JPH0894317A - Displacement gauge - Google Patents

Abstract

(57) [Abstract] [Purpose] A beam splitter using a birefringent crystal with a high extinction ratio can be used for splitting the measurement light and the reference light, and the reference light path optical system and the measurement light path optical system can be configured differently. The optical path lengths of the reference light and the measurement light can be made approximately the same without doing so, and a displacement meter with good measurement accuracy is provided. [Structure] A light wave emitted from a light source 1 and passing through a phase plate 1 is split by an orthogonal polarization beam splitter 3 into a measurement light beam of P polarization and a reference light beam of S polarization. The measurement optical path optical system that forms the optical path of the measurement light beam together with the orthogonal polarization beam splitter 3 includes a quarter wavelength plate 4, a moving plane mirror 5, and a corner cube prism 6. The reference optical path optical system that forms the optical path of the reference light beam together with the orthogonal polarization beam splitter 3 includes an orthogonal polarization beam splitter 23, a quarter-wave plate 24, a fixed plane mirror 25, and a corner cube prism 26.

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 splits a light beam into a reference light and a measurement light and measures a change in the optical path length of the measurement light by the interference between the reference light and the measurement light.

[0002]

2. Description of the Related Art In the precision industry, a precision length measuring machine is an important key part, and its performance greatly affects the performance of the entire device.

In a length measuring instrument using an interferometer, the most appropriate method is adopted depending on the environment for measurement and the required accuracy. For example, if the measurement is performed in an extremely stable environment, it is possible to increase the temporal coherence of the measurement laser as much as possible or introduce a squeezing method. On the other hand, in a very unstable environment, a method has been proposed in which environmental parameters that fluctuate with time, such as air fluctuations, do not affect the length measurement. Two-wavelength interferometry is a typical method.

Next, the principle of the displacement meter using the two-wavelength interferometry will be described. Consider a case where two different wavelength laser lights having the same phase characteristics are coaxially incident on a single interferometer. The optical path length observed at each wavelength is D1, D2
Then, the following equations (1) and (2) are established.

D1 = n1 (ρ, λ1) d (1) D2 = n2 (ρ, λ2) d (2) where ρ is an appropriate thermodynamic parameter, λ1 and λ2
Is the wavelength of each laser beam, n1 and n2 are the refractive indices of air at each wavelength of the laser beam, and d 1 is the distance. From the physical rule of thumb, the following formula is approximately established.

(N1-1) / (n2-1) = k (λ1, λ2) (3) Considering the equation (3), the simultaneous equations (1) and (2) are There are two and two unknowns, and the definite solution d is obtained.

As described above, the displacement meter employing the two-wavelength interferometry is a method of eliminating the error of the observation data due to the air fluctuation by utilizing the air dispersion.

Experimental examples of displacement meters employing the two-wavelength interferometry are described in detail in J. Jpn. Appl. Phys., Vol.28, L478 (1989) and Japanese Patent Laid-Open No. 1-98902. .

Next, with reference to FIG. 3, an example of a conventional displacement meter using the two-wavelength interferometry will be described.

FIG. 3 is a block diagram showing an example of a conventional displacement meter using the two-wavelength interferometry.

This conventional displacement meter, as shown in FIG.
A light source 1 that emits light fluxes having two different wavelengths (a light flux with an angular frequency of ω1 and a light flux with an angular frequency of ω2) that have the same coaxial phase characteristics, a phase plate 2, and an orthogonal polarization beam splitter 3
A quarter wavelength plate 4, a moving plane mirror 5, corner cube prisms 6 and 7, a reflecting mirror 8 and a dichroic mirror 9 for separating the light fluxes of the two different wavelengths.
, Wavelength selection filters 10 and 11, and polarizers 12 and 1
3 and photodetectors 14 and 15 for detecting an interference signal at each wavelength
And a signal processing system 16 for processing the detected signal to calculate the displacement of the movable plane mirror 5.

The phase plate 2 divides each of the two luminous fluxes emitted from the light source 1 into two luminous fluxes with substantially equal intensity by the orthogonal polarization beam splitter 3.
Each of the two light beams emitted from the light source 1 is circularly polarized light or linearly polarized light whose polarization plane is inclined by 45 degrees with respect to the paper surface.

The orthogonal polarization beam splitter 3 transmits each of the light fluxes of the two different wavelengths emitted from the light source 1 and passed through the phase plate 2 as a transmitted light flux of a P-polarized component whose polarization plane is parallel to the paper surface (in this example). Then, this light flux is a measurement light flux and a reflected light flux of an S-polarized component whose polarization plane is perpendicular to the paper surface (in this example,
This luminous flux is divided into a reference luminous flux), and the P-polarized luminous flux is transmitted while the S-polarized luminous flux is reflected. In order to improve the measurement accuracy in such polarization splitting, it is desirable to completely separate the P polarization component and the S polarization component. Therefore, since a beam splitter using a birefringent crystal has a better extinction ratio than a beam splitter using a thin film, a birefringent crystal such as a Glan-Taylor type orthogonal polarization beam splitter is used as the orthogonal polarization beam splitter 3. A beam splitter is used. And the orthogonal polarization beam splitter 3 is
The S-polarized light beam is reflected as an ordinary ray, and thus the P-polarized light beam is transmitted as an extraordinary ray. The direction of the optical axis of the birefringent crystal of the orthogonal polarization beam splitter 3 is
It is set so as to be orthogonal to the direction of the light flux emitted from the light source 1.

The quarter-wave plate 4, the moving plane mirror 5 and the corner cube prism 6 constitute a measuring optical path optical system which forms the optical path of the measuring light beam together with the orthogonal polarization beam splitter 3. The quarter-wave plate 4 is arranged so that the measurement light beam first emitted from the orthogonal polarization beam splitter 3 and the light beam traveling in the same direction as this light beam are incident. The moving plane mirror 5 is a quarter
The light flux emitted from the wave plate 4 in the direction opposite to the orthogonal polarization beam splitter 3 is arranged so as to be reflected in the original direction. The corner cube prism 6 is arranged so as to reflect the light flux emitted from the orthogonal polarization beam splitter 3 for the second time in the original direction.

The corner cube prism 7 constitutes a reference optical path optical system which forms the optical path of the reference light beam together with the orthogonal polarization beam splitter 3. The corner cube prism 7 is an orthogonal polarization beam splitter 3
It is arranged so as to reflect the reference light beam emitted from the light source in the original direction.

