Truly Distributed and Ultra-Fast Microwave Photonic Fiber-Optic Sensor

Abstract

Brillouin-based optical fiber sensing has gained considerable attention for the past few years thanks to its ability to offer distributed sensing. The major limitation of a Brillouin-based optical fiber sensor is its complexity because a time-consuming frequency-sweeping process is needed to obtain a local Brillouin gain spectrum (BGS) and to calculate the local Brillouin frequency shift (BFS). Thus, it is only suitable for static or slow-varying measurements. In this article, we propose an approach to achieve truly distributed and ultra-fast fiber-optic sensing based on an active and distributed bandpass microwave photonic filter (MPF) through stimulated Brillouin scattering (SBS). To obtain a truly distributed BFS, a counter-propagating single-shot pump pulse is launched into the fiber link and a microwave multi-tone (MMT) signal with a random initial phase distribution which is phase modulated on an optical carrier is launched into the fiber link from the other end. Due to the SBS effect, the −1st order sideband of the phase-modulated signal will experience Brillouin amplification while the +1st order sideband will experience Brillouin attenuation, and the phase-modulated signal is converted to an intensity-modulated signal. The entire operation is equivalent to a bandpass MPF. By detecting the optical signal at a photodetector (PD), a regenerated MMT signal with its magnitude and phase that are shaped by the MPF is obtained. By evaluating the regenerated MMT signal, the Brillouin information corresponding to the temperature or strain change at a specific location is revealed. The major advantage of the approach is that time-consuming frequency-sweeping process is avoided. Truly distributed strain, temperature, and vibration sensing with a 2 m spatial resolution over 49.5 m distance at a speed up to 83.3 kHz is experimentally demonstrated.