JPH06331898A - Objective lens - Google Patents

Abstract

PURPOSE:To provide the objective lens which has a high power and a high NA and has various aberrations, especially, the chromatic aberration corrected without using multiple cemented lenses or abnormal dispersion glass by including one diffraction type optical element and one cemented lens. CONSTITUTION:This objective lens includes at least one sheet of diffraction type optical element and one cemented lens. For the purpose of satisfactorily correcting the chromatic aberration, at least one sheet of diffraction type optical element satisfies conditions D1/D>0.8 and (hXf)/(LXI)>0.07 where D1 is the marginal luminous flux diameter to the position of the diffraction type optical element and D is the maximum marginal luminous flux diameter and (h) is the principal ray height on the surface of the diffraction type optical element and (f) is the focal length and I is the maximum image height on the specimen surface and L is the same focal length. Thus, the lens system is obtained, which has a high NA and a high power and has various aberrations, especially, the chromatic aberration satisfactorily corrected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens used in an optical system such as a microscope.

[0002]

2. Description of the Related Art Conventional objective lenses are particularly high in magnification and high in N.
In the case of A, a large number of cemented lenses were required to satisfactorily correct chromatic aberration among various aberrations, and it was necessary to use anomalous dispersion glass. Therefore, it is inevitably expensive, and it may not be possible to design an objective lens used with ultraviolet rays or infrared rays whose glass material is limited.

Recently, a diffractive optical element (DO
An optical system using E) is drawing attention. As an objective lens using this diffractive optical element, which is similar to the objective lens of the present invention, conventional examples are disclosed in JP-A-63-77003, JP-A-63-155432, and JP-A-59-33636. There are those described in JP-A-60-247611, JP-A-2-109, and JP-A-4-361201.

Further, a diffractive optical element utilizing the above-mentioned diffraction phenomenon, that is, a diffractive optical element [D
ifractive Optics Element
s (DOE)] are published in Optronics, "Optical Elements for Optical Designers", Chapters 6 and 7, and William C. "Newmet" by Sweet
HODS OF DESIGNING HOLOGRA
PHIC OPTICAL ELEMENTS "(SP
IE. VOL. 126, P46-53, 1977) and the like, but the principle thereof is briefly described as follows.

A normal optical glass refracts according to Snell's law represented by the following equation in FIG.

Nsin θ = n′sin θ ′ (1) where n is the refractive index of the incident side medium, n ′ is the refractive index of the exit side medium, θ is the incident angle of the light beam, and θ ′ is the outgoing angle of the light beam. is there.

On the other hand, in the diffraction phenomenon, light is bent according to the law of diffraction expressed by the following equation, as shown in FIG.

Nsin θ−n′sin θ ′ = mλ / d (2) where m is the order of diffracted light, λ is the wavelength, and d is the lattice spacing.

An optical element that refracts a light beam according to the above equation (2) is a diffractive optical element. Incidentally, FIG.
In FIG. 7, the shield part and the transmissive part are shown arranged side by side at a distance d. However, as shown in FIG. 18, a diffractive surface having a saw-toothed cross section is provided on the surface of the transparent body for blazing, or as shown in FIG. A high diffraction efficiency can be obtained by performing the binary approximation.

Next, advantages of using the above-mentioned diffractive optical element will be described.

In the case of a refracting thin lens, the following equation (3) is used.
The relationship shown in is established.

1 / f = (n−1) (1 / r ₁ −1 / r ₂ ) (3) where f is the focal length, r ₁ and r ₂ are the radii of curvature of the entrance and exit surfaces, and n Is the refractive index of the lens.

Differentiating both sides of the above equation (3) with respect to the wavelength λ yields the following equation (4).

Df / dλ = −f (dn / dλ) / (n−1) Δf = −f {Δn / (n−1)} (4) Excluding the coefficient multiplication effect, Δn / (n Since -1) represents the dispersion characteristic, the dispersion value ν can be defined as follows.

Ν≡ (n−1) / Δn (5) Therefore, the dispersion characteristic (Abbe number ν _d ) in the visible region is as follows.

