JP3306170B2 - Chromatic aberration correction element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an element for correcting chromatic aberration of an optical system, and more particularly to a chromatic aberration correcting element used in combination with an aspherical single lens in which aberration other than chromatic aberration has been corrected.

[0002]

2. Description of the Related Art In recent years, objective lenses for optical disks have
To reduce weight, single-sided aspherical lenses are being used. However, an aspheric single lens cannot correct chromatic aberration.

[0003] A semiconductor laser used as a light source of an optical disk device shifts its emission wavelength due to a change in output power or a change in temperature. For this reason, if the chromatic aberration of the objective lens is not corrected, the focusing position of the light beam changes due to the shift of the wavelength, and there is a possibility that an error occurs in reading and writing of information.

[0004] To solve this problem, the present inventors have
Invented a chromatic aberration correction element in which two or three glass lenses are bonded (Japanese Patent Application Laid-Open No. Hei 3-155514,
55515 publication). By using this chromatic aberration correcting element in combination with an aspherical single lens, a lens which is not affected by wavelength fluctuation can be provided in a relatively small size.

[0005] These chromatic aberration correction elements are arranged in an almost afocal portion of the optical system, and have the function of changing parallel light into divergent light or convergent light depending on the wavelength so as to cancel axial chromatic aberration of the objective lens. I have.

A lens whose chromatic aberration is to be corrected is generally a positive lens whose spherical aberration is corrected at a single wavelength. The focal length of the positive lens is short when the wavelength is short in the vicinity of visible light, and is long when the wavelength is long. Therefore, in order to cancel the axial chromatic aberration and prevent the focus position from moving, the light beam entering the positive lens may be divergent light in the case of short wavelength light and convergent light in the case of long wavelength light.

[0007]

However, an optical system using the above-described conventional chromatic aberration correcting element can correct axial chromatic aberration, but since the spherical aberration changes with a change in wavelength, the width of the wavelength change is particularly large. When is large, good performance could not be maintained at both wavelengths before and after the change.

The spherical aberration of the positive lens corrected at the reference wavelength is under for short-wavelength light having a high refractive index, and excessive for long-wavelength light having a low refractive index. This is the change in spherical aberration due to wavelength.

On the other hand, when the parallel light changes to divergent light or convergent light due to the operation of the conventional chromatic aberration correction element, the change is such that the object point at infinity from the positive lens side changes to a finite distance. Therefore, the spherical aberration changes. Due to this change, when the light incident on the positive lens becomes divergent light, the spherical aberration becomes under, and when it becomes convergent light, it becomes over. This is the change in spherical aberration caused by the action of the chromatic aberration correction element.

Since the changes in spherical aberration due to these two factors appear in the same direction, they cannot be corrected by an optical system using a conventional chromatic aberration correction element.

When the operating wavelength band is narrow, the change in the oscillation wavelength of the semiconductor laser is small, the change in the spherical aberration is also small, so that there is little problem. However, the change in the wavelength in a wider range, for example, the near-infrared semiconductor laser ( 780nm) and visible red semiconductor laser (680nm), or He-Ne laser (633nm) and YAG laser SHG wave (532nm) when switching and using light sources that are not close to each other, or simultaneously using multiple wavelengths In the case of using, for example, the change of the spherical aberration becomes large, so that some measure is required.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and is intended to correct axial chromatic aberration generated by a positive lens and to switch and use a light beam having a wavelength not approaching. Another object of the present invention is to provide a chromatic aberration correction element capable of suppressing a change in spherical aberration to a small value.

