JPH09257591A - Phase object measuring method and measuring apparatus - Google Patents

Abstract

(57) Abstract: A phase object measuring method and measuring apparatus capable of measuring with high accuracy and high resolution even when the refractive index difference between optical elements is large. SOLUTION: Coherent light from the same light source 1 is used to separate a reference wave a and a test wave b which transmits or reflects an object A, and a reference wave a and a test wave b are superimposed to form an interference fringe. The refractive index distribution of the phase object is measured by using the two methods of forming the interference wave and the method of forming the interference wave and dividing the inspection wave b that passes through the inspection object A and slightly overlapping them. Using a coherent light from the same light source, a Maha-Zehnder interferometer,
A sharing type interferometer and a single optical system were used, and it was possible to measure the transmitted wavefront simultaneously using two interferometers.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical element, a liquid,
More particularly, the present invention relates to a method and a measuring apparatus for measuring a refractive index distribution of an optical element by analyzing interference fringes.

[0002]

2. Description of the Related Art In recent years, molded articles made of plastic materials have become popular as optical lenses and other optical elements used in optical devices such as laser printers and cameras. This plastic molded lens is superior to an aspherical lens in terms of manufacturability and is less expensive than a glass-polished lens, but the distribution of the refractive index during manufacture is unstable, and unevenness often occurs inside the lens. . The non-uniformity of the refractive index inside the lens greatly affects the optical characteristics, leading to deterioration of image quality, resolution, and blurring. 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 for measuring the refractive index of an optical lens, there is a method such as using a precision differential refractometer to obtain the refractive index by measuring the refraction angle by the V-block method or the like, or a Twyman-Green interferometer or the like. 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, a two-beam interferometer such as a Fizeau interferometer or Mahachender interferometer is used. A method is known in which the transmitted wavefront is measured by the analysis of 1 and the optical homogeneity is obtained from the refractive index distribution.

However, in any of the above-mentioned methods, the object to be measured, that is, the optical element to be measured must be processed 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, and the three-dimensional spatial refractive index distribution cannot be measured. Therefore, it is not possible to specify the non-uniform portion of the refractive index three-dimensionally.

Therefore, the applicant of the present invention can measure the refractive index distribution of an optical element efficiently as a non-destructive three-dimensional spatial refractive index distribution regardless of the shape with high accuracy.
We have developed a refractive index distribution measuring method and device that can spatially identify a non-uniform portion of the refractive index.
This uses coherent light from the same light source, and uses this as a reference wave and a test wave that passes through a test object consisting of an optical element to be measured that is dipped in a test solution with almost the same refractive index. Divide and form an interference fringe image by superimposing the reference wave and the test wave, and rotate the sample around the axis orthogonal to the optical axis of the test wave while the sample is immersed in the test solution. The transmission wavefront is measured by analyzing the interference fringes at each rotation angle position, and the refractive index distribution of the optical element is measured from the measurement results of the transmission wavefront from a plurality of directions.

[0006]

According to the method and apparatus for measuring the refractive index distribution of the optical element by the applicant of the present invention, it is possible to measure with high accuracy even if there is a slight difference in refractive index. have.

In addition to the advantages of the above-described method and apparatus for measuring the refractive index distribution of an optical element, the present invention provides a phase that can be measured with high accuracy and high resolution even when the refractive index difference between optical elements is large. An object is to provide a measuring method and a measuring device for an object.

[0008]

In order to achieve the above object, the method for measuring a phase object according to claim 1 uses coherent light from the same light source to transmit a reference wave and an object to be inspected or A method of forming interference fringes by separating the reflected wave and the reference wave and the test wave, and a method of dividing the test wave that passes through the test object and slightly overlapping them to form interference fringes These two interference methods are used to measure the transmitted wavefront by analyzing interference fringes, 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, as in the second aspect of the present invention, the transmitted wavefront is measured one after another while rotating the test object, and an image is reconstructed from this transmitted wavefront data using CT analysis. However, the refractive index distribution of the optical element may be measured from this reconstructed image.

According to a third aspect of the present invention, the phase object measuring apparatus uses coherent light from the same light source to separate the reference wave and the test wave that transmits or reflects the test object, and separates the reference wave and the test wave. One optical system consists of a Maha-Zehnder interferometer that superimposes to form interference fringes, and a sharing interferometer that splits the test wave that passes through the test object and slightly overlaps them to form interference fringes. It is possible to simultaneously measure the transmitted wavefront by analyzing the interference fringes by the above two interferometers, and to measure the phase distribution of the object from the measurement results of the transmitted wavefront from multiple directions. To do.

