JPH08329471A - Optical disk reproducing method and reproducing apparatus - Google Patents

Abstract

(57) [Abstract] [Purpose] Less susceptible to optical disc tilt, substrate thickness error, substrate refractive index error, optical pickup wavefront aberration, defocus, etc., and reduce the manufacturing yield of optical disks and playback devices. Provided are a reproducing method and a reproducing apparatus for an optical disc on which high density land / groove recording has been performed without lowering. In a reproducing method of an optical disk for land / groove recording in which information is recorded on both a land and a groove, a beam spot for reproducing information is an ellipse or an ellipse having a major axis in a direction perpendicular to a track of the optical disk.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk reproducing method and reproducing apparatus, and more particularly to a reproducing method and reproducing apparatus for a land / groove recording type optical disk for recording information on both lands and grooves.

[0002]

2. Description of the Related Art An optical recording medium capable of storing information at high density is expected to expand its market as a large capacity external memory. For example, an optical disc has been attracting attention as an external memory of a computer because it can be reproduced at high speed, and various types of optical recording media having different information storage methods and different sizes have been proposed. For example, in the 5.25 inch diameter size, the write-once type that allows information to be written only once and the magneto-optical type that allows rewriting of information, and the 3.5 inch diameter in the ROM type that is read-only The magneto-optical type and the partial ROM type in which magneto-optical and ROM coexist are standardized by the ISO standard, and are expected to become more widely used in the future. In addition, recently, CD-ROM and
MD (Mini Disc) is rapidly spreading, and future trends are drawing attention.

Recording is performed on these optical disks by a laser beam from an optical pickup of a recording / reproducing apparatus.
The recorded portion is called a mark, and the area where the marks are continuously arranged is called a track. In order to increase the recording density, it is necessary to arrange these tracks in order, and for that purpose, it is necessary to guide the laser beam along the tracks. That is, we need a guide for tracking, which is generally in the form of a concave or convex groove,
The disk is formed in a spiral shape from the inner circumference to the outer circumference. This groove is called a guide groove.

Further, the guide groove will be described in detail. As defined in the ISO standard, a portion that is concave when viewed from the optical pickup, that is, a portion that is far away is called a land, and is viewed from the pickup. In this case, the part that becomes convex, that is, the part that becomes closer is called a groove. Information is recorded on either lands or grooves. Recording on a land is called a land recording method, and recording on a groove is called a groove recording method. A route for recording information is called a track. The track pitch from the track center to the adjacent track center is called the track pitch.

The width of the groove is defined as W = (Wtop + Wbottom) / 2, where Wtop is the width at the top of the groove and Wbottom is the width at the bottom of the groove. Also,
The height from the bottom of the groove to the top of the groove is called the groove depth. Among the optical discs currently on the market, the land recording type has a groove width of 0.3 to 0.6 μm, and the groove depth is the wavelength of the recording and reproducing laser λ and the refractive index of the optical recording medium substrate is n. Then, λ / (4 ・
n) to λ / (8 · n). Also, the track pitch is
1.6 μm. Recently, it has been reported that the track pitch is narrowed in order to record higher density information, and there are examples such as the track pitch of 1.4 μm, 1.2 μm and 1.0 μm.

The optical recording medium records information on either the land or the groove. The information recording area records a data area in which a user records information and address information such as a track number and a sector number which is not directly used by the user but is used by the recording / reproducing apparatus to manage information on an optical recording medium. Header area. These pieces of information are recorded as a mark string and a pit string.

The data area generally has different contents for each optical recording medium. On the other hand, the header area is generally the same in all optical recording media. Therefore, for mass production, the header region is generally provided in the form of a pit row at the stage of manufacturing the substrate of the optical recording medium. In this case, the header area may be called a preformatted area. The preformatted area is transferred from the stamper so as to form a pit row that is convex when viewed from the pickup.

By the way, to increase the recording density,
Various approaches have been taken for this purpose in order to further increase the advantages of the optical recording medium. For example, a method of narrowing the track pitch as described above has been proposed and studied. However, when a semiconductor laser beam with a wavelength of 780 to 830 nm is focused by an objective lens with a numerical aperture (NA) of 0.5 to 0.6, if the track pitch is made smaller than 1.4 μm, the information recorded on the adjacent track at the time of reproduction is recorded. There is a problem that the leakage (called crosstalk) is significantly increased, the tracking error signal necessary for the tracking operation is significantly decreased, and the tracking accuracy is deteriorated. Semiconductor lasers with wavelengths of 630 to 690 nm have been developed to avoid these problems.
The output is small and the price is high, so there was a problem in practicality.

