Abstract

Recently, mode-locking and frequency comb formation of semiconductor lasers [1], in particular, quantum cascade lasers (QCL) was reported independently by several groups [2]. Subsequently, these claims were also extended to dissipative soliton formation [3] and the observation of passive CEP stability [4]. While these reports were highly controversially received by the ultrafast community, neither party in this controversy was able to deliver a convincing explanation for the reported experimental findings so far. Here we apply the proven Haus Master Equation (HME) approach [5] to a system with dispersion, Kerr nonlinearity, and fast gain saturation to explain the experimental observation of apparent mode-locking. Nevertheless, as this locking process only provides a frequency lock rather than a phase lock, applications of mode-locked QCLs in the time domain or the claim for solitons remain elusive.

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