GaAIAs laser transmitter for optical fiber transmission systems

A feedback-stabilized GaAIAs injection laser optical communication source for transmission of NRZ data at 44.7 Mb/sec has been built and tested. Standard ECL input levels operate the emitter-coupled driver circuitry in which the stripe geometry double-heterostructure GaAIAs laser diode (λ = 825 nm) appears as a collector load. The laser is pre-biased near threshold by a separate feedback-stabilizing circuit. This circuit, utilizing an 800-kHz operational amplifier, compares the optical output measured from the back mirror of the laser using a PIN photodiode with data-pattern reference. In this way, closed-loop control of the prebias current makes the optical output insensitive to thermal effects and component aging. The driver–feedback components are assembled on thick-film hybrid integrated circuits. The laser, indium soldered to a gold-plated copper stud, and the thick-film driver circuit are mounted on an aluminum heat sink which forms the body of the driver package. A 40-cm Teflon-sheathed connectorized section of optical fiber is brought through the aluminum housing, strain relieved, and epoxied in place after positioning near the front mirror of the laser. Efficient coupling (50% or greater) between the laser and optical fiber is achieved by melting a spherical lens on the tip of the fiber. The driver package accessed through pins connected to the thick-film circuit is completed by filling the active volume of the heat sink with casting epoxy. The feedback package is of similar design but has only electrical inputs and outputs. The two-package subsystem launches a minimum of 1-mW peak power into a fiber with a ≈0.002-in. graded-index core. The extinction ratio is typically 15:1; peak-to-peak amplitude variation in the output is ~8%. The subsystem operates from 5°C to 55°C and tolerates ±5 % variations in the +5-V and -5.2-V supplies.