JPH059850B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is applicable to optical disks, magneto-optical disks, etc.
The present invention relates to a focusing error detection device used in an optical information detection device for reproducing information using an information recording disk provided with concentric or spiral information tracks and guide tracks.

[Conventional technology]

For example, as shown in FIG. 5, a conventional focusing error detection device includes a semiconductor laser 1 as a light source, a collimator lens 2 that converts the light beam emitted from the semiconductor laser 1 into a parallel light beam, and a device that records information on the parallel light beam. An objective lens 4 that focuses light on the disk 3 to form a beam spot and takes in the reflected light beam reflected by the information recording disk 3, and a beam splitter that changes the direction of the reflected light beam taken in by the objective lens 4. an optical system 6 consisting of a converging lens 6a and a cylindrical lens 6b, which images a beam spot using the reflected light flux whose direction has been changed as an astigmatic ray flux; The light receiving parts 7a, 7b are divided into four parts in a square shape by two boundary lines.
7c and 7d, which receives the beam spot imaged by the optical system 6 and detects a focusing error of the beam spot formed on the information recording disk 3 based on the image formation state. It consists of a detector 7.

The information recording disk 3 has an information track 3b and a guide track 3c formed in concentric or spiral grooves on a substrate 3a made of a light-transmitting material such as glass, and further includes an amorphous alloy of rare earth and transition metal. The information recording medium 3d made of a thin film or the like is
It is coated by vapor deposition, sputtering, etc. to enable high-density recording and reproduction.

Next, the principle for detecting a focusing error of the beam spot formed on the information recording disc 3 will be explained below.

First, the four light receiving parts 7a of the four-split photodetector 7,
The amount of light received at 7b, 7c, and 7d is expressed as Sa, Sb, and
Sc, Sd. Furthermore, the degree of focus f of the beam spot formed on the information recording disc 3 is expressed as (Sa+Sc)-(Sb+Sd).

Then, at the time of focusing, the beam spot 12 projected onto the 4-split photodetector 7 becomes circular as shown in FIGS. 6a and 6b, so that the received light amounts Sa, Sb, Sc and Sd are equal to each other. Therefore, the degree of focus f = (Sa + Sc) - (Sb + Sd)
becomes 0.

Further, as shown in FIG. 6c, when the distance between the information recording disk 3 and the objective lens 4 is short, the beam spot 12 projected onto the 4-split photodetector 7 is cylindrical as shown in FIG. It is an ellipse whose major axis is a direction b ₀ parallel to the generatrix of the lens 9. Therefore,
The focusing degree f=(Sa+Sc)−(Sb+Sd) is negative.

On the other hand, if the distance between the information recording disk 3 and the objective lens 4 is long as shown in FIG. 6e, the beam spot 12 projected onto the four-split photodetector 7 will be as shown in FIG. , it becomes an ellipse whose major axis is the information _a0 perpendicular to the generatrix of the cylindrical lens 9. Therefore, the degree of focus f=(Sa+Sc)−(Sb+Sd) is positive.

Therefore, the above focusing degree f=(Sa+Sc)−(Sb+
Sd) as a focusing error signal, it is possible to determine whether the distance between the information recording disk 3 and the objective lens 4 is appropriate, or whether it is long or short, depending on whether the value is positive or negative.

Incidentally, in the beam spot 12 projected onto the four-split photodetector 7, a shadow portion (hereinafter referred to as a diffraction pattern) 13 is generated due to diffraction by the guide track 3c. This diffraction pattern 1
3, even when in the focused state as shown in FIGS. 6a, b, the beam spot 11 formed on the information recording disc 3 and the information recording disc 3 as shown in FIGS. 7a, c, and e. Depending on the positional deviation with respect to the information track 3b, the values change as shown in b, d, and f in FIG. 7, respectively.

