Interrogation of Ultra-Weak FBG Array Using Double-Pulse and Heterodyne Detection

A high performance interrogation method for ultra-weak fiber Bragg grating (UWFBG) using double-pulse and a heterodyne detection method is proposed. The perturbation along the UWFBG array is located quickly through the use of double-pulsed optical input waveform. Then, the perturbation of fiber is quantified precisely by demodulating the phase of differential signals from a heterodyne configuration. The efficiency of measuring the perturbation is improved by more than 20 times than that of using a single probe pulse. In comparison with the conventional Rayleigh scattering-based approach, the proposed method is supreme in signal-to-noise ratio, approximately 18 dB higher. The use of differential signaling method can effectively remove the influence from frequency drift of the laser source, making this proposed method capable of measuring low frequency vibration. In our experiment, perturbations with both sinusoidal and triangle waveform were generated to quantitatively evaluate the performance of the proposed method. The minimum detectable fiber length variation is 14.63 nm, and the sensing frequency can be as low as 0.2 Hz.