JPH11160013A - Shearing interferometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a shearing interferometer having a reflection plane, whose reflectivity is not affected by the change in the polarization direction. SOLUTION: Coherent light from a same light source is transmitted through a measured object, two pencils of light 1, 2, which are slightly shifted from a direction being vertical to an optical axis by reflecting measured light at a first reflection plane 71b and a second reflection plane 72a, are created, and non-polarization coat comprising multilayer films is applied at two reflection planes 71b, 72b in an interferometer that arises interference of the two pencils of light by superposing them. As a result, even if measured object has optical rotary power, a reflectivity is constant and influence of ghost light is suppressed to a low level.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for measuring a refractive index distribution and the like of a test object by analyzing interference fringes.
It relates to the structure of a shearing interferometer.

[0002]

2. Description of the Related Art In recent years, plastics have been increasingly used as materials for optical lenses used in optical devices such as laser printers and cameras. Compared to a glass polished lens, a plastic molded lens has advantages in that it is superior in cost reduction and manufacturability of an aspherical lens and is inexpensive.

[0003] On the other hand, however, the refractive index distribution is unstable in production as compared with a glass lens, and non-uniformity may occur inside the lens. Non-uniformity inside the lens has a great effect on optical characteristics, leading to deterioration of image quality and blurring. Therefore, it is necessary to accurately measure the refractive index distribution inside the lens and evaluate the homogeneity of the optical lens.

Accordingly, the applicant of the present invention has filed a Japanese Patent Application No. Hei.
In No. 03502, the test object is immersed in the test solution and rotated about an axis orthogonal to the optical axis, and the interference fringes are analyzed at each of a plurality of rotation angle positions. Was calculated, and an image was reconstructed using CT (computer tomography) analysis, and a method of obtaining a refractive index distribution was proposed.

When the refractive index distribution is measured using such interference fringes, if the difference between the refractive indices is small, the pitch of the interference fringes is large and the fringe analysis can be sufficiently performed. However, when the refractive index difference is large, the pitch of the interference fringes becomes too fine and exceeds the resolution, and it becomes impossible to analyze the interference fringes.

Therefore, even when the refractive index difference is large, the pitch of the interference fringes does not become fine, and a shearing interferometer has been used in an improved measuring apparatus in order to enable fringe analysis.

A specific description will be given with reference to FIG. The device shown in FIG. 1 includes a light source 1 for emitting laser light as coherent light,
A mirror 3, a beam expander 5 for expanding a laser beam into a light beam of an appropriate size, a shearing generator 7, a beam splitter 9 for splitting a light beam, and a beam splitter 9
And a monitor 13 in the other light beam emitted from the beam splitter 9 and arranged in one of the light beams emitted from the light source. Interference fringe detector 1
The monitor 1 and the monitor 13 both have an imaging lens and a CCD sensor. The interference fringe detector 11 is connected to a computer or the like to perform fringe analysis. Project the fringe image.

The test object A is installed in a box-shaped container-like cell 15 provided between the beam expander 5 and the shearing generator 7. The cell 15 is filled with a test solution B whose refractive index is almost the same as the refractive index of the test object A. The test object A is placed on a turntable 17, and the turntable 17 is rotatable by an arbitrary angle about an axis O orthogonal to the optical axis by a servo motor (not shown) or the like. Both ends of the cell 15, that is, the laser beam entrance window 15a and the exit window 15b are parallel to each other, and
Optical flats 19 and 21 with high surface accuracy respectively
Is attached for liquid-tight shielding. Therefore, the cell 15 filled with the test object A and the test solution B becomes an object having a uniform refractive index as a whole, and the incident surface and the exit surface are parallel. The light is emitted as a parallel light flux.

In the shearing generating section 7, a first flat plate 71 and a second flat plate 72 are arranged in parallel, and FIG. 3 is an enlarged view of this portion. The inner two surfaces 71b and 72a of the first flat plate 71 and the second flat plate 72 are polished surfaces with high surface accuracy, and these become the first reflecting surface 71b and the second reflecting surface 72a, respectively. The two surfaces 71a and 72b are non-reflective surfaces.

