啁啾光纤光栅在光纤通信系统中的色散补偿

报道一个简单而新颖的、能将均匀光栅改变为线性啁啾光栅的方法,此方法可对光纤光栅的喃啾度和中心波长进行独立调节,实验表明,一长为１０ｃｍ的均匀光栅其啁啾度可增加至０．５ｎｍ／ｃｍ,而中心波长独立可调量达０．７ｎｍ利用这样的可调啁啾光栅在标准单模光纤（Ｇ６５２）中对一脉宽为２５ｐｓ的输入信号在１５５０ｎｍ窗口传输,成功地实现１００ｋｍ的色散补偿。     

基于级联相移线性啁啾光纤光栅的梳状滤波器设计

为了获得反射带宽大、信道间隔均匀的高性能梳状滤波器,提出一种基于级联相移线性啁啾光纤光栅的新型梳状滤波器的结构设计.利用传输矩阵法和法布里-珀罗谐振腔理论,数值分析了该梳状滤波器中啁啾光栅的级联方式、啁啾量、折射率调制量和级联处的相移量对光栅反射谱特性的影响.结果表明:对应不同的级联方式,反射峰存在疏密变化,即两个级联光栅的啁啾量同号时,反射峰分布均匀对称,可设计均匀滤波器;而两个啁啾量异号时,反射峰呈现出疏密渐变的特性,可设计非均匀滤波器.此外,光栅啁啾量增加,反射谱带宽增大,但是反射率降低,而增大折射率调制量,可以提高反射率;级联处的相移量改变时,每个反射峰位置发生移动,从而可以通过改变相移量选择滤波窗口.     

线性啁啾光纤光栅的数值分析

本文利用耦合模方程对线性啾光纤光栅的反射谱进行了计算,并讨论了方程中各参数对计算结果的影响.     

Novel B/Ge codoped photosensitive fiber and dispersion compensation in an 8×10 Gbit/s DWDM system

We used the Modified Chemical Vapor Deposition (MCVD) method to fabricate a B/Ge codoped fiber with a depressed cladding in which boron and germanium were also doped, the depressed cladding was photosensitive, which can greatly suppress the cladding mode coupling. The transmission spectrum of written grating in this fiber made by the phase mask method verified its larger photosensitivity and great suppression of the cladding mode coupling. We then analyzed the Bragg wavelength tuning theory in the phase mask method in detail which gave us a more exact equation. The ultra-violet (UV) exposure system was modified, and eight cascaded Fiber Bragg Gratings (FBG) centered at different wavelengths were written in this photosensitive fiber successfully using a 248 nm KrF excimer laser and one piece of 1067 nm phase mask. Each channel spacing was 100 GH according to the Densed Wavelength Division Multiplexing (DWDM) standards of the International Telecommunication Union (ITU-T). At the same time, we also wrote in chirp in each grating by tilting the phase mask to realize dispersion compensation. The dispersion in 44 km SMF was successfully compensated by the cascaded chirped gratings in the eight channel WDM system with each channel being 10 Gbit/s.     

Continuously wavelength-spacing-tunable and idler-output multiwavelength fiber optical parametric oscillator

We demonstrate a continuously wavelength-spacing-tunable and high-power multiwavelength fiber optical parametric oscillator based on the multiwavelength idler-output technique. The laser cavity for multiwavelength idler outputs is constructed by a pumped highly-nonlinear dispersion-shifted fiber as parametric gain medium, two highly-reflective chirped fiber Bragg gratings (CFBGs) and a superimposed CFBG as comb-like filter. At a pump power of 1.1 W, the idler output of 10 wavelengths around 1.56 μm is achieved with a wavelength spacing of 0.39 nm. The wavelength spacing can be continuously tuned from 0.39 to 1.0 nm by utilizing a cantilever beam-based chirp tuning method to change the FSR of the superimposed CFBG. Our experimental results show that the designed multiwavelength idler-output scheme can significantly increase the multiwavelength output power with a total output power of 98 mW and each idler-channel power of 16.3 mW.     

