Sadao Mizuno

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.

Dual Focus Optical Head for 0.6 mm and 1.2 mm Disks

We propose a dual focus optical head with a holographic optical element (HOE) which corrects spherical aberrations in order to read both 0.6 mm disks and 1.2 mm disks. The thin disks are read using transmitted light and the thick disks are read using +1st order diffracted light of the blazed HOE. The good quality of the focused spots and the focusing error signals was experimentally confirmed. No influence of unnecessary diffracted light was observed.

Lens design of compatible objective lens for blu-ray disc and digital versatile disk with diffractive optical element and phase steps

A two-element objective lens which is compatible with for digital versatile disk (DVD) and Blu-ray Disc (BD) and has adequate optical performances in terms of off-axis wavefront and on-axis chromatic aberration for both optical heads has been designed. The first-order diffracted light is used for DVD and the second-order one is used for BD. We analyzed chromatic spherical aberration and found that using first- and second-diffracted lights causes 20 mλ/nm chromatic spherical aberration at a wavelength of 400 nm. However, using 5λ and 10λ phase steps can reduce chromatic spherical aberration to 7 mλ/nm. The designed two-element lens consists of the first plastic lens, on which there is diffractive optical element, and the second plano-convex glass molded lens.

Compatible Objective Lens for Blu-ray Disc and Degital Versatile Disc Using Diffractive Optical Element and Phase-Step Element which Corrects both Chromatic and Spherical Aberrations

A compatible lens with a diffractive optical element that overcomes differences in disk thickness and longitudinal chromatic aberration and phase steps that eliminate chromatic spherical aberrations are proposed. Both blue and red parallel incident rays can be converged on both disks; therefore, the servo and readout signals can be detected by a single common photodetector. A compatible optical head, which can write Blu-ray Discs (BDs) and degital versatile discs (DVDs) with only one objective lens assembly and one photodetector, can be achieved. We show the convergence of the blue and red parallel incident rays and signal readout using the compatible objective lens.

Lens Design for Optical Head Compatible with Compact Disk and Digital Versatile Disk

This paper describes three approaches to lens design for reading different thickness disks with one optical system. In the case of an optical head using two different objectives with one actuator, the skew adjustments for both lenses are difficult. First we design objective lenses that are coma free against skew adjustment. The method that uses one objective lens with a liquid crystal shutter still results in a spherical aberration of 50 mλ for a 1.2 mm disk. Second, we design the inner area of the lens for a disk thickness of 0.9 mm, and then the spherical aberration becomes half. The third order spherical aberration of the entire aperture is made to be zero. Grating is integrated onto the surface of a dual focus lens, it causes stray light. We propose a grating profile that reduces stray light to one-fifth.

Inner Area Aberration Compensating Lens for Compact Disc (Compact Disc-Recordable)-Digital Versatile Disc Compatible Head

We have developed a dual-wavelength digital versatile disc(DVD)/compact disc(CD)-compatible inner area aberration compensating lens capable of reading compact disc-recordable (CD-R) disks. In order to be able to read CD and CD-R disks with an infrared laser, it is essential to increase the numerical aperture (NA) of the inner area, but the resulting deterioration of the read signal has been a problem. Performance levels that enable DVD disks to be read have been achieved by reducing the fifth-order spherical aberration (SA5) by means of a phase step.

Dual focus optical head for 0.6 mm and 1.2 mm disks

We propose a dual focus optical head with a holographic optical element (HOE) which corrects spherical aberrations in order to read both 0.6 mm disks and 1.2 mm disks. The thin disks are read using transmitted light and the thick disks are read using +1st order diffracted light of the blazed HOE. The good quality of the focused spots and the focusing error signals was experimentally confirmed. No influence of unnecessary diffracted light was observed.

Dual focus optical head for 0.6-mm and 1.2-mm disks

We propose a dual focus optical head with a holographic optical element (HOE) which compensates for spherical aberration, allowing it to read both 0.6 mm disks and 1.2 mm disks. The thin disk is read using transmitted light and the thick disk is read using +1st order diffracted light of the blazed HOE. The characteristics of the focused spot, the servo signal detection and the signal of the Compact Disc indicate that both disks can be read by the dual focus optical head.