Brillouin Optical Time Domain Reflectometry System Research Articles

Denoising of BOTDR Dynamic Strain Measurement Using Convolutional Neural Networks.

The Brillouin optical time domain reflectometry (BOTDR) system measures the distributed strain and temperature information along the optic fibre by detecting the Brillouin gain spectra (BGS) and finding the Brillouin frequency shift profiles. By introducing small gain stimulated Brillouin scattering (SBS), dynamic measurement using BOTDR can be realized, but the performance is limited due to the noise of the detected information. An image denoising method using the convolutional neural network (CNN) is applied to the derived Brillouin gain spectrum images to enhance the performance of the Brillouin frequency shift detection and the strain vibration measurement of the BOTDR system. By reducing the noise of the BGS images along the length of the fibre under test with different network depths and epoch numbers, smaller frequency uncertainties are obtained, and the sine-fitting R-squared values of the detected strain vibration profiles are also higher. The Brillouin frequency uncertainty is improved by 24% and the sine-fitting R-squared value of the obtained strain vibration profile is enhanced to 0.739, with eight layers of total depth and 200 epochs.

Probe pulse design in Brillouin optical time‐domain reflectometry

Brillouin optical time-domain reflectometry (BOTDR) is a branch of distributed fibre-optic sensors, and it can measure the strain and the temperature information, localised by the return time of the probe pulse, along the fibre based on the spontaneous Brillouin scattering process. Parameters of the BOTDR system, including the spatial resolution, the signal-to-noise ratio, the measurement speed, and the sensing range, have a mutually restrictive relationship. In order to improve the performance of the BOTDR system, researchers have focussed on improving the design of the probe pulse, for example, transforming the shape, the sequence, and the spectral properties of the pulse. This study summarises the recent progress in the design of detection pulses in BOTDR systems, and comprehensively demonstrates the improvement effects of various pulse modulation formats on the system performance.

Wavelet convolutional neural network for robust and fast temperature measurements in Brillouin optical time domain reflectometry.

In this paper, a wavelet convolutional neural network (WNN) consisting of a one-dimensional (1D) convolutional neural network and a self-adaptive wavelet neural network has been proposed and demonstrated experimentally for temperature measurement in a Brillouin optical time domain reflectometry (BOTDR) system. Based on the analysis of the system noise, it follows the Gaussian white noise distribution along the time-related sensing distance. The impact of the noise in time-domain on the measured Brillouin gain spectra (BGSs) could be neglected, so that the BGSs in the fiber can be regarded as a series of 1D input data of the proposed WNN. Different self-adaptive wavelet activation functions connected to each output of the full-connection network are adopted to realize the multi-scaled analysis and the scale translation, which can obtain more local characteristics in frequency-domain. The output extracted by the WNN is Brillouin frequency shift (BFS), which presents linearity correlation to the actual temperature. Considering the multi-parameters including different frequency ranges, signal-to-noise-ratios (SNRs), BFSs and spectral widths (SWs), a general model of the proposed WNN is trained to handle more extreme cases, in which it doesn't require retraining for different single-mode (SM) optical fibers in BOTDR sensing system. The performances of the WNN are compared with other two techniques, the Lorentzian curve fitting based on Levenberg-Marquardt (LM) algorithm and the basic neural network (NN) containing input and output layers together with two hidden layers. Both the simulated and measured results show that the WNN has better robustness and flexibility than the LM and the NN. Besides, the computational accuracy of the WNN is improved and the fluctuation of that is slighter, especially when the SNR is less than 11 dB. Moreover, the WNN takes approximately 0.54 s to measure the temperature from the 18,000 collected BGSs transmitted through the 18 km SM optical fiber. The calculating time of the WNN is greatly reduced by three orders of magnitude in comparison with that of the LM, and is comparable to that of the NN. It proves that the proposed WNN may provide a feasible or even better scheme for the robust and fast temperature measurement in BOTDR system.

Influence of pump pulse spectrum on the signal-noise-ratio of brillouin scattering in brillouin optical time domain reflectometry

Abstracts The influence of pump pulse spectrum launched into the sensing fiber on the spontaneous Brillouin scattering and Brillouin Optical Time Domain Reflectometry (BOTDR) sensing system, is investigated both theoretically and experimentally. The pump pulse power spectrum is the essential factor for the Brillouin scattering from physical mechanism, and it includes two ingredients: the laser linewidth and the modulated pulse waveform; they both subsequently affect the performance of the BOTDR system. Two different pump pulse spectra are generated in the experiment: (1) the output light of a distributed feedback (DFB) laser with 3 MHz linewidth, modulated to a rectangular pulse; and (2) the continuous output of a fiber laser with 4 KHz linewidth, modulated to a Lorentzian pulse. Experimental results indicate that the signal-noise-ratio (SNR) of the Brillouin scattering spectrum for the second pulse spectrum is 4 dB larger than that of the first, then the corresponding frequency fluctuation of the whole sensing fiber in BOTDR for the second pulse spectrum is 2 MHz smaller than that of the first. This is in accordance with numerical simulations. This study will likely prove to be beneficial for the design optimization of BOTDR sensing systems.

