JPH073505B2 - Optical disc lens - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a compact and high-performance optical disk lens, and more specifically, it uses an aspherical surface that is a quadric surface of revolution.
The present invention relates to an optical disc lens having a single-piece structure.

b. Prior art and its problems Lenses used for optical disks must have a large numerical aperture NA of 0.4 or more and a large aperture ratio, and must be corrected so that residual aberration is also within the diffraction limit. .
Also, in the actual usage of the lens, it is necessary to have good performance up to a field angle of about ± 1 °.

Conventionally, the lens for this purpose has been composed of three or more spherical glass lenses, but due to the demand for cost reduction, size reduction and weight reduction, this type of lens consists of a single lens and both surfaces are It has been proposed that the lens has an aspherical surface, or that it has two lenses and only one surface has a high-order aspherical surface. (JP-A-57-76512, 5)
However, in the case of using an aspherical single lens, it is not possible to correct aberrations to a practically sufficient amount of aberration unless both surfaces are made aspherical. And in the case of an aspherical lens with two surfaces, unlike a spherical lens, the tolerance for parallel movement and tilt of the surface becomes strict, and even if the design performance is excellent,
There is a problem that it is difficult to make a product that maintains design performance.

In addition, in the case of a double-sided aspherical single lens, lens performance cannot be satisfied unless at least one surface is an aspherical surface having a high-order aspherical coefficient. And
It has been generally difficult to measure the surface shape of a surface having such a high-order aspherical surface coefficient by highly accurate non-contact measurement such as interferometry.

In addition, the one having a two-lens configuration is also disclosed in
Since there is a term proportional to the sixth power of the incident height or more, such as −219511, an advantageous shape for surface shape measurement was not considered.

c. Objective In view of the above points, the present invention provides a compact lens for optical discs, which is less likely to cause performance deterioration due to eccentricity of the aspherical surface, and is capable of easily performing ultra-precision measurement of the aspherical surface shape, and having good mass productivity. Has an aim.

d. Means for Solving Problems As long as the lens is composed of one double-sided aspherical lens, the tolerance of the tilt of the lens and the ease of measuring the surface shape are not fundamentally solved. Each of the lenses is composed of two lenses, a first lens and a second lens, which are positive lenses having a convex surface on the incident side. The surface of the incident side surface (first surface) of the first lens is expressed by the following equation. Is a quadratic surface of revolution, Moreover, the optical disk lens is characterized by satisfying the following conditions (1) to (4).

(3) 0.0 <d ₂ /f<0.22 (4) -1 ≦ K <-0.3 where X (h): dropped from one point on the aspherical surface of height h from the optical axis to the tangent plane of the aspherical apex Vertical length h: Height from the optical axis r: Radius of curvature near the aspherical vertex (paraxial) K: Aspherical coefficient f: Focal length of the entire lens system f ₁ : Focal length of the first lens r ₁ : Paraxial radius of curvature of the first surface n ₁ : Refractive index of the first lens d ₂ : Air gap between the first lens and the second lens e. Action The optical disk lens is bright (NA0.5), and Since spherical aberration is required to be extremely small, a combination of three or more lenses is required when only spherical lenses are used. An aspherical surface is introduced in order to reduce the size of the lens. However, with one lens, the spherical aberration will always be under when the aspherical surface is not used.
Therefore, a large amount of aspherical surface is required to correct spherical aberration. When the amount of aspherical surface is shared by one surface, the occurrence of coma aberration is unavoidable. Therefore, in the end, a single-sided structure has to be a double-sided aspherical surface. Further, since the power can be dispersed only by the two surfaces, the radius of curvature of each surface naturally becomes small, and the eccentricity of the surfaces and the deterioration of the aberration due to the tilting tend to become large.

On the other hand, in the case of the two-lens configuration as in the present invention, the power can be dispersed to the two lenses and the aberration can be considerably reduced only by the spherical component of the lens. Although it is small, good performance can be obtained by making one of the four lens surfaces aspherical.

