Analysis of Tunable Internal Loss Caused by Franz–Keldysh Absorption in Transistor Lasers

The Franz–Keldysh (FK) absorption at the base-collector junction of a transistor laser (TL) is inevitable because of the reverse bias therein. The bias condition, thus, plays a crucial role in the determination of internal loss of TLs. In this study, effects from various facet coatings of edge-emitting TLs on the internal loss (
$\alpha _{{\rm int}}$
), which is influenced by the FK absorption, are investigated. Experimental analyses on the electrical and optical characteristics of these TLs at various temperatures are presented. The
$\alpha _{{\rm int}}$
of different coated devices at various bias voltages are then extracted from their light-versus-current curves. We demonstrate that the internal loss resulted from the FK absorption is bias dependent, and therefore tunable. By contrast, the intense field inside the cavity with highly reflective coatings on both facets may saturate the FK absorption and make it less bias controllable.