基于方差的奇异值分解域鲁棒零水印算法

 针对已提出的奇异值分解域鲁棒水印算法往往将水印嵌入在奇异值矩阵的奇异值上,利用奇异值分解产生的两个正交矩阵提出一种基于方差的鲁棒零水印算法.将图像分割成互不重叠的子块,对每个子块进行奇异值分解,通过判断每个子块U矩阵第一列元素平方的方差与V矩阵第一列元素平方的方差之间的大小关系产生零水印序列.算法实质上没有对原始图像做任何修改,具有非常好的不可见性.算法进行抵抗重采样、平滑处理、加噪音和JPEG压缩攻击实验测试鲁棒性.实验结果表明该算法在以上攻击中表现出很强的鲁棒性.     

Mode field profile and polarization dependence of long period fiber gratings written by CO2 laser

Mode field profiles and polarization dependence in the CO₂-laser-notched long period fiber gratings (LPFGs) were investigated in detail. Asymmetric field profile was demonstrated in the 2- and 3-dimensional far field patterns of the CO₂-laser-notched LPFG, which experimentally verifies the previously-presented asymmetry of refractive index modulation within the cross section of the gratings. Polarization dependent loss (PDL) of the LPFG was measured via Mueller matrix method. The maximum PDL of 1.68 dB and the maximum polarization extinction ratio of 1.07 dB are observed in the LPFG.     

三角形大带宽反射光谱光纤光栅的设计和制备

反射谱具有三角形形状的光纤光栅在光纤传感等领域中具有重要的应用前景。利用遗传算法设计出了产生三角形光谱分布的光纤光栅的耦合系数沿光纤的分布。计算表明,三角形光谱光纤光栅是可以实现的,其反射带宽可以通过光栅的啁啾量进行调节,常周期的三角光栅反射带宽小,变周期的三角光栅反射带宽大,其带宽同其啁啾量大致相等。实验中,采用光束扫描法制备了反射底宽为0．77nm三角形光谱的光纤光栅。     

啁啾叠栅光纤光栅反射特性和时延特性的研究

从耦合模理论出发,用矩阵分析法分析由两线性啁啾光纤光栅构成的啁啾叠栅光栅的反射特性和时延特性,分别得到光栅长度、啁啾系数和折射率变化峰值对反射谱及光栅时延的影响.     

Tunable filter based on a pair of special long-period fiber gratings and its application in fiber ring laser

A filter based on a Mach-Zehnder interferometer (MZI) formed by cascading a pair of longperiod fiber gratings (LPFGs) written by CO₂ laser in a twisted single-mode fiber is proposed. The transmission spectrum of the filter is tuned by modifying the bending curvature of the MZI, and thus wavelength tuning is achieved. And a tunable erbium-doped fiber (EDF) ring laser employing the filter as a wavelengthselection component is experimentally demonstrated. The laser can be tuned continuously from 1573.05 to 1595.75 nm with a side-mode suppression ration of ∼50 dB over the total tuning range.     

Transmission system over 3000 km with dispersion compensated by chirped fiber Bragg gratings

The limitation of the system with dispersion compensation by chirped fiber Bragg gratings is investigated in this paper. The transmission distance of the system based on chirped fiber Bragg gratings surpasses 3000 km. The bit error rate (BER) of the system is below 10⁻⁹ for as long as 2000 km. The BER is about 10⁻⁷ at 3000 km and, when forward error correction (FEC) is added, zero BER can be achieved. It is the longest transmission system with dispersion compensation by chirped fiber Bragg gratings.     

Controlling of symmetric and asymmetric mode coupling in long-period fiber gratings singe-side induced by long-pulse CO2 laser

We report a simple fabricating approach to control the mode couplings in long-period fiber gratings (LPFGs) through side exposing fiber to long-pulse-10.6-μm laser from a cheap, internally modulated CO₂ tube. By tuning focused-spot size on fibers, not only circularly symmetric mode coupling but also asymmetric mode couplings can be effectively achieved. Simulation of mode profiles in grating cross-section with Finite Element Method (FEM), and LPFG-cladding etching experiment with hydrofluoric acid (HF), support our explanation that asymmetric mode coupling in LPFGs depends on local refractive-index (RI) change within an azimuthally thin cladding layer, resulted from large-spot method induced deep melt flow on fiber surface during CO₂ laser irradiation.     

光纤光栅传输矩阵研究

利用模耦合理论给出并分析了一般坐标系下相移布喇格光纤光栅中的传输矩阵及其特性,所给出的传输矩阵具有分段不变性,而且不能被分解为一个矩阵和一个相移矩阵的乘积.利用该传输矩阵可以研究均匀、相移、啁啾、超结构等光纤光栅及光栅的级联等.计算了相移光栅的反射谱和相移量的关系,以及两个光栅级联时的反射谱.结果表明,同样相移量时的反射谱和已有文献不同,两个光栅级联时,也不同于已有文献,各自的谐振波长与光栅的级联没有关系.     

The ultra narrow-band fiber grating filter

In this paper, we propose an ultra narrow band fiber grating filter which is composed of two optical circulators and two fiber Bragg gratings (FBG). Through numerical simulation and experiment investigation, a narrow-band output spectrum with 1064 nm center wavelength and 0.017 nm bandwidth is obtained. It may find a certain application in the fields of free space optic communication, laser tuning and so on as we believe.     

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).     

Active phase control and frequency chirp effects on supercontinuum generation in high birefringence photonic crystal fiber

An acoustic-optics programmable dispersive filter (AOPDF) was first employed to actively control the linearly polarized femtosecond pump pulse frequency chirp for supercontinuum (SC) generation in a high birefringence photonic crystal fiber (PCF). By accurately controlling the second order phase distortion and polarization direction of incident pulses, the output SC spectrum can be tuned to various spectral energy distributions and bandwidths. The pump pulse energy and bandwidth are preserved in our experiment. It is found that SC with broader bandwidth can be generated with positive chirped pump pulses except when the chirp value is larger than the optimal value, and the same optimal value exists for the pump pulses polarized along the two principal axes. With optimal positive chirp, more than 78% of the pump energy can be transferred to below 750 nm. Otherwise, negative chirp will weaken the blue-shift broadening and the SC bandwidth.     

Investigation on the bistability of linearly chirped nonlinear Bragg gratings

Based on the nonlinear coupled mode theory, by using the reversely recursive transmission matrix method, the switching characteristics of linearly chirped nonlinear Bragg gratings (LC-NLBG) have been investigated numerically. The results show that, introducing spatial chirp in fiber gratings is helpful to widen the bistable wavelength range, and the influence of the chirp on the switching-on threshold and the on-off ratio of the bistable switching are obvious, the chirp-dependent bistability is due to the difference of the axial distribution of forward wave intensity in the gratings.     

啁啾双光纤光栅群时延特性

研究了变迹线性啁啾双光纤光栅的群时延特性及其在高次色散补偿中的应用.分析表明,群时延特性与周期差ΔΛ和不同的啁啾参量C有很大的关系.研究结果表明啁啾双光纤光栅在高次色散补偿和光纤CDMA中有应用价值.     

Actions of sodium nitrite on long period fiber grating with self-assembled polyelectrolyte films

In this paper, the ultrathin membranes of poly(sodium-p-styrenesulfonate) (PSS) and poly-dimethyl diallyl ammonium chloride (PDDA) are assembled on the surface of long period fiber gratings (LPFGs). The experiments indicate that the transmission spectra characteristics of LPFGs have greater change when bilayer number changed from 75 to 130. The action of sodium nitrite on the LPFGs is researched. The LPFGs without assembled films are not able to distinguish the concentrations of sodium nitrite solutions in the range of 0.001-0.2 mol/L. However, LPFGs assembled polyelectrolyte films of 130 bilayers have a higher distinguishability for these concentrations of sodium nitrite. With the increase of sodium nitrite concentrations, the center wavelengths of attenuation bands gradually shift towards the longer wavelength and represent a good linear relationship with concentrations. The method has the advantages such as low cost, no toxin, fast speed, no time limit. It is hopeful to be applied in real-time and on-line monitoring of sodium nitrite.