Femtosecond laser-induced glass refractive index change in tellurite glass

Tellurite glass has excellent infrared optical properties and is widely used in lasers, optical communications, and sensing. Femtosecond laser direct writing is a preferred method for manufacturing tellurite photonic devices, which requires precise control of the variable of refractive index. However, accurate testing of the refractive index changes induced by femtosecond laser in tellurite glass is challenging due to the small magnitude of change and localized region. In this study, we present a novel approach to measure the refractive index change of tellurite glass through the utilization of volume phase grating (VPG) diffraction. We first investigated the laserinduced damage threshold (LIDT) of tellurite glass and then proceeded to prepare VPGs inside the glass by using femtosecond laser direct writing technology, ensuring that the laser energy remained below the LIDT. By measuring the thickness and diffraction efficiency of VPGs prepared under different laser energies, we determined the associated change in refractive index. In our experiment, when subjected to laser incidence with wavelengths of 632.8 and 808 nm, the maximum values of refractive index change in the laser-modified region are measured to be 1.97 x 10-3 and 1.61 x 10-3, respectively. This method offers a precise means of measuring refractive index change within the microregion of tellurite glass induced by femtosecond laser. This capability holds great significance for the fabrication of high-quality tellurite glass photonic devices using lithographybased techniques.