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Distributed temperature measurement with millimeter-level high spatial resolution based on chaotic laser

Zhang Qian Wang Ya-Hui Zhang Ming-Jiang Zhang Jian-Zhong Qiao Li-Jun Wang Tao Zhao Le

Distributed temperature measurement with millimeter-level high spatial resolution based on chaotic laser

Zhang Qian, Wang Ya-Hui, Zhang Ming-Jiang, Zhang Jian-Zhong, Qiao Li-Jun, Wang Tao, Zhao Le
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  • The high-precision structural health monitoring of large civil structures and materials are increasingly demanded with widely using the distributed fiber sensors. A Brillouin optical correlation domain analysis for millimeter-levelhigh spatial resolution sensing using broadband chaotic laser is proposed and demonstrated. Through the analysis of the influence of polarization state and feedback strength on the chaotic laser, we experimentally achieve a broadband chaotic laser with a spectrum over 7.5 GHz in –3 dB which means that the theoretical spatial resolution is 3 mm, and we also successfully measure the distribution of fiber Brillouin gain spectrum with a temperature over 300 m measurement range with 7.05 mm spatial resolution, which is the first time that the sensor system based on chaotic laser has achieved the measurement with millimeter-level. However, there is still a difference in spatial resolution between the experimental and theoretical values. We can find that the chaotic laser has a time-delay feature; besides, with the broadening of chaotic laser, the threshold of stimulated Brillouin scattering in optical fibers increases while the Brillouin gain will weaken if the pump power is not enough here, and the cross-correlation peak of chaotic laser will narrow. All these problems cause the Brillouin gain signal to be easily submerged by noise, so the performance of the chaotic Brillouin optical correlation domain analysis system will decrease ultimately. Therefore, we also propose an optimization of Brillouin optical correlation domain analysis system by introducing the time-gated scheme into pump branch. It is obvious that the peak power of the pump wave is heightened by more than 9.5 dB after being amplitude-modulated by a square pulse with a pulse width of greater than acoustic phonon lifetime, and the signal-to-back ground noise ratio of the gain spectrum is improved effectively in theory; the cross correlation between chaotic pump wave and probe waveis locked within a pulse duration time, and the residual stimulated Brillouin scattering interactions existing outside the central correlation peak can be largely inhibited. In this optimized setup, the performance of the distributed temperature sensing is improved to 3.12 mm spatial resolution, which corresponds well to the theoretical value. The improved chaotic Brillouin optical correlation domain analysis technology will have a great potential application in high-precision structural health monitoring of large civil structures.
      Corresponding author: Zhang Ming-Jiang, zhangmingjiang@tyut.edu.cn
    [1]

    António B, Joan C, Sergi V 2016 Sensors 16 748

    [2]

    Bao X Y, Chen L 2011 Sensors 11 4152

    [3]

    Thévenaz L 2010 Front. Optoelectron. China 3 13

    [4]

    Kurashima T, Horiguchi T, Tateda M 1990 Opt. Lett. 15 1038

    [5]

    Hu J H, Zhang X P, Yao Y G, Zhao X D 2013 Opt. Express 21 145

    [6]

    Kim Y H, Song K Y 2017 Opt. Express 25 14098

    [7]

    Soto M A, Bolognini G, Pasquale F D 2011 Opt. Lett. 36 232

    [8]

    Li W H, Bao X Y, Li Y, Chen L 2008 Opt. Express 16 21616

    [9]

    Brown A W 2007 J. Lightw. Technol. 25 381

    [10]

    Hotate K, Arai H, Song K Y 2008 Sice J. Control Measur. Syst. Integrat. 1 271

    [11]

    Hotate K, Hasegawa T 2000 IEICE Trans. Electron. 83 405

    [12]

    Ryu G, Kim G T, Song K Y, Lee S B, Lee K 2017 J. Lightw. Technol. 35 5311

    [13]

    Zadok A, Antman Y, Primerov N, Denisov A, Sancho J, Thévenaz L 2012 Laser Photon. Rev. 6 L1

    [14]

