EXHIBIT D
UNITED STATES DISTRICT COURT FOR THE EASTERN DISTRICT OF TEXAS MARSHALL DIVISION
CERTIFIED COPY OF U.S. PATENT NO. 11,402,015
This is a true and correct copy of United States Patent No. 11,402,015 B2, entitled "Polarization-Diverse Grating Coupler for Silicon Photonic Transceivers," as issued by the United States Patent and Trademark Office on August 2, 2022. This document reproduces the patent in its entirety, including the cover page, specification, description of the drawings, and all claims as issued.
This document is filed as Exhibit D to the Original Complaint filed on April 22, 2024, in the above-captioned action.
Respectfully submitted,
CALDERWOOD & HARKNESS LLP
By: ________
Alan Ngo (Cal. Bar No. 287451) 350 South Grand Avenue, 48th Floor Los Angeles, CA 90071 Telephone: (213) 555-4200 Facsimile: (213) 555-4201
Email: ango@calderwoodharkness.com
Counsel for Plaintiff Optiwave Semiconductor Corp.
UNITED STATES PATENT
(12) United States Patent
Huang et al.
(54) POLARIZATION-DIVERSE GRATING COUPLER FOR SILICON PHOTONIC TRANSCEIVERS
(75) Inventors: Jian-Wei Huang, Santa Clara, CA (US); Nikolai Petrov, Santa Clara, CA (US); Mei-Lin Zhao, San Jose, CA (US)
(73) Assignee: Optiwave Semiconductor Corp., Santa Clara, CA (US)
(**) Notice: Subject to any disclaimer, the term of this patent is extended or adjusted under 35 U.S.C. § 154(b) by 0 days.
(21) Appl. No.: 16/555,841
(22) Filed: Aug. 30, 2019
(65) Prior Publication Data
US 2020/0064573 A1 Feb. 27, 2020
(60) Related U.S. Application Data
Provisional application No. 62/499,117, filed on Apr. 7, 2017.
(51) Int. Cl.
(52) U.S. Cl.
CPC — G02B 6/34 (2013.01); G02B 6/1245 (2013.01); G02B 2006/12107 (2013.01); H04B 10/2507 (2013.01)
(58) Field of Classification Search
CPC — G02B 6/34; G02B 6/124; G02B 6/1245; G02B 6/12; G02B 2006/12107; H04B 10/25; H04B 10/2507
USPC — 385/37, 385/39, 385/14, 385/31
See application file for complete search history.
(56) References Cited
U.S. PATENT DOCUMENTS
FOREIGN PATENT DOCUMENTS
OTHER REFERENCES
Taillaert, D. et al., "A Compact Two-Dimensional Grating Coupler Used as a Polarization Splitter," IEEE Photonics Technology Letters, Vol. 15, No. 9, pp. 1249–1251 (Sep. 2003).
Roelkens, G. et al., "High efficiency diffractive grating couplers for interfacing a single mode optical fiber with a nanophotonic silicon-on-insulator waveguide circuit," Applied Physics Letters, Vol. 92, No. 13, 131101 (2008).
Bogaerts, W. et al., "Silicon-on-Insulator Spectral Filters Fabricated With CMOS Technology," IEEE Journal of Selected Topics in Quantum Electronics, Vol. 16, No. 1, pp. 33–44 (Jan./Feb. 2010).
Mekis, A. et al., "A Grating-Coupler-Enabled CMOS Photonics Platform," IEEE Journal of Selected Topics in Quantum Electronics, Vol. 17, No. 3, pp. 597–608 (May/Jun. 2011).
Vermeulen, D. et al., "High-efficiency fiber-to-chip grating couplers realized using an advanced CMOS-compatible Silicon-On-Insulator platform," Optics Express, Vol. 18, No. 17, pp. 18278–18283 (2010).
Xiao, Z. et al., "Bandwidth analysis of waveguide grating coupler," Optics Express, Vol. 21, No. 5, pp. 5688–5700 (2013).
Cheng, L. et al., "Grating Couplers on Silicon Photonics: Design Principles, Emerging Trends and Practical Issues," in Proceedings of the Optical Fiber Communication Conference (OFC), Paper M3I.3, San Francisco, CA (Mar. 2014).
Song, J. et al., "High-efficiency two-dimensional grating coupler for polarization diversity in silicon photonic integrated circuits," IEEE Photonics Journal, Vol. 7, No. 6, 2700710 (Dec. 2015).
Marchetti, R. et al., "Coupling strategies for silicon photonics integrated chips," Photonics Research, Vol. 7, No. 2, pp. 201–239 (Feb. 2019).
(74) Attorney, Agent, or Firm — Cascade IP Advisors, LLC; Dr. Martin Chu
Primary Examiner — Jennifer L. Nguyen
Assistant Examiner — Patrick R. Donovan
(57) ABSTRACT
A polarization-diverse grating coupler for use in silicon photonic transceivers is disclosed. The grating coupler comprises a two-dimensional sub-wavelength grating structure formed on a silicon-on-insulator substrate, configured to simultaneously couple optical signals of two orthogonal polarization states—transverse-electric (TE) and transverse-magnetic (TM) modes—from an optical fiber into respective waveguide paths of a photonic integrated circuit. The coupler achieves polarization-independent coupling efficiency exceeding −2 dB across the C-band (1530–1565 nm) and L-band (1565–1625 nm) wavelength ranges, with polarization-dependent loss below 0.5 dB across the full operating bandwidth. The device is integrated into a coherent optical transceiver module for high-bandwidth data-center interconnect applications, and is fabricated using CMOS-compatible processes on a standard silicon-on-insulator wafer. The two-dimensional grating pattern combines the functions of fiber-to-chip coupling and polarization splitting in a single compact element, eliminating the need for separate off-chip polarization management components.
