Whispering-gallery modes in glass microspheres: optimization of pumping in a modified confocal microscope

Whispering-gallery modes (WGMs) on Nd3+-doped glass microspheres with a radius of ～15?μm were measured in a modified confocal microscope, where a dual spatial resolution in both excitation and detection zones was possible. As an alternative to the standard excitation mechanism by an evanescent wave, we used an efficient pumping/detecting scheme, focusing a laser in the microsphere and exciting the Nd3+ ions, whose fluorescent emission produces the WGMs. We have also measured the generated WGMs by changing the detection zone, where higher amplitude resonances were found when exciting in the center and detecting at the edge of the microsphere.