Patent Publication Number: US-2020301069-A1

Title: High-speed optical transmitter with a silicon substrate and multiple multiplexers

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 15/426,823, filed Feb. 7, 2017, entitled “High-Speed Optical Transmitter With A Silicon Substrate,” which application claims priority to U.S. Provisional Application No. 62/292,633, filed on Feb. 8, 2016, entitled “High-Speed Optical Transmitter with a Silicon Substrate,” U.S. Provisional Application No. 62/292,675, filed on Feb. 8, 2016, entitled “Stepped Optical Bridge for Connecting Semiconductor Waveguides,” and U.S. Provisional Application No. 62/292,636, filed on Feb. 8, 2016, entitled “Broadband Back Mirror for a III-V Chip in Silicon Photonics,” the disclosures of which are incorporated by reference for all purposes. 
     The entire disclosure of the following three U.S. patent applications are incorporated by reference into this application for all purposes:
         application Ser. No. 15/426,823, filed Feb. 7, 2017, entitled “High-Speed Optical Transmitter with a Silicon Substrate”;   application Ser. No. 15/426,366, filed Feb. 7, 2017, entitled “Stepped Optical Bridge for Connecting Semiconductor Waveguides”; and   application Ser. No. 15/426,375, filed Feb. 7, 2017, entitled “Broadband Back Mirror for a III-V Chip in Silicon Photonics.”       

    
    
     BACKGROUND OF THE INVENTION 
     Silicon integrated circuits (“ICs”) have dominated the development of electronics and many technologies based upon silicon processing have been developed over the years. Their continued refinement led to nano-scale feature sizes that can be important for making metal oxide semiconductor CMOS circuits. On the other hand, silicon is not a direct-bandgap material. Although direct-bandgap materials, including III-V semiconductor materials, have been developed, there is a need in the art for improved methods and systems related to photonic ICs utilizing silicon substrates. This application relates to optical transmitters and optical waveguides. More specifically, and without limitation, to optical lasers and/or optical waveguides in silicon. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments for a 400 Gb/s transmitter are disclosed. Example embodiments include:
         Example 1: A single optical output having sixteen different wavelengths, each wavelength transmitting at 25 Gb/s (1×16λ×25 G).   Example 2: Four optical outputs, each optical output having four different wavelengths, each transmitting at 25 Gb/s (4×4λ×25 G).   Example 3: A single optical output having eight different wavelengths, each transmitting at 50 Gb/s (1×8λ×50 G).   Example 4: A single optical output having four different wavelengths, each transmitting at 100 Gb/s (1×4λ×100 G).       

     In some embodiments, example 1, example 2, example 3, and example 4, each have four chips for lasers and/or four chips for modulators. In some embodiments, multiple waveguides per chip are used to reduce a number of chips used in a transmitter. 
     In some embodiments, a semiconductor chip comprises a first ridge and a second ridge; the first ridge is configured to guide a first optical mode in the semiconductor chip; the second ridge is configured to guide a second optical mode in the semiconductor chip. In some embodiments, the first optical mode is generated by a first laser; the second optical mode is generated by a second laser; and/or the semiconductor chip is a gain chip or a modulator chip. 
