sharply focus such a beam [1] makes it a versatile tool for various applications [2] such as plasmon excitation, optical trapping and laser material processing. In the field of cutting thick metal sheets with a radially polarized laser the benefits were outlined theoretically in 1999 by Niziev et al. [3]. In 2007 it was demonstrated by Meier et al. that the drilling speed of holes in mild steel with a Q-switched nanosecond laser can be increased by a factor of 1.5-4 by using azimuthally polarized laser radiation instead of linear or circular polarization [4]. Furthermore, it was experimentally shown that picosecond lasers with such polarization states are promising tools for the fabrication of micro holes [5].Axially symmetric polarized laser radiation can be either generated intra-or extra-cavity. While the latter was realized at high average powers in the multi kW range, e.g. by means of segmented half-wave plates and a multimode input beam [5], the former has been demonstrated with different approaches such as customized fibers [6], a tripleaxicon retroreflector unit [7], the so-called Giant Reflection to Zero Order (GIRO) mirror [8] or grating mirrors [9,10]. Up to now the listed intra-cavity approaches were only demonstrated in continuous wave (CW) operation. To generate pulsed beams with axially-symmetric polarization states at high average power so far only the aforementioned segmented half-wave plates were used to transform an incident linearly polarized fundamental-mode laser beam into a radially or azimuthally polarized LG01* mode. 85 W of average output power and radially polarized pulses as short as 750 fs were, for instance, achieved by means of three cascaded single-crystal fiber (SCF) amplifier stages starting from a linearly polarized seed beam with an average power of 1.5 W [11]. The polarization conversion was implemented between the second and the third amplification stage. An even higher output power was demonstrated Abstract We report on a single-stage high-power amplification of a radially polarized mode-locked laser beam in a single-crystal fiber (SCF) amplifier. The seed beam was amplified by a factor of 5.0 to an average output power of 66.3 W. The pulse duration of the amplified pulses was measured to be 909 fs at a repetition rate of 40.7 MHz, corresponding to a pulse energy of 1.63 µJ and a resulting pulse peak power of 1.58 MW. The output beam showed a very high quality of the doughnut-shaped intensity distribution and furthermore a high radial polarization purity.

We report on a high-power passively mode-locked radially polarized Yb:YAG thin-disk oscillator providing 125 W of average output power. To the best of our knowledge, this is the highest average power ever reported from a mode-locked radially polarized oscillator without subsequent amplification stages. Mode-locking was achieved by implementing a SESAM as the cavity end mirror and the radial polarization of the LG* mode was obtained by means of a circular Grating Waveguide Output Coupler. The repetition rate was 78 MHz. A pulse duration of 0.97 ps and a spectral bandwidth of 1.4 nm (FWHM) were measured at the maximum output power. This corresponds to a pulse energy of 1.6 µJ and a pulse peak power of 1.45 MW. A high degree of radial polarization of 97.3 ± 1% and an M-value of 2.16 which is close to the theoretical value for the LG* doughnut mode were measured.

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We report on the generation of kW-class, continuous-wave, radially polarized laser radiation in an Yb:LuAG thin-disk laser (TDL). Output powers of up to 980 W were achieved with optical efficiency of 50.5% with respect to the incident pump power of the TDL. The degree of radial polarization was measured to be 95.5% at maximum output power. This was achieved by the integration of a new generation of broadband, large-area grating-waveguide mirror with unprecedented high polarization discrimination as the end-mirror in the TDL resonator.

A simple and compact single-stage Yb:YAG single-crystal fiber amplifier was setup to amplify 784 fs long seed pulses to an output energy of 6 $$\upmu$$ μ J and an average output power of 290 W. The experimental results are verified by numerical models to estimate the limitations of the SCF technology with regards to beam quality and average output power.