JP3014311B2 - Objective lens system with variable disk substrate thickness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an objective lens system suitable for recording and reproducing a large-capacity optical information medium.

[0002]

2. Description of the Related Art It is effective to increase the NA of an objective lens for recording and reproduction of a high-density and large-capacity optical information medium. At this time, the amount of aberration increases due to the inclination of the optical axis of the lens. To prevent this, it is advantageous to reduce the thickness of the substrate of the optical disk. For the above reasons, attempts have recently been made to reduce the thickness of the disk substrate. On the other hand, a compact disk (CD) currently in wide use has a thick disk substrate of 1.2 mm. An optical disk device capable of recording and reproducing both the current compact disk and high density optical disk (SD) is required. However, when the substrate thickness of the disc changes, the spherical aberration changes remarkably, and it is impossible to satisfy both the compact disc (CD) and the high-density optical disc (SD) with one objective lens system.

Recently, a method of obtaining a multifocal point by adding a hologram lens to an objective lens is disclosed in
Although there are JP-A-98909 and JP-A-7-98431, the disadvantage of the decrease in light amount due to diffraction cannot be avoided. Japanese Patent Application Laid-Open No. Hei 7-153110 discloses a method for coping with a change in the thickness of a disk substrate by inserting a correction plate having a negative aspheric surface between a collimator lens and an objective lens. Correction plates corresponding to the number of disk substrates having different thicknesses are required, and the mechanism for taking in and out the correction plates becomes complicated. Also, it seems that a method of exchanging the set of the disk substrate and the objective lens seems to be performed. However, it is necessary to set the objective lens in a number different in the thickness of the disk substrate, and the economy and the complexity of the mechanism are avoided. Absent.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method capable of accommodating a large number of different disk substrate thicknesses with a simple mechanism, without losing the amount of light, and obtaining good performance.

[0005]

In recording and reproducing a large-capacity optical information medium, it is general to convert a divergent light from a laser light source into a parallel light flux from infinity by a collimator lens and form an image by an objective lens. However, the present invention eliminates the use of a collimator lens, and also eliminates aberrations that increase due to a change in the thickness of a disk substrate. It can be obtained with a simple configuration. That is, in the objective lens system having a variable disk substrate thickness according to the present invention, one or two positive lenses, a positive objective lens and a disk substrate are sequentially arranged from the light source side, and the objective lens system is caused by a change in the thickness of the disk substrate. In order to improve the aberration, the positive lens composed of one or two positive lenses is moved on the optical axis to improve the aberration, and the objective lens is moved in order to move the image point position due to a change in the thickness of the disk substrate. In an objective lens system in which the objective lens is focused by moving a very small amount on the optical axis and the aberration of the objective lens is improved in combination with a specific disk substrate thickness, the positive one or two positive lenses are replaced with a positive lens. When the focal length of the objective lens is f _M , the focal length of the positive single lens is f _C , and the radius of curvature of the positive single lens on the image side is r ₂ , 5.560 f _M = f _C or .705f _M = f _C or 6.710f _M = f _C or _{6.772f M = f C ···· (1} ) r 2 <0 ············ (2) The condition When the thickness of the disk substrate increases, the positive single lens is moved in a direction to approach the light source side, and when the thickness of the disk substrate decreases, the positive single lens approaches the image side. It is configured to move in a direction. Still another objective lens system having a variable disk substrate thickness according to the present invention includes a positive lens composed of one or two positive lenses, a positive objective lens, and a disk substrate sequentially arranged from the light source side. The aberration caused by the increase in aberration can be improved by moving the positive one or two positive lenses on the optical axis, and the objective lens can be moved by moving the image point position due to the change in the thickness of the disk substrate. Is moved by a small amount on the optical axis to focus the object lens, and the objective lens makes good aberration with respect to a convergent light beam (ultra-infinite light beam) directed toward the image side of the objective lens in combination with a specific disk substrate. in the objective lens system, the positive 1 or positive lens made of two a positive single lens, the focal length of the objective lens f _M, the focal length f _C of the positive single lens, said positive single Les Satisfies _{f C> 5f M ········ (6} ) r 2 <0 ·········· (2) becomes a condition when the radius of curvature of the image side's and r ₂ When the thickness of the disk substrate increases, the positive lens composed of one or two positive lenses is moved in a direction to approach the light source side, and when the thickness of the disk substrate decreases, the positive 1 or 2 It is configured to move a positive lens made of a sheet in a direction to approach the image side. Still another objective lens system having a variable thickness of a disk substrate according to the present invention includes a positive lens or a positive objective lens and a disk substrate, each of which comprises one or two positive lenses sequentially arranged from a light source side. The aberration caused by the increase in aberration can be improved by moving the positive one or two positive lenses on the optical axis, and the objective lens can be moved by moving the image point position due to the change in the thickness of the disk substrate. Is moved by a small amount on the optical axis to focus, and the objective lens is configured to improve the aberration in combination with a specific disk substrate thickness. The focal length of the composite positive lens is f _CT , the radius of curvature of the spherical surface is r ₁ , r ₂ , r ₃ , and r ₄ sequentially from the light source side, and the focal length of the objective lens is f _M. When f _CT > 4f _M (3) r ₂ <0, r ₄ <0 (4) The following condition is satisfied, and the thickness of the disk substrate is satisfied. When the thickness increases, the two positive lenses are moved in a direction to approach the light source side, and when the thickness of the disc substrate decreases, the positive lens is moved in a direction to approach the image side. I have.

