JPH1096601A - Light wave interference measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately correct measurement error caused by fluctuation of air density and humidity in light path by correcting the displacement of an object measured with a measuring optical system based on information on fluctuation of a refractive index in the air measured with a refractive index fluctuation measuring system. SOLUTION: A measuring optical system is an interferometer for length measurement for measuring the displacement of a moving mirror 140 along a specified direction. The lights of frequencies f10, f20 and f30 having different frequencies each other which are emitted along nearly the same light path from a light source 300 of the refractive index fluctuation measuring system are reflected by a dichroic mirror 110 and bound along with nearly the same light path as the lights from the light source 310 for length measurement. By almost coinciding the frequencies of light with frequency f10 and frequency f20 and almost coinciding those of frequency f20 and frequency f30, two interfered lights are produced and the fluctuation of refractive index accompanying the fluctuation of density and humidity of the air in the specific light path of the interfero meter for length measurement is measured based on those interfered lights to correct the measured displacement of the moving mirror 140.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical interference measuring apparatus, and more particularly to an optical interference measuring apparatus for measuring the length, displacement, density, etc. of an object with high accuracy.

[0002]

2. Description of the Related Art FIG. 9 is a diagram schematically showing a configuration of a conventional light wave interference measuring device. The light wave interference measurement device of FIG.
A heterodyne interferometric length measuring interferometer for measuring the displacement of the stage 131 in the optical axis direction (arrow direction in the figure);
It comprises a refractive index fluctuation measuring system for measuring the refractive index fluctuation of air (or other gas: hereinafter simply referred to as “air”) in the measuring optical path of the length measuring interferometer.

In the length measuring interferometer of FIG. 9, a length measuring light source 314 emits light of frequency f00 and light of frequency f01 having slightly different frequencies and orthogonal polarization directions. The lights of the two different frequencies are transmitted through the beam splitter 101, the dichroic mirror 118 and the dichroic mirror 128 to the polarization beam splitter 402.
And is separated into light of frequency f00 and light of frequency f01. That is, the light having the frequency f01 reflected by the polarization beam splitter 402 becomes reference light, is reflected by the fixed mirror 152, and returns to the polarization beam splitter 402 again.
The frequency f transmitted through the polarization beam splitter 402
The light of 00 becomes measurement light, is reflected by the moving mirror 142 attached to the stage 131, and returns to the polarization beam splitter 402 again.

The measurement light and reference light returned to the polarization beam splitter 402 are emitted from the polarization beam splitter 402 along substantially the same optical path, and are output from the dichroic mirror 1.
27, and enters the receiver 219 with the polarizer via the frequency filter 420. The interference light generated through the polarizer in this way is detected by the receiver 219 and input to the calculator 203 as a measurement beat signal for measuring the displacement of the stage 131.

On the other hand, a part of the light of frequency f00 and part of the light of frequency f01 emitted from the length measuring light source 314 is reflected by the beam splitter 101 and enters a receiver 218 with a polarizer. The interference light generated via the polarizer is detected by the receiver 218 and input to the arithmetic unit 203 as a reference beat signal for measuring the displacement of the stage 131. The arithmetic unit 203 obtains the displacement amount ΔD (f00) of the stage 131 without considering the influence of the refractive index variation based on the phase change of the measured beat signal from the receiver 219 with respect to the reference beat signal from the receiver 218.

[0006] On the other hand, the refractive index fluctuation measuring system uses a light having a frequency f1 and a light having a frequency f2 as a second harmonic thereof (f2 = 2).
f1) are emitted substantially parallel to each other. Note that the polarization direction of the light of frequency f1 and the frequency f2
And the polarization directions of the light of
It is almost the same as the polarization direction of the light of frequency f00 from No. 4.
The light having the frequency f1 and the light having the frequency f2 emitted from the light source 315 enter the frequency shifters 414 and 415, respectively. Frequency f1 incident on frequency shifter 414
Is frequency-modulated to light of frequency f10 (f10 = f1 + .DELTA.f1) slightly shifted in frequency from frequency f1.
The light having the frequency f2 incident on the frequency shifter 415 has a frequency f20 slightly shifted from the frequency f2.
(F20 = f2 + Δf2).

The light of frequency f10 and the light of frequency f20 are
The light enters a dichroic mirror 117 which is a frequency coupling element. Thus, the light of the frequency f10 and the light of the frequency f20 that are coupled along the substantially same optical path via the dichroic mirror 117 enter another dichroic mirror 118. The light having the frequency f10 and the light having the frequency f20 reflected by the dichroic mirror 118 are coupled along substantially the same optical path as the light having the frequency f00 and the light having the frequency f01 from the length measuring light source 314, and are transmitted to the polarization beam splitter 402. Incident.

The light of frequency f10 and the light of frequency f20 are
The light passes through the polarization beam splitter 402 and enters the movable mirror 142. The light of the frequency f10 and the light of the frequency f20 reflected by the movable mirror 142 pass through the polarization beam splitter 402, and the dichroic mirror 1 as a frequency separation element.
27. The light having the frequency f10 and the light having the frequency f20 reflected by the dichroic mirror 127 are converted into a second harmonic conversion element (hereinafter, referred to as "SHG conversion element") 185.
Incident on. In the SHG conversion element 185, a part of the light having the frequency f10 is subjected to the SHG conversion to obtain the frequency f10 '(f10' = 2).
f10). On the other hand, the light having the frequency f20 passes through the SHG conversion element 185 as it is. SHG conversion element 18
The light having the frequency f10 'and the light having the frequency f20 emitted from 5 enter the receiver 430 with a polarizer via the frequency filter 421. The interference light generated through the polarizer in this manner is detected by the receiver 430, and is used as a measurement beat signal for the refractive index fluctuation measurement by the arithmetic unit 20.
3 is input.

On the other hand, the light of the frequency f10 and a part of the light of the frequency f20 from the light source 315 are transmitted to the dichroic mirror 1
The light is reflected by 28 and enters the SHG conversion element 186. In the SHG conversion element 186, a part of the light having the frequency f10 is subjected to SHG conversion to become light having the frequency f10 '(f10' = 2f10). On the other hand, the light of the frequency f20 is
86 as it is. The light having the frequency f10 'and the light having the frequency f20 emitted from the SHG conversion element 186 are passed through the frequency filter 422 to the receiver 4 having the polarizer.
It is incident on 31. Thus, the interference light generated via the polarizer is detected by the receiver 431, and is input to the arithmetic unit 203 as a reference beat signal for measuring the refractive index fluctuation.

The arithmetic unit 203 measures the change in the refractive index of air in the measurement optical path of the length measuring interferometer based on the phase change of the measurement beat signal from the receiver 430 with respect to the reference beat signal from the receiver 431. Then, based on the refractive index fluctuation measured by the refractive index fluctuation measuring system, the displacement amount ΔD (f00) of the stage 131 measured by the length measuring interferometer is corrected, and the true displacement amount (geometric Distance) Δ
Find D. Hereinafter, the displacement amount D (f00) of the stage 131
Will be described below.

The optical path lengths D (fa), D (fb) and D (fc) for light of frequencies fa, fb and fc are represented by the following equations (1) to (3), respectively.

D (fa) = {1 + NF (fa)} D (1) D (fb) = {1 + NF (fb)} D (2) D (fc) = {1 + NF (fc) ｝ D (3)

Here, D is a geometric distance, and N is the density of air. Further, F (f) is a function that depends only on the frequency f of the light without depending on the density of the air if the composition ratio of the air is unchanged. Equations (1) to (3) above
Thus, the geometric distance D is given by the following equation (4).

D = D (fa) −A {D (fc) −D (fb)} (4) where A = F (fa) / {F (fc) −F (fb)}.

From equation (4), the amount of change in the geometric distance D, that is, the true displacement ΔD is given by the following equation (5).

ΔD = ΔD (fa) −A ｛ΔD (fc) −ΔD (fb)｝ (5) where ΔD (fa), ΔD (fb) and ΔD (f
c) is an optical path length change with respect to light of frequencies fa, fb, and fc.

