Grating Coupler Research Articles

Wavelength-tunable spatiotemporal mode-locking in a large-mode-area Er:ZBLAN fiber laser at 2.8 µm.

We report a tunable spatiotemporally mode-locked large-mode-area Er:ZBLAN fiber laser based on the nonlinear polarization rotation technique. A diffraction grating is introduced to select the operating wavelength. Under the spectral and spatial filtering effects provided by the grating and spatial coupling respectively, stable ps-level spatiotemporally mode-locked pulses around 2.8 µm with a repetition rate of 43.4 MHz are generated. Through a careful adjustment of the grating, a broad wavelength tuning range from 2747 to 2797 nm is realized. To the best of our knowledge, this is the first wavelength-tunable spatiotemporally mode-locked fiber laser in the mid-infrared region.

Photolithography Fabricated Broadband Waveguide Grating Couplers With 1 dB Bandwidth Over 100 nm

Photolithography Fabricated Broadband Waveguide Grating Couplers With 1 dB Bandwidth Over 100 nm

Inverse-designed broadband low-loss grating coupler on thick lithium-niobate-on-insulator platform

A grating coupler on 700-nm-thick Z-cut lithium-niobate-on-insulator platform with high coupling efficiency, large bandwidth, and high fabrication tolerance is designed and optimized by inverse design method. The optimized grating coupler is fabricated with a single set of e-beam lithography and etching process, and it is experimentally characterized to possess peak coupling efficiency of −3.8 dB at 1574.93 nm, 1 dB bandwidth of 71.7 nm, and 3 dB bandwidth of over 120 nm, respectively.

Plasmonic slanted slit gratings for efficient through-substrate light-plasmon coupling and sensing

We present an experimental study of plasmonic slanted slit gratings (PSSGs) designed to achieve directional coupling between an incident light beam and surface plasmon polaritons (SPPs) propagating along the surface of the structure. We also investigate mirrored PSSG pairs interconnected by a plasmonic slab waveguide. The structures are fabricated using direct milling by a gallium focused ion beam (FIB). In a mirrored pair arrangement, the first PSSG couples a perpendicularly-incident light beam to SPPs propagating in one direction along the waveguide, while the second PSSG decouples SPPs to perpendicularly-emerging light. This configuration shows promise for sensing applications due to the high sensitivity of the excited SPPs to changes in the refractive index of the bounding medium, and the separation of the optics from the fluidics by the substrate. The design also exhibits robustness to fabrication tolerances. The optical characteristics and sensing potential are investigated theoretically and experimentally, highlighting its potential for a wide range of applications.

Low-loss grating coupler with a subwavelength structure on a thin-film lithium niobate substrate.

We demonstrated a low-loss O-band grating coupler on an x-cut thin-film lithium niobate substrate by implementing subwavelength and apodized structures. The subwavelength gratings were used to mitigate the refractive index discontinuity between the input taper and grating region, which was the first application of such a structure for grating coupler optimization on a thin-film lithium niobate substrate. The coupling efficiency was measured to be -1.99 dB/coupler at a wavelength of 1312.8 nm, which was the lowest loss among the reported lithium niobate grating couplers that do not use metal mirrors. The proposed design does not require metal mirrors or any additional material layers and can be easily fabricated with a single-step lithography and etching processes.

Fully Connected Feedforward Neural Network for the Prediction of Amorphous Silicon Grating Couplers Efficiency

Photonic circuits are an enabling technology for the development of novel solutions in different fields such as healthcare, quantum computing, neural networks, communications, and manufacturing. Interconnections between devices and systems require low-loss light coupling strategies. Grating couplers are a promising solution to couple light between photonic circuits and optical fibres due to their off-plane coupling capabilities. Hydrogenated amorphous silicon (a-Si:H), which can be deposited by PECVD over a substrate of silica or glass, is a suitable low-cost solution for the production of such light coupling devices. In this work we developed, trained and tested a fully connected feedforward neural network for coupling efficiency prediction in a-Si:H grating couplers. The light coupling gratings were simulated by twodimensional finite-difference time-domain (FDTD) analysis and field distributions were analysed with the Finite Element Method (FEM). Simulated gratings include non-apodized, linear and quadratic refractive index variation designs featuring full or partial etching, operating at 1550 nm. Not featuring any type of bottom reflector, the couplers exhibit coupling efficiencies up to about 40 % (~ -4 dB). The neural network multiclass grating coupler efficiency classifier was trained with over 3000 simulation results, reaching an accuracy over 85%, for coupling efficiencies between 0 and 30%+.

