光纤传感器中布里渊增益系数与应变关系的理论研究

布里渊功率和应变的关系是分布式布里渊光纤温度和应变传感器的基础之一,而温度和应变又是通过改变布里渊增益系数来影响布里渊信号的功率。目前许多学者认为布里渊增益系数的变化与应变呈线性关系,但是我们通过理论研究和数值模拟发现,布里渊增益系数和应变之间并非一直为线性关系。因此在测量温度和应变时如果把布里渊增益系数的变化与应变近似地当作线性关系将会产生误差。本文基于此原因,对其进行理论研究并通过数值模拟得出各参数对布里渊增益系数变化的影响。     

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。     

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.     

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     

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。     

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.     

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。     

180 W单频全光纤激光器

高功率单频光纤激器在相干探测、功率光谱合成等方面具有广泛的应用前景。分析了高功率单频光纤激光器中受激布里渊散射效应的抑制方法,研究了放大级特性对受激布里渊散射效应的影响。采用线宽为70 kHz的单频光纤激光器作为种子源,经两级光纤放大,实现了中心波长1 064.1 nm、线宽70 kHz、最高功率为180 W的单频全光纤激光输出,光-光转换效率71.1%,光束质量Mx2=1.2,My2=1.21。分析了改变放大级特性前后输出功率提升的原因,认为改变放大级的温度分布减小了受激布里渊散射效应的增益系数,提高了输出激光的受激布里渊散射阈值,促使改变放大级温度分布后的输出功率大幅提高。该激光器的输出功率仅受限于泵浦功率,进一步提高泵浦功率,有望实现更高功率的单频光纤激光输出。     

掺Yb3+双包层单频光纤放大器中SBS的理论分析

为了研究掺Yb3+双包层单频光纤放大器中受激布里渊散射产生的规律,从含有受激布里渊散射的速率方程出发,采用模拟计算的方法进行了理论分析.结果表明,光纤长度、抽运方式、抽运功率和换热系数对受激布里渊散射有很大的影响,信号功率的影响较小.所得结论为从实验上研究单频光纤放大器中受激布里渊散射的变化规律提供了理论依据.     

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     

光学差分参量放大布里渊时域分析优化研究

为了优化设计基于光学差分参量放大的布里渊光时域分析抽运的脉冲编码形式以及斯托克斯光与反斯托克斯光脉宽差,采用微扰法和小信号近似法,获得了探测光与斯托克斯光脉冲、反斯托克斯光脉冲在较小作用区域内的脉冲响应,对其性能进行了理论分析与数值验证。结果表明,相位斯托克斯光脉冲可有效压缩布里渊增益谱宽,提高布里渊信号信噪比;当斯托克斯光与反斯托克斯光传感脉冲脉宽差为8ns时,探测光布里渊增益最大。这一结果对进一步分析基于光学差分参量放大的布里渊光时域分析系统性能以及开展相关实验研究是有帮助的。     

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.     

千瓦级低透射波前畸变端抽运板条增益模块技术研究

基于端抽运板条增益模块的主振荡器功率放大(MOPA)是高平均功率激光二极管抽运激光器(DPL)发展的有效途径之一,近年来受到广泛关注。通过理论分析和数值模拟,建立了板条增益模块的设计模型,分析了板条增益模块的小信号增益系数、输出特性、温度和热应力等关键参数。研制的模块尺寸为50cm(长)×30cm(宽)×20cm(等),静态波前畸变为0.22μm(不包括倾斜),实验研究获得了均匀性约94%的荧光分布,在占空比为60%条件下,输出平均功率1086.3W,斜率效率41.7%。     

正向抽运喇曼光纤放大器开关增益数值计算研究

采用数值计算的方法,对影响正向抽运喇曼光纤放大器(RFA)开关增益的几个因素——喇曼增益系数、抽运光功率、光纤长度、光纤损耗、有效纤芯面积等参数与正向抽运喇曼光纤放大器开关增益的关系进行了分析研究,并把数值分析的结果和小信号的解析结果进行了比较,然后在此基础上提出了有效提高正向抽运喇曼光纤放大器开关增益的几种方法。     

