JPH09178415A - Light wave interference measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide such a light wave interference measuring device with high measuring precision as being capable of reducing non-linear errors resulting from error light mixed in a reference light passage or a measurement light passage in a polarizing beam splitter. SOLUTION: A light splitting means 50 is arranged in a light passage between a light source 1 and a polarizing beam splitter 3 to polarizinglyl split first to third lights from the light source 1 into light with a first polarized condition and light with a second polarized condition and emit the light with the first polarized condition and the light with the second polarized condition along mutually parallel light passages at a preset space. A displacement amount of a moving mirror 7, measured in accordance with a first interference light is corrected, based on refraction factor variation information in the measurement light passages measured in accordance with a second interference light and a third interference light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light wave interference measuring device, and more particularly to a light wave interference measuring device for performing highly accurate displacement measurement.

[0002]

2. Description of the Related Art FIG. 14 is a diagram schematically showing a configuration of a conventional lightwave interference measuring apparatus. The light wave interferometer of FIG. 14 measures the true displacement amount D of the movable mirror 906 in the optical axis direction (arrow direction in the figure). The length measurement light source 912 emits light including the light of the frequency ω _{1 and} the light of the frequency ω ₁ ′ (ω ₁ ′ = ω ₁ + Δω ₁ ). The two lights have slightly different frequencies from each other and polarization directions orthogonal to each other.

These two lights are beam splitter 91.
It is incident on the polarization beam splitter 904 via 3 and is separated into light of frequency ω ₁ 'and light of frequency ω ₁ . Frequency ω
The light of ₁ 'becomes the reference light, and after being reflected by the fixed mirror 905,
It returns to the polarization beam splitter 904 again. The light of frequency ω ₁ becomes measurement light, which is reflected by the movable mirror 906 and then returns to the polarization beam splitter 904 again.

The measurement light and the reference light returning to the polarization beam splitter 904 are emitted from the polarization beam splitter 904 along the same optical path. The measurement light and the reference light emitted from the polarization beam splitter 904 along the same optical path interfere with each other via the polarizer 931. Polarizing element 931
Specifically, is a polarizing plate arranged at an angle of 45 ° with respect to each of the polarization direction of the reference light and the polarization direction of the measurement light. The interference light generated via the polarizing element 931 is received by the light receiving element 916. The interference beat signal (frequency Δω ₁ ) 934 converted by the light receiving element 916 is output by the phase meter 91.
7 is input.

On the other hand, 2 emitted from the length measuring light source 912
Some of the two lights are reflected by the beam splitter 913 and interfere with each other via the polarization element 930. Polarizing element 930
The interference light generated via the light is detected by the light receiving element 914 and input to the phase meter 917 as the reference signal (frequency Δω ₁ ) 933. The polarizing element 930 is a polarizing plate that causes two lights to interfere with each other in the same manner as the polarizing element 931. The phase meter 917 obtains the displacement amount Dm of the movable mirror 906 by measuring the phase change of the interference beat signal 934 with respect to the reference signal 933, and outputs the displacement amount information to the signal 94.
It is output as 0 to the calculator 935.

By the way, in order to perform the length measurement by the interference of light waves as shown in FIG. 14 with precision (high accuracy), the fluctuation of the refractive index of air (or other gas) in the optical path cannot be ignored. Therefore, the conventional light wave interferometer is shown in FIG.
As shown in, an air sensor 932 is provided as a means for correcting the measurement error due to the fluctuation of the refractive index of air in the measurement optical path. That is, the air sensor 932 is used to measure the temperature, pressure, and humidity of the atmosphere in the measurement optical path, and the wavelength of light is corrected based on the measurement result, thereby correcting the measurement error caused by the fluctuation of the refractive index of air. doing.

That is, assuming that the true displacement amount is D, the refractive index of air is n, and n is spatially uniform, the displacement amount Dm measured by the light wave interferometer is:

## EQU1 ## Dm = nD (1)

Here, when n is expressed using the wavelength of light,

## EQU2 ## Dm = λ ₀ D / λ (2) However, λ = c / ω. Here, λ ₀ is the wavelength of light in vacuum, and λ is the wavelength of light along the measurement optical path.

The wavelength λ is an amount that depends on the temperature, pressure and humidity of the air in the measurement optical path. Therefore, the temperature, pressure, and humidity of the air along the measurement optical path are measured by the air sensor 93.
The wavelength λ can be obtained by measuring at 2. That is, the calculator 935 detects the displacement Dm signal 940 from the phase meter 917 and the temperature from the air sensor 932.
The true displacement amount D of the movable mirror 906 can be obtained based on the pressure / humidity measurement signal 941 and the arithmetic expression shown in Expression (2).

[0010]

As described above, in the prior art, the air temperature,
Pressure and humidity are detected by an air sensor. Therefore, accurate correction is possible when the refractive index of air varies uniformly along the measurement optical path, but when the refractive index of air locally varies on the measurement optical path. However, there is a problem in that the measurement error due to the fluctuation of the refractive index of air cannot be accurately corrected.

Therefore, the present inventors have filed Japanese Patent Application No. 7-664.
In the specification and the drawing of No. 69, an optical wave interferometer capable of correcting a measurement error due to fluctuations in the refractive index of air by using light of two different frequencies is proposed. The lightwave interference measuring device disclosed in the specification and the drawings of Japanese Patent Application No. 7-66469 has not been disclosed at the time of filing this application and does not belong to the prior art of this application.

FIG. 13 is a specification of Japanese Patent Application No. 7-66469.
Schematic configuration of the light wave interferometer disclosed in the specification and drawings
FIG. The light wave interferometer of FIG. 13 will be described later.
2 on the same optical path as the light emitted from the length measurement light source 1.
Two different frequencies ω_Two(Fundamental wave) and ω_Three(2nd harmonic
Wave: 2ω _Two= Ω_Three) Laser light is coupled. like this
Then, using these two laser beams with different frequencies,
Refraction of air (or other gas, etc.) in the optical path of the optical interferometer
The rate fluctuation can be calculated.

In the light wave interferometer of FIG. 13, the light source 20 is used.
A part of the light 323 of the frequency ω ₂ emitted from the SHG is converted into the light 423 of the frequency ω ₃ (2ω ₂ = ω ₃ ) by the second harmonic generation element (hereinafter referred to as “SHG conversion element”) 22.
The converted light having the remaining frequency ω ₂ is converted into the SHG conversion element 22.
Is transmitted as it is. The polarization directions of the light 323 having the frequency ω ₂ and the light 423 having the frequency ω ₃ via the SHG conversion element 22 are relative to the polarization directions of the light (lights 302 and 402 described later) emitted from the length measurement light source 1. Angle of 45 °. Light 323 with frequency ω ₂ and frequency ω
₃ of the light 423, for example, dichroic light 302 of clock frequency coupling element frequency omega ₁ emitted from the long light source 1 measured by 24 consisting of a mirror and frequency omega ₁ 'of the light 402
Coupled to the same optical path as. The combined light enters the polarization beam splitter 3 via the same optical path.

The light 323 having the frequency ω ₂ and the light 423 having the frequency ω ₃ are reflected by the polarization beam splitter 3 toward the fixed mirror 5 (reference light) and transmitted through the movable mirror 7 (measurement light). Is divided into and Reference light and measurement light are
The polarization directions thereof are orthogonal to each other, but both contain light of frequency ω ₂ and light of frequency ω ₃ , respectively.
That is, the reference light includes the light 326 of the frequency ω ₂ and the light 426 of the frequency ω ₃ , and the measurement light is the light 327 of the frequency ω ₂ .
And light 427 of frequency ω ₃ .

The reference light reflected by the polarization beam splitter 3 is reflected by the fixed mirror 5 via the quarter-wave plate 4 and then the polarization beam splitter 3 via the quarter-wave plate 4.
Incident on. The reference light that has passed through the polarization beam splitter 3 is reflected by the corner cube prism 8 and then enters the polarization beam splitter 3. The reference light transmitted through the polarization beam splitter 3 is reflected again by the fixed mirror 5 via the quarter wavelength plate 4, and then returns to the polarization beam splitter 3 via the quarter wavelength plate 4. On the other hand, the measurement light transmitted through the polarization beam splitter 3 is reflected by the movable mirror 7 through the quarter wavelength plate 6 and then enters the polarization beam splitter 3 through the quarter wavelength plate 6. The measurement light reflected by the polarization beam splitter 3 is reflected by the corner cube prism 8 and then enters the polarization beam splitter 3. The measurement light reflected by the polarization beam splitter 3 is reflected again by the movable mirror 7 through the quarter wavelength plate 6 and then returns to the polarization beam splitter 3 through the quarter wavelength plate 6. Thus, the reference light (light 326 of frequency ω ₂ and light 426 of frequency ω ₃ ) and measurement light (frequency ω ₂
Light 327 and the light 427 of frequency ω ₃ are incident on the polarization beam splitter 3, are combined, and are emitted along the same optical path.

