-
基于φ-OTDR系统相干检测的原理为瑞利相干散射的光时域反射技术。φ-OTDR的光源为窄线宽激光器,φ-OTDR的传感系统与常规OTDR传感系统相比有多种优势,例如具有高灵敏度、长距离测量等等。在传感系统中,窄线宽激光器发出连续的光波经过声光调制器(acousto-optic modulator, AOM)调制成脉冲波,在通过掺铒光纤放大器(Er-doped fiber amplifier, EDFA)放大,经衰减器适当调节后,具有高相干的光脉冲序列进入环行器流入传感光纤作为光信号。探测器收到光信号将其转换为电信号后,通过采集卡进行数据采集,最后在工控机上进行数据处理。
当一个脉冲周期内,光纤上无扰动时,后向相干瑞利散射光的响应模型为1-D脉冲,但是在多个脉冲周期内,用φ-OTDR系统探测到的后向瑞利散射光的振幅e(t)来表达[17]:
$ \begin{array}{l} e(t) = \sum\limits_{i = 1}^N {{A_i}} \exp \left( { - \alpha \frac{{c{\tau _i}}}{{{n_{\rm{f}}}}}} \right) \times \\ \exp \left[ {{\rm{i}}2{\rm{ \mathsf{ π} }}f\left( {t - {\tau _i}} \right)} \right]{\mathop{\rm rect}\nolimits} \left( {\frac{{t - {\tau _i}}}{W}} \right) \end{array} $
(1) 式中,e(t)为散射光的振幅; t表示脉冲光在光纤中的传感时间; f为脉冲光频率; W为宽度; α为光纤损耗,Ai(i=1,2,3…)为入射光在第i个散射中心产生的脉冲光的振幅大小;τi为散射的时间延长;N表示传感光纤中散射点的总数; c是光在真空中的速度; nf表示折射率。当[(t-τi)/W]≤1时,矩形函数rect[(t-τi)/W]=1;在其它条件下,矩形函数为0。当光纤受到外界振动干扰时,瑞利散射的背向光可分为两部分: 其中一部分为光纤首端和扰动点之前的散射点,没有被扰动点干扰,光相位不变化;另一部分来自光纤末端与扰动点之间的散射点,受扰动点的影响,光相位发生变化。两部分散射光的表达式分别为[18]:
$ \begin{array}{l} \;\;\;\;\;\;\;\;{E_{\rm{a}}} = {A_{\rm{a}}}\exp \left( {{\rm{i}}{\varphi _{\rm{a}}}} \right) = \\ {E_0} \cdot \sum\limits_{k = m}^p {\left[ {\exp \left( { - 2\alpha {z_k}} \right)} \right]} {r_k}\exp \left( {{\rm{i}}{\varphi _k}} \right) \end{array} $
(2) $ \begin{array}{l} \;\;\;\;\;\;\;\;\;{E_{\rm{b}}} = {A_{\rm{b}}}\exp \left[ {{\rm{i}}{\varphi _{\rm{b}}} + \varphi (t)} \right] = \\ {E_0} \cdot \sum\limits_{k = p}^n {\exp } \left[ { - 2\alpha {z_k}} \right]{r_k}\exp \left\{ {{\rm{i}}\left[ {{{\bf{ \pmb{\mathsf{ φ}} }}_k} + \varphi (t)} \right]} \right\} \end{array} $
(3) 式中,Ea和Aa分别为扰动前的散射光强和幅度值,Eb和Ab分别为扰动后的散射光强和幅度值,E0为首端探测脉冲光幅值,φa和φb分别是在扰动前某一点的散射光相位和扰动后某一散射点相位,zk为光纤中第k个散射点距离光纤首端的距离,第p个散射点为扰动发生位置,rk和φk分别为第k个散射点的散射系数和散射光相位,φ(t)为扰动引起的光相位变化。
总背向瑞利散射光强E(t)为:
