The key aspect of working with F-Theta lenses is the importance of working in the Back Focal Length (BFL) of the lens as defined by the manufacturer. Only at this point will there be optimal consistency in marking performance across the working plane. If one works outside of the BFL, even when the best available focus is achieved in the centre of the field, there will be an increased variance of the feature or spot size across the field. This variance is lowest for high quality low M2 beams which exhibit a high Rayleigh length and hence depth of focus. Working with higher M2 beams out of focus means that there will be significant variance over the field. Thus in order to achieve the finest and most consistent feature size desired, always work in the vendor specified BFL plane of the lens. Any change in spot size must then be accomplished by either changing the input diameter of the beam or changing focal length of the F-theta lens. Simply varying working distance to change spot size will result in an inconsistent marking across the field.
Another important consideration is the state of collimation of the input beam from the laser to the scanner. If the beam is poorly collimated either diverging or converging this will affect the effective focal position of the beam and hence the working plane. Most consistent film ablation and dark (oxide) marks across a scan field require the collimation of the input beam to be adjusted so that the optimal focal condition corresponds with the specified BFL of the F-Theta lens being used.
Attempting to use a system where the focus turns out to be even 1% from the specified BFL will change both spot size and will vary power density at different points in the mark field and in some cases even shift the focus.
This study highlights the need to adjust the input of the beam into the scan head to focus at the specified BLF for the F-theta lens. Such adjustment allows users for example to fully benefit from advantages of higher powered, higher-mode lasers.