The reflection mirror 8, the dichroic mirror 9, the wavelength selection filters 10 and 11 and the polarizers 12 and 13 are associated with the luminous fluxes emitted from the measuring optical path optical system and the reference optical path for each of the luminous fluxes having two different wavelengths. It constitutes a light combining unit that combines the reference light beams emitted from the optical system. The wavelength selection filters 10 and 11 are
It is an optical filter that transmits only the light flux of each wavelength,
It is inserted to assist the function of the dichroic mirror 9 that selects only the light flux of a predetermined wavelength.
The polarizers 12 and 13 have a polarization direction of 45 degrees with respect to the paper surface.

The photodetectors 14 and 15 respectively receive the light fluxes combined by the photosynthesis section for the respective light fluxes of the two different wavelengths.

The signal processing system 16 includes photodetectors 14 and 1.
Based on the output signal of 5 (that is, the interference signal), the amount of change in the optical path difference between the optical path of the measurement light beam and the optical path of the reference light beam, that is, the displacement of the moving plane mirror 5 is obtained.

In the conventional displacement meter shown in FIG. 3 having such a configuration, each of the two light beams of different wavelengths emitted from the light source 1 and passing through the phase plate 2 is P-polarized by the orthogonal polarization beam splitter 3. It is split into a measurement light which is a transmitted light flux of the component and a reference light which is a reflected light flux of the S-polarized component.

The P-polarized measuring light that has passed through the orthogonal polarization beam splitter 3 passes through the quarter-wave plate 4 to become circularly polarized light, and then is reflected in the original direction by the moving plane mirror 5. The reflected light flux passes through the quarter-wave plate 4 again to become an S-polarized light flux, which is reflected by the orthogonal polarization beam splitter 3. After the reflected S-polarized light is reflected in the original direction by the corner cube prism 6,
It is reflected again by the orthogonal polarization beam splitter 3 toward the moving plane mirror 5. The reflected S-polarized light passes through the quarter-wave plate 4 to become circularly polarized light, and then is reflected in the original direction by the moving plane mirror 5. The reflected circularly polarized light passes through the quarter-wave plate 4 to become P-polarized light again, passes through the orthogonal polarization beam splitter 3, and is reflected by the reflecting mirror 8.
Head to.

On the other hand, the S-polarized reference light reflected by the orthogonal polarization beam splitter 3 is reflected in the original direction by the corner cube prism 7, and then is reflected by the orthogonal polarization beam splitter 3 to pass through the orthogonal polarization beam splitter 3. The light is made coaxial with the transmitted P-polarized measuring light and travels toward the reflecting mirror 8.

[0022]

In the conventional displacement meter shown in FIG. 3, as is apparent from the above description, the orthogonal polarization beam splitter 3 is provided between the optical path of the measurement light and the optical path of the reference light.
There is an optical path length difference of 4 times the distance between the moving plane mirror 4 and the moving plane mirror 4.

By the way, when measuring the displacement of the movable mirror by a general Michelson interferometer, assuming an ideal interferometer and environment, the measurement error ε related to the displacement δx of the movable mirror was used for the measurement. When the frequency of light is ν ₀ , the variation amount of the frequency of light is Δν ₀ , and the optical path length difference between the reference light and the measurement light is L, it is expressed by the following equation (4).

Ε = (L + δx) · σ (4) In formula (4), σ is optical frequency stability, and σ = Δν ₀ /
It is defined by ν ₀ .

From the equation (4), the measurement error caused by the fluctuation (instability) of the frequency of the light used for the measurement becomes larger as the optical path length difference between the reference light and the measurement light becomes larger, and the measurement error becomes smaller. In order to achieve this, it is desirable to make the optical path length of the reference light and the optical path length of the measurement light comparable. This also applies to the conventional displacement meter shown in FIG.

Therefore, in the conventional displacement meter shown in FIG. 3, since there is a large optical path length difference between the optical path of the measurement light and the optical path of the reference light as described above, the light used for the measurement (that is, the light source). However, there is a drawback in that the measurement error caused by the fluctuation of the frequency of the light emitted from No.

The corner cube prism 7 is moved so that the distance from the orthogonal polarization beam splitter 3 to the corner cube prism 7 is doubled between the orthogonal polarization beam splitter 3 and the moving plane mirror 4. It is possible to make the optical path lengths of the light and the measurement light comparable. However, when such a method is adopted, the portion forming the reference optical path optical system becomes a long interferometer, which is not practical.

Therefore, the inventor of the present invention sets the optical path length of the reference light and the optical path length of the measurement light to the same level in order to reduce the measurement error caused by the fluctuation of the frequency of the light used for the measurement.
In the conventional displacement meter shown in FIG. 4, the reference optical path optical system was made to have the same configuration as the measuring optical path optical system as shown in FIG. That is, as shown in FIG. 4, instead of the corner cube prism 7 in FIG. 3, a quarter-wave plate 104 and a plane mirror 105 are adopted.

FIG. 4 shows a measurement beam and an S-polarized beam, which are two beams of different wavelengths emitted from the light source 1 and passed through the phase plate 2 in FIG. The measurement light of P-polarized light is split into the reference light which is the reflected light flux of the component, passes through the quarter-wave plate 4 and is reflected in the original direction by the plane mirror 5, and then passes through the quarter-wave plate 4 again and returns to the S-polarized light. Then, the S-polarized reference light passes through the quarter-wave plate 104, is reflected in the original direction by the plane mirror 105, and passes through the quarter-wave plate 104 again. And a state of being returned to the orthogonal polarization beam splitter 3 as polarized light. The arrow X in FIG. 4 indicates the direction of the optical axis of the birefringent crystal of the orthogonal polarization beam splitter 3. Note that the light fluxes incident on the plane mirrors 5, 105 and the light fluxes reflected by the plane mirrors 5, 105 are not actually deviated from each other, but for easy understanding,
In FIG. 4, both light fluxes are expressed by being shifted.

However, as shown in FIG. 4, when the quarter-wave plate 104 and the plane mirror 105 are adopted instead of the corner cube prism 7 in FIG. 3, the interference between the measurement light and the reference light does not occur. , No longer works as a displacement meter. The reason is as shown in FIG.
It was found from the study by the present inventor that the light flux reflected at 5 is deviated from the original optical path direction in the orthogonal polarization beam splitter 3 due to the birefringence effect of the orthogonal polarization beam splitter 3.

