基于非相干光频域反射技术的高精度光纤网络健康在线监测系统 下载: 650次
Fibre optic fault affects fibre optic communication quality, data transmission rate and leads to network breakdown. Obtaining line fault information in real-time, accurately locating it, and improving fault maintenance efficiency is an important problem faced by the high-quality and rapid development of the fibre optic communication industry. Optical time-domain reflectometer (OTDR) is a common equipment for fault detection and quality analysis of optical fibre communication links. There are two unavoidable problems due to the limitation of its technical principle. First, the pulse modulation technology used for event location makes OTDR susceptible to the influence of the dispersion effect and widens the pulse width. The longer the measurement distance, the lower the spatial resolution. Second, for a spatial resolution of 1 m, the sampling frequency of OTDR should be at least 200 MHz, and the pulse laser with a pulse width of less than 10 ns and peak power of several watts should be emitted. The luminance efficiency of the light source decreases with high power after a long time of application. The performance of the passive optical devices inside the instrument is affected, leading to different degrees of photosensitive surface damage. Further, it reduces the photoelectric conversion efficiency, device life and overall performance of OTDR. Because of coherence and difference frequency detection, the coherent optical frequency domain reflector (COFDR) can allow light detection to have a larger amplitude range. Additionally, the receiver bandwidth can be very low, effectively reducing the noise and improving the dynamic range. However, its detection range is limited by the coherent length, frequency modulation rate and linearity of the tunable light source. Moreover, the development capacity of the ultra-narrow linewidth, single-frequency, coherent laser in China is weak, and the external purchase price is very high. Commercial COFDR costs nearly one million yuan, and the cost performance of engineering applications is not high. Therefore, this study conducts relevant research to solve these problems and realise the precise positioning and real-time online monitoring of optical fibre faults.
Based on the mechanism of incoherent optical frequency domain reflectometry (IOFDR) technique, this study proposes a cost effective, high-precision and distributed fibre quality detection method by employing Rayleigh backscattered light in the fibre as the signal light combined with its light wave conduction equation. The system structure is designed, and the system prototype is developed. The microwave signal source is used to perform step frequency modulation of the incoherent laser source. After the photoelectric conversion of the reflected light through the photoelectric detector, it beats with the local oscillator signal in the electrical domain. The amplitude-frequency and phase-frequency responses of the system at each modulation frequency constitute the frequency domain information of the system. Then, after Fourier inverse transformation, the time domain information of the event point distribution of the optical fibre is obtained. The theoretical mechanism and numerical model are derived in detail. The system is developed, and the key indexes of the system are verified through experiments.
The system can preliminarily realise distributed detection of 10 km (Fig. 4) optical fibre with very low optical power (<10 mW). It can also guarantee the spatial resolution of 0.1 m with no difference along the optical fibre (Figs. 5 and 6). The dynamic range is more than 34.5 dB [Fig. 6(a)], and the event blind area is very small (Fig. 7). Wavelength division multiplexing technology can also ensure the normal operation of the existing optical fibre communication network. It is an online health monitoring system for optical fibre networks with great development potential and popularisation value to realise a high-precision location of optical fibre fault information.
Based on the mechanism of the IOFDR technique, this study proposed a low-cost, high-precision and distributed fibre quality detection method by taking Rayleigh backscattering light in the fibre as the signal light combined with its light wave conduction equation. Additionally, its numerical model is derived in detail. The system structure is designed. A high-precision online health monitoring system for optical fibre networks is developed, which realises the quality and health monitoring of optical fibre networks by frequent-spatial transformation. The experimental verification shows that the system can realise distributed detection of 10 km optical fibre with very low optical power (<10 mW). It can also guarantee the spatial resolution of 0.1 m with no difference along the optical fibre. The dynamic range is better than 34.5 dB, and the event blind area is very low; thus it accurately reflects the position and quality of fibre failure points or joints. In future studies, we will improve the range of frequency modulation and frequency modulation rate, focussing on the suppression and solution of nonlinear problems in frequency modulation. The spatial resolution and measurement distance index of IOFDR will be further improved, which is a low-cost and high-precision optical fibre sensing and detection technology with great development potential and promotion value.
