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 12 cm disc with a medium thickness of 400 mu m is 1.09 terabytes, using a blue laser and an objective lens with a numerical aperture of 0.85. Optical aberrations are suppressed below 0.01 lambda(rms) on all layers (47 layers) 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 degrees and thus can endure very severe disc tilt.