JPH08122210A - Method and device for measuring refrative index distribution of optical element - Google Patents

Abstract

PURPOSE: To accurately measure the refractive index distribution of an optical element by a nondestructive method regardless of its shape by dividing the interferable light from the same light source into reference waves and inspection waves to be examined, and by forming an interference fringe image by superposing these waves. CONSTITUTION: The laser beam emitted from a laser beam source 1 is enlarged in its luminous flux diameter by a beam expander 3, and is divided into a laser luminous flux of reference waves (a) and a laser luminous flux of inspection waves (b) to be examined by means of a beam splitter 5. The reference waves (a) are reflected by a high reflecting mirror 7 and are incident on a beam splitter 11 without transmitting through a subject A to be inspected, and the inspection waves (b) are reflected by a high reflecting mirror 9, and transmit through the subject A and are incident on a beam splitter 11. The reference waves (a) and the inspection waves (b) are superposed to each other, and the interference fringe image is formed on the pickup surface of an interference fringe detector 15 by an image-formation lens 13. The interference fringe image formed on the pickup surface of the detector 15 becomes an image signal, and after having been converted by an A/D converter 33, it is input into an arithmetic unit 17, and is analyzed by a transmission liquid surface measuring part 35, so that the distribution of reflective factors can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for measuring a refractive index distribution of an optical element such as an optical lens, and more particularly to a method and apparatus for measuring a refractive index distribution of an optical element by analyzing an interference fringe image.

[0002]

2. Description of the Related Art In recent years, molded lenses made of plastics materials have become widespread as optical lenses used in optical devices such as laser printers and cameras. This plastics molded lens is superior in terms of manufacturability of an aspherical lens to a glass-polished lens and is inexpensive, but the distribution of the refractive index in manufacturing is unstable, and unevenness may occur inside the lens. Many. The non-uniformity of the refractive index inside the lens has a great influence on the optical characteristics and causes deterioration of image quality and resolution. For this reason, it is necessary to measure the distribution of the refractive index inside the optical lens with high accuracy and evaluate the homogeneity of the optical lens.

As a method of measuring the refractive index of an optical lens, there are two methods such as a method of measuring a refraction angle by a V-block method using a precision differential refractometer or the like to obtain the refractive index, and a method of Twyman-Green interferometer. There is a method of measuring the refractive index from the interference fringes using a light flux interferometer, and as a method of measuring optical homogeneity, an interference fringe using a two-beam interferometer such as a Fizeau interferometer or a Mach-Zehnder interferometer. A method is known in which the transmitted wavefront is measured by image analysis and the optical homogeneity is determined from the refractive index distribution.

If a more detailed explanation of these refractive index measuring methods and optical homogeneity measuring methods is required, see pages 63-68 of Optics Vol. 20, No. 2 (February 1991). See Refractive Index and Optical Homogeneity Measurements of Optical Materials.

[0005]

However, in any of the above methods, it is necessary to process the object to be measured, that is, the sample into a predetermined shape with high precision, and the optical element to be measured must be destroyed. Further, the refractive index distribution obtained from the transmitted wavefront is an average value integrated in the traveling direction of the optical path,
It is not possible to measure the three-dimensional spatial refractive index distribution.
This makes it impossible to specify the non-uniform refractive index in three-dimensional space.

The present invention has been made in view of the above-mentioned problems, and the refractive index distribution of an optical element is a non-destructive, three-dimensional spatial refractive index distribution efficiently regardless of the shape. It is an object of the present invention to provide a method of measuring a refractive index distribution of an optical element and a device therefor, which can be accurately measured and can specify a non-uniform refractive index portion three-dimensionally.

[0007]

In order to achieve the above-mentioned object, a method of measuring the refractive index distribution of an optical element according to the present invention comprises a reference wave based on coherent light from the same light source and an optical object to be measured. It divides into the test wave that passes through the test object consisting of elements, forms an interference fringe image by superimposing the reference wave and the test wave, and measures the refractive index distribution of the optical element from the formed interference fringe image. It has a feature.