[0013]

In order to achieve the above object, a chromatic aberration correcting element according to the present invention generates a divergent wavefront having a spherical aberration that is excessive with respect to the incidence of a parallel light beam having a wavelength shorter than a reference wavelength. In addition, a converging wavefront having an under spherical aberration is generated when a parallel light beam having a wavelength longer than the reference wavelength is incident. That is, the
Material with almost no difference in refractive index at reference wavelength and different dispersion
It consists of a positive lens and a negative lens
And the radius of curvature of the bonding surface is far away from the optical axis.
Therefore, it is characterized by an aspheric surface whose absolute value is small.
Or at least one of the light incident and exit end faces
A plane perpendicular to the axis is defined as a concentric annular zone with respect to the optical axis.
It is configured as a diffraction type formed in a step shape,
The base curve, which is the macroscopic curvature of the formed surface,
The absolute value decreases as the radius of curvature moves away from the optical axis
And a point at a distance h from the optical axis in the optical axis direction.
Assuming that the displacement amount of the base curve is ΔX (h),
Amount Δ of a point at a distance h from the optical axis of the surface formed in the shape of a circle
X '(h) is given by equation (3). ΔX ′ (h) = (mλ0 / (n−1)) Int ((ΔX (h) / (mλ0 / (n−1))) + 0.5) (3) where m is an integer and n is Refractive index, λ0,
A wavelength using the positive element or any one wavelength within the wavelength range,
Int (x) is a function that gives an integer that does not exceed x.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the chromatic aberration correcting element according to the present invention will be described.

In order to correct the change in spherical aberration due to the wavelength of the positive lens and the spherical aberration caused by the incidence of divergent light and convergent light on the positive lens, a surface having a chromatic aberration correction effect must be provided. What is necessary is just to make it the shape which generates a spherical aberration. Therefore, the chromatic aberration correction element according to the present invention generates a divergent wavefront having an excessive spherical aberration with respect to the incidence of a parallel light beam having a wavelength shorter than the reference wavelength, and generates a divergent wavefront with a parallel light beam having a wavelength longer than the reference wavelength. It is configured to generate a focused wavefront having an under spherical aberration.

As a specific type of the chromatic aberration correcting element, there are a refraction type formed by bonding a positive lens and a negative lens using two kinds of materials having almost no difference in refractive index at the reference wavelength and having different dispersions. A diffraction type in which at least one of the light incident and emission end faces is formed in a stepwise shape with a plane perpendicular to the optical axis as a concentric annular zone with respect to the optical axis is considered. And, in the case of the refraction type, the absolute value of the base curve, which is a macroscopic curve of the surface formed stepwise in the case of the diffraction type, becomes smaller as the radius of curvature becomes farther from the optical axis. By using an aspheric surface, the above-described spherical aberration can be generated.

In general, low-order spherical aberration has a form of a fourth-order function with respect to the incident height. Therefore, if the surface of the color correction element is a surface having a fourth-order asphericity, the change in spherical aberration is substantially reduced. Can be corrected. However, when an aspherical single lens is used as the positive lens to be corrected, if the aspherical surface of the chromatic aberration correction element is an aspherical surface close to a spheroid having a positive conical constant, a higher-order spherical aberration change component Can be corrected.

The aspherical surface close to the spheroidal surface is:
When the displacement ε (h) from the spheroid at a point at a distance h from the optical axis is expressed by the equation (1), for all distances h within the effective maximum radius of the passing light beam, the case of the refraction type To
In the case of the diffraction type, it is desirable to satisfy the condition of the expression (4).

[0019]

１ (h) = ΔX (h) −Ch ² / (1−√ (1- (1 + K) C ² h ² )) (1) | ε (h) | <λ / ΔnMAX (2) ) | Ε (h) | <λ / (n−1) (4) where ΔX (h) is the amount of sag of the aspheric surface, C is the paraxial curvature,
K is a conical constant, λ is the maximum working wavelength, ΔnMAX is the absolute value of the refractive index difference in the state where the refractive index difference of the medium before and after the bonding surface is the largest in the working wavelength band, and n is the refractive index It is.

In the case of a diffractive chromatic aberration correction element, the displacement of the base curve in the direction of the optical axis at a distance h from the optical axis is defined as ΔX (h) from the optical axis of the step-shaped surface. The displacement amount ΔX ′ (h) at the point of distance h is given by equation (3).

