JPH05120705A - Optical head error detector - Google Patents

Abstract

(57) [Summary] [Object] To provide an error detection device for an optical head, which is easy and inexpensive to manufacture, and can be manufactured at low cost. An optical means in which first and second Fresnel zone plates having first and second focal lengths having a linear fringe pattern for converging light reflected from a surface to be illuminated are alternately and integrally formed. Thus, the reflected light from the surface to be illuminated is made into an astigmatic ray bundle, the change in the focusing shape is detected by a photodetector, and the focus error of the emitted light is detected. [Effect] An error detection device for an optical head that is easy to manufacture and inexpensive can be realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head error detecting device for detecting a focus error and a track error of irradiation light on a surface to be irradiated.

[0002]

2. Description of the Related Art FIG. 4 shows an example of a conventional optical head error detecting device described in Japanese Patent Laid-Open No. 2-87336.

The laser light from the laser light source 1 is collimated into a bundle of parallel rays by a collimator lens 2 and is directed toward an information recording medium 5 via a beam splitter 3. The irradiated laser light is focused on the information recording medium 5 by the objective lens 4. The objective lens 4 is driven by a driving device (not shown) and is feedback-controlled so as to always focus on the information recording medium 5. The reflected light from the information recording medium 5 passes through the objective lens 4 and is split by the beam splitter 3 to be guided to an error detection system called an "astigmatism method". The focus error detection light split by the beam splitter 3 is divided into four Fresnel zone plates 20.
It is incident on the four-division photodetector 9 via. The 4-division Fresnel zone plate 20 converts the focus error detection light into an astigmatic ray bundle whose wavefront is not concentric by diffraction.

A four-division Fresnel zone plate 20 used here is shown in FIG. The four-division Fresnel zone plate 20 has four regions 20a, 20b, 20c, which are formed by two division lines 20l which are orthogonal to each other and pass through the center 20o.
It is divided into 20d. In addition, opposite diagonal areas 20a
And 20c and 20b and 20b are formed with curved irregularities such that the focal lengths of the first-order diffracted light are equal.

The two dividing lines 201 of the four-division Fresnel zone plate 20 thus constructed are arranged so as to overlap with the two dividing lines of the four-division photodetector 9 when viewed from the optical axis direction. .. As the detection signals A, B, C, and D of the 4-division photodetector 9, the focus error signal F processed by the adders 11 and 12 and the subtractor 3 is output, and
The track error signal T is output after being processed by the subtractor 15. The focus error signal F and the track error signal T are given by the following equations. F = (A + C)-(B + D) T = (D-B)

According to the position of the objective lens 4 with respect to the information recording medium 5, FIG.
Images as shown in (a '), (b), (b'), (c) and (c ') are formed. That is, when the information recording medium 5 is located in front of the focal point of the objective lens 4, the error detection light is detected by the photodetection sections 9a and 9c on the 4-split photodetector 9 as shown in FIG. The image is formed so that it becomes larger at and smaller at the photodetectors 9b and 9d. Therefore, the focus error signal F becomes positive.

When the information recording medium 5 is accurately positioned at the focal point of the objective lens 4, the error detection light becomes a circular beam on the four-division photodetector 9 as shown in FIG. 6 (b '). .. Therefore, the detection signals A of the photodetectors 9a, 9b, 9c, 9d,
The sizes of B, C and D are the same. Therefore, the focus error signal F becomes zero.

When the information recording medium 5 is positioned on the other side of the focal point of the objective lens 4, the error detection light is shown in FIG. 6 (c ').
As shown in FIG.
An image is formed such that it is small at c and large at the photodetectors 9b and 9d. Therefore, the focus error signal F becomes negative.

As described above, the focus error signal F changes according to the position of the information recording medium 5 with respect to the objective lens 4. By feeding back this focus error signal F to the drive device in the focus direction of the objective lens 4,
The laser light is controlled so as to focus on the information recording medium 5.

6 (a), (a '), (b),
(B '), (c), and (c') show the case where there is only a focus error and no track error, but if there is no focus error and there is a track error, an image as shown in FIG.
An image is formed on the split photodetector 9. That is, the shape of the image is shown in FIG.
This is the same as the case of the in-focus point as shown in (b '), but the brightness becomes different depending on the track error. Figure 6 (d)
Indicates a case where a horizontal track error occurs. By feeding back according to the track error, the laser light is controlled so as to be accurately placed on the track of the information recording medium 5.

