Steven Jackel

Efficient extracavity generation of radially and azimuthally polarized beams.

We demonstrate an efficient transformation of a linearly polarized Gaussian beam to a radially or an azimuthally polarized doughnut (0,1)* Laguerre-Gaussian beam of high purity. We use a spatially variable retardation plate, composed of eight sectors of a lambda/2 retardation plate, to transform a linear polarization distribution to radial/azimuthal distribution. We transformed an Nd:YAG Gaussian beam with M(2)=1.3 to a radially and azimuthally polarized (0,1)* Laguerre-Gaussian beams with M(2)=2.5 and degree of radial/azimuthal polarization of 96-98%.

Production of radially or azimuthally polarized beams in solid-state lasers and the elimination of thermally induced birefringence effects.

Production and amplification of radially and azimuthally (tangentially) polarized laser beams are demonstrated. Based on the different focusing between radially and tangentially polarized light in thermally stressed isotropic laser rods, Nd:YAG laser oscillators were developed to produce low-loss stable oscillation in a single polarization. Pure radially polarized light at 70 W with M2 = 2 and on-axis impure radially polarized light at 150 W with M2 = 2.5 were achieved. The radially polarized beams were then amplified while good beam quality and polarization purity were retained. Complete elimination of thermal-birefringence-induced aberrations was demonstrated. This should allow much better beam quality from rod-based high-power lasers.

Efficient pumping scheme for neodymium-doped materials by direct excitation of the upper lasing level

An efficient pumping scheme that involves direct excitation of the upper lasing level of the Nd(3+) ion is demonstrated experimentally. The results obtained for direct upper laser level pumping of Nd:YAG R2 (869 nm) and Nd:YVO(4) (880 nm) were compared with traditional approximately 808-nm pump band excitation. A tunable cw Ti:sapphire laser was used as the pump source. In Nd:YAG, the oscillator slope efficiency increased by 10% and the threshold decreased by 11%. In Nd:YVO(4), the slope efficiency increased by 5% and the threshold decreased by 11%. These results agree with theory. The increase in optical efficiency indicates that laser material thermal loading can be substantially reduced.

Thermally boosted pumping of neodymium lasers.

Pumping at 885 nm from thermally excited ground-state levels directly to the Nd:YAG upper lasing level is experimentally demonstrated by use of a Ti:sapphire pump laser. This approach utilizes thermal energy contained within the laser medium to provide part of the pump energy required to achieve population inversion. Slope efficiency increased by 12% compared with traditional pump band excitation (lambda(pump) = 808 nm) and by 7% compared with ground-state direct pumping (lambda(pump) = 869 nm). The combined transition from the first and second thermally excited Stark components of the ground state (4I(9/2)) to the upper lasing level (4F(3/2)) has characteristics that make thermally boosted pumping a suitable candidate for use with diode lasers: reasonable absorption (1.8 cm(-1)) and bandwidth (2.7 nm FWHM). A model suggests that, compared with traditional 808-nm pumping, heat could be reduced by 40% by use of thermally boosted pumping.

885 nm high-power diodes end-pumped Nd:YAG laser

Direct pumping from thermally excited ground levels directly to the upper lasing level of Nd:YAG was demonstrated using high-power, 885 nm, CW diodes. 14 W output power with 0.63 slope efficiency and 0.53 light-to-light efficiency were obtained.

Birefringence-induced bifocusing for selection of radially or azimuthally polarized laser modes

We develop a round-trip matrix diagonalization method for quantitative description of selection of radially or azimuthally polarized beams by birefringence-induced bifocusing in a simple laser resonator. We employ different focusing between radially and tangentially polarized light in thermally stressed laser rods to obtain low-loss stable oscillation in a radially polarized Laguerre-Gaussian, LG(0,1)*, mode. We derive a free-space propagator for the radially and azimuthally polarized LG(0,1)* modes and explain basic principles of mode selection by use of a round-trip matrix diagonalization method. Within this method we calculate round-trip diffraction losses and intensity distributions for the lowest-loss transverse modes. We show that, for the considered laser configuration, the round-trip loss obtained for the radially polarized LG(0,1)* mode is significantly smaller than that of the azimuthally polarized mode. Our experimental results, obtained with a diode side-pumped Nd:YAG rod in a flat-convex resonator, confirm the theoretical predictions. We achieved a pure radially polarized LG(0,1)* beam with M(2)=2.5 and tens of watts of output power.

2 kW, M2 < 10 radially polarized beams from aberration-compensated rod-based Nd:YAG lasers.

Radially polarized light in a 2.1 kW, good quality beam was obtained from a Nd:YAG rod-based master oscillator power amplifier. Several techniques were utilized: a pure radially polarized oscillator, efficient pump chambers, external compensation of lower-order aberrations, and higher-order aberration compensation by pairing of pump chambers.

Influence of birefringence-induced bifocusing on optical beams

We studied the influence of thermal birefringence-induced bifocusing on optical beams. An analytical model was developed to describe the bifocusing aberration. With this model we determined the circle-of-least-confusion location and the amount of beam quality degradation expected. A numerical model was developed to describe wave-front deformation by a bifocal lens and the intensity distribution at the foci of the polarization components - radial and tangential as well as at the circle of least confusion. Model predictions for beam quality degradation and intensity distributions along the focal range were confirmed experimentally using a strongly pumped Nd:YAG rod as a bifocal lens. It is shown that bifocusing is an important aberration in high-power rod-based lasers that must (and can) be corrected to achieve high beam quality.

Effects of thermally induced aberrations on radially and azimuthally polarized beams

We investigate the radial and azimuthal polarization degradation in high-power lasers induced by thermal aberrations. Thermal and propagation simulations, supported by measurements, show that thermally induced wavefront aberrations can strongly affect the polarization. Depolarization induced by primary aberrations and high-order azimuthal aberrations that arise in high-power rod-based lasers was analyzed along the beam propagation axis. Implications for pump-chamber design and amplifier architecture, in order to eliminate the depolarization effect, are discussed.

High-energy Nd:Cr:GSGG lasers based on phase and polarization conjugated multiple-pass amplifiers

Lasers based on Nd:Cr:GSGG low-energy oscillator/highenergy multiple-pass amplifiers produced 1.7 J pulses in a M2?2 divergence beam at 2.4% electrical efficiency. Thermal lensing and birefringence correction were major factors driving the amplifier design. Essential components in achieving a moderate average power output were an intra-amplifier telescope to correct for lowest order thermal focusing, a phase conjugate mirror to correct for higher order thermal lensing aberrations, and a linear optics derived polarization conjugator to correct for birefringence.