JPH05342619A - Optical pickup device - Google Patents

Abstract

PURPOSE:To reduce R-dependence or to miniaturize an optical system by making a light spot on a light receiving surface small. CONSTITUTION:Assuming that the thicknesses of a beam splitter 4, a compensation plate 8 and a cover glass 9 through which a light beam passes from a collimating lens 5 to a light receiving surface 10 are t1, t2, t3 and their refraction indices are n1, n2, n3, respectively, the reduced distance in air of a detecting system L1 is L1=t1/n1+t2/n2+t3/n3. Assuming that the thicknesses of a cover glass 2 and a grating grid 3 through which the light beam passes from a laser diode 1 to the collimating lens 5 are t4, t5 and their refraction indices are n4, n5, respectively, the reduced distance in air of a light source system L2 is L2=t4/ n4+t5/n5. In this optical pickup, L2>L1 is assumed. The correction of spherical aberration of the collimating lens 5 is matched with the condition of the cover glass corresponding to the reduced distance in air L2 of the light source system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk reproducing apparatus, and more particularly to an optical pickup for the same.

[0002]

2. Description of the Related Art In an optical pickup, light emitted from a laser diode serving as a light source passes through a cover glass of a plane parallel plate and a grating grid and is reflected by a beam splitter to reach a collimator lens. Further, the collimator lens forms a bundle of parallel rays, which is converged on the information recording surface by the objective lens.

Further, the light reflected on the information recording surface becomes a bundle of parallel rays when passing through the objective lens, becomes convergent light when passing through the collimator lens, and is received through the flat beam splitter, the correction plate and the cover glass. Converge on the surface.

Since there is a plane parallel plate such as a cover glass of a laser diode in the optical path from the light source to the collimator lens, the spherical aberration has been corrected by designing the collimator lens according to the corresponding cover glass conditions.

In the conventional optical pickup, the cover glass condition of the light source system from the light source to the collimator lens and the cover glass condition of the light receiving system from the collimator lens to the light receiving surface are made equal.

[0006]

As described above, since the cover glass of the plane parallel plate and the grating grid are provided between the light source of the optical pickup and the collimator lens, the collimator lens should be adjusted to meet these cover glass conditions. Spherical aberration is corrected by designing
The light spot on the information recording surface is made small.

However, the thickness of the cover glass of the light receiving system is equal to the thickness of the cover glass of the light source system, and in the light receiving system, the light passes through the beam splitter and the correction plate, and they are inclined with respect to the optical axis. On the surface including the tilt direction, spherical aberration becomes large, and the light spot on the light receiving surface cannot be reduced.

The present invention has been made in view of the above points, and an object thereof is to reduce the light spot on the light receiving surface to improve the R dependency, and
An object of the present invention is to provide an optical pickup whose optical system can be downsized.

[0009]

In the optical pickup of the present invention, the light from the light source is condensed on the information recording surface of the optical disc by the collimator lens and the objective lens, and the reflected light from the information recording surface is reflected by the objective lens and the objective lens. In an optical pickup that uses a collimator lens to collect light on the light-receiving surface of the light-receiving element, the sum of the numerical values obtained by dividing the thickness of each parallel-plane plate-shaped optical material in the optical path between the collimator lens and the light-receiving surface by its refractive index. And the thickness of each plane-parallel plate-shaped optical material in the optical path between the collimator lens and the collimator lens is smaller than the sum of the numerical values divided by the refractive index.

[0010]

According to the optical pickup of the present invention, the cover glass condition of the light receiving system is smaller than the cover glass condition of the light source system, and if the spherical aberration of the collimator lens is corrected in accordance with the cover glass condition of the light source system, the light receiving system receives light. In the system, the spherical aberration of the collimator lens is overcorrected.

According to the present invention, the aberration is corrected on the surface on the side where the beam splitter and the correction plate are inclined with respect to the optical axis, and on the surface orthogonal to that, the spherical aberration is corrected excessively. Inverse spherical aberration occurs.

