Reflection-type diffractive optical element employing SiC and Si for application to high power laser material processing

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.