JP3277140B2 - Objective lens and optical pickup device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical pickup device for optically recording or reproducing information on an information recording medium such as an optical disk. More specifically, the present invention relates to an optical pickup device capable of performing accurate recording / reproducing operations on optical recording media having different substrate thicknesses.

[0002]

2. Description of the Related Art In recent years, optical discs are capable of recording a large amount of information signals at high density, and thus have been used in many fields such as audio, video, and computers.

At present, compact discs (CDs), video discs, and mini discs (MD) are widely commercially available.
Also, a magneto-optical disk for a computer or the like uses a substrate having a thickness of 1.2 mm. For this reason, the objective lens of the optical pickup is also designed to correct the aberration generated by the substrate having a thickness of 1.2 mm.

On the other hand, various studies have been made to increase the recording capacity. Among them, there are a method of improving the optical resolution by increasing the numerical aperture (NA) of the objective lens, and a method of providing a multilayer recording layer.

[0005] When the NA of the objective lens is increased, the diameter of the condensed beam is reduced in proportion, so that the recording capacity can be increased.
Therefore, it is necessary to reduce the thickness of the disk substrate in order to keep the allowable error of the disk inclination at the same level. For example, the objective lens NA of 0.5 and the substrate thickness of 1.2 mm have the same disk tilt tolerance as the objective lens NA of 0.6 and the substrate thickness of 0.6.
mm.

However, when the substrate thickness of the disk is reduced as described above, compatibility with an optical disk having a conventional substrate thickness cannot be maintained.

[0007] Even in the case of a multi-layer disc provided with a plurality of recording layers via a transparent substrate having a certain thickness, the recording capacity of one disc is greatly increased. However, since the thickness of the substrate as viewed from the objective lens differs in each recording layer, accurate recording and reproduction of information cannot be performed with one optical pickup.

As a method for solving the above problems,
For example, Japanese Patent Application Laid-Open No. 7-182690 proposes a method of mechanically inserting and removing a conversion lens.

In this method, the optical system is set to be compatible with an optical recording medium having a certain substrate thickness when no conversion lens is inserted. Then, when accessing an optical recording medium having another substrate thickness, a conversion lens is inserted, and the optical path is changed so as to be compatible with the other optical recording medium. As a result, information can be recorded / reproduced / erased on / from an optical recording medium having at least two types of substrate thicknesses.

This conversion lens includes a concave lens, a convex lens,
Semiconductor laser (light source) formed by hologram lens etc.
It is arranged on the fixed side of the optical head between the and the objective lens, and does not move integrally with the objective lens.

In Japanese Patent Application Laid-Open No. 7-302439, the objective lens is divided into a plurality of regions, and each region has a lens surface shape having different specifications.
Focus objectives have been proposed.

The bifocal objective lens is, for example, a lens which is concentrically divided into a plurality of regions, and a light beam incident on each of the divided regions is focused on one of two types of optical recording media. It is supposed to be. For this reason, in this method, a light beam that always passes through two types of light paths is emitted, and each light beam is made to correspond to a condensed spot on an optical recording medium having a different substrate thickness.

[0013]

However, the above-mentioned method of taking in and out of the conversion lens itself (Japanese Patent Laid-Open No. 7-18 / 1994)
No. 2690), when the conversion lens is inserted, it is necessary to precisely match the optical axis of the objective lens with the optical axis of the conversion lens, and it is necessary to increase the positioning accuracy and the inclination accuracy of the conversion lens in the optical axis direction. There is. Therefore, a mechanism for moving the conversion lens in and out with high accuracy is required, which increases the size of the apparatus, increases the cost, and has a problem in mass productivity. Further, since the lens itself is movable, the lens position and the like easily change depending on the surrounding environment, and there is a problem in terms of reliability.

On the other hand, the method using a bifocal objective lens (Japanese Patent Application Laid-Open No. Hei 7-302439) does not require newly added parts and mechanisms, and is excellent in this respect. First, the light use efficiency decreases according to the divided area ratio. Further, by dividing (providing a discontinuous point in the shape of the objective lens), the main beam diameter of the focused spot is reduced, but the side lobe intensity is increased, and as a result, the substantial beam diameter is increased. There was a problem that it would.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a low-cost objective lens and an optical system which are excellent in mass productivity and excellent in reliability without deteriorating optical performance. It is an object to provide a pickup device.

