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Various means of increasing the extinction ratio in ultra short directional couplers using slight asymmetry in the coupled waveguide structure are investigated. It is shown that couplers with 10–200 µm coupling length and with high extinction ratios larger than 30 dB can be achieved. Shorter asymmetric couplers have an extinction ratio that is more fabrication tolerant.
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Kim et al. (1998) studied this question.
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