JPH11108837A - Measuring device for refractive index distribution - Google Patents

Abstract

(57) [Problem] To provide a refractive index measuring device capable of measuring even when the difference in refractive index is small and large, and capable of reducing the number of optical elements. SOLUTION: A laser beam from a light source 1 is split into a reference wave a and a test wave b by a first light splitter 5. The test wave b passes through the phase object as the test object O, and is split by the second light splitter 9 into a light beam b1 and a light beam b2. Luminous flux a
Is reflected by the first mirror 7 and overlaps with the light beam b1 by the first optical superimposer 11, causing interference. On the other hand, the light flux b2
Is divided into b21 and b22 by a second mirror so that there is a shift of δ between them. When the refractive index difference is small, a normal interference fringe is formed at a + b1, and when the refractive index difference is large, an interference fringe is formed at b21 + b22 due to sharing interference with a wide fringe interval. The polarization directions of the two interference fringes are orthogonal to each other, and one of the two interference fringes can be selected by rotating the third polarizer 21.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an optical element, a liquid,
Also, the present invention relates to an apparatus for measuring a phase object such as a gas, and particularly to an apparatus capable of measuring a refractive index distribution in a phase object by analyzing interference fringes.

[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 measure the refractive index distribution inside the lens three-dimensionally with high accuracy and evaluate the homogeneity of the optical lens.

As a method of measuring the refractive index of an optical lens, a method of measuring a refractive angle by a V-block method or the like using a precision differential refractometer or the like to obtain a refractive index, or an interference method such as a Twyman-Green interferometer or the like. There is a method of measuring the refractive index from the interference fringes using an interferometer, and the method of measuring optical homogeneity is based on the analysis of interference fringe images using an interferometer such as a Fizeau interferometer or a Mach-Zehnder interferometer. There is known a method of measuring a transmitted wavefront and obtaining optical homogeneity from a refractive index distribution.

However, in any of the above methods, the test object must be processed into a predetermined shape, and the optical element to be measured must be destroyed. Also, the refractive index distribution obtained from the transmitted wavefront becomes an average value integrated in the optical path traveling direction, and the three-dimensional spatial refractive index distribution is measured to specify the non-uniform refractive index part three-dimensionally. Can not.

The inventor of the present application has proposed that the test object is immersed in a test solution having substantially the same refractive index and rotated about an axis perpendicular to the optical axis, and interference fringes are detected at each of a plurality of rotation angles. And a method for calculating the amount of transmitted wavefront from these interference fringes, performing a first-order Fourier transform, and further performing a two-dimensional inverse Fourier transform to obtain a three-dimensional refractive index distribution is developed.

[0007]

According to the method and the apparatus for measuring the refractive index distribution according to the above technique, there is an advantage that the measurement can be performed with high accuracy even if the difference in the refractive index from the sample liquid is small. . However, when measuring a phase object having a large amount of transmitted wavefront, such as when the refractive index difference is large, the number of interference fringes increases, which may exceed the resolution of the detector. In view of this, the inventor has also developed that, when the refractive index difference is large, measurement is performed by using a sharing interferometer to increase the fringe interval.

However, when the refractive index difference is small,
If a normal interferometer such as a Mahachenda interferometer is used, and the refractive index difference is large, a sharing interferometer should be used, and two interferometers must be provided in the measurement device. The number of optical elements increases and the device becomes expensive. The present invention has been conceived from such a fact, and provides a refractive index measuring device that can measure even when the refractive index difference is small or large, and that can reduce the number of optical elements. It is intended to be.

[0009]

In order to achieve the above object, a measuring apparatus according to the present invention divides coherent light from a light source into a reference wave and a test wave transmitted through a test object, and splits both waves. A first interferometer for overlapping and interfering, and a second for dividing the test wave into two light beams, displacing the wavefront of one of the light beams in a direction perpendicular to the optical axis and then superimposing them to form interference fringes due to sharing interference. And an interferometer, wherein the refractive index distribution of the test object is measured by making both interferometers switchable and analyzing one of the interference fringes.

