远程干涉型光纤传感系统的非线性相位噪声分析

以远程干涉型光纤传感系统为背景, 研究了系统非线性相位噪声构成, 对各构成要素的具体影响进行了详细分析和综合评价, 简要讨论了噪声抑制方案. 研究表明, 系统相位噪声主要包括强度噪声转化而来的相位噪声、非线性效应引起激光 线宽展宽导致的相位噪声以及自相位调制和交叉相位调制引入的相位噪声. 受激布里渊散射和四波混频可引入强度噪声并转化为相位噪声, 对于探测带宽较窄的光纤传感系统, 四波混频引入的该部分噪声往往可以忽略. 受激布里渊散射、四波混频和调制不稳定性都可引起激光线宽展宽从而造成相位噪声的增大. 当系统信道数目较多时, 交叉相位调制对相位噪声的贡献不可忽略. 所得结论对远程干涉型光纤传感系统的实际应用具有重要的指导意义.     

Stimulated Brillouin scattering-induced phase noise in an interferometric fiber sensing system

Stimulated Brillouin scattering-induced phase noise is harmful to interferometric fiber sensing systems. The localized fluctuating model is used to study the intensity noise caused by the stimulated Brillouin scattering in a single-mode fiber. The phase noise structure is analyzed for an interferometric fiber sensing system, and an unbalanced Michelson interferometer with an optical path difference of 1 m, as well as the phase-generated carrier technique, is used to measure the phase noise. It is found that the phase noise is small when the input power is below the stimulated Brillouin scattering threshold, increases dramatically at first and then gradually becomes flat when the input power is above the threshold, which is similar to the variation in relative intensity noise. It can be inferred that the increase in phase noise is mainly due to the broadening of the laser linewidth caused by stimulated Brillouin scattering, which is verified through linewidth measurements in the absence and presence of the stimulated Brillouin scattering.     

Stimulated Brillouin scattering-induced phase noise in an interferometric fiber sensing system

Stimulated Brillouin scattering-induced phase noise is harmful to interferometric fiber sensing systems.The localized fluctuating model is used to study the intensity noise caused by the stimulated Brillouin scattering in a single-mode fiber.The phase noise structure is analyzed for an interferometric fiber sensing system,and an unbalanced Michelson interferometer with an optical path difference of 1 m,as well as the phase-generated carrier technique,is used to measure the phase noise.It is found that the phase noise is small when the input power is below the stimulated Brillouin scattering threshold,increases dramatically at first and then gradually becomes flat when the input power is above the threshold,which is similar to the variation in relative intensity noise.It can be inferred that the increase in phase noise is mainly due to the broadening of the laser linewidth caused by stimulated Brillouin scattering,which is verified through linewidth measurements in the absence and presence of the stimulated Brillouin scattering.     

Stimulated Brillouin scattering limited intensity-dependent four-wave mixing between pump and signal in a non-zero dispersion shifted fiber

The intensity-dependent four-wave mixing (FWM) efficiency is derived by the nonlinear Schrodinger equations including the self phase modulation and cross phase modulation between the two Fabry–Perot modes of a pump and a single-wavelength signal. The maximum FWM for mixing the pump with signal in non-zero dispersion shifted fiber is explained as the minimum phase mismatch for FWM due to equal group delay for the pump and signal. It was also found that the experimental measurements for the generated FWM power are limited by the stimulated Brillouin scattering (SBS) threshold. Under the SBS threshold, the experimental results are still coincided well with the theoretical calculations of the FWM generated power ratio to have the difference of 0.12 dB for the input signal power of 12 mW.     

Relative intensity noise induced by four-wave mixing in the case of two-wave transmission in an optical fiber

Four-wave mixing (FWM) is a significant nonlinear effect in wavelength division multiplexing (WDM) fiber-optic systems. For two-wave transmission, it is easily found that the FWM noise power decreases with frequency spacing and increases with signal power. However, the variation of relative intensity noise (RIN) with frequency spacing and signal power is only 2 dB at most. The intensity fluctuations induced by the energy exchange between the FWM generated new waves and the original ones are trivial and the influence of FWM on RIN can be neglected. It is also found that the increase of RIN with signal power is mainly attributed to stimulated Brillouin scattering (SBS) rather than FWM.     

