Amplified spontaneous emission

Amplified spontaneous emission (ASE) or superluminescence is light, produced by spontaneous emission, that has been optically amplified by the process of stimulated emission in a gain medium. It is inherent in the field of random lasers.

ASE is produced when a laser gain medium is pumped to produce a population inversion. Feedback of the ASE by the laser's optical cavity may produce laser operation if the lasing threshold is reached. Excess ASE is an unwanted effect in lasers, since it is not coherent, and limits the maximum gain that can be achieved in the gain medium. ASE creates serious problems in any laser with high gain and/or large size. In this case, a mechanism to absorb or extract the incoherent ASE must be provided, otherwise the excitation of the gain medium will be depleted by the incoherent ASE rather than by the desired coherent laser radiation. ASE is especially problematic in lasers with short and wide optical cavities, such as disk lasers (active mirrors).

ASE can also be a desirable effect, finding use in broadband light sources. If the cavity has no optical feedback, lasing will be inhibited, resulting in a broad emission bandwidth due the bandwidth of the gain medium. This results in low temporal coherence, offering reduced speckle noise when compared with a laser. Spatial coherence can be high, however, allowing for tight focusing of the radiation. These characteristics make such sources useful for fiber optic systems and optical coherence tomography. Examples of such sources include superluminescent diodes and doped fiber amplifiers.

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