Ν _d = (n _d −1) / (n _F −n _C ) (6) On the other hand, in the case of a diffractive optical element, the following equation holds. The focal length f of the diffractive optical element is the height h of the incident parallel light.
When the lattice spacing at is d _h , the following equation (7) is obtained.

F = h / (n′sin θ ′) = (d _h h) / (mλ) (7) In the case of an aberration-free diffractive optical element, d _h h is constant, so f = C / λ (C is a constant). Differentiating both sides of this f = C / λ by λ yields formula (8) as follows.

Df / dλ = −C / λ ² = −f / λ Δf = −f (Δλ / λ) (8) Since Δn / (n−1) = ν, equations (4) and (8) Therefore, ν = λ / Δλ. Therefore, the Abbe number ν _d in the visible range of the diffractive optical element is as follows.

Ν _d = λ _d / (λ _F −λ _C ) = − 3.453 (9) As described above, the diffractive optical element has a very large negative dispersion characteristic. Since the dispersion characteristic of ordinary glass is about 20 to 95, it can be seen that the diffractive optical element has a very large inverse dispersion characteristic. In addition, similar calculations show that the diffractive optical element has anomalous dispersion.

Among the above-mentioned conventional examples, JP-A-63-7700
3, JP-A-63-155432, and JP-A-59-33.
The lens system disclosed in Japanese Patent Laid-Open No. 636 and JP-A-60-247611 relates to a pickup lens of an optical disk, and includes one or two diffractive optical elements or one refractive optical element (lens) and a diffractive optical element. It consists of one optical element,
Basically, the light source is monochromatic, and the ability of the diffractive optical element to correct chromatic aberration is not utilized.

Further, Japanese Patent Laid-Open No. 2-1109 and Japanese Patent Laid-Open No. 4-109.
The lens system disclosed in Japanese Patent No. 361201 relates to a photographing lens used for a stepper or the like, and is composed only of quartz and does not use a cemented lens. In particular, the former lens system of Japanese Patent Laid-Open No. 2-1109 is characterized in that a diffractive optical system is arranged at the pupil position, and the latter lens system of Japanese Patent Laid-Open No. 4-36201 is a diffractive optical element. The peripheral portion is characterized by using higher-order diffracted light than the central portion.

However, in these conventional examples, the pickup lens type cannot support a microscope objective lens which requires a more complicated structure. The stepper lens type may be applicable to a low-magnification microscope objective lens, but cannot be applied to a high-magnification, high-NA microscope objective lens. That is, when the chromatic aberration of the objective lens is corrected only by the diffractive optical element, the power of the diffractive optical element must be increased, and the minimum pitch of the diffractive optical element becomes so small that it cannot be produced.

[0023]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an objective lens which has a high magnification and a high NA and which corrects various aberrations, particularly chromatic aberrations, without using a lot of cemented lenses or anomalous dispersion glass. .

[0024]

The objective lens of the present invention comprises:
It is characterized in that it includes at least one diffractive optical element and at least one cemented lens, and various aberrations, especially chromatic aberration, are satisfactorily corrected without using a special glass material with a small number of optical elements. .

In a normal objective lens, chromatic aberration is corrected by using a cemented lens in which lenses made of glass materials having different Abbe numbers are cemented. And the Abbe number of ordinary glass materials is 20-
95 are all positive values. On the other hand, the Abbe number of the diffractive optical element has a negative and small absolute value as described above. Therefore, if this diffractive optical element is combined with an ordinary glass lens, a strong chromatic aberration correction action can be provided.

Further, in a high-class objective lens, it is necessary to use apochromat for color correction, and therefore anomalous dispersion glass must be used frequently. However, since these glasses are expensive and often have poor workability, these objective lenses are made more expensive. This problem can also be solved by using a diffractive optical element and utilizing its large anomalous dispersion.

When the color correction of the objective lens of high NA and high magnification is performed only by the diffractive optical element, its power becomes too strong and the minimum pitch becomes too small. However, by using at least one cemented lens, the diffracted light is diffracted. It is possible to share the color correction with the mold optical element and loosen the minimum pitch.