With this configuration, the laser beam (test wave) transmitted through the test object A enters at 45 ° from the upper left in FIG. 3, passes through the outer surface 71a of the first flat plate 71, and A part of the light beam is reflected by the reflecting surface 71b, the remaining light is transmitted, and a part of the light beam is reflected by the second reflecting surface 72a, and the remaining light is transmitted through the second flat plate 72. Then, the light beam reflected by the first reflection surface 71b and the light beam reflected by the second reflection surface 72a become parallel light beams as shown by a solid line and a dotted line in FIG.

[0011] These parallel light beams are transmitted to the first reflecting surface 71b.
Shearing interference of a lateral shift amount √2S proportional to the distance S between the second reflection surface 72a and the second reflection surface 72a. Then, the image of the shearing interference fringe is formed on both the interference fringe detector 11 and the monitor 13, and is CT-analyzed by the interference fringe detector 11 and a computer (not shown) while being visually observed on the monitor 13. The refractive index distribution of the specimen A will be measured.

[0012]

However, the above-mentioned shearing interferometer has the following problems.

First, the luminous flux generated in the shearing portion is:
Other than the above, these become ghost light,
Affects the observation of interference fringes. That is, as shown in FIG. 3, the test wave transmitted through the test object A is first partially reflected on the outer surface 71a of the first flat plate 71 to generate a light beam. Further, the first reflection surface 71b and the second reflection surface 7
The light beam is multiple-reflected between the light beams 2a and 2a. Also, the second
The light is reflected by the outer side surface 72b of the flat plate 72 of FIG. The rest passes through the second flat plate 72 to become a light beam.
Of these, the light flux becomes ghost light. Further, the transmitted light beam may cause stray light in some cases.

Second, there is a problem due to a change in the direction of polarized light. That is, the test wave is usually P-polarized light or S-polarized light, but if the test object A is a medium having birefringence, the polarization component changes. Then, when the polarization component changes, the reflectances of the first reflection surface 71b and the second reflection surface 72a differ, and both reflectances deviate from the initially set relationship,
The effect of ghost light and the like becomes large.

Third, there is a problem due to an error in processing accuracy between the first reflecting surface and the second reflecting surface. If there is an error in the processing accuracy of the reflecting surface due to uneven coating or the like, the reflectance between the first reflecting surface and the second reflecting surface deviates from an ideal state, and the influence of ghost light increases.

The present invention has been made to solve the above-mentioned problem, and has as its first object to provide a shearing interferometer capable of generating shearing light which is not affected by a polarization component.

It is a second object of the present invention to provide a shearing interferometer capable of reducing the influence of ghost light and the like even if there is an error in the processing accuracy of the reflecting surface. It is a third object to provide a shearing interferometer in which stray light is not generated from transmitted light.

[0018]

SUMMARY OF THE INVENTION In order to achieve the first object, the present invention transmits coherent light from the same light source to a test object and transmits the transmitted test wave to a first and a second test object. In a shearing interferometer that reflects light from a reflecting surface to form two light beams slightly displaced in a direction orthogonal to the optical axis, and superimposes and interferes with the two light beams, the first and second reflecting surfaces have no polarization. It is characterized by a coat. Further, the non-polarization coat may be a multilayer coat.

In order to achieve the second object of the present invention, when the reflectance of the first reflecting surface is R1 and the reflectance of the second reflecting surface is R2, 0.10 ≦ R1 ≦ It is characterized in that 0.16 0.14 ≦ R2 ≦ 0.20.

In order to achieve the third object of the present invention, the present invention provides an alignment adjusting means for adjusting the parallelism between the first and second reflecting surfaces, and a surface of the first and second reflecting surfaces. It is characterized by having coarse adjustment means for roughly adjusting the interval and fine adjustment means for adjusting the surface interval on the order of the wavelength of the coherent light or less.

At this time, the alignment adjusting means includes:
It is desirable that the coarse adjustment unit and the fine adjustment unit are arranged outside the light beam of the coherent light.

[0022]

Embodiments of the present invention will be described below. In FIG. 2, a light beam used for shearing interference is a light beam reflected by the first reflection surface 71b and the second reflection surface 72a. Ideally, the ratio of the intensity to the light flux is 1: 1.
, Is desirably 0, but in practice this is not the case.