The reference light and the measurement light combined by the polarization beam splitter 3 are reflected by the frequency separation element 25 which is, for example, a dichroic mirror. Thus, the reference light (light 326 of frequency ω ₂ and light 42 of frequency ω ₃
6) and the measurement light (the light 327 having the frequency ω ₂ and the light 427 having the frequency ω ₃ ) are separated from the light near the frequency ω ₁ that passes through the frequency separation element 25, and enter the polarization beam splitter 28. The polarization beam splitter 28 transmits the measurement light (the light 327 of the frequency ω ₂ and the light 427 of the frequency ω ₃ ) reflected by the movable mirror 7, and the reference light (the light 326 of the frequency ω ₂ reflected by the fixed mirror 5). And light 426) of frequency ω ₃ is reflected.

[0017] Light of the light 327 of the frequency omega ₂ of the polarization beam splitter 28 the light 327 of the frequency omega ₂ transmitted through and frequency omega ₃ light 427, the frequency omega ₃ by SHG device 30 (2ω ₂ = _{ω 3)} 331.
On the other hand, the light 427 of the frequency omega ₃ is a light 431 of frequency omega ₃ and is transmitted through the SHG device 30. As a result, the light 331 converted from the frequency ω ₂ to the frequency ω ₃ by the SHG conversion element 30 interferes with the light 431 of the frequency ω ₃ reflected by the movable mirror 7, and the interference light is received by the light receiving element 3
Detected by 2. Further, the light 326 of the frequency ω ₂ reflected by the polarization beam splitter 28 and the frequency ω _{2
Also for} the light 426 of No. ₃ , the light 335 converted from the frequency ω ₂ to the frequency ω ₃ by the action of the SHG conversion element 34.
The light receiving element 36 detects the interference light with the light 435 of the frequency ω ₃ reflected by the fixed mirror 5.

The interference signals 110 and 109 detected by the light receiving elements 32 and 36, respectively, are input to the phase meter 37. In the phase meter 37, the interference signal 109 (reference signal)
The phase change of the interference signal 110 (measurement signal) with respect to is measured. Thus, the optical path length change D for the light of frequency ω ₃
(Ω ₃ ) and optical path length change D for light of frequency ω ₂
The difference from (ω ₂ ), that is, {D (ω ₃ ) −D (ω ₂ )} can be obtained. {D (ω
₃ ) -D (ω ₂ )} signal 111 is calculated by the calculator 38
Is supplied to.

Further, in the optical interference measuring apparatus of FIG. 13, the length for the light source 1 measurement, the light 302 and the frequency omega ₁ of the frequency omega ₁ '(omega
₁ '= ω ₁ + Δω ₁ ) and the light 402 are emitted.
The two lights have slightly different frequencies from each other and polarization directions orthogonal to each other.

These two lights are transmitted to the beam splitter 12
And incident on the polarization beam splitter 3 via the frequency coupling element 24 is separated into a light 302 of the light 402 and the frequency omega ₁ of the frequency omega ₁ '. The light 402 having the frequency ω ₁ 'becomes reference light, and returns to the polarization beam splitter 3 via the quarter wave plate 4, the fixed mirror 5, the quarter wave plate 4, the polarization beam splitter 3 and the corner cube prism 8.
Further, the light 302 having the frequency ω ₁ becomes the measurement light, and passes through the quarter-wave plate 6, the movable mirror 7, the quarter-wave plate 6, the polarization beam splitter 3, and the corner cube prism 8,
Return to the polarization beam splitter 3.

The measurement light and the reference light returning to the polarization beam splitter 3 are emitted from the polarization beam splitter 3 along the same optical path. The reference light and the measurement light emitted from the polarization beam splitter 3 along the same optical path are
After passing through the frequency separation element 25, the light 40 having the frequency ω ₁ 'is
9 and a light 309 of frequency ω ₁ and interfere via the polarizer 10. The interference light generated via the polarizing element 10 is received by the light receiving element 11. The interference beat signal (frequency Δω ₁ ) 107 converted by the light receiving element 11 is input to the phase meter 15.

On the other hand, a part of the two lights emitted from the length measuring light source 1 is reflected by the beam splitter 12,
Interference occurs via the polarizing element 13. The interference light generated through the polarizing element 13 is detected by the light receiving element 14 and input to the phase meter 15 as a reference signal (frequency Δω ₁ ). The phase meter 15 obtains the displacement amount D (ω ₁ ) of the movable mirror 7 by measuring the phase change of the interference beat signal 107 with respect to the reference signal 106, and outputs the displacement amount information to the calculator 38 as a signal 108.

The computing unit 38 corrects the displacement amount Dm of the movable mirror 7 measured by the length measuring interferometer using the length measuring light source 1,
The true displacement amount (geometrical distance) D is obtained. Less than,
The correction from the displacement amount (ω ₁ ) of the movable mirror 7 to the true displacement amount (geometrical distance) D will be described. Frequency ω ₁ , ω ₂
And the optical path length change D (ω ₁₎ for omega ₃ light, D (omega
₂ ) and D (ω ₃ ) are represented by the following equations (3) to (5), respectively.

[0024]

[Equation 3] D (ω ₁ ) = [1 + N · F (ω ₁ )] · D (3)

[Equation 4] D (ω ₂ ) = [1 + N · F (ω ₂ )] · D (4)

[Equation 5] D (ω ₃ ) = [1 + N · F (ω ₃ )] · D (5)

Where D is the geometric distance and N is the density of air. F (ω) is a function that depends only on the frequency ω of the light without depending on the density of the air if the composition ratio of the air is unchanged. From the above equations (3) to (5), the geometric distance D is given by the following equation (6).

[Equation 6] D = D (ω ₁ ) −A [D (ω ₃ ) −D (ω ₂ )] (6) where A = F (ω ₁ ) / [F (ω ₃ ) −F (Ω ₂ )].

{D (ω ₃ ) -D of the second term on the right side of the equation (6).
(Ω ₂ )} can be obtained by the phase meter 37 as described above. D (ω ₁ ) of the first term on the right side is
It can be determined by the phase meter 15. Therefore,
The calculator 38 calculates the displacement amount D (ω ₁ ) measured by the interferometer for length measurement by the calculation formula (6) based on the output signal 108 of the phase meter 15 and the output signal 111 of the phase meter 37. The true displacement amount D can be obtained by performing the correction.

In the light wave interferometer of FIG. 13, the light source 20 is used.
The light having the frequency ω _{2 and} the light having the frequency ω ₃ from the optical path travels through the same optical path as the light for measurement emitted from the light source 1 for length measurement.
Therefore, even if the refractive index fluctuation of the air is not uniform along the measurement optical path, the measurement error due to the refractive index fluctuation of the air is corrected, and the displacement amount of the movable mirror in the optical axis direction is measured with high accuracy. be able to.

In the light wave interference measuring apparatus of FIG. 13, the measuring light composed of the light 327 having the frequency ω ₂ and the light 427 having the frequency ω ₃ reflected by the corner cube prism 8 is
It is the light that should be reflected by the polarization beam splitter 3.
Further, the reference light composed of the light 326 having the frequency ω _{2 and} the light having the frequency ω ₃ 426 reflected by the corner cube prism 8 is the light to be transmitted through the polarization beam splitter 3.

By the way, it is difficult to construct and manufacture the polarization beam splitter so as to have the same function with respect to a plurality of lights having different frequencies, that is, the polarization beam splitter ideally functions. Therefore, a part of the measurement light to be reflected by the polarization beam splitter 3 is transmitted and mixed into the reference optical path, or a part of the reference light to be transmitted through the polarization beam splitter 3 is reflected and mixed into the measurement optical path. Or As a result, the light mixed in the reference optical path and the measurement optical path becomes error light, and a non-linear error occurs, which lowers the measurement accuracy.

