  The output spectrum of both gas and semiconductor lasers usually contains
more than one frequency. Multimode operation in gas versus semiconductor lasers
arises from different physics. In gas lasers, slow equilibration of the
electron populations at different energies makes each frequency an independent
single-mode laser. The slow electron diffusion in semiconductor lasers,
combined with the spatially varying optical intensity patterns of the modes,
makes each region of space an independent single-mode laser. We develop a rate
equation model for the photon number in each mode which captures all these
effects. Plotting the photon number versus pumping rate for the competing
modes, in both subthreshold and above threshold operation, illustrates the
changes in the laser output spectrum due to either slow equilibration or slow
diffusion of electrons.
