Free-space coupling efficiency in a high-Q deformed optical microcavity.

The free-space coupling technique provides a promising means to excite high-Q whispering gallery modes in deformed microcavities, but the precise quantification of the coupling efficiency remains challenging because of the non-Lorentzian spectral lineshape in the transmission and the partial collection in emission. Here, we experimentally identify the free-space coupling efficiency by measuring the threshold of stimulated Raman scattering in a slightly deformed microcavity. The measured efficiency is up to 30%. Furthermore, the dependence of the coupling efficiency on the incident angle is obtained by focusing the laser beam on the microcavity periphery, which is consistent with the prediction of the mode field distribution. Finally, it is experimentally demonstrated that free-space coupling efficiencies remain high even when the focusing beam has been translated several micrometers, both horizontally and vertically.

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