     In some embodiments, an optical transmitter using semiconductor lasers and wavelength-division multiplexing (WDM) comprises a substrate, four gain chips integrated on the substrate, a plurality of reflectors integrated on the substrate, four modulator chips integrated on the substrate, sixteen waveguides integrated on the substrate, four multiplexers integrated on the substrate, and four optical outputs integrated on the substrate, wherein: the substrate is silicon; the four gain chips and the plurality of reflectors form a plurality of lasers integrated on the substrate; the plurality of lasers are configured to transmit on predetermined optical channels of a WDM protocol; the four modulator chips modulate light generated by the plurality of lasers; the sixteen waveguides are configured to guide light from the four modulator chips to the four multiplexers; each of the four multiplexers is configured to receive light from four waveguides of the sixteen waveguides and combine the light from the four waveguides into an optical output of the four optical outputs; there is one optical output for each of the four multiplexers; and/or each of the four optical outputs is configured to transmit light of four different frequencies to an optical fiber. In some embodiments, there are four lasers per gain chip, and each laser per gain chip operates on the same predetermined optical channel of the WDM protocol; each gain chip, of the four gain chips, has a different bandgap; there is one laser per gain chip, and each modulator chip has four ridges to produce four modulated signals; the four gain chips comprise III-V material; the substrate is part of a silicon-on-insulator (SOI) wafer, the SOI wafer comprises a device layer of crystalline silicon, and the sixteen waveguides are formed in the device layer; and/or each of the modulator chips modulate each of the plurality of lasers to produce a plurality an optical beams, each optical beam of the plurality of optical beams modulated at 25 Gb/s plus or minus 20%. 
     In some embodiments, an optical transmitter comprises a substrate, a plurality of gain chips integrated on the substrate, a plurality of reflectors integrated on the substrate, a plurality of modulator chips integrated on the substrate, a plurality of waveguides integrated on the substrate, and one or more multiplexers integrated on the substrate, wherein: the substrate is silicon; the plurality of gain chips and the plurality of reflectors form a plurality of lasers; the plurality of modulator chips modulate light from the plurality of lasers; the plurality of waveguides guide light from the plurality of modulators to one or more multiplexers; and/or the one or more multiplexers combine light from the plurality of waveguides into one or more optical outputs. In some embodiments, the plurality of gain chips comprise four gain chips, the plurality of modulator chips comprise four modulator chips, and the plurality of waveguides comprise four waveguides; the plurality of waveguides comprise sixteen waveguides, the one or more multiplexers comprise four multiplexers, and the one or more optical outputs comprise four optical outputs; the one or more multiplexers is one multiplexer, the plurality of waveguides comprises sixteen waveguides coupled with the one multiplexer, and the one or more optical outputs is one optical output; the optical channels are spaced using a 20 nm channel spacing plus or minus 30%; the optical channels are spaced using a channel spacing between 3.5 nm and 13 nm; each gain chip, of the plurality of gain chips, has a different bandgap; the plurality of modulator chips are configured to modulate light from the plurality of lasers using a pulse-amplitude modulation (PAM) technique having more than two levels; each gain chip, of the plurality of gain chips, comprise III-V material, the substrate is part of a silicon-on-insulator (SOI) wafer, the SOI wafer comprises a device layer of crystalline silicon, each waveguide, of the plurality of waveguides, is formed in the device layer of the SOI wafer, and the one or more multiplexers are formed in the device layer of the SOI wafer. 
     In some embodiments, a method of operating an optical transmitter comprises: generating a plurality of laser beams using a plurality of lasers integrated on a substrate, wherein generating the plurality laser beams comprises applying electrical power to a plurality of gain chips; modulating the plurality of laser beams using a plurality of modulator chips to form a plurality of modulated beams; guiding the plurality of modulated beams to one or more multiplexers using a plurality of waveguides; and combining the plurality of modulated beams into one or more output beams using the one or more multiplexers. In some embodiments, modulating the plurality of laser beams comprises using a pulse-amplitude modulation (PAM) technique having more than two levels; the pulse-amplitude modulation technique is PAM4; modulating the plurality of laser beams comprises modulating each laser beam of the plurality of laser beams to transmit at 25 Gb/s plus or minus 20%; and/or a combined transmission rate of the one or more output beams is 400 Gb/s plus or minus 20%. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating various embodiments, are intended for purposes of illustration only and are not intended to necessarily limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is described in conjunction with the appended figures. 
         FIG. 1  depicts a simplified top view of an embodiment of a sixteen-wavelength transmitter. 
         FIG. 2  depicts a simplified top view of an embodiment of a second transmitter. 
