JPS61122938A - Optical pick-up device - Google Patents

Abstract

PURPOSE:To improve a sensitivity of a focus servo signal by irradiating, to a disk, the primary diffracting light of almost non-astigmatism by a diffracting means and photodetecting,a t a photodetecting element, the primary diffracting light having astigmatism diffracted by a diffracting means of a disk reflecting light. CONSTITUTION:By using a phase type diffracting plate 2 which separates incident light 6 from a light source 1 mainly into + primary diffracting light and -primary diffracting light which has almost non-astigmatism for one side and astigmatism for other side, making a prescribed angle with each other, irradiates the - primary diffracting light or the + primary diffracting light out of diffracting light of light which are almost non-astigmatism through an objective lens 3 on a disk 4. Out of the diffracting lights by a diffracting means 2 of a disk reflecting light, the - primary diffracting light or the + primary diffracting light 10 which has a non-point-astigmatism, is photodetected by a photodetecting element 5. By such a constitution, the device can be made small and the focus servo can be obtained by a highly sensitive astigmatism method.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical pickup device for recording and reproducing information on optical video discs, digital audio discs, and the like.

[Conventional technology]

When information is optically recorded and played back on a disc.

In order to make recording and reproducing light such as a laser beam incident on a disk and to receive the reflected light, it is necessary to separate the incident light and the reflected light. As such a separation means, a half mirror or 1
A combination of a /4 wavelength plate and a polarizing prism has been proposed, but if these are used.

There was a drawback that the device became large. Therefore, a diffraction grating can be used as the separation means, as disclosed in, for example, Japanese Patent Laid-Open Publication No. 119548/1988 proposed by the present applicant. However, in this embodiment, focus control, which is essential for an optical pickup, has not been considered. Although various proposals have been made for focus control methods, a method generally referred to as an astigmatism method using optical means such as a cylindrical lens is attracting attention from a practical standpoint. However, if a cylindrical lens is used in the technique disclosed in Japanese Unexamined Patent Publication No. 59-119548, the cost will increase and the device cannot be miniaturized.

As a solution to this problem, for example, Japanese Patent Application Laid-Open No. 56-5701
It is conceivable to use a single diffraction grating not only as a separation means, but also as an astigmatism means, as disclosed in Japanese Patent No. 3.

[Problem that the invention seeks to solve]

However, in the technology disclosed in JP-A-56-57013, the incident light that enters the disk from the light source is the zero-order diffracted light by the diffraction grating, and the reflected light from the disk toward the light receiving element is the diffraction grating. Since the first-order diffracted light is used, there is a drawback that the amount of light that can be received by the light-receiving element is small. As disclosed in the publication, even if the diffraction efficiency of the first-order diffracted light is 40%, for example, if the diffraction efficiency of the zero-order diffracted light is 20%, the amount of light received will be only 8% or less.

Furthermore, if the difference in the diffraction angle between the zero-order diffraction light and the first-order diffraction light is small, a sufficient amount of astigmatism cannot be obtained, and a stable four force error signal cannot be obtained.

[Means for solving problems].

FIG. 1 shows a schematic configuration of an optical pickup device of the present invention. In the figure, reference numeral 1 denotes a light source such as a semiconductor laser or a gas laser that emits recording and reproducing light of a predetermined wavelength λ. 2 is a phase type diffraction plate (diffraction grating) that diffracts incident light. The distribution state of the diffracted light is determined by the depth, interval, and direction of the grooves of the 0 phase type diffraction plate 2. d is set to approximately 1/2 of the wavelength λ, and approximately 1
Only the second-order diffraction light is derived. Furthermore, the spacing and direction of the grooves are set so that the diffraction angle is relatively large. 3 is an objective lens.

It is driven by a focus control means (not shown) to converge the light onto the disk 4. Reference numeral 5 denotes a light-receiving element that receives the light reflected by the disc 4 and outputs focus, error signals, and reproduction signals of information recorded on the disc 4.

[Effect]

The effect will now be explained. Light 6 emitted from the light source 1 is diffracted by the phase type diffraction plate 2.

