Digital coherent detection research on Brillouin optical time domain reflectometry with simplex pulse codes

The digital coherent detection technique has been investigated without any frequency-scanning device in the Brillouin optical time domain reflectometry （BOTDR）, where the simplex pulse codes are applied in the sensing system. The time domain signal of every code sequence is collected by the data acquisition card （DAQ）. A shift-averaging technique is applied in the frequency domain for the reason that the local oscillator （LO） in the coherent detection is fix-frequency deviated from the primary source. With the 31-bit simplex code, the signal-to-noise ratio （SNR） has 3.5-dB enhancement with the same single pulse traces, accordant with the theoretical analysis. The frequency fluctuation for simplex codes is 14.01 MHz less than that for a single pulse as to 4-m spatial resolution. The results are believed to be beneficial for the BOTDR performance improvement.

Influence of laser linewidth on performance of Brillouin optical time domain reflectometry

The effects of optical sources with different laser linewidths on Brillouin optical time domain reflectometry （BOTDR） are investigated numerically and experimentally. Simulation results show that the spectral linewidth of spontaneous Brillouin scattering remains almost constant when the laser linewidth is less than 1 MHz at the same pulse width; otherwise, it increases sharply. A comparison between a fiber laser （FL） with 4-kHz linewidth at 3 dB and a distributed feedback （DFB） laser with 3-MHz linewidth is made experimentally. When a constant laser power is launched into the sensing fiber, the fitting linewidths of the beat signals （backscattered Brillouin light and local oscillator （LO）） is about 5 MHz wider for the DFB laser than for the FL and the intensity of the beat signal is about a half. Furthermore, the frequency fluctuation in the long sensing fiber is lower for the FL source, yielding about 2 MHz less than that of the DFB laser, indicating higher temperature/strain resolution. The experimental results are in good agreement with the numerical simulations.

Photon counts modulation in optical time domain reflectometry

The quantum fluctuation of photon counting limits the field application of optical time domain reflection. A method of photon counts modulation optics time domain reflection with single photon detection at 1.55 μm is presented. The influence of quantum fluctuation can be effectively controlled by demodulation technology since quantum fluctuation shows a uniform distribution in the frequency domain. Combined with the changing of the integration time of the lock-in amplifier, the signal to noise ratio is significantly enhanced. Accordingly the signal to noise improvement ratio reaches 31.7 dB compared with the direct photon counting measurement.

Brillouin optical time-domain analysis for geotechnical monitoring

In this paper, we show some recent experimental applications of Brillouin optical time-domain analysis （BOTDA） based sensors for geotechnical monitoring. In particular, how these sensors can be applied to detecting early movements of soil slopes by the direct embedding of suitable fiber cables in the ground is presented. Furthermore, the same technology can be used to realize innovative inclinometers, as well as smart foundation anchors.

Terahertz Time-Domain Spectroscopy method for optical parameter extraction of plastic materials

A method for extracting optical parameters of plastics materials based on terahertz time domain spectroscopy is presented. The transmission-type Terahertz Time-Domain Spectroscopy(THz TDS) system is adopted to detect the refractive index and extinction coefficient on different plastic materials. Then the corresponding spectral information is obtained by Fourier transform of the terahertz time domain waveform of the sampling points, including the corresponding amplitude and phase information of the waveform. The optical parameter extraction model is built. By using the simplex optimization method, the curves of the refractive index and extinction coefficient for the plastic material are obtained. The experimental samples are made of different plastic parallel plate materials. The experimental results show that the optimization of optical parameters can improve their extraction accuracy, and the error of refractive index is ±0.005. Extraction technology with the simplex optimization method of optical parameter based on THz TDS can help to extract the optical parameters of engineering plastics. It is of great significance for the research of terahertz nondestructive testing.

