High-gain wavelength-selective amplification and cavity ring down spectroscopy in a fluoride glass erbium-doped microsphere.

We experimentally demonstrate a compact optical amplifier consisting of a rare-earth-doped whispering-gallery-mode microsphere coupled via a tapered fiber. A gain up to 20 dB is reported in an erbium-doped fluoride glass microsphere 135 µm in diameter. Below the amplification regime, the optical gain is used to compensate for unavoidable losses due to surface contamination or scattering. Quality factor as high as 2×10(9) has been measured by analyzing the transient response of the microsphere excited by a dynamically shifted frequency input signal.