JPH0735861A - Distance measuring equipment - Google Patents

Abstract

PURPOSE:To allow accurate measurement of phase difference which is not susceptible to the fluctuation in the quantity of reflected light even at high optical beat frequency. CONSTITUTION:In the distance measuring equipment, one linearly polarized light 41 of two adjacent longitudinal modes emitted from a gas laser source 2 having an internal resonator is passed through a quarter wavelength plate having orientation of 45 deg. and converted into a circularly polarized light 43. Linearly polarized light 42 of the other mode is passed through a half wavelength plate 28 and optically rotated by 45 deg.. Both lights 43, 44 are superposed and split by means of a beam splitter 22. One split beam passes through a polarized beam spiitter 30 and a quarter wavelength plate 32 of orientation 45 deg. and impinges on an object 10 and the reflected light therefrom impinges on an optical detector 60. The other split beam passes, as a reference light, through a polarized beam splitter 34 and split in two directions toward optical detectors 62, 64.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention measures the phase difference between the reflected light of an intensity-modulated laser beam reflected by an object to be measured and a reference light to determine the distance (absolute distance) to the object to be measured.
The present invention relates to a distance measuring device for measuring.

[0002]

2. Description of the Related Art A conventional example of this type of distance measuring device is shown in FIG.

The laser light source 2 is, for example, an internal mirror type He-N.
This is a gas laser light source having an internal resonator, such as an e-laser. Two adjacent longitudinal modes of the laser light source 2 are linearly polarized and their vibration directions are orthogonal to each other. When these two modes of linearly polarized light 31 and 32 are passed through the polarizer 6 having an azimuth of 45 °, the frequencies (eg, 1 to
An optical beat of 1.2 GHz) is generated, and the light 33 intensity-modulated in a sine wave is obtained.

This light 33 is split into two by the beam splitter 8, one light 34 is made incident on the object to be measured 10, and the reflected light 34 from it is reflected by the beam splitter 8 to be detected by the photodetector 1.
On the other hand, the light is guided to 2 and converted into an electric signal there, while the other light 35 split by the beam splitter 8 is guided to the photodetector 14 as a reference light and converted into an electric signal there. Both photodetectors 1
Between the two AC signals having the optical beat frequency, which are obtained in 2 and 14, the optical path length from the beam splitter 8 to the photodetector 14 and the light traveling back and forth between the beam splitter 8 and the DUT 10 are measured. Since there is a phase difference corresponding to the difference with the optical path length reaching the detector 12, the time during which light travels back and forth between the object to be measured 10 is measured by measuring the phase difference between both AC signals with the phase difference meter 16. Then, the distance (absolute distance) to the DUT 10 can be obtained.

[0005]

In the distance measuring device as described above, the higher the frequency of the two longitudinal mode intervals, the higher the sensitivity of the measurement becomes, which is preferable. However, the phase difference meter 16 has a high frequency. Since it is necessary to measure a slight phase difference at, the phase difference measurement becomes difficult, and therefore there is a limit in improving the measurement sensitivity in the conventional device configuration.

For example, the phase difference meter 16 may be a digital type or a vector voltmeter, but if the minimum scale of the phase difference angle is, for example, 2π / 1000, the digital type has a limit of about 80 MHz and more. It is necessary to use a vector voltmeter for the frequency of.
However, even in the case of the vector voltmeter, the phase measurement error becomes extremely large when the optical beat frequency approaches 1 GHz.
Moreover, in the above-described conventional device, the amount of light reflected by the object to be measured 10 varies depending on the distance to the object to be measured 10, the position where the light is incident on the object to be measured 10, the reflectance of the object to be measured 10, The level of the electrical signal output from the detector 12 also changes, which also makes it difficult to measure the phase difference at high frequencies.

Therefore, it is a main object of the present invention to provide a distance measuring device having an optical system that enables accurate phase difference measurement without being affected by changes in the amount of reflected light even at high optical beat frequencies. To do.

[0008]

SUMMARY OF THE INVENTION A distance measuring device of the present invention is a device for measuring a distance to an object to be measured by measuring a phase difference between a reflected light of an object to be measured and a reference light of intensity-modulated laser light. , A gas laser light source having an internal resonator, a phase shift means for converting linearly polarized light of one of two adjacent longitudinal modes of the laser light source into circularly polarized light, and linearly polarized light of the other mode by 45 ° Let or 4 of the same light
A polarizing means for extracting a component in the 5 ° direction and an interfering means for superimposing the light obtained from the both means to cause interference are provided, and the light from the interfering means is used as the intensity-modulated laser light. Characterize.

