JPH02238306A - Apparatus for measuring fine displacement - Google Patents

Abstract

PURPOSE:To measure fine displacement with high accuracy by grasping the displacement of an object to be measured in an optical axis direction as a change of the interference pattern of the light reflected by a mirror and calculating phase shift at every interference pattern. CONSTITUTION:The light emitted from a light source 1 is transmitted through or reflected by a beam splitter 2 and the transmitted light is reflected by the mirror 4 mounted on an object 3 to be measured to again return to the position of the beam splitter 2 while the light reflected by the beam splitter 2 is reflected by a reference mirror 5 to again return to the position of the beam splitter 2. The light synthesized by this method and becoming an interference pattern K is detected by a unidimensional photoelectric detector 6. Then, a change of the interference pattern K is sampled to calculate phase shift at every interference pattern and, by this method, the displacement quantity of the object 3 to be measured can be calculated from the phase shift quantity.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a minute displacement measuring device that measures the displacement of an object using the interference phenomenon of light.

Conventional technology Conventionally, a very common method of measuring minute displacements of an object (hereinafter referred to as the object to be measured) using the interference phenomenon of light is to measure the interference fringes that occur due to the displacement of the object. There is a method to measure by counting the number of movement.
In this case, the detection accuracy is about one interference fringe, that is,
The limit is accuracy on the order of wavelength. In addition, there is also a measurement interferometer that uses a heterodyne interferometer.In this case, the displacement measurement range is wide, but in order to achieve detection accuracy, a special light source (two-frequency laser) and a fairly complex Since it requires a computational processing system, the device itself is quite expensive.

Problems to be Solved by the Invention In recent years, there has been a need to measure minute displacements, particularly displacements of about 1/100th of the wavelength λ of light, with considerably high precision, as seen in the displacement measurement of electrostrictive elements, for example. It is being requested more and more. One of the ultra-high precision micro displacement measurement methods is the fringe scanning method, which requires applying a displacement of about λ to the mirror that makes up the interferometer with an accuracy of about λ/50 to λ/100. This accuracy determines the measurement accuracy of the interferometer itself, and an electrostrictive element is used to generate this displacement. However, in this case, the electrostrictive element is
In order to control this, it is necessary to determine and adjust the relationship between the voltage applied to the electrostrictive element and the displacement before actual measurement, which is extremely troublesome.

Means for Solving the Problems Therefore, in order to solve such problems, the present invention provides a method in which a beam splitter is disposed on the optical path of the emitted light from the light source, and the emitted light passes through the beam splitter. An object to be measured is arranged on the optical path, a mirror is attached to the object to be measured at a certain angle with the optical axis of the light incident on the object, and the output light is adjusted to the direction of the light passing through the beam splitter. A reference mirror is disposed on an optical path in a direction perpendicular to the optical path leading to the object to be measured, and the light reflected by the mirror and the reference mirror is combined via the beam splitter to form an interference pattern. A photodetector is disposed on the optical path consisting of a
A micro-displacement amount measuring means is provided having a digital signal processing section that calculates a change in the interference pattern based on the digital signal converted by the converter, calculates a phase shift for each interference pattern, and calculates the amount of displacement, and A driving power source was provided to drive the object and displace it in the optical path direction.

Further, the digital signal processing section calculates the phase shift using an arithmetic algorithm using the Fourier transform method.

Effect: Therefore, by driving the object to be measured using the drive power source, the object to be measured is displaced in the optical axis direction, and this displacement is captured as a change in the interference pattern of the light reflected by the mirror, and the minute displacement measuring means is used. The amount of displacement of the object to be measured can be determined by determining changes in these interference patterns using the digital signal processing section and calculating the phase shift for each interference pattern.

Example An embodiment of the present invention will be described based on the drawings.

First, the overall configuration of the minute displacement measuring device will be explained based on FIGS. 1 and 2.

A beam splitter 2 is disposed on the optical path of the light emitted from the light source 1, and an object to be measured 3 is disposed on the optical path a of the light transmitted through the beam splitter 2.

