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 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.     

Design and analysis of a vertical directional coupler between a three-dimensional plasmonic slot waveguide and a silicon waveguide

The design of a vertical directional coupler between a three-dimensional plasmonic slot waveguide and a silicon waveguide is theoretically investigated in detail. It consists of two steps: the design of isolated plasmonic slot waveguide and silicon waveguide and the determination of the gap between the two waveguides and the length of a coupling region. The designed structure transfers 70.8% of the power carried by the silicon waveguide mode to the plasmonic slot waveguide mode when the gap is 150 nm and the coupling length is 2.14 μm. The wavelength dependence of our vertical directional coupler is also studied. The analysis shows that the amount of the transferred power changes slightly over a very wide wavelength range between 1.40 μm and 1.61 μm. Moreover, if we employ the fabrication technology for silicon photonics, it is quite tolerant to the variation of the length of its coupling section. Finally, the vertical directional coupler is considered for a polarizer.     

Stratified waveguide grating coupler for normal fiber incidence

We propose a new stratified waveguide grating coupler (SWGC) to couple light from a fiber at normal incidence into a planar waveguide. SWGCs are designed to operate in the strong coupling regime without intermediate optics between the fiber and the waveguide. Two-dimensional finite-difference time-domain simulation in conjunction with microgenetic algorithm optimization shows that approximately 72% coupling efficiency is possible for fiber (core size of 8.3 microm and delta=0.36%) to slab waveguide (1.2-microm core and delta=3.1%) coupling. We show that the phase-matching and Bragg conditions are simultaneously satisfied through the fundamental leaky mode.     

掺半导体玻璃波导光栅耦合高速全光开关

非线性光学器件的主要应用目的之一是全光信号处理,非线性光波导象其它非线性光学器件一样,也已被广泛地应用于全光信号处理方面的研究,如人们已采用棱镜波导非线性分布耦合方式来实现全光限幅、全光开关和全光双稳等研究[1,2].在非线性光波导器件中,由于光被束缚在微米尺寸的光波导中传输,而大大增加了光功率密度,因此,在光与非线性光波导的相互作用过程中可以大幅度地降低非线性效应的阈值功率,提高全光信号处理的灵敏度.此外,非线性光波导器件的小型化、集成化等优点更吸引人们去进行深入的研究.     

BPM Simulation and Comparison of 1 × 2 Directional Waveguide Coupling and Y-Junction Coupling Silicon-on-Insulator Optical Couplers

For developing large area opto-electronic silicon-on-insulator (SOI) devices, the optical coupler is a basic key device. In this article, the authors design and simulate 1 2 2 directional waveguide coupling and Y-branch coupling optical couplers based on Unibond SOI rib waveguides. The beam propagation method (BPM) is used for light propagation analysis. The simulation results and comparisons of the two kinds of optical couplers are reported. The S-bend waveguide for attaching to the two kinds of SOI optical coupler is also analyzed by BPM. We find that the directional coupler has lower power loss, but the Y-junction coupler is more wavelength insensitive with the same device size and splitting angle. The fabrication tolerance analysis shows Y-junction coupler has better fabrication characteristics.     

Efficient fiber-to-waveguide coupling through the vertical leakage from a microring

A fiber-to-waveguide coupler is proposed to efficiently couple light from a single-mode fiber into a submicrometer semiconductor waveguide for integration with optoelectronic circuits. A microring with a specific refractive index is designed on the top of the semiconductor waveguide. The gradual vertical leakage from the microring forms steady coupling into the semiconductor waveguide. Coupling efficiency up to 93% is demonstrated using the three-dimensional finite-difference time-domain method. A tapered-waveguide or microring structure can be used to convert the lateral-mode size for coupling light into a single-mode semiconductor waveguide.     