As described above, ΔD (fa) of the first term on the right side of the equation (5) is calculated by the receiver 218 in the length measuring interferometer.
And the measured beat signal from the receiver 219. Further, {ΔD (fc) −ΔD (fb)} of the second term on the right side of Expression (5)
Is calculated based on the phase change between the reference beat signal from the receiver 431 and the measured beat signal from the receiver 430 in the refractive index fluctuation measurement system. Thus, arithmetic unit 2
In step 03, the displacement amount ΔD (fa) measured by the length measuring interferometer is corrected based on the arithmetic expression of equation (5), and the true displacement amount ΔD (fa) is corrected.
D can be determined.

[0015]

In general, a change in the refractive index of air in an optical path is caused by a change in air density (that is, a change in temperature).
Depends not only on the humidity but also on the humidity of the air. However, in the above-described conventional light wave interference measuring apparatus, the measured value of the length measuring interferometer is corrected in consideration of only the influence of the refractive index fluctuation caused by the density fluctuation of the air in the optical path.
As a result, when using a conventional light wave interference measurement device in an environment where the humidity of air in the optical path fluctuates, a measurement error occurs due to the refractive index fluctuation accompanying the humidity fluctuation, and the measurement accuracy is reduced. There was an inconvenience.

The present invention has been made in view of the above-mentioned problems, and has been made in consideration of the above-described problems, and has been made in consideration of a high-precision light wave interference capable of correcting a measurement error caused by a refractive index variation caused by a density variation and a humidity variation of a gas in an optical path. It is an object to provide a measuring device.

[0017]

According to the present invention, there is provided a measuring optical system for optically measuring an amount of displacement of an object, and a gas density variation in an optical path of the measuring optical system. And a refractive index fluctuation measuring system for measuring the refractive index fluctuation due to humidity fluctuation, the gas in the optical path measured by the refractive index fluctuation measuring system the amount of displacement of the object measured by the measurement optical system. A light wave interference measurement device is provided, wherein a geometric displacement amount of the object is measured by correcting based on refractive index fluctuation information.

According to a preferred aspect of the present invention, the measuring optical system is a length measuring interferometer for measuring a displacement amount of a movable mirror along a predetermined direction, and the refractive index fluctuation measuring system includes:
A first light for outputting the first light, the second light, and the third light having different frequencies from each other along substantially the same optical path.
A light source unit, by making the frequency of the first light substantially equal to the frequency of the second light among the first light and the second light via a predetermined optical path of the length measuring interferometer. A first interference light generation system for generating a first interference light, and a frequency of the second light of the second light and the third light passing through the predetermined optical path of the length measuring interferometer The third
A second interference light generation system for generating a second interference light by making the frequency substantially equal to the frequency of the first light.
Based on the interference light and the second interference light, the refractive index fluctuation accompanying the density fluctuation and humidity fluctuation of the gas in the predetermined optical path of the length measuring interferometer was measured, and the refractive index fluctuation was measured by the length measuring interferometer. Correct the displacement of the moving mirror.

According to a preferred aspect of the present invention, the measuring optical system is a length measuring interferometer for measuring a displacement amount of a movable mirror along a predetermined direction, and the refractive index fluctuation measuring system is A first light, a second light, and a third light having different frequencies from each other, a fourth light having a slightly different frequency from the first light, and a fourth light having a slightly different frequency from the second light. A second light source unit for outputting the fifth light and the sixth light slightly different in frequency from the third light along substantially the same optical path; and a reference light path of the length measuring interferometer. A first interference light generation system for generating a first interference light between the first light and the fourth light via a measurement optical path of the length measurement interferometer; The second through a reference optical path
A second interference light generating system for generating a second interference light between the first light and the fifth light via the measurement optical path of the length measuring interferometer, and a reference light path of the length measuring interferometer. A third interference light generating system for generating a third interference light between the third light and the sixth light via a measurement optical path of the length measuring interferometer, wherein the first interference light Measuring, on the basis of the second interference light and the third interference light, a refractive index variation accompanying a density variation and a humidity variation of a gas in a reference optical path and a measurement optical path of the length measuring interferometer, The displacement of the movable mirror measured by the interferometer is corrected.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION In the light wave interference measuring apparatus of the present invention,
For example, there is provided a refractive index fluctuation measuring system for measuring a refractive index fluctuation accompanying a density fluctuation and a humidity fluctuation of a gas in an optical path of a length measuring interferometer for measuring a displacement amount of a moving mirror along a predetermined direction. I have. Therefore, it is possible to correct a measurement error caused by a change in refractive index due to a change in density and a change in humidity in the optical path of the length measuring interferometer, and to measure the displacement amount of the movable mirror with high accuracy.

As a specific example, the refractive index fluctuation measurement system includes a first light source unit for outputting the first light, the second light, and the third light along substantially the same optical path. . Then, the first interference light is generated by making the frequency of the first light through the measurement optical path of the length measuring interferometer substantially equal to the frequency of the second light. Further, the second interference light is generated by making the frequency of the second light through the measurement optical path of the length measuring interferometer substantially equal to the frequency of the third light. Thus,
Measuring the displacement of the movable mirror with high accuracy, because it is possible to correct the measurement error caused by the refractive index fluctuation caused by the density fluctuation and the humidity fluctuation of the gas in the optical path using the light of three different frequencies. Can be.

Hereinafter, embodiments of the present invention will be described 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. Also,
FIG. 2 is a diagram schematically showing an internal configuration of the light source unit 300 of FIG. Further, FIG. 3 shows the measurement detection unit 320A of FIG.
FIG. 5 is a diagram schematically showing an internal configuration of a reference detection unit 320B.

The optical interference measuring apparatus shown in FIG.
A heterodyne interferometric length measuring interferometer for measuring the displacement in the optical axis direction of 0 (the direction of the arrow in the figure) is provided. In the length measuring interferometer, a length measuring light source 310 composed of, for example, a He-Ne laser light source has two lights whose frequencies are slightly different from each other and whose polarization directions are orthogonal to each other, that is, light of a frequency f00 and frequency f01 (f01 = f00 + Δf00). ) Are emitted along substantially the same optical path. As shown by the arrows in the drawing, it is assumed that the polarization direction of the light of frequency f00 is parallel to the plane of FIG. 1 and the polarization direction of the light of frequency f01 is perpendicular to the plane of paper.

Light of two different frequencies emitted from the length measuring light source 310 passes through the beam splitter 100,
The light enters a dichroic mirror 110 which is a frequency coupling element. The dichroic mirror 110 has a frequency f00
It has the property of transmitting only nearby light and reflecting light of other frequencies. Therefore, dichroic mirror 1
The light having the frequency f00 and the light having the frequency f01 transmitted through the dichroic mirror 120 are frequency-separating elements.
Through the polarization beam splitter 160. The polarization beam splitter 160 is arranged so as to transmit light having a polarization direction parallel to the plane of FIG. 1 and reflect light having a polarization direction perpendicular to the plane of FIG. Therefore, the light of frequency f01 is reflected by the polarizing beam splitter 160,
Fixed mirror 1 consisting of corner cube prism as reference light
It is led to 50. On the other hand, the light having the frequency f00 is transmitted through the polarization beam splitter 160, and is guided as measurement light to the movable mirror 140 formed of a corner cube prism. Moving mirror 14
0 denotes a stage 13 movable along the direction of the arrow in the figure.
It is attached to 0.

The reference light composed of the light having the frequency f01 is reflected by the fixed mirror 150, and is again reflected by the polarization beam splitter 16
Return to 0. The measurement light composed of the light having the frequency f00 is reflected by the movable mirror 140 and is again reflected by the polarization beam splitter 16.
Return to 0. Thus, the light having the frequency f01 emitted from the polarization beam splitter 160 via the reference optical path and the light having the frequency f00 emitted from the polarization beam splitter 160 via the measurement optical path are frequency-separated along substantially the same optical path. The light enters a dichroic mirror 121 as an element.
Like the dichroic mirror 110, the dichroic mirror 121 transmits only light having a frequency near f00,
It has the property of reflecting light of other frequencies.

