JPH0545140A - Method and device for measuring radius of curvature - Google Patents

Abstract

PURPOSE:To obtain a method and a device for obtaining a radius of curvature of a spherical surface to be inspected in a short time and highly accurately. CONSTITUTION:A light which can interfere from a light source 1 is emitted to a reference surface 4 and a calibration spherical surface with a known radius of curvature r0 which is located on a position 9, thus enabling a reference wave to interfere with a wave to be inspected. Then, a spherical surface to be inspected with a known radius of curvature r is placed at the position 9 instead of the calibration spherical surface for achieving interference similarly. At this time, by giving a periodical wavelength change to the light which can interfere and at the same time by giving a frequency difference of by modulation elements 16 and 17 between the reference wave and the wave to be inspected, normal interference and at the same time heterodyne interference are generated. An image-forming surface 11 or 13 of interference fringes receives a beat signal I from a photodiode 13 and can measure a light path difference highly accurately by performing processing with an operation device 23, thus obtaining the radius of curvature r of the spherical surface to be inspected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for measuring a radius of curvature of a spherical object such as an optical lens or a ball of a bearing.

When optical lenses, bearings, etc. are manufactured, it is necessary to measure the finished state of the spherical surface and evaluate whether or not the radius of curvature of the spherical surface is as designed. Conventionally, the following two methods have been used. First, the first uses the optical spherometer shown in FIG. This is the case where the coherent spherical wave is incident on the spherical surface a to be inspected, when the focal position of the objective lens coincides with the surface A of the spherical surface a to be inspected, and when it coincides with the center of curvature O of the inspected spherical surface. Since interference fringes are formed at some points, the radius of curvature r of the spherical surface to be measured is measured by measuring the distance S between these two points with an external linear encoder or the like. That is, the radius of curvature r of the spherical surface a to be tested can be obtained by S = r.

The second method is to scan the surface to be inspected with an optical probe or a stylus of the contact needle type, and calculate the radius of curvature r from depth information at that time.

[0004]

However, in the above-mentioned first method, it is necessary to move the spherical surface to be measured and to measure twice, and it takes a long time to measure a large number of pieces. , Inefficient. In addition, the second method scans the surface at points, which further takes time. The present invention has been made to solve the above problems, and an object of the present invention is to provide a method and an apparatus capable of obtaining the radius of curvature of a spherical surface to be inspected in a short time.

[0005]

To achieve the above object, the method of the present invention irradiates a reference surface and a surface to be inspected with coherent light from a light source, and superimposes the reference wave and the object to be detected. In a measurement method using an interference optical system that forms interference fringes, a reference wave from a reference surface and a calibration sphere with a known radius of curvature (ro) are irradiated with coherent light so that they are focused at the center of curvature. Then, the optical path difference when the interference fringes are formed by superimposing it with the reflected test wave is obtained as the reference optical path difference (Lo), and then the curvature of the spherical surface under test having an unknown radius of curvature (r) is measured. The optical path difference (L) when the interference wave is formed by irradiating the coherent light so as to focus on the center and superimposing the reference wave and the test wave is obtained, and the curvature of the spherical surface under test is calculated from the equation r = Lo-L + ro. It is characterized by a configuration for obtaining the radius (r).

At this time, a periodic wavelength change (Δλ) is given to the coherent light having the reference wavelength (λo), and a frequency difference (Δν) is given between the reference wave and the test wave by the modulator.
A beat signal (I) is created by superimposing both waves, and the following equation I = A · cos {2π (Δνt + 2LΔλ / λo ² + 2L / λo)} r = Lo −L + ro where A: beat signal amplitude t: time It is desirable to obtain the optical path difference (Lo, L) from the above.

Further, the apparatus of the present invention uses an interference optical system for irradiating the reference surface and the surface to be inspected with coherent light from the light source and superimposing the reference wave and the object wave to form interference fringes. In a measuring device, a calibration sphere having a known radius of curvature (ro) for reference positioning, and a reference optical path difference (Lo) between a reference surface and a constituent spherical surface.
) And a means for measuring the optical path difference (L) between the reference surface and the spherical surface to be inspected.

