JPH1090113A - Interferometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To highly accurately evaluate performance of a large numerical aperture(NA) lens by providing a reference wavefront generation means which generates a reference wavefront to interfere with a wavefront to be detected which runs an incident optical path in an opposite direction after being reflected at a reflecting means and passes again a lens to be detected. SOLUTION: A laser beam 11 is reflected at a polarizing reflecting face 13a of a polarization beam splitter(PBS) 13, runs along an optical axis 12 in a forward direction and penetrates a 1/4 wave plate 15 to be a circularly polarized light. The light passing the 1/4 wave plate 15 is reflected at a face 16a to be a reference wave R1 . The wave further penetrates a plane plate 16 and becomes a parallel light T1 . The parallel light T1 is refracted at a plane 19b to be a spherical wave-like wavefront and reaches a convex face 19a. The wavefront in a range S provided for reflection is directly reflected and consequently returns an original optical path to be a parallel plane wave again, which penetrates the plane plate 16, etc., and reaches a screen 21. A light and shade pattern because of an interference of the reference wave R1 and a wave R2 to be detected is projected onto the screen 21, and imaged by a CCD 22.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an interferometer for evaluating the performance of an optical lens, and more particularly to an interferometer suitable for evaluating a lens having a high numerical aperture (NA).

[0002]

2. Description of the Related Art One of the methods for evaluating an optical lens is a method using an interferometer. There are various types of such interferometers, such as a Fizeau interferometer and a Twyman-Green interferometer. All interferometers are common in that they require a reference wavefront (reference wavefront) to interfere with the test wavefront that has passed through the subject (optical lens), and how to accurately form that reference wavefront This is an important factor that affects the accuracy (that is, evaluation accuracy) of the interferometer.

FIG. 9 shows a schematic configuration of a conventional optical lens evaluation system using a Fizeau interferometer. Specifically, evaluation is performed using a so-called fringe scan method. This interferometer uses a laser beam 10 as a parallel light beam.
A polarizing beam splitter (hereinafter, referred to as PBS) 103 for reflecting light 1 toward the optical axis 102;
A quarter-wave plate 105, a plane plate 106, and an aperture 10 arranged between the lens 3 and the lens 104 to be inspected in the order from the one closer to the PBS 103 so as to be orthogonal to the optical axis 102.
7 and a correction plate 10 sequentially disposed behind the lens to be tested 104 (forward direction when focusing on the traveling direction of the laser beam).
8 and the concave reflecting mirror 109, and 1 /
A screen 111 and an image pickup device 112 using, for example, a CCD (charge-coupled device) are sequentially provided on the opposite side to the four-wavelength plate 105.

The laser light 101 is linearly polarized light,
100% of the light is reflected on the polarization reflection surface 103a in the forward direction 103. 1/4 wavelength plate 1
Numeral 05 is for converting linearly polarized light into circularly polarized light or circularly polarized light into linearly polarized light by giving a 90-degree phase difference between two orthogonal polarization components to light passing therethrough. The flat plate 106 is a quarter-wave plate 105
The surface 106a opposed to the lens 106a is subjected to anti-reflection processing, and the surface 106b opposed to the test lens 104 is a high-precision parallel flat plate having a reflectance of about 4%.
The light that is internally reflected by the light source and travels in the backward direction becomes a reference wave R ₁ as a reference plane wave that causes interference. The flat plate 106 is formed on the optical axis 1 by a piezo element (not shown) or the like.
It is possible to move in four steps in the direction of 02 by λ / 4 (１ ／ of the wavelength of light). The aperture 107 is a stop for restricting a light beam incident on the test lens 104.

The test lens 104 is an objective lens or the like for an optical pickup in an optical disk device, and has a high NA.
have. The correction plate 108 is for generating an optical path difference equivalent to an optical path difference generated when the test lens 104 is used in combination with a cover glass when the test lens 104 passes through the cover glass. is there. For example, when the test lens 104 is the above-described objective lens for an optical pickup, the transparent substrate portion of the optical disk corresponds to a cover glass.

[0006] The concave reflecting mirror 109 is used to
4 is provided with a high-precision concave spherical surface 109a for reflecting the wavefront which has been narrowed down to one point and then spread in a spherical wave shape in the reverse direction (return direction). This concave reflection mirror 1
Reference numeral 09 denotes an arrangement in which the center of curvature 110 of the concave spherical surface 109a matches the focal position of the lens 104 to be measured. Incidentally, the reflectance of the concave spherical surface 109a is where correction plate 108 after being reflected, substantially equal to the strength of the lens 104 and the plane plate 106 to be detected R ₂ of intensity reference wave R ₁ returned after passing through the It is set to be.

