Design and fabrication of a diffractive optical element for the objective lens of a small form factor optical data storage device

Abstract

The demand for small and high-capacity optical data storage devices has rapidly increased. The areal density of the optical disk is increased using a higher numerical aperture objective lens and a shorter wavelength source. We have designed and fabricated a wafer-scale stacked micro objective lens with a numerical aperture of 0.85 and a focal length of 0.467 mm for the 405 nm blue–violet laser. To compensate the chromatic aberration of the objective lens, a diffractive optical element (DOE) was used. Among the various fabrication methods for micro DOE, the UV-replication process is more suitable for mass-production. In this study, an eight-stepped DOE pattern as a master was fabricated by photolithography and reactive ion etching processes. A flexible mold was fabricated for improving the releasing properties and shape accuracy in the UV-molding process. In the replication process, the effects of exposing time and applied pressure on the replication quality were analyzed. Finally, the shapes of the master mold and the molded pattern were measured by an optical scanning profiler. The deviation between the master and the molded DOE was less than 0.1 µm. The efficiency of the molded DOE was measured by a DOE efficiency measurement system which consists of a laser source, a sample holder, an aperture and an optical power meter, and the measured value was 84.5%.