10 Claims, 10 Drawing Sheets
POLARIZATION-DIVERSE GRATING COUPLER FOR SILICON PHOTONIC TRANSCEIVERS
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No. 62/499,117, filed Apr. 7, 2017, the entire contents of which are incorporated herein by reference.
TECHNICAL FIELD
The present invention relates generally to silicon photonic integrated circuits (PICs) for optical communication systems. More specifically, the invention pertains to grating coupler devices that achieve polarization diversity for coupling light between optical fibers and silicon photonic waveguides in transceiver modules used in data-center interconnects. The invention addresses the design, structure, fabrication, and integration of polarization-diverse grating couplers that are compatible with complementary metal-oxide-semiconductor (CMOS) fabrication processes and are suitable for deployment in coherent optical transceivers operating at data rates of 400 Gbps, 800 Gbps, 1.6 Tbps, and beyond.
BACKGROUND OF THE INVENTION
The exponential growth of data traffic driven by cloud computing, artificial intelligence, machine learning, streaming media services, and the proliferation of connected devices has placed unprecedented demands on the bandwidth and energy efficiency of optical interconnects within hyperscale data centers. Modern data centers require optical links capable of transmitting hundreds of gigabits per second—and increasingly, terabits per second—over distances ranging from a few meters within a rack to tens of kilometers between data center facilities. To meet these requirements, the optical communications industry has turned to silicon photonics as a platform for integrating optical components on a silicon substrate using CMOS-compatible fabrication processes. Silicon photonics offers the promise of leveraging mature semiconductor manufacturing infrastructure to produce compact, low-cost, and highly integrated photonic devices at scale.
A silicon photonic integrated circuit (PIC) typically comprises range optical components monolithically integrated on a single silicon chip, including waveguides, modulators, photodetectors, multiplexers, demultiplexers, and optical couplers. These components are fabricated on a silicon-on-insulator (SOI) wafer, which provides a high-index-contrast waveguide platform well suited for dense optical integration. The silicon device layer of the SOI wafer, typically approximately 220 nm thick, serves as the optical waveguide core, while the underlying buried oxide (Box) layer, typically approximately 2 µm thick, serves as the lower cladding.
A critical challenge in silicon photonic systems is the efficient coupling of light between an external optical fiber and the on-chip silicon waveguides. Optical fibers used in telecommunications typically have a mode field diameter on the order of 10 µm, whereas silicon photonic waveguides have cross-sectional dimensions on the order of hundreds of nanometers. This large mode-size mismatch makes direct butt-coupling of the fiber to the waveguide inefficient without some form of mode-size adaptation. Grating couplers have emerged as a widely used solution for fiber-to-chip coupling, as they allow near-vertical coupling of light from a fiber positioned above the chip surface, facilitating wafer-level testing and packaging.
A grating coupler operates by diffracting light incident on a periodic structure in or on the surface of the silicon waveguide, redirecting the light into the plane of the chip and into the waveguide mode. Conventional one-dimensional grating couplers, consisting of a set of parallel lines or trenches etched into the silicon, can efficiently couple one polarization state of the incoming light—typically the transverse-electric (TE) mode—into the waveguide. However, single-mode optical fibers carry light in an arbitrary and time-varying state of polarization. The TE and transverse-magnetic (TM) polarization modes experience very different effective indices in silicon waveguides due to the strong birefringence of the high-index-contrast SOI platform. As a result, a one-dimensional grating coupler designed for TE-mode coupling will exhibit poor coupling efficiency for TM-mode light, and vice versa. This polarization sensitivity is a significant problem for coherent optical transceivers, which must demodulate signals regardless of their incoming polarization state.
Several approaches to managing polarization in silicon photonic systems have been explored in the prior art. Polarization-maintaining fibers can be used to preserve a known polarization state from transmitter to receiver, but such fibers are expensive, difficult to splice, and impractical for deployment at the scale required in data-center environments. Off-chip polarization beam splitters can be placed between the fiber and the chip to separate the TE and TM components before coupling, but these components are bulky, add insertion loss, require precise alignment, and are incompatible with the compact, low-cost integration goals of silicon photonics.
One-dimensional grating couplers have been combined with separate on-chip polarization-splitting elements, such as polarization beam splitters based on directional couplers or multimode interference (MMI) devices, to achieve polarization diversity. In such schemes, a one-dimensional grating couples both TE and TM light into the waveguide, and a downstream polarization splitter separates the two components. However, this approach has significant drawbacks. The one-dimensional grating coupler itself is inherently polarization-dependent, so the coupling efficiency for the two polarization states differs substantially, resulting in unacceptable polarization-dependent loss (PDL). Additionally, the on-chip polarization splitter and any required polarization rotator consume additional chip area, add insertion loss, and increase fabrication complexity.
