Grating couplers for thick SOI rib waveguides

The use of grating couplers in high index contrast waveguides like silicon on insulator (SOI) offers several advantages over other coupling approaches, including better alignment tolerances and allowing for wafer-scale testing. The grating couplers were developed for nanometric SOI waveguides (Si-wires), and recently also for micrometric rib waveguides. In this paper we review our work in fiber-to-chip grating couplers for thick SOI rib waveguides, where a coupling efficiency of ?2.2?dB was demonstrated experimentally. We also discuss the use of grating couplers to improve optical throughput (étendue) of a planar waveguide Fourier-Transform (FT) spectrometer implemented in thick rib waveguides.     

硅基光栅耦合器的研究进展

硅基光子集成芯片的研究近年来发展迅速, 已成为信息技术领域中最热门的研究方向之一, 光通信、光互连、光传感等相关研发应用机构高度关注其发展, 并积极介入. 硅基光子集成芯片中, 光栅耦合器作为光信号的输入和输出装置受到极大重视, 尤其在封装和测试等环节体现出极具价值的技术优势. 本文主要分析了光栅耦合器的工作原理、基本特性及国内外的发展现状和趋势, 同时也概括了本课题组近期在该方向上的研究成果. 关键词： 光栅耦合器 耦合效率 绝缘体上硅 光子集成     

Apodized fully-etched surface grating coupler using subwavelength structure for standard silicon-on-insulator waveguide

We design and demonstrate the fully-etched apodized grating couplers based on the silicon-on-insulator (SOI) platform using subwavelength structure for both transverse electric (TE) and transverse magnetic (TM) modes operation. The subwavelength grating (SWG) is used to engineer the refractive index using second-order effective medium theory (EMT). The whole designing procedure is given in details, especially a feasible and programmable method is developed to precisely manipulate the coupling strength of each grating cycle. A perfect Gaussian output beam can be synthesized for the TE mode operation, achieving a field overlap up to 98.3% with the Gaussian fiber mode. The simulated peak coupling losses are ?4.63 and ?2.99 dB for the TE mode and the TM mode, respectively, which are comparable with conventional shallowly etched grating couplers, realizing a fabrication simplification without performance penalty. The measured peak coupling loss is ?7.6 dB for the TE mode coupler with a 1 dB bandwidth of 45 nm, and ?6.1 dB for the TM mode coupler with a 1 dB bandwidth of 34 nm.     

High‐efficiency single etch step apodized surface grating coupler using subwavelength structure

Grating couplers are key elements enabling the coupling of light between planar waveguide circuits and optical fibers. In this work, it is demonstrated using simulations and experiments that a high coupling efficiency can be achieved for an arbitrary buried oxide thickness by judicious adjustment of the grating radiation angle. The coupler strength is engineered by subwavelength structure, allowing straightforward apodization and single etch step fabrication. The design has been implemented using Fourier‐eigenmode expansion and finite difference time domain methods. The measured coupling loss of a continuously apodized grating is −2.16 dB with a 3 dB bandwidth of 64 nm, therefore opening promising prospects for low‐cost and high‐volume fabrication using 193 nm deep‐ultraviolet lithography. It is also shown by simulations that a coupling loss as low as −0.42 dB is predicted for a modified coupler structure with bottom mirror.     

Design for broadband high-efficiency grating couplers

In this Letter, we propose general optimization methods to design broadband high-efficiency grating couplers for planar waveguides. We attribute the coupling bandwidth to the mismatch of effective indices between the diffracted beam and the actual grating structure around the operation wavelength for fiber to waveguide excitation. The coupling bandwidth formula is deduced. A simple parameter-separate optimization procedure is proposed for general layered grating couplers for high coupling efficiency. Using our principle, we optimized a grating coupler for a horizontal slot waveguide operating at wavelength 1.55 μm for TM polarization. The grating coupler has 1 dB bandwidth of 60 nm and coupling efficiency of 65% with incident light from single-mode optical fiber (SMF) at 8°.     

High efficiency silicon nitride grating coupler

In this paper, we have designed, fabricated and characterized silicon nitride grating couplers with high efficiency at 1490 nm. The devices are fabricated using deep UV photolithography with resolution requirement of ～500 nm. The grating coupler fabricated yields a peak coupling efficiency of ?5.1 dB. The 1-dB bandwidth of the grating coupler is 60 nm.     

Polarization-independent grating coupler for micrometric silicon rib waveguides

Grating couplers are a promising approach to implement efficient fiber-chip coupling. However, their strong polarization dependence makes dual-polarization operation challenging. In this Letter we propose, for the first time, a polarization-independent grating coupler for thick rib silicon-on-insulator (SOI) waveguides. Coupling efficiency is optimized by designing the grating pitch and duty cycle, without varying the bottom oxide thickness, which significantly simplifies practical implementation. Directivity of the grating coupler is enhanced by a high reflectivity layer under the bottom oxide after the selective removal of the Si substrate. Dual-polarization coupling efficiency of -2.8 dB is shown.     