First, the outer surfaces 71a and 72b of the first flat plate 71 and the second flat plate are antireflection surfaces, and a single-layer antireflection film was conventionally formed. In addition, since the reflectance can be arbitrarily set by appropriately selecting the anti-reflection film, the anti-reflection film has been formed aiming at a reflectance of 0.

However, in a single-layer antireflection film, S
The reflectance differs between the polarized light and the P-polarized light. Therefore, if the test object has birefringence, for example, even when the test object is incident on P-polarized light, the luminous flux transmitted through the test object contains the S-polarized light component, and the reflectance greatly deviates from 0. Will be. Therefore, the luminous flux is relatively increased, and the ghost is easily affected by the ghost light.

Therefore, in the present invention, these antireflection surfaces 71a and 72b are optical surfaces that do not depend on polarization (non-polarization). Specifically, a multilayer film was used instead of the conventional single-layer film. With such a configuration, antireflection surfaces 71a and 72b having an arbitrary reflectance and independent of polarization.
Could be obtained. As a result, in FIG. 3, the effect of the ghost light could be sufficiently reduced.

Next, first and second light sources for generating shearing light will be described.
The reflection surfaces 71b and 72a will be described. As described above, the ideal reflectance of the antireflection surfaces 71a and 72b is ideally 0. However, there is an error in processing accuracy such as non-polarization at an incident angle of 45 ° and unevenness of the antireflection film. Therefore, in actuality, it becomes about 0.5 to 0.2%, and although a little, a luminous flux remains.

On the other hand, the first and second reflecting surfaces 71b, 72a
It is desirable from the viewpoint of measurement that the reflectance is small. However, since the luminous flux remains as described above, if the reflectances of the first and second reflecting surfaces 71b and 72a are small, the ghost light becomes relatively stronger with respect to the luminous flux and interferes with the luminous flux. The contrast of the stripes is low.

Conversely, the first and second reflecting surfaces 71b, 72a
Is large, the first and second reflecting surfaces 71b and 72
The light flux repeatedly reflected during a increases, and the light flux due to multiple reflection increases, so that the influence due to this cannot be ignored.

[0029]

[Table 1]

Table 1 shows the reflectance of each surface and the intensity of each light beam in the embodiment when the incident light is 1. The right column in the table shows the intensity of each light beam when the light intensity is 1.000.

Table 2 below shows the reflectance of the first and second reflecting surfaces and the intensity of the light beam when the intensity of the light beam is 1 in each combination.

[0032]

[Table 2]

From the above, it is understood that the reflectance R1 on the first reflecting surface 71b is optimally in the range of 0.10 ≦ R1 ≦ 0.16. Second reflection surface 72a
It is ideal that the reflectance with respect to the luminous flux is 1: 1. However, assuming that ± 30% (0.7 to 1.3) is a desirable allowable value, 0.14 ≦ R2 ≦ 0.20

The range is appropriate. In the range surrounded by the thick line in Table 2, the intensity with the luminous flux is ± 30% (0.7 to 1.0).
3) is shown, and it can be said that a combination within this range is more desirable.

FIG. 1 is a diagram schematically showing a structure for adjusting the position of the first flat plate 71 and the second flat plate 72. Two flat plates 7
The reference numerals 1 and 72 are fixed to holders (not shown) and attached to a common base.

The first flat plate 71 on the incident light side is provided with an alignment adjusting means 21 and an auxiliary interval adjusting means 23. The alignment adjusting means 21 is provided for the first flat plate 7.
This is for adjusting the degree of parallelism with the second flat plate 72 by rotating 1. Further, the auxiliary interval adjusting means 23 is provided with the first
Is moved in the optical axis direction to roughly adjust the distance between the flat plates 71 and 72.

The second flat plate 72 has a coarse adjusting means 25 and a fine adjusting means 27. These are both the second flat plate 7
This is the same as moving the second flat plate 2 in the optical axis direction, but the coarse adjusting means 25 is for setting the interval S between the two flat plates 71 and 72 shown in FIG. 3, and the second flat plate is in the order of about 100 μm. Move 72. On the other hand, the fine adjustment means 27
The second flat plate 72 is moved in the optical axis direction on the order of the wavelength (nm) or less of the light source 1, and uses a piezo element. The number of the fine adjustment means 27 may be one.
2 may move in one direction, and the parallelism may go out of order. Therefore,
In order to prevent this, the circumference of the circular second flat plate is set to 12
It is desirable to provide at about three places equally divided by 0 °.

