Patent Publication Number: US-11387626-B1

Title: Integrated high-power tunable laser with adjustable outputs

Description:
RELATED APPLICATIONS 
     The present application is a continuation claiming the benefit under 35 U.S.C. § 120 of U.S. patent application Ser. No. 16/133,660 filed Sep. 17, 2018 entitled “INTEGRATED HIGH POWER TUNABLE LASER WITH ADJUSTABLE OUTPUTS;” which is a continuation claiming the benefit under 35 U.S.C. § 120 of U.S. patent application Ser. No. 15/383,555 filed Dec. 19, 2016 and issued as U.S. Pat. No. 10,079,472 on Sep. 18, 2018 entitled “INTEGRATED HIGH POWER TUNABLE LASER WITH ADJUSTABLE OUTPUTS;” which is a continuation claiming the benefit under 35 U.S.C. § 120 of U.S. patent application Ser. No. 14/797,018, filed Jul. 10, 2015 and issued as U.S. Pat. No. 9,559,487 on Jan. 31, 2017 entitled “INTEGRATED HIGH-POWER TUNABLE LASER WITH ADJUSTABLE OUTPUTS;” which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No. 62/023,483, filed Jul. 11, 2014 entitled “INTEGRATED HIGH-POWER TUNABLE LASER WITH ADJUSTABLE OUTPUTS;” all of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     Field 
     The present application relates to tunable lasers, optical amplifiers, and to optical communication systems. 
     Related Art 
     Tunable lasers conventionally consist of a tunable wavelength filter and a single optical gain medium inside a resonant laser cavity. A depiction of a conventional tunable laser  100  is shown in  FIG. 1 . The laser cavity may include an intracavity beam  102  that reflects between a high-reflector end-mirror  105  and partially-transmitting mirror  140  (referred to as an “output coupler”). The intracavity beam passes through the gain medium  110  and tunable wavelength filter  130  as it circulates between the end-mirror and output coupler. 
     Such lasers normally have only one output beam  104 , which emits from the output coupler  140 . For example, the output coupler may transmit about 10% of the optical power in the intracavity beam  102  outside the laser cavity to form the output beam  104 . Conventionally, the amount of power coupled outside the laser cavity cannot be adjusted while the laser is operating. Instead, the laser must be shut off, and a different output coupler  130  installed and aligned. 
     Because a conventional laser contains one gain medium, the laser power is limited by the saturation power of the gain medium  130 . Once the saturation power level is reached in the gain medium, no further substantial increase in output power from the laser cavity can be achieved. To increase available laser power, a conventional technique passes the output beam  104  through an optical amplifier located downstream of the laser  100 . 
     BRIEF SUMMARY 
     The present technology relates to tunable lasers, high-power lasers, and optical amplifiers. A plurality of optical amplifiers may be integrated in parallel into a laser cavity. Additionally, the laser may include a tunable filter and provide a plurality of power output ports, where power from each port is adjustable. According to some embodiments, a laser having a laser cavity may comprise a reflector at a first end of the laser cavity and an intracavity N×M coupler arranged to receive light from the reflector at a first port and distribute the light to N output ports. The laser may further include Q optical amplifiers arranged to amplify light from at least some of the N output ports and at least one reflector arranged to reflect the amplified light back to the N×M coupler. The number of optical amplifiers incorporated in the laser cavity may be greater than or equal to two. 
     Methods for operating a tunable laser having integrated optical amplifiers are also described. According to some embodiments, a method of generating coherent light may comprise acts of reflecting light from a first reflector, and distributing the reflected light, with an N×M coupler, to N optical paths. The method may further include producing amplified light by amplifying light in at least two of the N optical paths, and returning the amplified light to the N×M coupler and first reflector. A method of operating a tunable laser may also include adjusting a phase of an optical signal in at least one of the N optical paths to adjust an output power from one of multiple power output ports of the tunable laser. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Various aspects and embodiments of the application will be described with reference to the following figures. It should be appreciated that the figures are not necessarily drawn to scale. Items appearing in multiple figures are indicated by the same reference number in all the figures in which they appear. 
