Ultrafast Laser Direct Writing of Diffraction Gratings in PVA/AA Photopolymer with Tunable Phase Modulation Mechanisms.

Diffractive optical elements (DOEs) are essential components in optical and photonic technologies, motivating the need for rapid mass production with high-precision fabrication methods. Photopolymers are particularly attractive for DOEs production because their optical phase can be modified via both surface and refractive index modulations. In this work, we report the fabrication of DOEs on PVA/AA photopolymers using ultrafast laser direct writing. By combining surface topography measurements with a phase depth model, we extract the surface and bulk phase depth contributions and demonstrate a transition from surface-dominated modulation at 1 kHz to bulk refractive index modification under cumulative conditions at 100 kHz. These outcomes highlight ultrafast laser direct writing as a powerful, rapid and controlled method for the high-quality and rapid fabrication of DOEs.