JPH0462565B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] This invention relates to a large-diameter recording/reproducing objective consisting of two lenses suitable for focusing light source light directly onto the information recording surface of an optical information recording medium such as an optical disk. Regarding lenses. [Prior Art] The most common optical system used in recording and reproducing devices for information recording media such as optical disks in recent years is as shown in FIG. The parallel light is made into parallel light, and the light is focused onto the information recording surface 1 by the objective lens 2. In this optical system, focusing is performed by moving the objective lens 2 in the optical axis direction in response to surface wobbling of an optical disk or the like. This method has the advantage that the performance of the optical system remains unchanged even if the objective lens 2 is moved, but on the other hand, the optical system becomes expensive because it requires two lenses: the objective lens 2 and the collimator lens 3. There is a problem. Therefore, in order to reduce the cost of the optical system, a method has been developed in which the light from the light source 4 is focused directly onto the information recording surface 1 using the objective lens 2 without using a collimator lens, as shown in FIGS. 10 and 11. It is being actively researched. In the case shown in Fig. 10, focusing is performed by moving only the objective lens 2, but since the performance changes after the objective lens 2 is opened due to the movement, the imaging magnification cannot be increased too much, and the reference The imaging magnification was about -1/40 to -1/8. In recent years, regarding optical systems for compact disc playback, (1) There has been a demand for more compact optical systems. (2) By improving the quality of compact disks,
Even if the possible range of focusing is narrow, it is no longer a practical problem. As a result of reviewing the optical system for reasons such as
It has become clear that it is possible to use the optical system shown in Figure 0 at a standard imaging magnification of about -1/4. On the other hand, in the system shown in Fig. 11, focusing is performed by moving the unit 5 of the entire optical system including the light source 4 and the objective lens 2, and there is no change in the numerical aperture or performance deterioration due to focusing, but the unit is In order to make the light source 5 as light as possible, it is important to reduce the distance between the light source 4 and the information recording surface 1 while ensuring the necessary working distance. Therefore, the imaging magnification must be set to -1/6 to -1/2, which is larger than that of the optical system shown in FIG. Typical objective lenses used in the optical system shown in Figure 9 are:
JP-A-55-4048, JP-A-58-87521,
2nd group 3 described in Japanese Patent Application Laid-Open No. 59-174810
There is one that consists of two sheets. Furthermore, there are a wide variety of objective lenses that have been devised and proposed for these purposes. On the other hand, as an objective lens devised for use as the objective lens of the optical system shown in FIGS. 10 to 11, there is one described in Japanese Patent Laid-Open No. 59-86018. This is NA0.45 at imaging magnification -1/4.
According to the specifications, it has four elements in three groups, but since it is a three-group structure, the lens frame structure is complicated, and it can be said that it is an extremely difficult lens to make. Also, since the NA is 0.45, it can be used as an objective lens for compact disc playback, but its performance is insufficient for objects that require a numerical aperture of about 0.5 to 0.53, such as objective lenses for video disc playback. . Furthermore, the objective lens for DRAW and the objective lens for recording optical disks effectively utilize the energy of the light source, so the numerical aperture of the optical system shown in Figure 9 is
It is necessary to use a 0.25 to 0.3 collimator and to set the imaging magnification of the optical system, which includes the collimator lens and objective lens, to about -1/2. In other words, in order to omit the collimator lens in the optical system for recording DRAW or optical discs, the imaging magnification -
At 1/2, an objective lens with a very large diameter of NA0.5 is required, and the number of lens components must also be increased. Furthermore, as is well known, when the imaging magnification is -1x (equal magnification), the distance between the object and image is the minimum, so it is possible to realize the optical system shown in Figure 11 with an imaging magnification of -1x. desired. In recent years, attempts have been made to correct the spherical aberration inherent in spherical lenses by making the refractive surfaces of lenses aspheric, thereby reducing the number of lens components and reducing costs. Among these, the two-sheet structure is disclosed in Japanese Patent Application Laid-Open No. 55-45084,
58-219511, JP 59-7917, JP 59-9619, JP 59-48724, JP 59-49512, JP 59-49513 There are some listed. These lenses are lenses designed to be used as objective lenses in the optical system shown in Figure 9.
It cannot be used when the distance between the light source and the information recording surface is small. From the fact mentioned earlier about spherical lenses, if a lens with positive refractive power is added to the light source side of these objective lenses, the imaging magnification will be -1/4.