         FIG. 3  depicts a simplified top view of an embodiment of a third transmitter. 
         FIG. 4  depicts a simplified top view of an embodiment of a crossing for waveguides. 
         FIG. 5  depicts a an embodiment of a sharp bend for waveguides. 
         FIG. 6  illustrates a flowchart of an embodiment of a process for operating an optical transmitter. 
     
    
    
     In the appended figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The ensuing description provides preferred exemplary embodiment(s) only, and is not intended to limit the scope, applicability, or configuration of the disclosure. Rather, the ensuing description of the preferred exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing a preferred exemplary embodiment. It is understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope as set forth in the appended claims. 
     Embodiments generally relate to an optical transmitter having a silicon platform. In some embodiments, the optical transmitter provides 400 Gb/s transmission. A chip is bonded to a platform. In some embodiments, the chip is made of III-V material and the platform is a silicon-on-insulator (SOI) wafer. In some embodiments, bonding is used as described in U.S. application Ser. No. 14/509,914, filed on Oct. 8, 2014, which is incorporated by reference. In some embodiments, chips are bonded into recesses of the platform. In some embodiments, chips are formed from epitaxial layers of compound semiconductor material (e.g., III-V materials). In some embodiments, chips are used to perform functions that are difficult for silicon to perform (e.g., a chip with a direct bandgap is used as a gain medium or a modulator for a laser; silicon has an indirect bandgap making silicon a poor optical emitter). An example of a tunable laser using a chip for a gain medium, and reflectors in silicon, is given in U.S. application Ser. No. 14/642,443, filed on Mar. 9, 2015, which is incorporated by reference. In some embodiments, chips are formed by dicing a semiconductor wafer and/or bonding the chips to the substrate using template assisted bonding, such as described in U.S. application Ser. No. 14/261,276, filed on Apr. 24, 2014, and U.S. application Ser. No. 14/482,650, filed on Sep. 10, 2014, which are incorporated by reference. In some embodiments, thick silicon is used to more efficiently couple (e.g., butt couple) waveguides in silicon with waveguides in chips. In some embodiments, thick silicon is between 0.7 μm and 5 μm, or from 1 μm to 2.5 μm. In some embodiments, multiple ridges/lasers per chip are used to reduce a number of chips used in a transmitter. Reducing the number of chips improves fabrication yield and cost. In some embodiments, reducing the number of chips is accomplished without significant penalty to performance and/or wavelength tuning. In some embodiments, ridges on one chip channel light of different frequencies. 
     Referring first to  FIG. 1 , a simplified top view of an embodiment of a sixteen-wavelength transmitter  100  (a first transmitter) is shown. The sixteen-wavelength transmitter  100  transmits at 400 Gb/s by having sixteen lasers of different frequencies, each laser modulated at 25 Gb/s. Gain chips  104  and modulator chips  108  are bonded to a substrate  112  (e.g., of silicon). In some embodiments, the substrate  112  is part of a silicon-on-insulator (SOI) wafer. The SOI wafer comprising the substrate  112  (which is crystalline silicon), an insulating layer (e.g., SiO2), and a device layer (e.g., crystalline silicon). In some embodiments, bonding as described in the &#39;914 application is used to bond the gain chips  104  and/or the modulator chips  108  to the substrate  112 . Four gain chips  104  are bonded the substrate  112 ; a first gain chip  104 - 1 , a second gain chip  104 - 2 , a third gain chip  104 - 3 , and a fourth gain chip  104 - 4 . Four modulator chips  108  are bonded the substrate  112 ; a first modulator chip  108 - 1 , a second modulator chip  108 - 2 , a third modulator chip  108 - 3 , and a fourth modulator chip  108 - 4 . 
