JP2000065518A - Phase difference measurement method of optical heterodyne interferometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately obtain the position of a target in real time by obtaining the sum or the difference of the light beat signal waveforms between reference and measurement light, obtaining the maximum value of an envelope curve for connecting the extreme values of a beat signal and the amount of displacement at a certain point, and then performing the operation of both of them. SOLUTION: A laser 1 emits two frequency beams where polarized beams with a frequency difference of F cross each other and applies them to an optical heterodyne interferometer 2 and then emits reference light with a frequency difference of F and measurement light with a frequency difference of (f) being Doppler-shifted by the move of a target in the interferometer. They are detected by photodetectors 4 and 5, respectively, and light beat signals cos2πFt and cos2πft are outputted due to the square characteristics. Both amplitudes are aligned by an amplitude controller 3 and are added and subtracted by an adder/subtractor 6, thus outputting a beat waveform. Only a waveform a(t) of the envelope curve of the beat waveform is taken out by a demodulator 7 to obtain a maximum value D of a(t). A specific calculator 8 obtains the ratio of the both at time (t) and displays a phase difference ϕ(t) at the time (t) by an inverse trigonometric function calculator 9 and displays it on a phase difference display 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the position of a moving target in an optical heterodyne interferometer on a scale smaller than the wavelength of light.

[0002]

2. Description of the Related Art An optical heterodyne interferometer was originally developed as a method of determining the position of a target on a scale of 1/100 or less of the light of a light source. However, this relates only to the distance between the target movements that satisfies the two conditions of the movement and then the standstill, that is, the movement amount.

On the other hand, there is an increasing demand for reading the position of a target at a certain time point t on a scale equal to or smaller than the wavelength of light while keeping the movement of the interference arm of the optical heterodyne, that is, without stopping the target. .

As a means for approximately solving this problem, a frequency of an optical beat including a changing side in an optical heterodyne interferometer (for example, an optical beat frequency on a measuring side of a Zeeman laser) is increased by, for example, 256 times using a PLL and a VCO. In this case, the value multiplied by 256 is counted, and the position of the target can be read on a scale of 1/256 of the wavelength. However, this is based on the premise that the period of the light beat is one period before and that the period at the present time is exactly equal, and it is correct to assume that the target moves at a constant speed in the world of a wavelength of 256 minutes.

[0005] However, with the possibility of finer grasp of the movement of the target and the possibility of positively giving a fine speed change to the target, a method for determining the position of the target with higher real time is required.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method in which the position of a moving target can be known in principle in real time with the accuracy of the phase of an optical heterodyne signal in optical heterodyne interference.

[0007]

The basis of this problem is to sort out the problem that the image is not clear, that is, what is the phase difference between AC signals having different frequencies.

In the optical heterodyne interferometer, the difference between the optical beat frequencies is accurately taken as the optical beat frequency difference, and the fine change in the phase difference within one cycle caused by the different optical beat frequency difference is detected by the target position. Must be seen as a change in

Here, the terms used in this specification will be organized. That is, the difference between an optical beat and a beat. The optical beat is a beat of light having two different frequencies and is obtained by the square characteristic of the detector. A beat, on the other hand, is a "beat" obtained by overlapping two optical beats.

A schematic diagram of an optical heterodyne is shown in FIG.

FIG. 1 shows. Here, the frequency of the reference light beat is F, the Doppler shift due to the movement of the target (M ₂ ) is Δf, and the frequency on the measurement side is f. D ₁ and D ₂ are photodetectors, and the incident light is a laser beam having orthogonal polarization with a frequency difference of F. Now, if the target is moving at v, the Doppler shift is given by △ ω = 2π △ f = kv.

In order to obtain a beat, the wave of the reference light beat is cos2πFt, and the wave on the measurement side is cos2πft.

(Equation 1) If you add like

(Equation 2) Is obtained. The cosine in the second half of the equation is a beat signal whose amplitude changes, and the cosine in the first half indicates a change in the amplitude of the beat signal. What we want to pay attention to in this expression is that the sign is inverted before and after the first half cosine becomes zero. The fact that there is an inversion of the amplitude means that
This means that the phase of the second half cosine jumps 180 °. (

[Fig. 3]

[0012]

The first half of the cosine of Equation 1 is represented by a (t).

(Equation 3) However, this is considered when the peak of the beat signal is connected, and although it is called an envelope, it does not exist as it is. Further Doppler shift formula

(Equation 4) Using

(Equation 5) ,

(Equation 6) Is obtained.