Two-dimensional grating couplers have been proposed as a potential solution to the polarization diversity problem. A two-dimensional grating consists of a periodic pattern of features (such as holes or pillars) arranged in two orthogonal directions on the surface of the silicon waveguide. In principle, such a grating can diffract TE-polarized light into one output waveguide and TM-polarized light into a second output waveguide oriented orthogonally to the first, thereby performing both fiber-to-chip coupling and polarization splitting in a single compact element. However, prior implementations of two-dimensional grating couplers have suffered from significant limitations:
(1) Narrow bandwidth. Most prior two-dimensional grating couplers have been designed to operate within the C-band only (approximately 1530–1565 nm), with a 1-dB bandwidth of less than 40 nm. This limits their utility for modern transceivers that employ both C-band and L-band wavelength channels to maximize capacity.
(2) Insufficient coupling efficiency. Many prior designs achieve coupling efficiencies of −4 dB or worse for at least one of the two polarization states, resulting in unacceptable signal loss and degrading receiver sensitivity.
(3) Excessive polarization-dependent loss. Even when both polarization states are nominally coupled, the difference in coupling efficiency between TE and TM modes often exceeds 1 dB or more, degrading the performance of coherent detection algorithms that rely on balanced signal levels from both polarization tributaries.
(4) Large footprint. Some prior two-dimensional grating designs require a grating area of 30 µm × 30 µm or larger, consuming valuable chip real estate that could otherwise be used for additional transceiver channels or other photonic components.
(5) Incompatibility with standard CMOS fabrication. Certain proposed grating coupler designs require exotic materials (e.g., gold or silver reflectors) or non-standard etch processes that are not available in standard CMOS photonics foundries, limiting their manufacturability and increasing cost.
There exists, therefore, a long-felt need in the art for a compact, broadband, polarization-diverse grating coupler that achieves high coupling efficiency and low polarization-dependent loss for both TE and TM modes, operates across both the C-band and L-band, has a small footprint suitable for dense photonic integration, and can be fabricated using standard CMOS-compatible processes on a silicon-on-insulator wafer. Such a device would enable the realization of high-performance coherent optical transceivers for next-generation data-center interconnects at 400 Gbps, 800 Gbps, 1.6 Tbps, and beyond.
SUMMARY OF THE INVENTION
The present invention provides a polarization-diverse grating coupler for silicon photonic transceivers that addresses the foregoing limitations of the prior art. The coupler employs a two-dimensional sub-wavelength grating structure that simultaneously splits incoming light from an optical fiber into two orthogonal polarization components and directs each component into a separate silicon waveguide on the photonic integrated circuit.
In one aspect, the invention provides a polarization-diverse grating coupler comprising a two-dimensional grating pattern with sub-wavelength features formed in the silicon device layer of a silicon-on-insulator (SOI) substrate. The grating pattern is designed such that the diffraction conditions for TE-polarized and TM-polarized light are satisfied simultaneously, with TE-mode light being coupled into a first output waveguide oriented along a first direction (e.g., the x-axis) and TM-mode light being coupled into a second output waveguide oriented along a second direction orthogonal to the first (e.g., the y-axis). The sub-wavelength feature dimensions and grating periodicities are optimized to achieve broadband operation, with a 1-dB coupling bandwidth exceeding 80 nm, thereby enabling operation across both the C-band (1530–1565 nm) and the L-band (1565–1625 nm).
In another aspect, the invention achieves a coupling efficiency of better than −2 dB (greater than approximately 63%) for both the TE and TM polarization states across the full operating bandwidth, with a polarization-dependent loss of less than 0.5 dB. These performance metrics represent a substantial improvement over prior art two-dimensional grating couplers.
In yet another aspect, the grating coupler of the invention has a compact footprint of less than 20 µm × 20 µm, enabling dense integration with other photonic components on the transceiver chip.
In a further aspect, the grating coupler of the invention is designed for fabrication using standard CMOS-compatible processes, including deep-UV lithography and reactive ion etching, on standard SOI wafers. No exotic materials or non-standard fabrication steps are required.
The invention also provides a silicon photonic transceiver module incorporating the polarization-diverse grating coupler, wherein the coupler is monolithically integrated on the same silicon die as coherent detection circuitry, waveguide multiplexers and demultiplexers, modulators, photodetectors, and electronic driver and transimpedance amplifier circuits. The transceiver module supports aggregate data rates of 400 Gbps, 800 Gbps, and 1.6 Tbps and beyond per fiber using wavelength-division multiplexing.
The invention further provides a method of coupling light into a silicon photonic transceiver using the polarization-diverse grating coupler, and a method of fabricating the grating coupler.
By providing a single-element solution to polarization diversity in silicon photonic transceivers, the present invention addresses a long-felt need in the art, reduces system complexity, lowers manufacturing cost, and enables the realization of next-generation coherent optical transceivers for data-center interconnect applications.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:
FIG. 1 is a schematic top view of the polarization-diverse grating coupler according to an embodiment of the present invention, showing the two-dimensional sub-wavelength grating pattern and the output waveguide paths for the TE and TM polarization modes.
FIG. 2 is a cross-sectional view of the grating coupler formed on a silicon-on-insulator substrate, taken along the line A-A′ of FIG. 1, showing the buried oxide layer, the silicon device layer, the grating etch features, and the upper cladding layer.
FIG. 3 is a schematic diagram of a coherent optical transceiver module incorporating the polarization-diverse grating coupler of the present invention, showing the fiber input, the coupler, the polarization-separated waveguide paths, a polarization rotator, and downstream coherent detection circuitry.
