With the rapid development of high-tech fields such as gene sequencing, semiconductor chips, and life medicine, traditional microscopes are no longer able to meet the increasing technological needs. The microscopic objective lens, as the most core technical component in the microscopic system, is related to the imaging performance of the entire system and needs to meet conditions such as high numerical aperture, large field of view, and wide spectral range. When the imaging objective is located in the air, there is a theoretical limit to the numerical aperture of non-immersed structures. Therefore, non-immersed objective lenses with a high numerical aperture have become a technical challenge that Chinese surgical researchers urgently need to solve. The spectral range required in the field of gene sequencing is becoming shorter and shorter, moving towards ultraviolet and even deep ultraviolet. The wide spectral range causes an increasing color difference in the system, and ordinary application lenses do not need to correct the secondary spectrum. Microscopic objective lenses have extremely strict requirements for chromatic and spherical aberration and require correction. We focus on the technical requirements and current development trends of high-end microscopic objective lenses and design a catadioptric objective lens with a high numerical aperture and wide spectral band, with a numerical aperture of 0.91 and effective correction of color difference.

We have designed a microscopic objective lens that eliminates secondary spectra within a wide spectral range. Usually, in order to eliminate axial color differences, a combination of positive and negative focal lenses of different materials is required for correction. By taking two lenses as an example, there is a significant difference in Abbe numbers and a small difference in optical power between the two lens materials. In order to further correct the secondary spectrum, it is necessary to have similar dispersion coefficients and significant differences in Abbe numbers between the two lens materials. However, for conventional lens materials, two-piece lenses fail to eliminate residual secondary spectra. For a catadioptric structure, a negative lens element with an inner reverse side has a positive focal power and an axial color difference direction opposite to the positive lens element. When both are used simultaneously, the axial color difference in the system can be completely corrected. By studying achromatic theory, we design a microscopic objective lens that eliminates secondary spectra within a wide spectral range.

We use the optical design software ZEMAX to design a microscopic objective lens with a high numerical aperture and wide spectral range. Based on project requirements and optical system design indicators, the design results are analyzed. According to the primary aberration theory and the characteristics of apochromatic aberration in the catadioptric structure, the power of each light group in the initial structure is calculated, and the specific values are shown in Table 2. The aperture of the first group is 0.61, and the focal length is 17.44 mm; the aperture of the latter group is 0.91, and the focal length is 135.33 mm. After optimizing and analyzing the initial structure, the final optical path map of the optical system is obtained. As shown in Fig. 7, the system consists of 12 lenses with only one material. As shown in Fig. 9, the dipole spectrum value is 0.15 μm. This indicates that the secondary spectrum has been positively corrected. As shown in Fig. 10, the single segment field curvature of the system is less than 110 nm, and the maximum distortion of the system is less than 0.0001. As shown in Fig. 12, the color focal shift curves of all wavelengths in the system are within the diffraction limit radius range, indicating that the color difference at the system position has been well corrected.

With the development of science and technology and the progress of production processes, microscopy needs to meet the strict technical requirements of more industries, such as gene sequencing and semiconductor chip fields, which require microscopy objective lenses to have a high numerical aperture, large field of view, and wide spectral band achromatic ability. The optical system lens components designed in this article only use one material, eliminating the secondary spectrum in the range of 275-600 nm from the deep ultraviolet spectrum to the visible spectrum, solving the problem of traditional achromatic aberration requiring multiple materials to cooperate, and removing the pain point of less available glass materials in the ultraviolet band. Based on the existing research results of the project team, an optical system with a larger numerical aperture and shorter light transmission wavelength has been designed. From the design results, it can be seen that the system has a small volume and is convenient for actual production assembly. It can be widely used in fields such as semiconductor wafer defect detection and gene sequencing.