Whispering Gallery Mode (WGM) silica microresonators are a particularly unique group of microcavities in the sense that they can confine light inside the device for an extended period of time while maintaining a high quality (Q) factor due to the total internal reflection. As a result, WGM resonators have high circulating optical power, which can cause nonlinear optical processes such as stimulated Raman scattering (SRS). It has been demonstrated that SRS has been observed in various WGM silica microresonators with the sub-mW Raman lasing threshold. However, in case of the Raman lasing efficiency, it is limited by the intrinsic property of silica itself, which is the Raman gain coefficient. Therefore, in the present work, we introduce a hybrid silica toroidal microcavity in order to enhance the Raman lasing efficiency. First, we synthesize a suite of silica sol-gels doped with a range of Zirconium (Zr) concentrations and integrate the material with silica toroidal microresonator. The intrinsic Raman gain of the Zr-doped silica is measured using Raman spectroscopy, and the values show a clear dependence on Zr dopant concentrations. The lasing performance is characterized using a 765 nm pump source, and the Raman emissions for the coated devices are detected at 790 nm and longer. The lasing emission and characteristic threshold curves are quantified using both an optical spectrum analyzer and an optical spectrograph. The lasing slope efficiency of exhibits a marked increase from 3.37% to 47.43% as the Zr concentration increases due to the Raman gain improvement. These values are particularly notable as they are the unidirectional, not bidirectional, lasing efficiencies.
Hyungwoo Choi and Andrea M. Armani, "Impact of zirconium dopants on the lasing efficiency of Raman microcavity laser," Proc. SPIE 10090, Laser Resonators, Microresonators, and Beam Control XIX, 1009017 (Presented at SPIE LASE: February 01, 2017; Published: 27 February 2017); https://doi.org/10.1117/12.2249687.
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