JPS61281210A - Aspherical single lens - Google Patents

Abstract

PURPOSE:To correct the aberration with a high accuracy, to have the sufficient operating distance, to be small-sized and light and to have the shape characteristic suitable for molding by being an aplanatic single lens and specifying the refractive index and the central wall thickness. CONSTITUTION:The lens is an aplanatic single lens whose first surface is the projected aspheric surface and whose second surface is the plane, the refractive index of the lens is in the range of 1.7-2.0 for the use wavelength (usually, 0.63-0.83mum) and the central wall thickness is in the range of 1-5mm. When the refractive index (n) is <1.7, a central wall thickness d1 suitable for removing the aberration comes to be lower and simultaneously, an edge surface angle thetacomes to be higher, the edge surface thickness comes to be extremely small, and therefore, molding comes to be extremely hard. When the refractive index (n) exceeds 2.0, the optimum central wall thickness d1 is increased, the lens weight comes to be higher, in addition, an operating distance W.D comes to be short and the lens is hard to be supplied for the practical use.

Description

Detailed Description of the Invention "Industrial Application Field" The present invention consists of a small and lightweight plano-convex aspherical single lens whose aberrations are within the diffraction limit of light and are corrected with high precision, and which is molded. The present invention relates to an aspherical single lens having shape characteristics suitable for use in, particularly, an aspherical single lens suitable for use as an objective lens for optical discs.

"Conventional technology" Recently, optical disc technology using semiconductor lasers has been rapidly developing, but the objective lens, which is incorporated as a condensing lens in the signal detection section of the optical disc, is used to read information recorded in high density. , it is necessary that axial spherical aberration be almost completely eliminated and at the same time the sine condition be satisfied. Furthermore, depending on the tracking method, it is desirable to reduce astigmatism as well.

Furthermore, since the optical disc lens is used in a movable part of the device, it must have a sufficient working distance from the disc and be small and lightweight.

Furthermore, since this type of lens must meet the demand for large quantities, it is important that it is easy to manufacture and has excellent mass productivity.

In order to meet these demands, for example,
8512, Japanese Patent Application Laid-open No. 57-201210, and Japanese Patent Application Laid-open No. 59-28714, an abrasive lens with aspherical surfaces on both sides has been proposed, but these lenses generally have It tends to have a complicated shape with a large difference in wall thickness at the end face. When such l/lens are obtained by molding technology, that is, a technology in which a heat-softened optical material is pressurized in a mold with a high-precision molding surface and molded in a single contact, the amount of plastic deformation of the material is large. Therefore, there is a drawback that the surface accuracy of the molded product is reduced and the mold is easily worn out. In addition, since these lenses tend to have a large amount of aspherical surface on both sides, not only do mold processing and measurement and evaluation of the molding surface require more advanced technology and complicated work, but also The technique of aligning the optical axes of both sides of the mold during lens molding also involves many difficulties.

Although composite lenses in which a plastic aspherical layer is formed on the glass surface are also known, this type of lens has the drawback of complicating the manufacturing process and making it less suitable for mass production.

[Problems to be Solved by the Invention] The present invention has been made in view of the above-mentioned situation, and its purpose is to correct aberrations with high precision, and to
- Provides an aspherical single lens that has a working distance of 100-minutes, is small and lightweight, and has shape characteristics suitable for molding, especially an aspherical single lens suitable for use as an objective lens for optical discs. It's about doing.

``Means for Solving the Problems'' In order to achieve the above purpose, the present inventor has comprehensively studied not only from the standpoint of optical design but also from the aspects of molding technology, measurement technology, etc. , in a plano-convex aspherical single lens, the refractive index (labor) of the lens medium is 1.7 to 2.
If it is within the range of 0, the optimum center wall thickness for aberration correction found corresponding to the refractive index (7'L) is in the range of 1 to 5 layers, and the height of the aspherical part is Δh. ) and end face angle (
The present inventors have discovered that since the lens θ) is small, it is possible to obtain a lens that is small and lightweight, has shape characteristics suitable for molding, and has a sufficient working distance, and has accomplished the present invention.

That is, the aspherical single lens according to the present invention is characterized by being an aplanatic single lens in which the first surface is a convex aspherical surface and the second surface is a flat surface, and the refractive index of the lens is similar to the wavelength used (usually is 0.63~0.83g+s), 1
．． 7 to 2.0, and the center wall thickness is 1 to 5 m.
It is located in the range of m.

Here, the convex aspherical surface is expressed by the following formula.

However, X is the length of a perpendicular drawn from a point on the curved surface to the tangent plane of the vertex, C is the curvature of the tangent surface at the vertex, H is the incident height, K is the conic coefficient, D, E, F, and G are the respective aspherical coefficients of the 4th to 10th orders.

In the aspheric lens of the present invention, the relationship between the refractive index (-rL), the center thickness (dx), the numerical aperture (NA'), and the focal length (CH) is d1=g (,NA', ) There is a functional relationship, and there is a tendency for the center wall thickness (dl) to increase almost linearly as the refractive index shifts from 1.7 to 2.0, but within the specification range, NA = 0. 4~0,6,
=3 to 59Il, it can take a suitable value in the range of dl-L to 5ffl. In addition, the refractive index (7?) is 1
．． If it is less than 7, the center wall thickness is optimal for removing aberrations (
When dl) becomes too small, the end face angle (θ) becomes too large and the end face thickness (dl−Δh) becomes extremely small, making molding extremely difficult. In addition, the refractive index (7L
) exceeds 2.0, the optimum center wall thickness (dl) increases and the lens weight becomes excessive, as well as the working pressure a (W
, []) become short, making it difficult to put it into practical use.

