Multimode instabilities in mid-infrared quantum cascade lasers

We present experimental evidence for a mid-infrared frequency comb based on AlGaAs/GaAs quantum cascade laser. The comb-like emission spectra span over similar to 50cm(-1) at a center wavenumber similar to 1080cm(-1). The measured width of the comb lines is <= 20MHz. The instability sets in when the intracavity power is sufficiently large. In addition, intersubband transitions feature strong third-order optical nonlinearities X-(3), due to a large matrix element between the upper and lower laser states, allowing a parametric process, due to four-wave mixing. The observed spectral behavior is similar, in many ways, to the Risken-Nummedal-Graham-Haken (RNGH) instability.