A general variational optimization procedure is used to synthesize codirectional couplers with arbitrary sidelobe suppression down to −125 dB. Apodization shapes yielding sidelobe levels over the range of −40 dB to −75 dB are given by a single design formula. The limiting performance in terms of sidelobe suppression versus the bandwidth of passive evanescent-type couplers is found.
© 1995 Optical Society of America
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