    Cohen R, London Y, Antman Y, Zadok A 2014 Opt. Express 22 12070

    [15]

    Ji Y N, Zhang M J, Wang Y C, Wang P, Wang A B, Wu Y, Xu H, Zhang Y N 2014 Int. J. Bifurcat. Chaos 24 1450032

    [16]

    Zhang J Z, Zhang M T, Zhang M J, Liu Y, Feng C K, Wang Y H, Wang Y C 2018 Opt. Lett. 43 1722

    [17]

    Zhang J Z, Feng C K, Zhang M J, Liu Y, Wu C Y, Wang Y H 2018 Opt. Express 26 6962

    [18]

    Zhang J Z, Wang Y H, Zhang M J, Zhang Q, Li M W, Wu C Y, Qiao L J, Wang Y C 2018 Opt. Express 26 17597

    [19]

    Jeong J H, Lee K, Song K Y, Jeong J M, Lee S B 2012 Opt. Express 20 27094

    [20]

    王安帮 2014 博士学位论文 (太原: 太原理工大学)

    Wang A B 2014 Ph. D. Dissertation (Taiyuan: Taiyuan University of Technology) (in Chinese)

    [21]

    Zhang J Z, Wang A B, Wang J F, Wang Y C 2009 Opt. Express 17 6357

    [22]

    Zhang M J, Liu H, Zhang J Z, Liu Y, Liu R X 2017 IEEE Photon. J. 9 1943

    [23]

    Parker T, Farhadiroushan M, Handerek V A 1997 Proceedings of IEE Colloquium on Optical Techniques for Smart Structures and Structural Monitoring London, UK, February 17, 1997 p1

  • 图 1  两种典型偏振匹配态下混沌激光的特性 (a1), (b1) 光谱; (a2), (b2) 频谱; (a3), (b3)自相关曲线

    Figure 1.  The characteristics of the chaotic light at two typical polarization matching states: (a1), (b1) Optical spectra; (a2), (b2) power spectra; (a3), (b3) autocorrelation curve

    图 2  基于宽线宽混沌激光BOCDA系统的实验装置图

    Figure 2.  The experimental setup of broadband chaotic BOCDA

    图 3  不同温度下待测光纤末端的布里渊增益谱

    Figure 3.  The BGS at different temperature end of FUT

    图 4  待测光纤沿线布里渊频移分布图 (a)整条光纤沿线的布里渊频移分布; (b)加热位置附近的局部放大图

    Figure 4.  The map of BFS distribution along the FUT: (a) Measured along the entire FUT; (b) the local enlargement near heated zone

    图 5  待测光纤沿线布里渊频移分布曲线

    Figure 5.  Measured distribution of the Brillouin frequency shift along the FUT

    图 6  时间门控技术装置图

    Figure 6.  The setup of time-gated

    图 7  脉冲调制前后泵浦光时序图

    Figure 7.  The time series of the chaotic pump waves (red) and pulse amplitude-modulated (blue)

    图 8  引入时间门控技术前(a)后(b)两路光在待测光纤中发生受激布里渊散射示意图

    Figure 8.  The schematic diagram of SBS in the previous system (a) and the time-gated system (b)

    图 9  混沌布里渊增益谱和温度的关系 (a)待测光纤中随温度变化的布里渊增益谱; (b)加入时间门控技术前后待测光纤中随温度变化的布里渊频移量

    Figure 9.  The relationship of the Chaotic BGS with temperature: (a) Temperature-dependence of the BGS in the FUT; (b) that of the BFS in the chaotic BOCDA systems with (blue) and without (red) the time-gated scheme

    图 10  待测光纤沿线布里渊频移分布图 (a)整条光纤沿线的布里渊频移分布; (b)加热位置附近的局部放大图

    Figure 10.  The map of BFS distribution along the FUT: (a) Measured along the entire FUT:(b) the local enlargement near heated zone

    图 11  优化后系统中待测光纤沿线布里渊频移分布曲线

    Figure 11.  Measured distribution of the Brillouin frequency shift along the FUT in the setup after optimization