With the above arrangement, it is necessary to obtain the amount of the phase difference between the light reflected by the object to be measured and the reference light by the calculation of the trigonometric function and without being affected by the change in the amount of reflected light. Sine and cosine signals can be produced optically. As a result, even at high optical beat frequencies, without being affected by changes in the amount of reflected light,
Accurate phase difference measurement is possible, which can improve measurement sensitivity.

[0010]

1 is a block diagram showing a distance measuring device according to an embodiment of the present invention. The distance measuring device of this embodiment is configured as follows.

That is, the laser light source 2 is a gas laser light source having an internal resonator, as in the case of the conventional example, and the two linearly polarized light beams 41, 4 of adjacent two longitudinal modes of the laser light source 2 are adjacent to each other.
2 is separated by the first polarized beam splitter 18.

Linearly polarized light 41 of one mode separated
Is transmitted through the first quarter-wave plate 20 having an azimuth of 45 °, is given a retardation of 90 ° to be converted into circularly polarized light 43, and is then incident on the beam splitter 22. In this embodiment, the quarter-wave plate 20 constitutes the phase shifting means described above.

Linearly polarized light 42 of the other mode separated
Is reflected by the two reflectors 24 and 26 and is made incident on the beam splitter 22 from a direction perpendicular to the circularly polarized light 43 side, and in the meantime, it is transmitted through the ½ wavelength plate 28 and 45 °. Only the plane of polarization is rotated (that is, optical rotation). The linearly polarized light obtained thereby is indicated by reference numeral 44 in the figure. In this embodiment, the half-wave plate 28 constitutes the above-mentioned polarization means.

The circularly polarized light 43 and the linearly polarized light 44 incident on the beam splitter 22 are superposed there and split into two directions. One of the split lights 43 and 44 is made incident on the second polarized beam splitter 30 as measurement light, and the other light 43 and 44 is made incident on the third polarized beam splitter 34 as reference light.

The polarized beam splitter 30 transmits the p component of the circularly polarized light 43 and the p component of the linearly polarized light 44 inclined at 45 °.
There are only components, and both components cause interference. That is, the intensity-modulated linearly polarized light (optical beat) 45 is obtained from the polarized beam splitter 30. The beat frequency of the linearly polarized light 45 is the frequency between the linearly polarized lights 41 and 42 of the two longitudinal modes from the laser light source 2. In this example,
The polarized beam splitter 30 together with the polarized beam splitter 34 constitutes the above-mentioned interference means.

When this linearly polarized light 45 is transmitted through the second quarter-wave plate 32 having an azimuth angle of 45 °, it becomes circularly polarized light 46, which is incident on the object to be measured 10. The reflected light from the DUT 10 is also circularly polarized light 46, which is again converted into quarter wave plate 32.
Is transmitted, it becomes a linearly polarized light 47 whose vibration plane is shifted by 90 ° from the original linearly polarized light 45. Therefore, this linearly polarized light 47
Is reflected by the polarized beam splitter 30 and incident on the first photodetector 60, where it is converted into an electrical signal S ₁ .

On the other hand, the other light 43, 44 split by the beam splitter 22 is made incident on the third polarized beam splitter 34. In this polarized beam splitter 34, circularly polarized light 4
The s-component of 3 and the s-component of the linearly polarized light 44 inclined at 45 ° cause interference and produce intensity-modulated linearly polarized light (optical beat) 48, while the p-component of the circularly polarized light 43 and the linearly polarized light 44 The p component transmits and causes interference, and intensity-modulated linearly polarized light (optical beat) 49 is obtained. The beat frequencies of these linearly polarized light 48 and 49 are also the laser light source 2
Is a frequency between the linearly polarized light 41 and 42 of the two longitudinal modes. Both linearly polarized lights 48 and 49 then enter the second photodetector 62 and the third photodetector 64, respectively, where they are converted into electrical signals S ₂ and S ₃ , respectively. In this embodiment, the polarized beam splitter 34 also constitutes the above-mentioned interference means, like the polarized beam splitter 30.

Here, the angular frequency between the linearly polarized lights 41 and 42 of the two longitudinal modes from the laser light source 2 is ω, and the photodetector 6
Assuming that the phase difference between the reflected light 47 incident on 0 and the reference light 48 and 49 incident on the photodetectors 62 and 64 is φ and the time is t, from the photodetectors 60, 62 and 64, respectively, , The electric signals S ₁ , S ₂ and S _{3 of the} combination shown in the equation 1 or the equation ₂ are obtained. Electrical signal S ₁
The coefficient A that depends on V represents the change in the amount of reflected light described in the conventional example, and is unknown.

