Research On The Sensitivity Of Faraday Effect Based Orthogonally Polarized Dual-frequency Fiber-laser Magnetic Field Sensors

Recently, with many new results and discoveries in fiber-optics, various types of fiber-optic sensors with enhanced performance have been developed, which have found important applications in a variety of fields such as geological exploration, industrial detection, military guidance. They have presented advantages including immunity to electromagnetic interference,light-weight, small volume, narrow bandwidth. By measuring the change of the lightwave parameters such as phase, polarization, amplitude and wavelength, fiber-optic magnetic field sensors can measure a magnetic field by employing fiber-optic devices as the sensitive components. A number of mechanisms can be utilized to implement fiber-optic magnetic field sensors, such as Faraday effect, magnetostriction, magnetic force, Lorenz force and magnetic fluidic material and so on, among which Faraday effect is very popular due to its ability to measure magnetic field directly. In recent years, development of novel fiber-optic magnetic field sensors with high sensitivity, high detection precision and wide dynamic range has become a research focus in the field of optical fiber sensing.The main contents of this thesis are as follows:Firstly, the development of short cavity DBR fiber lasers is studied. By using phase masks and a 193 nm Ar F excimer laser, we directly write fiber grating pairs in Er doped fiber with their wavelength matching that of the template to fabricate fiber grating lasers with two orthogonal polarization modes in single longitudinal mode. The dual-frequency output of the laser with orthogonal polarizations provides a radio frequency monitoring signal for sensing.Secondly, based on Faraday effect, miniature and agile magnetic field sensor has been demonstrated by using a dual-polarization fiber grating laser. To develop a suitable package to optimize the sensitivity to magnetic field, it is desired to study the spatial response of a dual-polarization fiber grating laser to magnetic field based on Faraday effect. According to the birefringence and Faraday effect in the optical fiber, the spatial response of a dual-polarization fiber grating laser to a magnetic field is derived.Thirdly, we studied the sensitivity enhancement for the proposed magnetic field sensor. The rotation-fusion method can decrease the phase difference between the fast and the slow axis,which can therefore partially offset the optical path difference between the two polarizations and hence reduce the beat frequency. The proposed structure for sensitivity enhancement can covert the surrounding magnetic field into the intra-cavity birefringence more efficiently, which can therefore enhance the sensitivity of the proposed magnetic field fiber-optic sensor.Lastly, we briefly summarize the content of the thesis. Moreover, we outlook the future industrial application of the miniature fiber-laser magnetic-field sensor based on Faraday effect.