WO2006135053A1

WO2006135053A1 - Optical pickup device, reproducing device and birefringence correction plate

Info

Publication number: WO2006135053A1
Application number: PCT/JP2006/312146
Authority: WO
Inventors: Toshinori Sugiyama
Original assignee: Hitachi Maxell, Ltd.
Priority date: 2005-06-17
Filing date: 2006-06-16
Publication date: 2006-12-21
Also published as: CN101199010A; JPWO2006135053A1; CN101199010B

Abstract

An optical pickup device is provided with a light source; an objective lens for focusing an optical beam emitted from the light source on a recording plane; a polarizing beam splitter arranged on an optical path between the light source and the objective lens; and a birefringence correction plate arranged on an optical path between the objective lens and the polarizing beam splitter for giving a phase difference between two polarization components of the entered optical beam which are orthogonally intersecting each other. In the optical pickup device, the phase difference generated by the birefringence correction plate differs, depending on a position within the plane of the birefringence correction plate. Thus, influence of birefringence is more effectively reduced by using the simpler optical element, and a margin for birefringence fluctuation of an information recording medium is widened.

Description

Specification

Optical pickup device, reproduction device and birefringence correction plate

Technical field

The present invention relates to an optical pickup device, a reproducing device, and an optical pickup device which irradiate light to a recording surface of an information recording medium having a light transmitting medium and receive reflected light from the recording surface. A birefringence correction plate.

Background art

In the technical field of information recording media, improvement in recording density has become an increasingly important technical issue, and conventionally, improvement in linear recording density along information tracks and area recording by narrowing the track pitch The recording density has been improved in terms of both the improvement of the density.

[0003] A DVD, which is one of the information recording media, achieves a surface density of 4.7 GB in CD size by setting the laser wavelength to 650 nm and the numerical aperture NA of the objective lens to 0.6. In addition, in the HD DVD currently under development, the area density is improved by adopting a laser wavelength of 405 nm and a numerical aperture NAO. 65, and a capacity of 15 GB in CD size has been realized.

Generally, a transparent substrate used for an information recording medium is the same as in the case of the above-described DVD and HD DVD, which are often produced by injection molding with excellent mass productivity. Since the DVD and HD DVD have a disk shape, the flow of molten resin spreads concentrically from the inner circumference to the outer circumference at the time of injection, so the principal axis of birefringence generated on the transparent substrate after molding is the radius of the disk The direction, the circumferential direction orthogonal to that, and the thickness direction of the substrate. Assuming that the refractive index corresponding to the main axis in the radial direction is n, the refractive index corresponding to the main axis in the circumferential direction is n, and the refractive index corresponding to the thickness direction of the substrate is n, the polycarbonate most commonly used in CDs and DVDs

z

In the case of a molded substrate, the amount of birefringence in the in-plane direction n− n (hereinafter also referred to as in-plane birefringence) and the amount of birefringence in the thickness direction n−n (hereinafter referred to as cross-sectional birefringence). = (n +

Each of a z a t n) / 2 has the following values.

-4 x 10 " ⁵ ≤ nn 4 x 10" ⁵

nn = 4 x 10 ^_ ^{4 to} 7 x 10-4 The magnitude relationship between the refractive indices n and n in the in-plane direction is different depending on the molding process conditions.

The magnitude relationship between the refractive index n in the direction and the refractive index n in the in-plane direction is generally n> n, that is, n, >> η ζ a a ζ r t ζ. These refractive index values also change with resins used for molding, but the conditions to be satisfied as substrate materials of information recording media such as material cost, moldability, molded substrate mechanical characteristics, etc. are well balanced. There are currently no other materials besides polycarbonate resin. Therefore, at present, even if the density of the information recording medium is increased, it is unlikely that the substrate to be used will change, so the birefringence (the amount of birefringence) generated in the substrate remains the same as before. Therefore, as the recording capacity is increased due to the increase in density, the range of allowable birefringence (the margin for the fluctuation of the birefringence) becomes narrow, which may cause problems in recording and reproduction.

In order to solve the problem of reducing the allowable range of the amount of birefringence due to the increase in recording capacity (higher recording density) as described above, conventionally, for example, the influence of birefringence generated in the medium is corrected A pickup device provided with a liquid crystal correction element for the purpose is proposed (see, for example, Patent Document 1).

Further, in the information recording medium, in addition to the above-mentioned process conditions, birefringence is also generated by a stress caused by a centrifugal force accompanying the rotation of the medium at a high speed. Therefore, assuming that radial stress due to centrifugal force is σ and circumferential stress is σ, the total amount of birefringence (η-) generated in the information recording medium during rotation is generated due to the stress caused by the centrifugal force. Amount of birefringence and t—sum r—sum

It is represented by the sum of the amount of birefringence generated in the above-described process, and is represented by the following equation.

(n n) = C X — σ) + (n— n)

t—sum r—sum t r t r

The first term on the right side of the above equation is the amount of birefringence generated by centrifugal force, and the second term corresponds to the amount of birefringence generated in the substrate molding process. Here, C is a coefficient called a photoelastic constant, and in the case of polycarbonate resin, a value of C = ^{7.2 ×} 10 — ¹¹ [Pa — ¹ ] is known.

Conventionally, as a means for correcting the difference in the amount of birefringence at the radial position due to the rotation of the information recording medium, a pickup device provided with a wave plate for correcting the difference in the amount of birefringence at the radial position has been proposed. It has been proposed (see, for example, Patent Document 2).

Patent Document 1: Japanese Patent Application Laid-Open No. 2000-268398

Patent Document 2: Japanese Patent Application Laid-Open No. 2004-245957 Disclosure of the invention

Problem that invention tries to solve

[0010] In the HD DVD standard currently under development, as described above, the surface density is improved by adopting a laser wavelength of 405 nm and a numerical aperture of 0.65. Therefore, the problem of birefringence as described above is a bigger problem than the DVD, which is the main product of the present optical disc, and the HD DVD standard makes the margin for the fluctuation of the birefringence amount in the substrate more It is expected to be narrower.

The inventors of the present invention verified the influence of birefringence in HD DVD. According to the HD DVD standard, the area density is improved by shortening the laser wavelength and improving the numerical aperture (0.6 → 0.65). On the other hand, the effect of the birefringence of the light transmitting medium of the information recording medium, ie, the transparent substrate, existing between the information recording surface of the information recording medium and the objective lens of the optical pickup device It was necessary to increase the fluctuation of the signal modulation degree with respect to the fluctuation of the in-plane birefringence of the transparent substrate. That is, in the case of HD DVD, it has been found that the influence of double refraction of the transparent substrate becomes large, and the range of allowable in-plane birefringence (the margin for fluctuation of the amount of in-plane birefringence) narrows. The contents of this verification will be explained concretely below.

In the verification experiments of the present inventors, the signal amplitude change (degree of modulation) of 3T pit (short mark) and tracking error signal (DPP: Divided Push Pull signal) with respect to the in-plane birefringence amount of DVD and HD DVD Changes) were calculated by analysis of the reproduced signal taking into account the polarization effect. The results are shown in Figures 8 and 9. Fig. 8 shows the evaluation results of modulation degree change of 3T short mark and DPP signal with respect to in-plane birefringence amount of DVD. In-plane birefringence amount n-n (n: refractive index in circumferential direction of substrate, n : The refractive index of the substrate in the radial direction) was taken, and the degree of modulation of each signal was taken on the vertical axis. Also, FIG. 9 shows the evaluation results of the modulation degree change of the 3T short mark and the DPP signal with respect to the in-plane birefringence amount of the HD DVD. In addition, the change characteristics of the in-plane birefringence amount n− n with the rotation speed in the HD DVD are also evaluated, and the results are shown in FIG.

As apparent from FIGS. 8 and 9, in the DVD, although the in-plane birefringence amount n−n varies by about ± 6 × 10 — ⁵ , although the modulation degree of the 3T short mark and the fluctuation of the DPP signal are slight. On the other hand, in HD DVD, the modulation of 3T short marks and the fluctuation of DPP signal with respect to the in-plane birefringence amount Larger than in the case of DVD. From the results of FIGS. 8 and 9, in consideration of the performance of the recording and reproducing system of the present information recording medium, in order to ensure stable operation in the HD DVD, the fluctuation of the in-plane birefringence amount of the HD DVD is ± 3 × that there is a need to keep the ^10-5 or less was bought minute.

At the stage of mass production of a resin substrate by injection molding, the in-plane birefringence amount is

± 4 X 10 ^_5 is known to vary the degree, even when rotated at high speed information recording medium in lOOOOrpm about the limit of the recording and reproducing equipment, as shown in FIG. 10, the in-plane Fuku屈Oriryou There is increased by about ⁵ X 10_ 5. Therefore, considering the variation of the in-plane birefringence amount generated by injection molding and the variation amount of the in-plane birefringence amount when used at high speed rotation, the variation of the in-plane birefringence amount is ± 3 in HD DVD. It is practically impossible to keep it below X 10 ⁵

Therefore, the present invention has been made to solve the above-mentioned problems, and an object of the present invention is to reduce the influence of birefringence more effectively by using an optical element having a simpler configuration, It is an object of the present invention to provide an optical pickup device, a reproducing device (or a recording and reproducing device), and a birefringence correction plate capable of widening the margin for the fluctuation of the amount of internal birefringence.

Means to solve the problem

According to the first aspect of the present invention, the recording surface of the information recording medium having the light transmitting medium is irradiated with a light beam through the light transmitting medium, and the reflected light of the recording surface is received. An optical pickup device comprising: a light source; an objective lens for condensing a light beam emitted from the light source on the recording surface; and polarized light disposed on an optical path between the light source and the objective lens. A beam splitter, and a birefringence correction plate disposed on an optical path between the objective lens and the polarization beam splitter, for providing a phase difference between two orthogonal polarization components of an incident light beam; An optical pickup device is provided, wherein the phase difference generated by the birefringence correction plate differs depending on the position in the plane of the birefringence correction plate.

In the optical pickup device according to the first aspect of the present invention, the distribution force of the phase difference in the plane on which the light beam of the birefringence correction plate is incident is rotationally symmetric with respect to the center of the incident light beam. Is preferred.

The present inventors have reduced the influence of birefringence in the transparent substrate of the above-mentioned information recording medium. In order to make them According to the verification of the present inventors, the birefringence is affected by three components of the thickness direction, radial direction and circumferential direction (track direction) with respect to the substrate. It was necessary that refraction (cross-sectional birefringence) greatly affects the degree of modulation of the signal. That is, the cross-sectional birefringence of the transparent substrate n − n (where n = (n + n)

a z a t r

) Z2, n: The refractive index of the substrate in the thickness direction) is reduced to reduce the variation of the signal modulation degree with respect to the variation of the in-plane birefringence amount n−n z tr. However, the value of the cross-sectional birefringence n-n is different depending on the material in the injection molding process with good mass productivity.

a z

It is difficult to significantly reduce the amount of cross-sectional birefringence due to the improvement of the molding process.

Here, the influence on the signal modulation degree due to the cross-sectional birefringence amount n−n of the transparent substrate will be described.

a z

Do. According to the verification of the present inventors, of the reflected light from the information recording medium, light reflected almost perpendicularly to the surface of the information recording medium is not affected by the cross-sectional birefringence of the transparent substrate. However, it was found that the light reflected in the oblique direction with respect to the surface of the information recording medium, among the reflected light of the information recording medium, was greatly affected by the cross-sectional birefringence of the transparent substrate.

In general, in a transparent substrate used in an information recording medium, the refractive index n in the thickness direction of the transparent substrate is smaller than the refractive index n in the in-plane direction (cross sectional birefringence exists).