$ \begin{array}{l} E(t) = {E_{\rm{a}}} + {E_{\rm{b}}} = {A_{\rm{a}}}\exp \left( {{\rm{i}}{\varphi _{\rm{a}}}} \right) + \\ \;\;\;\;\;\;\;\;{A_{\rm{b}}}\exp \left[ {{\rm{i}}{\varphi _{\rm{b}}} + {\rm{i}}\varphi (t)} \right] \end{array} $
(4) 总功率P(t)为:
$ \begin{array}{l} \;\;\;\;\;\;P(t) = A_{\rm{a}}^2 + A_{\rm{b}}^2 + \\ 2{A_{\rm{a}}}{A_{\rm{b}}}\cos \left[ {\varphi (t) + {\varphi _{\rm{a}}} - {\varphi _{\rm{b}}}} \right] \end{array} $
(5) 扰动发生时会引起瑞利散射迹线的变化,通过扰动前后瑞利散射迹线的差分计算,可获得扰动点的位置。
本实验中提出了一种新颖的φ-OTDR结构,如图 1所示。分布反馈式激光器(distributed feedback laser,DFB)是窄线宽激光器,其线宽为3kHz,波长1550nm;AOM是声光调制器,调制带宽100MHz,上升沿为30ns;EDFA是Er3+的光纤放大器,放大增益为25dB,光衰减器(optical attenuator,OA)调节输出脉冲功率;光滤波器(optical filter, OF)是采用0.8mm信道间隔的波分复用滤波器;光环行器(optical circulator,OC)为多模光环行器;传感的光纤为多模光纤与单模光纤的复合,中间通过模式转换器(mode converter,MC)连接; 光纤的尾端加上光隔离器(optical isolator,OI), 从环行器3号端口输出的多模信号再经过模式转换器转为单模信号。返回的单模瑞利散射信号由光电探测器(photoelectric detection,PD)进行光电转换;数据采集卡(data acquisition card,DAQ)采集频率为20MHz;在工控机(industrical personal computer,IPC)上处理数据。图中,MMF(multi-mode fiber)为多模光纤, SMF(single-mode fiber)为单模光纤。
如系统结构图所示,探测距离为5km的多模光纤与25km的单模光纤,中间使用模式转换器连接,与传统仅使用单模光纤为传感介质的φ-OTDR系统相比,本实验中结构可以使用更高的探测脉冲功率,而不会产生不敏感区域,可以完成30km左右范围内的监测。
-
在实验中发现多模光纤明显比单模光纤的振动响应明显,尽管多模光纤与单模光纤具有相同的材料特性,但是多模光纤却比单模光纤高出近一个数量级的捕获效率。通过研究多模光纤的捕获率和模式耦合,进一步探索多模光纤的特性。在多模光纤中,以基本模式传播的入射光和由固定模式(ν, μ)捕获的散射光的功率分数由下式[19]给出:
$ {B_{\nu , \mu }} = \frac{{6{q_\nu }{{\rm{ \mathsf{ π} }}^2}\Delta }}{V}\int_0^\infty {{F_{0, 0}}} {(\rho )^2}{F_{\nu , \mu }}{(\rho )^2}{\rm{d}}\rho $
(6) 式中, ρ为径向变量,μ和ν分别表示径向与方位角,V是归一化的频率,Δ=(n02-n12)/(2n02), 其中n0和n1分别是纤芯与包层的折射率,F0, 0(ρ)表示基本模式,Fν, μ(ρ)表示径向和方位角分布(ν, μ)的固定模式。在这个表达式中引入qν以区分径向模式和方位模式:
$ {q_\nu } = \left\{ {\begin{array}{*{20}{l}} {2, (\nu = 0)}\\ {1, (\nu \ne 0)} \end{array}} \right. $
(7) 当ν≠0时,qν获值为1,它对应于方位角模式;当ν=0时,qν获值为2,它对应于径向模式。将信号发射到多模光纤的基本模式,则散射光纤会耦合到光纤中的所有允许的模式。目前已经确定高阶模式有良好的瑞利背向散射光,并且每个模式中的散射光独立传播且没有明显的交叉耦合。所以与单模光纤传感的背向散射光相比,多模光纤传感的背向散射光更强,振动响应也就更明显。另外,多模光纤有解决信号衰落的可能。在多模光纤传感过程中有多种模式,有一种模式的瑞利背向散射光衰落,但仍然有许多其它模式可以完成执行测量任务。
