COHERENCE-TUNED INTERROGATION OF A REMOTE ELLIPTIC-CORE, DUAL-MODE FIBER STRAIN SENSOR

Two dual-mode fibers in tandem act as unbalanced sensor and recovery i nterferometers. We investigate how coherence-tuned interrogation of th e remote sensor fiber is affected by the coherence properties of the m ultimode laser diode source and the modal group index difference of th e dual-mode fibers. The parameters for optimized sensor performance ar e ascertained. We focus particularly on the recovery of small optical phase modulations induced by periodic strain. Using homodyne phase tra cking, stability of the detected ac signal is achieved to within +/- 1 0(-3) in the presence of additional, quasistatic phase drifts of large amplitude. The minimum detectable differential modal phase modulation is 5 murad rms/ square-root Hz at 70 Hz. As an example, remote interr ogation of a piezoelectric quartz high-voltage transducer is demonstra ted.