A highly robust microsphere whispering-gallery-mode resonator in multimode fiber

We propose a robust whispering-gallery-mode resonator based on a microsphere embedded in the etched cavity at the multimode fiber end. Another section of slightly tapered multimode fiber with a flat end is inserted inside the cavity and in tight contact with the microsphere for fixing the position of the microsphere. The whole device is sheathed by a section of hollow fiber for environment protection. Such a device is compact, stable in operation and highly robust.