Spatial Sensitivity Characterization of Dual-Polarization Fiber Grating Laser Sensors

In this paper, the spatial sensitivity of a dual-polarization fiber grating laser sensor is characterized by measuring the response in terms of beat-frequency change to localized transverse loading at different longitudinal positions. The induced frequency shift is proportional to the induced local birefringence change and the normalized light intensity, as a result of the resonant nature of the cavity. Measured result suggests that the sensitivity profile is mainly determined by the cavity length, as well as the grating reflectivities and the gain. We found that higher mass (or force) sensitivity can be obtained by shortening the laser cavities. Sensitivity as high as 51.1 MHz/g (or 5.192 GHz/N) has been achieved with a laser which has an effective cavity length close to 1 mm. The corresponding mass resolution reaches $9.78 \times 10^{-5}$ g. The result offers useful guidance for the transducer design for fiber laser sensors toward the detection of extremely small mass or weak perturbations.