Objective for optical disk, optical pickup, optical disk writer-reader, and optical disk reader

Abstract

An objective for an optical disk is made of a single double-sided aspherical lens having a numerical aperture (NA) equal to or greater than 0.75 and capable of minimizing axial aberration, off-axis aberration, surface-to-surface eccentricity aberration, and chromatic aberration. The objective (11) has a first aspherical surface (1). The vertex of the first surface has a radius of curvature R1 defined as follows:0.95.A<R1<1.05.AA=B / CB=0.85f(n-1)C=n(0.60866-0.11.t / f-0.1272.d / f)(0.83+0.2.NA)where n is a refractive index of the lens, f is a focal length of the lens, t is a thickness along optical axis through the center of the lens and d is the thickness of a transmission layer of the optical disk.

Description

[0001] 1. Field of the Invention[0002] The present invention relates to an objective having a high numerical aperture (NA) to realize a large-capacity optical disk, an optical pickup with the objective, an optical disk writer-reader with the objective, and an optical disk reader with the objective.[0003] 2. Description of the Related Art[0004] Conventional objectives for compact disks (CDs) have numerical apertures (NAs) of 0.45 to 0.5 and employ laser beams of about 780 nm in wavelength to read and write the CDs. Objectives for digital versatile disks (DVDs) have NAs of about 0.6 and employ laser beams of about 650 nm in wavelength to read and write the DVDs.[0005] To handle high-capacity optical disks, now being developed are next-generation pickups with objectives that have high NAs and operate on short-wavelength beams.[0006] The short wavelength beams may include a blue laser beam of about 400 nm in wavelength.[0007] Examples of objectives having high NAs are reported in the fo...

AI Technical Summary

Benefits of technology

[0034] Desirably, the objective of any one of the above aspects has a manufacturable eccentricity tolerance between the first and second surfaces and minimizes off-axis aberration.

Problems solved by technology

Higher numerical apertures result in lowering system margins.

The system of the paper (B), however, needs an assembling process of two lens groups, and therefore, is disadvantageous to mass production and increases costs.

The short working distance increases a risk of colliding with an optical disk, thereby deteriorating the reliability of the system.

A lens having a high numerical aperture can be designed but is not always manufacturable.

It is nearly impossible for a lens having a numerical aperture of 0.75 or higher to simultaneously satisfy these requirements.

If the manufacturing tolerance is considered, the off-axis aberration worsens because the manufacturing tolerance, i.e., the eccentricity tolerance of the lens is securable only by sacrificing the axial aberration and off-axis aberration of the lens.

This lens causes a large aberration even on a slight eccentricity.

The disclosures cover a wide range of specifications, and therefore, are insufficient to actually design a good lens.

The two-lens-group system mentioned above involves a short working distance, and therefore, greatly increases a risk of colliding with an optical disk when the lens groups employ a higher numerical aperture.

Optical disks are generally made of plastic, which unavoidably involves warp.

No further improvement is expected in optical disk warp because the warp depends on disk material.

With such a short working distance, the lens system will collide with an optical disk if focus servo runs off due to disturbance, vibration, or defects during a disk write or read operation.

The system, however, involves a very short working distance of 0.1 mm to increase a risk of collision.

In this case, a minimum focal length of the objective is desirably defined as f>0. However, there is no means at present to manufacture such a very small lens.

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