Since the depth d of the groove is set to approximately 1/2 of the wavelength λ of the light, almost only the +1st-order diffracted light and the 11th-order diffracted light are derived at this time. The phase type diffraction plate 2 is, for example, -
It is designed to diffract only the first-order diffracted light (or +1st-order diffracted light) with almost no aberration, and this -1st-order diffracted light enters the objective lens 3 as incident light 7, and is converged and irradiated onto the disk 4. (+1st-order diffracted light is not shown). The reflected light 8 reflected by the disk 4 and incident on the objective lens 3 is further incident on the phase type diffraction plate 2, where it is diffracted again into +1st-order diffracted light and 11th-order diffracted light. Then, the +1st order diffracted light (or -1st order diffracted light) 9, which has almost no aberration, is returned to the light source 1, and the 1st order diffracted light (or +1st order diffracted light) -10, which has astigmatism, is irradiated onto the light receiving element 56. For example, if the diffraction efficiency of the phase type diffraction plate 2 is 50% and the reflectance of the disk 4 is 100%, the light receiving element 5
25% of the light emitted from the 1-phase diffraction plate 2
can receive light. Furthermore, the amount of astigmatism included in the light 10 incident on the light receiving element 5 increases as the opening angle of the +1st-order diffracted light of the phase type diffraction plate 2 increases, but if the angle is too large, the device becomes large. So,
The angle is set to such an extent that sufficient astigmatism can be obtained to perform focus servo. Note that the principle of focus servo based on the astigmatism method is well known, so a description thereof will be omitted.

FIG. 2 shows an example of the shape of the phase type diffraction plate 2, which provides no aberration to one side of the ±1st-order diffraction light and astigmatism to the other. As shown in the figure, the grooves 21 are arranged substantially symmetrically, are straight at the center, and curve toward the outer periphery. The spacing and direction of the grooves 21 are determined by the phase matching between the light 6 from the light source 1 on the surface of the phase diffraction plate 2 and the light 7 from the virtual light source 11 directed from the phase diffraction plate 2 toward the objective lens 3. Determined. That is, if the coordinates of the light source 1 are S, the coordinates of the virtual light source 11 are R1, the coordinates of the edge of the groove 21 on the surface of the phase type diffraction plate 2 are G, and the optical distances between the coordinates S and G and R and G are SG and RG, respectively. , the shape formula is (SG-RG)X2yc/λ=2mtc (however, m=0.±1.±2...).

As shown in FIGS. 3 and 4, the phase type diffraction plate 2 may be shaped like a wedge, or the phase type diffraction plate 2 may be provided with a wedge-shaped flat plate 3.
1 in combination, it is possible to emphasize astigmatism.

〔effect〕

As described above, in the present invention, incident light is mainly diffracted at a predetermined angle to each other, and is separated into +1st-order diffracted light and 11th-order diffracted light, one with almost no aberration and the other with astigmatism. By using means, one part of the diffracted light of the incident light is approximately aberration-free.
The first-order diffracted light (or +1st-order diffracted light) is irradiated onto the disk through an objective lens, and the 11st-order diffracted light (or +1st-order diffracted light) having astigmatism among the diffracted lights of one reflected light is received by the light receiving element. As a result, the device can be made smaller, the amount of astigmatism can be increased (approximately twice as much as the astigmatism of the first-order diffracted light for the zero-order light), and the amount of astigmatism can be increased. A focus servo signal can be obtained using the sensitivity/astigmatism method.

[Brief explanation of drawings]

FIG. 1 is a schematic side view of the optical pickup device of the present invention, FIG. 2 is a plan view of its phase type diffraction plate, and FIGS. 3 and 4 are schematic side views of other embodiments. 1... Light source 2... Phase type diffraction plate 3... Objective lens 4... Disk 5... Light receiving element 11.
... Virtual light source 21 ... Groove 31 ... Plane plate or more

Claims (2)

[Claims] (1) a light source, an objective lens that focuses incident light emitted from the light source and incident on the disk onto the disk, and a light receiving element that receives at least a portion of the reflected light reflected by the disk; The groove is arranged in the optical path of the incident light and the reflected light, and the depth of the groove is approximately 1/2 of the wavelength λ of the light emitted by the light source.
an optical pickup device having a diffraction means for separating incident light into +1st-order diffracted light and -1st-order diffracted light that form a predetermined angle with each other, the diffraction means separating the +1st-order diffracted light One of the second-order diffracted light and the -1st-order diffracted light becomes light with substantially no aberration, and the other becomes light with astigmatism, and substantially no of the diffracted light of the incident light diffracted by the diffraction means is arranged. Irradiating the disk with the −1st-order diffracted light (or the +1st-order diffracted light) of the aberration through the objective lens,
The −1st order diffracted light (or the +1st order diffracted light) having astigmatism among the diffracted light of the reflected light diffracted by the diffraction means
An optical pickup device characterized in that the light is received by the light receiving element. (2) The diffraction means has a lattice shape that gives a phase difference, and the grooves have the coordinates of the light source S, the coordinates of the edges of the grooves on the surface of the diffraction means G, and the incident light toward the objective lens. When the coordinates of the virtual light source are R, the optical distances between the coordinates S and G are SG and RG, and the values of m are 0, ±1, ±2, etc., the shape formula, (SG-RG )×2π/λ=2mπ.