Glass molding of all glass Fresnel lens with vitreous carbon micromold.

An all glass Fresnel lens (AGFL) was fabricated by glass molding with a vitreous carbon (VC) micro mold. In the glass molding process, a glass plate was heated up to its softening temperature and pressed against to the VC mold to replicate the Fresnel pattern. The VC molds having negative shape Fresnel profile were fabricated by carbonization of replicated Furan precursor using a diamond turning machined nickel master. During the carbonization process, the Furan precursor shrank due to the thermal decomposition, and this shrinkage must be compensated to obtain a precise AGFL. In this study, we examined the shrinkage ratio during the carbonization process using a preliminary experiment using the commercially available PMMA Fresnel lens as the master, and fabricated a nickel master with an enlarged Fresnel profile for shrinkage compensation. To verify the compensation method, the surface profiles of the fabricated VC mold and molded AGFL were measured and compared with the designed profile. The deviations between measured and designed profiles were less than 4 μm. In addition, the tip radii of the grooves and draft angle of the molded AGFL were within the acceptable tolerance for CPV applications.