The SL Undersea Lightwave System is a large capacity digital fiber-optic transmission system capable of spanning the world's largest oceans and seas. The system is now being manufactured for use in telecommunications systems planned for the Atlantic and Pacific Oceans. In this paper we will discuss how the risks associated with using relatively new lightwave technology were minimized to design a digital undersea communication system which excels in reliability, availability, and performance.

A digital optical fiber undersea cable system targeted for transatlantic service in 1988 is now under development at Bell Laboratories. The system uses single-mode fibers to carry data at a bit rate of 280 Mbits/s. Using digital speech compression techniques, a total system capacity of over 35 000 two-way voice channels can be realized. With laser transmitters at 1.3 μm, repeater spacings are expected to exceed 35 km. This paper discusses system parameters, repeaters, fiber and cable design, terminal equipment, and system measurements.

An experimental repeater for amplification and regeneration of 50 Mb/s fiber-optical pulses has been built and tested. For the receiver either Si p-i-n or avalanche photodiodes are used in conjunction with a high impedance FET input amplifier. The high voltage for the avalanche photodiode is generated internally and controlled by the received signal. This AGC circuit is capable of compensating for temperature changes of the avalanche gain over the range of-40 - +60\degC. The optical transmitter consists of either a GaAs light emitting diode or a GaA1As laser diode coupled to optical fibers and directly modulated by a current driver with 30 percent electrical efficiency. For 10-9error rate, the required average optical signal power for a pseudorandom signal is p-i-n diode: -41.5 dBm; avalanche diode: -56.6 dBm. The optical output power into a fiber with 1 percent index difference is LED: -17 dBm; GaAlAs laser: 0 dBm. The repeater power requirement is about 2 W.

Second-order phase-locked loops with signal injection (PLLI's) have orders of magnitude larger pull-in ranges than conventional second-order PLL's with identical natural frequencies and damping factors. The PLLI utilizes a little-known characteristic of the Van der Pol oscillator, and for practically no additional hardware the pullin range is increased sufficiently to eliminate in many cases the need for crystal control of the voltage-controlled oscillator (VCO). In nonlinear systems (e.g., a retiming chain of a digital repeater) where the in-phase and in-quadrature noise components are correlated, the PLLI in addition features a significant reduction in output phase jitter. A 44.6MHz PLLI, designed for timing recovery in a fiber optic repeater, has 8.4-kHz conventional minimum jitter bandwidth. To reduce time jitter accumulation in a chain of repeaters, the damping factor was increased to 4 resulting in aQof 1300. The pull-in range of this loop is 9.6 MHz compared to 18 kHz for the same loop without signal injection. The static phase error is\pm 0.6\degfor ± 0.3 MHz oscillator drift. By injecting with 18° phase lag, the PLLI can take advantage of the correla tion between amplitude and phase noise components and the total output phase jitter is reduced by 10.5 dB due to destructive interference.