JPH09281003A - Apparatus for measuring phase matter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable measurement of high accuracy and high resolving power even if the refractive index difference of phase matter is large by simultaneously measuring Mach-Zehnder interference and shearing interference by the coherence light from the same light source and to reduce the disturbance of a transmission wave surface by optical elements by reducing the number of optical elements. SOLUTION: The laser beam emitted from a laser beam source 1 is separated into reference waves (a) and waves (b) to be inspected by a beam splitter BS1 and the waves (b) to be inspected pass through matter A to be inspected to be split into transmitted waves and reflecting waves by the BS2. The reflecting waves are superposed on the reference waves (a) passed through a minutely displaceable high reflecting mirror 9 in a BS3 to be measured as Mach-Zehnder interference by a detector 15. The transmitted waves are split in light path by a shearing generating part 80 to be measured as shearing interference by a detector 75. If the BS1 and the BS3 are replaced with polarizing beam splitters, interference contrast or the like is further enhanced. Computer tomography analysis by the rotation of waves to be inspected is also possible.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical element, a liquid,
The present invention relates to a measuring device for a phase object such as gas, and is particularly suitable for measuring the refractive index distribution of an optical lens or other 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 a Maha-Zehnder 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 enables high-precision and high-resolution measurement even when the refractive index difference between optical elements is large. It is also an object of the present invention to provide a phase object measuring apparatus capable of reducing the influence of disturbance of a transmitted wave front due to the elements themselves by reducing the number of optical elements.

[0008]

According to the first aspect of the present invention,
Using coherent light from the same light source, the reference wave and the test wave are split, and the interferometer that superimposes the reference wave and the test wave that has transmitted or reflected the test object again, and the test wave before superimposing It is characterized in that it is possible to perform two types of interferometry using a dividing means and an interferometer that forms one of the divided wavefronts by laterally shifting and superposing them.

According to a second aspect of the present invention, in the first aspect of the invention, a polarization beam splitter that splits the reference wave and a test wave that transmits or reflects the test object, and a beam splitter that splits the test wave in two, or A half mirror, a polarization beam splitter that superimposes one reference wave and a test wave to form an interference fringe, and a means that forms another interference fringe by slightly shifting the other test wave And

According to a third aspect of the present invention, in the first aspect of the invention, the transmitted wave front is measured one after another while rotating the wave to be detected, and reconstructed using CT (Computer Tomography) analysis. To do.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a phase object measuring apparatus according to the present invention will be described below with reference to the drawings. The embodiment shown in FIG. 1 corresponds to the invention described in claim 1, and is configured by a combination of a Maha-Zehnder interferometer and a sharing interferometer. The measuring apparatus shown in FIG. 1 includes a laser light source 1 that emits laser light in the left-right direction (x direction) in the figure, a beam expander 3, a beam splitter BS1 for separating a light beam, and a beam splitter BS1 in the y direction. A high-reflecting mirror 9 that reflects the reflected light flux in the x direction, a beam splitter BS2 that splits the light flux that is transmitted through the beam splitter BS1 and that is transmitted or reflected by the inspection object A, which is a phase object, into two parts. A beam splitter BS3 that superimposes the light flux reflected by the beam splitter BS2 in the y direction and the light flux from the high-reflection mirror 9 and an imaging lens 13 that converges the superposed light flux.
And an interference fringe detector 15 by an area image sensor such as a CCD whose image is formed on the light receiving surface by the imaging lens 13.

Further, a shearing generator 80 composed of two parallel flat plates 81 and 82 for reflecting the other light flux transmitted through the beam splitter BS2 in the y direction and a light flux from the shearing generator 80 are converged. It has an image forming lens 73 and an interference fringe detector 75 by an area image sensor such as a CCD whose image is formed on the light receiving surface by the image forming lens 73.

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 beam splitter BS1 in the downward y direction in the figure, and the above-mentioned beam. The splitter BS1 is separated into a wave b to be detected which goes straight. The reference wave a is reflected by the high-reflecting mirror 9 in the right x direction in the drawing, and does not pass through the object A to be measured by the beam splitter BS.
Leads to 2. The test wave b passes through the test object A and is then split into two by the beam splitter BS2. The light beam reflected by the beam splitter BS2 at a right angle in the lower part of the drawing reaches the beam splitter BS3.

