Grating Coupler Research Articles

Monolithic Integrated Optical Receiver With a Metal Reflector-Assisted Surface Grating Coupler

High-speed optical receivers with high-efficiency power collection capability are highly demanded in an optical wireless communication system to achieve high sensitivity due to the power-limited eye safety regulation. A cascaded aperture optical receiver enabled by a waveguide photodetector and a grating coupler is a promising candidate because of its advantage of optimizing the electrical performance and the light collection independently. Here, we propose using a metal reflector (MR)-assisted grating coupler to increase the coupling efficiency further. A sensitivity-enhanced monolithic integrated optical receiver with an MR-assisted grating coupler is realized for the first time, which is demonstrated on the Indium Phosphide membrane on Silicon platform. The maximum −3 dB bandwidth and the external responsivity of the waveguide photodetector inside the proposed optical receiver are 52.2 GHz and 0.31 A/W, respectively. A back-to-back receiver sensitivity of −4.75 dBm for a 10 Gb/s OOK signal at the 7% FEC limit is achieved. With the enhancement of an MR, the achieved sensitivity is calculated to be 1.5 dB higher based on the measured results compared with the cascaded aperture optical receiver without an MR on the same platform.

1310 nm TM grating couplers to operate silicon nitride ring resonator biosensors

Photonic integrated circuits (PICs) fabricated on silicon nitride (SiN) wafers are witnessing a massive implementation because they can provide low-loss waveguides in a broad wavelength range (from the visible to the near infrared) whilst being fabricated in large volumes using standard silicon fabrication processes. One of the most relevant application fields of this technology is biosensing since SiN PICs enable label-free lab-on-a-chip systems for fast and sensitive detection of minute amounts of different biological entities. To this end, resonant structures such as ring resonators (RR) are highly appropriate, but they need suitable interfaces with optical fiber to be tested on-wafer and used in real applications. The interface that satisfies the two previous requirements is the grating coupler (GC). However, experimental evidence of GCs operating at 1310 nm wavelengths for the transverse magnetic (TM) mode, for which RR-based sensors perform better than for the widely employed transverse electric (TE) mode, has not been reported so far. In this work, the operation of fully-etched focusing GCs at 1310 nm wavelengths for the TM mode in a SiN PIC is demonstrated. Experiments show that the fabricated GCs display an ≈40 nm bandwidth and coupling losses around 13 dB per interface. In addition, we show successful biosensing experiments using bovine serum albumin and anti-bovine serum albumin as biological agents in a microfluidic environment. Further engineering for the interface should lead to lower coupling losses, which would pave the way to practical applications of SiN PICs based on RRs operated for the TM mode and at 1310 nm wavelengths.

Design of low-loss Nb2O5-based grating coupler in scanning differential laser-Doppler integrated probe for cross-sectional velocity distribution measurement

We present an Nb2O5-based grating coupler that employs multiple Nb2O5 / SiO2 layers to improve the optical input/output power efficiency of a scanning differential laser-Doppler integrated probe for two-dimensional cross-sectional velocity distribution measurements. The multiple Nb2O5 / SiO2 layers are designed for both the transmitting grating coupler and the receiving grating coupler. To demonstrate the effect of the designed multiple layers, the field distribution around the grating coupler is simulated as a feasibility study of the concept. The results indicated that the input/output power at the grating coupler can be improved using multiple Nb2O5 / SiO2 layers over the input/output angle of 0 to 20 deg and the wavelength of 1530 to 1570 nm. The increase of the input/output power and the focusing from the transmitting grating coupler were confirmed by the simulation using the finite-difference time-domain method and beam propagation method.

Lithium niobate thin film grating couplers optimized by particle swarm optimization and a neural network

Grating couplers (GCs) are a kind of critical device for integrated photonics, which connect on- and off-chip devices. In this paper, chirped GCs on Z-cut lithium niobate on insulator were designed and optimized using a backward propagation neural network (BPNN) combined with the particle swarm optimization (PSO) algorithm. The BPNN was proposed to predict the coupling efficiency (CE) of chirped GCs at hundreds of wavelengths simultaneously, which is 7400 times faster than finite difference time domain simulation. Furthermore, PSO was employed to search for the GC structures with high CE. The maximum CE that can be optimized through our trained network reaches 63% in 1550 nm. This work provides a fast and accurate method for designing efficient GCs at any central wavelength.

