Precision investigation of phase noise dynamics for fiber Brillouin laser

We introduce what we believe to be a novel architecture to suppress cavity resonance noise via common-mode rejection, enabling direct quantification of fundamental quantum noise and pump-induced noise. By generating primary and secondary Brillouin lasers within the same ultrahigh-Q fiber ring cavity, we conducted systematic experimental verification of the linewidth narrowing effect and the fundamental Schawlow-Townes linewidth (i.e., quantum noise) feature of Brillouin lasers generated in ultrahigh quality factor fiber cavity. Theoretical models align with observed 1/f2 phase noise scaling, confirming the inverse dependence of noise power on Brillouin laser output. This approach enables precise characterization of Brillouin laser intrinsic linewidths at the microhertz scale and large suppression ratio of the linewidth narrowing process, paving the way for high-coherence lasers in precision metrology.