We have proposed a novel magnetic field sensor based on orthogonally-polarized dual-frequency fiber laser and Faraday effect. In this paper, we propose a method to enhance the sensitivity of such Faraday effect based heterodyning fiber laser magnetic field sensor by tuning the intra-cavity intrinsic linear birefringence. We demonstrate that the sensitivity to magnetic field intensity is inversely proportional to the linear birefringence. A CO2-laser treatment is therefore proposed to tune the intra-cavity linear birefringence. With CO2-laser treatment to lower the intra-cavity linear birefringence, the sensitivities of heterodyning fiber laser sensors to magnetic field can be enhanced.
A dual-polarization fiber grating laser is proposed to sense a magnetic field by attaching the fiber laser to a copper wire. When an electrical current is injected into the copper wire and a perpendicular magnetic field is applied, the current generates Ampere force to squeeze the fiber laser and change the birefringence inside the laser cavity, resulting in beat note frequency change. The magnetic field induced beat note frequency change can be discriminated from environment disturbances by applying an alternating current, which therefore demonstrates a novel miniature fiber-optic magnetic field sensor with high sensitivity and inherent immunity to disturbances.
A silicon steel sheet is proposed in this paper to work as a magnetic field concentrator to enhance the sensitivity of a Faraday effect based magnetic field sensor using a dual-polarization fiber grating laser. When the silicon steel sheet is placed close to the cavity of the fiber grating laser, the magnetic field is concentrated around the silicon steel sheet and hence the fiber grating laser experience stronger magnetic field than the case without the silicon steel sheet, which results in a larger magnetic field induced beat note frequency change after photodetection of the two orthogonally polarized laser outputs. With the same axial magnetic field, the experiment results confirm that the sensitivity of the sensor with a silicon steel sheet is improved over the one without a silicon steel sheet, which validates our proposal.
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