Non-uniform Bias Magnetic Field Magneto-optical Bragg Devices Diffraction Characteristics

Magnetostatic wave (MSW) is a kind of slow dispersive wave propagating in magnetic medium, which is also a microwave electromagnetic wave. So the propagation characteristics of MSW can be controlled by bias magnetic fields. In magneto-optic (MO) waveguids, the interaction between MSWs and guided optical waves (GOWs) can lead to the Bragg diffraction of the GOWs. The resulting many new devices, such as MO modulator, optical switching, scanning, frequency spectrum analyzers, can be developed with applications to optical communication and optical information processing. However, the MSW-based MO devices haven't still been applied extensively up to now because of low Bragg diffraction efficiency.The generation and propagation characteristics of magnetostatic forward volume waves (MSFVWs) under nonuniform bias magnetic field and the Bragg diffraction efficiency of GOWs with the MSFVWs are theoretically studied in this paper.The dispersive characteristics and the dynamic magnetization of the MSFVWs in bismuth-doped YIG film under transversely nonuniform bias field are analyzed by the variational approach. The influence of MSFVW bandwidth on the diffraction efficiency is also considered. For the nonuniform bias magnetic field of U-type parabola profile, when the MSW frequency is less than the cut-off frequency at the edge of MO film, the effective MO interaction length reduces. The edge cut-off frequency is determined by the maximum of the nonuniform field.The Bragg diffraction efficiency of GOWs in bismuth-doped YIG film is caculated by considering the edge cut-off effect. The obtained theoretical curve is basically agreement with the experimental results. It is shown that the diffraction performance for MO Bragg cells can be greatly increased by using an appropriately nonuniform bias field. This theoretical method can also be applied to the optimization design of the MO Bragg devices.