An “in-fiber” Whispering-Gallery-Mode bi-sphere resonator, sensitive to nanometric displacements

A double-cavity, Whispering-Gallery-Mode (WGM) resonator, composed of two polystyrene microspheres, is formed inside the capillary of a microstructured optical fiber. The spectra are compared with the corresponding WGM patterns of the isolated microspheres. We collect scattering spectra from the contact area between the two beads, as the spacing between them is varied, and the evolution of the modes with sphere separation is discussed. Potential applications of the presented optical design are also proposed.