  Ultrahigh repetition rate lasers will become vital light sources for many
future technologies; however, their realization is challenging because the
cavity size must be minimized. Whispering-gallery-mode (WGM) microresonators
are attractive for this purpose since they allow the strong light-matter
interaction usually needed to enable mode-locking. However, the optimum
parameter ranges are entirely unknown since no experiments have yet been
conducted. Here, we numerically investigate pulsed operation in a toroidal WGM
microresonator with gain and saturable absorption (SA) to study the
experimental feasibility. We show that dispersion is the key parameter for
achieving passive mode-locking in this system. Moreover, the design guideline
provided in this work can apply to any small resonators with gain and SA and is
not limited to a specific cavity system.
