  We discuss a phase-sensitive technique for remote interrogation of passive
Bragg grating Fabry-Perot resonators. It is based on Pound-Drever-Hall laser
frequency locking, using radio-frequency phase modulation sidebands to derive
an error signal from the complex optical response, near resonance, of a
Fabry-Perot interferometer. We examine how modulation frequency and resonance
bandwidth affect this error signal. Experimental results are presented that
demonstrate when the laser is locked, this method detects differential phase
shifts in the optical carrier relative to its sidebands, due to minute fiber
optical path displacements.

Optics;Instrumentation and Detectors