Excitation of whispering gallery mode resonances by a phase-matched fiber taper

The values of propagation constants of the mode in a fiber taper and for the lowest radial mode of whispering gallery mode (WGM) resonance in silica microspheres with different sizes are theoretically calculated. Using phase-matching relation, silica microsphere diameter required to fiber tapers with different diameters is determinated. Then to match the propagation constant of the mode in the fiber taper with low loss and waist diameter about 2.8 μm, a silica microsphere with diameter 143.1 μm and good shape is chosen, and the taper-microsphere coupling system is set up for excitation of the lowest radial mode of WGM resonance. By injecting the laser power into the fiber taper, a transmission spectrum with narrow absorption peak and equal peak gaps is observed, and the absorption peak position is consistent with the lowest radial mode of WGM resonance position in the microsphere calculated by using the Mie theory. Based on the transmission spectrum, a high value of quality factor Q as 6 × 10⁴ is measured in our taper-microsphere system.