FIG. 4 is a graph of coupling efficiency (in dB) versus wavelength (in nm) for both the TE and TM polarization modes, demonstrating broadband operation across the C-band and L-band wavelength ranges.
FIG. 5 is a graph of polarization-dependent loss (PDL) in dB versus wavelength (in nm), showing that the PDL remains below 0.5 dB across the full operating bandwidth of 1530 nm to 1625 nm.
FIG. 6 is a scanning electron microscope (SEM) image of a fabricated grating coupler according to an embodiment of the present invention, showing the sub-wavelength feature dimensions and the two-dimensional periodic arrangement.
FIG. 7 is a block diagram of a complete silicon photonic transceiver system incorporating the polarization-diverse grating coupler, showing the grating coupler interfacing to a waveguide multiplexer/demultiplexer, an array of coherent modulators, an array of balanced photodetectors, and electronic driver and transimpedance amplifier circuits.
FIG. 8 is a detailed layout of the two-dimensional grating pattern according to an embodiment of the present invention, showing the grating periodicity in two orthogonal directions, the duty cycle, and the etch depth parameters.
FIG. 9 is a schematic view of an alternative embodiment of the grating coupler incorporating apodized grating features for enhanced directionality and reduced back-reflection.
FIG. 10 is a flowchart of the fabrication process for the polarization-diverse grating coupler according to an embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The following description of the preferred embodiments of the invention is provided with reference to the accompanying drawings. It is understood that the invention is not limited to the specific embodiments described herein, but encompasses all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims. Like reference numerals are used throughout the drawings to refer to like elements.
Overview
Referring generally to the drawings, and initially to FIGS. 1 through 3, a polarization-diverse grating coupler 100 according to an embodiment of the present invention is formed on a silicon-on-insulator (SOI) wafer 200. The SOI wafer 200 comprises a silicon substrate 210, a buried oxide (Box) layer 220 disposed on the silicon substrate 210, and a silicon device layer 230 disposed on the box layer 220. In a preferred embodiment, the silicon device layer 230 has a thickness of approximately 220 nm, which is a standard thickness for silicon photonic platforms and is widely available from commercial SOI wafer suppliers. The box layer 220 has a thickness of approximately 2 µm, which provides sufficient optical isolation between the silicon device layer 230 and the silicon substrate 210 to prevent leakage of guided optical modes into the substrate.
The polarization-diverse grating coupler 100 comprises a two-dimensional grating region 110 formed in the silicon device layer 230, a first output waveguide 120 extending from the grating region 110 in a first direction (e.g., the x-direction), and a second output waveguide 130 extending from the grating region 110 in a second direction orthogonal to the first (e.g., the y-direction). The two-dimensional grating region 110 is configured to receive an optical signal from an optical fiber 300 positioned above the chip surface at a coupling angle θ of approximately 8 to 12 degrees from the surface normal. The grating region 110 diffracts the transverse-electric (TE) polarization component of the input optical signal into the first output waveguide 120 and the transverse-magnetic (TM) polarization component of the input optical signal into the second output waveguide 130. In this manner, the grating coupler 100 performs both fiber-to-chip coupling and polarization splitting in a single integrated element, without the need for separate off-chip or on-chip polarization management components.
Grating Structure (FIGS. 1, 2, 6, and 8)
Referring now to FIGS. 1, 2, 6, and 8, the two-dimensional grating region 110 comprises a periodic array of sub-wavelength features 112 arranged in a two-dimensional pattern within the silicon device layer 230. In a preferred embodiment, the sub-wavelength features 112 comprise circular holes etched partially through the silicon device layer 230. In alternative embodiments, the sub-wavelength features 112 may comprise pillars, rectangular holes, elliptical holes, or other shapes, as described further below.
The grating region 110 has grating periods Λx and Λy in the x-direction and y-direction, respectively. In a preferred embodiment, the grating periods Λx and Λy are each in the range of approximately 580 nm to 630 nm. The specific values of Λx and Λy are selected based on the desired operating wavelength range, the coupling angle θ, and the effective indices of the TE and TM modes in the silicon waveguide. In one specific embodiment, Λx is approximately 610 nm and Λy is approximately 590 nm. The slight difference between Λx and Λy compensates for the difference in effective index between the TE and TM modes, ensuring that both polarization states are coupled with similar efficiency.
The sub-wavelength features 112 have a characteristic dimension (e.g., hole diameter) in the range of approximately 200 nm to 350 nm. In a preferred embodiment, the hole diameter is approximately 280 nm. The duty cycle of the grating (defined as the ratio of the feature dimension to the grating period) is in the range of approximately 0.35 to 0.60, and in a preferred embodiment is approximately 0.46.
The sub-wavelength features 112 are formed by partial etching of the silicon device layer 230 to an etch depth d in the range of approximately 70 nm to 150 nm. In a preferred embodiment, the etch depth d is approximately 100 nm, leaving approximately 120 nm of unetched silicon beneath each feature. The partial etch design enables precise control of the grating coupling strength and minimizes scattering losses. The etch depth d is a critical parameter that affects the coupling efficiency, bandwidth, and directionality of the grating coupler 100.
