An optical setup comprising an external Fabry-Pérot (FP) cavity that maintains ultrashort pulse spatial and temporal coherence, is proposed to generate continuous sub-terahertz repetition rate ultrashort pulses (USP). Analytical results from different sets of pulse-to-pulse carrier-envelop phase offset (Δ CEO) output conditions manifest the possibility to extract and utilize the reflected USP signal from an ultrastable Pound-Drever-Hall phase-locked external FP cavity to stabilize or reset the Δ CEO to 0 or multiple of 2π rad in real time. The generation of a steady, ultrahigh repetition rate output, from a low repetition rate ultrashort pulse laser requires management of carrier-envelope phase (CEP). The CEP management with the passive external FP cavity group delay dispersion control enables transition from usual quasi-to continuous repetition rate. The net control and management of these parameters allow the passive external FP cavity multiplication factor to be increased above 200.
