Novel intensity-modulated, temperature-independent FBG sensors

Intensity-modulated fiber-optic sensors have many advantages including simplicity and potential low cost. The developments of several intensity-modulated fiber Bragg grating (FBG) sensor systems are introduced in this paper. These include a displacement sensor, a tilt sensor (both based on chirped FBGs), and a dual-FBG based sensor which can measure lots of parameters such as strain. For the chirped FBG based sensors, the measurands are related to reflection optical powers of the sensing FBGs by changing their chirp rates (or reflection bandwidths) which are insensitive to temperature. While the dual-FBG based sensor is based on electrical beating of modulated radio-frequency (RF) signals reflected from the two FBGs, which act as the sensing element and reference respectively. The power of the RF beating signal depends on the wavelength separation of the two FBGs. By monitoring the RF signal power, intensity-modulated FBG sensing with the sensitivity depending on the modulation frequency of the light source is realized while the temperature effect is compensated by the reference FBG. Experimental measurement of strain, displacement and tilt angle are implemented and good results are achieved.