Picosecond gain dynamics of an actively mode-locked external-cavity laser diode

We report a new experimental approach to study the temporal gain dynamics of an actively mode-locked external-cavity laser diode. The experiment can be viewed as an electrical pump-optical probe technique. Picosecond time resolution is achieved by synchronizing the actively mode-locked semiconductor laser to a mode-locked Ti:sapphire laser. The transmission of the Ti:sapphire pulses through the active region of the laser diode is then measured as a function of photon energy and delay time. Our experiment allows to investigate the dynamics of the entire gain spectrum with picosecond time resolution since the Ti:sapphire laser is wavelength tunable. On the basis of these data we are able to determine directly the wavelength dependence of the differential gain. The results compare well with microscopic calculations. The implication for the mode-locking behavior of the diode laser is discussed.>