  • [1]

    António B, Joan C, Sergi V 2016 Sensors 16 748

    [2]

    Bao X Y, Chen L 2011 Sensors 11 4152

    [3]

    Thévenaz L 2010 Front. Optoelectron. China 3 13

    [4]

    Kurashima T, Horiguchi T, Tateda M 1990 Opt. Lett. 15 1038

    [5]

    Hu J H, Zhang X P, Yao Y G, Zhao X D 2013 Opt. Express 21 145

    [6]

    Kim Y H, Song K Y 2017 Opt. Express 25 14098

    [7]

    Soto M A, Bolognini G, Pasquale F D 2011 Opt. Lett. 36 232

    [8]

    Li W H, Bao X Y, Li Y, Chen L 2008 Opt. Express 16 21616

    [9]

    Brown A W 2007 J. Lightw. Technol. 25 381

    [10]

    Hotate K, Arai H, Song K Y 2008 Sice J. Control Measur. Syst. Integrat. 1 271

    [11]

    Hotate K, Hasegawa T 2000 IEICE Trans. Electron. 83 405

    [12]

    Ryu G, Kim G T, Song K Y, Lee S B, Lee K 2017 J. Lightw. Technol. 35 5311

    [13]

    Zadok A, Antman Y, Primerov N, Denisov A, Sancho J, Thévenaz L 2012 Laser Photon. Rev. 6 L1

    [14]

    Cohen R, London Y, Antman Y, Zadok A 2014 Opt. Express 22 12070

    [15]

    Ji Y N, Zhang M J, Wang Y C, Wang P, Wang A B, Wu Y, Xu H, Zhang Y N 2014 Int. J. Bifurcat. Chaos 24 1450032

    [16]

    Zhang J Z, Zhang M T, Zhang M J, Liu Y, Feng C K, Wang Y H, Wang Y C 2018 Opt. Lett. 43 1722

    [17]

    Zhang J Z, Feng C K, Zhang M J, Liu Y, Wu C Y, Wang Y H 2018 Opt. Express 26 6962

    [18]

    Zhang J Z, Wang Y H, Zhang M J, Zhang Q, Li M W, Wu C Y, Qiao L J, Wang Y C 2018 Opt. Express 26 17597

    [19]

    Jeong J H, Lee K, Song K Y, Jeong J M, Lee S B 2012 Opt. Express 20 27094

    [20]

    王安帮 2014 博士学位论文 (太原: 太原理工大学)

    Wang A B 2014 Ph. D. Dissertation (Taiyuan: Taiyuan University of Technology) (in Chinese)

    [21]

    Zhang J Z, Wang A B, Wang J F, Wang Y C 2009 Opt. Express 17 6357

    [22]

    Zhang M J, Liu H, Zhang J Z, Liu Y, Liu R X 2017 IEEE Photon. J. 9 1943

    [23]

    Parker T, Farhadiroushan M, Handerek V A 1997 Proceedings of IEE Colloquium on Optical Techniques for Smart Structures and Structural Monitoring London, UK, February 17, 1997 p1

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  • Received Date:  04 January 2019
  • Accepted Date:  18 February 2019
  • Available Online:  01 May 2019
  • Published Online:  20 May 2019

Distributed temperature measurement with millimeter-level high spatial resolution based on chaotic laser

    Corresponding author: Zhang Ming-Jiang, zhangmingjiang@tyut.edu.cn
  • 1. Key Laboratory of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, Taiyuan 030024, China
  • 2. Institute of Optoelectronic Engineering, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China