The means for measuring the optical path difference and the means for measuring the optical path difference measure the wavelength of the coherent light by a known reference value (λ
o)) a light source that can periodically change by a known value (Δλ), a modulator that gives a frequency difference (Δν) between the reference wave and the test wave, and a beat that is created by superimposing the reference wave and the test wave. Signal (I) receiving means and the following equation: I = A.cos {2π (Δνt + 2LΔλ / λo ² + 2L / λo)} r = Lo −L + ro where A: beat signal amplitude t: time to spherical surface to be detected It is desirable to have a configuration including an arithmetic device for obtaining the radius of curvature (r) of

[0009]

The coherent light emitted from the light source is split by the optical path splitting element into one that travels to the reference surface and one that travels to the test spherical surface. The one that reaches the reference surface and is reflected becomes a reference wave, and the one that reaches and is reflected by the spherical surface to be inspected becomes a wave to be inspected and is eventually superimposed. As a spherical surface to be inspected, a calibration sphere having a known radius of curvature and ro is placed at the beginning, and coherent light is irradiated on this calibration spherical surface. Move the calibration sphere back and forth on the optical axis,
When the coherent light is irradiated so as to be focused on the center of the radius of curvature of the calibration sphere, the test wave and the reference wave form an interference fringe. The optical path difference from the reference surface is measured from the position of the calibration sphere that forms this interference fringe, and is set as the reference optical path difference Lo.

Next, the calibration sphere is removed, the spherical surface to be measured is placed, interference fringes are similarly generated, and the optical path difference L at that time is measured. From Lo, L obtained as described above and the known value ro, the radius of curvature r of the spherical surface to be inspected is calculated by the equation r =
It can be obtained by Lo-L + ro.

The above coherent light having the reference wavelength λo is periodically changed within the range of Δλ to obtain Lo.
And L can be obtained, but they cannot be obtained with high accuracy. Therefore, Δν between the reference wave and the test wave is
By giving the frequency difference of, heterodyne interference is caused, and Lo and L can be obtained with high accuracy. The beat signal I (intensity signal) represented by the AC component of the superimposed coherent light is given by the following equation. I = A · cos {2π (Δνt + 2LΔλ / λo ² + 2L / λo)} where: A: amplitude of beat signal t: time In this equation, I can be measured as a beat signal from interference fringes, and λo, Δλ and Δν Is a known value, L can be calculated. In this electrical phase detection, the resolution is as high as about 2π / 10,000, and for example, if L is several tens cm, measurement on the order of μm becomes possible.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of the device of the present invention. The basic configuration of the apparatus shown in this embodiment is a Mach-Zehnder interferometer, but other interferometers can be similarly implemented.

Reference numeral 1 shown in the figure is a light source, and a semiconductor laser which emits laser light having high coherence is used. The coherent light from the light source 1 is linearly polarized light, and the collimator lens 2
Are made parallel to each other, and are partially reflected by the first half mirror 3 to proceed toward the reference surface 4 formed of the mirror, and the rest are transmitted to reach the optical isolator 5. Optical isolator 5
Is composed of a beam splitter 5a and a λ / 4 plate 5b, and the beam splitter 5a is provided with a reflecting surface 5c. The coherent light is reflected by the reflecting surface 5c of the optical isolator 5.
Transmitted through the λ / 4 plate 5b to be circularly polarized light, expanded by two beam expanders 6 and 7 into a parallel light beam having an appropriate thickness, passes through the objective lens 8, and becomes a spherical wave to be a spherical surface to be inspected. 9
Reaches and is reflected to become a test wave. The wave to be detected goes backward through the objective lens 8 and the beam expanders 6 and 7, is linearly polarized in the direction orthogonal to the going direction by the λ / 4 plate 5b of the optical isolator 5, and is then reflected by the reflecting surface 5c. Reaching a half mirror 10, part of which is reflected and enters an image sensor 11 for monitoring, and the other part is transmitted and off-axis light is eclipsed by an aperture 12, and a receiving means 13 including a photodiode is provided.
Reach The spherical surface 9 to be inspected is supported by a supporting means (not shown) and a driving means such as a linear actuator so as to be movable back and forth on the optical axis.

The coherent light reflected by the first half mirror 3 and directed to the reference surface 4 is reflected here to become a reference wave, which is bent 90 ° to the right and reaches the second half mirror 10. It is incident on the image sensor 11 and the receiving means 13 while being superimposed on the above-described test wave. The image sensor 11 and the receiving means 13 are installed at the same optical position, and when the spherical surface 9 to be inspected is moved back and forth on the optical axis, the center of curvature O of the spherical surface 9 to be inspected and the focal point of the objective lens 8 are aligned. When they match, the reference wave and the test wave interfere with each other to form an image of interference fringes on the image sensor 11. On the other hand, in the receiving means 13, a photodiode or the like takes out only the central portion of the image of the interference fringe with the aperture 12 and obtains an electric signal.

