单频大功率光纤放大器中抑制受激布里渊散射的理论分析

对单频大功率光纤放大器中的受激布里渊散射(SBS)抑制问题进行了分析和模拟。建立了双包层光纤放大器的含有受激布里渊散射效应的传输方程组,并考虑了温度差对受激布里渊增益系数的影响。通过数值求解方程组研究了前向、后向和双向抽运方式下,抽运功率、对流系数、光纤长度和斯托克斯频率偏移对受激布里渊散射增益的影响。在抽运功率、对流系数和光纤长度均相同的条件下,后向抽运方式的受激布里渊增益最小;对流系数或光纤长度的减少会降低受激布里渊增益。计算了总抽运功率为1kW,三段抽运方式下的受激布里渊增益,其结果远远大于增益阈值。因此,设计单频大功率光纤放大器宜采用后向抽运方式,尽量减小光纤外表面空气的对流速度以增加温度差,同时应该尽量缩短光纤长度。     

BOTDR的已敷设传感光纤温度和应变区分测量方法

为了解决已敷设传感光纤中布里渊谱峰功率初值难以获取,基于频移和功率双参量的温度和应变区分测量误差大等问题,提出了解决方法。通过标定实验确定布里渊频移和相对谱峰功率的温度和应变系数、频移初始值;根据布里渊散射功率特性方程,通过试探法,利用已敷设光路中温度和应变已知的参考光纤确定方程系数,建立了谱峰功率初始值;利用归一化方法克服了传感系统中乘性噪声导致的测量误差;利用谱宽变化消除了温度和应变突变点处的谱峰功率异常峰值;最后,根据光纤复合海底电缆的现场情况建立了模拟光路,并进行了温度和应变测量实验。结果表明,在5.6 km处可实现4.3℃和110 的测量精度,可实现已敷设传感光纤整条光路上的温度和应变区分测量,为工程应用提供了理论和实验依据。     

双光纤双参量布里渊光时域分析传感技术的研究

对于布里渊分布式光纤传感器(DOFS),温度或应变的变化都会引起布里渊频移谱改变,因此存在交叉敏感问题。在以往对布里渊光时域反射(BOTDR)计的双参量传感研究中,采用单根光纤,通过同时检测布里渊频移和功率变化,实现双参量传感。但对于布里渊光时域分析(BOTDA),由于受激布里渊散射的偏振相关性,不能实现对受激散射光功率的准确检测,因此难以实现单光纤的双参量传感。针对这一问题采用温度和应变系数不同的双光纤进行双参量传感。先测量了几种常用光纤的温度和应变布里渊频移系数,然后选择G652和G652成缆两种光纤,通过构建系数矩阵,由两根光纤的布里渊频移计算得出温度和应力,从而实现了温度分辨率25℃左右,应变分辨率约为200με的双参量传感。     

一种光纤传感器温度与应变同时解调的方法

文章分析了一种利用外差检测所得微波信号的电场振幅和频率来计算后向布里渊散射光功率及频移的方法.该方法通过对功率较强的微波信号进行数值拟合,利用得到的直流分量和交流分量计算出后向散射光功率、频移及其随温度、应变变化的系数矩阵.数值结果表明,温度、应变的解调精度分别为1℃、100 με,但也受到一定采样频率的限制.     

布里渊频移响应特性及其灵敏度调制方法研究

为研究基于布里渊光时域反射测量原理的分布式传感光纤在工程应用中的温度和应变交叉敏感问题,首先采用介质中声学声子非弹性光散射理论,建立了布里渊频移与光纤所受应变和温度的理论关系模型,并数值模拟了纤芯材料属性与内置调制膜层参数对光纤布里渊频移应变、温度灵敏度的影响,得到当纤芯折射率增大、纤芯泊松比减小或内置调制膜层与裸纤间隙增大时,布里渊频移应变和温度灵敏度均减小;而当纤芯折射率、纤芯弹性模量减小或内置调制膜层热膨胀系数增大时,布里渊频移应变和温度灵敏度均增大。在此基础上搭建了布里渊散射测量系统,并选择了同时具备温度增敏与应变减敏特性的内置调制膜层型分布式光纤进行验证试验,试验结果与仿真分析一致。     

分布式布里渊光纤应变传感信号的增敏检测方法

基于外差相干检测原理,摒弃了传统扫频构建布里渊散射频谱的方法,以布里渊散射光功率为应变监测参量,利用具有增敏作用的高频脉冲检波管对布里渊散射拍频信号进行包络检波,直接将拍频信号转换成布里渊散射功率信号,并对携带应变信息的布里渊散射功率信号进行应变增敏检测,利用高频脉冲检波管增益响应曲线对功率信号的调制提高散射功率应变敏感系数。经测试,增敏后布里渊散射光功率应变敏感系数为0.024%/με,与传统直接检测法得到的应变敏感系数-7.7×10-4%/με相比,敏感系数提高30倍,有效提高了系统信噪比;对增敏后的布里渊功率信号进行解调,在24.7km的光纤长度上,实现65με的应变测量标准差,测量时间由传统扫频方法的超过30s降为仅1s。本文系统可有效满足快速精确检测应变的监测需求,且不需要扫频模块等复杂器件,结构简单,稳定性高,对实际应用具有重要的意义。     