It is clear that it is easy to construct a lens with an NA of about 0.45. However, the lens has a three-group structure, which causes the problems mentioned above, and there was a desire for a bright objective lens for optical disks that can be optimally used when the distance between the light source and the information recording surface is small, while retaining the two-group structure. . [Problems to be solved by the invention] This invention was made in view of the above-mentioned situation.
It is an object of the present invention to provide an objective lens for an optical disk having a two-element structure that is optimally used when the distance between a light source and an information recording surface is short. [Means for Solving the Problems] The present invention provides an objective lens having, in order from the light source side, a first lens having a positive refractive power and at least one surface of which is an aspherical surface, and a first lens having a positive refractive power. The second lens has the following conditions: O<f/f ₁ <0.9 (1) 1.1<r ₃ /{(n ₂ −1) f ₂ }<3.0 ( 2) However, f: Composite focal length of the entire system f ₁ : Focal length of the first lens f ₂ : Focal length of the second lens r ₃ : Vertex radius of curvature of the light source side surface of the second lens n ₂ : Second lens The present invention provides an objective lens for recording and reproducing a large-diameter optical information recording medium, which is characterized by satisfying the refractive index of the lens. [Operation] When the numerical aperture and focal length on the image side are constant, the shorter the distance between the light source and the information recording surface, the larger the effective diameter of the lens becomes, making it difficult to correct aberrations. In objective lenses for optical disks, it is necessary to properly correct spherical aberration and sine conditions, but in the case of large-diameter objective lenses such as those mentioned above, aberration correction is insufficient with just one aspheric surface. . Therefore, it becomes necessary to make two or more of the four refractive surfaces of the two-element lens aspheric. In this invention, the first
Spherical aberration and sine conditions are corrected by making each of the lens and the second lens aspherical lenses. Condition (1) is a condition for determining the refractive power distribution of the first lens and indicates the range in which aberrations can be effectively corrected by making the lens aspheric. If the focal length of the first lens is shortened beyond the upper limit, the first lens will have a significantly large aperture compared to its focal length, resulting in large spherical aberrations and comatic aberrations. Even if you increase the number of aspherical surfaces to correct this, it will only increase the spherical aberration and unevenness of the sine condition, and even if the maximum value of spherical aberration can be kept small, it will not be possible to reduce the wavefront aberration. is possible in principle, but in practice it becomes more difficult to design. If the focal length of the first lens becomes longer than the lower limit, the second lens will conversely have a significantly larger aperture than its focal length, causing similar difficulties. When implementing the present invention by giving a specific imaging magnification m, the spherical aberration and comatic aberration occurring in the first lens and the second lens can be kept small if the following conditions are satisfied. 0.05<f/(1-|m|)<0.7 (1)' Condition (1)' indicates that the larger |m| becomes, the larger the refractive power of the first lens becomes. Furthermore, as condition (1)' shows, when |m| It is desirable to have a smaller shape. r ₃ is the vertex curvature radius of the light source side surface of the second lens,
The conditions shown below are for this purpose, where n ₂ is the refractive index of the second lens, and f ₂ is the focal length of the second lens. 1.1f ₂ <r ₃ /(n ₂ -1) < 3.0f ₂ (2) If the upper limit is exceeded, the second lens will have excessive comatic aberration. Moreover, when the lower limit is exceeded, the under coma aberration becomes large in the second lens. Of course, it is possible in principle to correct both spherical aberration and comatic aberration by making multiple surfaces aspheric, but since aberrations that occur beyond the range of condition (2) include many higher-order aberrations, , it is not easy to correct this well with an aspheric surface. Furthermore, the amount of aspherical surface becomes large, which is not preferable in terms of processing. The lens of this invention has a positive refractive power.