The light beams of the first interferometer and the second interferometer are incident on a common polarizer as linearly polarized lights in directions orthogonal to each other, and by rotating the polarizers, the two interferometers are rotated. Switching can be adopted.

Alternatively, a light source, a first light splitter for splitting coherent light from the light source into two, and a light beam placed in one of the split optical paths and transmitted through the test object into two light beams A second light splitter for splitting, a first mirror disposed in the other optical path split by the first light splitter, and a first mirror capable of moving the first mirror on the order of a wavelength or less. A first light superimposing device for superimposing one light beam split by the second light splitter and the light beam reflected by the first mirror to cause interference, and A second mirror in the other light beam split by the two light splitters and having two parallel reflecting surfaces, and one of the reflecting surfaces of the second mirror movable on the order of a wavelength or less. And second driving means. The reflected light of the second mirror and the first
A second optical superimposer for receiving the light emitted from the optical superimposer from a different direction.

[0012] A first polarizer is disposed between the first optical superimposer and the second optical superimposer, and a second polarizer is disposed between the second mirror and the second optical superimposer. The first and second polarizers are arranged so that their polarization directions are orthogonal to each other, and the third polarizer is rotatably provided around the optical axis in the light beam emitted from the second optical superimposer. It is desirable.

[0013] The first light splitter is a polarization beam splitter, and one light splitter is provided between the second light splitter and the first optical superimposer.
A configuration in which a half-wave plate is provided, or a configuration in which the test object is held rotatably about an axis orthogonal to the optical axis may be used.

[0014]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a view showing one embodiment of the refractive index distribution measuring device of the present invention. The apparatus shown in FIG. 1 is configured by a combination of a Mahazda interferometer as a first interferometer and a sharing interferometer as a second interferometer.

The overall configuration of the apparatus is as follows: a light source 1 for emitting laser light as coherent light, a beam expander 3, and a first beam splitting beam from the beam expander into a reference wave a and a test wave b. The light splitter 5, the first mirror 7 placed in the optical path of the reference wave a, and the test wave b into two light beams b1 and b
A second light splitter 9 for splitting the light into two light beams, and a first light superposition device 1 for superimposing the light flux b1 and the light a reflected by the first mirror 7
1 and two parallel reflecting surfaces 13 arranged in the light beam b2
a, 13 b, a second optical superimposer 15, a first polarizer 17, a second polarizer 19,
A rotatable third polarizer 21, an imaging lens 23 provided in an optical path from the third polarizer 21, and an interference fringe detector 25 provided at an imaging position of the imaging lens 23. Consists of The test object O is arranged between the first light splitter 5 and the second light splitter 9.

The first mirror 7 has a first driving device 7a.
Is provided, the reflecting surface 7b of the first mirror can be moved on the order of the wavelength or less, and the optical path length between the reference wave a and the test wave b is nπ.
/ 2 (n is a positive integer).

The second driving means 13 is provided on the second mirror 13.
c, the reflecting surface 13b is moved on the order of the wavelength or less to form a slight shift in the direction orthogonal to the optical axis between the light beam b21 reflected by 13a and the light beam b22 reflected by 13b.

The first polarizer 17 and the second polarizer 19 convert circularly polarized light incident thereon into linearly polarized light and emit the linearly polarized light. The directions of the linearly polarized light are orthogonal to each other. . The third polarizer 21 is capable of rotating at least 90 ° about the optical axis. For example, when the third polarizer 21 can transmit linearly polarized light from the first polarizer 17 at a certain rotation position, When rotated by 90 ° from the position, linearly polarized light from the second polarizer 19 can now be transmitted.

The interference fringe detector 25 uses a CCD image sensor. Further, the test object O is disposed between the first light splitter 5 and the second light splitter 9. Specimen O
Is supported by an object support device (not shown) and is rotatable about an axis orthogonal to the optical axis.

As is clear from the above configuration, in the apparatus of the present invention, a part of the optical element is shared between the Mach-Zehnder interferometer as the first interferometer and the sharing interferometer as the second interferometer. It has a coexisting configuration.

