Abstract

Fibre sensors that modulate optical intensity can be very simple but are subject to interference from attenuation variation in the downlead. We present a method (Q- modulation) of compensating such interference by performing a differential measurement. The method requires neither the mechanical precision of spectral modulation sensors /1/, nor the bandwidth of time of flight techniques /2/. In Q- modulation sensing, the intensity modulation produced by a sensor occurs in a fibre resonator as shown in Fig.1. The transfer function of the resonator for the intensity modulation subcarrier is peaked at the resonance frequencies. The light launched into the fibre downlead is intensity- modulated near the fundamental resonance frequency. The influence of loss induced by the sensor is detected by its influence on the quality factor (Q) of the resonator. As the attenuation of the sensor increases, the loop Q- factor decreases, and the frequency response peak broadens. The ratio of the modulation factor at resonance to that off resonance decreases with increasing sensor attenuation in the loop.

© 1992 Optical Society of America