Temporal and spectral gain dynamics in an actively modelocked semiconductor laser

We apply a new electrical pump, optical probe spectroscopic technique using a fs Ti:sapphire laser to study the temporal and spectral gain dynamics of a modelocked external cavity semiconductor laser. The temporal behaviour of the gain and the spectrum of the differential gain are directly obtained. The spectrum of the differential refractive index is determined via a Kramers-Kronig transformation enabling us to determine the wavelength dependence of the α parameter. From the discussion of these results, strategies, limits and solutions are proposed to improve the quality of modelocking of diode lasers consequently resulting in pulse shortening. The proposals are experimentally verified for wavelength-detuned operation and for synchronous pumping with ultrashort optical pulses instead of electrical modulation.