Next, formation of interference fringes by the first interferometer will be described. The laser beam emitted from the light source 1 is expanded in beam diameter by the beam expander 3 and is split by the first light splitter 5 into a reference wave a traveling straight and a test wave b refracting at a right angle. a: b is divided so as to be approximately 1: 3.

The test wave b passes through the phase object as the test object O, and is split by the second light splitter 9 into a light beam b1 and a light beam b2. b1: b2 is set to be approximately 1: 2. The light flux a of the reference wave emitted from the first light splitter 5 is reflected by the first mirror 7, and
Reach On the other hand, the light beam b1 emitted from the second light splitter 9 also reaches the first light superimposing device 11. The first optical superimposer 11 superimposes the light beam a of the reference wave and the light beam b1 of the test wave. The first driving unit 7a sets the optical path length of the light beam a and the light beam b1 to have a phase of nπ / 2. The two light beams will interfere with each other. The luminous flux of a + b1 emitted from the first optical superimposer 11 is passed by the first polarizer 17 only as a linearly polarized light component in a direction parallel to the plane of the paper as shown by the arrow in the figure.

The linearly polarized light transmitted through the first polarizer 17 is
The light goes straight on the reflection surface of the second optical superimposer 15 composed of a polarization beam splitter, and reaches the third polarizer 21. If the polarization direction of the third polarizer 21 is the same as that of the first polarizer 17, the light beam a + b1 passes through the third polarizer and forms an interference fringe on the interference fringe detector 25 by the imaging lens 23. Form an image. As the interference fringe detector 15, a linear CCD or an array sensor arranged in a direction orthogonal to the interference fringes is used.

Next, the formation of interference fringes by the second interferometer, ie, the sharing interferometer, will be described. Of the light beams that have passed through the beam expander 3 from the light source 1, the light beam b reflected by the first light splitter 5 passes through the test object O,
The light reaches the second light splitter 9. Then, the light beam b2 transmitted through the second light splitter 9 is reflected by the two reflecting surfaces 13a and 13b of the second mirror 13. Second mirror 13
Has a second driving means 13c, and the reflection surface 13b is changed to the reflection surface 13 by an electric-displacement conversion element such as a piezo element.
The light beam b21 reflected on the reflection surface 13a and the light beam b22 reflected on the reflection surface 13b are shifted from each other by a small distance δ in a direction orthogonal to the optical axis. When the light beams b21 and b22 are superimposed, sharing interference occurs, and the second light superimposer 1
After 5, an interference fringe image is formed on the interference fringe detector 25.

With the above configuration, in the apparatus shown in FIG.
Measurement of interference fringes by an interferometer (Mach-Zehnder interferometer) and measurement of interference fringes by a second interferometer (sharing interferometer) are possible. Therefore, when the change in the refractive index inside the test object O is small, the interference fringes of the Mach-Zehnder interferometer are imaged on the interference fringe detector 25, and the fringe analysis is performed by the phase shift method.

If the change in the refractive index inside the test object is large, the number of interference fringes is large and the pitch of the fringes is small, so that the resolution of the interference fringe detector 25 is exceeded. On the other hand, since the sharing interference is interference between the test waves, there is an effect of lowering the sensitivity. In other words, the transmitted wavefront on the interference fringe detector 25 is reduced, and it is possible to measure even a phase object for which fringe analysis cannot be performed by the Mach-Zehnder interferometer.

In the above interferometer, the polarization direction of the light beam a + b1 in the Mach-Zehnder interferometer and the polarization direction of the light beam b21 + b22 in the sharing interferometer are mutually orthogonal. Also, the third polarizer 21
Is rotatable about the optical axis, but cannot transmit any polarized light. Therefore, by rotating the third polarizer 21 by 90 ° in either direction, it is possible to easily switch between imaging the interference fringes of the Mach-Zehnder interferometer and imaging the interference fringes of the sharing interferometer. Can be.

FIG. 2 shows a measuring apparatus according to a second embodiment of the present invention. Since most of them are common to the embodiment of FIG. 1, the description will focus on the differences. The first light splitter 5 'uses a polarization beam splitter in this embodiment. And
The incident angle of the linear polarization plane is adjusted so that the split light beams a: b are approximately 1: 2. With this configuration, all the light beams in the device become linearly polarized light. The directions are indicated by arrows in the drawing and double circles with black circles at the center.

