Permanent index changes in Ge-SiO2 glasses by excimer laser irradiation

Ultraviolet (uv)-induced photochemical reactions related to Bragg gratings in Ge-doped SiO2 glasses were investigated. Pre-existing oxygen monovacancy-type defects in GeO2-SiO2 optical fiber preform were changed to GeE' centers via one-photon absorption process by illumination with an Hg lamp (4.9 eV, 16 mW cm(-2)). The irradiation with KrF or ArF excimer laser pulses (> 500 MW cm(-2) pulse(-1)) induced electron trapped centers associated with fourfold coordinated Ge ion (GEC). Concentration of GEC increased as the square of the laser power (two-photon absorption process). Intense uv absorptions due to GEC's might be related to the positive index change responsible for the formation of fiber gratings. Similar uv-induced photochemical reaction could be observed in GeO2-SiO2 thin glass films prepared by the sputtering method. A positive index change of similar to 1% was attained in the film of Ge/(Si + Ge) < 0.3 by the excimer laser irradiation. In the region of Ge/(Si + Ge) greater than or equal to 0.3, on the contrary, a negative index change of > 3% was caused by the irradiation accompanied with the positive volume change of > 18%. Bragg gratings with a surface relief pattern have been written upon the latter films by the irradiation through a phase mask. (C) 1998 Elsevier Science S.A. All rights reserved.