One-dimensional grating coupler on lithium-niobate-on-insulator for high-efficiency and polarization-independent coupling.

A metal-based one-dimensional grating coupler on an x-cut lithium-niobate-on-insulator wafer structure for a polarization-independent fiber interface is designed and demonstrated. By using a metal-based plasmonic mode, the diffractive angle for the two polarized modes in the lithium niobate ridge waveguide can be tuned to be the same. The polarization dependence of the grating coupler therefore can be effectively reduced. The fabricated device exhibits -3.56-dB and -4.08-dB peak coupling losses per coupler at 1573 nm for the TE and TM modes, respectively. The polarization-dependent losses are less than 0.69 dB in a 44-nm wavelength range. The demonstrated grating coupler can serve as a polarization-independent optical fiber interface on lithium-niobate-on-insulator and facilitate on-chip polarization diversity applications.

Desing of grating couplers for submicron optical waveguides


 
 
 Optical coupling gratings are strategic components of integrated optics that allow the interaction of a laser signal with optical interconnects, integrated photonic microdevices and biosensors. In this work, the design of binary and sinusoidal type coupling gratings for Al2O3/SiO2/Si submicron guides operating in the visible (633 nm) and infrared (1550 nm) is presented herein. In particular, the coupling efficiency is analyzed as a function of the main design parameters: waveguide thickness, period, etch depth and incidence angle. The results indicate that coupling efficiencies of 21 and 15 percent and decoupling efficiencies of 25 and 22 percent can be obtained for binary and sinusoidal gratings, respectively, at a wavelength of 1550 nm; coupling efficiencies of 7.8 and 7.6 percent and decoupling efficiencies of 53 and 34 percent can be obtained for a wavelength of 633 nm. The proposed designs have potential applications for the fabrication of integrated circuits.
 
 

Ultra-thin grating coupler for guided exciton-polaritons in WS2 multilayers

Abstract An ultra-thin transition metal dichalcogenide (TMDC) layer can support guided exciton-polariton modes due to the strong coupling between excitons and photons. Herein, we report the guided mode resonance in an ultra-thin TMDC grating structure. Owing to the strong exciton resonances in TMDCs, a TMDC grating structure shows guided-mode resonance even at a thickness limit of ∼10 nm and is capable of realizing polaritonic dispersion in a monolithic grating structure. We investigated the polarization and thickness dependence of the optical dispersion relations of the tungsten disulfide (WS2) grating structure. In addition, we confirmed that the monolithic WS2 grating coupler can be used to couple the near-field guided exciton-polariton out into the far field. We believe that ultra-thin TMDC layers can facilitate sub-wavelength nanophotonic applications.

Relaxed-tolerance subwavelength grating coupler

Short-wavelength mid-infrared (MIR) silicon photonics at 2–2.5 μm wavelengths has a wide range of applications in optical communications, chemical analysis, and environmental monitoring. In this spectral region, subwavelength grating (SWG) couplers can be fabricated by using commercial multi-project wafer (MPW) services, due to their larger device feature sizes compared with those in the telecommunication band. However, conventional SWG couplers are still sensitive to variations in dimensions produced by errors in fabrication processes, restricting the fabrication reproducibility of the devices. Here, we demonstrated a relaxed-tolerance SWG coupler that has relaxed fabrication tolerance to overcome this limitation. The approach is based on the design of the SWG coupler with dual-hole structures to reduce the influence of random fabrication variations on the grating effective refractive index. Theoretically, the slope of the center wavelength shift of the relaxed-tolerance SWG coupler to the variation of the subwavelength hole size is 0.58, while that is 0.63 for the conventional SWG coupler. We demonstrated the relaxed-tolerance SWG coupler with a peak coupling efficiency of −6.2 dB at a wavelength of 2.038 μm with a 1-dB bandwidth of about 30 nm. Additionally, fabrication reproducibility and optical fiber coupling tolerance are also presented. The study paves a promising way toward developing short-wavelength MIR grating couplers with high performance, excellent repeatability, and cost-efficiency.

Differential Sensing with Replicated Plasmonic Gratings Interrogated in the Optical Switch Configuration.

