01 What is thermal lens effect
After the optical element is continuously irradiated by the laser beam for a long time, thermal deformation will occur due to the temperature rise, which will cause the refractive index of the transmissive optical element and the reflection direction of the reflective optical element to change. The generation of thermal lens effect will affect the laser mode and focus position after focusing.
For fiber laser marking equipment, the laser output from the isolator head of the laser will mark the material at the focal point after being deflected by the galvanometer and focused by the field lens. Therefore, the thermal lens effect mainly occurs on the galvanometer mirror and the field mirror.
02 Effect of Thermal Lensing
The vibrating mirror is mainly composed of a drive board, a servo motor and an X-Y optical mirror. When the reflectivity of the mirror is low, there are impurities in the medium, and the surface is polluted, the mirror will be thermally deformed by the laser beam, and the flat mirror will become a convex mirror, resulting in deformation of the output spot and affecting the energy distribution of the focused spot.
The field lens plays a role in focusing the output spot in the fiber laser marking equipment, and its characteristic is that all the focal points are on a horizontal plane. The field lens is mainly composed of concave and convex lenses. When the transmittance of the lens group is low, there are impurities in the medium, and the surface is polluted, the lens will be thermally deformed by the laser beam, and the focal length and focal depth of the lens will change. And the focus is not on a horizontal plane, the marking effect of the center and the edge is inconsistent.
03 Thermal Lens Effect Test Method
Use a 250w laser to test and print three straight lines on the slope. The straight lines are long enough so that the two ends of the lines printed by the laser on the slope cannot be printed due to partial focus. After 30 minutes of light output with 90% power at the center frequency, the image file shifts to the left. Draw three straight lines at 1mm, measure the offset of the two sets of straight lines before and after light emission, and the focal length change caused by the thermal effect △H=d×sina. The greater the laser power, the more obvious the thermal penetration effect.
04 Comparison of thermal effects of different lenses
Using the HFM-250W laser with different lenses, through the thermal lens effect test principle just mentioned, the following experimental results are obtained as shown in the figure:
F160 Full quartz telecentric
F160 Full Quartz
F160 Ordinary glass
It can be seen that the lens made of ordinary glass has the largest offset and the thermal lens effect is the most obvious; the lens made of all-quartz has the smallest offset and the thermal lens effect is small.
05 Measures to reduce thermal lens effect
The thermal lens effect will affect the stability of fiber laser marking, especially for the marking of sensitive materials. In order to reduce the influence of thermal lens effect, combined with the above experimental results, we can take the following measures:
1. Choose good quality galvanometer and field mirror, and use all quartz material;
2. Perform daily maintenance on the vibrating mirror and the field mirror to prevent the lens from being polluted by impurities;
3. Replace the damaged galvanometer and field mirror.