When the collimator lens 2 and the objective lens 4 have no aberration, the change in the diffraction pattern 13 becomes symmetrical with respect to the axis corresponding to the direction perpendicular to the guide track 3c of the information recording disc 3.
Therefore, the light receiving parts 7a and 7b of the 4-split photodetector 7
If the four-split photodetector 7 is provided so that the boundary line Y-Y between 7c and 7d coincides with the axis corresponding to the direction perpendicular to the guide track 3c, the diffraction pattern 13 will be as shown in FIG. 7b. , d, and f, the degree of focus f = (Sa + Sc) - (Sb + Sd)
becomes 0, and it can be determined that the state is focused.

[Problem that the invention seeks to solve]

However, in the conventional system described above, if the collimator lens 2 and the objective lens 4 have aberrations, the positional relationship between the information track 3b of the information recording disc 3 and the beam spot 11 formed on the information recording disc 3 is When the diffraction pattern 13 is shifted as shown in FIGS. 8a, c, and e, the diffraction pattern 13 with respect to the axis corresponding to the direction orthogonal to the guide track 3c of the information recording disc 3 is shown as shown in FIGS. 8b, d, and f, respectively. (hereinafter referred to as crosstalk). The focusing degree f = (Sa + Sc) - (Sb + Sd) at this time is
Even when in focus, the value is positive in the case of b in FIG. 8, 0 in the case of d in the same figure, and negative in the case of f in the same figure, for example.

Therefore, as shown in FIG. 9, even though the distance between the information recording disk 3 and the objective lens 4 is close to the in-focus state, the focusing error signal becomes large due to a minute change in distance. This has caused a problem in that the focusing control may become unstable.

Therefore, by rotating the objective lens 4, the aberrations of the objective lens 4, the collimator lens 2, etc. can cancel each other out, and the crosstalk can be reduced.However, in general, the objective lens 4 and the information recording Since the distance from the medium 3 is only a few mm, it is very difficult to adjust the rotational position of the objective lens 4 and to fix it after adjustment while monitoring the focusing error signal. Furthermore, since the objective lens 4 is equipped with many parts such as driving devices for focusing and tracking,
It is also difficult to adjust the rotation without affecting their accuracy.

[Means for solving problems]

In order to solve the above problems, the focusing error detection device according to the present invention includes a light source, a collimator lens that converts the light beam emitted from the light source into a parallel light beam, and an information track and a guide track that converts the parallel light beam into a parallel light beam. A beam spot is formed by condensing the light onto a provided information recording disk, and an objective lens is provided to take in the reflected light beam reflected by the information recording disk, and a beam spot is formed from the reflected light beam taken in by this objective lens. An optical system that forms an image, and a light receiving section that receives the imaged beam spot into a plurality of parts, and detects focusing errors of the beam spot formed on the information recording disk based on the image formation state. In the focusing error detection device, the collimator lens and the objective lens have symmetry with respect to an axis corresponding to a direction orthogonal to the guide track of the diffraction pattern generated in the beam spot projected on the photodetector. This feature is characterized in that the collimator lens is installed at a rotational position that is small enough to be affected by aberrations.

[Effect]

With the above configuration, a diffraction pattern may occur in the beam spot projected on the photodetector due to diffraction by the guide track, or a deviation in the positional relationship between the beam spot formed on the information recording disk and the guide track may occur. Therefore, even if the diffraction pattern changes, the symmetry of this diffraction pattern with respect to the axis corresponding to the direction orthogonal to the guide track will not be impaired by the effects of aberrations of the collimator lens and the objective lens. Therefore, it is possible to stably detect focusing errors without causing deterioration in the quality of the focusing error signal.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 to 4.