The screen 111 includes a flat plate 106.
Interference pattern between the reference wave R ₁ is reflected by the surface 106b, and the detection R ₂ which is again converted into a plane wave (collimated light) by subject lens 104 after being reflected by the concave spherical surface 109a of the concave reflecting mirror 109 Is to be projected so as to be visually recognizable. The interference pattern projected on the screen 111 is imaged by the imaging device 112 and processed by a signal processing circuit (not shown).

Next, the operation of such a conventional evaluation system will be briefly described. The laser light 101, which is linearly polarized light, is reflected by the polarization reflection surface 103a of the PBS 103 at approximately 100%, travels in the forward direction along the optical axis 102, and
05 and becomes circularly polarized light. Part of the light transmitted through the wavelength plate 105 is reflected by the surface 106 a of the flat plate 106 and becomes a reference wave R ₁ . The reference wave R ₁ further passes through the quarter-wave plate 105 in the backward direction, and becomes linearly polarized light having a polarization direction orthogonal to the polarization direction of the original linearly polarized light. Therefore, reference wave R ₁ is substantially 100% PBS103
The light penetrates and reaches the screen 111.

On the other hand, the parallel light T ₁ transmitted through the quarter-wave plate 105 to become circularly polarized light, and further transmitted through the plane plate 106 is stopped down by the lens 104 to be inspected, and
After the focal point is formed behind 08, the light spreads again in a spherical wave shape and enters the concave reflecting mirror 109. Concave reflection mirror 10
Numeral 9 reflects the incoming spherical light wave in the direction opposite to the incident direction. As a result, the incident light passes through the original optical path, becomes a parallel plane wave again by the test lens 104, and sequentially transmits through the plane plate 106 and the 波長 wavelength plate 105. At this time, since the circularly polarized light becomes a linearly polarized light that is polarized in a direction orthogonal to the direction of the original linearly polarized light by passing through the 波長 wavelength plate 105, it passes through the PBS 103 approximately 100% and reaches the screen 111. I do.

As a result, interference fringes (bright and dark patterns) caused by interference between the reference wave R ₁ and the test wave R ₂ are projected on the screen 111, and the interference fringes are imaged by the imaging device 112. Then, the flat plate 106 is set to λ / 4
The light beam is moved in four steps in the optical axis direction to change the intensity of the interference projected on the screen 111, and the change in the intensity is processed by a signal processing circuit (not shown) to evaluate the performance by the fringe scan method.

[0011]

As described above, in the conventional interferometer, the spherical wave reflecting means for reflecting the wavefront which is narrowed down by the lens 104 to be inspected and then spread in a spherical wave shape in the opposite direction has a high spherical precision. The concave reflection mirror 109 was used. However, in the field of optical discs in recent years, an objective lens for an optical pickup having a high NA is being trial-produced in order to realize a higher density.
Lens evaluation may be required. In the case of such a high NA lens, since the aperture angle θ (FIG. 9) is large, the area of the concave spherical surface 109a of the concave reflecting mirror 109 that is actually used for reflection is also large. Therefore, a high-precision concave spherical surface is required in a wide range. However, it is actually extremely difficult to manufacture such a spherical mirror, and the manufacturing cost is extremely high.

Further, since the concave reflecting mirror 109 is large in weight and size, and has a thin central portion, even if the concave spherical surface 109a can be manufactured to have an ideal spherical shape, its own weight, temperature, or Distortion is likely to occur due to a fixing jig or the like when incorporated in the interferometer, and handling is not easy.

Further, as described above, when evaluating a test lens which is assumed to be used in combination with a cover glass, a correction plate 108 corresponding to the cover glass is required. If it is thin, it is difficult to manufacture, and it is easily deformed due to its low strength.
There is a problem with handling. In particular, when the test lens 104 is an objective lens for an optical pickup, in order to increase the NA, it is necessary to take measures against coma caused by skew of the optical disk substrate. Since it is proportional to the power and inversely proportional to the thickness of the disk substrate, it is necessary to make the disk substrate thinner. As a result, the thickness of the correction plate 108 used for the interferometer tends to be very thin. Since the thin correction plate has low strength and is easily distorted or deformed, it is expected that manufacturing and handling of the correction plate will become more difficult.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and an object of the present invention is to provide an interferometer which can evaluate the performance of a high NA lens with high accuracy and is easy to manufacture and handle. It is in.