As shown in FIG. 2, the grating region 110 may be covered by an upper cladding layer 240, which in a preferred embodiment comprises silicon dioxide (SiO₂) deposited by plasma-enhanced chemical vapor deposition (PECVD). The upper cladding layer 240 provides mechanical protection for the grating features, creates a symmetric refractive index environment around the silicon device layer 230 (which improves coupling efficiency), and provides a planar surface for subsequent metallization and packaging steps.
The two-dimensional periodic arrangement of the sub-wavelength features 112 creates a grating that diffracts incoming light in two orthogonal in-plane directions simultaneously. The diffraction condition for coupling into the first output waveguide 120 (TE mode, x-direction) can be expressed as:
neff,TE = nclad × sin(θ) + m × λ / Λ_x
where neff,TE is the effective index of the TE waveguide mode, nclad is the refractive index of the cladding, θ is the coupling angle, m is the diffraction order (typically m = −1 for first-order coupling), and λ is the free-space wavelength. An analogous condition applies for coupling into the second output waveguide 130 (TM mode, y-direction), with Λy and neff,TM substituted accordingly. By appropriately selecting Λx, Λy, the feature dimensions, and the etch depth, both diffraction conditions can be simultaneously satisfied across a broad wavelength range.
As shown in FIG. 8, the overall footprint of the grating region 110 in a preferred embodiment is approximately 15 µm × 15 µm. This compact footprint is a significant advantage of the present invention compared to prior art two-dimensional grating couplers, which often require 30 µm × 30 µm or larger grating areas. The compact footprint enables denser integration of multiple grating couplers and other photonic components on a single transceiver chip.
The sub-wavelength features 112 are referred to as "sub-wavelength" because their individual dimensions (e.g., hole diameter) are smaller than the operating wavelength of the light being coupled. This sub-wavelength character distinguishes the features from conventional grating lines, in which the feature dimensions are comparable to or larger than the wavelength. The sub-wavelength features interact with the incoming light collectively through the periodic arrangement, rather than individually, which enables precise engineering of the effective refractive index and dispersion properties of the grating.
Polarization Diversity Operation (FIG. 3)
Referring now to FIG. 3, the operation of the polarization-diverse grating coupler 100 within a coherent optical transceiver module 400 is described. An optical fiber 300, which may be a single-mode fiber (SMF) such as an ITU-T G.652-compliant fiber, carries an input optical signal 310 from an external optical network. The fiber 300 is positioned above the grating coupler 100 at a coupling angle θ of approximately 8 to 12 degrees from the surface normal. In a preferred embodiment, the coupling angle θ is approximately 10 degrees. The slight off-normal coupling angle is selected to minimize direct back-reflection into the fiber and to satisfy the phase-matching conditions for first-order diffraction.
The input optical signal 310 arriving at the grating coupler 100 has an arbitrary and generally time-varying state of polarization, comprising both TE and TM polarization components. The two-dimensional grating region 110 simultaneously diffracts the TE polarization component into the first output waveguide 120 and the TM polarization component into the second output waveguide 130. The first output waveguide 120 carries the TE-polarized signal in the x-direction toward a first signal processing path 410, and the second output waveguide 130 carries the TM-polarized signal in the y-direction toward a second signal processing path 420.
In a preferred embodiment, a polarization rotator 430 is disposed in the second output waveguide 130 (or in the second signal processing path 420) to convert the TM-polarized signal to the TE polarization state. This conversion simplifies downstream signal processing, as both signal paths then carry TE-polarized light, which can be processed by identical coherent detection circuits. The polarization rotator 430 may comprise a mode converter based on an asymmetric waveguide cross-section, a bi-level taper, or other known polarization rotation structures compatible with the SOI platform.
The first and second signal processing paths 410, 420 lead to a coherent detection module 440 that demodulates the data encoded on the optical signal. The coherent detection module 440 may comprise optical hybrids, balanced photodetector pairs, and transimpedance amplifiers (TIAs) configured for coherent reception of advanced modulation formats such as dual-polarization quadrature phase-shift keying (DP-QPSK), dual-polarization 16-quadrature amplitude modulation (DP-16QAM), or higher-order formats. The coherent detection module 440 recovers the amplitude, phase, and polarization information of the incoming optical signal, enabling high-spectral-efficiency data transmission.
Performance Characteristics (FIGS. 4 and 5)
Referring now to FIGS. 4 and 5, the performance characteristics of the polarization-diverse grating coupler 100 according to the present invention are described. The following performance data were obtained through a combination of three-dimensional finite-difference time-domain (3D-FDTD) electromagnetic simulations and experimental measurements of fabricated devices.
FIG. 4 shows the coupling efficiency (in dB, referenced to the fiber mode power) as a function of wavelength for both the TE and TM polarization modes. The coupling efficiency for the TE mode is shown as a solid line and for the TM mode as a dashed line. Over the C-band wavelength range of 1530 nm to 1565 nm, the TE coupling efficiency ranges from approximately −1.5 dB to −1.8 dB, and the TM coupling efficiency ranges from approximately −1.6 dB to −2.0 dB. Over the L-band wavelength range of 1565 nm to 1625 nm, the TE coupling efficiency ranges from approximately −1.8 dB to −2.3 dB, and the TM coupling efficiency ranges from approximately −1.9 dB to −2.4 dB. Across the combined C+L band (1530 nm to 1625 nm), the coupling efficiency remains better than −2.5 dB for both polarization states, and across the C-band and the majority of the L-band, the coupling efficiency exceeds −2 dB for both states.
