Fiber Bragg grating sensor interrogation system based on an optoelectronic oscillator loop

In this article, we propose and experimentally demonstrate a fiber Bragg grating (FBG) sensor interrogation technique based on an optoelectronic oscillator (OEO). The main components of the OEO loop in this proposed scheme contains an electro-optic modulator (EOM), a section of dispersive element, an electric filter, and a photodiode (PD). The reflection signal of the FBG sensor is functioning as the optical source of the OEO. The oscillating frequency of the OEO is determined by the overall time delay of the OEO loop. Due to the dispersive element in the loop, time delay of the OEO loop is a function of the OEO optical source wavelength. As a result, the wavelength change of the FBG can be converted into the oscillating frequency shift of the OEO. A proof-of-concept FBG based axial strain sensing experiment is carried out. The experimental results show that the frequency of the OEO generated microwave signals have a good linear relationship with the axial strain applied to the FBG. The sensitivity is about 58 Hz/με when using dispersion compensation fiber (DCF) with dispersion of -120 ps/(nm*km) as the dispersive medium and tracking the microwave signal with frequency near 2056.4 MHz, which is consistent with the theoretical calculation. The proposed method can also be applied to interrogate optical sensors based on detecting the wavelength change of the optical signals.