基于单模光纤的喇曼分布式温度测量的喇曼效应不明显,而使用多模光纤与喇曼结合用于分布式温度测量可以表现出良好的灵敏度。光纤喇曼分布式温度传感系统依靠接收光纤的斯托克斯与反斯托克斯喇曼背向散射光,但是喇曼信号一般比较弱(比输入抽运功率弱60dB~70dB)。为了喇曼温度测量系统有良好的信噪比,就需要传感光纤获取更高的发射功率,而多模光纤恰好有较高的非线性阈值与较大的有效面积接收入射功率。多模光纤支持较高的入射抽运功率,而不会改变感测系统的性能,所以, 喇曼分布式温度测量使用多模光纤作为传感介质是合适的。
由于多模光纤比单模光纤的成本低,在短距离的光纤传感系统中,多模光纤更常用。全世界已经有数千个油井中安装的多模光纤,并用于各种情况的监测,例如振动检测、温度测量、应力监测等等。多模光纤有多种模式,为实现多种信号同时感测提供了可能,如果实现安装一套传感系统可以监测多种参量信息,则将具有更好的成本效益。后面将进一步研究基于复合光纤实现分布式温度与振动同时测量。
复合光纤对φ-OTDR振动传感远程敏感
Composite optical fiber bring about remote sensitive to vibration of φ-OTDR
-
摘要: 为了避免在高功率脉冲下产生光纤非线性效应和前端振动不敏感,采用了一种复合光纤的新型光纤传感结构方法,进行了理论分析和实验验证。使用多模光纤与单模光纤的混合作为传感光纤的方法,通过处理不同功率脉冲下的传感信号,取得了光纤传感的距离数据,并针对多模光纤在喇曼系统的传感作用,对其传感特性进行了讨论。结果表明,该新型复合光纤传感结构可探测30km处的振动信号。此研究为多模光纤在相位敏感光时域传感领域的科学研究和工程应用提供了参考。Abstract: In order to avoid the nonlinear effect of the optical fiber and the insensitivity of the front-end vibration under the high power pulse, a new fiber sensing structure method of composite fiber was adopted, which was analyzed theoretically and verified experimentally. By using the hybrid of multi-mode fiber and single-mode fiber as the sensing fiber method, the distance data of the optical fiber sensing was obtained by processing the sensing signals under different power pulses. The results show that the new composite optical fiber sensor structure can detect the vibration signal of 30km. At the same time, the sensing function of multi-mode fiber in Raman system was further studied, and the sensing characteristics of multi-mode fiber were discussed. The new sensor structure provides an important reference for the scientific research and engineering application of the multimode fiber in the field of phase-sensitive optical time-domain sensing.
