Optical design of a single-sided microholographic data-storage system using a diffractive optical element and a blue laser

We introduce a compact optical design for a single-sided microholographic data-storage system, using a diffractive optical element (DOE) and a blue laser diode. The diffractive optical element is used to separate the disc-focusing servo beam from the reference beam in a single optical path to create a compact optical system. In conventional systems, three beam paths are generally required for the reference beam, signal beam and disc-focusing beam. Our design employs a single optical path for all three beams, utilizing a DOE. The total capacity of a 12cm disc with a medium thickness of 400 μm is 1.09 terabytes, using a blue laser and an objective lens with a numerical aperture of 0.85. Optical aberrations are suppressed below 0.01λ_rms on all layers (47layers) and the disc tilt margin is calculated based on tilt compensation by the objective lens. The final design has a disc tilt margin of ±0.1° and thus can endure very severe disc tilt.