The high-reflection mirror 9 is supported by an electric-displacement conversion element 19 composed of a piezo element or the like, and the optical path length of the test wave b is variably set in wavelength order in order to perform interference fringe analysis by the phase shift method. Therefore, it is arranged so that it can be finely displaced in the optical path direction. The test wave b reflected by the beam splitter BS2 is superposed on the reference wave a by the beam splitter BS3, passes through the imaging lens 13, and then the detector 15
An interference fringe is formed thereon. The optical system that forms the interference fringes constitutes a Maha-Zehnder interferometer.

On the other hand, the test wave transmitted through the beam splitter BS2 is reflected by the two parallel flat plates 81 and 82 arranged at a certain fixed interval. The inner two surfaces of the parallel flat plates 81 and 82 are polished surfaces with high surface accuracy, and the outer two surfaces are non-reflective surfaces. With this configuration, the optical paths are divided by the inner surfaces of the parallel plates 81 and 82, and the shearing interferometer interferes with two test waves laterally shifted in proportion to the distance between the two inner surfaces. The intensity signal corresponding to the interference fringes is converged by the imaging lens 73 and taken into the detector 75. Each detector 15, 75
Uses an array sensor such as a linear CCD or a two-dimensional CCD. Further, the use of an attenuating plate in the optical path on the side of the reference wave a has an advantage that it is easy to obtain an interference fringe with good contrast.

According to the embodiment shown in FIG. 1, it is possible to simultaneously measure Maha-Zehnder interference and sharing interference.
When measuring a relatively small phase object such that the amount of transmitted wavefront is several λ or less when the wavelength is λ, the detector 1
In step 5, fringe analysis is performed by the phase shift method. When measuring a phase object having a large amount of transmitted wavefront, the number of interference fringes is large and the resolution of the detector 15 may be exceeded. In this case, analysis is performed by sharing interference. Since the sharing interference is the interference between the waves to be detected b, it has the effect of reducing the sensitivity. That is, the amount of transmitted wavefront on the detector 75 becomes small, and even a phase object that cannot be fringe-analyzed by the detector 15 can be measured.

Next, an embodiment corresponding to the invention described in claim 2 will be described with reference to FIG. In the embodiment shown in FIG. 2, the beam splitter BS1 in the embodiment shown in FIG. 1 is replaced with a polarization beam splitter PBS1, the beam splitter BS3 is replaced with a polarization beam splitter PBS2, and the beam splitter BS3 is used.
The difference from the embodiment shown in FIG. 1 lies in that a polarizer 51 is arranged between the lens and the imaging lens 13. In FIG.
The incident angle of the linear polarization plane of the laser beam expanded by the beam expander 3 is adjusted by the polarization beam splitter PBS1 so that the ratio of the reference wave a to the test wave b becomes approximately 2: 1. The light reflected by the polarization beam splitter PBS1 becomes a reference wave a, which is reflected by a mirror 9 having a drive element (for example, a piezo element) 19 whose optical path can be variably set in the wavelength order, and the polarization beam splitter PBS.
Leads to 2.

On the other hand, the light transmitted through the polarization beam splitter PBS1 passes through the object A to be inspected and is split into two by the beam splitter BS2. Test wave b reflected by this BS2
And the reference wave a is superposed by the polarization beam splitter PBS2. At this time, the light quantity of the test wave a and the light quantity of the reference wave b have a substantially one-to-one relationship. Further, the reference wave a and the test wave b reach the polarizer 51 side without loss of light quantity due to the effects of the polarization plane and the polarization beam splitters PBS1 and PBS2. The reference wave a and the test wave b form an interference fringe on the detector 15 after passing through the polarizer 51 and the imaging lens 13. On the other hand, the test wave b that has passed through the beam splitter BS2 causes sharing interference, and is received by the detector 75 via the imaging lens 73.
The intensity signal corresponding to the interference fringes on the detectors 15 and 75 is converted into a digital signal by an AD converter (not shown) and processed by the arithmetic unit.