The refractive index distribution in the transparent substrate for light obliquely reflected to the surface has an elliptical shape. In this case, the major axis direction of the elliptical refractive index distribution is the slow axis with respect to light obliquely reflected to the surface of the information recording medium (the minor axis direction is the fast axis). Therefore, a phase difference occurs (birefringence occurs) between the polarization component in the major axis direction and the polarization component in the minor axis direction of the light obliquely reflected to the surface of the information recording medium.

When birefringence of light reflected in the oblique direction with respect to the surface of the transparent substrate as described above is present, the degree of modulation of the 3T short mark and the fluctuation of the DPP signal become large. In particular, in the case of an information recording medium recorded at high density such as HD DVD, the numerical aperture of the objective lens becomes large, so the aperture of the light beam becomes large and it is reflected obliquely to the surface of the transparent substrate. The effect of double refraction of light (the effect of cross-sectional birefringence) increases. Therefore, in the optical pickup device of the present invention, the light beam passing through the birefringence correction plate is obliquely reflected to the surface of the transparent substrate. The optical pickup device is provided with a birefringence correction plate that generates a predetermined phase difference with respect to the incident light and compensates for the birefringence of the obliquely reflected light.

According to the second aspect of the present invention, the recording surface of the information recording medium having the light transmitting medium is irradiated with a light beam through the light transmitting medium, and the reflected light of the recording surface is received. An optical pickup device comprising: a light source; an objective lens for condensing a light beam emitted from the light source on the recording surface; and polarized light disposed on an optical path between the light source and the objective lens. A beam splitter, and a birefringence correction plate disposed on an optical path between the objective lens and the polarization beam splitter, for providing a phase difference between two orthogonal polarization components of an incident light beam; The birefringence correction plate has a central region and an outer edge region provided so as to surround the central region, and the birefringence amount of the central region is different from the birefringence amount of the outer edge region. An optical pickup device characterized by It is subjected.

Further, in the optical pickup device according to the second aspect of the present invention, it is preferable that the fast axis of the outer edge region is directed in the direction of circling the central region.

In the optical pickup device according to the second aspect of the present invention, it is preferable that the outer edge area is equally divided into four areas in the direction of circling the central area. However, the present invention is not limited to this, and the outer edge area may be divided into five or more areas in the direction of circling the central area. According to the principle of compensation for cross-sectional birefringence described later, the phase advance axis of the outer edge region of the birefringence correction plate is ideally ideally in the circumferential direction of the incident light beam LV, so the number of divisions of the outer edge region There are more, more preferred.

In the optical pickup device according to the second aspect of the present invention, preferably, the central area is square, and the fast axis of the outer edge area is directed along the outer edge of the central area.

In the optical pickup device according to the second aspect of the present invention, the birefringence correction plate has a substrate and a birefringence correction piece provided on the substrate, and the birefringence correction piece has the outer edge region. It is preferable to be provided U ,.

As described above, according to the verification of the present inventors, among the reflected light from the information recording medium, the light reflected in the oblique direction with respect to the surface of the transparent substrate is affected by the cross-sectional birefringence of the transparent substrate. It is essential to receive the On the other hand, in the optical pickup device according to the second aspect A birefringence correction plate disposed on an optical path between the objective lens and the polarization beam splitter, wherein the central region has almost no birefringence with respect to passing light (the phase difference is almost 0); And an outer edge area which causes birefringence (generates a phase difference) with respect to passing light provided to surround the light source, and the fast axis of the outer edge area is directed in the direction of substantially circling the central area. Used a birefringence correction plate. As used herein, “little birefringence occurs in the central region” means that the amount of birefringence in the central region of the birefringence correction plate is about 1 Z5 or less of the amount of birefringence in the outer edge region. Further, the “birefringence amount” as used herein refers to the difference between the refractive index in the fast axis direction and the refractive index in the slow axis direction. For example, in the outer edge region of the birefringence correction plate, the difference between the refractive index in the fast axis direction and the refractive index in the direction perpendicular to the fast phase axis and in the in-plane direction (slow axis) is It becomes the amount of birefringence. That is, the amount of birefringence of the birefringence correction plate is the amount of birefringence in the in-plane direction.

The central region of the birefringence correction plate is preferably formed of a material having an isotropic refractive index in the in-plane direction so that birefringence does not occur substantially with respect to passing light. As such a material, for example, fused quartz, optical glass or the like can be used.

Further, the “rotational direction” of the birefringence correction plate as referred to in the present specification is the direction in the in-plane direction of the birefringence correction plate and surrounding the central region. However, the direction of orbiting includes the following cases, which are only along the outer edge of the central region. For example, in the birefringence correction plate 6 shown in FIG. 3, the central region (region where the optical path difference adjustment plate 61 is formed) is formed in a square shape, and the outer edge region is in the direction along the outer edge of the central region Force in which the fast axis 63 of (a region in which the birefringence correction pieces 62a to 62d are formed) is facing When the central region is formed in a circular shape in the birefringence correction plate 6 shown in FIG. Even when the direction of the axis 63 is along the outer edge of the central region, even if there is a region, the fast axis 63 of the outer region is the in-plane direction of the birefringence correction plate and the central region In this case as well, the direction of the fast axis is included in the direction of circling the central region.

In the birefringence correction plate used for the optical pickup device according to the second aspect of the present invention, the width of the central region is smaller than the diameter of the passing light beam, and therefore perpendicular to the surface of the information recording medium. The light reflected in the direction passes through the central area of the birefringence correction plate, and the light reflected in the oblique direction with respect to the surface of the information recording medium passes through the outer edge area of the birefringence correction plate. The configuration is as follows. Therefore, in the birefringence correction plate used in the optical pickup device according to the second aspect of the present invention, only the light reflected in the oblique direction with respect to the substrate surface of the information recording medium is phase corrected by the birefringence correction plate. Therefore, the influence of cross-sectional birefringence of the transparent substrate can be reduced.

Here, the principle by which the influence of the cross-sectional birefringence of the transparent substrate can be reduced by the optical pickup device of the second aspect of the present invention will be briefly described.

Usually, the cross-sectional birefringence of the disc-like transparent substrate produced by the injection molding process is n−n az

It becomes> 0. That is, since the refractive index n in the thickness direction of the transparent substrate is smaller than the average refractive index nza in the in-plane direction, the refractive index of the transparent substrate is, for example, elongated in the in-plane direction of the transparent substrate as distribution NS in FIG. Distribution in the shape of an ellipsoid. Therefore, for example, as in the light L2 in FIG. 5, the refractive index in the plane perpendicular to the light obliquely reflected in the radial direction of the transparent substrate has an elliptical distribution N2 as shown in the upper drawing of FIG. It becomes.

In the refractive index distribution N2 of the transparent substrate on the surface perpendicular to the light L2 obliquely reflected in the radial direction of the transparent substrate, as shown in the upper drawing of FIG. 6 (b), the refractive index in the major axis direction is the transparent substrate The refractive index n in the circumferential direction is smaller than the refractive index n in the circumferential direction of the transparent substrate in the direction (short diameter direction) perpendicular to the circumferential direction. Therefore, for light L2 that is obliquely reflected in the radial direction of the transparent substrate, the slow axis of the transparent substrate is in the circumferential direction (the major axis direction of the distribution N2 in FIG. 6B). Therefore, when the light L2 passes through the transparent substrate, the phase of the polarization component in the circumferential direction of the light L2 is delayed with respect to the polarization component in the direction perpendicular to that, and the light L2 is polarized between the mutually orthogonal polarization components. A difference occurs.

In the optical pickup device according to the second aspect of the present invention, the light L2 in which the phase difference is generated between the two polarization components as described above is incident on the outer edge region of the birefringence correction plate through the objective lens. . In the optical pickup device according to the second aspect of the present invention, the fast axis of the outer edge region of the birefringence phase plate is directed in the direction of circling the central region of the birefringence correction plate and obliquely reflected in the radial direction of the transparent substrate. The direction of the phase advance axis of the outer edge area when the light L2 passes through the outer edge area is the same as the circumferential direction of the transparent substrate, that is, the retardation of the transparent substrate relative to the light L2 when the light L2 passes through the transparent substrate The axis and the slow axis of the birefringent phase plate for the light L2 when the light L2 passes through the outer edge region of the birefringent phase plate are in the same direction. Therefore, it passes through the transparent substrate When light L2 having a phase difference between the two polarization components passes through the outer edge area of the birefringence correction plate, the phase of the polarization component of the light L2 whose phase is delayed when passing through the transparent substrate is It proceeds with respect to the vertical polarization component (polarization component whose phase advances when passing through the transparent substrate). As a result, the phase difference between the mutually orthogonal polarization components of the light L 2 generated when passing through the transparent substrate can be corrected by the birefringence correction plate. Thereby, the birefringence of the light L2 obliquely reflected in the radial direction of the transparent substrate can be compensated, and the cross-sectional birefringence of the transparent substrate can be equivalently reduced. The birefringence can also be compensated for light which is obliquely reflected in the circumferential direction of the transparent substrate as shown by light L3 in FIG. 5 according to the same principle. That is, in the optical pickup device according to the second aspect of the present invention, as described above, with respect to light obliquely incident on the transparent substrate, the direction of the slow axis of the transparent By making the directions approximately coincide with each other, the birefringence of light obliquely incident on the transparent substrate is compensated to reduce the influence of the cross-sectional birefringence of the transparent substrate.

In the present invention, the phase correction amount of the birefringence correction plate (the optical path difference between the polarization components generated by the birefringence correction plate) corresponds to the cross-section birefringence of the light transmitting medium (transparent substrate) of the information recording medium used. Accordingly, although it can be set appropriately, it is preferable to set the thickness to about 10 to 40 nm, particularly about 15 to 25 nm, for polycarbonate molded substrates that are most used at present. By setting the phase correction amount within this range, the influence of the cross-sectional birefringence can be sufficiently reduced with respect to the polycarbonate molded substrate that is most used at present and stable operation is possible even with the current recording and reproducing apparatus. Can be secured.

In the present invention, the dimensions of the central region of the birefringence correction plate depend on the diameter of the incident light beam, the numerical aperture of the objective lens, the material of the light transmitting medium (transparent substrate) of the information recording medium, etc. It can be set appropriately. When the conventional polycarbonate polycarbonate molded substrate is used as the light transmitting medium in the specification of HD DVD, the width of the central region should be about half the diameter of the light beam incident on the birefringence correction plate. As a result, it has been confirmed by the present inventors' verification experiments that the influence of cross-sectional birefringence can be sufficiently reduced.

[0037] In the optical pickup device according to the second aspect of the present invention described above, the correction mechanism for the optical disc being driven is used as in the conventional optical pickup device provided with a correction mechanism such as a liquid crystal correction element or a wave plate. It is not necessary to perform recording and playback while changing the birefringence correction value. Therefore, the influence of the cross-sectional birefringence of the transparent substrate can be reduced and the margin for the fluctuation of the in-plane birefringence of the optical disc can be substantially expanded only by providing the birefringence correction plate having a predetermined phase correction amount.

According to the third aspect of the present invention, the recording surface of the information recording medium having the light transmitting medium is irradiated with a light beam through the light transmitting medium, and the reflected light of the recording surface is received. A light source, an objective lens for condensing a light beam emitted from the light source on the recording surface, and a phase difference between two mutually orthogonal polarization components of the incident light beam. An optical pickup device is provided, characterized by comprising: a birefringence correction plate, and the birefringence correction plate is disposed on the information recording medium side of the objective lens.

According to the fourth aspect of the present invention, the recording surface of the information recording medium having the light transmitting medium is irradiated with a light beam through the light transmitting medium, and the reflected light of the recording surface is received. A light source, an objective lens for condensing a light beam emitted from the light source on the recording surface, and an incident light disposed on the information recording medium side of the objective lens. And a birefringence correction plate for giving a phase difference between two polarization components orthogonal to each other of the beam, wherein the slow axis of the birefringence correction plate is oriented in the thickness direction of the birefringence correction plate. An optical pickup device is provided.

