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 degrees C). For both SiO2 and 50SiO(2)-50Al(2)O(3) compositions, the normalized birefringence does not decrease when tested up to 1100 degrees C, while for the 4,6 mol% GeO2-SiO2 erased for 20% at 1000 degrees C.