[0015]

The interferometer of the present invention comprises a reflecting means for reflecting a spherical wave light wave after passing through a lens to be inspected while maintaining the spherical wave shape, and an incident light path after reflection by the reflecting means. And a reference wavefront generating means for generating a reference wavefront as a reference for causing interference with a test wavefront obtained by passing through the test lens again. The interference between the test wavefront and the reference wavefront is provided. An interferometer for evaluating the optical characteristics of an inspection lens, which is configured using an optical lens having at least a convex spherical surface as a reflection unit. As reflection means,
For example, using an optical lens composed of a convex spherical surface and a plane,
The plane side of the optical lens is arranged to be substantially perpendicular to the optical axis of the lens to be measured, facing the lens to be measured. Then, the light wave after passing through the lens to be measured is made incident on the plane side of the optical lens to be converted into a spherical wavefront centered on the center of curvature of the convex spherical surface of the optical lens, and this spherical wavefront is converted into an optical lens. The wavefront to be detected is obtained by reversing the incident optical path by reflecting the light on the inner surface of the convex spherical surface. When a hemisphere is used as the optical lens, the optical lens is arranged such that the center of curvature coincides with the focal position of the lens to be measured. Also, the test lens is
In the case where the optical lens is used in a state where another medium is arranged on the convergent light path, the optical lens is formed to be thicker than the hemisphere by a thickness optically equivalent to the medium, and the lens to be measured is formed. The light waves after passing are arranged so as to be focused at the center of curvature of the convex spherical surface. Further, the convex spherical surface side of the optical lens is arranged to face the test lens, and the light wave after passing through the test lens is reflected by the convex spherical surface of the optical lens to reverse the incident optical path so as to obtain the test wave surface. It is also possible to

In the interferometer of the present invention, the light wave after passing through the lens to be inspected is reflected by at least the optical lens having a convex spherical surface while maintaining its wavefront shape, and then travels backward in the incident optical path and is again returned to the lens to be inspected. , And becomes a test wavefront.
The test wavefront interferes with the reference wavefront generated by the reference wavefront generator to form an interference fringe.

[0017]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows a schematic configuration of an optical lens evaluation system using a Fizeau interferometer according to an embodiment of the present invention. This system performs evaluation using a fringe scan method. This interferometer reflects a laser beam 11 that is a parallel light beam and directs the laser beam 11 in the direction of an optical axis 12.
A quarter wavelength plate 15, a plane plate 16 and an aperture 17, which are arranged between the BS 13 and the PBS 13 and the lens 14 in order from the one closer to the PBS 13 so as to be orthogonal to the optical axis 12; (A forward direction when focusing on the traveling direction of the laser beam), and a screen 21 and a CCD, for example, which are sequentially arranged on the opposite side to the quarter wavelength plate 15 of the PBS 13. And an imaging device 22. Here, the hemispherical lens 19 corresponds to the reflecting means in the present invention, and the flat plate 16 corresponds to the reference wavefront generating means in the present invention.

The laser light 11 is linearly polarized light,
The third polarized light reflecting surface 13a reflects 100% of the light in the forward direction. The quarter-wave plate 15 is
By giving a 90-degree phase difference between two orthogonally polarized light components to light passing therethrough, linearly polarized light is converted into circularly polarized light or circularly polarized light is converted into linearly polarized light. The flat plate 16 is a high-precision parallel flat plate having a surface 16a facing the quarter-wave plate 15 subjected to antireflection treatment, and a flat surface 16b facing the test lens 14 having a reflectance of about 4%. light directed toward the backward direction by internal reflection at these planes 16b becomes the reference wave R ₁ as a reference plane wave causing interference. The flat plate 16 can be moved in four steps in the direction of the optical axis 12 by λ / 4 by a piezo element or the like (not shown). Aperture 17
Is a stop for restricting a light beam incident on the test lens 14. The test lens 14 is, for example, an objective lens for an optical pickup in an optical disk device, and has a high N
A.

