Heterodyne Systems for Optical Fibre Communications and a Proposed Electro-Optic Frequency Shifter
There has been a recent increase in interest in the possibilities of heterodyne or coherent communication systems. Such schemes have potential advantages over direct detection, principally in improved sensitivity, giving increased repeater spacings. Also different modulation formats, ie. frequency or phase modulation, light amplification and frequency division multiplexing schemes could be used. Improvements in semiconductor laser spectral qualities, ie. stable single mode, narrow linewidth operation, suggest that a heterodyne system may be possible. There are, however, a number of practical problems which must be solved.