Effect of four-wave mixing in WDM optical fibre systems

Wavelength division multiplexing (WDM) can both significantly enhance transmission capacity and provide more flexibility in optical network design. Through the use of erbium-doped fibre amplifiers (EDFAs), it is possible to build long-distance transparent optical transmission links without electrical regenerators. In such systems, fibre nonlinearities are likely to impose a transmission limit due to increased total interaction length. There are a number of optical nonlinear effects in optical fibres, such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), carrier-induced phase modulation and four-wave mixing (FWM). Out of these SRS and FWM are the dominant effects. In this paper, an algorithm has been suggested to study the effect of FWM in the total system noise considering the combined effect of SRS and FWM in the presence of amplified spontaneous noise (ASE). It has been found from the study that to maximize the signal-to-noise ratio of the transmitted signal in a WDM system FWM noise needs to be reduced as this is the dominant noise factor.     

基于法拉第旋镜的干涉型光纤传感系统偏振相位噪声特性研究

偏振诱导信号衰落现象的抑制是干涉型光纤传感系统的关键技术之一. 针对法拉第旋镜(FRM)法抑制偏振诱导信号衰落技术的残留偏振相位噪声问题进行了深入的理论和实验研究. 运用琼斯矩阵法建立了基于法拉第旋镜的干涉型光纤传感系统偏振相位噪声的理论模型; 分析了影响系统偏振相位噪声的主要原因: 法拉第旋镜的旋光角度偏差、入射光偏振态调制度、干涉仪两臂光纤双折射; 提出了相应的抑制偏振相位噪声的方法. 详细仿真分析了入射光偏振态调制度对干涉型光纤传感系统偏振相位噪声的影响, 仿真分析得出若法拉第旋镜旋光角度偏差为最大工艺制造误差1°, 当入射光偏振态调制度为1.84 rad时, 系统可能出现的最大偏振相位噪声为0.0815 rad. 最后, 搭建了基于M-Z型偏振态调制器的偏振相位噪声测试系统, 测试了在传输光纤受到外界偏振扰动的情况下, 干涉传感系统存在的偏振相位噪声, 实验测试结果与理论分析结果基本一致, 有力地证明了该偏振相位噪声理论分析模型的正确性.     

Analysis of bottlenecks in DWDM fiber optic communication system

This paper deals with the limiting factors like Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS) and Four Wave Mixing (FWM) in DWDM Fiber Optic Communication. The variation of SBS threshold power level for the spectral width of the source is analyzed and it is found that addition of spectral width in the source lowers the SBS effect. By reducing the SBS effect, we could transmit more optical power in a single channel and maintain high OSNR. Similarly the effect of phase mismatching (dispersion) and wavelength spacing on both FWM and SRS are analyzed. This work aims at finding the optimal value of spectral width, dispersion and channel spacing, thereby exploring the possibilities of increasing the overall capacity of the fiber optic trunk line.     

基于小波变换的长距离光正交频分复用系统中四波混频引起的相位噪声分析

四波混频（FWM）效应是相干光正交频分复用（CO-OFDM）系统中最主要的非线性作用。分析基于小波变换的相干光正交频分（WT-OOFDM）系统原理,研究小波变换对FWM效应和光放大器自发辐射（ASE）引起的相位噪声的改善效果。数值结果表明,在采用常规G. 652光纤,100 Gbit/s的WT-OOFDM传输1500 km时,非线性相位噪声降低20%,总相位噪声降低15%。     

Numerical analysis for four-wave mixing based wavelength conversion of differential phase-shift keying signals

We numerically investigate the main constrains for high efficiency wavelength conversion of differential phase-shift keying （DPSK） signals based on four-wave mixing （FWM） in highly nonlinear fiber （HNLF）. Using multi-tone pump phase modulation techniques, high efficiency wavelength conversion of DPSK signals is achieved with the stimulated BriIlouin scattering （SBS） effects effectively suppressed. Our analysis shows that there is a compromise between conversion efficiency and converted idler degradation. By optimizing the pump phase modulation configuration, the converted DPSK idler＇s degradation can be dramatically decreased through balancing SBS suppression and pump phase modulation degradation. Our simulation results also show that these multi-tone pump phase modulation techniques are more appropriate for the future high bit rate systems.     

