The performance of high power laser cutting systems is often degraded by thermal lensing of the cutting head optics. Thermal lensing occurs when a small fraction of the beam is absorbed by the lens during operation. The absorbed power increases the temperature of the lens, which raises its index of refraction in positive dn/dT materials. The increased index of refraction causes the beam to focus early, shifting the focus back towards the cutting head. With three or more lenses in most 1 µm laser cutting heads, thermal lensing effects can quickly accumulate, resulting in focus shifts of several millimeters or more in multi-kW systems [1].
Mirrors, alternatively, have the opposite effect on focus shift [2]. A mirror surface becomes more convex as its surface temperature rises from laser power absorption. Consequently this causes the laser beam to diverge and shift the focus away from the cutting head. Mirrors used with lenses in this way can help compensate for thermal lensing effects in high power laser systems.
The goal of this paper is to characterize thermal focus shift mechanisms for different lenses and mirrors. A high power cutting head will be designed using a combination of mirrors and lenses to minimize focus shift. A breadboard version of the cutting head will be tested.