HIGHLY NONLINEAR NEAR-RESONANT PHOTODARKENING IN A THULIUM-DOPED ALUMINOSILICATE GLASS-FIBER

Permanent photoinduced optical attenuation has been observed in Tm3+-d oped aluminosilicate glass fibers on exposure to near-resonance mode-l ocked 1064-nm radiation at 300 K. The rate of this darkening was obser ved to follow a 4.7 +/- 0.4 power dependence on the 1064-nm intensity. The result shows that absorption of infrared light of moderate intens ity can lead to photoionization in rare-earth-doped glass, provided th at a multiphoton stepwise excitation channel exists.