MEMS阵列光开关的光纤耦合设计

利用高斯光束传输理论进行了阵列光开关的光纤耦合设计计算。根据ABCD矩阵推导出了光束通过球透镜时的传播规律,计算了在光纤与光纤之间两个球透镜耦合的效率,分析了影响耦合效率的因素。通过设计计算,确定了光纤端部的位置,使得整个损耗达到了最小。从理论上来看,在波长λ=1 55μm时,4×4阵列光开关的耦合效率可以达到91%以上。     

Experimental demonstration of single-mode fiber coupling using adaptive fiber coupler

Coupling plane wave into a single-mode fiber(SMF) with high and steady coupling efficiency is crucial for fiberbased free-space laser systems, where random angular jitters are the main influencing factors of fiber coupling. In this paper, we verified a new adaptive-optic device named adaptive fiber coupler(AFC) which could compensate angular jitters and improve the SMF coupling efficiency in some degree. Experiments of SMF coupling under the angular jitter situation using AFC have been achieved. Stochastic parallel gradient descent(SPGD) algorithm is employed as the control strategy, of which the iteration rate is 625 Hz. In closed loop, the coupling efficiency keeps above 65% when angular errors are below 80 μrad. The compensation bandwidth is 35 Hz at sine-jitter of 15 μrad amplitude with average coupling efficiency of above 60%. Also, experiments with simulated turbulence have been studied. The average coupling efficiency increases from 31.97% in open loop to 61.33% in closed loop, and mean square error(MSE) of coupling efficiency drops from 7.43% to 1.75%.     

Coupling efficiency of single-mode fiber components using GRIN-rod lenses

This article analyzes the characteristics of single-mode fiber (SMF) components using graded-index-rod (GRIN-rod) lenses. The coupling efficiency of the SMF to GRIN-rod lens to SMF coupling system is derived in the general case. The problem is simplified by using the reciprocal field method. All solutions are in closed form. It is found that SMF position tolerance is about 3 μm; the angular tolerance is about 1°. The GRIN-rod lens position tolerance is much less critical while the angular tolerance is extremely small. These tolerances could be much less critical if the fiber is kept near the GRIN-rod lens axis.     

Coupling efficiency of single-mode fiber components using GRIN-rod lenses

This article analyzes the characteristics of single-mode fiber (SMF) components using graded-index-rod (GRIN-rod) lenses. The coupling efficiency of the SMF to GRIN-rod lens to SMF coupling system is derived in the general case. The problem is simplified by using the reciprocal field method. All solutions are in closed form. It is found that SMF position tolerance is about 3 μm; the angular tolerance is about 1°. The GRIN-rod lens position tolerance is much less critical while the angular tolerance is extremely small. These tolerances could be much less critical if the fiber is kept near the GRIN-rod lens axis.     

Application of fiber interferometer in coherent Doppler lidar

A novel coherent Doppler lidar (CDL) system based on single-mode fiber (SMF) components and instruments is presented to measure the speed of target. A fiber interferometer used in CDL system is reported.This fiber mixer is employed as a coherent receiver to resolve the shifts of signal-to-noise ratio (SNR) and mixing efficiency induced by backscattered field's wavefront error. For a certain wavelength, the maximum coupling efficiency between signal and SMF is determined by the ratio of pupil diameter to focal length of the coupling lens. The legible interference patterns and spectrum signals show that fiber interferometer is suitable to compensate for amplitude and phase vibrations. This robust coherent receiver can achieve improved CDL system performance with less transmitter power.     

半导体激光器与单模光纤耦合效率的分析

在高斯场分布近似的前提下,从模场半宽匹配的角度描述了半导体激光器（ＳＬＤ） 与单模光纤（ＳＭＦ） 之间的光耦合理论。对不同的ＳＬＤ ＳＭＦ 间光耦合的方式作了分析,并从耦合效率与成本双重角度给出了适合于实际工程应用的几种耦合方式的优选率     

Experimental investigation on coupling efficiency between semiconductor laser diodes and single-mode fibres by an etching technique

The coupling efficiency between semiconductor laser diodes (LDs) and single-mode fibres (SMFs) can be improved by matching their respective modes. This may be achieved by constructing a conical microlens on the end of an Al-coated vapour-axial deposition (VAD) SMF. A simple process to etch an SMF end selectively to produce such a conical microlens is described.A minimum coupling loss of 3 dB has been obtained using 1.3-m InGaAsP/InP buried heterostructure LDs. Experimental results on coupling efficiency and loss penalty due to lateral misalignment are also reported. This conical microlens is easy to manufacture and reproduce.     