Therefore, land / groove recording has been proposed as another approach for increasing the recording density. This is a method of increasing the recording capacity by recording the data, which has conventionally been recorded only on either the land or the groove, on both the land and the groove. In this method, the land width and the groove width are the same in order to read the data written in the land and the data written in the groove with the same C / N value. That is, when the track pitch is 1.4 μm, both the land width and the groove width are 0.7 μm. This method can make crosstalk between adjacent groove and land to zero or minimum value (crosstalk free) by selecting the groove depth appropriately, and it is very effective for high density. Is.

In the land / groove recording system, the first document relating to the groove depth at which crosstalk is free is JP-A-57-138065 (corresponding US Pat. No. 4,423,502). In this document, when the wavelength of the reproducing laser beam is λ and the refractive index of the optical disk substrate is n, the groove depth is λ /
It is described that if it is about (6n), the crosstalk becomes small. However, no optical analysis of the groove depth that makes crosstalk free is described, and a plurality of documents issued after that do not disclose.
Only the description similar to 38065 is seen.

By the way, the present inventor has previously analytically derived a conditional expression that makes crosstalk free in the data area. First, the derivation process will be briefly described below. Now, when data is recorded on an arbitrary land and there is no recording on the adjacent groove, the reproduction signal strength when reproducing the data recorded on the land is set to A. Next, when data is recorded in the adjacent groove and no data is recorded in the land, the reproduction signal strength (crosstalk signal strength from the adjacent groove) when reproducing the land is set to a. Assuming that the groove depth is d _G , the refractive index of the optical disk substrate is n, and the wavelength of the reproducing laser beam is λ, the reproduction signal strength when data is recorded on an arbitrary land and the groove adjacent thereto and the land is reproduced. I _L can be expressed by the following equation.

I _L = | A + a · exp (i4πnd _G / λ) | ² = A ² + a ² + 2Aa · cos (4πnd _G / λ) where I _L = A ² is a crosstalk-free condition, a ² + 2Aa · cos (4πnd _G / λ) = 0 (a) holds.

Contrary to the above case, let us consider the reproduction signal strength I _G when data is recorded on an arbitrary land and a groove adjacent thereto and the groove is reproduced. In this case, since the groove depth is −d _G , I _G = | A + a · exp (−i4πnd _G / λ) | ² = A ² + a ² + 2Aa · cos (−4πnd _G / λ) where I _G = A ² is a crosstalk-free condition, and therefore a ² + 2Aa · cos (4πnd _G / λ) = 0 holds. This is exactly the same as the expression (a). That is,
The crosstalk-free condition is the same regardless of whether data is recorded on an arbitrary land and a groove adjacent to the land, or whether the land is reproduced or the groove is reproduced.

However, this is true only when the recording mark is purely an intensity modulation mark. actually,
Not only magneto-optical discs, but also read-only optical discs such as CDs and phase-change optical discs, recording marks are mixed modulation marks of intensity and phase (for example, in magneto-optical discs, intensity modulation is phase modulation by Kerr rotation angle θ _K. Car ellipticity χ
According to _K), and the signal needs to be treated as a complex number. Therefore, the above relationship does not hold.

This will be described in more detail. When the data is recorded on the arbitrary land by the magneto-optical mark and there is no recording on the adjacent groove, the reproduction signal strength when reproducing the land is A + iB. A is the car rotation angle θ _K
And B is a modulation component due to the Kerr ellipticity angle χ _K. Next, when the data is recorded in the adjacent groove and the land is not recorded, the reproduction signal strength (crosstalk signal from the adjacent groove) when reproducing the land is a.
+ Ib. a is a modulation component due to the Kerr rotation angle θ _K , and b is a modulation component due to the Kerr ellipticity angle χ _K. Assuming that the groove depth is d _G , the refractive index of the optical disk substrate is n, and the wavelength of the reproduction laser beam is λ, the reproduction signal strength I when data is recorded on an arbitrary land and a groove adjacent thereto and the land is reproduced. _L can be expressed by the following formula.

