Novel approach for design of low index contrast multimode interference devices

Self-imaging theory is widely accepted as a good method in designing multimode interference (MMI) couplers, but it is also true that self-imaging theory is not suitable for low-contrast structures. An improved self-imaging theory is proposed in this paper for the optimal design of low-contrast 1 × N MMI couplers. The average effective width of the MMI waveguide and the average effective propagation constant of the MMI waveguide are used in the improved self-imaging theory. An approach is given to find the average effective width. We use this approach in the optimal design of a 1?×?4 silica MMI coupler, and the results show that the improved self-imaging theory is more accurate than conventional self-imaging theory for low-contrast structures.     

Tight focusing induces pulse delay and pulse compression of double-ring-shaped radially polarized ultrashort light pulses

The focusing of double-ring-shaped radially polarized ultrashort light pulses by a high-numerical aperture objective is investigated using vectorial Debye theory. After focusing, the double-ring-shaped radially polarized ultrashort light pulses slow down near the focus, and this pulse delay induces pulse compression in the propagation direction. That is, without changing the pulse duration, the spatial pulse length of the ultrashort light pulse is decreased near the focus. The velocity of the longitudinal component of the light pulse near the focus is lower than that of the radial component. The simulation results demonstrate that control of the ratio of the pupil radius to the beam radius affects not only the longitudinal component of the light pulse, but also the velocity and spatial pulse length of the light pulse because of the destructive interference between the longitudinal components of the inner and outer rings of the light pulse.     

Multimode polarization degree of mixture optical states in integrated directional couplers

Mixture multimode optical field classical states propagating in N?×?N integrated directional couplers are analyzed by using the density matrix formalism in a N-dimensional optical space. These mutimode optical fields present a kind of generalized polarization and accordingly a definition of a multimode polarization degree is proposed. It is based on the distance measure between a mixture state and an unpolarized state in a N-dimensional optical space so that in the case N=2 the standard polarization degree is recovered. It is shown that directional couplers can reduce or increase remarkably the multimode polarization degree of a mixture state. Likewise a simple measurement technique, based on Y junctions, of this multimode polarization degree is proposed. Finally all the results can be formally extended to the special case of multimode single photon quantum states.     

Failure analysis of three reconditioned rail car couplers

Three reconditioned rail car couplers, arbitrarily identified as couplers, failed during service due to cracking of the front face. The three couplers were reportedly upgraded by quenching and tempering to enhance strength and toughness. The objective of this investigation was to determine the mode of crack propagation and the root cause of the observed failures. Verl E. (Bud) Kahle, P.E., was a professional failure analyst for more than forty years. After receiving his Bachelor of Science degree in physical metallurgy from Washington State University 1957, Bud was employed by the General Electric Company at a facility in Hanford, Washington. He later was employed by the Aerojet General Corporation and eventually was head of the Southern Pacific Materials Science Laboratory in Sacramento. After retirement from Southern Pacific he owned and operated his own firm, Kahle Metallurgical Consulting Services, for fifteen years. He came to be known as “the man with the answers” to clients in a variety of industries. Bud was a Life Member of ASM International. Many of Bud’s clients remarked on the high quality of his failure analysis case histories and encouraged him to have them published. After Bud passed away in 1999, his wife, Suzanne, approached ASM International to see if the society would be interested in publishing any of his work. Mrs. Kahle was able to supply a large number of reports, which Bud had carefully archived over the years. Journal of Failure Analysis and Prevention Associate Editor Michael Stevenson read through the collected case histories, selected the ones to be included in this issue, and edited them for journal publication. ASM International wishes to thank Mrs. Kahle for providing these case histories for publication.     

超短激光脉冲腔外光谱展宽与压缩技术

超短激光脉冲是超快光学和强场激光物理研究领域的重要驱动光源,通常可以通过脉冲后压缩方法获得。本文详细介绍了超短脉冲后压缩的发展现状、原理和相关技术,包括块状固体材料压缩、薄片组压缩、多通腔压缩、中空波导压缩以及光子晶体光纤压缩等。并通过对现有的脉冲后压缩技术进行总结,为未来的研究与发展方向提供了理论指导。     

PC系统的电磁兼容检测及其软、硬件设计

本文叙述了计算机系统的电磁兼容检测技术，并从软、硬件两方面提出了抗干扰措施。     

Fast and accurate modeling of waveguide grating couplers

A boundary variation method for the analysis of both infinite periodic and finite aperiodic waveguide grating couplers in two dimensions is introduced. Based on a previously introduced boundary variation method for the analysis of metallic and transmission gratings [J. Opt. Soc. Am. A 10, 2307, 2551 (1993)], a numerical algorithm suitable for waveguide grating couplers is derived. Examples of the analysis of purely periodic grating couplers are given that illustrate the convergence of the scheme. An analysis of the use of the proposed method for focusing waveguide grating couplers is given, and a comparison with a highly accurate spectral collocation method yields excellent agreement and illustrates the attractiveness of the proposed boundary variation method in terms of speed and achievable accuracy.     

