Optical amplifier "erbium-doped waveguide amplifier" ("EDWA") is an optical amplifier that uses a waveguide to boost an optical signal. Amplification is achieved by stimulated emission of photons from dopant ions in the doped fiber. The pump laser excites ions into a higher energy from where they can decay via stimulated emission of a photon at the signal wavelength back to a lower energy level. The excited ions can also decay spontaneously (spontaneous emission) or even through nonradiative processes involving interactions with phonons of the glass matrix. These last two decay mechanisms compete with stimulated emission reducing the efficiency of light amplification. The
Sound amplification by stimulated emission of radiation of direct semiconductors. The reasoning behind that comes from the fact that, due to the k-selection rule in semiconductors, interband transitions with the production of only one phonon can be only those that produce an optical phonon. However, optical phonons have a short life-time (they split into two due to anharmonicity) and therefore they add some important complications. Here we can note that even in the case of multi-stage process of acoustic phonon creation it is possible to create SASER. Examples of narrow-gap indirect semiconductors that can be used are chalcogenides PbTe, PbSe and PbS with energy gap 0.15 –