A new plasmonic configuration is proposed for application in a sensor and demonstrated for the detection of variations in the bulk refractive index of solutions. The configuration consists of monitoring two diffracted orders resulting from the interaction of a TM-polarized optical beam incident on a grating coupler, operating based on an effect termed the "optical switch". The two monitored diffracted orders enable differential measurements which cancel the drift and perturbations common to both, leading to an improved detection limit, as demonstrated experimentally. The measured switch pattern associated with the grating coupler is in good agreement with theory. Bulk sensing is demonstrated under intensity interrogation via the sequential injection of solutions comprised of glycerol in water into a fluidic cell. A limit of detection of about 10-6 RIU was achieved. The optical switch configuration is easy to implement and is cost-effective, yielding a highly promising approach for the sensing and the real-time detection of biological species.

Directional Amplified Photoluminescence through Large-Area Perovskite-Based Metasurfaces.

Perovskite nanocrystals are high-performance, solution-processed materials with a high photoluminescence quantum yield. Due to these exceptional properties, perovskites can serve as building blocks for metasurfaces and are of broad interest for photonic applications. Here, we use a simple grating configuration to direct and amplify the perovskite nanocrystals' original omnidirectional emission. Thus far, controlling these radiation properties was only possible over small areas and at a high expense, including the risks of material degradation. Using a soft lithographic printing process, we can now reliably structure perovskite nanocrystals from the organic solution into light-emitting metasurfaces with high contrast on a large area. We demonstrate the 13-fold amplified directional radiation with an angle-resolved Fourier spectroscopy, which is the highest observed amplification factor for the perovskite-based metasurfaces. Our self-assembly process allows for scalable fabrication of gratings with predefined periodicities and tunable optical properties. We further show the influence of solution concentration on structural geometry. By increasing the perovskite concentration 10-fold, we can produce waveguide structures with a grating coupler in one printing process. We analyze our approach with numerical modeling, considering the physiochemical properties to obtain the desired geometry. This strategy makes the tunable radiative properties of such perovskite-based metasurfaces usable for nonlinear light-emitting devices and directional light sources.

Optimization and Experiments of On-Chip Silicon Nitride Grating Couplers

Optimization and Experiments of On-Chip Silicon Nitride Grating Couplers

Photolithography Fabricated Sub-Decibel High-Efficiency Silicon Waveguide Grating Coupler

Using the optimized shift-pattern overlay method, we present a new high-efficiency silicon waveguide grating coupler operating at 1550 nm center wavelength, which has a simulation predicted coupling loss of 0.68 dB and experimentally measured loss of 0.89 dB. This record result was achieved without any substrate bottom mirrors or use of electron beam lithography and was fabricated with the standard processes of a multi-project wafer fabrication service from a commercial foundry.

Low-Loss Apodized Grating Couplers Based on Lithium Niobate on Insulator

Low-Loss Apodized Grating Couplers Based on Lithium Niobate on Insulator

Influence of Fiber Bragg Grating Coupling Mode on Sensing Characteristics of Unidirectional CFRP Plate

Influence of Fiber Bragg Grating Coupling Mode on Sensing Characteristics of Unidirectional CFRP Plate

Dual-wavelength DFB Laser Array Based on Sidewall Grating and Lateral Modulation of the Grating Coupling Coefficient

A monolithic dual-wavelength DFB laser array based on sidewall gratings and a novel modulation of the grating coupling coefficient is proposed and demonstrated experimentally. The grating coupling coefficient distribution along the cavity is modulated by changing the alignment between the gratings on the two sidewalls. The frequency difference between the two lasing modes can be modulated by changing the cavity length and grating recess depth. A series of microwave signals in the range of 50 GHz to 59 GHz is observed after beating the two optical lines in a photodetector. The measured optical linewidths are 250 kHz and 850 kHz when the cavity length is 1200 μm and 1000 μm, respectively.

长波量子阱红外探测器的不同光栅耦合机制研究

长波量子阱红外探测器的不同光栅耦合机制研究

Silicon photonic receiver for satellite laser communication terminals.

