Characterization of Refractive Index Change Induced by Femtosecond Laser in Lithium Niobate

When femtosecond laser is focused into optical materials, their refractive index can be changed. Then various important optical elements can be fabricated by this technology. Herein, we report a simple and practical method to measure the refractive index changes induced by femtosecond laser in optical materials. By fabricating the gratings in a Z-cut LiNbO3 crystal and measuring the ratio of grating's different diffraction orders' energy, the refractive index change (3.4x10(-3)) in LiNbO3 crystal by femtosecond laser irradiation was obtained. This approach represents a simple way for measuring the refractive index change induced by femtosecond laser in a variety of transparent materials like sapphire, quartz and glass etc. In addition, this method would find the great technological importance in fabrication of laser written refractive index structures such as lens, DOEs and other optical structures.