Flat Filter Based on a Chirped Super Moiré Grating

In this paper, super Moiré grating (SMG) with multiple superimposed gratings, in the best of our knowledge, is studied at first time. A flat multichannel transmission filter based on a chirped SMG with three superimposed gratings can be achieved, which benefits wavelength-division multiplexing (WDM) system. At the same time, a reflection filter can also be obtained in a chirped SMG, which can be used as an optical add/drop multiplexer filter for WDM system.     

Virtual Gires-Tournois etalons realized with phase-modulated wideband chirped fiber gratings

We propose and demonstrate novel virtual Gires-Tournois (GT) etalons based on fiber gratings. By introducing an additional phase modulation in wideband linearly chirped fiber Bragg gratings, we have successfully generated GT resonance with only one grating. This technique can simplify the fabrication procedure while retaining the normal advantages of distributed etalons, including their full compatibility with optical fiber, low insertion loss, and low cost. Such etalons can be used as dispersion compensation devices in optical transmission systems.     

Spectral characterization of fiber gratings with high resolution

We demonstrate a high-resolution technique to measure the optical magnitude and phase responses of fiber gratings. The technique employs single-sideband modulation of an optical source and has spectral resolution in the hertz regime. The technique is demonstrated by measurement of the phase ripples of unapodized and apodized chirped gratings as well as the transmission spectrum of a pi-phase-shifted grating. Although it is demonstrated on fiber gratings, the technique is applicable to any optical device.     

测定固体线膨胀系数的一种方法

当两个光栅紧靠在一起时，从它们的透射光中将看到明暗相间的条纹。该条纹间距与两个光栅夹角有关，若我们把一个光栅固定在随金属丝的伸长而绕某一定轴转动的支架上，则根据该转轴到金属丝间距和条纹间距及其它的变化量，就很容易得出金属丝随温度升高而伸长的微小量与条纹间距变化量之间的关系，并通过它算出该金属丝的线膨胀系数。     

Epitaxial growth of fully relaxed Si0.75Ge0.25 on SOI substrate

A fully relaxed Si_0.75Ge_0.25 film with low dislocation densities is fabricated by epitaxial growth on SOI substrate without depositing graded buffers. The relaxation mechanism of the SiGe layer directly grown on SOI substrate is also analyzed. For SiGe grown on SOI with low Ge content, the strain is redistributed between SiGe and the top Si of SOI substrate, and the strain residing in SiGe layer can be fully relaxed by the formation and expansion of dislocation half-loops near the SiGe/Si interface. The surface morphology and crystal quality of all samples are analyzed by optical microscopy and transmission electron microscopy (TEM), respectively. Compared to the Si_0.75Ge_0.25 layer epitaxially grown on graded buffer, the Si_0.75Ge_0.25 directly grown on SOI substrate appears good surface morphology and perfect crystal quality.     

Photoacoustic operation modes for determination of absorption spectra of SiGe mixed crystals

This paper presents comparison of two photoacoustic modes of determination of optical absorption spectra of semiconductors illustrated with the results obtained for SiGe crystals. Experimental transmission and absorption photoacoustic spectra of SiGe crystals as well as appropriate models for determination of optical absorption coefficient spectra are given. The idea and experimental set-up of the analyzed methods are presented too. From the fitting procedure of theoretical characteristics to experimental transmission and absorption photoacoustic spectra and after computations of the optical absorption coefficient spectra, three components of the optical absorption coefficient spectra of SiGe crystals were identified, i.e., band to band transitions, Urbach tail and free carriers absorption. Their parameters are given and discussed in the paper. At the end, the advantages and disadvantages of both methods are discussed. To the best of our knowledge, such a comparison of the two PA methods of determination of the optical absorption spectra has not been done before.     

基于啁啾相移光纤光栅的滤波器设计及应用

相移光纤光栅作为透射型带通滤波器件降低了使用成本和插入损耗.在光通信和传感领域有着较高的应用价值.啁啾相移光纤光栅的透射峰波长和数量由相移点的位置和数量来确定,透射峰特性受相移量的影响.采用分段切趾的方法获得满足光通信要求的滤波特性,利用传输矩阵法分析了切趾前后透射峰特性的变化,结果表明:切趾改善了滤波器的幅度和相位响应,依据滤波器的设计要求选取不同的切趾函数以及切趾比例;切趾还大大减弱了透射峰波长和深度对相移量的敏感度,适当切趾后相移量的影响可以忽略.并提出一种应用双通道啁啾相移光纤光栅滤波器产生脉冲毫米波副载波信号的方案.     

