Fig. 2: Mode-locked pulses from an 8 μm wavelength QCL.
From: Mode-locked short pulses from an 8 μm wavelength semiconductor laser
a SWIFTS characterization of the QCL operated close to lasing threshold. The laser is modulated at the in-phase synchronization frequency and at 37 dBm power level. b Reconstructed time-domain signal of the QCL showing a train of 6.5 ps short pulses, which is 2.5 ps longer than the shortest pulses generated in THz QCLs25. c Simulation of the QCL using the coherent Master equation described in Supplementary Section 1. d Interferometric autocorrelation (IAC) of the QCL pulses close to threshold. Red dots: envelope of the IAC reconstructed using SWIFTS. e IAC at higher current. f IAC at the rollover current, still displaying the 8:1 ratio. The second burst at a delay equal to the cavity roundtrip time is due to the interference of subsequently emitted pulses. Its peak value of 8 provides another proof for the coherence of the pulses because phase-decoherence would smear out the fringes of the IAC and thus decrease the peak value to smaller than 8. Inset: zoom on the interferometric fringes. The DC bias applied to the modulation section was between 3 and 4 V for all measurements shown in this figure.