[0008] The reason why aberration correction can be satisfactorily performed with respect to a change in the thickness of the disk substrate of the present invention will be described below. Here, it is assumed that it is well known in the art that a change in the thickness of the disk substrate significantly changes the spherical aberration of the objective lens. (Explanation 1) The spherical aberration of the positive objective lens changes depending on the object distance of the incident light. In contrast to spherical aberration with respect to an object at infinity, as the object approaches a positive objective lens at a finite distance, the undercorrected spherical aberration increases. In other words, it is a change in the near point of the spherical aberration, and the same holds true for a light beam at a far-infinite distance. (Explanation 2) Undercorrected spherical aberration occurs for a positive single lens on which a divergent light beam from a finite distance object point as a light source is incident. The amount slightly varies depending on the degree of the refracting power sharing between the light source side and the image side of the positive single lens.

The present invention has been developed by integrating, considering, and compiling the above. For recording and reproduction of a large-capacity optical information medium, the objective lens is set to have a high NA, the thickness of the disk substrate is specified to be small, and aberrations are eliminated to the utmost under this combination. In this case, the light beam incident on the objective lens may be any of a parallel light beam from an object at infinity, a divergent light beam from an object at a finite distance, and a convergent light beam (ultra-infinite light beam) directed toward an object in the image side direction of the objective lens.

Next, the lens arrangement in the optical system of the present invention will be described with reference to FIG. If the objective lens is combined with the specific disc substrate thickness mentioned above,
The focal point of the positive single lens and the distance to the light source are set so that the image point where the divergent light flux from the object point that is the light source is created by the positive single lens is obtained near the incident light flux used when designing the objective lens. Set. For the sake of simplicity, a case will be described where the combination of the objective lens and the specific disk substrate thickness corrects the aberration with respect to a parallel light beam from an object at infinity.

(A) When the thickness of the disk substrate is a specific value The light source is arranged so as to be slightly outside the front focal position of the positive single lens. This is because the undercorrected spherical aberration of the positive single lens is taken into account (described above). (b) When the disc substrate thickness is larger than a specific value Spherical aberration is overcorrected. If the positive single lens is moved so that the image point of the light source by the positive single lens becomes a divergent light beam from an object point at a finite distance with respect to the objective lens, the aberration can be improved (the above-described explanation 1). That is, the positive single lens is brought closer to the light source side, and the distance from the objective lens is increased accordingly. Also in this case, the spherical aberration due to the positive single lens described in the above description 2 is also a target to be corrected. (c) When the thickness of the disk substrate is smaller than a specific value Spherical aberration is insufficiently corrected. If the positive single lens is moved so that the image point of the light source by the positive single lens becomes a convergent light flux (ultra-infinite light flux) directed to a point in the image side direction of the objective lens, the aberration is improved (the above-described explanation 1). . That is, the positive single lens is moved away from the light source, and the distance from the objective lens is reduced accordingly. Also in this case, the spherical aberration due to the positive single lens described in the above description 2 is also a target to be corrected.

The above methods (a), (b) and (c) are based on the design basis of the combination of the objective lens and the specific disk substrate thickness when the object distance is a divergent light beam from an object at a finite distance and on the image side of the objective lens. Convergent light beam (ultra-infinite light beam) heading toward an object in the direction
Holds true for the With the method described above, spherical aberration can be corrected extremely well even when the disk substrate thickness changes at high density, large capacity, and high NA, but there is a slight effect on coma, For example, it is necessary to consider the performance deterioration due to the movement (shift) of the objective lens in the direction perpendicular to the optical axis. Therefore, at a high NA (SD), the change in the thickness of the disk substrate is within 20% of a specific value. desirable. For example, NA
= 0.6 and the specific thickness of the disk substrate is 0.6 mm, it is better to keep it at about ± 0.12 mm.