[0016]

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above-mentioned problems.
It has been made to solve the, a first focal point of the objective lens corresponding to the two kinds of optical recording media having different substrate thicknesses, the outer peripheral portion is generated for the thinner optical recording medium having a substrate thickness The wavefront aberration becomes substantially zero, and
The wavefront aberration generated for an intermediate substrate thickness of the type of optical recording medium is set to be minimum, and is formed in a shape having no discontinuity.

The present invention is also an objective lens corresponding to two types of optical recording media having different substrate thicknesses, wherein the objective lens is represented by one aspherical formula shown in (Equation 1), and has a lens radius The low order aspherical coefficient at the position r is determined corresponding to an optical recording medium having a large substrate thickness, and the high order aspherical coefficient is determined according to an optical recording medium having a small substrate thickness. Objective lens. Z = r ² / R / (1+ (1− (1 + K) × (r / R) ² ) ^0.5 ) + B × r ⁴ + C × r ⁶ + D × r ⁸ + E × r ¹⁰ +... : Depth from surface vertex, r: lens radius position, R: paraxial radius of curvature, K: conical constant, B, C, D, E,...: Aspheric coefficient of r2n (n is 2, 3) , 4,5 ...)

An optical pickup device according to the present invention has a light source and a photodetector for receiving reflected light from the optical recording medium, and transmits information to two types of optical recording media having different substrate thicknesses. In an optical pickup device capable of recording and reproduction, an objective lens for condensing a light beam from the light source on the optical recording medium comprises the above-mentioned objective lens.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of an objective lens and an optical pickup device according to the present invention will be described with reference to the drawings. Here, a CD having a substrate thickness of 1.2 mm is used.
The objective lens and the optical pickup device corresponding to the two types of optical recording media of DVD and 0.6 mm DVD will be described.

FIG. 1 is an explanatory diagram illustrating a schematic configuration of an optical pickup device according to the present embodiment. Reference numeral 1 denotes a hologram laser unit including a semiconductor laser as a light source, a photodetector, and a hologram. The light beam emitted from the hologram laser unit 1 is
Is reflected by the rising mirror 3 and is condensed on the optical disk 5 by the objective lens 4. The light beam reflected by the optical disk 5 similarly enters the hologram laser unit 1 through the objective lens 4, the rising mirror 3, and the collimator lens 2, and the information signal and the pickup control signal (tracking error signal and focus error signal) are converted. Is detected. Then, the optical pickup device is driven by a control unit (not shown) based on the tracking error signal and the focus error signal, so that a light beam that has just been focused on a desired position on the optical disk 5 is irradiated. Here, in the present embodiment, an example is described in which a hologram laser unit capable of downsizing is adopted, but an optical system in which a general laser and a photodetector are separately configured may be used. Is the same.

Next, the objective lens 4 mounted on the present pickup will be described. Here, two types of optical recording media having different substrate thicknesses will be described as DVD and CD.

The objective lens 4 of this pickup device is t
₁ (= 0.6 mm) and t ₂ (= 1.2 mm)
VD, CD substrate thickness, NA ₁ (= 0.6), NA ₂ (=
0.38) are the numerical apertures of DVD and CD, respectively, and the outer peripheral portion (the area on the outer peripheral side from the portion corresponding to the numerical aperture NA ₂ ) has a wavefront aberration with respect to the substrate thickness t ₁ (DVD). The light beam is formed to be substantially zero (the wavelength of the light beam is 650 nm). The inner peripheral portion (region on the inner peripheral side of the portion corresponding to the numerical aperture NA ₂ ) is formed such that the wavefront aberration is minimized when the substrate thickness is larger than t _{1 and} smaller than t _2. (This substrate thickness is hereinafter referred to as an inner peripheral portion optimum substrate thickness). Further, the outer peripheral portion and the inner peripheral portion of the objective lens 4 change continuously so as to have one focal point (change so as not to have a discontinuous point).

