Research On Multi-mode Magneto-optical Coupling Theory

The mode converters are the key device in the mode divison multiplexing system,and the design and development of the mode conversion devices can be not separated from the corresponding theoretical guidance.In the paper,three different structures of the magneto-optical(MO)fiber,Bragg diffraction cell,and MO grating under the multi-mode MO coupling theoretical modal are studied.The coupled mode equations(CMEs)are established respectively,and their mode coupling characteristics are studied.The corresponding analysis method can also be generalized in other similar structures.The main research contents and innovations of this paper are as follows:1.By the linear polarization approximation,the magneto-optical CMEs of multimode in MO fiber are established according to the orthogonality and normalization conditions of fiber eigenmodes.The coupling characteristics between modes are analyzed by introducing the MO coupling coefficient related to the mode field,and the theoretical results are in agreement with the simulation data.Our research shows that mode coupling in MO fibers can be divided into intra-mode group(intra-MG)coupling and inter-mode group(inter-MG)coupling.The influence of the inter-MG coupling on the polarization conversion efficiency can be greatly reduced by appropriately adjusting the applied magnetic field and optimizing the fiber length,which is helpful for the design of polarization mode converters;By properly designing the MO fiber structure with larger specific Faraday rotation,the inter-MG MCE can also be further improved,and then a spatial mode converter based on multi-mode MO fibers might be developed.2.The theoretical modal suitable for multi-mode MO Bragg diffraction cell is established.The mode conversion and diffraction efficients(MC&Des)between the same modes can be up about 98% by analyzing the mode conversion characteristics of Bragg diffraction cell and optimizing the structure of Bragg cell.Further more,it can be resulted the mode conversion switches function.The multi-mode Bragg diffraction compared with traditional fundamental mode case as follows:(1)The higher-order TE/TM modes interact with the magneto-static waves(MSWs)in the MO Bragg cell,and even the relevant MSWs are also of higher-order modes.(2)Under the multi-mode case,the restriction of MO films on higher-order guided optical wave(GOW)modes is weaker than the fundamental mode case,and the corresponding modal field distribution can't be approximated as that of the fundamental mode,which will bring about a more complex deduction of the CMEs.(3)The MO coupling coefficients become dependent on the modal field distributions of GOWs and MSWs,it is more general and expandable compared with single mode,which is providing theoretical guidance for development of MO sensors,magnetic control mode conversion and selectors.3.The CMEs of the multimode MO grating are derived.The influence of the MO coupling coefficients and the mode conversion and diffraction efficiency are analyzed.The MC&DE can theoretically be up to about 99% by optimizing the film thickness and coupling length.Furthermore,the period of the MO grating can be adjusted by adjusting the magnetic field,which can be used for the design and development of integrated mode converters based on the fix the MO grating;certainly,the different order mode conversion is also available by modifying the amplitude of the magnetization and increasing the bias magnetic field.