Statistical properties of Rayleigh backscattering in single-modefibers

An analytical model that takes into account the influence of the laser linewidth on Rayleigh backscattering is given. For an unmodulated source the power spectral density is found to be identical to the delayed self-homodyne spectrum of the laser. The backscattered signal was measured for different laser linewidths, and the results are compared with the theory. The effects of Rayleigh backscattering noise on fiber gyroscopes and bidirectional optical systems are discussed     

Crosstalk penalities due to coherent Rayleigh noise inbidirectional optical communication systems

The statistical properties, i.e. power spectral densities and probability density functions, of the noise due to Rayleigh backscattered light and due to interference of backscattered and reflected light are derived from the autocorrelation function of the optical source field. The crosstalk penalties due to Rayleigh backscattering in single- and double-source bidirectional systems are calculated. It is shown that it is possible to realize a bidirectional system with commercially available connectors in which Rayleigh backscattering is the dominant crosstalk contribution. The measured crosstalk penalties and bit-error probabilities in a double-source bidirectional system confirm the theoretical results     

混沌脉冲激光雷达水下目标探测

海水对光波的吸收和散射,严重制约了激光雷达水下目标探测的性能。通过对激光在海水传输过程中产生后向散射的定量分析,说明了激光回波信号被海水后向散射影响的严重性。分析比较了距离选通技术和强度调制技术抑制海水后向散射的能力,提出了使用自身具有高频强度调制特性的混沌脉冲激光进行水下目标探测,设计了基于相关法测距的混沌脉冲激光雷达水下目标探测方案。通过对后向散射光以及带有不同后向散射强度的回波信号光的时域和频域特性的研究,使用互相关噪声水平算法判定混沌脉冲激光雷达抑制海水后向散射的能力。理论仿真分析表明,当后向散射光强度是混沌脉冲激光强度36倍时,仍能提取出目标信号。     

采用后向瑞利散射空间差分的周界安防分布式振动监测

提出了应用于周界安防的基于后向瑞利散射光空间差分干涉的光纤分布式振动传感监测方案。环境振动信号引起传感光纤的相位变化,含有相位信息的后向瑞利散射信号注入到非平衡迈克尔逊干涉仪,实现了后向瑞利散射信号的空间差分干涉,光纤分布式振动传感监测完成相位信号的解调,提高了系统的灵敏度。该系统能够实时监测入侵者的行走路线和速率以及模拟挖掘的距离的判断,实现了15 dB的信噪比以及35 m的探测距离。     

单模光纤中高相干光源的瑞利散射光的统计特性

基于光纤中高相干光源(相干长度大于100km)的瑞利散射光的统计特性,采用随机相向量模型,推导了光纤中瑞利散射光的功率分布和相位分布,指出瑞利散射光功率的概率密度函数(PDF)符合修正的莱斯分布,而瑞利散射光的相位分布近似为高斯分布。采用高灵敏度的光电探测器对不同长度的光纤的瑞利散射光功率进行了测试,实验结果与理论吻合,同时发现不同长度光纤的瑞利散射光功率的最大值随平均值线性增加。     

Spectral behavior of directly modulated laser diodes exposed to Rayleigh backscatter from a single-mode fiber

The spectral behavior of a laser diode exposed to Rayleigh backscattering (RB) from a single-mode fiber is investigated for arbitrary laser-fiber coupling efficiency, fiber length, and coherence time of the light of the free-running source. It is found that resonance effects in the spectral behavior of laser diodes exposed to RB from a single-mode fiber occur only for coupling efficiencies above a critical value eta /sub c/. This threshold is very low for unmodulated light but it is substantially increased when the light is modulated. A simple mathematical relation is given showing the dependence of the threshold on the fiber length and on the coherence time of the light.<>     

Quantitative analysis of Rayleigh crosstalk in Raman amplifiers

Exact and approximate analytical formulas for the inband crosstalk caused by discrete reflection followed by Rayleigh backscattering are derived. Also, a simplified expression for the double Rayleigh backscattered power is found and the total crosstalk in terms of elementary functions is calculated. Results of measurements performed in a nonzero dispersion-shifted fibre agree with theoretical predictions within 1 dB.     

Rayleigh Scattering Reduction by Means of Optical Frequency Dithering in Passive Optical Networks With Remotely Seeded ONUs

With the purpose of reducing the crosstalk caused by Rayleigh scattering in bidirectional centralized light systems, we propose and experimentally demonstrate an optical frequency modulation to broaden the optical spectrum effectively. Analytic expressions for the Q-parameter of the signal affected by Rayleigh backscattering are also given and experimentally checked, both for optical frequency stable and dithered sources     

无源光接入网中反射式ONU的研究

对光纤到户(FTTH)网络来说,一个低成本高效的灵活接入方案是非常重要的.在波分复用-无源光网络(WDM-PON)中,用户端的收发器件以及波长分配方案是系统的关键.单纤双向单波长方案可以大大简化网络结构,降低用户的接入成本,但这种结构存在很严重的瑞利反向散射的串扰.文章分析了各种不同ONU端光源的实现方式,并分析了若干种抑制瑞利反向散射串扰影响的技术.     