In order to satisfactorily correct chromatic aberration, it is desirable to satisfy the following conditions (1) and (2).

(1) D ₁ /D>0.8 (2) (h × f) / (L × I)> 0.07 where D ₁ is the marginal light beam diameter at the position of the diffractive optical element, and D Is the maximum marginal beam diameter, h is the chief ray height on the surface of the diffractive optical element, f is the focal length, I is the maximum image height on the sample surface, and L is the parfocal distance.

There are roughly two types of chromatic aberration, axial chromatic aberration and chromatic aberration of magnification. The former is a shift due to the wavelength of the focal position and the latter is a shift due to the wavelength of the focal length (magnification).

Of these chromatic aberrations, the most effective position for correcting the axial chromatic aberration is the pupil position in the objective lens, but it does not have to be the exact pupil position.
A large luminous flux diameter (axial marginal luminous flux diameter) near the pupil is effective for correcting axial chromatic aberration. The condition (1) is defined in consideration of this. In this condition (1), if the lower limit of 0.8 or less is reached, axial chromatic aberration that occurs in other refractive optical elements (lenses) cannot be completely corrected by the diffractive optical element, and many cemented lenses are used in the refractive optical element. In addition, the anomalous dispersion glass is required, and the effect of using the diffractive optical element is not sufficient.

On the other hand, the most effective position for correcting the chromatic aberration of magnification is not the position of the pupil but the position where the chief ray slightly away from it has a certain ray height. The condition (2) defines the arrangement position of the diffractive optical element for effectively correcting the chromatic aberration of this magnification. If the lower limit of 0.07 is not satisfied in this condition (2), chromatic aberration of magnification cannot be sufficiently corrected, and many cemented lenses are used in the refractive optical element,
It is necessary to use the anomalous dispersion glass, and the effect obtained by using the diffractive optical element cannot be sufficiently obtained. Condition (2)
In, f, L, and I are for normalizing this condition, f / I is a parameter of the chief ray angle, and L is a parameter for scaling the size of the entire optical system.

As described above, in the objective lens of the present invention, it is particularly preferable to dispose a diffractive optical element suitable for the application at a position satisfying the above conditions (1) and (2). It is more preferable for better correcting chromatic aberration.

In order to share the correction of chromatic aberration by using at least one cemented lens and at least one diffractive optical element in the lens system, the Abbe number difference Δν between both lenses of at least one cemented lens must satisfy the following condition. It is preferable to satisfy (3).

(3) When Δν> 20 Δν is smaller than the lower limit of 20 of the condition (3), the chromatic aberration correcting action of the cemented lens becomes insufficient, and the minimum pitch of the diffractive optical element cannot be increased too much.

Further, the diffractive optical element has a manufacturing feature that the lattice spacing can be set arbitrarily. Therefore, the diffractive optical element can obtain an action equivalent to that of an arbitrary aspherical lens by changing the lattice spacing in various ways, and moreover, the diffractive optical element may have a large number of inflection points, so that it can be designed more than an ordinary aspherical lens. The degree of freedom is great and the production accuracy is good. Moreover, it is possible to correct chromatic aberration that cannot be corrected by an aspherical lens. The gradient index lens can correct chromatic aberration, but the number of gradient index lenses that can be actually manufactured is limited, and it cannot sufficiently cope with ultraviolet rays and infrared rays. As described above, the diffractive optical element has an aberration correction capability superior to that of the aspherical lens or the gradient index lens, and is advantageous in manufacturing. Therefore, by using this as the objective lens as in the present invention, it is possible to improve the performance of the objective lens and reduce the cost, and to design a new objective lens which has been impossible in the past.

[0037]

EXAMPLES Next, examples of the present invention will be described. First, the diffractive optical element used in the examples of the present invention will be described in more detail. The diffractive optical element (DOE) used in the examples described later is as described above, but as a method of designing an optical system including such a diffractive optical element,
Ultra-high index method (ultrahigh
What is called index methods) is known. This is a virtual lens with an extremely high refractive index (Ultra High Index).
It is a method of designing by replacing with a lens). Regarding this, SPIE 126, pp. 46-53 (1977).
Year) but will be briefly described with reference to FIG. In FIG. 20, 1 is an ultra-high index lens and 2 is a normal line. In this ultra-high index lens, the relationship expressed by the following equation (11) is established.

