Reduction of Laser Frequency Noise and Intensity Noise in Phase-Shifted Fiber Bragg Grating Acoustic Emission Sensor System

The noise performance of a fiber-optic acoustic-emission (AE) sensor system is often limited by the laser intensity noise and the laser frequency noise. Here, we report a novel low-cost, high performance noise reduction method for fiber-optic AE sensor system based on a π-phase-shifted fiber Bragg grating. This method uses a reference channel built by an all-fiber Michelson interferometer formed by a 3×3 fiber-optic coupler and two Faraday mirrors. The reference channel has two outputs whose spectra are two sinusoidal fringes with a relative phase shift of 2π/3. Novel signal processing method is developed to extract both the laser intensity noise and the laser frequency noise from the outputs of the reference channel, which are subsequently removed from the output of the sensing channel. The results show that, for an AE sensor system using a distributed-feedback semiconductor laser as the laser source, this method achieves a noise reduction ratio of more than 20 dB. The reference channel possesses a broad operating spectral width and has the potential to be used in a multiplexed AE sensor system.