A beam expander, also known as a collimator, is a commonly used Galilean telescope system that usually consists of a negative lens and a positive lens, both of which are co-focused. These laser beam expanders have the advantages of being structurally simple, low cost, and small in size, and are widely used in laser marking.
Usually, we use the divergence parameter of the beam as the characteristic of a perfect Gaussian laser beam. Divergence refers to the spreading of the light wave at a certain angle during its spatial propagation process. Even perfectly straight rays can experience divergence due to diffraction effects. Diffraction refers to the bending effect of light rays when they are obstructed by opaque objects, such as knife edges. The spreading occurs due to the secondary wavefront array emitted at the edge of the cut. These secondary waves and the main wave will interfere with each other, sometimes forming complex diffraction patterns.
Galilean beam expanders are the most common type of beam expanders and originated from Galilean telescopes. They usually consist of a concave lens as the input lens and a convex lens as the output lens. The input lens delivers a defocused beam to the output lens. The general low-magnification beam expanders are manufactured based on this principle because it is simple, small in size, and low in cost. They are designed to have minimal spherical aberration, low wavefront distortion, and chromatic aberration. However, they cannot accommodate spatial filtering or perform high-magnification expansion.
Keplerian beam expanders are generally used when spatial filtering or high-magnification expansion is needed. Keplerian telescopes usually have a convex lens as the input lens, focusing the beam with the focal length onto the output element. Additionally, spatial filtering can be achieved by placing a pinhole at the focus of the first lens.
Laser beam expanders have two main purposes: to expand the diameter of a laser beam and to reduce the divergence angle of a laser beam. Therefore, they are used for long-distance illumination or projection, as well as for focusing systems. The divergence angle of a beam that has been expanded is inversely proportional to the expansion ratio. Compared to an unexpanded beam, an expanded beam can be focused to a smaller size. The magnification of a laser beam expander is the ratio of the beam diameter. The product of the spot size and the divergence angle of the laser beam output from the laser beam expander is an optical invariant, approximately a constant value. When the waist radius of the beam expands by a factor of x, the divergence angle compresses to 1/x of its original value. The compression of the divergence angle is actually the collimation of the laser. However, this compression of the divergence angle does not improve the beam quality, which depends on the product of the waist radius and the divergence angle; while compressing the divergence angle, the waist radius will increase, and the spot size will increase as well. Laser marking machines often use them in combination with a flat-field focusing lens.