There are significant differences in the actual optical characteristics of specific targets in different spectral bands. Thus, the two imaging systems shared an aperture for detection and identification. The internal light was separately imaged after passing through an optical device that separated the visible near-infrared and infrared light. In this way, the light from the two bands could be used to synchronously detect and image the target, allowing it to track the target more effectively. Thus, it could meet the all-day, wide-range, and high-resolution detection requirements. There have been few research reports on visible near-infrared and mid-infrared dual-band large-angle spectroscopy. Therefore, this research has an important reference value for dual-band infrared detection and imaging technology.

The width and reflectance of the cut-off band at large angles were studied using the evaporation coating method. After practical calculation, it was concluded that at least four groups of film stacks should be used as the initial film system. A spectral curve that met the requirements could be obtained based on this optimization of the initial film system. The stress of ZnS film is generally compressive, while the stress of YbF₃ film is generally tensile. The control variable method was used to optimize the deposition process and adjust the corresponding thickness to change the stress of the ZnS and YbF₃ single-layer films. Because the film system was formed by depositing alternate layers of high and low refractive index materials, the tensile stress and compressive stress could offset each other. Therefore, the deformation of the substrate was smaller when the single-layer stresses of the high and low refractive index materials with corresponding thicknesses were closer. The film system structure was analyzed based on the measured stress results, which showed that the two sides of the film could offset each other and tended to be smaller in the design.

Based on a study of the characteristics of the high and low refractive index materials, the deposition process parameters of the film material (Table 2) were selected as the auxiliary parameters of the ion source (Table 3). Before coating, a constant temperature was 1 h. Thereafter, the ion source was used to clean the substrate for 15 min. A constant temperature of 200 ℃ was maintained for 2 h after coating, then the temperature was naturally cool to 90 ℃ for venting. The stress changes before and after coating are listed in Table 4. It can be seen from the analysis of the parameters before and after the process adjustment that the stress of the single-layer ZnS did not match that of the single-layer YbF₃. The ZnS and YbF₃ deposition rates and auxiliary process parameters of the ion source were adjusted. The ZnS deposition rate was adjusted from 2 nm/s to 1.5 nm/s, and the deposition parameters of the YbF₃ ion source was adjusted from 200 V and 5 A to 220 V and 5 A. After adjusting the process parameters, the stress of the single-layer ZnS was close to that of the single-layer YbF₃ (Table 4). The antireflective film coated on the reverse side presented a tensile stress state, which could compensate for the stress on the spectral surface. The antireflective film coated on the reverse side met the requirements (Fig. 6). The spectrum of the deposited film was obtained and analyzed (Fig. 7), and the spectral data were imported into the film system design software for fitting. It was found that the actual thickness of the ZnS was 1.08 times the design thickness, and the actual thickness of the YbF₃ was 0.95 times the design thickness. After adjusting the film thickness ratio, the transmission spectrum curve of the spectral surface after deposition (Fig. 8) showed that the overall spectrum was well fitted with the design without obvious deviation (transmittance of 3.7-4.8 μm under the condition of the single-sided coating). The theoretical average transmittance in the band of 3.7-4.8 nm was 79.5%, and the average measured transmittance was 77%. The measured spectrum was fitted, and the fitting results showed that the spectral performance was not caused by the mismatch of the film thickness. After analysis, it was found that the addition of ion source-assisted deposition in the adjustment of the stress matching resulted in an increase in the absorption of the ZnS and YbF₃ films but still met the technical specifications.

In this study, the temperature, deposition method, and ion source parameters were adjusted using the control variable method. The film stress was calculated using the Stoney formula. Then, the stresses of specific-thickness single-layer ZnS and YbF₃ were adjusted to make these stresses match. When the stress of the antiantireflection surface was used to compensate for the stress of the spectral surface, the surface accuracy (PV value) of the spectral surface decreased from 0.63λ to 0.182λ. Then, the problem of the poor surface shape of the spectroscope could be solved. Based on a reverse analysis of the film deposition results, the film thickness was adjusted to improve the reflectivity and transmittance at an incidence of 55°. In the 0.6-0.9 μm wave band, the average reflectivity was 90.77%. In the 3.7-4.8 μm wave band, the average transmittance was 91.15%. The prepared film basically met the use requirements of the system components.