Performance analysis of Brillouin optical time domain reflectometry (BOTDR) employing wavelength diversity and passive depolarizer techniques

We propose and experimentally validate a wavelength diversity technique combined with a passive depolarizer in order to improve the performance of Brillouin optical time domain reflectometry (BOTDR). The wavelength diversity technique enables the maximization of the launch pump power and suppresses the nonlinear effects, the latter of which limits the conventional BOTDR performance. As a result, the signal-to-noise ratio increases, thus improving the measurement accuracy for strain and temperature. In addition, considering the complexity and expensive methods required for polarization noise suppression in BOTDR system, a simple, low-cost passive depolarizer is employed to reduce the polarization noise. The experimental results show that the signal-to-noise ratio is improved by 4.85 dB, which corresponds to 174% improvement compared to a conventional BOTDR system.

Performance Improvement of Brillouin Ring Laser Based BOTDR System Employing a Wavelength Diversity Technique

In this paper technique is employed in a Brillouin optical time domain reflectometry (BOTDR) using a Brillouin ring laser (BRL) as a local oscillator. In the wavelength diversity technique, multiple wavelengths are injected into the sensing fiber, while the peak power of each wavelength is set below the nonlinear threshold level. This technique significantly maximizes the overall launch pump power, without activating the nonnegligible nonlinear effects, which overcomes the limitation of the conventional BOTDR system. The BRL, which is simple and cost-effective, that can be used to reduce the receiver bandwidth in the order of few MHz. In addition, a passive depolarizer is used to reduce the polarization noise. The proposed system is validated experimentally over a 50 km sensing fiber with a 5 m spatial resolution. The experimental results demonstrate a signal-to-noise ratio improvement of 5.1 dB, which corresponds to 180% improvement compared to a conventional BOTDR system.

基于双脉冲探测的亚米级空间分辨率布里渊时域反射技术

In conventional Brillouin optical time domain reflectometry (BOTDR),mutual restraint between the spatial resolution and the width of spontaneous Brillouin scattering spectrum can not be avoided. To solve this problem, we ameliorated the approach using double ultra-short pulses as probe light. By detecting spectral envelope, the spatial resolution of BOTDR can be extracted. This method improves the spatial resolution of the BOTDR system and meanwhile avoids the influence of Brillouin gain spectrum broadening caused by narrowing pulse on measurement accuracy. The experimental results show that with this novel BOTDR system, the temperature measurement can be realized at a 0.5 m spatial resolution, and the large broadening of the spontaneous Brillouin scattering spectrum can be avoided.

Synthetic Spectrum Approach for Brillouin Optical Time-Domain Reflectometry

We propose a novel method to improve the spatial resolution of Brillouin optical time-domain reflectometry (BOTDR), referred to as synthetic BOTDR (S-BOTDR), and experimentally verify the resolution improvements. Due to the uncertainty relation between position and frequency, the spatial resolution of a conventional BOTDR system has been limited to about one meter. In S-BOTDR, a synthetic spectrum is obtained by combining four Brillouin spectrums measured with different composite pump lights and different composite low-pass filters. We mathematically show that the resolution limit, in principle, for conventional BOTDR can be surpassed by S-BOTDR and experimentally prove that S-BOTDR attained a 10-cm spatial resolution. To the best of our knowledge, this has never been achieved or reported.

Performance of a Distributed Simultaneous Strain and Temperature Sensor Based on a Fabry-Perot Laser Diode and a Dual-Stage FBG Optical Demultiplexer

A simultaneous strain and temperature measurement method using a Fabry-Perot laser diode (FP-LD) and a dual-stage fiber Bragg grating (FBG) optical demultiplexer was applied to a distributed sensor system based on Brillouin optical time domain reflectometry (BOTDR). By using a Kalman filter, we improved the performance of the FP-LD based OTDR, and decreased the noise using the dual-stage FBG optical demultiplexer. Applying the two developed components to the BOTDR system and using a temperature compensating algorithm, we successfully demonstrated the simultaneous measurement of strain and temperature distributions under various experimental conditions. The observed errors in the temperature and strain measured using the developed sensing system were 0.6 °C and 50 με, and the spatial resolution was 1 m, respectively.