Further, the first and second polarizers 17 and 19 are not provided, and a half-wave plate 27 is provided between the second optical splitter 9 and the first optical superimposer 11 instead. . In the case of forming an interference fringe by the Mahazenda interferometer, the light beam a of the reference wave and the light beam b of the test wave are orthogonal to each other. Therefore, a half-wave plate 27 is provided between the second optical splitter 9 and the first optical superimposer 11. That is, the light flux b reflected by the second light splitter 9
1 changes the polarization direction by 90 ° by passing through the half-wave plate 27, becomes linearly polarized light in the same direction as the light flux a of the reference wave, and can cause interference.

In the sharing interferometer, since the linearly polarized light in the direction orthogonal to that of the Mach-Zehnder interferometer is used, the switching can be performed by rotating the third polarizer 21.

FIG. 3 shows a measuring apparatus according to a third embodiment of the present invention.
This is an example using an X-ray CT (Computed Tomography) analysis technique. The measuring apparatus of this embodiment is the same as the apparatus of FIG. 2 except for the periphery of the test object O. The test object 0 is a container 31 immersed in a test solution A having a refractive index substantially equal to that of the test object O.
The optical flats 33 and 35 having good surface accuracy are used for the entrance window and the exit window of the light beam. Specimen O
Is held by a support device (not shown) having a rotation axis P perpendicular to the optical axis, and the container 31 is fixed so that the test object 0 can rotate about the axis P.

The sample liquid A has a property that its refractive index changes with temperature. Therefore, it is desirable to keep the temperature of the sample solution A constant. For this reason, an outer box 37 is provided outside the container 31, and a liquid B such as water kept at a constant temperature is put in the outer box 37 so that it can be circulated. With such a configuration, the temperature of the sample solution B is kept constant,
Accurate measurement becomes possible.

Next, an analysis method using computed tomography will be briefly described. The transmitted wavefront is measured without setting the test object O in advance, and a steady error component of the apparatus itself is eliminated. Next, the test object O is set in the container 31,
The transmitted wavefront is measured by irradiating the light beam from the light source 1. At this time, if the refractive index of the test object O is completely uniform and equal to the refractive index of the test solution A, the fringe analysis result becomes zero. However, when the test object O slightly deviates from the refractive index of the test solution A, the following relational expression holds.

Φ (y) = (2π / λ) ∫Δn (x, y) dx (3) where φ (y): transmitted wavefront (rad) Δn (x, y): sample O and sample A Λ: wavelength of laser light

A light source 1 is applied to an object O having a non-uniform refractive index distribution.
, And the interference fringes generated on the interference fringe detector 25 are taken in and fringe analysis is performed, whereby the refractive index distribution can be obtained from the obtained transmitted wavefront.

However, the result obtained by one measurement is a transmitted wavefront integrated in the thickness direction (x direction) of the test object O. Therefore, in order to determine the spatial position of the non-uniform portion, it is necessary to rotate the test object and perform the same fringe analysis a plurality of times. That is, the test object is rotated around the (relative) z-axis with respect to the optical axis of the interferometer, measured over a range of 180 degrees (or 360 degrees) in the incident direction, and re-combined on a computer. The three-dimensional refractive index distribution of the specimen can be measured. This is X-ray CT (Computed Tomog)
raphy).

First, the transmitted wavefront data P (y, φ) incident from the direction of the angle φ is subjected to one-dimensional Fourier transform. After converting the Fourier-transformed polar coordinate data of each section into rectangular coordinates, the two-dimensional inverse Fourier transform is performed, whereby the two-dimensional refractive index distribution of the test object can be reconstructed. By outputting the reconstructed two-dimensional refractive index distribution on a display or the like for display, or by using an appropriate output means, the refractive index distribution of the test object O can be measured.

