Direct phase reading in two-mode fiber sensors and sensor arrays

A novel signal-processing scheme is presented for two-mode fiber in-line interferometric sensors in which frequency shifters are used as directional couplers that provide mode coupling to form an interferometer and simultaneously transfer the optical phase shift to the interference signal at the beat frequency (a few kilohertz to hundreds of kilohertz). The phase shift can be read directly by using a two-channel lock-in amplifier or a time-interval counter. Several sensing regions can be cascaded in a single strand of fiber, and cross talk between them is below -34 dB.