The light output and the electron density within the active layer of a semiconductor laser have been calculated from the steady state solution . The assumed rate equation model takes into account the spontaneous emission into the lasing modes. Simple analytical approximation formulae have been found under the assumption of a power-law dependence between optical gain g and electron density n(g∝n'). The approximations are compared with numerical results from the rate equations. With the exception of a small region near the lasing threshold good agreement has been found. The theoretical results are compared with experimentally measured light outputs from single and double heterostructure lasers. From the comparison below threshold the power law exponent l may be evaluated