Passive modelocking of semiconductor lasers with tunable group velocity dispersion cavity

Passive (hybrid) modelocking of AlGaAs lasers in an external cavity with high tunable group-velocity-dispersion (GVD) is reported. An internal grating compressor is used to continuously vary the intracavity GVD while a second grating compressor serves to compress the output pulses. Mode-locking is only found to occur for positive (normal) GVD. At optimum pulse compression, the dispersion of the external compressor is measured to linearly increase with intracavity dispersion. The large chirp amplitude detected at nearly-zero intracavity GVD as well as the general evolution of pulse parameters are shown to be well described by a modelocked laser model including fast laser gain saturation.