The crystal type polarization beam splitter exhibits high performance for a plurality of frequencies. However, when a crystal-type polarization beam splitter is used in the optical interferometer with a double-pass configuration, if the polarization states of the measurement light and the reference light change based on the double-pass configuration, the separation function deteriorates due to crystal anisotropy. Will end up. As a result, the light mixed in the reference optical path and the measurement optical path becomes error light, and a non-linear error occurs, which lowers the measurement accuracy.

The present invention has been made in view of the above-mentioned problems, and a light wave interferometer with high measurement accuracy, which reduces a non-linear error caused by error light mixed in the reference optical path and the measurement optical path in the polarization beam splitter. The purpose is to provide.

[0032]

In order to solve the above-mentioned problems, in the present invention, the first light having a predetermined frequency and the second light and the third light having different frequencies are provided on the same optical path. And a reference having a first polarization state for guiding the first light to the third light output from the light source unit along a reference optical path to a fixed mirror. A polarization beam splitter for polarization-separating light and measurement light having a second polarization state guided along a measurement light path to the movable mirror; and the first beam passing through the measurement light path and the reference light path. A first interference light generation system for generating a first interference light between the measurement light passing through the measurement light path and the reference light passing through the reference light path based on light; and the second interference light generation system passing through the measurement light path. Light and one of the third lights Of the second light and the third light passing through the reference optical path, the second interference light generation system for generating the second interference light by making the frequency of the second light substantially match the frequency of the other light. Is disposed in the optical path between the light source unit and the polarization beam splitter, and a third interference light generation system for generating a third interference light by making the frequency of the light of the other light substantially match the frequency of the other light, The first from the light source unit
Light or the third light is polarized and separated into the light having the first polarization state and the light having the second polarization state, and the light having the first polarization state and the second light. A light separation means for emitting light having a polarization state along mutually parallel optical paths at a predetermined distance, and a displacement amount of the movable mirror measured based on the first interference light, There is provided a light wave interference measuring apparatus characterized by performing correction based on refractive index variation information in the measurement optical path measured based on the second interference light and the third interference light.

According to a preferred aspect of the present invention, the polarization beam splitter further comprises a removing means for removing the error light mixed in the reference light path and the error light mixed in the measurement light path. In this case, the removing means is disposed in an optical path between the polarization beam splitter and the fixed mirror, and includes a first spatial filter for passing only reference light along a predetermined reference optical path, and the polarization beam. The second spatial filter is arranged in the optical path between the splitter and the movable mirror and allows only the measuring light along a predetermined measuring optical path to pass therethrough.

The light separating means has a positive lens and a Wollaston prism positioned at the focal position of the positive lens, or has a polarizing beam splitter and a reflecting mirror such as a total reflection prism. Further, according to a preferred aspect of the present invention, the first interference light generation system is for coupling the reference light and the measurement light emitted from the polarization beam splitter along the mutually parallel optical paths on the same optical path. The optical coupling means is further provided. In this case, the optical coupling means has a positive lens and a Wollaston prism positioned at the focal position of the positive lens, or has a polarizing beam splitter and a reflecting mirror such as a total reflection prism.

[0035]

BEST MODE FOR CARRYING OUT THE INVENTION In the light wave interference measuring apparatus of the present invention,
The refractive index fluctuation information in the measurement optical path is measured based on _two lights having different frequencies, that is, a light having a frequency ω _{2 and} a light having a frequency ω ₃ . Then, the light of the frequency ω _{2 and} the light of the frequency ω ₃ which are to be separated into the reference light and the measurement light through the polarization beam splitter are spatially separated along the optical paths parallel to each other by the action of the light separating means. . That is, since the two lights to be separated into the reference light and the measurement light can be preliminarily parallel-shifted via the polarization beam splitter, the measurement light path and the reference light path are not coupled on the same light path, It is spatially separated.

Therefore, even if the polarization beam splitter mixes in the reference optical path and the measurement optical path, the influence of the mixed error light can be reduced. That is, it is possible to reduce the non-linear error caused by the mixed error light, and it is possible to perform highly accurate measurement. It should be noted that by providing the removing means for removing the error light mixed in the reference optical path and the error light mixed in the measurement optical path in the polarization beam splitter, it is possible to further reduce the non-linear error caused by the mixed error light, Further accurate measurement can be performed.

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a diagram schematically showing a configuration of an optical interference measuring apparatus according to a first embodiment of the present invention. In addition,
In the light wave interference measuring apparatus of the first embodiment, the so-called homodyne interference method is used to measure the refractive index fluctuation. Further, FIG. 2 is an enlarged view of the length measuring unit 200 of FIG.

The light wave interferometer of FIG. 1 is provided with a length measuring light source 1 for supplying light for measuring the displacement amount of the movable mirror 7 in the optical axis direction (the direction of the arrow in the figure). The length measuring light source 1 is
For example, He-N that supplies laser light having a wavelength of 633 nm
e-laser. The length light source 1 measuring the two linearly polarized light and polarized orientation different to the frequency slightly from each other are orthogonal to each other, i.e. the light 302 and the frequency omega ₁ of the frequency omega ₁ of the _{'(ω 1' = ω 1} + Δω 1) The light 402 is emitted along the same optical path. Below, light of frequency ω ₁ and frequency ω
_The 1'lights 402 are collectively referred to as dual frequency light. Further, for simplicity of explanation, the light 302 of the frequency omega ₁ is a P-polarized light having a polarization direction parallel to the paper surface, light 402 of the frequency omega ₁ '
Is S-polarized light having a polarization direction perpendicular to the paper surface.

[0039] Some of the measurement of length for the light source 1 a frequency omega ₁ emitted from the light 302 and the frequency omega ₁ 'of the light 402 is transmitted through the beam splitter 12, for example, enters the frequency coupling element 24 consisting of a dichroic mirror To do. The frequency coupling element 24 has a property of transmitting light near the frequency ω ₁ and reflecting light of other frequencies. Therefore,
Light 402 of the light 302 and the frequency omega ₁ 'frequency omega ₁ that has passed through the frequency coupling element 24 is incident on the light splitting element 50. The light separation element 50 is an optical element that spatially separates P-polarized light and S-polarized light that have entered along the same optical path and emits them along parallel optical paths, and is composed of, for example, a beam displacer. Here, the beam displacer can maintain the optical path of one of the S-polarized light and the P-polarized light as it is and shift the other optical path in parallel by utilizing the anisotropy of the crystal.

Therefore, the P-polarized light 302 and the S-polarized light 402 which have entered the light separating element 50 along the same optical path are emitted from the light separating element 50 along the optical paths 122 and 120 which are parallel to each other and enter the polarization beam splitter 3. To do. The polarization beam splitter 3 reflects S-polarized light and
It is arranged to transmit polarized light. Therefore, the light 402 having the frequency ω ₁ 'of the two-frequency light incident on the polarization beam splitter 3 is reflected by the polarization beam splitter 3 and guided to the fixed mirror 5 as reference light. On the other hand, the light 302 of the frequency ω ₁ of the two-frequency light passes through the polarization beam splitter 3 and is guided to the movable mirror 7 as measurement light.

The reference light consisting of a light 402 of frequency ω ₁ '(S
After passing through the spatial filter 52, the polarized light becomes circularly polarized light 55 through the quarter-wave plate 4. Circularly polarized light 55
After being reflected by the fixed mirror 5, it becomes P-polarized light through the quarter-wave plate 4. The P-polarized reference light 402 passes through the spatial filter 52, and then the polarization beam splitter 3
Incident on. The reference light 56 that has passed through the polarization beam splitter 3 in the P-polarized state is reflected by the corner cube prism 8 and then enters the polarization beam splitter 3.
The reference beam 56 that has passed through the polarization beam splitter 3 in the P-polarized state remains at 1 / after passing through the spatial filter 52.
It becomes circularly polarized light 57 through the four-wave plate 4. The circularly polarized light 57 is reflected by the fixed mirror 5 and then becomes S-polarized light through the quarter-wave plate 4. The S-polarized reference light 402 passes through the spatial filter 52 and then enters the polarization beam splitter 3. The reference light 58 reflected by the polarization beam splitter 3 in the S-polarized state is emitted from the length measuring unit 200 along the optical path 120.