基于瑞利散射的单端BOTDA系统温度特性研究

提出一种不受电光调制器传输曲线温度漂移现象影响的基于瑞利散射的单端布里渊光时域分析系统,并对系统所需的合成信号及温度特性进行分析,通过搭建单端布里渊光时域分析温度传感系统测量系统的温度特性.结果表明:通过测量布里渊增益谱获得的布里渊频移与温度呈良好的线性关系;由单端布里渊光时域分析温度测量系统获得的布里渊频移的温度系数为1.109 MHz/℃,与传统双端布里渊光时域分析系统获得的1.20 MHz/℃具有良好的一致性,在1.77 km光纤上可实现9.5m空间分辨率的温度传感测量.     

单模光纤中受激布里渊散射阈值研究

分析和讨论了受激布里渊散射(SBS)阈值计算的Smith模型和Küng模型,研究了更为准确估算光纤中布里渊散射阈值的方法,通过布里渊增益系数与光纤长度的关系,发现对于较短长度光纤,其布里渊增益系数随着光纤长度变化范围较大,仅在长距离光纤时,布里渊增益系数才可以近似为常数。实验测量了25 km单模光纤的受激布里渊散射阈值,推导出用布里渊时域反射仪(BOTDR)测量受激布里渊散射阈值计算公式,最后用布里渊时域反射仪测量了不同长度光纤受激布里渊散射阈值,实验结果与理论分析吻合。     

波长扫描型布里渊光时域反射仪

布里渊光时域反射仪(BOTDR)是一种具有广泛应用前景的分布式光纤传感器。对于特定的入射波长,自发布里渊散射光的布里渊频移与温度和应变成线性关系,通过测量光纤沿线布里渊频移分布可实现温度或应变的分布式传感。布里渊功率谱扫描是BOTDR获取布里渊频移的常用手段,已有光频差扫描与电频扫描两种方式。基于布里渊频移对波长的依赖性,提出一种波长扫描型BOTDR。采用可调谐激光器作为光源,通过扫描入射光波长,来获取布里渊功率谱,该方法兼具光频差扫描与电频扫描的优点。实验证明了该方法的可行性,对23.4km光纤进行测量,实现了5m的空间分辨率与2.2℃的温度测量精度。     

增益型受激布里渊相移谱宽范围功率特性

相移谱的功率依赖特性对矢量布里渊光时域分析系统的优化设计具有重要意义。对增益型受激布里渊散射（SBS）相移谱进行了建模分析；搭建了外差pump-Stokes系统，在5~90 mW泵浦光功率和5 W~9 mW斯托克斯光功率范围内测量了400 m标准单模光纤的增益型SBS相移谱；分析了Stokes光功率影响增益型SBS相移的机理。结果表明:当固定Stokes光功率时，增益型SBS相移范围与泵浦光功率成良好线性关系；因泵浦耗尽作用的影响，导致当Stokes光功率由5 W上升至8 mW时，增益型SBS相移范围的泵浦光功率灵敏度由1.448（）/mW下降至1.156（）/mW。根据理论和实验结果，对增益型VBOTDA系统进行了优化设计分析，为其在长距离和高精度传感领域的发展奠定了基础。     

Theoretical analysis of the relationship between the Brillouin gain coefficient and the strain in the optical-fiber sensors

The relation between the power of the Brillouin signal and the strain is one of the bases of the distributed fiber sensors of temperature and strain. The coefficient of the Bfillouin gain can be changed by the temperature and the strain that will affect the power of the Brillouin scattering. The relation between the change of the Brillouin gain coefficient and the strain is thought to be linear by many researchers. However, it is not always linear based on the theoretical analysis and numerical simulation. Therefore, errors will be caused if the relation between the change of the Brillouin gain coefficient and the strain is regarded as to be linear approximately for measuring the temperature and the strain. For this reason, the influence of the parameters on the Brillouin gain coefficient is proposed through theoretical analysis and numerical simulation.     

