Study Of Fiber-Optic Interferometric Magnetic Field Sensors Based On Magnetic Fluid

All-fiber sensor has been widely used in various fields because of its characteristics of low cost, high sensitivity, compact structure, corrosion resistance, good insulation and anti-electromagnetic interference. So far, it can measure of more than seventy Physical quantities including temperature, stress, velocity, acceleration, et al. It reveals a tendency that all-fiber sensors have gradually replaced the traditional sensors, and they are moving in the development direction of all-fiber structure and microstructure. since the magnetic field sensors have been widely used in military, medicine, modern industry as well as in various devices used in our daily lives, American Scientists first proposed the idea of using the optical fiber to detect the magnetic field in the nineteen seventies. Nevertheless, the fiber cannot respond to magnetic fields directly. Usually, the optical fiber magnetic field sensors are made using magneto-optic materials which are sensitive to magnetic fields to drive a fiber interferometer. Magnetic fluid is such a novel functional nanomaterial which possesses various critical optical properties, for instance, magneto-optical characteristics, refractive index tunability, birefringence, magnetic anhysteresis and so on, thus igniting booming international studies on it. In light of all the advantages of magnetic fluid and interferometric fiber-optic sensors, this thesis intends to carry out an experimental study into several novel magnetic-fluid filled interferometric fiber magnetic field sensors from the following aspects:Firstly, the existent problems and research tendency have been discussed when magnetic fluid and fiber-optic sensors are integrated. By virtues of magnetic fluid and fiber-optic interferometers, the principles of the magnetic-fluid filled fiber-optic interferometers including Fabry-Perot interference, Mach-Zehnder interference and Sagnac interference, together with the benefits, drawbacks and application prospect of each, have been discussed and analysed respectively.Secondly, two simple but effective methods of making magnetic-fluid filled fiber Mach-Zehnder magnetic field sensors have been proposed to avoid the bottleneck problem of absorption loss. The all-fiber magnetic field sensors based on the evanescent wave theory and refractive index tunability of magnetic fluid. The sensitivities of dual-S structure and mutimode-single mode-mutimode fiber structure are 0.2904 nm/mT and 0.1231nm/mT, respectively. Considering the light passed the open cavity directly will lead to rather large absorption loss and the enclosed cavities have troubles in measurement of refractive index of liquid, two means have been proposed to solve these problems above and have advantages of compact structure, easy fabrication, well stability, high sensitivity, et al. Thus, optical fiber interferometer and magnetically controlled refractive-index tunability of magnetic fluid are integrated well.Finally, considering the designed magnetic field sensors based on magnetically controlled birefringence of magnetic fluid are rarely reported and some problems still exist in reported papers, we designed the D-shaped fiber microstructure fabricated by fs laser for introducing the birefringence of magnetic fluid into a high birefringence Sagnac loop to achieve an in-line structure. The designed sensor broke through the limitation of tunable range of refractive-index of magnetic fluid, which is so small that limit the sensing range of magnetic field and the improvement of sensitivity. Therefore, the proposal no only has much of room for increasing the sensitivity but also has a rather stable extinction ratio. Because the dichroism absorption will lead to the amplitude of the interference spectrum decreased by the increasing of extro-magnetic field. Last but not least, the proposed sensor overcome the drawbacks that sensitive section can not be made longer to increase sensitivity, whereas the length or thickness of the magnetic fluid film was limited in the open cavity, especially for high concentration of black-brown magnetic fluid. The results of preliminary experiment shows that the sensor has a good performance, and the achieved sensitivity of 0.08264nm/mT can be further improved by increasing the depth and length of the D-shaped structure.