In the measuring device according to the present invention, as in the fourth aspect of the invention, an optical path splitting means composed of an optically anisotropic material and a polarization beam splitter is provided for the test wave transmitted through the test object. Good. Further, as in the invention according to claim 5, the transmitted wavefront is measured while rotating the test object,
Means may be provided to reconstruct the image using CT analysis.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a phase object measuring method and measuring apparatus according to the present invention will be described below with reference to the drawings. 1 shows a case of measurement by a Maha-Zehnder type interferometer, and FIG. 2 shows a case of measurement by a sharing interferometer. The measurement method and measurement device shown in FIG. 1 and the measurement method and measurement device shown in FIG. The phase object measuring method and the measuring apparatus according to the present invention are established.

The measuring apparatus shown in FIG. 1 comprises a laser light source 1 and
The beam expander 3, the beam splitter 5 for separating the light flux, the two high-reflecting mirrors 7 and 9, the polarization beam splitter 11 for superimposing the light flux, and the beam splitter 49 for separating the superposed light flux again are separated. And the polarizer 51
Imaging lens 13 for converging one of the light fluxes that have passed through, and a CCD whose image is formed on the light receiving surface by this imaging lens 13.
And an image pickup lens 57 for converging the other light flux that has been separated by the beam splitter 49 and has passed through the λ / 4 plate 53 and the polarizer 55. It has an interference fringe detector 59 by an area image sensor such as a CCD whose image is formed on the light receiving surface by the imaging lens 57.

The laser beam as the coherent light emitted from the laser light source 1 has a beam diameter expanded by the beam expander 3 and is reflected by the polarization beam splitter 5 at a right angle downward in the figure, and the above-mentioned reference wave a. The polarized beam splitter 5 is separated into a wave b to be detected which goes straight. The reference wave a is reflected by the high-reflection mirror 9 at right angles to the right side of the drawing, and reaches the polarization beam splitter 11 without passing through a test object A described later. The test wave b passes through the test object A and is then reflected at a right angle downward in the figure by the high-reflection mirror 7 to reach the polarization beam splitter 11. The high-reflection mirror 9 is supported by an electric-displacement conversion element 19 such as a piezo element, and in order to perform interference fringe analysis by the phase shift method, the optical path length of the wave to be detected b is variably set in wavelength order in the optical path direction. It is arranged so that it can be finely displaced.

A container-shaped cell 21 for accommodating the object to be inspected A is arranged in the optical path of the wave to be inspected b. In the cell 21, an object A which is a phase object is set in a fixed state as an object to be measured, and an axis line (with respect to the paper surface) orthogonal to the optical axis of the wave to be inspected b as shown by an arrow in the figure A rotary test table 23 is arranged so as to be rotatable around a vertical axis). The rotary inspection table 23 is connected to an output shaft of a motor (not shown) such as a servo motor or a stepping motor, and is rotationally driven to a predetermined rotation angle position.

Both end surfaces of the cell 21 which traverses the optical path of the wave to be detected b are an entrance window 25 and an exit window 27 for the light beam. These entrance window 25 and exit window 27 are formed by optical flats 29 and 31 having high surface accuracy. Liquid tightly shielded. 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 sample A on the rotary test table 23 is the test solution B. It is soaked in.

In the polarization beam splitter 11, the reference wave a and the test wave b are superimposed on each other. In this example, the polarization direction of the laser light emitted from the laser light source 1 is set to be 45 degrees with respect to the paper surface, and the beam splitters 5, 1
A polarization beam splitter is used as each 1. As described above, the reference wave a and the test wave b are overlapped by passing through the polarization beam splitter 11, but they do not interfere with each other because their polarization planes are orthogonal to each other.

