JP2001034994A - Confocal type optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical pickup which has a less decrease in detection sensitivity because of an inclination of an optical element which separates a light advancing to a recording medium and a light returning from the recording medium. SOLUTION: The optical pickup has a semiconductor laser 12 for emitting a laser light, a collimate lens 14 for collimating the laser light, and an objective lens 20 for collecting the collimated light into a recording medium 22. The optical pickup further includes a prism 16 of an anisotropic medium and a quarter-wave plate 18 between the collimate lens 14 and the objective lens 20 so as to separate the light advancing to the recording medium 22 and the light returning from the recording medium 22. The prism 16 is constituted of, e.g. a prism of a uniaxial optical crystal showing double refraction. The optical pickup is provided with a pin hole 26 arranged to a position conjugate to a collective point by the objective lens 20, and a photodetector 28 for detecting the light passing through the pin hole 26 on an optical path of the light returning from the recording medium 22.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup for optically recording and reproducing information on a recording medium, and more particularly, to selectively recording information on surfaces located at different depths in the recording medium. The present invention relates to a confocal optical pickup for performing recording and reproduction.

[0002]

2. Description of the Related Art A confocal optical pickup for selectively recording and reproducing information on a plurality of surfaces (recording surfaces) located at different depths in a recording medium is already known. As used herein, the term “recording / reproducing” means recording and / or reproducing information or both.

[0003] Japanese Patent No. 2624255 discloses an example of such a confocal optical pickup. The optical pickup includes a laser light source for emitting light for recording and reproduction, an objective lens for condensing the laser light in a recording medium, and a beam splitter for reflecting light traveling toward the recording medium and transmitting light returning from the recording medium. And a pinhole disposed at a position conjugate to the focal point of the objective lens, and a photodetector for detecting light passing through the pinhole.

In this optical pickup, a confocal optical system is formed between the converging point of the objective lens and the pinhole, so that return light from a point other than the converging point cannot pass through the pinhole and is condensed. Only the return light from the point passes through the pinhole and is detected by the photodetector. Therefore, by changing the depth of the condensing point, information can be selectively recorded and reproduced on a plurality of recording surfaces located at different depths in the recording medium.

[0005]

In this optical pickup, the laser light emitted from the laser light source is
The light is reflected by the beam splitter toward the recording medium. When this beam splitter is tilted due to temperature change, etc.,
Because the laser beam reflected by the beam splitter also tilts,
The laser light is applied to the recording medium in an inclined state. Further, light reflected by the recording medium is also reflected with the same inclination. The reflected light reflected by the recording medium with such an inclination passes through the beam splitter, but is condensed at a position shifted from the pinhole due to the inclination. As a result, when the beam splitter is tilted, the return light cannot pass through the pinhole, and the reflected light may not be detected by the photodetector.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide detection sensitivity caused by the inclination of an optical element that separates light traveling toward a recording medium from light returning from the recording medium. An object of the present invention is to provide a confocal optical pickup with a small decrease.

[0007]

According to one aspect of the present invention, there is provided a confocal optical pickup comprising a light source for emitting light for recording and reproduction, and an objective lens for condensing light from the light source on a recording medium. Located between the light source and the objective lens, a prism of an anisotropic medium that separates the irradiation light toward the recording medium and the return light from the recording medium, and is located on the optical path of the return light from the recording medium, It has a pinhole disposed at a position conjugate with the focal point of the objective lens, and a photodetector for detecting light passing through the pinhole.

In another aspect of the confocal optical pickup of the present invention, a light source for emitting light, an objective lens for condensing light from the light source in a recording medium, and a position between the light source and the objective lens are provided. A prism of an anisotropic medium that separates the irradiation light toward the recording medium and the return light from the recording medium, and is located on the optical path of the return light from the recording medium. A detector disposed at a location,
And a photodetector whose size of the light receiving portion is 100 times or less the wavelength of the irradiation light.

[0009] The prism of the anisotropic medium is composed of, for example, a Wollaston prism.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

First, the confocal optical pickup of the first embodiment will be described.

As shown in FIG. 1, the confocal optical pickup according to the first embodiment includes a semiconductor laser 12 for emitting laser light, a collimating lens 14 for collimating the laser light from the semiconductor laser 12, And an objective lens 20 that focuses the converted laser light on a recording position 24 in the recording medium.