[0021]

ΔX ′ (h) = (mλ0 / (n−1)) Int ((ΔX (h) / (mλ0 / (n−1))) + 0.5) (3) where m is an integer , N is the refractive index, λ0 is the wavelength at which the chromatic aberration correcting element is used, or any one wavelength within the wavelength range,
Int (x) is a function that gives an integer that does not exceed x.

Equation (2) is a condition in which the optical path length difference is 1λ or less when a refraction type chromatic aberration correction element is used. Similarly, equation (4) is a condition when a diffraction type chromatic aberration correction element is used. ,
The condition is that the optical path length difference is 1λ or less. If the difference exceeds the upper limit, the wavefront aberration rms value exceeds 0.1λ, so that it cannot be used for optical information recording / reproduction.

FIG. 1 is a lens diagram of a positive objective lens which is corrected by the chromatic aberration correction element of the embodiment 1-3. The specific numerical configuration is shown in Table 1. In the table, NA is the aperture ratio, f is the focal length, ω is the half angle of view, fb is the back focus, r is the radius of curvature, d is the lens thickness or air gap, n
i is the refractive index at the wavelength inm, and ν is the Abbe number. In addition,
The first surface and the second surface represent an objective lens having both aspheric surfaces, and the third surface and the fourth surface represent a cover glass of the optical disk.

The aspheric surface has a distance X from the tangent plane of the aspherical vertex of a coordinate point on the aspherical surface having a height from the optical axis Y, a curvature C of the aspherical vertex (1 / r), and a conical shape. Coefficients K, 4th, 6th, 8
Next, assuming that the tenth and tenth order aspherical coefficients are A4, A6, A8, and A10, they are expressed by the following equations.

[0025]

Equation 3] ^{X = (CY 2 / (1} + √ (1- (1 + K) C 2 Y 2))) + A4Y 4 + A6Y 6 + A8Y 8 + A10Y 10

Table 2 shows the conic coefficients and aspheric coefficients. Figure 2 shows spherical aberration SA, sine condition SC, wavelength 780n
m, chromatic aberration indicated by spherical aberration at 680 nm.

[0027]

[Table 1] NA = 0.55 f = 3.00 ω = 1.4 ° fb = 1.088 Surface number rd n588 ν n780 n680 1 1.894 2.200 1.49700 81.6 1.49282 1.49461 2 -4.186 1.088 3 ∞ 1.200 1.58547 29.9 1.57346 1.57834 4 ∞

[0028]

[Table 2]

[0029]

Embodiment 1 FIG. 3 shows an optical system in which a refraction type chromatic aberration correcting element according to Embodiment 1 of the present invention is combined with the objective lens shown in FIG. In the chromatic aberration correction element of Example 1, the bonding surface r2 is an elliptical surface, and ε (h) is 0 within the effective diameter. The specific numerical configuration of this optical system is shown in Table 3. The first to third surfaces are chromatic aberration correction elements, and the fourth and fifth surfaces are
The surface is an objective lens, and the sixth and seventh surfaces are cover glasses of the optical disk. In this example, the second, fourth, and fifth surfaces are aspherical surfaces, and their aspherical surface coefficients are shown in Table 4. FIG.
Indicates spherical aberration and chromatic aberration due to this configuration.

[0030]

[Table 3] FNO = 1: 0.9 f = 3.00 ω = 1.4 ° fb = 0.00 Surface number rd n588 ν n780 n680 1 ∞ 2.000 1.75500 52.3 1.74523 1.74940 2 -4.400 1.000 1.76182 26.5 1.74404 1.75132 3 ∞ Optional 4 1.894 2.200 1.49700 81.6 1.49282 1.49461 5 -4.186 1.088 6 ∞ 1.200 1.58547 29.9 1.57346 1.57834 7 ∞

[0031]

