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 108. 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.

: All LP modes of a few mode fiber are simultaneously characterized using phase-sensitive optical low-coherence interferometry. The differential modal group delay and absolute chromatic dispersion values of each mode are retrieved from a single measurement without spatial mode transformers.

et al.. Microcavity-quality-factor enhancement using nonlinear effects close to the bistability threshold and coherent population oscillations. Physical Review A, American Physical Society, 2012, 85 (6), pp.063824. 10.1103 Microcavity quality factor enhancement using nonlinear effects close to the bistability threshold and coherent population oscillations We analytically show that inserting a driven two level system inside a microcavity can improve its optical properties. In this approach, the strong dispersion induced by a pump via population oscillations increases the cavity lifetime experienced by a slightly detuned probe. We further predict that if the cavity is pumped through a resonant channel, optical absorptive or dispersive bistability can be combined with the population oscillation induced steep material dispersion to obtain a strong quality factor enhancement. Moreover differential amplification coming from the nonlinear feature of the pump transfer function can be used to drastically increase the probe transmission beyond intrinsic characteristics of the resonator. The Q-factor enhancement and the differential amplification can be advantageously combined with a frequency pulling effect to stabilize or readjust the microcavity resonance frequency.