The optical pickup separates the light traveling toward the recording medium 22 and the light returning from the recording medium 22 by interposing a prism 16 of an anisotropic medium and a quarter wavelength between the collimating lens 14 and the objective lens 20. It has a plate 18. The prism 16 is formed of, for example, a uniaxial optical crystal prism having birefringence.

Further, the optical pickup includes a pinhole 26 disposed on the optical path of the return light from the recording medium 22 at a position conjugate with the focal point of the objective lens 20, that is, the recording position 24, and passing through the pinhole 26. And a photodetector 28 for detecting the reflected light.

Further, although not shown, the optical pickup has a mechanism for changing the depth of the focal point formed in the recording medium 22, that is, the recording position 24. Specifically, a mechanism for moving the objective lens 20 in the optical axis direction,
A mechanism for moving the recording medium 22 in the optical axis direction may be used.

The semiconductor laser 12 emits a linearly polarized divergent beam. The divergent beam emitted from the semiconductor laser 12 is converted into a parallel beam by the collimator lens 14 and enters the prism 16. The polarization plane and the optical axis of the semiconductor laser 12 and the prism 16 are set so that the linearly polarized light incident on the prism 16 becomes ordinary light. Accordingly, the parallel beam incident on the prism 16 is transmitted therethrough, converted into circularly polarized light by a quarter-wave plate, and condensed by the objective lens 20 at a recording position 24 located inside the recording medium 22.

The rotation direction of the circularly polarized light of the return light reflected at the recording position 24 is reversed. The return light is the objective lens 2
After being incident on 0, the light is converted to linearly polarized light by the quarter-wave plate 18. Since this linearly polarized light has passed through the quarter-wave plate 18 twice, it has a polarization direction orthogonal to the polarization direction of the linearly polarized light immediately after being emitted from the semiconductor laser 12. Therefore,
This linearly polarized light becomes extraordinary light for the prism 16.

Since the refractive index of the ordinary light of the prism 16 is slightly different from that of the extraordinary light, the extraordinary light of the return light is refracted by the prism 16 in a direction slightly different from the ordinary light. The return light that has passed through the prism 16 is converted into a convergent beam by the collimator lens 14 and condensed on the pinhole 26.

More specifically, the pinhole 26 has a hole, that is, an opening formed in the pinhole 26 in a conjugate positional relationship with the recording position 24 in the recording medium 22.
Return light from other than 4 cannot pass through it, and only return light from the recording position 24 can exactly pass through it.

The return light passing through the pinhole 26 is detected by a photodetector 28. The return light detected by the photodetector 28 includes only the information of the recording position 24 located at a position conjugate with the pinhole 26, so that the information can be accurately recorded and reproduced with respect to the recording position 24.

In the optical pickup according to the present embodiment, both the light traveling toward the recording medium 22 and the light returning from the recording medium 22 pass through the prism 16 and are separated by the difference in the refractive index between ordinary light and extraordinary light. For this reason, even if the prism 16 is tilted due to a temperature change or the like, the influence acts on the ordinary light and the extraordinary light in the same manner, so that the return light cannot be prevented from passing through the pinhole.

Hereinafter, this advantage will be described with a specific example of the prism 16 with reference to FIG. The prism 16 shown in FIG. 2 is a prism made of a birefringent crystal of niobium lithium, having an apex angle of 10 °, a refractive index of ordinary light of 2.2732, and a refractive index of extraordinary light of 2.1915.

Light ray A indicates a light ray traveling from the light source via the prism 16 to the recording medium, and light ray B indicates a return light traveling from the recording medium via the prism 16 to the photodetector. The optical axis of the prism 16 is set parallel to the light beam A.
At this time, the angle between the light beam A and the light beam B is 6.39522
°.

It is assumed that the prism 16 is rotated counterclockwise by 0.05 ° about the intersection C between the ray A and the prism 16.
The light ray A is irrelevant to the rotation of the prism 16, but the light ray B of the return light slightly rotates according to the rotation of the prism 16.
The angle formed by the light beams A and B after the rotation of the prism 16 is 6.39568 °. Therefore, the amount of change is
6.39568−6.39522 = 0.046 °, which is very small.

