Maintaining radial-polarization and beam-quality in muli-kW rod-based

lasers through the use of polycrystalline Nd:YAG rods

Inon Moshe, Steven Jackel, Yaakov Lumer, Avi Meir, Revital Feldman, Yehoshua Shimony Applied Physics div., Soreq NRC, Yavne 81800, Israel

inon@soreq.gov.il

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.

Figure 1: Upper line: Measured WF deformations in STAR modules after spherical aberrations were corrected. Bottom line: FF intensity profile and beam-quality degradation calculated using the measured WFs. On the right: accumulated WF distortion and total beam quality

degradation calculated on the basis of the separate pump chamber WF measurements.

Important non-radially symmetric birefringence occurs in stressed single-crystal materials. Shear stresses at

the boundaries between the pumped and unpumped regions along the rod axis together with the non-isotropic

photo-elastic tensor induced by the cubic crystallography, result in birefringence with three-fold symmetry and

six-fold depolarization symmetry for radially or azimuthally polarized beams [2]. This birefringence was found

to become a strong effect at the pump powers used. Our method to eliminate this effect was to use

polycrystalline rods that macroscopically behave as an isotropic material and that eliminate such

depolarization[3]. Polarization and beam quality maintenance was measured through three STARs in a chain,

each pumped with 6kW of diode-light. The oscillator (probe) produced 60W in a 94.5% azimuthally polarized

beam with good beam quality (M2=2.5) [4]. The amplifiers were relay-imaged and spherical aberration in each

pump-chamber was compensated. The beam-quality was M2=4 and azimuthal polarization purity of 94%.

(Output power with the probe beam was 1620W.) The results demonstrate perfect polarization maintenance and

good beam quality preservation.

Power scaling was demonstrated with a more powerful oscillator. The oscillator generated a 550W

azimuthally-polarized beam with wavelength, tuned by a tilted etalon, to achieve maximum amplifier gain. 4kW

was measured from three amplifiers. More amplifiers provide higher powers and better extraction efficiency.

References [1] I. Moshe, S. Jackel, A. Meir, Production of radially or tangentially polarized beams in solid-state lasers and elimination of thermally

induced birefringence effects, Opt. Lett. 28, 807-809 (2003).

[2] Y. Lumer, I. Moshe, S. Jackel, Z. Horvitz, A. Meir, R. Feldman, and Y. Shimony, "Depolarization induced by pump edge effects in high average pump power laser rods", JOSA B 27 38-44 (2010). [3] I. Moshe, S. Jackel, Y. Lumer, A. Meir, R. Feldman, Y. Shimony, Use of polycrystalline Nd:YAG rods to achieve pure radially or

azimuthally polarized beams from high-average-power lasers, Opt. Lett. 35 2511-2513 (2010). [4] Y. Lumer, I. Moshe, S. Jackel, A. Meir, "Use of phase corrector plates to increase the power of radially polarized oscillators", JOSA B 27

1337-1342 (2010).

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