JPS59231738A - Optical pickup - Google Patents

Abstract

PURPOSE:To improve the accuracy of a focusing error signal by using a lens, in which the concave side of a plane/concave lens is used as the incident side of parallel luminous fluxes and a concentric diffraction grating is formed at the plane side, as a condenser/Fresnel lens. CONSTITUTION:The parallel luminous fluxes delivered from a collimator lens 3 are made incident to the concave side of a Fresnel lens 18 and converted into scattered beams. A 0-order diffracted beam 9 transmits directly through the lens 18 and is made obliquely incident to a disk 7. Thus the beam 9 is changed into a reflected beam 1a on the disk surface and never returns to the lens 18. A 0- order diffracted beam 9a obtained at a place near an optical axis is also reflected by the disk 7 and scattered with increment of the output angle on the concave side of the lens 18. Then the beam 9a is never made incident to a photodetector 13. Then a primary diffracted beam 8 is condensed at one point of the disk 7 and reflected by the disk 7. This reflected beam passes through the lens 18 and then a 1/4 wavelength plate 5 in the form of parallel luminous fluxes and is separated through a beam splitter 4. These split beams pass through a luminous flux form converting optical system 12 and are received by the detector 13.

Description

Detailed Description of the Invention The present invention optically reads information from recording pits recorded on a disc-shaped information recording medium (hereinafter referred to as a disc) in an optical disc player, etc., and reads information into the recording pits on the disc. This invention relates to an optical pickup that focuses light.

Optical disc players use an optical optical pickup to read out information signals from the recording pits recorded on the disc, and focus the information reading beam onto the disc surface where the recording pits are located. It is necessary to track the recording pits, to always accurately irradiate the recording pits on the disk surface with reading light, and to receive the reflected light from the recording pits.

The astigmatism method is known as a method for detecting focusing deviation in order to always focus the light beam on the disk surface.

FIG. 1 shows a conventional optical pickup of this type using the astigmatism method.

In FIG. 1, a laser beam (2) emitted from a laser light source (1) enters a collimator optical system (3) and then is converted into a parallel beam by a polarizing beam splitter +41. 1/4
The light passes through the wave plate (5) and enters the Fresnel lens (6).

Here, the Fresnel lens has a concentric diffraction grating formed on a parallel plate. The above Fresnel lens (6
)K Most of the incident parallel light flux is converted into first-order diffracted light (8) that is focused on one point P on the surface of the disk (7), but there is also zero-order diffracted light (9) that passes through as is.

Next, the first-order diffracted light condensed at one point P on the surface of the disk (7) is reflected here, follows the opposite optical path, passes through the Fresnel lens (6), becomes a backward light beam, and becomes a backward light beam that passes through the quarter-wave plate ( 5)
The light is transmitted through a polarizing beam splitter (4), transmitted through a beam shape conversion optical system a consisting of a convex lens 0 and a cylindrical lens 0, and is received by a photodetector αJ. The light beam shape conversion optical system is provided to convert a parallel light beam into a convergent astigmatic light beam.

On the other hand, the 0th order diffracted light (9) is also reflected by the disk (7) and follows the opposite optical path and enters the Fresnel lens (6).

A part of the incident light becomes first-order diffracted light and becomes focused light I,
After passing through the condensing point, the light becomes diverging light, enters the polarizing beam splitter (4), and its optical path is changed, but hardly enters the photodetector α.

Next, the rest of the light incident on the Fresnel lens (6) passes through the quarter-wave plate (5) as a parallel light beam, and is separated by the polarizing beam splitter (4).

After passing through the light beam shape conversion optical system α, the light beam is transferred to a photodetector (13
Inject into +. That is, the reflected light of the light focused on the disk (7) by the Fresnel lens (6) and the light reflected from the disk (7) as parallel light beams are superimposed.

