In phase-sensitive optical time domain reflectometer (Φ-OTDR), the existence of interference fading signal leads to misjudgment and missing judgment in vibration sensing. In this paper, a fading recognition model of back propagation (BP) neural network is constructed, features are extracted from beat frequency generated based on frequency shift delay loop. A fading recognition accuracy of 91.82% is achieved. Rotated-vector-sum algorithm is introduced to aggregate signal labelled non-fading. Vibration location and phase demodulation is realized and on this basis, this method improves the interference fading suppression probability.
The security pre-warning system of underground pipelines was traditionally designed by electronic sensors, usually suffering from intrinsic flawssuch as too scattered monitoring points and poor anti-interference performance. To overcome these disadvantages, a distributed optical fiber temperature and strain sensing system based on Brillouin Optical Time Domain Reflectometer (BOTDR) is designed and developed for underground pipelines security monitoring. In this paper, the sensing mechanism of temperature and strain sensing is analyzed and a BOTDR experimental scheme is presented. Meanwhile, the kernel software program is developed for signal demodulation. Then, the integrated BOTDR instrument is designed compactly and evaluated by temperature and strain measurement experiments, for respectively simulating pipeline leakage and deformation. The experimental results show that the sensing range of 10.268 km is achieved with a spatial resolution of 1.15 m. The nonlinear error is less than 1% FSS over the temperature and strain change of 30~90°C and 0-15000με.
Brillouin optical time domain reflectometer (BOTDR) has attracted extensive attention due to its advantages of one-end access and long-distance measurement. The measurement accuracy and sensing range of BOTDR are affected by the system signal-to-noise ratio (SNR), which is closely related to the characteristics of the probe light pulse. In this paper, the influence of two characteristics of probe light on SNR is theoretically analyzed and experimentally verified, including the extinction ratio (ER) and peak power. Firstly, the theoretical model of probe pulse extinction ratio (ER) and SNR in the BOTDR is established, and the SNR distribution versus different ER is analyzed. Both numerical simulation and experimental verification show that the SNR was enhanced with the increase of ER. Besides, the SNRs under different pulse peak power are also evaluated. The experimental results show that the SNR increases along with the peak power at the beginning but decreases when the peak power exceed a certain value. The research results provide a reference for selecting the appropriate probe pulse features in BOTDR for higher SNR.
A novel pattern recognition method based on relevant vector machine for optical fiber vibration sensor is proposed to meet the requirement of accurate identification of long-distance pipeline leakage. The sensing system is based on Sagnac interferometer, which is suitable for vibration signal detection. The initial vibration signals are firstly pretreated by the wavelet de-noising method. Then, the wavelet decomposition algorithm is utilized to obtain decomposition coefficients at all levels to build feature vectors. Furthermore, the relevant vector machine is presented as a classifier to identify three types of vibration signals, including pipeline leakage, human tapping and human walking. Experimental results demonstrate that the proposed method can discriminate three threat events with an average recognition rate above 85% with a 4-km-long sensing fiber, which can satisfy actual application requirements.
In this paper, the composite interferometric distributed optical fiber vibration sensing technology based on a two-path unbalanced Mach-Zehnder/Sagnac linear interferometric structure is proposed. Firstly, the laser interference mechanism and vibration positioning principle of this system are theoretically analyzed, and the vibration positioning algorithm with the time-delay estimation based on the improved generalized cross-correlation principle is proposed. Secondly, the experimental system is built and double photodetectors are used to obtain the two-path interferometric signal with a certain time delay, and the high-precision time delay signal is obtained with the help of conjugate multiplication and digital filtering algorithm. The experimental results show that the system can accurately locate different types of vibration signals with a 65 km sensing fiber, and the location limitation of broadband vibration signals for a single-channel interferometric structure can be resolved. This system is expected to be popularized and applied in fields such as leakage monitoring of coalbed methane pipelines.
The long-range pipelines for coalbed methane (CBM) transport are generally laid in the field with wide coverage and under harsh operating condition. Most conventional electronic sensing technologies are not appropriate for CBM pipeline safety monitoring featuring long distance, large-capacity measuring points, and severe working condition. In view of above problems, the multi-parameter optical fiber sensing is proposed. For preventing third-party damage, a vibration sensing scheme is designed based on Φ-OTDR principle. For leakage prewaring, a ROTDR-based temperature detection scenario is realized. For pipe deformation precaution, a BOTDR-based strain sensing solution is exploited. Additionally, a cloud database is built on the server for online monitoring. The experimental results demonstrate that: the sensing range of vibration can be 12km with the 20-m spacial resolution and 8-m positioning accuracy; the temperature measurement accuracy is ±1° within the temperature range -25°~200° over the range of 10 km optical fiber; the strain measurement range is 11000 με when the measured distance is about 10 km and the spatial resolution is 1.23m. The multi-parameter detection approach by optical fiber sensing provides a new monitoring method for the safety prewaring of long-range CBM pipelines.
Phase-sensitive optical time domain reflectometry (Φ-OTDR) is a promising approach for detecting and locating vibration signals along the whole fiber link. In this paper, a novel high-speed and complete signal transmission scheme based on field programmable gate array (FPGA) and universal serial bus (USB) 2.0 is proposed and applied in the Φ-OTDR system to obtain real-time vibration location. By using the novel scheme, only the effective data is selected and transmitted to the upper system for post-processing in real time. In the experiment, the vibration signals are simulated by hand-clapping, 50 Hz square waves and 400 Hz sine waves. The experimental results demonstrate that the proposed system exhibits good frequency response for vibrations below 1 kHz over 12 km fiber length. In addition, repetition vibration tests are carried out on 4.633 km and 10.022 km, and the location error is within the range of -7 m to +11 m. Therefore, the proposed system holds considerable potential for real-time vibration location and may have a wide application range in areas such as intruder detection, premier security, oil/gas pipeline leakage detection, and railway infrastructure monitoring, etc.
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