Laser sources can be classified according to their gain medium, including disk lasers, fiber lasers, semiconductor lasers, and YAG lasers. When modulating parameters and evaluating beam quality, the key parameters include M² definition, divergence angle, beam parameter product (BPP), and single-mode vs multi-mode definitions.
In a laser processing system, the optical path typically includes focusing lenses, collimating lenses, beam expanders, wedge mirrors, line collimation lenses, line diffusers, and cylindrical lenses, among others. Among these, the collimation and focusing head and the galvo scanner are core optical structures. This article focuses on the first and most commonly used structure — the collimation and focusing head, which has great engineering value.
1. Focusing Lens
A focusing lens converges a parallel laser beam into a small focal point. Single-element focusing lenses are widely used, but due to spherical aberration and chromatic aberration, special lenses are often designed for high-precision applications.
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Spherical Aberration: This occurs because traditional spherical lenses focus rays from the lens center and edge at slightly different points, resulting in a blurred focus and poor cutting quality.
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Aberration Correction: To correct this, compound lenses (two or three lenses combined) or aspherical lenses can be used. A compound achromatic lens combines a positive and a negative lens whose aberrations cancel each other out. Aspherical lenses offer the best correction but are costly, so they are mainly used in older YAG laser systems rather than modern fiber lasers.
2. Collimating Lens
A collimating lens transforms a point light source into a parallel beam. In simple terms, it works as a focusing lens used in reverse.
When a point light source is placed at one focal length from the lens, the outgoing beam on the other side becomes parallel. Collimating lenses are essential components in fiber laser cutting and welding heads. Depending on application needs, these lenses may also be designed with aberration correction, using compound or achromatic designs for higher precision.
3. Theoretical Design of Laser Collimators
In free-space optical systems, the input and output fiber ends are separated by a small distance to allow insertion of optical components that achieve specific functions.
Since the light emitted from a fiber is a Gaussian beam with a small waist and large divergence, a collimator is required to expand the beam waist and reduce the divergence angle. This ensures efficient coupling between fibers.
From a geometrical optics view, a lens should ideally focus all light to a single point; however, diffraction and aberration make this impossible. According to Gaussian beam theory, the divergence angle remains unchanged as light passes through a uniform medium, while the waist position shifts. When the incident light is positioned at the front focal point of the lens, the beam can be perfectly collimated.
Commonly used collimators include GRIN lenses (graded-index) and C-lenses (constant refractive index). C-lenses are more cost-effective and perform better for long working distances.
4. Practical Application — Collimation and Focusing Head
The collimation and focusing head is one of the most critical components in an external optical path. It typically contains two lenses:
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The collimating lens, which converts divergent light from the fiber into a parallel beam;
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The focusing lens, which focuses the parallel beam onto the workpiece.
For example, a configuration might use a 100 μm core fiber with 150 mm collimation and 250 mm focusing.
5. Structural Classification of Collimation and Focusing Heads
Based on their design, collimation and focusing heads can be divided into four categories:
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Basic type — pure optical collimation and focusing without CCD.
2–4. CCD-integrated types — used for alignment, path calibration, or process monitoring.
The choice of structure depends on space constraints and system interference conditions.
Additionally, nozzle structures differ by application; some are optimized using gas-flow simulations to improve shielding gas efficiency.
High-power and medium-power collimation heads mainly differ in lens material and coating. Performance indicators include:
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Thermal drift (focus shift) — typically ≤1 mm for high-quality heads.
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Power loss — ideally below 3%, with top models achieving 1%. Excessive loss leads to heat buildup and optical damage.
6. Functional Variants
A newer design, the dot-ring welding head, reshapes a laser beam from circular to ring-shaped or dot-ring form. This balances energy distribution, reduces spatter, and improves welding quality on steel materials. However, for highly reflective materials such as aluminum or copper, reduced central energy may cause incomplete penetration.
7. Summary
Optical components in a laser transmission system are divided into transmissive and reflective types.
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Transmissive materials (e.g., fused silica) are used for collimating and focusing lenses due to their high transmittance, heat resistance, and low thermal expansion.
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Protective mirrors are often made from K9 glass with reflective coatings.
Reflective optics do not produce chromatic dispersion, but the coating materials must ensure:
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Stable reflectivity,
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High thermal conductivity,
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High melting point.
These characteristics prevent damage or explosion of the coating under excessive heat absorption, ensuring reliability and performance in industrial environments.
Post time: Oct-31-2025