Abstract: The high-precision structural health monitoring of large civil structures and materials are increasingly demanded with widely using the distributed fiber sensors. A Brillouin optical correlation domain analysis for millimeter-levelhigh spatial resolution sensing using broadband chaotic laser is proposed and demonstrated. Through the analysis of the influence of polarization state and feedback strength on the chaotic laser, we experimentally achieve a broadband chaotic laser with a spectrum over 7.5 GHz in –3 dB which means that the theoretical spatial resolution is 3 mm, and we also successfully measure the distribution of fiber Brillouin gain spectrum with a temperature over 300 m measurement range with 7.05 mm spatial resolution, which is the first time that the sensor system based on chaotic laser has achieved the measurement with millimeter-level. However, there is still a difference in spatial resolution between the experimental and theoretical values. We can find that the chaotic laser has a time-delay feature; besides, with the broadening of chaotic laser, the threshold of stimulated Brillouin scattering in optical fibers increases while the Brillouin gain will weaken if the pump power is not enough here, and the cross-correlation peak of chaotic laser will narrow. All these problems cause the Brillouin gain signal to be easily submerged by noise, so the performance of the chaotic Brillouin optical correlation domain analysis system will decrease ultimately. Therefore, we also propose an optimization of Brillouin optical correlation domain analysis system by introducing the time-gated scheme into pump branch. It is obvious that the peak power of the pump wave is heightened by more than 9.5 dB after being amplitude-modulated by a square pulse with a pulse width of greater than acoustic phonon lifetime, and the signal-to-back ground noise ratio of the gain spectrum is improved effectively in theory; the cross correlation between chaotic pump wave and probe waveis locked within a pulse duration time, and the residual stimulated Brillouin scattering interactions existing outside the central correlation peak can be largely inhibited. In this optimized setup, the performance of the distributed temperature sensing is improved to 3.12 mm spatial resolution, which corresponds well to the theoretical value. The improved chaotic Brillouin optical correlation domain analysis technology will have a great potential application in high-precision structural health monitoring of large civil structures.

    • 目前, 基于受激布里渊散射的分布式光纤温度和应变传感技术已被广泛应用于各种大型土木建筑结构的健康监测[1-3], 然而在应用过程中仍存在一些技术瓶颈, 例如传感系统空间分辨率难以突破毫米量级, 导致无法精准定位和监测. 在众多基于受激布里渊散射的分布式光纤传感技术中, 布里渊光时域分析技术(Brillouin optical time domain analysis, BOTDA)[4-6]具有测量距离长的优势, 例如基于光脉冲编码的BOTDA技术[7], 其传感距离可高达120 km, 然而空间分辨率因受限于声子寿命难以突破1 m. 为了提高系统的空间分辨率, 研究者们先后提出差分脉冲对技术[8]、暗脉冲技术[9]等, 实现了亚米量级空间分辨率的测量, 但增加了系统的复杂度且测量耗时长.

      布里渊光相干域分析技术(Brillouin optical correlation domain analysis, BOCDA)[10,11]具有测量空间分辨率高的优势. 例如, 韩国Song等[12]在正弦调制光源的基础上引入时域数据处理技术进行多传感点的并行测量, 在1530 m传感光纤上实现了空间分辨率小于3 cm的分布式应变测量; 以色列Thevenaz等[13]通过使用高速的伪随机码对泵浦光和探测光进行相位调制, 在40 m传感光纤上实现了1 cm空间分辨率的分布式温度测量等. 然而此类通过对光源进行频率或相位调制的技术受被调制激光器特性的限制, 导致其空间分辨率难以突破毫米量级. 理论上, BOCDA系统的空间分辨率取决于光源的相干长度, 即光源的线宽越宽, 系统的空间分辨率越高. 所以, 研究者们提出了基于低相干态光源的BOCDA传感技术, 例如以色列AviZadok课题组[14]提出将自发辐射放大(Amplified Spontaneous Emission, ASE)噪声作为BOCDA系统的光源, 使得传感系统免受电光调制器、微波信号源、信号发生器带宽的影响, 实现了空间分辨率为4 mm的分布式温度测量, 但是由于ASE光源的光谱密度和输出功率较低, 导致系统信噪比较差, 传感距离不超过2 m. 我们课题组也提出将低相干态、类噪声的混沌激光应用于传感系统[15], 并在BOCDA系统中实现了传感距离为906 m、空间分辨率为4 cm的分布式温度测量[16], 后通过时延特征抑制[17]并引入时间门控技术, 最终在10.2 km的传感光纤上实现了9 cm空间分辨的分布式温度测量[18], 为解决传感距离与空间分辨的矛盾提供了方案. 但由于受限于光源的线宽较窄, 其空间分辨率仍没能突破毫米量级.