High efficiency grating couplers based on shared process with CMOS MOSFETs

Grating couplers are widely investigated as coupling interfaces between silicon-on-insulator waveguides and optical fibers.In this work,a high-efficiency and complementary metal-oxide-semiconductor(CMOS) process compatible grating coupler is proposed.The poly-Si layer used as a gate in the CMOS metal-oxide-semiconductor field effect transistor(MOSFET) is combined with a normal fully etched grating coupler,which greatly enhances its coupling efficiency.With optimal structure parameters,a coupling efficiency can reach as high as ～ 70% at a wavelength of 1550 nm as indicated by simulation.From the angle of fabrication,all masks and etching steps are shared between MOSFETs and grating couplers,thereby making the high performance grating couplers easily integrated with CMOS circuits.Fabrication errors such as alignment shift are also simulated,showing that the device is quite tolerant in fabrication.     

Novel grating couplers for diode-bars multi-point side-pumping double-clad fiber

Three grating couplers for diode-bars single-point and multi-point side-pumping double-clad fiber (DCF) are presented. The principles, characteristics and coupling efficiencies of these grating couplers are described and computed by a rigorous electromagnetic theory. We show computationally that the optimized gratings can couple the high-power TE- or TM-polarized pump lights with 94.7% and 79.7% into the DCF, respectively. For an un-polarized pump light, 72.4% efficiency can be reached. We also have analyzed the leakage of pump powers for diode-bars multi-point side-pumping DCF. Besides, the fabrication tolerances and other performances of the grating couplers are analyzed.     

Simulations of nanograting-assisted light coupling in GaN planar waveguide

The numerical simulations of nanogratings integrated with gallium nitride (GaN) planar waveguides as well as the experimental in-coupling results are presented. A simulation tool based on the eigenmode expansion method and advanced boundary conditions provided a rigorous model of 400-nm-period grating couplers. A full-vectorial Maxwell solver allowed performing a number of simulations with varying grating parameters, where coupling efficiency, reflection and transmission characteristics of device were calculated. Gratings with different etch depths and arbitrary shapes were simulated using a staircase approximation, with an optimized number of steps per single slope. For the first time, an impact of dry etch processing on GaN coupler efficiency was evaluated, due to the inclusion of the sloped sidewalls, with regard to the technological constrains. Finally, the experimental results in the visible spectrum region (?? = 633 nm), for 400-nm-deep gratings etched in GaN waveguide, were presented together with theoretical data for binary and trapezoidal profiles of a grating, for different optical mode profiles ( ${{\rm TE}_{0}\div {\rm TE}_{3}\,{\rm modes}}$ ).     

光纤耦合器的理论、设计及进展

系统总结了光纤耦合器的发展历程,归纳提炼出各个阶段的标志性事件;详细阐述了光纤耦合器的耦合类型、制作方法、性能参数;详细评述了光纤耦合器的理论分析方法;全面分析了X型、星型、光栅型、混合型等各种典型光纤耦合器的基本结构、工作原理及耦合特性;指出并展望了光纤耦合器的发展方向和应用前景。作者率先提出并设计了超长周期光纤光栅耦合器,实验上实现了两个超长周期光纤光栅之间的有效耦合。     

光纤耦合器的理论、设计及进展

系统总结了光纤耦合器的发展历程,归纳提炼出各个阶段的标志性事件;详细阐述了光纤耦合器的耦合类型、制作方法、性能参数;详细评述了光纤耦合器的理论分析方法;全面分析了X型、星型、光栅型、混合型等各种典型光纤耦合器的基本结构、工作原理及耦合特性;指出并展望了光纤耦合器的发展方向和应用前景。作者率先提出并设计了超长周期光纤光栅耦合器,实验上实现了两个超长周期光纤光栅之间的有效耦合。     

Ring fiber resonators based on fused-fiber grating add-drop filters:application to resonator coupling

We introduce a fiber ring optical resonator based on adiabatic fused-fiber grating couplers. The coupling of a through fiber to the resonator is controlled by the strength of the fiber Bragg gratings. By using two of these couplers and incorporating erbium-doped (ED) fiber in the ring, we control the internal loss of the ring by pumping the ED fiber. The transmission spectra of the through port and the drop port of a four-port configuration, a ring coupled to two waveguides, are measured. We show that the loss/coupling ratio of the ring-fiber system can be changed and thus that the transmission properties of the fiber can be controlled.     

A Novel Design of Grating Couplers for Efficient Broadband Coupling between Silicon-on-Insulator Nanophotonic Waveguides and Fibres

A novel design of out-of-plane grating couplers is proposed for coupling between silicon-on-insulator nanophotonic waveguides and single-mode fibres. The coupler with the first-order diffraction coupling to the optical fibre is actually a second-order reflected grating with two times of period of the first-order grating. To enhance outcoupled power, a back hole is designed to form in the silicon substrate and a kind of metals is placed on the top acting as a reflection layer. The coupler is optimized using coupled-mode-based simulations, showing that the coupling efficiency to and from tapered optical fibre can be as high as 85% with 1dB bandwidth about 23 nm.     

