Abstract

Semiconductor laser evolution throughout threshold is continuous in both power and linewidth showing a phase transition like behaviour. It is not correct to consider the below threshold behaviour as that of a light emitting diode and the above one as a fully coherent laser. The two regions obey to different mechanisms such as cavity filtered amplified spontaneous emission and carrier recombination below threshold and gain, photon lifetimes and linewidth enhancement above threshold. Few measurements of coherence behaviour have been made in the threshold region. We have previously developed a method using the Schawlow-Townes slopes below and above threshold in order to extract the linewidth enhancement factor α. [1]. Coherence evolution at threshold has been studied for classical detuned lasers based on analysis of the Fokker-Planck equation [2] but no useful expression adapted to semiconductor lasers has been given so far. We use here an analytical model enabling us, by fitting, to extract all major internal parameters of a semiconductor laser.

© 1996 IEEE