LAUSR

We report an Yb-doped fiber master-oscillator power-amplifier (MOPA) system with the capability of selectively generating doughnut-shaped radially and azimuthally polarized beams with user-defined temporal pulse shapes. The desired output polarization was generated with the aid of a nanograting spatially variant half-waveplate (S-waveplate). The latter was used to convert the linearly polarized fundamental (LP01) mode output from the preamplification stages to a doughnut-shaped radially polarized beam prior to the power amplifier stage. A maximum output pulse energy of ∼860  μJ was achieved for ∼100  ns pulses at 25 kHz with user-defined pulse shape for both radial and azimuthal polarization states. The polarization purity and beam propagation factor (M2) were measured to be >12  dB and 2.2, respectively.