The compensation principle of birefringence in the optical pickup device according to the fourth aspect will be briefly described with reference to FIGS. 21 and 22.

As described in the birefringence correction principle in the optical pickup device according to the second aspect of the present invention, the cross-sectional birefringence of the disc-like transparent substrate produced by the injection molding process is usually n−n Since it becomes> 0, the distribution of the refractive index of the transparent substrate is in-plane az as shown in FIG. 21 (a).

It is an ellipsoidal distribution NS extending in the direction.

Therefore, the refractive index of the transparent substrate in the plane perpendicular to the light L 2 obliquely reflected in the radial direction of the transparent substrate is an elliptical distribution N 2 as shown in the upper drawing of FIG. 22 (b), The refractive index in the major axis direction is the refractive index n in the circumferential direction of the transparent substrate, but the refractive index n ′ in the direction (third direction) perpendicular to the circumferential direction is smaller than the refractive index n in the circumferential direction of the transparent substrate . Therefore, for the light L2 reflected obliquely in the radial direction of the transparent substrate, the slow axis of the transparent substrate is in the circumferential direction (FIG. 22 (b)). Distribution NS2). Therefore, when the light L2 passes through the transparent substrate, the phase of the circumferential polarization component of the light L2 is delayed with respect to the polarization component in the direction perpendicular thereto, and the phase between the polarization components of the light L2 is orthogonal to each other. A difference occurs.

In the optical pickup device according to the fourth aspect of the present invention, as described above, the light L 2 in which the phase difference is caused between the two polarization components is a birefringence provided between the objective lens and the information recording medium. It is incident on the compensation plate. At this time, as shown in FIG. 21A, the light L2 is obliquely incident from the first direction (the radial direction of the transparent substrate) of the birefringence correction plate. In the optical pickup device of the fourth aspect of the present invention, the birefringence correction plate has a slow axis in the thickness direction, that is, the refractive index n in the thickness direction of the birefringence correction plate is the average refractive index in the in-plane direction. Because it is larger than η (η, η),

3 0 1 2

As shown in FIG. 21 (b), the refractive index of the refraction correction plate is an ellipsoidal distribution NP extending in the thickness direction.

Therefore, the refractive index of the birefringence correction plate in the plane perpendicular to the light L 2 obliquely incident from the first direction (the radial direction of the transparent substrate) of the birefringence correction plate is the lower side of FIG. 22 (b). An elliptical distribution NP2 like the following, and the refractive index n in the second direction (corresponding to the transparent substrate and the circumferential direction) is

2 Lower than the refractive index n 'in the vertical direction (the 1' direction in Fig. 22 (b)). Therefore, when the light L2 reflected in the radial direction from the transparent substrate is incident on the birefringence correction plate, the phase advance axis of the birefringence correction plate is in the second direction with respect to the light L2. In this case, the slow axis of the transparent substrate with respect to the light L2 when the light L2 passes through the transparent substrate, and the fast axis of the birefringent phase plate with respect to the light L2 when the light L2 passes through the birefringent phase plate Same direction.

Therefore, when the light L 2 having a phase difference between the two polarization components passes through the birefringence correction plate, the phase of the polarization component in the second direction of the light L 2 whose phase is delayed when passing through the transparent substrate , Advanced with respect to its vertical polarization component (polarization component whose phase advances when passing through the transparent substrate). As a result, the phase difference between the mutually orthogonal polarization components of the light L 2 generated when passing through the transparent substrate can be corrected by the birefringence correction plate. Thereby, the birefringence of the light L2 reflected obliquely in the radial direction of the transparent substrate can be compensated, and the cross-sectional birefringence of the transparent substrate can be equivalently reduced. Further, in the birefringence correction plate of the present invention, birefringence can be compensated for the light which is obliquely reflected in the circumferential direction of the transparent substrate as light L3 in FIG. 21 (a) according to the same principle. . That is, in the optical pickup device according to the fourth aspect of the present invention, As described above, with respect to light obliquely incident on the transparent substrate, the direction of the slow axis of the transparent substrate and the direction of the fast axis of the birefringence correction plate are made to coincide with each other. The birefringence is compensated to reduce the influence of the cross-section birefringence of the transparent substrate.

Therefore, also in the optical pickup device according to the fourth aspect of the present invention, with respect to the optical disc being driven, as in the optical pickup device provided with the correction mechanism such as the conventional liquid crystal correction element or wave plate. It is not necessary to perform recording / reproduction while changing the birefringence correction value by the correction mechanism. The effect of cross-sectional birefringence of the transparent substrate can be reduced simply by orienting the slow axis in the thickness direction of the birefringence correction plate in advance. It is possible to substantially expand the margin of the in-plane birefringence amount with respect to the optical disc.

In an optical pickup device according to a fourth aspect of the present invention, the birefringence correction plate includes a substrate and a birefringence plate provided on the substrate, and the birefringence plate in the thickness direction Assuming that the difference between the refractive index and the refractive index in the in-plane direction is Δη and the thickness of the birefringence plate is t, it is preferable to satisfy 180 nm≤A nX t≤300 nm. By setting the range of the parameter Δ n X t value to the above range, the influence of the cross-sectional birefringence can be sufficiently reduced with respect to the polycarbonate molded substrate currently most used at present, and the current recording Stable operation is ensured even with the playback device.

In the optical pickup device of the present invention, the wavelength of the light beam emitted from the light source is preferably 430 nm or less. As a light source used in the present invention, a light source that emits light having a short wavelength equal to or less than the blue wavelength is preferable. Among the blue lasers currently provided in the field, the longest wavelength is about 430 nm.

In the optical pickup device of the present invention, the numerical aperture of the objective lens is preferably 0.6 or more. The optical pickup apparatus of the present invention is suitable as an apparatus for high recording density media such as HD DVD. In such an apparatus, it is general that the numerical aperture NA of the objective lens increases as the recording density increases. is there. For conventional DVD specifications, NA = 0.6 The power HD DVD specifications for NA = 0.65. If the recording density is further increased in the future, it is predicted that the numerical aperture NA of the objective lens will be further increased.

In the optical pickup device of the present invention, the difference n−n between the refractive index n in the in-plane direction of the light transmitting medium and the refractive index n in the thickness a direction is a value of 2 × 10 — ⁴ or more. Is preferred. In particular, on zaz The light transmitting medium is preferably a molded substrate made of polycarbonate.

According to the fifth aspect of the present invention, a recording surface of a disk-shaped information recording medium having a light transmitting medium is irradiated with a light beam through the light transmitting medium, and the recording surface of the disk An optical pickup device according to any one of the first to fourth aspects, which is a reproduction device for receiving information and receiving reflected light, and a rotation device for rotationally driving the disc-shaped information recording medium. An apparatus is provided.

Note that “reproduction device” as used in the present specification means not only a device that performs only the reproduction operation, but also a device that performs the reproduction and recording operations.

In the reproduction apparatus of the present invention, it is preferable that the maximum number of revolutions of the above-mentioned rotation device is 6000 rpm or more. When the maximum number of revolutions of the disk-shaped information recording medium is set to 60 OO rpm or more by using the reproducing apparatus of the present invention, reproduction of high transfer rate becomes possible. Moreover, according to the inventors' verification, the lower limit of the number of rotations at which the influence of the change in the amount of in-plane birefringence due to the centrifugal force when the medium rotates can not be neglected is the number of rotations of 6000 rpm. The change in in-plane birefringence due to centrifugal force is negligible with respect to the in-plane birefringence generated by the molding process.

[0054] According to a sixth aspect of the present invention, there is provided a birefringence correction plate for providing a phase difference between two polarization components orthogonal to each other of an incident light beam, which has a central region and the central region. There is provided a birefringence correction plate comprising: an outer peripheral area provided so as to surround the two, wherein the birefringence of the central area is different from the birefringence of the outer peripheral area.

[0055] In the birefringence correction plate according to the sixth aspect of the present invention, it is preferable that the fast axis of the outer edge region is directed in the direction of circling the central region.

[0056] In the birefringence correction plate according to the sixth aspect of the present invention, the width of the central region is preferably half the diameter of the light beam to be incident. In the birefringence correction plate according to the sixth aspect of the present invention, it is preferable that the outer edge area is equally divided into four areas in the direction of circling the central area. Furthermore, in the birefringence correction plate according to the sixth aspect of the present invention, the central region is square, and the fast axis of the outer edge region is directed in the direction along the outer edge of the central region. Is preferred.

[0057] In the birefringence correction plate according to the sixth aspect of the present invention, the birefringence correction plate includes a substrate, Preferably, a birefringent plate is provided on the plate, and the birefringent plate is provided in the outer edge region. Further, in the birefringence correction plate according to the sixth aspect of the present invention, the birefringence plate is preferably quartz.

According to the seventh aspect of the present invention, in the birefringence correction plate for giving a phase difference between two polarization components orthogonal to each other of the incident light beam, the slow axis of the birefringence correction plate is a complex. A birefringence correction plate is provided, characterized in that it is oriented in the thickness direction of the refraction correction plate.

In a birefringence correction plate according to a seventh aspect of the present invention, the birefringence correction plate includes a substrate and a birefringence plate provided on the substrate, and the birefringence plate in the thickness direction of the birefringence plate Assuming that the difference between the refractive index and the refractive index in the in-plane direction is Δη and the thickness of the birefringence plate is t, it is preferable to satisfy 180 nm≤A nX t≤300 nm. Further, in the birefringence correction plate according to the seventh aspect of the present invention, the birefringence plate is preferably quartz.

Effect of the invention

As described above, in the optical pickup device, the reproducing device, and the birefringence correction plate of the present invention, it is necessary to adjust the refractive index of the outer edge region of the birefringence correction plate only appropriately in advance or It is possible to compensate the birefringence of light incident obliquely to the transparent substrate and light reflected obliquely to reduce the influence of the cross-sectional birefringence of the transparent substrate by simply orienting the slow axis of the light source in the thickness direction. It is possible to widen the margin for the fluctuation of the in-plane birefringence amount of the optical disc.

In the optical pickup device, the reproducing device, and the birefringence correction plate of the present invention, the retardation axis of the birefringence correction plate may be set by simply adjusting the refractive index of the outer edge region of the birefringence correction plate in advance. Since the birefringence of light obliquely incident on the transparent substrate and light obliquely reflected can be compensated for by reducing the effect of cross-sectional birefringence of the transparent substrate only by pointing in the thickness direction, the conventional optical pickup device Since it is not necessary to perform recording / reproduction while changing the correction value of the phase difference in the direction of eliminating birefringence by a correction mechanism such as a liquid crystal correction element or a wave plate as in the above, the configuration can be simplified.

Further, in the reproducing apparatus of the present invention, when the maximum number of revolutions of the disk-shaped information recording medium is set to 6000 rpm or more, high transfer rate recording and reproduction become possible.

Brief description of the drawings

[FIG. 1] FIG. 1 is a side view showing a schematic configuration of an optical pickup device used in Example 1. FIG. 3 is a view showing a cross section B-B in FIG.

[FIG. 2] FIG. 2 is a top view showing a schematic configuration of the optical pickup device used in Example 1.

FIG. 2 is a view showing a cross section A-A in FIG.

FIG. 3 is a perspective view of a birefringence correction plate used in Example 1.

[FIG. 4] FIG. 4 is a top view of a birefringence correction plate used in Example 1.

FIG. 5 is a view for explaining the principle of compensating birefringence with the birefringence correction plate used in Example 1.

6 (a) to 6 (c) are diagrams for explaining the principle of compensating birefringence with the birefringence correction plate used in Example 1. FIG.

[FIG. 7] FIG. 7 is a circuit diagram of a photodetector used in Example 1.