According to the embodiment shown in FIG. 2, it is possible to simultaneously measure Maha-Zehnder interference and sharing interference.
When measuring a phase object with a relatively small amount of transmitted wave front,
The detector 15 performs fringe analysis by the phase shift method. When measuring a phase object with a large amount of transmitted wave front, analysis is performed by shearing interference. As described above, the sharing interference is an interference between the waves b to be detected and therefore has an effect of reducing the sensitivity. That is, the amount of transmitted wavefront on the detector 75 becomes small, and even a phase object that cannot be fringe-analyzed by the detector 15 can be measured.

Next, an embodiment corresponding to the invention described in claim 3 will be described with reference to FIG. The embodiment shown in FIG. 3 is designed to be measured using CT analysis. As shown in FIG. 3, the object A to be measured in the embodiment shown in FIG. It is immersed in the container 21 filled with the test solution B which is almost the same as the above. The incident window 25 and the exit window 27 for the light flux are hermetically sealed using optical flats 29 and 31 having good surface accuracy. The inspection object A is installed on an inspection table 23 having a rotation axis perpendicular to the optical axis, that is, perpendicular to the paper surface in FIG. 3, and the container 21 is fixed and the inspection object A has the above rotation axis. It has a rotatable structure. Other configurations are the same as those of the embodiment shown in FIG.

The laser beam expanded by the beam expander 3 becomes the reference wave a and the test wave b which passes through the test object by the polarization beam splitter PBS1. Reference wave a
Is reflected by a mirror 9 having a drive element 19 such as a piezo element whose optical path can be variably set in the wavelength order, and reaches the polarization beam splitter PBS2. The test wave b is divided into two by the beam splitter BS2 after passing through the test object A. The light beam reflected by the beam splitter BS2 is superimposed on the reference wave a by the polarization beam splitter PBS2,
After passing through the imaging lens 13, interference fringes are formed on the detector 15. Therefore, this optical system constitutes a Maha-Zehnder interferometer. On the other hand, the test wave b that has passed through the beam splitter BS2 is reflected by the shearing generator 80 composed of two parallel flat plates 81 and 82 arranged at a certain interval, and the inside of the parallel flat plates 81 and 82 is reflected. Two test waves laterally shifted in proportion to the distance between the two surfaces interfere with each other. Therefore,
This optical system constitutes a sharing interferometer.

In the embodiment shown in FIG. 3, the transmitted wavefront is measured in a state where the object A to be inspected is not set on the table 23 to be inspected in advance, thereby eliminating the steady error component of the apparatus itself. Next, the inspection object A is set on the inspection table 23 and the transmitted wave front is measured. At this time, when the refractive index of the test object A is completely uniform and equal to the refractive index of the test solution, the fringe analysis result is 0. However, when the sample is slightly deviated from the refractive index of the test solution, 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 refractive index difference between the sample and the test solution λ is the laser beam Is the wavelength of.

The refractive index distribution can be obtained from the obtained transmitted wave front by transmitting light to an object having a non-uniform refractive index distribution, capturing the interference fringes generated on the detector, and performing fringe analysis. . However, the result obtained by one measurement is the transmitted wavefront accumulated in the thickness direction (x direction) of the test object A.
Therefore, in order to determine the spatial position of the non-uniform portion, it is necessary to rotate the test object A and perform the same fringe analysis a plurality of times. That is, the inspection object A is rotated around the Z axis relative to the optical axis of the interferometer, measured in a range of 180 ° (or 360 °) with respect to the incident direction, and recombined on a computer. Thereby, the three-dimensional refractive index distribution of the test object A can be measured.

As the processing method on the computer, the same method as the X-ray CT (Computed Tomography) analysis is used. There is a Fourier transform method as an example of a CT analysis method, and the calculation procedure is shown in FIG. 4 and the concept of the Fourier transform method is shown in FIG. 4 and 5, the transmitted wavefront data P (y, φ) incident from the direction of the angle φ
One-dimensional Fourier transform of (0 ≦ φ <2π (or π is acceptable)) is performed. The two-dimensional refractive index distribution of the test object can be reconstructed by converting the Fourier-transformed polar coordinate data of each section into rectangular coordinates and then performing the two-dimensional inverse Fourier transform. The refractive index distribution of the test object can be measured by outputting and displaying the reconstructed two-dimensional refractive index distribution on a display or the like, or by using an appropriate output means.