[Table 4] 4th surface 5th surface 2nd surface K = -0.5800 K = 0.0000 K = 0.2500 × 10 A4 = 0.7540 × 10 ^-3 A4 = 0.3250 × 10 ^-1 A6 = -0.3670 × 10 ^-4 A6 =- 0.1000 × 10 ^-1 A8 = 0.2800 × 10 ^-4 A8 = 0.2000 × 10 ^-2 A10 = -0.3600 × 10 ^-4 A10 = -0.1820 × 10 ^-3

FIG. 5 shows a configuration similar to that of the first embodiment.
FIG. 6 shows an optical system in which the bonding surface r2 has a spherical surface, and FIG. 6 shows spherical aberration and chromatic aberration by the optical system in FIG. 4 and 6
It can be understood from the comparison with that that by changing the bonding surface from a spherical surface to an elliptical surface, the amount of change in spherical aberration due to wavelength fluctuation is reduced.

[0033]

Embodiment 2 FIG. 7 shows an optical system in which a diffraction type chromatic aberration correcting element is combined with the objective lens shown in FIG. As shown in FIGS. 8A and 8B, the diffraction type chromatic aberration correction element is formed by forming a plane perpendicular to the optical axis in a stepwise manner as a concentric annular zone with respect to the optical axis. ing.

Table 5 shows the configuration of an optical system in which the diffraction type chromatic aberration correction element of the second embodiment is combined with the objective lens shown in FIG. In this chromatic aberration correction element, the base curve, which is the macroscopic curvature of the surface r1 formed in a stepped shape, is a quaternary aspheric surface. FIG. 9 shows spherical aberration and chromatic aberration according to this configuration, respectively.

In this example, the first, third and fourth surfaces are aspherical, and their aspherical coefficients are shown in Table 6.

[0036]

[Table 5] FNO = 1: 0.9 f = 3.00 ω = 1.4 ° fb = 0.00 Surface number rd n588 ν n780 n680 1 -104.400 1.000 1.51633 64.1 1.51072 1.51315 2 ∞ Optional 3 1.894 2.200 1.49700 81.6 1.49282 1.49461 4 -4.186 1.090 5 ∞ 1.200 1.58547 29.9 1.57346 1.57834 6 ∞

[0037]

[Table 6] Surface 3 Surface 4 Surface 1 K = -0.5800 K = 0.0000 K = 0.0000 A4 = 0.7540 × 10 ^-3 A4 = 0.3250 × 10 ^-1 A4 = -0.3400 × 10 ^-3 A6 = -0.3670 × 10 ^-4 A6 = -0.1000 × 10 ^-1 A8 = 0.2800 × 10 ^-4 A8 = 0.2000 × 10 ^-2 A10 = -0.3600 × 10 ^-4 A10 = -0.1820 × 10 ^-3

[0038]

[Embodiment 3] Table 7 shows the chromatic aberration correcting element of Embodiment 3 in FIG.
2 shows a configuration of an optical system combined with the objective lens shown in FIG.
The base curve, which is the macroscopic curvature of the step r1 of the chromatic aberration correcting element, is an elliptical surface, and ε (h) is 0 within the effective diameter. FIG. 10 shows the spherical aberration and the chromatic aberration of this configuration. In this example, the first,
The third and fourth surfaces are aspherical surfaces, and their aspherical coefficients are shown in Table 8
Is shown in

[0039]

[Table 7] FNO = 1: 0.9 f = 3.00 ω = 1.4 ° fb = 0.00 Surface number rd n588 ν n780 n680 1 -104.400 1.000 1.51633 64.1 1.51072 1.51315 2 ∞ Optional 3 1.894 2.200 1.49700 81.6 1.49282 1.49461 4 -4.186 5 ∞ 1.200 1.58547 29.9 1.57346 1.57834 6 ∞

[0040]

