DEMODULATION SCHEME FOR FIBER BRAGG GRATING SENSORS BASED ON ACTIVE CONTROL OF THE SPECTRAL RESPONSE OF A WAVELENGTH-DIVISION MULTIPLEXER

We present a closed-loop technique for measuring wavelength shifts ass ociated with fiber Bragg gratings by using a fused biconical wavelengt h division multiplexer (WDM). The spectral response of the WDM is acti vely tuned by stretching of the coupling region to maintain a fixed co upling ratio at the reflected Bragg wavelength. The closed-loop operat ion allows sensitivities usually associated with a highly selective WD M to be obtained without compromising the measurement range. A simple theoretical model is presented together with experimental results for temperature and strain measurements. (C) 1998 Optical Society of Ameri ca.