By arranging the light receiving surfaces in the vicinity of the circles of least confusion on both sides, the light spot can be made smaller than the conventional one.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical pickup which is an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is an optical path diagram for explaining an optical pickup which is an embodiment of the present invention. Reference numeral 1 shown in the figure is a laser diode, which emits a laser beam as a light source.

The light emitted from the laser diode 1 passes through a flat cover glass 2 and a grating grid 3 and reaches a beam splitter 4. The grating grid 3 diffracts and separates the laser beam so as to form a sub-spot on the information recording surface.

The laser light which is the light source is beam splitter 4
Is reflected by the collimator lens 5 to be a parallel light beam by the collimator lens 5 and further converged by the objective lens 6 to form a light spot on the information recording surface 7.

The laser light reflected by the information recording surface 7 is collimated by the objective lens 6 and converged by the collimator lens 5. The converged laser light passes through the beam splitter 4, the correction plate 8 and the cover glass 9 to form a light spot on the light receiving surface 10 of the light receiving element. The correction plate 8
Are arranged in order to correct the coma difference caused by the tilt of the beam splitter 4, and are tilted in the opposite direction to the beam splitter 4.

The thicknesses of the beam splitter 4, the correction plate 8 and the cover glass 9 that pass between the collimator lens 5 and the light receiving surface 10 are t ₁ , t ₂ and t _{3, respectively,} and the refractive indices are n ₁ , n ₂ and Let n ₃ be the air conversion distance L of the detection system
₁ is L ₁ = t ₁ / n ₁ + t ₂ / n ₂ + t ₃ / n ₃ .

When the thicknesses of the cover glass 2 and the grating grid 3 passing between the laser diode 1 and the collimator lens 5 are t ₄ and t _{5, respectively,} and the refractive indices are n ₄ and n ₅ , respectively, the air-equivalent distance of the light source system. L ₂ is L ₂ = t ₄ / n ₄ + t ₅ / n ₅ .

In this optical pickup, L ₂ > L ₁ . The spherical aberration of the collimator lens 5 is corrected according to the cover glass condition corresponding to the air conversion distance L ₂ of the light source system.

Therefore, in the light receiving system, the correction of the spherical aberration of the collimator lens becomes excessive in the direction in which the beam splitter 4 and the correction plate 8 are not tilted, and spherical aberration in the direction opposite to that in the tilted direction occurs. There is. The light-receiving surface 10 is arranged near the circle of least confusion on both sides. In this way, the light spot on the light receiving surface could be made smaller than the conventional one.

[0021]

According to the optical pickup of the present invention, the light spot on the light receiving surface of the light receiving element can be made smaller than the conventional one. Therefore, it is possible to shorten the distance between the main spot and the sub-spot of the three-beam method.

Between the spot interval P _PD on the light receiving surface and the spot interval P _{DISC on} the information recording surface, P _PD = P _DISC × (f _CL
/ F _obj ). However, f _CL is the focal length of the collimator lens, and f _obj is the focal length of the objective lens.

From the above equation, if the spot spacing P _PD on the light receiving surface is reduced, the spot spacing P _DISC on the information recording surface is reduced, and the change of the tracking error signal depending on the distance from the disc center, that is, the R dependency can be reduced. It will be possible.

On the other hand, if P _DISC is not changed, the focal length f _CL of the collimator lens can be reduced and the optical system can be downsized.

[Brief description of drawings]

FIG. 1 is an optical path diagram for explaining an optical pickup that is an embodiment of the present invention.

[Explanation of symbols]

 1 Laser diode 2 Cover glass 3 Grating grid 4 Beam splitter 5 Collimator lens 6 Objective lens 7 Information recording surface 8 Correction plate 9 Cover glass 10 Light receiving surface

Claims (1)

[Claims] 1. Light from a light source is condensed on an information recording surface of an optical disc by a collimator lens and an objective lens,
In an optical pickup that collects the reflected light from the information recording surface on the light receiving surface of the light receiving element by the objective lens and the collimator lens, the thickness of each parallel flat plate optical material in the optical path between the collimator lens and the light receiving surface. The optics characterized in that the sum of the values obtained by dividing the Ceremony pickup.