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An optical heterodyne receiver that can select any channel out of 2.5 Gbit/s frequency division multiplexed (FDM) signals with spacing as narrow as 5 GHz has been developed. Sharp filtering characteristics of the heterodyne receiver at the intermediate frequency stage enable FDM signals with such narrow channel spacing to be demultiplexed.
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Kikuchi et al. (2002) studied this question.
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