Fabrication of multilevel reflective diffractive optical elements by means of laser ablation lithography

Abstract

Multilevel Diffractive Optical Elements for high-power laser beam shaping have been designed and produced. The originality of the proposed approach rests on the realization of four phase levels reflection elements with a single etching step. Thus, if the phase change of 270° is obtained by etching a pixel element, intermediary phase difference is obtained by etching subwavelength structures of the same height with appropriate filling factor. Sizing of the subwavelength gratings required with this theory has been carry out for TE and TM polarized radiation with a rigorous electromagnetic model, the Finite Difference Time Domain method. In a first step, a test component has been realized using photolithography to validate the sizing of the subwavelength gratings. In a second step we have adapted a specific fabrication tool based on laser ablation and direct writing for greater flexibility. Characterization has been carried out with Coherence Probe Microscopy. The technique has been applied to the fabrication of a diffractive element used with a high-power CO2 laser beam for surface marking.