FREQUENCY-MODULATED MULTIMODE LASER-DIODE FOR FIBER BRAGG GRATING SENSORS

A demodulation scheme for fiber Bragg grating (FBG) sensors is present ed. It is based on the generation of an electrical carrier by using a modulated multimode laser diode to illuminate the fiber grating. The c hange in Bragg wavelength is measured by tracking the phase of the car rier at the detector output in either an open- or a closed-loop scheme . A theoretical analysis of the interrogation technique in terms of li nearity and dynamic range is presented. Experimental data were obtaine d for both strain and temperature measurements. Sensitivities of 0.7 m u epsilon/root Hz and 0.05 degrees C/root Hz were obtained over a dyna mic range of approximate to 60 dB, The application of this demodulatio n scheme to a multiplexed sensing system is also demonstrated.