Next, when used for a compact disk (CD), the wavelength used is as long as 780 nm and NA = 0.
45 is the current state. Since the wavelengths of the high density optical disk (SD) are 650 nm, 635 nm, etc., when these wavelengths are used, the NA required for the compact disk (CD) is: when the operating wavelength is 650 nm, NA _CD = 0.45 · (650 / 780) = 0.375 When the used wavelength is 635 nm, NA _CD = 0.45 · (635/780) = 0.366, and even if the disk substrate thickness changes greatly from 0.6 mm to 1.2 mm. Sufficiently high performance is obtained. For this purpose, it is advisable to insert a stop between the positive single lens and the objective lens. It should be noted that a change in the position of the entire system image point with respect to a change in the thickness of the disk substrate is performed by a minute movement of the objective lens.

Next, conditional expression (1) will be described. In the present invention, since the collimator lens is omitted, the positive single lens also has the function of the collimator lens. Conditional expression 1 defines the relationship between the focal length of the positive single lens and the focal length of the objective lens. Assuming that the focal length of the positive single lens is f _C , NA is NA _C , the focal length of the objective lens is f _M , and NA is NA _M , if there is no aberration in both, f _C / f _M = NA _M / NA _C ... (5) That is, if the objective lens is NA _M =
In the case of 0.6, the lower limit of NA _C of the positive single lens is such that the lower limit of conditional expression (1) is f _C > 5f _M and the objective lens has NA _M = 0.
In the case of 6, NA _c <0.12, which prevents the burden on the NA on the light source side from becoming excessive. Each upper limit of condition (1) is Example 1 than each embodiment, f _C / f _M = 6.710 Example 2, f _C / f _M = 6.705 Example 3, f _C / f _M = 6.772 In the fourth embodiment, f _C / f _M = 6.705 In the fifth embodiment, f _C / f _M = 5.560 Each NA _c is 0.089 to 0.108, and the light source is used. An excellent aberration characteristic, which will be described later, is provided without increasing the load on the NA on the side.

Conditional expression (2) is for preventing the performance from deteriorating due to mutual decentration between the positive single lens and the objective lens.
When the objective lens is moved (shifted) in the direction perpendicular to the optical axis by tracking or the like, there is no problem if the positive single lens can also be shifted at the same time. However, when only the objective lens is shifted to prevent complication of the mechanism, the performance due to eccentricity A drop occurs. This amount varies depending on the degree of refraction of the positive single lens on the light source side and the image side. When the image-side refractive power distribution is reduced, the performance decreases due to the shift of the objective lens.
When the value exceeds the range of the conditional expression (2), this tendency increases. The meaning of conditional expression (6) is not different from the meaning of the lower limit of conditional expression (1).

The conditional expression (3) is a conditional expression when the positive single lens is constituted by two positive lenses. By using the above-described two positive single lenses, it is possible to increase the amount of light by increasing the NA on the light source side. If the NA of the movable lens in this two-element configuration is N _CT , the N of the objective lens
When A _M = 0.6, NA _CT <0.
In the range of 15, the NA on the light source side can also be increased.

Conditional expression (4) is intended to prevent performance degradation due to mutual eccentricity between the movable lens and the objective lens when the positive single lens is constituted by two positive lenses. When the positive refracting power of the image-side surface of both lenses becomes weak, performance degradation due to the shift of the objective lens increases. When the value exceeds the range of the conditional expression (4), this tendency increases. The objective lens system according to the present invention includes a single positive lens or two positive lenses, and employs an aspheric surface as the movable lens that moves when the thickness of the disc substrate changes, thereby increasing the NA on the light source side. Improving performance does not depart from the scope of the present invention. However, when only the objective lens shifts due to tracking or the like, it is necessary to consider performance maintenance due to mutual eccentricity between the movable lens and the objective lens.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Tables 1 to 8 show Embodiments 1 to 8 of the objective lens system of the present invention having a variable thickness of the disk substrate. The symbols in the table are as follows.

