Abstract :
A high-capacity experimental optical fiber communications system operating at 44.7 Mb/s has been designed and set up in an environment approaching field conditions to test the applicability of optical fiber transmission systems in interoffice digital trunking. The basic components of the system are a 2000 ft. sheathed and protected cable 1.3 cm in diameter containing 144 fibers, a feedback stabilized GaAlAs injection laser transmitter, and a silicon avalanche photodiode receiver. To simulate field conditions, the optical fiber cable was pulled through standard plastic underground ducts at the Bell Laboratories facility in Atlanta. The cable was terminated in a mass splice and connected to an optical distribution frame via a cable fan out. Single fiber jumper cables with connectors were used to join the laser transmitters and photodetector receivers to the fibers in the optical distribution frame. Appropriate terminal equipment was used to interface the optical device components and regeneration circuitry to standard Bell System DS3 (44.7 Mb/s) digital signals. The fibers used in this experiment were multimode graded index fibers of germania doped borosilicate glass made by the modified CVD process by Western Electric. The transmitter subsystem employed a stripe geometry double heterostructure GaAlAs laser operating at 825 nm. Standard ECL input signals operated an emitter coupled driver circuit in which the laser appeared as a collector load. A feedback stabilizing circuit controlled the prebias current through the laser to make the optical output insensitive to temperature changes, component aging, and data pattern effects. The transmitter subsystem was capable of generating -3 dBm average optical power in protected fiber pigtail over the temperature range 0 to 50°C. The receiver subsystem employed an n+pπp+silicon avalanche photodiode detector (APD) fabricated from high resistivity epitaxial silicon. The APD output was coupled into a transimpedance preamplifier circuit providing a receiver sensitivity of -53 dBm for a 10-9bit error rate over the temperature range 0 to 50°C. The dynamic range of the APD receiver was in excess of 30 dB of optical power. Both transmitter and receiver su- bsystems operated from +5V, -5.2V power supplies and were packaged as self-contained modules which could be carried on circuit boards that plug into equipment bays. The results and overall performance of the optical fiber systems experiment at Atlanta will be reviewed. Particular emphasis will be placed on the operation and performance of the optical terminal devices including the GaAs laser, Si APD, circuit design and packaging.
