Characterization techniques for planar optical microresonators

Optical microresonators that are coupled to optical waveguides often behave quite similar to Fabry-Perot resonators. After summarising key properties of such resonators, three characterization methods will be discussed. The first involves the analysis of transmission and reflection spectra, from which important parameters like (waveguide) loss and coupling or reflection coefficients can be extracted. The second method is called transmission-based scanning near-field optical microscopy (T-SNOM) which allows to map out the intensity distribution inside a high-Q resonator with subwavelength resolution. For such resonators conventional SNOM suffers from inaccuracies introduced by the disturbing effect of the presence of the probe on the field distribution. T-SNOM avoids this problem by exploiting this disturbing effect. The third method is most applicable to large-size (many wavelengths across) resonators. It involves simultaneous analysis of scattered light and transmission/reflection spectra in order to relate spectral features to large-scale field distributions inside the resonator. Together, these techniques form a convenient toolbox for characterizing many different planar optical microresonators.