[0017] r _i: sequentially radius of curvature radius or aspheric vertex curvature of the spherical d _i: sequentially lens thickness or air distance n along the optical axis _i: refractive index at successively lens material of wavelength 650nm of t: Thickness on the optical axis of the disk substrate nb: Refractive index of the material of the disk substrate at a wavelength of 650 nm WD: Working distance L ₁ : Distance on the optical axis from the first lens to the light source f: Focal length of the whole system f _C : Positive F _CT : Focal length when two positive single lenses are formed f _M : Focal length of objective lens NA: NA of the whole system NA _{M M} : NA NA _{C of} objective lens: Light source side NA L _1M : Object distance used when designing the objective lens (when a divergent light beam from a finite distance object enters (-)) The aspherical surface equation is given by: x: at the vertex of the lens surface at a point on the aspherical surface Distance from tangent plane h: Height from the axis C: curvature of the aspheric vertex (C = 1 / r) K : conical constant A _2i: When an aspheric coefficients

(Equation 1) It is represented by Note that the effective diameter of the objective lens when the disk substrate thickness t = 0.6 is also used for t = 0.5 and t = 0.7, and when t = 1.2, the diaphragm (the front of the objective lens 2) is used.
Position). (Hereinafter referred to as margins.)

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[Table 5]

[Table 6]

[Table 7]

[Table 8]

The objective lenses in Examples 1 to 5 and Example 8 are common and have a good aberration with respect to an object at infinity (L _1M = ∞) at a specific disk substrate thickness of 0.6 mm. It is shown in FIG. FIGS. 3 and 4 show aberration curves of the first embodiment, FIGS. 5 and 6 show aberration curves of the second embodiment, FIGS. 7 and 8 show aberration curves of the third embodiment, and FIGS. 7 and 8 show aberration curves of the fourth embodiment. 9 and 10 show aberration curves of Example 5 in FIGS. 11 and 12. FIG. The objective lens in Example 6 has a good aberration with respect to a divergent light beam (L _1M = −300) from a finite object 300 mm before the first surface of the objective lens at a specific disk substrate thickness of 0.6 mm. An aberration curve is shown in FIG. 13, and aberration curves of the whole system are shown in FIGS. The objective lens in the seventh embodiment has an aberration with respect to a convergent light beam (ultra-infinite light beam) (L _1M = 300) directed to an object located 300 mm on the image side from the first surface of the objective lens at a specific disk substrate thickness of 0.6 mm. This aberration curve is shown in FIG. 16 under the condition of good, and the aberration curves of the whole system are shown in FIGS. 17 and 18. In Example 8, a positive single lens is composed of two positive lenses. FIG. 19 is a sectional view of the configuration, and FIGS. 20 and 21 show aberration curves of the entire system. It can be seen that good performance is obtained in any of the examples even when the thickness of the disk substrate changes.

[0019]

As described above, the objective lens system having a variable thickness of the disk substrate according to the present invention has a small number of components and has a very simple mechanism for recording / reproducing a high-density, large-capacity optical information medium. It can sufficiently cope with the continuous variation of the thickness of the disk substrate, and can have good performance. The present invention can be considered as a zoom objective lens system in which the thickness of the disk substrate is a variable, can cope with a large number of disk substrate thicknesses, and does not have the drawback of light quantity reduction due to diffraction.

[Brief description of the drawings]

FIG. 1 is a sectional view of a configuration of a first embodiment of a variable thickness substrate objective lens system according to the present invention.

FIG. 2 is an aberration curve diagram in Examples 1 to 5 and Example 8 when a disk substrate thickness of an objective lens is 0.6.

FIG. 3 is a diagram showing a case where the disk substrate thickness of Example 1 is (a) is not more than 0.
6B are aberration curve diagrams at 1.2.

FIG. 4 is a diagram showing a case where the disk substrate thickness in Example 1 is (a) is not more than 0.
5 (b) is an aberration curve diagram at 0.7.

FIG. 5 is a diagram showing a case where the thickness of the disk substrate in Example 2 is (a) is 0.
6B are aberration curve diagrams at 1.2.

FIG. 6 is a diagram showing a case where the thickness of the disk substrate in the second embodiment is (a) is 0.
5 (b) is an aberration curve diagram at 0.7.

FIG. 7 shows that the disk substrate thickness of (a) in Example 3 is 0.
6B are aberration curve diagrams at 1.2.

FIG. 8 is a diagram showing a case where the disk substrate thickness in Example 3 is (a) 0.
5 (b) is an aberration curve diagram at 0.7.

FIG. 9 shows that the disk substrate thickness of (a) in Example 4 is not more than 0%.
6B are aberration curve diagrams at 1.2.

FIG. 10 is a diagram showing a case where the disk substrate thickness of Example 4 is (a) is not more than 0.
5 (b) is an aberration curve diagram at 0.7.

FIG. 11 shows that the disk substrate thickness of (a) in Example 5 is not more than 0%.
6B are aberration curve diagrams at 1.2.

FIG. 12 shows that the disk substrate thickness of (a) in Example 5 is not more than 0%.
5 (b) is an aberration curve diagram at 0.7.

