Crystallization behaviors with pre-treatments of thermal annealing and ultraviolet laser irradiation in Ge-doped SiO₂ glass fiber preforms

Crystallization behaviors in Ge-doped SiO₂ fiber preforms fabricated by vapor-phase axial deposition (VAD) method have been investigated. Since X-ray diffraction (XRD) peaks at around 2θ = 22° in the crystallized glasses are reported to be strongly related with the induced second- and third-order optical nonlinear effects in poled glasses, enhancement of such crystalline phases is quite important and useful for fabrication of non-linear functional glasses. It has been found that crystallization behaviors are drastically affected by thermal annealing and ultraviolet (UV) laser irradiation performed prior to heat-treatments for crystallization. In particular, it has been clarified that a crystalline phase, which is known to be highly effective for increase of induced optical nonlinearity, can be increased by pre-treatments of UV-laser irradiation at λ = 193 nm wavelength. These results suggest that modification of initial defect states in Ge-doped SiO₂ fiber preforms could lead to fabricate crystallized glasses with optical nonlinearity induced by poling and/or crystallization.