High-Efficiency, Large Bandwidth Grating Coupler Based on Lithium Niobate Thin Film (Invited)

High-Efficiency, Large Bandwidth Grating Coupler Based on Lithium Niobate Thin Film (Invited)

Low-loss grating coupler with a gradient index-matching subwavelength structure

Low-loss grating coupler with a gradient index-matching subwavelength structure

Vertical Grating Coupler Based on Double-Layer Silicon Nitride Antireflection

Vertical Grating Coupler Based on Double-Layer Silicon Nitride Antireflection

Single-step-etched ultra-compact metamaterial grating coupler enabled by a hierarchical inverse design approach

Single-step-etched ultra-compact metamaterial grating coupler enabled by a hierarchical inverse design approach

Computational Study of the Coupling Performances for a Long-Distance Vertical Grating Coupler

We present a high-efficiency silicon grating coupler design based on a left–right mirror-symmetric grating and a metal mirror. The coupler achieves nearly perfect 90-degree vertical coupling. When two SOI chips are placed face to face with a vertical working distance of 50 μm, the chip-to-chip interlayer coupling efficiency reaches as high as 96%. When the vertical working distance ranges from 45 μm to 55 μm, the coupling loss remains below 1 dB. We also verified the effectiveness of our designed vertical coupler through 3D FDTD full-model simulation. The results demonstrate that our proposed vertical coupling structure represents a high-efficiency solution for 3D optical interconnects. The integration of multiple photonic chips in a 3D package with vertical optical and electrical interconnects is also feasible in the foreseeable future.

High-Efficiency Unidirectional Dual-Wavelength-Band Grating Coupler With 1D Subwavelength Structure

High-Efficiency Unidirectional Dual-Wavelength-Band Grating Coupler With 1D Subwavelength Structure

Reconfigurable phase change chalcogenide grating couplers with ultrahigh modulation contrast

In photonic integrated circuits, efficient coupling of light between fibers and waveguides is challenging due to mode area mismatch. In-plane grating couplers (GC) have become popular for their low cost, easy alignment, and design flexibility. While most GC designs have fixed coupling efficiencies, with a view to emerging adaptive neuromorphic and quantum integrated circuits and interposers that need ultra-compact memory/modulation components, we introduce a CMOS-compatible GC based on phase-change chalcogenide alloy germanium antimony telluride. The GC design optimized utilizing inverse design techniques achieves over 50% coupling efficiency at 1550 nm when amorphous, and near-zero efficiency when switched to a crystalline state. This design is non-volatile, reversible, and provides ultra-high transmission modulation contrasts of up to 60 dB. While the operational range can be adjusted across the telecommunication band by modifying the GC's etch depth or thickness. We show that such devices do not need global switching of their entire phase change volume and can achieve maximum modulation contrasts through switching precisely positioned phase change inclusions hinting at low-power and ultrafast modulation potential.

