Stability Limits of Self-Induced Transparency Modelocking with Bi-Directional Propagation

Abstract

In recent work [1, 2], we demonstrated that quantum cascade lasers (QCLs) that have interleaved gain and absorbing periods can be modelocked using the self-induced transparency (SIT) effect, and that pulses on the order of the coherence time T₂ (∼ 100 fs) can thereby be obtained. The absorbing periods help shape the pulses and make the lasers stable by absorbing the continuous waves. An extensive analytical and computational study suggests that SIT modelocking is robust for QCLs [1, 2]. In prior analysis, we assumed uni-directional propagation of pulses, as is usually assumed in the modeling of modelocked lasers. However, in QCL cavities, pulses bounce back and forth between the edges and so propagate in both directions. Since QCLs have a very fast gain recovery time T₁, interference between forward and backward continuous traveling waves may create standing waves and hence spatial hole burning [3]. While this effect is not expected to be important for modelocked pulses, we expect that as the pulses bounce back from the edges, a portion of the cavity near the edges will not have recovered, which may affect the stability of SIT modelocking.