A mirror 4 is attached to the object to be measured 3 at a constant angle with respect to the optical axis of the incident light. In this case, the mirror 4 is attached at an angle θ with respect to a direction perpendicular to the optical axis of the light incident on the object 3 to be measured. Further, a reference mirror 5 is provided on the optical path b of the light reflected by the beam splitter 2 among the emitted light.
is provided. Furthermore, on the optical path C consisting of the light of the interference pattern K that is reflected by the mirror 4 and the reference mirror 5 and combined via the beam splitter 2, there is a one-dimensional photoelectric detector 6 as a photodetector. is installed. This one-dimensional photoelectric detector 6 is connected to an A/D converter 7, and this A/D converter 7 is connected to a computer circuit 8 as minute displacement measuring means. This computer circuit 8 has a digital signal processing section (not shown), and this digital signal processing section determines the change in the interference pattern K, calculates the phase shift for each interference pattern K, and calculates the phase shift of the object to be measured 3. It is equipped with a calculation algorithm function that calculates the amount of displacement. Further, the computer circuit 8 is connected to a programmable power source 9 as a driving power source, and the programmable power source 9 is connected to the object to be measured and can displace the object to be measured 3 in the direction of the optical path a. It looks like this.

In such a configuration, first, the overall flow of this device will be explained. The light emitted from the light source 1 is transmitted or reflected by the beam splitter 2, and the transmitted light is reflected by the mirror 4 attached to the object to be measured 3 and then returns to the position of the beam splitter 2. The light reflected by the reference mirror 5 returns to the position of the beam splitter 2 again. As a result, the light reflected by the mirror 4 and the reference mirror 5 and combined by passing through the beam splitter 2 to form an interference pattern K is detected by the one-dimensional photoelectric detector 6. This detected signal is converted into a digital signal by the A/D converter 7 and then sent to the computer circuit 8. In this computer circuit 8, the digital signal processing unit samples the change in the interference pattern K that occurs as the object to be measured 3 is displaced in the direction of the optical path a, and calculates the phase shift for each interference pattern. The amount of displacement of the object to be measured 3 is determined from the value of the phase shift (detailed explanation will be given later). Thereafter, the amount of displacement determined in this manner is sent to the programmable power source 9, and the programmable power source 9 controls the drive of the object to be measured 3 based on the amount of displacement.

Next, the functions of the arithmetic algorithm of the digital signal processing section of the computer circuit 8, which constitutes the main part of the present invention, will be explained. Now, the object to be measured 3 is moving along the optical path a for a distance Q
It is assumed that the displacement occurs by

At this time, the intensity distribution (power spectrum) of the interference pattern K on the one-dimensional photoelectric detector 6, which is the observation surface, is as follows: (X, y) = a (X, y) /λ)−[f,x+φ
(x, y)+2Q]...(1).

Here, a(x, y) is background noise, b(
x, y) indicates contrast unevenness, and φ(x, y) indicates an error due to the surface precision of the mirror 4.

Now, for simplicity, if we consider equation (1) in one dimension, I(x
)=a(x)+b(x)・cosk(f,x+d)
...(2) becomes.

However, d;φ(x)+2Q (d: phase shift) k=2π/λ (λ: wavelength) shall be.

Expression (2) is generally expressed in the form of Fourier expansion with respect to X with kf as the fundamental wave: (3) where Cr and Sr are r-order Fourier coefficients.

Here, when formula (2) is transformed, I(x) = a(x) + b(x)・coskd−c
oskf, x+ b(x)・sinkd−sinkf,
x − (4).

Comparing equations (3) and (4), we find that the Fourier coefficient of the component that changes in synchronization with kf of I(x), that is, (3)
From the formula, when r==1, C, . S, is (::,=b(
x) −coskd −= (5) S,= b(x
)・sink d − (6) What can be done?

By the way, if we pay attention to equation (2), a(X) is caused by f. It shows a low frequency component compared to b
(x) is f. such as speckle. It shows a higher frequency component than f
This is sufficiently small compared to the 0 component (interference fringes generated by tilting the mirror 4 by an angle θ). Therefore, taking these things into consideration, the power spectrum of I(x) is f. It has a sharp peak nearby, and this f. can be well distinguished from other components. This situation is shown in waveforms as shown in FIGS. 3 to 5. In addition, how to read these waveforms is as follows, taking FIG. 3 as an example. When the displacement of the object to be measured 3 is Q, the phase shift is (2π/λ).
2Q, the interference pattern K is as shown in Figure 3(a), and the sampling signal is as shown in Figure 3(b).
The Fourier-transformed power spectrum is as shown in FIG. 3(C).

