Application Research Of Sensitivity-enhanced Sensing Based On All-fiber Birefringent Filter

As the field of optical fiber sensing still needs to break through the problems of high precision and high sensitivity for practical application scenarios,exploring appropriate measurement schemes and improving sensing performance are the main research directions at present.In the background of optical fiber sensing system,the birefringent filter based on polarization maintaining fiber is studied theoretically,and numerically analyzed and simulated.Taking the study of filter performance as a breakthrough point,the conventional optical fiber filter was optimized and experimentally tested for the application requirements of high sensitivity of the measured parameters in the field of optical fiber sensing.On this basis,the research work on the sensitivity-enhanced sensing application of the proposed birefringent optical fiber filter is carried and the performance of the optical fiber sensor is improved.The main research contents are as follows:1.A PM Lyot birefringent fiber filter based on 45°splicing is proposed.Based on this filter,an all-fiber high sensitivity displacement sensor is developed,and its sensing performance is experimentally studied.The transmission spectrum function of this filter is derived using Jones matrix,and the simulation results show that the birefringence and fusion angle of PM fiber have obvious effects on the displacement sensitivity and spectral modulation depth,respectively.Moreover,the Lyot filter-based sensor has a good linear response.The sensitivity of 122.81 pm/μm was finally obtained in the range of 200μm displacement variation,and its R~2 was greater than 0.997.In addition,the sensor is compatible with the intracavity system,which can narrow the linewidth and enhance the signal-to-noise ratio(SNR)to achieve high-resolution sensing.The experiment shows that the sensitivity of the intracavity displacement sensor can reach60pm/μm with a linewidth narrower than 0.05 nm and a SNR higher than 55 d B.And the difference of sensitivity and wavelength stability between passive mode and intracavity system is analyzed.2.An all-fiber temperature sensor based on 45°spliced PM Lyot filter is studied and built.The influence of thermo-optic effect and thermal expansion effect on temperature sensitivity is described.The temperature sensitivity obtained in the experiment is-1.1 nm/℃and R~2 is greater than 0.998.The characteristics of temperature sensitive hydrogel and its enhancement mechanism in the field of optical fiber sensing are introduced in detail.In order to improve the temperature sensitivity,a temperature sensor based on Lyot filter coated with hydrogel is fabricated,and the final sensitivity obtained is-2.72 nm/°C with R~2 of 0.996.Analyzing and comparing the experimental results before and after hydrogel coating,the temperature sensitivity is improved by about 2.5 times after coating,and the wavelength stability is better.3.An all-fiber Lyot-like filter is proposed and built by optimizing the 45°spliced PM Lyot filter.The spectral characteristics of the filter are simulated using Jones matrix,and the analysis indicates that the filter does not need to reconfigure the fiber,and the filter bandwidth can be tuned by adjusting the polarization controller,which is verified experimentally.Based on the Lyot-like filter,an optical fiber sensor is designed and developed to achieve simultaneous sensing measurements according to the different responses of polarization maintaining fiber to external displacement and temperature.In addition,the sensing performance under different filter bandwidths is experimentally investigated,and the results show that the sensitivity of displacement and temperature can be improved to 119.45 pm/nm and-1385.12 pm/°C through bandwidth tuning.