[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Thin-Disk Laser Emitting Beams with 980 W of CW-Output Power and Radial Polarization

摘要： Within the past decade, a variety of scientific groups have demonstrated the potential of radially and azimuthally polarized laser beams in many applications [1-3]. Experimental investigations in high-power operation indicate additional beneficial properties for material processing, such as drilling and deep-penetration-welding, which have been demonstrated to lead to an increase of the process efficiency by using such a polarization state. Cutting of metal sheets with radially polarized beams at 1 micron wavelength have been reported recently to increase the process efficiency [4]. In the present work, we report on the generation of radially polarized laser beams in cw operation by using grating-waveguide mirror (GWM) [5]. The grating design is based on the leaky-mode coupling mechanism, which allows for sufficient reduction of the reflectivity of the polarization state (azimuthal or radial) to be suppressed in the resonator. It consists of a circular sub-wavelength diffraction grating integrated into a highly or partially reflective dielectric multilayer mirror. At the central wavelength of the Yb:LuAG TDL, i.e., at 1030 nm, the absolute values of the reflectivities for the TE- and TM-polarized light were measured to be (55.2 ± 0.2)% and (99.8 ± 0.2)%, respectively. This corresponds to a reflectivity difference of approximately 45%. At wavelengths of 1023, 1042, and 1059 nm, the reflectivity of the TE-polarized beam was measured to be <1%, making the grating attractive for other applications with lasers operating within this wavelength range. The grating was implemented as cavity end mirror in an Yb:LuAG thin-disk laser setup, pumped at its “zero-phonon” line, i.e. λ = 969 nm. The implementation of the GWM led to the extraction of a radially polarized beam with an output power of 980 W and an optical efficiency of 50.5%. Single transversal mode operation of a radially polarized LG01 mode was sustained for the whole output power range with beam quality factors of Mx2 = 2.0 and My2 = 2.3. Figure 1(a) shows a photograph of the GWM. In Fig. 1(b), the performance of the laser and the far field intensity distribution of the emitted laser beam is shown. A qualitative analysis of the polarization was carried out by recording the intensity distributions transmitted through a rotating polarizer at different orientations of its transmission axis. Furthermore, the purity of polarization (degree of polarization) was measured with a custom-made camera-based 2D Stokes-polarimeter. An average degree of radial polarization of 95% was measured.