Laser beam expander is a lens component that changes the diameter of laser beam and divergence Angle. The laser beam emitted from the laser has a certain divergence angle. For laser processing, a laser beam expander can make the laser beam collimated and obtain a high power density spot after focusing.
【1】Definition of laser beam expander
The principle of laser beam expander and optical telescope (refractor telescope) are essentially the same. The laser beam expander is equivalent to a refracting telescope after the positions of the objective lens and the imaging lens are exchanged. Like refracting telescopes, laser beam expanders can be divided into two Keplerian type and Galilean type.
Keplerian beam expander consists of two lenses with positive focal length. The focal points of the two lenses coincide and are respectively located at two positions of the sum of focal lengths; the lens on close to the object is called the objective lens, and the lens on the other side is called the image-forming lens. There is a real image surface between the two lenses (the place of the rays converge).
Galilean beam expander consists of a positive lens and a negative lens, also located at the position of the sum of the focal lengths of the two lenses. Since the focal length of the negative lens is negative, the distance between the two lenses is shorter than that of Keplerian type.
Keplerian beam expander will form an energy focusing area between two lenses and heat the surrounding air, which may lead to wavefront error; high-power lasers can even cause ionization of the air. Therefore, Galilean beam expander is more suitable for using in most application scenarios.
Schematic Diagram of Galilean Beam Expander
Keplerian beam expanders are usually used in some scenarios that need to change the beam quality of spatial filtering, because there is a focal point for placing filters between lenses in the beam expander.
Schematic Diagram of Keplerian Beam Expander
【2】How to select laser beam expander
In the process of laser processing, the formula for calculating the focus spot of laser processing equipment is:
Where, D is the size of the focusing spot, f is the effective focal length of the focusing system, θ Is the full angle divergence angle of the incident laser beam. After inserting a beam expander of m times between the laser outlet and the focusing system, the laser spot will be expanded by m times, and the corresponding divergence angle will become θ/ m. For example, when a beam expander of 3 times is added to the laser system, the focus spot is D/3, which is 1/3 of the original, and it is significantly reduced. Using the above formula, a focusing system with a focal length of f/m can also be used to obtain a focus spot of the same size. What are the advantages of using a eam expander? Next, SPT will analyze the laser cutting machine and laser marking machine respectively.
Advantages of beam expander for laser cutting machine:
The moving range of the focusing head of the laser cutting machine is larger than that of the laser outlet. After using the beam expander, the divergence angle of the laser beam becomes smaller, and the change of the light spot incident on the focusing mirror is relatively smaller within the processing range, so that a more uniform processing effect can be obtained. Moreover, the working distance of the laser focusing head is long, which is conducive to protecting the focusing lens.
ZAMIA Q150i RF Excited Laser - Diffuse Reflection Polarizer Cutting
The following points must be paid attention to when matching the beam expander to the laser cutting machine:
1. The coating wavelength of the beam expander should match the wavelength of the laser.
2. Attention shall be paid to the allowable incident and outgoing light spots of the beam expander. The laser spot shall be smaller than the incident spot, and after beam expansion, the laser spot shall not be larger than the outgoing spot of the beam expander.
3. The spot size after beam expansion should match the 45︒ reflector of the subsequent optical path and be smaller than the diameter of the focusing lens.
4. The light spot after beam expansion shall not be greater than 30% of the focal length of the focusing lens; due to spherical aberration, the light spot will become larger for plano-convex lens and meniscus lenses.
CO2 Laser - Automotive Interior Cutting
Advantages of beam expander for laser marking machine:
Compared with the laser cutting machine, the focusing head of the laser marking machine is relatively fixed relative to the laser outlet. After the beam expander is added to the laser marking machine, the working distance of the laser focusing head is basically unchanged, and the marking range is not reduced.
CO2 Laser - Laser Marking
The following points must be paid attention to when matching the beam expander to the laser marking machine:
1. The coating wavelength of the beam expander should match the wavelength of the laser.
2. Attention shall be paid to the allowable incident and outgoing light spots of the beam expander. The laser spot shall be smaller than the incident spot, and after beam expansion, the laser spot shall not be larger than the outgoing spot of the beam expander.
3. The spot size after beam expansion shall match with the galvanometer lens, and shall not be greater than the maximum incident light spot allowed by the galvanometer lens.
4. The spot size after beam expansion shall match with the incident light spot allowed by the field lens F-θ, and shall not be greater than the maximum incident light spot allowed by the field lens.
5. For the front focusing marking machine, the laser spot after beam expansion shall not be greater than the incident spot allowed by the front focusing system.
【3】How to install laser beam expander
In laser operation, aiming to obtain better focusing effect of laser spot, it is necessary to pay attention to the installation position of the beam expander. Limited by the internal transmission optical path of the CO2 RF excited laser, the laser beam at the laser outlet is not a perfect fundamental-mode Gaussian beam. Taking F-series RF excited laser as an example, the optical field at the laser outlet does not reach the Gaussian distribution, so it is inappropriate to install a beam expander at this position.
ZAMIA Series CO2 RF Excited Laser
According to the coherent propagation characteristics of the laser beam, the laser beam gradually changes into Gaussian optical field distribution after free transmission in space for a certain distance. Adding a bean expander after the beam becomes Gaussian optical field distribution is conducive to obtaining a better focused spot. The experiments have shown that a better focusing spot can be obtained by installing a beam expander at a position about 10cm away from the optical outlet of the F series RF excited laser.
F series RF Excited Laser - Installation Diagram of Beam Expander
Due to the built-in beam shaping system (click to learn), the output of Q series lasers is quasi-fundamental mode circular symmetric Gaussian beam. Therefore, the position of the beam expander is not strict.
ZAMIA Q Series RF Excited Laser - Integrated Beam Shaping System
N30Pro+ RF excited lasers provide built-in beam expander (3/4/5 times optional), which solves the tedious process of matching a beam expander for users, and improves the assembly time of the whole machine by more than 30%; moreover, it makes the laser beam energy distribution more uniform and the processing accuracy higher.
ZAMIA N30Pro+RF Excited Laser - Red Light + built-in beam expander