Further, the test object is dipped in a test solution having substantially the same refractive index, and while the test object is dipped in the test solution, it is rotated about an axis orthogonal to the optical axis of the test wave, and at least two The transmitted wavefront is measured by analyzing the interference fringe image at each of the two rotation angle positions, and the refractive index distribution of the optical element is measured from the measurement results of the transmitted wavefront from a plurality of directions.

In the measuring method according to the present invention, the object to be inspected is rotated around an axis orthogonal to the optical axis of the wave to be inspected, the transmitted wave front is measured at each rotation angle position, and CT ( The detailed feature is that an image is reconstructed by a computer tomography method and the refractive index distribution of the optical element is measured from the reconstructed image.

Further, in the measuring method according to the present invention,
The superimposed light flux of the reference wave and the test wave is divided into two, and a phase difference of nπ / 2 is provided between the one superposed light flux and the other light flux to form an interference fringe image by each of the light fluxes. The test object is rotated around an axis orthogonal to the optical axis of the test wave, and the change in brightness of the two interference fringe images is counted to calculate the phase displacement amount of the interference fringes. The transmitted wavefront may be measured by calculating the wavefront amount.

In order to achieve the above-mentioned object, the measuring apparatus of the refractive index distribution of an optical element according to the present invention comprises a reference wave based on coherent light from the same light source and an optical element to be measured. A two-beam interferometer that forms an interference fringe image by superimposing a reference wave and a test wave, and an interference fringe image detector that is arranged on an image forming surface of the interference fringe image. And a transmitted wavefront measuring means for measuring the transmitted wavefront by analyzing the interference fringe image detected by the interference fringe image detector.

Further, a rotating means may be provided for immersing the test object in a test solution having substantially the same refractive index and rotating the test object around an axis line orthogonal to the optical axis. In the measuring apparatus according to the present invention, the transmitted wavefront measuring means reconstructs an image by the CT method from the transmitted wavefront data at each rotation angle position.
A T processing unit may be included.

In the measuring device according to the present invention,
A beam splitter that splits a superposed light flux of a reference wave and a test wave into two, an optically anisotropic member that gives a phase difference of nπ / 2 between the superposed light fluxes split by the beam splitter, and each superposition Two interference fringe image detectors, one each being arranged on the image plane of the interference fringe image due to the light flux, and the change in brightness of the interference fringe image detected by each of the two interference fringe image detectors to be inspected In the above, the counting means for counting at the observation points corresponding to the mutually same part and the phase displacement amount of the interference fringes are calculated from the change in brightness and darkness of the interference fringe image counted by the counting means, and from this, the transmission at each rotation angle is calculated. It may have a transmitted wavefront aberration amount calculation means for calculating the wavefront amount. In this case, the interference fringe image detector may be a linear image sensor. Further, one of the superimposed lights is rotated so that the two interference fringe image detectors are arranged in parallel.

[0014]

[Operation] The light from the light source is divided into a reference wave and a test wave,
The divided test wave is transmitted through the test object composed of the optical element to be measured, and after passing, the reference wave and the test wave are superimposed to form an interference fringe image, and the refractive index of the optical element is determined from the formed interference fringe image. Measure the distribution. Further, the test object is dipped in a test solution having almost the same refractive index to measure the refractive index distribution. This makes it possible to measure the refractive index with a high degree of freedom regardless of the shape of the test object.

Further, the transmitted wavefront is measured by analyzing the interference fringe image at each of at least two rotation angle positions while rotating around the axis orthogonal to the optical axis of the wave to be detected. As a result, the transmitted wavefront of the test wave incident on the test object from multiple directions is measured, and the refractive index distribution in the depth direction of the test object is also measured by combining the plurality of transmitted wavefront data. , It becomes possible to specify the non-uniform portion of the refractive index in a three-dimensional space.

