Temporal behaviour of the focal shift of beam forming optics for high power single mode lasers

Abstract

New generations of thin disk and fibre lasers are combining excellent beam quality, typically having M² between 1.2 and approximately 8, with output power in the kilowatt range. This combination of power and beam quality brings the imaging performance of beam-forming optics into focus. Former research into beam quality degradation and focal shift revealed that the high beam qualities increase the requirements on the optical system of laser processing heads [1], [2]. In many cases the degradation in beam quality and the focal shift inhibits stable process conditions. Furthermore, the classical spot size calculation on the work piece doesn’t work for high power diffraction limited lasers.To pre-estimate the impact of the laser induced focal shift on a specific process, one has to know the temporal response of the optical system to changes in the laser power used. Therefore, the step function response of the laser induced focal shift of beam forming optics was measured with high temporal resolution. These sophisticated measurements in a wide range of different time scales of beam forming optics brought up a comprehensive basis to estimate the impact of the laser induced focal shift on specific laser based processes.New generations of thin disk and fibre lasers are combining excellent beam quality, typically having M² between 1.2 and approximately 8, with output power in the kilowatt range. This combination of power and beam quality brings the imaging performance of beam-forming optics into focus. Former research into beam quality degradation and focal shift revealed that the high beam qualities increase the requirements on the optical system of laser processing heads [1], [2]. In many cases the degradation in beam quality and the focal shift inhibits stable process conditions. Furthermore, the classical spot size calculation on the work piece doesn’t work for high power diffraction limited lasers.To pre-estimate the impact of the laser induced focal shift on a specific process, one has to know the temporal response of the optical system to changes in the laser power used. Therefore, the step function response of the laser induced focal shift of beam forming optics was measured with high temporal resolution. These so...