Therefore, the reference light having the frequency f01 and the measurement light having the frequency f00 pass through the dichroic mirror 121 and enter the frequency filter 170. The frequency filter 170 transmits only light having the frequency f00 and light having the frequency f01, and blocks light having other frequencies, that is, error light. The reference light having the frequency f01 and the measurement light having the frequency f00 transmitted through the frequency filter 170 are incident on a receiver 211 having a polarizer. The polarizer of the receiver 211 is arranged at an angle of 45 ° with respect to the polarization direction of the measurement light having the frequency f00 and the polarization direction of the reference light having the frequency f01. Therefore, the measurement light having the frequency f00 and the reference light having the frequency f01 interfere via the polarizer, and the receiver 211 outputs a beat signal (frequency Δf00 = f01−f00) based on the interference light.
Is supplied to the arithmetic unit 200 as a measurement beat signal for measuring the displacement of the stage 130.

On the other hand, a part of the light having the frequency f01 and the light having the frequency f00 emitted from the length measuring light source 310 are reflected by the beam splitter 100, and thereafter, enter the receiver 210 having a polarizer. The polarizer of the receiver 210 is
The polarization direction of the measurement light having the frequency f00 and the polarization direction of the reference light having the frequency f01 are arranged at an angle of 45 degrees. Therefore, the measurement light having the frequency f00 and the frequency f
01 interferes with the reference light via the polarizer, and the receiver 210
Supplies a beat signal (frequency Δf00) based on the interference light to the arithmetic unit 200 as a reference beat signal for measuring the displacement of the stage 130. In the arithmetic unit 200, the receiver 21
Based on the phase change of the measured beat signal from the receiver 211 with respect to the reference beat signal from 0, the displacement amount ΔD (f0
0) is required.

The optical interference measuring apparatus shown in FIG. 1 also has a refractive index fluctuation measuring system for measuring a refractive index fluctuation caused by a fluctuation in air density and air humidity in a measuring optical path of the length measuring interferometer. In the refractive index fluctuation measurement system, the light source 311 of the light source unit 300 emits light of the frequency f1 (for example, the wavelength 1064n).
m) and the light of the second harmonic, frequency f2 (f
2 = 2f1: for example, light having a wavelength of 532 nm) are emitted along optical paths almost parallel to each other. Note that both the light having the frequency f1 and the light having the frequency f2 have a polarization direction parallel to the plane of FIG. 1 and FIG. The light having the frequency f1 and the light having the frequency f2 have substantially different frequencies from the light having the frequency f00 and the light having the frequency f01 from the length measuring light source 310.

The light having the frequency f1 is slightly frequency-modulated through the frequency shifter 220, and the frequency f10 (f10 = f
1 + Δf1). On the other hand, the light of frequency f2 is S
A part of the light having the frequency f2 which is incident on the HG conversion element 180 is converted by the SHG conversion element 180 into a frequency f3 (f3 = 2f2).
), And the rest is converted to the SHG conversion element 180.
Is transmitted as it is. Note that a nonlinear optical crystal such as KTP (KTiOPO ₄ ) can be used for the SHG conversion element 180. The light of frequency f2 and the light of frequency f3 emitted from the SHG conversion element 180 enter the dichroic mirror 122 as a frequency separation element along substantially the same optical path.

The light of frequency f 2 transmitted through the dichroic mirror 122 enters the frequency shifter 221, and the light of frequency f 3 reflected by the dichroic mirror 122 enters the frequency shifter 222. In the frequency shifter 221, the light of the frequency f2 is slightly frequency-modulated to become light of the frequency f20 (f20 = f2 + .DELTA.f2). In the frequency shifter 222, the light of the frequency f3 is slightly frequency-modulated to light of the frequency f30 (f30 = f3 + .DELTA.f3). Here, Δf2 ≠ 2Δf1 and Δf3 ≠ 2Δf2
It is. Frequency-modulated light of frequency f10, frequency f20
And the light of frequency f30 are coupled along substantially the same optical path by the action of dichroic mirrors 111 and 112, which are frequency coupling elements, and emitted from the light source unit 300.

The light of frequency f10, the light of frequency f20, and the light of frequency f30 emitted along the substantially same optical path from the light source unit 300 are reflected by the dichroic mirror 110, and the light (frequency) Are coupled along substantially the same optical path as the light near f00). The light having the frequency f10, the light having the frequency f20, and the light having the frequency f30 reflected by the dichroic mirror 110 enter the polarization beam splitter 160 via the dichroic mirror 120. The polarization beam splitter 160 is, as described above,
It is arranged to transmit light having a polarization direction parallel to the paper surface of FIG. 1 and reflect light having a polarization direction perpendicular to the paper surface of FIG. Therefore, the light of frequency f10, the light of frequency f20, and the frequency f
The light of 30 passes through the polarizing beam splitter 160 and is reflected by the moving mirror 140, and then is returned to the polarizing beam splitter 1 again.
Return to 60.

Thus, the light of the frequency f10, the light of the frequency f20, and the light of the frequency f30 emitted from the polarization beam splitter 160 via the measurement optical path enter the dichroic mirror 121. The dichroic mirror 121 is
As described above, only light having a frequency near f00 is transmitted,
It has the property of reflecting light of other frequencies. Therefore,
Due to the operation of the dichroic mirror 121, the frequency f10
, The light of the frequency f20, and the light of the frequency f30 are separated from the light from the length measuring light source 310 (light near the frequency f00). Thus, the light of frequency f10, the light of frequency f20, and the light of frequency f20 reflected by the dichroic mirror 121.
The light of 30 enters the measurement detection unit 320A.

Referring to FIG. 3, the light of the frequency f10, the light of the frequency f20, and the light of the frequency f30 incident on the measurement detection unit 320A along substantially the same optical path are partially reflected by the beam splitter 104, The light enters a dichroic mirror 440 which is a frequency separation element. The light of the frequency f30 reflected by the dichroic mirror 440 enters the receiver 431, and the light of the frequency f10 and the light of the frequency f20 transmitted through the dichroic mirror 440 enter the dichroic mirror 441, which is another frequency separating element. The light of frequency f20 reflected by the dichroic mirror 441 enters the receiver 432, and the light of frequency f10 transmitted through the dichroic mirror 441 is received by the receiver 43.
3 is incident. The receivers 431 to 433 are receivers for detecting optical path fluctuations of each light beam, and a signal related to the detected optical path fluctuations is supplied to the arithmetic unit 200. In addition,
As the receivers 431 to 433, for example, a high-resolution P
Elements such as an SD (Position Sensitive Detector), a CCD, and a four-segment detector that can measure the displacement of the beam in the x direction and the displacement in the y direction can be used.

On the other hand, the light having the frequency f10, the light having the frequency f20, and the light having the frequency f30 transmitted through the beam splitter 104 are converted into dichroic mirrors 128 serving as frequency separation elements.
Incident on. The dichroic mirror 128 has a frequency f
It has a characteristic of reflecting almost all of the light of frequency 10 and a part of the light of frequency f20, and transmitting almost all of the light of frequency f30 and the rest of the light of frequency f20. Dichroic mirror 1
The light having the frequency f10 and the light having the frequency f20 reflected at 28 enter the SHG conversion element 181. Note that a nonlinear optical crystal such as KTP (KTiOPO ₄ ) can be used for the SHG conversion element 181.

In the SHG conversion element 181, the light of the frequency f20 is transmitted as it is, and a part of the light of the frequency f10 is SHG converted to light of the frequency f10 '(f10' = 2f10). S
The light having the frequency f10 'and the light having the frequency f20 emitted from the HG conversion element 181 enter the receiver 212 with a polarizer via the frequency filter 171. The frequency filter 171 transmits only light of the frequency f10 'and light of the frequency f20, and blocks light of other frequencies, that is, error light. The light having the frequency f10 'and the light having the frequency f20 interfere via the polarizer of the receiver 212, and the receiver 212 generates a beat signal (frequency f10'-f20 = 2Δf) based on the interference light.
1 -Δf2) is supplied to the arithmetic unit 200 as a measurement beat signal for measuring the refractive index fluctuation.

