  We report on the first direct measurement of the emission character of
quantum cascade laser based frequency combs, using intensity autocorrelation.
The correlation technique is based on fast electro-optic sampling, with a
bandwidth optimized to match the emission spectra of the comb laser, centered
around 2.5~THz. We find the output light to be amplitude and frequency
modulated at the same time, with intensity modulation depth as high as 90\%,
occuring on timescales as short as few picoseconds. We compare these findings
with intensity autocorrelation measurements of pulsed Terahertz light
originating from a photoconductive antenna and find very different results. We
observe no significant difference for the comb and dispersed regime. The
technique will be of significant importance in future for the measurement of
ultra-short pulses from quantum cascade lasers.
