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°.     

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.     

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.     

SOI基纳米光栅耦合器的结构改进与仿真验证

针对硅基光波导与单模光纤在光耦合过程中存在耦合效率低,对准难度大的问题,提出了一种新型基于绝缘体上硅(SOI)结构的纳米光栅耦合器。运用有限时域差分法探究了纳米光栅耦合器的结构特性,结合光纤准直器的准直原理,系统分析了纳米光栅耦合器的结构模型。研究了入射光波长、入射角度、光栅占空比等因素对光栅耦合效率的影响,得出了基本纳米光栅耦合器的最佳结构参数;在基本耦合光栅结构的基础上进行了改进,增加了后反射器、增透膜和底层介质膜,并运用Optiwave OptiFDTD 8.0软件优化了各部分结构的设计参数,得到了优化的耦合光栅结构。纳米光栅耦合器的耦合效率提高至59.37%,3dB带宽达到65nm。     

A novel highly efficient grating coupler with large filling factor used for optoelectronic integration

A novel highly efficient grating coupler with large filling factor and deep etching is proposed in silicon-on-insulator for near vertical coupling between the rib waveguide and optical fibre.The deep slots acting as high efficient scattering centres are analysed and optimized.As high as 60% coupling efficiency at telecom wavelength of 1550-nm and 3-dB bandwidth of 61 nm are predicted by simulation.A peak coupling efficiency of 42.1% at wavelength 1546-nm and 3-dB bandwidth of 37.6 nm are obtained experimentally.     

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.     

Compact and high-efficient wavelength demultiplexing coupler based on high-index dielectric nanoantennas

Wavelength demultiplexing waveguide couplers have important applications in integrated nanophotonic devices. Two of the most important indicators of the quality of a wavelength demultiplexing coupler are coupling efficiency and splitting ratio. In this study, we utilize two asymmetric high-index dielectric nanoantennas directly positioned on top of a silicon-on insulator waveguide to realize a compact wavelength demultiplexing coupler in a communication band, which is based on the interference of the waveguide modes coupled by the two nanoantennas. We add a Au substrate for further increasing the coupling efficiency. This has constructive and destructive influences on the antenna's in-coupling efficiency owing to the Fabry-Perot(FP) resonance in the SiO_2 layer. Therefore, we can realize a wavelength demultiplexing coupler with compact size and high coupling efficiency. This coupler has widespread applications in the areas of wavelength filters,on-chip signal processing, and integrated nanophotonic circuits.     

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。     

Apodized grating coupler using fully-etched nanostructures

A two-dimensional apodized grating coupler for interfacing between single-mode fiber and photonic circuit is demonstrated in order to bridge the mode gap between the grating coupler and optical fiber. The grating grooves of the grating couplers are realized by columns of fully etched nanostructures, which are utilized to digitally tailor the effective refractive index of each groove in order to obtain the Gaussian-like output diffractive mode and then enhance the coupling efficiency.Compared with that of the uniform grating coupler, the coupling efficiency of the apodized grating coupler is increased by 4.3% and 5.7%, respectively, for the nanoholes and nanorectangles as refractive index tunes layer.     

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。     

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.     

Polarization beam splitter using a binary blazed grating coupler

A novel polarization beam splitter using a two-layer grating coupler is proposed and demonstrated. It can directly couple the normally incident light from fiber into two separate waveguides according to their polarization states while splitting them. It realizes high coupling efficiency and a good extinction ratio by using binary blazed grating couplers. The coupling length is less than 14 microm. The extinction ratio is better than 20 dB for both polarizations over a 40 nm wavelength range, and the coupling efficiencies for the two layers are 58% and 50%.     

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.     

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.     

Focusing subwavelength grating coupler for mid-infrared suspended membrane waveguide

A mid-infrared (mid-IR)-focusing subwavelength grating (SWG) coupler and suspended membrane waveguide (SMW) on a silicon-on-insulator wafer are studied. For a transverse-electric mode uniform SWG, finite-difference time-domain simulation predicts 44.2% coupling efficiency with 1 dB bandwidth of about 220 nm and backreflection of 0.78% at 2.75 μm. Then the uniform SWG is curved to a focusing SWG using a phase-matching formula. The SMWs are analyzed by the finite element method and fabricated. An Er3+-Pr3+ co-doped mid-IR fiber laser is used for device characterization. The fabricated mid-IR SWG coupler has 24.7% coupling efficiency.     

用于光纤光栅解调的波长敏感光纤耦合器

为了拓宽光纤耦合器的使用范围,开发光纤耦合器的新功能,采用熔锥技术制作波长敏感耦合器,该耦合器在分光的同时对波长敏感。通过耦合理论验证实验结果,实验数据与理论值相符合。实验中得到波长灵敏度最大值为17.86%/nm的耦合器。采用拉锥工艺制作波长敏感耦合器工艺简单,耦合比峰值对应波长控制易于实现。该耦合器可用于光纤光栅布拉格波长漂移解调。令待解调光纤光栅布拉格波长与耦合器波长灵敏度最大值对应的波长一致,当波长发生漂移时,耦合器输出耦合比发生变化。自制的波长敏感耦合器实现了对布拉格波长为1566.71 nm光纤光栅波长漂移的解调,波长漂移1.80 nm,耦合比变化20.34%。此种解调方式具有光路简单,易于与光纤匹配的优点,可以应用在大型建筑中光纤光栅的健康监测。     

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.     

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.     

300 W侧面分布式抽运掺Yb全光纤放大器

采用角度磨抛的方式, 在纤芯/包层为20/400 μm双包层掺Yb光纤上制作了侧面抽运耦合器. 该耦合器对975 nm的半导体二极管抽运光的耦合效率最高可达97%, 对1080 nm信号光的泄漏比小于2%. 分析了侧面抽运耦合器的性能以及多个侧面抽运耦合器的级联分布对抽运耦合效率的影响; 同时, 在前向抽运和双向抽运方式下, 分析了级联耦合器的分布及信号光泄漏比对激光器整体效率的影响, 并进行了数值模拟. 采用自行研制的侧面抽运耦合器, 搭建了侧面耦合分布式抽运、掺Yb双包层全光纤主振荡功率放大器, 获得了波长为1080 nm、功率为303 W 的基模激光输出. 进一步增加抽运点个数, 提高抽运功率, 可获得更高的输出功率.     

High efficiency grating coupler between silicon-on-insulator waveguides and perfectly vertical optical fibers

A high-efficiency waveguide-to-fiber grating coupler for silicon-on-insulator waveguides was designed. Perfectly vertical fiber coupling is achieved by using an asymmetric grating structure to suppress the second-order Bragg reflection from the grating. The ability to use a perfectly vertical positioned optical fiber simplifies the packaging of the photonic integrated circuit. A coupling efficiency of 80% at a wavelength of 1.55 microm is obtained.