Optical Beam Steering Using Vertically Actuated MEMS Grating Coupler

Optical Beam Steering Using Vertically Actuated MEMS Grating Coupler

Wideband-Efficient SOI Uniform Subwavelength Grating Couplers by Effective-Index and Leakage-Factor Matching at Multiple Wavelengths

Wideband-Efficient SOI Uniform Subwavelength Grating Couplers by Effective-Index and Leakage-Factor Matching at Multiple Wavelengths

Broadband Port-Selective Silicon Beam Scanning Device for Free-Space Optical Communication

Indoor free space optical (FSO) communication technology that provides high-speed connectivity to edge users is expected to be introduced in the near future mobile communication system, where the silicon photonics solid-state beam scanning device is a promising tool because of its low cost, long-term reliability, and other beneficial properties. However, the current two-dimensional beam scanning devices using grating coupler arrays have difficulty in increasing the transmission capacity because of bandwidth regulation. To solve the problem, we have introduced a broadband surface optical coupler, “elephant coupler,” which has great potential for combining wavelength and spatial division multiplexing technologies into the beam scanning device, as an alternative to grating couplers. The prototype port-selective silicon beam scanning device fabricated using a 300 mm CMOS pilot line achieved broadband optical beam emission with a 1 dB-loss bandwidth of 40 nm and demonstrated beam scanning using an imaging lens. The device has also exhibited free-space signal transmission of non-return-to-zero on-off-keying signals at 10 Gbps over a wide wavelength range of 60 nm. In this paper, we present an overview of the developed beam scanning device. Furthermore, the theoretical design guidelines for indoor mobile FSO communication are discussed.

Silicon nitride TM-pass polarizer using inverse design.

Integrated silicon nitride polarizers play a critical role in the design of complex integrated devices such as filters, switches, and large Mach-Zehnder interferometer networks. These devices require precise control of both polarizations on a single circuit. In addition, polarizers are essential to accurately characterize these devices, primarily due to the low efficiency and polarization extinction ratio (PER) of the surface coupling gratings used in CMOS-compatible silicon nitride platforms for test-specific optical I/O. In this article, we present the design and experimental performance of six prototypes of TE-reflector/TM-pass polarizers specifically optimized for the C-band. These prototypes resemble subwavelength gratings with several additional intricate aspects. In particular, the longer prototypes feature two distinct regions, one representing non-intuitive tapers and the other showcasing a more distinct subwavelength grating. We achieve a high TM transmission efficiency of -0.28 dB along with a PER of 18.2 dB. These results are obtained with a device occupying an area as low as 11 µm × 2 µm, setting a new performance benchmark for compact polarizers compatible with standard silicon nitride platforms.

High-performance grating couplers on 220-nm thick silicon by inverse design for perfectly vertical coupling

Efficient grating couplers (GCs) for perfectly vertical coupling are difficult to realize due to the second-order back reflection. In this study, apodized GCs (AGCs) are presented for achieving perfectly-vertical coupling to 220 nm thick silicon-on-insulator (SOI) waveguides in the C-band. We compare the performance of the AGCs to that of uniform GCs (UGCs) and demonstrate the superiority of the former. The AGCs were obtained through inverse design using gradient-based optimization and were found to effectively suppress back reflection and exhibit better matching to the Gaussian beam profile. The design and measurement results show that AGCs have a 3 dB lower coupling loss than UGCs. We fabricated focusing AGCs by electron beam lithography with a single, 70 nm shallow etch and a minimum feature size of 100 nm, which makes them compatible with CMOS technology. The AGCs achieved a coupling efficiency of −5.86 dB for perfectly vertical coupling. Overall, our results demonstrate the potential of AGCs for achieving high-performance coupling in the C-band on the SOI platform.

Solid-state Lidar with wide steering angle using counter-propagating beams

In a solid-state photonics-based Lidar, all essential components can be integrated into a silicon chip. It is simple and effective to use a tunable laser source to implement Lidar’s beam steering. However, how to effectively increase the steering angle in a small wavelength tuning range is usually a key challenge due to the limited material and waveguide dispersion. In Silicon-on-insulator waveguide, we design a novel solid-state Lidar using two trans-electrical (TE) polarized beams counter-propagating towards each other. Two corresponding output beams from just a single grating coupler (GC) can be seamlessly combined to double the beam steering angle. Furthermore, a low-priced solid-state Lidar is designed for TE polarized beams counter-propagating towards each other by using wavelength division multiplexed laser array.

Efficient and perfectly vertical binary blazed grating coupler with a slit

Efficient and perfectly vertical binary blazed grating coupler with a slit