      为了实现毫米级高分辨率的测量, 本文提出了一种基于宽线宽混沌激光的布里渊光相干域分析的分布式温度传感技术, 实验通过改变光反馈混沌的外部参数, 在最佳偏振匹配状态且反馈强度为0.12时获得了–3 dB线宽为7.5 GHz宽线宽混沌光源, 其理论空间分辨率为3 mm, 并在300 m的传感光纤上实现了7.05 mm空间分辨率的分布式温度测量, 即混沌布里渊光相干域系统突破了毫米量级高分辨率的测量, 但与系统空间分辨率的理论值之间存在差异. 进一步, 我们通过引入时间门控技术使系统测量增益谱的信号背景噪声比(signal-to-background ratio, SBR)[19]由约2.28 dB提升为4.55 dB, 最终实现了3.12 mm空间分辨率的分布式温度测量, 与系统的理论空间分辨率一致.

    2.   实验原理
    • 本文采用光反馈法产生宽线宽的混沌激光, 其动态特性满足Lang-Kobayashi速率方程[20,21], 理论表达为:

      其中, (2a)式中最后一项${k_{\rm{f}}}E\left( {t - {\tau _{_{\rm{f}}}}} \right)\exp \left( { - {\rm{i}}\omega t} \right)$为反馈光; EN分别是半导体激光腔中的复合电场振幅和载流子密度; α, G分别是线宽增强因子、微分增益系数; ${\tau _{\rm{p}}}$是光子寿命, ω是半导体激光器的输出角频率; (2b)式中q, V, ${\tau _n}$分别是电荷量、有源区体积和载流子寿命, I是半导体激光器的泵浦电流密度. 实验中可通过改变反馈光的偏振匹配态及反馈光强度得到不同混沌状态, 在偏振匹配态不变时, 改变反馈光强度得到一系列不同状态的混沌激光, 同时发现其自相关曲线中时延特征信号的大小会随反馈强度的改变而改变. 由于时延特征信号在非峰值引起的受激布里渊放大会引起额外的噪声, 在很大程度上限制了混沌BOCDA系统的性能, 所以取时延特征信号最小时的反馈强度为最佳反馈强度[17]. 如图1即为最佳反馈强度0.12时两种典型偏振匹配态下的混沌激光特性, 可以看出从偏振匹配态a到b, 光谱 –3 dB线宽由约3.12 GHz变为7.59 GHz, 频谱变得更加平坦且–3 dB带宽由3.88 GHz变为8.53 GHz, 自相关曲线内中心相关峰的半高全宽(Full Width at Half Maximum, FWHM)从0.114 ns减小到0.029 ns, 即该系统的理论空间分辨率由约1 cm提高到约3 mm, 意味着我们通过改变反馈光偏振匹配态和反馈强度, 使混沌BOCDA系统的理论空间分辨率实现了从厘米量级到毫米量级的重要突破.

      Figure 1.  The characteristics of the chaotic light at two typical polarization matching states: (a1), (b1) Optical spectra; (a2), (b2) power spectra; (a3), (b3) autocorrelation curve

