As the optical technology develops, in order to adapt to complex and changeable battlefield environments, composite guided modes have received more and more attention. Long-wave infrared (LWIR)/laser dual-mode seeker can provide complementary benefits such as all-weather operation, anti-electronic interference, and high hit accuracy. Currently, common-aperture dual-band systems with a laser of 1.064 μm and an LWIR of 8-12 μm mostly use a half-reflecting mirror to realize beam splitting. The image space resolution and field of view of the infrared optical system still need to be improved. Aiming at improving the feature resolution ability and all-weather working capability of the guidance structure, this paper proposed a new dual-mode guided optical system based on a secondary mirror splitting method. The passive infrared module was used to search for the target, and the active lidar module was utilized to lock the target and realize guidance accurately, and thus the high-precision scanning and patrol in a compact structure could be realized.

The Ritchey-Chretien (R-C) structure was used as a common component to address the issue of the seeker's limited optical system size, and the secondary mirror was coated with a beam splitter film to combine the long-wave infrared reflection optical module with the laser transmission optical module. In addition, the paper studied the initial structure solution method of the catadioptric system, as well as the effect of various optical obscuration conditions on the diffraction limit of the modulation transfer function of the incoherent imaging system. An illustration of a common-aperture dual-mode guided system with an optical obscuration ratio of 1/3 and an F-number of 0.98 was presented. The residual aberrations of the primary and secondary mirrors were compensated for by using several refractive lenses, and an optical passive athermalization method was used to complete the long-wave infrared athermalization in the range of -40-60 ℃. By using Monte Carlo analysis, the assembly tolerance and optical component tolerance were simulated. The resulting tolerance distribution was reasonable and workable.

According to the optical transfer function (OTF) of the annular obstructing optical system (Fig. 4), a reasonable obstruction ratio was determined, and the lens aperture of the system with a small F-number and large aperture was weighed against the light-gathering ability of the optical structure. First, the secondary imaging structure was used to reduce the light height of the edge field of view, and the optical aperture after being obstructed was fully utilized. The long-wave infrared imaging structure, with good image quality, consists of two reflectors and five refractive lenses (Fig. 6), and the MTF of this structure at 42 lp/mm is higher than 0.32 (Fig. 7). The reflectivity in the long-wave infrared band exceeds 90%, and the transmittance in the laser band exceeds 80%, which allows for high-precision target scanning and precise guidance. Then, through the selection of optical and structural materials and the distribution of optical power, the athermalized design of the long-wave infrared module in the temperature range of -40-60 ℃ was realized, which shows good thermal stability (Fig. 11 and Fig. 12) and processability (Table 3). Finally, this paper kept the infrared optical module's common aperture part, used the left side of the secondary mirror as a refracting material, and optimized the design of the laser-receiving optical module. The light in the receiving system with a laser of 1.064 μm was parallel after optimization (Fig. 9), and a narrow-band filter was added to avoid wavelength shift caused by large-angle incidence. The light of all fields of view was focused within 0.5 mm of the detector target surface (Fig. 10), which can enhance the signal-to-noise ratio of the laser module.

A design method for the dual-mode guided optical structure was proposed and demonstrated in this paper. In addition, the effect of different obscuration ratios on the MTF diffraction limit of the catadioptric optical structure was studied, and a dual-mode guided imaging system with a small F-number and a common aperture of an LWIR of 8-12 μm and a laser of 1.06 μm was designed. The method of secondary mirror splitting simplifies the system structure effectually, and the secondary imaging structure increases the field of view and reduces stray light. In order to enhance the signal-to-noise ratio effectively, a narrow-band filter was introduced into the laser module's small-angle light. The long-wave infrared optical module has good imaging quality in the 4°×3° field of view, and the modulation transfer function (MTF) at 42 lp/mm is higher than 0.32. The optical obstruction ratio is only 1/3. Through the combination of different materials, an athermalization design under a temperature of -40-60 ℃ was achieved. The overall size is reflected by only 98 mm (length)×70 mm (width)×70 mm (height), and the structure is compact, which meets the application requirements of lightweight and engineering and can provide a certain reference for the design of multi-mode guided optical structures.