Free-space coupled, ultralow-threshold Raman lasing from a silica microcavity

We report Raman lasing in a slightly deformed silica microcavity using an efficient free-space coupling technique, with an absorbed pump threshold as low as 100 μW in the 1550 nm wavelength band. This ultralow threshold originates from the ultrahigh quality factors of whispering gallery modes, exceeding 10⁸, even though the microcavity is deformed from a circular symmetry. Furthermore, the threshold of free-space coupled Raman lasing is experimentally demonstrated to show a strict inverse square dependence on the intrinsic quality factor of the cavity mode, which agrees well with the theoretical prediction.