The positions of the dividing lines of the four-division photodetector 9 and the four-division Fresnel zone plate 20 do not have to be the positions shown in FIG. 4 as long as they coincide with each other when viewed from the optical axis direction. For example, the four-division photodetector 9 and the four-division Fresnel zone plate 20 may be arranged at positions rotated by 45 degrees about the optical axis. In this case, the track error signal T is T = (A−C) + (D−B).

[0012]

In the conventional optical head as described above, the four-division Fresnel zone plate 20 is used in order to focus the focus error detection light on the four-division photodetector 9 to form an image on the four-division photodetector 9. , Which is manufactured by the microfabrication technique used in the semiconductor manufacturing process. In this process, first, it is necessary to draw a mask by using an electron beam direct drawing device or a pattern generator.

However, the conventional four-division Fresnel zone plate 20 is composed of a curved line, and in order to accurately draw the curved line, the beam diameter of the electron beam direct writing apparatus or the pattern generator is made small and the writing position is made fine. It had to be set and controlled. Therefore, there is a problem in that a huge amount of calculation time and exposure time are required to manufacture a mask having such a curve, and the production cost is increased.

The present invention has been made to solve the above problems, and provides an error detection device for an optical head which can be manufactured at a low cost with a short writing time by an electron beam direct writing device or a pattern generator. It is a thing.

[0015]

The above object is to provide an optical means for converting the reflected light from the surface to be illuminated into an astigmatic ray bundle, and to the irradiated surface based on the first and second focal positions of the reflected light. In an error detecting device for an optical head, which comprises a detecting means for detecting an error of irradiation light, the optical means comprises a first one-dimensional Fresnel zone plate for converging the reflected light to the first focus and the optical head. A second one-dimensional Fresnel zone plate for focusing on a second focal point, and the first one-dimensional Fresnel zone plate and the second one-dimensional Fresnel zone plate are alternately and integrally formed, The detection means has a photodetector for detecting a focusing shape of the reflected light focused by the first one-dimensional Fresnel zone plate and the second one-dimensional Fresnel zone plate,
This is achieved by an error detection device for an optical head, which detects an error between the converging position of the irradiation light and the irradiation surface based on the detection signal detected by the photodetector.

[0016]

According to the present invention, the first and second Fresnel zone plates having the first and second focal lengths having the linear fringe pattern for focusing the reflected light from the surface to be illuminated are alternately and integrally formed. By the formed optical means, the reflected light from the surface to be illuminated is made into an astigmatic ray bundle, the change of the focusing shape is detected by the photodetector, and the focus error of the emitted light is detected.

[0017]

FIG. 1 shows an optical head error detecting device according to an embodiment of the present invention. The same components as those of the conventional optical head shown in FIG. 4 are designated by the same reference numerals.

In this embodiment, the four curves formed by the conventional curve are used.
Instead of the split Fresnel zone plate 20, an 8-split Fresnel zone plate 30 having a linear fringe pattern is used. Moreover, the photodetector divided into the same number as the number of divisions of the Fresnel zone plate is used.

Eight-division Fresnel zone plate 30 having a linear fringe pattern used in this embodiment.
Is shown in FIGS. 2 (a) and 2 (b). The 8-division Fresnel zone plate 30 having this linear fringe pattern has four division lines 31l, 32l, 33l passing through the center 30o.
34l for eight areas 30a, 30b, 30c, 30d,
It is divided into 30e, 30f, 30g, and 30h. Here, in the conventional Fresnel zone plate 20, the radius is constituted by concentric circles whose proportion is proportional to the square root of an integer.
The 8-division Fresnel zone plate in this embodiment is constituted by the tangent lines of the concentric circles. Specifically, 30a
And 30e are composed of a straight line that bisects an acute angle formed by the dividing lines 31l and 32l and a tangent line that passes through the intersection of the concentric circles,
30b and 30f make the acute angle made by the dividing lines 32l and 33l 2
30c and 30g are composed of a straight line that divides equally and a tangent line that passes through the intersection of the concentric circles. 30h is dividing lines 34l and 31
It is composed of a straight line that bisects the acute angle formed by l and a tangent line that passes through the intersection of the concentric circles. In addition, every other diagonal regions 30a and 30c and 30e and 30g have the same focal length of the first-order diffracted light, and 30b, 30d, and 3 have the same focal length.
Respective linear fringe patterns are configured such that 0f and 30h have the same focal length of the first-order diffracted light, and the focusing action of each Fresnel zone plate occurs only in the direction perpendicular to the fringe pattern, and in other directions, For example, it has no focusing action in the direction parallel to the pattern. Hereinafter, for the sake of simplicity, the regions 30a, 30c, 30e, and 30g having the same focal length will be referred to as a region A, and the regions 30b, 30d, 30f, and 30h will be referred to as a region B.