[FIG. 8] FIG. 8 shows signal amplitude characteristics due to birefringence in the conventional DVD detection optical system.

[FIG. 9] FIG. 9 shows signal amplitude characteristics due to birefringence in the conventional HD DVD detection optical system.

[Fig. 10] Fig. 10 is a view showing the influence of centrifugal force on the in-plane birefringence amount in HD DVD.

[11] FIG. 11 is for HD DVD sectional birefringence amount is 6 X 10- ^4, a 3T mark of the modulation degree for the in-plane birefringence in the case of applying the optical pickup apparatus of Example 1 It is the figure which showed the change.

FIG. 12 is for HD DVD sectional birefringence amount is 6 X 10- ^4, the modulation degree of the DPP signal to the in-plane birefringence in the case of applying the optical pickup apparatus of Example 1 It is the figure which showed the change.

[13] FIG. 13 is for HD DVD sectional birefringence amount is 4 X 10- ^4, a 3T mark of the modulation degree for the in-plane birefringence in the case of applying the optical pickup apparatus of Example 1 It is the figure which showed the change.

FIG. 14 is for HD DVD sectional birefringence amount is 4 X 10- ^4, the modulation degree of the DPP signal to the in-plane birefringence in the case of applying the optical pickup apparatus of Example 1 It is the figure which showed the change.

FIG. 15 is for HD DVD sectional birefringence amount is 2 X 10- ^4, Example 1 light pit It is a figure showing change of the degree of modulation of 3T mark to the amount of in-plane birefringence at the time of applying a Kuup device.

FIG. 16 is for HD DVD sectional birefringence amount is 2 X 10- ^4, the modulation degree of the DPP signal to the in-plane birefringence in the case of applying the optical pickup apparatus of Example 1 It is the figure which showed the change.

[FIG. 17] FIG. 17 is a side view showing a schematic configuration of the optical pickup device used in Example 2, and is a view showing a cross section taken along line '-' in FIG.

[FIG. 18] FIG. 18 is a top view showing a schematic configuration of the optical pickup device used in Example 2, and is a view showing a cross section A'-A 'in FIG.

FIG. 19 is a perspective view of a birefringence correction plate used in Example 2.

FIG. 20 is a cross-sectional view of a birefringence correction plate used in Example 2.

21 (a) and 21 (b) are diagrams for explaining the principle of compensating birefringence with the birefringence correction plate used in Example 2. FIG.

22 (a) to 22 (c) are diagrams for explaining the principle of compensating birefringence with the birefringence correction plate used in Example 2. FIG.

FIG. 23 is for HD DVD sectional birefringence amount is 6 X 10- ^4, a 3T mark of the modulation degree for the in-plane birefringence in the case of applying the optical pickup apparatus of Example 2 It is the figure which showed the change.

FIG. 24 is for HD DVD sectional birefringence amount is 6 X 10- ^4, the modulation degree of the DPP signal to the in-plane birefringence in the case of applying the optical pickup apparatus of Example 2 It is the figure which showed the change.

FIG. 25 is for HD DVD sectional birefringence amount is 4 X 10- ^4, a 3T mark of the modulation degree for the in-plane birefringence in the case of applying the optical pickup apparatus of Example 2 It is the figure which showed the change.

FIG. 26 is for HD DVD sectional birefringence amount is 4 X 10- ^4, the modulation degree of the DPP signal to the in-plane birefringence in the case of applying the optical pickup apparatus of Example 2 It is the figure which showed the change.

FIG. 27 is for HD DVD sectional birefringence amount is 2 X 10- ^4, Example 2 Light pitch It is a figure showing change of the degree of modulation of 3T mark to the amount of in-plane birefringence at the time of applying a Kuup device.

FIG. 28 is for HD DVD sectional birefringence amount is 2 X 10- ^4, the modulation degree of the DPP signal to the in-plane birefringence in the case of applying the optical pickup apparatus of Example 2 It is the figure which showed the change.

Explanation of sign

1 light source

2 collimate lens

3 compound prism

4 λ Ζ 4 board

5 Start-up lens

6, 6 'birefringence correction plate

7 Lens Honorable One

8 Objective lens

9 Cylindrical Norens

10 condenser lens

11 light detector

20 disc-shaped information recording medium

21 Recording surface

61 Optical path difference adjustment plate

62a, 62b, 62c, 62d birefringence correction pieces

62 'birefringent plate

63 Advanced axis

100, 200 optical pickup device

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the optical pickup device, the reproducing device, and the birefringence correction plate of the present invention will be specifically described by way of Examples, but the present invention is not limited thereto.

Example 1 [Optical Pickup Device]

The schematic configuration of the optical pickup device of Example 1 is shown in FIGS. 1 is a view showing a cross section B-B in FIG. 2, and FIG. 2 is a view showing a cross section A-A in FIG.

As shown in FIGS. 1 and 2, the optical pickup device 100 of Example 1 mainly includes a semiconductor laser 1 (light source) that emits a light beam with a wavelength of 405 nm, a collimating lens lens 2, and a compound prism. 3 (polarization beam splitter), λΖ 4 plate 4, raising mirror 5, birefringence correction plate 6, objective lens 8, lens holder 7 for holding the objective lens, cylindrical lens 9, focusing It comprises a lens 10 and a light detector 11. In this example, as shown in FIG. 1, the birefringence correction plate 6 is disposed on the optical path between the compound prism 3 and the objective lens 8 and supported by the lens holder 7. The components and devices other than the birefringence correction plate 6 were the same as those used in the conventional optical pickup device. Therefore, the explanation of the components and devices other than the birefringence correction plate 6 is omitted here.

The schematic configuration of the birefringence correction plate 6 of this example is shown in FIGS. As shown in FIG. 3, the birefringence correction plate 6 includes a substrate 60 which is a square plate-like member, an optical path difference adjustment plate 61 provided on one surface of the substrate 60, and four birefringence correction pieces 62a to 62a. And 62d. The substrate 60 was formed of fused quartz and had a thickness of 1 mm. The optical path difference adjusting plate 61 and the four birefringence correction pieces 62a to 62d are formed of quartz. Further, as shown in FIG. 3, the surface shape of the optical path difference adjusting plate 61 is a square, and the surface shapes of the birefringence correction pieces are all the same, and are trapezoidal. Then, the fast axis 63 of each birefringence correction piece was formed in the longitudinal direction of each birefringence correction piece. As the birefringence correction piece, lithium niobate, a high molecular weight film (for example, a polyimide film produced by rolling) or the like can be used other than quartz.

In this example, the optical path difference adjusting plate 61 and the birefringence correction pieces 62a to 62d are formed by bonding two quartz plates having the same shape and in which the directions of the fast axes are orthogonal to each other. . Specifically, an optical path difference adjusting plate 61 and birefringence correction pieces 62a to 62d were produced as follows. First, a quartz plate was cut out so as to have a thickness of 0.31 mm and a length of 10 mm and a length of 10 mm, respectively, along an anisotropic axis, of an artificial synthetic quartz crystal having uniaxial anisotropy. Then, the cut quartz plate was polished to a thickness of 0.30 mm. Next, the polished quartz plate was divided into four equal pieces of about 10 mm square to prepare four quartz plates. Next, two out of the four cut out quartz plates were pasted so that the anisotropic axes were orthogonal to each other. Next, while measuring with a known birefringence measuring device, one side of the bonded quartz plate was polished so as to obtain a phase correction amount Onm (birefringence amount is almost 0) of the bonded quartz plate. Next, a square plate member having a bonded laminated plate force of 1.6 mm was cut out. Thus, the optical path difference adjusting plate 61 of this example was obtained.

Each of the birefringence correction pieces 62a to 62d was manufactured as follows using the remaining two quartz plates out of the four cut out sheets. First, two quartz plates were bonded so that the anisotropic axes were orthogonal to each other. Next, the phase correction amount of the bonded crystal plate is 20 nm (birefringence amount is 0. 009596: corresponding to n'- n and n'- n in the lower diagrams of FIGS. 6B and 6C described later) Become

1 2 2 1

Thus, while measuring with a known birefringence measuring device, one side of the bonded crystal plate was polished. More specifically, polishing was performed so that the difference in thickness between two bonded quartz plates was about 2 m. Then, from the polished bonded crystal plate, a plate-like member having an upper base 1.6 mm, a lower base 4.8 mm and a height 1.6 mm and the fast axis parallel to the trapezoidal upper base and the lower base I cut out at least four or more. Thereby, birefringence correction pieces 62a to 62d of this example were obtained.

Next, as shown in FIG. 3, the four birefringence correction pieces 62a to 62d manufactured as described above are formed so that the longitudinal ends of the birefringence correction pieces are in contact with each other. It was installed on top. Then, the optical path difference adjustment plate 61 is fitted in a square area defined by the four birefringence correction pieces 62a to 62d. In this example, the optical path difference adjusting plate 61 and the birefringence correction pieces 62a to 62d are mounted on the substrate 60 by an optical UV adhesive. As a result, as shown in FIG. 3, at the center of the birefringence correction plate 6, a central region with almost zero birefringence is formed by the optical path difference adjustment plate 61, and birefringence correction pieces 62a to 62 An outer edge area consisting of 62d is formed. In this way, birefringence correction plates 6 having different birefringence amounts in the central region and the outer edge region were produced. Furthermore, in this example, the birefringence correction plate 6 in which the phase correction amount of the outer edge region is 40 nm (birefringence power: SO 0. 01912) was produced in the same manner as the above-mentioned production method.

Incidentally, since the fast axis 63 of each birefringence correction piece constituting the outer edge region is directed in the longitudinal direction of each birefringence correction piece, the birefringence correction plate 6 of this example is shown in FIG. Thus, the direction of the fast axis 63 of the outer edge region along the outer edge of the central region (optical path difference adjustment plate 61), ie, the center Turn around the area. Although the light beam passing through the outer edge region of the birefringence correction plate 6 has a predetermined phase difference between two polarization components orthogonal to each other of the light beam due to birefringence, a phase correction amount SOnm is generated in the central region. Since the optical path difference adjusting plate 61 is provided, the phase difference generated in the central region is almost zero. That is, in the birefringence correction plate 6 of this example, the in-plane distribution of the phase difference between the two polarization components generated for the light beam 32 passing through the birefringence correction plate 6 is rotated with respect to the center of the light beam 32. It becomes symmetrical.

Further, in this example, as shown in FIG. 4, the width a of the central region of the birefringence correction plate 6 is half the diameter (2a) of the light beam 32 incident on the birefringence correction plate 6. I did it. More specifically, in this example, since the diameter of the light beam 32 incident on the birefringence correction plate 6 is about 3.2 mm, the width a of the central region of the birefringence correction plate 6 is 1.6 mm. Thus, the optical path difference adjusting plate 61 and the four birefringence correction pieces 62a to 62d were formed.

Further, the optical path difference adjusting plate 61 in this example is provided to eliminate the optical path difference between the light passing through the outer edge area and the central area, and the surface height of the central area and the outer edge area is I tried to be the same. Furthermore, in the birefringence correction plate 6 of this example, in order to avoid the influence of moisture, a glass substrate may be provided on the surface on which the birefringence correction pieces 62a to 62d are formed.

[Operation of Optical Pickup Device]

Next, the operation of the optical pickup device 100 of this example will be described. A laser beam 30 (a light beam of linear polarization) emitted from the semiconductor laser 1 is converted into a parallel beam 31 by the collimator lens 2 and is incident on the composite prism 3. The collimated light 31 is converted into a substantially circular light beam by being refracted by the composite prism 3. Thereafter, after the polarization functional film 3a is equalized, it is transmitted through the λ 4 plate 4 and converted into a circularly polarized light beam 32. Thereafter, the light beam 32 is changed its optical path by the rising mirror 5 and passes through the birefringence correction plate 6.

