Laser beam processing is a powerful technique used in a variety of materials processing applications, including cutting, welding, drilling, and surface modification. In this article, we will explore the different ways in which laser beams can be used for materials processing, and discuss the advantages and limitations of each approach. We will also examine the factors that influence the effectiveness of laser beam processing, such as laser power, beam profile, wavelength, and pulse duration.
Additionally, we will discuss the importance of laser beam profiling in materials processing, and how accurate characterization of laser beams can help to optimize process parameters and improve the quality and efficiency of materials processing operations. Whether you are a enginer, researcher, laser manufacturer, or simply interested in the world of lasers, understanding the use of laser beams in materials processing is essential for achieving optimal results and unlocking the full potential of laser technology.
Materials processing applications that use laser beams
Laser beams are widely used in materials processing due to their ability to deliver high energy, high power, and highly focused beams of light to a specific location. Additionally, it is relatively easy to manipulate the amount of energy deposited in the interaction area on the target to change the amount of the removed material or adapt to the thickness of the metal when welded. Also the position of the beam can be changed easily during the process. When compared to CNC machines, the laser does not use any tool that removes the desired material. Instead, a light beam is used and, obviously, the light does not wear as mechanical tools do. Due to this fact lots of cost can be saved on tooling.
In the medical applications there are numerous advantages, too. Using the laser beam that e.g. cuts the tissue does not require any physical contact of the medical device with the patient making the device highly aseptic.
Some examples of materials processing applications that use laser beams include:
Cutting: Lasers can be used to cut a wide range of materials, from metals and plastics to ceramics, glass or even diamonds. The high-energy laser beam melts or vaporizes the material, creating a clean, precise cut with minimal heat-affected zone.
Welding: Lasers can be used to weld a wide range of materials, including metals, plastics, and ceramics. The laser beam melts the material, creating a weld that is strong and has minimal distortion.
Drilling: Lasers can be used to drill small, precise holes in a wide range of materials, including metals, plastics, and ceramics. The laser beam melts or vaporizes the material, creating a clean, precise hole.
Surface modification: Lasers can be used to change the surface properties of materials, such as surface hardening, surface cleaning, and surface texturing. The laser beam can be used to heat the surface, creating a change in the surface micro or nanostructure.
3D printing: Lasers can be used to fuse powders or melt plastics to create 3D structures. The laser beam is used to melt or fuse the material, layer by layer, to create the final 3D structure. This process is often called: sintering.
Marking and engraving: Lasers can be used to mark or engrave a wide range of materials, including metals, plastics, and ceramics. The laser beam can be used to remove or change the surface color of the material, creating a permanent mark or engraving.
Surface cleaning: Various surfaces can be cleaned using lasers. E.g. Historical artifacts can be renovated using pulsed lasers as it was done by the scientific team of Institute of Optoelectronics, Military University of Technology at Wawel castle in Cracow.
The choice of laser and the specific processing method will depend on the material and the desired end result:
- Nd:YAG pulsed lasers are used in cutting diamonds and renovation of arts.
- CO2 CW lasers are commonly used to cut plastics
- Fiber, CW lasers operating roughly at 1100 nm wavelength are commonly used to cut metals
- Nd: YAG is also used in marking applications
Diffraction limited focal spot - what does it mean?
A diffraction-limited focal spot refers to the smallest spot that can be formed by a laser beam using a lens or a mirror system. The size of this spot is determined by the diffraction of light, which is a fundamental physical phenomenon that occurs when light passes through an aperture or is reflected by a mirror.
The size of the diffraction-limited focal spot can be described by the Airy disk, which is the pattern formed by the superposition of the diffraction patterns produced by the individual points in the aperture of the lens or mirror. The size of the Airy disk is determined by the wavelength of the light and the numerical aperture (NA) of the lens or mirror system. The smaller the wavelength and the NA, the smaller the diffraction-limited focal spot will be.
It’s important to note that the diffraction-limited focal spot is the smallest spot that can be achieved using a lens or mirror system, but there are other factors that can affect the size of the focal spot in practice. For example, aberrations in the lens or mirror system, or the presence of dirt or dust on the optics can cause the focal spot to be larger than the diffraction limit. Additionally, thermal effects can also cause the focal spot to change in size over time.
A diffraction-limited focal spot is important in many applications that require high-resolution imaging, such as microscopy, or high-precision material processing. In these applications, a small focal spot can provide a high intensity at the focal point, which can increase the resolution and precision of the process.
Diffraction limited focal spot formula
Imagine your collimated laser beam has 1/e2 diameter D. It passes a lens with a focal length f and has wavelength lambda. In this case the smallest possible size of the focal spot is given by the formula:
d = 2.44*lambda*f/D
This is also called Airy disk size.