    3.   实验装置
    • 基于宽线宽混沌激光毫米级分辨率分布式光纤测温技术的实验装置如图2所示, 其中红色虚线框内为宽线宽混沌激光源, 是由无内置隔离器的分布式反馈半导体激光器(distributed feedback laser diode, DFB-LD, WTD, E21239)、光环形器(optical circulator, OC1)、光纤偏振控制器(polarization controller, PC1)、可调光衰减器(variable optical attenuator, VOA)和50:50光纤耦合器四个分立光学器件构成的单反馈外腔. 其中, DFB-LD的输出通过OC1进入反馈环, 通过调节DFB-LD的偏置电流以及反馈光路中的光纤PC1, VOA, 改变反馈光的偏振匹配状态和反馈强度, 使得DFB-LD进入混沌状态并产生宽线宽混沌激光, 随后宽线宽的混沌激光经光隔离器(isolator, ISO1)进入90:10的光纤耦合器后分成两路, 其中90%的一路为探测光路, 经过光纤PC2进入由微波信号源(KEYSIGHT N5173B, scan range 9 kHz—13 GHz)驱动的电光调制器(electro-optical modulator, EOM, EOSPACE, 12.5 Gb/s)进行双边带调制以及载波抑制, 其中正弦信号的调制频率约等于布里渊频移${\nu _{\rm{B}}}$. 经调制后的宽线宽混沌激光依次经过可编程光延迟发生器(programmable optical delay generator, PODG, General Photonics ODG-101, MDL-002)、掺饵光纤放大器(erbium doped fiber amplifier, EDFA1, Connet VLSS-980-B)、扰偏器(polarization scrambler, PS, General Photonics PCD-104)及光隔离器(ISO2)注入到待测光纤(fiber under test, FUT)的一端. 其中, PODG用于相关峰的定位, EDFA1将探测光功率放大为11 dBm, PS用于消除布里渊增益信号对偏振态的依赖性. 10%一路为泵浦光路, 先后经过光纤PC3, 掺饵光纤放大器(EDFA2, Keopsys CEFA-C-PB-HP)以及OC2注入到FUT的另一端, 其中EDFA2将泵浦光功率放大为33 dBm. 两路光在待测光纤中发生受激布里渊放大后, 经OC2输出端进入可调带通滤波器(band pass filter, BPF, Yenista XTM-50, 带宽4—80 GHz), 滤出的斯托克斯光功率由带有积分球光电二极管功率探头(Thorlabs S145C, power range 1 μW—3 W, resolution 1 nW)的数字光功率计(optical power meter, OPM, Thorlabs PM100D)进行采集. 其中, FUT为单模光纤(SMF, G.655), 总长约为300 m(末端20 cm放置在恒温箱内), 实验可通过调节可编程PODG将待测点定位于光纤不同位置处, 从而进行FUT沿线温度信息的采集.

      Figure 2.  The experimental setup of broadband chaotic BOCDA

    4.   实验结果与分析
    • 实验选择DFB-LD在最佳反馈强度0.12且偏振匹配态b时的输出作为BOCDA系统光源, 此时混沌激光的–3 dB线宽约为7.5 GHz, 系统的理论空间分辨率约为3 mm. 设置探测光的扫频范围为10.55—10.75 GHz, 扫频步进为1 MHz, 获得待测光纤末端布里渊增益谱(Brillouin gain spectrum, BGS)随温度变化的结果如图3所示. 显然, 与室温下(23 ℃)测量得到的布里渊增益曲线(蓝色)相比, 加热到55 ℃时的布里渊增益曲线(红色)产生了约32 MHz的频移, 和布里渊频移(Brillouin frequency shift, BFS)对温度的灵敏度(1 MHz/℃)相吻合, 且此时测量增益谱的信号背景噪声比约为2.28 dB.

      Figure 3.  The BGS at different temperature end of FUT

      进一步测量得到待测光纤沿线布里渊频移的分布如图4所示. 其中, 图4(a)为300 m待测光纤沿线布里渊频移的整体分布图, 可以看到在室温区(23 ℃)布里渊频移稳定在约10.653 GHz, 光纤末端加热区(269 m处, 55 ℃)见图中红色曲线标示, 布里渊频移发生明显变化; 图4(b)为加热位置附近的局部放大图, 其中加热区(20 cm)内平均布里渊频移约为10.685 GHz, 布里渊频移变化量约为32 MHz, 与前述实验结果一致, 而且布里渊频移的标准差(standard deviation, Std)[18]约为 ± 1.8 MHz即测量温度误差约为 ± 1.8 ℃, 说明系统具有较好的测量准确性.

