Patent Publication Number: US-2023152460-A1

Title: Dual lens receive path for lidar system

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 62/720,350, filed Aug. 21, 2018, the disclosure of which is incorporated herein in its entirety. 
    
    
     FIELD 
     This disclosure relates generally to laser scanning and, more particularly, to using a fiber optic cable in the receive path of a laser scanning system. 
     BACKGROUND 
     Light detection and ranging (LiDAR) systems use light pulses to create an image or point cloud of the external environment. Some typical LiDAR systems include a light source, a pulse steering system, and light detector. The light source generates light pulses that are directed by the pulse steering system in particular directions when being transmitted from the LiDAR system. When a transmitted light pulse is scattered by an object, some of the scattered light is returned to the LiDAR system as a returned pulse. The light detector detects the returned pulse. Using the time it took for the returned pulse to be detected after the light pulse was transmitted and the speed of light, the LiDAR system can determine the distance to the object along the path of the transmitted light pulse. The pulse steering system can direct light pulses along different paths to allow the LiDAR system to scan the surrounding environment and produce an image or point cloud. LiDAR systems can also use techniques other than time-of-flight and scanning to measure the surrounding environment 
     SUMMARY 
     The following presents a simplified summary of one or more examples to provide a basic understanding of the disclosure. This summary is not an extensive overview of all contemplated examples, and is not intended to either identify key or critical elements of all examples or delineate the scope of any or all examples. Its purpose is to present some concepts of one or more examples in a simplified form as a prelude to the more detailed description that is presented below. 
     A dual lens assembly positioned along an optical receive path within a LiDAR system is provided. The dual lens assembly is constructed to reduce a numerical aperture of a returned light pulse and reduce a walk-off error associated with one or more mirrors of the LiDAR system. 
     In some embodiments, a light detection and ranging (LiDAR) system is provided that includes a light source configured to generate a pulse signal that is transmitted by the LiDAR system, one or more mirrors configured to steer a returned light pulse associated with the transmitted pulse signal along an optical receive path, a dual lens assembly positioned along the optical receive path, wherein the dual lens assembly is constructed to reduce a numerical aperture of the returned light pulse and reduce a walk-off error associated with the one or more mirrors, and a fiber configured to receive the returned light pulse along the optical receive path from the dual lens assembly. 
     In another embodiment, a light detection and ranging (LiDAR) system is provided that includes a steering system operative to steer a plurality of returned light pulses along an optical receive path, a fiber configured to receive the plurality of returned light pulses along the optical receive path, the fiber comprising a core, and a dual lens assembly positioned along the optical receive path in between the steering system and the fiber, wherein the dual lens assembly optimizes a spot beam produced by the plurality returned light pulses for entry into the core. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present application can be best understood by reference to the figures described below taken in conjunction with the accompanying drawing figures, in which like parts may be referred to by like numerals. 
         FIG.  1    illustrates an exemplary LiDAR system using pulse signal to measure distances to points in the outside environment. 
         FIG.  2    illustrates the exemplary LiDAR system using pulse signal to measure distances to points in the outside environment. 
         FIG.  3    illustrates the exemplary LiDAR system using pulse signal to measure distances to points in the outside environment. 
         FIG.  4    depicts a logical block diagram of the exemplary LiDAR system. 
         FIG.  5    depicts a light source of the exemplary LiDAR system. 
         FIG.  6    depicts a light detector of the exemplary LiDAR system. 
         FIG.  7    depicts a steering system of the exemplary LiDAR system including a field lens and a fiber. 
         FIG.  8    depicts an exemplary configuration of the field lens and the fiber. 
         FIG.  9    depicts beam walk-off (in dashed line) that may occur in a steering system without the field lens and beam bending (in solid line) that may occur in a steering system with the field lens. 
         FIG.  10    illustrates the relationships between a returned optical signal and the angle change of a polygon in the exemplary steering system, with or without a field lens. 
         FIG.  11    depicts an exemplary optical coupling of the fiber to a photodetector of the exemplary LiDAR system. 
         FIG.  12    depicts a steering system of the exemplary LiDAR system including a dual lens and a fiber. 
         FIGS.  13 A- 13 D  illustrate the relationship between the numerical apertures of a returned optical signal with respect to polygon scanner geometry. 
         FIGS.  14 A- 14 C  depict beam walk-off with and without a field lens. 