Further, the object is rotated around an axis orthogonal to the optical axis of the wave to be detected, and an image is reconstructed by the CT method from the transmitted wavefront data at each rotation angle position. As a result, the three-dimensional spatial refractive index distribution of the optical element can be measured more strictly and highly accurately.

Further, the phase shift amount of the interference fringes is calculated by counting the change in brightness and darkness of each interference fringe image formed by the two superimposed light beams provided with a phase difference of nπ / 2. By calculating the transmitted wavefront amount at each rotation angle and measuring the transmitted wavefront from this, the transmitted wavefront can be continuously measured in a rotated state of the test object without stopping.

Further, one of the two superposed light beams is rotated,
The detector for detecting the interference fringe image is installed in parallel.
This reduces the positional error between the two detectors,
Measurement accuracy can be improved. In this case, if a linear image sensor is used as the interference fringe image detector, the interference fringe image can be detected with high resolution and at high speed.
Moreover, it can be obtained as a serial output.

[0019]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows one embodiment of the refractive index distribution measuring apparatus for an optical element according to the present invention. This measuring apparatus has a basic configuration of a Mach-Zehnder interferometer, and includes a laser light source 1, a beam expander 3, a beam splitter 5 for splitting a light beam, two high-reflecting mirrors 7,
9, a beam splitter 11 for superimposing a light flux, an imaging lens 13, an interference fringe detector 15 by an area image sensor such as a CCD, a high speed image processing device, and an arithmetic processing device 17 including a microcomputer and the like. There is.

A laser beam as a coherent beam emitted from the laser light source 1 has a beam diameter expanded by a beam expander 3 and goes straight through the beam splitter 5 to become a reference wave a, and FIG. Then, it is refracted downward at a right angle to proceed and is split into another laser beam which becomes the detected wave b. The reference wave a is reflected by the high-reflecting mirror 7 and does not pass through an object A to be described later, and the beam splitter 11
The incident wave b is reflected by the high-reflection mirror 9, passes through the object A, and enters the beam splitter 11.

Reference wave a incident on the beam splitter 11
And the wave b to be detected are mutually superposed by the beam splitter 11, and the imaging fringe 13 forms an interference fringe image on the imaging surface of the interference fringe detector 15. The high-reflecting mirror 9 is supported by an electric-displacement conversion element 19 such as a piezo element. In order to perform interference fringe analysis by the phase shift method, the high-reflection mirror 9 is slightly moved in the optical path direction in order to variably set the optical path length of the test wave b in the wavelength order. It is arranged so that it can be displaced.

A container-shaped cell 21 for accommodating the object A to be inspected is arranged in the optical path of the wave to be inspected b. An object A, which is an optical element to be measured, is set in a fixed state in the cell 21, and as shown by an arrow in the figure, the axis A orthogonal to the optical axis of the wave b to be measured (perpendicular to the paper surface A rotatable test table 23 is arranged so as to be rotatable about its axis. The rotary inspection table 23 is drivingly connected to a servo motor (not shown), and is rotationally driven to a predetermined rotation angle position by the servo motor.

Both end surfaces of the cell 21 which cross the optical path of the wave b to be detected are an entrance window 25 and an exit window 27 for the light beam. Is shielded by. The cell 21 is filled with a test solution (contact solution) B having a refractive index almost equal to that of the test object A, and the test object A on the rotating test table 23 is the test solution B. It is soaked in.

The interference fringe image formed on the image pickup surface of the interference fringe detector 15 by the imaging lens 13 as described above is photoelectrically converted by the interference fringe detector 15 into an electric image signal, and the A / D After being A / D converted by the converter 33, it is input to the arithmetic unit 17. The arithmetic unit 17
Includes a transmitted wavefront measuring unit 35 that performs a calculation calculation of a transmitted wavefront by analyzing an interference fringe image by a phase shift method or the like.

