Effect of Brillouin slow light on distributed Brillouin fiber sensors

The effect of Brillouin slow light on distributed Brillouin fiber sensors (DBFSs) is studied. We demonstrate Brillouin slow light for a 1.2 ns pulse with peak powers (PS) from 3.3 to 56.2 mW on depletion of the pump power (PP) ranging from 1.3 to 83.2 mW in conventional optical fibers (SMF-28). Experiments show that, when pump power depletion is not negligible, for a given PP the Brillouin gain and delay time of a pulse decrease when PS increases in a long (> or =10 km) sensing fiber. The optimum pump beam depletion resulting from strong interaction of the pump and the probe in the fiber provides accurate temperature and strain information at a high spatial resolution. Our study reveals that at low PP the spatial resolution error caused by the pulse delay for a DBFS with centimeter spatial resolution is less than 5% of the pulse length.     

陡前沿脉冲产生的受激布里渊散射稳定性研究

受激布里渊散射转换效率及相位共轭保真度的稳定性与泵浦脉冲的前沿上升时间有关，对前沿上升时间与介质声学声子寿命接近的泵浦脉冲，在泵浦能量超过受激布里渊散射阈值较多时，会造成严重的相位共轭保真度不稳定。本文针对上升前沿时间为２ｎｓ的泵浦脉冲及声学声子寿命的１ｎｓ和ＣＣｌ４介质，在泵浦脉冲前沿根部引入缓慢上升的小信号后，采用振放双池受激布里渊散射相位共轭镜，在泵浦激光能量达到５００倍受激布里渊散射阈值时     

毫米级高分辨率的混沌激光分布式光纤测温技术

近年来,随着分布式光纤传感技术在各大基础设施健康监测领域的广泛应用,人们对能够实现毫米量级精准定位和监测技术的需求日益增长.本文提出了一种基于宽线宽混沌激光的高分辨率分布式光纤测温技术.实验通过改变光反馈混沌源的偏振匹配态和反馈强度等外部参数,产生了–3 dB线宽约为7.5 GHz的宽线宽混沌激光,并在300 m传感光纤实现了空间分辨率为7.05 mm的分布式温度测量.同时,为了抑制光源线宽增加造成的布里渊增益谱恶化,在泵浦路中引入了时间门控技术,其中经脉冲调制后的泵浦光峰值功率提高了约9.5 dB,同时脉冲调制使混沌互相关锁定于脉冲持续时间内,从而布里渊增益谱的信号背景噪声比由约2.28 dB提升为4.55 dB,最终实现了空间分辨率为3.12 mm的分布式温度测量.     

Generation of short light pulses under the stimulated Brillouin scattering in fibers with an optimal feedback

The generation of short light pulses (1 ns) in single mode fibers under pumping by wide laser pulses (of a microsecond duration) due to the backward stimulated Brillouin scattering (SBS) is numerically investigated. The influence of the acoustic diffraction is taken into account. The cases of acoustic waveguide and anti-waveguide fibers are considered. For an acoustic anti-waveguide fiber, a dependence of overlap integral S on the acoustic mode number n has a sharp peak in the region of n 100. Computer simulations have demonstrated the energy conversion of the pump wave into short pulses of the signal (Stokes) wave in the case of synchronous pumping. The optimal length of the fiber should be approximately equal to the half-length of the pump pulse. The bypass time of the Stokes pulse of the optimal circuit fiber and the feedback loop must be equal to the repetition period of the pump pulse. An importance of acoustic mode structure of the fiber for the process of forming pulse train in shown. We have found that the acoustic anti-waveguide fibers with a small core (a < 3 m) can be preferable for obtaining the stable train of compressed pulses.     

Controllable optical delay line using a Brillouin optical fiber ring laser

A controllable optical delay line using a Brillouin optical fiber ring laser is demonstrated and a large timedelay is obtained by cascading two optical fiber segments. In experiment, a single-mode Brillouin opticalfiber ring laser is used to provide Stokes wave as probe wave. We achieve a maximum tunable time delayof 61 ns using two cascading optical fiber segments, about 1.5 times of the input probe pulse width of 40ns. In the meantime, a considerable pulse broadening is observed, which agrees well with the theoreticalprediction based on linear theory.     