         FIG. 1  depicts a conventional tunable laser cavity; 
         FIG. 2  depicts a tunable laser that includes N optical amplifiers arranged in parallel, according to some embodiments; 
         FIG. 3A  depicts an N×M coupler, according to some embodiments; 
         FIG. 3B  depicts an N×M coupler, according to some embodiments; 
         FIG. 4  depicts a thermo-optic phase shifter, according to some embodiments; 
         FIG. 5  depicts a tunable wavelength filter, according to some embodiments; 
         FIG. 6A  depicts a waveguide loop mirror, according to some embodiments; 
         FIG. 6B  depicts a waveguide mirror, according to some embodiments; 
         FIG. 7A  depicts butt-coupled waveguides with mode size adapting regions, according to some embodiments; 
         FIG. 7B  depicts butt-coupled waveguides with mode size adapting regions, according to some embodiments; 
         FIG. 8  depicts a semiconductor optical amplifier, according to some embodiments; 
         FIG. 9  depicts a tunable laser that includes N optical amplifiers coupled to a coherent optical receiver and optical transmitter, according to some embodiments; 
         FIG. 10A  and  FIG. 10B  depict an alternate embodiment for coupling N optical amplifiers to N ports of an N×M coupler in a tunable laser cavity; 
         FIG. 11A  depicts an embodiment of a coupled optical amplifier chip and silicon photonics chip in which a laser cavity is distributed between the two chips; 
         FIG. 11B  depicts an embodiment of a coupled optical amplifier chip and silicon photonics chip in which a laser cavity is distributed between the two chips; and 
         FIG. 11C  depicts an embodiment of a coupled optical amplifier chip and silicon photonics chip in which a laser cavity is distributed between the two chips. 
     
    
    
     DETAILED DESCRIPTION 
     The present technology pertains to tunable lasers that may be used in optical communication systems, among other applications. Aspects of the application include apparatus and methods to provide a tunable laser that includes a plurality of optical amplifiers in a parallel configuration and that can provide output power from multiple adjustable power ports. Additionally, the tunable laser is readily scalable to higher powers and additional power ports. According to another aspect of the application, methods of manufacturing a tunable laser of the types described herein are disclosed. 
     According to some embodiments, a tunable laser, such as tunable laser  200  described below in connection with  FIG. 2 , may be microfabricated and used in integrated optical systems, such as photonic integrated circuits (PICs). The PICs may be used in optical communication systems or optical coherent tomography systems, for example. In some cases, the tunable laser may be used for supplying an optical carrier wave and/or local oscillator to optical transmitters and receivers. In some embodiments, a tunable laser (such as tunable laser  200 ) may be fabricated in a fiber-optic system, e.g., as a tunable fiber laser. A tunable fiber laser may include a plurality of fiber amplifiers arranged in parallel and coupled into a laser cavity using fiber couplers. 
     The aspects and embodiments described above, as well as additional aspects and embodiments, are described further below. These aspects and/or embodiments may be used individually, all together, or in any combination of two or more, as the application is not limited in this respect. 
     Referring to  FIG. 2 , a tunable laser  200  in accordance with an aspect of the present application may comprise an array of optical amplifiers  230 - 1 ,  230 - 2  . . .  230 -N (referred to collectively as  230  and individually as  230 - m ) that are coupled to an intracavity N×M optical coupler  250  and to a first cavity reflector  207  at a first end of the tunable laser cavity. The N optical amplifiers may be coupled to the N×M coupler through a plurality of optical paths  212  and  222 . In some embodiments, the optical paths may comprise integrated photonic waveguides, e.g., fabricated from semiconductor and/or oxide material on a substrate. In some implementations, the optical paths  212  and/or  222  may comprise fiber-optic waveguides. A second end of the laser cavity may comprise a second reflector  205  arranged to reflect light back through the optical amplifiers  230 . Additionally, the laser may include P output power ports  260 - 1  . . .  260 -P (referred to collectively as  260  and individually as  260 - m ). As described below, in some embodiments N and P are integers and P may be equal to or less than M−1. 
     Although  FIG. 2  depicts N optical amplifiers coupled to N input ports of the N×M coupler, some embodiments may have fewer than N optical amplifiers. For example, some embodiments may have Q optical amplifiers coupled to a portion of the N input ports of the N×M coupler, where Q is fewer than N. An input port that does not have an optical amplifier coupled to it may be used as an intracavity power monitor in some embodiments. 
     The tunable laser  200  may further include a tunable wavelength filter  130  and at least one intracavity phase shifter. At least one laser cavity optical path  224  extending between the N×M coupler  250  and the first cavity reflector  207  may be provided, and may include the tunable wavelength filter  130 . In some embodiments, the laser cavity optical path  224  may include a phase shifter. In the illustrated embodiment, a plurality of intracavity phase shifters  240 - 1 ,  240 - 2  . . .  240 -N are provided (referred to collectively as  240 , and individually as  240 - m ), one corresponding to each of the optical amplifiers  230 . The phase shifter(s)  240  may be located in the optical paths  222  connected to the N×M coupler  250 . The wavelength filter  130  may be tuned to select a lasing wavelength that circulates in between the first cavity reflector  207  and second reflector  205 , passing through the optical amplifiers  230 . The phase shifter(s)  240  may be tuned to adjust an amount of output power delivered from one or more of the power ports  260 - 1  . . .  260 -P. The tuning (or adjusting) of the phase shifters may be dynamic (during operation of the laser) in some embodiments. Accordingly, a lasing wavelength for the tunable laser  200  may be selected by providing a control signal to the tunable wavelength filter  130 . Additionally, power from one or more of the power ports may be adjusted while the laser is operating by providing one or more control signals to one or more of the phase shifters. 