[Table 8] 3rd surface 4th surface 1st surface K = -0.5800 K = 0.0000 K = 0.2000 × 10 ⁺⁴ A4 = 0.7540 × 10 ^-3 A4 = 0.3250 × 10 ^-1 A6 = -0.3670 × 10 ^-4 A6 = -0.1000 × 10 ^-1 A8 = 0.2800 × 10 ^-4 A8 = 0.2000 × 10 ^-2 A10 = -0.3600 × 10 ^-4 A10 = -0.1820 × 10 ^-3

FIG. 11 shows a structure similar to the second and third embodiments.
4 shows spherical aberration and chromatic aberration when a base curve of a surface formed in a step shape is a spherical surface. By comparing with FIGS. 9 and 10, it can be understood that the wavelength variation of the spherical aberration can be suppressed to be smaller than that in the case of the spherical surface by using the quaternary aspherical surface and the elliptical surface.

FIG. 12 shows a double-sided aspherical positive single lens to be corrected by the chromatic aberration correction element of Example 4-5. Specific numerical configurations are shown in Tables 9 and 10. Spherical aberration of this lens alone and its wavelength 633n
m, chromatic aberration represented by spherical aberration at 532 nm
Is shown in

[0043]

[Table 9]

[0044]

[Table 10]

[0045]

Fourth Embodiment FIG. 14 is an explanatory diagram of an optical system in which a refractive chromatic aberration correction element according to a fourth embodiment of the present invention is combined with the objective lens shown in FIG. Specific numerical configurations are shown in Tables 11 and 12. The bonding surface r2 of the chromatic aberration correction element is an elliptical surface, and ε (h) is 0 within the effective diameter. FIG. 15 shows spherical aberration and chromatic aberration due to this configuration.

[0046]

[Table 11] FNO = 1: 0.9 f = 3.29 ω = 1.7 ° fb = 0.00 Surface number rd n588 ν n633 n532 1 ∞ 0.800 1.74077 27.8 1.73541 1.74959 2 2.280 2.000 1.74100 52.7 1.73804 1.74567 3 ∞ Optional 4 2.180 2.250 1.54358 55.6 1.54151 1.54680 5 -6.250 1.332 6 ∞ 1.200 1.58547 29.9 1.58156 1.59194 7 ∞

[0047]

[Table 12] 4th surface 5th surface 2nd surface K = -0.3265 K = 0.0000 K = 0.6000 A4 = -0.2263 × 10 ^-2 A4 = 0.1670 × 10 ^-1 A6 = -0.5014 × 10 ^-3 A6 = -0.5080 × 10 ^-2 A8 = -0.7162 × 10 ^-5 A8 = 0.8000 × 10 ^-3 A10 = -0.3194 × 10 ^-4 A10 = -0.4848 × 10 ^-4

FIG. 16 shows an optical system having a configuration similar to that described above and in which the bonding surface r2 of the chromatic aberration correction element is spherical.
7 shows the spherical aberration. By adopting an elliptical surface, the shape of spherical aberration at both wavelengths can be made closer and the amount of spherical aberration can be reduced as a whole.

[0049]

Fifth Embodiment FIG. 18 is an explanatory diagram of an optical system in which a diffraction type chromatic aberration correction element according to a fifth embodiment of the present invention and the objective lens shown in FIG. 12 are combined. 11 and 12. In this example, the base curve of the step-shaped surface of the chromatic aberration correction element is an elliptical surface, and ε (h) is 0 within the effective diameter. FIG. 19 shows the spherical aberration and chromatic aberration due to this configuration.