         FIGS.  15 A- 15 C  depict field lens design characteristics with and with respect to focus of a field lens. 
         FIGS.  16 A- 16 B  depicts control of numerical aperture by a concave lens an its benefits to signal throughput. 
         FIGS.  17 A- 17 B  illustrate a dual lens system and how the concave and field lenses perform on two perpendicular planes. 
         FIG.  18    depicts LiDAR detection system data loss improvement with various lens systems compared to a no lens system. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of examples, reference is made to the accompanying drawings which form a part hereof, and in which it is shown by way of illustration specific examples that can be practiced. It is to be understood that other examples can be used and structural changes can be made without departing from the scope of the disclosed examples. 
     Some LiDAR systems use an open air optical path or optical path with one or more lenses to receive and optimize detection of returned pulse signals. This has a disadvantage in that the detection mechanism needs to either be close to where the returned pulse enters the system or a potentially complex system needs to be in place to redirect the returned pulse signal to the detector. In some embodiments of the present technology, an optical fiber is used to direct returned light pulses to a light detector. This way, the detector can be placed in an arbitrary location relative to the signal steering system that receives the return signal. Further, the detector can be placed fairly close to the exit end of the fiber, thus improving the integrity and amount of the detected light signals. Depending on how the returned light pulse is received by the LiDAR system, errors (e.g., walk-off error) that reduce signal strength or place more stringent tolerances on the system may be reduced. Some embodiments of the present technology use a field lens to redirect returned light pulses into an optical fiber core or directly into a light detector, thus reducing the errors due to walk-off of the pulses. Embodiments discussed herein use a dual lens assembly to maximize light transfer into an optical fiber core by controlling the numerical aperture (NA) of the light and by minimizing walk-off errors. 
     Some LiDAR systems use the time-of-flight of light signals (e.g., light pulses) to determine the distance to objects in the path of the light. For example, with respect to  FIG.  1   , an exemplary LiDAR system  100  includes a laser light source (e.g., a fiber laser), a steering system (e.g., a system of one or more moving mirrors), and a light detector (e.g., a photon detector with one or more optics). LiDAR system  100  transmits light pulse  102  along path  104  as determined by the steering system of LiDAR system  100 . In the depicted example, light pulse  102 , which is generated by the laser light source, is a short pulse of laser light. Further, the signal steering system of the LiDAR system  100  is a pulse signal steering system. However, it should be appreciated that LiDAR systems can operate by generating, transmitting, and detecting light signals that are not pulsed and/use derive ranges to object in the surrounding environment using techniques other than time-of-flight. For example, some LiDAR systems use frequency modulated continuous waves (i.e., “FMCW”). It should be further appreciated that any of the techniques described herein with respect to time-of-flight based systems that use pulses also may be applicable to LiDAR systems that do not use one or both of these techniques. 
     Referring back to  FIG.  1    (a time-of-flight LiDAR system that uses light pulses) when light pulse  102  reaches object  106 , light pulse  102  scatters and returned light pulse  108  will be reflected back to system  100  along path  110 . The time from when transmitted light pulse  102  leaves LiDAR system  100  to when returned light pulse  108  arrives back at LiDAR system  100  can be measured (e.g., by a processor or other electronics within the LiDAR system). This time-of-flight combined with the knowledge of the speed of light can be used to determine the range/distance from LiDAR system  100  to the point on object  106  where light pulse  102  scattered. 
     By directing many light pulses, as depicted in  FIG.  2   , LiDAR system  100  scans the external environment (e.g., by directing light pulses  102 ,  202 ,  206 ,  210  along paths  104 ,  204 ,  208 ,  212 , respectively). As depicted in  FIG.  3   , LiDAR system  100  receives returned light pulses  108 ,  302 ,  306  (which correspond to transmitted light pulses  102 ,  202 ,  210 , respectively) back after objects  106  and  214  scatter the transmitted light pulses and reflect pulses back along paths  110 ,  304 ,  308 , respectively. Based on the direction of the transmitted light pulses (as determined by LiDAR system  100 ) as well as the calculated range from LiDAR system  100  to the points on objects that scatter the light pulses (e.g., the points on objects  106  and  214 ), the surroundings within the detection range (e.g., the field of view between path  104  and  212 , inclusively) can be precisely plotted (e.g., a point cloud or image can be created). 