Simultaneous distributed measurement of strain and temperature fromnoise-initiated Brillouin scattering in optical fibers

The simultaneous determination of strain and temperature distributions from the measurement of noise-initiated Brillouin scattering (NIBS) power and frequency shift in optical fibers is discussed. Equations governing the growth of the NIBS signal are derived and from these, we calculate the dependence of the Brillouin power on temperature and strain. We study the potential problem given by the need to normalize the nonlinear Brillouin signal and present a new technique that solves this problem by mathematically combining the values of the Stokes and anti-Stokes powers to produce a linear effective power. Experimental results are presented that support this theory and allow the verification of the coefficients governing the dependence of the Brillouin power and frequency shift on temperature and strain. The signal-to-noise ratio of the sensor is discussed, and it is found that the noise associated with the field statistics plays a limiting role in the sensor performance and that an optimum value for the Brillouin gain factor can be determined. A simultaneous distributed temperature and strain sensor is demonstrated; preliminary results show a strain resolution of 100-μm strain, a temperature resolution of 4°C, and a spatial resolution of 40 m, over a sensing length of 1200 m     

Characterization of Brillouin fiber generator and amplifier for optimized working condition of distributed sensors

The Brillouin fiber amplifier (BFA) has been used in Brillouin optical time and frequency domain analyzer based sensors. For BFA based distributed sensor the optimized working condition is to have the highest Brillouin gain, i.e., highest SNR, which avoids high pump depletion induced gain saturation. We have found that the optimum working condition for distributed sensor system is associated with the stimulated Brillouin scattering (SBS) threshold for BFA, which can be experimentally determined with Stokes power inflexion and/or Stokes spectrum linewidth minimum methods. This threshold depends on both pump and probe power instead of just the pump power as in Brillouin fiber generator (BFG), as well as on sensing length as confirmed by our experimental results and theoretical simulation. This was achieved by introducing the concept of absorption coefficient of the sensing medium defined as the ratio of the total output power to the total input power. We find that the medium absorption is minimized when input Stokes power is an order of magnitude lower than BFA threshold. This minimum is a signature of the balance between maximum Stokes gain and pump depletion which is also the reason why Stokes spectrum linewidth goes through a minimum.     

In situ spatially-resolved thermal and brillouin diagnosis of high-power ytterbium-doped fibre laser by brillouin optical time domain analysis

An in situ spatially-resolved diagnosis of thermal and Brillouin characteristics of a double-clad ytterbium-doped fibre (YDF) laser operating at 1.09 μm is demonstrated. For this a Brillouin optical time domain analysis technique based on 1.55 μm Brillouin pump and probe beams is utilised. A 2.4 K temperature difference across the YDF laser when it was running with 6.5 W pump power has been measured and resolved. Based on the measured thermal and Brillouin characteristics of it is expected the YDF, that the effective Brillouin gain coefficient would decrease by 20% for 1.09 μm radiation if an 80 K temperature variation is built up across it, as a result of the quantumheating by the pump power.     

Effects of Intensity Modulation of Light Source on Brillouin Optical Correlation Domain Analysis

The effects of the intensity modulation (IM) of light source on fiber-distributed sensors based on Brillouin optical correlation domain analysis (BOCDA) are analyzed by numerical simulation, and the results are experimentally confirmed. We show that the shape of the Brillouin gain spectrum in the BOCDA system has a particular dependence on the optical spectrum of light source and that it can be controlled and tailored by proper modification of the optical spectrum using an additional IM. In the experiments, we applied several IM waveforms based on the simulation results for confirmation. Additionally, a distributed strain measurement along a 1-km optical fiber with a 30-cm spatial resolution is demonstrated by applying a proper IM scheme, which is the longest range reported using the BOCDA system     

光纤参量放大器增益特性的理论与仿真分析

为了优化光纤参量放大器的增益特性,利用理论分析,对FOPA在不同光纤长度、不同泵浦光功率及不同光纤非线性系数下的增益与带宽特性进行了比较与分析。结果表明:使用较长的高非线性光纤、较大的泵浦光、或较大的非线性系数,均可提高FOPA的增益,增益带宽也可随着泵浦光功率和非线性系数的增大而增大。此外,利用仿真模拟对理论分析结果进行了验证。虽然,仿真结果与理论分析结果间略有偏差,但仿真结果依然较好的验证了理论结果的正确性与可行性。     

A quantitative comparison of the classical rate-equation model withthe carrier heating model on dynamics of the quantum-well laser: therole of carrier energy relaxation, electron-hole interaction, and Augereffect

In this paper, a quantitative theoretical comparison of the classical rate-equation model with the carrier heating model for large signal dynamic response of 1.5-μm InGaAs-InGaAsP single-mode quantum-well (QW) lasers Is performed. The contributions of carrier energy relaxation, electron-hole interaction, and Auger effect to the nonlinear gain are inspected in detail by a numerical comparison of the two models at room temperature (293 K) and low temperature (50 K). It can be shown that contribution of the carrier heating to the nonlinear gain coefficient is proportional to an effective carrier energy relaxation time, and the contribution of the electron-hole energy exchange time shows a nonlinear relation. Furthermore, the influence of Auger heating on the modulation dynamics is also considered and is found to be indescribable by a single phenomenological nonlinear gain coefficient. The dependence of the nonlinear gain coefficient on the laser emission wavelength of distributed feedback lasers is also demonstrated quantitatively for the first time