  4050  may include hardware, software, or both providing one or more interfaces for communication (such as, for example, packet-based communication) between computer system  4000  and one or more other computer systems  4000  or one or more networks. As an example, communication interface  4050  may include a network interface controller (NIC) or network adapter for communicating with an Ethernet or other wire-based network or a wireless NIC (WNIC); a wireless adapter for communicating with a wireless network, such as a WI-FI network; or an optical transmitter (e.g., a laser or a light-emitting diode) or an optical receiver (e.g., a photodetector) for communicating using fiber-optic communication or free-space optical communication. Computer system  4000  may communicate with an ad hoc network, a personal area network (PAN), an in-vehicle network (IVN), a local area network (LAN), a wide area network (WAN), a metropolitan area network (MAN), or one or more portions of the Internet or a combination of two or more of these. One or more portions of one or more of these networks may be wired or wireless. As an example, computer system  4000  may communicate with a wireless PAN (WPAN) (such as, for example, a BLUETOOTH WPAN), a WI-FI network, a Worldwide Interoperability for Microwave Access (WiMAX) network, a cellular telephone network (such as, for example, a Global System for Mobile Communications (GSM) network), or other suitable wireless network or a combination of two or more of these. As another example, computer system  4000  may communicate using fiber-optic communication based on 100 Gigabit Ethernet (100 GbE), 10 Gigabit Ethernet (10 GbE), or Synchronous Optical Networking (SONET). Computer system  4000  may include any suitable communication interface  4050  for any of these networks, where appropriate. Communication interface  4050  may include one or more communication interfaces  4050 , where appropriate. 
     In particular embodiments, bus  4060  may include hardware, software, or both coupling components of computer system  4000  to each other. As an example, bus  4060  may include an Accelerated Graphics Port (AGP) or other graphics bus, a controller area network (CAN) bus, an Enhanced Industry Standard Architecture (EISA) bus, a front-side bus (FSB), a HYPERTRANSPORT (HT) interconnect, an Industry Standard Architecture (ISA) bus, an INFINIBAND interconnect, a low-pin-count (LPC) bus, a memory bus, a Micro Channel Architecture (MCA) bus, a Peripheral Component Interconnect (PCI) bus, a PCI-Express (PCIe) bus, a serial advanced technology attachment (SATA) bus, a Video Electronics Standards Association local bus (VLB), or another suitable bus or a combination of two or more of these. Bus  4060  may include one or more buses  4060 , where appropriate. 
     In particular embodiments, various modules, circuits, systems, methods, or algorithm steps described in connection with the implementations disclosed herein may be implemented as electronic hardware, computer software, or any suitable combination of hardware and software. In particular embodiments, computer software (which may be referred to as software, computer-executable code, computer code, a computer program, computer instructions, or instructions) may be used to perform various functions described or illustrated herein, and computer software may be configured to be executed by or to control the operation of computer system  4000 . As an example, computer software may include instructions configured to be executed by processor  4010 . In particular embodiments, owing to the interchangeability of hardware and software, the various illustrative logical blocks, modules, circuits, or algorithm steps have been described generally in terms of functionality. Whether such functionality is implemented in hardware, software, or a combination of hardware and software may depend upon the particular application or design constraints imposed on the overall system. 
     In particular embodiments, a computing device may be used to implement various modules, circuits, systems, methods, or algorithm steps disclosed herein. As an example, all or part of a module, circuit, system, method, or algorithm disclosed herein may be implemented or performed by a general-purpose single- or multi-chip processor, a digital signal processor (DSP), an ASIC, a FPGA, any other suitable programmable-logic device, discrete gate or transistor logic, discrete hardware components, or any suitable combination thereof. A general-purpose processor may be a microprocessor, or, any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     In particular embodiments, one or more implementations of the subject matter described herein may be implemented as one or more computer programs (e.g., one or more modules of computer-program instructions encoded or stored on a computer-readable non-transitory storage medium). As an example, the steps of a method or algorithm disclosed herein may be implemented in a processor-executable software module which may reside on a computer-readable non-transitory storage medium. In particular embodiments, a computer-readable non-transitory storage medium may include any suitable storage medium that may be used to store or transfer computer software and that may be accessed by a computer system. Herein, a computer-readable non-transitory storage medium or media may include one or more semiconductor-based or other integrated circuits (ICs) (such, as for example, field-programmable gate arrays (FPGAs) or application-specific ICs (ASICs)), hard disk drives (HDDs), hybrid hard drives (HHDs), optical discs (e.g., compact discs (CDs), CD-ROM, digital versatile discs (DVDs), blu-ray discs, or laser discs), optical disc drives (ODDs), magneto-optical discs, magneto-optical drives, floppy diskettes, floppy disk drives (FDDs), magnetic tapes, flash memories, solid-state drives (SSDs), RAM, RAM-drives, ROM, SECURE DIGITAL cards or drives, any other suitable computer-readable non-transitory storage media, or any suitable combination of two or more of these, where appropriate. A computer-readable non-transitory storage medium may be volatile, non-volatile, or a combination of volatile and non-volatile, where appropriate. 