Another beam splitter 49 is provided on the light emitting surface side of the polarization beam splitter 11. The beam splitter 49 is the polarization beam splitter 1
The superimposed light flux of the reference wave a from 1 and the test wave b is split into two. One of the superimposed light beams split into two by the beam splitter 49 causes interference by passing through the polarizer 51, and the interference fringes are imaged on the imaging surface of the first interference fringe detector 15 by the imaging lens 13. The other superimposed light flux passes through the λ / 4 plate 53, which is an optically anisotropic material, so that a phase difference of π / 2 is generated with respect to the one superimposed light flux. The other superimposed light flux then causes interference by passing through the polarizer 55, and the interference fringes are imaged on the imaging plane 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 will be 0 degrees out of phase. For example, if the interference fringes imaged on the first interference fringe detector 15 are in the state shown in FIG. 5A, the interference fringes imaged on the second interference fringe detector 59 are shown in FIG. 5B. The state becomes as shown in, and the interference fringe image is shifted by half. In this case, the first interference fringe detector 15
The second interference fringe detector 59 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.

FIG. 6 shows an example of 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. The calculation device shown in FIG. 6 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. The first interference fringe detector 15 and the second interference fringe detector 59 have amplification circuits 61 and 63, respectively.
Square wave conversion circuits 65 and 67 are connected. The square waves output from the square wave conversion circuits 65 and 67 are input to the pulse counting circuit 69, and the count data from the pulse counting circuit 69 is input to the phase displacement amount calculating device 71.

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 the objects to be inspected A
Is an output signal waveform in the case where the interference fringes change at a constant speed by rotating about the axis line orthogonal to the optical axis, and the output signal waveform of the first interference fringe detector 15 is the first
This is because the change in light intensity at the point P1 of the interference fringe detector 15 (see FIG. 5) is observed, and the output signal waveform of the second interference fringe detector 59 corresponds to the same observation point as the point P1. This is because the change in light intensity due to the change in light intensity at the point P2 (see FIG. 5) of the second interference fringe detector 15 is observed, and both have a phase shift of 90 degrees.

These output signals are sent to the square wave conversion circuit 6
The signals 5 and 67 are converted into a square wave signal as shown in FIGS. 7C and 7D at a predetermined threshold level.
The two square wave signals are input to the pulse count circuit 69, and 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. To do. Data on the moving direction and the number of moving interference fringes are input to the phase shift amount calculating device 71. The phase displacement amount calculating device 71 calculates the amount of phase displacement in the optical axis direction from these data, and adds the initial value previously measured by the phase shift method to this amount of phase displacement to calculate the amount of aberration of the transmitted wavefront. 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.

Next, a method of measuring the refractive index distribution of the test object A which is a phase object using the measuring device having the above-mentioned 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 the phase shift method. Next, the rotary inspection table 23 is rotationally driven in the arrow direction, and the inspection object A is rotated around the axis line orthogonal to the optical axis. 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.

Next, the phase shift amount calculating 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 in the incident direction ranging from 180 degrees to 360 degrees, the measurement data is analyzed by using the CT (Computer Tomography) method, and the refractive index distribution is calculated.

The case of the measurement with the Maha-Zehnder interferometer has been described above. However, the same optical system as this optical system is used, and a part of this optical system is changed, so that the test wave b that passes through the test object b Is divided, and these are slightly shifted and overlapped to form an interference fringe to form a sharing type interferometer, which also measures the refractive index distribution of the test object. Is able to.

FIG. 2 shows an example of a sharing type interferometer, in which a shield plate 85 is provided so as to be capable of advancing and retreating on the optical path of the reference wave a separated by the polarization beam splitter 5 for separating the light beam. When a sharing type interferometer is configured, the shield plate 85 advances into the optical path of the reference wave a to block the reference wave a, and the configuration as the Maha-Zehnder interferometer is eliminated. Further, instead of the high-reflecting mirror 7 in the Mach-Zehnder interferometer shown in FIG. 1, a sharing generating section 80 is arranged.

As shown in FIG. 3, the shearing generator 80 is composed of two parallel plates 81 and 82 arranged in parallel with each other and at a constant interval. The two inner surfaces of the two parallel flat plates 81 and 82 are polished surfaces with high surface accuracy, and the outer two surfaces 83 and 84 are non-reflective surfaces. With this configuration, a part of the test wave b is reflected by the inner surface of the parallel plate 81 and the rest is transmitted, and a part of the test wave b transmitted through the parallel plate 81 is reflected by the inner surface of the parallel plate 82. The light passes through the parallel plate 81 while refracting. The part of the test wave b reflected by the inner surface of the parallel plate 81 and the remaining part of the test wave b reflected by the inner surface of the parallel plate 82 are parallel to each other from the shearing generator 80. Emit.

