Interrogation of low finesse optical fibre Fabry - Pérot interferometers using a four wavelength technique

A passive signal processing technique for miniature interferometric sensors is reported. It is based on phase stepping techniques and utilizes four adjacent axial modes from a multimode laser diode to provide four phase shifted intensity outputs. It is shown that the effects of multiple reflections on the recovered phase are reduced by approximately two orders of magnitude compared to methods employing two and three phase shifted outputs. Demultiplexing is accomplished using a monochromator and line scan CCD combined with digital processing. Demonstration as a vibration and temperature sensor for a 174 m low finesse fibre Fabry - Pérot cavity produced 50 dB dynamic range and 9 mrad sensitivity at 40 Hz, and 70 range and 0.07 resolution respectively. Systematic errors are also discussed.