Fig. 8
From: Marine reef soundscape monitoring with fiber-optic distributed acoustic sensing
Schematic diagram of the experimental DAS setup for underwater monitoring of reef soundscapes. A narrow-linewidth continuous-wave (CW) laser (green arrows) was modulated into 50 ns pulses at a 50 kHz repetition rate using an acousto-optic modulator (AOM). The pulses were amplified by a pulsed erbium-doped fiber amplifier (EDFA) and launched into a 1 km optical fiber via Circulator 1 (Cir. 1). The final 70 m of the fiber were deployed in the reef environment, while the remaining length remained onshore as a reference for background noise. Each pulse generated Rayleigh backscatter (red arrows) that returned toward the interrogator and was routed by Cir. 1. Because the backscattered signal is weak, it was amplified by a CW EDFA and filtered using a fiber Bragg grating (FBG) with Circulator 2 (Cir. 2) to suppress amplified spontaneous emission (ASE) noise. The cleaned signal was converted to an electrical signal by a photodetector operating at 200 MS/s, then digitized for analysis. Timing and intensity of the backscatter enabled precise localization of acoustic events along the fiber.
