Chirp in a transistor laser: Franz-Keldysh reduction of the linewidth enhancement

A quantum well (QW) transistor laser with the capability of high current density, a collector current ensuring minimal operational change (beyond laser threshold) in base QW carrier population, a favorable long narrow emitter laser geometry, and built-in base-collector Franz-Keldysh absorption (with $IE+IB+IC=0$⁠), is measured for chirp-related behavior. A relatively low transistor laser linewidth enhancement factor $αe∼0.7$ is obtained despite the resolution limitations of the measurement apparatus.