     The tunable laser  200  may also include a wavelength locker  270 . The wavelength locker  270  may comprise an integrated photonic circuit configured to sense an operating wavelength of the tunable laser. The wavelength locker  270  may comprise an interferometer, a Bragg grating structure, a resonator, or a combination thereof, and may produce a signal detected by wavelength locking circuitry  280  that is indicative of a lasing wavelength for the tunable laser. An output from the wavelength locking circuitry  280  may be provided to the intracavity tunable wavelength filter  130 , so as to stabilize an operating wavelength of the tunable laser. In some embodiments, a wavelength locker  270  may be fabricated from a material having a low thermo-optic coefficient. In some cases, the wavelength locker and/or at least a portion of the chip or chips on which the tunable laser is fabricated may be temperature controlled using a thermo-electric cooler or heater. 
     In operation, the tunable laser  200  may produce laser light that reflects from the first cavity reflector  207 , passes through the N×M coupler  250  where it is distributed to, and amplified by, the N optical amplifiers  230 , and then proceeds to the second reflector  205  where it is reflected back through the amplifiers and laser cavity. As the light circulates back and forth in the laser cavity, each of the N optical amplifiers  230  contributes gain to the intracavity laser power. Additionally, a portion of the intracavity power is tapped out of the cavity through the P output power ports  260 . In various embodiments, N, M, and P are integers. N may be greater than, or equal to, 2. M may be less than, equal to, or greater than N. P may be less than, or equal to M. In some embodiments, N=M, and P=N−1. 
     Providing an array of optical amplifiers  230  in parallel, rather than just one larger amplifier in a laser cavity, improves thermal and optical performance of the tunable laser  200 . The array spreads heat generated by the amplifiers  230  over a larger area, where it can be dissipated more easily. For example, an injection current to drive the amplifiers to obtain a given amount of power is spread over N separate regions of a substrate rather than being concentrated in a single region. An array of optical amplifiers also permits higher optical saturation power in the laser. In a semiconductor optical amplifier, the amount of available carriers per unit volume for optical gain may have an upper limit. Having multiple amplifiers in parallel increases the amplifier volume, while maintaining optical single-mode operation in each amplifier, and therefore increases the amount of available carriers for optical gain. 
     According to some embodiments, the optical amplifiers  230  may be located on a first semiconductor chip  210 . The N×M coupler  250  and tunable filter  130  may be located on a second semiconductor chip  220 . The phase shifters  240  may be located on the first or the second semiconductor chip. The first semiconductor chip  210  may comprise any suitable first semiconductor material, e.g., indium phosphide and/or any of its alloys (collectively referred to as indium phosphide), gallium arsenide and/or any of its alloys, or gallium nitride and/or any of its alloys. The material of the first semiconductor chip  210  may be different from a second semiconductor material of the second semiconductor chip  220 . For example, the second semiconductor chip  220  may comprise silicon, silicon dioxide, silicon oxynitride, and/or silicon nitride, and include integrated silicon photonic devices. 
     When a tunable laser, such as tunable laser  200 , is distributed across two semiconductor chips, mode-size adapters may be formed at the junction of the optical paths between the two semiconductor chips. For example, mode-size adapters may be formed at the ends of integrated waveguides running to the edge of a chip. In the example of  FIG. 2 , mode size adapters  215 - 1 ,  215 - 2  . . .  215 -N are provided (collectively referred to herein as  215 ), one for each of the optical paths  212 . The mode-size adapters may improve coupling efficiency of optical radiation from an optical path (e.g., a waveguide)  212  on the first semiconductor chip  210  to an optical path (e.g., a waveguide)  222  on the second semiconductor chip  222 . 
     One example of an optical N×M coupler  350  which may be used as the N×M coupler  250  of  FIG. 2  is depicted in  FIG. 3A , although the various aspects of the present application are not limited to only this type of coupler. In some implementations, an optical coupler may comprise a multi-mode interference (MMI) coupler or a star coupler. An optical coupler may comprise N first ports on one side of a coupling region  320  and M second ports on a second side of the coupling region. In the illustrated example, N equals four, such that first ports  310 - 1  . . .  310 - 4  (collectively referred to as  310 ) are provided, while M equals two such that second ports  330 - 1  and  330 - 2  (collectively referred to as  330  and individually as  330 - m ) are provided. It should be appreciated that other numbers of ports may be provided. In some embodiments, the coupling region  320  may comprise an integrated slab waveguide in which optical modes entering from the N first ports  310  expand and interfere optically before exiting through the M second ports  330 . In some implementations, an optical coupler  350  may be formed as an integrated silicon optical device wherein the N and M ports and coupling region  320  are fabricated as silicon waveguide structures. The N and M ports may comprise single-mode optical waveguides each having transverse dimensions between approximately 50 nm and approximately 700 nm. In some embodiments, a single-mode waveguide may have a height between approximately 50 nm and approximately 300 nm and a width between approximately 200 nm and approximately 700 nm. The coupling region  320  may comprise a multimode slab waveguide, and have a same height as waveguides of the N and M ports. A width of the coupling region may be between approximately 1 micron and approximately 50 microns. 
