Comparison of the performance of optically amplified transmissionsystems

In this paper, we show a comparison among the performance of single- and multichannel optical systems considering NRZ and soliton signals. The results have been obtained by means of numerical simulations, taking into consideration the chromatic dispersion, the Kerr nonlinearity, the fiber loss, and the ASE noise of the optical amplifiers. The performance of IM-DD and coherent asynchronous ASK systems have been evaluated in terms of the Q factor. We have considered the propagation in links encompassing conventional step-index fibers, DS fibers and in links with two different dispersion management techniques in which the chromatic dispersion is varied along the propagation distance both in randomly and in deterministically way. The effects of the in-line filtering process are mainly investigated in soliton propagation     

Measurement of the spatial distribution of birefringence in opticalfibers

We describe a technique for the measurement of the birefringence spatial distribution in a single-mode optical fiber with a resolution of 1 m. This technique is based on a polarization optical time-domain reflectometer using a rotary linear polarizer. We report results performed on different types of fibers: standard step-index and dispersion shifted fibers     

Modes in divergent parabolic graded-index optical fibers

The authors discuss an optical fiber characterized by a core of divergent parabolic graded-index profile. A general formulation is developed to study this fiber as well as other types, including standard step-index and convergent parabolic profiles. An analytical approach to solve the basic differential propagation equation is developed. The solution takes the form of a summation of Bessel functions. An algorithm is programmed and simulations are carried out. Main characteristics of divergent profile fibers, such as propagation constant, dispersion, and power distribution, are presented and compared to those of standard fibers. It is found that the principal mode LP₀₁ exhibits a flat top distribution that better fills core's fiber, suggesting possible applications for laser fibers and amplifiers     

Measurement of dispersion using short lengths of an optical fiber and picosecond pulses from semiconductor film lasers

Group velocity dispersion spectra have been determined for 7.8-m lengths of a single-mode optical fiber. The dispersion spectra are derived from measurement of the transit delay time of picosecond pulses (4-10 ps) from optically pumped ultrashort cavity semiconductor film lasers. The delay time spectra have been measured to 1 ps precision over the range from 1.11 to 1.53 μm for single-mode step-index and quadruple-clad fibers. This is the highest temporal resolution achieved for any pulse delay measurement in optical fibers. The resulting dispersion spectra are in good agreement with measurements made for the same fibers by other techniques.     

三类色散渐减光纤环镜光脉冲压缩特性的比较

文章比较了3类色散渐减光纤环镜的光脉冲压缩特性,结果发现,不考虑高阶效应时,线性渐减光纤环镜需要的最佳光纤长度最短,压缩后的光脉冲质量较高;考虑高阶效应后,情况恰恰相反,高斯渐减光纤环镜需要的最佳光纤长度最短,压缩质量较高,但此时3种类型的光纤构成的非线性光纤环镜压缩特性差别不大.     

An efficient method for determining the chromatic dispersioncharacteristics of nonlinear single-mode optical fibers

To determine the dispersion characteristics of nonlinear single-mode optical fibers, the various orders of derivative of the propagation constant with respect to frequency must be calculated. Recent studies have shown that high nonlinearity changes the guided modal field and then modifies the chromatic dispersion characteristics. However, the methods used either were approximate or were based on rigorous formulation but requiring numerical differentiation. A rigorous method that eliminates the need of numerical differentiation entirely is proposed. It is based on a variational finite-element formulation. The propagation constant and the modal field are obtained by solving a set of nonlinear algebraic equations, and this part dominates the computational efforts. The derivatives of the propagation constant can be easily obtained by solving sets of linear algebraic equations. Several examples are examined to show the change of chromatic dispersion characteristics resulting from the change in the modal field under the influence of nonlinearity     

Frequency characteristics of self-sustained intensity oscillation of a laser diode using a delayed electrical feedback

Frequency characteristics of self-sustained intensity oscillation of a laser diode, introduced by a delayed electrical feedback using an optical fiber, is investigated. A step-index multimode fiber is utilized as an optical delay line, and it is found that the spectral profile of the self-sustained intensity oscillation follows the baseband frequency response of the fiber.     

Characteristics of dispersion-shifted dual shape core single-mode fibers

The structural design, bending loss, and dispersion characteristics of the dual shape core (DSC) single-mode fiber with zero dispersion at 1.55 μm are described. It is clarified that the bending loss of the DSC fiber is less than one tenth of that of a step-index fiber, and that the dispersion sensitivity due to a small change of the core radius is less than half of that of a step-index fiber. The explanation for the small bending loss is presented. A greater reduction of bending loss is expected from the replacement of the center core profile with a Gaussian or triangular profile.     

Dispersion compensation optical fiber modules for 40 Gbps WDM communication systems

Dispersion compensation was originally proposed to equalize pulse distortion.With the development of wavelength division multiplexing (WDM) techniques for large capacity optical communication systems,dispersion compensation technologies have been applied into the field.Fiber-based dispersion compensation is an attractive technology for upgrading WDM communication systems because of its dispersion characteristics and good compatibility with transmission optical fibers.Dispersion compensation fibers and the modules are promising technologies,so they have been receiving more and more attention in recent years.In this work,high performance dispersion compensation fiber modules (DCFMs) were developed and applied for the 40 Giga bit-rate systems.First,the design optimization of the dispersion optical fibers was carried out.In theory,the better the refractive index profile is,the larger the negative dispersion we could obtain and the higher the figure of merit (FOM) for the dispersion optical fiber is.Then we manufactured the fiber by using the plasma chemical vapor deposition (PCVD) process of independent intellectual property rights,and a high performance dispersion optical fiber was fabricated.Dispersion compensation fiber modules are made with the dispersion compensating fibers (DCFs) and pigtail fibers at both ends of the DCFs to connect with the transmission fibers.The DCFMs present the following superior characteristics:low insertion loss (IL),low polarization mode dispersion,good matched dispersion for transmission fibers,low nonlinearity,and good stability for environmental variation.The DCFMs have the functions of dispersion compensation and slope compensation in the wavelength range of 1525 to 1625nm.The experiments showed that the dispersion compensation modules (DCMs) met the requirements of the GR-1221-CORE,GR-2854-CORE,and GR-63-CORE standards.The residual dispersions of the G.652 transmission lines compensated for by the DCM in the C-band are less than 3.0ps/nm,and the dispersion slopes are also compensated for by 100%.With the DCFMs,the 8×80km unidirectional transmission experiments in the 48-channel 40Gbps WDM communication system was successfully made,and the results showed that the channel cost was smaller than 1.20dB,without any bit error.     

手征光纤中导模的色散特性

从手征介质中的电磁场本构关系出发，利用麦克斯韦方程组，对纤芯和包层都是手征介质构成的阶跃型手征光纤进行了解析求解，导出了模式场解和特征方程。用数值方法研究了纤芯和包层的手征导纳对手征光纤中导模的归一化传播常数、群延迟、波导色散等特性的影响。     

In-field comparison among polarization-mode-dispersion measurementtechniques

We report polarization mode dispersion measurements on a 168 km long undersea optical cable, performed with two techniques. From experimental results, statistical behavior of polarization parameters and their time fluctuations have been extracted. An average differential group delay of about 13 ps shows that dispersion-shifted fibers can be affected by a polarization dispersion significantly larger than standard step-index fibers. The correlation length of the polarization-coupling perturbation in the tested fibers is not negligible with respect to the link length     

Distributed measurement of the chromatic dispersion of an optical fiber using a wavelength-tunable OTDR

In this paper, a distributed measurement of the chromatic dispersion (CD) of an optical fiber is presented. The measurement is based on the analysis of a set of bidirectional optical time domain reflectometer traces collected over the wavelength range of interest. To calculate the distributed CD of the fiber, several approximations are required. The validity of these approximations is verified via a numerical simulation for both step-index and dispersion-shifted fibers (DSFs). These simulations allow the placement of an upper bound on the error of the measurement. Experimentally, measurements of both step-index and DSFs are reported.     

Prediction of Laser Wavelength for Minimum Total Dispersion in Single-Mode Step-Index Fibers

Pulse dispersion in single-mode optical fibers with step-index profiles has been analyzed in the past using asymptotic methods. One of these methods is based on the approximate characteristic equation for the dominant mode of propagation in these structures, obtained using the "weakly guided" condition. Other methods use approximations for certain parameters of this equation. Utilizing numerical methods of differentiation and interpolation, we have developed a method for the analysis of pulse dispersion in these fibers that is based on solutions of the exact characteristic equation. Exact formulas for the parameters necessary for this study have been established and developed to the point where the steps that would follow, involving extensive analytical effort, are replaced by computational procedures. We make comparisons between our method and those that, although based on asymptotic expressions, present the best theoretical characteristics. The differences found are discussed. This method permits greater precision in prediction of the ideal laser wavelength for use with a given single-mode optical fiber.     

Transmission characteristics of a single-mode fiber in the 1.3-µm wavelength region

Transmission characteristics of 1.3-μm-band single-mode optical fibers were studied experimentally and theoretically. OH-ion content dependence of optical loss in the 1.3-μm region was investigated on single-mode fibers fabricated by the VAD method. The optical loss was evaluated for OH-ion content by calculating correlation factors between the optical loss and 1.39-μm OH-peak absorption loss. It was clarified that the optical loss calculated by correlation factors agreed approximately with the loss given by Lorentzian absorption in case of 0.0253-μm absorption half width, Next, the relationship between chromatic dispersion and fiber parameters was studied for step-index single-mode fibers. It was found that the zero-dispersion wavelength and the incline at the wavelength was expressed as simple functions of effective cutoff wavelength and relative index difference. Experimentally obtained zero-dispersion wavelengths on single-mode fibers fabricated by the VAD method were found to be in excellent agreement with the calculated values.     

Modulation instability of wave packets in inhomogeneous optical waveguides

Conditions for the modulation instability of a wave packet in an optical waveguide with the exponential profile of the group velocity dispersion along the waveguide’s length is investigated. With allowance for the Kerr nonlinearity and the nonlinearity dispersion, analytical expressions for the integral increment of amplification of a harmonic perturbation and for other important characteristics determining the dynamics of the modulation instability in the waveguide with the specified inhomogeneity type are obtained.     

基于光纤差分相移键控的色散补偿方案的研究

为了研究光差分相移键控(DPSK)调制格式在光纤高速传输系统中的色散补偿, 利用色散补偿光纤(DCF)的色散补偿原理, 对40Gbit/s光纤传输系统进行色散补偿, 分析了40Gbit/s单通道光纤传输系统中3种DPSK调制格式信号的频谱特性; 仿真了3种码型的色散容忍度以及3种调制格式在考虑光纤的非线性下的色散补偿方案。结果表明, 光非归零码差分相移键控(NRZ-DPSK)信号具有最好的色散容忍度, 但其受非线性的影响比较大; 33%归零码差分相移键控(33%RZ-DPSK)信号的色散容忍度差, 但其色散补偿后的效果优于NRZ-DPSK; 而载波抑制归零码差分相移键控信号对色散和非线性效应都有较好的抑制; 3种DPSK调制格式均在对称补偿2方案中色散补偿的效果最佳。此仿真研究对光DPSK信号在光纤中的色散补偿具有参考意义。     

光子晶体光纤的原理、应用和研究进展

光子晶体光纤（POF）与普通光纤在光纤结构、单模特性、色散特性和非线性特性等方面有着显著的差别。本文将简要分析PCF的管理，并探讨其重要特性以及应用价值 ，最后回顾了近来PCF的研究进展。     

Uniform Core Field in Symmetrical Planar Waveguides and Circular Fibers with Nonlinear Claddings

It is known that no uniform electric field profile exists in a planar linear waveguide. The uniform core field can be shown to exist in symmetrical planar waveguides and circular fibers with nonlinear claddings. Theoretical analysis and numerical calculations are carried out to show that when the modal index equals to the core refractive-index, the core field becomes uniform at an appropriate optical power. Analysis for a step-index circular fiber with nonlinear cladding have also shown that the core field becomes uniform under similar conditions. The occurrence of a uniform field in a waveguide core may believe to have promising applications in waveguide, optoelectronic and photonic devices.     

色散缓变光纤中五阶非线性调制不稳定性

在三五阶非线性共存时,研究了有损耗单模光纤中基于两光波交叉相位调制的不稳定条件和增益谱.在色散缓变光纤正色散区,分析了五阶非线性系数、色散纵向变化参量以及两扰动的频率大小关系对交叉相位调制不稳定增益谱的影响.结果表明,在五阶非线性下,色散的纵向渐减仍有利于展宽调制不稳定增益谱;正五阶非线性可使增益谱的谱宽和谱峰值增大,并使谱峰位置远离主波频率,负五阶非线性的作用则相反;两扰动频率大小关系不同,色散纵向参量的变化对增益谱的谱峰大小和位置的影响也不同,五阶非线性对交叉相位调制不稳定性的加强或抑制程度也不同.     

图像处理在光纤光斑微位移传感中的应用

基于多模光纤的多模干涉原理以及图像处理算法,对多模光纤输出端面光斑图像相位谱的灰度共生矩阵特征值进行提取,提出一种新型光斑图样处理算法,并且将其应用于微位移传感。详细分析了以阶跃型多模光纤为基础的单模-多模光纤结构在两光纤径向产生微位移时,位移量与端面光斑图像相位谱特征值之间的关系。对多种结构参数光斑图样的分析表明,两光纤径向微位移与相关性及一致性两特征值成近似线性关系,可以用光斑图像处理的方法进行微位移测量,从而实现传感。实验结果表明与目前常用的计算光斑图像归一化强度内积处理算法相比,多模光纤芯径较小时,线性度、动态范围比较接近;芯径较大时,线性度提高的同时动态范围扩大约一倍。因此所提出的算法稳定性更好,适用范围更广。