Laser Beam Expander: Parameter, Principle, and Product Selection
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Publish time:
2025-12-04
Laser Beam Expander: Parameter, Principle, and Product Selection
Laser beam expanders are designed to alter the laser beam diameter, divergence angle, and intensity distribution. They are commonly used in applications such as laser scanning, laser resonators, laser interferometry, and remote sensing to achieve functions like reducing spot energy density, decreasing beam diameter at a specific distance, minimizing focused spot size, and compensating for laser beam size. They are one of the most widely used components in laser systems.
Figure 1. Aspheric high-magnification beam expander by CASTECH
1. Parameters of Laser Beam Expanders
Wavelength: Laser wavelength refers to the spatial frequency of the light wave emitted by the laser, primarily measured in nm and μm. Beam expanders are designed for specific laser wavelengths. Unless the beam expander itself is achromatic, different wavelengths require different expanders. CASTECH's beam expanders cover wavelengths in the ultraviolet (193nm, 266nm, 355nm), visible (532nm, 632.8nm), and near-infrared (1030nm, 1064nm, 1550nm) ranges. Additionally, a series of products suitable for broad bandwidths are available.
Power Density or Energy Density: This refers to the optical power/energy per unit area, typically measured in MW/cm² or J/cm². Higher power/energy per unit area results in greater density. To prevent damage to laser system components and air ionization caused by high power/energy density, beam expanders are often used to increase the spot area, thereby reducing density. This necessitates that the beam expander itself has a high damage threshold. Our company imposes high damage threshold requirements on beam expanders for different wavelength bands and employs damage testing equipment to verify this threshold.
Magnification of the Beam Expander: Determined by the focal lengths of the object-side lens and image-side lens:
MP= f object-side/ f image-side
Output Divergence Angle of the Beam Expander: Inversely proportional to the magnification: θout = θin / MP
The divergence angle is a key parameter describing the performance of a beam expander. It quantifies the spread of the beam relative to its waist due to diffraction over long distances. The beam divergence angle is typically defined by the half-angle of the laser. For a Gaussian beam, the divergence angle (θ) is defined as: θ = λ / (πω0), where λ is the laser wavelength and ω0 is the beam waist radius.
2. Basic Principles of Laser Beam Expanders
The principle of a laser beam expander is similar to that of a telescope, both being afocal systems where collimated light enters and exits the optical system parallel. Laser beam expanders can be classified into two main types: Keplerian and Galilean.
The Keplerian type consists of two positive lenses. Their focal points coincide, and the distance between the lenses is approximately equal to the sum of their focal lengths. When collimated light enters, an energy convergence focus point is formed between the lenses. Placing a pinhole aperture at this focus allows for spatial filtering, improving laser beam quality. However, in high-power applications, this design carries risks of self-focusing and air ionization.
Figure 2. Schematic of a Keplerian beam expander
The Galilean design utilizes one positive and one negative lens. The distance between the lenses equals the difference of their focal lengths. Compared to the Keplerian type, this design often has smaller optical power per lens, is more compact, and generally less expensive. Furthermore, because the spherical aberration effects of the positive and negative lenses are opposite, the Galilean design can partially cancel spherical aberration, making it more suitable for large apertures and non-rotationally symmetric beams. The absence of an internal focal point makes the Galilean structure more suitable for high-power laser applications.
Figure 3. Schematic of a Galilean beam expander
3. Product Types of CASTECH's Laser Beam Expanders
1. Standard Products
CASTECH offers a wide variety of standard beam expander products with short delivery times. These include fixed-magnification, non-adjustable divergence expanders; fixed-magnification, adjustable divergence expanders; and manual zoom beam expanders (e.g., 1-3X, 2-10X) for common wavelengths like 266nm, 355nm, 532nm, and 1064nm.
Figure 4. Variable Magnification Beam Expander by CASTECH
2. Customized Products
CASTECH also provides customized solutions for beam expansion and shaping based on specific application requirements. Examples include high-magnification expanders, achromatic expanders, aspheric expanders, cylindrical expanders, motorized zoom expanders, water-cooled expanders, and beam homogenizing & collimating expanders.
Motorized and Water-Cooled Zoom Expanders: Applications like 3D printing and laser scanning may require continuously adjustable spot sizes. Leveraging our engineering team's expertise in motor programming and mechanical design, CASTECH offers motorized zoom and water-cooled zoom beam expanders. Zoom expanders adjust the spot size over a fixed working distance, while the addition of water cooling mitigates thermal lensing effects under high laser power.
Figure 5. Motorized Zoom Expander by CASTECH
Figure 6. Water-cooled Zoom Expander by CASTECH
Cylindrical and Aspheric Expanders: For applications requiring beam collimation with optimized spot shape, cylindrical or aspheric lens solutions are typically employed to optimize the beam's fast and slow axis dimensions.
Cylindrical Expanders: In this approach, a cylindrical lens expands the beam along its narrower dimension (slow axis). The advantage is that it focuses the slow axis without amplifying the orthogonal component. The relationship between the two-axis divergence angles and focal lengths is given by:
θ_fast = θ_fast f2 / f1
Figure 7. Cylndrical Expander by CASTECH
Aspheric Expanders: Benefiting from CASTECH's exceptional capabilities in aspheric manufacturing and optical design, this solution provides strong support for customizing beam homogenizing and collimating optics.
Figure 8. Aspheric Expander by CASTECH
This approach not only performs traditional collimation and expansion but also homogenizes the energy distribution of the laser beam. The output intensity distribution is uniform, resembling a top-hat profile, which delivers excellent performance in laser engraving and welding.
Figure 9. Comparison of beam intensity before and after homogenization/collimation
4. Strict Quality Management
CASTECH emphasizes stringent quality control, ensuring every beam expander undergoes rigorous production and inspection processes before shipment. To guarantee surface quality, assembly and inspection are conducted in a high-level cleanroom environment. During inspection, parameters such as surface cleanliness, wavefront error, and beam quality are strictly monitored and controlled using equipment including but not limited to Zygo interferometers, metallurgical microscopes, autocollimators, and beam profilers, striving to provide customers with the best possible user experience.
Figure 10. Zygo testing platform of CASTECH
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