基于达曼光栅的动态光耦合器

提出了一种基于达曼光栅的动态光耦合器,通过控制装置中达曼光栅位移参量,可实现入射光束的分束或合束以及两者之间的动态转换。适当选择达曼光栅类型可实现任意N×M的动态光耦合。实验中以1550 nm光波长为例,对1×8达曼动态光耦合器进行测量,测得其实现光开关功能时插入损耗为0.43 dB,实现光分束功能时均匀性达到0.03,单路插入损耗均值为10.5 dB。该实验装置易于调节、体积小、能耗低,且关键元件达曼光栅制作工艺成熟,易于批量化生产。特别是在实现中大规模光交换阵列时,该方案就具有更明显的优越性,有实用意义。     

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 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.     

CMOS compatible highly efficient grating couplers with a stair-step blaze profile

A novel grating coupler with a stair-step blaze profile is proposed.The coupler is a CMOS process compatible device and can be used for light coupling in optical communication.The blaze profile can be optimized to obtain a high efficiency of 66.7% for the out-of-plane coupling at the centre wavelength of 1595 nm with a 1 dB bandwidth of 41 nm.Five key parameters of the stair-step blaze grating and their effects on the coupling are discussed for the application in L band telecommunication.     

An ultra‐compact multimode interference coupler with a subwavelength grating slot

Multimode interference couplers (MMIs) are fundamental building blocks in photonic integrated circuits. Here it is experimentally demonstrated, for the first time, a two‐fold length reduction in an MMI coupler without any penalty on device performance. The design is based on a slotted 2 × 2 MMI fabricated on a commercial silicon‐on‐insulator (SOI) substrate. The slot is implemented with a subwavelength grating (SWG) comprising holes fully etched down to the oxide cladding, thereby allowing single etch step fabrication. The device has been designed using an in‐house tool based on the Fourier Eigenmode Expansion Method. It has a footprint of only 3.5 μm x 23 μm and it exhibits a measured extinction ratio better than 15 dB within the full C‐band (1530 nm‒1570 nm). SWG engineered slots thus offer excellent perspectives for the practical realization of MMIs couplers with substantially reduced footprint yet with outstanding performance.     

Subwavelength grating Fourier‐transform interferometer array in silicon‐on‐insulator

A planar waveguide Fourier‐transform spectrometer with densely arrayed Mach‐Zehnder interferometers is demonstrated. Subwavelength gratings are used to produce an optical path difference without waveguide bends. The fabricated device comprises of an array of 32 Mach‐Zehnder interferometers, which produce a spatial interferogram without any moving parts, yielding a spectral resolution of 50 pm and a free‐spectral range of 0.78 nm. As a result of similar propagation losses in subwavelength grating waveguides and conventional strip waveguides, loss imbalance is minimized and high interferometic extinction ratio of −25 to −30 dB is obtained. Furthermore, phase and amplitude errors arising from normal fabrication variation are compensated by the spectral retrieval process using calibration measurements.     

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.     

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 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.     

Design and fabrication of planar blazed grating for LC rear-projection display system

Design and fabrication of planar blazed grating for single-panel LC rear-projection display system are presented. The structure of planar blazed grating is designed to disperse the incident visible light and achieve a high diffraction efficiency in a specified direction corresponding to a specified order. Two economical fabrication methods are studied. By ultrahigh-precision cutting technology, the continuous-relief planar gratings with the structure line width 0.96 and 1.5 μm are fabricated. In addition, other grating with the structure line width 2.5 μm is fabricated by using excimer laser dragging. Comparing the two fabrication results, the second one has smoother surface and smaller spacing error but the vertex angle is not enough sharp and the structure line width is limited.     

V-band directional tandem coupler of monolithic uniplanar structure

We present a uniplanar coplanar-waveguide 3-dB tandem coupler operating at V-band frequencies. The uniplanar structure is monolithically fabricated by using two-section parallel-coupled lines and air-bridge crossovers replacing the conventional multilayer or bonded structures. Due to an optimized tandem structure and non-bonded crossovers minimizing the parasitic components, a maximum coupling of 2.5 dB is measured at 62 GHz with a 2 dB bandwidth of 83%, while a high directivity factor of 33 dB is simultaneously obtained at 58–62 GHz. Over the entire design frequency range of 30–90 GHz, we achieve good phase unbalance of 90 ± 6.0°, as well as return loss and isolation lower than −23 and −16 dB, respectively.     

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.     

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°.     

A novel compact wavelength-division multiplexer using highly dispersive waveguide-to-waveguide coupling

A novel compact wavelength-division multiplexer using highly dispersive waveguide-to-waveguide coupling is designed, simulated and analyzed. The device consists of two very close single-moded waveguides that are periodically connected to form a mode-dependent dispersive grating. It is demonstrated that the wavelengths over the edges of the photonic band gap contributed by the grating can be separated in a very short propagation distance. Using the finite-difference time-domain method, the result shows that the wavelengths of 1570 and 1530 nm are separated by the grating in a coupler length of 57 μm which is much shorter than the required length of about 340 μm without grating assistance. The channel contrast of 20 dB and the insertion loss about 2 dB are achieved.     

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°.     

Efficient broadband silicon-on-insulator grating coupler with low backreflection

We design and fabricate an efficient broadband grating coupler on a 400 nm thick silicon-on-insulator wafer. The measured coupling loss is 3 dB when coupling to a single-mode fiber at 1310 nm wavelength with TE polarization. The spectral FWHM and backreflection are determined to be 58 nm and -27 dB, respectively.     

Design of a polymer directional coupler electro-optic switch using two-section reversed electrodes

By using the coupled mode theory, electro-optic modulation theory, conformal transforming method and image method, the structure is designed, the parameters are optimized, and the characteristics are analyzed for a polymer directional coupler electro-optic switch with two-section reversed electrodes. Simulation shows that the designed device exhibits excellent switching functions. Under the operation wavelength of 1550 nm, the electro-optic coupling region length is 4751 μm, the cross-state and bar-state voltages are about 1.22 and 2.65 V, and the insertion loss and crosstalk are less than 2.21 and −30 dB, respectively. By slightly adjusting the state voltages, the blight of the fabrication errors on the switching characteristics can be easily eliminated. The calculation results of the presented technique are in good agreement with those of the beam propagation method (BPM).     

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。     

Acrylic-based asymmetric and variable coupling ratio Y-branch plastic optical fiber coupler

Acrylic-based asymmetric and variable couplers have been developed using a single structured Y-branch design with a high-index-contrast waveguide taper and a void structure for fiber attenuation using the lateral displacement of two fibers. Device fabrication is performed by producing the device structure on an acrylic block using a computer numerical control (CNC) machine tool. The fabricated device has an excess loss of 5.85 dB, while the coupling ratios are 56.86 and 43.14% when the device is operated as a 3 dB coupler. In the asymmetric coupler mode, the coupling ratio ranges from 44.84 to 8.01% for port 1 and 55.16 to 91.99% for port 2. The excess loss of this device varies from 5.42 to 7.64 dB. In the variable coupler mode, the coupling ratio ranges from 10.09 to 32.88% for port 1 and from 89.91 to 67.12% for port 2. The excess loss of the device varies from 5.85 to 8.49 dB.     

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.