Laser circumferential detection systems actively detect all-round targets by emitting laser beams and feature good initiative, good directionality, and less susceptibility to electronic interference. The emission optical system is an important part that determines the detection range of the system and affects the detection accuracy. In the emission optical system, cylindrical lenses and aspherical lenses are usually adopted to change the divergence of semiconductor laser light sources, and the complex surface spliced by prisms and cylindrical mirrors and aspherical cylindrical lens arrays make the energy uniform within the active area. However, the outgoing light field of the above method is still a linear beam, and the areas where the emission fields meet are prone to non-crossing or excessive crossing, which will damage the energy uniformity within the entire field of view (FOV) and affect the anti-interference ability and detection accuracy of laser circumferential detection. To improve the uniformity of the outgoing light field and enhance its effective area, we propose a forward-tilt detection scheme using a conical FOV.

The partition scheme of the laser circumferential detection system generally places the transmitting and receiving system evenly in the radial missile direction, usually divided into four to eight quadrants. We use a six-partition layout and set the beam's forward tilt angle to 60°,detecting the target in advance and obtaining relevant information. A single partition is mainly divided into three parts: fast-axis and slow-axis collimation, slow-axis beam expansion and homogenization, and forward-tilt detection. Firstly, the output beam of the semiconductor laser is collimated in both meridional and sagittal directions by employing an aspheric lens. Then, a Powell prism is utilized to realize beam expansion and homogenization in the sagittal direction. Finally, a deflection prism is leveraged to ensure that the forward tilt angle of the beam is 60°, and a complete conical FOV is assembled by a cylindrical lens in the sagittal direction. Additionally, we describe how to obtain the initial parameters of the aspherical lens, the principle of the Powell prism for beam expansion and homogenization, and the beam deflection in the deflection prism.

Based on the principle of equal optical paths, the initial parameters of the aspherical lens are obtained by ray tracing in the meridian and sagittal directions (Table 2). The optimized fast axis divergence angle and slow axis divergence angle of the beam are ±0.6° and ±0.5°. By adopting the interactive design of ZEMAX and LightTools, the laser beams approximately achieve a flat-top distribution in the sagittal direction on the four target planes, with irradiance uniformity exceeding 86%. According to the refraction law and total reflection conditions to be met when the beam deflects in the prism, the low melting point glass D-ZLAF85A is selected as the prism material, and the apex angle of the prism is set to 27.2°. Meanwhile, an extended polynomial surface is employed for optical path compensation to ensure that the angle between the missile beam and axis at different apertures is the same as 60° to generate a conical FOV. The optimized extended polynomial [Eq. (7)] is obtained. After the deflection prism, a beam expanding and broadening cylindrical lens is added in the sagittal direction. When the radius curvature of the cylindrical lens and its distance from the deflection prism are adjusted, the six partitions will be spliced into a complete circular FOV. The entire system [Fig. 8(b)] has a FOV angle of ±0.75° on the meridian plane, and each partition covers a FOV of ± 30° on the sagittal plane, and the six partitions realize a 360° FOV without blind spot detection. The irradiance uniformity of the annular FOV in the circumferential direction can reach more than 91%, and the energy utilization rate can reach over 98%.

Aiming at the requirements of the circumferential detection system for the detection distance, emission divergence angle, and energy uniformity, we propose a six-quadrant partition scheme based on the conical detection FOV and design a set of emission optical system with a forward tilt angle of 60°. The entire system can emit at a divergence angle of ±0.75° on the meridian plane, and cover a 360° FOV on the sagittal plane to achieve circumferential detection. It can be placed horizontally on a plane perpendicular to the missile axis. Meanwhile, the irradiance uniformity of the beam on the four target planes is greater than 90%, and the energy utilization rate is as high as 98%. Considering the actual processability, the Powell prism and the deflection prism are made into one piece by molding to achieve a more compact structure. The optical system consists of only three lenses to meet the requirements of engineering and light weight applications.