Next, a method of measuring the refractive index distribution of the object A to be measured by using the measuring device having the above-mentioned structure will be described. First, before setting the inspection object A on the rotating inspection table 23, the image signal of the interference fringe image output from the interference fringe detector 15 is taken into the arithmetic unit 17 and the transmission wavefront measuring unit 35 causes the interference fringes. Analyze the image and measure the transmitted wavefront in the initial state.
Based on this measurement result, initial processing is performed to eliminate the steady error component of the measuring device itself.

Next, the inspection object A is set on the rotating inspection object table 23, and the interference fringe image output from the interference fringe detector 15 in a state where the rotating inspection object table 23 is positioned at the initial rotation position. The image signal of 1 is taken into the arithmetic unit 17, the interference fringe image is analyzed by the transmitted wavefront measuring unit 35, and the transmitted wavefront is measured.

Here, when the refractive index of the test object A is completely uniform and this refractive index is equal to the refractive index of the reagent solution B filled in the cell 21, an interference fringe image by the phase shift method is obtained. The analysis should be zero. On the other hand, when the refractive index of the test object A is slightly different from the refractive index of the test solution B, the following relational expression holds.

Φ (y) = (2π / λ) ∫Δn (x, y) dx where φ (y) is the transmitted wavefront (rad) Δn (x, y) is the refraction between the sample A and the test solution B. Index difference λ: wavelength of laser light

In the measurement of the transmitted wavefront only under the condition that the rotating object table 23 is located at the initial rotation position, the analysis result of the interference fringe image is integrated in the x direction (optical path traveling direction), and this is enough. It is not possible to specify the spatial position of the nonuniform refractive index portion.

Therefore, the rotary inspection table 23 is rotated by a predetermined angle, for example, 90 degrees from the initial rotation position, and the direction of the inspection wave b of the inspection object A on the rotational inspection table 23 is rotated with respect to the optical axis. The rotation angle from the initial rotation position of the inspection table 23 is changed. Even when the test object A is rotationally displaced in this way, the test object A is immersed in the test solution B, and therefore, as before the rotational displacement,
An interference fringe image is formed on the imaging surface of the interference fringe detector 15 by superimposing the reference wave a and the test wave b. Under this condition, the image signal of the interference fringe image output from the interference fringe detector 15 is used as the arithmetic unit 1
Then, the transmitted wavefront is measured by the transmitted wavefront measuring unit 35.

As a result, in a plurality of directions with respect to the inspection object A,
In this case, the transmitted wavefronts are respectively measured by the test wave b incident from two directions, and the spatial position of the non-uniform refractive index portion of the test object A is specified by the combination of the plurality of transmitted wavefront data. Will be possible.

When the refractive index distribution of the test object A is measured as a complete three-dimensional spatial distribution, the test object A is rotated around an axis orthogonal to the optical axis of the test wave b. The incident direction of the test wave b with respect to A is changed within a range of 180 degrees or 360 degrees, measurement data of the transmitted wave front at each rotation angle position is collected, and an image is reconstructed by a computer. Reconstruction of this image is performed by a known CT (Computer Tomography) method.

FIG. 2 shows the principle of the CT method, which should be obtained by performing a one-dimensional Fourier transform on the variable X for the transmitted wavefront data p (X, φ) of the test wave incident from the direction of the angle φ. The φ direction component in the polar coordinate representation of the two-dimensional Fourier transform of the refractive index distribution Δn (x, y) can be obtained.

That is, 0 ≦ φ ≦ 2π or 0 ≦ φ ≦ π
After measuring the transmitted wavefront over the angle range of, the transmitted wavefront data is subjected to one-dimensional Fourier transform, and the Fourier-transformed polar coordinate data of each cross section is converted into rectangular coordinate data, and then 2
By performing the three-dimensional inverse Fourier transform, the three-dimensional refractive index distribution of the test object A can be reconstructed.