Fiber optic monitoring of an anti-slide pile in a retrogressive landslide

Anti-slide piles are one of the most important reinforcement structures against landslides,and evalu-ating the working conditions is of great significance for landslide mitigation.The widely adopted analytical methods of pile internal forces include cantilever beam method and elastic foundation beam method.However,due to many assumptions involved in calculation,the analytical models cannot be fully applicable to complex site situations,e.g.landslides with multi-sliding surfaces and pile-soil interface separation as discussed herein.In view of this,the combination of distributed fiber optic sensing(DFOS)and strain-internal force conversion methods was proposed to evaluate the working conditions of an anti-sliding pile in a typical retrogressive landslide in the Three Gorges reservoir area,China.Brillouin optical time domain reflectometry(BOTDR)was utilized to monitor the strain distri-bution along the pile.Next,by analyzing the relative deformation between the pile and its adjacent inclinometer,the pile-soil interface separation was profiled.Finally,the internal forces of the anti-slide pile were derived based on the strain-internal force conversion method.According to the ratio of calculated internal forces to the design values,the working conditions of the anti-slide pile could be evaluated.The results demonstrated that the proposed method could reveal the deformation pattern of the anti-slide pile system,and can quantitatively evaluate its working conditions.

Two-Dimensional Terahertz Time-Domain Spectroscopy and Its Applications

In this work, we review the developing progress of two-dimensional terahertz time-domain spectroscopy（THz-TDS） and its diverse applications, including analyzing the polarization of THz radiation from a laser-induced plasma source and studying the corresponding physical mechanism, and characterizing the optical properties of crystals, etc.

Parallel-Computing Wavelet-Based FDTD Method for Modeling Nanoscale Optical Resonator

An efficient wavelet-based finite-difference time-domain(FDTD)method is implemented for analyzing nanoscale optical devices,especially optical resonator.Because of its highly linear numerical dispersion properties the high-spatial-order FDTD achieves significant reduction in the number of cells,i.e.used memory,while analyzing a high-index dielectric ring resonator working as an add/drop multiplexer.The main novelty is that the wavelet-based FDTD model is extended in a parallel computation environment to solve physical problems with large dimensions.To demonstrate the efficiency of the parallelized FDTD model,a mirrored cavity is analyzed.The analysis shows that the proposed model reduces computation time and memory cost,and the parallel computation result matches the theoretical model.

Recent Advances in Phase-Sensitive Optical Time Domain Reflectometry (Ф-OTDR)

Phase-sensitive optical time domain reflectometry(Ф-OTDR)is an effective way to detect vibrations and acoustic waves with high sensitivity,by interrogating coherent Rayleigh backscattering light in sensing fiber.In particular,fiber-optic distributed acoustic sensing(DAS)based on theФ-OTDR with phase demodulation has been extensively studied and widely used in intrusion detection,borehole seismic acquisition,structure health monitoring,etc.,in recent years,with superior advantages such as long sensing range,fast response speed,wide sensing bandwidth,low operation cost and long service lifetime.Significant advances in research and development(R&D)ofФ-OTDR have been made since 2014.In this review,we present a historical review ofФ-OTDR and then summarize the recent progress ofФ-OTDR in the Fiber Optics Research Center(FORC)at University of Electronic Science and Technology of China(UESTC),which is the first group to carry out R&D ofФ-OTDR and invent ultra-sensitive DAS(uDAS)seismometer in China which is elected as one of the ten most significant technology advances of PetroChina in 2019.It can be seen that theФ-OTDR/DAS technology is currently under its rapid development stage and would reach its climax in the next 5 years.

Experimental validation for time-domain fluorescence diffuse optical tomography of linear scheme

We present a full three-dimensional, featured-data algorithm for time-domain fluorescence diffuse optical tomography that inverts the Laplace-transformed time-domain coupled diffusion equations and employs a pair of appropriate transform-factors to effectively separate the fluorescent yield and lifetime parameters. By use of a time-correlation single-photon counting system and the normalized Born formulation, we experimentally validate that the proposed scheme can achieve simultaneous reconstruction of the fluorescent yield and lifetime distributions with a reasonable accuracy.

Interferometric Distributed Sensing System With Phase Optical Time-Domain Reflectometry

We demonstrate a distributed optical fiber sensing system based on the Michelson interferometer of the phase sensitive optical time domain reflectometer （q0-OTDR） for acoustic measurement. Phase, amplitude, frequency response, and location information can be directly obtained at the same time by using the passive 3 ×3 coupler demodulation. We also set an experiment and successfully restore the acoustic information. Meanwhile, our system has preliminary realized acoustic-phase sensitivity around -150 dB （re rad/μPa） in the experiment.

