Optical heterodyne communication systems1 have been proposed as a possible transmission method to obtain ultralong repeater spacings. Such systems require signal and local oscillator highly coherent light sources. A He-Ne gas laser or external cavity laser diode has been used in prototype heterodyne experiments. However, a practical system will need single-longitudinal-mode (SLM) semiconductor lasers because they have many advantages, such as the small device size, spectral stability to ambient temperature fluctuation, and high reliability.
© 1985 Optical Society of AmericaPDF Article