Highly Precise Birefringence Demodulation to Overcome the Sensing Limitation From the Free Spectral Range on the Fiber Lyot Filter

A high precision demodulation method based on least squares fitting for expanding the temperature measurement range of the fiber Lyot filter has been proposed and experimentally demonstrated. And a compact reflective Lyot filter has also been proposed to verify this method. The filter consists of a polarizer and a section of polarization maintaining fiber (PMF) coated with a gold film at the end. The birefringence of the PMF can be obtained through the least squares method, so that the temperature measurement range is not limited to the free spectral range (FSR) of the periodic interference spectrum. Experimental results show that the proposed method can extend the measurement range by 5.5 times compared to the dip wavelength tracking method. And the demodulation fluctuation of the birefringence is within 10^-7 .