The light of frequency f20 and the light of frequency f30 transmitted through the dichroic mirror 128
The light enters the SHG conversion element 182 via 45. SH
In the G conversion element 182, the light of the frequency f30 is transmitted as it is, and a part of the light of the frequency f20 is subjected to the SHG conversion and converted to the frequency f2.
The light becomes 0 '(f20' = 2f20). Note that a nonlinear optical crystal such as KDP (KH ₂ PO ₄ ) can be used for the SHG conversion element 182. SHG conversion element 182
The light having the frequency f20 'and the light having the frequency f30, which are emitted from the light source, enter the receiver 213 with the polarizer via the frequency filter 172. The frequency filter 172 transmits only light having the frequency f20 'and light having the frequency f30, and blocks light having other frequencies, that is, error light. Frequency f2
The light of 0 'and the light of frequency f30 interfere via the polarizer of the receiver 213, and the receiver 213 converts a beat signal (frequency f20'-f30 = 2Δf2-Δf3) based on the interference light into a measurement of refractive index fluctuation. Calculator 2 as the measurement beat signal of
Supply to 00.

On the other hand, the light of the frequency f10, the light of the frequency f20, and a part of the light of the frequency f30 emitted along the substantially same optical path from the light source unit 300 are reflected by the dichroic mirror 12
The light is reflected by 0 and enters the reference detection unit 320B.
In addition, as shown in FIG. 3, the configuration of the reference detection unit 320B is exactly the same as the configuration of the measurement detection unit 320A. Therefore, also in the reference detection unit 320B, the receiver 43
Signals relating to optical path fluctuations detected at 1 to 433 are supplied to the arithmetic unit 200. Further, the receiver 212 outputs a beat signal (frequency f10'-f20 = 2.DELTA.f1-) based on the interference light.
Δf2) is supplied to the arithmetic unit 200 as a reference beat signal for measuring the refractive index fluctuation. Further, the receiver 213
Is a beat signal (frequency f20'-f30 =
2Δf2−Δf3) is supplied to the arithmetic unit 200 as a reference beat signal for measuring the refractive index fluctuation.

As described above, in the arithmetic unit 200, the receiver 2
12, the optical path length change ΔD (f2) for the light of frequency f2 and the optical path length change ΔD (f1) for the light of frequency f1 based on the phase change between the reference beat signal for measuring the refractive index fluctuation and the measured beat signal. ), Ie, ｛ΔD
(F2) -ΔD (f1)} is obtained. Also, receiver 2
13, the optical path length change ΔD (f3) for the light of frequency f3 and the optical path length change ΔD (f2) for the light of frequency f2 based on the phase change between the reference beat signal for measuring the refractive index fluctuation and the measured beat signal. ), Ie, ｛ΔD
(F3) -ΔD (f2)} is obtained. Further, based on signals from the receivers 431 to 433, the light source 3
The change of each optical path resulting from the change of the exit angle of No. 11 is detected. The correction of the measurement error caused by the fluctuation of each optical path is well known, and a detailed description thereof will be omitted in the following embodiments.

In the first embodiment, in the arithmetic unit 200,
A signal relating to the displacement amount ΔD (f00) and ｛ΔD (f2) −
A signal relating to ΔD (f1)｝ and {ΔD (f3) −ΔD
(F2) The true displacement .DELTA.D of the stage 130 in which the measurement error caused by the refractive index fluctuation caused by the density fluctuation and the humidity fluctuation of the air in the measuring optical path is corrected based on the signal related to .SIGMA.
Ask for. Hereinafter, the displacement amount ΔD (f00) of the stage 130
Will be described below.

The optical path lengths D (fa), D (fb), D (fc) and D (fd) for the lights of frequencies fa, fb, fc and fd are calculated by the equations of B. Edlen (Metrologia, vol. 2, p.
71, 1966), the following equations (6) to
It is represented by (9).

D (fa) = {1 + NF (fa) + pG (fa)} D (6) D (fb) = {1 + NF (fb) + pG (fb)} D (7) D (fc) = {1 + NF (fc) + pG (fc)} D (8) D (fd) = {1 + NF (fd) + pG (fd)} D (9)

Here, D is a geometric distance, N is the density of air, and p is the humidity of the air (water vapor partial pressure). Further, F (f) and G (f) are functions that depend only on the frequency f of light without depending on the density and humidity of the air if the composition ratio of the air is unchanged. From the above equations (6) to (9), the geometric distance D is calculated by the following equation (10).
Given by

[0042]

D = D (fa) −A ｛D (fc) −D (fb)｝ − B ｛D (fd) −D (fc)｝ (10) where A = [F (fa) (G (fd) -G (fc))-G (fa) (F (fd) -F (fc)] / [(F (f
c) -F (fb)) (G (fd) -G (fc))-(F (fd) -F (fc)) (G (fc) -G (fb))] B = [G (fa) (F (fc) -F (fb))-F (fa) (G (fc) -G (fb)] / [(F (f
c) -F (fb)) (G (fd) -G (fc))-(F (fd) -F (fc)) (G (fc) -G (fb))]

From equation (10), the true displacement ΔD, that is, the change in the geometric distance D is given by the following equation (11).

ΔD = ΔD (fa) −A ｛ΔD (fc) −ΔD (fb)｝ − B ｛ΔD (fd) −ΔD (fc)｝ (11) where ΔD (fa) and ΔD (fb ), ΔD (fc) and ΔD (fd) are the frequencies fa, fb, fc and f
This is an optical path length change for the light of d.

As described above, ΔD (fa) of the first term on the right side of the equation (11) is based on the phase change of the measured beat signal from the receiver 211 with respect to the reference beat signal from the receiver 210, and the displacement ΔD (fa) f00). Also, ｛ΔD (fc) of the second term on the right side of equation (11)
-ΔD (fb)｝ is a difference in optical path length change ΔΔD (f2) −Δ based on a phase change between a reference beat signal for measuring refractive index fluctuation from the receiver 212 and a measured beat signal.
D (f1)}. Further, {ΔD (fd) −ΔD (fc)} of the third term on the right side of equation (11).
Can be calculated as the difference {ΔD (f3) −ΔD (f2)} in optical path length change based on the phase change between the reference beat signal for measuring the refractive index fluctuation from the receiver 213 and the measured beat signal. . Thus, in the first embodiment,
The measurement error caused by the change in the refractive index caused by the density change and the humidity change of the air in the measurement optical path is corrected, and the true displacement ΔD of the movable mirror 140 and the true displacement Δ of the stage 130 are corrected.
D can be determined with high accuracy.

FIG. 4 is a diagram schematically showing a configuration of an optical interference measuring apparatus according to a second embodiment of the present invention. FIG. 5 is a diagram schematically showing an internal configuration of the light source unit 301 in FIG. FIG. 6 is a diagram schematically showing the internal configurations of the measurement detection unit 321A and the reference detection unit 321B of FIG. The second embodiment has the same configuration as the first embodiment, but the internal configuration of the light source unit 301 and the detection unit 321 is the same as that of the first embodiment.
This is basically different from the embodiment. Therefore, in FIG. 4, elements having the same functions as those of the first embodiment are denoted by the same reference numerals as in FIG. Hereinafter, the second embodiment will be described focusing on the differences from the first embodiment.

The optical interference measuring apparatus shown in FIG.
A heterodyne interferometric length measuring interferometer for measuring the displacement in the optical axis direction of 0 (the direction of the arrow in the figure) is provided. The configuration of the length measuring interferometer in the second embodiment is exactly the same as in the first embodiment. Therefore, in the arithmetic unit 201, based on the phase change of the measured beat signal from the receiver 211 with respect to the reference beat signal from the receiver 210, the stage 130 does not consider the influence of the refractive index fluctuation.
Of the displacement ΔD (f00) is obtained.