Next,

Equation 7 It is more convenient to rewrite the real function with a complex function to obtain a vector on a complex plane, and to stop the rotation of one of the vectors in addition. Here the angular frequency is 2π
The vector of F is fixed on the real axis. The result is

FIG. 2 is as follows.

[Fig. 2]

(Equation 8) ,

[Equation 9] ,

(Equation 10) Holds, and 2a (t) represents the displacement of the envelope.
Here, the amplitude of the signal is assumed to be 1. Tor.

[0014] It can be seen that the displacement is a phase difference. And

Since φ = kvt as shown in Equation 9, if the speed of the target is constant, φ changes linearly until a (t) becomes 0, but v is not constant and v = v ( When t), φ is

[Equation 11] It changes like

The above phase difference φ is

As seen from Equation 10, it is given by cos φ = a (t) / D. Therefore, by measuring D and a (t), φ = c
By calculating os ⁻¹ a (t) / D, the value of ψ = kvt can be known. Alternatively, a difference may be obtained instead of the sum of two optical beat waveforms. In that case,

(Equation 12) The cosine changes to a sine like.

[0016]

DETAILED DESCRIPTION OF THE INVENTION Reference side of an optical heterodyne interferometer,
The two optical beats on the measurement side are taken out by the photodetector, and the amplitudes of both beats are controlled to be equal, and the addition and subtraction of the optical beat signals are performed to create a beat signal of the optical beat signal.
The shape of the envelope a (t) indicating the change in the amplitude of the beat signal
Ask for. Although there are various ways to determine the shape of the envelope, the extreme value of the beat signal near time t is directly read.

FIG. 3 shows that high-speed A / D conversion is preferable, but it can also be obtained by performing square detection, synchronous detection, and rectification detection. The maximum value D is obtained simultaneously with the form of a (t).

When both are known, φ = cos ⁻¹ [a (t)
/ D] (addition) or φ ′ = sin ⁻¹ [a (t)
/ D] (subtraction) to determine the phase difference φ. And

It is better to use the right half of the circumference in FIG. Therefore, φ ′ = φ + 90 ° or φ−90 ° is used.

Here, the period of the beat signal may not be constant. That is, the shape of the envelope may vary in the width of the time axis scale during a half cycle, that is, there may be fine variations in the speed of the target.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A laser 1 has a frequency difference F and a polarization 2 orthogonal to each other.
Emits high frequency light. This light is incident on the optical heterodyne interferometer 2, and from the interferometer 2, light having a frequency difference of F and measurement light having a frequency difference f subjected to Doppler shift due to movement of a target in the interferometer are extracted. These lights are detected by the photodetectors 4 and 5, respectively, but due to the square property of the photodetectors, the optical beat signals cos2π having frequencies of F and f, respectively.
Ft, cos2πft is obtained. An amplitude controller 3 is provided to equalize the amplitudes of the two optical beat signals. This control may be performed optically using a Pockels cell or a liquid crystal, or may be performed electrically for an optical beat signal. The waveforms of the two optical beat signals are added or subtracted by the adder / subtractor 6 to obtain a "beat", that is, a beat waveform. This obtained beat waveform

FIG. 3 shows that only the envelope waveform a (t) of FIG. 3 is extracted by the demodulator 7 and the maximum value D of a (t) is determined. further,
The ratio between the two at the time point t is determined by the ratio calculator 8, and the phase difference ｔ (t) at the time point t is indicated by the inverse trigonometric function calculator 9 by the phase difference indicator 10.

[0020]

The length of a half wavelength or less is determined in real time by an optical heterodyne interferometer. In this case, the target to be measured may be moving.

[0021]

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing the basics of the present invention.

FIG. 2 is an explanatory diagram for developing an equation according to the present invention.

FIG. 3 is a diagram of a beat used in the present invention.

FIG. 4 is an embodiment of the present invention.

[Explanation of symbols]

 k, wave number 1, quadrature two-frequency laser 2, optical heterodyne interferometer 3, amplitude controller 4, 5, photodetector 6, adder / subtractor 7, doubletoner 8, ratio calculator 9, inverse trigonometric function calculator 10, Phase difference display

Claims (1)

[Claims] After controlling the reference light beat signal and the measurement light beat signal having different frequencies to have the same amplitude,
Addition or subtraction of two signal waveforms is performed, and the resulting waveform beats, that is, the maximum value D of the envelope a (t) obtained by connecting the extreme values of the beat signal, and the displacement amount at a certain time t That is, a (t) is obtained and a
An optical heterodyne phase difference measuring method, wherein a phase difference is determined at a time point t by obtaining an arc cosine or an arc sine of (t) / D.