Elongating depth of field by focusing radiation of multimode lasers using multi-focus beam shaping optics

Abstract

Performance of laser technologies like welding, cutting of thick metal sheets, which are realized using multi-kW multimode fiber lasers, fiber-coupled solid-state and diode lasers, can be essentially enhanced by elongated effective depth of field (DOF) of focused laser beam. Increasing of zone along optical axis with minimized spot size makes it possible to distribute laser energy along axis and get additional processing effects from delivery the energy portion in depth of a workpiece. Another positive effect is in widening the tolerances of a workpiece dimensions and positioning a laser processing head. Because of relatively low optical quality of multimode laser radiation, characterized by high values of specifications Beam Parameter Product (BPP) or M2, the DOF by ordinary focusing is narrow. To elongate the caustic of focused beam it is suggested to apply refractive optics splitting an initial laser beam into several beamlets, which are focused in different foci separated along optical axis. This optics can operate both as a focusing lens and as a collimator. With the multi-focus optics, the length of zone of material processing along optical axis is defined rather by distances between separate foci, which are determined by optical design of the optics and can be chosen according to requirements of a particular laser technology. Due to stability of the distances between foci there is provided stability of a technology process. This paper describes some design features of refractive multi-focus optics, examples of real implementations and experimental results are presented as well.Performance of laser technologies like welding, cutting of thick metal sheets, which are realized using multi-kW multimode fiber lasers, fiber-coupled solid-state and diode lasers, can be essentially enhanced by elongated effective depth of field (DOF) of focused laser beam. Increasing of zone along optical axis with minimized spot size makes it possible to distribute laser energy along axis and get additional processing effects from delivery the energy portion in depth of a workpiece. Another positive effect is in widening the tolerances of a workpiece dimensions and positioning a laser processing head. Because of relatively low optical quality of multimode laser radiation, characterized by high values of specifications Beam Parameter Product (BPP) or M2, the DOF by ordinary focusing is narrow. To elongate the caustic of focused beam it is suggested to apply refractive optics splitting an initial laser beam into several beamlets, which are focused in different foci separated along optical axis. This opti...