Total internal reflection-evanescent coupler for fiber-to-waveguide integration of planar optoelectric devices

We present a method for parallel coupling from a single-mode fiber, or fiber ribbon, into a silicon-on-insulator waveguide for integration with silicon optoelectronic circuits. The coupler incorporates the advantages of the vertically tapered waveguides and prism couplers, yet offers the flexibility of planar integration. The coupler can be fabricated by use of either wafer polishing technology or gray-scale photolithography. When optimal coupling is achieved in our experimental setup, the coupler can be packaged by epoxy bonding to form a fiber-waveguide parallel coupler or connector. Two-dimensional electromagnetic calculation predicts a coupling efficiency of 77% (- 1.14-dB insertion loss) for a silicon-to-silicon coupler with a uniform tunnel layer. The coupling efficiency is experimentally achieved to be 46% (-3.4-dB insertion loss), excluding the loss in silicon and the reflections from the input surface and the output facet.     

Optimization of polishing parameters for optical coupler based on side-polished photonic crystal fiber

The rapid development in areas such as fiber-optic communication, fiber lasers, and high power transmission has generated an increasing demand for photonic crystal fiber (PCF). However, there is no well-proven approach for PCF coupler fabrication because most approaches proposed introduce defects that are non-negligible in practice. Previously, we have made an attempt by applying side-adhering technique to PCF coupler fabrication. This paper presents our recent effort in establishing a model of side-polished PCF (SPPCF) coupler and tuning various fabrication parameters in search for its desirable characteristics. It is found that the optimal coupling effect is achieved when the residual radius is set 5.7 μm and the polishing angle is set 0°. The optimizations presented in this paper may serve as a foundation for further research and development of SPPCF coupler.     

基于LiNbO_3的长周期波导光栅可调谐耦合器的设计

设计了一种基于LiNbO_3的长周期波导光栅可调谐耦合器.该耦合器利用长周期光栅的独有特性将输入波导的导模经包层模耦合至输出波导导模.由于LiNbO_3的电光效应,波导光栅芯层与包层的有效折射率随外加电压变化,从而耦合器的谐振波长及耦合效率可由外加电压调谐.分析了光栅周期与耦合器的长度对耦合器带宽和耦合效率调谐范围的影响,以及波导尺寸对谐振波长调谐灵敏度的影响.结果表明光栅周期越短,耦合器长度越长,则耦合器的带宽越窄,耦合效率调谐范围也越大.此外,谐振波长调谐灵敏度随波导宽度的增加而减小,而波导厚度对谐振波长调谐灵敏度的影响可以忽略.对光栅周期为94μm、长度为3.52cm的耦合器进行仿真,结果表明,谐振波长灵敏度为26.2pm/V,3dB带宽可达4.5nm,当外加电压从0变化到200V时,谐振波长变化5.24nm,耦合效率可在1到0.15之间进行调谐.     

Compact efficient broadband grating coupler for silicon-on-insulator waveguides

We have designed a high-efficiency broadband grating coupler for coupling between silicon-on-insulator (SOI) waveguides and optical fibers. The grating is only 13 microm long and 12 microm wide, and the size of the grooves is optimized numerically. For TE polarization the coupling loss to single-mode fiber is below 1 dB over a 35-nm wavelength range when using SOI with a two-pair bottom reflector. The tolerances to fabrication errors are also calculated.     

基于光子集成芯片的可穿戴光纤光栅解调研究

为了实现光纤光栅传感器在可穿戴系统中的应用,提出了一种基于硅基光子集成芯片的可穿戴光纤光栅传感解调系统。基于比利时iSiPP50G工艺的光子集成芯片由4×1长波长VCSEL阵列、1×8阵列波导光栅、2×2 MMI耦合器、4×1光纤光栅耦合器阵列、Ge-on-Si波导光电探测器、直波导和弯曲波导等组成。在完成对VCSEL光源金线键合和光子集成芯片光纤耦合封装的基础上,设计了手环式解调电路,对人体温度和心音信号进行了实时测量。实验结果表明:解调系统的动态波长检测范围为1 540 nm～1 560 nm,波长分辨率为0.08 pm,解调精度为5 pm,温度监测范围为35℃～42℃,误差为±0.1℃;可检测50 Hz～100 Hz频率范围内的心音信号,可识别出第一心音和第二心音,并计算出心动周期、心率、第一心音时限、第二心音时限和心力等特征参数。     