It is shown in the graphics below
Please, note that the diffraction-limited focal spot size can be defined as:
d = 1.22*lambda/NA.
NA is a parameter defining an optical system that the light passes through and it is named: Numerical Aperture. In the very advanced optical systems it is possible to tune NA to the level that effectively the focal spot will be smaller than the diffraction limit of a regular setup. This is a method employed in the lithography systems which are used to produce microprocessors. In these setups excimer lasers are used to produce structures much smaller than their wavelength.
How beam imperfections affect the focal spot and process parameters?
Beam divergence: An increase in beam divergence will cause the focal spot to be larger and less intense. This can result in a reduction of resolution and precision in material processing. Also if the ablation is the main mechanism, then its efficiency may be reduced (e.g. in marking applications).
Beam pointing stability: A lack of beam pointing stability can cause the focal spot to move around, making it difficult to maintain a consistent focus on the material. This can lead to variations in the process parameters and a reduction in precision.
Beam mode quality: A low-quality beam mode, such as a high-order transverse mode, can cause the focal spot to be uneven and have a non-uniform intensity distribution. This can lead to variations in the process parameters and a reduction in precision.
Spatial and temporal coherence: A low coherence can cause the focal spot to be larger and less intense, and can also affect the stability of the process. It can also affect the ability of the laser to focus the beam to a small spot, and the ability to create interference patterns.
Power distribution: A non-uniform power distribution can cause the focal spot to be uneven and have a non-uniform intensity distribution. This can lead to variations in the process parameters and a reduction in precision.
Spectral properties: Spectral properties such as a broad spectrum, can cause the focal spot to be larger and less intense, and can also affect the stability
Aberrations of the optical systems: imperfection of the optical systems used to focus the beam will increase the size of the focus and will scatter the power over a greater area. This phenomenon may result in the reduction of process precision or loss of imaging resolution.
As an example this image shows chromatic aberration:
And this image shows spherical aberration, which is very common in setups where the laser beam is focused by only one (especially spherical) lens:
To conclude, it is always true that the laser beam quality is critical from the perspective of the process where it is being used. It has to be monitored and the maintenance works should be planned to keep it as good as possible. One good example could be using femtosecond lasers in the medical procedure of cataract removal. In such an operation a human eye retina is cut by a femtosecond laser to allow removal of a natural lens. The size of a spot has a direct impact on the size of a scar forming after the procedure. This scar later scatters the light causing side effects. The relation is: the greater focal spot, the greater the risk of the side effects. This example very clearly presents how important it is to take care of the laser beam quality.
Why laser beam monitoring is essential in the process quality management?
Laser beam monitoring is essential in process quality management as it allows for real-time analysis of the laser beam properties, such as beam profile, power, and energy, ensuring that the laser process is operating optimally and within specified parameters. By continuously monitoring the laser beam, operators can detect potential issues early, allowing for quick corrective action and minimizing the risk of product defects or downtime. Furthermore, laser beam monitoring allows for accurate characterization of the laser beam, which is crucial for process optimization and control. Ultimately, laser beam monitoring is essential for ensuring high-quality products and optimizing manufacturing processes in industries such as medical, aerospace, and automotive. Huaris AI Cloud is predictive maintenance for laser systems.
Some of the reasons why laser beam monitoring is essential include:
Process control: By continuously monitoring the laser beam parameters, such as power, beam width, and pointing, it is possible to detect and correct any variations or changes that may occur, which can affect the process quality. This can help to ensure that the process is consistent and produces parts with the desired quality.
Safety: By monitoring the laser beam, it is possible to detect any unexpected changes in the beam that could indicate a problem with the laser or its optics. This can help to prevent damage to the equipment and potential safety hazards.
Efficiency: By continuously monitoring the laser beam, it is possible to detect any variations or changes in the beam that may affect the efficiency of the process. For example, a decrease in the beam power can result in a decrease in the cutting speed, or an increase in the beam pointing stability can result in an increase in the cutting precision.
Predictive/preventive maintenance: By monitoring the laser beam over time, it is possible to detect any changes or variations that may indicate a problem with the laser or its optics. This can help to identify potential issues before they lead to a failure or a significant reduction in the process quality.
Traceability: By monitoring the laser beam, it is possible to collect data about the process and the beam parameters, which can be used to trace potential changes back if the quality assurance needs to check the reasons of the process malfunction.
Numerous process parameters can be affected if the laser beam does not meet quality acceptance criteria. Huaris Cloud is the first solution of this kind to monitor the beam parameters over a long period of time and supports the laser owner in automatic detection of the laser misbehavior.