     In other embodiments, an optical coupler  350  may be formed using any suitable semiconductor material, dielectric material, or material compositions. Material compositions may include metallic layers in some embodiments. Dielectric materials may include insulators such as oxides or nitrides. Optical couplers formed from other materials or material compositions may have different dimensions than those listed above. 
     For an optical coupler  350 , the first ports  310  may be referred to as “input” ports, and the second ports  330  may be referred to as “output” ports. However, an N×M optical coupler may exhibit reciprocity and operate in both directions. For example, an optical coupler may distribute light received at N “input” ports among M “output” ports. The received light may have a distribution of N different intensities. The light distributed among the M output ports may have a distribution of M different intensities. The total of the output intensities may or may not be approximately equal to a total of the input intensities, depending on the design of the optical coupler. In some implementations, the direction of the light may be reversed, so that the optical coupler distributes light received at the M ports among the N ports having the same distribution of N different intensities. Although  FIG. 3A  depicts a 4×2 optical coupler, there may be any other number of first ports  310  and second ports  330 . 
     Another example of an N×M optical coupler  352  is depicted in  FIG. 3B . In the illustrated example N equals two such that first ports  310 - 1  and  310 - 2  are provided, and M equals four such that second ports  330 - 1  . . .  330 - 4  are provided. These numbers are merely examples. According to some embodiments, an optical coupler may comprise a plurality of single-mode optical waveguides that interact along their length. For example, two or more waveguides may run parallel and in close proximity to each other at coupling regions  315  and  325  (e.g., an optical directional coupler or an optical adiabatic coupler). The coupling regions may be regions where two or more waveguides are spaced near each other so that at least an evanescent field from one waveguide extends into at least one adjacent waveguide. As an optical mode travels along a waveguide, power will couple from one waveguide into at least one adjacent waveguide. 
     As noted above, one or more of the optical paths extending between the N×M optical coupler  250  and the N optical amplifiers  230  may include one or more phase shifters  240 . In some implementations, there is one phase shifter in each optical path  222 . A phase shifter  240 - m  may be configured to adjust a phase of an optical signal traveling along the optical path  222 , and to affect the optical interference of fields at the N×M coupler  250 . By adjusting the phase of an optical signal in one or more optical paths  222 , an amount of power emitted from the output power ports  260  and in the laser cavity optical path  224  can be altered. For example, adjusting a phase in one of the optical paths  222  can change the way in which the optical fields interfere at the N×M coupler  250  and deliver power to each of the M ports. As an example, according to one phase setting all of the intracavity power may flow through the laser cavity optical path  224 . Another phase setting may distribute some of the intracavity power among the power ports  260 . 
     For the phase shifters  240  to affect optical interference in the N×M coupler  250  consistently over a wide wavelength range, the optical path lengths between the N×M coupler and second reflector  205  through each optical amplifier may be approximately equal. In practice, the optical path lengths may differ, provided they do not differ by more than the temporal coherence length of the laser radiation. Having different optical path lengths may result in wavelength dependence, and may be used in some embodiments to provide optical wavelength filtering in the laser cavity. In some implementations, the phase shifters  240  may be adjusted by control signals that are varied manually and/or automatically. For example, each power port  260 - m  may include an optical tap and a power detector, so that an operator can provide a control signal to adjust the phase shifters to obtain a desired power ratio from the power ports. Additionally or alternatively, feedback circuitry or any suitable control circuit may provide control signals to the phase shifters  240  responsive to detected power at one or more ports, so as to stabilize power from one or more ports  260 . The feedback circuit may be any suitable circuit or may be implemented via digital signal processing. A feedback circuit may receive at least one power signal from a detector arranged to monitor a power from a power port, and provide a control signal to a phase shifter  240 - m  to alter a phase responsive to the received power signal. The feedback circuit may compare the received power signal to a second signal to determine a value for the control signal. 
       FIG. 4  depicts a non-limiting example of a phase shifter  440  that may be used in a tunable laser, for example as a phase shifter  240 - m  of the tunable laser  200 . According to some embodiments, the phase shifter  440  may be a thermo-optic phase shifter, as described by M. R. Watts et al., in “Adiabatic Thermo-Optic Mach-Zehnder Switch,” Opt. Lett. Vol. 38, No. 5, 733-735 (2013), which is incorporated herein by reference. Such thermo-optic phase shifters can achieve efficient optical phase modulation of up to 2π in a length of waveguide less than 20 microns. In other embodiments, the phase shifter  440  may comprise a semiconductor-based phase shifter that alters phase by current injection into a waveguide. For an optical fiber implementation, the phase shifter may comprise piezoelectric material that stretches a length of fiber. Regardless of the type of phase shifter  440 , it may be controlled by an electrical bias to adjust the phase of an optical signal traversing the optical path  222 . 
