Active Modelocking Improvement of MIR-QCL by Integrating Graphene as a Saturable Absorber

The active mode-locking technique of quantum cascade lasers is highly affected by the bias current level and the magnitude of the modulated signal because of the spatial hole burning effect. The laser generates a stable pulse train near the threshold current and for a high modulation magnitude. We show in this paper an improvement of these conditions with an integration of a single-layer graphene which works as a saturable absorber. The light-matter interaction in the laser active region is described by the two-level Maxwell-Bloch equations and in graphene layer by the Maxwell-Ampere equation and Maxwell-Bloch equations. These system equations are solved using the Finite-Difference Time-Domain (FDTD) method which has the main advantage of not involving any approximation. The weakly coupled splitting scheme is adopted for the time discretization of Maxwell and Bloch equations. The graphene saturable absorption is modelled by a saturable conductivity in Maxwell-Ampere equation and with the dipole moment and carrier density in Maxwell-Bloch equations. For a QCL structure with a gain recovery time of 50 ps, simulation results show better stability of the mode-locking over a broad range of injection current and for smaller magnitude of the modulated signal. QCL structure with gain recovery time close to 1 ps reveals the generation of two pulses per roundtrip linked to spatial hole burning. The carrier grating strength can be reduced by changing the boundary conditions of the cavity such as with a high reflectivity mirror in front of graphene layer.

I. Introduction

Quantum cascade lasers (QCL) are based on quantum wells engineering to build radiative transitions between two energy levels within the same band. The operating wavelengths can cover a wide spectral band from mid-infrared to far-infrared regions including the terahertz one. Cascading several quantum wells leads to the generation of several photons from a single electron [1]. Different QCL structures were proposed and one technology has undergone significant advances such as the fabrication of a room temperature mid-infrared (MIR) QCL emitting watt-level power [2]. However, mode-locking in QCLs remains one of the biggest challenges because of the fast gain recovery time on the order of 1 to 50 ps compared to the roundtrip time of around 40–60 ps typically for 1–3 mm cavity length. This can affect the stability of mode-locked pulses because the gain can be saturated before the pulse completes a roundtrip. Actively mode-locked QCL was achieved with the applied DC current close to the laser threshold in the MIR region and through an active gain modulation which is one of the most efficient method used until now for the generation of ultrashort pulses [3], [4]. However, the active mode-locking is very sensitive to the spatial hole burning (SHB) effect which is favoured by the QCL physical dynamics in Fabry-Perot cavity. A necessary design modification of the QCL structure wells was necessary to increase the excited level lifetime around 50 ps limited thus the mode-locked operation to cryogenic temperatures [3]. Recently, the generation of short pulses has been investigated with other methods. In [5], active mode-locking of a QCL in an external ring cavity was reported. The effect of SHB is mitigated and an external modulation is applied on the entire QCL instead of a short section. Another approach concerns the use of frequency comb QCL with a system compensation of the linear chirp induced by the dispersion of the group delay [6], [7]. Dispersion compensation is achieved with an external grating compensator and results in relatively long pulse duration because of the non-linear chirp. The synchronization states in frequency comb QCL was investigated with an optimal modulation frequency [8]. This combined at the same time in-phase (AM modulation) and anti-phase (FM modulation) synchronizations in bi-functional quantum designs adjusted by the modulation frequency.