Beam shaping for multi-kW single mode lasers

The power of modern TEM₀₀ CW and pulse lasers reaches several kW, which causes specific requirements to beam shaping optics from the point of view of resistance to high-power laser radiation, stable transformation of irradiance distribution. Various industrial applications, such as additive manufacturing, laser drilling, require creating flat-top, “inverse-Gauss”, “doughnut” intensity distributions of focused spots in the working plane, which is achieved by manipulating the beam wavefront using refractive aspherical or diffractive optical elements (DOE). Gradient heating of optical elements in the case of high-power single mode CW fiber lasers or solid-state pulse lasers leads to thermal lensing resulting in thermal focus shift and thermally induced spherical aberration. The cause of this thermal lensing is the distortion of the beam wavefront, which is practically a wavefront aberration; this aberration disturbs optical effects of beam shaping optics through uncontrolled transformation of intensity profile, leads to instabilities in conditions of material processing. Avoidance or minimization of undesirable thermo-optical effects is an actual practical task.

The proposed solution presents refractive beam shapers with smooth optical surfaces that transform the Gaussian profile of a TEM₀₀ beam into a beam with the Airy disk intensity distribution and a specific phase profile in a control manner with minimum wavefront deformation. When focusing this collimated beam with a lens, optimal conditions of interference are created near the focal plane to create flat-top, “doughnut”, “inverse-Gauss”, “spot with ring” spot profiles. Implementing these beam shaping optics from such crystals as sapphire or crystalline quartz, characterized by high thermal conductivity or self-compensating thermo-optical effects, makes it possible to minimize or exclude thermal lensing and avoid its negative impact on the stability and quality of the resulting spot profiles. This approach allows operation with high power CW and ultra-short pulse lasers, use F-theta lenses and objectives and mirror scanners, with depth of focus similar to the Rayleigh length of a comparable TEM₀₀ beam, easy integration in industrial equipment, simple adjustment procedure and switching between profiles. There will be considered design basics of beam shapers, analysis of profile behaviour near focal plane, examples of implementations, results of profile measurements and material processing.