Characteristics of Fused Optical Fiber Grating Couplers with Tapered Shapes

Fused optical fiber grating couplers (FGCs), in which Bragg gratings are written in the coupling regions of fused optical fiber couplers with tapered shapes along them are theoretically and experimentally investigated. By coupled mode analysis considering these tapered shapes, the drop efficiencies of the FGCs, which are inherently zero for the normal fiber couplers without grating, are calculated. The grating offset, which is the difference between the centers of grating and the coupler taper, is found to be effective to increase the drop efficiency of the FGC. It is determined that the tapered shapes influence the wavelength dependence of the drop efficiency and tolerate the grating offset precision. Apodizations for the grating of FGC are found to be effective in suppressing side lobes of the drop efficiency. Measurement of the wavelength characteristics of drop efficiency and the temperature characteristics of drop wavelength of the fabricated fused FGC confirm that the lightwave with specific wavelength can be dropped by the FGC and that the temperature dependence of the drop wavelength is similar to that of Bragg wavelength changes of usual fiber Bragg gratings.     

Fiber grating couplers for silicon nanophotonic circuits: Design modeling methodology and fabrication tolerances

Silicon nanophotonic circuits can exhibit a very high level of functional integration due to the very small cross sections of the silicon waveguides. However, to be implemented in data transmission networks, such circuits still must be interfaced with optical fibers having much larger dimensions. Due to this mismatch in size, a coupling structure is required in order to minimize the coupling loss. Diffraction grating coupler structures are one of the best candidates to perform this mode size conversion with good performances. However, they are also very sensitive to fabrication tolerances that may require an adaptation of the coupling conditions. In this paper, we present an iterative numerical method to optimize the design of a grating coupler by analyzing the out coupled beam from the waveguide towards the fiber. Using this method we show in details the sensitivity of the grating couplers to the principal fabrication variabilities in order to maximize the robustness of the design. A grating with 53% fiber to waveguide coupling efficiency is designed. Considering the dispersion of the modern CMOS fabrication processing, it appears that the optimal fiber coupling ratio remains rather constant but the optimal coupling angle at a given wavelength may vary by as much as ±10°.     

Grating coupler efficiency enhancement by double layer interference

A simple method for improving grating couplers' coupling efficiency without any extra microfabrication processes is proposed. This method can improve the coupling efficiency with 1.69 dB by utilizing the combined interference in the cladding layer and air gap between the cladding surface and the paralleled angle polished fiber facet. The proposed method can be applied to various kinds of on-chip grating couplers. Back reflection,1 dB bandwidth, and fiber alignment tolerance have also been improved at the same time.     

Design method for high performance grating couplers in photonic integrated circuits

The optimization of grating couplers is usually realized by multiple simulations using specific computational software for this task. Many grating parameters must be analyzed and designed to get the maximum coupling efficiency and the transmission spectrum centred at the wavelength of operation. However, these simulations may take a long time and consume high computational resources depending on the simulation resolution. This work is focused on finding a method to optimize the grating parameters with the lowest number of simulations. In this way, closed-form expressions are presented to get the optimal values for the period and fill-factor, which are the main parameters in the grating design. The usefulness of the proposed approach is shown for the design of silicon grating couplers operating at 1.31 µm and 1.55 µm and both TE and TM polarizations.     

Fourier-modal methods applied to waveguide computational problems

Rigorous coupled-wave analysis (also called the Fourier-modal method) is an efficient tool for the numerical analysis of grating diffraction problems. We show that, with only a few modifications, this method can be used efficiently for the numerical analysis of aperiodic diffraction problems, including photonic crystal waveguides, Bragg mirrors, and grating couplers. We thus extend the domain of applications of grating theories.     

Imaging errors in arrayed waveguide gratings

A detailed assessment of imaging errors in arrayed waveguide gratings (AWG) is presented, focussing on possible design related imaging errors as well as on typical imaging errors mediated by the fabrication process. From a design point of view, special interest is drawn to the effect of coupling within the array of grating waveguides and to the impact of the paraxial approximation for the star couplers which is commonly used in the design of AWGs. Both become important design issues affecting the performance of AWG devices when moving to high numbers of narrowly spaced channels. For the technology related imaging errors we focus on one side on the impact of almost uncorrelated phase errors at high spatial frequencies. They can typically be attributed to fluctuations in the index profile and waveguide dimensions in the grating region. On the other side correlated long range phase errors are treated which occur in the grating region as well as within the star couplers and which are typical for stress induced index changes in silica-on-silicon waveguides. It is shown that only modelling tools which include all types of the above imaging effects can provide reliable boundary conditions for a comprehensive design of high end AWG concerning spectral shape and chromatic dispersion.