Laser Transmitters for 70-MHz Entrance Links

An experiment to evaluate lightwave technology as a means for providing inexpensive, wideband, reliable entrance links between remote satellite earth stations and video/telephone operating centers is being performed at Bell Laboratories locations at Holmdel, N.J. and Naperville, 111.1 In this experiment, either light-emitting diodes (LEDS) or injection-laser diodes (ILDS) are intensity-modulated with a 70-MHz electrical signal that is frequency-modulated by baseband signals (IM/ FM). Analog receivers using either p-i-n photodiodes or avalanche photodiodes (APDS) convert the modulated light back into electrical format. The frequency-modulated 70-MHz carrier occupies the band 70 ± 20 MHz. There is also a possibility of sending several narrowband FM carriers occupying the same band. The baseband signal would be either a single 6-MHz diplexed TV channel or 1200 multiplexed voice channels occupying a similar bandwidth. Others have also reported the application of fiber optics to satellite entrance links.2, 3 The design objective for the transmitter was to achieve a carrier-tonoise ratio(cNR) of at least 35 dB. If an injection laser were used as the source, a simple analysis of signal-to-noise ratio showed that, to meet this requirement, an average power of --10 dBm must be launched into the optical fiber.4 This assumes 50-percent modulation index, 10 dB of 1617