High-speed optical transmitter with a silicon substrate

A 400 Gb/s transmitter is integrated on a silicon substrate. The transmitter uses four gain chips, sixteen lasers, four modulators to modulate the sixteen lasers at 25 Gb/s, and four multiplexers to produce four optical outputs. Each optical output can transmit at 100 Gb/s to produce a 400 Gb/s transmitter. Other variations are also described.

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%.

DETAILED DESCRIPTION OF THE INVENTION

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 toFIG. 1, a simplified top view of an embodiment of a sixteen-wavelength transmitter100(a first transmitter) is shown. The sixteen-wavelength transmitter100transmits at 400 Gb/s by having sixteen lasers of different frequencies, each laser modulated at 25 Gb/s. Gain chips104and modulator chips108are bonded to a substrate112(e.g., of silicon). In some embodiments, the substrate112is part of a silicon-on-insulator (SOI) wafer. The SOI wafer comprising the substrate112(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 '914 application is used to bond the gain chips104and/or the modulator chips108to the substrate112. Four gain chips104are bonded the substrate112; a first gain chip104-1, a second gain chip104-2, a third gain chip104-3, and a fourth gain chip104-4. Four modulator chips108are bonded the substrate112; a first modulator chip108-1, a second modulator chip108-2, a third modulator chip108-3, and a fourth modulator chip108-4.

Waveguides130are integrated on the substrate112(e.g., in the device layer of the SOI wafer). Reflectors140are integrated on the substrate112(e.g., Bragg gratings in the device layer of the SOI wafer). Ridges142are formed on the gain chips104and/or the modulator chips108to guide light transmitted through the gain chips104and/or the modulator chips108. In some embodiments, more than one ridge142is formed on each gain chip104and/or modulator chip108. In the embodiment shown, there are four ridges142formed on each gain chip104and modulator chip108.

Reflectors140are integrated on the substrate to be on two sides of a gain chip104. In some embodiments, a mirror is formed in the gain chip104. Two reflectors140, optically coupled with a ridge142, form an optical resonator for a laser144. In the embodiment shown, each gain chip104supports four lasers144. Each modulator chip108modulates light received from four lasers144.

InFIG. 1, there are sixteen lasers144and sixteen waveguides130. 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 chip104-3comprises 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 chip104-4will understand that the third gain chip104-3also has four ridges142. Similarly, a person of skill in the art will understand that there are four lasers144, supported by the third gain chip103-4, even though only one laser144is labeled in the figure.

The sixteen waveguides130route light from the sixteen lasers144to a multiplexer160. The multiplexer160combines light from the sixteen lasers144to an optical output164. In some embodiments, the optical output164comprises a crystalline silicon core. The optical output164is optically coupled with an optical fiber168.

In some embodiments, one gain chip104is used for one laser144. In some embodiments one gain chip104is used to support, two, three, five, or more lasers144. InFIG. 1, there are eight chips: four gain chips104and four modulator chips108. Each chip has four ridges142patterned on the chip. In some embodiments, only one, two, or three ridges142, or more than four ridges142, are patterned on each chip. In some embodiments, gain chips104have different bandgaps for different lasing frequencies.

In some embodiments, a 400 Gb/s transmitter has a single optical output164and four different wavelengths, each laser144transmitting at 100 Gb/s. There are four gain chips104, and each gain chip104has only one ridge (thus each gain chip104supports only one laser144). In some embodiments, a 400 Gb/s transmitter has a single optical output164and eight different wavelengths, eight lasers144transmitting at 50 Gb/s. There are four gain chips104, and each gain chip104has two ridges (thus each gain chip104supports two lasers144). In some embodiments, four gain chips104and four modulator chips108are integrated on the substrate112with the multiplexer160, wherein each gain chip104supports 1, 2, or 4 lasers for 4, 8, or 16λ wavelength-division multiplexing (WDM) and each modulator chip108supports 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't use multiplexer160tuning or laser144wavelength tuning.8 channels can be supported with 10 nm channel spacing. 10 nm channel spacing uses some multiplexer160tuning and/or some laser144wavelength tuning.16 channels can be supported with 5 nm channel spacing. 5 nm channel spacing uses more multiplexer160tuning and/or laser144wavelength 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 multiplexer160tuning and/or laser144wavelength tuning.

