Multimode fiber Φ-OTDR with holographic demodulation

We propose and demonstrate a method to perform quantitative phase-sensitive optical time domain reflectometry (Φ-OTDR) using multimode fiber. While most Φ-OTDR sensors use single-mode fiber, multimode fiber exhibits higher thresholds for non-linear effects, a larger capture fraction of Rayleigh backscattered light, and the potential to avoid signal fading by detecting many spatial modes in parallel. Previous multimode fiber based OTDR sensors discarded most of the backscattered light and thus failed to take advantage of these noise-reducing factors. Here, we show that by performing off-axis holography with a high-speed camera, we can record the entire Rayleigh backscattered field, maximizing the detected light level and making the sensor immune to fading. The sensor exhibits a high degree of linearity, a minimum phase noise of -80 dB [rel. rad²/Hz], and 20 kHz bandwidth.