Application Of Freeform Surface In Imaging Optical Systems

Traditional optical systems usually consist of spherical or simple aspherical elements for the advantages of manufacturing and testing. Along with the expanding application scenarios and performance upgrading, the traditional optical systems face great difficulty for meeting the requirements of intelligence, miniaturization and low cost. The emergence of optical freeform surface provides a novel idea to solve this problem. Optical freeform surface provide optical designers more design freedom for their non-rotational symmetry and strong surface characterization ability. With the help of freeform surface, optical designers could design an optical system with flexible space layout to meet the specific application scenario. With the ongoing improvement of optical manufacture, testing and adjustment technology, freeform surface has become the important developing direction of future optical system design.This paper mainly focuses upon the applications of freeform surface in the imaging optical systems and the following aspects are studied: the representation forms of freeform surface and its characterization in different describe basis functions; vector aberration theory(nodal theory) in the optical system with freeform surface; image quality evaluation of the optical systems with non-rotationally symmetric and freeform surface; application of freeform surface in the reflective optical system design; application of freeform surface in NA1.35 projection lithographic lens design and compensation of the refractive index homogeneity. The following is our main work in this paper:(1) Expound the different representation forms of freeform surface, and the transformation method between these different freeform surfaces. The transformation method provides a thought to get the initial structure of freeform surface optical systems. The nature of freeform surface under different representation forms is summarized and compared and it provides the basis for the selection of freeform surface type in the optical system design process.(2) Expound the vector aberration theory in non-rotational symmetric optical systems in the third order form, and analyze the node behavior of aberrations. Vector aberration theory in freeform optical system is also expounded in detail. Astigmatism is taken as the example to show the influence of freeform surface on node behavior. The study of vector aberration for freeform surface optical system provides a powerful theoretical guidance in optical system design.(3) The mainstream optical design software is usually uncapable of evaluating the non-rotationally optical system, especially, when the freeform surface is introduced to the system. This paper puts forward the concept of the best reference grid and establishes the corresponding mathematical model. The parameters of the best reference grid can be obtained through the combination use of optical design software and mathematic software. With the help of the best reference grid, the effective focal length and distortion can be accurately calculated. We can also optimize the system based on the best reference grid.(4) Research the applications of freeform surface in the reflective optical systems. Zernike freeform surface has been applied successfully designed a focal length of 1200 mm, F # 4, field of view 20 ° × 4 °, off-axis three mirror systems for space optical remote sensing. The best reference grid is applied to evaluate the image quality of the freeform surface system. XY freeform surface and Zernike freeform surface has been applied successfully for designing a focal length 50 mm, exit pupil diameter 8 mm, field of view of 50 °×25 °, off-axis two mirror system for helmet display. The design is compact in structure, small in volume, light in weight, and achromatic.(5) Research the applications of freeform surface in the NA1.35 lithographic system. Firstly, Q-type freeform surface has been applied successfully to lithographic optical system design. Both wave aberration RMS and distortion are within 1 nm after optimization with Q-type freeform surface. Meanwhile, the lithographic system satisfies the requirements of optical work length, field of view, imaging telecentricity, NA consistency. The second, apply the radial basis function freeform surface compensate the refractive index inhomogeneity in NA1.35 lithography objective. The basic compensation principle is deduced in detail and a good compensation result is obtained.