For pt. I, see ibid., vol.6, 451 (1974). The light output powers of a semiconductor laser diode within an external cavity and of two optical coupled diodes are evaluated as a function of the system parameters (drive currents and coupling coefficient). A power law dependence between optical gain and electron density has been assumed and spontaneous emission into the amplified light field is taken into account in this approach. Analytical approximations are evaluated for the photon number within the diode cavity. With the exception of a small region near threshold there is good agreement between approximations and numerically calculated photon numbers. The theoretical results are compared with experimental data and good quantitative agreement is found in both cases, the diode in the external cavity and the optical coupled diodes.