The cross polarization coupling (CPC) between orthogonally polarized modes in a single whispering-gallery microresonator can lead to electromagnetically induced transparency (EIT) like effects. Depending on the CPC strength, coupled mode induced transparency (CMIT), coupled mode induced attenuation (CMIA), or Autler-Townes splitting (ATS) can be observed. Previously, the values of CPC strength were found by fitting the experimental throughput spectra to a steady-state model. However, our dynamical analysis suggests an independent way of estimating the CPC strength by sinusoidal modulation at the input. From experimentally determined parameters, we first find one estimate of the CPC strength by model fitting as before. From the modulation frequency that gives the minimum throughput amplitude on resonance, we find another estimate of the CPC strength. Our preliminary experimental results show that the two values agree quite well, which means we have an independent way of finding the CPC strength.
Adiabatically tapered fibers are often used to excite whispering gallery modes (WGMs) of microresonators used as chemical sensors. Recently it was demonstrated that using a non-adiabatic tapered fiber can enhance refractive index sensing. The incoming light is distributed between fundamental and higher-order fiber modes, whereas only the fundamental mode is detected because the uptaper is adiabatic. The interference effect between these fiber modes when exciting a WGM leads to the sensitivity enhancement. We have shown theoretically that even greater enhancement is possible for absorption sensing. For a given WGM, the predicted enhancement can be calculated by measuring the throughput power when the two fiber modes are in and out of phase at the input. Enhancement can be confirmed by sending the light in the reverse direction through the asymmetrically tapered fiber so that only one fiber mode is incident on the microresonator. Using a carefully designed asymmetrically tapered fiber, we have demonstrated this enhancement in experiments using a hollow bottle resonator (HBR) with an internal analyte. Absorption in the analyte causes a change in the WGM throughput fractional dip depth; these changes were studied with varying analyte concentration for forward and reverse propagation to evaluate the absorption sensitivity. For both liquid and gaseous analytes, our measured sensitivity enhancements are not inconsistent with the predicted enhancements of at least a factor of 100.