Nonlinear optical crystals Sr2TiSi2O8 are written arbitrarily in a stoichiometric SrO-TiO2-SiO2 glass by using a high repetition rate femtosecond laser. The mechanism of nucleation and subsequent crystal growth is assumed to the thermal accumulation effect and the laser-induced reduction of Ti4+ to Ti3+. Additionally, the threshold time for inducing crystal formation decreases, while the corresponding dot size increases with increasing laser power. The threshold variation is also discussed according to the mechanism of nucleation. We suggest that this technique can be used in light-wave switching, 3D optical memory and laser display. (C) 2007 Published by Elsevier B.V.