On the other hand, the measurement light (P-polarized light) composed of the light 302 having the frequency ω ₁ passes through the spatial filter 53,
It becomes circularly polarized light 60 through the / 4 wavelength plate 6. The circularly polarized light 60 is reflected by the movable mirror 7 and then becomes S-polarized light via the quarter-wave plate 6. The measurement light 302 that is S-polarized is
After passing through the spatial filter 53, it enters the polarization beam splitter 3. The measurement light 61 reflected by the polarization beam splitter 3 in the S-polarized state enters the polarization beam splitter 3 after being reflected by the corner cube prism 8. The measurement light 61 reflected by the polarization beam splitter 3 in the S-polarized state passes through the spatial filter 53 and then becomes circularly polarized light 62 via the quarter-wave plate 6. The circularly polarized light 62 is reflected by the moving mirror 7 and then 1
It becomes P-polarized light through the / 4 wavelength plate 6. The P-polarized measurement light 302 passes through the spatial filter 53 and then enters the polarization beam splitter 3. The measurement light 63 transmitted through the polarization beam splitter 3 in the P-polarized state has an optical path 12
It is ejected from the length measuring unit 200 along the line 2.

In this way, the light 402 of the frequency ω ₁ 'which is emitted from the polarization beam splitter 3 via the reference optical path,
The light 302 of the frequency ω ₁ emitted from the polarization beam splitter 3 via the measurement optical path is different from the optical path 12
It is incident on the frequency separation element 25 along 0 and 122. The frequency separation element 25 has a characteristic of transmitting only light having a frequency near ω ₁ and reflecting light of other frequencies. Therefore, the reference light 402 having the frequency ω ₁ 'and the frequency ω ₁
The measurement light 302 of is transmitted through the frequency separation element 25. Reference light 402 of frequency ω ₁ 'transmitted through the frequency separation element 25
And the measurement light 302 having the frequency ω ₁ enters the optical coupling element 51. The optical coupling element 51 is an optical element that combines P-polarized light and S-polarized light that have entered along mutually parallel optical paths and emits them along the same optical path.
It can be configured by arranging 0 in the opposite direction to the traveling direction of light.

Therefore, the P-polarized light 30 incident on the optical coupling element 51 along the optical paths 120 and 122 which are parallel to each other.
The 2 and S-polarized light 402, after being coupled to the same optical path, reference light frequency omega ₁ of the measuring light 309 and the frequency omega ₁ '40
9 is emitted from the optical coupling element 51. The reference light 409 having the frequency ω ₁ ′ and the measurement light 309 having the frequency ω ₁ emitted from the optical coupling element 51 interfere with each other via the polarization element 10. The polarizing element 10 is composed of, for example, a polarizing plate arranged at an angle of 45 ° with respect to the polarization direction of the dual-frequency light. The interference light generated through the polarizing element 10 is received by the light receiving element 11, and the light receiving element 11 outputs a measurement beat signal (frequency Δω ₁ ) 107 based on the interference light to the phase meter 15
To supply.

On the other hand, part of the light 402 and the frequency omega ₁ of the light 302 of the measuring emitted from the long light source 1 two-frequency light or frequency omega ₁ ', after being reflected by the beam splitter 12, the polarization element 13 Incident. The polarizing element 1
Like the polarizing element 10, 3 is a polarizing plate for interfering two-frequency light. Accordingly, the light 30 of the light 402 and the frequency omega ₁ of the polarizing element 13 frequency omega ₁ that is generated through a '
The interference light with 2 is detected by the light receiving element 14. Light-receiving element 14, the reference signal based on the interference light of the light 402 and the frequency omega ₁ of the light 302 of the frequency omega ₁ '(frequency [Delta] [omega ₁₎ 1
06 is supplied to the phase meter 15. The phase meter 15 measures the phase change of the measurement beat signal 107 with respect to the reference signal 106 to obtain the displacement amount D (ω ₁ ) of the movable mirror 7 that does not consider the influence of the refractive index variation, and the displacement amount D (ω ₁ ω
_The signal 108 relating to ₁ ) is supplied to the arithmetic unit 38.

The light wave interferometer of FIG. 1 also uses, for example, a wavelength of 1064n as the light for measuring the refractive index fluctuation.
A light source 20 for supplying m YAG laser light is provided.
Linearly polarized light 32 of frequency ω ₂ emitted from the light source 20
3 is incident on the SHG conversion element 22 and the light 32 of frequency ω ₂
Part of 3 is generated by the SHG conversion element 22 at a frequency ω ₃ (ω ₃
= 2ω ₂ ) which is SHG converted into the light 423, and the remaining light having the frequency ω ₂ passes through the SHG conversion element 22 as it is. The SHG conversion element 22 is, for example, a nonlinear optical material KT.
It can be configured by iOPO ₄ .

The light 323 having the frequency ω ₂ and the light 423 having the frequency ω ₃ emitted from the SHG conversion element 22 are reflected by the frequency coupling element 24 and the light from the length measuring light source 1 (light near the frequency ω ₁ ). And the same optical path. As described above, the frequency coupling element 24 has the frequency ω
It has a characteristic of transmitting only light in the vicinity of ₁ and reflecting light of other frequencies. Thus, the light source 20 and the SHG
The conversion element 22 constitutes a light source unit 201 that outputs two lights having different frequencies along the same optical path.

The frequency ω reflected by the frequency coupling element 24
The light 323 of ₂ and the light 423 of frequency ω ₃ enter the light separation element 50. Frequency ω incident on the light separation element 50
_Of the light 323 of ₂ and the light 423 of frequency ω ₃ , the P-polarized component 54 (light 323a of frequency ω ₂ and light 423a of frequency ω ₃ ) is emitted from the light separation element 50 along the optical path 122, and the polarization beam splitter 3 Incident on. Furthermore, S-polarized light component 59 (the frequency omega ₂ of the light 323b and the frequency omega ₃ light 423b) of the frequency omega ₂ of the light 323 and the frequency omega ₃ light 423 incident on the light separation element 50 is further parallel to the optical path 122 The light is emitted from the light separation element 50 along the optical path 120 and is incident on the polarization beam splitter 3.

As described above, the polarization beam splitter 3 is arranged so as to reflect S-polarized light and transmit P-polarized light. Therefore, the light 323b having the frequency ω ₂ and the light 423b having the frequency ω ₃ incident on the polarization beam splitter 3 in the S-polarized state along the optical path 120 are reflected by the polarization beam splitter 3 and guided to the fixed mirror 5 as reference light. . On the other hand, the light 323a having the frequency ω ₂ incident on the polarization beam splitter 3 in the P-polarized state along the optical path 122.
The light 423a having the frequency ω ₃ passes through the polarization beam splitter 3 and is guided to the movable mirror 7 as measurement light.

Light 323b of frequency ω ₂ and frequency ω ₃
After passing through the spatial filter 52, the reference light (S-polarized light) composed of the light 423b of the above-mentioned light 423b becomes circularly-polarized light 55 through the quarter-wave plate 4. The circularly polarized light 55 is reflected by the fixed mirror 5 and then becomes P-polarized light via the quarter-wave plate 4. The reference lights 323b and 423b that have become P-polarized light enter the polarization beam splitter 3 after passing through the spatial filter 52. The reference light 56 that has passed through the polarization beam splitter 3 in the P-polarized state is reflected by the corner cube prism 8 and then enters the polarization beam splitter 3. The reference light 56 that has passed through the polarization beam splitter 3 in the P-polarized state passes through the spatial filter 52 and then becomes circularly polarized light 57 via the quarter-wave plate 4. Circularly polarized light 57
After being reflected by the fixed mirror 5, it becomes S-polarized light through the quarter-wave plate 4. Reference lights 323b and 4 which are S-polarized
After passing through the spatial filter 52, the light beam 23b enters the polarization beam splitter 3. The reference light 58 reflected by the polarization beam splitter 3 in the S-polarized state is emitted from the length measuring unit 200 along the optical path 120.

