Abstract
The almost universal acceptance of monomode fiber as the prime contender for future long distance transmission systems has recently reawakened interest in optical heterodyne and homodyne detection. Coherent systems appear particularly attractive for the low-loss 1500-nm fiber transmission window, where predicted1 improvements in receiver sensitivity of 20 dB over direct detection could result in an increase in regenerator spacing by more than 50 km. A key prerequisite for development of such systems is the availability of lasers with exceptional spectral stability and sub megahertz line width for use as the transmitter and local oscillator within the receiver. This requirement was provided in an initial 8-Mbit/sec heterodyne experiment by 1523-nm He-Ne lasers both at the transmitter and receiver.2 The further development of this technique for higher bit rates was limited by the narrow gain/frequency profile of the gas lasers, which restricted the tuning range of the receiver local oscillator and thus limited the maximum stable IF to 110 MHz. This limitation.
© 1984 Optical Society of America
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