JPH0462563B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention particularly relates to a recording/reproducing objective lens having a two-lens configuration suitable for focusing light from a light source directly onto the information recording surface of an optical information recording medium such as an optical disk. [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's 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. 9 and 10. It is being actively researched. In the case shown in Fig. 9, focusing is performed by moving only the objective lens 2, but the movement changes the performance after the objective lens 2 opens, so the imaging magnification cannot be increased too much, and the reference The imaging magnification was about -1/40 to -1/8. In recent years, optical systems for compact disc playback have required (1) a more compact optical system; (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 above optical system for the following reasons,
It has become clear that the optical system shown in FIG. 9 can be used at a standard imaging magnification of about -1/4. On the other hand, in the system shown in Fig. 10, 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. The lens best suited for use as an objective lens in the optical system shown in Figures 9 and 10 was published in Japanese Patent Publication No.
-There is one described in Publication No. 86018. However, the above-mentioned known example had a four-layer configuration and was expensive. In addition, in recent years, attempts have been made to reduce the number of lens components and reduce costs by correcting the spherical aberration inherent in spherical lenses by making the refractive surfaces of lenses aspheric. Among these, the one with two sheets is JP-A-Sho.
55-45084, JP 58-219511, JP 59-7917, JP 59-9619, JP 59-48724, JP 59-49512 ,
There is one described in JP-A-59-49513. However, these lenses were designed to be used as objective lenses in the optical system shown in FIG. Therefore, it is difficult to use when the distance between the light source and the information recording surface is small, and as shown in FIGS.
There is a need to develop an objective lens with good performance that can be used in the optical system shown in the figure. [Problems to be solved by the invention] This invention uses a two-element structure that is optimally used when the distance between the light source and the information recording surface is short, a smaller number of elements than the conventional one, and a minimum number of aspherical surfaces. The present invention aims to provide an objective lens for optical disks. [Means for Solving the Problems] The present invention has an objective lens configured such that the convex surface faces the light source side in order from the light source side, and the first lens has a positive refractive power, and the objective lens has a convex surface facing the light source side. The surface of the second lens opposite to the light source side is an aspherical surface, and the following condition (1) −0.4<f/r ₂ <0.06 (2) 0.32 <r ₃ /n ₂ f<0.5 (3) 1.7f<f ₁ <2.5f However, f: Combined focal length of the entire system f ₁ : Focal length of the first lens f ₂ : What is the light source side of the first lens? Radius of curvature of the opposite surface r ₃ : Radius of curvature of the light source side surface of the second lens n ₂ : Provides an objective lens for recording and reproducing an optical information recording medium, which satisfies the refractive index of the second lens. It is something to do. [Operation] In the objective lens of the present invention, it is necessary to convert divergent light with a large numerical aperture into convergent light using only two single lenses. The first lens is a spherical lens that has the function of converting diverging light into convergent light, and the negative spherical aberration that occurs at this time must be made as small as possible. Condition (1) is for this purpose; if the upper limit is exceeded and the surface of the first lens on the side opposite to the light source becomes concave and tight, the surface of the first lens on the side of the light source becomes convex in order to maintain the necessary refractive power. It gets tight. Therefore, the angle of incidence of the outermost ray with respect to the axial object point becomes large on each surface of the first lens, and a large amount of negative high-order aberration occurs. Conversely, if the surface of the first lens opposite to the light source side is convex and stiff, the refraction angle on this surface becomes large, and similarly, large negative higher-order aberrations occur. In order to correct this with the aspherical surface of the second lens on the side opposite to the light source, the amount of deformation of the aspherical surface becomes significantly large, making it difficult to process and manufacture the aspherical surface with high precision. Also, if the refractive power of the first lens is weakened, condition (1) can be satisfied.
Although the negative spherical aberration that occurs outside the range will be reduced, the burden on the refractive power of the second lens will increase, and if only the light source side surface of the second lens is made aspherical, it will not be possible to correct the sine condition. I can't. Condition (2)
is related to the apex radius of curvature r ₃ of the light source side surface of the second lens, which is an aspheric surface. In this invention, only one aspherical surface can be used, so in order to maintain good sine conditions, the second lens needs to have a shape that causes as little comatic aberration as possible. If it becomes larger than the upper limit, the sine condition becomes over, and if it exceeds the lower limit, it becomes under. Furthermore, the present invention requires that the focal length f ₁ of the first lens falls within the following range. 1.7f<f ₁ <2.5f (3) When the focal length of the first lens becomes longer than the upper limit, the sine condition cannot be corrected simply by making the light source side surface of the second lens aspheric. If the focal length of the first lens exceeds the lower limit and the focal length of the first lens becomes shorter, the height of the rays on the surface of the second lens opposite to the light source side becomes smaller, so the amount of deformation of the aspherical surface becomes larger, and the sine condition becomes smaller when the aperture is large. It becomes under. If this is forcibly corrected, the spherical aberration will be undervalued in the intermediate annular zone. [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.

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〔Effect of the invention〕

Figures 2 to 7 are examples 1 to 6.
It is a diagram of various aberrations. As is clear from these figures, aberrations have been sufficiently corrected as an objective lens for optical disks. In Examples 1 to 5, aberrations were corrected by setting the imaging magnification to -1/4, but these examples were used as objective lenses for both optical disk optical systems shown in FIGS. 9 and 10. The fact that such a lens can be realized with a two-element lens as shown in the cross-sectional view shown in Figure 1, and with a minimum number of aspherical surfaces, will help reduce the cost of optical disk optical systems. This can be said to make a major contribution. 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. Note that in optical systems for optical discs, optical elements such as polarizing beam splitters are often arranged on the light source side of the objective lens, but this can be accommodated by slight design changes to the above embodiments.

[Brief explanation of the drawing]

FIG. 1 is a sectional view including a cover glass of the objective lens of the present invention. Figure 2, Figure 3, Figure 4, Figure 5
6 and 7 are aberration diagrams of the first to sixth embodiments, respectively. FIG. 8 is a layout diagram of a conventional optical disk optical system. 9 and 10 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 Consisting of, in order from the light source side, a first lens with a convex surface facing the light source side and having a positive refractive power, and a second lens having a convex surface facing the light source side and having a positive refractive power,
The surface of the second lens opposite to the light source side is an aspherical surface, and the following conditions (1) −0.4<f/r ₂ <0.06 (2) 0.32<r ₃ /n ₂ f<0.5 (3) 1.7f＜f ₁ <2.5f However, f: Composite focal length of the entire system f ₁ : Focal length of the first lens f ₂ : Radius of curvature of the surface of the first lens opposite to the light source side r ₃ : Second lens An objective lens for recording and reproducing an optical information recording medium, characterized in that the radius of curvature n ₂ of the surface of the lens on the light source side satisfies the refractive index of the second lens.