It has long been established that the combs emitted by quantum cascade lasers (QCLs) cannot be described as a train of short pulses, separated by the cavity roundtrip time. Instead, simulations made for typical device parameters suggest that, in steady state, these four-wave mixing driven combs have a constant temporal envelope, and undergo periodic rapid and complicated swings in frequency.
Recent work in characterising the modal phases has revealed a state which, somewhat unexpectedly, has a simple parabolic phase profile, corresponding to a linearly chirped output field. Moreover, this phase relationship was shown to be stable over time, and to be recoverable after the laser’s power had been cycled; from the perspective of a fixed external pulse compression scheme, these last two properties are critical.
In this work, we use a pair of gratings and lenses in a 4-f Martinez-type scheme to modify the phase of a high-power (~1 W) QCL comb emitted at 8.2 um with more than 100 cm-1 spectral bandwidth. By changing the position of the second grating, a parabolic phase can be added to or subtracted from the field. Employing this scheme, we demonstrate a compression of the QCL output from a 133 ps continuous wave waveform, to a train of pulses of width < 20 ps, and a peak power more than 10x that of the original. With this proof-of-principle work, we highlight the potential of the QCL system to deliver short, powerful pulses, with applications in nonlinear spectroscopy, for example.