[0050]

[Table 13] f = 3.29 ω = 1.7 ° fb = 0.00 Surface number rd n588 ν n633 n532 1 ∞ 2.000 1.51633 64.1 1.51462 1.51900 2 41.000 Optional 3 2.180 2.250 1.54358 55.6 1.54151 1.54680 4 -6.250 1.341 5 ∞ 1.200 1.58547 29.9 1.58156 1.59194 6 ∞

[0051]

[Table 14] 3rd surface 4th surface 1st surface K = -0.3265 K = 0.0000 K = 0.2450 × 10 ⁺³ A4 = -0.2263 × 10 ^-2 A4 = 0.1670 × 10 ^-1 A6 = -0.5014 × 10 ^-3 A6 = -0.5080 × 10 ^-2 A8 = -0.7162 × 10 ^-5 A8 = 0.8000 × 10 ^-3 A10 = -0.3194 × 10 ^-4 A10 = -0.4848 × 10 ^-4

FIG. 20 shows the spherical aberration and chromatic aberration in the case where the base curve of the step-shaped surface of the chromatic aberration correction element has a spherical surface in the same configuration as in the fifth embodiment. By comparing FIG. 19 and FIG. 20, it can be understood that the variation of the spherical aberration can be suppressed by making the base curve an elliptical surface.

[0053]

As described above, according to the present invention, not only the longitudinal chromatic aberration of the converging lens due to the wavelength fluctuation can be corrected, but also the fluctuation of the spherical aberration can be suppressed. Thus, a change in performance of the optical system due to a change in wavelength can be suppressed.

Therefore, a lens whose chromatic aberration is not corrected can be used in an optical information recording apparatus using two wavelengths which are relatively separated from each other, and an information reading apparatus using a light emitting diode or a white light source. be able to.

[Brief description of the drawings]

FIG. 1 is a lens diagram of a positive objective lens corrected by a chromatic aberration correction element according to Example 1-3.

FIG. 2 is a diagram of spherical aberration and chromatic aberration of the objective lens shown in FIG. 1 alone.

FIG. 3 is a lens diagram of an optical system in which the lens shown in FIG. 1 is combined with a refractive chromatic aberration correction element of Example 1 in which a bonding surface is an elliptical surface.

FIG. 4 is a diagram showing spherical aberration and chromatic aberration of the optical system shown in FIG. 3;

FIG. 5 is a lens diagram of an optical system in which the objective lens shown in FIG. 1 is combined with a refractive chromatic aberration correction element having a spherical bonding surface.

6 is a diagram showing spherical aberration and chromatic aberration of the optical system shown in FIG. 5;

7 is a lens diagram of an optical system in which the lens shown in FIG. 1 and a diffraction type chromatic aberration correction element are combined.

FIGS. 8A and 8B are explanatory diagrams showing a configuration of a step-like surface of the diffraction type chromatic aberration correction element, where FIG. 8A is a side view and FIG. 8B is a plan view.

9 is a diagram showing spherical aberration and chromatic aberration when the diffraction type chromatic aberration correction element of Example 2 in which the base curve of the stepped surface is an elliptical surface is used in the optical system shown in FIG. 7;

10 is a diagram showing spherical aberration and chromatic aberration when the diffractive chromatic aberration correction element of Example 3 in which the base curve of the stepped surface is a quaternary aspheric surface in the optical system shown in FIG. 7;

11 is a diagram showing spherical aberration and chromatic aberration when a diffraction type chromatic aberration correction element having a spherical base curve with a stepped surface is used in the optical system shown in FIG. 7;

FIG. 12 is a lens diagram of a positive objective lens corrected by the chromatic aberration correction elements of Examples 4 and 5.

13 is a diagram of spherical aberration and chromatic aberration of the objective lens alone shown in FIG.

14 is a lens diagram of an optical system in which the objective lens shown in FIG. 12 and a refractive chromatic aberration correction element having a bonding surface of a spherical surface are combined.

15 is a diagram of spherical aberration and chromatic aberration of the optical system shown in FIG.

FIG. 16 is a lens diagram of an optical system in which the lens shown in FIG. 12 is combined with a refractive chromatic aberration correction element of Example 4 in which the bonding surface is an elliptical surface.

17 is a diagram of spherical aberration and chromatic aberration of the optical system shown in FIG.

18 is a lens diagram of an optical system in which the lens shown in FIG. 12 and a diffraction type chromatic aberration correction element are combined.