DWDM系统非简并四波混频串扰的分析

考虑到DWDM系统中的比特序列的随机性、信道间脉冲走离效应等多种因素的影响,完善了归零调制格式下的四波混频效率的计算方法,给出了相应的非简并四波混频噪声标准差的计算公式,并利用该公式进行实例分析,计算结果表明,当占空比和信道间隔变小时,不同信道内的随机比特序列的相对初始时延及光纤色散值的取值组合对四波混频效率的影响变得更大,为保证系统的低色散和低四波混频串扰特性,必须对这些量取值组合进行优化. 关键词： 非简并四波混频 脉冲走离 密集波分复用     

Extension of measurement range in Brillouin optical correlation domain analysis by pump-probe switching

We propose and experimentally demonstrate a novel method for the extension of the measurement range in Brillouin optical correlation domain analysis by switching the pump and probe waves for selectively inducing the stimulated Brillouin scattering (SBS) in the sensing fiber. In this scheme, two correlation peaks are located in the sensing fiber and SBS is alternatively induced at each position by pump/probe selective switching and optical attenuation. In the experiment, twice enlargement of measurement range is successfully achieved with 146-m fiber with a 10-cm spatial resolution.     

Reduction of fiber-nonlinearity-enhanced amplifier noise by means of phase-sensitive amplifiers

In optical fiber transmission systems near the zero-dispersion wavelength that use in-line erbium-doped fiber amplifiers (EDFA's), the enhancement of optical amplifier noise caused by four-wave mixing (FWM) in transmission fibers degrades signal-to-noise ratio (SNR) excessively. We theoretically show that the enhancement of amplifier noise by the FWM in transmission fibers can be effectively eliminated by implementing in-line phase-sensitive amplifiers (PSA's). Small-signal analysis of the nonlinear Schr?dinger equation shows that the transmission distance limited by the SNR of an in-line PSA system is expanded four times more than that of an in-line EDFA system.     

外调制光纤CATV中SBS与MPI的激光器高频微扰抑制理论分析

受激布里渊散射(SBS)与多径干涉噪音(MPI)是光纤AM-CATV外调制传输系统中主要的非线性现象,本文分析了激光器高频扰动抑制SBS与MPI的原理,导出了扰动对系统SBS阈值提高量及MPI抑制程度的理论公式,通过对扰动引起的激光器频率啁啾及附加强度调制而导致的色散的理论分析与计算,得到了扰动信号频率及幅度的选取原则。     

高非线性光纤中四波混频的磁控机理研究

将磁光效应和光纤非线性效应作为微扰,采用导波光耦合理论分析了高非线性光纤中导波光发生磁光四波混频的机理;实验测试了磁场对四波混频偏振依赖性的影响,理论分析与实验结果符合.研究表明,当输入的探测光和抽运光为相互正交的线偏振光时,四波混频的磁控效果最为明显,输出的闲频光功率随磁光耦合系数振荡变化,施加适当的外加磁场可使四波混频效率得到进一步提高.选择适当费尔德常数的光纤材料,利用磁光效应对四波混频偏振依赖性的影响,可实现不同范围的磁场测量.     