FIG. 13 is an aberration curve diagram in Example 6 when the disk substrate thickness of the objective lens is 0.6.

FIG. 14 shows that the disk substrate thickness of (a) in Example 6 is not more than 0.5.
6B are aberration curve diagrams at 1.2.

FIG. 15 shows that the disk substrate thickness of (a) in Example 6 is equal to 0.
5 (b) is an aberration curve diagram at 0.7.

FIG. 16 is an aberration curve diagram at the disk substrate thickness of 0.6 of the objective lens in the seventh embodiment.

FIG. 17 is a diagram showing a case where the disk substrate thickness of Example 7 is (a) 0.
6B are aberration curve diagrams at 1.2.

FIG. 18 is a diagram showing a case where the disk substrate thickness of Example 7 is (a) is not more than 0.5;
5 (b) is an aberration curve diagram at 0.7.

FIG. 19 is a configuration sectional view of an eighth embodiment.

FIG. 20 shows that the disk substrate thickness of (a) in Example 8 is 0.5.
6B are aberration curve diagrams at 1.2.

FIG. 21 is a diagram showing a case where the disk substrate thickness of Example 8 is (a) is not more than 0%.
5 (b) is an aberration curve diagram at 0.7.

Claims (3)

(57) [Claims] 1. A positive lens comprising one or two positive lenses, a positive objective lens, and a disk substrate are sequentially disposed from a light source side, and the positive lens is provided for an increase in aberration due to a change in thickness of the disk substrate. By moving one or two positive lenses on the optical axis, the aberration can be improved, and the objective point can be moved by a small amount on the optical axis to move the image point position due to the change in the thickness of the disk substrate. In an objective lens system in which the objective lens is focused and the aberration is improved in combination with a specific disk substrate thickness, the positive lens composed of one or two positive lenses is a single positive lens, and the focal length of the objective lens is Is f _M , the focal length of the positive single lens is f _C , and the radius of curvature of the positive single lens on the image side is r _2. 5.560 f _M = f _C or 6.705f _M = f _C or 6. 710f _M = f _C or It is to satisfy the _{6.772f M = f C ······ (1} ) r 2 <0 ············ (2) The condition, the thickness of the disc substrate increases When, the positive single lens is moved in a direction to approach the light source side, and when the thickness of the disc substrate is reduced, the positive single lens is configured to move in a direction to approach the image side. Objective lens system with variable disk substrate thickness. 2. A positive lens comprising one or two positive lenses, a positive objective lens, and a disk substrate are sequentially arranged from a light source side, and the positive lens is provided for an increase in aberration caused by a change in thickness of the disk substrate. By moving one or two positive lenses on the optical axis, the aberration can be improved, and the objective point can be moved by a small amount on the optical axis to move the image point position due to the change in the thickness of the disk substrate. In an objective lens system which focuses and makes the objective lens have good aberration with respect to a convergent light beam (ultra-infinite light beam) directed toward the image side of the objective lens in combination with a specific disk substrate, When the positive lens made of a single lens is a positive single lens, the focal length of the objective lens is f _M , the focal length of the positive single lens is f _C , and the image-side radius of curvature of the positive single lens is r ₂ f _C > 5f _M (6) r ₂ <0 (2) When the following condition is satisfied and the thickness of the disk substrate increases, Positive one
Alternatively, the positive lens composed of two lenses is moved in a direction to approach the light source side, and when the thickness of the disk substrate decreases, the positive lens composed of one or two positive lenses is moved in a direction approaching the image side. An objective lens system having a variable thickness of a disk substrate. 3. A positive lens comprising one or two positive lenses, a positive objective lens, and a disk substrate are sequentially disposed from a light source side, and the positive lens is prevented from increasing aberration caused by a change in thickness of the disk substrate. By moving one or two positive lenses on the optical axis, the aberration can be improved, and the objective point can be moved by a small amount on the optical axis to move the image point position due to the change in the thickness of the disk substrate. In an objective lens system in which the objective lens is focused and the aberration is improved in combination with a specific disk substrate thickness, the positive one or two positive lenses are composed of two positive lenses. When the focal length of the lens is f _CT , the radius of curvature of the spherical surface is r ₁ , r ₂ , r ₃ , and r ₄ sequentially from the light source side, and the focal length of the objective lens is f _M , f _CT > 4f _M. (3) r ₂ <0, r ₄ <0... (4) When the following condition is satisfied and the thickness of the disk substrate is increased, the two positive lenses are moved in a direction approaching the light source side, and the disk substrate is moved. An objective lens system having a variable thickness of a disk substrate, wherein the positive lens is moved in a direction to approach the image side when the thickness of the lens decreases.