The 1-dB coupling bandwidth—defined as the wavelength range over which the coupling efficiency remains within 1 dB of its peak value—is approximately 85 nm for the TE mode and approximately 82 nm for the TM mode. This broadband performance is a significant improvement over prior art two-dimensional grating couplers, which typically exhibit 1-dB bandwidths of less than 40 nm.
FIG. 5 shows the polarization-dependent loss (PDL) of the grating coupler 100 as a function of wavelength. The PDL is defined as the difference (in dB) between the TE and TM coupling efficiencies at each wavelength. Across the full operating bandwidth of 1530 nm to 1625 nm, the PDL remains below 0.5 dB, with a maximum PDL of approximately 0.4 dB occurring near 1600 nm. Over the C-band, the PDL is less than 0.2 dB. This low PDL is a critical advantage for coherent transceiver applications, as high PDL degrades the performance of digital signal processing algorithms used for polarization demultiplexing and equalization.
Additional performance metrics of the grating coupler 100 include:
Back-reflection: less than −20 dB (i.e., less than 1% of the input power is reflected back into the fiber) across the operating bandwidth. Low back-reflection is important to avoid interference with upstream laser sources and to meet the return-loss requirements of optical transceiver standards.
Polarization extinction ratio: greater than 15 dB in each output waveguide. This means that the power of the unwanted polarization component in each output waveguide is at least 15 dB below the power of the desired polarization component, ensuring clean polarization separation.
Fiber alignment tolerance: the coupling efficiency remains within 1 dB of its peak value for fiber position offsets of up to ±2 µm in both the x and y directions. This relaxed alignment tolerance simplifies the fiber attachment and packaging process.
Integration with Transceiver Module (FIG. 7)
Referring now to FIG. 7, the integration of the polarization-diverse grating coupler 100 into a complete silicon photonic transceiver system 500 is described. The transceiver system 500 is formed on a single silicon photonic die 510, which is fabricated on an SOI wafer as described above. The transceiver system 500 comprises the following functional blocks, all monolithically integrated on the die 510:
(a) One or more polarization-diverse grating couplers 100 as described herein, each configured to receive an optical signal from a respective fiber in a fiber array.
(b) A waveguide multiplexer/demultiplexer 520, which separates the incoming optical signal into individual wavelength channels. The multiplexer/demultiplexer 520 may comprise an arrayed waveguide grating (AWG), an echelle grating, a cascade of ring resonators, or other wavelength-selective elements known in the art. In certain embodiments, the multiplexer/demultiplexer 520 incorporates technology related to U.S. Patent No. 10,234,847 (the '847 Patent), which is separately owned by the assignee of the present invention and describes advanced waveguide multiplexer architectures for silicon photonic platforms.
(c) A coherent modulator array 530, comprising a plurality of Mach-Zehnder modulators or IQ modulators configured to encode data onto optical carrier signals for transmission. The modulator array 530 operates on TE-polarized light in the silicon waveguides.
(d) A balanced photodetector array 540, comprising a plurality of germanium photodiodes configured for coherent detection of the received optical signals. The photodetector array 540 receives the polarization-separated and wavelength-demultiplexed signals from the grating couplers 100 and the multiplexer/demultiplexer 520.
(e) Electronic driver circuits 550 for the modulators and transimpedance amplifier (TIA) circuits 560 for the photodetectors, which may be integrated on the same die (in a monolithic approach) or on a separate electronic integrated circuit (EIC) die that is flip-chip bonded or wire-bonded to the photonic die 510.
(f) Thermal management structures 570 for dissipating heat generated by the active components on the photonic die 510. In certain embodiments, the thermal management structures 570 incorporate micro-channel heat sink technology related to U.S. Patent No. 9,678,562 (the '562 Patent), which is separately owned by the assignee of the present invention and describes advanced thermal management architectures for high-power photonic integrated circuits.
The complete transceiver system 500 supports aggregate data rates of 400 Gbps (e.g., using 4 wavelength channels at 100 Gbps per channel, or 8 wavelength channels at 50 Gbps per channel), 800 Gbps (e.g., using 8 wavelength channels at 100 Gbps per channel), and 1.6 Tbps and beyond (e.g., using 16 wavelength channels at 100 Gbps per channel or 8 wavelength channels at 200 Gbps per channel). The wavelength channels are spaced within the C-band and/or L-band, and the broadband operation of the grating coupler 100 of the present invention enables all wavelength channels to be coupled through a single grating coupler with uniformly high efficiency.
Alternative Embodiments (FIG. 9)
Referring now to FIG. 9 and to the general principles described above, several alternative embodiments of the polarization-diverse grating coupler are contemplated within the scope of the present invention.
In a first alternative embodiment, the grating coupler 100 includes an apodization region in which the dimensions of the sub-wavelength features 112 vary gradually from the center of the grating to the periphery. Specifically, the hole diameter (or pillar width) and/or the etch depth may be made smaller near the center of the grating and larger toward the edges, or vice versa. This apodization technique modifies the grating coupling strength as a function of position, resulting in a more uniform illumination of the fiber mode field and improved coupling efficiency. In simulations, the apodized grating coupler achieves a peak coupling efficiency of approximately −1.2 dB for the TE mode and −1.4 dB for the TM mode, compared to −1.5 dB and −1.6 dB for the non-apodized design. The apodized design also exhibits reduced back-reflection, with back-reflection less than −25 dB.
