Thermal lens effect is a phenomenon observed in laser systems where the temperature gradient generated by the absorbed laser energy causes changes in the refractive index of the laser medium, leading to a change in the focusing properties of the laser beam. This effect can significantly impact the performance and stability of high-power laser systems, such as the HFM-250W pulsed fiber laser.
Understanding the Thermal Lens Effect
When optical components are exposed to continuous laser irradiation, as seen in fiber laser marking machines, elevated temperatures can lead to thermal deformation. This, in turn, affects the refractive index of transmitting optical elements and the reflection direction of reflective optical elements, thus impacting laser beam characteristics.
In the context of fiber laser marking, the output laser from the isolator module passes through a beam steering mirror and a focusing lens before material marking at the focal point. Therefore, the thermal lens effect is predominantly noticeable on the galvanometer and the field lens.
Effects on Optical Elements
The galvanometer, composed of a driver board, servo motor, and X-Y optical reflector, can experience thermal deformation due to low reflectivity, impurities, or surface contamination. This leads to a transformation from a planar to a convex mirror, resulting in distorted output spots and energy distribution alterations post-focusing.
Comparison of Thermal Lens Effects on Galvanometer
The field lens, made up of convex and concave lenses, plays a role in focusing the output spot. When the transmittance is low, impurities are present in the medium, or the lens surface is contaminated, the lens undergoes thermal deformation, altering its focal length and depth. This effect can lead to focal points not residing in a single horizontal plane, causing inconsistent marking effects between the central and edge regions.
Comparison of Thermal Lens Effects on Field Lenses
Testing the Thermal Lens Effect
To assess the thermal lens effect, a 250W laser was utilized to create three straight lines on an inclined surface. After emitting light at 90% power for 30 minutes, the file was shifted 1mm to the left, and another set of lines was created. Measuring the displacement differences between the two sets of lines allows calculation of the thermal effect-induced focal length change (∆H = d × sinθ), with larger laser powers magnifying the thermal lens effect.
Thermal Lensing Effect Testing Method
Comparing Thermal Effects with Different Lenses
Using the HFM-250W fiber laser with various lenses, experimental results were obtained by employing the thermal lens effect testing principle. It was observed that lenses made from ordinary glass exhibited the largest displacement, indicating a pronounced thermal lens effect. In contrast, lenses crafted from full quartz material exhibited minimal displacement, signifying a lesser thermal lens effect.
Thermal Effects Comparison of Different Lenses
The thermal lens effect significantly impacts the stability of fiber laser marking, particularly for sensitive materials. To mitigate these effects, quality beam steering mirrors and focusing lenses crafted from full quartz material are recommended. Routine maintenance to prevent impurity contamination and the replacement of damaged optical elements are essential strategies to minimize the influence of thermal lens effects.