As shown in FIG. 2, in actual measurement, first, a reference calibration spherical surface 9a indicated by a dotted line is placed at the position of the spherical surface 9 to be inspected, and interference fringes are formed as described above. .. The radius of curvature of the calibration spherical surface 9a is known and is set to ro. Next, an inspected spherical surface 9 to be measured is placed instead of the calibration spherical surface 9a, and interference fringes are formed in the same manner. In addition, assuming that the reference light and the test light are superposed along the same optical axis, the point P (the point that is virtually obtained) that is the end of the shorter one (reference light in this case) is the optical path difference. The reference point is calculated, but the coordinate position of this P point can be obtained in advance. From the above, the reference optical path difference Lo can be obtained from the coordinate position where the calibration spherical surface 9a has caused interference, and the optical path difference L can be obtained from the coordinate position of the test spherical surface 9.

By substituting each value thus obtained into the following equation, r = Lo-L + ro (1), the radius of curvature r of the surface to be inspected can be obtained. In the actual measurement, first measure on the calibration spherical surface and then once Lo
Then, it is only necessary to measure L for each spherical surface to be measured, and since it is only necessary to generate interference fringes once for one spherical surface to be measured, the measurement can be performed very easily. become able to.

Next, a heterodyne interference method for obtaining Lo and L in the above equation with higher accuracy will be described.
A bias current is injected into the semiconductor laser constituting the light source 1 to emit laser light. At this time,
The current sinusoidally modulated by the oscillator 15 is the modulator 1
4, it is added to the bias current, and a wavelength variation of Δλ can be caused with respect to the wavelength of the laser and the reference λ o.
When this light is used for the above measurement, a phase difference occurs due to the optical path difference of Lo or L between the reference wave and the test wave.

A modulation element 16 such as an acousto-optic element is provided in the optical path of the test light divided by the first half mirror 3, and a similar modulation element 17 is provided in the optical path of the other reference light. , Each modulates the frequency of the coherent light by the oscillators 18 and 19 so that a frequency difference of Δν can be generated between the two.

The coherent light forms the above-mentioned interference fringes. At this time, the reference wave and the test wave simultaneously cause heterodyne interference, and emit a beat signal of frequency Δν. When this is received by the receiving means 13 including a photodiode and photoelectrically converted, the AC component I of the intensity signal is determined by the following equation. I = A · cos {2π (Δνt + 2LΔλ / λo ² + 2L / λo)} (2) where: A: beat signal amplitude t: time In the above equation, the first term of the phase term depends on the frequency difference of Δν. The phase component of the beat signal of the generated heterodyne interference, the second term represents the phase component of the beat signal due to the light source wavelength Δλ, and the third term represents the initial phase.

Now, if only the wavelength variation of Δλ is given and there is no frequency difference of Δν, the above equation becomes I = A · cos {2π (2LΔλ / λo ² + 2L / λo)} (3) ..

Since λo and λ are known, the optical path difference L (or Lo) can be obtained by measuring I.
However, the accuracy of measurement of the optical path difference L is determined by the phase detection accuracy and the wavelength shift magnitude Δλ. By the way, in a commercially available GaAlAs laser, if the change of the injection current is made too large, the oscillation mode of the laser jumps to another mode. The limit of the wavelength shift amount Δλ that does not jump is about 0.05 nm at the maximum. On the other hand, if 780 nm is used as λo, the combined wavelength of one cycle is λo ² / Δλ = (780 × 10 ^-6 ) ² / (0.05 × 10 ^-6 ) = 12 mm. That is, it can be read as a scale with a step of about 12 mm, but normally, the radius of curvature requires a resolution on the order of μm, which is not sufficient.
Therefore, in the embodiment of the present invention, in order to improve the resolution, the above-mentioned heterodyne interference method that gives the frequency difference of Δν is introduced, and the first term is added to the phase term as shown in equation (2). ..

In FIG. 1, the reference wave and the test wave are superposed by the second half mirror 10 and decomposed into two light beams,
On the one hand, an image of the interference fringes is formed on the monitor image sensor 11, and on the other hand, only the central portion near the optical axis is taken out by the aperture 12 and reaches the receiving means 13.

The center of curvature of the spherical surface 9 to be inspected and the focal position of the objective lens need to coincide with each other. This is done by monitoring the interference fringe image on the image sensor 11 to reduce the number of fringes as much as possible. This can be achieved by setting the surface to be inspected so that the accuracy at this time can be adjusted with high accuracy of the reference wavelength λo or less.

The intensity signal of the interference light in the receiving means 13 is given by the above equation (2), which is photoelectrically converted and replaced as the electric signal I. The electric signals of the oscillators 18 and 19 are combined by the mixer 20 to form a reference signal for phase detection, which is input to the phase comparator 21 together with the electric signal I.