In a second alternative embodiment, a bottom metal reflector 250 is disposed beneath the box layer 220 (or between the box layer 220 and the silicon substrate 210) to enhance the upward directionality of the grating coupler 100. The bottom reflector 250 redirects downward-diffracted light back toward the fiber, increasing the net upward coupling efficiency. The bottom reflector 250 may comprise an aluminum, copper, or gold layer deposited on the underside of the box layer using wafer bonding or back-side processing techniques. In simulations, the addition of a bottom reflector increases the coupling efficiency by approximately 1 to 2 dB compared to the design without a reflector.
In a third alternative embodiment, the grating coupler 100 incorporates a focusing grating design, in which the grating lines are curved rather than straight. The curved grating lines focus the diffracted light directly into the output waveguides 120, 130, eliminating the need for separate taper or focusing elements between the grating and the waveguides. This reduces the overall footprint of the coupler and shortens the optical path length, which may reduce propagation losses.
In a fourth alternative embodiment, the sub-wavelength features 112 have asymmetric shapes (e.g., elliptical holes with different major and minor axis dimensions) to enhance the polarization extinction ratio in each output waveguide. The asymmetry introduces a directional preference in the diffraction of each polarization state, reducing the crosstalk between the TE and TM output channels.
Fabrication Process (FIG. 10)
Referring now to FIG. 10, a method of fabricating the polarization-diverse grating coupler 100 according to an embodiment of the present invention is described. The fabrication process is fully compatible with standard CMOS foundry processes and does not require any exotic materials or non-standard processing steps.
Step S1: A starting SOI wafer 200 is provided, having a silicon device layer 230 of approximately 220 nm thickness on a buried oxide (Box) layer 220 of approximately 2 µm thickness. The SOI wafer 200 may be obtained from any of several commercial suppliers.
Step S2: A photoresist layer is deposited on the surface of the silicon device layer 230, and the two-dimensional grating pattern is defined in the photoresist using deep-UV lithography. In a preferred embodiment, 193 nm ArF immersion lithography is used, which provides sufficient resolution to define the sub-wavelength features 112 with dimensions in the 200–350 nm range. Alternatively, 248 nm KrF lithography may be used for larger feature sizes, or electron-beam lithography may be used for prototyping and research applications.
Step S3: The exposed silicon device layer 230 is partially etched using reactive ion etching (RIE) or inductively coupled plasma (ICP) etching to the specified etch depth d (approximately 70–150 nm, preferably approximately 100 nm). The etch chemistry is selected to provide high selectivity between silicon and the photoresist, and to produce smooth, vertical sidewalls on the sub-wavelength features 112. A suitable etch chemistry includes a mixture of SF₆ and C₄F₈ gases. The etch depth is controlled by time-based or endpoint-detection-based process control.
Step S4: The photoresist is stripped, and a second lithography and etch step is performed to define the output waveguides 120, 130 and any other waveguide structures on the chip. This step uses a full etch through the silicon device layer 230 to form the strip waveguides. In an alternative process flow, the waveguides and grating may be defined in a single lithography step using a multi-level etch process.
Step S5: An upper cladding layer 240 of silicon dioxide (SiO₂) is deposited over the patterned silicon device layer 230 by plasma-enhanced chemical vapor deposition (PECVD). The cladding layer 240 has a thickness of approximately 1 to 3 µm. The cladding layer 240 provides mechanical protection, environmental passivation, and a symmetric optical cladding for the grating and waveguide structures.
Step S6: Contact vias are etched through the cladding layer 240, and metal interconnects and bond pads are formed using standard CMOS metallization processes (e.g., aluminum or copper interconnects). These metal structures provide electrical connections to active devices (modulators, photodetectors, heaters) on the photonic die.
Step S7: The processed wafer is diced into individual photonic die, each die is packaged in a suitable transceiver module package, and one or more optical fibers are aligned and attached to the grating coupler(s) on each die. Fiber attachment may be performed using active alignment with UV-curable epoxy, or using passive alignment features such as V-grooves or micro-lenses formed on the chip or in the package.
The fabrication process described above is compatible with standard CMOS photonics foundry processes offered by foundries such as Drayton Holdings, Thackeray Holdings, and Ridgeline & Co., enabling volume manufacturing at low cost.
While the invention has been described with reference to specific embodiments, it is understood that the scope of the invention is defined by the appended claims, and that all modifications, equivalents, and alternatives falling within the scope of the claims are intended to be covered thereby. Specific dimensions, materials, configurations, and numerical values described above are exemplary and are not intended to limit the scope of the claims.
What is claimed is:
1. A polarization-diverse grating coupler for a silicon photonic transceiver, comprising:
a silicon-on-insulator substrate having a silicon device layer and a buried oxide layer;
a two-dimensional grating structure formed in the silicon device layer, the grating structure comprising a periodic array of sub-wavelength features arranged in two orthogonal directions;
wherein the two-dimensional grating structure is configured to simultaneously couple a first polarization component of an input optical signal into a first waveguide and a second polarization component, orthogonal to the first, into a second waveguide;
wherein the first polarization component comprises a transverse-electric (TE) mode and the second polarization component comprises a transverse-magnetic (TM) mode;
wherein the grating coupler achieves a coupling efficiency of at least −3 dB for each of the first and second polarization components over an operating wavelength range spanning at least 60 nm within the 1530 nm to 1625 nm band.
