The Influence of Laser Linewidth on the Brillouin Shift Frequency Accuracy of BOTDR

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.

Narrow-Linewidth Laser Linewidth Measurement Technology

“…Only 3 km of fiber was used to measure a linewidth of 150 Hz. In 2018, Bai [58] successfully measured a laser with a linewidth of 98 Hz using the CDSPST method with a fiber delay of 2,950 m. In 2019, He [59] reported a linewidth demodulation scheme that achieved linewidth measurement by demodulating the coherent envelope of a short-delay self-heterodyne interference spectrum, and used this method to demodulate a 2.53-kHz linewidth. In 2020, Wang [60] reported a dual-parameter acquisition method and used it to calculate a 458-Hz linewidth by obtaining the frequency difference and amplitude difference of the coherence envelope.…”

Recent Advances in Brillouin Optical Time Domain Reflectometry

“…The experimental results indicated that the BFS accuracy improves with the laser linewidth narrowing. However, the BFS accuracy will deteriorate when the laser linewidth decreases to a certain value (reported as 98 Hz in [47]), which may arise from the increasing coherent Rayleigh noise (CRN) related closely with the narrowing linewidth. The above reports will be helpful to choose an appropriate laser for BOTDR.…”