The hemispherical lens 19 has a convex spherical surface 19a and a plane 19b, of which the plane 19b is opposed to the lens 14 to be measured, and is arranged so as to be orthogonal to the optical axis 12. The center of curvature 19c (FIG. 2) of the convex spherical surface 19a is the optical axis 1
2 are arranged. The light converged by the test lens 14 is incident on the plane 19b. 2, the hemispherical lens 19 is formed to be thicker than the hemisphere by d, and the arrangement position in the optical axis direction is such that the spherical light wave incident from the plane 19b is a convex spherical surface 19b.
Fine adjustment is made so that light is condensed at one point at the center of curvature 19c of a. The light gathered at one point at the center of curvature 19c is
Further, it spreads in a spherical wave shape, and at the inner surface of the convex spherical surface 19a, regular reflection is performed while maintaining the wavefront shape, and the light travels backward in the backward direction. Here, the thickness d portion of the hemispherical lens 19 is based on the assumption that the test lens 14 is used in combination with a cover glass (in the case of an objective lens for an optical pickup, a transparent substrate portion of an optical disk). In this case, the portion is added to generate an optical path difference equivalent to the optical path difference generated when the light passes through the cover glass, and serves as a substitute for the conventional correction plate 108 (FIG. 9). Incidentally, the reflectance of the convex spherical surface 19a is where the reflected subject lens 14 and the detection intensity of R ₂ is the reference wave R that the planar plate 16 has returned to transmission
It is set to be approximately equal to the intensity of ₁ .

The test lens 14 has two lenses 14a,
14b, which is a high NA lens, and ideally converts a plane wave into a perfect spherical wave. The test lens 14 may have a single lens configuration or a configuration including two or more lens groups. The screen 21 includes the reference wave R ₁ internally reflected by the plane 16 b of the plane plate 16 and the hemispherical lens 1.
The interference pattern with the test wave R ₂ , which is reflected by the convex lens 19 a and converted into a plane wave (parallel light) by the test lens 14 again after being reflected by the test lens 14, is projected so as to be visually recognized. The interference pattern projected on the screen 21 is imaged by the imaging device 22 and processed by a microcomputer (not shown).

Next, the operation of such a lens evaluation system will be described. Laser light 11 which is linearly polarized light is PBS1
The third polarized light reflecting surface 13a reflects approximately 100%, travels in the forward direction along the optical axis 12, and passes through the quarter-wave plate 15 to become circularly polarized light. Part of the light transmitted through the quarter-wave plate 15 becomes the reference wave R ₁ is reflected by the surface 16a of the flat plate 16, and further transmitted through the quarter wavelength plate 15 in the backward direction, the original linearly polarized light It becomes linearly polarized light having a polarization direction orthogonal to the polarization direction. For this reason, the reference wave R ₁ is approximately 1
The light reaches the screen 21 by transmitting 00%.

On the other hand, the parallel light T transmitted through the quarter-wave plate 15 to become circularly polarized light and further transmitted through the flat plate 16
₁ is condensed by the test lens 14 and enters the plane 19b of the hemispherical lens 19 before focusing. The light incident on the hemispherical lens 19 is refracted on the plane 19b to form a spherical wavefront centered on the center of curvature 19c. After traveling through the thickness d, the light is focused on the center of curvature 19c, and further focused on P. The light travels through the hemispherical lens 19 while spreading on a spherical wavefront centered at the center, and reaches a convex spherical surface 19a. In the convex spherical surface 19a, the range S provided for reflection is formed as a high-precision spherical surface, so that the incident light wave receives specular reflection (vertical reflection) there, and keeps the incident wavefront as it is and moves backward in the incident direction. As a result, a parallel plane wave is formed again by the test lens 14 through the original optical path, and the plane plate 16
And １ ／ wavelength plate 15 in order. At this time, 1
Since the circularly polarized light becomes a linearly polarized light that is polarized in a direction orthogonal to the direction of the original linearly polarized light by transmitting through the 波長 wavelength plate 15, the screen 21 is transmitted by almost 100% through the PBS 13.
Reach the top.

As a result, a light and dark pattern due to the interference between the reference wave R ₁ and the test wave R ₂ appears on the screen 21, and this is picked up by the image pickup device 22. here,
Since the plane plate 16 is a high-precision parallel plane plate, the reference wave R ₁ has a substantially perfect (no distortion) plane wave shape. Therefore, assuming that the test lens 14 has ideal optical performance, the test wave R ₂ also has a substantially perfect plane wave shape, and the brightness distribution on the screen 21 is substantially uniform (the entire surface is substantially the same). Lightness). In this state, the center of curvature 1 of the convex spherical surface 19a of the hemispherical lens 19 is
When 9c is slightly shifted from the optical axis 12, a substantially linear bright and dark interference fringe is projected on the screen 21. FIG. 3 shows an example of such an interference fringe.
In this example, the interference fringe of the lens to be inspected having an NA of 0.8 is 0.028λ when its wavefront aberration is represented by rms (root mean square), and it has good optical performance. I understand.