From the above facts, it can be seen that the optical pickup of this embodiment is not substantially affected by the inclination of the prism. That is, in the optical pickup according to the present embodiment, the detection sensitivity hardly decreases due to the inclination of the optical element that separates the light traveling toward the recording medium and the light returning from the recording medium.

The prism 16 need not be a uniaxial birefringent prism, but may be a biaxial birefringent prism.

FIG. 3 shows another prism 30 applicable to the optical system of FIG. 1 in place of the prism 16 described above. The prism 30 is a quartz prism having a natural optical rotation, and has an optical axis parallel to a light beam passing through the prism. In the application of such a prism 30, 1/4
The wave plate 18 is arranged on the light source side with respect to the prism 30.

The light beam A from the light source is converted into, for example, clockwise circularly polarized light by passing through the quarter-wave plate 18 and enters the prism 30. Also, the return light beam B
Is reflected by the recording medium, is converted into left-handed circularly polarized light, and enters the prism 30.

The prism 30 has different refractive indices for clockwise circularly polarized light and counterclockwise circularly polarized light. Therefore, the light beam A and the light beam B are refracted by the prism 30 at different angles and are separated from each other.

FIG. 4 shows another prism 32 applicable to the optical system of FIG. 1 in place of the prism 16 described above. The prism 32 is a uniaxial birefringent prism, and has an optical axis perpendicular to an optical path shared by the light beams A and B in the prism.

The light beam A from the light source is totally reflected inside the prism 32 and goes to the recording medium, and the light beam B of the return light from the recording medium is totally reflected inside the prism 32 and goes to the photodetector. The light beam B becomes a linearly polarized light having a polarization direction orthogonal to the polarization direction of the linearly polarized light of the light beam A by passing through the quarter-wave plate twice.

The prism 32 has different refractive indices for the light beams A and B. Therefore, the light beams A and B are reflected at different angles inside the prism 32 and are separated from each other.

Next, a confocal optical pickup according to a second embodiment will be described with reference to FIG. In this embodiment,
Most parts are common to the first embodiment, and the common members are indicated by the same reference numerals, and description thereof will be omitted.

As shown in FIG. 5, the confocal optical pickup of the second embodiment does not have the pinhole 26 and the photodetector 28 in comparison with the configuration of FIG. Another photodetector 34 is arranged at a position conjugate with the middle recording position 24. The photodetector 34 has a light receiving portion having a size substantially functioning as a pinhole. Stated another way, the photodetector 34 has a light receiving portion as large as a pinhole.

The photodetector 34 is provided by the collimator lens 14.
The size of the light spot converged on the top depends on the wavelength (λ) of the light of the semiconductor laser 12 and the numerical aperture of the collimator lens 14.
(NA). Its size is approximately λ / NA. N
If A is 0.1, the size of the spot is 10 of wavelength λ.
It is twice. Considering the positioning accuracy of the spot and the light receiving portion, it is desirable that the light receiving portion is about 10 times as large as the spot size.

For this reason, the size of the light receiving portion of the photodetector 34 is desirably 100 times or less the wavelength of the laser light emitted from the semiconductor laser 12. Although "the size of the light-receiving portion" is originally pointed out by a two-dimensional parameter, the term "size of the light-receiving portion" herein is a one-dimensional parameter that characteristically represents the shape thereof.
For example, let it mean its diameter relative to a circle.

In this embodiment, the photodetector having the light receiving portion having a size similar to that of the pinhole is arranged so that the light receiving portion and the recording position have a conjugate positional relationship.
Eliminates the need for pinholes.

Next, a confocal optical pickup according to a third embodiment will be described with reference to FIG.

As shown in FIG. 6, the confocal optical pickup according to the third embodiment has a semiconductor laser 12 for emitting a laser beam, a collimating lens 14 for collimating the laser beam, and a laser beam for storing the laser beam in a recording medium. And an objective lens 20 for condensing light at the recording position 24 of the recording medium.

The optical pickup separates the Wollaston prisms 36 and 1 made of an anisotropic medium between the collimating lens 14 and the objective lens 20 in order to separate the light traveling toward the recording medium 22 and the light returning from the recording medium 22. Quarter wave plate 18
And

The optical pickup includes a condensing lens 3 for condensing the return light on the optical path of the return light from the recording medium 22.
8, a pinhole 26 disposed at a position conjugate with the converging point 24, and a photodetector 28 for detecting light passing through the pinhole 26.

