Measuring the whispering gallery mode volume of a CaF2 disk resonator using half-tapered fibers

We experimentally measure the axial field profile of an optical whispering gallery mode in a CaF2 disk resonator. We use half of a tapered fiber to couple light into the resonator and the other half to sense the evanescent field at the surface of the resonator. This sensing half taper is scanned axially to measure the axial profile of the field. We also numerically calculate the mode volume, and show good agreement between the numerical calculations and experimental measurements. The numerical calculations are enabled by the measurements of the surface axial profile of the disk resonator, which is performed using white-light interferometry.