The condition (1) shows the power distribution of the two power lenses, and the conditional expression (2) shows the power component in the first lens. By satisfying these conditions (1) and (2), it is possible to reduce the amount of spherical aberration generated for each surface, reduce the amount of aspherical surface, and at the same time reduce the change in aberration with respect to decentering.

Conditional expression (3) is a condition that determines the air gap between the first lens and the second lens. By satisfying this condition (3), the sensitivity of the aberration change to the lateral deviation of the two lenses and the compactness of the lens system can be improved. Can be achieved.

When considering high-precision aspherical surface shape measurement technology, there are mainly contact type measurement and interferometry measurement. Interferometry is desirable for non-contact measurement, but at present, in general aspherical surfaces with a large amount of aspherical surface, the reflection on the surface does not return to the interferometer, and therefore contact measurement is often used. However, with an aspherical molding die, an aspherical lens made of plastic, or the like, the contact type may cause scratches, and it is not sufficient if the contact type can be used for measurement. Therefore, in the objective lens for the optical disk, it is necessary to shape the surface shape with high accuracy, so an aspherical surface shape that can be used in the non-contact type interferometry is adopted, and high-speed surface accuracy measurement, superdyne interferometry, etc. It is advantageous in manufacturing that high-precision measurement is possible.

In general, hyperboloid of revolution, paraboloid of revolution, ellipsoid of revolution, sphere,
For curved surfaces called planes of quadratic rotation and surfaces with asphericity within a few λ (lambda: wavelength of light beam used during measurement) from those surfaces, measure the aspherical surface shape by the interferometry method. Is possible.

Therefore, in the lens according to the present invention, the aspherical surface is configured by a quadric surface of revolution.

The relationship between the quadric surface of revolution and the aspheric coefficient K is as follows: K <−1: hyperboloid of revolution K = −1: paraboloid of revolution 1 <K <0: ellipsoid of revolution (focus is aligned on the optical axis) K = 0: spherical surface 0 <K: spheroidal surface (based on an ellipse whose focus is perpendicular to the optical axis rotated about the optical axis). It does not have advantageous characteristics for surface accuracy measurement.

Next, considering the asphericity required from the viewpoint of aberration of the lens, when only one surface of the two positive lenses is an aspherical surface, it is necessary to make the aspherical surface so that the spherical aberration is over. There is. This means that the lens has asphericity in the direction in which the edge of the lens becomes thicker than that of a spherical lens. When the aspherical surface is a quadratic surface of rotation, the incident side surface (first surface or third surface) of the lens is used. If the surface has an aspherical surface, K <
0, and if the fourth surface has an aspherical surface, k> 0. When the second surface, which has a large radius of curvature, is made aspherical, the aspherical surface of the quadratic surface of rotation cannot have a sufficient amount of aspherical surface.

Therefore, the surface which becomes an aspherical surface for the purpose of the present invention is limited to the first surface or the third surface. Furthermore, since the ease of making the aspherical measurement system depends on the divergence angle of the light beam incident on or reflected on the non-measurement surface, the first surface having a gentler curvature than the effective diameter of the lens is more advantageous. .

In this way, when the aspherical surface is the rotational quadric surface of the first surface,
When the lens is constructed so as to reduce the occurrence of aberrations due to decentering of the lens, particularly decentering between the first and second surfaces which are aspherical surfaces, it is desirable that the range of the aspherical surface coefficient K is approximately -0.9 <K <-0.3. However, performance deterioration due to eccentricity becomes greater at more or less than this.

However, considering the measurement by the interferometry, it is very advantageous to set the aspherical surface as a paraboloid of revolution with K = -1.

For example, when assembling an interferometer for measuring the aspherical surface (K = -0.56) of the first embodiment and the aspherical surface (K = -1.00) of the third embodiment, as shown in FIGS. 7 and 8, respectively. Although the type may be considered, the interferometer of FIG. 8 for a parabolic surface can be used because the incident light is parallel light, so the position of the incident light and the surface to be measured can be used only by bending the surface. There are advantages such as no vignetting of light.

