Abstract
This paper analyzes the influence of laser linewidth on the measurement accuracy of a frequency-scanning Brillouin optical time domain reflectometer (FS-BOTDR), allowing for both the width of Brillouin gain spectrum and the signal-to-noise ratio (SNR) of the BOTDR system. The measurement accuracy of the Brillouin frequency shift (BFS) is theoretically investigated versus the duration of the probe pulse and the linewidth of the laser source, by numerically simulating how a FS-BOTDR works and evaluating the Brillouin gain spectrum (BGS) width and the system SNR. The simulation results show that the BFS accuracy is improved as the laser linewidth becomes narrower when the probe pulse width is fixed. We utilize five types of lasers with respective linewidths of 1.05 MHz, 101 kHz, 10.2 kHz, 3.1 kHz, and 98 Hz to compare the BFS measurement accuracy over a ~10 km optical sensing fiber. The experimental results demonstrate that the root-mean-square error (RMSE) of BFS decreases with the laser linewidth narrowing from 1.05 MHz to 3.1 kHz, which is in good agreement with the numerical simulation. However, the RMSE of BFS increases when the laser linewidth is less than 3.1 kHz, which may arise from the coherent Rayleigh noise due to a too narrow laser linewidth. The results can provide a theoretical basis and experimental guidance for choosing the appropriate laser linewidth in BOTDR.
Highlights
Brillouin optical time domain reflectometer (BOTDR) was firstly proposed as continuouslydistributed optical fiber sensing technology in 1993 [1]
The mixed signal was filtered by a band-pass filter (BPF) with the center source to perform frequency scanning, the output frequency of which increased from 11.2 GHz to frequency of 600 MHz and bandwidth of 87 MHz
For the linewidth less than Hz, the accurate measured linewidth was calculated by utilizing the value of ΔS by the the laser linewidth, the measured linewidths of five lasers were, respectively, 1.05 MHz, kHz, 10.2 kHz, and 3.1k Hz, which were close to the nominal specifications
Summary
Brillouin optical time domain reflectometer (BOTDR) was firstly proposed as continuouslydistributed optical fiber sensing technology in 1993 [1]. The measurement accuracy of BFS can be affected by numerous factors, such as the fitting algorithm for Brillouin gain spectrum (BGS), the features of probe pulses, and even the characteristics of the laser source. A series of time-frequency analysis methods called Cohen’s class were proposed for the signal processing of BOTDR, reducing the BFS fluctuation by three times [12] For improving both the measurement accuracy and data processing speed, a similarity matching method was proposed, making the standard derivation of BFS results three times better [13]. The influence of the laser linewidth on the BFS accuracy of BOTDR needs to be further discussed in detail, considering both the BGS width and the system SNR. The broadening effect of BGS was analyzed, allowing for both the pulse width of the probe light and the frequency-scanning process in an FS-BOTDR. The results of this research will be helpful to choose the laser linewidth for the BOTDR
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