Assuming that the focal length of the first-order diffracted light in the area A is fa and the focal length of the first-order diffracted light in the area B is fb, the astigmatic difference Δ is Δ = fa-fb. For each of the areas A and B at this time,
The distances ra _k and rb _k from the center 30o of the k-th boundary line counting from the center 30o are: ra _k ² = k · fa · λ rb _k ² = k · fb · λ (k = 1, 2, 3 · ...) is given. Here, λ is the wavelength of the reflected light of the information recording medium 5. When the astigmatic difference Δ is converted into the distance Δ ′ on the information recording medium 5, Δ ′ = Δ / β ² . Here, β is the lateral magnification from the information recording medium 5 of this optical system to the 8-split photodetector 9, and β ² is called the longitudinal magnification. Astigmatic difference Δ'in the information recording medium 5
Is generally 10 to 25 μm. Therefore, the astigmatic difference Δ and the lateral magnification β are determined from these values, the focal lengths fa and fb of the regions A and B are determined, and ra _k ,
rb _k is determined.

In the present invention, the photodetectors 9 have the same number of photodetectors 9a, 9b, 9c, 9d and 9 as the Fresnel zone plates.
It is divided into e, 9f, 9g, and 9h. Here again, for the sake of brevity, the photodetectors 9a, 9c, 9e and 9g are referred to as the photodetector A, and the photodetectors 9b, 9d, 9f and 9h are referred to as the photodetector B.

Eight-division Fresnel zone plate 30 having a linear fringe pattern obtained as described above.
Both of the four dividing lines and the four dividing lines of the 8-division photodetector 9 are arranged so as to overlap each other when viewed from the optical axis direction.

Each detection signal A, B, from the 8-split photodetector 9
C, D, E, F, G, H are adders 11a, 11b, 12
a, 12b, 13a, 13b and the subtractor 15 to output the focus error signal F, and the adder 11a, 11b, 12a, 12b, 14a, 14b and the subtracter 16 process the track error signal T. Is output. Focus error signal F and track error signal F
Is as follows: F = (A + C + G + E)-(B + D + F + H) T = (A + B + G + H)-(C + D + E + F)

An image as shown in FIG. 3 is formed on the 8-division photodetector 9 according to the position of the objective lens 4 with respect to the information recording medium 5. Since the number of regions of the Fresnel zone plate of this embodiment is 8, and the image becomes complicated, for simplicity, in FIG. 3, region A is representative of regions 30a and 30e, and region B is representative of 30d. Only the image of the light flux passing through 30h is shown, but since the shape of the image of the light flux passing through the region having the same focal length is the same as described above, the image formation shape of the light flux shown in FIG. The operation of this embodiment can be understood from.

The case where the information recording medium 5 is located in front of the focal point of the objective lens 4 is shown in FIGS. 3 (a) and 3 (a ').
Shown in. The error detection light passing through the area B becomes linear near the focal point as shown in the figure, and is detected by the photodetector B on the 8-split photodetector 9. Further, since a part of the light flux passing through the area A is also detected by the photodetector B, the signal is large in the photodetector B and small in the photodetector A. Therefore, the focus error signal F becomes negative.

When the information recording medium 5 is accurately positioned at the focal point of the objective lens 4 (FIG. 3 (b)), the error detection light is emitted as shown in FIG.
As shown in (b '), images are formed so that the light amounts of the respective photodetection sections of the 8-division photodetector are equal. Therefore, the focus error signal F becomes zero.

The case where the information recording medium 5 is located on the far side from the focal point of the objective lens 4 is shown in FIGS.
It is shown in (c '). The error detection light passing through the area A becomes linear near the focal point as shown in the figure, and is detected by the photodetector A on the 8-split photodetector 9. Further, since a part of the light flux passing through the region B is also detected by the photodetector A, the signal is large in the photodetector A and small in the photodetector B. Therefore, the focus error signal F becomes positive. An example of the focus error signal F obtained in this embodiment is shown in FIG.

If the tracks of the information recording medium 5 are arranged in the meridional direction in FIG. 1, the shape of the image on the photodetector 9 does not change, but a light amount difference occurs in the sagittal direction depending on the track error. Therefore, the photodetectors 9a, 9
It is possible to detect the track error by the difference between the signals of b, 9g, 9h and 9c, 9d, 9e, 9f.

As described above, according to this embodiment, it is possible to detect the focus error and the track error using the 8-division Fresnel zone plate having the linear fringe pattern.

