Observation of Self Mode-Locking in Terahertz Quantum Cascade Lasers through Intra-Cavity Self-Detection

Abstract

We report an experimental study of terahertz (THz) quantum cascade lasers (QCLs) exploiting an intra-cavity self-detection process, where we demonstrate the generation of trains of self-mode-locked pulses. The detection technique is based on a well-known property of these devices, namely the fact that their gain dynamics unfolds on an ultra-fast time scale, in the order of tens of ps (for THz QCLs), determined by intersubband relaxation [1]. The resulting modulation of the population inversion can therefore be observed as a function of time over very large bandwidths, in the ∼10–100GHz range, simply by monitoring the QCL current [2]. To fully exploit this possibility, in this work we processed an exceptionally long single-plasmon ridge waveguide of 15mm, based on an ultra-low threshold, 4.2THz active-region design [3,4]. The resulting multimode emission spectrum consists of ∼60 longitudinal modes with a free spectral range (FSR) of ∼2.4GHz. The QCL was driven in free-running in continuous wave (CW) at a temperature of 30K, and we monitored the ac current gain-induced modulation with the help of a microwave probe connected to a real-time oscilloscope with and overall 3-dB measurement-system bandwidth >45GHz [2]. In Fig.1(a) we report an example of measured time-trace, showing a train of ∼85-ps long pulses separated by T = 205ps =1/(2×FSR) (not corrected by the measurement-system frequency response). The corresponding Fourier transform on the right shows that at least up to 25 modes (∼60GHz/2.4GHz) are coherently coupled and participate to the pulse train. This number is limited by the bandwidth of our system. Another striking finding is that the inter-mode beatings in Fig.1(b) are extremely narrow (<30kHz at 4.8GHz), leading to a very stable measured pulse train over time scales of tens of μs (not shown), a clear signature of self-mode locking operation. Despite several demonstrations of spontaneous inter-mode beat-note narrowing and active modelocking [3–6], to the best of our knowledge this is the first time that self-mode-locked pulses are reported for a THz QCL. This finding contradicts the presently established theory according to which an unfavourable ratio “gain-recovery time/roundtrip time” prevents spontaneous mode-locking in QCLs (although this conclusion is somewhat relaxed for THz QCLs) [7]. Depending on the drive current, the regime of stable self-mode locking with two pulses per roundtrip (Fig.1), alternates with wider current regions characterised by a more unstable regime, with one pulse per roundtrip, and more pronounced amplitude noise and timing jitter. A complete analysis of the laser dynamics vs drive current will be given, allowing to discriminate between amplitude and phase noise.