Miniature Fiber-laser Magnetic-field Sensor Based On Faraday Effect

Optical fiber sensors are new-style sensors emerged from the late70’s. They use light tocarry sensitive information and optical fiber to transmit sensitive information. They can measureelectric field, magnetic field, pressure, temperature, strain and other physical measurands,showing many significant advantages such as immunity to electromagnetic interference, highprecision and sensitivity, easy realization of remote monitoring and large-scale sensor networksOptical fiber magnetic-field sensor can accurately measure a magnetic field in real time. Itis very promising in the smart grid, current monitoring, and other magnetic field related fieldsmeasurement. In this thesis we propose a miniature fiber-laser magnetic-field sensor based onfaraday effect, which is applied to the measurement of magnetic field. Through optimization ofthe laser, the sensitivity of the proposed sensor is improved.The main contents of this thesis are as follows:Firstly, for the development of short cavity DBR fiber laser, by using phase masks and a193nm excimer laser, we directly write fiber grating pairs in Er doped fiber with theirwavelength matching that of the template on which fiber grating lasers with single longitudinalmode are then developed consequently The dual-frequency output of the laser with orthogonalpolarizations provides a radio frequency monitoring signal for sensing. The overall length of thelaser can be less than20mm with a signal to noise ratio of the output of the beat signal betterthan70dB.Secondly, we propose a miniature fiber-laser magnetic-field sensor based on faraday effect.According to the birefringent and Faraday effect in the optical fiber, the dependence of the beatfrequency of a fiber laser on the magnetic field is derived. Experiments on the measurement ofmagnetic field have been conducted, which confirm the validity of the theory model.Thirdly, we have propose a method to directly increase the sensitivity of the sensor byplacing a silicon steel sheet in parallel to a laser. The silicon steel sheet works as a magneticcollector, which collects the surrounding magnetic field to enhance the strength of the magneticfield near the laser resonant cavity, and hence to increase the sensitivity of the sensor. In addition,we have proposed a method to actively control the beat frequency of a laser through, the heat treatment by CO2laser. Exposure of the fiber laser between two gratings under CO2laser, onecan change the resonant cavity birefringence effectively, which therefore changes the beatfrequency of the laser outputs. This method has realize a tuning range from19MHz to1.7GHz,greatly expanding the beat frequency tuning range of the laser and lay the foundation for furtherimproving the sensitivity of the sensor.Lastly, we briefly summarize the content of the whole thesis. We also forecast a scheme toincrease the sensitivity of the proposed optical fiber magnetic field sensor, that is, the method ofsplicing with rotation to actively tune the beat frequency. Moreover, we outlook the futureindustrial application of the miniature fiber-laser magnetic-field sensor based on faraday effect.