Hereinafter, this objective lens 4 will be described with reference to FIGS.
This will be described in more detail with reference to FIGS.

FIG. 2 is a diagram showing the relationship between the optimum inner peripheral substrate thickness of the objective lens 4 of the present embodiment and the wavefront aberration of only the inner peripheral portion during CD and DVD reproduction. From this figure, when the inner peripheral portion is adapted to DVD (inner peripheral portion optimal substrate thickness = 0.6), 0.07λ (rms
It can be seen that the wavefront aberration of (value) is obtained, but the wavefront aberration at the time of reproducing the CD decreases as the inner peripheral portion optimum substrate thickness increases. In general, it is said that the aberration of the entire imaging optical system of the optical pickup needs to be 0.07λ or less. In particular, in the case of a high-density optical recording medium such as a DVD, the wavefront aberration may be smaller. Required. Therefore, it is preferable that the objective lens itself has a wavefront aberration of 0.04λ or less, and in consideration of this, it is desirable to set the inner peripheral portion optimum substrate thickness to be greater than 0.06 mm and 0.95 or less. However, here, polycarbonate is assumed as the substrate.

FIG. 3 is a diagram comparing an objective lens (comparative example) suitable only for a DVD with an objective lens according to an embodiment of the present invention. In the figure, the horizontal axis indicates the distance from the center of the objective lens to the incident position of light, and the vertical axis indicates the optical path length difference at that position (the optical path length difference from the light transmitted through the center of the objective lens). I have. In the objective lens of the comparative example of FIG. 3A, the optical path length difference is set to be substantially 0 in all the regions. However, in FIG. 3B, the difference is larger than the portion corresponding to the numerical aperture NA _2. The value is set to be substantially constant on the outer peripheral side and to be continuously changed on the inner peripheral side. Here, in a portion where the optical path length difference on the outer peripheral side is substantially constant, the wavefront aberration is almost zero with respect to the DVD.
It is set to be.

FIG. 4 is a diagram showing an optical path length difference when a CD is reproduced using the two types of objective lenses shown in FIG. 4A shows a comparative example, and FIG. 4B shows the objective lens of the present embodiment. As shown in this figure, in the case of the present embodiment, the optical path length difference during CD reproduction is small. That is, the amount of wavefront aberration generated during CD reproduction is suppressed.

FIG. 5 is a diagram showing an optical path length difference when it is assumed that an optical recording medium having an optimum inner peripheral substrate thickness is reproduced by the objective lens of the present embodiment. 4A shows a comparative example, and FIG. 4B shows the objective lens of the present embodiment. As shown in this figure, in the case of the present embodiment, the optical path length difference is very small in the inner peripheral portion (the portion on the inner peripheral side from the portion corresponding to NA ₂ from the center) with respect to the inner peripheral portion optimum substrate thickness. become.

As described above, the objective lens according to the present embodiment has such a shape that the wavefront aberration generated on the outer peripheral side, which is a major problem during DVD reproduction, becomes substantially zero, and the inner peripheral side, which does not cause a major problem, is used. The shape is such that the wavefront aberration is minimized when the substrate is thicker than a DVD (DV
D has a wavefront aberration), so for a CD, the reproduction characteristics can be improved by using only this lens without adding any optical means, and the characteristics at the time of DVD can be improved. Is not impaired, that is, DV
The reproduction operation can be stably performed for both D and CD.

Further, since the outer peripheral portion and the inner peripheral portion change continuously (change so as not to have a discontinuity), the side rope does not become large and the increase in the light beam spot diameter is suppressed. it can.

Hereinafter, a specific example of such an objective lens will be described.

Generally, the objective lens 4 has an aspherical shape, and the aspherical expression of the lens is as follows: Z = r ² / R / (1+ (1− (1 + K) × (r / R) ² ) )
^0.5 ) + B × r ⁴ + C × r ⁶ + D × r ⁸ + E × r ¹⁰ +... Here, Z: Depth (distance in the optical axis direction) from the surface vertex (intersection between the light exit surface and the optical axis) r: Lens radial position (radial distance) R: Paraxial radius of curvature K : Conical constants B, C, D, E,...: Aspherical coefficients of r ²ⁿ (n is 2, 3, 4, 5,...).