(N _U −1) dz / dh = nsin θ−n′sin θ ′ (11) where n _U is the refractive index of the ultra-high index lens and z is the optical axis direction of the ultra-high index lens. Coordinates, h is the distance from the optical axis, n and n'are the refractive indices of the incident side medium and the exit side medium, and θ and θ ', respectively.
Is the angle of incidence and the angle of exit of the ray. In the data of the examples shown below, n _U = 1001 or 10001.

From equations (2) and (11), the following equation (1
2) is obtained.

(N _U −1) dz / dh = mλ / d (12) That is, between the surface shape of the ultra-high index lens (refractive lens having an extremely large refractive index) and the pitch of the diffractive optical element. Satisfies the equation (12), and the pitch of the diffractive optical element can be determined from the data designed by the ultra-high index method through this equation.

A general axisymmetric aspherical surface is expressed as follows.

Z = ch ² / [{1-c ² (k + 1) h ² } ^1/2 ] + Ah ⁴ + Bh ⁶ + Ch ⁸ + Dh ¹⁰ + ... (13) where z is the optical axis (the direction of the image) Is positive), h is the coordinate axis in the meridional direction among the coordinate axes orthogonal to the z axis with the origin at the intersection of the surface and the z axis, c is the curvature of the reference surface, and k is the conic constant A, B, C, D ...・ 4th, 6th, 8th, 10th
Next is the aspherical coefficient of ...

From equations (12) and (13), the pitch d of the diffractive optical element equivalent to the aspherical surface at a certain ray height is
It is expressed by the following equation (14). d = mλ / [(n- 1) {ch / (1-c 2 (1 + k) h 2) 1/2 + 4Ah 3 + 6Bh 5 + 8Ch 7 + 10Dh 9 + ····}] (14) Note that the following embodiments In the example, the aspherical terms are up to 10th order, but 12th, 14th, ... Aspherical terms may be used.

The data of each example are shown below. Example 1 Focal length = 3.6 mm, NA = 1.1 (water immersion), Magnification = 100, Parfocal distance = 45 mm Maximum image height of sample surface = 0.05 mm r ₀ = ∞ (object surface) d ₀ = 0.17 n ₀ = 1.521 ν ₀ = 56.02 r ₁ = ∞ d ₁ = 0.12 n ₁ , ν ₁ (water) r ₂ = ∞ d ₂ = 2.5814 n ₂ = 1.596 ν ₂ = 39.3 r ₃ = -2.0016 d ₃ = 0.15 r ₄ = -6.6313 _{d 4 = 2.2727 n 3 = 1.678} ν 3 = 55.34 r 5 = -4.4815 d 5 = 0.15 r 6 = 7.2872 d 6 = 3.7582 n 4 = 1.488 ν 4 = 70.21 r 7 = -5.6995 d 7 = 1.0 n 5 = 1.678 ν ₅ = 55.34 r ₈ = 8.2626 d ₈ = 3.3432 n ₆ = 1.497 ν ₆ = 81.14 r ₉ = -9.3735 d ₉ = 0.15 r ₁₀ = ∞ d ₁₀ = 1.5 n ₇ = 1.516 ν ₇ = 64.14 r ₁₁ = ∞ d ₁₁ = 0 r ₁₂ = -5.5443 x 10 ⁶ (DOE) d ₁₂ = 0.15 [n _U = 10001] r ₁₃ = 8.6431 d ₁₃ = 1.0 n ₈ = 1.678 ν ₈ = 55.34 r ₁₄ = 4.7497 d ₁₄ = 4.2904 n ₉ = 1.497 ν ₉ = 81.14 r ₁₅ = -5.7355 d ₁₅ = 1.0 n ₁₀ = 1.596 ν ₁₀ = 39.29 r ₁₆ = -55.2542 d ₁₆ = 0.15 r ₁₇ = 12.1862 d ₁₇ = 2.2382 n ₁₁ = 1.497 ν ₁₁ = 81.14 r ₁₈ = -14.3804 d ₁₈ = 7.0 n ₁₂ = 1.596 ν ₁₂ = 39.29 r ₁₉ = 7.3769 d ₁₉ = 8.5761 r ₂₀ = -2.9707 d ₂₀ = 2.0598 n ₁₃ = 1.678 ν ₁₃ = 55.34 r ₂₁ = 1.11.1831 d ₂₁ = 7.0 n ₁₃ = 1.596 ν ₁₄ = 39.3 r ₂₂ = -7.1268 (DOE surface) K = -1, A = 0.282845 × 10 ^-8 , B = -0.695088 × 10 ^-10 C = 0.643649 × 10 ^-11 , D = -0.321846 × 10 ^-12 D ₁ /D=0.99, (h × f) / (L × I) = 0.064, minimum pitch = 130 μm

