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Advances of key technologies on optical reflectometry with ultra-high spatial resolution
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摘要:
作为分布式光纤传感器核心技术，光反射仪能够对光纤进行非破坏性检测，获取沿光纤长度的反射率、折射率和偏振态等分布信息来判断光纤链路各类异常“事件”。在一些高端监测领域，例如光纤到户(fiber-to-the-home, FTTH)接入网的故障诊断、大型发电机组和大型变压器内部的热点和形变监测以及大飞机的机翼结构安全监测等应用，对传感器空间分辨率、测量距离等性能提出了非常高的要求。本文总结了光反射仪技术国内外的研究现状，并针对应用需求，回顾了几种实现长距离高空间分辨率光反射仪的关键技术及其在实现更高性能时所面临的技术难点。针对各类技术难点，分别提出三种创新性方案，从三种不同角度加以改善，推动光反射仪技术在分布式传感系统中的应用。
Abstract:As the core technology of distributed fiber-optic sensing, optical reflectometry may realize the non-destructive measurement at a remote position. It can be used to retrieve the distributed information such as reflectance, refractive index, polarization state along the optical fiber, and to diagnose the irregular "event" on fiber-optic links. For some high-end fields, such as the fault diagnosis on the fiber-to-the-home (FTTH) access network, the deformation monitoring on large generating units and large transformers, and the security monitoring on structures of airplane wings, the requirements on spatial resolution and measurement range of the sensing technologies are very high. In this paper, we summarized the research status on state-of-art optical reflectometry technologies, and reviewed the advances of key technologies on optical reflectometry with ultra-high spatial resolution and long measurement range. We proposed three different methods to improve the performance, and tried to promote their applications on distributed fiber-optic sensing systems.
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Overview: As the core technology of distributed fiber-optic sensing, optical reflectometry may realize the non-destructive measurement at a remote position. It can be used to retrieve the distributed information such as reflectance, refractive index, polarization state along the optical fiber, and to diagnose the irregular "event" on fiber-optic links. In this paper, we summarized the research status on state-of-art optical reflectometry technologies, and reviewed the advances of key technologies on optical reflectometry with ultra-high spatial resolution and long measurement range. We proposed three different methods to improve the performance, and tried to promote their applications on distributed fiber-optic sensing systems.
Firstly, we propose and demonstrate a millimeter-resolution long-range optical frequency domain reflectometry (OFDR) using an ultra-linearly 100 GHz swept optical source realized by injection-locking technique and cascaded four-wave-mixing (FWM) process. The ultra-linear swept source is realized using an external modulation method with a linearly swept radio frequency (RF) signal. By using the injection-locked frequency swept laser as the optical source of OFDR, we obtain a spatial resolution of 4.2 mm over 10 km measurement range. To further improve the spatial resolution, FWM process is used to broaden the frequency sweeping span. A frequency sweeping span of ~100 GHz is achieved with cascaded FWM. We demonstrate a 1.1 mm spatial resolution over 2 km measurement range with the proposed ultra-linearly swept optical source.
Then, we demonstrate an ultra-high-resolution optical time domain reflectometry (OTDR) system by using a mode-locked laser as the pulse source and a linear optical sampling technique to detect the reflected signals. Taking advantage of the ultrashort input pulse, the large detection-bandwidth, as well as the low timing jitter of linear optical sampling system, a sub-mm spatial resolution is achieved. As the pulse-width is broadened with the increase of distance due to the chromatic dispersion and the large bandwidth of the ultrashort pulse, by adopting digital chromatic dispersion compensation, we achieved a spatial resolution of 340 mm when measuring the reflector at 10 km.
The final method is based on linear optical sampling and pulse compression method. We propose an all-optic sub-THz-range linearly chirped optical source and a high-bandwidth detection system to characterize it. Taking advantage of the chromatic dispersion effect, ultrashort optical pulses are stretched to be ~10-ns linearly chirped pulses with sub-THz range, which yields a large time-bandwidth product of 4500, a high compression ratio of 4167 and a chirp rate of 45 GHz/ns. A ultra-high spatial resolution of 120 μm with 150 m measurement range and 20.4 dB extinction ration is finally demonstrated.
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图 2 基于注入锁定的OFDR系统实验框图[17]
Figure 2. Experimental setup of the injection-locking scheme. FL: fiber laser; IM: intensity modulator; VOA: variable optical attenuator; PC: polarization controller; DFB: distributed feedback diode laser; Amp: RF amplifier; AWG: arbitrary waveform generator; FUT: fiber under test; BPD: balanced photodetector; A/D: analog-to-digital converter[17]
图 3 (a) 注入锁定后的光谱图；(b)扫频信号时频关系[17]
Figure 3. (a) Optical spectrum of the slave laser which is injection locked to the 8th-order sideband of the master laser, and the inset is spectrum of the generated optical comb after IM; (b) Relative optical frequency as a function of time after the injection-locking[17]
图 10 (a)~(b)分别是补偿前10 km处反射点的时域信号和时频图；(c)~(d)分别是补偿后10 km处反射点的时域信号和时频图；(e)补偿前后反射点对比图
Figure 10. (a) Reflection peak without chromatic dispersion (CD) compensation; (b) Time-frequency map of the reflection peak without CD compensation; (c) Reflection peak with CD compensation; (d) Time-frequency map of the reflection peak with CD; (e) Details of the reflection peak at 10 km with/without CD compensation[18]
图 11 脉冲压缩方案实验结果。(a)产生并被接收到的线性扫频信号，脉冲宽度为10 ns，左边图是在25 TS/s采样率下采集信号的放大展示，右边图展示的残余信号是由于脉冲选择器消光比较低留下的旁瓣；(b)线性扫频信号的时频展示，产生扫频信号带宽为450 GHz；(c)实行匹配滤波之后的压缩脉冲[19]
Figure 11. Experimental results. (a) A temporal frame of 10 ns linearly chirped pulse; (b) Shot time Fourier transformation (STFT) analysis of the linearly chirped pulse; (c) Calculated autocorrelation of the linearly chirped pulse[19]
图 12 相位编码的实验结果。(a) 5个周期脉冲编码后的时域图；(b)相位编码后脉冲的时频图；(c) 5个脉冲的压缩结果；(d) 386个脉冲的压缩结果[19]
Figure 12. Experimental results. (a) 20 ns measurement of linearly chirped pulse with phase modulation; (b) STFT analysis of the linearly chirped pulse with phase modulation; (c) Calculated autocorrelation of the 20 ns linearly chirped pulse with phase modulation; (d) Autocorrelation of the 1544 ns time-aperture pulse[19]
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