THE SEMICONDUCTOR-LASER LINEWIDTH - A GREENS-FUNCTION APPROACH

The linewidth problem of a semiconductor laser is studied on the basis of a non-equilibrium Green's function method. Starting from a continu ous mode description of the radiation field, the low quality factor of semiconductor laser resonators is taken into account. For the case of a single lasing mode a linewidth formula is derived and related to th e Schawlow-Townes expression. The linewidth is found to exceed the cor responding Schawlow-Townes value significantly. The reason for this ef fect consists in the fact that gain saturation prevents large inversio n even far above threshold. This phenomenon is usually ignored and is shown to yield laser linewidths comparable with experiments.