-
Key words:
- fiber optics /
- vibration measurement /
- remote sensitive /
- multi-mode fiber
-
[1] TIAN Q, YANG D, ZHANG Y, et al. Detection and recognition of mechanical, digging and vehicle signals in the optical fiber pre-warning system. Optics Communications, 2018, 412: 191-200. doi: 10.1016/j.optcom.2017.11.017 [2] JIAN C Z, YANG Zh, HONG X X, et al. Pipeline leak detection technology based on distributed optical fiber acoustic sensing system. IEEE Access, 2020, 8: 30789-30796. doi: 10.1109/ACCESS.2020.2973229 [3] ROMAIN Z S, XIN L, YU W, et al. Recent progress in the perfor-mance enhancement of phase-sensitive OTDR vibration sensing systems. Sensors, 2019, 19(7): 1709-1718. doi: 10.3390/s19071709 [4] PENG F, WU H, JIA X H, et al. Ultra-long high-sensitivity U-OTDR for high spatial resolution intrusion detection of pipelines. Optics Express, 2014, 22(11): 13804-13810. doi: 10.1364/OE.22.013804 [5] WU M S, FAN X Y, LIU Q W, et al. Quasi-distributed fiber-optic acoustic sensing system based on pulse compression technique and phase-noise compensation. Optics Letters, 2019, 44(24): 5969-5972. doi: 10.1364/OL.44.005969 [6] CHEN Q, LIU T, LIU K, et al. An elimination method of polarization-induced phase shift and fading in dual Mach-Zehnder interfero-metry disturbance sensing system. Journal of Lightwave Technology, 2013, 31(19): 3135-3141. doi: 10.1109/JLT.2013.2276942 [7] QIAN H, TAO Zh. All fiber distributed vibration sensing using modulated time-difference pulses. Photonics Technology Letters, 2013, 25(20): 1955-1957. doi: 10.1109/LPT.2013.2276124 [8] LU Y, ZHU T, CHENG L, et al. Distributed vibration sensor based on coherent detection of phase-OTDR. Journal of Lightwave Technology, 2010, 28(22): 3243-3249. [9] XU G, HE Ch Ch, ZHANG L, et al. Research of positioning techno-logy of Mach-Zehnder interferometer. Laser Technology, 2019, 43(2): 195-200(in Chinese). [10] ZABIHI M, CHEN Y S, ZHOU T, et al. Continuous fading suppre-ssion method for Φ-OTDR systems using optimum tracking over multiple probe frequencies. Journal of Lightwave Technology, 2019, 37(14): 3602-3610. doi: 10.1109/JLT.2019.2918353 [11] YUAN Q, WANG F, LIU T, et al. Using an auxiliary Mach-Zehnder interferometer to compensate for the influence of laser-frequency-drift in Φ-OTDR. IEEE Photonics Journal, 2019, 11(1): 1-9. [12] WANG X, LIU Y, LIANG Sh, et al. Event identification based on random forest classifier for Φ-OTDR fiber-optic distributed distur-bance sensor. Infrared Physics & Technology, 2019, 31(97): 319-325. [13] KOTOV O, CHAPALO I, PETOR A, et al. Distributed interference multimode fiber sensor with disturbances localization ability. IEEE International Conference on Electrical Engineering and Photo-nics, 2018, 67(45): 257-260. [14] KE T, MENG Zh, GERALD F, et al. Highly sensitive strain sensor based on composite interference established within S-tapered multimode fiber structure. Optics Express, 2018, 26(26): 33982-33992. doi: 10.1364/OE.26.033982 [15] MAO Y, ASHRY I, HVEDING F, et al. Simultaneous distributed acoustic and temperature sensing using a multimode fiber. IEEE Journal of Selected Topics in Quantum Electronics, 2020, 26(4): 1-7. [16] MARKIEWIEZ K, KACZOROWSKI J, YANG Z, et al. Frequency scanned phase sensitive optical time-domain reflectometry interrogation in multimode optical fibers. APL Photonics, 2020, 5(3): 031302. doi: 10.1063/1.5138728 [17] SHENG Y, YANG Y H, WANG C, et al. Optical fiber distributed acoustic sensing based on the self-interference of Rayleigh backsca-ttering. Measurement, 2016, 79: 222-227. doi: 10.1016/j.measurement.2015.09.042 [18] LI Q, ZHANG C, LI L, et al. Localization mechanisms and location methods of the disturbance sensor based on phase-sensitive OTDR. Optik, 2014, 125(9): 2099-2103. doi: 10.1016/j.ijleo.2013.10.036 [19] MA B, OI K, AH H, et al. Rayleigh backscattering from the fundamental mode in step-index multimode optical fibers. Applied Optics, 2017, 56(2): 354-364. doi: 10.1364/AO.56.000354