JPH05314509A - Focus error detection device for optical head - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a focus error detection device for an optical head that is small and lightweight, and whose optical axis can be easily adjusted. [Structure] A two-divided Fresnel zone plate for detecting a focus error of an optical head for recording and reproducing information on an information recording medium is provided with an optical axis, and the divided two regions are line-symmetric with respect to a dividing line. Two Fresnel zone plates corresponding to the positions are configured, two split photodetectors are provided at two focal positions, and the optical axes are arranged so as to extend on the extension lines of the split lines of the two split photodetectors. Thus, focus error detection that is not affected by wavelength fluctuations is performed.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art In recent years, compact discs (CDs),
An optical disk that reproduces recorded information using a semiconductor laser, such as a laser disk (LD), or write once (WORM), phase change (PC), or magneto-optical (MO) as an external storage medium of a computer. Optical discs capable of additional recording have been commercialized.

An optical head is used to record and reproduce information on such an optical disc.

FIG. 6 shows an example of a conventional optical head. The laser light emitted from the laser light source 1 is emitted by the irradiation optical system 100.
Thus, the information recording medium 5 is irradiated. That is, the laser light emitted from the laser light source 1
Is converted into a parallel light flux by the laser beam and is irradiated onto the information recording medium 5 via the 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 that the objective lens 4 is always focused accurately on the information recording medium 5. The reflected light from the information recording medium 5 passes through the objective lens 4 again and is split by the beam splitter 3 to obtain a focus error detection system 20 called “Foucault method”.
Lead to zero.

The focus error detection light reflected by the beam splitter 3 is received by the two-split photodetectors 8a and 8b by the Foucault prism 6 and the convex lens 7. The Foucault prism 6 splits a light beam into two split photodetectors 8a and 8b,
The light is deflected, and the convex lens 7 collects the divided light beams on the two-divided photodetectors 8a and 8b. 2-split photodetector 8
a and 8b are light receiving parts 8h, 8i, 8j and 8 by dividing lines.
It is divided into k parts, and operational amplifiers 9h, 9i, 9j and 9k are provided for each of the two divided light receiving parts. The signals detected by the light receiving units 8h, 8i, 8j, and 8k are converted into current-voltage by the operational amplifiers 9h, 9i, 9j, and 9k, and the adders 10a and 10b and the subtractor 1 are performed.
1 and is output as a focus error signal F.

If the detection signals of the light receiving portions 8h, 8i, 8j, and 8k are H, I, J, and K, respectively, the focus error signal F can be described by the following equation. F = (H + K)-(I + J)

A focus error signal F is output according to how far the information recording medium 5 is from the focal plane of the objective lens 4.

When the information recording medium 5 is positioned in front of the focal plane of the objective lens 4, the focus error detection light is as shown in FIG.
In the state as shown in (a) and (a '), the two-division photodetector 8
The light is received by a and 8b. The center of the light flux comes to the center side of the light receiving portions 8h and 8k outside the respective dividing lines. Therefore,
The detection signals H and K of the light receiving units 8h and 8k are large,
The detection signals I and J of i and 8j become small, and the focus error signal F becomes positive according to the above equation.

When the information recording medium 5 is accurately positioned on the focal plane of the objective lens 4, the focus error detection light is as shown in FIG.
In the state shown in (b) and (b '), the two-division photodetector 8
The light is received by a and 8b. The center of the light flux is just above each dividing line. Therefore, the light receiving portions 8h, 8i, 8
The detection signals H, I, J, and K of j and 8k become equal, and the focus error signal F becomes 0.

When the information recording medium 5 is located beyond the focal plane of the objective lens 4, the focus error detection light is as shown in FIG.
The two-division photodetector 8 in the state shown in (c) and (c ').
The light is received by a and 8b. The center of the light flux comes to the center side of the light receiving portions 8i and 8j inside the respective dividing lines. Therefore,
Since the detection signals H and K of the light receiving units 8h and 8k are small,
The detection signals I and J of i and 8j become large, and the focus error signal F becomes negative.

FIG. 8 shows the relationship between the position of the information recording medium 5 with respect to the objective lens 4 and the focus signal F. Focus error signal F
When the position p of FIG. 7 (a ') at which the maximum is located is the center of the light flux of the focus error detection light at the center of the light receiving portions 8h and 8k outside the dividing lines on the respective two-divided photodetectors 8a and 8b. so,
The position r in FIG. 7 (c ') at which the focus error signal F is minimized is such that the centers of the light fluxes of the focus error detection light are the light receiving portions 8i and 8j inside the dividing lines on the respective two-division photodetectors 8a and 8b. When it comes to the center. The position q is the in-focus position shown in FIG. 8 (b ').

As described above, the focus error signal F is generated by the objective lens 4
It changes according to the position of the information recording medium 5 with respect to. By performing feedback control of the focus error signal F to the driving device of the objective lens 4, it is possible to control so that the focus of the irradiation laser light coincides with the information recording medium 5.