Example 2 Focal length = 9 mm, NA = 0.6 (water immersion), Magnification = 20, Parfocal distance = 45 mm Maximum image height on sample plane = 0.25 mm r ₀ = ∞ d ₀ = 0.17 n ₀ = 1.521 ν ₀ = 56.02 r ₁ = ∞ d ₁ = 0.12 n ₁ , ν ₁ (water) r ₂ = ∞ d ₂ = 1.0 n ₂ = 1.516 ν ₂ = 64.15 r ₃ = ∞ d ₃ = 1.3056 r ₄ = -3.8226 d ₄ = 1.5 n ₃ = 1.744 ν ₃ ＝ 44.79 r ₅ ＝ -96.9912 d ₅ ＝ 2.2154 n ₄ ＝ 1.755 ν ₄ ＝ 27.51 r ₆ ＝ -5.4626 d ₆ ＝ 0.15 r ₇ ＝ -19.8772 d ₇ ＝ 2.2181 n ₅ = 1.487 ν ₅ = 70.21 r ₈ = -6.7810 d ₈ = 0.15 r ₉ = -8.6681 d ₉ = 1.5 n ₆ = 1.639 ν ₆ = 34.48 r ₁₀ = 61.1760 d ₁₀ = 3.7076 n ₇ = 1.487 ν ₇ = 70.21 r ₁₁ = -7.8096 d _{11 = 0.15 r 12 = 58.3918 d} 12 = 3.5924 n 8 = 1.487 ν 8 = 70.21 r 13 = -9.6113 d 13 = 1.5 n 9 = 1.749 ν 9 = 34.96 r 14 = -41.0966 d 14 = 0.15 r 15 = 19.2027 d ₁₅ = 1.5 n ₁₀ = 1.603 ν ₁₀ = 42.32 r ₁₆ = 11.9017 d ₁₆ = 3.6322 n ₁₁ = 1.487 ν ₁₁ = 70.21 r ₁₇ = -76.0706 d ₁₇ = 0.15 r ₁₈ = -800896.0617 (DOE) d ₁₈ = 0 [n _U = 1001] r ₁₉ = ∞ d ₁₉ = 2.0 n ₁₂ = 1.516 ν ₁₂ = 64.15 r _{20 = ∞ d 20 = 14.1927 r} 21 = 23.9945 d 21 = 2.9486 n 13 = 1.762 ν 13 = 40.1 r 22 = -31.1050 d 22 = 1.5 n 13 = 1.487 ν 14 = 70.21 r 23 = 10.0681 (DOE surface) K = -1, A = 0.263441 × 10 ^-8 , B = -0.964788 × 10 ^-11 C = -0.315285 × 10 ^-13 , D = -0.299622 × 10 ^-15 D ₁ /D=0.99, (h × f) / ( L × I) = 0.205, minimum pitch = 87 μm

