Frequency-modulated multimode laser diode for fiber Bragg gratingsensors

A demodulation scheme for fiber Bragg grating (FBG) sensors is presented. It is based on the generation of an electrical carrier by using a modulated multimode laser diode to illuminate the fiber grating. The change in Bragg wavelength is measured by tracking the phase of the carrier at the detector output in either an open- or a closed-loop scheme. A theoretical analysis of the interrogation technique in terms of linearity and dynamic range is presented. Experimental data were obtained for both strain and temperature measurements. Sensitivities of 0.7 μϵ/√Hz and 0.05°C/√Hz were obtained over a dynamic range of ≈60 dB. The application of this demodulation scheme to a multiplexed sensing system is also demonstrated