Fiber Laser Based Magnetic Field Sensor Using Magnetostrictive Composite Material

Fiber-optic magnetic sensors have been attracting considerable research effort for their advantages in immunity to electromagnetic interference, light weight, compact size and large bandwidth. Many mechanisms, such as Faraday effect, magnetic force, etc, can be used to implement fiber-optic magnetic field sensors, among which magnetostrictive material based implementations are very popular. However, bulk magnetostrictive materials are normally fragile and show limited upper working frequency due to the strong eddy current at high frequencies. Therefore, magnetostrictive composite materials are proposed to improve performance.In this thesis, we demonstrate a novel miniature fiber-optic magnetic field sensor by embedding a DBR fiber grating laser into an epoxy resin bonded magnetostrictive composite material with Terfenol-D particles doped. The DBR fiber grating laser is inscribed by a 193 nm excimer on an Er-doped fiber with grating lengths of 6.5 mm and 5.5 mm respectively, and a grating spacing of 5 mm. The magnetostrictive composite material is prepared by mixing Terfenol-D particles with E44 epoxy resin and 650 polyamide curing agent according to a volume ratio of 1:10:10. When a transversal magnetic field is applied, the magnetic field induces strain in the magnetostrictive composite material. A mechanical structure is designed to convert this strain to transversal stress which is applied to the embedded heterodyning fiber laser. The birefringence inside the laser cavity is then changed by the magnetic field which results in beat note frequency change. By discriminating this beat note frequency change, the strength of the applied magnetic field can be measured. The response of the proposed sensor is measured, which shows a quite good directivity with a sensitivity of 10.5 Hz/μT to magnetic field and a large measurable range up to about 0.3 T.The main contents of this thesis are as follows:Firstly, we study the fabrication of dual-polarization DBR fiber laser. By using phase masks and a 193 nm excimer laser, we directly write the fiber grating pairs in Er doped fiber with their wavelengths matching those of the phase mask templates, which then fabricate fiber grating lasers operating in single longitudinal mode with two orthogonally polarized polarization modes. We also use anneal to improve the performance of the DBR fiber laser.Secondly, we mix Terfenol-D particles with E44 epoxy resin and 650 polyamide to develop the magnetostrictive composite material. A mechanism model for resin based magnetostrictive composite material is derived and set up. Then, we present a brief discussion for the optimization and considerations of magnetostrictive composite material fabrication.Thirdly, we propose a DBR fiber laser based magnetic field sensor using magnetostrictive composite material. Based on the birefringence in the optical fiber, the dependence of the beat frequency of a fiber laser on the magnetic field is derived. Experiments on the proposed fiber laser magnetic field sensor to measure magnetic field have been conducted, which confirm the validity of the theory model.Finally, we briefly summarize the content of the entire thesis. We also outlook the future industrial application of fiber laser based magnetic field sensor using magnetostrictive composite material.