In optical communication systems, coherent detection permits significant improvements in receiver sensitivity and wavelength selectivity compared with direct detection and allows a variety of modulation/demodulation formats to be used in practical applications. In the 1.3-1.6-µm lightwave band, an ideal coherent receiver requires only 10-20 photons/bit to obtain a bit error rate (BER) of 10−9, while direct detection requires ~1000 photons/bit. Because of its improved wavelength selectivity, the advanced technology of frequency multiplexing with channel spacings of only 100 MHz can be used as in microwave systems instead of the 100 GHz required with conventional optical wavelength-division-multiplexing technology.

© 1988 Optical Society of America

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