The local-field corrective effect on Rabi oscillation of ultrashort pulse excitation in semiconductor GaAs

The Rabi oscillation of the thin bulk semiconductor GaAs, which takes into account the effect of the local-field correction induced by the interacting excitons, is investigated by numerically solving the semiconductor Bloch equations. It is found, for a 2π few-cycle pulse excitation, that two incomplete Rabi-floppings emerge due to the competition between the Rabi frequency of the incident pulse and the internal-field matrices. Furthermore, for a sub-cycle 2π pulse excitation a complete Rabi-flopping can occur because of the absolute phase effect. We ascribe these characteristics of the Rabi oscillation to the renormalized Rabi frequency.     

All fiber on-axis coupling scheme between single mode fiber and GRIN fiber

In this paper we present a simple and novel method to maximize on-axis coupling efficiency to radially symmetric fibers without the need for extra free space optical elements. The method is based on inserting a segment of step-index multimode fiber (MMF), cleaved to a particular length, between the input fiber and the output fiber (OF). The MMF segment modifies the input field to match the guided modes in the OF. Using this technique we show that, by inserting an appropriate length MMF segment, it is theoretically possible to obtain a coupling coefficient as high as ?0.8 dB between a single mode fiber and a graded index ring-shaped fiber and ?0.32 dB for a multi-shell fiber. Our experimental measurements showed good agreement with theoretical predictions for the ring fiber.     

Characterization of a compact silicon-based slot-to-strip waveguide crossing

A compact crossing scheme for a silicon-based slot-to-strip (or vice versa) waveguide is proposed and analyzed by using a finite-difference time-domain method, where a strip–multimode waveguide (SMW) crossing is used at the center and two logarithmically tapered slot-to-strip mode converters are incorporated into the ports with slot waveguides. For the input ports with slot waveguides and output ports with strip waveguides, the guided modes are efficiently transformed through the mode converter, and then enter into the SMW, where the fields converge at the center of the intersection due to the self-imaging effect. Hence, the size of the input beam is much smaller than the width of the SMW at the crossing center, leading to significant reductions of both crosstalk and radiation loss. The numerical results show that a slot-to-strip waveguide crossing operating at a wavelength of 1.55?μm can be achieved with insertion loss, crosstalk, and reflection of 0.134/0.182, ?36.18/?38.6, and ?35.8/?42.02?dB for input ports with slot/strip waveguides, respectively.     

Design of vibration-insensitive Sagnac fiber-optic current sensors using spun high-birefringence fibers

A novel fiber-optic current sensing element is proposed to enhance sensor performance using spun high-birefringence fibers. Such element includes three fiber sections. Two terminal sections with a varying spin rate along the fiber are utilized to replace the fiber quarter-wave plates, each converting the light polarization state from linear to circular and vice versa. The middle section with a uniform spin rate is utilized as the current sensing fiber that maintains the circular polarization state during the light propagation. The fiber is also wound into a special geometric structure so that the Sagnac phase shift can be inherently eliminated, and the sensing result does not depend on the position of the current conductor. The evolution of the light polarization state was analyzed using coupled-mode theory with different polarization state incidents in the sensing fiber. A sensor scheme based on this type of spun fiber is also proposed.     

SMS光纤结构特性的数值模拟分析

采用光束传播法(BPM),对单模-多模-单模(SMS)光纤结构内部传输光场分布情况,进行数值模拟;分析了SMS光纤结构中不同的多模光纤长度,多模光纤纤芯直径和输入光波长对光纤结构内部传输光场分布情况的影响,以及所得透射谱的变化规律;进一步探讨了基于多模干涉理论的SMS光纤结构传感器在各传感领域的应用潜力.最后,以基于多模干涉理论的SMS光纤结构折射率传感器为例,分析了多模光纤纤芯外界折射率的改变对光纤结构输出透射谱的影响,并进行实验加以验证.实验结果与仿真模拟结论一致,从而验证了模拟的准确性.     

Design and fabrication of an optical interleaver with cascaded multimode interference couplers

A lattice-form optical interleaver was designed and fabricated with a silicon-based silica waveguide. Cascaded multimode interference couplers were first employed in the lattice circuit and helped to relax the fabrication tolerance. The device shows good performance, the insertion loss is less than 2.25 dB, the passband ripple is less than 0.15 dB, the cross talk is less than -18.9 dB, and the 0.5 dB passband is more than 100 GHz.     

