PRE-FOCUSING DEFLECTION UNITS

Pre-focusing deflection units are developed due to cost and size limitations on the output lens and the beam aperture limitations with 2-axis deflection units, which limit their ability to produce smaller spot sizes over medium size scanning fields. Pre-focusing deflection units meet customers application requirements for large processing fields with smaller spot sizes and allow the user to change the working distance, field and spot size with the same deflection unit. 3-axis technology also benefits application-on-the-fly with moving targets, while in 3D-applications, the setup enables processing of non-flat parts or uneven surfaces and high power products. These solutions are now available for Nd:YAG, diode, and CO₂-lasers.

In a pre-focusing deflection unit, the laser beam first enters a moving lens - the Linear-Translator-Module. A moving lens diverges the beam rapidly before it passes one or two focusing lenses. Then the beam hits a deflection unit that directs the laser towards any X-Y position inside the marking field. 

In a deflection unit without focus correction, the focused laser spot at the centre of the field describes an arc when moved in either axis, creating a sphere of focused points above the working field. At locations away from the centre of the working field, the laser beam is not focused. This is due to the increased length from the lens to the workpiece, as the scanners direct the beam away from the centre of the field. 

In a pre-focusing deflection unit, focus compensation is accomplished by slight adjustments in the distance between the moving lens and the focusing lens, as the scanners direct the beam across the working field by a third moving Z-axis, hence the name “pre-focusing deflection unit”.