In some embodiments, gain chips104have different bandgaps to produce different wavelength ranges for lasers. In some embodiments, modulator chips108have different bandgaps. In some embodiments, an echelle grating is used for the multiplexer160.

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.

InFIG. 2, an embodiment of a second transmitter200is shown. The second transmitter200is similar to the sixteen-wavelength transmitter100, except instead of having one multiplexer160, the second transmitter200comprises four multiplexers160and four optical outputs164coupled with four optical fibers168. The second transmitter200comprises a first multiplexer160-1, a second multiplexer160-2, a third multiplexer160-3, and a fourth multiplexer160-4. The second transmitter200comprises a first optical output164-1, a second optical output164-2, a third optical output164-3, and a fourth optical output164-4. Each multiplexer160inFIG. 2receives four optical inputs and combines those four optical inputs into an optical output164. One optical input to the multiplexer160comes from each of the gain chips104. For example, the first multiplexer160-1receives on optical input from a waveguide130coupling light from the first gain chip104-1; the first multiplexer160-1receives on optical input from a waveguide130coupling light from the second gain chip104-2; the first multiplexer160-1receives on optical input from a waveguide130coupling light from the third gain chip104-3; and the first multiplexer160-1receives on optical input from a waveguide130coupling light from the fourth gain chip104-4. The first multiplexer160-1combines light from four inputs to the first optical output164-1. The first optical output164-1transmits the light combined from the first multiplexer160-1to the first optical fiber168-1. The second multiplexer160-2receives four inputs, one from each of the gain chips104, combines the four inputs to the second optical output164-2, for transmission to the second optical fiber168-2. The third multiplexer160-3receives four inputs, one from each of the gain chips104, combines the four inputs to the third optical output164-3, for transmission to the third optical fiber168-3. The fourth multiplexer160-4receives four inputs, one from each of the gain chips104, combines the four inputs to the fourth optical output164-4, for transmission to the fourth optical fiber168-4.

Thus the second transmitter200generates four output beams, each output beam with four different wavelengths. Each laser144is 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 output164transmits at 100 Gb/s, and the second transmitter200transmits at 400 Gb/s. In some embodiments, the second transmitter200is for reverse compatibility with 100 Gb/s systems. In some embodiments, the second transmitter200can 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).

InFIG. 2, lines representing waveguides130are coded for different wavelengths. Waveguides130that are black and solid represent a first wavelength of light being transmitted. Waveguides130that are black and dashed represent a second wavelength of light being transmitted. Waveguides130that are gray and solid represent a third wavelength of light being transmitted. Waveguides130that are gray and dashed represent a fourth wavelength of light being transmitted. In some embodiments, a multiplexer160combines more than one waveguide130from a modulator chip108.

The second transmitter200uses WDM over a wavelength span of 80 nm. The second transmitter200can be used in a temperature range from 0 degrees C. to 70 degrees C. without tuning lasers144and/or multiplexers160. 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 chips104, the modulator chips108, the waveguides130, and the multiplexers160are integrated (e.g., monolithically) on the substrate112, wherein the substrate112is silicon. Each gain chip104covers a different wavelength band (e.g., having different bandgaps). Similarly, each modulator chip108covers a different wavelength band from other modulator chips108.

InFIG. 3, an embodiment of a third transmitter300is shown. The third transmitter300is similar to the second transmitter200, except instead of having four lasers144per gain chip104, the third transmitter300has one laser144per gain chip104, each of which is split into four waveguides130before transmission to a modulator chip108. Other configurations are possible. For example, in some embodiments there are two lasers144per gain chip104, which are each split into two waveguides130before the modulator chip108. In some embodiments, there are four lasers144per gain chip104and only two ridges124or one ridge124per modulator chip108(less ridges124per modulator chip108than gain chip104; thus there are more modulator chips108than gain chips104on the substrate112). Other numbers of ridges124(e.g., 3, 5, 6, 8, 12, etc.) can be made on chips based on a size of a chip and sizes of ridges124and waveguides130.