      Figure 4.  The map of BFS distribution along the FUT: (a) Measured along the entire FUT; (b) the local enlargement near heated zone

      根据上述实验结果解调出待测光纤沿线布里渊频移的分布曲线如图5所示. 由于BOCDA系统的实验空间分辨率可以用上升沿和下降沿10%—90%所对应的光纤长度的平均值来表示[8], 如图中上升沿和下降沿对应的光纤长度分别为6.62 mm和7.48 mm, 取其平均值为7.05 mm. 所以基于宽线宽混沌激光BOCDA系统的空间分辨率约为7.05 mm, 突破了毫米量级分辨率的分布式温度测量.

      Figure 5.  Measured distribution of the Brillouin frequency shift along the FUT

    • 上述基于宽线宽混沌激光的BOCDA系统, 其空间分辨率的理论值为3 mm, 但实验测量值仅有7.05 mm与理论值相比存在较大的误差. 从误差来源分析, 考虑到本文中所使用的光源为宽线宽混沌激光, 与窄线宽的混沌激光相比其在光纤中的受激布里渊散射阈值变大[22], 这里受激布里渊散射的阈值可表示为[23]

      式中, ${P_{{\rm{th}}}}$为布里渊散射阈值;${A_{{\rm{eff}}}}$为光纤有效面积; 修正因子b介于1和2之间, 取决于泵浦光于斯托克斯光的相对偏振方向; $\Delta {\nu _{{\rm{source}}}}$为光源线宽;${L_{{\rm{eff}}}}$为光纤有效作用长度;${g_{\rm{B}}}$为布里渊增益系数. 此时如果入射泵浦光功率较低则会导致测量布里渊增益信号减弱; 同时随着光源线宽的增加, 泵浦光和探测光在光纤中作用产生的相关峰变窄; 而且混沌信号本身含有时延特征, 其在非峰值引起的受激布里渊散射放大会引起额外的噪声且在沿光纤传播过程中不断积累, 即光纤越长噪声积累越多. 这些均导致布里渊增益信息在传输过程中极易被噪声淹没, 从而影响系统的测量性能. 因此, 为了提高系统的测量精度, 实现理论空间分辨率值的测量, 一方面通过引入时间门控技术使泵浦光功率提高的同时实现时延特征的有效抑制, 另一方面通过缩短光纤, 将光纤从300 m变为160 m, 以减少噪声沿光纤的积累, 进而提高系统的测量精度.

      图6所示为时间门控技术[17]的实验装置图, 泵浦光经过EDFA3和PC3进入另一个由脉冲信号驱动的EOM进行强度调制, 后经EDFA2将功率放大为33 dBm, 最终经OC2入射到FUT的一端.

      Figure 6.  The setup of time-gated

      实验设置调制脉冲的持续时间∆τ = 100 ns, 周期为T = 1.5 μs, 幅值电压为2.8 V, 此时被调制EOM的消光比达到最大约为20 dB[18]. 脉冲调制前后泵浦光的时序变化如图7所示, 其中蓝色曲线为脉冲调制前泵浦光的时序, 红色曲线为脉冲调制后的泵浦光时序, 可以看到脉冲调制后泵浦光的峰值功率提高了约9.5 dB, 理论上有效提高了测量增益谱的信号背景噪声比.

      Figure 7.  The time series of the chaotic pump waves (red) and pulse amplitude-modulated (blue)

      图8所示为引入时间门控技术前后待测光纤中发生受激布里渊散射的示意图. 图8(a)为前述系统待测光纤中发生受激布里渊散射示意图, 可以看到两路光在待测光纤中间位置产生稳定的相关峰(即受激布里渊散射的声波场), 但由于混沌信号在外腔时延处发生弱幅自相关导致在主峰附近存在残余次峰[17], 并在受激布里渊散射放大时会引起额外的噪声并沿着光纤不断积累, 最终导致增益信号被噪声淹没而影响测量精度. 图8(b)为引入脉冲调制后两路光在待测光纤中发生受激布里渊散射示意图, 由于脉冲调制使两路光的相互作用被限制于脉冲持续时间内, 非中心峰放大和非零基底噪声被有效抑制.

