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.
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.
The Risley-prism-based light beam steering apparatus delivers superior pointing accuracy and it is used in imaging LIDAR and imaging microscopes. A general model for pointing error analysis of the Risley prisms is proposed in this paper, based on ray direction deviation in light refraction. This model captures incident beam deviation, assembly deflections, and prism rotational error. We derive the transmission matrixes of the model firstly. Then, the independent and cumulative effects of different errors are analyzed through this model. Accuracy study of the model shows that the prediction deviation of pointing error for different error is less than 4.1×10<sup>-5</sup>° when the error amplitude is 0.1°. Detailed analyses of errors indicate that different error sources affect the pointing accuracy to varying degree, and the major error source is the incident beam deviation. The prism tilting has a relative big effect on the pointing accuracy when prism tilts in the principal section. The cumulative effect analyses of multiple errors represent that the pointing error can be reduced by tuning the bearing tilting in the same direction. The cumulative effect of rotational error is relative big when the difference of these two prism rotational angles equals 0 or π, while it is relative small when the difference equals π/2. The novelty of these results suggests that our analysis can help to uncover the error distribution and aid in measurement calibration of Risley-prism systems.