This can be expressed by the following equation. The relationship between the Cartesian coordinate system (ξ, η) and the polar coordinate system (r, φ) is expressed as follows: ξ = r cos θ η = r sin θ The transmitted wavefront is p (X, φ), and the refractive index of the sample A and the test solution B is If the difference is Δn (x, y), the two-dimensional Fourier transform F
(Ξ, η) is expressed as follows.

[0036]

[Equation 1]

Δn (x, y) = (1 / 4π ² ) ∫∫F
(Ξ, η) exp i (ξx + ηy) dξdη

FIG. 3 shows the configuration of the arithmetic unit 17 in the case of reconstructing an image by the CT method to obtain the three-dimensional refractive index distribution of the test object A. In this case, the arithmetic unit 17 inputs the image signal from the interference fringe detector 15 for each rotation angle φ and calculates the amount of aberration of the transmitted wavefront, and the wavefront aberration amount for each rotation angle φ. A one-dimensional Fourier transform unit 39 for performing a one-dimensional Fourier transform, a polar coordinate-orthogonal coordinate transform unit 41 for transforming the Fourier-transformed polar coordinate data of each section into rectangular coordinate data, and a two-dimensional inverse Fourier transform of the orthogonal coordinate data 2 Dimensional inverse Fourier transform unit 43, refractive index conversion unit 45 that converts the result of two-dimensional inverse Fourier transform into a refractive index, and refractive index that outputs a refractive index distribution based on the refractive index converted by the refractive index conversion unit 45. And a distribution output unit 47.

In the case of the CT method, the refractive index distribution has no regularity, and even for an object whose distribution is unknown, the refractive index distribution is determined by the non-uniform portion of the refractive index of the object. It becomes possible to specify the spatial position.

FIG. 4 shows another embodiment of the refractive index distribution measuring apparatus for an optical element according to the present invention. Note that, in FIG. 4, portions corresponding to those in FIG. 1 are denoted by the same reference numerals as those in FIG. In this embodiment, the polarization direction of the laser light emitted from the laser light source 1 is set to be about 45 degrees with respect to the paper surface, and the beam splitters 5 and 1 are used.
A polarization beam splitter is used as each 1. The reference wave a and the test wave b are overlapped by passing through the beam splitter 11, but they do not interfere with each other because their polarization planes are orthogonal to each other.

Further, in this embodiment, another beam splitter 49 is provided, and the beam splitter 4
Reference numeral 9 divides the superposed light flux of the reference wave a and the test wave b from the beam splitter 11 into two. One of the superimposed light beams split into two by the beam splitter 49 passes through the polarizer 51 to cause interference, and the interference fringes form the imaging lens 13
An image is formed on the imaging surface of the first interference fringe detector 15 by.
The other superimposed light flux passes through the λ / 4 plate 53, which is an optically anisotropic member, so that a phase difference of π / 2 is generated with respect to the one superimposed light flux. The other superposed light flux is then transmitted to the polarizer 5
As a result, the interference fringes are imaged on the image pickup surface of the second interference fringe detector 59 by the imaging lens 57.

The interference fringe images formed on the first interference fringe detector 15 and the second interference fringe detector 59 are for the same object A, but the second interference fringe detector is the same. The presence of the λ / 4 plate 53 in the optical path leading to
It is 0 degrees out of phase. For example, assuming that the interference fringe image formed on the first interference fringe detector 15 is in the state shown in FIG. 5A, the interference fringe image formed on the second interference fringe detector 59 is shown in FIG. ), And the interference fringe image is shifted by half.

In this case, the first interference fringe detector 15 and the second interference fringe detector 59 are arranged so as to receive optical information from the same point on the object A to be inspected. The deviation of the interference fringe image is not limited to 90 degrees = π / 2, and may be nπ / 2 in general, where n is a natural number.

FIG. 6 shows an apparatus for calculating the phase displacement amount of the interference fringes from the output signals of the first interference fringe detector 15 and the second interference fringe detector 59. This calculation device obtains a Lissajous waveform from the output signals of the first interference fringe detector 15 and the second interference fringe detector 59 and counts the change in brightness of the interference fringes.

