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20 August 2015 Design and fabrication of multilayer dielectric gratings for spectral beam combining
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Metal gratings are mostly used on low energy optics, but it has low laser damage threshold. Spectral beam combining is a method to make high power laser beam using diffraction gratings. Multilayer dielectric (MLD) high reflectance mirror is designed for high efficiency gratings using HfO2 and SiO2 for high laser damage threshold. On the top of the mirror, polarization dependent SiO2 grating structure is simulated by finite domain time division (FDTD) method at 1055nm for spectral beam combining. To estimate the far field diffraction characteristics, we first calculate near field electromagnetic wave properties at the substrate region and these are transformed to angular diffraction characteristics at about 1 meter apart from the grating. Multilayer dielectric mirror is deposited by electron beam evaporation method at the substrate temperature 250°C. Four types of high efficiency MLD gratings are selected and these are fabricated by lithography and reactive ion etching method. To fabricate the designed submicron structure, 4X stepper is used for pattern formation on the photo resistor. We use fused silica as a substrate and additional dummy silicon wafer substrates are used for grating structure confirmation using scanning electron microscope. The diffraction efficiencies are measured and these are compared with simulated results.
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Hyun-Ju Cho, Hyun-Tae Kim, and Yong-Soo Lee "Design and fabrication of multilayer dielectric gratings for spectral beam combining", Proc. SPIE 9556, Nanoengineering: Fabrication, Properties, Optics, and Devices XII, 955615 (20 August 2015);

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