     A thermo-optic phase shifter may include resistive elements  410  located adjacent an optical path (assumed to be a waveguide in this example)  222 . In some embodiments, there may be just one resistive element  410  adjacent to the waveguide. A resistive element  410  may be located beside and/or above and/or below the optical waveguide. A resistive element may be formed of a resistive semiconductor material, metal, or any other suitable material that converts electrical current into heat. A thermo-optic phase shifter  440  may further include electrically conductive traces that extend to a first terminal  425  and a second terminal  427 . The first and second terminals may be contact pads. Electrical current may be applied across the resistive element  410  via the first and second terminals. As current flows through the resistive element, the resistive element may dissipate heat that couples to at least a portion of the optical waveguide  222  and thereby change the refractive index within the optical waveguide. This change in refractive index can change the phase of an optical signal traveling through the waveguide  222 . A thermo-optic phase shifter may extend along a waveguide over a distance between approximately 2 microns and approximately 400 microns, according to some embodiments. Other embodiments may include other lengths. 
     The various aspects described herein are not limited to thermo-optic phase shifters  440 . In some implementations, a phase shifter  240 - m  may comprise an electro-optic phase shifter. An electro-optic phase shifter may comprise a semiconductor junction (e.g., a p-n or p-i-n) formed in a portion of an integrated waveguide. The semiconductor junction may be configured to inject carriers into a region of the waveguide through which an optical mode travels. The injection of carriers increases optical absorption and can change the refractive index in the waveguide through Kramers-Kronig relations applied to the optical absorption. An electro-optic phase shifter may extend along a waveguide over a distance between approximately 50 microns and approximately 800 microns, according to some embodiments. Other embodiments may include other lengths. 
       FIG. 5  depicts a tunable wavelength filter  530  that may be included in a high-power, tunable laser cavity, according to some embodiments. For example, the tunable wavelength filter  530  may serve as the tunable wavelength filter  130  of  FIG. 2 . The tunable wavelength filter  530  may include a pair of integrated photonic ring resonators  510 ,  520  located adjacent to a laser cavity optical path  224 . The ring resonators  510 ,  520  may be circular, elliptical, or have a race track pattern, and may be of different sizes. Each ring resonator may have a free spectral range that can be adjusted thermo-optically through resistive heating elements  540 . By adjusting the free spectral range of each ring resonator, it is possible to select a wavelength of an optical signal that can couple to the first ring resonator  510  from the laser cavity optical waveguide  224 - 1 , couple to an intermediate waveguide  224 - 2 , couple to the second ring resonator  520 , and couple to an end waveguide  224 - 3 , where the optical signal travels to and reflects from the laser cavity reflector  207 . 
     The ring resonators  510 ,  520  may be formed as integrated optical waveguides, according to some embodiments. The ring resonator waveguides may have a transverse profile approximately the same as a transverse profile of the cavity optical waveguide  224 - 1 , and described above. In some embodiments, the ring resonator waveguides may be formed from a same material as the cavity optical waveguide  224 - 1  (e.g., silicon). 
     According to some embodiments, the first laser cavity reflector  207  may comprise any suitable reflector that can be integrated on a PIC. One example of a reflector is illustrated in  FIG. 6A . According to this embodiment, a reflector may comprise a waveguide loop mirror. For example, an end of the cavity optical path (e.g., waveguide)  224  may extend into a loop  610  at an end of the laser cavity that circles back on the cavity optical waveguide. In some embodiments, the loop  610  may comprise a single mode waveguide, having a same transverse profile and formed of the same materials as the cavity optical waveguide  224 . The loop may extend in any suitable shape, e.g., a teardrop shape. 
     Another example of a reflector  207  that can be implemented at an end of a waveguide on a chip is depicted in  FIG. 6B . According to this embodiment, a reflector may comprise a multi-mode interference reflector having an expanded region  630  of a waveguide  224 . The expanded region  630  may comprise a slab waveguide region or optical cavity into which an optical mode traveling along the waveguide  224  may expand, optically interfere, and reflect back into the waveguide  224 . 
     According to some embodiments, a second reflector  205  at an opposite end of the laser cavity may be implemented as a reflective coating deposited on facets on optical waveguides  212 . For example, the first semiconductor chip may be cleaved or cut, exposing facets of the waveguides  212  that pass through the optical amplifiers. A reflective coating may then be deposited on the exposed facets. The reflective coating may comprise a multi-layer dielectric coating having high reflectivity at the lasing wavelength. In other embodiments, a plurality of second reflectors may be used. For example, each waveguide  212  may include a loop mirror or multi-mode interference reflector, so that the second reflector  205  comprises an array of reflectors. 
