We successfully prepared reflection-type diffractive optical elements (R-DOEs) using SiC and Si wafers for application to high-power laser material processing. SiC and Si, whose thermal conductivity are 200 W/mꞏK, are preferred materials to be cooled by water to prevent thermal lens effect. Our calculation revealed that these R-DOEs achieved 30-kW laser processing by using water cooling on the backside of these R-DOEs. Measured energy conversion efficiencies were 75% using a single-mode laser with a wavelength of 1.064 μm. Moreover, the integrated intensities of a reconstructed image measured using both single-mode and multi-mode lasers were almost the same. We also succeeded in designing a simple cooling unit. Distortion of these R-DOEs caused by water pressure was also measured to prevent any change in focusing length and distortion of the shaped beam. The measured curvature radius was 100 mm, in which there was a -0.3-mm change in focusing length. The measured reconstructed image was not distorted. We experimentally confirmed that the laser irradiation tolerance of the combined R-DOE and cooling unit was more than 10-kW. These results corresponded well with our theoretical estimation. These results suggest reflection-type DOEs are a good beam shaper for high power laser processing using more-than-10-kW laser sources.
A KTa1-xNbxO3 (KTN) varifocal lens achieves a fast response of microseconds, a large aperture of 3 mm, a random change of focusing length and a high transmission exceeding 99%. However, the variable lens power is smaller than that of other varifocal lenses. We successfully revealed the best way to obtain a variable lens power of 5 m-1 with a KTN varifocal lens. We simulated the relationships between the length of a KTN lens and both wave aberration and variable lens power. The wave aberration increased with increases in variable lens power. We employed the ratio of the wave aberration to the variable lens power to evaluate the scanning resolution, which improved as the ratio decreased. The best length for a KTN lens was 5.3 mm for an octagonal structure whose aperture size, thickness, and electrode angle were 3 mm, 4 mm and 26.5 degrees, respectively. Our experimental results agreed with the simulation. A wave aberration of less than λ/ 10 (λ=1064 nm) and a variable lens power of 0-2.5 m-1 were obtained. We concluded that combining a pair of optimized structures was the best way to obtain a variable lens power of 0-5 m-1, which is comparable to that of varifocal lenses using other principles.