Investigations on ring-shaped pumping distributions for the generation of beams with radial polarization in an Yb:YAG thin-disk laser.

Tom Dietrich,Martin Rumpel,Thomas Graf,Marwan Abdou Ahmed

doi:10.1364/oe.23.026651

Abstract

We present experimental investigations on the generation of radially polarized laser beams excited by a ring-shaped pump intensity distribution in combination with polarizing grating waveguide mirrors in an Yb:YAG thin-disk laser resonator. Hollow optical fiber components were implemented in the pump beam path to transform the commonly used flattop pumping distribution into a ring-shaped distribution. The investigation was focused on finding the optimum mode overlap between the ring-shaped pump spot and the excited first order Laguerre-Gaussian (LG(01)) doughnut mode. The power, efficiency and polarization state of the emitted laser beam as well as the thermal behavior of the disk was compared to that obtained with a standard flattop pumping distribution. A maximum output power of 107 W with a high optical efficiency of 41.2% was achieved by implementing a 300 mm long specially manufactured hollow fiber into the pump beam path. Additionally it was found that at a pump power of 280 W the maximum temperature increase is about 21% below the one observed with standard homogeneous pumping.

Highlights

An optimized polarization distribution can significantly improve the absorption of the laser radiation during material processing
We present experimental investigations on the generation of radially polarized laser beams excited by a ring-shaped pump intensity distribution in combination with polarizing grating waveguide mirrors in an Yb:YAG thin-disk laser resonator
A Grating waveguide mirrors (GWM) consists of a sub-wavelength diffraction grating integrated into a highly reflective (HR) multilayer mirror

Summary

Introduction

An optimized polarization distribution can significantly improve the absorption of the laser radiation during material processing. Theoretical and experimental studies have shown a considerable increase of the process efficiency, e.g. for drilling, cutting and deep-penetration welding, using cylindrically (radially or azimuthally) polarized laser radiation as compared to other polarization states [1,2,3]. Such beams offer interesting focusing properties [4,5,6,7] which can be used for a variety of applications such as optical tweezers [8] or particle acceleration [9]. We show that using ring-shaped pumping leads to a significant reduction of the temperature (i.e by approximately 21%) in the central area of the disk while keeping the laser efficiency as high as in the case of the flattop pumping (i.e around 41% at an output power higher than 100 W)

Generation of ring-shaped pumping distributions

Experimental set-up

Thin-disk laser performance

Temperature distribution across the thin-disk laser crystal

Findings

Conclusion