     As illustrated in  FIG. 2 , the optical amplifiers  230  may be located on a first semiconductor chip  210  and the N×M coupler  250  may be located on a second semiconductor chip  220 . The optical amplifiers  230  may connect to the N×M coupler through N optical paths  212 ,  222  that comprise integrated optical waveguides. The integrated waveguides may, in some implementations, be butt-coupled to one another at edges of the semiconductor chips, as depicted in  FIG. 7A  and  FIG. 7B . Such butt-coupled waveguides can provide efficient transfer of power from one waveguide  212  on one chip to another waveguide  222  on an adjacent chip. Where the waveguides meet at the edge of the chip, there may be mode-size adapters  215 . One example of a mode-size adapter  715  is depicted in  FIG. 7A . 
     In some implementations, a mode-size adapter  715  may comprise a portion of an integrated optical waveguide that changes in structure as it approaches the edge of a semiconductor chip or a region where optical coupling to another waveguide will take place. For example, a waveguide  212  for an optical amplifier  230 - 1  may expand gradually in size and/or curve at a mode-size adapting region  720  near an edge of the first semiconductor chip  210 . The expansion in size may increase a transverse dimension of a waveguide up to 2 microns or more, in some cases. The expansion may allow the optical mode in the waveguide to expand in a direction transverse to the waveguide near the edge of chip, and thereby make coupling of the optical mode from one waveguide on the first chip to a second waveguide  222  on the second semiconductor chip  220  less sensitive to misalignment between the waveguides. 
     Mode-size adapting regions  720 ,  730  of each waveguide may follow any suitable curved path, such that an optical mode travelling along an optical axis  703  in the first adapting region  720  and exiting the first semiconductor chip  210  is aligned with an optical axis of the second adapting region  730  and makes an angle α with respect to a normal of a facet of each waveguide at the chip edges. The angle α may be between approximately 5° and approximately 40°, according to some embodiments. Butt-coupling using such angled optical axes can reduce deleterious effects of potential reflections from the chip edges. For example, potential reflections from the chip edge may reflect into a direction that is not readily coupled back into the optical waveguide  212 . The reflected light may be in a direction that is outside the numerical aperture of the waveguide, and therefore is not captured and guided by waveguide  212 . An optical adhesive or index-matching adhesive may be used in some cases to bond the butt-coupled waveguides. In some implementations, the optical axes of the butt-coupled waveguides may be normal to the chip edges, and optical adhesive or index-matching adhesive may be used to bond the butt-coupled waveguides. According to some embodiments, an anti-reflection coating (e.g., a multi-layer dielectric stack) may be formed on facets of but-coupled waveguides to reduce interface reflections (e.g., when going from an InP chip in which the cladding may comprise InP to a Si chip in which the cladding may comprise an oxide). 
     Another example of a mode-size adapter  717  is depicted in  FIG. 7B . In some embodiments, the optical waveguides may taper and reduce in lateral and/or vertical dimensions in the mode-size adapting regions  722 ,  732  at edges of the respective chips. In some implementations, a transverse dimension of a waveguide may reduce to about 50 nm. By reducing a transverse dimension of an optical waveguide, an optical mode within the waveguide is confined less strongly and expands out into the surrounding dielectric or air as the waveguide becomes smaller. This can increase the lateral dimension of an optical mode traveling along the waveguide as it approaches the edge of the semiconductor chip. 
     According to some embodiments and referring again to  FIG. 2 , the optical amplifiers  230  may comprise any suitable type of optical amplifier. In a fiber-optic system, an optical amplifier may comprise an erbium-doped fiber, for example. When implemented in a PIC, an optical amplifier may comprise a semiconductor optical amplifier (SOA). In some embodiments, an SOA  800  may have a structure as depicted in the elevation view of  FIG. 8 . For example, an SOA may be formed on a semiconductor substrate  805 , which may be a silicon (Si) substrate or an indium phosphide (InP) substrate, though other semiconductor substrates may be used in other embodiments. There may, or may not, be a dielectric or insulating layer  810  (e.g., an oxide or nitride layer) on the substrate. For example, the substrate may comprise a semiconductor on insulator (SOI) substrate, according to some embodiments. The insulation layer  810  may be between approximately 50 nm thick and approximately 4 microns thick. 
     In some implementations, a semiconductor optical amplifier  800  may comprise InP material and include a first n-doped InP base layer  820  formed on the substrate. The base layer  820  may be between 100 nm thick and approximately 2 microns thick. A buffer layer  825  comprising n-doped InP may be formed on the base layer  820 . The buffer layer  825  may be formed by epitaxial growth or ion implantation, according to some embodiments, and may be between approximately 5 nm and approximately 50 nm thick. An intrinsic layer  830  of InP may be subsequently grown epitaxially on the buffer layer. The intrinsic layer  830  may be between approximately 50 nm and approximately 200 nm thick, according to some embodiments. A p-doped layer  840  may be formed on the intrinsic layer to form a p-i-n junction. The intrinsic layer  830  and p-doped layer  840  may be epitaxially grown. The SOA  800  may additionally include electron-blocking and hole-blocking layers (not shown) in some embodiments. A first electrical contact (not shown) may be formed on the p-doped layer  840 , and a second electrical contact may connect to the base layer  820  so that a current can be applied across the p-i-n junction. 