Full investigation of the backscattering in resonator fiber optic gyro

Resonator Fiber optic gyro (RFOG) is a high accuracy inertial rotation sensor based on the Sagnac effect. The optical fiber ring resonator (OFRR) is the key rotation sensing element. The backscattering characteristics of the OFRR are fully investigated. The Rayleigh backscattering dominates the backscattering spectrum with the input power below the threshold. High carrier suppression is crucial to reduce this Rayleigh backscattering error. Being different from the intrinsic Rayleigh scattering of the fiber, the stimulated Brillouin scattering (SBS) in the OFRR should be avoided. This is because the finesse and the resonance notch depth of the OFRR decrease for the pump depletion through the SBS process. The shot noise limited sensitivity (SLS) of the RFOG is improved by increasing the input power. Through analysis, it is found that the threshold input power is improved after phase modulation. The SLS of the RFOG is analyzed at different modulation parameters and its relevant SBS threshold. Accordingly, the optimized modulation frequency and the corresponding maximum input power are all obtained. A simple method of testing the frequency shift in the SBS is also proposed. In this method, the central frequency of the laser source is locked to the resonance frequency of the OFRR in one direction. A steady SBS light is observed and its frequency shift is measured together.     

Reduction of FWM noise in WDM-based QKD systems using interleaved and unequally spaced channels

To extensively deploy quantum key distribution(QKD) systems, copropagating with classical channels on the same fiber using wavelength division multiplexing(WDM) technology becomes a critical issue. We propose a user-based channel-interleaving WDM scheme with unequal frequency spacing(UFS-i WDM) to reduce the impairment on the quantum channels induced by four-wave mixing(FWM), and theoretically analyze its impact on quantum bit error rate(QBER). Numerical simulation results show that a UFS-i WDM can significantly reduce the FWM noise and improve QBER compared with the corresponding WDM scheme with equal frequency spacing(EFS), especially in the case of nonzero dispersion shifted fiber.     

A comparative study of modified PUSCA and paired PUSCA strategies in WDM system

In G.653 compliant fiber, four-wave mixing (FWM) is a dominant non-linear effect. Performance of optical wavelength division multiplexing (WDM) network based on G.653 fiber is affected by the generated FWM components. The paper aims to discuss the methods used to reduce the number of FWM components and make efficient utilization of bandwidth. We have earlier proposed two methods (a) modified periodically unequally spaced channel allocation (modified PUSCA) and (b) paired PUSCA. In this paper, we have compared both the methods by considering the ratio of increase in weighted average signal to FWM noise ratio to increase in bandwidth as a figure of merit. Further, the performance of paired PUSCA scheme on G.653 fiber has been compared with equally spaced channel allocation (ESCA) scheme on G.652 fiber in which FWM is not a dominant non-linear effect. We demonstrate that paired PUSCA on G.653 fiber performs better than ESCA on G.652 fiber when the number of channels is less than 30.     

Stimulated Brillouin scattering and its dependences on strain and temperature in a high-delta optical fiber with F-doped depressed inner cladding

Stimulated Brillouin scattering (SBS) in a high-delta fiber with F-doped depressed inner cladding is studied through considering the interaction of acoustic and optical modes in the fiber. It is found that the number of acoustic modes in the fiber is reduced and the frequency spacing between neighboring modes is enlarged because of the F doping. The dependences of SBS on strain and temperature are measured and compared for each acoustic mode to investigate the feasibility of discriminative sensing of strain and temperature by use of the fiber.     

Generation and transmission simulation of 60 G millimeter-wave by using semiconductor optical amplifiers for radio-over-fiber systems

A novel method of the millimeter-wave (MMW) generation is proposed for Radio-over-fiber systems with low phase noise by using semiconductor optical amplifiers (SOAs). The simulation results show that the method is correct. When a pump light and a signal light are input into the SOA, the converted light induced by four-wave mixing (FWM) effect is generated. The optical MMW is generated between the signal light and converted light after filtering the pump light. The electrical MMW is obtained by beating the signal light and converted light via a photo-detector. The phase noise of the system is low because the phase difference between the signal light and converted light is stable for the FWM effect. The power of MMW is increased with the increase of the pump light power and the pump current of the SOA, is decreased with the increase of the polarization azimuths of the pump light.