Distributed multimode fiber Φ-OTDR sensor using a high-speed camera

Abstract

While the vast majority of Φ-OTDR sensors use single mode fiber, multimode fiber is also widely deployed by the telecom industry. From a sensor design perspective, multimode fiber also offers advantages compared with single mode fiber, such as higher nonlinear thresholds and immunity to interference fading. However, most attempts to perform distributed strain sensing in a multimode fiber rely on interrogation systems designed for single mode fiber. As a result, these systems discard most of the backscattered light by coupling the multimode fiber under test to a single mode fiber based receiver. In this work, we present a technique that combines a high-speed camera with a time-gated local oscillator to construct a distributed multimode fiber sensor capable of using the entire backscattered speckle field. We demonstrate quantitative, fully distributed strain sensing across a 2 km multimode fiber with a spatial resolution of 20 m, a bandwidth of 400 Hz, and a noise floor of −61 dB re rad 2 /Hz (4.9 pε/√Hz). The same system can be electronically reconfigured to probe any single sensor position with a bandwidth of up to 20 kHz and a noise floor of −86 dB re rad 2 /Hz (0.27 pε/√Hz).