The optical interference measuring apparatus shown in FIG. 4 is further provided with a refractive index fluctuation measuring system for measuring a refractive index fluctuation caused by a change in air density and air humidity in the reference light path and the measurement light path of the length measuring interferometer. ing. In the refractive index fluctuation measuring system of the second embodiment, the light source 312 of the light source unit 301 includes light having a frequency f1 and light having a frequency f2 which is a second harmonic thereof (f2 = 2).
f1) are respectively emitted along optical paths substantially parallel to each other. The light of frequency f1 and the light of frequency f2 both have a polarization direction parallel to the plane of FIG. 4 and FIG. The light having the frequency f1 and the light having the frequency f2 are the light having the frequency f00 and the light having the frequency f01 from the length measuring light source 310.
Has a frequency substantially different from that of the light.

The light having the frequency f 1 is incident on the polarization beam splitter 161 via the wave plate 190. Wave plate 1
Numeral 90 appropriately rotates the polarization direction of the incident light of frequency f1 according to the arrangement of the polarization splitting surface of the polarization beam splitter 161. As a result, the light of the frequency f1 is
And the polarization beam splitter 161, the light is separated into two lights having substantially equal intensities and orthogonal polarization directions. That is, the light of the frequency f1 reflected by the polarization beam splitter 161 (the light having the polarization direction perpendicular to the plane of FIG. 5) is transmitted to the frequency shifter 223 by the light of the frequency f1 transmitted through the polarization beam splitter 161 (the plane of FIG. 5). The light having a polarization direction parallel to the light is incident on the frequency shifter 224.

In the frequency shifter 223, the light having the frequency f1 is slightly frequency-modulated, and the frequency f11 (f11 = f1 +
Δf1 ′). In the frequency shifter 224,
The light of the frequency f1 is slightly frequency-modulated, and the frequency f10
(F10 = f1 + Δf1). On the other hand, the frequency f2
Is incident on the SHG conversion element 183, and a part of the light having the frequency f2 is converted by the SHG conversion element 183 into the frequency f3 (f
3 = 2f2), and the remainder passes through the SHG conversion element 183 as it is. The light of frequency f2 and the light of frequency f3 emitted from the SHG conversion element 183 are
The light enters the dichroic mirror 123 as a frequency separation element along substantially the same optical path.

The light having the frequency f 2 transmitted through the dichroic mirror 123 is incident on the polarization beam splitter 163 via the wave plate 191. The light of the frequency f2 is generated by the action of the wave plate 191 and the polarization beam splitter 163.
The light is separated into two lights having substantially equal intensities and orthogonal polarization directions. That is, the polarization beam splitter 16
The light of frequency f2 (light having a polarization direction perpendicular to the plane of FIG. 5) reflected at 3 is transmitted to the frequency shifter 226 by the light of frequency f2 transmitted through the polarization beam splitter 163 (polarization direction parallel to the plane of FIG. 5). Is a frequency shifter 22
5 respectively. In the frequency shifter 226, the light having the frequency f2 is slightly frequency-modulated, and the frequency f21 (f
21 = f2 + .DELTA.f2 '). In addition, frequency shifter 2
At 25, the light of frequency f2 is slightly frequency modulated,
It becomes light of frequency f20 (f20 = f2 + Δf2).

The light having the frequency f 3 reflected by the dichroic mirror 123 enters the polarization beam splitter 165 via the wave plate 192. Due to the action of the wave plate 192 and the polarizing beam splitter 165, the light of the frequency f3 is substantially equal in intensity and orthogonal in the polarization direction.
Separated into two lights. That is, the light of the frequency f3 reflected by the polarization beam splitter 165 (the light having the polarization direction perpendicular to the plane of FIG. 5) is transmitted to the frequency shifter 228 and the light of the frequency f3 transmitted through the polarization beam splitter 165 (the plane of FIG. 5). The light having a polarization direction parallel to the light is incident on the frequency shifter 227. In the frequency shifter 228,
The light of the frequency f3 is slightly frequency-modulated, and the frequency f31
(F31 = f3 + .DELTA.f3 '). Further, the frequency shifter 227 slightly modulates the light of the frequency f3 into light of the frequency f30 (f30 = f3 + .DELTA.f3).

The light having the frequency f10 and the light having the frequency f11 are coupled along substantially the same optical path by the action of the polarization beam splitter 162. Further, the light having the frequency f20 and the light having the frequency f21 are coupled along substantially the same optical path by the action of the polarization beam splitter 164. Further, the light having the frequency f30 and the light having the frequency f31 are coupled along substantially the same optical path by the action of the polarization beam splitter 166. Thus, the light of frequency f10, the light of frequency f20, and the light of frequency f30 having the polarization direction parallel to the paper surface of FIGS. 4 and 5, the light of frequency f11 having the polarization direction perpendicular to the paper surface, the light of frequency f21, and the like. The light of frequency f31 is
The light is coupled along substantially the same optical path by the action of dichroic mirrors 113 and 114, which are frequency coupling elements, and emitted from the light source unit 301.

The light having the frequency f10, the light having the frequency f20 and the light having the frequency f30, the light having the frequency f11, the light having the frequency f21 and the light having the frequency f31 emitted from the light source 301 along substantially the same optical path are dichroic. The light enters the polarization beam splitter 160 via the mirror 110 and the dichroic mirror 120. Polarizing beam splitter 16
Numeral 0 is arranged so as to transmit light having a polarization direction parallel to the plane of FIG. 4 and reflect light having a polarization direction perpendicular to the plane of FIG. Therefore, the light having the frequency f10, the light having the frequency f20, and the light having the frequency f30 pass through the polarizing beam splitter 160, are reflected by the movable mirror 140, and return to the polarizing beam splitter 160 again. Also, the frequency f11
, The light of frequency f21, and the light of frequency f31 are reflected by the polarization beam splitter 160, reflected by the fixed mirror 150, and then return to the polarization beam splitter 160 again.

Thus, the light of the frequency f10, the light of the frequency f20, and the light of the frequency f30 via the measurement optical path, and the light of the frequency f11, the light of the frequency f21, and the light of the frequency f31 via the reference optical path are polarized light beams. The light is emitted from the splitter 160 along substantially the same optical path, and the dichroic mirror 12
Incident on 1. The light having the frequency f10, the light having the frequency f20, the light having the frequency f30, the light having the frequency f11, the light having the frequency f21, and the light having the frequency f31 reflected by the dichroic mirror 121 have a frequency from the length measurement light source 310 near f00. And is incident on the measurement detection unit 321A.

Referring to FIG. 6, the light of frequency f10, the light of frequency f20, and a part of the light of frequency f30 incident on the measurement and detection section 321A along substantially the same optical path are reflected by the polarization beam splitter 403. The light enters a dichroic mirror 442 which is a frequency separation element. The light of the frequency f30 reflected by the dichroic mirror 442 enters the receiver 434, and the light of the frequency f10 and the light of the frequency f20 transmitted through the dichroic mirror 442 enter the dichroic mirror 443, which is another frequency separation element. The light of the frequency f20 reflected by the dichroic mirror 443 enters the receiver 435, and the light of the frequency f10 transmitted through the dichroic mirror 443 is received by the receiver 43.
6 is incident. In this way, the signal regarding the optical path fluctuation detected by the receivers 434 to 436 is supplied to the arithmetic unit 201.