The cause will be described in detail. As described above, the P-polarized light is transmitted by the orthogonal polarization beam splitter 3
Is an extraordinary ray, and S-polarized light is an ordinary ray of the orthogonal polarization beam splitter 3. As is clear from the above description, there are both cases where the measurement light becomes S-polarized light and enters the orthogonal polarization beam splitter 3, and there is a case where it becomes P-polarized light and enters the orthogonal polarization beam splitter 3. In this way, the measurement light may become an extraordinary ray and enter the orthogonal polarization beam splitter 3. Even in this case, the optical axis of the birefringent crystal of the orthogonal polarization beam splitter 3 is adjusted so that the optical path of the measurement light reflected by the moving plane mirror 5 does not deviate from the original optical path in the orthogonal polarization beam splitter 3. The direction X is set. That is, as described above, the direction X of the optical axis of the birefringent crystal of the orthogonal polarization beam splitter 3 is set to be orthogonal to the direction of the light beam emitted from the light source 1 (see FIG. 4). In the conventional displacement meter shown in FIG. 3, as the reference light, only the S-polarized reference light (that is, the ordinary ray) is incident on the orthogonal polarization beam splitter 3, so that the reference light reflected by the corner cube prism 7 is orthogonal. There is no deviation from the original optical path in the polarization beam splitter 3. However, when the quarter-wave plate 104 and the plane mirror 105 are adopted in place of the corner cube prism 7 in FIG. 3 (see FIG. 4), the quarter-wave plate 104 is reflected by the plane mirror 105 and the quarter-wave plate 104 is reflected. The passed reference light becomes P-polarized light (that is, extraordinary light) and enters the orthogonal polarization beam splitter 3. Since the incident direction is tilted with respect to the direction orthogonal to the direction X of the optical axis set as described above, it becomes P-polarized light and becomes the orthogonal polarization beam splitter 3.
As shown in FIG. 4, the reference light incident on is deviated from the original optical path in the orthogonal polarization beam splitter 3. In this way, the light beam reflected by the plane mirror 105 is deviated from the original optical path direction in the orthogonal polarization beam splitter 3 due to the birefringence effect of the orthogonal polarization beam splitter 3.

Therefore, in the conventional displacement meter shown in FIG. 3, since the quarter-wave plate 104 and the plane mirror 105 cannot be used in place of the corner cube prism 7, the optical path of the reference light split by the orthogonal polarization beam splitter 3 is used. The length and the optical path length of the measuring light cannot be made equal, and therefore
There is a drawback that the measurement error caused by the fluctuation of the frequency of the light used for the measurement is large.

When a beam splitter that does not use a birefringent crystal such as a thin film orthogonal polarization beam splitter is used as the orthogonal polarization beam splitter 3,
Since the problem due to the birefringence effect described above does not occur, a quarter cube is used instead of the corner cube prism 7 in FIG.
The wave plate 104 and the plane mirror 105 can be adopted,
The measurement error caused by the fluctuation of the frequency of the light used for the measurement can be reduced. But in this case
The extinction ratio of beam splitters that do not use birefringent crystals such as thin film orthogonal polarization beam splitters is poor compared to beam splitters that use birefringent crystals.
P compared to a beam splitter using a birefringent crystal
The polarization component and the S polarization component cannot be completely separated, and therefore, from this point, the measurement accuracy deteriorates.

The circumstances explained above are not limited to the displacement meter employing the two-wavelength interferometry method, but also one arbitrary light beam is split into the measurement light and the reference light by the orthogonal polarization beam splitter, and the optical paths of the reference light and the measurement light are divided. The same applies to the displacement meter that measures the change in the difference by the interference between the reference light and the measurement light.

The present invention has been made in view of the above circumstances, and a beam splitter using a birefringent crystal having a high extinction ratio can be used for splitting the measurement light and the reference light, and a reference optical path optical system can be used. An object of the present invention is to provide a displacement meter with good measurement accuracy, which can make the optical path lengths of the reference light and the measurement light comparable without making the measurement optical path optical system different.

[0036]

In order to solve the above problems, the displacement gauge according to the first aspect of the present invention has at least two displacement sensors which are coaxial or are parallel in a spatially close state.
A light source that emits light fluxes of two or more different wavelengths, and a light flux emitted from the light source of two or more different wavelengths, respectively, are polarized with a first light flux whose polarization plane is a first polarization plane. A second plane of light which is a second plane of polarization orthogonal to the first plane of polarization and a plane of polarization that transmits the light flux of the first plane of polarization and the plane of polarization is A first light splitter that reflects a light beam that is a second plane of polarization;
A measurement optical path optical system that forms an optical path of a measurement light beam, which is one of the first and second light beams of each of the two or more light beams having different wavelengths, together with the first light splitter; A reference optical path optical system for forming an optical path of a reference light beam, which is the other of the first and second light beams of two or more light beams having different wavelengths, together with the first light splitter; With respect to each of the two or more light beams having different wavelengths, a light combining unit for respectively combining the measurement light beam emitted from the measurement optical path optical system and the reference light beam emitted from the reference optical path optical system, and the two or more different ones from each other. With respect to each of the light fluxes of the wavelengths, between the light detection unit that receives the light fluxes combined by the photosynthesis unit, and the optical path of the measurement light flux and the reference light flux based on the output signal of the light detection unit. Light of Signal processing means for obtaining the amount of change in the differential is obtained by the configuration with a. Then, one of the measurement optical path optical system and the reference optical path optical system receives the first light beam first emitted from the first light splitter and a light beam traveling in the same direction as this light beam. First
A quarter-wave plate, and a first reflector that reflects in the original direction a light beam emitted from the first quarter-wave plate in a direction opposite to the first light splitter, A second reflector that reflects the light beam emitted from the first light splitter for the second time in the original direction. Further, the other of the measurement optical path optical system and the reference optical path optical system receives the second light flux first emitted from the first light splitter, and has a polarization plane of the first polarization plane. And a second light splitter which transmits a light flux having a polarization plane which is the second polarization plane, and the second light flux which is first emitted from the second light splitter. And a second quarter-wave plate on which a light flux traveling in the same direction as this light flux is incident, and emitted from the second quarter-wave plate in a direction opposite to the second light splitter. A third reflector for reflecting the light flux in the original direction and a fourth reflector for reflecting the light flux emitted from the second light splitter for the second time in the original direction are provided.

