Femtosecond-laser direct-writing volume phase gratings inside Ge–As–S chalcogenide glass

In this study, volume-phase gratings (VPGs) were fabricated inside Ge–As–S chalcogenide glasses (ChG) with femtosecond-laser writing technology. VPGs with high thicknesses were obtained through single-step writing by exploiting the self-focusing effect of highly nonlinear ChGs. The clear diffraction patterns and high diffraction efficiency of the prepared gratings indicated their high optical performance. Testing results showed that the diffraction efficiency of VPGs first increased with laser pulse energies and then decreased when the laser damage threshold of the glass reached 9.42 nJ. Grating thickness also exhibited the same trend as laser pulse energies and reached the maximum value of 28 μm at 9.42 nJ. The changes in the refractive indexes of volume gratings fabricated under different laser pulse energies were carefully investigated. The changes in the refractive indexes of the gratings remained at the same level while laser pulse energy was increased. Meanwhile, grating thickness played a key role in diffraction efficiency. The maximum diffraction efficiency of the prepared VPGs reached 50% at 808 nm under optimized laser parameters.