On the other hand, the light having the frequency f10, the light having the frequency f20, and the light having the frequency f30 transmitted through the polarization beam splitter 403.
And the light of the frequency f11, the light of the frequency f21, and the light of the frequency f31 enter the dichroic mirror 124 as a frequency separation element. The dichroic mirror 124 has a characteristic of reflecting light of the frequency f10 and light of the frequency f11 and transmitting light of other frequencies. Therefore, the frequency f reflected by the dichroic mirror 124
The light of 10 and the light of frequency f11 are passed through the frequency filter 173.
To the receiver 214 with a polarizer. The frequency filter 173 includes the light having the frequency f10 and the frequency f11.
, And blocks light of other frequencies, that is, error light. The light having the frequency f10 and the light having the frequency f11 interfere with each other via the polarizer of the receiver 214.
Is a beat signal based on the interference light (frequency f10−f11 = Δf
1−Δf1 ′) is supplied to the arithmetic unit 201 as a measurement beat signal for measuring the refractive index fluctuation.

The light having the frequency f20 and the light having the frequency f30 transmitted through the dichroic mirror 124 and the light having the frequency f21
And the light having the frequency f31 enter a dichroic mirror 125 which is a frequency separation element. The dichroic mirror 125 has a characteristic of reflecting light having the frequency f20 and light having the frequency f21 and transmitting light having other frequencies. Therefore, the light of the frequency f20 and the light of the frequency f21 reflected by the dichroic mirror 125 enter the receiver 215 with the polarizer via the frequency filter 174. The frequency filter 174 transmits only the light of the frequency f20 and the light of the frequency f21, and blocks the light of other frequencies, that is, the error light. The light of the frequency f20 and the light of the frequency f21 interfere via the polarizer of the receiver 215, and the receiver 215 generates a beat signal (frequency f20-f) based on the interference light.
21 = Δf 2 −Δf 2 ′) is supplied to the arithmetic unit 201 as a measurement beat signal for measuring the refractive index fluctuation.

The light having the frequency f 30 and the light having the frequency f 31 transmitted through the dichroic mirror 125 are bent by the mirror 146, and then enter the receiver 216 with the polarizer via the frequency filter 175. Frequency filter 1
Reference numeral 75 transmits only light having the frequency f30 and light having the frequency f31, and blocks light having other frequencies, that is, error light. The light of frequency f30 and the light of frequency f31 are
6, and the receiver 216 receives a beat signal (frequency f30-f31 = .DELTA.f3-.DELTA.f) based on the interference light.
3 ′) is supplied to the arithmetic unit 201 as a measurement beat signal for measuring the refractive index fluctuation.

On the other hand, light of frequency f10, light of frequency f20 and light of frequency f30, light of frequency f11, light of frequency f11, and frequency f21 emitted from light source 301 along substantially the same optical path.
Is reflected by the dichroic mirror 120 and enters the reference detection unit 321B. As shown in FIG. 6, the reference detection unit 321B
Is exactly the same as the configuration of the measurement detection unit 321A. Therefore, also in the reference detection unit 321B, the signal relating to the optical path fluctuation detected by the receivers 434 to 436 is supplied to the arithmetic unit 201. Also, the receiver 214
Is a beat signal based on the interference light (frequency f10−f11 = Δ
f1−Δf1 ′) is supplied to the arithmetic unit 201 as a reference beat signal for measuring the refractive index fluctuation. Further, the receiver 215 outputs a beat signal (frequency f20-f) based on the interference light.
21 = Δf 2 −Δf 2 ′) is supplied to the arithmetic unit 201 as a reference beat signal for measuring the refractive index fluctuation. Further, the receiver 216 outputs a beat signal (frequency f30) based on the interference light.
−f31 = Δf3−Δf3 ′) is supplied to the computing unit 201 as a reference beat signal for measuring the refractive index fluctuation.

As described above, in the arithmetic unit 201, the receiver 2
An optical path length change .DELTA.D (f1) with respect to the light of frequency f1 is obtained based on the phase change between the reference beat signal for measuring the refractive index variation from 14 and the measured beat signal. Further, based on the phase change between the reference beat signal for measuring the refractive index fluctuation from the receiver 215 and the measured beat signal, the optical path length change ΔD (f2) for the light having the frequency f2 is obtained.
Further, based on the phase change between the reference beat signal for measuring the refractive index fluctuation from the receiver 216 and the measured beat signal, the optical path length change ΔD (f3) for the light of frequency f3.
Ask for. Further, based on signals from the receivers 434 to 436, a change in each optical path mainly due to a change in the emission angle of the light source 312 is detected.

Therefore, in the above equation (11), the displacement amount ΔD (f00) is expressed as ΔD (fa) by ΔD (fa).
(Fb) as an optical path length change ΔD for light of frequency f1
By substituting (f1) for .DELTA.D (fc) with .DELTA.D (f2) for light of frequency f2 and .DELTA.D (fd) for the change in optical path length .DELTA.D (f3) for light of frequency f3, respectively, the reference optical path and the measurement optical path The true displacement amount ΔD of the stage 130 in which the measurement error caused by the change in the refractive index caused by the density fluctuation and the humidity fluctuation of the air inside can be obtained. Thus, in the second embodiment, the measurement error caused by the refractive index fluctuation caused by the density fluctuation and the humidity fluctuation of the air in the reference light path and the measurement light path is corrected, and the true displacement amount ΔD of the movable mirror 140 and the stage 130 are corrected. True displacement Δ
D can be determined with high accuracy.

FIG. 7 is a diagram schematically showing the configuration of an optical interference measuring apparatus according to a third embodiment of the present invention. FIG. 8 is a diagram schematically showing an internal configuration of the light source unit 302 of FIG. The third embodiment has the same configuration as the first embodiment. However, the light source 31 for length measurement in the first embodiment is used.
0 and one light source unit 300 in the second embodiment.
The difference is that it is replaced by 02. 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. Hereinafter, the third embodiment will be described focusing on the differences from the first embodiment.

The light wave interference measuring apparatus shown in FIG.
It has. Referring to FIG. 8, the light source 313 of the light source unit 302 emits light having a frequency f1 and light having a frequency f2 (f2 = 2f1), which is the second harmonic thereof, along optical paths substantially parallel to each other. . Note that both the light having the frequency f1 and the light having the frequency f2 have a polarization direction parallel to the plane of FIG. 7 and FIG. Hereinafter, for simplicity of explanation, light having a polarization direction parallel to the paper surface of FIGS. 7 and 8 is referred to as p-polarized light, and light having a polarization direction perpendicular to the paper surface is referred to as s-polarized light. The light of the frequency f1 is transmitted to the wave plate 195.
, And enters the polarization beam splitter 400. The wave plate 195 appropriately rotates the polarization direction of the incident light of the frequency f1 according to the arrangement of the polarization splitting surface of the polarization beam splitter 400. As a result, the light having the frequency f1 is generated by the action of the wave plate 195 and the polarizing beam splitter 400.
The light is separated into two lights whose polarization directions are orthogonal, that is, p-polarized light and s-polarized light.

That is, the s-polarized light having the frequency f 1 reflected by the polarization beam splitter 400 is reflected by the frequency shifter 41.
1, the frequency f transmitted through the polarizing beam splitter 400
The 1 p-polarized light enters the frequency shifter 410. The intensity ratio between the s-polarized light having the frequency f1 reflected by the polarization beam splitter 400 and the p-polarized light having the frequency f1 transmitted through the polarization beam splitter 400 is determined by the combination of the wave plate 195 and the polarization beam splitter 400. It can be adjusted appropriately. As will be described later, the p-polarized light having the frequency f1 transmitted through the polarization beam splitter 400 is frequency-modulated into light having the frequency f10, and then SH is measured by the measurement detection unit 320A and the reference detection unit 320B.
The frequency is converted by the G conversion element 181. Since the frequency conversion requires a large light intensity, the intensity of the p-polarized light having the frequency f1 transmitted through the polarization beam splitter 400 is smaller than the intensity of the s-polarized light having the frequency f1 reflected by the polarization beam splitter 400. Wave plate 1
It is preferable to combine the polarization beam splitter 95 with the polarization beam splitter 400.

The frequency shifter 411 slightly modulates the light of the frequency f 1 in the s-polarized state,
11 (f11 = f1 + Δf1 '). In the frequency shifter 410, the light having the frequency f1 in the p-polarized state is slightly frequency-modulated, and the frequency f10 (f10 = f1 + Δf)
1) Be light. The drive output of the driver of the frequency shifter 411 and the drive output of the driver of the frequency shifter 410 are supplied to the arithmetic unit 202 as reference signals for measuring the displacement of the movable mirror 141.