Amplifier circuits 61 and 63 and square wave conversion circuits 65 and 67 are connected to the first interference fringe detector 15 and the second interference fringe detector 59, respectively. First interference fringe detector 1
5, the output signals output from the second interference fringe detector 59 are amplified by amplifier circuits 61 and 63, respectively.

FIG. 7A shows the output signal waveform of the first interference fringe detector 15, and FIG. 7B shows the output signal waveform of the second interference fringe detector 59. These are output signal waveforms when the interference fringes change at a constant speed by rotating the test object A around an axis orthogonal to the optical axis, and the output signal waveforms of the first interference fringe detector 15 Is due to the observation of the light intensity change at the point P1 of the first interference fringe detector 15 (see FIG. 5), and the output signal waveform of the second interference fringe detector 59 is the same observation point as the point P1. This is because the change in the light intensity due to the change in the light intensity at the point P2 (see FIG. 5) of the second interference fringe detector 15 corresponding to the above is observed, and both have a phase shift of 90 degrees.

These output signals are converted into a square wave conversion circuit 65,
67 according to a predetermined threshold level shown in FIG.
It is converted into a square wave signal as shown in (c) and (d). These two square wave signals are used for the pulse counting circuit 6
9, the pulse count circuit 69 detects the moving direction and the number of moving interference fringes by detecting the input timing of the two square wave signals and counting the up pulse and the down pulse.

The data of the moving direction and the number of moving interference fringes are input to the phase shift amount calculating device 71, and the phase shift amount calculating device 71 calculates the phase shift amount in the optical axis direction from these data, and the phase shift amount is calculated. The amount of aberration of the transmitted wavefront is calculated by adding the initial value previously measured by the phase shift method to the amount. By performing this operation for each pixel of each interference fringe detector, the amount of phase displacement and the amount of transmitted wave front of the entire test object can be obtained.

A method for measuring the refractive index distribution of the object A to be measured using the measuring device having the above structure will be described. First,
The electric-displacement conversion element 19 is driven and the initial value of the transmitted wavefront distribution is measured using an analysis method such as a phase shift method.

Next, the rotary inspection table 23 is rotationally driven in the direction of the arrow, and the inspection object A is rotated around the axis line orthogonal to the optical axis (around the axis line perpendicular to the paper surface). The rotation of the object A moves the interference fringe images formed on the first interference fringe detector 15 and the second interference fringe detector 59, and the movement direction and movement amount (number of movements) of the interference fringes are determined. It is detected by the pulse counting circuit 69 as described above.

Next, the phase shift amount calculation device 71 calculates the phase shift amount in the optical axis direction from the moving direction of the interference fringes and the number of moving lines, and adds the initial value previously measured by the phase shift method to this phase shift amount. , The aberration amount p (x, θ) of the transmitted wavefront at a certain rotation angle φ is calculated. This operation is performed within a range in which the incident direction extends over 180 degrees or 360 degrees, the measurement data is analyzed using the CT method, and the refractive index distribution is calculated.

In the measuring device shown in FIG.
The first interference fringe detector 15 and the second interference fringe detector 59 are 1
A linear image sensor such as a three-dimensional CCD may be used. A linear image sensor such as a one-dimensional CCD can serially output an image signal at high speed, and thus real-time measurement of interference fringes becomes possible.

For example, when a high-resolution one-dimensional CCD of 7 μm × 5000 pixels is used, the resolution is 70 μm even if the number of images required for one fringe is 10 pixels.
Then, a maximum of 500 fringes, that is, a wavefront aberration amount of 500
It can handle up to λ (approximately 300 μm), the size of the test object d = 50 mm, the refractive index difference Δn = 0.001 from the test solution.
However, Δnd is approximately 79λ, which is sufficiently acceptable.

