초정밀 레이저 하이브리드 가공 기술을 이용한 미세 광학 소자 제작 및 특성 연구

Abstract

This research is focused on optical characterizations of optical elements such as diffraction grating, Fresnel zone plate (FZP), and microlens arrary (MLA) fabricated directly using a femtosecond laser. First, FZP has been fabricated using a femtosecond laser and characterized optically. Lithography processing with a femtosecond laser which can be replaceable with phase mask has been applied to fabricate a FZP on silica surface and optical characteristics of the FZP are compared to those of FZP on the silica surface fabricated conventional femtosecond technology using laser ablation processes. Without phase mask used in the conventional lithography processing to generate pattern, a femtosecond laser can be used to generate patterns from direct illumination of a femtosecond laser beam on photoresist (PR). Diffraction efficiencies of those FZPs fabricated using two different methods have been measured and compared. The diffraction efficiency of the FZP from the femtosecond lithography processing is around 12.1% and above around 3.4% than that of the FZP fabricated using the femtosecond ablation method. Secondly, diffraction gratings have been fabricated on the surface of silica surface using a femtosecond laser. To enhance diffraction efficiency of the diffraction gratings, CO2 laser surface treatment processing is used. A femtosecond laser focused on the surface with an objective lens and precise moving stage is used to fabricate the diffraction grating, Due to surface roughness of fabricated diffraction grating diffraction efficiency is limited to around 17.6%, but number of the surface treatments make it possible to have enhance diffraction efficiency to around 32.7% until 4th treatments. However, more treatments make profiles of the diffration grating changed due to melting of the diffraction grating so that the diffration efficiency is reduced. Such CO2 laser surface treatment technology together with a femtosecond fabrication technology will provide high quality optical elements with high diffraction efficiency on various transparent materials surface. Third, MLA on silica surface has been fabricated using a femtosecond and CO2 lasers and optical characterizations have been compared with MLA fabricated with different methods. Line profiles on the silica surface using a femtosecond laser have been used to generate profiles of the MLA and to generate radius of curvatures of lens CO2 laser surface treatment processing is used. Dome and cylinder types of initial profiles of the lens array have been fabricated using a femtosecond and CO2 lasers and the profiles fabricated are measured through optical profile measurements. Initial deep profiles of the MLA to control the radius curvatures of the lens array makes it difficult to remove surface roughness inside fabricated valleys of the MLA. To overcome the problem, instead of line profiles lens profiles of the MLA have been fabricated varying the depths of the profiles using a femtosecond laser. Also CO2 laser surface treatments make it possible to remove the surface roughness. In addition to these methods described above to overcome lack of large yields of fabrication outputs using a laser micromachining MLA basic mold patterns generated on the base have been applied. The characteristics of the fabricated MLA have been compared with commercial MLA. A femtosecond laser with beam shaping with the fabricated MLA makes it possible to have similar laser fabrication results of ITO coated with the thickness of 1800 Å to those from laser fabrication using a commercial MLA, which are confirmed from optical microscope measurements of the fabricated profiles.