In the measuring apparatus of the present invention, it is possible to perform measurement by using a Mahachenda interferometer and a sharing interferometer (first and second detectors). When the number of interference fringes is small, the interference fringes are imaged by a Mach-Zehnder interferometer and measured by the phase shift. When the number of interference fringes is large and exceeds the resolution of the interference fringe detector 25, the analysis is performed by sharing interference.

Therefore, according to the present invention, an object having a large difference in refractive index can be measured with high resolution by combining two interferometers.

[0040]

As described above, according to the present invention, even in the case where the number of fringes exceeds the resolution due to the large number of fringes in the ordinary interference fringe measuring method, the measurement can be performed by sharing interference. be able to. Also, when measuring while rotating the test object around the rotation axis perpendicular to the optical axis,
The three-dimensional refractive index of the test object can be measured by the CT analysis.

Further, since the first interferometer (Mach-Zehnder interferometer) and the second interferometer (sharing interferometer) are switched and used, a part of the optical element can be shared, and the whole apparatus is inexpensive. Can be made.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a first embodiment of an apparatus for measuring a refractive index distribution according to the present invention.

FIG. 2 is a diagram showing the configuration of a second embodiment of the apparatus for measuring the refractive index distribution of the present invention.

FIG. 3 is a diagram showing a configuration of a third embodiment of the apparatus for measuring a refractive index distribution according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source 5 1st light splitter 5 '1st light splitter (polarization beam splitter) 7 1st mirror 7a 1st drive means 9 2nd light splitter 11 1st light superposition device 13 2nd 13c Second drive means 15 Second optical superimposer 17 First polarizer 19 Second polarizer 21 Third polarizer (common polarizer) 23 Imaging lens 25 Interference fringe detector O Specimen P Axis orthogonal to optical axis a Reference wave b Test wave

Claims (7)

[Claims] 1. A first interferometer that divides coherent light from a light source into a reference wave and a test wave that passes through a test object and superimposes and interferes both waves, and splits the test wave into two light beams. A second interferometer that shifts the wavefront of one of the light beams in the direction perpendicular to the optical axis and then superimposes them to form an interference fringe due to sharing interference, and switches between the two interferometers to make any interference An apparatus for measuring a refractive index distribution, which measures a refractive index distribution of an object by analyzing fringes. 2. A light beam of the first interferometer and a light beam of the second interferometer are incident on a common polarizer as linearly polarized lights in directions orthogonal to each other, and the two interferometers are rotated by rotating the polarizers. 2. The apparatus for measuring a refractive index distribution according to claim 1, wherein switching is performed. 3. A light source, a first light splitter for splitting coherent light from the light source into two, and a light beam placed in one of the split optical paths and transmitted through the test object into two light beams. A second light splitter for splitting, a first mirror disposed in the other optical path split by the first light splitter, and a first mirror capable of moving the first mirror on the order of a wavelength or less. A first light superimposing device for superimposing and interfering one light beam split by the second light splitter with the light beam reflected by the first mirror; and A second mirror in the other light beam split by the two light splitters and having two parallel reflecting surfaces, and one of the reflecting surfaces of the second mirror movable on the order of a wavelength or less. And a second drive unit. 4. The light reflected by the second mirror and the light reflected by the first mirror.
The refractive index distribution measuring device according to claim 3, further comprising a second optical superimposing device that receives the light emitted from the optical superimposing device from a different direction. 5. A first polarizer is disposed between the first optical superimposer and the second optical superimposer, and a second polarized light is disposed between a second mirror and the second optical superimposer. And the third polarizer is provided rotatably about the optical axis in the light beam emitted from the second optical superimposer, with the polarization directions of the first and second polarizers being orthogonal to each other. The apparatus for measuring a refractive index distribution according to claim 4, characterized in that: 6. The method according to claim 1, wherein the first light splitter is a polarization beam splitter, and a half-wave plate is provided between the second light splitter and the first optical superimposer. Item 3 or 4
An apparatus for measuring the refractive index distribution described in the above. 7. The apparatus according to claim 1, wherein the test object is held rotatably about an axis orthogonal to the optical axis.
The measurement device for refractive index distribution according to any one of the above.