When the difference in the refractive index from the test solution is large, the interference fringes change rapidly due to the rotation of the test object.
Since D can serially output the image signal at a high speed with a response speed of several milliseconds or less, it is possible to calculate the transmitted wavefront amount in real time while rotating.

FIG. 8 shows another embodiment of the present invention, which is the same as described with reference to FIG. 4 except for the high reflection mirror 72. The high-reflection mirror 72 reflects the superimposed light, and the second interference fringe image detector 59 is arranged in parallel with the first interference fringe image detector 15. By arranging the two detectors in parallel with each other in this way, the positional deviation error of the detectors is reduced, and the measurement accuracy can be improved.

[0056]

As described above, according to the present invention, the following effects can be obtained. The reference light and the test wave are split from the same light source, the test wave and the test object consisting of the optical element to be measured are transmitted, and then superimposed to form an interference fringe image. Since the refractive index distribution of the element is measured, it is possible to measure the refractive index distribution with high accuracy.

Further, since the test object is immersed in the test solution having almost the same refractive index, the refractive index distribution can be measured regardless of the shape of the test object. In addition, while measuring the transmitted wavefront of the test wave incident on the test object in multiple directions while the test object is immersed in the test solution, a combination of the plurality of transmitted wavefront data sets Since the refractive index distribution in the depth direction is also measured with respect to the test object, it becomes possible to specify the non-uniform refractive index portion three-dimensionally.

Further, by rotating the test object around an axis orthogonal to the optical axis of the test wave and reconstructing an image by the CT method from the transmitted wavefront data at each rotation angle position, the three-dimensional shape of the optical element is obtained. The spatial refractive index distribution is measured more strictly and highly accurately based on a larger amount of information.

Further, the phase shift amount of the interference fringes is calculated by counting the change in brightness and darkness of each interference fringe image by the two superimposed light fluxes provided with the phase difference of nπ / 2, and the transmitted wavefront aberration at each rotation angle is calculated from this. By calculating the amount and measuring the transmitted wavefront, the transmitted wavefront can be continuously measured without rotating in the rotated state of the test object, and the measurement time can be shortened. .

In this case, if a linear image sensor is used as the interference fringe image detector, the interference fringe image can be detected with high resolution at high speed and as a serial output, so that the transmitted wave front is measured in real time. It becomes possible. Further, since one of the superposed light beams is rotated and the two interference fringe image detectors are arranged in parallel, the position accuracy of the detectors is improved and the measurement accuracy can be improved.

[Brief description of drawings]

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

FIG. 2 is a diagram illustrating the principle of the CT method.

FIG. 3 is a block diagram showing an embodiment of an arithmetic unit based on the CT method.

FIG. 4 is a configuration diagram showing another embodiment of the refractive index distribution measuring apparatus for an optical element according to the present invention.

5A and 5B are explanatory diagrams showing examples of interference fringe images.

FIG. 6 is a block diagram showing an embodiment of an apparatus for calculating a phase displacement amount of interference fringes.

7A and 7B are explanatory views showing waveform examples of output signals of the interference fringe detector, and FIGS. 7C and 7D are waveform diagrams of square waves.

FIG. 8 is a configuration diagram showing another embodiment of the refractive index distribution measuring apparatus for an optical element according to the present invention.

[Explanation of symbols]

 1 laser light source 1 beam splitter 11 beam splitter 15 interference fringe detector (first interference fringe detector) 17 arithmetic processing unit 21 cell 23 rotating object table 29 optical flat 31 optical flat 35 transmitted wavefront measuring unit 37 transmitted wavefront amount Calculation unit 39 One-dimensional Fourier transform unit 41 Polar coordinate-Cartesian coordinate transform unit 43 Two-dimensional inverse Fourier transform unit 45 Refractive index conversion unit 47 Refractive index distribution output unit 49 Beam splitter 53 λ / 4 plate 59 Second interference fringe detector 69 Pulse counting circuit 71 Phase displacement amount calculation device