Further, although not shown, the optical pickup has a mechanism for changing the depth of the focal point 24 formed in the recording medium 22. Specifically, a mechanism for moving the objective lens 20 in the optical axis direction or a mechanism for moving the recording medium 22 in the optical axis direction may be used.

The divergent beam emitted from the semiconductor laser 12 is converted into a parallel beam by the collimator lens 14 and enters the Wollaston prism 36. The linearly polarized laser light passes through the Wollaston prism 36 as, for example, ordinary light-extraordinary light. Wollaston prism 3
After passing through the 1/4 wavelength plate, the light transmitted through 6 is condensed by the objective lens 20 at a recording position 24 located inside the recording medium 22.

The return light reflected at the recording position 24 enters the objective lens 20 and passes through the quarter-wave plate 18.
The light enters the Wollaston prism 36. This return light is also linearly polarized light and has a polarization direction orthogonal to the polarization direction of the linearly polarized light immediately after being emitted from the Wollaston prism 36 because it has passed through the quarter-wave plate 18 twice. Since the order of the going light and the returning light passing through the prisms constituting the Wollaston prism 36 is reversed, the linearly polarized light of the returning light passes through the Wollaston prism 36 as ordinary light and extraordinary light.

The return light having passed through the Wollaston prism 36 is condensed by the condenser lens 38 onto the pinhole 26 arranged at a position conjugate with the recording position 24. The pinhole 26 accurately passes only the return light from the recording position 24, and the return light passing through the pinhole 26 is detected by the photodetector 28.

In this embodiment, the Wollaston prism 3
6 is used to separate the going light and the returning light,
The opening angle between the outward and return trips can be increased. As a result, the semiconductor laser 12 and the photodetector 28 can be arranged with a sufficient space.

Although several embodiments have been described in detail with reference to the drawings, the present invention is not limited to the above-described embodiments, but may be performed within the scope of the invention. Including all implementations.

[0048]

According to the confocal optical pickup of the present invention, since the light traveling toward the recording medium and the light returning from the recording medium are separated by using the prism of the anisotropic medium, the inclination is caused by the inclination of the prism. Low decrease in detection sensitivity.

[Brief description of the drawings]

FIG. 1 shows an optical system of a confocal optical pickup according to a first embodiment.

FIG. 2 is a schematic diagram for explaining the influence of the rotation of the prism shown in FIG.

FIG. 3 shows another prism applicable to the prism of FIG. 1;

FIG. 4 shows yet another prism applicable to the prism of FIG. 1;

FIG. 5 shows an optical system of a confocal optical pickup according to a second embodiment.

FIG. 6 shows an optical system of a confocal optical pickup according to a third embodiment.

[Explanation of symbols]

 Reference Signs List 12 semiconductor laser 14 collimating lens 16 prism of anisotropic medium 18 quarter-wave plate 20 objective lens 26 pinhole 28 photodetector

Claims (3)

[Claims] 1. A confocal optical pickup for optically recording and reproducing information on and from a recording medium, comprising: a light source for emitting light for recording and reproduction; and light from the light source condensed on the recording medium. An objective lens, a prism of an anisotropic medium, which is located between the light source and the objective lens and separates irradiation light toward the recording medium and return light from the recording medium, and on an optical path of return light from the recording medium 1. A confocal optical pickup, comprising: a pinhole located at a position conjugate with a focal point of an objective lens, and a photodetector for detecting light passing through the pinhole. 2. A confocal optical pickup for optically recording and reproducing information on and from a recording medium, comprising: a light source for emitting light; an objective lens for condensing light from the light source into the recording medium; A prism of an anisotropic medium which is located between the light source and the objective lens and separates irradiation light toward the recording medium and return light from the recording medium; and an objective located on an optical path of the return light from the recording medium. A confocal light, comprising: a detector arranged at a position conjugate to the focal point of the lens, wherein the size of the light receiving portion is 100 times or less the wavelength of the irradiation light. pick up. 3. The confocal optical pickup according to claim 1, wherein the prism of the anisotropic medium is a Wollaston prism.