It is composed of sheets. Therefore, in the case of a spherical lens, the spherical aberration is insufficiently corrected in any case, and at least one of the aspherical surfaces must have the effect of overcoming the spherical aberration. This is Δi
is the difference in the optical axis direction between the aspherical surface at the outermost periphery of the effective diameter of the i-th refractive power and the reference spherical surface having the apex radius of curvature ri, and the case where the aspherical surface is displaced toward the light source as it moves away from the optical axis is When positive, condition (3)−
It is necessary to satisfy any one of 1 to (3)-4. Δ ₁ >0 (3)−1 Δ ₂ <0 (3)−2 Δ ₃ >0 (3)−3 Δ ₄ >0 (3)−4 Furthermore, to suppress the amount of asphericization of each refractive surface , it is desirable that all aspherical surfaces have the function of correcting spherical aberration. Condition (4) is for this purpose. Δ ₁ ≧0 Δ ₂ ≦0 Δ ₃ ≧0 Δ ₄ ≦0 (4) Even if condition (4) is not satisfied, if condition (3) is satisfied, the necessary condition for spherical aberration correction is satisfied, but in this case This makes the design unnecessarily difficult, and the total amount of asphericalization becomes large, which is not desirable. In addition, in order to properly correct astigmatism, d ₁ should be set as the first
It is desirable to satisfy condition (5) by setting the axial thickness of the lens, _d3 , as the axial thickness of the second lens. d ₁ +d ₃ /f>0.4 (5) When implementing the present invention by giving a more specific imaging magnification m, (d ₁ +d ₃ )/(1+|m|)f>0.9 (5)' It is desirable to do so. When the lower limit of (5) and (5)' is exceeded, the astigmatism becomes under-represented, and the end thickness of the lens becomes small, which causes great difficulty when manufacturing an aspherical lens by plastic injection molding or the like. [Example] Examples of the objective lens of the present invention will be shown below. The symbols in the table are: ri: radius of apex curvature of the i-th lens surface from the light source side di: spacing between the i-th lens surfaces from the light source side ni: refractive index of the i-th lens material from the light source side νi: light source Atsbe number for the d-line of the i-th lens material from the side In addition, in an orthogonal coordinate system with the apex of the surface as the origin and the optical axis direction as the X axis, the apex curvature is C, the conic constant is K, When the aspherical coefficient is Ai and Pi is the power of the aspherical surface (Pi>2.0) It is expressed as φ=√ ² + ² , (C=1/r). In addition, the value of cover glass G is also shown in the table. Hj is the effective radius of the j-th refracting surface and represents the height of the peripheral ray with respect to the on-axis object point. Aspheric amount Δj
When the aspherical shape is expressed as above, Δj=Xsp・j−XAS・j (j=1, 2, 3,
4) However Cj=1/rj Kj: Conic constant of the j-th surface Ai ^(j) : Aspheric coefficient of the j-th surface Pi ^(j) : Aspheric power of the j-th surface. When the refracting surface is a spherical surface, Δj=0.

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〔Effect of the invention〕

FIGS. 2 to 8 are diagrams of various aberrations of Examples 1 to 7. As is clear from these figures, sufficient aberration correction has been made for the objective lens for optical disks. Examples 1 to 5 have an imaging magnification of -1/2, an NA on the image side of 0.49 to 0.5, and an NA on the light source side of 0.25, making them ideal for the optical system shown in Figure 11 for recording DRAW and optical discs. This is the objective lens used. In Example 6, the imaging magnification is −1/
It is designed to have a 4x magnification, has an NA of 0.54, and is brighter than known objective lenses, so it can be applied to optical systems for video disc playback. Embodiment 7 is designed to have an imaging magnification of -1x and an object image distance that is the shortest. As described above, when the objective lens of the present invention is used, it is possible to make the optical system compact, so that it can sufficiently meet specifications that will be required in the future. Furthermore, since there are no restrictions on aberration correction for the lens constituent materials, it is possible to further reduce costs by using plastic injection molding technology or the like. In optical systems for optical discs, an optical element such as a polarizing beam splitter is often placed on the light source side of the objective lens, but this can be accommodated by making some design changes to the above embodiments. Also, when m=0, the present invention is effective if condition (1)' is followed, and it is possible to design a bright lens that can be used as an objective lens in the optical system shown in FIG.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of an objective lens and a cover glass according to an embodiment of the present invention. Figure 2, Figure 3, Figure 4,
FIG. 5, FIG. 6, FIG. 7, and FIG. 8 are various aberration diagrams for Examples 1 to 6, respectively. FIG. 9 is a layout diagram of a conventional optical disk optical system. FIGS. 10 and 11 are optical layout diagrams of an optical system using the objective lens of the present invention. 1: Optical disk (optical information recording medium), 2: Objective lens, 3: Collimator lens, 4: Light source, 5:
Light source unit.

Claims (1)

[Claims] 1. In order from the light source side, a first lens having positive refractive power and having at least one aspherical surface; and a first lens having positive refractive power and having at least one aspherical surface. It is composed of a certain second lens, and the following conditions are O<f/f ₁ <0.9 (1) 1.1<r ₃ /{(n ₂ −1) f ₂ }<3.0 (2) where f: composite focal point of the entire system Distance f ₁ : Focal length of the first lens f ₂ : Focal length of the second lens r ₃ : Radius of curvature of the apex of the light source side surface of the second lens n ₂ : Satisfies the refractive index of the second lens. Objective lens for recording and reproducing large-diameter optical information recording media.