Variations of linewidth enhancement factor and linewidth as a function of laser geometry in (AlGa)As lasers

A quantitative analysis of the laser geometry dependence of the linewidth enhancement factor α and linewidth Δf in GaAs/AlGaAs lasers is presented. Analytical expressions for the refractive index change and α are presented. Variations of α and Δf depending on active layer thickness and laser length are estimated by using the expressions and previous experimental data of absorption spectra. Considering small variations in the reported experimental data depending on laser type, it is shown that the calculated values of α agree well with the experimental data. The magnitude of α decreases gradually with the decrease of active layer thickness and laser length. Since the change of active layer thickness affects only α, neglecting small variations of other terms in the linewidth expression, the reduction of active layer thickness is suggested to be a practical method for the reduction of linewidth.