Permanent index changes in Ge–SiO 2 glasses by excimer laser irradiation

Abstract

Ultraviolet (uv)-induced photochemical reactions related to Bragg gratings in Ge-doped SiO 2 glasses were investigated. Pre-existing oxygen monovacancy-type defects in GeO 2–SiO 2 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 GeO 2–SiO 2 thin glass films prepared by the sputtering method. A positive index change of ∼1% was attained in the film of Ge/(Si+Ge)<0.3 by the excimer laser irradiation. In the region of Ge/(Si+Ge)≧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.