All-Optical Tuning of WGMs in Microspheres Made of Er/Yb Codoped Optical Fiber

All-optical, pump-assisted, thermal tuning of high-Q whispering-gallery mode (WGM) resonances of microspheres is demonstrated. The microspheres were made of an Er/Yb codoped single-mode silica optical fiber. The pump light absorbed by the active ions heats the microsphere, which leads to the shift of the resonances. A tuning range in excess of 2 nm in the C-band is achieved. The presence of active ions is restricted to the core of the microsphere. The WGM fields, which are confined in a region close to the surface of the microsphere, do not overlap with the dopant. Thus, high-Q resonances (>10⁷) are achieved, as in pure silica microspheres, even at optical wavelengths within the absorption bands. We explored the use of the microsphere as a photonic scanning receiver for RF signals.