Effect of Pump Light Polarization and Beat Note Stabilization for Dual-Polarization Fiber Grating Laser Sensors

Dual-polarization fiber grating lasers have been exploited as high-sensitivity photonic sensors for a variety of measurands by monitoring the frequency of the output beat signal. In this paper, we demonstrate that the environmentally induced perturbations on polarization orientation of the pump light can cause considerable changes in output beat frequency, which can significantly affect the resolution of the sensor especially for static or quasi-static measurements. We found that the beat frequency change with the pump orientation is mainly caused by the resonantly enhanced nonlinearity associated with the generation of green fluorescent light, rather than thermal effect or Kerr electro-optic effect. In order to obtain stable beat signal and improve the accuracy of the fiber laser sensor, the fiber lasers are annealed in advance and then connected to polarization-maintaining fibers and devices to avoid the influence of external perturbations. The frequency fluctuation has been reduced from 1.5 to about 0.1 MHz, and the corresponding resolution of the sensor to static transverse load is improved from 1.76 × 10 ^-4 to 1.17 × 10 ^-5 N/mm.