Thermal lensing mitigation for high power laser processing

We have developed an optical model capable of designing and evaluating lenses for high power lasers. The model is used to design two types of optical lenses suited for high power laser applications. A three element objective lens with a focal length of 150mm and NA of 0.2 is capable of achieving diffraction limited performance. However, the performance is greatly compromised when conventional fused silica is used. Both the spot size and waist location are too sensitive to lens temperature variation. To overcome the issue, an athermal lens with both fused silica and CaF2 elements can perform well over a wide range of temperature variation from 20 °C to 300 °C. Similarly, a f-theta lens with a focal length of 200mm, scanning angle of ±18° is designed. Both thermal lensing caused by material as well as environment temperature are analyzed. Overall, the f-theta lens is not as sensitive as the objective due to its small NA. Nevertheless, thermal lensing can still affect the process and needs to be addressed. With a similar approach, an athermal f-theta lens with a combination of fused silica and CaF2 elements can perform well over a wide range of temperature variation from 20 °C to 300 °C.