Objective  Photoelectric imaging system serves as the “eye” of all kinds of equipment, which plays an indispensable role in scene detection and target recognition. To acquire more abundant target information, one of the development directions is multi-band fusion detection. However, the existing multi-band imaging system mostly adopts the discrete structure, with large system volume architecture, high manufacturing cost, and lack of spatial consistency due to parallax between the discrete systems. The challenges pose difficulties in image fusion and other back-end processing. Multi-band common aperture, also a common configuration, is generally used to split the front optical path with optical components, and subsequently respond to the detection requirements of different bands through the discrete rear optical path. To address these issues, coaxial folded mirrors for visible/near-infrared imaging systems are designed in this paper.

  Methods  To guarantee the surface accuracy and relative orientation accuracy for multiple mirrors, an interferometric null test with a computer-generated hologram (CGH) is proposed (Fig.5). Diamond turning technology is applied to machining the mirrors. In this approach, two CGHs are designed for the null test of the monolithic primary/tertiary mirrors and the monolithic secondary/fourth mirrors (Fig.6, Fig.9). Ghost image of disturbance orders of diffraction is effectively separated by properly choosing the power carrier and the axial position of the CGH. A single CGH is capable of simultaneously measuring both the surface error and the relative orientation error of multiple mirrors (Fig.8). The result of the interferometric null test shows multiple mirrors are measured with nearly null fringes, indicating high accuracy in terms of surface form and orientation. Moreover, no ghost disturbance is observed.

  Results and Discussions   The optical components undergo the diamond turning process, and the mirror blank is shared among the primary mirror and the three additional mirrors, allowing for simultaneous processing (Fig.12). After processing, a CGH is used to conduct zero compensation measurements on both mirrors (Fig.13). The measured surface shape error is shown (Fig.14), and the primary mirror and the three mirrors demonstrate a combined surface shaper error of PV 0.87λ, RMS 0.12λ; Interference diagram reveals that the ghost image stripes only exist outside the main mirror and the three mirror stripes, and they do not form interference. The primary mirror and the three mirrors reach a near-zero fringe state at the same time, indicating a high level of surface shape accuracy and mutual pose accuracy (reaching the sub-wavelength level), which meets the imaging requirements of the system.

  Conclusions  The study proposes an interferometric null test with a CGH for the coaxial folded mirrors in visible/near-infrared imaging systems. The method involves the creation of multiple holographic regions with different functions on the same CGH substrate, which allows for the generation of the aspheric wavefronts of different shapes after the diffraction of the incident test wavefront. Consequently, the zero position of different mirror shapes can be tested at the same time. Following ultra-precision machining based on CGH compensation measurement, the mirror shape accuracy and pose accuracy attain a sub-wavelength level, which realizes direct assembly without additional assembly and adjustment for optimal imaging performance. Similarly, by positioning reference processing, multiple similar systems are nested coaxially, which enables multi-band coaxial imaging from visible light to near-infrared. Such capability holds obvious advantages for unmanned platform target detection and fast image fusion processing.