The semiconductor laser linewidth: a Green´s function approach

The linewidth problem for a semiconductor laser is studied on the basis of a nonequilibrium Green´s function method. Starting from a continuous-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 the Schawlow-Townes expression. The linewidth is found to exceed the corresponding Schawlow-Townes value significantly. The reason for this is that gain saturation prevents large inversion even far above threshold. This phenomenon is usually ignored and is shown to yield laser linewidths comparable with experiments