     If a corresponding light pulse is not received for a particular transmitted light pulse, then it can be determined that there are no objects within a certain range of LiDAR system  100  (e.g., the max scanning distance of LiDAR system  100 ). For example, in  FIG.  2   , light pulse  206  will not have a corresponding returned light pulse (as depicted in  FIG.  3   ) because it did not produce a scattering event along its transmission path  208  within the predetermined detection range. LiDAR system  100  (or an external system communication with LiDAR system  100 ) can interpret this as no object being along path  208  within the detection range of LiDAR system  100 . 
     In  FIG.  2   , transmitted light pulses  102 ,  202 ,  206 ,  210  can be transmitted in any order, serially, in parallel, or based on other timings with respect to each other. Additionally, while  FIG.  2    depicts a 1-dimensional array of transmitted light pulses, LiDAR system  100  optionally also directs similar arrays of transmitted light pulses along other planes so that a 2-dimensional array of light pulses is transmitted. This 2-dimensional array can be transmitted point-by-point, line-by-line, all at once, or in some other manner. The point cloud or image from a 1-dimensional array (e.g., a single horizontal line) will produce 2-dimensional information (e.g., (1) the horizontal transmission direction and (2) the range to objects). The point cloud or image from a 2-dimensional array will have 3-dimensional information (e.g., (1) the horizontal transmission direction, (2) the vertical transmission direction, and (3) the range to objects). 
     The density of points in point cloud or image from a LiDAR system  100  is equal to the number of pulses divided by the field of view. Given that the field of view is fixed, to increase the density of points generated by one set of transmission-receiving optics, the LiDAR system should fire a pulse more frequently, in other words, a light source with a higher repetition rate is needed. However, by sending pulses more frequently the farthest distance that the LiDAR system can detect may be more limited. For example, if a returned signal from a far object is received after the system transmits the next pulse, the return signals may be detected in a different order than the order in which the corresponding signals are transmitted and get mixed up if the system cannot correctly correlate the returned signals with the transmitted signals. To illustrate, consider an exemplary LiDAR system that can transmit laser pulses with a repetition rate between 500 kHz and 1 MHz. Based on the time it takes for a pulse to return to the LiDAR system and to avoid mix-up of returned pulses from consecutive pulses in conventional LiDAR design, the farthest distance the LiDAR system can detect may be 300 meters and 150 meters for 500 kHz and 1 Mhz, respectively. The density of points of a LiDAR system with 500 kHz repetition rate is half of that with 1 MHz. Thus, this example demonstrates that, if the system cannot correctly correlate returned signals that arrive out of order, increasing the repetition rate from 500 kHz to 1 Mhz (and thus improving the density of points of the system) would significantly reduce the detection range of the system. 
       FIG.  4    depicts a logical block diagram of LiDAR system  100 , which includes light source  402 , signal steering system  404 , pulse detector  406 , and controller  408 . These components are coupled together using communications paths  410 ,  412 ,  414 ,  416 , and  418 . These communications paths represent communication (bidirectional or unidirectional) among the various LiDAR system components but need not be physical components themselves. While the communications paths can be implemented by one or more electrical wires, busses, or optical fibers, the communication paths can also be wireless channels or open-air optical paths so that no physical communication medium is present. For example, in one exemplary LiDAR system, communication path  410  is one or more optical fibers, communication path  412  represents an optical path, and communication paths  414 ,  416 ,  418 , and  420  are all one or more electrical wires that carry electrical signals. The communications paths can also include more than one of the above types of communication mediums (e.g., they can include an optical fiber and an optical path or one or more optical fibers and one or more electrical wires). 
     LiDAR system  100  can also include other components not depicted in  FIG.  4   , such as power buses, power supplies, LED indicators, switches, etc. Additionally, other connections among components may be present, such as a direct connection between light source  402  and light detector  406  so that light detector  406  can accurately measure the time from when light source  402  transmits a light pulse until light detector  406  detects a returned light pulse. 