In the displacement meter according to the second aspect of the present invention, a light source for emitting a light beam and the light beam emitted from the light source are polarized with the first light beam whose polarization plane is the first polarization plane. A second plane of light which is a second plane of polarization orthogonal to the first plane of polarization and a plane of polarization that transmits the light flux of the first plane of polarization and the plane of polarization is A measurement in which a first light splitter that reflects a light beam that is a second polarization plane and an optical path of a measurement light beam that is one of the first and second light beams is formed together with the first light splitter. An optical path optical system, a reference optical path optical system that forms the optical path of a reference light beam that is the other of the first and second light beams together with the first light splitter, and a measurement emitted from the measurement optical path optical system. A light combining unit for combining the light flux and the reference light flux emitted from the reference optical path optical system; A signal for obtaining the amount of change in the optical path difference between the optical path of the measurement light beam and the optical path of the reference light beam based on the output signal of the light detection section that receives the light beam combined by the light combining section and the photodetection section. And a processing means. Then, one of the measurement optical path optical system and the reference optical path optical system receives the first light beam first emitted from the first light splitter and a light beam traveling in the same direction as this light beam. A first quarter-wave plate and a first reflector for reflecting in the original direction a light beam emitted from the first quarter-wave plate in a direction opposite to the first light splitter. And a second reflector that reflects the light beam emitted from the first light splitter for the second time in the original direction. Further, the other of the measurement optical path optical system and the reference optical path optical system receives the second light flux first emitted from the first light splitter, and has a polarization plane of the first polarization plane. And a second light splitter that transmits a light beam having a polarization plane that is the second polarization plane;
From the second quarter-wave plate and the second quarter-wave plate on which the second light flux first emitted from the light splitter and the light flux traveling in the same direction as this light flux are incident. The second
A third reflector for reflecting the light beam emitted in the direction opposite to the light splitter in the original direction, and a third reflector for reflecting the light beam emitted the second time from the second light splitter in the original direction. 4 reflectors.

Further, the displacement meter according to the third aspect of the present invention is the displacement meter according to the first or second aspect, wherein the first optical splitter is a beam splitter using a birefringent crystal, The beam splitter reflects the light flux whose polarization plane is the second polarization plane as an ordinary ray, and makes the direction of the optical axis of the birefringent crystal of the beam splitter orthogonal to the direction of the light flux emitted from the light source. The measurement light flux and the reference light flux have substantially the same optical path length.

[0039]

According to the present invention, each of the two or more light fluxes having different wavelengths emitted from the light source is converted by the first light splitter into the first light flux whose polarization plane is the first polarization plane. The polarization plane is split into a second light flux which is a second polarization plane orthogonal to the first polarization plane.

For convenience of explanation, it is assumed that the first light flux is the measurement light flux and the second light flux is the reference light flux. Further, it is assumed that the measurement optical path optical system includes a first quarter-wave plate, a first reflector, and a second reflector. Further, it is assumed that the reference optical path optical system includes a second light splitter, a second quarter-wave plate, a third reflector and a fourth reflector.

First, the optical path of the measurement light beam formed by the measurement optical path optical system and the first light splitter will be described.
The first light beam (that is, here, the measurement light beam) first emitted from the first light splitter is circularly polarized through the first quarter-wave plate and then the first reflector. Is reflected in the original direction. The reflected light flux again passes through the first quarter-wave plate to become a light flux whose polarization plane is the second polarization plane and is reflected by the first light splitter. The light beam whose polarization plane is the second polarization plane which is reflected by the first light splitter and is emitted from the first light splitter for the second time is reflected in the original direction by the second reflector, It is again reflected by the first light splitter in the direction of the first reflector. The light beam whose reflected plane of polarization is the second plane of polarization passes through the first quarter-wave plate, becomes circularly polarized light, and then is reflected in the original direction by the first reflector. The reflected circularly polarized light passes through the first quarter-wave plate, and again becomes a light beam having a polarization plane of the first polarization plane and passes through the first light splitter. That is,
The measurement light flux is emitted from the measurement optical path optical system with its polarization plane becoming the first polarization plane.

Next, the optical path of the reference light beam formed by the reference optical path optical system and the first light splitter will be described. The second light beam (that is, here, the reference light beam) first emitted from the first light splitter is reflected by the second light splitter. The reference light beam reflected by the second light splitter and first emitted from the second light splitter passes through the second quarter-wave plate to become circularly polarized light, and then is converted to the original light by the third reflector. Is reflected in the direction of. The reflected light flux is again the second quarter
By passing through the wave plate, the polarization plane becomes a light flux having the first polarization plane and passes through the second light splitter. The light flux having the first polarization plane, which is the second polarization plane transmitted through the second light splitter and emitted from the second light splitter, is reflected in the original direction by the fourth reflector, It again passes through the second light splitter.
The light flux whose transmitted plane of polarization is the first plane of polarization is
After passing through the half-wave plate to become circularly polarized light, it is reflected in the original direction by the third reflector. The reflected circularly polarized light passes through the second quarter-wave plate to become a light flux whose polarization plane is the second polarization plane again, and is reflected by the second light splitter. A light beam whose reflected polarization plane is the second polarization plane is reflected by the first light splitter. That is, the reference light becomes a light beam whose polarization plane is the second polarization plane and exits from the reference optical path optical system.

As can be seen from the above description, the measurement light beam has a polarization plane of the first polarization plane and enters the first light splitter, and the measurement light beam has the second polarization plane. There are both cases where it becomes incident on the first light splitter,
As for the reference light flux, only the light flux whose polarization plane is the second polarization plane is incident on the first light splitter.

Therefore, according to the present invention, even when the beam splitter using the birefringent crystal is used as the first light splitter, the light flux which is the second polarization plane is divided into the first light splitter. Setting so that it becomes an ordinary ray of the optical system (that is, a beam splitter that reflects a light beam whose polarization plane is the second polarization plane as an ordinary ray) is used.
Even if the measurement light flux that has become the light flux (the extraordinary ray of the first light splitter) that is the plane of polarization of is incident on the first light splitter, its optical path is the direction of the original optical path within the first light splitter. The direction of the optical axis of the first light splitter is set so as not to deviate (that is, the direction of the optical axis of the birefringent crystal is made orthogonal to the direction of the light beam emitted from the light source). Therefore, the problem due to the birefringence effect described above does not occur. Therefore, the first
A beam splitter using a birefringent crystal with a high extinction ratio can be used as the optical splitter of
It is possible to relatively completely separate the component of which the polarization plane is and the component of which the polarization plane is the second polarization plane.

Further, according to the present invention, since the optical path of the measurement light beam and the optical path of the reference light beam are formed as described above, the optical path lengths of the reference light and the measurement light can be made approximately the same.

