Analytical solution of the rate equation in direct modulation of semiconductor laser

We studied the rate equations of a semiconductor laser operating in a direct modulation mode for the purpose of obtaining the direct dependence of output photon density on modulating current. We show that the system of differential equations may be reduced to a special case when the spontaneous carrier decay rate is equal to the photon decay rate. The unique solution is obtained for this case. We also show that a laser operating in direct modulation exhibits an inertia in switching between the optical and electrical fields which introduces a corresponding loss in the modulating current. The output photon density and the input carrier density depend directly on this loss. For the systems with a minimal and maximal loss, the output photon density can be obtained analytically.