       FIG.  5    depicts a logical block diagram of one example of light source  402  that is based on a laser fiber, although any number of light sources with varying architecture could be used as part of the LiDAR system. Light source  402  uses seed  502  to generate initial light pulses of one or more wavelengths (e.g., 1550 nm), which are provided to wavelength-division multiplexor (WDM)  504  via fiber  503 . Pump  506  also provides laser power (of a different wavelength, such as 980 nm) to WDM  504  via fiber  505 . The output of WDM  504  is provided to pre-amplifiers  508  (which includes one or more amplifiers) which provides its output to combiner  510  via fiber  509 . Combiner  510  also takes laser power from pump  512  via fiber  511  and provides pulses via fiber  513  to booster amplifier  514 , which produces output light pulses on fiber  410 . The outputted light pulses are then fed to steering system  404 . In some variations, light source  402  can produce pulses of different amplitudes based on the fiber gain profile of the fiber used in the source. Communication path  416  couples light source  402  to controller  408  ( FIG.  4   ) so that components of light source  402  can be controlled by or otherwise communicate with controller  408 . Alternatively, light source  402  may include its own controller. Instead of controller  408  communicating directly with components of light source  402 , a dedicated light source controller communicates with controller  408  and controls and/or communicates with the components of light source  402 . Light source  402  also includes other components not shown, such as one or more power connectors, power supplies, and/or power lines. 
     Some other light sources include one or more laser diodes, short-cavity fiber lasers, solid-state lasers, and/or tunable external cavity diode lasers, configured to generate one or more light signals at various wavelengths. In some examples, light sources use amplifiers (e.g., pre-amps or booster amps) include a doped optical fiber amplifier, a solid-state bulk amplifier, and/or a semiconductor optical amplifier, configured to receive and amplify light signals. 
     Returning to  FIG.  4   , signal steering system  404  includes any number of components for steering light signals generated by light source  402 . In some examples, signal steering system  404  may include one or more optical redirection elements (e.g., mirrors or lens) that steer light pulses (e.g., by rotating, vibrating, or directing) along a transmit path to scan the external environment. For example, these optical redirection elements may include MEMS mirrors, rotating polyhedron mirrors, or stationary mirrors to steer the transmitted pulse signals to different directions. Signal steering system  404  optionally also includes other optical components, such as dispersion optics (e.g., diffuser lenses, prisms, or gratings) to further expand the coverage of the transmitted signal in order to increase the LiDAR system  100 &#39;s transmission area (i.e., field of view). An example signal steering system is described in U.S. Patent Application Publication No. 2018/0188355, entitled “2D Scanning High Precision LiDAR Using Combination of Rotating Concave Mirror and Beam Steering Devices,” the content of which is incorporated by reference in its entirety herein for all purposes. In some examples, signal steering system  404  does not contain any active optical components (e.g., it does not contain any amplifiers). In some other examples, one or more of the components from light source  402 , such as a booster amplifier, may be included in signal steering system  404 . In some instances, signal steering system  404  can be considered a LiDAR head or LiDAR scanner. 
     Some implementations of signal steering systems include one or more optical redirection elements (e.g., mirrors or lens) that steers returned light signals (e.g., by rotating, vibrating, or directing) along a receive path to direct the returned light signals to the light detector. The optical redirection elements that direct light signals along the transmit and receive paths may be the same components (e.g., shared), separate components (e.g., dedicated), and/or a combination of shared and separate components. This means that in some cases the transmit and receive paths are different although they may partially overlap (or in some cases, substantially overlap). 
       FIG.  6    depicts a logical block diagram of one possible arrangement of components in light detector  404  of LiDAR system  100  ( FIG.  4   ). Light detector  404  includes optics  604  (e.g., a system of one or more optical lenses) and detector  602  (e.g., a charge coupled device (CCD), a photodiode, an avalanche photodiode, a photomultiplier vacuum tube, an image sensor, etc.) that is connected to controller  408  ( FIG.  4   ) via communication path  418 . The optics  604  may include one or more photo lenses to receive, focus, and direct the returned signals. Light detector  404  can include filters to selectively pass light of certain wavelengths. Light detector  404  can also include a timing circuit that measures the time from when a pulse is transmitted to when a corresponding returned pulse is detected. This data can then be transmitted to controller  408  ( FIG.  4   ) or to other devices via communication line  418 . Light detector  404  can also receive information about when light source  402  transmitted a light pulse via communication line  418  or other communications lines that are not shown (e.g., an optical fiber from light source  402  that samples transmitted light pulses). Alternatively, light detector  404  can provide signals via communication line  418  that indicate when returned light pulses are detected. Other pulse data, such as power, pulse shape, and/or wavelength, can also be communicated. 