Example 3 Focal length = 3.6 mm, NA = 0.75, Magnification = 50, Parfocal distance =
Maximum image height of 45 mm sample surface = 0.265 mm r ₀ = ∞ d ₀ = 0.9498 r ₁ = -2.2690 d ₁ = 4.0409 n ₁ = 1.678 ν ₁ = 55.34 r ₂ = -3.4762 d ₂ = 0.1 r ₃ = 16.9526 d ₃ = _{3.3974 n 2 = 1.487 ν 2 =} 70.21 r 4 = -9.1604 d 4 = 0.1 r 5 = -28.7293 d 5 = 1.8 n 3 = 1.596 ν 3 = 39.29 r 6 = 6.5830 d 6 = 4.4396 n 4 = 1.487 ν 4 = 70.21 r ₇ = -15.6408 d ₇ = 0.1 r ₈ = ∞ d ₈ = 1.0 n ₅ = 1.516 ν ₅ = 64.15 r ₉ = ∞ d ₉ = 0 r ₁₀ = -5.7585 × 10 ⁶ (DOE) d ₁₀ = 0.1 [ n _U = 10001] r ₁₁ = 20.2252 d ₁₁ = 3.6611 n ₆ = 1.487 ν ₆ = 70.21 r ₁₂ = -10.5398 d ₁₂ = 1.8 n ₇ = 1.596 ν ₇ = 39.29 r ₁₃ = 10.7283 d ₁₃ = 3.4719 n ₈ = 1.487 ν ₈ = 70.21 r ₁₄ = -28.0713 d ₁₄ = 9.5097 r ₁₅ = 24.5125 d ₁₅ = 3.5584 n ₉ = 1.596 ν ₉ = 39.21 r ₁₆ = -8.897 d ₁₆ = 1.8 n ₁₀ = 1.498 ν ₁₀ = 65.03 r ₁₇ =- 18.2382 d ₁₇ = 4.0127 r ₁₈ = -7.0308 d ₁₈ = 1.8 n ₁₁ = 1.498 ν ₁₁ = 65.03 r ₁₉ = 15.1697 (DOE surface) K = -1, A = 0.885874 x 10 ^-9 , B = -0.373681 x 10 ^-10 C = 0.171463 x 10 ^-11 , D = -0.455259 x 10 ^-13 D ₁ / D = 0.98 , (H × f) / (L × I) = 0.071, minimum pitch = 80 μm

Example 4 Focal length = 3.6 mm, NA = 0.75, Magnification = 50, Parfocal distance =
Maximum image height of 45 mm specimen surface = 0.265 mm r ₀ = ∞ d ₀ = 0.9145 r ₁ = -2.6605 d ₁ = 4.1491 n ₁ = 1.678 ν ₁ = 55.34 r ₂ = -3.3609 d ₂ = 0.1 r ₃ = 59.4216 d ₃ = 3.9495 n ₂ = 1.617 ν ₂ ＝ 62.8 r ₄ ＝ −4.8439 d ₄ ＝ 1.8 n ₃ ＝ 1.596 ν ₃ ＝ 39.29 r ₅ ＝ 8.9186 d ₅ ＝ 4.4558 n ₄ ＝ 1.439 ν ₄ ＝ 94.96 r ₆ ＝ -11.2459 d ₆ ＝ 0.1 r ₇ = ∞ d ₇ = 1.0 n ₅ = 1.516 ν ₅ = 64.15 r ₈ = ∞ d ₈ = 0 r ₉ = -7.2979 × 10 ⁶ (DOE) d ₉ = 0.1 [n _U = 10001] r ₁₀ = 19.9999 _{d 10 = 3.4234 n 6 = 1.439} ν 6 = 94.96 r 11 = -21.6675 d 11 = 14.4364 r 12 = 103.5371 d 12 = 6.0 n 7 = 1.596 ν 7 = 39.29 r 13 = -11.7643 d 13 = 3.0881 r 14 = - 6.5185 d ₁₄ = 2.7072 n ₈ = 1.498 ν ₈ = 65.03 r ₁₅ = 17.4388 (DOE surface) K = -1, A = 0.136333 × 10 ^-8 , B = -0.205407 × 10 ^-10 C = 0.275330 × 10 ^-12 , _{^{D = -0.502831 × 10 -14 D 1}} /D=0.96,(h×f)/(L×I)=0.076, minimum peak Ji = 157 μm

