Thermal Lens Effects in Laser Applications: Solutions and Insights

Thermal Lens Effects And Solutions For 3 Major Laser Applications

What is thermal lens effect

The laser absorptivity increases due to poor quality, dirt, and damage of the optical elements. Continuous exposure to laser beams for a prolonged period of time can cause thermal deformation, altering the refractive index of the transmission 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. The transparent optical elements have a higher laser absorptivity and a greater temperature increase, 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 lens effect in the application of laser

The faster the thermal expansion of an optical element, the more prominent its thermal lens effect, as a result of high power and advanced processing of inverse materials.

The thermal lens 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 blackening:

When the thermal lens effect takes place, the focal length shortens, the surface energy density of the material decreases, and blackening of alumina becomes impossible. In severe cases, the blackening effect at the center and edge may become inconsistent.

Alumina blackening

(2) Deep metal carving:

Metal deep sculptures typically utilize field mirrors with a shallow focal length (depth of focus).

During high-power deep carving, the thermal lens effect causes the energy density of the material to rapidly decrease, resulting in a shallower depth in the metal.

Due to the greater expansion at the center of the optical element compared to the edge, the center is shallower and the depth around it is uneven.

Deep metal carving

(3) Sheet cutting:

Pulsed fiber lasers use either 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 retraction that occurs in a very short time frame (< 1s), the starting position of the cutting can be successfully cut through, while other positions may not be able to be cut.

2. Influence on welding

(1) Metal sheet spot welding:

When using a high-power fiber laser for spot welding metal sheets, the thermal lens effect can result in inconsistent joint sizes, unstable welding, and insufficient drawing force.

(2) Continuous metal welding:

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 portion of the weld to be normal, while the back portion may be shallow or completely impenetrable.

The thermal lens effect is particularly pronounced when welding aluminum and copper, which are high-reflective materials.

Continuous metal welding

3. Influence on cutting

(1) Stainless steel cutting:

When cutting stainless steel, the thermal lens effect can lead to inconsistent cutting surfaces, an increase in slag, and even difficulties in maintaining a continuous cut.

(2) Carbon steel cutting:

When cutting carbon steel, the thermal lens effect can result in an increase in slag at the bottom and difficulties in maintaining a continuous cut.

Carbon steel cutting

Solution to thermal lens effect

  1. The solution to the thermal lens effect caused by the lenses of various optical elements in vibrating lens processing equipment is:
Galvanometer

Galvanometer

Optical elementCauses of thermal lensSolution of thermal lens effect
Laser output headDirty or damaged output head lens or crystalInspection and cleaning before installation
Collimator (for CW lasers)Poor quality, dirt and damage of collimating lensClean the collimating lens or replace the collimator with good lens quality
Combined lensPoor quality, dirt and damage of combined lensSelect the self-contained red laser or replace the beam combining lens with good quality
Field mirrorPoor quality, dirt and damage of field lensClean the field lens or replace the quartz field lens
Comparison of red photosynthetic lens and thermal lens

Comparison of red photosynthetic lens and thermal lens

Comparison of field lens and thermal lens

Comparison of field lens and thermal lens

  1. The solution to the thermal lens effect caused by the lenses of various optical elements in cutting head/welding head processing equipment is:
Laser welding head

Laser welding head

Laser cutting head

Laser cutting head

Optical elementCauses of thermal lensSolution of thermal lens effect
Laser output headDirty or damaged output head lens or crystalInspection and cleaning before installation
Cutting head / welding headUpper protective lensPoor quality, dirt and damage of lensClean 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 comparison of lower protective lens and thermal lens

Front and rear contrast of focusing lens and thermal lens

Front and rear contrast of focusing lens and thermal lens

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Shane
Author

Shane

Founder of MachineMFG

As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.

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