Gain-Induced Evolution of Mode Splitting Spectra in a High- $Q$ Active Microresonator

We investigate the mode splitting phenomenon in a fiber taper-coupled active microresonator excited below the lasing threshold. Effects of optical gain on the split modes are studied by monitoring the transmission spectra of the resonator at various excitation levels. We demonstrate Q-factor enhancement as well as amplification of the split modes by optical gain. It is observed that the responses of the split modes to the gain vary at different taper-resonator coupling conditions. Mode splitting may reflect itself as two dips, two peaks, or one dip and one peak in the transmission spectra depending on the coupling conditions, the interactions of the scatterers with the optical modes, and the levels of optical gain. Experimental results match well with the theoretical predictions. Our investigation shows that optical gain in the microresonator can be utilized not only to manipulate the mode splitting profiles but also to help improve the resolvability of the split modes.