Then, the light beam 32 transmitted through the birefringence correction plate 6 is condensed by the objective lens 8 on the recording surface 21 of the disk-shaped information recording medium 20. The objective lens 8 is supported by the lens holder 7 as in the conventional optical pickup device, and the lens holder 7 is driven by an electromagnetic force (not shown), and the recording surface 21 of the disk-shaped information recording medium 20 The lens position is controlled so that the laser spot is focused at a predetermined position of. At this time, since the birefringence correction plate 6 is supported by the lens holder 7, the birefringence correction plate 6 is also moved together with the objective lens 8. Move.

The light beam reflected by the recording surface 21 of the disk-shaped information recording medium 20 is again converted into substantially parallel light by the objective lens 8, and the birefringence correction plate 6, the rising mirror 5, the λΖ 4 plate 4 The light enters the composite prism 3 through the Next, the light beam incident on the composite prism 3 is reflected by the polarization function film 3 a of the composite prism 3, passes through the cylindrical lens 9 and the condenser lens 10, and reaches the light detector 11. The light detector 11 is configured as shown in FIG. 7 and is the same as the known light detector configuration. That is, the photodetector 11 is divided into four, and has a configuration capable of detecting a focus error signal according to the astigmatism method and a tracking error according to the push method. Further, the push-pull signal is a known signal, that is, a sum signal. It is configured to obtain a DPP signal (Divided Push Pull signal) that is normalized.

[0079] [Principle of correcting birefringence]

Next, the principle of correction of birefringence by the birefringence correction plate 6 of the present embodiment will be described with reference to FIGS. In the following description, the principle of correcting birefringence with respect to light reflected from a transparent substrate will be described, but birefringence is also corrected with the same principle with respect to light incident on the transparent substrate.

FIG. 5 is a diagram showing the relationship between the light beam passing between the birefringence correction plate 6 and the disk-shaped information recording medium 20 and the refractive index of the transparent substrate of the disk-shaped information recording medium 20. The ellipsoid NS shown in the upper part of FIG. 5 indicates the distribution of the refractive index in the transparent substrate (polycarbonate substrate) of the disk-shaped information recording medium 20. In general, in a transparent substrate made of polycarbonate manufactured by an injection molding process, the refractive index n in the radial direction and the refractive index n in the circumferential direction have substantially the same value (n ^), and the thickness direction of the transparent substrate The refractive index η is the average refractive index in the in-plane direction 面 rtza

Since the refractive index of the transparent substrate is smaller than (= (+ _nt ) Z2, the refractive index of the transparent substrate becomes an ellipsoidal distribution NS extending in the in-plane direction of the transparent substrate as shown in the upper drawing of FIG.

Further, the first direction described in the birefringence correction plate 6 in the lower view in FIG. 5 is a direction corresponding to the radial direction of the transparent substrate and the objective lens 8, and the second direction is the transparent substrate and the objective lens. Corresponds to the circumferential direction of 8 (track direction).

First, the light L1 reflected substantially perpendicularly to the surface of the transparent substrate will be described.

First, since the light L1 is reflected in a direction substantially perpendicular to the surface of the transparent substrate, the light L1 is The refractive index Nl in the transparent substrate in the plane perpendicular to the reflection direction of is as shown in FIG. 6 (a). In this case, in the refractive index N1 of the transparent substrate for the light L1, as shown in FIG. 6A, the refractive index n in the radial direction and the refractive index n in the circumferential direction have substantially the same value (n = n). Therefore, when the light L1 passes through the transparent substrate, there is almost no phase difference between the radial polarization component and the circumferential polarization component of the light L1. Here, the fact that the phase difference hardly occurs means that the phase difference is smaller than the phase difference generated with respect to the light L 2 described later, and the refractive index n in the radial direction according to the forming condition of the transparent substrate This is a meaning including the case where there is a slight difference between the refractive index n and the circumferential refractive index n, and that a slight phase difference thereby occurs.

Next, light L 1 reflected substantially perpendicularly to the surface of the transparent substrate passes through the center of objective lens 8 and is incident on the central region of birefringence correction plate 6 as shown in FIG. . The optical path difference adjusting plate 61 provided in the central region hardly generates birefringence (birefringence amount O), so when the light L1 passes through the birefringence correction plate 6, the first direction of the light L1 (transparent substrate (transparent substrate) There is no phase difference between the polarization component in the second direction (corresponding to the radial direction of the transparent substrate) and the polarization component in the second direction (corresponding to the radial direction of the transparent substrate). That is, no birefringence occurs to the light L1 reflected substantially perpendicularly to the surface of the transparent substrate.

Next, the light L 2 which is also obliquely reflected in the radial direction of the transparent substrate force will be described. As described above, the cross-sectional birefringence of the transparent substrate of the disk-shaped information recording medium manufactured by the molding process is n−n> 0 (the refractive index n in the thickness direction of the transparent substrate is the average refraction in the in-plane direction) Because the index nazza is smaller), the refractive index NS of the transparent substrate becomes an elliptical distribution extending in the in-plane direction as shown in the upper drawing of FIG. Therefore, the refractive index N2 in the transparent substrate on the plane perpendicular to the reflection direction of the light L2 is elliptical as shown in the upper drawing of FIG. 6 (b), and the refractive index in the major axis direction (circumferential direction) is transparent. The refractive index n in the circumferential direction of the substrate is obtained, but the refractive index n 'in the direction perpendicular to the circumferential direction (third direction in the upper figure in FIG. 6B) is smaller than the refractive index n in the radial direction of the transparent substrate . Therefore, for the light L2 obliquely reflected in the radial direction of the transparent substrate, the slow axis of the transparent substrate is in the circumferential direction. Therefore, when the light L2 passes through the transparent substrate, the phase of the circumferential polarization component of the light L2 is delayed with respect to the polarization component in the direction (third direction) perpendicular thereto, and the light L2 is orthogonal to each other. A phase difference occurs between the two polarization components.

Then, as shown in FIG. 5, light L 2 in which a phase difference has occurred between the two polarization components is an objective lens. It is incident on the birefringence correction piece 62 a in the outer edge region of the birefringence correction plate 6 through the lens 8. At this time, the light L 2 is incident substantially perpendicularly to the surface of the birefringence correction plate 6. In the birefringence correction plate 6 of this example, the fast axis 63 of the outer edge region with respect to incident light is directed in the direction of circling around the central region of the birefringence correction plate 6, so birefringence correction of the outer edge region is performed. The direction of the fast axis 63 with respect to the light L2 incident on the piece 62a is the same as the direction (circumferential direction) of the slow axis of the transparent substrate when the light L2 passes through the transparent substrate. Therefore, when light L2 having a phase difference between two polarization components on the transparent substrate passes through the birefringence correction piece 62a in the outer edge region of the birefringence correction plate 6, light L2 whose phase is delayed when passing through the transparent substrate The phase of the polarized light component advances with respect to the perpendicular polarized light component (the polarized light component whose phase advances when passing through the transparent substrate). As a result, the phase difference between the mutually orthogonal polarization components of the light L 2 generated when passing through the transparent substrate can be corrected by the birefringence correction plate 6. This makes it possible to compensate for the birefringence of the light L2 obliquely reflected in the radial direction of the transparent substrate, and to equivalently reduce the cross-sectional birefringence of the transparent substrate.

Next, the light L 3 which is also reflected obliquely in the circumferential direction will be described. As described above, the refractive index of the transparent substrate of the disk-shaped information recording medium manufactured by the forming process becomes an ellipsoidal distribution NS extending in the in-plane direction, as shown in the upper drawing of FIG. Therefore, the refractive index in the transparent substrate in the plane perpendicular to the reflection direction of the light L 3 is elliptical as shown in the upper view of FIG. 6C, and the refractive index in the major axis direction (radial direction) is the transparent substrate The refractive index n 'in the radial direction of the transparent substrate is smaller than the refractive index n in the circumferential direction of the transparent substrate in the direction perpendicular to the radial direction (the fourth direction in FIG. 6C). . Therefore, for the light L3 that is obliquely reflected in the circumferential direction of the transparent substrate, the slow axis of the transparent substrate is in the radial direction. Therefore, when the light L3 passes through the transparent substrate, the phase of the polarization component in the radial direction of the light L3 is delayed with respect to the polarization component in the direction (fourth direction) perpendicular to that. A phase difference occurs between the polarization components.

Then, as shown in FIG. 5, light L 3 having a phase difference between the two polarization components is incident on birefringence correction piece 62 d of the outer edge region of birefringence correction plate 6 through objective lens 8. Be done. At this time, the light L 3 is incident substantially perpendicularly to the surface of the birefringence correction plate 6. At this time, the direction of the fast axis 63 of the outer edge area with respect to the light L 3 incident on the birefringence correction piece 62 d of the outer edge area is the light L 3 is in the same direction as the direction (radial direction) of the slow axis of the transparent substrate when passing through the transparent substrate. Therefore, when light L3 having a phase difference between the two polarization components on the transparent substrate passes through the birefringence correction piece 62d in the outer edge region of the birefringence correction plate 6, the light whose phase is delayed when passing through the transparent substrate The phase of the polarization component of L3 advances with respect to the perpendicular polarization component (the polarization component in which the phase advances when passing through the transparent substrate). As a result, the phase difference between mutually orthogonal polarization components of the light L 3 generated when passing through the transparent substrate can be corrected by the birefringence correction plate 6. This makes it possible to compensate for the birefringence of the light L3 obliquely reflected in the radial direction of the transparent substrate, and to equivalently reduce the cross-sectional birefringence of the transparent substrate.

In the present embodiment, as described above, the direction of the slow axis of the transparent substrate and the direction of the fast axis of the birefringence correction plate are substantially equal to the light obliquely reflected to the surface of the transparent substrate. By matching, the birefringence of light obliquely reflected to the surface of the transparent substrate is compensated, thereby reducing the influence of the birefringence of the cross section of the transparent substrate.

[Evaluation experiment]

In the evaluation experiment of this example, first, the change of the 3T short pit modulation degree with respect to the in-plane birefringence amount and the modulation of the DPP signal when the HD DVD having various cross-sectional birefringence amounts is attached to the optical pickup device of this embodiment. The change in degree is calculated by ellipsometry.

[0090] and shows the evaluation results of HD DVD having a cross-sectional birefringence 6 X 10- ⁴ to 11 and 12. Further, the evaluation results for the HD DVD having a cross-sectional birefringence 4 X 10- ⁴ to 13及beauty 14. Furthermore, showed evaluation results for HD DVD having a cross-sectional birefringence 2 X 10- ⁴ to 15 and 16. The characteristics shown by the solid line in FIGS. 11 to 16 are the results when the birefringence correction plate is not used, and the characteristics shown by the alternate long and short dash line set the phase correction amount of the birefringence correction plate to 20 nm. In the case where the birefringence amount of the outer peripheral region of the birefringence correction plate is 0. 0095, and the characteristic shown by the broken line is 40 nm when the phase correction amount of the birefringence correction plate is This is the result of the case where the birefringence amount of the outer edge region of the refraction correction plate is 0. 01912). Figures 11, 13 and 15 show the changes in 3T short pit modulation with respect to the in-plane birefringence, and Figures 12, 14 and 16 show changes in the modulation of the DPP signal with the in-plane birefringence. FIG. The amount of in-plane birefringence taken along the horizontal axis in FIGS. 11 to 16 includes the amount of change in the amount of in-plane birefringence due to centrifugal force. As apparent from the results in FIGS. 11 to 16, the in-plane birefringence amount can be obtained by providing the birefringence correction plate of the present example on the optical path between the objective lens and the compound prism (λ / 4 plate). It is important to be able to reduce the change in the degree of short pit modulation and the change in the degree of modulation of the DPP signal when H changes. That is, by using the birefringence correction plate of this embodiment, the birefringence of light obliquely reflected (or incident) to the surface of the transparent substrate is corrected to reduce the influence of the cross-birefringence of the transparent substrate. It has been possible to substantially expand the margin of fluctuation of the allowable in-plane birefringence amount.