     According to some implementations, a SOA, such as SOA  800 , may be patterned in a waveguide structure. A cross-section of the waveguide structure may have a profile as depicted in  FIG. 8 . An optical mode may be confined to the waveguide and pass primarily through the intrinsic region  830  of waveguide where carrier recombination and optical amplification can occur. Although  FIG. 8  depicts a ridge waveguide, some embodiments may include a buried waveguide (e.g., a waveguide comprising semiconductor material surrounded on two or more sides by a dielectric having a lower refractive index). 
     Although a semiconductor optical amplifier is described as being indium-phosphide based (which includes alloys of InP) in connection with  FIG. 8 , other materials may be used in other embodiments for a SOA. For example, a SOA may comprise gallium arsenide and/or its alloys. In some implementations, a SOA may comprise gallium nitride and/or its alloys. In some embodiments, a SOA may comprise alloys of indium-aluminum. 
     According to some embodiments, a tunable laser that includes a plurality of optical amplifiers may be implemented in a PIC for optical communications, as depicted in  FIG. 9 . A PIC  900  may comprise the tunable laser  901  that includes, for example, a 4×4 optical coupler  250 , four SOAs  230 - 1 ,  230 - 2 ,  230 - 3 , and  230 - 4 , four mode size adapters  215 - 1 ,  215 - 2 ,  215 - 3 , and  215 - 4 , and four phase shifters  240 - 1 ,  240 - 2 ,  240 - 3 , and  240 - 4 . The tunable laser  901  may include three power output ports  904 - 1 - 904 - 3  in addition to a laser cavity path  244  that directs intracavity power to a tunable wavelength filter  130  and a reflector  207 . The laser cavity path  244  may be the same in nature as the previously described path  224 . Laser light from a first power port  904 - 1  may couple to a coherent receiver  902 . This laser light may provide a local oscillator signal for the coherent receiver. Laser light generated by the tunable laser  901  may also be provided through two other power ports  904 - 2 ,  904 - 3  to an optical transmitter  903 . 
     The coherent receiver  902  may be formed on a same semiconductor chip  220  as a portion of the tunable laser  901 , or may be formed on a different semiconductor chip. In some embodiments, a coherent optical receiver may include an optical surface coupler  910  that is configured to receive signal light, on which information is encoded, from an optical fiber and couple the signal light into integrated optical waveguides of the coherent receiver  902 . The coherent receiver may further include one or more integrated coherent receiver photonic circuits  920 - 1 ,  920 - 2  that process the received signal light and produce a plurality of electrical signals that can be detected through contact pads  915 . A local oscillator signal provided from the tunable laser  901  may be split with an optical splitter  912  to provide a local oscillator signal for each of the integrated coherent receiver circuits  920 - 1 ,  920 - 2 . The integrated coherent receiver circuits  920 - 1 ,  920 - 2  may include phase-diverse and polarization-diverse photonic circuits. 
     The optical transmitter  903  may comprise a pair of nested Mach-Zehnder interferometers  950 . A nested Mach-Zehnder interferometer may include a plurality of optical splitters  912  and a plurality of thermo-optic phase shifters  940 . A nested Mach-Zehnder interferometer may also include high-speed electro-optic phase modulators  952 . The nested Mach-Zehnder interferometers may be used for quadrature signal modulation and/or dual polarization modulation of optical signals. An optical transmitter  903  may further include an output surface coupler  980  that is configured to couple optical radiation from one or more waveguides to an optical fiber. 
     According to some embodiments, a nested Mach-Zehnder interferometer  950  may also include a 2×2 optical coupler  914  from which one exit port may provide an optical reference signal to an on-chip photodetector. The photodetector may convert the optical reference signal to an electrical signal that can be detected at a signal pad  915 . The electrical signal can be monitored to determine relative powers from the two nested Mach-Zehnder interferometers  950 . 
     Although a tunable laser has been described as having a portion including optical amplifiers formed on a first semiconductor chip  210  and a second portion formed on a second semiconductor chip  220 , in some implementations a tunable laser may be formed on a single semiconductor chip, as depicted in  FIG. 10A  for tunable laser  1000  on chip  1010 . An elevation view of the structure shown in  FIG. 10B . For example, an N×M optical coupler  320  and integrated optical waveguides  310 - 1  . . .  310 - 4 ,  330 - 1  . . .  330 - 4  may be formed in a first semiconductor layer  1022  on a substrate  1005 . In some implementations, the first semiconductor layer may comprise a silicon semiconductor layer (e.g., a silicon-on-insulator layer). Additionally, the phase shifters  240 - 1  . . .  240 - 4  for one or more of the N input ports to the coupler  320  may be formed on the first semiconductor layer  1022 . A second layer  1024  of semiconductor material, for example InP may be formed over the first semiconductor layer  1022  as depicted in  FIG. 10B . According to some embodiments, the second layer of semiconductor material may be formed by a bonding process. For example, a wafer bonding and etch-back process may be employed as described in U.S. Pat. No. 9,020,001, which is incorporated herein by reference. 
