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We have observed a new type of infrared-induced photodarkening in high-numerical-aperture fluoride fibers doped with 3000 and 10,000 parts in 106 by weight of Tm. The loss induced in the visible region by 1140-nm radiation is very strong (as high as 25 dB in a 50-cm piece) and broadband; it can be removed by irradiation with the same pump wavelength at lower powers.
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Barber et al. (1995) studied this question.
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