We report detailed measurements of the pump-current dependency of the self-
pulsating frequency of semiconductor CD lasers, A distinct kink in this dep
endence is found and explained using a rate-equation model. The kink denote
s a transition between a region where the self-pulsations are weakly sustai
ned relaxation oscillations and a region where Q-switching takes place. Sim
ulations show that spontaneous emission noise plays a crucial role in the c
ross-over.