Densification involved in the UV-based photosensitivity of silicaglasses and optical fibers

A comprehensive survey of photosensitivity in silica glasses and optical fiber is reviewed. Recent work on understanding the mechanisms contributing to germanium or aluminum doped fiber photosensitivity is discussed within the framework of photoelastic densification models     

Nanoscale Phase Separation in Lithium Niobium Silicate Glass by Femtosecond Laser Irradiation

Understanding the phase transformation in glass and the morphology of related nanostructure after femtosecond laser irradiation is of great importance for fabricating functional optics, in which glass crystallization is involved to obtain nonlinear optical properties. We report on the crystallization inside lithium niobium silicate glass induced by fs laser irradiation. Energy‐dispersive X‐ray spectroscopy coupled to scanning transmission electron microscopy (STEM/EDS) and transmission electron microscopy confirm a nanoscale phase separation whereby LiNbO₃ crystals are embedded in lamella‐shaped frames of amorphous SiO₂. The obtained nanostructure may have applications in fabricating second‐order nonlinear optical devices.     

Fictive-Temperature Mapping in Highly Ge-Doped Multimode Optical Fibers

In this paper, a Fourier transform infrared focal-plane-array detector was used to image the "bond-stretching" vibration mode observed near sigma=1120 cm^-1 of highly Ge-doped graded-index multimode optical fibers (GI-MMFs). We show that the distribution of the peak wavenumber sigma is nonuniform across the core and cladding of the MMF, i.e., sigma is smaller in the core due to the Ge-doping. Next, as calibration curves between sigma and the fictive temperature T_f are not available in the literature for highly Ge-doped glasses (above 7w%), we have determined calibration curves from 1w% to 30w% in Ge. Then, we have applied these corrections in order to estimate, for the first time to our knowledge, the fictive-temperature distribution within multimode-fiber cross section. Using these curves, we show that T_f is higher at the fiber edges, presumably due to faster cooling. Furthermore, there is also a T_f increase in the center of the core (i.e., higher Ge content)     

Systematic Control of Structural Changes in GeO2 Glass Induced by Femtosecond Laser Direct Writing

We report the structural changes inside germania glass induced by femtosecond laser pulses. Inspection by polarization microscopy and secondary electron microscopy indicate that the periodic nanostructures consist of oxygen defects such as ODCs (oxygen deficient centers) and NBOHCs (nonbridging oxygen hole centers) for laser pulse energy less than 0.2 μJ. However, the glass network was dissociated and O₂ molecules were generated for laser pulse energy greater than 0.4 μJ. Two different structural changes, form‐birefringence and dissociation, were induced in GeO₂ glass, depending on the laser pulse energy. The form‐birefringence exhibited by the nanogratings in GeO₂ glass is larger than that in SiO₂ glass for pulse energy less than 0.2 μJ, as the density of nanovoids enclosed by ODCs in GeO₂ glass is higher than that in SiO₂ glass. Arrhenius plots of the phase retardation caused by the nanogratings in GeO₂ and SiO₂ indicate that the oxygen defects are relaxed at a temperature 100°C above the glass‐transition temperature.     

Femtosecond laser direct writing in SiO2-Al2O3 binary glasses and thermal stability of Type II permanent modifications

We investigate the potential of fabricating thermally stable refractive index contrasts using femtosecond (fs) near-infrared (IR) radiation in aluminosilicate glasses. A set of pure SiO₂-Al₂O₃ glasses are manufactured, characterized (density and Raman), and investigated after being irradiated by fs laser within the Type II regime. The formation of nanogratings is identified and studied using quantitative birefringence measurements. Their thermal stability is then investigated through 30 minutes step isochronal annealing (up to 1250°C). For both SiO₂ and 50SiO₂-50Al₂O₃ compositions, the normalized birefringence does not decrease when tested up to 1100°C, while for the 4,6 mol% GeO₂-SiO₂ erased for 20% at 1000°C.     

Ultrashort pulse laser processing of silica at high repetition rates—from network change to residual strain

We report on the analysis of silica-based glasses after processing with ultrashort laser pulses at high repetition rates. Heat accumulation leads to strong local heating of the glass. The subsequent quenching results in a fictive temperature rise that scales with the repetition rate. Consequently, the relative volume change leads to residual tensile strain within the modified volume of larger than 10⁻³, which is confirmed by wide-angle X-ray scattering (WAXS) measurements. Studying the surface topography after cleaving of laser-modified regions allows for quantification of the corresponding elastic strain as well as the glass density behavior on the fictive temperature.