Five wavelength interrogation technique for miniature fibre optic Fabry-Perot sensors

Five equally spaced wavelengths from a multimode semiconductor laser diode source are used to provide five phase shifted intensity outputs from a low finesse Fabry-Perot interferometer, from which the absolute phase, module 2 pi, can be determined. The use of five measurements is shown to reduce the phase errors arising from multiple reflections by two orders of magnitude compared to techniques using two or three intensity outputs. In addition, errors arising from miscalibration of the cavity are also reduced by up to two orders of magnitude compared to two, three and four phase shifted intensity outputs. Phase errors are discussed and the technique is demonstrated for temperature measurement. A 70 degrees C range and 0.07 degrees C resolution are obtained.