[0013]

In the conventional optical head as described above, the Foucault prism 6 and the convex lens 7 are used to divide and polarize the focus error detection light on the two two-division photodetectors 8a and 8b. Focus error detection system 2
00 is difficult to reduce in size and weight. In addition, the Foucault prism 6 is difficult to obtain the flatness of the prism surfaces constituting the Foucault prism 6 and the angular accuracy of each, resulting in high production cost. There is a problem that it becomes difficult to adjust the optical axis between the photodetectors 8a and 8b.

The present invention has been made in order to solve the above problems, and its main object is to provide a focus error detecting device for an optical head which is small and lightweight and whose optical axis can be easily adjusted.

[0015]

The above object is to provide a two-division Fresnel zone plate for diffracting light reflected from a surface to be illuminated into first and second light beams, and the first and second dichroism Fresnel zone plates for diffracting the light. In a focus error detection device for an optical head, which comprises a detection means for detecting a focus error of irradiation light on the irradiation surface based on the first and second light fluxes, the two-division Fresnel zone plate is used to detect the reflected light. A first Fresnel zone plate that diffracts into the first light flux at a dividing line that intersects the optical axis that is substantially the center of the light flux;
Of the second Fresnel zone plate diffracted into the light flux of the second Fresnel zone plate, and the centers of the respective annular zones of the first and second Fresnel zone plates are line-symmetric with respect to the dividing line of the two-division Fresnel zone plate. And the detection means detects a first light beam split by the first Fresnel zone plate, and a second light beam split by the second Fresnel zone plate. A second two-division photodetector for detecting, and is arranged such that the optical axis of the reflected light exists on an extension of each division line of the first and second two-division photodetectors. It is achieved by a focus error detection device for an optical head.

[0016]

With the first Fresnel zone plate diffracting into the first light flux and the second Fresnel zone plate diffracting into the second light flux with the dividing line intersecting with the optical axis which is the approximate light flux center of the reflected light as a boundary. A two-divided Fresnel zone plate is formed, and the centers of the respective annular zones of the first and second Fresnel zone plates are arranged so as to be line-symmetric with respect to the dividing line of the two-divided Fresnel zone plate. 2 of 2
Arranged so that the optical axis of the reflected light exists on the extension of each division line of the divided photodetector, the first light flux by the first Fresnel zone plate is detected by the first two-division photodetector, The second light flux from the second Fresnel zone plate is detected by the second two-division photodetector, and the first detector and the second detector
The focal length of the irradiation laser beam is detected based on the detection signal of the detector of.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an optical head focus error detection apparatus according to an embodiment of the present invention. The same components as those of the conventional optical head shown in FIG. 6 are designated by the same reference numerals, and the description thereof will be omitted or simplified.

In this embodiment, the conventional Foucault prism 6 is used.
And a two-division Fresnel zone plate 20 is used instead of the convex lens 7, and two-division light detection in which the division lines are arranged so as to face the optical axis instead of the two conventional two-division photodetectors 8a and 8b. The vessels 21a and 21b are used.

A typical Fresnel zone plate is shown in FIG.
As shown in (a) and (b), it is composed of concavo-convex annular zones with a concentric circle whose radius is proportional to an integer square root from the center o of the annular zone as a boundary, and acts like a lens by light diffraction. Is. If the focal length of the first-order diffracted light is f, then k
The radius of the th boundary is given by r _k ² = k × f × λ. λ is the wavelength of the irradiation laser light. The Fresnel zone plate can be manufactured by finely processing glass or the like by the technique used for semiconductor manufacturing. It should be noted that, although it is actually composed of a large number of ring zones of several hundred, only a few ring zones are shown in a simplified manner in FIG.

A two-part Fresnel zone plate 20 used in this embodiment is shown in FIG. This two-part Fresnel zone plate 20 has a parting line 20L passing through the center,
It is divided into two areas 20a and 20b. Each area 2
The annular zones are finely processed so that the focal lengths of the first-order diffracted light are equal in 0a and 20b, and the respective annular zone centers 20ac and 20bc of the regions 20a and 20b are drawn with respect to the dividing line 20L of the two-divided Fresnel zone plate 20. They are arranged symmetrically, and are virtually outside the two-divided Fresnel zone plate 20, just like the complete Fresnel zone plate shown in FIG.

The two-division Fresnel zone plate 20 thus obtained is arranged so that the division line 20L of the two-division Fresnel zone plate 20 intersects the optical axis, and the first-order diffracted light of each of the regions 20a and 20b is 2 at the focus position
The split photodetectors 21a and 21b are arranged so that the optical axis of the reflected light is located on the extension of each split line.

Light receiving section 2 of two-divided photodetectors 21a and 21b
1h, 21i, 21j, 21k detection signals H, I, J,
The K is processed by the adders 10a and 10b and the subtractor 11, and the focus error signal F is output. The focus error signal F is given by the following equation. F = (H + K)-(I + J)

When the information recording medium 5 is located in front of the focal plane of the objective lens 4, the focus error detection light is as shown in FIG.
As shown in (a) and (a ′), the two-division photodetector 21
The centers of the light beams formed on a and 21b come to the center side of the light receiving portions 21h and 21k outside the respective dividing lines. Therefore, the detection signals H and K of the light receiving portions 21h and 21k are large, the detection signals I and J of the light receiving portions 21i and 21j are small, and the focus error signal F is positive.

When the information recording medium 5 is accurately positioned on the focal plane of the objective lens 4, the focus error detection light is as shown in FIG.
As shown in (b) and (b '), the two-part photodetector 21
The centers of the light beams formed on a and 21b are located just above the respective dividing lines. Therefore, the light receiving parts 21h, 21
The detection signals H, I, J, and K of i, 21j, and 21k become equal, and the focus error signal F becomes zero.

When the information recording medium 5 is located beyond the focal plane of the objective lens 4, the focus error detection light is as shown in FIG.
As shown in (c) and (c '), the two-part photodetector 21
The centers of the light fluxes formed on a and 21b come to the centers of the light receiving portions 21i and 21j inside the respective dividing lines. Therefore, the detection signals H and K of the light receiving portions 21h and 21k are small,
The detection signals I and J of the light receiving portions 21i and 21j become large, and the focus error signal F becomes negative.

4 (a), (a '), (b),
(B '), (c), and (c') show the case where there is no fluctuation in the wavelength of the irradiation laser light due to only the focus error, but there is no focus error and the wavelengths of the irradiation laser light are λ ₁ and λ _2. , Λ ₃ (λ ₁ <λ ₂
If it changes to <λ ₃ ), it moves to s ₁ , s ₂ , s ₃ or t ₁ , t ₂ , t ₃ on the dividing line on the two-division photodetectors 21a and 21b as shown in FIG. Therefore, it does not affect the focus error signal F.

As described above, according to the present invention, the focus error F can be obtained by using the two-divided Fresnel zone plate 20 instead of using the conventional Foucault prism 6 and the convex lens 7. Moreover, it is not affected by the wavelength variation of the irradiation laser light.

[0028]

As is apparent from the above description, according to the present invention, since the optical system for detecting the focus error is integrally formed, the optical system is small and lightweight and the optical axis can be easily adjusted. A focus error detection device for the head can be realized.

[Brief description of drawings]

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

2A and 2B are a plan view and an aa sectional view of an ordinary Fresnel zone plate.

FIG. 3 is a diagram showing a two-division Fresnel zone plate used in the focus error detection device of the optical head of FIG.

4 (a), (a '), (b), (b'),
6C and 6C are diagrams showing the luminous flux of the focus error detection light of the focus error detection device of the same optical head on the two-divided photodetector.

FIG. 5 is a diagram showing a luminous flux on a two-divided photodetector of focus error detection light of a focus error detection device of the same optical head.

FIG. 6 is a diagram showing a conventional focus error detection device for an optical head.

FIG. 7 is a diagram showing a luminous flux on a two-divided photodetector of focus error detection light of a conventional focus error detection device for an optical head.

FIG. 8 is a graph showing a focus error detection signal of a conventional optical head.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 laser light source 2 collimator lens 3 beam splitter 4 objective lens 5 information recording medium 6 Foucault prism 7 convex lens 8a, 8b 2-split photodetector 9h, 9i, 9j, 9k operational amplifier 10a, 10b adder 11 subtractor 20 2-split Fresnel zone Plate 21a21b Two-division photodetector 100 Irradiation optical system 200 Focus error detection system

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tadashi Takeda 5329 Shimosuwa Town, Suwa County, Nagano Sankyo Seiki Co., Ltd. Shimosuwa Factory

Claims (1)

[Claims] 1. Reflected light from a surface to be illuminated is first and second reflected light.
A two-division Fresnel zone plate that diffracts into a light beam of, and a detection unit that detects a focus error of the irradiation light with respect to the irradiation surface based on the first and second light beams diffracted by the two-division Fresnel zone plate. In a focus error detection device for an optical head, the first Fresnel that diffracts the first split light beam at the split line that crosses the split Fresnel zone plate intersecting the optical axis that is the approximate light flux center of the reflected light. It is composed of a zone plate and a second Fresnel zone plate diffracting into the second light flux, and the center of each ring zone of the first and second Fresnel zone plates is a division of the two-division Fresnel zone plate. The first detecting element is arranged so as to be line-symmetric with respect to a line, and the detecting means detects the first light flux from the first Fresnel zone plate. A split photodetector and a second two-split photodetector for detecting the second light flux by the second Fresnel zone plate, and each of the first and second split photodetectors. A focus error detecting device for an optical head, wherein the optical axis of the reflected light is arranged on an extension of a division line.