     Returning to  FIG.  4   , controller  408  contains components for the control of LiDAR system  100  and communication with external devices that use the system. For example, controller  408  optionally includes one or more processors, memories, communication interfaces, sensors, storage devices, clocks, ASICs, FPGAs, and/or other devices that control light source  402 , signal steering system  404 , and/or light detector  406 . In some examples, controller  408  controls the power, rate, timing, and/or other properties of light signals generated by light source  402 ; controls the speed, transmit direction, and/or other parameters of light steering system  404 ; and/or controls the sensitivity and/or other parameters of light detector  406 . 
     Controller  408  optionally is also configured to process data received from these components. In some examples, controller determines the time it takes from transmitting a light pulse until a corresponding returned light pulse is received; determines when a returned light pulse is not received for a transmitted light pulse; determines the transmitted direction (e.g., horizontal and/or vertical information) for a transmitted/returned light pulse; determines the estimated range in a particular direction; and/or determines any other type of data relevant to LiDAR system  100 . 
       FIG.  7    depicts pulse steering system  700 , which can be used to implement pulse steering system  404  discussed above. Pulse steering system  700  receives returned pulses along paths  701  and includes mirror  702 , polygon scanner  703  (e.g., a reflective polygon rotating around the x-axis as shown in the figure), parabolic mirror  704  (e.g., a mirror focusing the pulse paths), mirror  705 , and mirror  706 . Part of pulse steering system  700  is also used to direct transmitting light pulses. For example, a fiber positioned with mirror  705 , mirror  702 , or in some location respective to polygon scanner  703  optionally provides light pulses that can be directed along different paths outside of the LiDAR system by pulse steering system  700 . 
     In some embodiments, returned light pulses collected by pulse steering system  700  are redirected into an optical fiber (e.g., fiber  710 ), which carries the returned light pulses to a photodetector. This allows the pulse steering system to be located in an arbitrary position with respect to the light detector. 
     In some embodiments of the present technology, a lens or other optical element is used in the optical receive path to increase the tolerance of walk-off error of the returned pulses. For example, in  FIG.  7   , field lens  708  is placed in the path of return pulses traveling from mirror  706  to fiber  710 . The use of a lens (e.g., a cylindrical lens) or other types of optical element mitigates beam walk-off in receiving channel. 
       FIG.  8    depicts field lens  708  positioned to redirect returned light pulses traveling along paths  701  into the fiber core  802 , which is surrounded by fiber cladding  804 . Without field lens  708  (or other optical elements in other embodiments), light pulses may hit fiber cladding  804  or miss fiber  710  altogether. In  FIG.  8   , field lens  708  is a cylindrical lens that can be shaped into spherical, conic, or aspherical shapes. The cylindrical field lens can be made using traditional lens fabrication process such as glass polishing and grinding, precision glass molding, or precision plastic molding. 
     In some embodiments of the present technology, the field lens  708  is configured to redirect returned light pulses traveling along paths  710  directly to a detector (e.g., an avalanche photodiode). In these embodiments, the steering system does not include a fiber. The returned light pulses are directed via the mirrors of the steering system to reach the detector. The detector can be placed fairly close to or directly on the field lens to improve the integrity of the detected signals. 
       FIG.  9    depicts paths  901  of returned light pulses that are redirected by field lens  708  (or other elements). In contrast, paths  902  show that returned light pulses that are not redirected will completely miss fiber  710 . These paths may be misaligned to fiber  710  (and more specifically to fiber core  802 ) because of various errors, process variations, environmental conditions, fabrication of the hardware such as the mirrors, and other effects that are difficult or impossible to fully account for. For example, polygon scanner  703  has a certain amount of jitter in its rotation speed, thus introducing walk-off of the pulses during normal operation of the steering system. When polygon scanner  703  transmits a pulse, the polygon continues to rotate, which means the optical receive path through the steering system is going to be slightly offset from the optical transmit path through the steering system. The amount of offset depends in part on how much the polygon rotated between when the pulse was transmitted and the return pulse was received. The speed of rotation can be controlled only within a certain margin. The distance (and therefore time) a pulse must travel depends on the distance to an object that scatters the pulse. These two variations determine, in-part, the amount of rotation that occurs after a pulse is transmitted and when a corresponding return pulse is received. The use of field lens  708  (or other components) in the optical path allows the system to tolerate more variation in the rotation (and other sources in error) by redirecting more of the returned pulses into fiber core  802 . 
