Micro-D-shaped Fiber Laser for High-Resolution Refractive Index Sensing via Frequency Analysis

High-resolution refractive index (RI) detection is of great significance in many fields such as biomedical detection. Traditional optical fiber RI sensors based on spectral wavelength analysis are limited by the device resolution of the optical spectrum analyzer (OSA). In this paper, a micro-D-shaped fiber laser sensor based on beat frequency analysis for RI sensing is proposed and demonstrated to be with high sensitivity and high resolution. Compared with ordinary side-polished fiber, the birefringence of micro-D-shaped fiber is more significant and has much higher response sensitivity. The beat frequency signal response to the external RI change is observed with a low-cost oscilloscope (range of 0-500 MHz). The experimental results show that the polarization mode beat frequency based on a micro-D-shaped fiber laser has a sensitivity of -1.35 GHz/RIU and a demodulation resolution of 7.40×10-6 RIU. The RI detection resolution of the proposed sensing system is superior to various optical fiber RI sensors based on spectral wavelength shift, light intensity resolution, and speckle demodulation reported in recent years, showing broad application prospects in the fields of high precision RI detection, such as biochemical analysis and tumor marker detection.