We have designed a compatible objective lens for HD DVD/DVD/CD drives utilizing a multiple diffractive
structure. In this case, if priority has been given to a high HD DVD diffraction efficiency, the CD diffraction
efficiency is reduced. Therefore, plenty of noise light is generated at the entrance pupil diameter of a CD
inner area which may cause a false signal in the focus error (FE) signal during CD playback. Furthermore, the
quality of the spot and the side-lobe intensity gets worse due to the light at the entrance pupil diameter of a CD
outer area, and hence, may cause recording and playback problems. To solve these problems, we improved the
objective lens applying a scalar diffraction simulation. We separated the position of the false signal far away
from the regular signal in the FE signal, and suppress the amplitude of the false signal much smaller than the
regular signal in FE signal during CD playback to achieve the reliable focus servo. In addition, we managed to
improve the spot and the side-lobe quality to a practical level during CD playback with providing an aperture
filtering function on the objective lens.
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