Efficient excitation and phase matching of fiber-coupled degenerate whispering gallery modes

Abstract

The ability of optical whispering gallery mode (WGM) resonators to store energy in a very small volume for a long time enables their use in practical applications as low-threshold lasers, high-sensitivity sensors, and tunable add-drop filters. In this paper, we express in detail the tapered fiber asymmetric coupling mechanism to multimode microsphere resonators in order to selectively excite or collect WGMs. It is shown that due to the close effective refractive indices of degenerate spherical WGMs, depending on the amount of phase matching and overlap of fiber and resonator modes, a combination of WGMs with certain intrinsic losses is excited. By precisely designing the taper dimensions, critical coupling to one or a group of modes can be satisfied. This paper provides a general figure of merit for designing taper-coupled microresonators that can be employed in nonlinear optics and lasers in which the intra-cavity field distribution determines the characteristics of the device.