Zemax软件在取样光栅对设计中的应用

 光束取样光栅(BSG)是一种重要的用于光束取样诊断的衍射光学元件。以2块取样光栅代替单块光栅作为初始光学结构,运用Zemax光学设计软件采用分步优化的方法设计了具有消像差功能的光栅对结构,此方法比采用Matlab语言编程计算的方法消像差更快捷、更灵活,同时可以验证程序计算结果的正确性。设计结果显示：再现的会聚光经2块光栅衍射后在像平面上点列图中的均方根半径仅为0.506μm,单块取样光栅的均方根半径则为7.284μm,说明2块光栅能够做到像差互相矫正,其像质明显好于单块取样光栅。另外,设计出的双光栅光学系统,可进行远场光斑质量检测,为激光束性能的在线诊断提供了行之有效的技术支持。     

基于光纤光栅波长路由的光纤自愈网

介绍了一种基于光纤光栅波长路由的全光双纤单向自愈环网,环内四节点具有16×10Gbit/s的传输能力。利用啁啾光纤光栅(CFBG)的波长可选择特性,实现了分布式的上下话路和波长选择功能,同时有效地补偿了传输色散。网络管理参照了网络管理协议(SNMP)的网络管理模型,通过对网络的重要参数进行实时监测来控制网络运行。实验结果表明,当环上任一处光缆发生故障时,通信业务能够自动恢复,恢复时间小于50ms,可实现网络的快速自愈,符合ITU-T G.841的建议规定。     

Birefringent azopolymer long period fiber gratings induced by 532 nm polarized laser

Based on the photoinduced birefringence in azo materials, the model of the birefringent long period fiber gratings (BLPFGs) has been proposed and the transmission of such birefringent fiber gratings was further simulated by the coupled mode theory. In order to obtain the optimum condition for making birefringent azopolymer optical fiber (APOF) gratings the photoinduced birefringence of azo dye and methyl methacrylate (MMA) copolymers was systematically studied. After photosensitivity study of azo samples, using Teflon technique, azo dye containing single mode polymer optical fiber (POF) was fabricated. Finally, BLPFGs can be fabricated in azopolymer fiber using polarized 532 nm laser. A 120 μm LPFG with a 50% duty cycle has been successfully written within the core of the fiber with 532 nm laser by an amplitude mask method. Under polarized microphotography, the grating was observed when the optical axis was set at 45° with the direction of the first polarizing lens of the polarization microscope.     

Optical limiting of high-repetition-rate laser pulses by carbon nanofibers suspended in polydimethylsiloxane

The mechanism of optical unidirectional (OUD) transmission in parallel subwavelength dual-metal gratings was investigated. It was found that this kind of OUD phenomenon originates from the coupling of the surface plasmon polaritons (SPPs) between the front grating and a layer of metal film which replaces the rear grating. The higher the intensity of the coupled SPPs at the entrances of the rear grating, the higher the transmittance can be achieved. Basing on this property, an effective OUD example was achieved by exploring the intensity difference at the entrances of the rear gratings between the two incidences of opposite directions. In this kind of OUD, the positive transmittance can exceed 80 % and the difference between the transmittances of the two opposite directions can be as large as 63 %. The detailed design process was also presented.     

Surface plasmon resonance transmission filters at 1053nm based on metallic grating with narrow slit

Metallic gratings with narrow slits can lead to special optical properties such as strongly enhancing the transmission and considerably strengthening the polarized effect. A narrow-band filter suitable for application in optical communication is designed by sandwiching a metallic grating between two identical dielectric films. The maximum transmission can reach 96% after optimizing the parameters of films and grating at a central wavelength of 1053 nm. It is the first time, to our knowledge, that such high transmission has been reported since the discovery of the extraordinarily high transmission through periodic holes or slits; moreover, the extremely polarized effect is also found in P mode of this symmetric grating.     

Optimisation of superimposed chirped fibre Bragg gratings for the generation of ultra-high speed optical pulse bursts

Compact superstructure fibre Bragg gratings for the generation of ultra-high speed optical pulse bursts are reported. The gratings are equivalent in response to multi-pass superimposed chirped fibre Bragg gratings (SI-CFBGs), but an arbitrary sequence of consecutive gratings can be replicated in a single fabrication step using a simple trigonometric amplitude-phase function. SI-CFBGs are fabricated in photosensitive optical fibre and the time-resolved reflection of a 10 GHz picosecond pulse train is measured via frequency-resolved optical gating and optical sub-harmonic sampling detection techniques. The experimental results are compared to the theoretically simulated grating response and good agreement is found.     