As shown in FIG. 1, the focusing error detection device includes a semiconductor laser 21 as a light source, a collimator lens unit 22 including a collimator lens 22a that converts the light beam emitted from the semiconductor laser 21 into a parallel light beam, and An objective lens unit 24 includes an objective lens 24a that focuses a light beam onto an optical disk 23, which is an information recording disk, to form a beam spot, and captures a reflected light beam reflected by the optical disk 23;
A beam splitter 25 changes the direction of the reflected light flux taken in by the beam splitter 25, and the reflected light flux whose direction has been changed is used as an astigmatism ray flux to form a beam spot image. An optical system 26 consisting of a convergent lens 26a and a cylindrical lens 26b;
and a 4-minute accurate photodetector 27 which receives the beam spot imaged by the optical disc 23 and detects a focusing error of the beam spot formed on the optical disk 23 based on the image formation state.

The collimator lens unit 22 is rotatably provided in a recess 28a provided in the housing 28, and is held by a presser spring 29.

The optical disk 23, which is an information recording disk, is provided with a substrate 23a made of a light-transmissive material such as glass, with a concentric or spiral shape having a depth of about 1/8 of the wavelength λ of the light emitted from the semiconductor laser 21. An information track 23b and a guide track 23c are formed in the groove,
Further, the information recording disk 23d, which is made of an amorphous alloy thin film of rare earth elements and transition metals, is coated by vapor deposition, sputtering, etc., so as to enable high-density recording and reproduction.

Further, the 4-split photodetector 27 is oriented at 45° with respect to the generatrix of the cylindrical lens 26b, and
Boundary lines X-X and Y-Y corresponding to directions parallel and perpendicular to the guide track 23c of the optical disk 23
The light receiving part 27a is divided into four parts in a square shape by
27b, 27c, and 27d.

On the other hand, as shown in FIG. direction). That is, inside one end of the cylindrical fixed support 31, there are a focus magnet 32, a focus yoke 33, and a focus magnetic gap 3 for driving in the focus direction.
A focus magnetic circuit consisting of 4 is formed. Further, near the other end of the fixed support 31, a pair of tracking magnets 35 facing each other,
35 forms a tracking magnetic circuit.

Inside the fixed support 31, an intermediate support 38 is swingably supported by focus direction movable parallel springs 37 backed by rubber-like elastic bodies 36.
is provided. A focus coil 39 is provided at the end of the intermediate support 38 to be inserted into the focus magnetic gap 34 with a predetermined gap maintained therebetween.

Further, inside the intermediate support body 38, an objective lens barrel 42 is further provided which is swingably supported by tracking direction movable parallel springs 41, 41 which are backed by rubber-like elastic bodies 40, 40. This objective lens barrel 42 has tracking coils 43, 43 attached to its side wall in proximity to the tracking magnets 35, 35, and a lens holder 44 for holding the objective lens 24a at one end. On the other hand, a counterbalance weight 45 is provided on the other end side.

In the above configuration, as shown in FIGS. 3b, d, and f, in the beam spot 52 projected onto the 4-split photodetector 27, there is a portion (hereinafter referred to as 53 (referred to as a diffraction pattern). This diffraction pattern 5
3, even when in focus, Fig. 3 a, c,
As shown in e, it changes depending on the positional deviation between the beam spot 51 formed on the optical disk 23 and the information track 23b of the optical disk 23. However, by rotating the collimator lens unit 22, adjustments are made so that the aberrations of the collimator lens 22a, objective lens 24a, etc. cancel each other out.
By setting, as shown in FIG. 3b, d, and f, no matter what the positional relationship between the beam spot 51 and the information track 23b, the diffraction pattern 53 will be aligned with respect to the boundary line Y-Y. It becomes a symmetrical shape. In other words, no crosstalk occurs.

Therefore, the four light receiving sections 2 of the 4-split photodetector 27
Let the amounts of light received at 7a, 27b, 27c, and 27d be Sa, Sb, Sc, and Sd, respectively, and express the focusing degree f of the beam spot 51 formed on the optical disk 23 as (Sa+Sc)−(Sb+Sd). The degree of focus f is always 0 when in focus, regardless of the positional relationship between the beam spot 51 and the information track 23b.

