PMD compensation using a fuzzy automatic polarization controller

A new first-order polarization mode dispersion (PMD) compensation technique is experimentally demonstrated in a 2.5 Gbit/s optical transmission system. It achieves adaptive compensation over a wide range of differential group delay (DGD) values, since it does not try to compensate it, but to eliminate one of the principal states of polarization by means of a liquid crystal polarization controller and a polarizer. The PMD parameters of the received signal are measured in real time through its spectrum analysis, and used as the feedback signal in a fuzzy logic algorithm controlling the liquid crystal device. The algorithm achieves a response time faster than the PMD variations in real links, also featuring endless operation. Moreover, a deterministic compact PMD emulator has been implemented in order to test the PMD compensator system.     

偏振模色散补偿控制中偏振度技术的性能

基于主偏振态理论，导出了光信号偏振度的解析表达式.并对40 Gb/s光传输系统中的信号偏振度受各种因素的影响进行了数值模拟，包括啁啾、脉冲形状、脉冲宽度、自发辐射噪声以及消光比等.结果表明，偏振度技术能有效地监测和控制40 Gb/s系统中小于37.5 ps的偏振模色散.而且发现就最大化偏振度技术对差分群延时的容许范围而言，脉冲的1/e强度半宽取0.45个位宽（11.25 ps）是最优的.     

偏振模色散补偿中的偏振主态与分束器主轴的对准

光纤通信系统的比特率超过10Gb/s或更高时,偏振模色散引起的脉冲信号展宽成为主要障碍.分离光纤线路中的两个偏振模式对于提高偏振模色散补偿的精度和速度有重要意义.讨论了光纤线路中两偏振主态与补偿器中偏振分束器主轴的对准问题,给出了偏振分束器任何一个主轴上光强的表示并推导出相应的电功率信号表示式,建立了电功率与光信号两个模式之间的延迟时间以及偏振主态与分束器主轴相对角度的变化关系.初步实验表明,可以通过偏振控制器或可转动的光纤连接器实现偏振主态与偏振分束器的对准.     

飞秒激光脉冲的频率分辨偏振光学开关法测量研究

建立了一台频率分辨偏振光学开关（PG FROG）法飞秒脉冲测量装置，利用该装置对“极光Ⅱ号”飞秒激光放大系统进行了测量.在利用偏振光学开关法测得的时域和频域信号基础上，结合对信号光强度分布的计算机迭代处理，得到了有关飞秒激光电场、光谱及其相位的信息；并且对系统工作在不同状态时的激光脉冲进行了测量和比较分析，给出了有关该系统较详细的电场、光谱、相位以及啁啾状况.结果显示，当系统工作在零啁啾附近时，该系统输出的激光脉冲的电场、光谱和相位分布较规则，相位起伏较小；当系统偏离零啁啾状态时，虽然电场和光谱变化不很明显，但相位分布变化剧烈. 关键词： 频率分辨偏振光学开关(PG FROG)法 飞秒激光 自相关     

Enhanced soliton robustness to polarization mode dispersion by using degenerate four-wave mixing

We propose a new method to strengthen the nonlinear pulses robustness to polarization mode dispersion through modulating the breath oscillation behavior of two principal polarization components of optical pulses. The optical realization of the breath behavior modulation is based on the degenerate four-wave mixing effect in fibers with high birefringence. The breath intensity increases and the periodical breath oscillation distances shorten in the process of breath behavior modulation. It is proved that the breath-modulated pulses have stronger adaptive abilities to polarization mode dispersion than common solitons when propagating in conventional single-mode fibers with random birefringence.     

Optically compensated dual-polarization interferometric fiber-optic gyroscope

We first introduce optical compensation into the field of interferometric fiber-optic gyroscopes (IFOGs), using the opposite polarity of the disturbance between two orthogonal polarization states to suppress polarization-fluctuation-induced noise. A dual-polarized IFOG, in which the fast and slow axes of polarization-maintaining fiber work simultaneously, is implemented and tested. Interference signals of the two axes are added to achieve optical compensation. Experiments show that the IFOG's sensitivity is effectively enhanced in compensated output: all Allan variance indices are improved, among which bias instability is reduced from 0.335 and 0.227 deg/h(fast and slow axes output, respectively) to 0.061 deg/h.     

Polarization locked vector solitons and axis instability in optical fiber

We experimentally observe polarization-locked vector solitons in optical fiber. Polarization locked-vector solitons use nonlinearity to preserve their polarization state despite the presence of birefringence. To achieve conditions where the delicate balance between nonlinearity and birefringence can survive, we studied the polarization evolution of the pulses circulating in a laser constructed entirely of optical fiber. We observe two distinct states with fixed polarization. This first state occurs for very small values birefringence and is elliptically polarized. We measure the relative phase between orthogonal components along the two principal axes to be +/-pi/2. The relative amplitude varies linearly with the magnitude of the birefringence. This state is a polarization locked vector soliton. The second, linearly polarized, state occurs for larger values of birefringence. The second state is due to the fast axis instability. We provide complete characterization of these states, and present a physical explanation of both of these states and the stability of the polarization locked vector solitons. (c) 2000 American Institute of Physics.     

20Gb/s系统中偏振模色散的偏振度反馈控制分析

基于主偏振态理论和一阶偏振模色散近似,推导出光信号偏振度的一个简洁解析表达式。并利用该解析表达式和快速傅里叶变换,对20Gb／s系统的偏振模色散补偿中,偏振度监测法的灵敏度和跟踪范围受多种因素(如脉冲啁啾、占空比、消光比、放大自发辐射噪声、自相位调制等)的影响程度进行了数值模拟分析。发现利用偏振度监测法反馈控制偏振模色散补偿时,监测灵敏度和跟踪范围是折中的,较小的占空比有利于提高偏振度监测法的灵敏度,而且总可以对高达1．5个位周期(75ps)的差分群延时进行跟踪,但当占空比低于0．5时,跟踪范围迅速缩小,消光比和放大自发辐射噪声不会明显改变偏振度监测法的跟踪范围,自相位调制效应显著地影响偏振度监测法的性能。     

Accurate Jones matrix expansion for all orders of polarization mode dispersion

I present a simple method for extracting the dispersive elements from a general unitary Jones matrix in such a way that the parts comprising the phase delays and higher-order dispersion in the two principal states of polarization are separated from those describing the frequency-dependent coupling between them. I then rigorously expand these two parts into power series in relative optical frequency, wherein the coefficients are explicit functions of the polarization mode dispersion vectors of various orders. This accurate expansion of the Jones matrix reveals in particular the amplitudes and phases of the cross-coupled components of the principal states of polarization, which are incorrectly described in most polarization mode dispersion models.     

10 Gb/s光纤通信系统一阶偏振模色散动态补偿系统反馈控制的实现

在偏振模色散(PMD)自动补偿技术中,如何根据反馈信号得到相应的控制信号,使补偿速度跟随偏振模色散变化始终是该技术的一个核心问题。提出了一种新颖的自适应抖动跟踪算法,完成了以微波信号为反馈的多自由度的一阶偏振模色散自动反馈补偿系统的跟踪补偿实验。算法成功地解决了传统算法在跟踪搜索过程中易陷入局部极值的问题,有效地克服了系统中的重要控制器件偏振控制器的磁滞现象以及动态补偿时跟踪搜索过程中易出现的瞬间恶化现象。实验结果表明该算法在对出现突发偏振模色散扰动后自动进行补偿的响应速度在ms量级,最快能达到1~2 ms。     

Two-photon fourth-order polarimetery

We demonstrate a fourth-order polarimeter that employs two-photon detection to measure the fourth-order polarization coherency matrix (FOCM) of an optical signal. We measure the FOCM of picosecond pulses subjected to polarization mode dispersion (PMD). The fourth-order polarimeter distinguishes different PMD states even when the state of polarization, including the degree of polarization, is constant.     

基于光信号偏振度的偏振模色散补偿系统的研究

偏振模色散已成为当前发展高速长距离光纤传输系统的主要限制因素。理论上分析了光纤一阶偏振膜色散效应对高速伪随机非归零码／归零码光信号偏振度的影响，并利用数值模拟的方法分析了信号不同输入偏振态以及高阶偏振膜色散效应对非归零码光信号偏振度的影响。最后对利用信号偏振度作为反馈控制信号的自适应偏振膜色散补偿系统的补偿性能进行了分析，大量统计分析结果表明对于10Gbit／s的非归零码光纤传输系统，当传输线路的平均偏振膜色散值小于43ps时，利用极大化输出信号偏振度的偏振膜色散补偿系统对信号眼图的补偿概率可以达到99．99％．     

Waveform Measurement of Ultra-Short Optical Pulses Based on Two-Photon Absorption in Si-Image Sensor

We demonstrate a novel waveform mesurement system of ultra-short optical pulses based on the two-photon absorption process in a Si-image sensor. Using an interferometer with a tilt mirror in the reference path, the relative time difference between the signal and reference pulses is spatially distributed on the Si-image sensor, so the intensity auto-correlation is monitored as an image at a time without using moving parts. This system can create or remove an interference fringe pattern overlapped on the auto-correlation image by controlling the polarization states of the laser pulse, which is useful for measuring the crossing angle between the signal and reference beams as well as avoiding contamination of the interference fringe into the auto-correlation image. We successfully measured optical pulses less than 100 fs from a fiber ring laser with a temporal resolution of 0.4 fs. The obtained waveform agreed well with that observed with a conventional second harmonic generation (SHG) based auto-correlator.     

An experiment of PMD compensation in 40-Gb/s PSBT transmission system

An adaptive polarization mode dispersion （PMD） compensation experiment is reported in a 40-Gb/s phase shaped binary transmission （PSBT） communication system, with the use of a new digital signal processor （DSP）-based optical PMD compensator. PMD tolerance is found to be enhanced by 8 ps after PMD compensation with 1-dB optical signal-to-noise ratio （OSNR） penalty. Under the condition of fast change of states of polarization up to 85 rad/s in the fiber link, the performance of our PMD compensator undergoes the bit error ratio （BER） test for as long as 10 h.     

Characteristics Measurement of A FLM-Grating Cavity WDM Fiber Laser

In this paper, we study an FLM-grating cavity WDM fiber laser with dynamic polarization compensation technique for maintaining the output polarization states. We observe the characteristics and obtain the optimal experimental results.     

道威棱镜的偏振特性及偏振补偿研究

为了避免机载光电吊舱中共口径光学系统内部由于道威棱镜旋转引起的激光照射脉冲偏振态的变化,利用琼斯矩阵对道威棱镜的偏振特性与四分之一波片、半波片补偿道威棱镜旋转引起的激光脉冲偏振态变化进行了理论分析和实验验证。结果表明:线偏振的激光脉冲通过旋转一定角度的道威棱镜时,激光脉冲偏振态变为椭圆偏振,偏振态发生变化;而激光脉冲首先通过旋转一定角度的四分之一波片与半波片时,可使通过道威棱镜系统的激光脉冲偏振态保持不变,且两波片旋转角度与道威棱镜旋转角度之间存在一种非线性关系。采用偏振补偿方法可有效避免机载共口径光学系统中道威棱镜引起的激光脉冲偏振态变化,提高激光脉冲能量利用率,降低激光脉冲后向散射抑制难度。     

Polarization orthogonalization properties of optical systems

We investigate the conditions under which an optical system (or device) may transform polarization states at its input into orthogonal states at its output. We find that such polarization orthogonalization is possible if the Jones matrix of the optical system satisfies a specific inequality. One, two, or an infinite number of input polarization states may be orthogonalized. In the latter case, the locus of input states is a circle in the complex plane (and on the Poincaré sphere) of polarization. Several examples are given for illustration.     

单腔双光梳拍频信号包络提取方法

对单腔双光梳拍频信号的包络提取方法进行了实验研究。在单个光纤激光器中引入具有强双折射的保偏光纤,使脉冲沿偏振正交的两个方向进行复用传输和锁模。精细调节腔内偏振态,实现了重频差在337 Hz～2.33 kHz范围内连续可调的双光梳生成。通过激光器腔外偏振态调节和偏振分束,获得了消光比分别为28.5 dB和38.2 dB的两路光频梳。两路光频梳经过异步采样后,采用所设计的包络检波电路对拍频信号的包络进行提取,与采用基于样条插值和希尔伯特变换的包络提取算法计算结果相比,所提取的包络峰值位置基本保持一致,验证了该方法的可行性。双光梳拍频信号包络提取实验研究可以快速实时提取包络形状和包络峰值位置,可进一步应用于包络信号触发和精密测距等领域。     

On nonlinear propagation of extremely short pulses in optically uniaxial media

Nonlinear wave equations describing the propagation of optical pulses of duration up to a period of electromagnetic oscillations in transparent media with uniaxial optical anisotropy are derived on the basis of a quantum-mechanical model of material response. The electron and electron-vibrational nonlinearities, electron and ion dispersion, and diffraction are taken into account. It is shown that the inclusion of the electron response alone leads to a system of two constitutive equations for the ordinary and extraordinary polarization components. When a pulse propagates across the optical axis, this system is reduced to an inhomogeneous model of the Henon-Heiles type and, hence, generalizes the Lorentz classical electron model. In order to take into account stimulated Raman scattering (SRS) processes, an anisotropic analog of the Bloembergen-Shen quantum-mechanical model taking into account the population dynamics of SRS sublevels is obtained. The generation of an extraordinary wave video pulse with the help of the high-frequency ordinary component in the Zakharov-Benney resonance mode is investigated. Some analytic soliton-like solutions in the form of propagating bound states of ordinary and extraordinary video pulses corresponding to different birefringence modes are considered and their stability to self-focusing is analyzed.     

Optical coherent transient continuously programmed continuous processor

A novel technique for continuously programming an optical coherent transient spatial-spectral signal processor is proposed. The repeated application of two spatially distinct optical programming pulses to a nonpersistent hole-burning material writes an accumulated spatial-spectral population grating. An optical data stream is introduced on a third beam, resulting in a processor output signal that is spatially distinct from all the input pulses. Programming and processing take place simultaneously, asynchronously, and continuously. In the case of true-time delays, the efficiency that is achievable with currently available materials is of the order of that predicted for a perfect photon-gated device.