Modeling Dispersion and Cavity Engineering Effects on Mode-Locked Quantum Cascade Laser Operation

Quantum cascade lasers (QCLs) are a key building block in molecular spectroscopy, sensing, communications, and quantum technologies in the mid-infrared and terahertz wavelength region [1]. The lasing action involves intersubband transitions, and the center frequency can be custom-tailored by the targeted design of the quantum well heterostructure. The emerging gain properties (maximum, bandwidth, etc.) have been extensively studied and optimized [2], and their dynamics can even lead to frequency comb operation [3]. Apart from self-generated combs, which mostly exhibit frequency modulation, pulsed operation can be achieved by active and in the THz range also by passive mode-locking techniques [4], [5]. While for the passive case a distributed saturable absorber model accounts for the crucial dynamics [6], the active modulation can be modeled by the addition of a co-propagating microwave pump modulation to the Maxwell-Bloch equations [7]. Once the fundamental mechanisms of pulse formation and stability are understood and correctly modeled, the influence of the waveguide on the pulse shape can be investigated. In our contribution, we will discuss advanced modeling techniques to achieve favorable comb and pulse properties, e.g. in terms of spectral bandwidth, pulse duration, or maximum intensity.