Research On The Impact Of Optical Fiber Birefringence On Optical Fiber Sensors And Its Applications

Optical fiber sensors are used in many engineering fields because of their numerous advantages such as high sensitivity, high resolution, low transmission loss, fast response, easy networking. Birefringence is the important physical properties of optical fiber with its characteristics and magnitude closely related to the pressure, torsion, magnetic field and other physical quantities of optical fiber. This thesis study the impact of optical fiber birefringence on optical fiber sensors and its applications with the focus on the impact of nonreciprocal circular birefringence effect caused by magnetic field and the impact of linear birefringence due to lateral pressure on fiber laser magnetic field sensors. A theoretical model of the relationship between the beat frequency and the sensitivity of fiber laser magnetic field sensor has been built. This thesis propose an optical tuning scheme for beat frequency tuning by tuning pump laser polarization, which can therefore be used to tune the sensor sensitivity. Besides, the thesis has also studied the impact of reciprocal circular birefringence due to fiber torsion on Brillouin distributed fiber sensing.The main contents of this thesis are as follows:Firstly, this thesis introduces the theory of birefringence and its representation method with classification and elaboration of the origin of birefringence phenomenon. The thesis has studied the effect of optical fiber birefringence and its applications mainly for optical fiber laser magnetic field sensors and Brillouin distributed fiber sensing.Secondly, we have raised four methods, namely CO2 laser heat treatment method, ultraviolet laser illumination method, optical fiber torsion method and lateral pressure method, to reduce the intra-cavity linear birefringence of fiber laser magnetic field sensors. Based on the analysis and comparison between the four methods, the study has found that the lateral pressure method can minimize the intra-cavity linear birefringence of fiber laser magnetic field sensors. By lowering the beat frequency to 2 MHz, the maximum sensitivity of about 43 Hz/μT to magnetic field has been achieved. Furthermore, for permanent intra-cavity linear birefringence reduction, we proposed a post-processing technique by ultraviolet laser illumination after CO2 laser heat treatment of the fiber laser magnetic field sensor cavity, which should be able to reduce the linear birefringence to the maximum extent in a permanent way.Thirdly, we improve the theoretical model proposed by previous works and obtain a rigorous theoretical model by abandoning the approximation adopted by previous works. Therefore, the new model can be used to explain the phenomenon for very tiny birefringence and very small beat frequency for magnetic field sensing. Based on the model, we therefore point out that an upper limit determined by the Verdet constant exists for the sensitivity of the fiber laser magnetic field sensor. We also propose an optical tuning scheme for beat frequency tuning by tuning pump laser polarization, which can therefore be used to tune the sensitivity of the fiber laser magnetic field sensor.Lastly, we study the impact of reciprocal circular birefringence due to fiber torsion on Brillouin distributed fiber sensing and its application. We set up an automatic control system based on LABVIEW procedure of virtual instrument to realize Brillouin optical time-domain analyzer(BOTDA) for distributed optical fiber sensing measurement, which enables us to explore the interaction between fiber birefringence and the Brillouin scattering.