Amplified Spontaneous Emission
Atoms in an excited state will spontaneously emit photons when they fall back to their lower, or ground state. If the active medium is pumped hard, then population inversion will occur, and a high degree of spontaneous emission will occur, which is in effect amplified over that seen at lower pumping levels. In this scheme the wavelength of the output is allowed to run free, and amplified spontaneous emission (ASE) sources often emit across the entire emission waveband of the active medium. A length of erbium doped fiber can be used as an active medium to produce a wideband incoherent ASE light source in the 1530-1565nm wavelength region.