Development of Optical Fiber Sensors Based on Brillouin Scattering and FBG for On-Line Monitoring in Overhead Transmission Lines

A distributed on-line temperature and strain fiber sensing system based on the combined Brillouin optical time domain reflectometry (BOTDR) and fiber Bragg grating (FBG) technology is presented and investigated experimentally for monitoring the overhead transmission lines. The BOTDR sensing system can be used to measure the temperature of transmission lines (Optical Phase Conductor, OPPC or Optical Power Ground Wire, OPGW) which is helpful for monitoring the dynamic ampacity and icing forecasting. In order to effectively monitor the distortion of transmission line induced by the climatic fluctuation and natural disaster, e.g., ice coating and hurricane wind, a quasi-distributed FBG strain sensing system is connected in series with the insulator, integrated into the BOTDR system. The results showed the proposed system was effective and reliable for the monitoring of overhead transmission lines.

Design of Wide-Band Frequency Shift Technology by Using Compact Brillouin Fiber Laser for Brillouin Optical Time Domain Reflectometry Sensing System

A novel wide-band optical frequency shift technology by using a compact single-frequency Brillouin fiber laser (BFL) configured with high-doped Er3+ fiber and high nonlinear fiber (HNLF) is proposed and investigated experimentally. By adjusting variable optical attenuator (VOA) to eliminate the free-running oscillation in the fiber ring cavity, a stable single-frequency BFL will be considered as local oscillator (LO) for coherent detection in Brillouin optical time domain reflectometry (BOTDR) sensing system. The BFL operates at 1549.06 nm red-shifted 0.084 nm (about 10.5 GHz) from the pump seed laser. The laser signal-to-noise ratio (SNR) is larger than 50 dB and the laser linewidth is about 3.1 kHz. Applying the BFL to the BOTDR system, the measured results show that the beat frequency between the spontaneous Brillouin scattering (SBS) and the LO is about 420 MHz and the frequency fluctuation is less than ± 2 MHz for 20-km sensing fiber.

Determination of thermal residual strain in cabled optical fiber with high spatial resolution by Brillouin optical time-domain reflectometry

The thermal residual strain induced in the cabled optical fiber is a very important factor for evaluating the reliability of optical fiber cables. In order to determine the distributed thermal residual strain in cabled optical fiber, a measurement method based on Brillouin optical time-domain reflectometry (BOTDR) system is proposed in the article. Thermal characteristics of residual strain along cabled optical fibers are investigated theoretically and experimentally based on Brillouin frequency shift (BFS) detection. The thermal residual strain along the cabled fiber, if any, can be determined with a high spatial resolution that is equal to that of the BOTDR system and can be less than a few meters. A double-coated fiber in loose optical cable was used as the test cabled optical fiber, and the experimental results were in good agreement with those predicted from the theory. It has been found that the fiber residual strain increases linearly with decreasing temperature in the range from 50 to −50 °C.

Novel Technique to Improve Spatial Resolution in Brillouin Optical Time-Domain Reflectometry

A novel Brillouin optical time-domain reflectometry (BOTDR) system, called a double-pulse BOTDR (DP-BOTDR) system, is proposed for measuring distributed strain and temperature in a fiber with a sub-meter spatial resolution. Our experiment confirmed that the DP-BOTDR system enables us to measure the distributed Brillouin frequency shift, i.e., the distributed strain and temperature, with a spatial resolution of 20 cm. This spatial resolution is five times better than that provided by the conventional single-pulse BOTDR system.

OS09W0194 Detection of strain variation based on Brillouin spectrum shape changing

Fiber-optic distributed sensors, which can measure physical variables as a function of position along an optical fiber, are regarded as attractive sensors for structural health monitoring (SHM). Brillouin optical time domain reflectometry (BOTDR) can be used for measurements of strain distributions on structures, such as aircrafts, ships and bridges. In the BOTDR system for strain measurements, a pulsed light is launched into an optical fiber and the Brillouin backscattered light is measured. A short pulse is required to improve the spatial resolution. However, the measurement accuracy of BOTDR extremely becomes worse when the pulse width is less than 10 ns. When using 10 ns pulse, the accuracy and the spatial resolution are about ±30με and 1 m, respectively. This fact puts limitations on its applicability to SHM, because it is difficult to detect strain perturbation within the length of the spatial resolution. In this study, we developed a new technique with BOTDR to detect inhomogeneous strain fluctuating sharply within the spatial resolution. This technique is based on the fact that the profile of the Brillouin spectrum changes depending on strain distributions. We confirmed the dependency of the Brillouin spectrum on the strain distributions theoretically and experimentally.