In the above embodiment, the 8-division Fresnel zone plate was used, but the division number is not limited to 8. Even if the number of divisions is any, each Fresnel zone plate is formed by a linear fringe pattern, and error signal detection similar to that in the above embodiment can be performed. As a result of an experiment by the present inventor, the number of divisions is 4 × (n, where n is an integer (n = 0, 1, 2, 3 ...
In the case of -1), it was found that the focus error detection characteristic was good.

Further, the initial phase at the center of the Fresnel zone plate may be any number of times, and a bright and dark Fresnel zone plate, a phase inversion Fresnel zone plate or the like may be used, which is the same as the conventional configuration, and the merit of the linear fringe pattern is lost. I will not be told.

Furthermore, if the dividing line of the Fresnel zone plate and the dividing line of the photodetector are overlapped when viewed from the optical axis direction,
The area A and the photodetector A do not necessarily have to correspond to each other and the area B and the photodetector B do not necessarily correspond to each other. In this case, the polarity of the focus error signal is inverted.

[0033]

As described above, according to the present invention, the first and second Fresnel zones having the first and second focal lengths having the linear fringe pattern for focusing the reflected light from the illuminated surface are provided. By means of the optical means integrally forming the plates alternately, the reflected light from the surface to be illuminated is made into an astigmatic ray bundle, the change in the focusing shape is detected by a photodetector, and by detecting the focus error of the emitted light, An error detection device for an optical head that is easy to manufacture and inexpensive can be realized.

[Brief description of drawings]

FIG. 1 is a perspective view showing an error detection device for an optical head according to an embodiment of the present invention.

FIG. 2A is a plan view of an 8-division Fresnel zone plate having a linear fringe pattern used in the error detecting device for an optical head according to the present embodiment, and FIG.
It is the sectional view seen along the II line.

3 (a), (a '), (b), (b')
A portion, a portion (c), and a portion (c ') are views showing images of error detection light in the error detection device for the optical head of the present embodiment.

FIG. 4 is a perspective view showing a conventional error detection device for an optical head.

5A is a plan view of a four-division Fresnel zone plate composed of a curve used in a conventional optical head error detection device, and FIG. 5B is a cross-sectional view taken along the line V-V. is there.

FIG. 6 is a portion (a), a portion (a ′), a portion (b), and a portion (b ′).
, (C) part, (c ') part, and (d) part are diagrams showing images of error detection light in a conventional error detection device for an optical head.

FIG. 7 is a graph showing a focus error detection signal of the optical head of this embodiment.

[Explanation of symbols]

 1 laser light source 2 collimator lens 3 beam splitter 4 objective lens 5 information recording medium 9 photodetector 10 operational amplifiers 11, 12, 13, 14 adder 15, 16 subtractor 20 4 split Fresnel zone plate 30 8 split Fresnel zone plate

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Musashi Ricardo 2-6-3 Otemachi, Chiyoda-ku, Tokyo Within Nippon Steel Corporation (72) Inventor Shohei Hashiguchi 5-10-1, Fuchinobe, Sagamihara-shi Shin Made in Japan Electronics Research Laboratory

Claims (3)

[Claims] 1. Optical means for converting reflected light from a surface to be irradiated into an astigmatic ray bundle, and detection for detecting an error of the light to be irradiated onto the irradiated surface based on first and second focal positions of the reflected light. An optical head error detection device comprising: a first one-dimensional Fresnel zone plate for focusing the reflected light on the first focal point and a second focal point for focusing the reflected light on the second focal point. Two one-dimensional Fresnel zone plates, and the first one-dimensional Fresnel zone plates and the second one-dimensional Fresnel zone plates are alternately and integrally formed, and the detection means includes the reflected light. A photodetector for detecting a focusing shape focused by the first one-dimensional Fresnel zone plate and the second one-dimensional Fresnel zone plate, and based on a detection signal detected by the photodetector. Serial error detection system of an optical head and detecting an error between the condensing position of the illumination light and the surface to be illuminated. 2. The first one-dimensional Fresnel zone plate is a Fresnel zone plate member having a linear fringe pattern having the first focal point,
The second one-dimensional Fresnel zone plate is the second
Is a Fresnel zone plate member having a linear fringe pattern having focal points, and the first one-dimensional Fresnel zone plate member and the second one-dimensional Fresnel zone plate member are alternately and integrally formed. The optical head error detection device according to claim 1, wherein the optical means is configured by the above. 3. The photodetector comprises, by a dividing line passing through the center thereof, the number of constituents of the plurality of first Fresnel zone plates having the linear fringe pattern and the plurality of second second lines having a linear fringe pattern. 3. The error detecting device for an optical head according to claim 1, wherein the error detecting device is divided into a number equal to the total number of the Fresnel zone plates.