Tunable Magneto-optical Wavelength Filter Of Long-period Fiber Grating With Magnetic Fluids

Tunable wavelength filter of long-period fiber grating is an important optical filter for automatic switch optical networks. Due to its sensitivity to the refractive index of the surrounding material, coated with magneto-optical index tunable materials, the long-period fiber gratings can be fabricated into the magneto-optical tunable wavelength filters. The refractive index of magnetic fluids can be tuned by varying the intensity of the external magnetic field, and thus demonstrate potential use in fabricating tunable optical devices. In this thesis, utilizing the magneto-optical tunability of the magnetic fluids'refractive index, we have proposed and analyzed the tunable magneto-optical wavelength filter of long-period fiber grating with magnetic fluids.The principle for the sensitivity of long-period fiber grating to the ambient refractive index is the foundation for the design and fabrication of tunable wavelength filter of long-period fiber grating. Based on the coupled-mode theory, we introduced the vector field distribution of the core-mode and cladding modes, the coupling between modes and the coupled-mode equations in long-period fiber grating. Besides, the transmission spectra of long-period fiber grating with different surrounding refractive indices are numerically calculated. After investigating the relation of the resonant wavelength with the surrounding material, we analyzed the methods to improve the tuning range of the long-period fiber grating.The magneto-optical tunability of the magnetic fluids'refractive index directly determines the tuning range of the tunable magneto-optical wavelength filter of long-period fiber grating with magnetic fluids. In order to know the magneto-optical tunability of the magnetic fluids, we need to measure the magnetic field-dependent refractive index. By using the method-retroreflection on the fiber-optic end face, the field-dependent refractive index of magnetic fluids was measured and the magneto-optical tunability was analyzed.On the basis of the sensitivity of long-period fiber grating to the surrounding refractive index and the magneto-optical tunability of the magnetic fluids, we proposed the tunable magneto-optical wavelength filter of long-period fiber grating with magnetic fluids. The transmission spectra and the tuning range of the tunable magneto-optical wavelength filter were experimentally investigated. By comparing the experimental results with the theoretical ones, the experimental results were verified. In addition, the extinction-ratio of the tunable magneto-optical wavelength filter was measured and the methods to improve the performance of the filter were briefly analyzed.In our experiment, the water-based Fe₃O₄ magnetic fluids with the density of 1.2 g/ml are used, and the intensity range of the external magnetic field is between 0 Oe and 1661 Oe. While the intensity of the magnetic field increases, the refractive index of the magnetic fluids decreases from 1.4471 to 1.4246, realizing the maximum shift of 0.025. Coated with the magnetic fluids, the tunable magneto-optical wavelength filter of long-period fiber grating has a wavelength tuning range of 7.4 nm and an extinction-ratio of 3.28 dB. Through finding the magnetic fluids with the refractive index closer to the cladding index of long-period fiber grating or using new structured long-period fiber grating with higher sensitivity to ambient index, the wavelength tuning range of the tunable magneto-optical wavelength filter of long-period fiber grating with magnetic fluids can be expanded.