On the other hand, the measurement light (P-polarized light) consisting of the light 323a of the frequency ω ₂ and the light 423a of the frequency ω ₃ passes through the spatial filter 53 and then passes through the quarter-wave plate 6 and then the circularly-polarized light 60. Becomes The circularly polarized light 60 is reflected by the movable mirror 7 and then becomes S-polarized light via the quarter-wave plate 6.
The measurement lights 323a and 423a that have become S-polarized light enter the polarization beam splitter 3 after passing through the spatial filter 53. Polarization beam splitter 3 in S polarization state
The measurement light 61 reflected at is reflected by the corner cube prism 8 and then enters the polarization beam splitter 3. Polarization beam splitter 3 with S polarization state
After passing through the spatial filter 53, the measurement light 61 reflected by becomes a circularly polarized light 62 through the quarter-wave plate 6. The circularly polarized light 62 is reflected by the moving mirror 7 and then 1 /
It becomes P-polarized light through the four-wave plate 6. The measurement lights 323a and 423a that have become P-polarized light enter the polarization beam splitter 3 after passing through the spatial filter 53. Measurement light 6 transmitted through the polarization beam splitter 3 in the P-polarized state
3 is emitted from the length measuring unit 200 along the optical path 122.

As described above, the measurement light and the reference light composed of the light of the frequency ω _{2 and} the light of the frequency ω _{3 have} the same measurement optical path and reference optical path as the measurement light and the reference light composed of the light in the vicinity of the frequency ω _1. It is ejected from the length measuring unit 200 via the. Thus, the light 323b having the frequency ω ₂ and the light 423b having the frequency ω ₃ emitted from the polarization beam splitter 3 via the reference optical path, and the light 323a having the frequency ω ₂ emitted from the polarization beam splitter 3 via the measurement optical path and The light 423a having the frequency ω ₃ is different from the optical path 1
It is incident on the frequency separation element 25 along 20 and 122. As described above, the frequency separation element 25 has a characteristic of transmitting only light having a frequency near ω ₁ and reflecting light of other frequencies. Therefore, the measurement light and the reference light composed of the light of frequency ω _{2 and} the light of frequency ω ₃ are reflected by the frequency separation element 25, respectively.

The frequency ω reflected by the frequency separation element 25
₂ light and light of frequency ω ₃ , that is, light 326 of frequency ω ₂ and light 426 of frequency ω ₃ through the reference optical path.
And the light 327 having the frequency ω ₂ and the light 427 having the frequency ω ₃ via the measurement optical path enter the polarization beam splitter 28 along different optical paths. The polarization beam splitter 28 uses the measurement light in the P-polarized state (light 3 of frequency ω ₂
27 and the light 427 of the frequency ω ₃ are transmitted, and the reference light of the S polarization state (the light 326 of the frequency ω ₂ and the light 426 of the frequency ω ₃ ) is reflected. By inserting the aforementioned spatial filter between the polarization beam splitter 28 and the SHG conversion element 30, it is possible to prevent the reference light from going to the SHG conversion element 30 for measurement light. Similarly, by inserting a spatial filter between the polarization beam splitter 28 and the SHG conversion element 34, it is possible to prevent the measurement light from going to the SHG conversion element 34 for reference light.

[0054] Light of the light 327 of the frequency omega ₂ of the polarization beam splitter 28 the light 327 of the frequency omega ₂ transmitted through and frequency omega ₃ light 427, the frequency omega ₃ by SHG device 30 (2ω ₂ = _{ω 3)} 331.
On the other hand, the light 427 of the frequency omega ₃ is a light 431 of frequency omega ₃ and is transmitted through the SHG device 30. As a result, the light 331 converted from the frequency ω ₂ to the frequency ω ₃ by the SHG conversion element 30 interferes with the light 431 of the frequency ω ₃ reflected by the movable mirror 7, and the interference light is received by the light receiving element 3
Detected by 2. Further, the light 326 of the frequency ω ₂ reflected by the polarization beam splitter 28 and the frequency ω _{2
Also for} the light 426 of No. ₃ , the light 335 converted from the frequency ω ₂ to the frequency ω ₃ by the action of the SHG conversion element 34.
The light receiving element 36 detects the interference light with the light 435 of the frequency ω ₃ reflected by the fixed mirror 5.

The interference signals 110 and 109 detected by the light receiving elements 32 and 36, respectively, are input to the phase meter 37. In the phase meter 37, the interference signal 109 (reference signal)
The phase change of the interference signal 110 (measurement signal) with respect to is measured. Thus, the optical path length change D for the light of frequency ω ₃
(Ω ₃ ) and optical path length change D for light of frequency ω ₂
The difference from (ω ₂ ), that is, {D (ω ₃ ) −D (ω ₂ )} can be obtained. {D (ω
₃ ) -D (ω ₂ )} signal 111 is calculated by the calculator 38
Is supplied to. The arithmetic unit 38 corrects the measurement error due to the refractive index variation based on the signal 108 from the phase meter 15 and the signal 111 from the phase meter 37 and the arithmetic expression shown in the equation (6). The displacement amount D of the moving mirror 7
Then, the true displacement amount D of the moving table fixed to is determined and output.

As described above, in the first embodiment, the two lights to be separated into the reference light and the measurement light through the polarization beam splitter 3 are guided by the action of the light separating element 50 along the optical paths parallel to each other. Are spatially separated. That is, by preliminarily parallel-shifting the two lights to be separated into the reference light and the measurement light through the polarization beam splitter 3, the measurement light path and the reference light path are not coupled to each other on the same light path, and are mutually separated. Be separated. As a result, the light 32 of the frequency ω ₂ reflected by the corner cube prism 8
3a and measuring light 61 consisting of light 423a of frequency ω ₃
Should originally be reflected by the polarization beam splitter 3, but even if part of the measurement light 61 passes through the polarization beam splitter 3 and enters the reference optical path, the mixed error light is blocked by the spatial filter 52. .

The light 323b having the frequency ω ₂ and the light 4 having the frequency ω ₃ reflected by the corner cube prism 8 are also included.
Although the reference beam 56 composed of 23b should originally pass through the polarization beam splitter 3, even if a part of the reference beam 56 is reflected by the polarization beam splitter 3 and is mixed in the measurement optical path, the mixed error light is a space. It is blocked by the filter 53. In addition, the light 3 of the frequency ω ₂ reflected twice by the moving mirror 7
The measurement light composed of the light 423a having the frequency 23a and the frequency ω ₃ should originally pass through the polarization beam splitter 3,
A part of this measurement light may be reflected by the polarization beam splitter 3. In this case, even if part of the measurement light is reflected by the corner cube prism 8 and then passes through the polarization beam splitter 3 and enters the reference optical path, the mixed error light is blocked by the spatial filter 52.

The frequency ω reflected twice by the fixed mirror 5
_The reference light composed of the _second light 323b and the frequency ω ₃ light 423b should originally be reflected by the polarization beam splitter 3, but a part of this reference light may pass through the polarization beam splitter 3. In this case, even if a part of the reference light is reflected by the corner cube prism 8 and then reflected by the polarization beam splitter 3 and mixed into the measurement optical path,
The mixed error light is blocked by the spatial filter 53. As described above, in the first embodiment, it is possible to reduce the non-linear error due to the error light mixed in the reference optical path and the measurement optical path in the polarization beam splitter 3 and perform highly accurate measurement by the double pass configuration.

In the first embodiment described above, the frequency separation element 25 is arranged closer to the polarization beam splitter 3 than the optical coupling element 51. However, the optical coupling element 51
The polarization beam splitter 3 rather than the frequency separation element 25.
It can also be placed on the side. Further, in the above-described first embodiment, the polarization beam splitter 3 and the spatial filters 52 and 53 are configured separately. However, a part of the corresponding surface of the polarization beam splitter 3 may be formed as a rough surface to integrally configure the polarization beam splitter and the spatial filter.

Further, when the parallel shift amount of the light separation element 50 has frequency dependency in the above-mentioned first embodiment, the parallel shift amount is different between the refractive index fluctuation measuring light and the length measuring light. Occurs. In this case, the parallel shift amount can be corrected by disposing glass or the like having a predetermined characteristic.

FIG. 3 is a diagram schematically showing the structure of a lightwave interference measuring apparatus according to the second embodiment of the present invention. In the light wave interference measuring apparatus of the second embodiment, the so-called heterodyne interference method is used to measure the refractive index fluctuation.

In the light wave interference measuring apparatus of FIG. 3, in order to detect the interference light between the light of frequency ω _{2 and} the light of frequency ω ₃ by using the heterodyne method, the SHG conversion element 22 and the frequency coupling element 24 are connected. Are provided with frequency filters 64 and 67 and a frequency shifter 66. However, the other configuration of the device of FIG. 3 is basically the same as that of the device of FIG. 1 of the first embodiment. Therefore, in FIG. 3, elements having the same functions as those of the first embodiment are designated by the same reference numerals as those in FIG. Hereinafter, the second embodiment will be described by focusing on the differences from the first embodiment.