基于虚拟仪器的慢速烤燃系统的设计及应用

为了满足不敏感炸药的发展需求,在GJB 772A-1997及行业内标准烤燃装置的基础上,设计并开发了一套慢速烤燃系统。该系统基于LabVIEW程序的控制软件,由以控制器、数据采集卡、热电偶及数字I/O卡等为主的典型硬件构成。参照美军标MIL-STD-2105D和国内行业标准,对该系统的适用性开展了试验研究;采用RDX基与HMX基两种典型的混合炸药装药进行了慢速烤燃试验。结果表明,该慢速烤燃系统具有广泛的温度适用范围,试验数据拟合相关系数不低于0.999 8,在0.055、0.200、1.000、2.000℃/min和3.000℃/min等几种升温速率条件下,炸药均能实现较高精度的线性升温。与RDX基混合炸药相比,HMX基混合炸药响应程度有所缓和。该慢速烤燃系统满足试验需求,可为研究炸药的热不敏感性提供有效手段。     

Determination of the Coordinates of the Center of a Gaussian Beam by Means of a Matrix Photodetector Using a Weighting Method

A weighting algorithm to determine the coordinates of the center of a Gaussian laser beam projected onto a matrix photodetector is considered. The influence of the internal noise of the photodetector, the maximum brightness of the signal at the beam maximum, and the beam radius on the precision of the algorithm is investigated. Recommendations on image processing are presented.     

A class of generalized Birnbaum-Saunders distributions for reliability modeling

In this paper, we generalize the Birnbaum-Saunders (BS) distribution by two ways. One is based on the mixture representation of BS distribution, and a flexible weight is adopted to describe the kurtosis of the distribution. The other way is based on the transformation property of BS distribution, and we incorporate a power parameter in the transformation to describe the skewness of the distribution. Then a four-parameter BS distribution including skewness and kurtosis parameters is induced by combining the two ways. The properties of these generalized BS distributions are investigated. Then, the expectation maximization (EM) algorithm is proposed to estimate the parameters. Real data analysis is performed to illustrate the superiority of the generalized BS distributions. Finally, some potential generalizations are discussed.     

Precise etch-depth control of microlens-integrated intracavity contacted vertical-cavity surface-emitting lasers by in-situ laser reflectometry and reflectivity modeling

We studied the etch-depth control of 980 nm intracavity contacted vertical-cavity surface-emitting laser (VCSEL) structures with GaAs/AlGaAs distributed Bragg reflectors by in-situ laser reflectometry and reflectivity modeling in SiCl₄/Ar inductively coupled plasmas. Highly accurate etch-depth control can be achieved by counting the number of oscillation peaks in the experimental reflectance signal through the fitting of the reflectivity data calculated theoretically using a transfer matrix method. The fits provide a very good agreement, allowing us to distinguish individual layers precisely and stop the etching at a desired depth. After confirmation of the validity of in-situ dry etch monitoring, this technique was employed in the fabrication of microlens-integrated intracavity contacted VCSELs including composition-graded digital alloy AlGaAs for high precision control of the etch depth in intracavity region. The etch-depth difference between calculated and experimental results was kept below 20 nm, indicating a good etch performance. The spatial uniformity of ~ 5% was obtained over 1 × 1 cm² sample size.     

Material Design and Fabrication Strategies for Stretchable Metallic Nanocomposites

Stretchable conductive nanocomposites fabricated by integrating metallic nanomaterials with elastomers have become a vital component of human‐friendly electronics, such as wearable and implantable devices, due to their unconventional electrical and mechanical characteristics. Understanding the detailed material design and fabrication strategies to improve the conductivity and stretchability of the nanocomposites is therefore important. This Review discusses the recent technological advances toward high performance stretchable metallic nanocomposites. First, the effect of the filler material design on the conductivity is briefly discussed, followed by various nanocomposite fabrication techniques to achieve high conductivity. Methods for maintaining the initial conductivity over a long period of time are also summarized. Then, strategies on controlled percolation of nanomaterials are highlighted, followed by a discussion regarding the effects of the morphology of the nanocomposite and postfabricated 3D structures on achieving high stretchability. Finally, representative examples of applications of such nanocomposites in biointegrated electronics are provided. A brief outlook concludes this Review.     

防振球托盘的设计与应用

针对精度要求很高,产品的附加值很大的工业器械,在满足堆码、搬运货物、安全运输等基本功能下,增加托盘的缓冲性能。以这个理念为出发点,选择合适的缓冲材料和木料,设计了托盘结构和结合方式。以医疗器械的包装设计为例,通过实验室测试,验证了设计理念和设计流程的合理性,检验了防振球托盘的实用性,总结了防振球托盘的优缺点,为防振球托盘的应用提供依据。     

Design and demonstration of direct UV-written small angle X couplers in silica-on-silicon for broadband operation

Experimental demonstration of small angle (0.8 degrees-5 degrees ) direct UV-written X couplers in silica-on-silicon is presented. Maximum and minimum coupling ratios of 95%(+/-0.8%) and 1.9% (+/-1%), respectively, were recorded. The structures also display very low polarization and wavelength dependence. A typical excess loss of 1.0 dB(+/-0.5 dB) was recorded. Device modeling using the beam propagation method and an analytical model showed good agreement with experimental results over a broad crossing angle and wavelength range.