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A simple method for realizing the desired light coupling for an optical directional coupler is presented. This method consists of loading a metal film on one of the waveguides in the undesired coupling region. The effective coupling length, fabricated in c-plate LiNbO3, could be changed from 0 to 3L0 (L0: 0-dB coupling length) by controlling the length of the metal film.
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Mikami et al. (1978) studied this question.
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