Controling the coupling properties of active ultrahigh-Q WGM microcavities from undercoupling to selective amplification

Abstract

Ultrahigh-quality (Q) factor microresonators have a lot of applications in the photonics domain ranging from low-threshold nonlinear optics to integrated optical sensors. Glass-based whispering gallery mode (WGM) microresonators are easy to produce by melting techniques, however they suffer from surface contamination which limits their long-term quality factor to a few 10 8. Here we show that an optical gain provided by erbium ions can compensate for residual losses. Moreover it is possible to control the coupling regime of an ultrahigh Q-factor three port microresonator from undercoupling to spectral selective amplification by changing the pumping rate. The optical characterization method is based on frequency-swept cavity-ring-down-spectroscopy. This method allows the transmission and dispersive properties of perfectly transparent microresonators and intrinsic finesses up to 4.0 × 107 to be measured. Finally we characterize a critically coupled fluoride glass WGM microresonator with a diameter of 220 μm and a loaded Q-factor of 5.3 × 109 is demonstrated.