The output of the phase comparator 21 is further passed through a bandpass filter 22 having the modulation frequency of the oscillator 15 as the center, whereby fluctuations and vibration noise of the optical system are cut, and the result is input to a computer 23 comprising a computer. To be done. Known values of λ o and Δλ are input to the arithmetic unit 23 in advance, and the phase term of the second term of the equation (2) is calculated from these values and the value of I from the bandpass filter 22. Then, the optical path differences L and Lo can be obtained from the phase component of the second term, and the radius of curvature r can be obtained from the equation (1).

In the above-mentioned heterodyne interference method, since the beat signal can be frequency-shifted only by Δν, electrical phase detection becomes easy and a resolution of about 2π / 10,000 can be obtained. This is a resolution of about 12 / 10,000 mm = 1.2 μm in the above example. By appropriately setting λ o and Δλ, the order of μm or less is possible.

[0027]

As described above, according to the invention of claim 1 or 4, the radius of curvature can be measured by setting the spherical surface to be inspected only once, especially in mass production. It can be efficiently measured when a large number of analytes are measured. According to the inventions of claims 2, 3, 5, and 6, in addition to the above effects, the radius of curvature of the spherical surface to be inspected can be measured with high accuracy on the order of μm or less.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a curvature radius measuring apparatus according to the present invention.

FIG. 2 is a diagram showing a principle of calculating a radius of curvature according to the present invention.

FIG. 3 is a diagram illustrating a conventional method of measuring a radius of curvature.

[Explanation of symbols]

 1 Light Source 4 Reference Surface 8 Objective Lens 9 Test Spherical Surface 9a Calibration Spherical Surface 11 Image Sensor 12 Aperture 13 Receiving Means 14 Modulator 15, 18, 19 Oscillator 16, 17 Modulation Element 21 Phase Comparator 23 Arithmetic Device

Claims (6)

[Claims] 1. A measuring method using an interference optical system for irradiating a coherent light from a light source onto a reference surface and a surface to be inspected, and superimposing the reference wave and the wave to be inspected to form interference fringes, When the interference wave is formed by superimposing the reference wave from the target wave on the calibration sphere with a known radius of curvature (ro) and the test wave reflected by irradiating the coherent light so that it is focused on the center of curvature. Is obtained as a reference optical path difference (Lo), and then the coherent light is irradiated onto the spherical surface to be inspected having an unknown radius of curvature (r) so as to focus on the center of the curvature, and the reference wave and the wave to be inspected. A method for measuring a radius of curvature, which is characterized in that the optical path difference (L) when the interference fringes are formed is superposed by and, and the radius of curvature (r) of the spherical surface to be measured is obtained from the equation r = Lo-L + ro. 2. A periodic wavelength change (Δλ) is given to coherent light having a reference wavelength (λ o), and a frequency difference (Δ) is generated between a reference wave and a test wave by a modulator.
ν) is given and the two waves are superimposed to form a beat signal (I), and the following equation I = A · cos {2π (Δνt + 2LΔλ / λo ² + 2L / λo)} r = Lo −L + ro where A: beat signal The method for measuring a radius of curvature according to claim 1, wherein the optical path difference (Lo, L) is obtained from the amplitude t: time. 3. An interference fringe image is branched into two optical paths, only the central portion is taken out from one of the interference fringe images by an aperture to receive the beat signal, and the other interference fringe image is formed on an image sensor for monitoring. The radius of curvature measuring method according to claim 2, wherein the spherical surface to be inspected is set so that the number of interference fringes on the image sensor is reduced as much as possible. 4. A measuring device using an interference optical system in which coherent light from a light source is applied to a reference surface and a surface to be inspected and an interference fringe is formed by superimposing the reference wave and the wave to be inspected. A calibration sphere having a known radius of curvature (ro), and means for measuring the reference optical path difference (Lo) between the reference surface and the constituent spherical surface and the optical path difference (L) between the reference surface and the test spherical surface. A radius of curvature measuring device characterized by. 5. A light source capable of periodically changing the wavelength of coherent light by a known value (Δλ) from a known reference value (λ o) to a reference wave and a test wave. A modulation element for providing a frequency difference (Δν) between them, and a receiving means for receiving a beat signal (I) formed by superimposing a reference wave and a test wave,
The following equation, I = A · cos {2π (Δνt + 2LΔλ / λo ² + 2L / λo)} r = Lo −L + ro where A: Beat signal amplitude t: Calculation of the radius of curvature (r) of the spherical surface under test from time An apparatus for measuring a radius of curvature according to claim 4, wherein the apparatus comprises a device. 6. An optical path splitting element for splitting a coherent light beam in which a reference wave and a test wave are superimposed into two, a beat signal receiving means provided on one of the split image planes, and splitting. 6. The radius of curvature measuring device according to claim 5, further comprising an image sensor for monitoring provided on the other image forming surface.