[0019]

## EQU1 ## S ₁ = A sin (ωt + φ) S ₂ = sinωt S ₃ = cosωt

[Equation 2] S ₁ = A cos (ωt + φ) S ₂ = cosωt S ₃ = sinωt

When the electric signals S ₁ and S ₂ and the electric signals S ₁ and S ₃ are multiplied by the first and second mixers 66 and 68, respectively, the electric signals as shown in Formula 3 or Formula 4 are obtained. The signals S ₄ and S ₅ are obtained.

[0021]

[Equation 3] S ₄ = S ₁ · S ₂ = A sin (ωt + φ) · sin ωt S ₅ = S ₁ · S ₃ = A sin (ωt + φ) · cosωt

[Equation 4] S ₄ = S ₁ · S ₂ = Acos (ωt + φ) · cosωt S ₅ = S ₁ · S ₃ = Acos (ωt + φ) · sin ωt

And such electrical signals S ₄ and S ₅
Based on the above, the arithmetic circuit 70 calculates an amount related to the phase difference φ. The example is as follows.

That is, the electric signals S ₄ and S ₅ can be converted as shown in Expression 5 or Expression 6 according to the relationship between the sum and product of trigonometric functions, and the electric signals S ₄ thus converted,
If only the DC component is extracted from S ₅ , for example, by calculating the average value, electric signals S ₆ and S ₇ as shown in Equation ₇ are obtained.

[0024]

[Equation 5] S ₄ = (A / 2) {cosφ−cos (2ωt + φ)} S ₅ = (A / 2) {sin (2ωt + φ) + sinφ}

## EQU6 ## S ₄ = (A / 2) {cosφ + cos (2ωt + φ)} S ₅ = (A / 2) {sin (2ωt + φ) -sinφ}

[Equation 7] S ₆ = (A / 2) cosφ S ₇ = (A / 2) sinφ

Then, tan φ can be obtained by dividing both electric signals S ₆ and S ₇ of the equation ( ₇ ). By doing so, the coefficient A representing the change in the amount of reflected light described above can be obtained.
Disappears, and is completely unaffected by changes in the amount of reflected light. The target phase difference φ may be indicated in the form of this tan φ, or the phase difference φ can be calculated by further obtaining the inverse trigonometric function of the tan φ.

Even if the frequency ω is high (for example, about 1 to 1.2 GHz as described above), the calculation of such a trigonometric function can be performed with extremely high accuracy and at high speed, so that accurate phase difference measurement can be performed. This makes it possible to improve the measurement sensitivity.

FIG. 2 is a block diagram showing a distance measuring device according to another embodiment of the present invention.

The difference from the embodiment of FIG. 1 will be mainly described. In this embodiment, a half-wave plate 28 as in the embodiment of FIG. 1 is provided between the reflectors 24 and 26. Absent. Therefore, the linearly polarized light 42 separated by the polarized beam splitter 18 enters the beam splitter 22 as it is, and the linearly polarized light 42 and the circularly polarized light 43 described above enter the polarized beam splitters 30 and 34.

However, in this embodiment, the polarization beam splitters 30 and 34 are placed in the azimuth 4 with respect to the plane of vibration of the linearly polarized light 42.
Therefore, the polarized beam splitter 30 transmits only the p components of the linearly polarized light 42 and the circularly polarized light 43,
As a result, intensity-modulated linearly polarized light (optical beat) 45 is obtained as in the case of the embodiment of FIG. Further, in the polarization beam splitter 34, the linear polarization 42 and the circular polarization 43
The component reflects and the p component transmits, which results in two intensity-modulated linearly polarized light (optical beats) 48 and 49, as in the embodiment of FIG. That is, in this embodiment, the polarized beam splitters 30 and 3 tilted as described above are used.
The polarization means and the interference means described above are integrally formed by each of the four.

According to this embodiment, the half-wave plate 28 used in the embodiment of FIG. 1 is unnecessary, so that the same object can be achieved with a simpler structure.

In each of the above embodiments, the phase difference φ
One optical beat that includes the optical beat and two optical beats that do not include the phase difference φ and are different in phase by 90 ° are formed. However, if the phase shift means, the polarization means, and the interference means described above are used, , It is also possible to make two optical beats including the phase difference φ and different in phase from each other by 90 °, and one optical beat not including the phase difference φ, and in that case, the same triangular shape as above. By calculating the function, the amount related to the phase difference φ can be calculated quickly and accurately.

[0032]

[Equation 8] Asin (ωt + φ) Acos (ωt + φ) sinωt (or cosωt)

[0033]

As described above, according to the present invention, the amount of the phase difference between the light reflected by the object to be measured and the reference light is calculated by a trigonometric function, and is affected by the change in the amount of reflected light. The sine and cosine signals needed to obtain the desired signal can be generated optically. As a result, even at a high optical beat frequency, accurate phase difference measurement can be performed without being affected by changes in the amount of reflected light, thereby improving the measurement sensitivity.

If the above-mentioned polarization means and interference means are integrated with each other, the same object can be achieved with a simpler structure.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a distance measuring device according to an embodiment of the present invention.

FIG. 2 is a configuration diagram showing a distance measuring device according to another embodiment of the present invention.

FIG. 3 is a configuration diagram showing an example of a conventional distance measuring device.

[Explanation of symbols]

 2 laser light source 10 DUT 18 first polarized beam splitter 20 first quarter wavelength plate 22 beam splitter 28 1/2 wavelength plate 30 second polarized beam splitter 32 second quarter wavelength Plate 34 Third polarized beam splitter 60 First photodetector 62 Second photodetector 64 Third photodetector 66 First mixer 68 Second mixer 70 Arithmetic circuit

Claims (5)

[Claims] 1. An apparatus for measuring a distance to an object to be measured by measuring a phase difference between reflected light of an intensity-modulated laser beam by an object to be measured and a reference light, a gas having an internal resonator. A laser light source, a phase shift means for converting linearly polarized light in one of two adjacent longitudinal modes of the laser light source into circularly polarized light, and linearly polarized light in the other mode is rotated by 45 ° or 45 ° of the same light. Polarizing means for extracting a directional component and interference means for causing interference by superimposing light obtained from both means are used, and the light from the interference means is used as the intensity-modulated laser light. Distance measuring device. 2. The distance measuring device according to claim 1, wherein the polarization means and the interference means are integral with each other. 3. The first polarization beam splitter separates two adjacent linear polarizations of a longitudinal mode of a gas laser light source having an internal resonator, and the separated linear polarization of one mode is divided into a first polarization beam having an azimuth of 45 °. It is transmitted through the 1/4 wavelength plate of 1 to be converted into circularly polarized light, and the separated linearly polarized light of the other mode is transmitted through the 1/2 wavelength plate and rotated by 45 °, and both of these lights are divided into a common beam. And split the light into two directions by passing it through a second polarization beam splitter and a second quarter-wave plate with an azimuth of 45 °, and make it enter the object to be measured. The reflected light from it is transmitted again through the second quarter-wave plate and reflected by the second polarized beam splitter to enter the first photodetector, and the other light split by the beam splitter is reflected. Is split in two directions by a third polarization beam splitter, A distance measuring device characterized in that one light is made incident on the second photodetector and the other light is made incident on the third photodetector. 4. A first polarization beam splitter separates two adjacent linearly polarized lights of a longitudinal mode of a gas laser light source having an internal resonator, and the separated linearly polarized light of one mode has an azimuth of 45 °. The light is transmitted through the 1/4 wave plate of 1 to be converted into circularly polarized light, and both this light and the linearly polarized light of the other mode separated as described above are superposed by a common beam splitter and split into two directions. Then, one of the split lights is transmitted through the second polarization beam splitter having an azimuth of 45 ° and the second quarter wavelength plate having an azimuth of 45 ° to be incident on the object to be measured, and the reflected light from the object is reflected. The light is again transmitted through the second quarter-wave plate, reflected by the second polarized beam splitter, and made incident on the first photodetector, and the other light split by the beam splitter has an azimuth of 45 °. The third polarized beam splitter splits the beam in two directions, One light is incident on the second photodetector, and the distance measuring apparatus characterized by being configured so as to be incident the other light to the third light detector. 5. A first mixer for multiplying electric signals from the first photodetector and the second photodetector with each other, and electricity from the first photodetector and the third photodetector. A second mixer for multiplying signals by each other, and an arithmetic means for calculating an amount related to a phase difference between the reflected light by the object to be measured and the reference light, based on electric signals from both mixers. The distance measuring device according to 3 or 4.