SNR improvement in self-heterodyne detection Brillouin optical time domain reflectometer using Golay pulse codes

The application of Golay pulse coding technique in spontaneous Brillouin-based distributed temperature sensor based on self-heterodyne detection of Rayleigh and Brillouin scattering is theoretically and experimentally analyzed. The enhancement of system signal to noise ratio(SNR) and reduction of temperature measurement error provided by coding are characterized. By using 16-bit Golay coding, SNR can be improved by about 2.77 d B, and temperature measurement error of the 100 m heated fiber is reduced from 1.4 °C to 0.5 °C with a spatial resolution of 13 m. The results are believed to be beneficial for the performance improvement of self-heterodyne detection Brillouin optical time domain reflectometer.

A performance enhanced Rayleigh Brillouin optical time domain analysis sensing system

Aiming at the problem of large fading noise in Rayleigh Brillouin optical time domain analysis system, a wavelength scanning technique is proposed to enhance the performance of the temperature sensing system. The principle of the proposed technique to reduce the fading noise is introduced based on the analysis of Rayleigh Brillouin optical time domain analysis system. The experimental results show that the signal-to-noise ratio(SNR) at the end of optical fiber with length of 50 m after 17 times wavelength scanning is 5.21 d B higher than that with single wavelength, the Brillouin frequency shift(BFS) on the heated fiber with length of 70 m inserted at the center of sensing fiber can be accurately measured as 0.19 MHz, which is equivalent to a measurement accuracy of 0.19 °C. It indicates that the proposed technique can realize high-accuracy temperature measurement and has huge potential in the field of long-distance and high-accuracy sensing.

Dispersion-Induced Waveform Distortion Detection in 42.7 Gbps CS-RZ Signals by Optical Time Domain Level Monitoring

We propose a technique for chromatic dispersion monitoring based on optical time domain level monitoring. Experimental and simulation results show that the technique is effective for the monitoring of dispersion in 42.7-Gbps CS-RZ signals for dynamic dispersion compensation.

相干衰落抑制Φ-OTDR的分布式光纤周界安防技术

基于相位敏感光时域反射计(Φ-OTDR)的分布式光纤振动传感系统在铁路周界安防中有着重要的应用前景.为降低Φ-OTDR中固有的相干衰落对相位解调的影响,提高扰动信号识别率,提出一种基于矢量旋转滑动平均(MVRA)的相干衰落抑制方法.首先,对探测信号复矢量化,并对各位置的复矢量信号按初相角进行旋转对齐;然后,采用滑动平均的方法缓和信号幅度起伏以及减小噪声功率,提高信噪比,进而抑制相干衰落;其次,从衰落抑制信号解调出扰动信号,将MVRA与频谱提取重组(SERM)、数字向移变换(DPST)方法进行对比,通过差分相位标准差验证抗衰落效果;最后,通过搭建分布式光纤周界入侵检测实验平台,模拟环境噪声、应力破坏、攀爬、剪网4种防护扰动信号,以解调的相位灰度图作为特征图像,使用卷积神经网络进行模式识别.实验结果表明:相比SERM、DPST,MVRA能更高效地抑制衰落,当滑动窗长50 ns时,MVRA提高11.2 dB信噪比;扰动信号的识别率由衰落抑制前的88%提高到衰落抑制后的92%.

基于经验模态分解的单端BOTDA系统降噪方法研究

针对单端布里渊光时域分析(BOTDA)系统存在噪声大、信噪比较低等不足,提出一种基于经验模态分解的降噪方法。理论分析经验模态分解的降噪原理和少模光纤单端布里渊光时域分析传感原理,通过搭建的单端结构布里渊光时域分析温度传感系统,对经验模态分解的降噪效果进行对比分析。实验和仿真结果表明,经验模态分解算法对温度传感系统具有良好的降噪效果,降噪后信噪比提升了约3.06dB,温度测量精度提升了约0.98℃。

