Blue NA 0.85/DVD/CD compatible optical head

In high density optical disk systems employing blue laser diodes, high NA (numerical aperture) objective lenses and thin cover layer disks, compatibility with conventional DVDs and CDs is strongly required. The authors have previously proposed a blue/DVD/CD compatible optical head, whose NA for the blue optical disk is 0.7. On the other hand, NA 0.85 single objective lenses have also become available, which are suitable for the compatibility requirements because of the long working distance. So, this paper further proposes a blue/DVD/CD compatible optical head, whose NA for the blue optical disk is 0.85. In order to realize the compatibility between different types of disks, it is necessary to correct the spherical aberration and control the NA for each disk. In this three-wavelength optical head, the spherical aberration is corrected by variable magnification, and the NA is controlled by a wavelength selective aperture.