The light 368 having the frequency ω ₂ and the light 465 having the frequency ω ₃ emitted from the SHG conversion element 22 enter the frequency filter 64. Frequency filters 64 and 6
7 has a characteristic of reflecting light near the frequency ω ₃ and transmitting light of the frequency ω ₂ . Light 465 of the frequency omega ₃ which is reflected by the frequency filter 64, the frequency omega ₃ which slightly shifted in frequency from the frequency omega ₃ through the frequency shifter 66 '
Light (ω ₃ '= ω ₃ + Δω ₃ ) 469. The frequency shifter 66 is, for example, an acousto-optic device or the like. The light having the frequency ω ₃ ′ through the frequency shifter 66 enters the frequency filter 67.

On the other hand, the light 368 having the frequency ω ₂ that has passed through the frequency filter 64 enters the frequency filter 67 as it is. Thus, the light 469 having the frequency ω ₃ ′ and the light 368 having the frequency ω ₂ are recombined on the same optical path by the action of the frequency filter 67. Then, the combined light 368 of frequency ω ₂ and light 469 of frequency ω ₃ 'is
The light 323 having the frequency ω ₂ and the frequency ω ₃ in the embodiment
After passing through the same optical path as the light 423 of
It is reflected by 5 as measurement light and reference light.

Of the measurement light composed of the light 371 of the frequency ω ₂ and the light 471 of the frequency ω ₃ ′, the light 371 of the frequency ω ₂
The next light 373 of frequency omega ₃ are converted by the SHG device 30, the frequency omega ₃ 'light 471 of frequency omega ₃ and is transmitted through the SHG device 30' becomes the light 473. Thus, unlike the first embodiment, only the frequency [Delta] [omega ₃ two different light or frequency omega ₃ light 3
73 and the light 473 having the frequency ω ₃ 'perform heterodyne interference. Similarly, the reference light composed of the light 370 having the frequency ω ₂ and the light 470 having the frequency ω ₃ ′ is also transmitted through the SHG conversion element 34 to the two lights having the frequencies different from each other by Δω _3, that is, the light 372 having the frequency ω _3. The light 472 having the frequency ω ₃ 'becomes and causes heterodyne interference.

Therefore, the reference signal 10 from the light receiving element 36
9 and the measurement signal 110 from the light receiving element 32 are both interference beat signals. In the phase meter 37, as in the first embodiment, {D (ω ₃ ') -D (ω ₂ )} is obtained, and the signal 111 related to this value is supplied to the calculator 38. Calculator 38
Then, based on the signal 108 from the phase meter 15 and the signal 111 from the phase meter 37 and the arithmetic expression shown in Expression (6), the true displacement amount of the movable mirror 7 in which the measurement error due to the refractive index fluctuation is corrected. D is obtained and output.

As described above, also in the light wave interference measuring apparatus according to the second embodiment, the nonlinear error caused by the error light mixed in the reference light path and the measurement light path in the polarization beam splitter 3 is reduced, and the double path configuration provides high accuracy. A measurement can be made. Further, in the second embodiment, the refractive index fluctuation measurement using the light of frequency ω _{2 and} the light of frequency ω ₃ is performed by the heterodyne interference method. Therefore, the error due to the fluctuation of the output of the light source 20 is less likely to occur, and the phase difference {D (ω ₃ )
−D (ω ₂ )} can be accurately detected. As a result, the accuracy of detecting the true displacement amount D can be improved.

FIGS. 4 to 6 show the light splitting / coupling portion 202 (the light splitting element 50 and the light coupling element 5) in the first embodiment.
It is a figure which shows roughly the structure of the modification of 1). In the modification shown in FIG. 4, the positive lens 77 and the Wollaston prism 76 arranged at the focal position thereof are combined to form a light separation element. Further, the positive lens 79 and the Wollaston prism 78 arranged at the focal position thereof are combined to form an optical coupling element.

The Wollaston prism is an optical element having a function of separating P-polarized light and S-polarized light incident along the same optical path at a predetermined separation angle. Therefore, the P-polarized light and the S-polarized light that have entered the Wollaston prism 76 along the same optical path are separated by a predetermined separation angle, and then, by the action of the positive lens 77, the light separating / combining portion 202 is formed along the parallel optical paths. Is ejected from. Similarly, by combining the positive lens 79 and the Wollaston prism 78, P-polarized light and S-polarized light incident along mutually parallel optical paths may be combined on the same optical path and emitted from the light separation / coupling unit 202. it can.

If the separation angle of the Wollaston prism 76 in FIG. 4 is frequency-dependent, there will be a difference in the separation angle of the two lights having greatly different frequencies, and the parallelism of the two lights via the positive lens 77 will occur. Collapses. In this case, FIG.
By disposing the dispersion glass 80 on the rear side (on the side of the polarization beam splitter 3) of the positive lens 77 as shown in FIG.
Even if the separation angle of the Wollaston prism 76 has frequency dependence, the parallelism of the two lights through the positive lens 77 can be maintained. In this case, the dispersion glass 81 is also provided on the front side of the positive lens 79 (on the side of the polarization beam splitter 3).
It is necessary to place. Furthermore, dispersed glass 80
Even without using, the parallelism can be maintained by adding chromatic aberration to the positive lens according to the frequency dependence of the Wollaston prism.

In the modification shown in FIG. 6, the polarization beam splitter 82 and the total reflection prism 582 are combined to form a light separation element. Further, the polarization beam splitter 83 and the total reflection prism 583 are combined to form an optical coupling element. Note that, in FIG. 6, the total reflection prisms 582 and 583 may be replaced with another reflection mirror. In addition, the polarization beam splitters 82 and 83
May be a crystal type or a film type.

FIGS. 7 to 9 are schematic views showing the configuration of a modification of the length measuring unit 200 in the first embodiment. FIG.
The length measuring unit 200 is similar to the length measuring unit 200 in the first embodiment. However, in order to reduce the influence of overlapping error light, the polarization beam splitter 84, the polarization beam splitter 85, and the corner cube prism 87.
Differs from the first embodiment only in that Therefore, in FIG. 7, elements having the same functions as those of the first embodiment are denoted by the same reference numerals as those in FIG.
The modification of FIG. 7 will be described below, focusing on the differences from the first embodiment.

First, referring to FIG. 1 again, the influence of overlapping error light will be described. In FIG. 1, the measurement light composed of the light 323 a having the frequency ω _{2 and} the light 423 a having the frequency ω ₃ reflected twice by the movable mirror 7 should originally pass through the polarization beam splitter 3, but the polarization beam splitter 3
Part of this measurement light may be reflected by the polarization beam splitter 3 due to imperfections in manufacturing performance of.
In this case, part of the measuring light is the corner cube prism 8
After being reflected by, the light is reflected by the polarization beam splitter 3 and becomes overlapping error light. The overlapping error light makes another round trip of the measurement optical path (resultingly, four round trips). As a result, a non-linear error occurs due to the overlapping error light, making it impossible to perform highly accurate measurement.

Further, in FIG. 1, the reference light reflected twice by the fixed mirror 5 should originally be reflected by the polarization beam splitter 3, but a part of this reference light should pass through the polarization beam splitter 3. There is. In this case, a part of the reference light is reflected by the corner cube prism 8 and then transmitted through the polarization beam splitter 3 to become overlapping error light.
The overlapping error light makes two additional round trips in the reference optical path (resultingly four round trips). As a result, a non-linear error occurs due to the overlapping error light, making it impossible to perform highly accurate measurement. Therefore, in the modification of FIG. 7, a polarization beam splitter 84, a polarization beam splitter 85, and a corner cube prism 87 are additionally provided in order to reduce the influence of overlapping error light.

Therefore, in the modified example of FIG. 7, the reference light reflected by the polarization beam splitter 3 is reflected by the fixed mirror 5 through the spatial filter 52 and the quarter wavelength plate 4, and then the quarter wavelength. Returning to the polarization beam splitter 3 via the plate 4 and the spatial filter 52. The reference light transmitted through the polarization beam splitter 3 is reflected by the corner cube prism 8 via the polarization beam splitter 84, and then enters the polarization beam splitter 3 via the polarization beam splitter 84. The reference light that has passed through the polarization beam splitter 3 is reflected again by the fixed mirror 5 through the spatial filter 52 and the quarter wavelength plate 4, and then is polarized through the quarter wavelength plate 4 and the spatial filter 52. Return to 3.