In this case, f. The real part and imaginary part of the Boulier transform of I(x) at the beak position are calculated by C. of equations (5) and (6), respectively. , corresponds to S1, C,=Re [Ia(f',)] ・= (7)
S,=Im[Ia(f,)]−(8).

However, Ia indicates the value at the peak position of I(x) after Fourier transformation. Also, Re. Im is the real part,
Indicates an operator that takes an imaginary part.

Therefore, the phase shift d is d=(1/k)Jan-'[S,/C,]=(9
) can be expressed as

For example, now when the mirror 4 is displaced to Q IJ < Q , d. = d, and mirror 4 has Q of Ω
When the displacement reaches , (1=d, then ΔQ = (d, - d,)/2...(
10), from which the minute displacement ΔQ can be determined.

By providing the computer circuit 8 having a digital signal processing unit equipped with the above-mentioned calculation algorithm function, changes in the interference pattern K can be sampled one-dimensionally in the direction of the optical path a where the mirror 4 is given an inclination θ. The amount of displacement of the object to be measured 3 can be determined by performing a one-dimensional Fourier transform based on the sampling signal obtained thereby and calculating the phase shift for each interference pattern. .

Effects of the Invention The present invention captures the displacement of the object to be measured in the optical axis direction as a change in the interference pattern of light reflected by a mirror, and determines the change in the interference pattern using the digital signal processing section of the minute displacement measurement means. , the amount of displacement of the object to be measured is determined by calculating the phase shift for each interference pattern.
It is possible to obtain an inexpensive device that does not require a special optical system for measuring minute displacements as in the past, and is not affected by noise such as uneven illumination of the interference pattern and speckles as in the past. There is nothing like that. Also,
The digital signal processing unit uses the Freelier transform method for its calculation algorithm, which improves the reading accuracy to λ/100.
(λ is one wavelength), making it possible to measure minute displacements with high precision.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a perspective view showing how an interference pattern is detected by the photodetector, and Fig. 3 (a), (b), and (c) are Interference pattern when the displacement of the object to be measured is α and the phase shift is (2π/λ>2Q), the sampling signal, and the waveform diagram showing the respective waveforms of the worth vector, Fig. 4 (a) (b) (
In c), when the displacement of the object to be measured is Q0Q, the phase shift is (
2π/λ)2(Q+α,), waveform diagrams showing the waveforms of the interference pattern, sampling signal, and power spectrum when the displacement of the object to be measured is When Q+Q, the phase shift is (2π/λ
)2(Q+Q,), the interference pattern is
FIG. 3 is a waveform diagram showing the respective waveforms of a sampling signal and a power spectrum. ■... Light source, 2... Beam splitter, 3... Measured object, 4... Mirror 5... Reference mirror 6...
・Photodetector, 7...A/D converter, 8...Minute displacement measurement means, 9...Drive power source Figure 3Z diagram

Claims (1)

[Claims] 1. A light source, a beam splitter disposed on the optical path of the emitted light from the light source, and an object to be measured disposed on the optical path through which the emitted light passes through the beam splitter. , a mirror attached to the object to be measured at a constant angle with the optical axis of the light incident on the object to be measured, and an optical path of the emitted light passing through the beam splitter that leads to the object to be measured. and a reference mirror disposed on an optical path in a direction orthogonal to the reference mirror, and a reference mirror disposed on an optical path consisting of light of an interference pattern obtained by combining the lights reflected by the mirror and the reference mirror via the beam splitter. a photodetector,
An A/D converter connected to this photodetector and converting the detected signal into a digital signal; and an A/D converter connected to this A/D converter and detecting a change in the interference pattern based on the converted digital signal. micro-displacement measurement means having a digital signal processing unit that calculates the phase shift for each interference pattern and obtains the displacement; and a drive connected to the micro-displacement measurement means to drive the object to be measured and displace it in the optical path direction. A minute displacement measuring device characterized by comprising a power source for use. 2. The minute displacement measuring device according to claim 1, wherein the digital signal processing section calculates the phase shift by an arithmetic algorithm using a Fourier transform method.