基于布里渊光时域峰值边沿分析的变压器绕组局部热点检测

变压器绕组的温度分布一直是电网运行人员重点关注的对象。与传统传感器相比,分布式光纤传感具有抗电磁干扰能力强、可实现分布式测量等优势,其中布里渊光时域分析技术(BOTDA)的性能稳定,适用于大多数场景,但BOTDA的空间分辨率为m级,难以分辨绕组运行中的局部热点。该文从布里渊散射增益谱的机理入手,利用双峰拟合算法,并结合对异常峰值曲线的边沿分析,提出了一种在不增加系统硬件复杂度的基础上提升BOTDA空间分辨率的方法,以满足局部热点检测要求。通过对光纤沿线不同位置、不同热点长度及不同热点温度下的模拟试验验证了该方法的有效性,在实验室条件下可将准确传感的热点长度提升至原空间分辨率的一半以内。基于变压器实际绕组的局部升温试验,实现了绕组异常热点的精确定位和长度感知,并准确解调了绕组局部热点的温度,进一步验证了该方法的实际可行性和检测优越性,温度相对误差控制在5%以内,为提高变压器绕组温度在线监测的经济性和可行性提供新的思考,同时也为变压器绕组局部故障的提前感知和预警提供了新的思路。

基于时频域联合数据辅助MIMO均衡算法的多维度复用千公里级相干传输系统

基于少模光纤的模式复用系统利用单根纤芯中的正交模式实现各独立信道的并行传输,能够有效提升系统容量,解决单模光纤系统的容量危机.本文联合使用模式复用、波分复用与偏振复用技术,基于六模渐变型光纤实现了80个波长通道下32 Gbaud四模双偏振16阶正交振幅调制信号的1000 km相干传输,并在接收端使用时频域联合数据辅助8×8多输入多输出最小均方算法,以补偿由光纤传输导致的模式耦合及模间色散等线性损伤.与传统的纯时域或纯频域多输入输出最小均方均衡算法相比,该算法分别实现了57.1%的收敛速度提升与25.1%的收敛误差下降.经过1000 km少模光纤传输后,80通道每个模式及偏振态的误码率均低于开销为20%的软判决前向纠错编码阈值2.4×10^(-2),系统净速率达68.2 Tbit/s.

离散色散光域傅里叶变换的延时和带宽扩展研究

光域傅里叶变换为射频频谱分析提供了一种新的解决方案。光域傅里叶变换主要受限于大的二阶色散的获得,而离散傅里叶变换只需要控制离散点的相位,理论上可以解决大色散的获得问题。文章利用光纤环作为离散色散器件,用线性调频信号作为待测信号,进行了光域离散傅里叶变换的研究。该系统是一个快速傅里叶变换系统,每次变换用时约2.07ns。用示波器观测线性调频信号变换前与变换后的波形延迟,可以测出该系统的变换延时,约为100ns,其延时大小主要取决于系统中的光纤长度。该系统可以实现对信号的不间断处理。为了扩展系统的瞬时带宽,对片上集成光微环组方案的可实现性进行了分析。

基于BOTDR的区段煤柱水平变形监测

针对煤柱内部微裂纹萌生、变形破坏等潜在安全隐患,利用布里渊光时域反射(BOTDR)分布式光纤传感技术进行煤柱水平变形监测。首先,根据煤柱破坏碎胀特性理论分析,建立了光纤轴向应变与水平变形的转化方法;其次开展室内试验确定了金属基索状传感光纤轴向拉伸响应情况,并根据煤柱变形破坏规律修正了应变系数,最后以陕西大柳塔煤矿活鸡兔井为工程背景,对22206工作面区段煤柱水平变形实现现场监测。监测结果表明:区段煤柱采动侧和未采动侧存在明显的水平变形差异,采动侧变形量最大,约为未采动侧的5倍,但煤柱整体水平变形较小,仅出现弹性变形,煤柱较稳定;煤柱变形分区明显,呈现中部为弹性核区,两侧为塑性破坏区的分区特征;工作面从测点前20m推进至测点后40m时,煤柱水平应变呈指数增长,远离测点之后变形趋于稳定,反映煤柱变形与采动应力扰动之间的动态关联;通过分析煤柱水平应变与垂直应力,发现两者呈指数相关关系,定量建立了煤柱水平应变与垂直应力关系式,说明煤柱变形与破坏很大程度上取决于采动应力的分布与演化。利用BOTDR技术对区段煤柱内部变形进行监测可以精确获取区段煤柱内部的微观变形信息,对于煤柱留设尺寸确定、围岩控制及采空区压力评估等具有重要指导意义。