On the other hand, the measurement light transmitted through the polarization beam splitter 3 is the spatial filter 53 and the quarter wave plate 6.
After being reflected by the moving mirror 7 via the, the beam returns to the polarization beam splitter 3 via the quarter-wave plate 6 and the spatial filter 53. The measurement light reflected by the polarization beam splitter 3 is reflected by the corner cube prism 87 via the polarization beam splitters 84 and 85, and then enters the polarization beam splitter 3 via the polarization beam splitters 85 and 84. The measurement light reflected by the polarization beam splitter 3 is reflected again by the moving mirror 7 through the spatial filter 53 and the quarter wavelength plate 6, and then is polarized by the quarter wavelength plate 6 and the spatial filter 53. Return to splitter 3.

The overlapping error light generated from the measurement light returning to the polarization beam splitter 3 after reciprocating the measurement optical path up to the movable mirror 7 twice is a component that should be originally transmitted through the polarization beam splitters 3, 84 and 85. Therefore, most of the overlapping error light passes through the polarization beam splitter 84. The overlapping error light that has passed through the polarization beam splitter 84 loses its optical path to travel and therefore has no effect. If the overlapping error light that has passed through the polarization beam splitter 84 is reflected by the corner cube prism 8 and affects the measurement accuracy, a spatial filter 584 that allows only the reference light to pass through is attached and the overlapping error light It is possible to prevent adverse effects.

Since the performance of the polarization beam splitter 84 is not perfect, part of the overlapping error light is reflected by the polarization beam splitter 84. However, most of the overlapping error light reflected by the polarization beam splitter 84 passes through the polarization beam splitter 85,
It loses the optical path to go, so it has no effect. Further, even when a part of the overlapping error light is reflected by the polarization beam splitter 85, it is reflected by the corner cube prism 87 and re-enters the polarization beam splitter 85. As a result, most of the overlapping error light is transmitted through the polarization beam splitter 85 and loses the optical path to be traveled, so that there is no effect.

Similarly, the overlapping error light generated from the reference light returning to the polarization beam splitter 3 after making two round trips in the reference light path to the fixed mirror 5 is a component that should be originally reflected by the polarization beam splitters 3, 84 and 85. is there. Therefore,
Most of the overlapping error light is reflected by the polarization beam splitter 84. The overlapping error light reflected by the polarization beam splitter 84 passes through the polarization beam splitter 85. The overlapping error light that has passed through the polarization beam splitter 85 loses its optical path and therefore has no effect. If the overlapping error light reflected by the polarization beam splitter 85 is reflected by the corner cube prism 87 and affects the measurement accuracy, only the measurement light can pass through the spatial filter 5.
By attaching 85, it is possible to prevent the adverse effect of overlapping error light.

Since the performance of the polarization beam splitter 84 is not perfect, part of the overlapping error light will pass through the polarization beam splitter 84. However,
Most of the overlapping error light transmitted through the polarization beam splitter 84 is reflected by the corner cube prism 8 and re-enters the polarization beam splitter 84. as a result,
Most of the overlapping error light is reflected by the polarization beam splitter 84 and loses the optical path to be traveled, so that there is no influence.

As described above, even if the duplicated error light is generated from the reference light and the measurement light that have returned to the polarization beam splitter 3 after going back and forth through the reference light path and the measurement light path, the actions of the three polarization beam splitters 3, 84 and 85 Thereby, the overlapping error light can be sequentially reduced. That is, it is possible to reduce the non-linear error due to the overlapping error light and perform highly accurate measurement by the double pass configuration. Further, if the polarization beam splitters 84 and 85 are added and the polarization beam splitters are sequentially connected to the rear side (corner cube prism side) of the polarization beam splitters 84 and 85, the error light can be further reduced. Therefore, it is possible to perform more accurate measurement.

The length measuring unit 200 of FIG. 8 is similar to the length measuring unit 200 of the modification of FIG. 7, but is constructed such that the reflecting surface of the movable mirror and the reflecting surface of the fixed mirror are parallel to each other. The only difference is that Therefore, in FIG. 8, elements having the same functions as the components of the modification of FIG. 7 are designated by the same reference numerals as in FIG. 7. The modification of FIG. 8 will be described below, focusing on the differences from the modification of FIG. 7.

In the modification of FIG. 8, 1 is set in the reference optical path.
A reflecting mirror 88 is attached between the / 4 wavelength plate 4 and the fixed mirror 89 so that the reflecting surface of the fixed mirror 89 and the reflecting surface of the movable mirror 7 are parallel to each other. A total reflection prism may be used instead of the reflecting mirror 88. For example, when the present invention is applied to an optical wave interferometer for detecting the position of a two-dimensional stage of an exposure apparatus (lithography stepper) for manufacturing a semiconductor element or a liquid crystal display element, a movement fixed to the two-dimensional stage is performed. It is necessary to make the reflecting surface of the mirror parallel to the reflecting surface of the fixed mirror fixed to the projection optical system. Therefore, the configuration of the length measuring unit 200 shown in the modified example of FIG.
For example, it is suitable for an exposure apparatus.

The length measuring unit 200 in FIG. 9 is similar to the length measuring unit 200 in the first embodiment, but the arrangement of the quarter wavelength plate and the spatial filter is reversed in the measurement optical path and the reference optical path. Only the points are basically different. Note that FIG.
Also in FIGS. 7 and 8, a configuration in which the quarter wavelength plate and the spatial filter are reversed in at least one of the measurement optical path and the reference optical path is possible.

FIGS. 10 to 12 are schematic views showing the configuration of a modification of the light source unit 201 in the second embodiment of FIG. In the modification of FIG. 10, the frequency shifter 66 of FIG.
Instead of this, a frequency shifter 190 is provided between the frequency filters 64 and 67. Therefore, in the modification of FIG. 10, without frequency shifting the light 92 of a frequency omega _3, and only light 90 of a frequency omega ₂ and the frequency shifted light 91 of a frequency omega ₂ '.

Further, in the modification of FIG. 11, in addition to the frequency shifter 191 corresponding to the frequency shifter 66 of FIG. 3, a frequency shifter 192 is provided between the two frequency filters 64 and 67. Accordingly, the light 95 in the modification of FIG. 10, the frequency omega ₃ light 92 of a frequency omega ₃ '
Along with the frequency shift in, and the light 90 of frequency ω ₂ and the frequency shift to light 94 of frequency ω ₂ '.

In the modification of FIG. 12, the light 90 having the frequency ω ₂
And a light source 97 which emits the light 92 having the frequency ω ₃ along different optical paths. Light of frequency ω ₃ 92
Is frequency-shifted to the light 98 having the frequency ω ₃ ′ by the action of the frequency shifter 193. Light 98 of the light 92 and the frequency omega ₃ 'of the frequency omega ₃ is coupled to the same optical path through the frequency filter 67.

[0088] In the modification of FIG. 12, and only light of the frequency omega ₃ and frequency shift in the optical frequency omega ₃ '. However, only light of the frequency omega ₂ frequency omega ₂ as shown in FIG. 10 'may be frequency shifted to the light of the frequency omega ₂ of the optical frequency omega ₂ of the light and the frequency omega ₃ as shown in FIG. 11' And the light of frequency ω ₃ 'may be frequency-shifted. When measuring the refractive index variation in the homodyne as in the first embodiment, without performing frequency modulation, through a frequency filter 67 and a light 92 of the light 90 and the frequency omega ₃ frequency omega ₂ It suffices to combine them on the same optical path.

In each of the above-mentioned embodiments, the light of the frequency ω _{2 and} the light of the frequency ω ₃ having twice the frequency of ω ₂ are used for measuring the refractive index fluctuation. However,
In the present invention, it is important to make the frequency of one of the two lights having different frequencies, which have passed through the reference optical path and the measurement optical path, substantially coincide with the frequency of the other light to cause interference. Therefore, for example, by using the harmonic conversion element, by using the third harmonic or higher-order light higher than that,
The refractive index fluctuation may be measured.

