In the photon density rate equation, the first term ?GP is the rate at which photon density increases due to stimulated emission ( the same term in carrier rate equation, with positive sign and multiplied for the confinement factor ? ), the second term is the rate at which photons leave the cavity, for internal absorption or exiting the mirrors, expressed via the decay time constant { \ tau _ p } and the third term is the contribution of spontaneous emission from the carrier radiative recombination into the laser mode.
