Study On The Birefringence Fiber Loop Mirror Sensor And Its Application In Structural Health Monitoring

Birefringence fiber loop mirror exhibits high extinction ratio, high resistance to environmental changes, easy fabrication and the good stability. It has evoked more and more interests from researchers in optical fiber sensing. The birefringence fiber loop mirror sensor and its application in structural health monitoring were studied in this paper. The primary work and innovation are as following:(1) The sensitivity is important to birefringence fiber loop mirror sensor. The theoretical expression for the axial strain sensitivity of birefringence fiber loop mirror is developed in this paper. It is proposed that a high axial strain sensitivity depends on a long wavelength of monitoring point, a small birefringence, and a high strain dependent birefringence coefficient of birefringence fiber loop mirror, and the axial strain sensitivity is unrelated to birefringence fiber length and sensing length. The above results are verified by the numerical stimulation and experiment.(2) The splicing of birefringence photonic crystal fiber and conventional single-mode fiber is a difficult problem because birefringence photonic crystal fiber has microhole structure. So it is studied that birefringence photonic crystal fiber and conventional single-mode fiber is spliced by online spectrum monitoring.The strain sensitivity of birefringence photonic crystal fiber loop mirror is greatly lower than the one of conventional birefringence fiber loop mirror. It is difficult to measure the axial strain by monitoring the wavelength change of birefringence photonic crystal fiber loop mirror. The measured spectrum is analyzed. The results show that the absolute integrate of the monitoring peak decreases with increasing strain and the absolute integration is linear versus strain. Based on above results, it is proposed that the axial strain can be measured by monitoring the absolute integrate of the monitoring peak in this paper. The absolute integrate of the monitoring peak may be used as a comprehensive index which can indicate the light intensity of different wavelength. Therefore, it is also proposed that the axial strain can be measured by monitoring the light power of birefringence photonic crystal fiber loop mirror.(3) Demodulation system applied to practical project may reduce the cost. So the axial strain demodulation of birefringence fiber loop mirror is studied in this paper. Firstly, the demodulation principle of birefringence fiber loop mirror is analyzed. Secondly, the theoretical analysis for the sensitivity between the light intensity and the strain is performed. Lastly, the demodulation experiment system inputted by the laser light source and narrowband light source from FBG is studied respectively. It is proposed that the sensitivity of the demodulation experiment system inputted by the laser light source is 29 times higher than the one of the demodulation experiment system inputted by narrowband light source from FBG.(4)To ensure the structure safety, it is necessary to monitor the vibration. The wing shape aluminum alloy test panel is taken as research object in this paper. The laser light source is inputted into the birefringence fiber loop mirror sensor. The birefringence fiber loop mirror sensor is used to measure high frequency and low frequency vibration, the natural frequency and the damping.The experimental results agree with the ones from laser displacement sensor. The sensitivity of birefringence fiber loop mirror sensor is higher than the one of FBG in vibration. To ascertain anti-interference performance of the birefringence fiber loop mirror sensor, the birefringence fiber loop mirror sensor is tested in different interference conditions. To ascertain the influence of the different length birefringence fiber on the vibration monitoring, the birefringence fiber loop mirror with different length birefringence fiber is studied and calibrated. Lastly, the vibration signal of other forms is also analyzed.These results are useful to improve the sensitivity of birefringence fiber loop mirror sensor and its application system in structural health monitoring.The ability of the sensor and its application system to discriminate small singal can be improved by improving the sensitivity.The dependence on the precision of test equipment can be reduced and the application cost can be decreased by improving the sensitivity.