LINEWIDTH EVOLUTION IN SEMICONDUCTOR-LASERS THROUGHOUT THRESHOLD

A detailed experimental and theoretical investigation on the evolution of linewidth Delta v(I) function of injection current for different s tructures of semiconductor lasers has been undertaken. Linewidth shows a continuous transition and tends asymptotically on either side to Sc hawlow-Townes inverse power behaviours. In a small current zone at thr eshold warping of the linewidth is observed depending on linewidth enh ancement factor alpha and laser structure. Different models including fluctuations are used in order to describe the coherence evolution : a symptotic behaviours by simple linear response model of Van der Pol eq uation (VdP), continuous evolution by phase transition Landau model (L ) and complete description including alpha using Fokker-Planck field p robability density resolution (FP). All models show the same asymptoti c behaviour. (FP) seems to be the best-adapted one because it includes intrinsically phase-amplitude coupling. These models lead to a new de termination method of spontaneous emission rate R, alpha, number of ph otons at saturation S-s and cavity photon lifetime tau(p) by fitting o nly two direct measurements (linewidth and optical power).