Polarization properties of Rayleigh backscattering in single-modefibers

The state and degree of polarization (SOP, DOP) of Rayleigh backscattered light in low birefringent optical fibers was calculated using Stokes calculus. It was found that the SOP of Rayleigh backscattered light in low birefringent fiber is constant, viz. the same as the SOP of the incoming light. The DOP of Rayleigh backscattered light is one-third of the DOP of the incoming light, and not just anywhere between 0 and 100% as often mentioned in the literature. The experimental results agree with the calculations     

Narrow linewidth fiber laser for 100-km optical frequency domain reflectometry

A diode-pumped single-frequency piezoelectrically tuned fiber laser with narrow spectral linewidth has been used as a light source in applications for long-range coherent frequency-domain reflectometry. Frequency-modulated continuous-wave (FMCW) measurements of Rayleigh back-scattering and Fresnel reflection from a 95-km-long fiber have been demonstrated without the use of an optical amplifier. This is, to our knowledge, the longest distance measurement with FMCW. The high sensitivity and dynamic range of the long-range backscattering measurements benefit from the extremely long coherence length of the narrow linewidth fiber laser, which has been estimated to be 210 km in air.     

Linewidth of an externally modulated laser subjected to Rayleighbackscattering from a fiber

It is found that the linewidth of a semiconductor laser that is externally phase modulated and connected to a fiber with Rayleigh backscattering is broadened. At low modulation rates the linewidth can be several orders of magnitude greater than the free-running linewidth, but it converges to the free-running linewidth at sufficiently high modulation rates. As a result, the isolation needed to attain a specific linewidth can be calculated     

Rayleigh backscattering in a fiber gyroscope with limited coherence sources

Noise due to temporal fluctuations of Rayleigh baekscattered light in fiber-optical gyroscopes is studied experimentally with various sources whose coherence length is less than the fiber length. The reduction of the coherent fraction of backscattered light and its fluctuation frequencies with reduced source coherence is demonstrated and fit to an analytical model. Measured backscatter parameters for the fiber and sources used are presented.     

External Optical Feedback Induced Noise in DFB Fiber Laser Sensor System

External optical feedback effects due to reflection, Rayleigh backscattering and coherent Rayleigh backseattering in fiber distributed feedback （DFB） fiber laser sensor system have been investigated. If the feedback intensity exceeds critical amount, excess noise would be induced in the demodulator. The maximum tolerable intensity back-reflection coefficient Rc and backscattering coefficient Sc into a fiber DFB laser with lead fiber length from I m to 37.5 km before the onset of instabilities are shown. Rc is found to decrease with increasing lead fiber length while Sc was relatively invariable with varying fiber length. The coherent Rayleigh backscattering （CRBS） would induce neglectable noise with a lead fiber exceeding 13.5 kin. To eliminate these noises, one or two isolators should be incorporated in the system.     

水体散射对气泡后向光散射特性的影响

采用Lambert-Beer定律对水中气泡幕前向、后向散射激光功率进行了理论推导,得出了接收信号信噪比的计算公式.采用光学遮挡的方法对近距离水体后向光散射信号进行屏蔽,采用激光功率计对后向光散射信号进行测量.结果表明,近距离水体后向光散射对气泡的后向光散射影响显著,对近距离水体的后向光散射信号进行抑制,可以显著提高接收信号的信噪比.     

Interferometric Crosstalk Reduction in an RSOA-Based WDM Passive Optical Network

Reflection and Rayleigh backscattering-induced interferometric crosstalk in a link employing a reflective semiconductor optical amplifier (RSOA) may cause significant power penalty and, thus, limit the performance of the system. In this paper, we investigate interferometric crosstalk suppression in a centralized light generation wavelength division multiplexing-passive optical network (WDM-PON) by single-tone phase modulation either by utilizing the nonlinear behavior of the RSOA at the optical network unit (ONU) or by applying an external phase modulator at the source side. 6- and 7-dB reduction in power penalty for reflection-induced crosstalk is achieved, respectively. For Rayleigh backscattering-induced crosstalk power penalty is improved with 3 and 4.5 dB, respectively. The results show that an RSOA is very sensitive to reflections and backscattering and the tolerance to these impairments can be significantly improved by appropriate phase modulation. A comparison with advantages and disadvantages of both methods together with final recommendation is also given in the paper.     

掺铒光纤中的后向瑞利散射的研究

本文对掺饵光纤的瑞利散射进行了理论分析，计算了后向瑞利散射光与泵浦功率、掺饵光纤长度、信号光功率的关系，并提出了用隔离器减小后向瑞利散射光的方法。     

一种具有迈克耳逊结构的新型光时域反射仪

利用传输光纤中后向瑞利散射光携带有光纤位置、外界振动等信息的特点,采用光纤迈克耳逊结构构成干涉型光时域反射仪,有望以分布方式传感沿光纤的微小振动信号.文中设计了一种新型结构的分布式光纤振动传感器,该传感器用3dB耦合器将波长为1310nm的红外激光分为两路相干光:参考光和传感光,它们中的后向瑞利散射光在耦合器处发生干涉,干涉信号的变化点对应于振动点所在的位置.将没有振动信号、加微小振动信号和加损耗干扰三种情况分别作用于传感光纤并进行了试验对比.结果表明,在微小振动的情况下,光强变化微弱;加损耗时,光强变化显著,定位准确.     

OTDR in monomode fibres at 1.3 ?m using a semiconductor laser

Optical time domain reflectometry (OTDR) in monomode optical fibre using a 1.3 ?m semiconductor laser source is reported for the first time. A nonreflecting fibre defect may be accurately located at a range exceeding 17 dB one-way loss by observing the cessation of the Rayleigh backscattered signal.