We report the optical performance of a photonic receiver for laser communication applications. The receiver is composed of 14 × 12 grating coupler arrays. The received optical signal power will be combined electrically via germanium photodiodes. The photonic receiver is designed for 20-µm to 30-µm mode field diameter (MFD) input sources. To maximize the fill factor of the 200 µm × 200 µm light-receiving area, a design strategy has been proposed. (1) Grating couplers are customized for compactness. (2) Periods of grating couplers are designed to work as end-fire and back-fire grating couplers for the same incident angle of the input laser source. (3) Different widths of waveguides are routed to minimize cross talk. The photonic receiver is evaluated with a 10-µm MFD source. As a result of the evaluation, the receiving area considering the minimum efficiency of -10.5 dB is 95% of the designed area when illuminating 20-µm to 300-µm MFD laser sources.

High‐performance binary blazed grating coupler with a mixed structure of germanium and zinc selenide

AbstractA high‐performance binary blazed grating coupler with a mixed germanium and zinc selenide structure on a Ge‐on‐Si platform is proposed for a perfectly vertical coupling. To obtain high coupling efficiency, grating parameters are repeatedly and carefully optimized. For the perfectly vertical coupler with transverse‐magnetic polarized incident light, the coupling efficiency is 49.5% at the wavelength of 8 μm with a 1‐dB bandwidth of 110 nm. Thus, the coupling efficiency‐bandwidth product is beyond 54.5 nm. Furthermore, the manufacturing route of the grating is discussed, which adopts the combination of electron‐beam lithography and three‐dimensional nanoprinting.

Novel High‐Resolution and Large‐Bandwidth Micro‐Spectrometer Using Multi‐Input Counter‐Propagating Arrayed Waveguide Grating and Dual‐Wavelength Grating Coupler on Silicon on Insulator

AbstractMiniaturized micro‐spectrometers with high‐resolution and large‐bandwidth are of great importance for on‐site spectroscopic analysis applications. Here, a novel high‐resolution and large‐bandwidth micro‐spectrometer is presented, which operates at dual‐wavelength band by utilizing a (N + 3) × (N + 3) arrayed waveguide grating (AWG) and three dual‐wavelength grating couplers on a silicon‐on‐insulator (SOI) platform. The AWG working in a counter‐propagating manner can be viewed as two 3 × N AWGs, and the dual‐wavelength grating coupler acts as a prefilter. Three time‐division‐multiplexing inputs with wavelength spacings of 4/3 times that of N output waveguides are employed to improve the resolution of the micro‐spectrometer. Combining the corresponding outputs of three inputs, a micro‐spectrometer with resolution enhanced to one‐third wavelength spacing of output waveguides and effective channel number enlarged to 3 × (N − 2) is achieved, only using N output channels at each wavelength band. A micro‐spectrometer with 63 effective channels and resolutions of 0.5 and 0.442 nm at 1550 and 1310 nm bands, respectively, is experimentally demonstrated. The footprint of the fabricated micro‐spectrometer is 2 × 0.81 mm2, with measured on‐chip losses of −4.6 to −2.8 dB (−7.0 to −3.1 dB) and crosstalk levels < −18.5 (−16.8) dB at the 1550 (1310) nm band.

Mid-infrared polarization-insensitive grating coupler.

Mid-infrared (Mid-IR) (2-20 µm) silicon photonics has attracted much attention in the past few years due to its application potential in free-space optical communications, light detection and ranging, and molecular analysis. The grating coupler technology is one of the most widely employed approaches for light coupling between optical fibers and waveguides. In the mid-IR spectral region, due to the lack of reliable chalcogenide-fiber or ZBLAN-fiber polarization controllers, grating couplers usually suffer from huge insertion losses induced by the arbitrary polarization states of light coupled out of mid-IR fibers. As a result, it is significant to explore polarization-insensitive grating coupling techniques in mid-IR wavelengths. However, the study is currently still in its infancy. Here, we demonstrate an ultra-thin mid-IR polarization-insensitive grating coupler. The grating coupler has a maximum coupling efficiency of -11.5 dB at a center wavelength of ∼2200 nm with a 1-dB bandwidth of ∼148 nm. Compared with conventional subwavelength grating couplers, the polarization-dependent loss was improved from 9.6 dB to 2.1 dB. Moreover, we demonstrated a polarization-insensitive grating coupler at 2700-nm wavelength with a maximum coupling efficiency of -12.0 dB. Our results pave the way for the development of mid-IR photonic integrated circuits.