What is thermal lens effect ？
Laser absorptivity increases due to poor quality, dirt and damage to optical elements. Continuous exposure to laser beams for a prolonged period of time can cause thermal deformation, changing the refractive index of the transmitting optical element and changing the reflection direction of the reflective optical element.
This results in a change in the position of the laser focus (beam waist), affecting its application. Transparent optical elements have higher laser absorptivity and higher temperature rise, making the thermal lens effect more pronounced.
In this article, we will analyze the optical elements that are susceptible to thermal lens problems in various laser application equipment.
The influence of thermal effect on the application of metal
The faster the thermal expansion of an optical element, the more prominent its thermal lensing effect will be as a result of high power and advanced inverse materials processing.
The thermal lensing effect occurs when heat causes the optical element to expand, leading to an increase in focusing power, a reduction in focal spot size, and a shorter focal length and depth.
These phenomena can lead to instability during laser processing, ultimately impacting its effectiveness.
1. The influence on marking
(1) Alumina darkening:
When the thermal lensing effect occurs, the focal length decreases, the surface energy density of the material decreases, and darkening of the alumina becomes impossible. In severe cases, the dimming effect at the center and edge may become inconsistent.
(2) Deep metal carving:
Deep metal sculptures typically utilize field mirrors with a shallow focal length (depth of focus).
During high-power deep carving, the thermal lensing effect causes the energy density of the material to decrease rapidly, resulting in a shallower depth in the metal.
Due to the greater expansion in the center of the optical element compared to the edge, the center is shallower and the depth around it is uneven.
(3) Sheet cutting :
Pulsed fiber lasers use a single slow speed or multiple fast cutting methods, depending on the type of material being processed.
Due to the rapid thermal expansion and cooling shrinkage that occurs in a very short period of time (<1s), the initial cutting position may be cut successfully, while other positions may not be cut.
2. Influence on welding
(1) Spot welding of sheet metal:
When using a high-power fiber laser for spot welding of sheet metal, the thermal lens effect can result in inconsistent joint sizes, unstable welding, and insufficient pulling force.
(2) Continuous welding of metal:
The reflectivity of metal during continuous light welding is higher compared to that during cutting.
In high-power welding, the thermal lens effect can cause the front part of the weld to be normal, while the back part can be shallow or completely impenetrable.
The thermal lens effect is particularly pronounced when welding aluminum and copper, which are highly reflective materials.
3. Influence on the cut
(1) Stainless steel cutting :
When cutting stainless steel, the thermal lensing effect can lead to inconsistent cutting surfaces, increased slag, and even difficulty maintaining a continuous cut.
(2) Carbon steel cutting :
When cutting carbon steel, the thermal lens effect can result in increased slag at the bottom and difficulties in maintaining a continuous cut.
Solution for thermal lens effect
- The solution to the thermal lensing effect caused by the lensing of various optical elements in vibrating lens processing equipment is:
Galvanometer
|
Optical element |
Causes of thermal lenses |
Thermal Lens Effect Solution |
|
laser output head |
Dirty or damaged output head lens or crystal |
Inspection and cleaning before installation |
|
Collimator (for CW lasers) |
Poor quality, dirt and damage to collimation lenses |
Clean the collimating lens or replace the collimator with a good quality lens |
|
Combined lens |
Poor quality, dirt and damage to combined lenses |
Select independent red laser or replace beam combiner lens with a good quality one |
|
Field mirror |
Poor quality, dirt and damage to field lenses |
Clean the field lens or replace the quartz field lens |
Comparison of red photosynthetic lenses and thermal lenses
Comparison of Field Lenses and Thermal Lenses
- The solution to the thermal lensing effect caused by the lenses of various optical elements in cutting head/welding head processing equipment is:
laser welding head
laser cutting head
| Optical element | Causes of thermal lenses | Thermal Lens Effect Solution | |
|---|---|---|---|
| laser output head | Dirty or damaged output head lens or crystal | Inspection and cleaning before installation | |
| cutting head/welding head | Superior protective lens | Poor quality, dirt and damage to lenses | Clean the lens or replace the lens with a good quality one |
| collimating lens | |||
| Focus lens group | |||
| Lower protective lens | |||
Front and back comparison of lower protection lens and thermal lens
Front and back contrast of focus lens and thermal lens