Further, as apparent from the results in FIGS. 11 to 16, by setting the phase correction amount of the birefringence correction plate to about 20 nm, any of the cross-sectional birefringence amounts of 2 × 10 1 to ⁴ × 6 10 ⁴ In the case of the HD DVD, it is possible to sufficiently reduce the change in the degree of 3T short pit modulation and the change in the modulation of the DPP signal, which can change the phase correction amount of the birefringence correction plate. Therefore, in the optical pickup device of the configuration of the present embodiment, the configuration can be made simpler, and by adopting the optical pickup device of the configuration of the present embodiment, a wide variety of information recording media can be obtained. We have been able to construct an optical information recording and reproducing system that can be applied to (information recording media having various in-plane birefringence amounts).

Also, in this example, the cross-sectional birefringence amount is about 2 ×

4 X 10_ is ⁴ and 6 X 10_ ⁴ HD

Indeed each to prepare a DVD, further 〖this, the HD DVD of the respective cross-sectional birefringence amount, the in-plane birefringence from the inner periphery to the outer periphery to about 4. 0 X 10 one ^⁵ ~ 4. 0 X 10- ⁵ A modified HD DVD was produced. That is, various HD DVDs having different combinations of cross sectional birefringence amount and in-plane birefringence amount were produced. The cross-sectional birefringence of the transparent substrate was adjusted by adjusting the mold temperature at the time of injection molding and by changing the substrate beta temperature thereafter. The in-plane birefringence was adjusted by adjusting the injection speed of the molten resin at the time of injection molding and the mold holding time.

The various HD DVDs produced as described above were mounted on the optical pickup device of this example, and changes in the degree of 3T short pit modulation and changes in the modulation of the DPP signal with respect to the amount of in-plane birefringence were measured. In this experiment, as in the polarization analysis, when the birefringence correction plate is not used, the phase correction amount of the birefringence correction plate is 20 nm, and the phase correction amount of the birefringence correction plate is 40 nm. We went about each case. Also, in this experiment, HD DV The experiment was performed under the conditions where the rotation speed of D was set to 600 rpm to 1800 rpm, and the fluctuation of the in-plane birefringence amount caused by the centrifugal force was almost negligible. The measurement results are also shown in FIGS. The white circles in FIGS. 11 to 16 are the results when the birefringence correction plate is not used, and the white triangles are the results when the phase correction amount of the birefringence correction plate is 20 nm. The white squares indicate the results when the phase correction amount of the birefringence correction plate is 40 nm.

As apparent from the results of FIGS. 11 to 16, the measurement points of the actually manufactured HD DVD have characteristics (solid lines, dashed dotted lines and dashed lines in FIGS. 11 to 16) obtained by polarization analysis. It is almost riding. That is, from the evaluation results of both the polarization analysis (simulation analysis) and the measurement results of the actually manufactured HD DVD, the influence of the cross-section birefringence of the transparent substrate is reduced by using the birefringence correction plate of this example. It has been confirmed that it is possible to substantially expand the margin of variation of the allowable in-plane birefringence amount.

Further, as is clear from the evaluation results of this example, even if the in-plane birefringence amount changes by about ± 6 × 10 — ^{5, the} change in the degree of 3T short pit modulation and the change in the modulation of the DPP signal are Since it is small, it can be used without any problem even if the in-plane birefringence amount fluctuates by about 2 × 10 — ⁵ by centrifugal force by rotating the disc-like information recording medium at a rotational speed of 6000 rpm or more. In fact, even if the HD DVD was rotated at a rotational speed of 6000 rpm or more, the information could be recorded and reproduced without any problem.

Although the central region (optical path difference adjustment plate 61) of the birefringence correction plate 6 is square in the first embodiment, the present invention is not limited to this, and for example, the shape of the central region may be circular, It may be a polygon or the like. Further, in the first embodiment, an example in which the outer edge area of the birefringence correction plate 6 is formed by four birefringence correction pieces 62a to 62d, that is, an example in which the outer edge area is divided into four in the direction around the central area However, the present invention is not limited to this, and may be divided into five or more.

Further, although the example of the birefringence correction plate in which the birefringence correction piece is provided on the substrate has been described in the first embodiment, the present invention is not limited to this. A central region and an outer edge region may be provided on the substrate itself, and a birefringent correction plate may be used in which the fast axis of the outer edge region is directed in the direction of circling the central region. Such a birefringence correction plate can be produced, for example, as follows. First, injection molding of molten resin in the circumferential direction in the transparent substrate molding process Then, the transparent substrate with its fast axis directed in the circumferential direction is formed, and then the central portion of the transparent substrate is hollowed out, and a transparent member with small birefringence in the hollowed portion (for example, a glass member, an acrylic member, etc.) Can be obtained to obtain a birefringence correction plate having the above-mentioned configuration. Example 2

[Optical Pickup Device]

The schematic configuration of the optical pickup device of Example 2 is shown in FIGS. FIG. 17 is a view showing a cross section B'-B 'in FIG. 18, and FIG. 18 is a view showing a cross section A'-A' in FIG.

As shown in FIGS. 17 and 18, the optical pickup apparatus 200 of Example 2 mainly includes a semiconductor laser 1 (light source) that emits a light beam with a wavelength of 405 nm, a collimating lens 2, and a compound prism. 3 (polarization beam splitter), λΖ4 plate 4, rising mirror 5, birefringence correction plate 6 ', objective lens 8, lens holder 7 for holding objective lens 8, cylindrical lens 9 and , A condenser lens 10, and a light detector 11. In this example, as shown in FIG. 17, the birefringence correction plate 6 ′ is disposed on the optical path between the objective lens 8 and the disk-shaped information recording medium 20, and is supported by the lens holder 7. The elements and devices other than the birefringence correction plate 6 ′ were the same as those used in the conventional optical pickup device. Therefore, the description of the components and devices other than the birefringence correction plate 6 'is omitted here.

The schematic configuration of the birefringence correction plate 6 of this example is shown in FIGS. As shown in FIG. 19, the birefringence correction plate 6 ′ of this example is formed of a substrate 60 ′ which is a square plate member and a birefringence plate 62 ′ provided on one surface of the substrate 60 ′. Configured. As the substrate 60 ', a small birefringence optical glass with a thickness of 0.3 mm was used. Quartz was used for the birefringent plate 62 ', and as shown in FIG. 20, it was attached onto the substrate 60' via an adhesive 64 (UV adhesive for optics).

In the birefringence plate 62 ′ of this example, as shown in FIG. 19, the in-plane first direction (as will be described later, this direction corresponds to the radial direction of the disk-shaped information recording medium 20) The refractive index n and the in-plane second direction (the direction corresponds to the circumferential direction of the disc-like information recording medium 20 as described later) are substantially the same, and the thickness direction (in FIG. 19) Index of refraction n in the third direction of

A quartz plate having a larger average refractive index n (= (n + n) Z2) in the 2 3 directions was used. That is, lagging A quartz plate with a uniaxial (one-axis anisotropy) axis was used in the direction of thickness. Such a quartz plate can be manufactured by cutting out so that the anisotropic axis of the artificial synthetic quartz crystal is perpendicular to the surface of the quartz plate and then polishing it. Besides the crystal, a crystal of lithium niobate, a crystal of magnesium fluoride or the like can be used as the birefringent plate 62 '.

The birefringence correction plate 6 ′ of this example was produced as follows. First, a birefringent plate 62 'having a slow axis directed in the thickness direction (uniaxial anisotropy) was prepared and prepared by the method described above. In this example, the refractive index n in the thickness direction at a wavelength of 405 nm is 1.5667, and in-plane

3

A quartz plate having a refractive index η (η,) of 1. 55714 in the direction was used as a birefringent plate 62 ′. That

0 1 2

Therefore, the difference between the refractive index η in the thickness direction of the birefringence plate 62 ′ and the average refractive index η in the plane,

3 0

That is, the magnitude Δ η (the amount of cross-sectional birefringence) of the cross-sectional birefringence of the birefringence correction piece 62 ′ is 0.00956. Then, after the birefringence plate 62 ′ was adhered onto the optical glass 60 ′ with the adhesive 64, the birefringence plate 62 was polished to make the thickness t of the birefringence plate 62 about 25 m. Therefore, in the birefringent plate 62 of this example, the phase correction parameter Δ n X t value is 240 nm.

In the present embodiment, the phase correction parameter Δ n X t of the birefringent plate 62 ′ is 180 ππ! It is preferable that it is -30 Onm. When the An x t value is smaller than 180 nm, when polycarbonate having excellent mass productivity is used as a substrate material of the information recording medium, the correction of the cross-sectional birefringence is not sufficiently completed. As a result, the in-plane birefringence of the information recording medium And the reproduction margin of the information recording medium is reduced. On the other hand, when the A n X t value is larger than 300 nm, the cross-sectional birefringence correction is over-corrected when the cross-sectional birefringence is relatively small as the substrate material of the information recording medium, and polyolefin is used. Therefore, if the AnxT value is not in the above range, the compatibility of the optical pickup device with various information recording media is lost. The basis for that will be described later.

[Operation of Optical Pickup Device]

Next, the operation of the optical pickup device 200 of this example will be described with reference to FIGS. A laser beam 30 (a linearly polarized light beam) emitted from the semiconductor laser 1 is converted into a collimated beam 31 by the collimator lens 2 and is incident on the composite prism 3. The collimated light 31 is converted into a substantially circular light beam by being refracted by the composite prism 3. Thereafter, the light is transmitted through the polarizing film 3a, and then transmitted through the λ 4 plate 5 to be converted into a circularly polarized light beam 32. Then the light The beam 32 is changed its optical path by the raising mirror 5 and is incident on the objective lens 8. Then, the light beam focused by the objective lens 8 is focused on the recording surface 21 of the disk-shaped information recording medium 20 via the birefringence correction plate 6 ′. The objective lens 8 is supported by the lens holder 7 as in the conventional optical pickup device, and the lens holder 7 is driven by an electromagnetic force (not shown), and the recording surface 21 of the disk-shaped information recording medium 20 The lens position is controlled so that the laser spot is focused at a predetermined position of. At this time, since the birefringence correction plate 6 ′ is supported by the lens holder 7, the birefringence correction plate 6 ′ is also moved together with the objective lens 8.

The light beam reflected by the recording surface 21 of the disk-shaped information recording medium 20 is incident on the objective lens 8 through the birefringence correction plate 6 ′, and the reflected light is re-parallelized by the objective lens 8 again. The light is converted into light, and the light is incident on the composite prism 3 through the raising mirror 5 and the λΖ4 plate 4. Then, the light beam incident on the composite prism 3 is reflected by the polarization functional film 3 a of the composite prism 3, passes through the cylindrical lens 9 and the condenser lens 10, and reaches the light detector 11. The photodetector 11 is configured as shown in FIG. 7 and is the same as the known photodetector configuration.

[0107] [Principle of correcting birefringence]

Next, the principle of correction of birefringence by the birefringence correction plate 6 'of this embodiment will be described with reference to FIGS. In the following description, the principle of correcting birefringence with respect to light reflected from a transparent substrate is described, but birefringence is also corrected with the same principle with respect to light incident on the transparent substrate.

FIG. 21 (a) shows the relationship between the light beam passing between the birefringence correction plate 6 ′ and the disk-shaped information recording medium 20 and the refractive index of the transparent substrate of the disk-shaped information recording medium 20. FIG. 21A is a diagram showing the relationship, and the ellipsoid NS in the upper diagram in FIG. 21A shows the distribution of the refractive index in the transparent substrate (polycarbonate substrate) of the disk-shaped information recording medium 20. In general, in the case of a transparent substrate made of polycarbonate made of a polycarbonate-based injection molding process, the refractive index n in the radial direction and the refractive index n in the circumferential direction have substantially the same value (n ^), and the thickness direction of the transparent substrate The refractive index η is the average refractive index in the in-plane direction

r t z

Since the refractive index is smaller than n (= (n + n) Z 2), the refractive index NS of the transparent substrate is shown in the upper drawing of FIG. 21 (a) a r t

As shown in the figure, it becomes an ellipsoidal distribution extending in the in-plane direction. The first direction shown in the lower part of the birefringence correction plate 6 'in FIG. 21 (a) is the direction corresponding to the radial direction of the transparent substrate, The second direction corresponds to the circumferential direction (track direction) of the transparent substrate.

Further, FIG. 21 (b) shows the relationship between the light beam passing between the birefringence correction plate 6, and the objective lens 8 and the refractive index in the birefringence plate 62 'of the birefringence correction plate 6'. FIG. 21B is a diagram showing the relationship, and the ellipsoid NP in the upper diagram in FIG. 21 (b) shows the distribution of the refractive index in the birefringence plate 62 ′ of the birefringence correction plate 6 ′. As described above, the retardation plate 62 'of the birefringence correction plate 6' of this example has the slow axis directed in the thickness direction (the refractive index n in the thickness direction is the refractive index η in the in-plane direction). , n) because

3 1 2

The refractive index NP in the birefringence plate 62 'is an ellipsoidal distribution extending in the thickness direction (third direction), as shown in the upper drawing in FIG. 21 (b).

First, since the light L1 is reflected in a direction substantially perpendicular to the surface of the transparent substrate, the refractive index NS1 in the transparent substrate of the plane perpendicular to the reflection direction of the light L1 is the upper view of FIG. become that way. In this case, in the refractive index NS1 of the transparent substrate for the light L1, as shown in the upper part of FIG. 22 (a), the refractive index n in the radial direction and the refractive index n in the circumferential direction have substantially the same value (nn) Therefore, when the light L1 passes through the transparent substrate, almost no phase difference occurs between the radial polarization component and the circumferential polarization component of the light L1. Here, the fact that the phase difference hardly occurs means that the phase difference is smaller than the phase difference generated with respect to the light L2 described later, and the refractive index n in the radial direction according to the forming condition of the transparent substrate There is a meaning including the case where a difference with the circumferential refractive index n is somewhat generated, and thereby a phase difference is generated.

Next, the light L 1 reflected substantially perpendicularly to the surface of the transparent substrate is made incident on the birefringence correction plate 6. At this time, as shown in FIG. 21A, the light L1 is incident substantially perpendicularly to the birefringence correction plate 6 ′. In this case, in the refractive index NP1 of the birefringent plate 62 'for the light L1, as shown in the lower part of FIG. 22 (a), the refractive index n in the first direction (corresponding to the radial direction of the transparent substrate) Since the refractive index n of the transparent substrate (corresponding to the circumferential direction) has almost the same value, the light L

2

When the light passes through the birefringence correction plate 6 ′, there is almost no phase difference between the polarization component in the first direction of the light L1 and the polarization component in the second direction. That is, no birefringence occurs for light L1 reflected (or incident) almost perpendicularly to the surface of the transparent substrate!

Next, the light L 2 reflected obliquely in the radial direction of the transparent substrate force will be described. As described above, the refractive index of the transparent substrate of the disk-shaped information recording medium produced by the molding process is As shown in the upper part of Fig. 21 (a), it becomes an ellipsoidal distribution NS extending in the in-plane direction. Therefore, the refractive index NS2 in the transparent substrate in the plane perpendicular to the reflection direction of the light L2 becomes elliptical as shown in the upper drawing of FIG. 22 (b), and the refractive index in the major axis direction (circumferential direction) is the transparent substrate The refractive index n ′ in the circumferential direction is smaller than the refractive index n in the radial direction of the transparent substrate in the direction perpendicular to the circumferential direction (third direction in FIG. 22B). Therefore, for the light L2 obliquely reflected in the radial direction of the transparent substrate, the slow axis of the transparent substrate is in the circumferential direction. Therefore, when the light L2 passes through the transparent substrate, the phase of the circumferential polarization component of the light L2 is delayed with respect to the polarization component in the direction (third direction) perpendicular thereto, and the light L2 is orthogonal to each other 2 A phase difference occurs between two polarization components.

Next, the light L2 in which a phase difference has occurred between the two polarization components is incident on the birefringence correction plate 6 ′. At this time, as shown in FIG. 22 (a), the light L2 is incident on the surface of the birefringence correction plate 6 ′ from an oblique direction (first direction). Since the slow axis of the birefringent plate 62 'in this example is directed in the thickness direction, the refractive index NP2 in the birefringent plate 62' in the plane perpendicular to the incident direction of the light L2 is as shown in the lower view of FIG. As shown in the figure, it is elliptical, and the refractive index n in the second direction (corresponding to the circumferential direction of the transparent substrate) is perpendicular to that (the 1 'direction in FIG. 22 (b): the third direction of the transparent substrate). Corresponding)

2

It becomes smaller than the refractive index n '. Therefore, the direction of the fast axis with respect to the light L2 incident on the birefringence correction plate 6 'is the second direction, and the direction (circumferential direction) of the slow axis of the transparent substrate when the light L2 passes through the transparent substrate. In the same direction. Therefore, when the light L2 having a phase difference between the two polarization components on the transparent substrate passes through the birefringence correction plate 6 ′, the phase of the polarization component of the light L2 whose phase is delayed when passing through the transparent substrate is It advances with respect to the perpendicular polarization component (polarization component in which the phase advances when passing through the transparent substrate). As a result, the phase difference between the mutually orthogonal polarization components of the light L2 generated when passing through the transparent substrate can be corrected by the birefringence correction plate 6 '. Thereby, the birefringence of the light L2 obliquely reflected in the radial direction of the transparent substrate can be compensated, and the cross-sectional birefringence amount of the transparent substrate can be equivalently reduced.

Next, the light L 3 which is also reflected obliquely in the circumferential direction will be described. As described above, the refractive index of the transparent substrate of the disk-shaped information recording medium manufactured by the molding process is an ellipsoidal distribution NS extending in the in-plane direction as shown in the upper drawing of FIG. 21 (a). become. Therefore, the refractive index NS3 in the transparent substrate in the plane perpendicular to the reflection direction of the light L3 is shown in the upper figure of FIG. 22 (c). Thus, it becomes elliptical and the refractive index in the major axis direction (radial direction) is the refractive index in the radial direction of the transparent substrate! However, the refractive index n _t 'in the direction perpendicular to the radial direction (the fourth direction in FIG. 22C) becomes smaller than the refractive index n in the circumferential direction of the transparent substrate. Therefore, for the light L3 reflected obliquely in the circumferential direction of the transparent substrate, the slow axis of the transparent substrate is in the radial direction. Therefore, when the light L3 passes through the transparent substrate, the phase of the polarization component in the radial direction of the light L3 is delayed with respect to the polarization component in the direction (fourth direction) perpendicular to that. A phase difference occurs between the polarization components.

Next, light L 3 in which a phase difference has occurred between the two polarization components is incident on the birefringence correction plate 6 ′. At this time, as shown in FIG. 22 (a), the light L3 is incident on the surface of the birefringence correction plate 6 ′ from an oblique direction (second direction). Since the slow axis of the birefringent plate 62 'in this example is directed in the thickness direction, the refractive index NP3 in the birefringent plate 62' in the plane perpendicular to the incident direction of the light L3 is as shown in the lower view of FIG. As shown in Fig. 22, it becomes elliptical, and the refractive index n in the first direction (corresponding to the radial direction of the transparent substrate) is perpendicular to that (the 2 'direction in FIG. 22 (c): the fourth direction of the transparent substrate). Smaller than the index of refraction n 'of Therefore, for the light L3 incident on the birefringence correction plate 6 '

2

The direction of the fast axis is the first direction, and is the same as the direction (radial direction) of the slow axis of the transparent substrate when the light L 3 passes through the transparent substrate. Therefore, when light L3 having a phase difference between two polarization components on the transparent substrate passes through the birefringence correction plate 6 ', the phase of the polarization component of the light L3 whose phase is delayed when passing through the transparent substrate is It proceeds with respect to the vertical polarization component (polarization component whose phase advances when passing through the transparent substrate). As a result, the phase difference between the mutually orthogonal polarization components of the light L 3 generated when passing through the transparent substrate can be corrected by the birefringence correction plate 6 ′. This makes it possible to compensate for the birefringence of the light L3 obliquely reflected in the radial direction of the transparent substrate, and to equivalently reduce the cross-sectional birefringence of the transparent substrate.

In the present embodiment, as described above, the direction of the slow axis of the transparent substrate and the phase advance axis of the birefringence correction plate with respect to light obliquely reflected (or incident) to the surface of the transparent substrate. By compensating the birefringence of light obliquely reflected (or incident) to the surface of the transparent substrate to reduce the influence of the cross-sectional birefringence of the transparent substrate.

[Evaluation experiment]

In the evaluation experiment of this example, as in Example 1, first, various cross-sectional birefringence amounts are provided. The change in the degree of 3T short pit modulation and the change in the degree of modulation of the DPP signal with respect to the in-plane birefringence amount when the HD DVD was attached to the optical pickup device of this example was calculated by polarization analysis.

[0118] and shown in Figure 23 and 24 the evaluation results for the HD DVD having a cross-sectional birefringence 6 X 10- ^4. Further, the evaluation results for the HD DVD having a cross-sectional birefringence 4 X 10- ⁴ to 25及beauty 26. Furthermore, showed evaluation results for HD DVD having a cross-sectional birefringence 2 X 10- ⁴ to 27 and 28. The characteristics shown by the solid line in FIGS. 23 to 28 are the results when the birefringence correction plate is not used, and the characteristics shown by the alternate long and short dash line are the results when the birefringence correction plate of this embodiment is used. It is. 23, 25 and 27 show the change in the degree of 3T short pit modulation with respect to the in-plane birefringence, and FIGS. 24, 26 and 28 show the modulation of the DPP signal with respect to the in-plane birefringence. It is a figure showing change. The amount of in-plane birefringence along the horizontal axis in FIGS. 23 to 28 includes the amount of change in the amount of in-plane birefringence due to centrifugal force.

As apparent from the results of FIGS. 23 to 28, by providing the birefringence correction plate of the present example on the optical path between the objective lens and the disc-shaped information recording medium, the in-plane birefringence amount can be increased. The ability to reduce changes in the 3T short pit modulation degree and the DPP signal modulation degree at the time of the change is a force. That is, by using the birefringence correction plate of the present embodiment, the birefringence of light obliquely reflected (or incident) to the surface of the transparent substrate is corrected to reduce the influence of the birefringence of the cross section of the transparent substrate. It has been possible to substantially expand the margin of fluctuation of the allowable in-plane birefringence amount.

[0120] Further, as is clear from the results of FIG. 23-28, by the correction parameter one capacitor delta n X t of the birefringence compensation plate in this example about 240 nm, cross-sectional birefringence 2 X 10 one ⁴ Change the correction parameter (phase correction amount) of the birefringence correction plate even for HD DVDs of ~ 6 X 10 ⁴ ! Change in 3T short pit modulation degree and DPP signal modulation degree sufficiently. It is important to be able to reduce it. Therefore, also in the optical pickup device of the configuration of the present embodiment, the configuration can be simplified as in the first embodiment, and by adopting the optical pickup device of the configuration of the present embodiment. The ability to construct an optical information recording / reproduction system that can be adapted to a wide variety of media is essential.

