Photodarkening is the effect that the optical losses in a medium can grow when the medium is irradiated with light at certain wavelengths.
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It's of particular significance in systems which use lasers to transmit information along optical fibres. The super-transparent fibres can gradually darken when exposed to light - particularly at short wavelengths. This can (eventually) compromise the efficiency of fibre-optic systems (and also high-powered laser welding equipment).

The phenomenon has not yet been explained.

Further details International Society for Optics and Photonics