Atlanta Fiber System Experiment: GaAIAs Laser Transmitter for Lightwave Transmission Systems

An experimental lightwave communication system has been designed and set up in an environment approaching field conditions at the Bell Laboratories facility in Norcross, Georgia to study the feasibility of optical fiber transmission systems in interoffice digital trunking. 1 The experimental system operates at the DS-3 signal rate (44.736 Mb/s, the third level of the Bell System digital hierarchy) using a binary (on-off) nonreturn-to-zero (NRZ) signal format. Terminal transmitters and line regenerators require an optical source to convert the ECL-level logic signals to optical signals of 0.5 mW average power into a transmission fiber. GaAIAs injection lasers are sources well suited for this application, as they can be on-off modulated at high speeds (with rise times of less than 1 ns) with low drive power. In addition, the laser light can be effi1823 ciently coupled into low N.A. fibers, and the laser wavelength and linewidth are nearly optimum for digital transmission in low-loss glass fibers. However, some inherent disadvantages must be circumvented before GaAlAs lasers can be used in practical optical fiber systems, the most important of which is the temperature sensitivity of the laser. A two-package GaAlAs laser source subsystem was designed and built to operate at 44.7 Mb/s over the temperature range 5° to 55°C. One package contains the GaAlAs double-heterostructure injection laser operating at 825 nm and the driver modulation circuitry. This package also interfaces the laser to the fiber.