Mode-locking System of Coupled Microcavities with Gain and Nonlinear Loss

Ultrahigh repetition rate lasers are vital light sources, and a mode-locked laser with whispering-gallery-mode (WGM) microresonators would provide a cost-effective on-chip solution. We have already reported numerically on the mode-locking in an erbium (Er) ion-doped resonator that was coated with polymer-dispersed carbon nanotubes (CNTs) [1]. Although mode-locking is possible, the required
$Q$
is still high, so the experiment remains challenging. In this work, we show that a coupled microresonator system with a gain resonator coupled to a nonlinear loss resonator will solve this problem. It is based on the idea of employing the
$Q$
revival of the gain cavity when the loss is increased at the cavity with additional loss, which is known to exist in the vicinity of the exceptional point [2].