Mode excitation of optical strip waveguides and fibers by end-fire mutual coupling

Abstract

Excitation of different modes of both strip waveguides and fibers by their end-fire mutual coupling has been thoroughly investigated theoretically. It is shown that quasi-single mode excitation may be achieved by a suitable choice of their parameters and the conditions of coupling. Maximum excitation efficiency of the fundamental mode of a strip waveguide higher than 96% accompanied with simultaneous decrease of the intensity of high-order modes (not less than-20 dB as compared with the fundamental mode) may be achieved. Maximum excitation efficiency of the fundamental mode of a fiber higher than 90% is obtainable while over-all excitation efficiency of high-order modes is less than a few percent. In each case the conditions of optimum coupling are different. As our evaluations show, if high-order modes of exciting wave-guide are taken into account the results of our calculations will not be changed essentially. From the point of view of reaching high excitation efficiency the case when only the fundamental mode of multimode strip waveguides or multimode fibers excited by their end-fire mutual coupling is of considerable interest for designing multimode strip waveguide structures, integrated optical circuits and elements operating in the quasi-single mode regime. Experimental investigation of the mode excitation of strip waveguides and fibers by their end-fire mutual coupling is being carried out and the results will be published [19].