Improving fiber to waveguide coupling efficiency by use of a highly germanium-doped thermally expanded core fiber

Possessing a gradient refractive index (RI), thermally expanded core (TEC) fibers have been studied to be able to improve coupling efficiency of waveguide-fiber or LD-fiber according to the qualitative analysis of geometrical optics. The experiments show that the mode field radius (MFR) expansion of single-mode fiber with highly Germanium-doped (HGD) is more effective than that of conventional single-mode fiber (SMF). The coupling efficiency of a fabricated TEC fiber with HGD can enhance 5%–6% compared with that of a conventional-SMF. So the TEC fiber with HGD is a good candidate for the coupling of waveguide-fiber or LD-fiber.     

Sample-inverted reflow technique for fabrication of a revolved-hyperboloid microlens array in hybrid solgel glass

We propose a novel fabrication method, which is referred to as the sample-inverted reflow technique, to fabricate a refractive microlens array (MLA) with a revolved-hyperboloid profile in a solgel material. The fabricated solgel MLA demonstrates an excellent smooth profile with a fabrication error much less than the difference between the revolved hyperboloid and the spherical surface. In an application of coupling a laser diode (LD) to a single-mode fiber (SMF), we propose a two-MLA coupling scheme in which two revolved-hyperboloid MLAs are used between the LD and the SMF. In this configuration the coupling efficiency achieves 81.7% (-0.88 dB).     

Designing a coupler for the intersatellite optical communication system

Coupling a single-mode laser diode with 200 mW to a single-mode fiber (SMF) through an orthonormal aspherical cylindrical lens and a GRIN lens for the intersatellite optical communication system is proposed and demonstrated. We experimentally studied how the coupling efficiency changes with the SMF's position displacement and axial angle variation, and obtained 80 mW output power at the end of the SMF, which shows that the coupling units have satisfied the designed request.     

ABCD matrix model of quadric interface-lensed fiber and its application in coupling efficiency calculation

In order to simplify the theoretical coupling efficiency calculation of a laser diode (LD) to a single-mode fiber (SMF) with a quadric lens formed on the fiber tip, based on the Gaussian beam approximation, using the ABCD matrix method the relationship between the coupling efficiency and the structure parameters of quadric surface-lensed fiber is studied and analyzed in detail, and the coupling efficiency can reach as high as 99.87% with optimizing parameters of lensed fiber in theoretical simulation. Meanwhile, under paraxial approximation, the ABCD matrix for refraction of different quadric surface micro-lens can be formulated as a unified expression. This result can give some guidance suggestions for fabricating the quadric interface-lensed fiber.     

Mode matching and insertion loss in ultrabroadband Cr-doped multimode fibers

We investigate the fundamental insertion loss due to mode mismatch in an optical link involving a single-mode fiber-optic (SMF) transmission with the insertion of a segment of a multimode Cr(4+)-doped fiber (MMCDF). With an MMCDF core diameter of approximately 15.5 microm that matches the SMF, we obtained coupling efficiencies of 60.3% to 87.5% in the entire transmission spectral range of 1,300 to 1,600 nm. The high coupling efficiency (or low insertion loss) makes it possible for the employment of the MMCDF as an inline ultrabroadband optical amplifier in an optical link for the entire transmission spectral range.     

Photopolymer microtips for efficient light coupling between single-mode fibers and photonic crystal fibers

A novel method for light coupling between single-mode fibers (SMFs) and small-core photonic crystal fibers (PCFs) is demonstrated. The method is based on growing photopolymer microtips directly on the end faces of SMFs. The shape and size of the tips can be controlled by adjusting the laser power and the exposure time for polymerization to match the mode field to the small-core PCFs. A 5 dB improvement in coupling efficiency between a SMF and a commercial small-core, highly nonlinear PCF is experimentally demonstrated. This compact and efficient butt-coupling method is particularly suitable for PCF gas sensor applications.     

光子晶体光纤的耦合分析

利用等效折射率模型对光子晶体光纤与普通单模光纤的耦合情况进行了分析,并用有限元分析模型对等效折射率法得到的模场半径随着跨距的增大而增大的模场特性进行了验证,得到了当光子晶体光纤的填充比固定时,随着跨距的增大,耦合损耗会随之减小到最小值,然后又再度增大这一结论.并通过合理地选择光子晶体光纤的填充比以及跨距来提高耦合效率.     

