Two interferometric techniques for converting a linearly polarized laser beam into a radially polarized beam with uniform azimuthal intensity are described. The techniques are based on the linear combination of orthogonally polarized beams, which have tailored intensity and phase profiles. Linearly polarized beams with intensity profiles tailored using a modified laser or an apodization filter are combined in separate experiments to produce radially polarized light. A beam with an extinction ratio of -21.7 dB and azimuthal intensity variations of less than +/-12% is produced using the modified laser output. The second technique uses circularly polarized light and a unique spiral phase delay plate to produce the required phase profile. When focused, a radially polarized beam has a net longitudinal field useful for particle acceleration and, perhaps, other unique applications.
Conversion of a linearly polarized CO(2) laser beam into a radially polarized beam is demonstrated with a novel double-interferometer system. The first Mach-Zehnder interferometer converts the linearly polarized input beam into two beams with sin(2) θ and cos(2) θ intensity profiles, where θ is the azimuthal angle. This is accomplished by using two spiral-phase-delay plates with opposite handedness in the two legs of the interferometer to impart a one-wave phase delay azimuthally across the face of the beams. After these beams are interfered with, the resulting beams are sent directly into the second Mach-Zehnder interferometer, where the polarization direction of one beam is rotated by 90°. The beams are then recombined at the output of the second interferometer with a polarization-sensitive beam splitter to generate a radially polarized beam. The output beam is ≈92% radially polarized and contains ≈85% of the input power. This system will be used in upcoming laser particle acceleration experiments.
We have demonstrated a diode-end-pumped Nd:YAG laser that produces an output power of 60 W in a near-diffraction-limited beam (i.e., M(2) < 1.3). In multimode operation, the laser produces an output power of 92 W. The optical-to-optical efficiency (i.e., the ratio of laser power to diode power) is 26% for TEM(00) operation and 44% for multimode operation.
The design and performance of a 15-W output-power, diode-pumped cw Nd:YAG laser is described. With the use of geometrically multiplexed end pumping, the combined power of four 10-W diode-laser bars is deposited in the active-medium mode volume with an efficiency of 80%. While operating on several transverse modes the laser has a threshold of 3 W, an optical slope efficiency of 60%, and an overall electrical efficiency of 10.5%. Thermally induced higher-order spherical aberration of 1.2 waves limits the output power in the TEM(00) mode to approximately 6 W.
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