Example 5 Focal length = 36 mm, NA = 0.20, Magnification = 10, Parfocal distance = 10
₀ mm Maximum image height on specimen surface = 0.8 mm r ₀ = ∞ d ₀ = 10.8729 r ₁ = -10.1528 d ₁ = 7.0 SiO ₂ r ₂ = -12.8207 d ₂ = 0.2 r ₃ = 1185.9548 d ₃ = 7.0 SiO ₂ r ₄ = -20.8861 d ₄ = 9.6560 r ₅ = 166976.3323 (DOE) d ₅ = 0 [n _U = 1001] r ₆ = ∞ d ₆ = 3.0 SiO ₂ r ₇ = ∞ d ₇ = 36.8382 r ₈ = 18.3786 d ₈ = 5.2879 CaF ₂ r ₉ = -14.5637 d ₉ = 5.2019 SiO ₂ r ₁₀ = 15.9489 d ₁₀ = 3.5421 r ₁₁ = -9.8983 d ₁₁ = 6.6329 SiO ₂ r ₁₂ = -12.7649 d ₁₂ = 0.2 r ₁₃ = -107.5879 d ₁₃ = 3.4872 SiO ₂ r ₁₄ = -38.6725 D ₁ /D=0.69, (h × f) / (L × I) = 0.344, minimum pitch = 8.9 μm, where r ₀ , r ₁ , r ₂ ... Radius, d ₀ ,
d ₁ , d ₂ ... Are the distances between the surfaces, n ₁ , n ₂ , ... Are the refractive indices of the lenses, and ν ₁ , ν ₂ , ... Are the Abbe numbers of the lenses.

In the above data, r ₀ is the object plane. Further, Examples 1 and 2 are water immersion objective lenses, n ₀ and ν ₀ are cover glasses, n ₁ and ν ₁ are water, and d ₀ of Examples 3 to 5 is a working distance. Further DO in the data
Reference numeral E denotes a diffractive optical element, on which the diffractive surface of the bitch d obtained by the equation (14) is formed.

The above first embodiment is shown in FIG. 1, and the second embodiment is shown in FIG.
In addition, the third embodiment has a configuration as shown in FIG. 7 and the fourth embodiment has a configuration as shown in FIG. The diffractive optical elements used in these examples are those having an aspherical effect, whereby spherical aberration,
The coma aberration etc. are corrected well. In addition, Example 5 is shown in FIG.
Since the structure shown in (1) has a low NA and a low magnification, it is not necessary to give the diffractive optical element an aspherical effect, and the diffractive optical element corrects only chromatic aberration.

Example 1 is an objective lens which has been color-corrected from near ultraviolet to visible, and a diffractive optical element is arranged mainly at a large light beam diameter for correction of axial chromatic aberration. Example 2 is an objective lens that has been color-corrected from near-ultraviolet to visible, and has a large beam diameter and a high chief ray height in order to correct axial chromatic aberration and lateral chromatic aberration with a single diffractive optical element. A diffractive optical element is arranged at. Example 3 is an objective lens that performs color correction in the visible range, and Example 4 also corrects axial chromatic aberration and lateral chromatic aberration with one diffractive optical element. Furthermore, in Example 5, the off-axis chromatic aberration is mainly corrected by the diffractive optical element, and the on-axis chromatic aberration is mainly corrected by the lens in which quartz and fluorite are cemented.

In each of the sectional views of the embodiments, the right side is the object side. In addition, reference numeral B in FIGS. 1, 4, 7 and 10 indicates a body-attached position, and the lens final surface (the leftmost surface in the drawing), respectively.
More toward the object side, 3.6, 0.3526, 0.6477,
It is 1.2300. The position on the body of Example 5 is 1.0808 on the image side of the final lens surface. Further, the aberration curve diagrams of the respective examples are drawn by reverse tracing.

[0053]

The objective lens of the present invention is a lens system having a high NA and a high magnification by using a diffractive optical element, and various aberrations, particularly chromatic aberration, are favorably corrected.

[Brief description of drawings]

FIG. 1 is a sectional view of a first embodiment of the present invention.

