JPH07105566A - Optical pickup device - Google Patents

Abstract

PURPOSE:To decrease the wave front aberrations generated by a thickness error of an optical disk substrate and to enable the use of an objective lens having a large numerical aperture by disposing two pieces of transparent plates which are variable in the spacing therebetween and are filled with a liquid material therein between the objective lens and.an optical information recording medium. CONSTITUTION:A laser beam 1 is made into a convergent beam by the objective lens 2. The convergent beams forms a beam spot 5 on the reflection film 4 of the optical disk 3. At this time, the convergent beam 6 passes the substrate 7 and, therefore, if an error exists in the thickness of the substrate 7, a spherical aberration is generated. The spherical aberration is removed by using a member constituted of the transparent plates 8, 9 and the transparent liquid material 10 and changing the thickness of the liquid material 10. The optical pickup device including the transparent plates 8, 9, the optical disk substrate 7 and the liquid material 10 is previously so designed that the beam spot can be converged down to the diffraction threshold to enable correction by increasing and decreasing the thickness of the liquid material 10 in correspondence to an increase or decrease of the substrate thickness from a standard value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup device for reading information from an optical disk by using a laser beam, and more particularly to an optical pickup device using an objective lens having a large numerical aperture.

[0002]

2. Description of the Related Art With the progress of high-density recording on optical discs, there has been a need for an optical pickup capable of forming a smaller beam spot on the optical disc. At present, research and development of a short-wavelength light source necessary for such an optical pickup are being conducted, and use of an objective lens having a large numerical aperture is considered.
In the case of an objective lens with a numerical aperture of 0.4, which is used for ordinary optical pickups, the allowable amount of substrate thickness error is ± 0.1 mm.
If so, the generated wavefront aberration hardly caused a problem during reproduction.

[0003]

[Problems to be Solved by the Invention] However, more
If an objective lens with a large numerical aperture is used to form a small beam spot, the wavefront aberration generated by the substrate thickness error becomes large and the reproduction becomes unstable. Further, there is a drawback in that the reproduction beam spot is not reduced and the reproduction signal is deteriorated due to the wavefront aberration. Conventionally, there is no solution for suppressing the wavefront aberration corresponding to the thickness error of each of the plurality of substrates, which hinders the practical application of the objective lens having a large numerical aperture. Therefore, an object of the present invention is to reduce the amount of wavefront aberration that occurs when the substrate thickness changes in order to solve these drawbacks.

[0004]

Therefore, in the present invention, a laser beam generated from a laser light source is converged and irradiated onto an optical information recording medium through an objective lens, and reflected and returned by being diffracted by a row of minute pits on the optical recording medium. In an optical pickup device that detects a change in light and reads out information, the distance between two transparent plates facing each other during converging light between an objective lens and the optical information recording medium is variable, and a liquid substance is interposed therebetween. Is provided to control the distance between the two transparent plates of the member to cancel the wavefront aberration caused by the thickness error of the substrate of the optical recording medium.

[0005]

The wavefront aberration that occurs when a parallel substrate having a thickness d is inserted into a convergent light beam that is converged geometrically at one point is approximately proportional to the fourth power of the numerical aperture, and the substrate thickness error d is Proportional.

This is equal to the wavefront aberration that occurs when there is a thickness error d on the optical disc substrate. Aberration caused by the substrate thickness error d ₁ is primarily to the numerical aperture of the spherical aberration W _{4 0} a and the objective lens to the refractive index of the NA substrate and _{n 1, W 4 0 = (} n 1 2 -1) d 1 (NA) can be expressed by ⁴ / 8n ₁ ^3.

Further, even if the parallel plate is inserted into the convergent light, the numerical aperture thereof does not change. Therefore, spherical aberration W ₄ which can be corrected by changing the thickness of the liquid substance by d ₂ in the present invention. _{0 'when} the refractive index of the liquid substance and n _2, W _{4 0'} expressed as ^{_{= (n 2 2 -1) d}} 2 (NA) 4 / 8N 2 3. That is, d ₂ so that W _{4 0} + W _{4 0 '} = 0
The spherical aberration can be completely removed by changing For example, n ₁ = 1.51, when the n ₂ = 1.33, it is sufficient and d ₂ = -1.14d _1.

Therefore, the spherical aberration can be removed by changing the refractive index or the thickness of the transparent liquid substance inserted in the convergent optical path of the laser of the objective lens and the optical disk.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic block diagram of an embodiment of the present invention. In the figure, the laser beam 1 is converged by the objective lens 2 and forms a beam spot 5 on the reflection film 4 of the optical disc 3. At this time, since the convergent light 6 passes through the substrate 7, if there is an error in the thickness of the substrate 7, a spherical error occurs. This is corrected by the transparent plates 8 and 9 installed between the objective lens 2 and the substrate 7 and the transparent liquid substance 10.
Is.

As a basic physical property of the liquid substance 10, a liquid that has a sufficiently high transmittance for the laser light used and a refractive index of not 1.0 and is chemically stable, such as ethyl alcohol, benzene, and water, is preferable. The spherical aberration can be removed by changing the thickness of the liquid substance 10. The objective lens 2 includes transparent plates 8 and 9 and an optical disk substrate 7.
In addition, the beam spot including the liquid substance 10 is designed to be narrowed down to the diffraction limit, and it can be corrected by increasing or decreasing the thickness of the liquid substance 10 even if the substrate thickness is smaller or larger than the standard value. .

The change in the thickness of the liquid substance 10 is shown in FIG.
As shown in the sectional view of the embodiment of the present invention, the transparent plate 9 is fixed to the side wall surface 11, and the transparent plate 8 is slidable in the vertical direction. Then, the transparent plates 8 and 9 are attached to the bellows 12.
The transparent plate 9 which is connected and fixed by means is provided with an inlet / outlet port for the liquid substance 10, and is connected to the infusion pipe 13 and the pump 14. Then, the liquid substance 10 is moved by the pump 14 and the liquid substance 1
Control the thickness of 0.

In addition, as shown in FIG. 3 as another embodiment, instead of the pump 14 of FIG.
May be connected between the fixed part and the movable transparent plate 8 and the piezoelectric element 15 may be driven to change the thickness.

[0013]

According to the present invention, the wavefront aberration caused by the thickness error of the optical disk substrate can be reduced. Therefore, an objective lens having a large numerical aperture can be used, and the tolerance range of the substrate thickness can be widened, so that stable reproduction of an optical disc recorded at high density can be performed.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention.

FIG. 2 is a cross-sectional view of an essential part showing an embodiment of the present invention.

FIG. 3 is a cross-sectional view of essential parts showing another embodiment of the present invention.

[Explanation of symbols]

 1 Laser Beam 2 Objective Lens 3 Optical Disc 4 Reflective Film 5 Beam Spot 6 Convergent Light 7 Substrate 8, 9 Transparent Plate 10 Liquid Substance 11 Sidewall 12 Bellow 13 Infusion Pipe 14 Pump 15 Piezoelectric Element

Claims (1)

[Claims] 1. A laser beam generated from a laser light source is converged and irradiated onto an optical information recording medium through an objective lens,
In an optical pickup device that detects a change in reflected return light diffracted by a row of minute pits on the optical recording medium and reads information, the optical pickup device faces the converged light between the objective lens and the optical information recording medium. The distance between the two transparent plates is variable, and a member filled with a liquid substance is placed between them.
An optical pickup device, characterized in that the distance between two transparent plates of the member is controlled to cancel a wavefront aberration caused by a thickness error of the substrate of the optical recording medium.