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Directional couplers for wavelength division multiplexing using l.e.d.s as transmitters have been realised without imaging elements. Their total insertion loss (MUX + DEMUX) is of the order of 4 dB; their crosstalk attenuation and near-end crosstalk attenuation are 30 dB and >60 dB, respectively.
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Mahlein et al. (1980) studied this question.
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