Coherence and spectral properties of radiation emitted by a semiconductor laser with an external reflector

The Langevin equation for the phase is used to investigate the influence of an external reflector on the coherence properties of single-mode radiation emitted by a semiconductor laser in the case when the coupling between the laser and the reflector is weak. The conditions are found under which the influence of the reflector is important. The explicit form of the coherence function is found for the most interesting cases. The dependence of the emission spectrum of a laser with an external resonator on the pump current is investigated. It is shown that if the coherence length of the laser by itself is much less than the distance to the external mirror, the output spectrum is of multimode nature.