Therefore, as shown in FIG.
If the distance between the objective lens 24a and the objective lens 24a is close to the in-focus state, the in-focus state can be reliably detected based on the positive or negative value of the degree of focus f, and stable focusing control can be performed.

Furthermore, the collimator lens unit 22 is not close to the optical disk 23 rotating at high speed.
Further, unlike the objective lens unit 24, many parts such as a driving device are not provided, so the above adjustment can be easily performed.

Note that once the rotational position of the collimator lens unit 22 is adjusted, there is no need to change it.
After adjustment, it may be fixed to the housing 28 together with the collimator lens holder 22 using an adhesive or the like.

〔Effect of the invention〕

As described above, the focusing error detection device according to the present invention includes a light source, a collimator lens that converts the light beam emitted from the light source into a parallel light beam, and an information recorder provided with an information track and a guide track to convert the parallel light beam into a parallel light beam. An objective lens that focuses light on the disc to form a beam spot and captures the reflected light flux reflected by the information recording disc, and an optical system that forms an image of the beam spot from the reflected light flux captured by the objective lens. and a photodetector that receives the imaged beam spot with a plurality of divided light receiving sections and detects a focusing error of the beam spot formed on the information recording disk based on the image formation state. In the focusing error detection device, the symmetry of the diffraction pattern generated in the beam spot projected on the photodetector with respect to the axis corresponding to the direction perpendicular to the guide track is caused by aberrations of the collimator lens and the objective lens. This is a configuration in which the collimator lens is installed at a rotational position where it is less affected.

As a result, the symmetry of the diffraction pattern generated in the beam spot projected onto the photodetector with respect to the axis corresponding to the direction perpendicular to the guide track is lost due to the influence of aberrations of the collimator lens and objective lens. Therefore, there is an effect that stable focusing control can be performed without deteriorating the quality of the focusing error signal.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the configuration of a focusing error detection device according to the present invention, FIG. 2 is a longitudinal sectional view showing the structure of an objective lens unit, and FIGS. Fig. 4 is a graph showing the relationship between the distance between the optical disk and the objective lens and the focusing error signal, and Fig. 5 is the configuration of a conventional focusing error detection device. FIGS. 6 a to 6 f are explanatory diagrams showing the principle of focus detection in a conventional focusing error detection device, and FIGS. 7 a to 7 f are explanatory diagrams showing a beam spot in a conventional focusing error detection device, respectively. FIGS. 8a to 8f are explanatory diagrams showing the positional relationship between the beam spot and the information track, and the diffraction pattern, respectively. FIGS. FIG. 9 is a graph showing the relationship between the distance between the optical disk and the objective lens and the focusing error signal in a conventional focusing error detection device. 21 is a semiconductor laser (light source), 22a is a collimator lens, 23 is an optical disk (information recording disk),
23b is an information track, 23c is a guide track, 24a is an objective lens, 26 is an optical system, 27 is a 4-split photodetector (photodetector), 27a, 27b,
27c and 27d are light receiving sections, 51 and 52 are beam spots, and 53 is a diffraction pattern.

Claims (1)

[Claims] 1. A light source, a collimator lens that converts the light beam emitted from the light source into a parallel light beam, and a collimator lens that focuses the parallel light beam onto an information recording disk provided with an information track and a guide track to form a beam spot, and records the information. An objective lens that takes in the reflected light flux reflected by the disk, an optical system that forms a beam spot from the reflected light flux taken in by this objective lens, and a light receiving section that divides the imaged beam spot into a plurality of parts. A focusing error detection device includes a photodetector that detects a focusing error of a beam spot formed on an information recording disk based on the image formation state of the beam received by a The collimator lens is placed at a rotational position where the symmetry of the diffraction pattern generated in the beam spot with respect to an axis perpendicular to the guide track is small enough to be affected by aberrations of the collimator lens and the objective lens. A focusing error detection device characterized by: