Radially Polarized Picosecond MOPA System Based on Double-Clad Ytterbium-Doped Spun Tapered Fiber with Ring-Shaped Active Core

Active large mode area (LMA) tapered double-clad fibers (T-DCF) have been widely used in high-power pulsed fiber Master Oscillator Power Amplifier (MOPA) systems as a result of the capability of such systems to deliver a few MW-level peak power and several hundred watts of average power with several tens of microjoules pulse energy [1]. Cylindrical-vector beams perform various advantages for industrial and scientific applications, such as particle acceleration and trapping, high-resolution microscopy, optical data storage and material processing. Up to the present time, CW laser, nanosecond MOPA and high-power MOPA systems delivering radially and azimuthally polarized output beams have been demonstrated [2]. Typically, they are represented by a MOPA system in which an isotropic LMA fiber is used for the last amplification stage. However, there are some limitations that come with choosing isotropic LMA fiber - in particular, uncontrolled distortion of the spatial distribution and low contrast between the bright and dark zones of the doughnut-shaped beam. Recently, our group has demonstrated a new type of T-DCF - so-call spun T-DCF (sT-DCF) with small birefringence supporting the propagation and amplification of radially polarized light, which we propose for the solution to existing problems [3].