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 interferometers. We investigate how coherence-tuned interrogation of the remote sensor fiber is affected by the coherence properties of the multimode laser diode source and the modal group index difference of the dual-mode fibers. The parameters for optimized sensor performance are ascertained. We focus particularly on the recovery of small optical phase modulations induced by periodic strain. Using homodyne phase tracking, stability of the detected ac signal is achieved to within +/- 10(-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 interrogation of a piezoelectric quartz high-voltage transducer is demonstrated.