Therefore, in the lens of the present invention, it is desirable that the aspherical surface coefficient of the first surface should satisfy the condition (4), that is, -1≤K <-0.3.

f. Examples Hereinafter, examples of the present invention (including a cover glass) will be described. Here, NA is the numerical aperture, f is the focal length, r is the radius of curvature of each surface, d is the distance between the surfaces, n is the refractive index, WD is the working distance, and k is the aspherical coefficient of the first surface.

[Example 1] NA = 0.47 f = 4.5 Angle of view ± 1.0 ° rd n (wavelength 780 nm) 6.800 1.10 1.78565 23.205 0.10 3.177 1.10 1.78565 6.367 2.48 (WD) ∞ 1.20 1.50000 ∞ aspherical coefficient K = 0.560 [Example 2] NA = 0.47 f = 4.5 Angle of view ± 1.0 ° r dn (wavelength 780 nm) 4.445 1.60 1.48479 −174.866 .10 2.798 1.10 1.48479 8.154 2.25 (WD) ∞ 1.20 1.50000 ∞ Aspheric coefficient K = −0.70 [Example 3] NA = 0.47 f = 4.5 Angle of view ± 1.0 ° rd n (wavelength 780 nm) 6.442 1.80 1.48479 −57.638 0.10 3.021 1.10 1.78565 5.838 2.37 (WD) ∞ 1.20 1.50000 ∞ Aspheric coefficient K = −1.000 g. Effect As explained above, The optical disk lens having an aspherical shape according to the present invention has an aspherical shape as shown in FIGS.
With an interferometer having an optical path as shown in the figure, it is possible to measure much more easily than with an ordinary aspherical lens, and the aberration variation due to decentering of the aspherical surface is extremely small as shown in FIG. , Is highly mass-producible as compared with conventional lenses for optical disks that use aspherical surfaces.

[Brief description of drawings]

FIG. 1 is a lens configuration diagram of Example 1 of the present invention, FIG. 2 is an aberration diagram of Example 1, FIG. 3 is a lens configuration diagram of Example 2 of the present invention, and FIG. 4 is an aberration of Example 2. 5 and 5 are lens configuration diagrams of Example 3 of the present invention, FIG. 6 is an aberration diagram of Example 3, and FIG. 7 is an interferometer for measuring an aspherical surface of Example 1 at K = −0.560. Schematic diagram, FIG. 8 is a schematic diagram of an interferometer for measuring the aspherical surface of K = −1.000 in Example 3, and FIG. 9 is Example 1.
5 is a graph showing a change in the amount of aberration with respect to decentering of the first surface of FIG. M1: Half mirror M2: Spherical reflection mirror M3: Thin film half mirror or mirror M4: Spherical reflection mirror O ₁ : Test aspheric surface K = −0.560 O ₂ : Test aspheric surface K = −1.000 R ₁ : Reference plane R ₂ : Reference plane L ₁ : Lens

Claims (1)

[Claims] 1. A first lens element of a positive lens having a convex surface on the light beam incident side and a second lens element of a positive lens having a convex surface on the light beam incident side. The light flux incident surface of the lens is a quadratic surface of revolution whose surface shape is represented by the following equation: X (h) = (h ² / r) / {1+ [1− (1 + K) (h ² / r ² ) ] ^1/2 } And the lens for optical discs characterized by satisfying the following conditional expressions (1), (2), (3) and (4). (1) 0.33 <f / f ₁ <0.55 (2) 0.25 <(n ₁ −1) ・ f / r ₁ <0.66 (3) 0.0 <d ₂ /f<0.22 (4) −1 ≦ K <−0.3 However, X (h): length of a perpendicular line drawn from a point on the aspherical surface at a height h from the optical axis to the tangent plane of the aspherical apex, h: height from the optical axis, r: near the aspherical apex (Paraxial) radius of curvature, K: aspherical coefficient, f: focal length of the entire lens system, f ₁ : focal length of the first lens, r ₁ : paraxial radius of curvature of the light-incident surface of the first lens, n _{1 is} the refractive index of the first lens, d _{2 is} the air gap between the first lens and the second lens.