On the other hand, the light having the frequency f2 is incident on the SHG conversion element 184, and a part of the light having the frequency f2 is converted by the SHG conversion element 184 into light having the frequency f3 (f3 = 2f2), and the rest is converted into the SHG conversion light. The light passes through the element 184 as it is. The SHG conversion element 184 has KDP (KH
₂ PO ₄₎ can be used a non-linear optical crystal such as. The light of frequency f2 and the light of frequency f3 emitted from the SHG conversion element 184 enter the dichroic mirror 126 as a frequency separation element along substantially the same optical path.

The light of the frequency f 2 transmitted through the dichroic mirror 126 enters the frequency shifter 412, and the light of the frequency f 3 reflected by the dichroic mirror 123 enters the frequency shifter 413. In the frequency shifter 412, the light of the frequency f2 is slightly frequency-modulated to become light of the frequency f20 (f20 = f2 + .DELTA.f2). In the frequency shifter 413, the light having the frequency f3 is slightly frequency-modulated to become light having the frequency f30 (f30 = f3 + .DELTA.f3).

The light having the frequency f10 and the light having the frequency f11 are coupled along substantially the same optical path by the action of the polarizing beam splitter 401. Further, the light having the frequency f20 and the light having the frequency f30 are coupled along substantially the same optical path by the action of the dichroic mirror 115 which is a frequency coupling element. Thus, light of frequency f10 in the p-polarized state,
The light of the frequency f20, the light of the frequency f30, and the light of the frequency f11 in the s-polarized state are coupled along substantially the same optical path by the action of the dichroic mirror 116, which is a frequency coupling element, and are emitted from the light source unit 302. .

The light having the frequency f10, the light having the frequency f11, and the light having the frequency f emitted from the light source 302 along substantially the same optical path.
The light having the frequency of 20 and the light having the frequency of f30 enter the polarization beam splitter 167 via the dichroic mirror 102 which is a frequency separation element. Polarizing beam splitter 167
Are arranged to transmit p-polarized light and reflect s-polarized light. Therefore, the light of the frequency f10, the light of the frequency f20, and the light of the frequency f30 in the p-polarized state pass through the polarizing beam splitter 167, and the light of the frequency f11 in the s-polarized state is reflected by the polarizing beam splitter 167.

The s-polarized light having the frequency f 11 reflected by the polarization beam splitter 167 becomes circularly polarized light via the wave plate 193, and then enters the fixed mirror 151 formed of a plane mirror via the frequency filter 176. The circularly-polarized light having the frequency f11 reflected by the fixed mirror 151 along the same optical path as the incident optical path becomes p-polarized light via the frequency filter 176 and the wavelength plate 193, and returns to the polarization beam splitter 167. The p-polarized light having the frequency f11 returned to the polarization beam splitter 167 passes through the polarization beam splitter 167,
The light enters the polarization beam splitter 168. Like the polarization beam splitter 167, the polarization beam splitter 168 is arranged to transmit p-polarized light and reflect s-polarized light. Therefore, the polarization beam splitter 1
The p-polarized light having the frequency f11 transmitted through the light 68 enters the corner cube 135 via the frequency filter 177.

The p-polarized light having the frequency f 11 reflected by the corner cube 135 returns to the polarization beam splitter 167 via the frequency filter 177 and the polarization beam splitter 168. The p-polarized light having the frequency f11 returned to the polarization beam splitter 167 is
After passing through 67 and becoming circularly polarized light via the wavelength plate 193, the light enters the fixed mirror 151 again via the frequency filter 176. The circularly polarized light having the frequency f11 reflected by the fixed mirror 151 is transmitted to the frequency filter 176 and the wave plate 193.
, And returns to the polarization beam splitter 167. Frequency f11 returned to polarization beam splitter 167
The s-polarized light is reflected by the polarization beam splitter 167 and is a dichroic mirror 103 serving as a frequency separation element.
It is injected toward. Note that the polarization beam splitter 1
The reference optical path is surrounded by an air tube 330 in order to keep the refractive index fluctuation in the reference optical path between the 67 and the fixed mirror 151 small.

The p transmitted through the polarizing beam splitter 167
The light of the frequency f10, the light of the frequency f20, and the light of the frequency f30 in the polarization state are converted into circularly polarized light via the wave plate 194, and then enter the moving mirror 141 formed of a plane mirror. In addition,
The movable mirror 141 is attached to a stage (not shown) that can move along the arrow direction in the figure. The light of the frequency f10, the light of the frequency f20, and the light of the frequency f30, which are reflected by the movable mirror 141 along the same optical path as the incident optical path, become the s-polarized light via the wave plate 194, and become the polarization beam splitter 167. Return to Polarizing beam splitter 167
The light of frequency f10 in the s-polarized state and the frequency f20
And the light of the frequency f30 are polarized beam splitter 1
The light is reflected by 67 and enters the polarization beam splitter 168. The light having the frequency f10, the light having the frequency f20, and the light having the frequency f30 in the s-polarized state reflected by the polarization beam splitter 168 enter the polarization beam splitter 169.
The polarization beam splitter 169 transmits p-polarized light and transmits light of s, like the polarization beam splitters 167 and 168.
It is arranged to reflect polarized light. Therefore, the light having the frequency f10, the light having the frequency f20, and the light having the frequency f30 in the s-polarized state reflected by the polarization beam splitter 169 enter the corner cube 136.

The light of the frequency f10, the light of the frequency f20, and the light of the frequency f30, which are in the s-polarized state, reflected by the corner cube 136, are polarized by the polarization beam splitters 169 and 1.
Returning to the polarizing beam splitter 167 via 68. The light having the frequency f10, the light having the frequency f20, and the light having the frequency f30 in the s-polarized state that have returned to the polarization beam splitter 167 are reflected by the polarization beam splitter 167, and
After becoming circularly polarized light via 94, it enters the movable mirror 141 again. The light having the frequency f10, the light having the frequency f20, and the light having the frequency f30 in the circularly polarized state reflected by the movable mirror 141 become p-polarized light via the wave plate 194, and return to the polarization beam splitter 167. The light of the frequency f10, the light of the frequency f20, and the light of the frequency f30, which are in the p-polarized state and returned to the polarization beam splitter 167,
7 and is emitted toward the dichroic mirror 103 along substantially the same optical path as the light of the frequency f11.

Thus, the light of the frequency f10, the light of the frequency f20, and the light of the frequency f30 in the p-polarized state via the measurement optical path.
And the frequency f11 in the s-polarized state via the reference optical path
Is incident on the dichroic mirror 103 along substantially the same optical path. The dichroic mirror 103
It has the property of transmitting substantially all of the light of the frequency f11 and part of the light of the frequency f10 and reflecting light of other frequencies. Therefore, the s-polarized light having the frequency f11 and the p-polarized light having the frequency f10 transmitted through the dichroic mirror 103 are:
The light enters the frequency filter 178. Frequency filter 17
8 transmits only the light of the frequency f11 and the light of the frequency f10, and blocks the light of other frequencies, that is, the error light.
The reference light having the frequency f11 and the measurement light having the frequency f10 transmitted through the frequency filter 178 are transmitted to the receiver 21 having the polarizer.
7 is incident.

The polarizer of the receiver 217 is arranged at an angle of 45 ° with respect to the polarization direction of the measurement light having the frequency f10 and the polarization direction of the reference light having the frequency f11. Therefore, the measurement light having the frequency f10 and the reference light having the frequency f11 interfere with each other via the polarizer, and the receiver 217 generates a beat signal based on the interference light (frequency f10−f11 = Δf1−Δf1 ′).
Is supplied to the arithmetic unit 202 as a measurement signal for measuring the displacement of the movable mirror 141. In the arithmetic unit 202, the light source unit 302
Is compared with the measurement signal from the receiver 217, and the effect of the refractive index fluctuation is not taken into account of the movable mirror 14
As a result, the displacement .DELTA.D (f1) of the stage is obtained.