窄线宽纳秒脉冲光纤拉曼放大器的理论模型和数值分析

窄线宽纳秒脉冲光纤拉曼放大器在非线性频率变换、遥感探测和量子信息等领域有广泛的应用前景.综合考虑受激拉曼散射(stimulated Raman scattering,SRS)、受激布里渊散射(stimulated Brillouin scattering,SBS)、自相位调制(self-phase modulation)和交叉相位调制(cross-phase modulation)等非线性效应,建立了窄线宽纳秒脉冲光纤拉曼放大器的非线性动力学模型.仿真分析了放大器中脉冲激光的时频演化特性,对比研究了抽运脉冲宽度、光纤长度和信号光功率等因素对放大器性能的影响.研究发现,上述因素会影响放大器的SRS阈值、SBS阈值、输出激光线宽、激光转换效率等.例如,当脉冲宽度为800 ns时,SBS随着抽运功率的增加而发生,限制了激光功率的提升;减短抽运脉宽可以抑制SBS,但是输出激光的线宽易于展宽到数百MHz以上;增加光纤长度可以获得更低的SRS阈值和更高的转换效率,但是SBS效应和光谱展宽程度也随之增强.系统搭建中需要平衡各非线性效应,选择合适的系统参数.研究内容可以为窄线宽纳秒脉冲光纤拉曼放大器的设计搭建提供参考.     

Characterization of the Brillouin-loss spectrum of single-mode fibers by use of very short (<10-ns) pulses

The characterization of the Brillouin-loss spectrum of single-mode fibers with very short (<10-ns) pulses has been studied. It was found that the Brillouin-loss signal intensity is linearly related to the duration of the pump pulse used to obtain the spectrum. In contrast with the uniform trend of the signal, three distinct behaviors were observed in the spectral linewidth. At long pulse durations the linewidth was constant at approximately 40 MHz. Pulse durations of the order of the phonon lifetime resulted in a broader spectrum, reaching a maximum width of ~100 MHz at 5 ns. Reducing the pulse duration further resulted in a sudden narrowing of the Brillouin line.     

掺镱光纤激光器自脉冲与自脉冲内的自锁模研究

采用单端连续抽运方式, 对自由运转的双包层掺镱光纤激光器的输出特性进行了详细的实验研究. 实验中不但观察到了自脉冲, 而且首次在自由运转的光纤激光器中观察到自锁模现象, 对它们产生的物理机理进行了相应的理论分析. 分析表明: 增益光纤的弱(未) 抽运部分对信号光的吸收导致光纤激光器内自脉冲的出现, 轴向模之间的拍频和自相位调制导致自锁模现象的出现, 而受激拉曼散射、 受激布里渊散射等非线性效应使它们进一步增强. 当抽运光功率略高于阈值时, 自脉冲宽度比较宽, 随抽运光功率增加自脉冲的脉宽变窄; 自脉冲包络面内的自锁模脉冲的宽度随抽运光功率增加也变窄, 进一步增加抽运光功率, 自脉冲和自脉冲包络面内的自锁模现象消失. 实验测得自锁模脉冲的间隔为224 ns, 最大(小) 自锁模脉冲的半高全宽约为35.0 ns (6.3 ns); 测得信号光的中心波长为1090 nm, 谱线半高全宽的最大(小) 值约为7.05 nm (2.01 nm).     

光子晶体光纤结构与掺杂对受激布里渊散射快光的影响

由受激布里渊散射三波耦合方程导出了在小信号条件下的快光时间提前量,通过全矢量有限元法模拟了光子晶体光纤占空比和GeO₂掺杂质量分数对布里渊频移、时间提前量、脉冲展宽因子及脉冲形变的影响。结果表明,布里渊频移随着占空比和掺杂质量分数的增大而减小。在保持泵浦功率为20 mW和快光传输长度为10 m的条件下,时间提前量随着占空比的增大而增大,随着掺杂质量分数的增大而减小。脉冲展宽因子与时间提前量变化趋势相反。当占空比为0.8,Ge掺杂质量分数为18%时,能够实现快光时间提前量为29.7 ns,脉冲展宽因子为0.88。布里渊阈值随着占空比的增大而减小,随着掺杂质量分数的增大而增大。     

