| In magneto-optic (MO) film waveguides, the interaction between guided-optical waves (GOWs) and magnetostatic waves (MSWs) excited by the RF signal applied into microstrip line may lead to the Bragg diffraction effects and mode conversion of the GOWs. The MSW devices based on MO Bragg diffraction effects have a great application to optical communication and optical information processing such as RF spectrum analyzers, optical broadcast switches and optical modulators. In comparisons to the now prevalent acoustooptic (AO) Bragg devices which has the similar principal with MO Bragg modules, the former possesses the unique advantages such as much larger range of tunable carrier frequencies (0.5G~40GHz) readily obtained by varying bias DC magnetic field, larger MO bandwidth and much higher and electronically tunable modulation/switching and scanning speeds. So, it has a realistic significance to introduce MSW-based MO diffraction theory into the field of optical signal processing. With more complicated electromagnetic environment, it is necessary for the signal processing devices to own parallel processing capability. Obviously, parallel signal processing is also a developing trend for MO and AO Bragg devices. However, the existing microwave MO coupled-mode theory is only applied to the case of single-frequency MSW, and multi-frequency MO coupled-mode theory is seldom referenced to. Only several demonstrated experiments are reported. It is necessary to develop multi-frequency MO coupled-mode theory to provide a theoretical guide for the related experiments and applications.Based on the traditional MO coupled-mode theory, multi-frequency MO coupled-mode theory between multiple-frequency magnetostatic forward volume waves (MSFVWs) and GOWs was investigated in this paper. The following works and innovations were done:1. The specific diffracted process of GOWs with multi-frequency MSFVWs was studied. Taking three-frequency MSFVWs for example, the light state transfer figure was shown, which indicated the producing and transferring rule of each diffracted light. Multi-frequency MO coupled-mode equations in third-order MO interaction approximation (TA) was deduced in detail and the analytical expression of principal diffraction efficiency (PDE) for small signal case was obtained, which are the basis of studying multi-frequency MO diffraction characters. For two-frequency MO interaction with the MSWs of 7.302GHz and 7.245GHz, the numerical results from the multi-frequency coupled-mode equations were in agreement with the experimental data.2. The nonlinear phenomena in multi-frequency MO interaction, such as cross modulation, intermodulation and signal compression, were analyzed. The small signal limit of the signal compression less than 1dB corresponding to the source depletion of less than 40% was determined and in small signal range it's enough to only consider the first-order MO interaction. Imitating the calculations of the diffraction efficiencies (DEs) of the high-order products in multifrequency AO interaction, the DEs of the analytical expressions of the second- and third-order intermodulations were given under phase-matching condition and the results were in good agreement with the numerical analysis.3. The applications in the field of RF spectrum analysis and constructing optical broadcast switch were studied. For RF spectrum analysis, microwave RF frequencies can be obtained through the focused position of diffracted light beams in the focal plane of Fourier lens. In order to reproduce the relative RF intensity, a revision obtained through single-frequency diffraction spectrum should be made on the basis of the relative intensities of the diffracted lights detected by photodetector array. For optical broadcast switch, through the control of the multiple RF signals, the deflection angles and intensities of multiple principle diffracted lights can be regulated and sent to different ports simultaneously according to different multicast demands. |
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