Maintaining radial-polarization and beam-quality in muli-kW rod-based lasers through the use of polycrystalline Nd:YAG rods

MOPA configurations rely on initial beam parameters set by the oscillator (beam quality, polarization) that must be preserved during amplification. The main factors that degrade beam quality in rod based lasers are: birefringence induced bifocusing, thermal spherical aberrations, and pump-nonuniformity induced azimuthal aberrations. Thermally induced bifocusing was totally bypassed using cylindrically (radially or azimuthally) polarized beams [1]. Cylindrical polarizations are, however space variant, so attention must be paid to factors that degrade such polarizations. Main factors are: a non-concentrically aligned beam and pumped rods axes; and non-radially symmetric optical perturbations, such as azimuthal aberrations, non-radially symmetric absorption/amplification, and non-radially symmetric birefringence. Our STAR pump chambers were designed to side-pump rods while producing perfect radially-symmetric pump distributions and demonstrated low azimuthal aberrations. Spherical aberrations were corrected for each STAR by using specially designed wave-plates produced by Asphericon. Maximum optical pump power per STAR was 7kW (at 806nm) and short-cavity output-power was 3.1kW. Figure 1 presents the measured wave-front deformations in radially-polarized beam after single passes through each pump chamber that we tested, and the beam quality degradation calculated based on the measured WFs. The total WF distortion accumulated from all of the amplifiers and the predicted beam quality degradation appears on the right. The remaining azimuthal aberrations resulted from variations between diode arrays can be corrected using a single free-form phase-plate.