I _L = | A + iB + (a + ib) · exp (i4πnd _G / λ) | ² = A ² + B ² +2 (aA + bB) ・ cos (4πnd _G / λ) +2 (aB-Ab) ・ sin (4πnd _G / lambda) where, I _L = from a ² + B ² is a condition for crosstalk-free, 2 (aA + bB) · cos (4πnd G / λ) +2 (aB-Ab) · sin (4πnd G / λ) = 0 (B) holds.

Contrary to the above case, let us consider the reproduction signal strength I _G when data is recorded on an arbitrary land and a groove adjacent thereto and the groove is reproduced. In this case, since the groove depth is −d _G , I _G = | A + iB + (a + ib) · exp (−i4πnd _G / λ) | ² = A ² + B ² +2 (aA + bB) · cos (4πnd _G / λ) −2 (aB−Ab) · sin (4πnd _G / λ) Here, since I _G = A ² + B ² is a crosstalk-free condition, 2 (aA + bB) · cos (4πnd _G / λ) −2 ( aB−Ab) · sin (4πnd _G / λ) = 0 (c) holds. As is clear here, since the equation (b) is not equal to the equation (c), the condition equation for crosstalk free is different between when the land track is reproduced and when the groove track is reproduced, and the optimum groove depth is obtained. I'm sorry.

However, the Kerr ellipticity angle χ _K = 0 °, that is,
If B = b = 0, the equations (b) and (c) are both I _L = I _G = A ² + 2aA · cos (4πnd _G / λ) = 0, and cos (4πnd _G / λ) = -A / (2a) (d) is established. That is, the equation (b) = the equation (c), and there is a solution of the optimum groove depth, and the land / groove recording can achieve a high density.

[0019]

However, the above description is merely a theory, and in practice, the tilt of the optical disk, the unevenness of the substrate thickness, the unevenness of the refractive index of the substrate, the wavefront of the optical pickup of the reproducing apparatus, etc. If there is some variation in aberration, defocus amount, etc., -A / (2
a) changes significantly, which causes a deviation from the optimum groove depth. That is, crosstalk increases.

Therefore, in order to realize high density land / groove recording, it is necessary to ensure compatibility of a plurality of optical discs, so that tilt, tilt error of substrate, error of refractive index of substrate, etc. It is necessary to manufacture a small optical disc, a pickup with a small wavefront aberration, an actuator in which defocus is unlikely to occur, and this causes a problem that the yield of the product deteriorates and the cost increases.

It is an object of the present invention to solve the above problems and to provide a reproducing method and a reproducing apparatus for an optical disk on which high density land / groove recording is performed without lowering the manufacturing yield of the optical disk and the reproducing apparatus. And

[0022]

In order to solve the above problems, the inventors of the present invention set a beam spot for reproducing information to an ellipse or an ellipse having a major axis in a direction perpendicular to a track of an optical disc. As a result, the inventors have found that the wavefront aberration can be suppressed and high density land / groove recording can be favorably performed, and the present invention has been completed.

Accordingly, the first aspect of the present invention is, "In a method for reproducing an optical disk for land / groove recording in which information is recorded on both a land and a groove, a beam spot for information reproduction is a long axis in a direction perpendicular to a track of the optical disk. And a second method for reproducing information in an optical disk reproducing apparatus for land / groove recording which records information in both lands and grooves. The optical disc reproducing apparatus is characterized in that the beam spot of is an ellipse or an ellipse having a long axis in the direction perpendicular to the track of the optical disc, and third, "to form a beam spot for reproducing information. 3. An optical disk reproducing apparatus according to claim 2, wherein an oval or elliptical opening is provided in a part of the optical system of The "optical disc reproducing apparatus according to claim 2, wherein the pupil of the objective lens is an ellipse in the optical system for forming the beam spot for reproducing information" is provided. In an optical system for forming a beam spot for reproduction, the numerical aperture of the objective lens in the direction perpendicular to the track is set to be smaller than the numerical aperture in the direction parallel to the track. And a sixth object of the present invention is to provide a reproducing apparatus for an optical disk described above, and sixthly, in an objective lens of an optical system for forming a beam spot, the relationship between the wavelength λ of the information reproducing beam and the numerical aperture NA in the direction perpendicular to the track The optical disk reproducing apparatus according to claim 2, 3, 4, or 5, wherein λ / NA> 1.35 μm is satisfied.

[0024]

The above-described crosstalk between the adjacent land and groove of the land / groove recording type optical disk is related to the wavefront aberration. This wavefront aberration is classified into spherical aberration, astigmatism, coma, etc. Spherical aberration occurs when there is a thickness error or refractive index error on the optical disk substrate, and when there is tilt, astigmatism occurs. Aberration and coma are generated.

The amount of wavefront aberration is caused by an optical system such as an optical component manufacturing error of an information reproducing optical pickup, an optical axis adjustment error, a mounting error, a semiconductor laser manufacturing error, a defocus of a drive device, and an optical disk. It is greatly affected by both of tilt, tilt error of the substrate, error of the refractive index of the substrate, and the like which are caused when the optical disc is reproduced.

In order to reduce the crosstalk, the occurrence of wavefront aberration should be suppressed as much as possible. Therefore, if the beam diameter is increased, errors in the optical components of the optical pickup for reproducing information, optical axis adjustment errors, mounting errors, semiconductor laser errors, defocus, optical disc tilt (tilt), substrate thickness errors, substrate The influence of the refractive index error or the like becomes relatively small, and as a result, the wavefront aberration can be made small. However, increasing the beam in the track direction makes it difficult to reproduce high-density recorded information. Therefore, it is sufficient to increase the beam only in the direction perpendicular to the track and reduce the wavefront aberration by that amount.

The problem here is that it is expected that the crosstalk from the adjacent track (adjacent land or groove) will increase if the beam is enlarged only in the direction perpendicular to the track. Can be solved by setting the optimum groove depth as described above. As a concrete method to increase the beam spot diameter in the direction perpendicular to the track, insert an oval or elliptical aperture stop into the optical system (the insertion point may be anywhere in the optical path, but immediately before or after the objective lens). However, the numerical aperture NA of the objective lens may be reduced only in the direction perpendicular to the track. The relationship between the wavelength λ of the information reproducing beam and the numerical aperture NA in the direction perpendicular to the track is λ / NA> 1.35 μm = 1
The effect is more remarkable if the wavelength is set to 350 nm. For example, when λ = 685 nm, NA = 0.5 or less is preferable. When the beam spot is an ellipse, the ratio f / g of the major axis f and the minor axis g is preferably 1.05 to 1.38, and 1.1 to 1.
2 is more preferred.

In addition to this, the pupil of the objective lens is an ellipse or an ellipse, and the effective aperture diameter (pupil diameter) D and the beam spot diameter when entering the objective lens (the diameter which becomes 1 / e ^{2 of the} central intensity). ) It is also effective to make the ratio D / W (vignetting coefficient) of W close to 1.0 in the direction perpendicular to the track so that the emission beam spot is elliptical. Contrary to this, the vignetting coefficient in the track direction may be set to 1.0 or less, for example, 0.8 or 0.7.

In reality, even if the beam spot diameter is not strictly an ellipse or an ellipse, if the beam spot diameter is large in the direction perpendicular to the track, the influence of the above-mentioned error is reduced, and the high density land / groove recording system. It becomes possible to reproduce the optical disc. Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited thereto.

[0030]

EXAMPLE A magneto-optical disk recording / reproducing apparatus equipped with an optical pickup having an objective lens with an oval pupil is prepared. As shown in FIGS. 1 and 2, this optical pickup forms an elliptical beam spot having a major axis in a direction perpendicular to the track. The elliptical beam spot size is 1.29 μm in the direction perpendicular to the track and 1.17 μm in the track direction.
The ratio f / g of the major axis f to the minor axis g is about 1.1.

A magneto-optical disk is set in this apparatus to evaluate crosstalk. First, a method of measuring crosstalk will be described. When data is recorded in an arbitrary land and no data is recorded in the adjacent groove, the signal component strength of the reproduction signal when reproducing the data recorded in the land, that is, the carrier level is C _LM (dBm). Next, if the signal component strength of the reproduced signal when reproducing the adjacent groove having no recording, that is, the carrier level leaking from the recording of the land track to the groove is C _GO (dBm), from the land track to the groove track. The crosstalk CT _{L of} is expressed as

CT _L = C _GO -C _LM (dBm) Similarly, when data is recorded in an arbitrary groove and no data is recorded in the adjacent land, the reproduction signal of the data recorded in the groove is reproduced. The signal component strength, that is, the carrier level is C _GM (dBm). Next, the signal component strength of the reproduction signal when reproducing the adjacent land having no recording, that is,
If the carrier level leaking from the recording of the groove track to the land is C _LO (dBm), the crosstalk CT _G from the land track to the groove track is expressed as follows.

CT _G = C _LO -C _GM (dBm) The wavelength of the light source of the optical pickup is 680 nm, the NA of the objective lens in the direction perpendicular to the track is 0.55, and the vignetting coefficient D / W 0.
8. The polarization state is linearly polarized, the direction of which is parallel to the groove, and the optical system has almost no aberration. The magneto-optical disk has a Kerr rotation angle θ _K = 0.8 °, track pitch = land width = groove width 0.8 μm, and groove depth 72.
There are two types, one with a wavelength of nm (λ / 6.3 n) and the other with a track pitch of 0.6 μm and a groove depth of 85 nm (λ / 5.35 n).
Two types of tilts are available: 0 mrad and 3 mrad.

[0034] First, one of the land of the magneto-optical disk, the mark length 2 [mu] m, the mark pitch 4 [mu] m, after the recording mark width 0.6 [mu] m, the C _LM with a spectrum analyzer to reproduce the recorded information It was measured. Next, adjacent grooves were reproduced to measure C _GO , and CT _L was calculated from these values. The results are shown in Table 1. For comparison with the prior art, a magneto-optical reproducing device having an optical pickup with a circular pupil is prepared. As a magneto-optical disk, the track pitch was 0.8 μm and the groove depth was 67 nm (λ / 6.75).
n) and groove depth with a track pitch of 0.6 μm
Two types of 79 nm (λ / 5.75n) were prepared, and crosstalk evaluation was performed as in the example. Table 2 shows the results.

From these results, the embodiment is 7 to 8d.
It can be seen that the crosstalk difference is small by about B and is not easily affected by various errors.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

As described above, according to the present invention, it is possible to provide a reproducing method and a reproducing apparatus for an optical disc on which high density land / groove recording is performed without lowering the manufacturing yield of the optical disc and the reproducing device.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an elliptical beam spot which is a reproducing method of the present invention.

FIG. 2 is an explanatory diagram showing an elliptical beam spot which is a reproducing method of the present invention.

[Explanation of symbols]

 1 ... Land 2 ... Groove 3 ... Optical pickup 4 ... Mark 5 ... Elliptical beam spot

Claims (6)

[Claims] 1. A reproducing method of an optical disk for land / groove recording for recording information on both a land and a groove, wherein a beam spot for reproducing information is an ellipse or an ellipse having a major axis in a direction perpendicular to a track of the optical disk. A method for reproducing an optical disc, comprising: 2. A reproducing apparatus of a land / groove recording optical disk for recording information on both a land and a groove, wherein a beam spot for reproducing information is an ellipse or an ellipse having a major axis in a direction perpendicular to a track of the optical disk. An optical disk reproducing apparatus characterized by: 3. The optical disk reproducing apparatus according to claim 2, wherein an oval or elliptical opening is provided in a part of an optical system for forming a beam spot for information reproduction. 4. The optical disk reproducing apparatus according to claim 2, wherein in the optical system for forming the beam spot for reproducing information, the pupil of the objective lens is an ellipse. 5. An optical system for forming a beam spot for information reproduction, wherein the numerical aperture of the objective lens in the direction perpendicular to the track is set to be smaller than the numerical aperture in the direction parallel to the track. The optical disk reproducing apparatus according to claim 2. 6. In the objective lens of an optical system for forming a beam spot, the relationship between the wavelength λ of the information reproducing beam and the numerical aperture NA in the direction perpendicular to the track is λ / NA>
2. The composition according to claim 2, characterized in that it satisfies 1.35 μm.
An optical disk reproducing apparatus described in 4 and 5.