As shown in FIG. 6, the objective lens 4 has a CD area A having a numerical aperture of 0.38 and a DVD area B having a numerical aperture of 0.6 (including the area of A). Generally, in the case of a CD, the laser wavelength is about 780 nm and the numerical aperture is about 0.45. Therefore, when the laser wavelength used for DVD is 650 nm, the numerical aperture is about 0.38.) It is designed as follows using the equation.

(1) First, the lens shape in the area B is adjusted to DVD (substrate thickness: 0.6 mm), and each coefficient is optimized. At this stage, the objective lens 4 is
The wavefront aberration is set to be extremely small with respect to D.

(2) Next, based on the optimized lens, only the paraxial radius of curvature R is optimized in the area A so as to match the CD, and the wavefront aberration is reduced as much as possible.

(3) In the lens after the end of the step (2), since the coefficient R is set in the area A so as to conform to the CD, a large aberration occurs when reproducing the DVD. Then, next, in the area B (entire area), a higher order (here, 8th to 16th order) is set so as to fit the DVD.
Only the aspheric coefficients up to are optimized.

By operating only the higher order coefficients as described above,
Even if the coefficient changes, the shape change amount of the inner peripheral portion of the lens becomes small. Therefore, even after the operation (3), the inner peripheral portion of the lens still has a shape suitable for CD reproduction. Therefore, the influence upon reproducing the CD is small.

Conversely, even if the inner peripheral portion of the lens has a shape corresponding to a CD, the amount of wavefront aberration generated thereby is small. For this reason, by optimizing only the higher-order coefficients as described above, the shape of the outer peripheral portion having a larger area can be changed to a shape suitable for DVD reproduction, and DVD reproduction can be performed well.

The focal length of the objective lens designed as described above satisfies the above condition. As an example of the wavefront aberration when DVD and CD are reproduced by this objective lens, for example, DVD: 0.02λ, CD:
It becomes about 0.05λ.

When optimized only for DVD (CD) like an objective lens used in a conventional pickup device, when reproducing DVD (CD), the wavefront aberration is 0.01 λ or less, and CD (DVD) )), The wavefront aberration was about 0.07λ, so that only one of them is used for reproduction. However, according to the objective lens of the present embodiment, both DVD and CD can be reproduced well. Can be.

In this embodiment, although the paraxial R is adjusted in (2) and the higher order coefficient of the aspherical surface is adjusted in (3), the present invention is not limited to this. Alternatively, the lower-order coefficients in the aspherical surface equation may be determined so as to be suitable for CD reproduction, and the higher-order coefficients may be determined so as to be suitable for DVD reproduction.

[0041] (Reference Example) Next, FIG. 7 is a diagram showing the structure of an objective lens used in pickups device as a reference example of the present invention. This objective lens 4 ′ has a ring-shaped light shielding band 6 provided outside the boundary of the CD area A in the first embodiment. In particular,
The portion where the ring-shaped light-shielding band is provided is a portion that has an adverse effect when reproducing a CD, and by removing the light in this portion, the recording / reproducing characteristics can be improved. Hereinafter, the objective lens 4 'will be described.

First, the above-mentioned adverse effects will be described using the objective lens described in the first embodiment (assuming that there is no light-shielding band 6). As shown in FIG. 7, the objective lens is denoted by A (inner area (CD area)), and C outside the boundary of the inner area
(Boundary area) and further outside D (outer peripheral area), the light flux that has passed through the CD area A during CD playback becomes a focused beam with less aberration on the signal surface of the disc. , The information signal is correctly received and returned to the photodetector. On the other hand, the light beam that has passed through the outer peripheral area D becomes a blurred beam with a large aberration during CD reproduction, and does not enter the photodetector. The luminous flux that has passed through the boundary area C is C
The luminous flux that has passed through the D area A is not so narrow but is not as blurry as it has passed through the outer peripheral area D, so that it enters the photodetector in the form of noise.

Therefore, in this embodiment, the boundary area C (the area outside the area corresponding to the numerical aperture corresponding to the CD) is covered with the light shield 5 to block the passage of the light beam.
Thus, by blocking the light passing through the boundary area C, the noise component can be removed, and the recording / reproducing characteristics can be further improved.

