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Centimeter-Sized Aplanatic Hybrid Diffractive-Refractive Lens

The design and the fabrication of a hybrid diffractive-refractive lens (HDRL) by the femtosecond laser direct writing technique at 343-nm wavelength are demonstrated. The diffractive part is a 10-mm-diameter aplanatic element inside a Z-cut sapphire wafer. The refractive part is a spherical planoconvex lens with a 9.3-mm radius of curvature. These two parts are jointly combined to constitute a diffractive-refractive singlet. The hybrid singlet realizes aplanatic action for the aspherical lens, which reduces the focal spot size of the spherical lens by 1/4. The modulation transfer function of the hybrid singlet shows a better imaging resolution performance than that of the single spherical lens. The diffractive elements inside the spherical lens also were made and not only further improve thermal stability of the optical system but also reduce the volume and weight. This HDRL can find important applications in diffractive reflective hybrid optics, super-resolution imaging, and integrated optics.