Quantum walks: another view on mode-locking

It has been shown that ring quantum cascade lasers can emit solitons when the driving current is large enough such that spontaneous symmetry breaking occurs between the clockwise and counter-clockwise modes . We show that ring devices with very low backscattering will emit an optical frequency comb when injected with RF close to the round trip frequency. This result is interpreted as a new form of active mode-locking with a fast saturable gain. The transient behavior of the device is studied by time resolved spectroscopy.