  We propose in this communication an experimental study of the relaxation
oscillations behavior in mode-locked lasers. The semiconductor self-pulsating
laser diode is composed by two gain sections, without saturable absorber. It is
made of bulk structure and designed for optical telecommunication applications.
This specific device allows two different regimes of optical modulation: the
first one corresponds to the resonance of the relaxation oscillations and the
second one, to the mode-locking regime at FSR value. This singular behavior
leads us to characterize the self-pulsations which are coexisting in the laser
and to describe two regimes of output modulation: the first one appears thanks
to the resonance of the oscillation relaxation and the other one corresponds to
the FSR of the Fabry-Perot laser at 40 GHz.
