Minimal group refractive index dispersion and gain evolution in ultra-broad-band quantum cascade lasers

The group refractive index dispersion in ultra-broad-band quantum cascade (QC) lasers has been determined using Fabry-Perot spectra obtained by operating the lasers in continuous wave mode below threshold. In the wavelength range of 5-8 mum, the global change of the group refractive index is as small as +8.2 X 10(-3) mum(-1). Using the method of Hakki and Paoli, the subthreshold gain of the lasers has furthermore been measured as a function of wavelength and current. At the wavelength of best performance, 7.4 mum, a modal gain coefficient of 16 cm(.)kA(-1) at threshold and a waveguide loss of 18 cm(-1) have been estimated. The gain evolution confirms an earlier assumption that cross-absorption restricted laser action to above 6 mum wavelength.