DUAL FOCUS OPTICAL HEAD WITH A HOLOGRAM-INTEGRATED LENS

A dual focus optical head with a holographic optical element (HOE) tha t can read both 0.6 mm disks and 1.2 mm disks has been developed. To e nsure practicality and mass-producibility of the head, we examined ser vo-signal characteristics and evaluated allowable ranges for the misal ignment 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 allowab le ranges are wide enough for mass-production. When an objective lens having initial coma aberration is used with a plane-type HOE, coma abe rration still remains for the 1.2 mm disk after performing skew adjust ment 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.