With the continuous expansion of the application range of fiber lasers in the field of industrial processing, there is a greater demand for high-power fiber lasers.
In the application of higher power laser, a series of new problems will appear, which will affect the stability of laser processing, such as thermal lens effect.
What is thermal lens effect？
The laser absorptivity increases due to the poor quality, dirt and damage of the optical element, and the thermal deformation occurs when the optical element is irradiated continuously by the laser beam for a long time, which leads to the change of the refractive index of the transmission optical element and the reflection direction of the reflective optical element.
The thermal lens effect will change the position of the laser focus (beam waist), and then affect the application effect.
Because the transparent optical element has a higher laser absorptivity and a greater rise in temperature, the thermal lens effect is more obvious.
Next, we mainly analyze the optical elements which are prone to thermal lens problems in different laser application equipment.
The influence of thermal lens effect in the application of laser
The higher the power and the processing of the high inverse material, the faster the thermal expansion of the optical element is, and the more obvious the thermal lens is.
When the thermal lens effect is produced, the optical element expands by heat, the focusing ability becomes stronger, the focal spot size becomes smaller, and the focal length and focal depth become shorter.
The occurrence of these phenomena will eventually cause the instability of laser processing and affect the effect of laser processing.
1. The influence on marking
(1) Alumina blackening:
When the thermal lens effect occurs, the focal length becomes shorter, the surface energy density of the material decreases, alumina can not be blackened, and in serious cases, the blackness of the center and edge effect is inconsistent.
(2) Deep metal carving:
Metal deep sculptures generally use field mirrors with short focal length (depth of focus).
When high-power deep carving, due to the thermal lens effect, the energy density of the material decreases rapidly, so that the metal is not deep.
Because the expansion of the center of the optical element is larger than that of the edge, the center is shallow and the depth around is not the same.
(3) Sheet cutting:
According to different materials, pulsed fiber lasers usually use single slow speed or multiple fast cutting methods.
Thermal expansion and cooling retraction can occur in a very short time (< 1s), so the starting position of the cutting can be cut through, and the material can not be cut in other positions.
2. Influence on welding
(1) Metal sheet spot welding:
For the spot welding metal sheet of high power fiber laser, the thermal lens effect will cause inconsistent solder joint size, unstable welding and insufficient drawing force.
(2) Continuous metal welding:
The reflectivity of continuous light welding metal is higher than that of cutting.
In high power welding, the thermal lens effect will cause the part before welding to be normal, and the back part to be welded with shallow depth or completely impenetrable.
When welding aluminum and copper high reflective materials, the thermal lens effect is more obvious.
3. Influence on cutting
(1) Stainless steel cutting:
When cutting stainless steel, the thermal lens effect will cause inconsistent cutting surfaces, more and more slag, and even continuous cutting problems.
(2) Carbon steel cutting:
When cutting carbon steel, the thermal lens effect will cause more and more slag at the bottom and continuous cutting problems.
Solution to thermal lens effect
- The solution to the thermal lens effect produced by the lenses of different optical elements in the vibrating lens processing equipment:
Galvanometer
| Optical element | Causes of thermal lens | Solution of thermal lens effect |
| 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 of collimating lens | Clean the collimating lens or replace the collimator with good lens quality |
| Combined lens | Poor quality, dirt and damage of combined lens | Select the self-contained red laser or replace the beam combining lens with good quality |
| Field mirror | Poor quality, dirt and damage of field lens | Clean the field lens or replace the quartz field lens |
Comparison of red photosynthetic lens and thermal lens
Comparison of field lens and thermal lens
- Solution of thermal lens effect caused by different optical element lenses of cutting head/welding head processing equipment
Laser welding head
Laser cutting head
| Optical element | Causes of thermal lens | Solution of thermal lens effect | |
| Laser output head | Dirty or damaged output head lens or crystal | Inspection and cleaning before installation | |
| Cutting head / welding head | Upper protective lens | Poor quality, dirt and damage of lens | Clean the lens or replace the lens with good quality |
| collimating lens | |||
| Focusing lens group | |||
| Lower protective lens | |||
Front and rear comparison of lower protective lens and thermal lens
Front and rear contrast of focusing lens and thermal lens