2. The grating coupler of claim 1, wherein the sub-wavelength features have a period in each orthogonal direction of between 550 nm and 650 nm.
3. The grating coupler of claim 1, wherein the etch depth of the sub-wavelength features is between 50 nm and 180 nm.
4. The grating coupler of claim 1, further comprising a polarization rotator disposed in one of the first and second waveguides to convert the TM mode to a TE mode.
5. A silicon photonic transceiver module comprising:
an optical input port configured to receive an optical signal from an optical fiber;
a polarization-diverse grating coupler formed on a silicon-on-insulator substrate, the grating coupler comprising a two-dimensional sub-wavelength grating that splits the optical signal into a TE-polarized component directed to a first on-chip waveguide and a TM-polarized component directed to a second on-chip waveguide;
a coherent detection module optically coupled to the first and second on-chip waveguides and configured to detect data modulated on the optical signal;
wherein the polarization-diverse grating coupler provides a polarization-dependent loss of less than 1 dB across an operating bandwidth of at least 60 nm.
6. The transceiver module of claim 5, further comprising a waveguide multiplexer coupled to the coherent detection module for wavelength-division multiplexing of multiple optical channels.
7. The transceiver module of claim 5, wherein the grating coupler and the coherent detection module are monolithically integrated on a single silicon die.
8. A method of coupling light into a silicon photonic transceiver, comprising:
directing an optical signal from an optical fiber onto a two-dimensional sub-wavelength grating formed in a silicon device layer of a silicon-on-insulator substrate;
diffracting, by the two-dimensional sub-wavelength grating, a transverse-electric (TE) polarization component of the optical signal into a first silicon waveguide;
diffracting, by the two-dimensional sub-wavelength grating, a transverse-magnetic (TM) polarization component of the optical signal into a second silicon waveguide oriented at an angle to the first silicon waveguide;
processing the TE and TM components in respective signal paths of a coherent optical receiver.
9. The method of claim 8, further comprising rotating the polarization of the TM component to a TE mode before processing.
10. The method of claim 8, wherein the diffracting steps achieve a coupling efficiency of at least −2 dB for each polarization component.
11. The method of claim 8, wherein the two-dimensional sub-wavelength grating comprises features having a period of between 550 nm and 650 nm in each of two orthogonal directions.
12. A polarization-diverse grating coupler, comprising:
a two-dimensional sub-wavelength grating formed on a silicon photonic substrate, the grating comprising an array of etched features arranged in a two-dimensional periodic pattern with sub-wavelength periodicity in at least two directions;
a first output waveguide coupled to the grating and configured to receive a first polarization state of an input optical signal;
a second output waveguide coupled to the grating and configured to receive a second polarization state of the input optical signal, the second polarization state being orthogonal to the first polarization state;
wherein the two-dimensional sub-wavelength grating provides polarization splitting and fiber-to-chip coupling in a single integrated element;
wherein the coupler achieves a coupling efficiency of at least −2.5 dB for each polarization state over a wavelength range of at least 80 nm.
13. The grating coupler of claim 12, wherein the etched features comprise circular holes or pillars.
14. The grating coupler of claim 12, further comprising an apodization region wherein the dimensions of the etched features vary gradually from a center of the grating to a periphery.
15. The grating coupler of claim 12, further comprising a reflective layer disposed beneath the silicon photonic substrate to enhance upward coupling efficiency.
16. An integrated optical transceiver for data-center interconnect, comprising:
a photonic integrated circuit formed on a silicon-on-insulator substrate;
at least one polarization-diverse grating coupler integrated on the photonic integrated circuit, each grating coupler comprising a two-dimensional sub-wavelength grating configured to couple both transverse-electric and transverse-magnetic polarization states of an input optical signal into respective on-chip waveguide paths;
a wavelength-division multiplexing element integrated on the photonic integrated circuit and optically coupled to the at least one polarization-diverse grating coupler;
a plurality of coherent receiver channels, each channel configured to detect a respective wavelength channel of the input optical signal;
wherein the integrated optical transceiver supports an aggregate data rate of at least 400 Gbps.
17. The integrated optical transceiver of claim 16, wherein the aggregate data rate is at least 800 Gbps.
18. The integrated optical transceiver of claim 16, further comprising a thermal management structure for dissipating heat generated by the photonic integrated circuit.
19. The integrated optical transceiver of claim 16, wherein each polarization-diverse grating coupler achieves a polarization-dependent loss of less than 0.5 dB.
20. The integrated optical transceiver of claim 16, wherein the two-dimensional sub-wavelength grating of each grating coupler has a footprint of less than 20 µm × 20 µm.
Filed as Exhibit D to the Complaint in Optiwave Semiconductor Corp. v. Lumicore Technologies, Inc., Case No. 2:24-cv-00319-JRG (E.D. Tex.), filed April 22, 2024.
Counsel for Plaintiff:
Alan Ngo, Marcus Delgado, Wendy Schultz, Omar Siddiqui Calderwood & Harkness LLP 350 South Grand Avenue, 48th Floor Los Angeles, CA 90071
Local Counsel for Plaintiff:
James "Jim" Callahan Whitmore & Callahan LLP 110 North College Avenue, Suite 800 Tyler, TX 75702
OPTIWAVE SEMICONDUCTOR CORP.,
Plaintiff,
Civil Action No. 2:24-cv-00319-JRG
v.