Cascading-Induced Fundamental Linewidth Enhancement of a Microcavity Brillouin Laser

Stimulated Brillouin scattering (SBS) in microresonators provides a simple way to achieve chip-scale narrow-linewidth lasers. While theoretical studies suggest that the cascading effect enhances the fundamental linewidth of the first-order Brillouin laser, this phenomenon has yet to be experimentally validated. In this work, we investigate the fundamental linewidth behaviors of the Brillouin lasers in both the non-cascaded and cascaded cases using an ultrahigh-Q factor microrod resonator. In the non-cascaded case, we achieve a Brillouin laser with a 188 mHz fundamental linewidth without using an erbium-doped fiber amplifier. In the cascaded case, we observe, for the first time, that the cascading effect enhances the fundamental linewidth of the first-order Brillouin laser. These findings highlight the importance of mitigating or avoiding the cascading effect to achieve the narrowest possible linewidths of Brillouin lasers.