Fast wavelength-tunable hybrid laser with a single-channel gain medium

A tunable laser includes a semiconductor optical amplifier (SOA) having a reflective end coupled to a shared reflector and an output end, which is coupled to a demultiplexer through an input waveguide. The demultiplexer comprises a set of Mach-Zehnder (MZ) lattice filters, which function as symmetric de-interleaving wavelength splitters, that are cascaded to form a binary tree that connects an input port, which carries multiple wavelength bands, to N wavelength-specific output ports that are coupled to a set of N reflectors. A set of variable optical attenuators (VOAs) is coupled to outputs of the MZ lattice filters in the binary tree, and is controllable to selectively add loss to the outputs, so that only a single favored wavelength band, which is associated with a favored reflector in the set of N reflectors, lases at any given time. An output waveguide is optically coupled to the lasing cavity.

FIELD

The disclosed embodiments generally relate to the design of a semiconductor-based laser. More specifically, the disclosed embodiments relate to the design of a fast wavelength-tunable hybrid semiconductor laser having a single-channel gain medium.

RELATED ART

Silicon photonics is a promising new technology that can potentially provide large communication bandwidth, low latency and low power consumption for inter-chip and intra-chip connections. In order to achieve such low-latency, high-bandwidth optical connectivity, a number of optical components are required, including: optical modulators, optical detectors, wavelength multiplexers, wavelength demultiplexers, optical sources and optical switches.

Energy-efficient and cost-effective optical switches are required to make such optical connections practical in data centers and high-performance, data-intensive computing systems. One promising optical-switching approach is to use the unique wavelength routing capability of arrayed-waveguide-grating-routers (AWGRs) with carrier wavelength switching at the source node. (See K. Kato, et al., “32×32 full-mesh (1024 path) wavelength routing WDM network based on uniform loss cyclic-frequency arrayed-waveguide grating,” Electron. Lett., vol. 36, pp. 1294-1295, 2000.) However, to make this approach practical, a laser with fast wavelength tuning is needed to facilitate such source-originated optical switching.

SUMMARY

The disclosed embodiments relate to a system that provides a tunable laser, which includes a gain medium (such as a semiconductor optical amplifier) having a reflective end coupled to a shared reflector and an output end. The gain medium is coupled to a demultiplexer through an input waveguide, wherein the demultiplexer comprises a set of wavelength splitters that are cascaded to form a binary tree that connects an input port, which carries multiple wavelength bands, to N wavelength-specific output ports. The tunable laser also includes: a set of N reflectors coupled to the N output ports of the demultiplexer; and a set of variable optical attenuators (VOAs) coupled to outputs of the wavelength filters in the binary tree, which are controllable to selectively add loss to the outputs. A controller selectively activates the set of VOAs to add loss to unwanted wavelength bands in the demultiplexer, so that only a single favored wavelength band, which is associated with a favored reflector in the set of N reflectors, lases at any given time. Finally, an output waveguide is optically coupled to a lasing cavity formed by the shared reflector, the gain medium, the input waveguide, the demultiplexer and the favored reflector.

In some embodiments, the demultiplexer is a symmetric de-interleaving wavelength splitter.

In some embodiments, the symmetric de-interleaving wavelength splitter is a Mach-Zehnder (MZ)-lattice-based demultiplexer, wherein the wavelength splitters are MZ lattice filters.

In some embodiments, the wavelength-specific narrow-band reflectors comprise: narrow-band waveguide distributed Bragg reflectors (DBRs); ring reflectors; or ring reflectors coupled with loop mirrors.

In some embodiments, the set of N reflectors comprises broadband reflectors, and the input waveguide is optically coupled to the input port of the demultiplexer through an intervening shared ring filter, wherein the shared ring filter has a free spectral range (FSR) that matches a desired wavelength channel spacing.

In some embodiments, broadband reflectors comprise: broadband waveguide DBRs; loop mirrors with Y-junctions; or loop mirrors with directional couplers.

In some embodiments, the shared ring reflector includes: a thermal phase tuner to facilitate an initial alignment with a cavity mode; and an electro-optical (EO) phase tuner to facilitate subsequent fine alignment with the cavity mode.

In some embodiments, the input waveguide includes a thermo-optic coefficient (TOC) compensator comprising a section of compensation material. In these embodiments, the lasing cavity includes a length lSithrough silicon, a length lCthrough the compensation material, and a length lOGMthrough the optical gain material, wherein the effective refractive index of silicon is nSi, the effective refractive index of the compensation material is nC, and the effective refractive index of the optical gain material is nOGM. Moreover, the effective TOC of silicon is dnSi/dT, the effective TOC of the compensation material is dnC/dT, and the effective TOC of the optical gain material is dnOGM/dT. Finally, lC≈lOGM*(dnOGM/dT−dnSi/dT)/(dnSi/dT−dnC/dT), whereby the effective TOC of a portion of the lasing cavity that passes through the optical gain material and the compensation material is substantially the same as the TOC of silicon.

In some embodiments, the gain medium comprises a reflective semiconductor optical amplifier (RSOA), and the shared reflector comprises a reflective facet of the RSOA.

In some embodiments, the reflective facet of the RSOA is partially reflective, and unreflected light from the reflective facet feeds into the output waveguide.

In some embodiments, the shared reflector comprises a waveguide loop mirror with a first end coupled to the reflective end of the gain medium.

In some embodiments, a second end of the waveguide loop mirror is coupled to the output waveguide.

In some embodiments, the output waveguide is optically coupled to the input waveguide through a directional coupler.

In some embodiments, the gain medium is located on a gain chip, which is separate from a semiconductor chip that includes the input waveguide, the demultiplexer, the set of N reflectors and the set of VOAs.

Throughout this specification and in the appended claims we use the term “gain medium” to refer any device, which contains active gain material and can be used to power a laser. This can include but is not limited to: a semiconductor optical amplifier (SOA); an active device fabricated using a quantum-dot gain material; and an active device fabricated in a nonlinear fiber gain medium. We also a refer to a gain medium having a reflective end coupled to a “shared reflector.” The term “shared reflector” can include but is not limited to: a reflective facet coupled to the reflective end of the gain medium, whereby the gain medium comprises a reflective semiconductor optical amplifier (RSOA); a waveguide loop mirror coupled to the reflective end of the gain medium; and a distributed Bragg waveguide (DBR) mirror coupled to the reflective end of the gain medium.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled in the art to make and use the present embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present embodiments. Thus, the present embodiments are not limited to the embodiments shown, but are to be accorded the widest scope consistent with the principles and features disclosed herein.

Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

Implementation Details

We have developed a silicon-assisted hybrid laser with fast wavelength tuning that operates by turning “on” and “off” the individual semiconductor optical amplifiers (SOAs) located on the separate III-V gain medium, while maintaining the silicon components of the laser in a static state, or by performing only minimal resonance adjustments. (See U.S. patent application Ser. No. 15/047,090, entitled “Ring-Resonator-Based Laser with Multiple Wavelengths,” by inventors Jock T. Bovington, et al., which is incorporated by reference herein.) The disadvantage of this approach is that a multi-channel gain medium is required with channel counts equal to the number of tunable wavelength channels, N. When N is large, the cost of the III-V gain media becomes high, and associated manufacturing-defect rates can create problems. In addition, although the process of turning an SOA “on” and “off” may be high speed, associated current-injection-induced thermal effects can be slow, which can potentially limit the tuning speed.

We also developed a fast-tunable silicon-assisted hybrid laser using a single-channel gain medium, wherein the fast wavelength tuning is achieved by using a fast MEMS switch connected to a set of reflectors, while minimizing required tuning control. (See U.S. patent application Ser. No. 15/341,691, entitled “Scalable Fast Tunable Si-Assisted Hybrid Laser with Redundancy,” by inventors Xuezhe Zheng, et al., filed on 2 Nov. 2016, which is incorporated by reference herein.) Unfortunately, the process of integrating the SOI MEMS switch with the other silicon-photonic components has yet to be perfected.

To overcome the drawbacks of the above-described fast-tunable lasers, we have developed a fast-tunable hybrid laser source that uses a single-channel III-V gain medium and a cascaded set of MZI lattice filters. This laser source uses a passive thermo-optic compensator for each channel. Moreover, fast wavelength tuning is achieved by using electro-optic (EO) silicon variable optical attenuators (VOAs). In addition, by attaching a broadband modulator to the laser output, a hybrid optical transmitter can be implemented that provides fast wavelength tuning, and requires only minimal tuning power.

Wavelength tuning in a silicon-assisted hybrid laser is typically achieved by tuning the center wavelength of a silicon filter inside the laser cavity. Due to the weak EO effect of silicon, thermal tuning is commonly used. However, such thermal tuning is usually slow, with time constants on the order of a few microseconds. To achieve fast wavelength tuning, one possible technique is to reduce the required tuning range of the filters so that a faster EO tuner can be used.

Fast wavelength switching can be accomplished by using a silicon-based demultiplexer. In particular, a WDM demultiplexer that provides both low loss and flat transmission pass-bands can be produced by using cascaded Mach-Zehnder lattice filters. (For example, see Folkert Host, et al., “Cascaded Mach-Zehnder wavelength filters in silicon photonics for low loss and flat pass-band WDM de-multiplexing,” Optics Express, 21(20) 11652-11658, 2013.)FIG. 1illustrates such a 1× 8 demultiplexer100constructed using simple four-port Mach-Zehnder-interferometer (MZI) lattice filters101-107that serve as symmetric de-interleaving wavelength splitters. By cascading seven of these wavelength splitters in a binary tree as inFIG. 1, an input port110with eight wavelength bands λ1-λ8can be demultiplexed into a set of dedicated output ports120for each wavelength band. The lattice filters101-107comprise a lattice of optical waveguides of varying lengths, joined by optical directional couplers. Note that waveguides and directional couplers, apart from scattering loss on the waveguide side walls, are inherently loss-less components, resulting in low-loss devices. Moreover, in lattice filters101-107, a flat pass-band can be designed into the filter curve by adding extra lattice stages.

The lattice-filter-based WDM demultiplexer100illustrated inFIG. 1can be modified and incorporated into a tunable laser200as is illustrated inFIG. 2A. As depicted inFIG. 2A, tunable laser200includes a 1×8 demultiplexer comprising MZI lattice filters221-227, which has been modified by adding silicon variable optical attenuators (VOAs)231-244to the outputs of the MZI filters221-227. Note that each of the VOAs231-244can be implemented using a silicon waveguide integrated with a PIN diode. These integrated silicon VOAs231-244can be controlled to add additional loss to unwanted channels, so that only one favored wavelength lases at any given time.

The outputs of this modified demultiplexer are connected to a set of narrow-band reflectors251-258. As illustrated inFIG. 2A, these narrow-band-reflectors251-258can be implemented using a number of different structures, including a narrow-band waveguide DBR261, a ring reflector262and a ring reflector with a loop mirror263.

The input of this modified demultiplexer is connected through an input waveguide219to an RSOA204located on a separate III-V gain chip202, which is connected (e.g., via edge or surface-normal coupling) to the SOI chip210that includes the other components of tunable laser200. The HR facet of RSOA204and a favored narrow-band reflector from the set of narrow band reflectors251-258form a lasing cavity. Moreover, a directional coupler (DC) is used to couple the lasing light to an output waveguide213to produce a laser output214.

Because of the different thermo-optic coefficients (TOCs) of silicon and the III-V gain material, the position of the laser cavity modes will drift at a different rate from those of the narrow band reflectors251-258as the ambient temperature changes. This can cause “walk-offs” between the aligned reflector peaks and the laser cavity modes if the ambient temperature changes significantly, which will result in mode-hopping that is potentially fatal in high-speed communication links. This mode-hopping problem can be solved by using an active closed-loop feedback control system. However, this will not prevent drift of the entire array as the ambient temperature changes because each of the wavelength channels in the array will vary with temperature at a rate of approximately 0.08 nm/° C. This drift can potentially create a large tuning-range requirement for each narrow-band reflector.

An elegant solution to remove such drift and eliminate related tuning requirements is to add a simple TOC compensator211having a properly selected length to the input waveguide219, which can effectively eliminate temperature-induced mode-hopping. (See U.S. patent application Ser. No. 15/292,501, entitled “Surface-Normal Optical Coupling Interface with Thermal-Optic Coefficient Conversion,” by inventors Ying L. Luo, Xuezhe Zheng and Ashok V. Krishnamoorthy, filed 13 Oct. 2016, which is incorporated by reference herein.)

Note that TOC compensator211can be implemented using a SiON waveguide (or another material with a thermo-optic coefficient lower than silicon) with proper low-loss transition to the silicon waveguides. Assume the effective lengths of the three materials Si, SiON and III-V in the hybrid cavity are L1, L2, and L3, their refractive indices are n1, n2, and n3, and their thermo-optic coefficients are dn1/dT, dn2/dT and dn3/dT, respectively. The changes in optical path length of the cavity mode ΔnL due to temperature variation ΔT can be expressed as
ΔnL=(dn1/dT*L1+dn2/dT*L2+dn3/dT*L3)*ΔT.

We can make the average dn/dT of the hybrid cavity equal to dn1/dT by choosing
L2=(dn3/dT−dn1/dT)/(dn1/dT−dn2/dT)*L3.

With this TOC compensator waveguide design, the cavity modes will drift at the same pace as the silicon filter. Once the initial alignment is done, no further active tuning control is needed to keep the hybrid laser from mode-hopping due to thermal mismatch. Furthermore, by using a lookup table for the fine phase adjustment required for each channel to achieve reflector resonance alignment with the corresponding cavity mode, no active tuning control is needed for wavelength channel switching. Hence, fast wavelength switching can be achieved by turning on and off the VOAs and adjusting an EO phase tuner accordingly.

By adding a broad-band modulator215at laser output214that modulates an electrical input signal216to produce a modulated output218, a tunable transmitter with fast wavelength tuning can be produced. For example, broadband modulator215can be implemented using an MZI modulator or an electro-optic (EO) modulator, such as a SiGe Franz-Keldish modulator.

Note that tunable laser200essentially comprises a hybrid laser with a single gain medium shared by multiple lasing cavities. This design will not work well using an existing demultiplexer design that uses MZI filters due to mode competition from the multiple lasing modes determined by the lattice filter. However, the VOAs231-244between the MZI filters221-228can be controlled to add additional loss to unwanted wavelength channels and to thereby select a favored wavelength channel, which is associated with only one of the narrow-band reflectors251-258. Moreover, by using VOAs that operate through PIN current injection, this wavelength channel switching can be very fast, for example on the order of a few nanoseconds.

However, the system illustrated inFIG. 2Arequires initial static tuning for each of the narrow-band reflectors251-258to align the reflector with its corresponding lattice filter and cavity mode. This can be problematic because in order to compensate for manufacturing variations, this initial static tuning can consume a significant amount of power, especially when many wavelength channels are required.

To overcome these problems, an alternative embodiment shown inFIG. 2Buses broadband reflectors271-278instead of narrow-band reflectors251-258at each output port of the demultiplexer, wherein the band spacing of the broadband reflectors271-278is designed to be the same as the laser wavelength channel spacing. As illustrated inFIG. 2B, these broadband reflectors271-278can be implemented using a number of different structures, including a broadband waveguide DBR281, a loop mirror with a Y-junction282, and a loop mirror with a 50/50 directional coupler (DC)283.

In the embodiment of laser271illustrated inFIG. 2B, the input of the demultiplexer is connected to RSOA204through an intervening shared ring filter270, whose FSR is also the same as the laser wavelength channel spacing. The advantage of this design over the design illustrated inFIG. 2Ais that only the shared ring filter270needs to be tuned statically during an initial alignment operation. Moreover, a thermal phase tuner291, which is integrated into shared ring filter270, can be used for such tuning. Furthermore, due to cavity length uncertainty, the ring filter resonance may not be precisely aligned with the desired cavity mode of each channel. This problem can be solved by using an integrated EO phase tuner292(e.g., a PIN carrier injection phase tuner) to perform fine alignment of cavity modes, as long as the hybrid cavity is designed to ensure that the cavity spacing is much smaller than the FSR of the shared ring filter270.

Although the amount of attenuation needed at the VOAs231-244to enable wavelength switching with desired extinction is relatively small, the total power consumption of all of the “on” VOAs can be significant if the total number of wavelength channels is large. The solution to this problem is to add higher-level VOAs231-236to upper lattice-filter stages. For example, for the 8-wavelength case shown inFIG. 2B, only three VOAs need to be turned “on” at any given time.

In another embodiment, a fast wavelength-tunable III-V/Si hybrid laser300can be built as depicted inFIG. 3. This embodiment is the same as the embodiment illustrated inFIG. 2B, except that RSOA204inFIG. 2Bis replaced with a semiconductor optical amplifier302, which does not includes an HR facet. Instead, a waveguide-based loop mirror306coupled to an end of the SOA302is used as the shared mirror for all of the laser cavities. Also, instead of using directional coupler (DC)212to produce the laser output214as inFIG. 2B, a directional coupler ratio associated with loop mirror306can be adjusted to produce a laser output304from a free end of loop mirror306.

Operation

During operation, the tunable laser system described above operates as illustrated in the flow chart that appears inFIG. 4. First, the system generates an optical signal by powering a semiconductor optical amplifier (SOA) (step402). Next, the system channels the generated optical signal through an input waveguide into an input port of a demultiplexer, which comprises a set of Mach-Zehnder (MZ) lattice filters, which function as symmetric de-interleaving wavelength splitters, that are cascaded to form a binary tree that connects an input port, which carries multiple wavelength bands, to N wavelength-specific output ports, which are coupled to a set of N reflectors (step404). The system then controls a set of variable optical attenuators (VOAs) coupled to outputs of the MZ lattice filters in the binary tree to selectively activate the set of VOAs in a manner that adds loss to unwanted wavelength bands in the demultiplexer, so that only a single favored wavelength band, which is associated with a favored reflector in the set of N reflectors, lases at any given time (step406). Finally, the system optically couples light from a lasing cavity, which is formed by the shared reflector, the SOA, the input waveguide, the demultiplexer and the favored reflector, into an output waveguide (step408).

System

One or more of the preceding embodiments of the tunable laser may be included in a system or device. More specifically,FIG. 5illustrates a system500that includes an optical source502, such as a tunable laser. System500also includes a processing subsystem506(with one or more processors) and a memory subsystem508(with memory).

In general, components within optical source502and system500may be implemented using a combination of hardware and/or software. Thus, system500may include one or more program modules or sets of instructions stored in a memory subsystem508(such as DRAM or another type of volatile or non-volatile computer-readable memory), which, during operation, may be executed by processing subsystem506. Furthermore, instructions in the various modules in memory subsystem508may be implemented in: a high-level procedural language, an object-oriented programming language, and/or in an assembly or machine language. Note that the programming language may be compiled or interpreted, e.g., configurable or configured, to be executed by the processing subsystem.

Components in system500may be coupled by signal lines, links or buses, for example bus504. These connections may include electrical, optical, or electro-optical communication of signals and/or data. Furthermore, in the preceding embodiments, some components are shown directly connected to one another, while others are shown connected via intermediate components. In each instance, the method of interconnection, or “coupling,” establishes some desired communication between two or more circuit nodes, or terminals. Such coupling may often be accomplished using a number of photonic or circuit configurations, as will be understood by those of skill in the art; for example, photonic coupling, AC coupling and/or DC coupling may be used.

In some embodiments, functionality in these circuits, components and devices may be implemented in one or more: application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), and/or one or more digital signal processors (DSPs). Furthermore, functionality in the preceding embodiments may be implemented more in hardware and less in software, or less in hardware and more in software, as is known in the art. In general, system500may be at one location or may be distributed over multiple, geographically dispersed locations.

System500may include: a switch, a hub, a bridge, a router, a communication system (such as a wavelength-division-multiplexing communication system), a storage area network, a data center, a network (such as a local area network), and/or a computer system (such as a multiple-core processor computer system). Furthermore, the computer system may include, but is not limited to: a server (such as a multi-socket, multi-rack server), a laptop computer, a communication device or system, a personal computer, a work station, a mainframe computer, a blade, an enterprise computer, a data center, a tablet computer, a supercomputer, a network-attached-storage (NAS) system, a storage-area-network (SAN) system, a media player (such as an MP3 player), an appliance, a subnotebook/netbook, a tablet computer, a smartphone, a cellular telephone, a network appliance, a set-top box, a personal digital assistant (PDA), a toy, a controller, a digital signal processor, a game console, a device controller, a computational engine within an appliance, a consumer-electronic device, a portable computing device or a portable electronic device, a personal organizer, and/or another electronic device.

Moreover, optical source502can be used in a wide variety of applications, such as: communications (for example, in a transceiver, an optical interconnect or an optical link, such as for intra-chip or inter-chip communication), a radio-frequency filter, a bio-sensor, data storage (such as an optical-storage device or system), medicine (such as a diagnostic technique or surgery), a barcode scanner, metrology (such as precision measurements of distance), manufacturing (cutting or welding), a lithographic process, data storage (such as an optical-storage device or system) and/or entertainment (a laser light show).