Phase masks for mode conversion and other laser beam transformation are usually produced by surface profiling using spatially selective etching or deposition. Such fabrication techniques for making complex phase masks take significant time, effort, and expense. Surface damages and contaminations restrain the wide application of such phase elements as well. We propose an alternative, where volume phase masks were produced by spatially selective refractive index change in the bulk of plane-parallel plate of photo-thermo-refractive (PTR) glass. Those phase masks show high tolerance to harsh environments and high-power laser radiation. The approach uses a light amplitude modulating element called digital micromirror device (DMD) which has millions of micro-mirrors that tilt to ‘on’ or ‘off’ position based on the voltage applied to them. Selecting different time intervals for the mirrors to be ‘on’ or ‘off’, allows for grey level images to be generated and projected by the DMD device. Using a broadband UV light source, the desired amplitude image was projected onto a PTR glass plate for a designated period of time. The variable light amplitude exposure of the glass is converted to a refractive index change after thermal treatment step. The phase masks are recorded in the volume and are practically unsusceptible to surface damage, changes in the environment and other factors from which surface created phase masks suffer. The simplicity in fabricating grey level phase masks, the flexibility to computer design, and the robustness are the main advantages of the approach when compared to the standard phase mask fabricating techniques.
Duc-Quy Nguyen, Zachary J. Labossiere, Fedor Kompan, Ivan Divliansky, David Guacaneme, and Leonid B. Glebov, "Beam transforming phase masks in PTR glass by using digital micromirror device (Conference Presentation)," Proc. SPIE 10513, Components and Packaging for Laser Systems IV, 1051315 (Presented at SPIE LASE: February 01, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2290648.5751419081001.
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