In this way, the center of curvature 19c of the convex spherical surface 19a of the hemispherical lens 19 is slightly displaced from the optical axis 12 to project interference fringes on the screen 21, and the degree of bending of the interference fringes is visually checked. Although performance judgment is possible, in order to make this judgment with higher accuracy, the present system performs performance evaluation by the fringe scan method. In this fringe scanning method, the plane plate 16 is moved in four steps in the optical axis direction by λ / 4 to change the interference pattern (light and dark pattern) projected on the screen 21. Te, those that determine the wavefront state of the detection R ₂ quantitatively, the more measurement of the following contents in detail.

The fringe scanning method will be briefly described. Now, assuming that the wavefront shape of the test wave R ₂ is h (x, y) and the optical path length to be changed is l, the intensity distribution I (x, y, I) of the interference fringes that appears on the screen 21 and is captured by the imaging device 22. l) is represented by the following equation (1).

I (x, y, l) = a (x, y) + b (x, y) cos {2π / λ [h (x, y) −1]} (1)

Here, a (x, y) is a bias component of the interference fringe intensity distribution, and b (x, y) is a term representing a change in contrast of the interference fringe. Since the equation (1) is a periodic function with respect to the variable l, Fourier transform is performed on l, and a real part and an imaginary part corresponding to the first-order term of the Fourier series are extracted from the result. Is transformed, the following equation (2) is obtained.

[0029] h (x, y) = ( λ / 2π) · tan -1 {ΣI (x, y, l n) sin (2π n / N) / ΣI (x, y, l n) cos (2πn / N)｝ …… (2)

Here, l _n = n / N (n = 0, 1, 2, 2, 3)
.., N-1), and Σ represents the sum from n = 0 to n = N-1.

Here, if N = 4, the following equation (3) is obtained from the equation (2).

H (x, y) = (λ / 2π) · tan ⁻¹ {(I ₁ −I ₃ ) / (I ₀ −I ₂ )} (3)

Here, I _{0 to} I ₃ are I (x,
y, l ₀ ) to I (x, y, l ₃ ), which are determined by the imaging device 22 at each stage when the plane plate 16 is moved by 0, λ / 4, λ / 2, 3λ / 4, respectively. 4 shows the obtained interference fringe intensity distribution.

[0034] From this equation (3), the wavefront shape h (x, y) of the detection R ₂ is obtained, and each of the detection R ₂ (x,
By taking the square root of the average of the square of h (x, y) at the position y), a value corresponding to the rms of the spherical aberration is obtained.

Next, the spherical accuracy of the hemispherical lens 19, which is a feature of the present embodiment, will be described with reference to FIGS.

FIG. 4 shows a schematic configuration of an interferometer system for measuring the spherical accuracy of a spherical lens from which the hemispherical lens 19 is manufactured. In this figure, FIG.
The same components as those described above are denoted by the same reference numerals. In this system, an infinite null lens 25 is arranged in place of the test lens 14 in FIG.
9 is replaced by a spherical lens 19 ′ serving as a base of the hemispherical lens 19. Here, the null lens 25 is a special lens manufactured to convert infinite light (parallel light) into an ideal convergent spherical wave and focus on one point on the optical axis. The spherical lens 19 ′ is arranged such that the center of curvature 19 c coincides with the focal position of the null lens 25. Other configurations are the same as those in FIG.

In this interferometer system, the null lens 25
The ideal convergent spherical wave produced by the above is incident on the spherical lens 19 'to be inspected, and is specularly reflected (perpendicularly reflected), reverses the original optical path, passes through the null lens 25 again, and is subjected to the test wave R _2.
This test wave R ₂ is the plane 1 of the plane plate 16.
Interferes with the reference wave R ₁ reflected by 6b forming interference fringes on a screen 21 shape. Therefore, by analyzing this interference fringe, the spherical accuracy of the spherical lens 19 'can be measured.

FIG. 5 shows an example of measuring the spherical accuracy of the spherical lens 19 'obtained by the interferometer system of FIG. However, this figure shows a state where the center of curvature 19c of the spherical lens 19 'in FIG. 4 is slightly displaced from the optical axis so as to be convenient for visual confirmation.
This figure shows the NA of the convex spherical surface 19a of the spherical lens 19 '.
Are interference fringes obtained for a region corresponding to 0.8, and each interference fringe is substantially linear. When the wavefront aberration is represented by rms, the value is 0.008λ, which indicates that the wavefront aberration is formed on a highly accurate spherical surface (ideal spherical surface).