As described above, according to the present invention, a beam splitter using a birefringent crystal having a high extinction ratio can be used for splitting the measurement light and the reference light, and the reference light path optical system and the measurement light path optical system can be used. The optical path lengths of the reference light and the measurement light can be made approximately the same without having different configurations, and therefore a displacement meter with high measurement accuracy can be provided.

The two-wavelength interferometry is adopted in the first aspect of the present invention, whereas only the light flux of any one wavelength is used in the second aspect of the present invention. Is different. However, also in the second aspect of the present invention, similarly to the first aspect of the present invention, a beam splitter using a birefringent crystal having a good extinction ratio for splitting the measurement light and the reference light can be used, The optical path lengths of the reference light and the measurement light can be made approximately the same without making the reference optical path optical system and the measurement optical path optical system different configurations, and therefore a displacement meter with high measurement accuracy can be provided.

[0048]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on the embodiments shown in the drawings.

FIG. 1 is a block diagram showing a displacement gauge according to an embodiment of the present invention. In FIG. 1, the same components as those shown in FIG. 3 are designated by the same reference numerals.

This displacement meter, as shown in FIG. 1, is a light beam of two different wavelengths with uniform coaxial phase characteristics (it is not required to be coaxial, but parallel as long as they are spatially close). (Light source with angular frequency ω1 and angular frequency ω2), light source 1, phase plate 2, orthogonal polarization beam splitter (first optical splitter) 3, first quarter wavelength Board 4
Moving plane mirror (first reflector) 5, corner cube prism (second reflector) 6, orthogonal polarization beam splitter (second light splitter) 23, second quarter-wave plate 24 , Fixed plane mirror (third reflector) 25, corner cube prism 26 (fourth reflector), and reflector 8
A dichroic mirror 9 for separating the light fluxes of the two different wavelengths, and wavelength selection filters 10 and 11
, The polarizers 12 and 13, photodetectors (photodetection units) 14 and 15 that detect interference signals at respective wavelengths, and a signal processing system 16 that processes the detected signals and calculates the displacement of the moving plane mirror 5.
It consists of and.

The phase plate 2 divides each of the two light fluxes emitted from the light source 1 into two light fluxes with substantially equal intensity by the orthogonal polarization beam splitter 3.
Each of the two light beams emitted from the light source 1 is circularly polarized light or linearly polarized light whose polarization plane is inclined by 45 degrees with respect to the paper surface.

The orthogonal polarization beam splitter 3 transmits each of the light fluxes of the two different wavelengths emitted from the light source 1 and passed through the phase plate 2 as a transmitted light flux of a P-polarized component whose polarization plane is parallel to the paper surface (in this embodiment). In this example, this luminous flux is the measurement luminous flux)
And a reflected light beam of an S-polarized component whose polarization plane is perpendicular to the plane of the paper (in this embodiment, this light beam is a reference light beam), and transmits the P-polarized light beam and reflects the S-polarized light beam. In order to improve the measurement accuracy in such polarization splitting, it is desirable to completely separate the P polarization component and the S polarization component. Therefore, since a beam splitter using a birefringent crystal has a better extinction ratio than a beam splitter using a thin film, a Glan-Taylor type orthogonal polarization beam splitter or a Glan-Thomson type orthogonal polarization beam splitter is used as the orthogonal polarization beam splitter 3. A beam splitter using a birefringent crystal such as is used. And the orthogonal polarization beam splitter 3 is
The S-polarized light beam is reflected as an ordinary ray, and thus the P-polarized light beam is transmitted as an extraordinary ray. The direction of the optical axis of the birefringent crystal of the orthogonal polarization beam splitter 3 is
It is set so as to be orthogonal to the direction of the light flux emitted from the light source 1.

In this embodiment, the quarter-wave plate 4, the moving plane mirror 5 and the corner cube prism 6 constitute a measurement optical path optical system which forms the optical path of the measurement light beam together with the orthogonal polarization beam splitter 3. There is. The quarter-wave plate 4 is arranged so that the measurement light beam first emitted from the orthogonal polarization beam splitter 3 and the light beam traveling in the same direction as this light beam are incident. The moving plane mirror 5 is arranged so as to reflect the light flux emitted from the quarter-wave plate 4 in the direction opposite to the orthogonal polarization beam splitter 3 in the original direction. The corner cube prism 6 is arranged so as to reflect the light flux emitted from the orthogonal polarization beam splitter 3 for the second time in the original direction.

In the present embodiment, the orthogonal polarization beam splitter 23, the quarter-wave plate 24, the fixed plane mirror 2 are used.
5 and the corner cube prism 26 constitute a reference optical path optical system that forms the optical path of the reference light beam together with the orthogonal polarization beam splitter 3. The orthogonal polarization beam splitter 23 receives the reference light beam first emitted from the orthogonal polarization beam splitter 3, transmits the P polarized light beam, and reflects the S polarized light beam. Since the P-polarized light beam and the S-polarized light beam are individually incident on the orthogonal polarization beam splitter 23 and the incident light is not divided into the P-polarized component and the S-polarized component, the orthogonal polarization beam splitter 23. For this, it is not always necessary to use a beam splitter using a birefringent crystal having a good extinction ratio, and for example, a thin film orthogonal polarization beam splitter can be used. The quarter-wave plate 24 is arranged so that the reference light beam first emitted from the orthogonal polarization beam splitter 23 and a light beam traveling in the same direction as this light beam are incident. The fixed plane mirror 25 is 4
From the half-wave plate 24 to the orthogonal polarization beam splitter 23
It is arranged so as to reflect the light beam emitted in the direction opposite to that in the original direction. Corner cube prism 26
Are arranged so as to reflect the light flux emitted from the orthogonal polarization beam splitter 23 for the second time in the original direction.

The reflection mirror 8, the dichroic mirror 9, the wavelength selection filters 10 and 11 and the polarizers 12 and 13 are associated with the luminous fluxes of the two different wavelengths, the measuring luminous flux emitted from the measuring optical path optical system and the reference optical path. It constitutes a light combining unit that combines the reference light beams emitted from the optical system. The wavelength selection filters 10 and 11 are
It is an optical filter that transmits only the light flux of each wavelength,
It is inserted to assist the function of the dichroic mirror 9 that selects only the light flux of a predetermined wavelength.
The polarizers 12 and 13 have a polarization direction of 45 degrees with respect to the paper surface.

The photodetectors 14 and 15 respectively receive the light fluxes combined by the photosynthesis section for the respective light fluxes of the two different wavelengths.

