In the precise optical path of the laser marking machine, there is a seemingly simple yet crucial component - the beam expander. This article will delve into the working principle, technical parameters of the beam expander, and its decisive influence on the laser marking process.
First, to understand why a beam expander is necessary, we must first recognize that the light beam directly output from the laser has two inherent characteristics:
Beam divergence angle: During the transmission process of the laser, it is not a perfectly parallel beam. As the distance it travels increases, it gradually spreads out. This spreading angle is known as the divergence angle (unit: mrad).
Gaussian beam characteristics: The energy distribution of the laser beam in its cross-section follows a Gaussian pattern, meaning it is brightest at the center and gradually diminishes towards the edges.
If this kind of divergent beam is directly used for marking, a serious problem will arise: the size of the focused spot will change with the working distance.
When the distance is close: The light beam has not yet fully spread out. After being focused by the field lens, the light spot is small, the power density is high, and the marking is clear.
At a long distance: The beam has significantly spread. Even after being focused by the same lens, the resulting light spot will become larger, the power density will decrease, causing the marking lines to become thicker, blurry, and even unable to reach the material threshold.
This severely restricts the effective working range (depth of field) of the marking machine, and makes it extremely difficult to maintain consistent marking results on workpieces at different heights or on curved surfaces.
II. The fundamental function of the beam expander is to address the aforementioned issues. Its core function is:
Convert the incident laser beam with a smaller diameter into an output laser beam with a larger diameter, a smaller divergence angle (closer to parallel).
This process is referred to as "beam collimation" in optics.
Technical principle: Based on an inverted telescope system
The most common beam expanding lenses are Keplerian or Galilean structures, consisting of a pair of lenses: a short-focal-length collimating lens (the input lens) and a long-focal-length output lens.
Incidence: A beam of light with a divergence angle first passes through a collimating lens. According to geometric optics, this beam of light will be initially focused and converge to a single point of focus.
Translation: Before the beam has re-diverged, allow it to pass through the output lens. Due to the longer focal length of the output lens, it will "pull back" the beam and make it exit in a more parallel manner.
The final output beam not only has a larger diameter but also a significantly reduced divergence angle.
Key formula:
The performance of the beam expander is defined by two key parameters:
Expansion ratio (M):
Among them, f2 represents the focal length of the output lens, f1 is the focal length of the collimating lens, D out is the diameter of the output beam, and D in is the diameter of the input beam. For instance, a 3x beam expander can increase the diameter of the input beam by 3 times.
Divergence angle compression:
The divergence angle θ out of the output beam is compressed to 1/M of the input divergence angle θ in. This means that a 3x beam expander can reduce the divergence angle to one-third of its original value.
III. After the light beam is collimated by the beam expander, it enters the scanning mirror and the field lens (F-θ lens), and a qualitative leap will occur:
1,To obtain a smaller focused spot and improve the marking accuracy and resolution
According to the theory of optical diffraction, the diameter d of the focused spot is approximately:
Among them, λ represents the laser wavelength, f is the focal length of the field lens, and D is the diameter of the light beam that enters the field lens.
The conclusion is obvious: The expanding lens increases the diameter D of the incident beam on the field lens, thereby directly reducing the size of the focused spot d. A smaller spot means finer lines, higher graphic resolution and sharper edges, which is crucial for marking QR codes, micro text, and complex logos.
2. Increase depth of field and expand the effective working range
Depth of field refers to the axial distance range within which an acceptable focused spot size can be maintained. After being collimated, the beam has a very small divergence angle, so the spot size changes very little over a long propagation distance. This enables the laser marking machine to mark on workpieces at different heights and even on surfaces with certain curvatures without frequently adjusting the focal length, while still achieving clear and uniform results. This is particularly important when handling uneven workpieces on trays in an automated production line.
3. Protect optical components and increase power density
Reducing power density: The laser beam is expanded before entering the lens of the scanning mirror, causing its cross-sectional area to increase and the power density (power per unit area) to decrease. This reduces the thermal load on the mirror lens and potential damage, prolonging its service life, especially in high-power laser marking applications.
Improve energy utilization: A smaller focused spot means more concentrated energy, resulting in a higher power density on the material when the laser power is the same. This makes the marking process more efficient, faster, or enables the same marking effect to be achieved with lower power, saving energy and extending the lifespan of the laser.
The beam expander, which is not only the technical foundation for achieving high-precision and high-resolution marking, but also the key to expanding the adaptability of the equipment, protecting the core optical components, and enhancing the overall process stability. It can be said that without a beam expander in a laser marking machine, its performance will be greatly reduced. Understanding and correctly applying the beam expander is an indispensable part of optimizing any laser marking system.