As described above, in the present embodiment, the hemispherical lens 19 is used as a means for specularly reflecting the wavefront of the spherical wave transmitted through the lens 14 to be detected and returning the wavefront to the original optical path. It can be made smaller and lighter than the concave reflection mirror. For this reason, distortion due to its own weight, temperature, mounting jig and the like is small, handling is easy, and stable measurement can be performed for a long time. Further, the hemispherical lens 19 can be manufactured relatively easily by cutting a commercially available inexpensive spherical lens and polishing the flat portion to a mirror surface, and also has a sufficient spherical precision as shown in FIG. Obtainable. Therefore, a highly accurate spherical wave reflecting means can be manufactured more easily and at lower cost than a conventional concave reflecting mirror, and the cost of the entire interferometer can be reduced.

Further, as is apparent from FIG. 2, the opening angle θ 'of the light beam in the hemispherical lens 19 is smaller than the opening angle θ of the light beam before entering the hemispherical lens 19. Therefore, the area of the range S of the convex spherical surface 19a that is provided for reflection is smaller than that of the related art.
That is, the area in which high precision (ideal spherical shape) must be ensured is small, and this also facilitates the manufacture of the hemispherical lens 19.

If the lens under test 14 is intended to be used in combination with a cover glass (in the case of an objective lens for an optical pickup, a transparent substrate portion of an optical disk), a hemispherical lens is used. If the correction plate 108 is not formed as a complete hemisphere but is formed to be thicker than the hemisphere by a value d determined in consideration of an optical path difference generated when passing through the cover glass, the correction plate 108 conventionally required
(FIG. 9) becomes unnecessary. For this reason, even if the cover glass (the transparent substrate portion of the optical disk) is further thinned in the future, the compensator which is difficult to manufacture and handle because the thickness is extremely thin is omitted, and the manufacture and handling are performed. This can be dealt with simply by changing the thickness of the hemispherical lens 19 easily.

Next, how to determine the thickness of the hemispherical lens 19 in FIG. 1 (in other words, the thickness d of the hemispherical lens 19 beyond the hemisphere) will be briefly described.

As described above, the portion having a thickness d of the hemispherical lens 19 is formed by combining the lens under test 14 with a cover glass (in the case of an objective lens for an optical pickup, a transparent substrate portion of an optical disk). In the case where it is assumed to be used, it is a portion added to generate an optical path difference equivalent to an optical path difference generated when passing through a cover glass, and is a part of the conventional correction plate 108 (FIG. 9). It is an alternative. Here, the test lens 14 is designed to correct spherical aberration generated by the cover glass itself.

If the thickness d of the portion of the hemispherical lens 19 that exceeds the hemisphere is set to the same amount as the thickness of the cover glass, the spherical aberration caused by the difference in the refractive index between the cover glass and the hemispherical lens 19 is assumed. Will remain. Here, assuming that the refractive index of the cover glass is n and the refractive index of the hemispherical lens 19 is n + Δn, the residual spherical aberration W of the hemispherical lens 19 when the thickness is larger than that of the cover glass is d. _40n is represented by the following equation (4).

W _40n = {d (NA) ⁴ / 8n ³ } − (n ² −3) (Δn / n) + (n ² −6) (Δn / n) ² } (4)

On the other hand, the residual spherical aberration W _40d due to the thickness error Δd of the hemispherical lens 19 is expressed by the following equation (5).

W _40d = {(n + Δn) ² −1} (NA) ⁴ Δd / 8 (n + Δn) ³ (5)

Therefore, the condition for correcting the spherical aberration is W _40n + W _40d = 0. For example, if the thickness of the cover glass is 0.1 mm and the refractive index is 1.57 and the refractive index of the hemispherical lens 19 is 1.52, Δn decreases by 0.05. Therefore, spherical aberration can be completely corrected by increasing the thickness of the hemispherical lens 19 from the hemisphere by d + Δd = 0.014 mm. And
The hemispherical lens 19 having the thickness determined in this manner is used, and the convergent light passing through the lens to be measured 14 is arranged so as to be focused at the center of curvature 19c of the convex spherical surface 19a of the hemispherical lens 19. , The spherical aberration can be minimized.

However, in an actual evaluation system, in addition to the above-mentioned spherical aberration due to the difference in refractive index, chromatic aberration due to the difference between the working wavelength of the interferometer and the working wavelength of the device in which the lens 14 to be tested is incorporated must be considered. Must. However, since this chromatic aberration differs for each lens depending on the design specification of the lens 14 to be inspected, in practice, a simulation of ray tracing is performed to correct the aberration combining the above spherical aberration and chromatic aberration. It is necessary to set the thickness of the hemispherical lens 19. That is, after all, the optimum thickness of the hemispherical lens 19 is determined by the simulation result of the ray tracing program.