Interference grating structures in photonic crystal circuits

Photonic Crystal Circuits have been widely investigated for various optoelectronics device applications. And the gratings in photonics devices are indispensable tool, which is very common for light manipulation. In this study, an interference grating structure formed by Gaussian beam interferences in photonic crystal waveguide such that having Kerr type nonlinearity is proposed and its transmission characteristics are investigated. Finite-Difference Time-Domain method is used to analyze the device characteristics. According to simulation results, the interference grating has shown special transmission characteristics that can bring tunability for photonic crystal devices. This can be an effective method for controlling optical signals in the photonic crystal for all optical switching/routing applications as a part of add/drop multiplexing.     

Dispersion Compensators Based on Overlaped Linearly Chirped Fiber Gratings for DWDM Systems

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Dispersion Compensators Based on Overlaped Linearly Chirped Fiber Gratings for DWDM Systems

By introducing a convergent lens in the phase mask setup and by tilting appropriately the fiber from the phase mask, a linearly chirped fiber grating with an appropriate chirp can be obtained conveniently. Using this method, two groups of fiber gratings, each of them is composed of 8 overlaped linearly chirped fiber gratings but with different center reflection wavelength separations (～0.8 nm and ～1.6 nm, respectively) are fabricated. Experimental results show that, the compensators based on the two groups of fiber gratings can be used to compensate for the dispersion of 8×2.5 Gb/s transmission over 100 km of standard single-mode fiber for multiple channels in optical time-division multiplexing (OTDM) and densed wavelength division multiplexing (DWDM).     

Optical transmission gratings by one driven three-level atom and a microtoroidal resonator

Based on the strong coherent interaction between a three-level ladder-type atom and a fiber-taper-coupled microtoroidal resonator, we present a scheme for optical transmission gratings. Using experimentally accessible parameters, it is shown that alternating regions of high transmission and absorption can be created in the fiber-taper channel by spatially modulating an external coupling field. The model shows an obvious effect which has a direct analogy with the phenomenon of electromagnetically induced grating (EIG) in quantum systems.     

Tailoring Chirped MoirÉ Fiber Bragg Gratings for Wavelength-Division-Multiplexing and Optical Code-Division Multiple-Access Applications

The design and fabrication of chirped Moiré fiber Bragg gratings (CMGs) are presented, which can be used in either (1) transmission as passband filters for providing wavelength selectivity in wavelength-division-multiplexed (WDM) systems or (2) reflection as encoding/decoding elements to decompose short broadband pulses in both wavelength and time in order to implement an optical code-division multiple-access (OCDMA) system. In transmission, the fabricated CMGs have single or multiple flattened passbands (< 0.5 dB ripple) and exhibit > 12 dB isolation and near constant in-band group delay. It is shown that these filters do not produce any measurable dispersion-induced power penalties when used to provide wavelength selectivity in 2.5 Gbit/s systems. It is also demonstrated how CMGs can be used in reflection to encode/decode short pulses from a wavelength-tunable mode-locked Er-doped fiber laser.     

含有光纤光栅的萨尼亚克环的透射特性分析

详细分析了由光纤光栅和萨尼亚克（Sagnac)环构成的透射滤波器的特性,给出了在均匀光纤光栅的情况下,该滤波器透过率的解析公式,结果表明该滤波器的透射光谱是余弦函数调制后的光纤光栅反射光谱,当光纤光栅是“强”耦合时,该滤波器可以构成多通带滤波器,而当纤光栅为“弱”耦合时,该滤波器可以构成单通带滤波器,这些滤波器可应用于现代光纤通信系统中。     

Design and analysis of nano-deep corrugated waveguide grating-based dual-resonant filters in visible and infra-red regions

A theoretical analysis of nano-deep corrugated long-period waveguide gratings on a SU-8 polymer-based channel waveguide with NOA61 optical epoxy coated upper- and lower cladding is presented. The transmission spectra of the gratings show strong rejection bands both at visible (at wavelength region of 450?460 nm) and infra-red (at wavelength region of 1530?1540 nm) regions when a grating period of ?68 μm with optimized grating tooth height is considered. Phase-matching graphs are studied to find the relationship between resonance wavelength and grating period. These results show that the grating parameters significantly affect the characteristics of transmission spectra as well as the resonance wavelength of the grating. Long-period waveguide grating-based band pass filter made by use of same polymer materials are also designed and analyzed. These types of waveguide grating-based filters can widely be used for visible and infra-red wavelength sensing applications.