A circuit designed for direct modulation of semiconductor lasers at frequencies up to 2.5 GHz has been built in order to study the high speed modulation characteristics of injection lasers. The circuit provides flat (±1.0 dB) frequency response from 20 kHz to 2.6 GHz. An integral bias T network provides a dc bias port for laser prebias. A versatile experimental mounting fixture incorporating this circuit provides thermoelectric cooling of stud mounted lasers and allows for simultaneous measurements from both laser facets. The laser can be cooled or heated and an active feedback controller holds the stud temperature to ±0.05°C. This mounting fixture and circuit were recently employed in an experimental single mode fiber communications system employing a new Bell Labs mode stabilized laser at 1.5 μm. The system set a new length record of 119 km for unrepeatered transmission at a bit rate of 420 Mb/s. Distortion free modulation of injection lasers at speeds up to 2.0 Gb/s has been achieved.