Claims (12)

[Claims] 1. Coherent light from the same light source is divided into a reference wave that serves as a reference and a test wave that passes through a test object formed of an optical element to be measured, and interference fringes are formed by superimposing the reference wave and the test wave. A method for measuring the refractive index distribution of an optical element, which comprises forming an image and measuring the refractive index distribution of the optical element from the formed interference fringe image. 2. The method for measuring the refractive index distribution of an optical element according to claim 1, wherein the test object is immersed in a test solution having a refractive index that is substantially the same. 3. The transmitted wavefront is measured by analyzing the interference fringe image at each of at least two rotation angle positions by rotating the test object around an axis orthogonal to the optical axis, and measuring the transmitted wavefront from a plurality of directions. 3. The method for measuring the refractive index distribution of an optical element according to claim 1 or 2, wherein the refractive index distribution of the optical element is measured from the measurement result of the transmitted wave front. 4. The transmitted wavefront is measured at each rotation angle position by rotating the test object around an axis orthogonal to the optical axis of the test wave, and an image is reconstructed by CT method from the transmitted wavefront data. The method for measuring the refractive index distribution of an optical element according to claim 3, wherein the refractive index distribution of the optical element is measured from the reconstructed image. 5. A superposed light flux of a reference wave and a test wave is divided into two, and nπ / n is provided between one superposed light flux and the other light flux.
An interference fringe image is formed by each light flux with a phase difference of 2 and the test object is rotated around an axis orthogonal to the optical axis of the test wave to count the change in brightness between the two interference fringe images. The phase shift amount of the interference fringes is calculated, and the transmitted wavefront at each rotation angle is measured from the calculated phase shift amount.
2. The method for measuring the refractive index distribution of the optical element according to 2, 3, or 4. 6. Coherent light from the same light source is divided into a reference wave serving as a reference and a test wave that passes through a test object formed of an optical element to be measured, and interference fringes are formed by superimposing the reference wave and the test wave. A two-beam interferometer that forms an image, an interference fringe image detector arranged on the image plane of the interference fringe image, and an interference fringe image detected by the interference fringe image detector to analyze the transmitted wavefront. A device for measuring the refractive index distribution of an optical element, comprising: 7. The apparatus for measuring the refractive index distribution of an optical element according to claim 6, wherein the test object is immersed in a test solution having a refractive index that is substantially the same. 8. The apparatus for measuring the refractive index distribution of an optical element according to claim 6, further comprising rotating means for rotating the object under test around an axis orthogonal to the optical axis. . 9. The CT for reconstructing an image by the CT method from the transmitted wavefront data at each rotation angle position by the transmitted wavefront measuring means.
The apparatus for measuring the refractive index distribution of an optical element according to claim 8, further comprising a processing unit. 10. A beam splitter for splitting a superposed light flux of a reference wave and a test wave into two, and an optical anisotropy for giving a phase difference of nπ / 2 between the superposed light fluxes split by the beam splitter. A member, two interference fringe image detectors, one each being arranged on the image plane of the interference fringe image formed by each superimposed light flux, and the brightness of the interference fringe image detected by each of the two interference fringe image detectors. Counting means for counting the changes at the observation points corresponding to the same part on the surface to be inspected, and calculating the phase shift amount of the interference fringes from the change in brightness of the interference fringes image counted by the counting means, The device for measuring the refractive index distribution of an optical element according to claim 6, 7, 8 or 9, further comprising: transmitted wavefront amount calculation means for calculating the transmitted wavefront amount at each rotation angle. 11. The optical element according to claim 10, wherein one of the superposed light beams incident on the interference fringe image detector is rotated so that the two interference fringe image detectors can be arranged in parallel. Device for measuring the refractive index distribution of. 12. The apparatus for measuring the refractive index distribution of an optical element according to claim 10, wherein the interference fringe image detector is a linear image sensor.