      Figure 8.  The schematic diagram of SBS in the previous system (a) and the time-gated system (b)

      此时待测光纤中布里渊增益谱随温度变化的测量结果如图9所示. 因为在泵浦路中引入时间门控技术后, 测量的布里渊增益谱是泵浦光光谱和与待测光纤中布里渊谱的卷积, 经脉冲调制的泵浦光光谱在一定程度上被展宽, 导致布里渊增益谱被展宽, 最终布里渊频移量出现了1 MHz的测量偏差[18], 如图9(a)所示, 此时的布里渊频移量约为33 MHz. 但该偏差对温度的测量结果没有影响, 图9(b)所示为加入时间门控技术前后待测光纤中混沌布里渊频移随温度的变化关系, 可以看出加入时间门控技术后, 系统的温度系数由原来的1.03 MHz/℃变为1.09 MHz/℃, 所以33 MHz的频移量与32 ℃的实际温差相匹配. 同时相比于前述系统此时测量增益谱的信号背景噪声比由约2.28 dB提升为4.55 dB.

      Figure 9.  The relationship of the Chaotic BGS with temperature: (a) Temperature-dependence of the BGS in the FUT; (b) that of the BFS in the chaotic BOCDA systems with (blue) and without (red) the time-gated scheme

      进一步得到此时系统中待测光纤沿线布里渊频移的分布如图10所示. 图10(a)为布里渊频移沿130 m待测光纤的整体分布情况, 可以看到, 在室温区(23 ℃)布里渊频移稳定在约10.653 GHz, 在加热区(125 m处, 55 ℃)布里渊频移发生明显变化. 图10(b)为加热位置附近的局部放大图, 在加热区(约3 m)内布里渊频移约为10.686 GHz, 且此时布里渊频移的标准差约为 ± 1.7 MHz.

      Figure 10.  The map of BFS distribution along the FUT: (a) Measured along the entire FUT:(b) the local enlargement near heated zone

      同样根据上述实验结果解调出待测光纤沿线布里渊频移的分布曲线如图11所示. 图中上升沿和下降沿对应的光纤长度分别为3.06 mm和3.15 mm, 取其平均值得到此时系统的空间分辨率为3.12 mm. 如前所述, 本文所提出的基于宽线宽混沌激光BOCDA系统的理论空间分辨率约为3 mm, 可见在系统中引入时间门控技术后有效提高了系统的测量精度, 使系统的实验空间分辨率达到了其理论值.

      Figure 11.  Measured distribution of the Brillouin frequency shift along the FUT in the setup after optimization

    5.   结 论
    • 本文提出了一种基于宽线宽混沌激光布里渊光相干域分析的分布式温度传感系统. 实验通过改变光反馈混沌源的反馈光强度及偏振匹配态获得了–3 dB线宽为7.5 GHz的宽线宽混沌光源并将其应用于BOCDA系统中, 在300 m传感光纤上实现了7.05 mm空间分辨率的测量. 后通过引入时间门控技术提出了一种优化方案, 有效地提高了系统的测量精度, 最终实验实现了3.12 mm高分辨率的分布式温度测量. 与前面提到的基于ASE噪声的布里渊光相干域系统相比, 本文中的宽线宽混沌光源与ASE噪声源类似, 均具有低相干、类随机的特性, 可使系统免受被调制激光器特性的限制, 从而易实现毫米级高分辨率的测量, 但宽带混沌光源不受光谱密度和输出功率低的限制, 所以传感距离不局限于2 m. 然而在该系统中采用数字光功率计进行信号的采集与解调, 会引入较高的测量增益谱背景噪声导致传感距离仅有数百米. 所以在接下来的工作中, 一方面通过对采集系统的改进和优化, 旨在数公里传感光纤上实现毫米级高分辨率的分布式温度或应变的测量. 另一方面通过连续光注入混沌激光器或者利用混沌激光器产生的混沌信号注入另一个自由运行激光器, 产生带宽进一步加强的混沌信号, 旨在进一步提高分布式温度测量的精度. 综上, 该研究结果为一些基础设施结构的高精度监测提供了一种新思路.

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