Optical amplifiers involving single crystal waveguides

A device suitable for amplifying an optical signal at a desired wavelength, said device comprising means for introducing said signal, means for introducing optical power for said amplification, output means, and a region suitable for guiding said signal and said optical power characterized in that said region comprises 1) a single crystal core including a dopant and 2) adjacent cladding of lower refractive index than said core to provide said guiding, wherein said dopant is an atom in an oxidation state having an electronic structure that interacts with said single crystal material to provide amplification at said wavelength.