In each of the above-mentioned embodiments, the length measuring light source 1 is used.
Emits two lights whose polarization directions are orthogonal to each other and whose frequencies are slightly different from each other, and the displacement of the movable mirror 7 is measured based on the two-frequency light by using the heterodyne interference method. However, based on the light having a single frequency from the length measuring light source, the moving mirror 7 is moved by the homodyne interference method.
May be measured.

[0091]

As described above, according to the present invention, the two lights to be separated into the reference light and the measurement light are parallel-shifted in advance via the polarization beam splitter, so that the measurement light path and the reference light path are the same. They are spatially separated from each other without being coupled into the optical path. As a result, even if the polarized beam splitter mixes in the reference optical path and the measurement optical path, the influence of the mixed error light can be reduced. Further, by providing the removing means for removing the error light mixed in the reference light path and the error light mixed in the measurement light path in the polarization beam splitter, it is possible to further reduce the nonlinear error caused by the mixed error light, Further accurate measurement can be performed.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration of an optical interference measuring apparatus according to a first embodiment of the present invention.

FIG. 2 is an enlarged view of the length measuring unit 200 in FIG.

FIG. 3 is a diagram schematically showing a configuration of a lightwave interference measuring apparatus according to a second embodiment of the present invention.

FIG. 4 is a diagram schematically showing a configuration of a modified example of the light splitting / coupling unit 202 (the light splitting element 50 and the light coupling element 51) in the first embodiment.

FIG. 5 is a diagram schematically showing a configuration of a modified example of the light splitting / coupling unit 202 (the light splitting element 50 and the light coupling element 51) in the first embodiment.

FIG. 6 is a diagram schematically showing a configuration of a modified example of the light splitting / coupling unit 202 (the light splitting element 50 and the light coupling element 51) in the first embodiment.

FIG. 7 is a diagram schematically showing a configuration of a modified example of the length measuring unit 200 in the first embodiment.

FIG. 8 is a diagram schematically showing a configuration of a modified example of the length measuring unit 200 in the first embodiment.

FIG. 9 is a diagram schematically showing a configuration of a modified example of the length measuring unit 200 in the first embodiment.

FIG. 10 is a diagram schematically showing the configuration of a modification of the light source unit 201 in the second embodiment of FIG.

11 is a diagram schematically showing the configuration of a modification of the light source unit 201 in the second embodiment of FIG.

FIG. 12 is a diagram schematically showing the configuration of a modification of the light source unit 201 in the second embodiment of FIG.

FIG. 13 is a diagram schematically showing a configuration of a lightwave interference measuring device disclosed in the specification of Japanese Patent Application No. 7-66469 and the drawings.

FIG. 14 is a diagram schematically showing a configuration of a conventional lightwave interference measuring apparatus.

[Explanation of symbols]

 1, 20 Light source 3, 28 Polarization beam splitter 4, 6 1/4 wavelength plate 5 Fixed mirror 7 Moving mirror 8 Corner cube prism 10, 13 Polarizing plate 11, 14 Light receiving element 12 Beam splitter 15, 37 Phase meter 21, 23 Frequency Filters 22, 30, 34 SHG conversion element 24 Frequency coupling element 25 Frequency separation element 32, 36 Light receiving element 38 Operator 50 Optical coupling element 51 Optical separation element 52, 53 Spatial filter 66 Acousto-optic element

Claims (14)

[Claims] 1. A light source section for outputting a first light having a predetermined frequency and a second light and a third light having different frequencies along the same optical path, and an output from the light source section. The reference light having the first polarization state, which is guided along the reference light path to the fixed mirror, and the second light, which is guided along the measurement light path to the movable mirror, A polarization beam splitter for respectively polarization-separating into measurement light having a polarization state of, and the measurement light and the reference through the measurement light path based on the first light through the measurement light path and the reference light path. A first for generating a first interference light with the reference light through the optical path
An interference light generation system, and for generating a second interference light by causing the frequency of one of the second light and the third light that has passed through the measurement optical path to substantially match the frequency of the other light. The second interference light generation system and the third interference light are generated by making the frequency of one of the second light and the third light passing through the reference optical path substantially equal to the frequency of the other light. And a third interference light generation system for controlling the first polarized light, which is disposed in an optical path between the light source unit and the polarization beam splitter, and which converts the first light to the third light from the light source unit into the first polarized light. The light having a state and the light having the second polarization state are polarized and separated from each other, and the light having the first polarization state and the light having the second polarization state are separated from each other by a predetermined distance. And a light separating means for emitting along a parallel optical path, Correcting the displacement amount of the movable mirror measured based on the first interference light based on the refractive index variation information in the measurement optical path measured based on the second interference light and the third interference light. A light wave interferometer measuring device. 2. The removing unit for removing the error light mixed in the reference light path and the error light mixed in the measurement light path in the polarization beam splitter, further comprising: Optical interferometer. 3. The first spatial filter, which is disposed in an optical path between the polarization beam splitter and the fixed mirror, for passing only reference light along a predetermined reference optical path, The second spatial filter arranged in the optical path between the polarization beam splitter and the movable mirror, for allowing only the measurement light along a predetermined measurement optical path to pass therethrough. Optical interferometer. 4. The light wave according to claim 1, wherein the light separating unit has a positive lens and a Wollaston prism positioned at a focal position of the positive lens. Interferometer. 5. The light wave interference measuring apparatus according to claim 1, wherein the light separating unit has a polarization beam splitter and a reflecting mirror. 6. The first interference light generation system further includes an optical coupling means for coupling the reference light and the measurement light emitted from the polarization beam splitter along the optical paths parallel to each other on the same optical path. Claim 1 characterized by the above.
6. The lightwave interference measurement device according to any one of items 1 to 5. 7. The lightwave interference measuring apparatus according to claim 6, wherein the optical coupling unit includes a positive lens and a Wollaston prism positioned at a focal position of the positive lens. 8. The light wave interference measuring apparatus according to claim 6, wherein the optical coupling means includes a polarization beam splitter and a reflecting mirror. 9. A quarter wave plate arranged in an optical path between the polarization beam splitter and the fixed mirror, and a 1/4 wavelength plate arranged in an optical path between the polarization beam splitter and the movable mirror. A four-wave plate and the reference light path travels back and forth once to reflect the reference light that has passed through the polarization beam splitter and guide it to the fixed mirror through the polarization beam splitter.
9. A reflection means for reciprocatingly reflecting the measurement light from the polarization beam splitter and guiding the measurement light to the movable mirror via the polarization beam splitter, further comprising: The lightwave interferometer according to item. 10. The reference light passing through the polarization beam splitter after reciprocating once in the reference light path and the measurement light path
It further has a 2nd polarization beam splitter for reciprocating and polarization-separating with the measurement light which passed the said polarization beam splitter, The said reflection means reciprocates the said reference optical path 1 time, and the said polarization beam splitter and the said 2nd. A first reflection unit for reflecting the reference light through the polarization beam splitter and guiding it to the fixed mirror through the second polarization beam splitter and the polarization beam splitter, and the measurement optical path 1
A second reflection means for reciprocating and reflecting the measurement light passing through the polarization beam splitter and the second polarization beam splitter and guiding it to the movable mirror through the second polarization beam splitter and the polarization beam splitter. The optical wave interferometer according to claim 9, characterized in that it has. Wherein said light source unit outputs a light containing the light of the frequency omega ₁ of the light and the frequency omega ₁ 'to and polarization orientation different to the frequency slightly from each other are perpendicular to each other as the first light, the polarization beam splitter, the frequency omega ₁ of light to the measurement light, the light of the frequency omega ₁ 'to any one of claims 1 to 10, characterized in that the polarization separating each said reference light The lightwave interferometer described. 12. The light source unit includes a first light source unit for outputting the first light and a second light source for outputting the second light and the third light.
A light source unit, a frequency coupling element for coupling the second light and the third light from the second light source unit, and the first light from the first light source unit on the same optical path. The lightwave interference measuring apparatus according to claim 1, further comprising: 13. The second light source section converts a part of the second light from the light source for outputting the second light and a part of the second light into a second harmonic, and the second harmonic. A second frequency conversion means for outputting a wave as the third light, and the second interference light generation system converts the second light into the second harmonic through the measurement optical path. And a third frequency conversion means for converting the second light into the second harmonic through the reference optical path. The lightwave interference measurement apparatus according to claim 12, which is characterized in that. 14. The second light source unit further includes frequency shift means for slightly shifting the frequency of light of at least one of the second light and the third light. The lightwave interference measurement device according to claim 12 or 13.