InFIGS. 2 and 3, waveguides130intersect at crossings304to route to the multiplexers160. InFIG. 4, an embodiment of a crossing304is shown. The crossing304has a first input404-1, a second input404-2, a first output408-1, and a second output408-1. A first optical beam propagates from the first input404-1to the first output408-1. A second optical beam propagates from the second input404-2to the second output408-2.

The first input404-1comprises a first taper412-1, an expanding taper, to expand an optical mode of the first optical beam. The first output408-1comprises a second taper412-2, a narrowing taper, to constrict an optical mode of the first optical beam. The second input404-2comprises a third taper412-3, an expanding taper, to expand an optical mode of the second optical beam. The second output408-2comprises a fourth taper412-4, a narrowing taper, to constrict an optical mode of the second optical beam. The first taper412-1, the second taper412-2, the third taper412-3, and the fourth taper412-4, widen into each other in a direction toward a center of the crossing304. Thus two waveguides130cross each other. In some embodiments, two waveguides130cross each other perpendicularly.

Referring next toFIG. 5, an embodiment of a sharp bend500is shown. The modulator chips108are connected to high-speed drivers. Sharp bends500in waveguides130can be used for dense optical routing. Modulator chips108are on opposite sides of the substrate112than optical outputs164. In some embodiments, sharp bends500are used so that modulator chips108can be positioned near an edge of the substrate112so that high-speed drivers can be electrically connected with the modulator chips108.

In some embodiments, two 90-degree turns are used (e.g., to guide light from a modulator chip108to a multiplexer160. In some embodiments, one turn (e.g., a continuous curvature bend) is used to turn waveguides130through 180 degrees. In some embodiments, the sharp bend500has a radius of curvature less than 50 μm. In some embodiments, the sharp bend500is used for large-core waveguides (waveguides having a thickness greater than 1 μm).

Shallow-etched ridge waveguides504are used to maintain single-mode operation. Before bending, an adiabatic taper508converts the shallow-etched ridge waveguide504to a deep-etched channel waveguide514. The adiabatic taper508minimizes exciting higher-order modes (higher than a fundamental mode) in the deep-etched channel waveguide514. A continuous-curvature (CC) bend with small radius (radius <50 μm) is used to make a 90-degree waveguide turn in the deep-etched channel waveguide514. The CC bend is made with a deep-etched channel waveguide514to reduce radiation loss. The CC bend's curvature starts from zero, which reduces coupling loss with the adiabatic taper508. Then the CC bend's curvature gradually increases to maximum at a midpoint of the bend; then it gradually decreases to zero for coupling with an adiabatic taper508. The adiabatic changing of the CC bend's curvature can be linear, or with different orders of non-linearity, depending on the design.

FIG. 6illustrates a flowchart of an embodiment of a process600for operating an optical transmitter. Process600begins in step604with generating a plurality of laser beams using a plurality of lasers144. The plurality of lasers144are integrated on the substrate112. Generating the plurality of laser beams comprises applying electrical power the plurality of gain chips104.

In step608, the plurality of laser beams are modulated to form a plurality of modulated beams. The plurality of laser beams are modulated using the modulator chips108. 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 step612, the plurality of modulated beams are guided to one or more multiplexers160using a plurality of waveguides130. In step616, the modulated beams are combined into one or more output beams using the one or more multiplexers160. Each output beam is guided by an optical output164to an optical fiber168.

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 output164could 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 substrate112. Integrating techniques disclosed in other applications concurrently filed with this application are also envisioned. For example, instead of two reflectors140formed external to the gain chip104, a reflector140can be formed in the gain chip104as disclosed in the '___application, entitled “Broadband Back Mirror for a III-V Chip in Silicon Photonics.”

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.