Fibre optic sensor multiplexing by FMAMCW

An intrinsic quasi-distributed optical fibre sensor network composed of three intensity-modulated fibre sensors has been assembled to study the sensor response signal multiplexing performances. A frequency-modulated RF continuous wave F_t is used to modulate the intensity of the optical carrier emitted by a laser diode. The performances and operating conditions of two detection schemes, one digital and one analogue, have been presented and discussed. The first method makes use of three narrow parallel digital filters centred on each sensor signal frequency. The authors have obtained a signal-to-noise ratio of 40 dB and a cross-talk level lower than the noise for a signal frequency separation ratio of 1.2. The sensor modulation bandwidth is limited by the data acquisition time and the computing time to about 10 Hz. The analogue detection consists of an analogue filter and a linear phase-locked loop. The synchrone detection delivers a signal-to-noise ratio of 40 dB with an integration time constant of 100 ms. With sensor signals of equal intensity the cross-talk is lower than -40 dB. The sensor modulation bandwidth is limited by the sensor signal frequency separation to about a tenth of that value.