The signal processing system 16 includes photodetectors 14 and 1
Based on the output signal of 5 (that is, the interference signal), arithmetic processing is performed according to the well-known principle of the two-wavelength interferometry, and the change amount of the optical path difference between the optical path of the measurement light beam and the optical path of the reference light beam, that is, , The displacement of the moving plane mirror 5 is obtained.

In the displacement meter shown in FIG. 1 constructed in this way, each of the light fluxes of the two different wavelengths emitted from the light source 1 and passing through the phase plate 2 is converted into a P-polarized component by the orthogonal polarization beam splitter 3. It is divided into a measurement light beam that is a transmitted light beam and a reference light beam that is a reflected light beam of the S-polarized component.

The P-polarized measuring light transmitted through the orthogonal polarization beam splitter 3 passes through the quarter-wave plate 4 to become circularly polarized light, and then is reflected in the original direction by the moving plane mirror 5. The reflected light flux passes through the quarter-wave plate 4 again to become an S-polarized light flux, which is reflected by the orthogonal polarization beam splitter 3. The reflected S-polarized light is reflected in the original direction by the corner cube prism 6, and then again moved by the orthogonal polarization beam splitter 3 into the moving plane mirror 5.
Is reflected in the direction of. The reflected S-polarized light passes through the quarter-wave plate 4 to become circularly polarized light, and then is reflected in the original direction by the moving plane mirror 5. The reflected circularly polarized light is
After passing through the quarter-wave plate 4, it becomes P-polarized light again, passes through the orthogonal polarization beam splitter 3, and travels toward the reflecting mirror 8.

On the other hand, the S-polarized reference light reflected by the orthogonal polarization beam splitter 3 is reflected again by the orthogonal polarization beam splitter 23. The reflected reference light is 4
After passing through the half-wave plate 24 to become circularly polarized light, it is reflected in the original direction by the fixed plane mirror 25. The reflected light beam passes through the quarter-wave plate 24 again to become a P-polarized light beam and passes through the orthogonal polarization beam splitter 23. The transmitted P-polarized light is reflected in the original direction by the corner cube prism 26, and then passes through the orthogonal polarization beam splitter 23 again. The transmitted P-polarized light passes through the quarter-wave plate 24 to become circularly polarized light, and then is reflected in the original direction by the fixed plane mirror 25. The reflected circularly polarized light passes through the quarter-wave plate 24 to become S-polarized light again, and is reflected by the orthogonal polarization beam splitter 23. The reflected S-polarized reference light is reflected again by the orthogonal polarization beam splitter 3, is made coaxial with the P-polarized measurement light that has passed through the orthogonal polarization beam splitter 3, and travels to the reflecting mirror 8.

Coaxial and orthogonal polarization beam splitter 3
The P-polarized measurement light and the S-polarized reference light emitted from are reflected by the reflecting mirror 8 and then separated by the dichroic mirror 9 for each wavelength. The P-polarized measurement light having an angular frequency of ω1 and the S-polarized reference light are transmitted through the dichroic mirror 9 and the wavelength selection filter 10 that transmits only the light beam having an angular frequency of ω1, and the polarizer 12 is used to After the polarization planes are aligned and combined (interfered), the light is received by the photodetector 14. On the other hand, the P-polarized measurement light and the S-polarized reference light having an angular frequency of ω2 are reflected by the dichroic mirror 9, and the angular frequency is ω.
After passing through the wavelength selection filter 11 that transmits only the two light fluxes, the polarization planes of the two light beams are aligned by the polarizer 13 and multiplexed, and then received by the photodetector 15. Photo detector 1
The detection signals output from 4, 15 are arithmetically processed by the signal processing system 16 according to the well-known principle of two-wavelength interferometry,
The displacement amount of the moving plane mirror 5 from which an error due to air fluctuation is removed is obtained.

As can be seen from the above description, in this embodiment, the measurement light beam is P-polarized and is incident on the orthogonal polarization beam splitter 3, and is S-polarized light is incident on the orthogonal polarization beam splitter 3. Although there are both, as for the reference light flux, only the S-polarized light flux enters the orthogonal polarization beam splitter 3.

Therefore, according to this embodiment, even when a beam splitter using a birefringent crystal is used as the orthogonal polarization beam splitter 3, the S-polarized light beam is an ordinary ray of the orthogonal polarization beam splitter 3. (That is, a beam splitter that reflects the S-polarized light flux as an ordinary ray is adopted), and the measurement light flux that has become P-polarized light (an extraordinary ray of the orthogonal polarization beam splitter 3) is an orthogonal polarization beam splitter. Three
Incident light on the orthogonal polarization beam splitter 3
The direction of the optical axis of the orthogonal polarization beam splitter 3 is set so as not to deviate from the original optical path direction (that is, the direction of the optical axis of the birefringent crystal is the direction of the light beam emitted from the light source 1). By making them orthogonal to each other), the problem due to the birefringence effect described above does not occur. Therefore, a beam splitter using a birefringent crystal having a high extinction ratio can be used as the orthogonal polarization beam splitter 3, and the P-polarized component and the S-polarized component can be relatively completely separated.

Further, according to the present embodiment, as described above, since the optical path of the measurement light beam and the optical path of the reference light beam are formed, the optical path lengths of the reference light and the measurement light can be made approximately the same.

As described above, according to this embodiment, a beam splitter using a birefringent crystal having a high extinction ratio can be used for splitting the measurement light and the reference light, and the reference light path optical system and the measurement light path optics can be used. It is possible to make the optical path lengths of the reference light and the measurement light comparable without making the system different configurations,
Therefore, it is possible to provide a displacement meter with high measurement accuracy.

Next, a displacement meter according to another embodiment of the present invention will be described.

FIG. 2 is a block diagram showing a displacement meter according to this embodiment. In FIG. 2, the same components as those shown in FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted.

The embodiment shown in FIG. 2 is different from the embodiment shown in FIG. 1 only in the configurations of the light source, the light combining section and the signal processing system.

In this embodiment, a light source 31, a reflecting mirror 32 and a dichroic mirror 33 are used as the light source instead of the light source 1 in FIG. The light source 31 includes two light beams having different wavelengths (a light beam having an angular frequency of ω1 and a light beam having an angular frequency of ω1).
Two light fluxes) are emitted from spatially different places. The two emitted light beams having different wavelengths are made coaxial by the reflecting mirror 32 and the dichroic mirror 33. The light source 31
For example, a semiconductor laser-excited Nd: YAG laser beam and a second harmonic generated by condensing part of the laser beam into a resonator made of MgO: LiNbO3 (magnesium oxide-doped lithium niobate) crystal And are emitted 2
A wavelength light source can be used. In this case, the light source 31
From, the light of wavelength 1064nm output 150mW, wavelength 53
Light of 2 nm is oscillated with an output of 20 mW.