     Semiconductor optical amplifiers  230  and their respective waveguides  212  may be formed in the second semiconductor layer  1024 . In some embodiments, an insulating dielectric, for example an oxide,  1030  may be deposited between the layers. An insulating layer may also be deposited as an overlayer to passivate the device. Power from the semiconductor optical amplifiers  230  on the second layer may couple to the lower ports (e.g., silicon waveguides)  310  by evanescent coupling. In this manner, the power in the laser cavity can travel from one cavity reflector  205  through the semiconductor optical amplifiers, into the underlying silicon waveguides, and through the N×M optical coupler  320  to another cavity mirror  207  (not shown) coupled to one port  330 - m  of the optical coupler  320 . 
     In some embodiments, it may be preferable to form the optical amplifiers  230  on a different substrate from other components of the tunable laser.  FIGS. 11A - FIG. 11C  depict embodiments in which the SOAs may be formed on a first substrate or semiconductor chip  210  and coupled to a second semiconductor chip  220  on which the N×M coupler, phase shifters, and tunable wavelength filter are located. In some implementations, the SOAs may be formed on a “process side” or “device side”  1110  of the first semiconductor chip  210 . As shown in the configuration  1101  of  FIG. 11A , the first chip may be flipped and bonded to a first sub-mount  1115 . Such flip-chip bonding can improve heat dissipation from the SOAs. For example the sub-mount may comprise a material (e.g., aluminum nitride) having higher thermal conductivity than the material of the first semiconductor chip  210  (e.g., indium phosphide). The first sub-mount  1115  may be bonded to a base mount  1105 . The second semiconductor chip  220  may then be aligned and bonded to the base mount. In some embodiments, the components on the second semiconductor chip  220  are formed on a process side  1120 . In some embodiments, the second semiconductor chip  220  may be manipulated with a positioning device and its alignment to the first chip adjusted until a correct alignment is achieved. According to some embodiments, correct alignment may be detected by monitoring optical power transferred from one chip to the other. In some cases, a magnified image of the chip interface may be viewed or processed to determine correct alignment. The magnified image may be obtained through a combination of optical and electronic magnification using optical lenses and CCD or CMOS imaging array. Once aligned, an epoxy or UV-curable adhesive  1130  may then be cured to affix the second semiconductor chip  220  and preserve the alignment. In some embodiments, a UV curable adhesive or optical adhesive may additionally be located between the first semiconductor chip  210  and the second semiconductor chip  220  to provide both adhesion and index matching between the optical paths (e.g., waveguides)  212 ,  222  on each chip. 
     According to some embodiments, both the first semiconductor chip  210  and the second semiconductor chip  220  may be flip-chip bonded to a base mount  1105 , as depicted in configuration  1102  of  FIG. 11  B. In some cases, one or both of the chips may be solder bonded (e.g., using bump bonds) to the base mount  1105 . For example, the solder may be heated before bonding, the chips aligned, and then the solder may be cooled to bond the chips and preserve the alignment. In some embodiments, a UV-curable or optical adhesive may be used additionally between the chips and/or between the chips and base mount  1105  to aid in permanently fixing the chips after alignment has been achieved. 
     According to some implementations, the first semiconductor chip  210  containing the SOAs may be flip-chip bonded to the second semiconductor chip  220 , as depicted in configuration  1103  of  FIG. 11C . According to this embodiment, the second semiconductor chip  220  may include a trench  1150  or other receiving feature to receive the first semiconductor chip  210 . For example, the trench may have a depth between approximately 500 nm and approximately 10 microns, such that optical paths (e.g., waveguides)  212 ,  222  on the two chips become essentially coplanar when the chips are bonded together. The chips may be aligned and bonded using solder bonding and/or adhesive bonding as described above. 
     Having thus described several aspects and embodiments of the technology of this application, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those of ordinary skill in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the technology described in the application. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described. In addition, any combination of two or more features, systems, articles, materials, and/or methods described herein, if such features, systems, articles, materials, and/or methods are not mutually inconsistent, is included within the scope of the present disclosure. 
     Also, as described, some aspects may be embodied as one or more methods. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments. 
     All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms. 
     The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” 
     The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. 
     As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. 
     The terms “approximately” and “about” may be used to mean within ±20% of a target value in some embodiments, within ±10% of a target value in some embodiments, within ±5% of a target value in some embodiments, and yet within ±2% of a target value in some embodiments. The terms “approximately” and “about” may include the target value. 
     In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. The transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively.