HETEROGENEOUS INTEGRATION OF FREQUENCY COMB GENERATORS FOR HIGH-SPEED TRANSCEIVERS

Abstract

A photonics frequency comb generator includes two integrated dies: an indium phosphide die laser of a first wavelength is grown on from, and a silicon photonics die having a microring resonator connected to the laser and frequency modulators. The microring resonator converts the first wavelength into a number of second wavelengths. One type of the microring resonator is a hybrid non-linear optical wavelength generator, comprising non-silicon materials, such as SiC or SiGe built on silicon to yield a non-linear wavelength generation. The second wavelengths are generated by adjusting the ring's geometric size and a distance between the ring and the traverse waveguide. Another type of microring resonator splits the first wavelength into a plurality of second wavelengths and transmits the multiple second wavelengths to filters and modulators, and each selects and modulates one of the second wavelengths in a one-to-one relationship. This frequency comb generator has applications in WDM/CWDM and multi-chip modules in high speed transceivers.

Claims

1. A photonics frequency comb generator, comprising: a first die, wherein the first die comprises a light source grown on an indium phosphide material; a second die, wherein the second die comprises an optical wavelength generator placed on a silicon photonics substrate, wherein the first die and the second die are integrated together; an input terminal connecting to the first die; and an output terminal connected to the second die.

2. The photonics frequency comb generator, according to claim 1, wherein the light source is a laser grown on the indium phosphide material, and wherein the laser emits a first wavelength.

3. The photonics frequency comb generator, according to claim 2, wherein the optical wavelength generator is a microring resonator interconnected to the laser.

4. The photonics frequency comb generator, according to claim 3, wherein the silicon photonics substrate comprises: filters and modulators connected to the microring resonator; wherein the microring resonator is configured to convert the first wavelength into a plurality of second wavelengths; and wherein one of the filters selects one of the plurality of second wavelengths and transmits said wavelength to one of the modulators in a one-to-one relationship.

5. A photonics frequency comb generator, comprising: a first die, wherein the first die comprises a light source grown on an indium phosphide material; a second die, wherein the second die comprises a non-linear optical wavelength generator placed on a silicon photonics substrate, wherein the first die and the second die are integrated together; an input terminal connecting to the first die; and an output terminal connected to the second die.

6. The photonics frequency comb generator, according to claim 5, wherein the light source is a laser grown on the indium phosphide material, and wherein the laser emits a first wavelength.

7. The photonics frequency comb generator, according to claim 6, wherein the non-linear optical wavelength generator is a microring resonator comprising hybrid materials, and wherein the microring resonator is interconnected to the laser.

8. The photonics frequency comb generator, according to claim 7, wherein the hybrid materials comprise non-silicon materials, such as silicon carbide (SiC) or silicon germanium (SiGe).

9. The photonics frequency comb generator, according to claim 8, wherein the SiC material is fabricated using techniques comprising: 1) growing SiC on top of a silicon photonics substrate through an intermediate layer or PVD deposition; 2) performing flip-chip of a SiC die directly on the silicon photonics substrate; 3) performing flip-chip of the SiC die directly on the silicon photonics substrate, 4) attaching the SiC die through a polymer glass interposer, where the SiC die and the silicon photonics substrate are next to each other; and 5) attaching the SiC die to the silicon photonics substrate through a fiber, which is connected to the SiC die and the silicon photonics substrate through waveguide tapers.

10. The photonics frequency comb generator, according to claim 7, further comprising a plurality of microring modulators connected with the microring resonator with a waveguide bus, wherein the microring resonator is configured to convert the first wavelength into a plurality of second wavelengths, wherein the plurality of second wavelengths from the microring resonator is distributed into the waveguide bus, and wherein one of the plurality of microring modulators selects and transmits one of the plurality of second wavelengths from the waveguide bus into said microring modulators in a one-to-one relationship.

11. The photonics frequency comb generator, according to claim 10, wherein converting the first wavelength into the plurality of second wavelengths is achieved by adjusting a geometric size of the microring and a distance between the microring and the waveguide bus.

12. The photonics frequency comb generator, according to claim 5, wherein the light source grown on the indium phosphide material is a semiconductor optical amplifier (SOA) configured to provide gain to the first wavelength emitted by a laser oscillator cavity on the second die.

13. A multi-chip module, comprising: a photonics frequency comb generator, comprising: a first die, wherein the first die comprises a light source grown on an indium phosphide material; a second die, wherein the second die comprises an optical wavelength generator placed on a silicon photonics substrate; wherein the first die and the second die are connected by an optical waveguide; an input terminal connecting to the first die; and an output terminal connected to the second die.

14. The multi-chip module, according to claim 13, wherein the optical waveguide connecting the first die and the second die comprises one of an optical fiber, a polymer waveguide, or a glass waveguide.

15. The multi-chip module, according to claim 14, wherein the light source is a laser grown on the indium phosphide material, and wherein the laser emits a first wavelength.

16. The multi-chip module, according to claim 15, wherein the optical wavelength generator is a microring resonator connected to the laser.

17. The multi-chip module, according to claim 16, wherein the silicon photonics substrate comprises: multiple channels of filters and modulators connected to the microring resonator; wherein the microring resonator is configured to convert the first wavelength into a plurality of second wavelengths; and wherein one of the filters selects one of the plurality of second wavelengths and transmits said wavelength to one of the modulators in a one-to-one relationship.

18. The multi-chip module, according to claim 17, further comprising a third die connected to the second die, wherein the third die is an electronics die comprising circuits for electronic manipulation of signals, including equalization, coding, switching, and logic operations.

19. A multi-chip module, comprising N photonics frequency comb generators according to claim 13, wherein each of the N photonics frequency comb generators comprises a pair of the first die and the second die.

20. The multi-chip module, according to claim 19, further comprising an electronics die connecting to the N photonics frequency comb generators respectively via a multi-lane waveguide, wherein the electronics die comprises circuits for electronic manipulation of signals, including equalization, coding, switching, and logic operations.

21. An M-channel WDM module, comprising the photonics frequency comb generators according to claim 4, wherein a number of filters and a number of modulators are both M×4, wherein M is a positive integer 1, 2, . . . M and M×4 is 4, 8, . . . M×4.

22. An M-channel CWDM module, comprising the photonics frequency comb generators according to claim 4, wherein a number of filters and a number of modulators are both M′×4, wherein M′ is a positive integer 1, 2, . . . M′×4.

Description

BRIEF DESCRIPTION OF THE DRAWING(S)

[0043] Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale.

[0044] FIG. 1A shows a schematic diagram of an existing frequency comb generator based on silicon photonics technology only;

[0045] FIG. 1B shows a schematic diagram of an existing frequency comb generator based on indium phosphide technology only;

[0046] FIG. 2 shows a hybrid frequency comb generator using an indium phosphide die flip-chip mounted on a silicon photonics die, according to some embodiments of the current disclosure;

[0047] FIG. 3 shows a schematic diagram of a frequency comb generator having a bus waveguide followed by multiple microring modulators, according to some embodiments of the current disclosure;

[0048] FIG. 4A shows a schematic diagram of an exemplary configuration with the InP laser die interconnected to the frequency comb generator on the silicon photonics die, according to some embodiments of the current disclosure;

[0049] FIG. 4B shows a schematic diagram of another exemplary configuration with a hybrid InP die containing a laser/amplifier source attached to the silicon photonics die containing a laser cavity and a frequency comb generator, according to some embodiments of the current disclosure;

[0050] FIG. 5A shows a schematic block diagram of a frequency comb generator of 4-channel scheme, according to some embodiments of the current disclosure;

[0051] FIG. 5B shows a schematic block diagram of a frequency comb generator of N-channel scheme, according to some embodiments of the current disclosure;

[0052] FIGS. 6A-6C show schematic diagrams of three types of frequency comb generators implemented in multi-chip modules (MCM) where an indium phosphide die, a silicon photonics die, and an additional electronics die are used, according to some embodiments of the current disclosure;

[0053] FIG. 7 shows a schematic diagram of a frequency comb generator for wavelength-division multiplexing (WDM) operation, according to some embodiments of the current disclosure; and

[0054] FIG. 8 shows a schematic diagram of a frequency comb generator for coarse Wavelength Division Multiplexing (CWDM) dual operation, according to some embodiments of the current disclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0055] The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. As used herein, terms such as “top,” “bottom,” “front,” etc. are used for explanatory purposes in the examples provided below to describe the relative position of certain components or portions of components. Accordingly, as an example, the term “top current spreading layer” may be used to describe a current spreading layer; however, the current spreading layer may be on the top or on the bottom, depending on the orientation of the particular item being described.

[0056] With the introduction of photonics integrated circuits, frequency comb generators now have the possibility to be integrated into photonics chips and achieve small sizes that are, therefore, capable of being used in integrated transceivers.

[0057] FIG. 2 shows a hybrid frequency comb generator according to some embodiments of the current disclosure. In FIG. 2, an indium phosphide die is flip-chip mounted on a silicon photonics die, and a number of fibers or waveguides serve as the inputs and outputs.

[0058] FIG. 3 illustrates a schematic diagram of a frequency comb generator, according to some embodiments of the current disclosure.

[0059] In FIG. 3, the frequency comb generator includes a laser, a microring resonator, a bus waveguide of multiple wavelength lines followed by a number of microring modulators matching individual wavelength lines, such that each microring modulator modulates one wavelength line directly. Therefore, specific wavelength lines are extracted by corresponding microring resonators and are modulated individually.

[0060] The indium phosphide die includes a laser source and/or a semiconductor optical amplifier (SOA). The laser has a narrow frequency band emitting UV, visible, or IR light. The laser can be a fiber laser or a VCSEL laser grown on an III-V semiconductor substrate, such as the InP die, but not limited to the InP die only.

[0061] The laser light is guided by an integrated coupling to an input of a microring resonator on the silicon photonics die. The microring resonator broadens the wavelength from the laser by a nonlinear property that expands the narrow wavelength bandwidth from the laser source into a number of wavelengths, wavelength 1 to wavelength N shown in FIG. 3, or simply wavelengths in a broader spectrum.

[0062] To achieve that, a mechanism of generating non-linear light from the single wavelength from the laser output is required. In order to increase the nonlinearity of the microring resonator, the microring resonator may be built with a heterogeneous structure, i.e., on silicon photonics using non-silicon material, for example, silicon carbide, or silicon germanium (SiGe) rather than only silicon material. In this way, the microring resonator may yield high non-linear mixing and generate a spread over several wavelengths of the incoming wavelength signal from the indium phosphide laser/amplifier. This hybrid non-linear microring resonator on the silicon based photonics involves a fabrication process that allows or is compatible with growing the non-silicon materials on the silicon die, or at least can be integrated with the non-silicon die.

[0063] The choice of SiC comes from a few favorable characters of its material properties. SiC has a wide bandgap (2.4 to 3.2 eV), corresponding to a wide low-loss transmission window (400-5000 nm), which covers the operational transmission band for fiber optic communications. SiC fabrication is compatible with Complementary metal-oxide-semiconductor (CMOS), compared to non-compatible indium gallium arsenide (InGaAs), Aluminium nitride (AlN), and Lithium niobate (LiNbO3). SiC has both second-order and third-order optical nonlineatiry, as frequency comb generation can rely partly on its high third-order nonlinearity. In addition, silicon carbide has good thermal conductivity, with its thermal conductive coefficient 3 times higher than that of silicon.

[0064] A microring resonator can be fabricated with SiC using a CMOS compatible process in the temperature range of 800 degrees C. to 1000 degrees C. for single crystalline 3C-SiC or 400 degrees to 800 degrees for amorphous and nanocrystaline 3C-SiC. Processes such as chemical vapor deposition (CVD), physical vapor deposition (PVD), or plasma enhanced chemical vapor deposition (PECVD) may be used.

[0065] In some cases, crystalline SiC on SiO.sub.2 is required, and SiC cannot be grown directly on SiO.sub.2. A new SiC die made out of only SiC can be attached to the silicon photonics die through a die-to-die connecting method, such as transferring the SiC thin film to a SiO.sub.2 film on a substrate, such as a silicon wafer or a silicon-on-insulator (SOI) wafer having a very thin silicon top layer, or performing surface treatment to form a buffer layer before the SiC layer growth on a silicon based substrate.

[0066] In summary, various techniques of fabricating the hybrid microring resonator with SiC include: 1) growing SiC on top of silicon photonics through an intermediate layer plus PVD deposition; 2) performing flip-chip of SiC die directly on the silicon photonics with light inputs and outputs through waveguides; 3) performing flip-chip of SiC die directly on the silicon photonics with light inputs and outputs through vertical grating couplers; 4) attaching through a polymer glass interposer, where the silicon carbide die and the silicon photonics die are next to each other; and 5) attaching the SiC die through a fiber, which is connected to each die through waveguide tapers.

[0067] The techniques of fabricating the hybrid microring resonator with SiC are not limited to the above listed methods, and other techniques may be use as long as they allow high fabricating yield while ensuring maximum transfer of optical light from the silicon carbide die to the silicon photonics die.

[0068] As shown in FIG. 3, the broad wavelength output (wavelength 1 to wavelength N) of the microring resonator is coupled to a transverse bus waveguide of multiple wavelengths, followed by a number of microring modulators matching individual wavelengths, so each microring modulator modulates one wavelength directly. Therefore, specific wavelengths are extracted by corresponding microring resonators and are modulated individually. Each of the N microring modulators is tailored to a different wavelength.

[0069] By adjusting the diameter of the microring resonator and the decoupling distance between the ring and the traverse waveguide, the free spectral range and, hence, the wavelength difference of N channels in the frequency comb can be selected.

[0070] FIG. 4A shows a schematic diagram of an exemplary configuration with the InP laser die interconnected to the frequency comb generator on the silicon photonics die, according to some embodiments of the current disclosure.

[0071] Partitioning of the different system functionalities among different materials leads to different configurations. As shown in FIG. 4A, the laser is implemented in the InP die and the microring resonator for comb generation is implemented in the silicon photonics (SiPh) die. Other components, such as the optical filters which generate wavelength 1 to wavelength N, and the microring modulators are also implemented on the silicon photonics die. It is possible to also include a nonlinear non-silicon material, such as SiC on the silicon substrate for comb generation.

[0072] FIG. 4B shows a schematic diagram of another exemplary configuration with a hybrid InP die containing a laser/amplifier source attached to the silicon photonics die containing a laser cavity and a frequency comb generator, according to some embodiments of the current disclosure.

[0073] In another configuration shown in FIG. 4B, the laser cavity can be hybrid. The laser oscillator is built on the side of the silicon photonics die, while the gain block that provides amplification by the semiconductor optical amplifier (SOA) is grown on the InP die. The other optical filtering components and feedback mechanisms to generate wavelength 1 to wavelength N are also fabricated on the silicon photonics die, via applying techniques, for example, a Bragg grating or an arrangement of a microring resonator and optical waveguide couplers.

[0074] A transverse bus waveguide, as shown in FIG. 3, is applied in FIG. 4A and FIG. 4B, following the microring resonator. Modulation of each individual wavelength is performed similarly as well. Therefore, the functions and structures of those modulation parts will not be repeated here.

[0075] There are different techniques of implementation to modulate the light coming out of the frequency comb generator. In another exemplary embodiment, the microring resonator creates a number of signals at equidistantly spaced wavelengths, then the different wavelengths are filtered by dedicated filters. Each filter has a central wavelength designed to match one of the signals at that wavelength only. The signal from that filter, which is a narrow wavelength band (sometimes called a single wavelength, even though it has a bandwidth), is then sent to an optical modulator, for example, a Mach-Zehnder modulator, which modulates the optical signal by an electrical signal. The electrical signal may be a non-return to zero (NRZ) modulator, a four-level pulse amplitude modulator (PAM-4), or a modulator applying any multi-level signal conveying device technique, for example, like coding digital information into any format.

[0076] The filtering technique shown in FIG. 5A and FIG. 5B, implements a dedicated filter for each frequency comb line responding to one wavelength band. This dedicated filter extracts the wavelength band and sends it to one comb line, and a modulator performs phase modulation or amplitude modulation according to applications. The modulator can be a Mach-Zehnder modulator or a microring modulator.

[0077] FIG. 5A shows a schematic block diagram of a frequency comb generator having a four-channel scheme, according to some embodiments of the current disclosure. In the four-channel scheme of FIG. 5A, the four-channel frequency comb generator system includes a light source on an indium phosphide die. The light source can be a laser and/or an amplifier combined into the laser block. The emitted single wavelength laser light is guided into a microring resonator on the silicon photonics die. The microring resonator combs out multiple lines of different frequencies, a number of filters are used to separate the lines of the frequency comb, and a number of modulators, each following one of the filters, are used to independently modulate that frequency line. In the exemplary case of four channels depicted in FIG. 5A, the microring resonator connects to four channels, and each channel is formed of one filter and one modulator connected in series. The outputs of the four channels are combined into optical fiber cables.

[0078] FIG. 5B discloses another system similar to FIG. 5A but has N-channels. The descriptions of the N-channeled system should be similar to the above four-channel scheme and thus will not be repeated here.

[0079] In some other applications, the frequency comb generator is used in large multi-chip modules (MCM). Typically, the indium phosphide dies and the silicon photonics dies do not have to be co-integrated. Instead, they are interconnected through a fiber connection or polymer flex waveguides.

[0080] FIGS. 6A-6C show schematic diagrams of three types of frequency comb generators implemented in multi-chip modules (MCM), which include an indium phosphide die, a silicon photonics die, and an additional electronics die, according to some embodiments of the current disclosure.

[0081] The three types of frequency comb generators in large MCM chips may use separates dies, which do not need to be integrated as described with respect to the above-mentioned systems. Examples of these systems are described below and illustrated in FIGS. 6A to 6C.

[0082] In FIG. 6A, there are two dies—the InP Die 1 and silicon photonics Die 2. The laser source is fabricated on an indium phosphide material (Die 1) and a second integrated circuit is fabricated in a silicon photonics chip (Die 2). The two dies are interconnected through a conduit made of a fiber, a polymer waveguide, a glass waveguide, or other interconnecting lines. The silicon photonics Die 2 includes a microring resonator to complete the frequency comb operation, as well as filtering and modulation blocks to select and modulate each separate frequency line.

[0083] In FIG. 6B, the system in FIG. 6A is extended with a third chip (die 3), which is an electronics die containing all the systems for electronic manipulation of signals, including but not limited to equalization, coding, switching, and logic operations.

[0084] In FIG. 6C, the multichip module (MCM) implementation is further extended. A central Die 4 (electronics) containing all signal manipulation electronics is surrounded by N pairs of photonics dies, each pair comprising a Die 1 of indium phosphide for light source and a Die 2 of silicon photonics for frequency combing, and the Die 1 and Die 2 are interconnected by a conduit of optical fibers, a polymer waveguide, or a glass waveguide, similar to the system described in FIG. 6A. The central electronics Die 4 is connected to each of the photonics pair Die 1 and Die 2 through M lanes of electrical connections, which may comprise power lanes and low frequency and high frequency lanes.

[0085] The integration of frequency comb generators into transceivers also offers the opportunity to extend a transceiver from a wavelength WDM source, for example, a four-wavelength channel scheme, to coarse-WDM (CWDM) as in eight wavelength channels. The similar frequency comb generators can be used to generate either four wavelengths to feed a WDM link or eight wavelengths to feed a CWDM link. The following figures describe these extensions.

[0086] FIG. 7 shows a schematic diagram of a frequency comb generator for WDM operation, according to some embodiments of the current disclosure.

[0087] In FIG. 7, a laser is built on the indium phosphide die, followed by a microring resonator, filters, and modulators built on the silicon photonics die. The output of the laser is non-linearly broadened or split by the microring resonator and separated into four channels by wavelength-division-multiplexing techniques. The outputs of the four channels are assembled into waveguide or fiber optical cables.

[0088] FIG. 8 illustrates a schematic diagram of a frequency comb generator for CWDM dual operation, according to some embodiments of the current disclosure.

[0089] FIG. 8 shows an extension of the four-channel WDM system into an eight-channel system. A coarse wavelength division multiplexing is a wavelength multiplexing technology for city and access networks. Typical transmission is realized using 8 channels with wavelengths between 1270 nm and 1610 nm. Due to the channel spacing of relative coarse 20 nm, cost-effective lasers can be used. In FIG. 8, the eight channel CWDM combines two of the four-channel WDMs illustrated in FIG. 7. A laser is built on the indium phosphide die, and the output of the laser is non-linearly broadened or split by the microring resonator, generating eight wavelength bands (4+4), and separated into two different buses by wavelength-division-multiplexing techniques. The laser and the microring resonator are shared by all channels of the WDMs. The following filters and modulators are built on two silicon photonics dies, each responsible for one four-channel WDM. The outputs of the four channels are assembled into waveguide or fiber optical cables. This scheme can be used in combination with the approaches in FIG. 3 or FIG. 5B.

[0090] Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed, and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.