Object lens for optical pickup, optical pickup and optical information processing device

Abstract

As an object lens for an optical pickup, applied is a single-lens and both-side-convex configuration is applied. Further, specific conditional formulas are created with respect to a particular numerical aperture NA, for a paraxial curvature radius R1 on the surface on the light source side; the working distance WD, the refractive index nd with respect to the d-line, and the focal length f. Furthermore, specific conditional formulas are created for the refractive index nd with respect to the d-line and the Abbe's number νd.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to an object lens for an optical pickup, an optical pickup, and an optical information processing device. [0003] 2. The Description of the Related Art [0004] For “an optical recording medium” such as a CD (compact disk) or a DVD (digital video disk), “an optical information processing device” which performs recording, reproduction, and deletion of information using an optical pickup has been spread widely. In this technical field, high-density recording onto the optical recording medium is demanded. [0005] Since the light spot formed on the recording surface of the optical recording medium is formed of a beam waist of a laser beam which converges by means of the object lens of the optical pickup, and the diameter of the beam waist is proportional to the wavelength of laser beam, and, also, is inverse proportional to the numerical aperture (NA) of the object lens, increase in NA of the o...

AI Technical Summary

Benefits of technology

The present invention aims to provide an optical pickup with a small-sized beam spot and a large NA, which is advantageous for high-density information recording. The objective lens has been designed to control chromatic aberration and improve the accuracy of the optical pickup. Additionally, the objective lens has been corrected for spherical aberration caused by substrate thickness error to enhance the reliability of the optical pickup. The invention also aims to achieve satisfactory recording, reproduction, or deletion onto each recording surface of the multilayer optical recording medium. The objective lens has been designed using specific conditional formulas and can be used for both new standard and conventional CD and DVD optical recording media. The invention provides an improved optical pickup with a single-lens and both-side-convex configuration that can effectively reduce the light spot diameter and has a large manufacture tolerance.

Problems solved by technology

Compared with a single-lens configuration, such a type of lens of two-lens-combined configuration requires complicated assembling process, high precision in assembling work, and, also, may not be driven at a high speed due to increase in the weight itself.

Moreover, as a working distance, i.e., the distance between the object lens and the optical recording medium surface is shortened in case of employing such a two-lens-combined configuration, there increases a risk of hitting of the object lens onto the recording medium surface, resulting in a series damage occurring thereon.

As a result, it becomes difficult to achieve high reliability in this scheme.

Although such a problem does not occur in case of usage of a single lens configuration in the object lens, it may not be possible to achieve large NA.

In the optical pickup, when the wavelength emitted from the semiconductor laser changes from the original design value due to the variation for every individual, or due to the mode hopping, chromatic aberration arises in the optical pickup's optical system, the diameter of beam spot increases on the medium recording surface, and thus, there is a possibility of causing a problem on recording / reproducing operation in the device.

Especially the chromatic aberration occurring in the object lens by the wavelength change when using the short wavelength semiconductor laser not more than the emission wavelength around 440 nm may causes a nonpermissible problem.

On the other hand, manufacture error on the order of ±10 nm is not avoidable with respect to the incident side substrate thickness of 0.1 mm.

Such a substrate thickness error may cause spherical aberration in imaging function of the object lens designed according to the standardized substrate thickness.

Thereby, the beam spot diameter may be increased, and, thus, the proper operation may not be expected in the optical pickup.

As well-known, the spherical aberration is proportional to the forth power of NA of the object lens, the substrate thickness error may cause a larger problem as NA of the object lens increases.

However, as described above, such a two-lens-combined configuration object lens even having high NA may be problematic in comparison to a single lens configuration.

However, these conventional examples have a low implementability in terms of manufacture.

Moreover, actual utilization thereof is difficult when the implementability on the condition of the manufacture tolerance is low even when the wavefront performance in the design median is satisfactory.

As can be seen therefrom, the above-mentioned conditions cannot be actually satisfied.

However, it is difficult to attain a miniaturization and cost reduction of the device according to such a scheme.

Images