In laser beam expanders, the concave lens, through its combination of diverging rays and recollimation by a convex lens, becomes a core component for optimizing beam divergence angle. Its working principle is based on the refractive properties of lenses in geometric optics: when a parallel laser beam passes through the concave lens, due to its thin-in-the-middle, thick-at-the-edges structure, the light rays are deflected away from the principal axis, forming a diverging spherical beam. This divergence process provides the foundation for subsequent beam parameter optimization; in conjunction with a convex lens, beam diameter enlargement and precise control of the divergence angle can be achieved.
In a typical Galilean beam expander system, the concave lens, as the input lens, undertakes the primary divergence task. Its divergence effect enlarges the diameter of the original laser beam and changes its wavefront curvature. At this point, although the beam divergence angle initially decreases due to the increased diameter, it has not yet reached the collimation standard required for application. The divergence effect of the concave lens needs to be coordinated with the collimation effect of the subsequent convex lens; beam parameter optimization is achieved through matching the focal lengths of the two lenses. For example, when a diverged beam from a concave lens enters a convex lens, the convex lens refocuses and adjusts the diverged beam into a parallel beam, ultimately outputting a laser beam with an enlarged diameter and a significantly reduced divergence angle.
The focal length of the concave lens directly affects the performance of the beam expander system. A focal length that is too short will result in insufficient reduction of the divergence angle, failing to meet the requirements of long-distance transmission or high-precision machining; a focal length that is too long may make the system too large, increasing design complexity. In practical applications, the focal length parameters of the concave lens must be calculated comprehensively based on the initial divergence angle of the laser beam, the target beam expansion ratio, and system space constraints. For example, in a system requiring an 8x beam expansion ratio, the focal lengths of the concave lens and the convex lens must be matched in a specific ratio to ensure that while the beam diameter is increased by 8 times, the divergence angle is compressed to near the theoretical diffraction limit.
The radius of curvature of the concave lens is a key parameter determining its divergence capability. A larger radius of curvature results in weaker divergence from the lens, but a higher tolerance for manufacturing errors. Conversely, a smaller radius of curvature leads to stronger divergence, but may introduce additional aberrations. Therefore, the choice of radius of curvature must balance divergence efficiency and beam quality. Modern laser beam expanders often employ aspherical concave lenses, optimizing surface curvature distribution to reduce spherical aberration, coma, and other aberrations while maintaining strong divergence, thus improving the collimation and energy concentration of the output beam.
Material properties are equally crucial to the performance of concave lenses. Laser beam expanders have stringent requirements for the refractive index uniformity, thermal stability, and laser damage threshold of the lens material. For example, high-power laser systems require low-absorption materials such as fused silica or calcium fluoride to avoid beam distortion caused by thermal lensing effects; while ultraviolet or deep ultraviolet laser systems require ultraviolet-grade fused silica or magnesium fluoride to ensure high transmittance at short wavelengths. Furthermore, the concave lens surface needs to be coated with an anti-reflection film to reduce reflection loss and improve beam energy utilization, especially in multi-band composite light scenarios, where the application of multilayer dielectric films can significantly improve system performance.
The installation orientation of the concave lens also needs strict control. In beam expanding systems, the concave surface of the concave lens usually faces the laser source to ensure that the light diverges primarily through the concave surface. Incorrect installation orientation may cause the light propagation path to deviate from the design expectation, leading to divergence angle control failure or beam quality degradation. In addition, the coaxial adjustment of the concave lens and convex lens requires precision clamping tools and optical inspection equipment to ensure that the optical axis coincidence of the two lenses is better than micrometer level, avoiding beam offset or increased aberrations due to assembly errors.
In laser beam expanding systems, the concave lens, through its synergistic effect with the convex lens, optimizes the beam divergence angle and enlarges the beam diameter. The comprehensive design of its focal length, radius of curvature, material properties, and installation accuracy directly determines the performance ceiling of the beam expanding system. With the continuous development of laser technology, the manufacturing process and materials science of concave lenses have made continuous progress, providing more reliable beam control solutions for high-power, high-precision laser applications.