Next, we will explain the effect of the above-mentioned beam shape conversion optical system α.
Ru. If the axis of the cylindrical lens (11) is oriented vertically in the figure, and the cylindrical lens aυ
Assuming that has the property of converging a light beam in a direction perpendicular to the plane of the drawing, the laser light beam incident on the light beam shape conversion optical system (2) is converted into a convergent light beam having rotational symmetry by the convex lens, and further, It is focused by the cylindrical lens OD in the direction perpendicular to the paper surface of the figure to form a first focal line Q5a parallel to the paper surface of the figure.
If there is no focal line, a second focal line (15b) perpendicular to the plane of the paper is formed.

The photodetector (2) detects the first focal line Qsa) and the second focal line (Qsa) when the disk (7) is at the best focus position.
jab) is placed at the position of the circle of least confusion of the astigmatic light beam.

FIG. 1(B) is a diagram showing an example of the shape of the photodetector α and the cross-sectional shape of the light beam on the photodetector when it is in the best focus state. The photodetector Q31 is divided into four by orthogonal linear electrically insulating regions. In the best focus state, the beam cross section (II is circular, but as mentioned above, the reflected light of the light focused on the disk (7) by the Fresnel lens (6) and the parallel beam remaining on the disk (7)
The two reflected lights cause interference on the photodetector (2) and produce interference fringes within the cross section of the beam, resulting in a non-uniform intensity distribution. Therefore, the photodetector (13a) (13b) (
13c) Nonuniformity occurs in the output of (13d), and the differential amplification output does not become zero, causing an offset.

FIG. 1 (Q is a diagram showing the relationship between the photodetector α (to) and the beam cross-sectional shape when the disk (7) is closer to the Fresnel lens (6) than the best focus state. In this case, the first
The focal line (15a) of the hook 1 in Figure (4) is the photodetector 09
In order to approach , the beam cross section (1
61 indicates a vertically long shape. However, the light beam cross section a9 due to the light reflected by the disk (7) as a parallel light beam remains circular, and interference fringes are similarly present in the light beam cross section αQ.

FIG. 1 is a diagram showing the relationship between the photodetector (13) and the cross-sectional shape of the beam when the disk (7) is further away from the best focus state. In this case, the focal line (15a
) moves away from the photodetector α (to) and becomes the second focal line (12b
) approaches, the beam cross section αQ has a horizontally elongated shape. However, as in the case where the disk (7) approaches, the beam cross section αη remains circular, and interference fringes are present in the beam cross section (IB).

The sum of the outputs of the photodetectors (15a) and (15c) divided from the above FIG. It can be seen that the focusing error signal can be obtained by differentially amplifying the sums of .

Now, since the conventional optical pickup shown in FIG. 1 (4) uses a Fresnel lens (6) in which a concentric diffraction grating is fabricated on a parallel plate, it is impossible to avoid the influence of the 0th order diffracted light. It causes unnecessary background light on the photodetector and interference with the light beam that causes a focus error signal, resulting in uneven intensity distribution.

Along with reducing the accuracy of the focus error signal.

When the disk (7) is tilted, the distribution of reflected light due to the 0th order diffracted light on the photodetector changes, resulting in a drawback that the accuracy of the focus error signal is reduced.

The present invention was made in order to eliminate the above-mentioned drawbacks of the conventional optical pickup using a parallel plate-shaped Fresnel lens, and will be described below with reference to the drawings.

FIG. 2 is a diagram showing an embodiment of the present invention.

In the figure, (1B is a Fresnel lens in which a concentric diffraction grating is fabricated on the plane side of a plano-concave lens.The parallel light beam exiting the collimator lens (3) enters the concave side of the Fresnel lens (IB). It is converted into diverging light.The 0th order diffracted light (9) passes through the disk (7) as it is.
The reflected light (1
It becomes a). This reflected light does not return to the Fresnel lens aυ again. Furthermore, the 0th order diffracted light (9a) near the optical axis similarly passes through the Fresnel lens (18) and obliquely enters the disk (7), where it is reflected and the reflected light (la
a), the light enters the Fresnel lens again, but the outgoing angle is widened by the concave surface of the Fresnel lens and the light diverges. Therefore, this reflected light also does not enter the photodetector side.