The width of the ring portion of the light-shielding band 6 formed in the boundary area C is in the range of about 0.05 mm to 0.2 mm. The larger the width, the better the recording / reproducing characteristics. When recording / reproducing a DVD or the like that uses the entire surface of the lens, loss of light use efficiency increases. Therefore, it is preferable to set the optimum width in consideration of both characteristics.

In the first and second embodiments, the case where the two types of optical recording media are a CD and a DVD has been described, but the present invention is not limited to this.

[0046]

According to the objective lens of the present invention, the outer peripheral portion is set so that the wavefront aberration is substantially zero with respect to an optical recording medium having a small substrate thickness, and the inner peripheral portion is formed of an optical recording medium having two types of substrate thicknesses. Since the minimum value is set for the intermediate optical recording medium, both of the two types of recording media can be recorded and reproduced favorably by one single-focus objective lens. In addition, since it is formed so as not to have a discontinuity point, the expansion of the side rope can be suppressed.

Further, the lower order coefficient of the aspherical surface type is set so as to correspond to an optical recording medium having a large substrate thickness, and the higher order coefficient is set so as to correspond to an optical recording medium having a small substrate thickness. Therefore, it is possible to cope with both types of substrates having different thicknesses, and it is possible to suppress the expansion of the side rope.

[0048]

Further, according to the optical pickup device using the objective lens as described above, a low cost optical pickup which is small, excellent in mass productivity, and excellent in reliability, does not require any newly added parts and mechanisms. Equipment can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an optical pickup device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between an inner peripheral portion optimum substrate thickness and a wavefront aberration.

FIG. 3 is a diagram illustrating a DV using the objective lenses according to the first embodiment and the comparative example.
FIG. 9 is a diagram illustrating an optical path length difference when reproducing D.

FIG. 4 shows a CD using the objective lenses of the first embodiment and the comparative example.
FIG. 6 is a diagram showing an optical path length difference when reproducing the image.

FIG. 5 is a diagram illustrating a difference in optical path length when an objective lens according to the first embodiment reproduces an optical recording medium having an inner peripheral portion having an optimum substrate thickness.

FIG. 6 is a schematic diagram of the objective lens of the present invention.

FIG. 7 is a schematic diagram of an objective lens used in an optical pickup device according to a reference example of the present invention.

[Explanation of symbols]

 4, 4 'objective lens 6 light-shielding zone A CD area B DVD area C boundary area D outer peripheral area

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) G11B ⁷ /12-7/135 G02B 3/00-3/10 G02B 13/00-13/18

Claims (3)

(57) [Claims] 1. A single-focus objective lens corresponding to two types of optical recording media having different substrate thicknesses, wherein the outer peripheral portion has substantially zero wavefront aberration generated for an optical recording medium having a smaller substrate thickness. The inner peripheral portion is set so that the wavefront aberration generated with respect to the intermediate substrate thickness of the two types of optical recording media is minimized, and is formed in a shape having no discontinuity. Objective lens featured. 2. An objective lens corresponding to two types of optical recording media having different substrate thicknesses, wherein the objective lens is represented by one aspherical formula shown in (Equation 1), and has a low lens radius position r. An objective lens wherein the next aspheric coefficient is determined according to an optical recording medium having a thick substrate, and the higher aspheric coefficient is determined according to an optical recording medium having a thin substrate. Z = r ² / R / (1+ (1− (1 + K) × (r / R) ² ) ^0.5 ) + B × r ⁴ + C × r ⁶ + D × r ⁸ + E × r ¹⁰ +... : Depth from surface vertex, r: lens radius position, R: paraxial radius of curvature, K: conical constant, B, C, D, E,...: Aspheric coefficient of r2n (n is 2, 3) , 4,5 ...) 3. An optical pickup device having a light source and a photodetector for receiving light reflected from the optical recording medium, and capable of recording and reproducing information on two types of optical recording media having different substrate thicknesses. The optical pickup device according to claim 1, wherein an objective lens for condensing a light beam from the light source on the optical recording medium comprises the objective lens according to claim 1 or 2 .