Abstract: An apparatus and method for thermally tuning an optical amplifier comprises an optical waveguide doped with a fluorescent material, a thermal device for either heating or cooling the optical waveguide, and a pump light for exciting the fluorescent material. The apparatus shapes, shifts, and/or flattens the gain curves of the doped optical amplifier. Thulium doped fiber is cooled to shift the gain curve into the C-band. Erbium doped fiber is heated to flatten the gain curve in the C-band and is cooled to shift the gain curve above the L-band. The apparatus similarly shapes the gain curves of other fluorescent materials. The thermal device comprises three types of optical cooling devices. The apparatus is a component in communications systems, lasers, medical lasers and the like. The method comprises either heating or cooling optical waveguides doped with fluorescent materials to achieve the desired shaping, shifting, and flattening of the gain curves.