High efficiency diffractive optical element design and fabrication in a two-stage photopolymer

We show the design and fabrication of high diffraction efficiency, optically recorded gradient-index Fresnel lenses in a two-stage photopolymer. A design analysis reveals that lens f/# is limited by the material refractive index contrast, motivating use of recent high contrast polymers. The number of pixels required for the optical exposure is typically well beyond available spatial light modulator resolutions, motivating the use of a photolithographic mask. Thus, we present a photolithographic technique by which a single exposure into a self-developing photopolymer can directly print single custom high efficiency DOEs with freeform phase profiles, in contrast to holographic optical elements that are limited to the interference of two propagating fields. We use a dithered binary chrome mask with 9000 x 9000 pixels of 2.5 μm diameter to write lenses up to 23 mm in diameter. Lenses down to f/44 with 76% diffraction efficiency and f/79 with 83% diffraction efficiency are demonstrated.