     In particular embodiments, certain features described herein in the context of separate implementations may also be combined and implemented in a single implementation. Conversely, various features that are described in the context of a single implementation may also be implemented in multiple implementations separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination may in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. 
     While operations may be depicted in the drawings as occurring in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all operations be performed. Further, the drawings may schematically depict one more example processes or methods in the form of a flow diagram or a sequence diagram. However, other operations that are not depicted may be incorporated in the example processes or methods that are schematically illustrated. For example, one or more additional operations may be performed before, after, simultaneously with, or between any of the illustrated operations. Moreover, one or more operations depicted in a diagram may be repeated, where appropriate. Additionally, operations depicted in a diagram may be performed in any suitable order. Furthermore, although particular components, devices, or systems are described herein as carrying out particular operations, any suitable combination of any suitable components, devices, or systems may be used to carry out any suitable operation or combination of operations. In certain circumstances, multitasking or parallel processing operations may be performed. Moreover, the separation of various system components in the implementations described herein should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems may be integrated together in a single software product or packaged into multiple software products. 
     Various embodiments have been described in connection with the accompanying drawings. However, it should be understood that the figures may not necessarily be drawn to scale. As an example, distances or angles depicted in the figures are illustrative and may not necessarily bear an exact relationship to actual dimensions or layout of the devices illustrated. 
     The scope of this disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example embodiments described or illustrated herein that a person having ordinary skill in the art would comprehend. The scope of this disclosure is not limited to the example embodiments described or illustrated herein. Moreover, although this disclosure describes or illustrates respective embodiments herein as including particular components, elements, functions, operations, or steps, any of these embodiments may include any combination or permutation of any of the components, elements, functions, operations, or steps described or illustrated anywhere herein that a person having ordinary skill in the art would comprehend. 
     The term “or” as used herein is to be interpreted as an inclusive or meaning any one or any combination, unless expressly indicated otherwise or indicated otherwise by context. Therefore, herein, the expression “A or B” means “A, B, or both A and B.” As another example, herein, “A, B or C” means at least one of the following: A; B; C; A and B; A and C; B and C; A, B and C. An exception to this definition will occur if a combination of elements, devices, steps, or operations is in some way inherently mutually exclusive. 
     As used herein, words of approximation such as, without limitation, “approximately, “substantially,” or “about” refer to a condition that when so modified is understood to not necessarily be absolute or perfect but would be considered close enough to those of ordinary skill in the art to warrant designating the condition as being present. The extent to which the description may vary will depend on how great a change can be instituted and still have one of ordinary skill in the art recognize the modified feature as having the required characteristics or capabilities of the unmodified feature. In general, but subject to the preceding discussion, a numerical value herein that is modified by a word of approximation such as “approximately” may vary from the stated value by ±0.5%, ±1%, ±2%, ±3%, ±4%, ±5%, ±10%, ±12%, or ±15%. The term “substantially constant” refers to a value that varies by less than a particular amount over any suitable time interval. For example, a value that is substantially constant may vary by less than or equal to 20%, 10%, 1%, 0.5%, or 0.1% over a time interval of approximately 10 4  s, 10 3  s, 10 2  s, 10 s, 1 s, 100 ms, 10 ms, 1 ms, 100 μs, 10 μs, or 1 μs. The term “substantially constant” may be applied to any suitable value, such as for example, an optical power, a pulse repetition frequency, an electrical current, a wavelength, an optical or electrical frequency, or an optical or electrical phase. 
     As used herein, the terms “first,” “second,” “third,” etc. may be used as labels for nouns that they precede, and these terms may not necessarily imply a particular ordering (e.g., a particular spatial, temporal, or logical ordering). As an example, a system may be described as determining a “first result” and a “second result,” and the terms “first” and “second” may not necessarily imply that the first result is determined before the second result. 
     As used herein, the terms “based on” and “based at least in part on” may be used to describe or present one or more factors that affect a determination, and these terms may not exclude additional factors that may affect a determination. A determination may be based solely on those factors which are presented or may be based at least in part on those factors. The phrase “determine A based on B” indicates that B is a factor that affects the determination of A. In some instances, other factors may also contribute to the determination of A. In other instances, A may be determined based solely on B.