Prior to the measurement of the shearing interference, the first and second flat plates 71 and 72 are adjusted in position as follows. First, the interval between the two flat plates 71 and 72 is roughly determined by the auxiliary interval adjusting means 12. Next, the first flat plate 71 is rotated by the alignment adjusting means 11 to be parallel to the second flat plate 72. Next, the coarse adjustment means 13 sets the shearing amount to the set value S. The order adjusted here is on the order of microns.

Thereafter, the interference fringes are imaged and observed.
The phase shift method is used for analyzing the interference fringes. Therefore, the fine adjustment means 14 using the piezo element moves the second flat plate 72 in the direction of the optical axis on the order of the wavelength (submicron) or less to form interference fringes, and performs analysis.

In the above configuration, the alignment adjusting means 21, the auxiliary interval adjusting means 23, the coarse adjusting means 25 and the fine adjusting means 27 all have the light beam L incident on the flat plates 71 and 72.
It is arranged outside. With such a configuration, members other than the optical element are eliminated in the optical path of the light beam L, and the effect of stray light generated by reflection of transmitted light (the light beam in FIG. 3) can be suppressed, and highly accurate analysis can be performed. Becomes

Although the auxiliary interval adjusting means 23 roughly determines the interval between the flat plates 71 and 72 as described above, it is not always necessary, and the coarse adjusting means 25 may also serve the role. .

[0042]

According to the present invention as described above,
Since the first and second reflection surfaces as shearing generation surfaces are non-polarization reflection surfaces, the reflectance does not change even if the polarization direction changes. Therefore, even when the test object has birefringence, the transmitted wavefront can be measured under the same conditions.

The reflectance R1 of the first reflecting surface is
0.10 ≦ R1 ≦ 0.16, the reflectance R of the second reflecting surface
2 satisfies 0.14 ≦ R2 ≦ 0.20, so that even if there is an error in processing accuracy on each reflecting surface, ghost is small, the reflection ratio of shearing light is close to 1: 1 and contrast is good Interference fringes can be obtained. Therefore, S / N
Measurement with a high ratio becomes possible.

Alignment adjusting means for adjusting the parallelism between the first and second reflecting surfaces, coarse adjusting means for roughly adjusting the surface spacing between the first and second reflecting surfaces, and adjusting the surface spacing. By providing fine adjustment means for adjusting the order of the wavelength of the interference light or less, and by arranging these outside the shearing light beam (with the light beam), it is possible to suppress the influence of stray light and perform highly accurate analysis. become.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a position adjusting structure of a first flat plate and a second flat plate.

FIG. 2 is a diagram showing a configuration of a measuring apparatus of a refractive index distribution by a shearing interferometer.

FIG. 3 is a diagram illustrating a configuration of a shearing unit and each reflected light beam.

[Explanation of symbols]

 A test object 1 light source 21 alignment adjusting means 25 coarse adjusting means 27 fine adjusting means 71b first reflecting surface 72a second reflecting surface

Claims (5)

[Claims] 1. A coherent light beam from the same light source is transmitted to a test object, and the transmitted test wave is reflected by first and second reflection surfaces and slightly shifted in a direction orthogonal to an optical axis. A shearing interferometer for producing a light beam and superimposing and interfering these two light beams, wherein the first and second reflection surfaces are coated with a non-polarizing light. 2. The shearing interferometer according to claim 1, wherein the non-polarization coat is a multilayer coat. 3. The reflectance of the first reflecting surface is R1, the second reflectance is
The shearing interferometer according to claim 1, wherein when the reflectance of the reflecting surface is R2, 0.10 ≦ R1 ≦ 0.16 0.14 ≦ R2 ≦ 0.20. 4. An alignment adjusting means for adjusting the parallelism between the first and second reflecting surfaces, a coarse adjusting means for roughly adjusting the surface interval between the first and second reflecting surfaces, and the surface interval 4. A shearing interferometer according to claim 1, further comprising: fine adjustment means for adjusting the wavelength of the coherent light on the order of not more than the wavelength of the coherent light. 5. The shearing interferometer according to claim 4, wherein the alignment adjusting means, the coarse adjusting means, and the fine adjusting means are arranged outside the light beam of the coherent light.