Research On Orthogonally Polarized Dual-frequency Fiber-Laser Based Sensors For Weak Magnetic Field Detection

Fiber-optic sensors can measure many parameters such as strain, temperature, pressure, magneticfield, current, humidity, by use of fiber devices or optical fiber as sensing elements. They havedemonstrated many advantages including high resolution, high sensitivity, immunity toelectromagnetic interference, and the ability to work under harsh environments. Recently, withmany new results and discoveries in fiber-optics, various types of fiber-optic sensors withenhanced performance have been developed, which have found important applications in avariety of fields such as Geological exploration, industrial detection, military guidance. However,the existing optical fiber magnetic field sensors are usually unable to satisfy the requirement forweak magnetic field measurement. To solve this problem, a fiber magnetic field sensor based ondual-polarization fiber grating laser and Ampere force is proposed and demonstrated in thisthesis. The magnetic field sensor is proposed by placing a dual-polarization fiber grating laserunder a copper wire. Different from existing technologies, the beat signal by mixing the twoorthogonally polarized laser outputs of a DBR fiber-grating laser is used as the sensing signal.With a perpendicular magnetic field applied, an electrical current flowing through the copperwire can generate Ampere force to squeeze the fiber grating laser, resulting in the birefringencechange inside the laser cavity and hence the change of the beat note frequency. When analternating current is injected into the copper wire, the magnetic field induced beat notefrequency change can be discriminated from environment disturbances. A novel fiber-opticmagnetic field sensor is therefore demonstrated with high sensitivity and inherent immunity todisturbances.The main content of the thesis is as follows:(1) The fabrication of dual-polarization fiber grating laser is investigated. The fabrication isperformed by using phase-mask method with a scanning193nm-excimer laser-beam. Theinscription parameters including the exposure dosage, the lengths of the two gratings and gratingspacing are optimized to ensure single-longitudinal laser output along the two orthogonallypolarizations. The laser fabricated on an Erbium/Ytterbium co-doped fiber has a total length lessthan10mm. The output beat signal presents a signal-to-noise ratio higher than70dB. (2) We propose a novel magnetic field sensor based on dual-polarization fiber grating lasersand magnetic field induced Ampere force. The Ampere force is generated by an electric currentin a perpendicular magnetic field, which presses the dual-polarization fiber grating laser to shiftthe beat signal frequency. We derived the relationship between the optical fiber current sensorand the magnetic field, and confirmed the measurement of magnetic field in the experiment.(3) We studied the sensitivity enhancement for the proposed magnetic field sensor.Theproposed structure for sensitivity enhancement can covert the surrounding magnetic field into theintra-cavity birefringence more efficiently, which can therefore enhance the sensitivity of theproposed magnetic field fiber-optic sensor. We demonstrate when an alternating electric currentis employed, the proposed scheme can discriminate the magnetic field from environmentdisturbance very well, showing an inherent ability of anti-disturbance.