Figure 2.

(a) A view from underneath the dome of St. Paul’s Cathedral in London. The gallery underneath the dome is an example of an acoustic whispering gallery. Whispering gallery mode optical resonators were named after the acoustic whispering galleries as follows a similar principle, but with electromagnetic instead of acoustic waves. Photo by David Iliff. License: CC BY-SA3.0. (b) Photograph of light orbiting within a microsphere optical resonator. The sphere has been doped with erbium and is approximately 70 µm in diameter. Light has been evanescently coupled into the microsphere using an optical fiber. Reprinted by permission from Macmillan Publishers Ltd.: [7], copyright (2003).