     Waveguides  130  are integrated on the substrate  112  (e.g., in the device layer of the SOI wafer). Reflectors  140  are integrated on the substrate  112  (e.g., Bragg gratings in the device layer of the SOI wafer). Ridges  142  are formed on the gain chips  104  and/or the modulator chips  108  to guide light transmitted through the gain chips  104  and/or the modulator chips  108 . In some embodiments, more than one ridge  142  is formed on each gain chip  104  and/or modulator chip  108 . In the embodiment shown, there are four ridges  142  formed on each gain chip  104  and modulator chip  108 . 
     Reflectors  140  are integrated on the substrate to be on two sides of a gain chip  104 . In some embodiments, a mirror is formed in the gain chip  104 . Two reflectors  140 , optically coupled with a ridge  142 , form an optical resonator for a laser  144 . In the embodiment shown, each gain chip  104  supports four lasers  144 . Each modulator chip  108  modulates light received from four lasers  144 . 
     In  FIG. 1 , there are sixteen lasers  144  and sixteen waveguides  130 . Not all features are labeled in order to reduce clutter in the figures. The features will be clear to a person of ordinary skill in the art. For example, the third gain chip  104 - 3  comprises four ridges, yet the four ridges on the third gain chip are not labeled. But a person of ordinary skill comparing the ridges labeled “ 142 ” on the fourth gain chip  104 - 4  will understand that the third gain chip  104 - 3  also has four ridges  142 . Similarly, a person of skill in the art will understand that there are four lasers  144 , supported by the third gain chip  103 - 4 , even though only one laser  144  is labeled in the figure. 
     The sixteen waveguides  130  route light from the sixteen lasers  144  to a multiplexer  160 . The multiplexer  160  combines light from the sixteen lasers  144  to an optical output  164 . In some embodiments, the optical output  164  comprises a crystalline silicon core. The optical output  164  is optically coupled with an optical fiber  168 . 
     In some embodiments, one gain chip  104  is used for one laser  144 . In some embodiments one gain chip  104  is used to support, two, three, five, or more lasers  144 . In  FIG. 1 , there are eight chips: four gain chips  104  and four modulator chips  108 . Each chip has four ridges  142  patterned on the chip. In some embodiments, only one, two, or three ridges  142 , or more than four ridges  142 , are patterned on each chip. In some embodiments, gain chips  104  have different bandgaps for different lasing frequencies. 
     In some embodiments, a 400 Gb/s transmitter has a single optical output  164  and four different wavelengths, each laser  144  transmitting at 100 Gb/s. There are four gain chips  104 , and each gain chip  104  has only one ridge (thus each gain chip  104  supports only one laser  144 ). In some embodiments, a 400 Gb/s transmitter has a single optical output  164  and eight different wavelengths, eight lasers  144  transmitting at 50 Gb/s. There are four gain chips  104 , and each gain chip  104  has two ridges (thus each gain chip  104  supports two lasers  144 ). In some embodiments, four gain chips  104  and four modulator chips  108  are integrated on the substrate  112  with the multiplexer  160 , wherein each gain chip  104  supports 1, 2, or 4 lasers for 4, 8, or 16λ wavelength-division multiplexing (WDM) and each modulator chip  108  supports 1, 2, or 4 modulators for 4, 8, or 16λ WDM. 
     In some embodiments, for WDM over an 80 nm wavelength span and temperature range from 0 degrees C. to 70 degrees C.:
         4 channels can be supported with 20 nm channel spacing. 20 nm channel spacing doesn&#39;t use multiplexer  160  tuning or laser  144  wavelength tuning.   8 channels can be supported with 10 nm channel spacing. 10 nm channel spacing uses some multiplexer  160  tuning and/or some laser  144  wavelength tuning.   16 channels can be supported with 5 nm channel spacing. 5 nm channel spacing uses more multiplexer  160  tuning and/or laser  144  wavelength tuning than 10 nm channel spacing.       

