A frequency spectrum of a semiconductor laser is shown below. The Fabry-Perot modes are clearly shown below threshold. Above threshold, a single mode become dominant and becomes the lasing frequency. Let us now move on to examine the rate equations governing the gain below and above threshold, by considering the injection of carriers and the mechanisms for loss. In the above equation, R sp is the rate of spontaneous emission, R l is the current leakage from the active layer, R nr is the non-radiative recombination rate and finally, R st is the rate of stimulated emission.
Stimulated emission requires the presence of photons to take place.
Alternatively, the recombination terms can be written in terms of the carrier density N . Below threshold the rate of stimulated emission is negligible and can be neglected. Above threshold the main photon generation term is the rate of stimulated emission. This rate must be adjusted because the photons can occupy a volume larger than that occupied by the electrons which are confined to the active medium. The ratio of the volume of the cavity to the volume occupied by the photons in the cavity is called the confinement factor and is given the symbol G .