     The improved walk-off characteristics of embodiments of the present technology are shown in  FIG.  10   . Curve  1004  shows that the window of polygon angles that provide for little to no degradation in the optical signal is about twice as large as the window provided without the field lens, as shown by curve  1002 . 
       FIG.  11    shows one example of how the returned light pulses from the exit end of fiber  710  can be coupled to photodetector  604  ( FIG.  6   ) by a plurality of lenses, which may be spherical, conic, aspherical, or ball lens. Coupling  1100  includes aspherical lens  1102 , bandpass filter  1104  (which is matched to the light source frequency), and aspherical lens  1106 . The narrow bandpass filter is positioned between the lens stack to suppress light whose wavelength is outside the signal wavelength band. The multi-layer bandpass film can also be deposited onto the plano surface of either the first lens  1102  or the second lens  1106 . Cover window  1108  protects photodetector  604 , which can detect when a returned light pulse is received. The coupling optic shown in  FIG.  11    is applicable in cases where the detecting area of the photodetector is different from fiber core area, especially where the detecting area of the photodetector is smaller than the fiber core area. Small area detectors are generally desirable because of fast transient response and lower cost. In some examples, the detecting surface of the photodetector has a similar or identical diameter (e.g., 200 μm) as the optical fiber core. In other cases where the detecting area of the photodetector can be chosen to be larger than the fiber core area, detector can be placed or glued directly onto the end facet of the fiber. 
       FIG.  12    depicts pulse steering system  1200 , which can be used to implement pulse steering system  404  discussed above in a similar manner as pulse steering system  700 . Pulse steering system  1200  receives returned pulses along paths  1201  and includes mirror  1202 , polygon scanner  1203  (e.g., a reflective polygon rotating around the x-axis as shown in the figure), parabolic mirror  1204  (e.g., a mirror focusing the pulse paths), mirror  1205 , and mirror  1206 . Part of pulse steering system  1200  is also used to direct transmitting light pulses. For example, a fiber positioned with mirror  1205 , mirror  1202 , or in some location respective to polygon scanner  1203  optionally provides light pulses that can be directed along different paths outside of the LiDAR system by pulse steering system  1200 . 
     In some embodiments, returned light pulses collected by pulse steering system  1200  are redirected into an optical fiber (e.g., fiber  1210 ), which carries the returned light pulses to a photodetector. This allows the pulse steering system to be located in an arbitrary position with respect to the light detector. 
     In some embodiments, a dual lens assembly or other optical element is used in the optical receive path to improve walk-off characteristics, increase the tolerance of walk-off error, and reduce numerical aperture (NA). For example, in  FIG.  12   , dual lens assembly  1220  is placed in the path of return pulses traveling from mirror  1206  to fiber  1210 , and may include concave lens  1205  and field lens  1208 . In this example, concave lens  1205  is placed upstream of field lens  1208 . Concave lens  1209  may improve numerical aperture while field lens  1208  may improve walk-off characteristics and increase the tolerance of walk-off error. In some embodiments, field lens  1208  can be a convex lens or positive lens that is operative to bend light towards a desired optical axis. A dual lens system allows the benefits of both the concave lens and the field lens in the light transmission system. 
       FIG.  13 A  illustrates the physical shape of light beams being directed to a dual lens assembly in a pulse steering system such as pulse steering system  1200 . A polygon scanner such as polygon scanner  1330  may rotate, which may reflect light from individual facets throughout its rotation. Since each facet may be longer (x-direction in  FIG.  12   ) than it is wide (chordal width in the Y-Z plane of  FIG.  12   ), as the return light is focused by mirror system  1320  for delivery to a fiber optic cable, there are different design concerns for each plane.  FIGS.  13 A and  13 B  compare physical differences in light angles as light signals reflect off a facet of polygon scanner  1330  and are focused. The chordal width of the facet in the direction illustrated in  FIG.  13 A  may be significantly smaller than the facet length illustrated in  FIG.  13 B . As a result, the light reflecting off the final mirror in the pulse control system along the plane shown in  FIG.  13 A  will have a significantly lower numerical aperture than the light reflecting off the plane shown in  FIG.  13 B . An illustration of narrow numerical aperture light angles entering dual lens system  1310  from  FIG.  13 A  is shown in  FIG.  13 C . An illustration of wide numerical aperture light angles entering dual lens system  1310  from  FIG.  13 B  is shown in  FIG.  13 D . 
