Coherent Brillouin Optical Time-Domain Analysis Fiber Sensor With Intradyne-Based Multistage Coherent Detection

Frequency shift measurement of Brillouin phase shift spectrum (BPS) by Brillouin optical time-domain analysis (BOTDA)-based optical fiber sensors is an attractive approach to realizing fast and high-resolution distributed strain, vibration, and temperature monitoring. This article proposes a system that employs a multistage coherent detection technique based on an intradyne optical phase-diversity receiver at the front stage and an electrical coherent receiver at the second stage. In the sensing optical fiber, probe light within the Brillouin scattering bandwidth co-propagates with a pilot light that has the same optical phase as the probe light at the input of the sensing optical fiber. Those lights are converted into complex electrical signals with the help of a local light at the optical polarization- and phase-diversity receiver. The second-stage electrical coherent receiver, implemented on a digital processing platform, cancels optical phase fluctuation between probe/pilot light and local light and detects the BPS by comparing the phase of the complex electrical signals extracted from the probe and pilot light. Proof-of-concept experiments of BPS measuring BOTDA conducted on a 1-km-long sensing section of a 10-km transmission fiber are performed and highly sensitive detection with the total power of probe and pilot light power as low as -32-dBm, which is 10 dB lower than that attained in the previous study, is demonstrated; the standard deviation of measured Brillouin frequency shift is 2.1 MHz.