Investigation of type I modifications in fused silica for recording phase diffractive optical elements by femtosecond laser

Abstract

In this paper we demonstrate the application of direct laser writing technology for writing phase diffractive optical elements under a glass surface. This technology is based on writing refractive index change modifications in volume of transparent materials. Different parameters were studied to determine the type of modifications: pulse repetition rate, pulse energy, pulse density, focusing depth and polarization. The modulation of the refractive index change of these modifications was investigated by using two different methods: Bragg gratings method and Michelson interferometer method. Further development was made to form 1D and 2D gratings and even more difficult elements such as Fresnel lens. It has been observed that the diffraction efficiency of the Bragg gratings was up to 97 %, ∼ 96 % for 1D gratings, 75 % for 2D gratings and 64 % for the Fresnel lens.