FIG. 2 is a spherical aberration, astigmatism, and distortion aberration curve diagram of Example 1 of the present invention.

FIG. 3 is a coma aberration curve diagram of the first embodiment of the present invention.

FIG. 4 is a sectional view of a second embodiment of the present invention.

FIG. 5 is a spherical aberration, astigmatism, and distortion aberration curve diagram of Example 2 of the present invention.

FIG. 6 is a coma aberration curve diagram of the second embodiment of the present invention.

FIG. 7 is a sectional view of a third embodiment of the present invention.

FIG. 8 is a spherical aberration, astigmatism, and distortion aberration curve diagram of Example 3 of the present invention.

FIG. 9 is a coma aberration curve diagram of Example 3 of the present invention.

FIG. 10 is a sectional view of a fourth embodiment of the present invention.

FIG. 11 is a curve diagram of spherical aberration, astigmatism, and distortion of Example 4 of the present invention.

FIG. 12 is a coma aberration curve diagram of Example 4 of the present invention.

FIG. 13 is a sectional view of a fifth embodiment of the present invention.

FIG. 14 is a curve diagram of spherical aberration, astigmatism, and distortion of Example 5 of the present invention.

FIG. 15 is a coma aberration curve diagram of Example 5 of the present invention.

FIG. 16 is a view showing a refraction state of ordinary glass.

FIG. 17 is a diagram showing a refraction state of light due to a diffraction phenomenon.

FIG. 18 is a sectional view of a diffractive optical element in a blazed state.

FIG. 19 is a sectional view of a diffractive optical element subjected to binary approximation.

FIG. 20 is a diagram showing a refraction state of light in an ultra-high index lens.

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[Procedure amendment]

[Submission date] November 1, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 2

[Correction method] Change

[Correction content]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0004

[Correction method] Change

[Correction content]

A diffractive optical element utilizing the above-mentioned diffraction phenomenon, that is, a diffractive optical element
[Diffractive Optical Elem
ents (DOE)] are published by Optronics in "Optical Elements for Optical Designers", Chapters 6 and 7,
And William C. "NEW" by Sweet
METHODS OF DESIGNING HOL
OGRAPHIC OPTICAL ELEMENT
S ”(SPIE. VOL. 126, P46-53, 19
77) and the like, the principle of which is briefly described as follows.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction content]

Df / dλ = −f (dn / dλ) / (n−1) ∴Δf = −f {Δn / (n−1)} (4) Excluding the coefficient multiplication effect, Δn / ( Since n-1) represents the dispersion characteristic, the dispersion value ν can be defined as follows.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0019

[Correction method] Change

[Correction content]

Ν _d = λ _d / (λ _F −λ _C ) = − 3.453 (9) As described above, the diffractive optical element has a very large negative dispersion characteristic. Since the dispersion characteristic of ordinary glass is about 20 to 95, it can be seen that the diffractive optical element has a very large inverse dispersion characteristic. In addition, similar calculations show that the diffractive optical element has anomalous dispersion .

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0028

[Correction method] Change

[Correction content]

In order to further correct chromatic aberration, it is desirable to satisfy at least one of the following conditions (1) and (2) .

[Procedure correction 6]

[Document name to be amended] Statement

[Correction target item name] 0033

[Correction method] Change

[Correction content]

As described above, in the objective lens of the present invention, a diffractive optical element suitable for the intended use is required to satisfy at least one of the above conditions (1) and (2). It is more preferable to dispose it in a stationary position in order to satisfactorily correct chromatic aberration.

Claims (2)

[Claims] 1. An objective lens comprising at least one diffractive optical element and at least one cemented lens. 2. The objective lens according to claim 1, wherein at least one of the diffractive optical elements satisfies the following conditions (1) and (2). (1) D ₁ /D>0.8 (2) (h × f) / (L × I)> 0.07 where D ₁ is the diameter of the marginal light beam at the position of the diffractive optical element, and D is the maximum. Marginal beam diameter, h is the chief ray height at the position of the diffractive optical element, f is the focal length of the objective lens,
L is the parfocal distance, and I is the maximum image height on the sample surface.