Fabrication of concave refractive microlens arrays in solgel glass by a simple proximity-effect-assisted reflow technique

We report a simple method for fabricating a concave refractive microlens array (MLA) in solgel glass by using a proximity-effect-assisted reflow technique. The solgel concave refractive MLA that we fabricated had excellent surface smoothness; good dimensional conformity, with an 8.23% nonuniformity of the microlens elements; and structural perfection, with a biggest deviation of 1% from a perfect concave spherical crown. The relative error between the measured and the designed values of the concave MLA's focal length was only 1.83%. Compared with the conventional fabrication techniques for concave MLAs, the proposed method has significant advantages including simplicity, low cost, good element conformity, and smooth device surface.     

Self-processing solgel material for one-step fabrication of micrometer-period sinusoidal phase gratings using the Lloyd's mirror scheme

A novel self-processing silica-zirconia hybrid solgel material has been developed and employed in one-step fabrication of micrometer-period sinusoidal phase gratings. In the process, the gratings with a sinusoidal profile were corrugated on the surface of the solgel film by UV exposure using the Lloyd's mirror setup. No further development or etching step is needed to reveal the sinusoidal profile because the corrugation is formed due to the self-processing property of the solgel material, which is robust enough to be used as an end product. The period, amplitude, diffraction efficiency of the +/-1 order, and surface roughness of one of the fabricated gratings are 0.99 microm, 330 nm, 30.56%, and 1.27 nm, respectively. The new self-processing material is practical and promising for fabrication of micro-optical elements.     

Microlens fabrication on single-mode fibres for efficient coupling from laser diodes

An analysis of the etching process for forming a truncated conical microlens on the end surface of a single-mode fibre (SMF) so as to improve its coupling efficiency with a laser diode (LD) has been carried out. This is followed by a simple geometric-optics and electromagnetic analysis to calculate this coupling efficiency, when the lens is formed on a SMF fabricated by the MCVD technique. The theory is supported by measurements which gave a maximum coupling efficiency of 57%.     

Volume growth initiated by point-to-point ultraviolet-laser direct writing in hybrid solgel glass for three-dimensional microfabrication

Volume-growth solgel glass acting as self-developing material is used for three-dimensional (3D) fabrication of micro-optical elements. The solgel glass is exposed to a focused laser beam (lambda = 325 nm), and surface corrugation is created as a result of surface expansion in the exposed area. The profile depth is related to the laser beam intensity, which is controlled by an acousto-optic modulator, which makes it possible to fabricate 3D structures in the material. Preliminary results show that the controllable volume-growth effect will be of considerable use for production of 3D micro-optical elements with excellent surface smoothness in solgel materials.     

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。     

Phase perturbation investigation of laser diode for the laser diode to single-mode fiber coupling system

A novel phase-perturbation method as well as an experimental system were established to study the deviation of the laser diode (LD) far-field intensity distribution from the ideal Gaussian. An analytical expression of the LD far-field intensity distribution and a numerical method based on the Monte Carlo calculation were developed to fit the measured results. The loss of the LD to the single-mode fiber (SMF) coupling system caused by this kind of deviation of the LD far-field intensity distribution was quantitatively analyzed.     

Semiconductor laser diode to single-mode fiber coupling using diffractive optical elements

An integrated mode converter consisting of two diffractive optical elements (DOEs) and a Silicon slab is presented for low-loss coupling between a semiconductor laser diode (LD) and a single-mode fiber (SMF). The phase structures of the DOEs are designed using iterative phase retrieval algorithm. We introduce a new far-field amplitude constraint into the iteration to provide very high mode conversion quality. Compared with previously published mode converters, the scheme is more universal because it’s applicable for any semiconductor LD. In simulation, coupling losses lower than 0.02 dB are predicted for all the discussed LDs with aspect ratios of the elliptical fields from 1 to 9. The requirements on axial displacement and rotation angle have been removed. The tolerance for 1-dB loss increment for lateral misalignment is 0.9 μm. And the coupling loss is insensitive to tilt angle.     

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.     

Effect of random coupling of orthogonal modes on polarization characteristics of single-mode fiber optical waveguides (SMF) and SMF-based ring interferometers. I. Change in the degree of polarization of nonmonochromatic radiation propagating through a single-mode fiber optical waveguide

We propose a method for calculating the degree of polarization of nonmonochromatic radiation as it propagates through a single-mode fiber optical waveguide (SMF), based on dividing the length of the fiber into segments equal to the depolarization length of the radiation in the SMF. We have obtained the dependences of the degree of polarization of the radiation on the length of the fiber for an SMF with strong and weak anisotropy. In the case of anisotropic SMF's characterized by weak coupling of orthogonal modes, the results match those obtained previously by other calculation methods. The results for weakly anisotropic SMF's characterized by strong coupling of orthogonal modes are obtained here for the first time.Institute of Applied Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, No. 10, pp. 1311–1320, October, 1994.     

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°.     

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%.     

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     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: VII. Zero drift in a fiber ring interferometer with a circuit formed by a twisted optical fiber with weak natural linear birefringence

Zero drift and fading of the interference signal determined by polarization nonreciprocity at the exit from a fiber ring interferometer (FRI) of great length designed for recording a number of general relativity (GR) effects and made on the basis of a uniformly twisted single-mode optical fiber (SMF) with weak natural birefringence are considered. The calculations are carried out by numerical simulation using the model of random coupling between orthogonal polarization modes in an SMF, which was proposed in the first part of this work. The so-called minimum arrangement of an FRI with a nonmonochromatic radiation source, including a linear polarizer located between two beam splitters, is considered. Numerical estimates are made. It is shown that the FRI with a circuit on the basis of a twisted SMF has a significantly smaller zero drift and significantly smaller fading of the interference signal than the FRI with a circuit formed by an untwisted SMF. It is shown that the FRI produced on the basis of a twisted SMF provides the polarization sensitivity necessary for detecting the GR effects under study.     

Simulation and optimization of a polymer directional coupler electro-optic switch with push-pull electrodes

Structural model and design technique are proposed for a polymer directional coupler electro-optic switch with rib waveguides and push-pull electrodes, of which the electric field distribution is analyzed by the conformal transforming method and image method. In order to get the minimum mode loss and the minimum switching voltage, the parameters of the waveguide and electrode are optimized, such as the core with, core thickness, buffer layer between the core and the electrode, coupling gap between the waveguides, electrode thickness, electrode width and electrode gap. Switching Characteristics are analyzed, which include the output power, insertion loss, and crosstalk. To realize normal switching function, the fabrication error, spectrum shift, and coupling loss between a single mode fiber (SMF) and the waveguide are discussed. Simulation results show that the coupling length is 3082 μm, push-pull switching voltage is 2.14 V, insertion loss is less than 1.17 dB, and crosstalk is less than −30 dB for the designed device.     

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.     

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.     

Radiation-induced attenuation effect in hydroxyl-rich pure-silica-core photonic crystal fiber

In order to evaluate the vulnerability of photonic crystal fiber (PCF) in radiation environment, radiation-induced attenuation (RIA) characteristics of a pure-silica-core PCF with high content of hydroxyl was examined using a ⁶⁰Co radiation source. Its dose dependent RIA profile at 1310 nm was measured and compared with Ge-doped single mode fiber (SMF). Beyond our expectation, PCF showed significantly higher RIA and its recovery was also slower. To interpret the non-ideal RIA characteristics of PCF, hypotheses involving nonbridging oxygen hole centers (NBOHC) or near-infrared absorption (NIR) defect were given and the contribution of PCF's unique fabrication technology to these color centers’ high infrared absorption was discussed.     

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。     

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.     

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.