Quasicritical coupling in a few-mode tapered-fiber coupled whispering-gallery-mode system

A quasicritical coupling state was demonstrated both in theory and experiment with a few-mode tapered-fiber (TF) coupled whispering-gallery-mode (WGM) cavity. In such a configuration, the coupling mode to activate the WGM and the drop mode are in the same TF and of almost the same decay rate with the variation of the gap between the TF and WGM cavity. Consequently, when the gap decreases to zero, the transmission of the coupling mode decreases monotonously to zero at the resonance wavelength, while the transmission of the drop mode increases monotonously to unit, which means that neither the coupling efficiency in the coupling mode nor the transmission of the drop mode will decrease in the traditional overcoupling region. Thus a steadily high coupling efficiency can be achieved accordingly, which we defined as quasicritical coupling. Such a coupling state will be helpful to develop WGM-cavity-based filters and benefit the fabrication of ultranarrow-linewidth lasers.