Next, another embodiment of the present invention will be described. FIG. 6 is an enlarged view of a hemispherical lens used for the interferometer according to the present embodiment. In the present embodiment,
Instead of the hemispherical lens 19 shown in FIGS. 1 and 2, a hemispherical lens 29 formed as a complete hemisphere (that is, a thickness d = 0) is used. A conventionally used correction plate 108 is disposed between the side and the test lens 14. Then, the convergent light from the test lens 14 is projected onto the convex spherical surface 29 of the hemispherical lens 29.
a hemispherical lens 2 so as to focus on the center of curvature 29c of a
9 is adjusted and arranged.

Even in the case of such a configuration, the above-described embodiment (see FIG. 3) can be used to easily manufacture and handle the hemispherical lens 29, reduce costs, prevent distortion and deformation, and enable long-term stable measurement. 1 and FIG. 2). Further, since the opening angle θ ′ of the light beam in the hemispherical lens 29 is smaller than the opening angle θ of the original incident light beam,
The same applies to the point that the area S of the convex spherical surface 29a of the hemispherical lens 29 that is provided for reflection needs to be small. However, this configuration differs from FIGS. 1 and 2 in that a correction plate 108 is required.

Of course, when the lens under test 14 is not intended to be used in combination with a cover glass, that is, in actual use, another medium member is placed between the hemispherical lens 19 and the beam irradiation target. If not present, as shown in FIG. 7, the hemispherical lens 29 is formed as a complete hemisphere, and the convergent light from the lens 14 to be examined is focused on the center of curvature 29 of the convex spherical surface 29a of the hemispherical lens 29.
It is only necessary to adjust and arrange the position of the hemispherical lens 29 so as to focus at c, and the correction plate 108 in FIG. 6 is not required.

Next, still another embodiment of the present invention will be described with reference to FIG. In the present embodiment, the directions of the convex spherical surface 19a and the flat surface 19b of the hemispherical lens 19 in FIG. 1 are reversed, the convex spherical surface 19 is arranged toward the lens 14 to be measured, and the center of curvature 19c of the convex spherical surface 19a. Are adjusted and arranged so as to coincide with the focal position of the test lens 14. In addition, a correction plate 108 is disposed between the hemispherical lens 19 and the test lens 14. Other configurations and arrangements are the same as those in FIG.

In this embodiment, the convergent wavefront that has passed through the lens to be inspected 14 passes through the correction plate 108, then is specularly reflected by the convex spherical surface 19a of the hemispherical lens 19, and passes through the original incident optical path. a detection wave R ₂ passes through the 14 in the opposite direction. In the above-described embodiment (FIG. 1 and the like), the test lens 1
4, the light wave that is expanding after being focused once is reflected internally by the convex spherical surface 19 a such as the hemispherical lens 19, whereas in the present embodiment, the focal point passes through the lens 14 to be measured. Is converged on the convex spherical surface 19a of the hemispherical lens 19 so as to be externally reflected.

Also in this embodiment, the hemispherical lens 1
9, the same effects as those of the above-described embodiment (FIGS. 1 and 2) are obtained, such as easiness of manufacture and handling, low cost, prevention of distortion and deformation, and possibility of stable measurement for a long time. However, when the test lens 14 is based on the premise that a cover lens is used, as shown in FIG.
08 is required.

Although the present invention has been described with reference to the embodiment, the present invention is not limited to this embodiment, and can be variously modified within an equivalent range. For example, in each of the above embodiments, a Fizeau-type interferometer is used, but another type of interferometer (for example, a Twyman-Green interferometer) may be used. Further, the objective lens 14 for an optical pickup has been described as an example of the lens 14 to be inspected. However, it is needless to say that the objective lens 14 can also be applied to evaluation of an objective lens of a microscope.

[0057]

As described above, claims 1 to 5
According to the interferometer according to any one of the above, the light wave of the spherical wave shape after passing through the test lens is reflected by an optical lens having at least a convex spherical surface while maintaining its wavefront shape, and then the incident light path is reversed. Then, the test wavefront having passed through the test lens again and the reference wavefront generated by the reference wavefront generating means interfere with each other to form an interference fringe. Therefore, an optical lens having a convex spherical surface is used as the reflecting means. It can be made smaller and lighter than conventional concave reflecting mirrors. For this reason, distortion due to its own weight, temperature, mounting jig and the like is small, handling is easy, and stable measurement can be performed for a long time. Further, an optical lens having a convex spherical surface can be manufactured relatively easily by cutting a commercially available inexpensive spherical lens and polishing the flat portion to a mirror surface, and can obtain a lens having a sufficient spherical precision. Therefore, the highly accurate spherical wave reflecting means can be manufactured more easily and at lower cost than the conventional concave reflecting mirror, and the cost of the entire interferometer can be reduced.

