Dual-frequency Optoelectronic Oscillator for Thermal-Insensitive Interrogation of a FBG Strain Sensor

We propose and experimentally demonstrate an approach to perform high-speed and high-resolution thermal-insensitive interrogation of a fiber Bragg grating (FBG) strain sensor based on a dual-frequency optoelectronic oscillator (OEO). Two phase-shifted FBGs (PSFBGs) are incorporated in the OEO loop to implement a microwave photonic filter with two passbands based on phase modulation and phase-modulation to intensity-modulation conversion, to generate two microwave signals with their frequencies determined by the center frequencies of the two passbands. When one of the PSFBG is experiencing a strain, a beat frequency between the two microwave signals that is linearly proportional to the strain applied to the sensing PSFBG is obtained. By monitoring the beat frequency using a digital signal processor, the strain is measured. The proposed approach is experimentally demonstrated. High-resolution sensing with a resolution of $0.83~\mu \varepsilon $ that is thermal insensitive is demonstrated.