Low frequency relaxation oscillations in class B lasers with feedback

Relaxation oscillations have been studied both theoretically and experimentally in a multimode YAG laser in the presence of a derivative feedback. In addition to the standard relaxation oscillations, discrete low frequency components are present in the noise spectrum of each individual mode but they compensate almost exactly in the total intensity. The influence of a combined derivative feedback involving both the total intensity and that of one selected mode on the behavior of the relaxation oscillations and nonlinear dynamics of the laser is investigated. It is shown that the compensation nature of the low frequency relaxation oscillations of the multimode class B lasers can be disturbed in this case and, as a consequence, the selected relaxation oscillations appear in the noise spectrum of the total intensity. A higher feedback level can induce a destabilization of the laser and eventually chaos. An excellent qualitative agreement between experiments and theory has been obtained.