High-power focused beams for industry

Multi-kilowatt beams with high brightness offer advantages for fast and precise material processing applications such as remote cutting, where the beam must propagate over large distances. Thin-disk lasers, like the one shown in Figure 1, are available commercially and are widely used for material processing in the automotive, solar, and microelectronic industries. They can provide the desired combination of high-power and highly focused beams because the thermally induced aberrations of the laser are orders of magnitude lower than in conventional rodor slab-shaped crystal lasers. Furthermore, thin-disk crystals can be pumped intensively and still be cooled very homogenously and efficiently due to the high surface-to-volume ratio. In recent years, researchers have proved the theoretical and practical scalability of thin-disk lasers to very high power levels.1 With high-quality crystals and optimized heat sinks, it is possible to generate beams with several kilowatts of power and very high efficiencies (typically 60–65%)2 by simply increasing the pump spot diameter. However, air turbulence in the cavity and a thermally-induced phase distortion (on the order of a tenth of the wavelength) in the disk itself limit the beam quality at higher powers. We have designed a high-power, high-quality thin-disk laser suitable for remote cutting. We avoided the air turbulence by operating the laser cavity in a helium atmosphere and developed a mirror with an aspherical profile (see Figure 2). This compensated for the distortion of the optical phase caused by the temperature difference between the pump spot and the un-pumped region in the laser disk. Calculations confirmed by precise interferometric measurements showed a phase distortion in the laser experiments of about 100nm with a supergaussian edge profile. Our design used an ytterbium-doped yttrium aluminum garnet (Yb:YAG) thin-disk laser crystal attached to a diamond heat sink. A pump cavity with 20 passes through the thin-disk crystal ensured an efficient absorption of the diode laser pump Figure 1. View into the pumping optics of a vacuum-proof thin-disk laser cavity.