"Example" Next, Examples (Fb, 1 to 6) in which the aspherical single lens of the present invention is used as an objective lens for an optical disk.
and Reference Example A are shown in Table 1, and the positional relationship between the center cross section of the lens of the example and the cross section of the optical disk is shown in FIG.
Furthermore, the aberration curve diagrams of each lens of Examples and Reference Examples are shown below.
They are shown in FIGS. 2 to 8, respectively.

In these examples and reference examples, the refractive index (7L) of the semiconductor laser with respect to the wavelength 0.781Lw is 1.7184.
゜1.78B43, 1.89?68, 2. (10
and 1.60, and the lens specifications are numerical aperture (NA'); 0.53 or 0.
47. Optical calculations were performed for the cases where the focal length (f) was 3.79 or 4.5■■, and the effective beam aperture (a) was 4.0 or 4.2■. , the thickness of the coating material (polycarbonate) is 1
．． 2), and its refractive index was set to 1.55.

As shown in Fig. 1, dl, Δh, W, D, and θ in Table 1 indicate the center wall thickness, height of the aspherical surface, working distance, and end face angle, respectively, and R1 indicates the apex of the aspherical surface. R2 indicates the radius of curvature of the second surface.

As is clear from Table 1 and FIGS. 2 to 7, the aberrations of the lenses of the examples are all corrected to within the diffraction limit of light, and the aberrations are sufficiently corrected within the range of about 0.8 to 2.21 It has a working distance (LD), and a center wall thickness (dl) of approximately 1
It has a suitable value in the range of ~5 dragons, and is small and lightweight.

Furthermore, all of the lenses of the above examples have a small height Δh) of the aspherical part in the range of 0.5 to G, 8■■, and a small end face angle (θ) of 40° or less and 0. .8～
The lens has a simple shape and has a moderate end face thickness (dx-Δh) in the range of 3.7 mm. Although not shown in Table 1, the amount of aspherical surface itself is small. Therefore, when molding lenses, it is possible to easily form a high-precision surface by applying a small amount of plastic deformation to a simple-shaped optical material such as a plate. In this case, since the weight applied to the mold during pressurization is small, there is an advantage that the mold can be used repeatedly over a long period of time without damaging the precision molding surface of the mold. Furthermore, since Δh and θ are small, it is easy to process the precision molding surface of the mold, and furthermore, it is easy to measure the aspherical shapes of the mold and molded lens. When manufacturing these lenses, the optical axes of both surfaces can be easily aligned using a simple molding device. Furthermore, the signal detection section can be easily incorporated into the lens barrel by using the flat surface.

On the other hand, the lens of Reference Example A (n proverb 1.8) cannot satisfy the sine condition as shown in FIG.
As can be seen, the height of the aspherical part (Δh) is 0.87.
and a large end face angle (θ) of 47° and 0.03m.
Since it has an extremely small end face thickness (dl-Δh) of 5, the shape is inappropriate for molding.

Note that the lens of the example of the present invention has a refractive index (7t) of 1.
．． When it is 75 or more, the end face thickness (dx - Δh) is about 1.
When the refractive index (71) is 0 or more, the moldability is even better, and when the refractive index (71) is 1.9 or less, a large working distance (91,0)e of about 1.2 or more can be obtained. Therefore, when T·1.75 to 1.9, it is more suitable as an objective lens for an optical disc.

In the practice of the present invention, the refractive index (?1) is between 1.7 and 2.
Optical materials in the range of 0 are usually LaF-based, La5
It is preferable to appropriately select and use a glass suitable for pressure molding from among various types of optical glasses and tellurite glasses called F series and SF series.

"Effects of the Invention" As described above, the aspherical single lens of the present invention is an aplanatic single lens in which the first surface is a convex aspherical surface and the second surface is a flat surface, and the refractive index of the lens is It is in the range of 1.7 to 2.0 for the wavelength used, and the center thickness is 1 to 2.0.
5 am, and the end face angle is 40° or less, so aberrations are corrected with high precision, and it has a sufficient working distance, is small and lightweight, and can be molded and molded. It has suitable shape characteristics for measurement and evaluation.

Therefore, the aspheric single lens of the present invention is suitable for use as an objective lens for optical discs, and is highly suitable for mass production.

Note that the aspheric single lens of the present invention is not limited to the above-mentioned applications, and can be widely used, for example, as microlenses for electronic devices, lenses for optical transmission fibers, and the like.

(Margin below)

[Brief explanation of drawings]

FIG. 1 is a sectional view showing the positional relationship between an aspheric single lens and a disk according to an example of the present invention, and FIGS. 2 to 8 are aberration curve diagrams of each lens of the example and reference example. Patent applicant Saihara Co., Ltd. Figure 2 HA ” x O, 53 + IA' to, 531M,
tl −0,0[181 spherical convergence
Sine condition Non-heterosophical spherical aberration
Sine condition Astigmatism diagram 4 Spherical aberration Sine condition
Astigmatism Spherical aberration Sine condition Astigmatism diagram 6 Spherical aberration Sine condition
Astigmatism' dew 0.47 Spherical aberration Figure 8 1'-0,51 Spherical aberration Sine condition IN, 9-0.
0785 IN, H-0,0691 Astigmatism

Claims (3)

[Claims] (1) An aplanatic single lens in which the first surface is a convex aspherical surface and the second surface is a flat surface, and the refractive index of the lens is in the range of 1.7 to 2.0 for the wavelength used. 1. An aspherical single lens having a central wall thickness in the range of 1 to 5 mm. (2) The aspheric single lens according to claim 1, which has a refractive index of 1.75 to 1.9. (3) The aspherical single lens according to claims 1 and 2, in which the aspherical single lens is used as an objective lens for an optical disc.