       FIGS.  14 A- 14 C  are referenced in connection with a description of walk-off in a lens system. As light signals reflect from an object, the time light takes to travel from emitter to object and back to receiver is ΔT. Within this time, the polygon rotates an angle determined by Δθ=ωΔT where ω is the rotational speed of the polygon. Once the return light is bounced off the polygon facet, it propagates at 2Δθ angle with respect to the optical axis of the receiving optical system. As a result, the beam spot on the receiver is shifted by 2Δθ·f where f is the effective focus length of the receiving optic.  FIG.  14 A  depicts narrow numerical aperture light reaching field lens  1401  with small amount of walk off (in the case of short object distance), and the light is focused on fiber cable  1402 .  FIG.  14 B  depicts narrow numerical aperture light reaching field lens  1401  with walk-off (in the case of long object distance); the field lens redirects and focuses light onto fiber cable  1402 .  FIG.  14 C  depicts narrow numerical aperture light with walk-off in the absence of field lens  1401 ; since the light signal is not redirected towards fiber cable  1402 , data is lost. A field lens improves walk-off errors by redirecting the signal to the target even when the light signal fluctuates in space. 
       FIGS.  15 A- 15 C  different sized spot beams entering a fiber optic cable. It is desirable for the spot beam exiting the dual lens system to have a specified range of spot sizes relative to the cross-section of the core of the fiber optic cable.  FIG.  15 A  illustrates an example where the spot size  1504  is substantially similar to the cross-sectional area of fiber optic cable  1503 .  FIG.  15 B  illustrates a spot size  1505  that is substantially greater than the cross-section of fiber optic cable  1503 . This results in light falling outside of the fiber optic cable, possibly resulting in lost data.  FIG.  15 C  illustrates a spot size  1506  that is substantially smaller than the cross-sectional area (e.g., less than 30% of the available area). The dual lens system according to embodiments discussed herein can be designed to yield a spot beam size that falls between spot size  1504  and  1506 . For example, the desired spot size may range between 90% and 33%, between 95% and 50%, or between 80% and 60% of the cross-sectional area of the fiber optic cable. 
     With reference to  FIGS.  16 A and  16 B , numerical aperture correction of a concave lens is now described. Numerical aperture is the maximum acceptance angle within which light can transmit through fiber without loss. In  FIG.  16 A , light energy with high NA  1610  passes through a concave mirror  1600  with a resulting NA reduction angle  1620  before light enters fiber  1630 .  FIG.  16 B  depicts the normalized throughput of light energy through a fiber optic cable such as fiber  1630  with respect to the numerical aperture of light hitting the fiber optic cable. Normalized throughput, expressed as a ratio of light signal entering the fiber optic to the light signal exiting the fiber optic cable, is a measure of loss through the cable. Representative experimental data shown in  FIG.  16 B  illustrates that as numerical aperture increases, losses in the fiber optic cable increase. These losses are undesirable in a LiDAR system. 
       FIGS.  17 A and  17 B  show different views of a dual concave cylindrical lens/field lens system.  FIG.  17 A  shows the top view dual lens system  1730 , which receives high numerical aperture light such as shown in view  1320  (of  FIG.  13 B ). The data loss from the high numerical aperture light represented by  FIG.  17 A  may be improved by concave lens  1710  of a dual lens system while field lens  1720  may not affect light in this plane.  FIG.  17 B  shows the side view of dual lens system  1730 , which represents narrower numerical aperture light such as shown in view  1310  (of  FIG.  13 A ). The susceptibility to walk-off from the low numerical aperture light represented by  FIG.  17 B  may be improved by field lens  1720  of a dual lens system while concave lens  1710  may not affect light in this plane. The dual lens system described in this embodiment allows the benefits of two lens designs to be designed to improve walk-off characteristics, increase the tolerance of walk-off error, and reduce numerical aperture depending on the angle of light presented in either perpendicular plane. 