According to the interferometer of the second or third aspect, the light wave after passing through the lens to be measured is reflected while maintaining the wavefront shape by utilizing the internal reflection on the convex spherical surface of the optical lens. Therefore, in addition to the effect of the interferometer according to claim 1, the aperture angle of the convergent light is smaller than that in the conventional case of external reflection, and the area of the convex spherical region provided for reflection is correspondingly reduced. Thus, there is an effect that it is easier to manufacture a highly accurate spherical surface.

According to the fourth aspect of the present invention, in the case where the test lens is used in a state where another medium is disposed on the converging optical path, the optical lens is replaced with the medium. 3. The interferometer according to claim 2, wherein the interferometer is formed so as to be thicker than the hemisphere by a thickness optically equivalent to that of the hemisphere, so that the light wave after passing through the lens to be tested is focused at the center of curvature of the convex spherical surface. In addition to the effect described above, there is an effect that the conventionally required correction plate can be eliminated.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a configuration of an interferometer according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing the hemispherical lens in FIG. 1 in an enlarged manner.

FIG. 3 is an explanatory diagram showing interference fringes obtained for a test lens using the interferometer shown in FIG. 1;

FIG. 4 is an explanatory diagram showing a configuration of an interferometer system for measuring the spherical accuracy of the hemispherical lens in FIG.

5 is an explanatory diagram showing interference fringes of a hemispherical lens obtained by the interferometer system shown in FIG.

FIG. 6 is an explanatory view showing a hemispherical lens and a correction plate used in an interferometer according to another embodiment of the present invention.

FIG. 7 is an explanatory view showing a hemispherical lens used for an interferometer according to still another embodiment of the present invention.

FIG. 8 is an explanatory diagram showing a configuration of an interferometer according to still another embodiment of the present invention.

FIG. 9 is an explanatory diagram illustrating a configuration of a conventional interferometer.

[Explanation of symbols]

Reference numeral 12: optical axis, 13: polarizing beam splitter, 14: test lens, 15: quarter-wave plate, 16: flat plate, 1
9: hemispherical lens, 19a: convex spherical surface, 19b: flat surface

Continuing from the front page (72) Inventor Toshio Watanabe 6-7-35 Kita-Shinagawa, Shinagawa-ku, Tokyo Inside Sony Corporation (72) Inventor Kiyoshi Osato 6-35, Kita-Shinagawa, Shinagawa-ku, Tokyo Sonny shares In company

Claims (5)

[Claims] 1. A reflecting means for reflecting a spherical wave light wave after passing through a lens to be inspected while maintaining the spherical wave shape, and after reflecting by said reflecting means, reversing an incident optical path and passing through the lens again. Reference wavefront generation means for generating a reference wavefront of a reference for causing interference with the test wavefront obtained by
An interferometer for evaluating the optical characteristics of a test lens by using interference between a test wavefront and a reference wavefront, wherein an optical lens having at least a convex spherical surface is used as the reflection unit. Interferometer. 2. An optical lens having a convex spherical surface and a flat surface is used as the reflecting means, and the flat surface side of the optical lens is disposed substantially perpendicular to the optical axis of the test lens with the flat surface facing the test lens. The light wave after passing through the lens to be measured is made incident from the plane side of the optical lens to be converted into a spherical wavefront centered on the center of curvature of the convex spherical surface of the optical lens, and this spherical wavefront is converted into the optical lens. 2. The interferometer according to claim 1, wherein the wavefront to be detected is obtained by refracting the incident optical path by reflecting the light on the inner surface of the convex spherical surface. 3. The optical lens according to claim 1, wherein the convex spherical surface is a hemisphere having a hemispherical surface, and a center of curvature thereof is arranged to coincide with a focal position of the lens to be measured. Item 2. The interferometer according to Item 2. 4. In a case where the test lens is used in a state where another medium is arranged on a converging optical path, the optical lens has a thickness optically equivalent to the medium. 3. The interference lens according to claim 2, wherein the lens is formed to be thicker than a hemisphere, and the light wave after passing through the test lens is arranged so as to be focused at the center of curvature of the convex spherical surface. 4. Total. 5. The optical lens according to claim 1, wherein a convex spherical surface side of the optical lens is arranged to face the lens to be measured, and a light wave after passing through the lens to be measured is reflected by the convex spherical surface of the optical lens to reverse an incident optical path. 2. The interferometer according to claim 1, wherein said wavefront to be detected is obtained.