JPH0755418A - Displacement gauge - Google Patents

Abstract

PURPOSE:To measure a microdisplacement by determining the phase of spatial frequency spectrum repetitively based on the signal from a detector detecting an interference fringe repetitively as a function of traversal position and then determining the displacement of an object in the direction of optical axis based on one phase variation. CONSTITUTION:A one-dimensional image sensor 26 is disposed at a light receiving position 16 in order to detect interference fringe pattern repetitively. Signals representative of a detected interference fringe are fed to a spectral operating section 28 where the signals are subjected to A/D conversion and the spectrum of interference fringe is determined based on thus converted signal. Spectrums having highest and second highest power are then extracted and the information relating to the frequency and phase of these two spectrums is fed to a displacement operating section 30. The operating section 30 determines the displacement of an object 14 by accumulating the phase variation for each spectrum.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a displacement meter which applies the principle of an interferometer for detecting the displacement of an object to be measured.

[0002]

2. Description of the Related Art There are various known methods for measuring the displacement of an object to be measured. One of the methods is to use an interferometer. FIG. 3 is a schematic configuration diagram of a conventional displacement meter using an interferometer, and FIG. 4 is a diagram showing interference fringes formed at the light receiving position of the displacement meter shown in FIG.

Laser light 1 emitted from a laser light source 10
1 is incident on the beam splitter 12, and this splitter 1
2 divides the light into a transmitted light 11a and a reflected light 11b.
The transmitted light 11 a is reflected by the device under test 14 and enters the beam splitter 12 again, and is reflected by the beam splitter 12 to reach the light receiving position 16. On the other hand, the reflected light 11b is reflected by the reference body 18 such as a mirror and reaches the light receiving position 16 via the beam splitter 12. Here, the reference body 18 is slightly tilted with respect to the optical axis as shown in the figure, so that the light receiving position 1
The interference fringes as shown in FIG.

In the interferometer (displacement meter) thus constructed, the object 14 to be measured is in the optical axis direction (arrow A shown in FIG.
When it is displaced in the B direction), the interference fringes shown in FIG.
Move in the direction. Therefore, the slit 20 is arranged at the light receiving position 16, the light passing through the opening 20a (see FIG. 2) of the slit 20 is received by the optical sensor 22 and the number of light and dark is counted by the counter 24, whereby the measured object 14 is measured. Is detected.

However, in the above displacement meter, the resolution of displacement of the object to be measured is only about 1/2 of the wavelength of the laser beam 11,
It is impossible to measure a displacement smaller than that with sufficient accuracy. In order to solve this, instead of arranging the slit 20 and the optical sensor 22 at the light receiving position 16, the one-dimensional image sensor 26 with a sufficiently fine pitch is provided there (see FIG. 4).
It is conceivable to measure the amount of displacement of the object to be measured by performing Fourier transform of the light and dark pattern of the interference fringes obtained by arranging the two-dimensional image sensor 26 and accumulating the phase change of the peak frequency. In this case, compared to the method of counting the number of interference fringes shown in FIG. 3, it is possible to measure a very small displacement of the measured object 14.

[0006]

However, the object to be measured 1
Since 4 moves in the optical axis direction (directions of arrows A and B shown in FIG. 1), there is a possibility that the measured object 14 may be tilted with respect to the optical axis. When the device under test 14 is tilted with respect to the optical axis, the pitch of the interference fringes as shown in FIG. 4 changes, and therefore the peak frequency thereof changes.

On the other hand, the above method of accumulating the phase change of the peak frequency can detect the phase change of the peak frequency and accumulate the phase change only when the peak frequency is constant. There is a problem that it is not possible to deal with the inclination of the body. The present invention has been made in view of the above circumstances, and an object thereof is to provide a displacement meter capable of measuring a minute displacement of an object to be measured and capable of coping with the inclination of the object to be measured.

[0008]

DISCLOSURE OF THE INVENTION A displacement gauge of the present invention which achieves the above object divides a laser beam and irradiates one of the laser beams onto a measured object and the other reference object inclined with respect to the optical axis, and the measured object is measured. A displacement meter for measuring displacement of the measured object in the optical axis direction by generating interference fringes by interfering light reflected from the body and the reference body with each other, and detecting a change in the interference fringes. ) A detector for repeatedly detecting the light and shade of the fringe as a function of the transverse position across the fringe;
(2) Spectrum calculation means for repeatedly obtaining the phases of a plurality of spatial frequency spectra in the transverse direction of the interference fringes based on the signal detected by the detector (3) A plurality of spaces obtained by the spectrum calculation means It is characterized in that it comprises a displacement calculating means for obtaining the displacement of the object to be measured in the optical axis direction based on the phase change of at least one spatial frequency spectrum of the frequency spectrum.

[0009]

Since the displacement meter of the present invention repeatedly obtains the phases of a plurality of spatial frequency spectra, the maximum power spectrum and the second power spectrum are adopted as the plurality of spatial frequency spectra. By doing so, even if the DUT tilts with respect to the optical axis, it is possible to follow the phase change of the spatial frequency spectrum that is different sequentially.
The displacement of the object to be measured is required with high accuracy.

[0010]

EXAMPLES Examples of the present invention will be described below. FIG. 1 is a schematic configuration diagram of an embodiment of the displacement meter of the present invention.
The corresponding components of the conventional displacement meter shown in FIG. 3 are indicated by the same numbers as those shown in FIG. 3, and only the differences will be described.