     In some embodiments, the wavelength span for WDM can be greater than 80 nm by controlling temperature of the transmitter. In some embodiments, a temperature of the transmitter is controlled so that there can be less multiplexer  160  tuning and/or laser  144  wavelength tuning. 
     In some embodiments, gain chips  104  have different bandgaps to produce different wavelength ranges for lasers. In some embodiments, modulator chips  108  have different bandgaps. In some embodiments, an echelle grating is used for the multiplexer  160 . 
     In some embodiments, pulse-amplitude modulation (PAM) is used (e.g., with two levels). In some embodiments, PAM with more than two levels is used (e.g., PAM4; having four levels and/or not returning to zero for each pulse). Having more than two levels is used to reduce a baud rate for each wavelength. One tradeoff of using PAM4 techniques is that PAM4 techniques are more susceptible to noise than binary pulse-amplitude modulation. 
     In  FIG. 2 , an embodiment of a second transmitter  200  is shown. The second transmitter  200  is similar to the sixteen-wavelength transmitter  100 , except instead of having one multiplexer  160 , the second transmitter  200  comprises four multiplexers  160  and four optical outputs  164  coupled with four optical fibers  168 . The second transmitter  200  comprises a first multiplexer  160 - 1 , a second multiplexer  160 - 2 , a third multiplexer  160 - 3 , and a fourth multiplexer  160 - 4 . The second transmitter  200  comprises a first optical output  164 - 1 , a second optical output  164 - 2 , a third optical output  164 - 3 , and a fourth optical output  164 - 4 . Each multiplexer  160  in  FIG. 2  receives four optical inputs and combines those four optical inputs into an optical output  164 . One optical input to the multiplexer  160  comes from each of the gain chips  104 . For example, the first multiplexer  160 - 1  receives on optical input from a waveguide  130  coupling light from the first gain chip  104 - 1 ; the first multiplexer  160 - 1  receives on optical input from a waveguide  130  coupling light from the second gain chip  104 - 2 ; the first multiplexer  160 - 1  receives on optical input from a waveguide  130  coupling light from the third gain chip  104 - 3 ; and the first multiplexer  160 - 1  receives on optical input from a waveguide  130  coupling light from the fourth gain chip  104 - 4 . The first multiplexer  160 - 1  combines light from four inputs to the first optical output  164 - 1 . The first optical output  164 - 1  transmits the light combined from the first multiplexer  160 - 1  to the first optical fiber  168 - 1 . The second multiplexer  160 - 2  receives four inputs, one from each of the gain chips  104 , combines the four inputs to the second optical output  164 - 2 , for transmission to the second optical fiber  168 - 2 . The third multiplexer  160 - 3  receives four inputs, one from each of the gain chips  104 , combines the four inputs to the third optical output  164 - 3 , for transmission to the third optical fiber  168 - 3 . The fourth multiplexer  160 - 4  receives four inputs, one from each of the gain chips  104 , combines the four inputs to the fourth optical output  164 - 4 , for transmission to the fourth optical fiber  168 - 4 . 
     Thus the second transmitter  200  generates four output beams, each output beam with four different wavelengths. Each laser  144  is modulated at 25 Gb/s. In some embodiments, modulation is plus or minus 30%, 20%, 10%, and/or 5% of a rate. Thus each optical output  164  transmits at 100 Gb/s, and the second transmitter  200  transmits at 400 Gb/s. In some embodiments, the second transmitter  200  is for reverse compatibility with 100 Gb/s systems. In some embodiments, the second transmitter  200  can communicate with a 100 G course wavelength division multiplexing (CWDM) module by breaking the 400 G signal into four lanes of 100 G signal (e.g., by using a breakout cable). 
     In  FIG. 2 , lines representing waveguides  130  are coded for different wavelengths. Waveguides  130  that are black and solid represent a first wavelength of light being transmitted. Waveguides  130  that are black and dashed represent a second wavelength of light being transmitted. Waveguides  130  that are gray and solid represent a third wavelength of light being transmitted. Waveguides  130  that are gray and dashed represent a fourth wavelength of light being transmitted. In some embodiments, a multiplexer  160  combines more than one waveguide  130  from a modulator chip  108 . 