In this way, the test wave b transmitted through the test object A has its optical path divided by the shearing generator 80, and two parallel test objects having a lateral shift amount proportional to the distance between the inner two surfaces of the parallel flat plates 81 and 82. Detection is generated, resulting in sharing interference. This sharing interference image is split by the beam splitter 49 and detected by either the first interference fringe detector 15 or the second interference fringe detector 59, but detected by only one of the detectors. Interference fringes can be analyzed. The interference fringes can be analyzed by using a known method.

The above operation is carried out when the incident direction is 180 degrees to 36 degrees.
The measurement is performed in a range of 0 degree, the measurement data is analyzed by using a CT (Computer Tomography) method, and the refractive index distribution is calculated.

According to the embodiment described above, a Mahachender type interferometer and a sharing interferometer are configured by using almost the same optical system, and the characteristics of each interferometer are determined by using each interferometer. It is possible to make measurements of the transmitted wavefront and the refractive index profile of the fitted test object. That is, in the case of measuring an object (phase object) having a relatively small transmitted wavefront amount of about several λ or less, a Mahachender type interferometer as shown in FIG. 1 is configured and measured. In that case, a phase shift method or the like is used as an analysis method. When measuring an object (phase object) with a large amount of transmitted wave front,
Since the number of interference fringes is large, a shearing type interferometer as shown in FIG. 2 is constructed and measured. The sharing type interferometer has an advantage that sensitivity can be reduced due to interference between test waves, that is, the number of interference fringes can be reduced and analysis can be easily performed.

Therefore, when the number of interference fringes is small, the first interference fringe detector 15 analyzes by the phase shift method, and the number of interference fringes is large and exceeds the resolution of the first interference fringe detector 15. In this case, it is preferable to use the third interference fringe detector 75 for analysis by sharing interference. Further, when the difference in the refractive index distribution of the test object A is large or the difference in the refractive index with the matching liquid B is large, the amount of the transmitted wave front becomes large. Therefore, the configuration as shown in FIG. CT measurement is performed by measuring with an interferometer.

Next, the embodiment shown in FIG. 4 will be described. In this embodiment, the polarization beam splitter 11 and the high reflection mirror 7 in the Mahachender type interferometer shown in FIG.
A λ / 2 plate 88 is arranged between the and. As a result, the linearly polarized light that was parallel to the paper surface of FIG. 4 is rotated and has a component in the direction perpendicular to the paper surface. The component in the horizontal direction with respect to the paper surface travels straight through the polarization beam splitter 11 and interferes with the reference wave a, and the interference fringes are imaged by the imaging lens 13 on the imaging surface of the first interference fringe detector 15. On the other hand, the component perpendicular to the plane of the drawing is bent by the polarization beam splitter 11 and guided to the shearing generator 80 composed of two parallel flat plates.
As described above, the sharing generation unit 80 divides the test wave, and slightly shifts and superimposes the divided test waves to form interference fringes.
The test wave divided by means of shearing interference, and the interference fringes are imaged on the imaging surface of the third interference fringe detector 75 by the imaging lens 73. The division ratio can be changed by changing the angle of the crystal axis of the λ / 2 plate 88 as the optically anisotropic material.

The embodiment shown in FIG. 4 makes it possible to simultaneously perform the measurement by the Maha-Zehnder type interferometer and the measurement by the sharing type interferometer, and the first interference fringe detector 15 is used. Since it is possible to perform simultaneous measurement by the Maha-Zehnder interferometer and the shearing interferometer using the third interference fringe detector 75, by combining these measuring methods and measuring devices, the refractive index The inspection object A having a large difference can be measured with high resolution.

The CT analysis method is a known method as described in the specification and the drawings attached to the application for the above-mentioned application, and therefore its explanation is omitted. In the illustrated embodiment, an example in which a Maha-Zehnder interferometer and a sharing interferometer are combined is shown, but a combination of a Michelson or Fizeau interferometer and a sharing interferometer is also possible. Further, if the reflected wavefronts are made to interfere with each other, the surface shape can be measured.

[0035]

According to the method for measuring a phase object according to the first aspect, the coherent light from the same light source is used to separate the reference wave and the test wave which passes through or reflects the test object, and the reference wave. And the test wave are overlapped with each other to form an interference fringe, and the test wave that passes through the test object is divided, and the interference wave is formed by slightly overlapping them to form an interference fringe. High-resolution measurement by the method of forming interference fringes by superimposing the test wave and the test wave, and high dynamic range measurement by the method of dividing the test wave that passes through the test object and slightly overlapping them to form the interference fringes. Switching is possible, and there is an advantage that the applicable range of the measurable object is widened.