1 引言
光纤以其传输频带宽、通信容量大,体积小、质量轻、易布放,材质无源、本征安全、抗电磁干扰,无辐射、保密性好、无串扰,以及可在高温、高湿、高腐蚀等恶劣环境下长期稳定工作等诸多优点,在现代高速、大容量、宽带网络和军用通信系统中发挥着不可替代的关键性作用。然而光纤光缆属于一次铺设、长时间使用的传输媒介,在使用过程中,会因违规施工、车辆碾压、自然灾害、鼠害虫害、射击爆破、操作错误、老化等人为或外部环境变化影响出现各种故障,影响光纤通信质量、数据传输速率,甚至导致网络瘫痪,如何实时获取线路故障信息并精准定位,提高故障检修效率,是光纤通信行业高质量、快速发展所面临的重要问题[1-2]。
光时域反射仪(OTDR)作为光纤通信链路故障检测与质量分析设备,通过检测光在光纤中传输时产生的瑞利背向弹射光和菲涅耳反射光的功率,实现光纤链路的弯折、扭曲、断裂、损耗等故障事件的位置和衰减情况的离线检测,已成为通信运维部门的通用设备,绝大部分商用设备的事件空间分辨率为1 m,为进一步提高空间分辨,解决其与动态范围的相互制约的问题[3-4],国内外研究人员先后提出了脉冲编码调制法[5-6]、累加移动平均法、小波变换法[7]、短时分数阶傅里叶变换法[8]、线性光学采样法、混沌光时域反射法[9-10]等新方法,不同程度地提高了空间分辨率和动态范围指标,但是由于其技术原理限制仍存在以下不可回避的两个问题:1)用于事件定位的脉冲调制技术,使OTDR易受色散效应影响,脉宽展宽,测量距离越远空间分辨率越低;2)对于1 m的空间分辨率,OTDR采样频率至少要为200 MHz,且需要发射脉冲宽度小于10 ns、峰值功率为数瓦的脉冲激光,功率较高,长时间使用后光源自身发光效率会下降,且影响仪器内部光无源器件性能,甚至导致光电探测器光敏面受到不同程度的损伤,降低光电转换效率、器件使用寿命以及OTDR整体性能,同时较强的探测光功率会对正常通信及通信系统器件造成一定影响,因此,在测量时多为离线检测。相干光频域反射仪[11-12](COFDR)是通过光外差探测方式实现的,可调谐光源在时间上线性扫描,然后分成两束,一束作为参考光,另一束作为探测光,反射回来的信号光与参考光在耦合器中发生干涉,分析其在频域产生的干涉光信号,再通过傅里叶变换,就可以获得光纤信息,由于相干检测和差频检测的本质,可允许更大的探测光幅度范围、更低的接收器带宽,有效降低噪声和提高动态范围。但其探测范围受到可调谐光源的相干长度、频率调制速率及线性度的限制,且我国目前超窄线宽、单频、相干激光器的研制能力较弱,对外购置价格非常高,一台商业化的COFDR价格近百万,工程应用性价比不高。
为解决上述问题,本文基于非相干光频域反射(IOFDR)技术机理[13-16],以光纤中后向瑞利散射光作为信号光,并结合光波传导方程,提出了一种低成本、高精度、分布式光纤质量检测方法,并对其数值模型进行了详细推导,设计系统结构,研制系统样机。采用微波信号源对非相干激光光源进行步进频率调制,反射光经过光电探测器光电转换后与本振信号在电域进行拍频,每个调制频率下的系统的幅-频响应和相-频响应组成了系统的频域信息,经傅里叶逆变换后,可获得光纤事件点分布的时域信息。对其理论机理和数值模型进行详细推导,并搭建系统,通过实验验证,该系统可以以极低的光功率(<10 mW)实现10 km光纤的分布式检测,并可保证光纤沿线无差异的空间分辨率为0.1 m,动态范围≥34.5 dB,事件盲区极小,采用波分复用技术可保证在光纤通信网络正常工作的同时实现光纤故障信息的高精度定位,是一种极具发展潜力和推广价值的光纤网络在线健康监测系统。
2 IOFDR基本原理
IOFDR技术原理如
很明显,PP(z,ωm,t)是一个正实简谐函数,为使其物理意义更加明显且方便计算,将之改写为
光电探测器接收的光信号是光纤中位置不同、频率相同、相位不同的后向瑞利散射光信号的叠加,探测器上探测到的光纤位置为z处、长度为dz的光纤微元的瑞利散射光功率为
对于每一个调制频率ωm,都可以获得一个交流信号
3 IOFDR性能分析
将光时域检测系统中的光脉冲看成是一个单位冲激函数[17]
系统在单位冲激函数激励下引起的零状态响应被称为该系统的冲激响应,用h(t)来表征,将单位冲激响应进行傅里叶变换即可得到频响函数H(jω),其傅里叶逆变换即为h(t),表示为
实际IOFDR系统中测量的频率响应的频率范围是有限的,并且频率是步进离散的,光源调制频率范围-fmax≤Δfm≤+fmax,调制的步进频率为Δfm=Δω/2π,Δω为角频率调制间隔。IOFDR系统频率响应可表示为
对系统频响进行离散傅里叶变化(DFT),得到周期函数hp(t),表示为
利用频率响应函数的共轭对称性,即Hd(-jω)=
4 实验结果
4.1 实验装置
实验装置如
4.2 实验结果及分析
如
测试结果如
图 4. IOFDR测试结果图和尾端细节放大图。(a)整体测试图;(b)尾端细节放大图
Fig. 4. Diagrams of IOFDR test resultsand enlargement of tail details. (a) Overall test drawing; (b) enlargement of tail details