     The second transmitter  200  uses WDM over a wavelength span of 80 nm. The second transmitter  200  can be used in a temperature range from 0 degrees C. to 70 degrees C. without tuning lasers  144  and/or multiplexers  160 . In some embodiments, the second transmitter uses 20 nm channel spacing. In some embodiments, channel spacing is plus or minus 30%, 20%, 10% and/or 5% of the channel spacing. For example, the 20 nm channel spacing of the second transmitter is 20 nm plus or minus 6 nm, 4 nm, 2 nm, and/or 1 nm. The gain chips  104 , the modulator chips  108 , the waveguides  130 , and the multiplexers  160  are integrated (e.g., monolithically) on the substrate  112 , wherein the substrate  112  is silicon. Each gain chip  104  covers a different wavelength band (e.g., having different bandgaps). Similarly, each modulator chip  108  covers a different wavelength band from other modulator chips  108 . 
     In  FIG. 3 , an embodiment of a third transmitter  300  is shown. The third transmitter  300  is similar to the second transmitter  200 , except instead of having four lasers  144  per gain chip  104 , the third transmitter  300  has one laser  144  per gain chip  104 , each of which is split into four waveguides  130  before transmission to a modulator chip  108 . Other configurations are possible. For example, in some embodiments there are two lasers  144  per gain chip  104 , which are each split into two waveguides  130  before the modulator chip  108 . In some embodiments, there are four lasers  144  per gain chip  104  and only two ridges  124  or one ridge  124  per modulator chip  108  (less ridges  124  per modulator chip  108  than gain chip  104 ; thus there are more modulator chips  108  than gain chips  104  on the substrate  112 ). Other numbers of ridges  124  (e.g., 3, 5, 6, 8, 12, etc.) can be made on chips based on a size of a chip and sizes of ridges  124  and waveguides  130 . 
     In  FIGS. 2 and 3 , waveguides  130  intersect at crossings  304  to route to the multiplexers  160 . In  FIG. 4 , an embodiment of a crossing  304  is shown. The crossing  304  has a first input  404 - 1 , a second input  404 - 2 , a first output  408 - 1 , and a second output  408 - 1 . A first optical beam propagates from the first input  404 - 1  to the first output  408 - 1 . A second optical beam propagates from the second input  404 - 2  to the second output  408 - 2 . 
     The first input  404 - 1  comprises a first taper  412 - 1 , an expanding taper, to expand an optical mode of the first optical beam. The first output  408 - 1  comprises a second taper  412 - 2 , a narrowing taper, to constrict an optical mode of the first optical beam. The second input  404 - 2  comprises a third taper  412 - 3 , an expanding taper, to expand an optical mode of the second optical beam. The second output  408 - 2  comprises a fourth taper  412 - 4 , a narrowing taper, to constrict an optical mode of the second optical beam. The first taper  412 - 1 , the second taper  412 - 2 , the third taper  412 - 3 , and the fourth taper  412 - 4 , widen into each other in a direction toward a center of the crossing  304 . Thus two waveguides  130  cross each other. In some embodiments, two waveguides  130  cross each other perpendicularly. 
     Referring next to  FIG. 5 , an embodiment of a sharp bend  500  is shown. The modulator chips  108  are connected to high-speed drivers. Sharp bends  500  in waveguides  130  can be used for dense optical routing. Modulator chips  108  are on opposite sides of the substrate  112  than optical outputs  164 . In some embodiments, sharp bends  500  are used so that modulator chips  108  can be positioned near an edge of the substrate  112  so that high-speed drivers can be electrically connected with the modulator chips  108 . 
