Dual-Channel Fiber Optic Differential Interferometer With Square-Wave Modulation for Advanced Sensing Applications

The fiber optic differential interferometer (FODI) is a sensor designed to directly detect the temporal derivatives of physical quantities. We propose a novel dual-channel FODI (DC-FODI) with square-wave modulation, which enhances sensitivity and improves the dynamic range compared with previous designs. This system uses two optical fibers for simultaneous detection, each embedded within a Sagnac interferometer. The fiber delay lines in both interferometers are of equal length. A pair of equal-amplitude, reverse-phase square wave signals modulates the two interferometers. In this study, the mechanism for suppressing relative intensity noise (RIN) is thoroughly analyzed. In addition, the two delay lines are wound into coils of equal diameter and oriented in opposite directions to mitigate parasitic phase shifts caused by the Earth's rotation and environmental vibrations. The DC-FODI employs two 1-km optical fibers as delay lines. In static tests, DC-FODI demonstrates an over 20-fold improvement in phase sensitivity compared with the single-channel system, with the phase sensitivity estimated at $3 \times 10^{-8} \, \text{rad}/\sqrt{\text{Hz}}$. The system exhibits an essentially flat self-noise spectrum within the frequency range from 0.002 Hz to 100 Hz. Dynamic experiments confirm its sensing capability, demonstrating a dynamic range exceeding 112 dB. High-performance FODI is expected to drive advancements in measurement fields such as acceleration and strain rate measurement.