Coupling and crosstalk characteristics of hybrid silicon plasmonic waveguides

In this paper, the design and analysis of an ultracompact coupler based on a hybrid silicon plasmonic waveguide (HSPW) is proposed and its coupling and crosstalk characteristics have been theoretically investigated for the development of optical interconnects that can be realized using well-established complementary metal-oxide-semiconductor-compatible fabrication techniques. To determine the minimum horizontal separation distance and efficient coupling length for the designed coupler, the symmetric and antisymmetric supermodes are obtained and their characteristics are studied at a wavelength of 1.55 μm. Efficient light coupling is exhibited by the HSPW coupler with 75 % of power transfer between the two HSPWs with ultrashort coupling length of 2.14 μm when the separation distance is 50 nm. Further, it is shown that the crosstalk is significantly reduced with the insertion of metallic strip between the two HSPWs for realizing highly dense integrated plasmonic circuits.     

Vertical coupling characteristics between hybrid plasmonic slot waveguide and Si waveguide

For development of complementary metal–oxide–semiconductor (CMOS)-compatible integrated optical circuits, vertical directional coupling between a hybrid plasmonic slot waveguide and a Si waveguide is theoretically investigated in detail. To determine the vertical separation gap and efficient coupling length, we investigate the characteristics of the even and odd supermodes at a wavelength of 1.55 μm. The vertical coupler transfers 90% of the power carried by the Si waveguide to the hybrid plasmonic slot waveguide after normalizing to reference waveguides when the gap is 60 nm and the coupling length is 2.6 μm. Because of the lossy hybrid guided mode in the plasmonic waveguide, the transmitted power exhibits damped sinusoidal behavior depending on the overlapping length. The proposed vertical coupler shows more efficient light coupling between a dielectric and plasmonic waveguide in comparison to the other types of hybrid coupler, and can be exploited further for on-chip integrated opto-electronic circuits.     

Compact multimode interference coupler with tapered waveguide geometry

In this paper, a novel MMI coupler, based on general interference, with tapered waveguide geometry has been proposed for reduction of coupling length. The coupling characteristics and power imbalance of the proposed structure are compared with conventional MMI structures by using a mathematical model based on sinusoidal modes. It is seen that the beat length for tapered MMI coupler with angle of taper ∼1.05° is reduced by ∼24% of that of conventional MMI coupler and the coupling characteristics obtained with the mathematical model, match well with those obtained by more sophisticated BPM computer aided design software. The power imbalance for tapered 3 dB MMI coupler is more sensitive to the wavelength than that for conventional 3 dB MMI coupler and variation of power imbalance with fabrication tolerance for both the MMI coupler is almost same.     

High-efficiency optical coupling to planar photodiode using metal reflector loaded waveguide grating coupler

For the realization of optoelectronic integrated circuits, it is required to incident light perpendicularly to a planar Si photodiode. We propose a high-efficient vertical optical coupler using an amorphous Si optical waveguide grating coupler with top reflector, which is transparent at 850 nm wavelength range. The optical waveguide (width of 300 nm $\times $ height of 100 nm) coupler is analyzed by using finite element method. The coupling efficiency of 80 % is calculated at the grating period of 380 nm, the duty ratio of 0.75 and the depth of 35–65 nm with top metal reflector.     

Effective index method for planar lightwave circuits containing directional couplers

A simple and intuitive effective index method for planar lightwave circuits containing directional couplers is proposed. The optimal cladding refractive index of the effective two-dimensional planar waveguide is found according to the coupling length of the directional coupler involved. Numerical examples indicate that this proposed effective index method gives accurate results while saving significant computation time in simulation and design of planar lightwave circuits containing directional couplers.     

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。