The one-dimensional image sensor 2 is provided at the light receiving position 16.
6 are arranged, and the one-dimensional image sensor 26 repeatedly detects an interference fringe pattern as shown in FIG. A signal representing the interference fringe detected by the one-dimensional image sensor 26 is input to the spectrum calculation unit 28. In the spectrum calculator 28, the input signal is first converted into a digital signal, and the spectrum of the interference fringe is obtained based on the digital signal. The maximum power spectrum and the second power spectrum are extracted from this spectrum, and the frequency and phase information of these two spectra is input to the displacement calculator 30. In the displacement calculation unit 30, the 2 input from the spectrum calculation unit 28 is input.
The displacement of the object to be measured 14 is obtained by accumulating the phase change of one spectrum for each spectrum.

Here, when the measured object 14 is tilted with respect to the optical axis,
The case will be described. Figure 2 shows the space when the measured object is tilted.
It is the figure which showed the power P of cuttles typically. Object to be measured
When 14 is in a stable state without inclination, the light receiving position 1
The interference fringes formed in 6 are the space shown by the solid line in FIG.
It has a frequency distribution. Spectrum calculation unit 28
Then, since a discrete spectrum is obtained, at this time
At each frequency f₁ , F₂ , F₃ Spectrum power of
And the phase (not shown) is required. Spectrum calculation unit 2
In 8, the two spectra with the largest power, immediately
In this example, the frequency f₂ Spectrum and frequency f₃ of
The spectrum is extracted and the information thereof is used as the displacement calculator 30.
Entered in. Stable state of DUT 14 without tilt
, The maximum power frequency f₂ Of the spectrum of
By accumulating the phase changes, the displacement of the DUT 14 is
The frequency f of the second power that is obtained ₃ Spect of
The phase change of the monitor is also monitored.

Here, it is assumed that the object 14 to be measured begins to tilt slightly, and the spectral distribution at that time is entirely shifted as indicated by the alternate long and short dash line in FIG. At this point, the spectrum of frequency f ₃ has the maximum power, and the spectrum of frequency f ₄ has the second power. Therefore, this time, the displacement of the DUT 14 which has been obtained based on the phase change of the spectrum of the frequency f ₂ is obtained based on the phase difference between the previous phase and the current phase of the frequency f _3. The current displacement of the measured object 14 is added. At the same time, the phase of the spectrum of the frequency f ₄ , which is the spectrum of the second power, is monitored.

After that, it is assumed that the object 14 to be measured further tilts and shifts to a spectral distribution shown by a broken line in FIG. This time, just before the comparison frequency f ₃ and exchanges the power distribution of a frequency f _4, the spectrum of the power of the frequency f ₄ is the largest, the spectral power of the frequency f ₃ is the second. At this point, the current displacement of the measured object 14 is obtained based on the phase change of the spectrum of the frequency f ₄ , and this displacement is added to the accumulated displacement up to that point.

As described above, in this embodiment, by constantly monitoring both the maximum power spectrum and the second power spectrum, the displacement of the measured object can be measured even if the measured object is tilted. Although two spectra are always monitored in the above embodiment, it goes without saying that the number of spectra to be monitored is not limited to two. Further, FIG. 1 shows an example of a so-called Michelson interferometer, but the interferometer applied to the displacement meter of the present invention is not limited to the Michelson type, and the reflected light of the object to be measured and the reflection of the reference object are reflected. Any interferometer principle may be used as long as an interference fringe with light can be formed at the detection position.

Further, although the one-dimensional image sensor is arranged at the detection position in the above embodiment, the present invention is not limited to the one using the image sensor. For example, a mirror deflecting with time and a single optical sensor are used. A combination of and may be combined to scan and detect interference fringes.
Others The present invention is not limited to the above-mentioned embodiments, but can be variously constructed.

[0017]

As described above, since the displacement meter of the present invention monitors the phases of a plurality of spectra, the displacement of the measured object is high even if the measured object is tilted with respect to the optical axis. Measured with precision.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of a displacement meter of the present invention.

FIG. 2 is a diagram schematically showing the spectrum power when the measured object is tilted.

FIG. 3 is a schematic configuration diagram of a conventional displacement meter using an interferometer.

FIG. 4 is a diagram showing interference fringes formed at a light receiving position of the displacement meter shown in FIG.

[Explanation of symbols]

 10 Laser Light Source 11 Laser Light 12 Beam Splitter 14 Object to be Measured 16 Light Receiving Position 18 Reference Object 26 One-Dimensional Image Sensor 28 Spectral Calculator 30 Displacement Calculator

Claims (1)

[Claims] 1. Interference is achieved by dividing laser light and irradiating one of the measured object and the other with a reference object inclined with respect to the optical axis, and causing the light reflected from the measured object and the reference object to interfere with each other. In a displacement meter that generates a fringe and measures the displacement of the measured object in the optical axis direction by detecting a change in the interference fringe, the lightness and darkness of the interference fringe is a function of a position in a transverse direction across the interference fringe. And a spectrum calculation means for repeatedly obtaining the phases of a plurality of spatial frequency spectra in the transverse direction of the interference fringes based on the signal detected by the detector, and the spectrum calculation means. Displacement calculation means for determining the displacement in the optical axis direction of the object to be measured based on the phase change of at least one of the plurality of spatial frequency spectra. Displacement meter.