Further, in this embodiment, the position where the light beam passing through the phase plate 2 is incident on the orthogonal polarization beam splitter 3 is different from that of the embodiment shown in FIG. The emitted P-polarized measurement light and S-polarized reference light are spatially separated and emitted from the orthogonal polarization beam splitter 3.

In the present embodiment, the heterodyne method is applied by using the optical combiner and the signal processing system 44 having a configuration different from that of the embodiment shown in FIG.

That is, in this embodiment, the light combining section combines the reflecting mirrors 34, 35, the acousto-optic elements 36, 37 for changing the optical frequencies by different constant values, and the spatially separated luminous flux. Reflecting mirrors 38, 40 for wave
And orthogonal polarization beam splitters 39 and 41 and polarizers 42 and 43. Polarizer 42,43
Has a polarization direction of 45 degrees with respect to the paper surface. As the acousto-optic element 36.37, for example, PbMoO3 (lead molybdate) driven by a piezoelectric element, or tellurium dioxide, glass, or the like driven by a piezoelectric element can be used.

The P-polarized measuring light and the S-polarized reference light, which are spatially separated from the orthogonal polarization beam splitter 3 and finally emitted, are reflected by the reflecting mirrors 34 and 35, respectively, and then separated into different acoustic waves. It is incident on the optical elements 36 and 37.
The P-polarized measurement light of each wavelength (that is, each of the angular frequencies ω1 and ω2) is modulated by the acoustooptic device 36 by a constant value, that is, the angular frequency ω3, and the angular frequency ω1 + ω
3 and a P-polarized light beam having an angular frequency ω2 + ω3. At this time, since the diffraction angle of the acousto-optic element 36 varies depending on the wavelength, the measurement light that is a P-polarized light flux with an angular frequency ω1 + ω3 and the measurement light that is a P-polarized light flux with an angular frequency ω2 + ω3 are spatially separated. And enters the orthogonal polarization beam splitters 41 and 39, respectively. Similarly, the S-polarized reference light of each wavelength (that is, each of the angular frequencies ω1 and ω2) is modulated by the acousto-optic element 37 by a constant value, that is, the angular frequency ω4, and the angular frequency ω1 + ω4.
And the S-polarized light beam of angular frequency ω2 + ω4. At this time, since the diffraction angle of the acousto-optic element 37 varies depending on the wavelength, the reference light that becomes the S-polarized light flux of the angular frequency ω1 + ω4 and the reference light that becomes the S-polarized light flux of the angular frequency ω2 + ω4 are spatially separated. And the orthogonal polarization beam splitter 4 after being reflected by the reflected lights 40 and 38, respectively.
It is incident on 1, 39 respectively.

The measuring light, which is a P-polarized light beam having an angular frequency ω1 + ω3, and the reference light, which is an S-polarized light beam having an angular frequency ω1 + ω4, are coaxial with each other by the orthogonal polarization beam splitter 41, and are polarized by the polarizer 42. After the surfaces are aligned and multiplexed (interfered), the light is received by the photodetector 14. Note that the difference between ω3 and ω4 is appropriately determined, and the detector 14 receives the light beat. Similarly, the measurement light, which is a P-polarized light flux of angular frequency ω2 + ω3, and the reference light, which is an S-polarized light flux of angular frequency ω2 + ω4, are coaxial with each other by the orthogonal polarization beam splitter 39, and are polarized by the polarizer 43. After the surfaces are aligned and multiplexed (interfered), the light is received by the photodetector 15. The detector 15 also receives the beat of light.

The detection signals output from the photodetectors 14 and 15 are arithmetically processed by the signal processing system 44 in accordance with the well-known principle of the two-wavelength interference method and the principle of the heterodyne method, and the movement is performed by eliminating the error due to the fluctuation of the air. The amount of displacement of the plane mirror 5 is obtained.

Also in this embodiment, as in the embodiment shown in FIG. 1, a beam splitter using a birefringent crystal having a good extinction ratio can be used for splitting the measurement light and the reference light, and the reference optical path optics can be used. The optical path lengths of the reference light and the measurement light can be made approximately the same without making the system and the measurement optical path optical system different configurations, and therefore a displacement meter with good measurement accuracy can be provided.

Although the respective embodiments of the present invention have been described above, the present invention is not limited to these embodiments.

For example, in the embodiment shown in FIG. 2, two light fluxes having different wavelengths are produced from a semiconductor laser excitation ring Nd: YAG laser light and a part of the light flux is made of MgO: LiNbO3 (magnesium oxide-doped lithium niobate) crystal. The case of using the second harmonic obtained by condensing on the resonator is explained, but it can be obtained by condensing a part of the semiconductor laser light and its luminous flux on KNbO3 (potassium niobate) crystal. Second harmonics and the like can also be used. Here, if necessary, the oscillation wavelength of the light source may be fixed by using a light absorption line of a molecule, for example, an absorption line of iodine (I 2).

Further, the plane mirrors 5 and 25 shown in FIGS.
And each of the corner cube prisms 6 and 26 can be replaced by other reflectors such as plane mirrors, corner cube prisms, corner mirrors and right angle prisms.

Further, the measurement optical path optical system and the reference optical path optical system are distinguished from each other. For example, FIG. 1 and FIG.
The plane mirror 5 in the inside may be fixed and the plane mirror 25 in FIGS. 1 and 2 may be movable. In this case, the measurement optical path optical system and the reference optical path optical system are interchanged with respect to the case of the above embodiment. become.

Further, any one of the plane mirrors 5 and 25 and the corner cube prisms 6 and 26 in FIGS. 1 and 2 may be made movable.

Further, in the embodiment shown in FIG. 2, the optical combiner and the signal processing system 44 are applied so that the heterodyne method is applied.
However, in the present invention, the photosynthesis unit, the photodetection unit, and the signal processing system may be modified so that the 90-degree phase difference method or the like is applied. Such a modification is apparent from the above description and well-known matters regarding the 90-degree phase difference method and the like.

Further, in each of the above embodiments, an example of the displacement meter to which the two-wavelength interferometry is applied has been explained, but in the present invention,
The above embodiments may be modified to use three or more light beams different from each other, or may be modified to use only one light beam.

For example, when only one light beam is used without applying the two-wavelength interference method, FIGS. 1 and 2 may be modified as follows.

That is, in FIG. 1, instead of the light source 1, a light source which emits only a light flux of an arbitrary wavelength is used, the dichroic mirror 9, the wavelength selection filter 11, the polarizer 13 and the photodetector 15 are removed, and signal processing is performed. The calculation method in the system 16 may be changed. In addition, in FIG.
Instead of the light source 31, the reflecting mirror 32, and the dichroic mirror 33, a light source that emits only a light beam having an arbitrary wavelength is used, the orthogonal polarization beam splitter 39, the polarizer 43, and the photodetector 15 are removed, and calculation in the signal processing system 44 is performed. Change the method.

[0086]

According to the present invention, a beam splitter using a birefringent crystal having a high extinction ratio can be used for splitting the measurement light and the reference light, and the reference light path optical system and the measurement light path optical system are different. It is possible to make the optical path lengths of the reference light and the measurement light comparable without configuring them, and thus,
It is possible to provide a displacement meter with high measurement accuracy.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a displacement meter according to an embodiment of the present invention.

FIG. 2 is a configuration diagram showing a displacement meter according to another embodiment of the present invention.

FIG. 3 is a configuration diagram showing a conventional displacement meter.

FIG. 4 is a diagram showing a modification of the conventional displacement meter shown in FIG.

[Explanation of symbols]

 1, 31 Light source 2 Phase plate 3, 23, 39, 41 Orthogonal polarization beam splitter 4, 24 Quarter wave plate 5 Moving plane mirror 6, 7, 26 Corner cube prism 8, 32, 34, 35, 38, 40 Reflection Mirror 9,33 Dichroic mirror 10,11 Wavelength selection filter 12,13,42,43 Polarizer 14,15 Photodetector 16,44 Signal processing system 25 Fixed plane mirror 36,37 Acousto-optic element

Claims (3)

[Claims] 1. A light source that emits at least two light beams having different wavelengths so as to be parallel to each other in a coaxial or spatially close state, and the two or more light beams emitted from the light source. Each of the light fluxes having different wavelengths is divided into a first light flux whose polarization plane is the first polarization plane and a second light flux whose polarization plane is the second polarization plane orthogonal to the first polarization plane. At the same time, a first light splitter which transmits a light beam whose polarization plane is the first polarization plane and reflects a light beam whose polarization plane is the second polarization plane, and the two or more different from each other. A measuring light path optical system that forms an optical path of a measuring light beam, which is one of the first and second light beams of each wavelength, together with the first light splitter, and the two or more different wavelengths. Which is the other of the first and second luminous fluxes of each of the luminous fluxes of A reference light path optical system that forms an optical path of a light beam together with the first light splitter, and a measurement light beam emitted from the measurement light path optical system and the reference light path optical system for each of the two or more light beams having different wavelengths. A light combining unit that combines the reference light beams emitted from the system, and a light detection unit that receives the light beams combined by the light combining unit for each of the two or more light beams having different wavelengths; Signal processing means for obtaining an amount of change in the optical path difference between the optical path of the measurement light beam and the optical path of the reference light beam based on the output signal of the detection section, the measurement optical path optical system and the reference optical path optical system One of them is a first quarter-wave plate on which the first light beam first emitted from the first light splitter and a light beam traveling in the same direction as this light beam are incident; 1/4 of 1 A first reflector for reflecting the light beam emitted from the wave plate in the direction opposite to the first light splitter in the original direction and a light beam emitted for the second time from the first light splitter A second reflector that reflects light in the direction of the second light flux, wherein the other of the measurement optical path optical system and the reference optical path optical system is the second light flux first emitted from the first light splitter. And a second light splitter which transmits a light beam whose polarization plane is the first polarization plane and reflects a light beam whose polarization plane is the second polarization plane, and the second light. A second quarter-wave plate on which the second light beam first emitted from the splitter and a light beam traveling in the same direction as this light beam are incident, and from the second quarter-wave plate to the first quarter-wave plate. From the second light splitter, a third reflector that reflects the light beam emitted in the direction opposite to the second light splitter in the original direction; And a fourth reflector that reflects the light flux emitted for the second time in the original direction. 2. A light source that emits a light flux, and a first light flux whose polarization plane is a first polarization plane and a light flux emitted from the light source that is orthogonal to the first polarization plane. And a second light flux that is a second polarization plane, and transmits a light flux whose polarization plane is the first polarization plane,
A first light splitter that reflects a light beam whose polarization plane is the second polarization plane, and an optical path of a measurement light beam that is one of the first and second light beams is set to the first light beam. A measuring optical path optical system formed together with a splitter, a reference optical path optical system forming an optical path of a reference luminous flux which is the other of the first and second luminous fluxes together with the first optical splitter, and the measuring optical path An optical combining unit that combines a measurement light beam emitted from an optical system and a reference light beam emitted from the reference optical path optical system, a light detecting unit that receives the light beam combined by the light combining unit, and an output of the light detecting unit A signal processing unit that obtains a change amount of an optical path difference between the optical path of the measurement light beam and the optical path of the reference light beam based on a signal, and one of the measurement optical path optical system and the reference optical path optical system. Was first emitted from the first light splitter A first quarter-wave plate on which the first light flux and a light flux traveling in the same direction as the first light flux are incident, and a side opposite to the first light splitter from the first quarter-wave plate. A first reflector that reflects the light beam emitted in the original direction in the original direction, and a second reflector that reflects the second light beam emitted from the first light splitter in the original direction. In the other of the measurement optical path optical system and the reference optical path optical system, the second light flux first emitted from the first light splitter is incident, and the polarization plane is the first polarization plane. And a second light splitter which transmits a light flux having a polarization plane which is the second polarization plane, and the second light flux which is first emitted from the second light splitter. And a second quarter-wave plate on which a light beam traveling in the same direction as this light beam is incident, and from the second quarter-wave plate The third reflector for reflecting the light beam emitted in the direction opposite to the second light splitter in the original direction, and the light beam emitted for the second time from the second light splitter in the original direction And a fourth reflector that reflects the displacement. 3. The first light splitter is a beam splitter using a birefringent crystal, and the beam splitter reflects a light beam whose polarization plane is the second polarization plane as an ordinary ray, The optical path length of the birefringent crystal of the splitter is orthogonal to the direction of the light beam emitted from the light source, and the optical path lengths of the measurement light beam and the reference light beam are substantially the same. Displacement meter according to 2.