多波长布里渊掺铒光纤激光器的结构优化研究

基于受激布里渊散射效应(SBS)和掺铒光纤(EDF)的线性增益机理研究了一种环形腔多波长布里渊掺铒光纤激光器(BEFL)。在该激光器中使用单模光纤作为布里渊增益介质,使用掺铒光纤来放大产生的斯托克斯(Stokes)信号,使该激光器在室温下产生稳定的多波长输出。通过对激光器结构中的环行器和耦合器的位置以及耦合器的接入方式的研究与分析发现:耦合器的接入方式、环行器与耦合器的位置均对激光器的输出有影响。在可调谐光源TLS(布里渊泵浦激光器)的功率大小为14dBm,980nm泵浦激光器的功率大小为23dBm,单模光纤长度为10km的情况下进行了实验测试,结果表明:耦合器的接入方式对BEFL的输出影响很明显,最后给出了详细的测试结果和相应解释。     

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     

Stimulated Brillouin Scattering in a Single-Mode Tellurite Fiber for Amplification, Lasing, and Slow Light Generation

In this paper, Brillouin gain performances of tellurite fiber are investigated for photonics applications. We demonstrate stimulated Brillouin amplification and lasing and the simulated performance of slow light generation in a single-mode tellurite fiber. A Brillouin gain of 29 dB is achieved in a 100-m tellurite fiber with a pump power of 10 mW at 1550 nm. A peak value of Brillouin gain coefficients of 1.6989 X 10^-10 m/W is measured on the base of gain characteristics. An all-fiber Brillouin laser with the maximum unsaturated power of 54.6 mW at 1550 nm and a slope efficiency of 38.2% is achieved from a 200-m tellurite fiber by employing a ring cavity. Furthermore, widely tunable (~27 nm) Brillouin comb laser with 26 lines spaced at 7.97 GHz is obtained from the ring laser cavity including an erbium-doped fiber amplifier (EDFA). A simple theoretical model based on laser threshold theory successfully explains the properties of Brillouin comb lasers. Stimulated Brillouin scattering (SBS)-induced time delay per unit power and per unit length is also calculated using the measured data of Brillouin gain coefficients. A peak value of 0.09246 ns/mW/m and a time delay slope efficiency of 1.75 ns/dB are obtained for this tellurite fiber. Potential performance of a tellurite fiber for slow light generation is clarified on the base of Brillouin gain characteristic. Our results show that tellurite fiber is a promising gain medium for Brillouin fiber amplifiers, lasers, and slow light generation due to its low background loss and large Brillouin gain coefficient.     

基于受激布里渊散射的可调谐多波长激光器

可调谐多波长布里渊掺铒光纤激光器将光纤中的SBS非线性放大同掺铒光纤的线性放大相结合得到室温稳定的多波长输出,具有波长间隔一致、线宽窄、功率谱相对平坦等优点。设计了一种基于光纤布拉格(FBG)反射的线性可调谐多波长布里渊掺铒光纤激光器。该线性腔激光器的一端利用光纤布拉格光栅作为反射镜,有效抑制了腔内自激模的影响,增加激光器输出波长数。布里渊泵浦信号进入布里渊增益介质之前经过掺铒光纤放大器的两次放大,降低了布里渊增益的阈值。该多波长激光器实现了1 530～1 560 nm之间30 nm可调谐范围的输出。在布里渊泵浦信号功率2 mW,980 nm泵源抽运功率60 mW情况下,1 540～1 554 nm范围内,获得了波长间隔0.088 nm的16个波长的输出。     

Numerical model of stimulated Brillouin scattering excited by amultiline pump

A theoretical formulation and numerical simulation of the process of stimulated Brillouin scattering (SBS) excited by a multiline incident beam is presented. The one-dimensional code includes pump depletion and contributions from resonant and off-resonant acoustic gratings. For line separations up to the order of the spontaneous Brillouin linewidth, gain enhancement is accompanied by phase shifts between lines due to complex, off-resonant terms, which may result in a degradation of conjugation fidelity. For larger frequency separations, the degree of amplification is dependent on the ratio of the coherence length of the pump line spectrum to the SBS gain length. Comparisons with experimental data on reflectivity versus line separation show very good agreement     

Computational Investigation of the Electronic and Thermoelectric Properties of Strained Bulk Mg2Si

The purpose of this work was to investigate, by numerical simulation, the effect of isotropic and anisotropic strain on the transport properties of Mg₂Si. Analysis of the effects of temperature and charge-carrier concentration on evolution of the energy gap and on the thermoelectric properties of strained Mg₂Si is also reported in this paper. Gap evolution is highly dependent on the type of strain applied to the structure. The Seebeck coefficient (S) and power factor (PF) are strongly modified; a gain of up to 40% can be obtained for S and up to 100% for PF under specific conditions of strain. In most cases the temperature corresponding to the maximum value of PF was found to shift downward under the effect of strain.