High-power laser systems are capable of outputting nanosecond pulses with power exceeding megajoule levels, allowing high-temperature and high-density conditions to be achieved in laboratory settings. As a result, they are widely employed in various fields such as high-energy-density physics experiments, inertial confinement fusion, astrophysics, and materials science. Solid-state laser systems based on neodymium glass with long optical path lengths are typically used in high energy laser system, which warrant precise control of beam quality and placement. Researchers have conducted extensive studies to maintain a large field-of-view while improving collimation accuracy. However, owing to the limited resolution and field-of-view of optical systems, it is generally not feasible to achieve high accuracy and a large field-of-view in the same collimation system. Therefore, a larger field-of-view is often compromised to ensure high automatic collimation accuracy. In such cases, precise mounting and a highly stable mechanical structure are required to ensure that the collimated beam enters the field-of-view. However, given that the limits of mounting accuracy and long-term stability of the mechanism can only reach a few hundred microradians, the outcomes of this approach are limited. In this study, we proposed a dual-trace far-field automatic alignment scheme using a bifocal lens that enables us to achieve microradian collimation accuracy and a field-of-view of several milliradians in the same optical system. This approach significantly reduces the need for precision mounting and long-term stability of the mechanical structure.

Based on the ghost imaging theory of bifocal lenses, this study proposed a scheme for acquiring far-field feedback images with two different angular resolutions on a single camera. The core of the scheme is a bifocal-thick lens with different curvature radii on the left and right surfaces, coated with a 0° spectral dielectric film on both sides. For the off-axis incident beam, owing to the spectral effect of the dielectric film, a second-order ghost image is generated by the secondary reflection of the beam on the right inner surface of the lens; that is, a brighter main spot and a dimer ghost spot can be obtained simultaneously on the imaging plane. Owing to the multiple reflection and transmission processes, the angular deflection sensitivity of the ghost spot is much greater than that of the main spot; therefore, it exhibits a higher regulation accuracy in the feedback process. The low angular deflection sensitivity of the main spot ensures a field-of-view of several milliradians, whereas the high angular deflection sensitivity of the ghost spot ensures the collimation accuracy of microradians. A single imaging system that incorporates this bifocal lens is sufficient for far-field alignment. This study provided a design example of a bifocal lens and conducted a numerical analysis based on matrix optics theory and ZEMAX. The equivalent transfer matrix of the bifocal lens was used to demonstrate its focusing properties.

The ray tracing results for the main spot, as obtained from the ZEMAX simulation, are shown in Fig. 4. The offset in the image plane is about 2.11 mm. The ray tracing results for a second-order ghost image in the system are shown in Fig. 5. The offset in the image plane is about 1.86 mm. For the same incident-angle offset, the angular magnification of the second-order ghost image is approximately 6.6 times that of the main spot under the given design parameter. To further verify the feasibility and effectiveness of the design, an experimental optical path was built based on the theoretical design. The experimental optical path is illustrated in Fig. 6. The spot offsets for the same step values obtained in the experiment are shown in Fig. 8. When determining the angular magnification of the spot at the same offset, the angular magnification of the ghost spot is observed to be approximately 6.9 times that of the main spot, which is a deviation of approximately 4.5% compared to the simulated result of 6.6 times.

This paper proposed a dual-trace far-field auto-alignment scheme using a bifocal lens and provided a design example to illustrate the implementation of the system. The theoretical results for the lens were analyzed through calculations and simulations, while its imaging performance was experimentally verified. Experimental verification confirms that the imaging performance is consistent with the design specifications. The proposed design leverages the ghost image of an optical lens, thereby enabling multiple imaging with a single bifocal lens in a lightweight and highly integrated manner. Combining this design with existing automatic alignment schemes is expected to reduce the requirements for mechanical mounting and long-term stability, minimize redundancy, and enable practical applications in high-power laser systems.