     In some embodiments, two 90-degree turns are used (e.g., to guide light from a modulator chip  108  to a multiplexer  160 . In some embodiments, one turn (e.g., a continuous curvature bend) is used to turn waveguides  130  through 180 degrees. In some embodiments, the sharp bend  500  has a radius of curvature less than 50 μm. In some embodiments, the sharp bend  500  is used for large-core waveguides (waveguides having a thickness greater than 1 μm). 
     Shallow-etched ridge waveguides  504  are used to maintain single-mode operation. Before bending, an adiabatic taper  508  converts the shallow-etched ridge waveguide  504  to a deep-etched channel waveguide  514 . The adiabatic taper  508  minimizes exciting higher-order modes (higher than a fundamental mode) in the deep-etched channel waveguide  514 . A continuous-curvature (CC) bend with small radius (radius &lt;50 μm) is used to make a 90-degree waveguide turn in the deep-etched channel waveguide  514 . The CC bend is made with a deep-etched channel waveguide  514  to reduce radiation loss. The CC bend&#39;s curvature starts from zero, which reduces coupling loss with the adiabatic taper  508 . Then the CC bend&#39;s curvature gradually increases to maximum at a midpoint of the bend; then it gradually decreases to zero for coupling with an adiabatic taper  508 . The adiabatic changing of the CC bend&#39;s curvature can be linear, or with different orders of non-linearity, depending on the design. 
       FIG. 6  illustrates a flowchart of an embodiment of a process  600  for operating an optical transmitter. Process  600  begins in step  604  with generating a plurality of laser beams using a plurality of lasers  144 . The plurality of lasers  144  are integrated on the substrate  112 . Generating the plurality of laser beams comprises applying electrical power the plurality of gain chips  104 . 
     In step  608 , the plurality of laser beams are modulated to form a plurality of modulated beams. The plurality of laser beams are modulated using the modulator chips  108 . In some embodiments, modulating the plurality of laser beams comprises using a pulse-amplitude modulation (PAM) technique having more than two levels (e.g., 3, 4, 5, or 6 levels; in some embodiments, less than 5, 6, 8, or 10 levels). In some embodiments the pulse-amplitude modulation technique used is PAM4. 
     In step  612 , the plurality of modulated beams are guided to one or more multiplexers  160  using a plurality of waveguides  130 . In step  616 , the modulated beams are combined into one or more output beams using the one or more multiplexers  160 . Each output beam is guided by an optical output  164  to an optical fiber  168 . 
     The specific details of particular embodiments may be combined in any suitable manner without departing from the spirit and scope of embodiments of the invention. However, other embodiments of the invention may be directed to specific embodiments relating to each individual aspect, or specific combinations of these individual aspects. 
     The above description of exemplary embodiments of the invention has been presented for the purposes of illustration and description. The embodiments were chosen and described in order to explain the principles of the invention and its practical applications to enable others skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. For example, the embodiments shown can be scaled up or down. A transmitter with one gain chip with four ridges (for four lasers), a modulator with four ridges, four waveguides, one multiplexer, and one optical output  164  could be integrated the substrate to form a 100 Gb/s transmitter, with the four lasers being modulated at 25 Gb/s each. Or there could be two, three, five, or more 100 Gb/s transmitters integrated on the substrate  112 . Integrating techniques disclosed in other applications concurrently filed with this application are also envisioned. For example, instead of two reflectors  140  formed external to the gain chip  104 , a reflector  140  can be formed in the gain chip  104  as disclosed in the &#39;______ application, entitled “Broadband Back Mirror for a III-V Chip in Silicon Photonics.” 
     It is noted that the embodiments may be described as a process which is depicted as a flowchart, a flow diagram, a data flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed, but could have additional steps not included in the figure. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. 
     A recitation of “a”, “an”, or “the” is intended to mean “one or more” unless specifically indicated to the contrary. 
     All patents, patent applications, publications, and descriptions mentioned here are incorporated by reference in their entirety for all purposes. None is admitted to be prior art.