Abstract

A miniature low finesse fibre Fabry-Pérot interferometric temperature sensor using a passive digital signal processing algorithm is reported. The demodulation technique relies on multiple wavelength interrogation to produce a series of phase shifted intensity outputs. It is shown that by utilising five wavelengths, the maximum phase errors associated with higher order reflection from LFFFPI's are reduced to ~1mrad; approximately two orders of magnitude less than the more usually employed two wavelength technique. In addition, the technique is shown to produce much lower phase errors due to inaccuracies in the cavity size; for a 1% length error the maximum phase error is reduced from 20 mrad to ~ 50 µrad. The use of a single multimode laser diode to provide the five wavelengths reduces the very high stability required from individual sources to a much lower relative stability between laser modes.

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