Further, in this example, as in Example 1, the cross-sectional birefringence amount is about 2 ^× 10 ⁴ , 4 × 10 ⁴ and Indeed each to prepare a HD DVD as a 6 X 10_ ^4, further, in the HD DVD of the respective cross-sectional birefringence amount, the in-plane birefringence toward the inner circumferential force periphery to about -4 X 10 ^_5 ~4 X 10_ ⁵ A modified HD DVD was produced. That is, various HD DVDs having different combinations of cross sectional birefringence amount and in-plane birefringence amount were produced. The cross sectional birefringence amount and the in-plane birefringence amount of the transparent substrate were adjusted in the same manner as in Example 1.

The various HD DVDs manufactured as described above were mounted on the optical pickup device of this example, and changes in the degree of 3T short pit modulation and changes in the modulation of the DPP signal with respect to the amount of in-plane birefringence were measured. This experiment was conducted in the case where no birefringence correction plate was used and in the case where a birefringence correction plate was used, as in the polarization analysis. Moreover, in this experiment, the rotation speed of HD DVD was set to 600 rpm to 1800 rpm, and the experiment was performed under the condition that the fluctuation of the in-plane birefringence amount due to the centrifugal force can be almost ignored. The measurement results are also shown in FIGS. The white circles in FIGS. 23 to 16 are the results when the birefringence correction plate is not used, and the white squares are the results when the birefringence correction plate is used.

As apparent from the results of FIGS. 23 to 28, the measurement points of the actually produced HD DVD are substantially on the characteristics (on the solid line and the dashed dotted line in FIGS. 23 to 28) on the characteristics obtained by polarization analysis. ing . Therefore, from the evaluation results of both the polarization analysis (simulation analysis) and the measurement results of the actually produced HD DVD, the influence of the cross-sectional birefringence of the transparent substrate is reduced by using the birefringence correction plate of this example. It has been confirmed that it is possible to substantially expand the amount of variation in allowable in-plane birefringence amount.

Further, in this example, the following analysis experiment was performed to obtain a suitable range of the phase correction parameter Δ n X t of the birefringent plate 62 ′. Phase correction parameters A n X t have birefringence amounts of 180 nm and 30 O nm using birefringence correction plates having different cross-sectional birefringence amounts H The modulation degree change of the DPP signal with respect to the in-plane birefringence amount in D DVD was calculated by ellipsometry. Here, was ellipsometry against HD DVD that cross birefringence amount is 2 X 10_ ⁴ and 6 X 10_ ^4. That is, polarization analysis was performed on an HD DVD having a relatively small amount of cross-sectional birefringence and an HD DVD having a large amount of cross-sectional birefringence. The results are shown in Figures 24 and 28. The characteristic of the broken line in FIGS. 24 and 28 is the result in the case of using a birefringence correction plate having a birefringence plate with AnXt of 180 nm, and the characteristic of the two-dot chain line has a birefringence plate with a birefringence plate of AnXt of 300ηm. It is a result at the time of using a correction board.

As apparent from FIG. 24, when the cross-sectional birefringence amount is relatively large (6 × 10 ⁴ ), the AnXt force decreases, and the fluctuation of the modulation of the DPP signal with respect to the in-plane birefringence amount increases. This is because the phase correction amount becomes insufficient as AnXt becomes smaller. And, in the case of AnXt force Sl 80nm, the ratio of the maximum value to the minimum value of the DPP signal modulation degree in the range of the in-plane birefringence amount ± 6 x 10_ ⁵ is close to 2, and as a result, When the force is smaller than 180 nm, the ratio of the maximum value to the minimum value of the modulation of the DPP signal in the range of in-plane birefringence amount ± 6 × 10 ⁵ is 2 or more. When the ratio of the maximum value to the minimum value of the modulation of the DPP signal is 2 or more, there is an adverse effect that the track following performance of the optical pickup device is deteriorated.

[0127] Further, as apparent from FIG. 28, when cross-sectional birefringence amount is relatively small (2Χ10 ^_4), the delta nXt increases, variations in the modulation degree of the DPP signal for plane birefringence amount increases. This is because when AnXt becomes large, it becomes overcorrection. And in the case of AnXt force S 300 nm, the ratio of the maximum value to the minimum value of the modulation of the DPP signal in the range of the in-plane birefringence amount ± 6 × 10 — ⁵ is close to 2, and as a result, When it becomes larger than ΔΧΐΧΐ300ηπι, the ratio of the maximum value to the minimum value of the modulation degree of the DPP signal in the range of the in-plane birefringence amount ± 6 × 10 — ⁵ becomes 2 or more. Therefore, from the results of FIGS. 24 and 28, it was found that the preferable range of the phase correction parameter AnXt of the birefringence correction plate of this example is 180 nm≤AnXt≤3 OO nm.

In the above-described first and second embodiments, the optical pickup apparatus having one light source has been described. However, the present invention is not limited to this, and it may be a recording and reproducing apparatus compatible with a medium such as a DVD-CD drive. The same applies to optical pickup devices having light sources of multiple wavelengths. It is

In the first and second embodiments, the case where the wavelength of the light beam emitted from the light source is 405 nm has been described, but the present invention is not limited to this, and the present invention is used for high density recording. The same is applicable to light beams having a wavelength of 430 nm or less. In fact, when the same verification experiments as in Examples 1 and 2 were conducted using light sources having various wavelengths of 430 nm or less, the present inventors obtained the same results as in Examples 1 and 2. The

Industrial applicability

[0130] With the birefringence correction plate of the present invention, and the optical pickup device and the reproduction device (or the recording and reproduction device) using the same, various cross-sectional birefringence can be obtained without changing the phase correction amount of the birefringence correction plate. Birefringence can be compensated for the information recording medium of the quantity. Therefore, the birefringence correction plate of the present invention, and the optical pickup device and reproduction device using the same, the birefringence correction plate excellent in medium compatibility, and the optical pickup device and reproduction using the same. It is an apparatus, and is suitable as a birefringence correction plate applicable to a wide variety of information recording media, and an optical pickup apparatus and a reproduction apparatus using the same.

Claims

The scope of the claims

[1] An optical pickup device for irradiating a light beam onto a recording surface of an information recording medium having a light transmitting medium via the light transmitting medium, and receiving reflected light from the recording surface, comprising: ,

An objective lens for condensing a light beam emitted from the light source on the recording surface, a polarization beam splitter disposed on an optical path between the light source and the objective lens, the objective lens, and the polarization beam splitter And a birefringence correction plate for providing a phase difference between two mutually orthogonal polarization components of the incident light beam.

An optical pickup device characterized in that the phase difference generated by the birefringence correction plate is different depending on the in-plane position of the birefringence correction plate.

[2] The distribution of the phase difference in the plane where the light beam of the birefringence correction plate is incident is rotationally symmetric with respect to the center of the incident light beam. Optical pickup device.

[3] An optical pickup device for irradiating a light beam to a recording surface of an information recording medium having a light transmitting medium via the light transmitting medium and receiving reflected light from the recording surface, comprising: ,

The birefringence correction plate has a central region and an outer edge region provided so as to surround the central region, and the birefringence amount of the central region is different from the birefringence amount of the outer edge region. An optical pickup device characterized by

[4] An optical pickup device characterized in that the phase advance axis of the outer edge area is directed in the direction of circling the central area.

[5] The outer edge area is equally divided into four areas in the direction of circling the central area. The optical pickup device according to claim 4, characterized in that

[6] The optical pickup device according to [5], wherein the central region has a square shape, and the fast axis of the outer edge region extends in a direction along the outer edge of the central region.

[7] The birefringence correction plate includes a substrate and a birefringence correction piece provided on the substrate, and the birefringence correction piece is provided in the outer edge region. 3. The optical pickup device according to any one of 3 to 6.

[8] An optical pickup device for irradiating a light beam onto a recording surface of an information recording medium having a light transmitting medium via the light transmitting medium, and receiving reflected light from the recording surface, comprising: ,

An objective lens for condensing the light beam emitted from the light source on the recording surface; and a birefringence correction plate for providing a phase difference between two orthogonal polarization components of the incident light beam.

An optical pickup device, wherein the birefringence correction plate is disposed on the information recording medium side of the objective lens.

[9] An optical pickup device for irradiating a light beam onto a recording surface of an information recording medium having a light transmitting medium via the light transmitting medium, and receiving reflected light from the recording surface, comprising: ,

An objective lens for condensing the light beam emitted from the light source on the recording surface, and two polarization components disposed on the information recording medium side of the objective lens and orthogonal to each other of the incident light beams And a birefringence correction plate that gives a phase difference to the

An optical pickup device characterized in that the slow axis of the birefringence correction plate is directed in the thickness direction of the birefringence correction plate.

[10] The birefringence correction plate includes a substrate and a birefringence plate provided on the substrate, and the difference between the refractive index in the thickness direction of the birefringence plate and the refractive index in the in-plane direction is Δ The optical pickup device according to claim 9, characterized in that 180 nm 、 An x t t 300 nm is satisfied, where n is the thickness of the birefringence plate t.

[11] The optical pickup device according to any one of claims 1 to 10, wherein the wavelength of the light beam emitted from the light source is 430 nm or less.

[12] The optical pickup device according to any one of [1] to [1], wherein the numerical aperture of the objective lens is not less than 0.6.

[13] The difference n−n between the refractive index n in the in-plane direction of the light transmitting medium and the refractive index n in the thickness direction is a z a z

The optical pickup device according to any one of claims 1 to 12, which has a value of 2 X ^10-4 or more.

[14] The above-mentioned light transmitting medium is a molded substrate made of polycarbonate.

The optical pick-up apparatus of Claim 13.

[15] A recording surface of a disk-shaped information recording medium having a light transmitting medium is irradiated with a light beam through the light transmitting medium, and light reflected from the recording surface is received to reproduce information. An optical pickup device according to any one of claims 1 to 14;

And a rotating device for rotationally driving the disk-shaped information recording medium.

[16] The playback device according to Claim 15, wherein the maximum rotation speed of the rotating device is 6000 rpm or more.

[17] A birefringence correction plate for providing a phase difference between two polarization components of an incident light beam, which are orthogonal to each other,

With the central region,

An outer edge area provided to surround the central area;

A birefringence correction plate characterized in that a birefringence amount of the central region and a birefringence amount of the outer edge region are different.

[18] The birefringence correction plate according to claim 17, wherein the fast axis of the outer edge area is directed in the direction of circling the central area.

[19] The birefringence correction plate according to claim 17 or 18, wherein the width of the central region is half the diameter of the incident light beam.

[20] The birefringence correction plate according to any one of claims 17 to 19, characterized in that the outer edge area is equally divided into four areas in the direction of circling the central area.

[21] The central region is square, and the fast axis of the outer edge region is directed along the outer edge of the central region! 21. The birefringence correction plate according to claim 20, which is characterized in that

[22] The birefringence correction plate includes a substrate and a birefringence correction piece provided on the substrate, The birefringence correction plate according to any one of claims 17 to 21, wherein a refraction correction piece is provided in the outer edge region.

[23] The birefringence correction plate according to claim 22, wherein the birefringence correction piece is quartz.

[24] A birefringence correction plate for giving a phase difference between two mutually orthogonal polarization components of an incident light beam,

A birefringence correction plate characterized in that the slow axis of the birefringence correction plate is directed in the thickness direction of the birefringence correction plate.

[25] The birefringence correction plate includes a substrate and a birefringence plate provided on the substrate, and the difference between the refractive index in the thickness direction of the birefringence plate and the refractive index in the in-plane direction is Δ n The birefringence correction plate according to claim 24, wherein 180 nm ≤ An x t ≤ 300 nm is satisfied, where t is the thickness of the birefringence plate.

[26] The birefringence correction plate according to claim 25, wherein the birefringence plate is quartz.