| Mode division multiplexing (MDM) proposes a new dimension of multiplexing ways in fiber communication system, which is one of the methods with the most prospects for increasing the capacity of optical communication system. The mode excitation and mode conversion are the key technologies to solve the basic problems in MDM. Much frontier researches are to realize the excitation of arbitrary higher modes with high efficiency, low complexity, high resolution and the conversion of different modes with high efficiency and high speed, which are the prerequisites for the application of MDM to optical communications.In recent years, there are several new trends in the study of mode excitation and mode conversion, to name a few, long period fiber grating, planar optical waveguide, photonic lantern, fiber coupler, etc., which share the waveguide structure, and phase plate, Spatial Light Modulator (SLM), spot-based, adaptive optics, etc., which are the methods of free space optics. It is well known that the approaches based on the fiber waveguide structure are demanding for experimental crafts, while the approaches based on free space optics like SLM are not that demanding with respect to the objects aforementioned. Therefore, this dissertation aims at the excitation of arbitrary mode based on the principle of phase conjugation and SLM-based conversion of arbitrary mode.The dissertation is organized as follows. Firstly, the research background of MDM and its development at home and abroad are presented. Then the elements of the mode control theory in mode-multiplexing system based on free-space optical are introduced, which are the theoretical foundations for the foregoing mode excitation and mode conversion. Then, based on the principle of phase conjugate, combining the distribution of optical field and the structure of cavity resonator, the structure of the parallel plane cavity with square mirrors proposed and direct mode excitations are achieved with SLM, and the simulation results are given. Next, based on the spatial phase modulation with4f system, the method of arbitrary mode conversion using the optimal mode profile matching is proposed, and the experimental results of conversion using optimal grating aperture from low-order mode to high-order mode are given. Finally, a method for accuracy mode conversion based on compressed sensing and simulated annealing algorithm is proposed, and the simulation results are obtained.The main attributions and innovations of this dissertation are as follows:1.Methods of arbitrary mode excitation based on phase conjugationThe most effective method to excite laser mode is to utilize laser resonance cavity, which is far more prior to the method using external light field results in the mode’s stability, temporal coherence, spatial coherence and mode distortion degree, etc.. By the analysis of the distribution patterns of light field, the principle of SLM, the Fox-li numerical iteration method and the structure of resonance cavity, this dissertation proposes an approach of arbitrary mode excitation using phase-conjugate-based extra-cavity resonant square mirror parallel plane cavity. The feasibility of this approach are verified via Matlab simulation results, which shows that the target mode becomes the intrinsic mode of the resonant cavity when passing the end of the special design of the SLM, which means arbitrary mode excitation is available.2. Arbitrary mode conversion based on spatial phase modulationFirstly, the characteristics of mode field distribution of the pre-converting mode and objective mode are studied in spatial, spatial-frequency and Fourier transformed plane spatial domain. Then, based on spatial frequency spectrum filtering, the transfer function model for conversion between arbitrary two modes is built theoretically. Finally, a mode conversion optics system is constructed using phase SLM. In the dissertation, the mode field matching principle of the Fourier transform plane spatial domain is obtained by adapting its mode field waist radius to mode field radius, where in the adapting relationship of mode field waist radius and mode profile, as well as the analytic method of the mode field radius matching with respect to pre-conversing mode and objective mode, are formulated. The simulations show that this approach is able to realize the conversion between arbitrary two modes, and the normalized correlation coefficient of mode conversion is high. The proposed experiment platform for optimal aperture size conversion is able to convert the fundamental LP01mode to higher-order modes such as LPlla、LP11b、LP02、LP03、LP21a、LP21b、LP12a、LP12b.3.Analysis of mode features and algorithms of precise control of mode conversionBased on the analysis of mode features, a new algorithm for accuracy mode conversion is proposed combining compressed sensing algorithm with simulated annealing algorithm. The methodology of the proposed algorithm lies in the synthesis of the aforementioned image processing algorithms:after processed by the simulated annealing algorithm, a sparse optical field distribution is obtained in an effective way, then the distribution of phase SLM is adjusted using compressed sensing algorithm, and then the light field distribution model is shown on CCD. The proposed algorithm is able to make the converted mode more and more close to the ideal mode. Furthermore, not merely the calculation is greatly reduced but also more precise control of mode conversion is achieved. |
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