19 is a diagram of spherical aberration and chromatic aberration in the case where the diffractive chromatic aberration corrector of Example 5 in which the base curve of the stepped surface is an elliptical surface is used in the optical system shown in FIG. 18;

20 is a diagram of spherical aberration and chromatic aberration in the case where a diffraction-type chromatic aberration correction element having a spherical base curve with a stepped surface is used in the optical system shown in FIG. 18;

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-155514 (JP, A) JP-A-61-3110 (JP, A) JP-A-62-178916 (JP, A) 8134 (JP, A) JP-A-6-76346 (JP, A) JP-A-3-93048 (JP, A) JP-A-5-34642 (JP, A) JP-A-62-269922 (JP, A) JP-B-52-33501 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 5/18 G02B 9/00-17/08 G02B 21/02-21/04 G02B 25 / 00-25/04

Claims (4)

(57) [Claims] 1. A divergent wavefront having an excessive spherical aberration is generated for a parallel light beam having a wavelength shorter than a reference wavelength.
It is characterized by generating a condensed wavefront having an under-spherical aberration with respect to the incidence of a parallel light beam having a wavelength longer than the reference wavelength, and having a substantially different refractive index difference and a different dispersion at the reference wavelength.
A positive lens and a negative lens made of different materials
The bonding surface has a radius of curvature away from the optical axis.
Aspherical surface whose absolute value decreases as
Chromatic aberration correction element. 2. The bonding surface is an aspherical surface close to a spheroidal surface having a positive elliptic constant, and a displacement ε (h) from the spheroidal surface at a distance h from the optical axis is represented by (1). ) when represented by formula, in all of the distance h in the effective maximum radius of the optical beam passing through, characterized in that satisfies the condition (2) 請
Chromatic aberration correcting device according to Motomeko 1. ε (h) = ΔX (h) −Ch ² / (1−√ (1− (1 + K) C ² h ² )) (1) | ε (h) | <λ / ΔnMAX (2) where ΔX (h) is the sag amount of the aspheric surface, C is the paraxial curvature,
K is a conical constant, λ is the maximum operating wavelength, and ΔnMAX is the absolute value of the refractive index difference in the state where the refractive index difference of the medium before and after the bonding surface is the largest in the operating wavelength band. 3. The method according to claim 1, wherein a parallel light beam having a wavelength shorter than the reference wavelength is incident.
In contrast, a divergent wavefront with excessive spherical aberration is generated,
For incidence of a parallel light beam having a wavelength longer than the reference wavelength
Generates a focused wavefront with under spherical aberration.
As a feature , at least one surface of the light incident and exit end faces is configured in a diffraction type in which a plane perpendicular to the optical axis is formed in a step shape as a concentric annular zone with respect to the optical axis , and is formed in the step shape. Is an aspheric surface whose absolute value becomes smaller as the radius of curvature is away from the optical axis, and the base curve in the optical axis direction at a point at a distance h from the optical axis. The displacement amount ΔX ′ (h) at a point at a distance h from the optical axis of the stepped surface is given by Expression (3), where the displacement amount is ΔX (h). . ΔX ′ (h) = (mλ0 / (n−1)) Int ((ΔX (h) / (mλ0 / (n−1))) + 0.5) (3) where m is an integer and n is Refractive index, λ0 is a wavelength using the chromatic aberration correcting element or any one wavelength in the wavelength range,
Int (x) is a function that gives an integer that does not exceed x. Wherein said base curve is non-spherical near spheroidal surface having a positive conic constant, the amount of deviation from spheroidal at a point of the distance h from the optical axis ε a (h) (1) When represented by the expression, the condition of the expression (4) is satisfied at all distances h within the effective maximum radius of the passing light flux.
The chromatic aberration correction element according to claim 3 . ε (h) = ΔX (h) −Ch ² / (1−√ (1− (1 + K) C ² h ² )) (1) | ε (h) | <λ / (n−1) (4) Where C is the paraxial curvature, K is the conic constant, and λ is the maximum wavelength used.