       FIGS.  17 A and  17 B  also illustrate the design parameters related to the shapes of a concave cylindrical lens and a field lens that may be used in a dual lens system. In the top view shown in  FIG.  17 A , the concave cylindrical lens may have a flat, transparent surface on the upstream side and a transparent concave shape on the downstream side. In the side view shown in  FIG.  17 B , the concave lens may have a flat, transparent surface on the upstream side and a flat, transparent surface on the downstream side. In this way the concave lens may affect the focus of light in only one plane. Further design parameters of the concave lens may involve overall surface dimensions, radius of the concave surface on the downstream surface, material, anti-reflective coatings, and optical surface dimensions. 
     In the side view shown in  FIG.  17 B , the field lens may be illustrated as a convex cylindrical shape. The top view shown in  FIG.  17 A  illustrates the field lens viewed at 90 degrees from the convex shape of the lens, and is shown as a rectangular surface. In this way the field lens may affect the focus and walk-off characteristics of incoming light in only one plane. Further design parameters of the field lens may involve convex curve radius, overall dimensions, material, anti-reflective coatings, and optical surface dimensions. 
       FIG.  18    illustrates the benefits of the dual lens system as well as its design parameters.  FIG.  18    plots example experimental normalized receiver signal strength with respect to the angle of a polygon scanner such as polygon scanner  1203 . Due to walk off and data loss from high numerical aperture, as polygon scanner angle changes normalized signal strength through the system changes. Data trace  1810  represents a normalized receiver strength signal if no lens correction is made, and data is lost at relatively low polygon scanner angles. Data trace  1820  represents a normalized receiver signal strength if a sample off-the-shelf convex lens is used upstream of the fiber optic cable and shows an increased robustness to data loss for high polygon scanner angles compared to data trace  1810 . Data trace  1830  represents a normalized receiver signal strength if a non-optimized dual lens system including a concave lens and field lens is used, and illustrates significant improvement in robustness to data loss for high polygon scanner angles compared to data traces  1810  and  1820 . Data trace  1840  represents a normalized receiver signal strength if an optimized dual lens system (such as those presented herein) is designed for the application and represents further increased robustness to data loss for high polygon scanner angles compared to data trace  1830 .  FIG.  18    demonstrates that a dual lens system such as those discussed herein results in improvements in overall LiDAR system performance by mitigating data loss possible by using a dual lens system including a concave lens and a field lens. 
     It is believed that the disclosure set forth herein encompasses multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. Each example defines an embodiment disclosed in the foregoing disclosure, but any one example does not necessarily encompass all features or combinations that may be eventually claimed. Where the description recites “a” or “a first” element or the equivalent thereof, such description includes one or more such elements, neither requiring nor excluding two or more such elements. Further, ordinal indicators, such as first, second or third, for identified elements are used to distinguish between the elements, and do not indicate a required or limited number of such elements, and do not indicate a particular position or order of such elements unless otherwise specifically stated. 
     Moreover, any processes described with respect to  FIGS.  1 - 18   , as well as any other aspects of the invention, may each be implemented by software, but may also be implemented in hardware, firmware, or any combination of software, hardware, and firmware. They each may also be embodied as machine- or computer-readable code recorded on a machine- or computer-readable medium. The computer-readable medium may be any data storage device that can store data or instructions which can thereafter be read by a computer system. Examples of the computer-readable medium may include, but are not limited to, read-only memory, random-access memory, flash memory, CD-ROMs, DVDs, magnetic tape, and optical data storage devices. The computer-readable medium can also be distributed over network-coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. For example, the computer-readable medium may be communicated from one electronic subsystem or device to another electronic subsystem or device using any suitable communications protocol. The computer-readable medium may embody computer-readable code, instructions, data structures, program modules, or other data in a modulated data signal, such as a carrier wave or other transport mechanism, and may include any information delivery media. A modulated data signal may be a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. 
     It is to be understood that any or each module or state machine discussed herein may be provided as a software construct, firmware construct, one or more hardware components, or a combination thereof. For example, any one or more of the state machines or modules may be described in the general context of computer-executable instructions, such as program modules, that may be executed by one or more computers or other devices. Generally, a program module may include one or more routines, programs, objects, components, and/or data structures that may perform one or more particular tasks or that may implement one or more particular abstract data types. It is also to be understood that the number, configuration, functionality, and interconnection of the modules or state machines are merely illustrative, and that the number, configuration, functionality, and interconnection of existing modules may be modified or omitted, additional modules may be added, and the interconnection of certain modules may be altered. 
     Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. Therefore, reference to the details of the preferred embodiments is not intended to limit their scope.