In this measuring device, the two detectors 1
Simultaneous measurement is possible with the Maha-Zehnder interferometer using 5,75 and the sharing interferometer. When the number of interference fringes is small, the detector 15 measures by the phase shift method. When the number of interference fringes is large and exceeds the resolution of the detector 15, the detector 75 is used to analyze by sharing interference. By combining two interferometers, it is possible to measure an object with a large difference in refractive index with high resolution.

The beam splitter BS shown in FIG.
Beam splitter BS2 shown in FIG. 1, FIG. 1, FIG. 2 and FIG.
Are each formed of a semi-transmissive prism, but a half mirror may be used instead.

[0027]

According to the first aspect of the invention, the coherent light from the same light source is used to split the reference wave and the test wave,
Measurement by a sharing interferometer that re-superimposes the reference wave and the test wave transmitted or reflected by the test object, and a means for dividing the test wave before superimposing is provided, and one of the divided wavefronts is laid sideways. When two types of interferometric measurement, that is, a measurement with a Maha-Zehnder interferometer that superimposes and forms interference fringes, can be performed at the same time, high resolution and high dynamic range measurement can be performed with one device, and when the refractive index difference is large However, it became possible to measure the refractive index distribution. Further, since the number of optical elements is small, there is an advantage that the disturbance of the wavefront due to the optical elements is small and highly accurate measurement can be performed.

According to the invention described in claim 2, in the invention described in claim 1, the beam splitter for dividing the reference wave and the detection wave is a polarization beam splitter, and one reference wave and the detection wave are superposed to cause interference. Since the beam splitter that forms the fringe is a polarization beam splitter, the split ratio between the reference wave and the test wave can be adjusted to an appropriate amount, and an interference fringe with very good contrast can be easily obtained. There is also an advantage that the loss of light amount is small and the light utilization efficiency is high.

According to the invention of claim 3, in the invention of claim 1, the transmitted wave front is measured one after another while rotating the wave to be inspected, and reconstructed using CT analysis. The refractive index distribution of can be easily measured.

[Brief description of drawings]

FIG. 1 is an optical layout diagram showing an embodiment of a phase object measuring apparatus according to the present invention.

FIG. 2 is an optical layout diagram showing another embodiment of the phase object measuring apparatus according to the present invention.

FIG. 3 is an optical layout diagram showing still another embodiment of the phase object measuring apparatus according to the present invention.

FIG. 4 is a flowchart showing an example of a CT analysis method applicable to the present invention.

FIG. 5 is a conceptual diagram of the same CT analysis method as above.

[Explanation of symbols]

 1 light source a reference wave b test wave A test object BS1 beam splitter BS2 beam splitter BS3 beam splitter PBS1 polarization beam splitter PBS2 polarization beam splitter 15 detector 75 detector 80 sharing generator

Claims (3)

[Claims] 1. A coherent light beam from the same light source is used to divide into a reference wave and a test wave, and the reference wave and the test wave transmitted or reflected by the test object are superimposed again with an interferometer. A phase is characterized in that it is possible to perform two types of interference measurement using a means for dividing the wave to be detected in front, and an interferometer that forms an interference fringe by laterally superposing one of the divided wavefronts. Measuring device for objects. 2. A polarization beam splitter that splits a reference wave and a test wave that transmits or reflects the test object, a beam splitter or a half mirror that splits the test wave into two, and one of the reference wave and the test wave is superimposed. 2. The phase object measuring apparatus according to claim 1, further comprising a polarization beam splitter for forming interference fringes and means for forming the interference fringes by superimposing the other test wave with a slight shift. 3. The phase object measuring apparatus according to claim 1, wherein transmitted wave fronts are measured one after another while rotating the test wave and reconstructed by using CT (Computer Tomography) analysis.