Photodarkening
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Photodarkening is an optical effect observed in the interaction of laser radiation with amorphous media (glasses) in optical fibers. Until the 2000s, such creation of color centers was reported only in glass fibers.[1][2] Photodarkening limits the density of excitations in fiber lasers and amplifiers. The experimental results suggest that operating at a saturated regime helps to reduce photodarkening.[3]
One could expect the term photodarkening to refer to any process when any object becomes non-transparent (dark) due to illumination with light. Formally, the darkening of the photo-emulsion also could be considered as photodarkening. However, recent papers use this term meaning reversible creation of absorbing color centers in optical fibers. One may expect that the effect is not specific for fibers; therefore, the definition should cover wide class of phenomena, excluding, perhaps, non-reversible darkening of photographic emulsions.[citation needed]
According to the Encyclopedia of Laser Physics and Technology,[4] photodarkening is the effect that the optical losses in a medium can grow when the medium is irradiated with light at certain wavelengths. Photodarkening can also be defined as reversible creation of absorption centers in optical media at the illumination with light.
Rate
The inverse of the timescale at which photodarkening occurs can be interpreted as photodarkening rate.[2]