Next, the first-order diffracted light (8) is focused on one point on the disk (7). The focused laser beam is reflected by the disk (7), travels backwards, passes through the Fresnel lens, becomes a backward beam, passes through the 1/4 wavelength plate (5), and is sent to the beam splitter (4).
), transmitted through the beam shape conversion optical system 0α, and received by the photodetector 13. The focus error signal is obtained as described with respect to FIG.

However, in this case, background light and interference fringes due to the O-order diffracted light do not appear, and a highly accurate focus error signal is obtained, and the focus error signal is not affected by the inclination of the disk.

FIG. 3 is a diagram schematically showing an example of the shape of a Fresnel lens. In this example, the refractive index is 1 on the plano-concave lens ■.
It has a shape similar to the phase type Fresnel zone plate in which the transparent material Qυ described above is attached in an annular shape. The plano-concave lens (4) and the transparent material υ may be the same member.

FIG. 4 is a diagram schematically showing another example of the shape of the Fresnel lens. In this case, the transparent medium (21) attached to the plano-concave lens (4) is blazed. Of course, the plano-concave lens (a) and the transparent material (211) may be the same member.

As described above, according to the optical pickup according to the present invention, since a Fresnel lens in which a concentric diffraction grating is fabricated on the plane side of a plano-concave lens is used as a condensing Fresnel lens, the disk (7) using 0th-order diffraction light is The reflected light does not return to the photodetector, and background light based on interference fringes and 0th order diffracted light on the photodetector, which are seen in conventional optical pickups, can be removed, resulting in highly accurate focus errors. I get a signal. Furthermore, even if the disk is tilted, it will not be affected by the zero-order diffracted light.

[Brief explanation of the drawing]

Fig. 1 (4) is a diagram showing an example of a conventional optical pickup using the astigmatism method, and Fig. 1 (B) is a diagram showing an example of the shape of a photodetector and the cross-sectional shape of the light beam in the best focus state. , Figure 1 (Q is a diagram showing the relationship between the photodetector and the cross-sectional shape of the light beam when the disk is close, and Figure 1 (D) is a diagram showing the relationship between the photodetector and the cross-sectional shape of the light beam when the disk is far away. The figure shown,
FIG. 2 is a diagram showing an embodiment of the present invention, FIG. 3 is a diagram schematically showing an example of the shape of a Fresnel lens, and FIG. 4 is another diagram schematically showing an example of the shape of a Fresnel lens. It is. In the figure, (1) is a laser light source, and (3) is a collimator optical system. (4) is a polarizing beam splitter, (5) is a quarter-wave plate, (6) is a Fresnel lens, (7) is a disk, (8)
02 is a first-order diffraction light, (9) is an O-order diffraction light, and 02 is a beam shape conversion optical system. The 0th floor is a photodetector, the decoy is a Fresnel lens, and (4) is a plano-concave lens. Note that in FIG. 1, the same or corresponding parts are designated by the same reference numerals. Agent Masuo Oiwa Figure 1 (α) Figure 2 Figure 3 (a) Figure 4 (d)

Claims (1)

[Scope of Claims] A laser light source, a collimator optical system that converts diverging light from the laser light source into a parallel light beam, and a condensing optical system that focuses the parallel light beam onto an information recording medium. a polarizing beam splitter and a 1/4 wavelength plate for separating a parallel light flux that travels in the opposite direction from the parallel light flux produced when the laser light flux that has been reflected by the information recording medium passes through the condensing optical system again; a beam shape conversion optical system that converts the parallel beam separated by the polarizing beam splitter into a desired beam shape; In an optical pickup equipped with a photodetector that receives a laser beam whose shape has been changed, the condensing optical system is a Cuff Fresnel lens, and the Fresnel lens has a concave side of a plano-concave lens as a side on which the parallel beam is incident. , an optical pickup characterized in that it is a Fresnel lens in which a concentric diffraction grating is formed on the plane side of the plano-concave lens.