According to the second aspect of the present invention, the transmitted wavefront is measured one after another while rotating the test object, the image is reconstructed from this transmitted wavefront data using CT analysis, and the optical image is reconstructed from this reconstructed image. Since the refractive index distribution of the element is measured, the refractive index distribution can be measured even when the difference in the refractive index of the test object is large.

According to the third aspect of the present invention, there is provided a phase-object measuring apparatus in which the Maha-Zehnder interferometer and the shearing-type interferometer are formed by one optical system, and the transmitted wavefront is analyzed by analyzing the interference fringes by the two interferometers. Since I was measuring at the same time,
It is possible to switch between high resolution measurement by the Maha-Zehnder type interferometer and high dynamic range measurement by the sharing type interferometer, and there is an advantage that the applicable range of measurable objects is widened.

According to the invention described in claim 4, in the invention described in claim 3, for the test wave transmitted through the test object,
Since the optical path splitting means consisting of the optically anisotropic material and the polarization beam splitter is provided, it is possible to efficiently split the test wave without loss and to use it for the measurement of interference fringes. The division ratio can be changed by adjusting, and measurement can be performed under optimum conditions.

According to the invention described in claim 5, since means for reconstructing an image using CT analysis is provided while measuring the transmitted wavefront while rotating the object to be inspected, various objects to be inspected having a refractive index distribution are provided. It is possible to measure an object with high accuracy.

[Brief description of drawings]

FIG. 1 shows an embodiment of a phase object measuring method and measuring apparatus according to the present invention, and is an optical layout diagram showing a state in which a Mahachender type interferometer is configured.

FIG. 2 is a diagram showing an embodiment of a method and apparatus for measuring a phase object according to the present invention, and is an optical layout diagram showing a state in which a shearing type interferometer is configured.

FIG. 3 is a side view showing an example of a sharing generation unit used in the above embodiment.

FIG. 4 is an optical layout diagram showing another embodiment of the method for measuring a phase object and the measuring apparatus according to the present invention.

FIG. 5 is a field-of-view showing an example of interference fringes generated by the above embodiment.

FIG. 6 is a block diagram showing an example of an interference fringe phase shift amount calculation apparatus applicable to the above-described embodiment.

7A and 7B are waveform charts showing an example of a waveform of an output signal of an interference fringe detector, and FIGS. 7C and 7D are waveform charts showing a waveform example obtained by shaping the waveform.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 laser light source 3 beam expander A test object 11 polarization beam splitter 15 interference fringe detector 23 rotating means 49 beam splitter 53 λ / 4 plate as an optically anisotropic material 59 interference fringe detector 75 interference fringe detector 80 share Ring generator

Claims (5)

[Claims] 1. A method of separating coherent light from the same light source into a reference wave and a test wave that transmits or reflects a test object, and superimposing the reference wave and the test wave to form an interference fringe. A method for measuring a phase object, which comprises using two interference methods, in which a test wave that passes through a test object is divided and slightly overlapped to form interference fringes. 2. A transmission wavefront is measured one after another while rotating an object to be inspected, an image is reconstructed from this transmission wavefront data using CT (computer tomography) analysis, and a phase object of the phase object is reconstructed from this reconstructed image. The method for measuring a phase object according to claim 1, wherein a refractive index distribution is measured. 3. A Maha-Zehnder type that uses coherent light from the same light source to separate a reference wave and a test wave that passes through or reflects a test object and superimposes the reference wave and the test wave to form an interference fringe. The interferometer and the shearing type interferometer that splits the test wave that passes through the test object and slightly overlaps them to form interference fringes are configured with one optical system, and two interferometers are used for transmission. An apparatus for measuring a phase object, which is capable of simultaneously measuring a wavefront. 4. The phase object measuring apparatus according to claim 3, further comprising an optical path splitting unit including an optically anisotropic material and a polarization beam splitter for a test wave transmitted through the test object. 5. The measurement of a phase object according to claim 3, further comprising means for measuring a transmitted wave front while rotating a test object and reconstructing an image using CT analysis. apparatus.