Frequency scanning is a common method of BOTDR demodulation Brillouin gain spectrum (BGS). In the frequency scanning process, the electrical band-pass filter is important, and the bandwidth of the electrical filter has an effect on the performance of BOTDR. This paper focuses on the influence of band-pass filter (BPF) bandwidth on the measurement spectral width of BGS and spatial resolution of the frequency-scanning Brillouin optical time domain reflectometer (FSBOTDR), which further affects the measuring performance of BOTDR. The law of BFS accuracy and spatial resolution (SR) with filter bandwidth is theoretically investigated by analyzing the work principle of FS-BOTDR in numerical and evaluating the Brillouin gain spectrum width. Five filters with bandwidths of 40MHz, 55MHz, 70MHz, 85MHz, 100MHz in 700MHz center frequency is selected to compare the spatial resolution and BFS accuracy, which depend on the BGS spectral width. The analyzing and experimental results show that the spatial resolution improves with increasing the filtering bandwidth. When filter bandwidth increases, the BGS spectral width could be broadened, means the sensitivity of the filter output power to the frequency shift is decreased, which caused the reducing of measurement accuracy. The research can be a theoretical basis and experimental guidance on the choosing of filter bandwidth for FSBOTDR.
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.