布里渊散射谱拟合的混合优化算法

基于布里渊光时域分析仪的全分布式光纤传感系统中,光纤沿途的探测信号含有噪声导致被测量的温度或应变信息难以识别,光谱拟合的精确度对传感信息的识别非常重要。在传感系统低信噪比的情况下,提出了一种提取高精度布里渊散射谱特征的拟合方法,利用小波去噪结合莱文伯-马奈特(LM)算法调节权值后向传输(BP)网络对布里渊散射谱进行特征提取。克服了传统BP神经网络易陷入局部极值的缺点,保证求解的精度。数值仿真表明,该方法适合不同权重比、不同线宽和低信噪比以及大测量范围的情况进行光谱拟合,并且在信噪比为10dB的情况下得到拟合度均超过0.96。实验结果表明,该方法适用于多种泵浦功率情况下的布里渊散射谱的特征提取,优于传统BP神经网络算法且具有较高的拟合精度。     

18 W single-stage single-frequency acoustically tailored Raman fiber amplifier

A single-mode polarization-maintaining fiber doped to increase the Raman gain while suppressing stimulated Brillouin scattering (SBS) was utilized in a single-stage counter-pumped Raman fiber amplifier. The SBS suppression was achieved through the acoustic tailoring of the core. A pump probe experiment was conducted to characterize the Brillouin gain and indicated the existence of multiple Brillouin peaks. When the amplifier was seeded with approximately 15 mW of 1178 nm light, 11.5 W of cw output power was obtained with a linewidth ≤2 MHz. The application of a thermal gradient to further mitigate the SBS process increased the output power to 18 W, thus providing a net amplifier gain >30 dB.     

石墨烯的声子热学性质研究

声子是石墨烯导热过程中的主要载体,而声子的弛豫时间又是其中最基本、最重要的物理量.本文采用简正模式分解法研究了石墨烯声子的弛豫时间,并且借此分析了不同声子在导热过程中的贡献.该方法通过平衡分子动力学模拟实现,首先通过模拟得到单个声子的能量自相关函数衰减曲线,并进一步采用拟合和积分两种方法得到单个声子的弛豫时间.然后,研究了弛豫时间与波矢、频率和温度的关系.结果发现,弛豫时间随波矢的变化与对应的色散关系相近,弛豫时间与频率和温度的关系符合理论模型:1/τ=νnTm,其中声学支的n为1.56,而光学支结果较为发散,指数m对于不同声子支结果略有不同.最后,还研究了不同频率声子对导热的贡献,发现低频声子在态密度上占有绝对优势,并且其弛豫时间整体高于高频声子,所以低频声子对导热的贡献占据主导地位.     

Long Optical Delay Lines Enhanced by Ring Configuration in Optical Fibres

A long optically controlled delay line enhanced by ring configuration is demonstrated by using the group-velocity control of signal pulses based on stimulated Brillouin scattering. In experiment, two optical fibre ring cavities are used： one is used as the Brillouin laser, providing single-mode Stokes wave as probe wave; the other is used as the Brillouin amplifier, working as slow light medium. We achieve a maximum time delay of 215ns using the ring Brillouin amplifier, five times larger than the input probe pulse width of 40ns. In the meantime, a considerable pulse broadening is observed, which agrees well with the theoretical prediction based on linear theory.     

Cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber

Copropagating fundamental-wavelength and second-harmonic femtosecond pulses of Cr: forsterite laser radiation are used to study cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber. Parametric instability of the second-harmonic probe pulse induced through cross-phase modulation by the fundamental-wavelength pump pulse gives rise to distinct sidebands in the spectrum of the probe field transmitted through the fiber. The wavelength of these sidebands was tuned in our experiments within approximately 100 nm by varying the peak power and the delay time of the pump pulse, suggesting a convenient way of controlled parametric spectral transformation of ultrashort laser pulses.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Guided acoustic wave Brillouin scattering in photonic crystal fibers

We experimentally investigate guided acoustic wave Brillouin scattering in several photonic crystal fibers by use of the so-called fiber loop mirror technique and show a completely different dynamics with respect to standard all-silica fibers. In addition to the suppression of most acoustic phonons, we show that forward Brillouin scattering in photonic crystal fibers is substantially enhanced only for the fundamental acoustic phonon because of efficient transverse acousto-optic field overlap. The results of our numerical simulations reveal that this high-frequency phonon is indeed trapped within the fiber core by the air-hole microstructure, in good agreement with experimental measurements.     