All-fiber wavelength- and mode-selective coupler for optical interconnections

We experimentally demonstrate a new wavelength- and mode-selective coupler utilizing intermodal coupling between a standard single-mode fiber (SMF) and a hollow optical fiber (HOF). The fabricated device routes optical signals such that a 1.3-microm signal is directed to the HOF port, adiabatically converting the LP(01) mode into a ring-shaped mode, which can be further connected to a gigabit ethernet link, reducing the differential mode-dispersion penalty. Optical signals near 1.5microm , meanwhile, stay in the LP(01) mode of the SMF arm for further connection to conventional wavelength-division multiplexing links based on erbium-doped fiber amplifiers. The performance of the device is characterized in terms of insertion loss, channel isolation, and mode-conversion efficiency.     

基于ABCD矩阵的楔形柱面光纤微透镜的耦合技术分析

运用激光模式耦合理论分析了半导体激光器与楔形柱面光纤微透镜的连接损耗,设计并制作了楔形柱面光纤微透镜用来实现两者的模斑匹配。运用ABCD矩阵方法,对该组件的耦合效率进行了仿真计算和分析,得到了高耦合效率下楔形柱面光纤微透镜的结构参数。该方法可有效地用于楔形柱面光纤微透镜的优化设计。     

楔形光纤与半导体多量子阱平面光波光路芯片的耦合分析

采用楔形光纤(WSF)实现了与半导体多量子阱(MQW)平面光波光路(PLC)芯片的高效耦合。在多量子阱-平面光波光路前置模斑转换器(SSC)和不加模斑转换器的情况下,用阶梯串联法(SCM)数值模拟并优化设计了楔形光纤-平面光波光路间最佳耦合参量:楔形光纤楔角45°、端面圆柱透镜曲率半径2.5μm、模斑转换器-多量子阱-平面光波光路出射椭圆光斑长半轴3.5μm、纵横比5、楔形光纤-平面光波光路间垂直方向和水平方向无偏移、纵向间距5.5μm。用反向推演法(IDM)实验分析了楔形光纤样品的出射光场,与阶梯串联法(SCM)计算结果相比长轴误差为3.125%,短轴误差为0.8%。建立楔形光纤-平面光波光路-单模光纤(SMF)的耦合实验系统,在1.55μm波长处以单模光纤作为出纤的相同条件下,发现楔形光纤激励入射平面光波光路比单模光纤和锥形透镜光纤(TLF)作为入纤的耦合效率分别提高了24.827 dB和16.22 dB,为多量子阱-平面光波光路芯片尾纤封装技术提供了实验原型。     

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°.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: VI. Evolution in the degree of polarization of nonmonochromatic radiation traveling in a twisted optical waveguide

An analysis is made of the evolution of polarization of nonmonochromatic radiation travelling through single-mode optical fibers in the presence of random coupling between orthogonally polarized modes, which is caused by the random twist of axes of natural linear birefringence of an SMF, and in the presence of a regular axis twist. It is shown that the depolarization length of nonmonochromatic radiation in an SMF increases with increasing regular SMF twist, regardless of the presence of a random axis twist. Using the Monte Carlo method, the dependences of the mathematical expectation and the mean-square deviation of the degree of polarization of nonmonochromatic radiation on the fiber length for SMFs with different linear birefringences and regular axis twists are calculated. It is shown that the fiber length on which the degree of polarization of nonmonochromatic radiation reaches its limiting value increases and the limiting value itself decreases with an increasing regular twist. It is also shown that an induced SMF twist is able to improve parameters of a fiber interferometer, in particular, to decrease random phase changes and intensity fadings of the interference signal, which are caused by random coupling between polarization modes.     

空间激光与单模光纤和光子晶体光纤的耦合效率

为了设计最优光纤耦合系统,利用高斯模场近似单模阶跃光纤的模场和大模面积光子晶体光纤的模场,推导出了理想情况下空间激光与这两种光纤的耦合效率解析表达式以及光纤端面相对于耦合系统存在横向偏移和端面倾斜时的耦合效率解析表达式。基于上述理论表达式计算了空间激光与光纤的耦合效率,并通过实验验证了此理论表达式的有效性。理论计算和实验均证实了单模阶跃光纤对于横向偏移更敏感,当横向偏移量等于单模光纤的纤芯半径时所对应的耦合效率只有20.25%,为理论最大值的1/4;而大模面积光子晶体光纤对于端面倾斜更加敏感,当端面倾斜2°时对应的耦合效率只有40.5%,为理论最大值的1/2。所提出理论表达式和实验方法完全可以为设计光纤耦合系统提供准确的参数。