基于上述测试条件,采用截断法,用千分尺度量尾纤长度,依次在尾端减去长度为30 cm、20 cm、20 cm的尾纤,观测尾端反射峰值位置变化情况。如
图 5. 尾端反射峰细节图。(a)第一次,尾纤减去30 cm;(b)第二次,尾纤减去20 cm;(c)第三次,尾纤减去20 cm
Fig. 5. Detail diagrams of tail reflection peak. (a) The first time, the 30 cm long tail is subtracted; (b) the second time, the 20 cm long tail is subtracted; (c) the third time, the 20 cm long tail is subtracted
随后,采用相对测量法,采用米尺度量光纤跳线长度,依次在
图 6. 第一盘和第二盘光纤连接处反射峰测试情况。(a)接入7 m光纤跳线;(b)接入3 m光纤跳线
Fig. 6. Reflection peak tests at the first and second optical fibre connections. (a) 7 m long optical fibre jumper is accessed;(b) 3 m long optical fibre jumper is accessed
如
图 7. 不同情况下的反射峰细节图。(a)起始端未接入光纤跳线;(b)起始端接入3 m长光纤跳线
Fig. 7. Detail diagrams of reflection peaks under different conditions. (a) Starting end is not connected to optical fibre jumper cable; (b) starting end is connected to 3 m long optical fibre jumper cable
多次测量,由软件测试曲线结果计算动态范围为10log(幅值最大值/幅值平均值),由上述结果可以验证IOFDR由于在频域测量其本底噪声水平极低,归一化后的幅值平均值约为0.06 mV,而上述测试结果中反射峰最大值为162.7 mV,非系统响应最大值,则系统的动态范围优于34.5 dB。
采用波分复用技术,将中心波长为1650 nm的IOFDR系统接入双通信波长分别为1310 nm和1550 nm的某网络保障中心用于关键光纤网络链路质量的实时在线监测近两个月,系统运行稳定且对当前通信业务无影响,在光纤网络状态无异常情况下,各事件点反射强度值几乎没有变化;在实验室中,多次改变
图 8. 改变LC接头连接质量三次得到IOFDR系统的测量结果
Fig. 8. Measurement results of IOFDR system by changing connection quality of LC connector three times
5 结论
基于非相干光频域反射(IOFDR)技术机理,以光纤中后向瑞利散射光作为信号光,并结合其光波传导方程,提出了一种低成本、高精度、分布式光纤质量检测方法,并对其数值模型进行了详细推导。设计系统结构,研制了一种高精度光纤网络健康在线监测系统,利用频域-空域变换实现光纤网络的质量健康检测。通过实验验证,该系统可使用成本较低的非相干光源,以极低的光功率(<10 mW)实现10 km光纤的分布式检测,并保证光纤沿线无差异的空间分辨率0.1 m,动态范围优于34.5 dB,事件盲区极低,能够精准反映光纤故障点或接头位置及质量情况,是一种极具发展潜力和推广价值的低沉本、高精度光纤传感与检测技术。在后续的研究工作中,可进一步提高频率调制范围和频率调制速率,重点关注频率调制非线性问题的抑制与解决,以提高IOFDR空间分辨率和测量距离指标。
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Article Outline
于淼, 吉顺兵, 刘海, 杨光, 黄圣军, 刘军, 何禹潼, 孙铭阳. 基于非相干光频域反射技术的高精度光纤网络健康在线监测系统[J]. 中国激光, 2022, 49(4): 0406003. Miao Yu, Shunbing Ji, Hai Liu, Guang Yang, Shengjun Huang, Jun Liu, Yutong He, Mingyang Sun. High-Precision Optical Fibre Network Health Online Monitoring System Based on Incoherent Optical Frequency Domain Reflection Technology[J]. Chinese Journal of Lasers, 2022, 49(4): 0406003.