Energy relaxation by hot carriers in wurtzite GaN epilayers

In this paper, we report hot carrier energy relaxation processes studied by acoustic phonon emission in wurtzite GaN epilayers, using the heat pulse technique. In this method, the carriers were heated up by means of short (≈ 10 ns) voltage pulses and emitted phonons were detected by Al bolometers biased at their superconducting transition. Obtained phonon signals indicate that the optical phonon emission threshold has not been reached and longitudinal acoustic and transverse acoustic modes can be clearly resolved. This paper specifically concentrates on the electron temperature dependence of the energy relaxation rates and compares the experimental results with the existing theory.     

高非线性光纤中受激布里渊散射快光提前及脉冲形变

基于受激布里渊散射三波耦合方程组,应用小信号分析,研究了高非线性光纤的信号光脉冲提前及形变。结果表明,光纤中信号光脉冲的时间提前量在一定范围内随着输入信号光功率和传输距离的增长而近似线性增长,同时脉冲压缩程度加强;在相同条件下,非线性光纤比普通单模光纤的时间提前量更大,在50 m长的高非线性光纤中,信号光能量在50 m W时实现了203 ns的时间提前量和较小的脉冲形变。高非线性光纤的快光提前效率显著提高,脉冲形变显著减小。研究结果对快光的实现和应用具有参考意义。     

Analysis and measurement of GAWBS spectrum in a nonlinear fiber ring

The spectral density of the phase noise of an optical pulse in a fiber produced by guided acoustic wave Brillouin scattering (GAWBS) is derived from first principles. The predictions are in good agreement with the experimental results in a fiber interferometer. The experiments show that the lower limit on the quantum noise reduction is set by GAWBS. The GAWBS are of sufficiently low level to permit 5 dB squeezing.     

单纵模Nd∶YAG激光抽运CH_4中强后向受激拉曼散射

用单纵模Nd∶YAG二倍频激光[波长532 nm,线宽Δνp<100 MHz,脉宽(半峰全宽)6.5 ns]抽运CH4气体,观察到很强的后向一级斯托克斯(BS1)受激拉曼散射,这与前人采用脉宽30 ns的单纵模抽运激光得到的绝大部分为后向受激布里渊散射(SBS)完全不同,其原因是脉宽6.5 ns与本实验条件下CH4的受激布里渊散射声子寿命接近,受激布里渊散射处于瞬态。理论计算表明,这时的受激布里渊散射瞬态增益系数已略小于后向一级斯托克斯的增益系数,而被其竞争抑制。当脉冲重复频率为2 Hz,抽运能量为95 mJ时,在1.1 MPa CH4中,后向一级斯托克斯的量子转换效率高达73%,其时间波形出现张弛振荡,脉宽被压窄到1.2 ns,从而使后向一级斯托克斯峰值功率达到了抽运激光功率的2.7倍,而且其光束质量要大大优于抽运激光的光束质量。用编制的准二维计算机模型程序相当好地再现了实验中后向一级斯托克斯的时间波形张弛振荡。     

光纤中受激Brillouin散射动态弛豫振荡特性及其抑制方法

为了避免在高功率光纤放大器和光纤相位共轭镜等实际应用中因受激Brillouin散射（SBS）造成的光纤损伤,根据描述SBS动态弛豫振荡特性的振幅耦合方程,利用有限差分的数值模拟方法研究了光纤中SBS的动态弛豫振荡特性,并对其抑制进行了初步探讨.得到了光纤中SBS弛豫振荡在不同的脉冲上升时间的时空三维图;同时利用方波和脉冲光作抽运光进行了相应实验研究,实验结果与理论模拟结果符合很好.结果表明,增大抽运光脉冲上升时间可以有效抑制因SBS而产生的弛豫振荡,进而避免因其造成的光纤损伤. 关键词： 受激Brillouin散射 动态弛豫振荡特性 有限差分法