펨토초 레이저로 제작된 회절격자의 CO2 레이저 표면 연마 특성 연구

Abstract

We fabricated a two-layer diffraction grating inside fused silica glass by using a femtosecond laser direct writing method. The femtosecond laser with a wavelength of 515 , a pulse width of 250 , a repetition rate of 100 , and an average output power of 6 was used. The two layer diffraction grating with a grating period of 6 was successfully fabricated with the layer gap of 0.5, 1, 2, 3, and 5 , respectively. Also, we investigated the diffraction pattern by illuminating a He-Ne laser beam. Finally, we demonstrated the diffraction grating with a grating period of 3 by adjusting the gap of each layer with a grating period of 6 . Femtosecond laser direct writing technology of multi-layer has a potential to fabricate the diffraction grating with a grating period of below 1.5 . When writing grating inside fused silica using femtosecond laser, we find filament phenomenon which is engraved toward Z-axis direction. To reduce this phenomenon, we use objective lenses which have ×20 (NA=0.4), ×50 (NA=0.42), and ×100 (NA=0.5) magnification. We do experiment with 4.3 fixed pulse energy and changing of processing speed from 1 to 10 . As lens magnification and processing speed were increased, filament phenomenon is decreased and its length is 4 5 with 100 magnification of objective lens. After fabrication of diffraction grating on the surface of fused silica by using a femtosecond laser, its surface with CO2 laser was polished. We measured a surface and cross-section with optical microscope and SEM by changing polishing repetition rate of CO2 laser, respectively. We found that the efficiency of the diffraction grating with surface treatment less than 1 3 steps of the surface treatment by using a CO2 laser is increased by 7 %. A laser beam array is generated by using a phase hologram (Kinoform) and spatial light modulation (SLM). The beam size was 2.78 for the objective lens of ×40 magnification. This method will be applied to make complex 3D shapes by using a femtosecond laser beam.