Effect of Laser Polarization on Fiber Bragg Grating Fabry-Perot Interferometer for Ultrasound Detection

High-finesse fiber Bragg gratings (FBGs) Fabry-Perot interferometers (FPIs) have shown great promise as sensitive ultrasonic sensors. However, the fabrication process of the sensors usually introduces birefringence to the fiber, which makes the sensor operation sensitive to the polarization of the probe laser. Here, we theoretically study the effect of laser polarization on the sensitivity of the sensor with birefringence. We show that, to maintain the sensitivity above half of its maximum of a high-finesse FBG-FPI sensor, the spectral notch separation caused by sensor birefringence should be less than ~61% of the spectral notch width of the sensor. We fabricate an FBG-FPI sensor with negligible overall birefringence by rotating the fiber by 90° between the fabrication of the two FBGs. As a result, the birefringence introduced during the fabrication of the two FBGs cancels out each other. For comparison, a regular FBG-FPI fabricated without fiber rotation was also tested. While the regular FBG-FPI exhibited large variations in detection sensitivity with laser polarization, the polarization-insensitive showed little changes. As a result, no control on the laser polarization is needed during the operation of the polarization-insensitive FBG-FPI for ultrasonic detection, an important attribute required in many practical applications of the sensor.