Dual focus optical head with a hologram-integrated lens

A dual focus optical head with a holographic optical element (HOE) that can read both 0.6 mm disks and 1.2 mm disks has been developed. To ensure practicality and mass-producibility of the head, we examined servo-signal characteristics and evaluated allowable ranges for the misalignment of the HOE, the wavelength deviation, the disk tilt angle and the defocus distance. It was found that stable servo operation can be obtained and that except for the issue of coma aberration, the allowable ranges are wide enough for mass-production. When an objective lens having initial coma aberration is used with a plane-type HOE, coma aberration still remains for the 1.2 mm disk after performing skew adjustment for the 0.6 mm disk. We found that the aberration can be reduced by integrating an HOE with the objective lens on its first surface.