The light of the frequency f10, the light of the frequency f20, and the light of the frequency f30 in the p-polarized state reflected by the dichroic mirror 103 are incident on the measurement detection unit 320A. Note that the measurement detection unit 320A and the reference detection unit 32
The configuration of 0B is the same as the configuration of the first embodiment, and is as shown in FIG. Therefore, in the measurement detection unit 320 </ b> A, the signal regarding the optical path fluctuation detected by the receivers 431 to 433 is supplied to the arithmetic unit 202. Further, the receiver 212 outputs a beat signal (frequency f10'−) based on the interference light.
f20 = 2Δf1−Δf2) is supplied to the arithmetic unit 202 as a measurement beat signal for measuring the refractive index fluctuation. Further, the receiver 213 supplies a beat signal (frequency f20'-f30 = 2.DELTA.f2 -.DELTA.f3) based on the interference light to the arithmetic unit 202 as a measurement beat signal for measuring a refractive index fluctuation.

On the other hand, the light of the frequency f10, the light of the frequency f20, and a part of the light of the frequency f30, which are in the p-polarized state and emitted from the light source 302 along substantially the same optical path, are reflected by the dichroic mirror 102. Reference detection unit 32
0B is incident. Therefore, also in the reference detection unit 320B, the signal regarding the optical path fluctuation detected by the receivers 431 to 433 is supplied to the arithmetic unit 202. Further, the receiver 212 outputs a beat signal (frequency f1) based on the interference light.
0′−f20 = 2Δf1−Δf2) is supplied to the arithmetic unit 202 as a reference beat signal for measuring the refractive index fluctuation.
Further, the receiver 213 supplies the arithmetic unit 202 with a beat signal (frequency f20'-f30 = 2.DELTA.f2-.DELTA.f3) based on the interference light as a reference beat signal for measuring the refractive index fluctuation.

As described above, the arithmetic unit 202 includes the receiver 2
12, the optical path length change ΔD (f2) for the light of frequency f2 and the optical path length change ΔD (f1) for the light of frequency f1 based on the phase change between the reference beat signal for measuring the refractive index fluctuation and the measured beat signal. ), Ie, ｛ΔD
(F2) -ΔD (f1)} is obtained. Also, receiver 2
13, the optical path length change ΔD (f3) for the light of frequency f3 and the optical path length change ΔD (f2) for the light of frequency f2 based on the phase change between the reference beat signal for measuring the refractive index fluctuation and the measured beat signal. ), Ie, ｛ΔD
(F3) -ΔD (f2)} is obtained. Further, based on signals from the receivers 431 to 433, the light source 3
The change of each optical path resulting from the change of the exit angle of No. 12 is detected.

Thus, in the third embodiment, the displacement ΔD
(F1) and ｛ΔD (f2) −ΔD (f1
)｝, And ｛ΔD (f3) −ΔD (f2
) It is necessary to obtain the true displacement ΔD of the movable mirror 141 in which the measurement error caused by the refractive index fluctuation caused by the density fluctuation and the humidity fluctuation of the air in the measurement optical path is corrected based on the signal regarding｝. That is, in the first embodiment and the second embodiment, the measurement error caused by the refractive index fluctuation is corrected based on the light having three different frequencies from the light used in the length measuring interferometer. In the example, one of the three light beams having different frequencies used for correcting the measurement error caused by the refractive index fluctuation is used in combination with the length measuring interferometer.

Therefore, in the third embodiment, the true displacement ΔD is obtained according to the following equation (12) obtained by modifying the arithmetic equation (11) used in the first and second embodiments.

ΔD = ΔD (fb) −A ′ ｛ΔD (fc) −ΔD (fb)｝ − B ′ ｛ΔD (fd) −ΔD (fc)｝ (12) where A ′ = [F (fb ) (G (fd) -G (fc))-G (fb) (F (fd) -F (fc)] / [(F
(fc) -F (fb)) (G (fd) -G (fc))-(F (fd) -F (fc)) (G (fc) -G (f
b))] B '= [G (fb) F (fc) -F (fb) G (fc)] / [(F (fc) -F (fb)) (G (f
d) -G (fc))-(F (fd) -F (fc)) (G (fc) -G (fb))]

As described above, ΔD (fb) of the first term on the right side of the equation (12) is calculated based on the comparison between the reference signal from the light source 302 and the measurement signal from the receiver 217.
D (f1). Equation (12)
{ΔD (fc) −ΔD (fb)} of the second term on the right-hand side of the right-hand side indicates the change in the optical path length based on the phase change between the reference beat signal for measuring the refractive index fluctuation from the receiver 212 and the measured beat signal. The difference {ΔD (f2) −ΔD (f1)} can be obtained. Further, ｛Δ of the third term on the right side of equation (12)
D (fd) −ΔD (fc)｝ is a difference ΔΔD of an optical path length change based on a phase change between a reference beat signal for measuring refractive index fluctuation from the receiver 213 and a measured beat signal.
(F3)-. DELTA.D (f2)}.

As described above, also in the third embodiment, the measurement error caused by the refractive index fluctuation caused by the density fluctuation and the humidity fluctuation of the air in the measuring optical path is corrected, and the true displacement ΔD of the movable mirror 141 and the stage The true displacement ΔD can be obtained with high accuracy. In the third embodiment, the displacement of the movable mirror 141 is measured by using the light having the frequency f1. However, in principle, the light having the frequency f2 can be measured by using the light having the frequency f2.
The measurement can be performed using the light of No. 3. However, 3
The movable mirror 14 is formed by using light having the lowest frequency (light having the frequency f1 in the third embodiment) of the light having two frequencies.
It is preferable to measure the displacement amount of No. 1. Hereinafter, the reason will be described.

In general, in a length measuring interferometer, there is a problem that the measurement accuracy is reduced due to the extinction ratio of the polarizing beam splitter. Therefore, in the third embodiment, the extinction ratio as a whole is improved by combining the three polarization beam splitters 167, 168 and 169. However, since the extinction ratio of the polarizing beam splitter is finite, light that should originally pass through the polarizing beam splitter is reflected or light that should be reflected by the polarizing beam splitter is transmitted. As a result, the light reflected at the place where it should be transmitted and the light transmitted at the place where it should be reflected become error light, which lowers the measurement accuracy of the interferometer for length measurement.

In the third embodiment, the frequency f which is reflected at the place where the polarization beam splitter 167 should be transmitted.
The 10 p-polarized lights reach the receiver 212 via the SHG conversion element 181 of the measurement detection unit 320A. Since the intensity of the SHG light converted by the SHG conversion element 181 is proportional to the square of the intensity of the incident light, the conversion efficiency of the light of the frequency f10 is at most about 1% (the light source 313 converts the light of the frequencies f1 and f2 In each case, injection is performed at an intensity of 200 mW.) As a result, the measurement accuracy of the length measuring interferometer is not significantly affected.

On the other hand, when the displacement of the movable mirror 141 is measured using the light of the frequency f2 (or the light of the frequency f3), the error light reflected by the polarization beam splitter 167, that is, the p-polarized light of the frequency f20 ( Or p at frequency f30
The polarized light reaches the receiver 212 (or the receiver 213) without being subjected to the SHG conversion by the SHG conversion element 181 (or the SHG conversion element 182). As a result, the measurement accuracy of the length measuring interferometer is greatly affected. As described above, in the third embodiment, in order to suppress a decrease in measurement accuracy due to the extinction ratio of the polarizing beam splitter, the light of the frequency f1 having the lowest frequency among the lights of the three frequencies is used to move the movable mirror 141. Preferably, the displacement is measured.

In each of the embodiments described above, the length measuring interferometer of the heterodyne interference system is used, but a length measuring interferometer of the homodyne interference system may be used. In the above-described first and third embodiments, the refractive index fluctuation in the measurement optical path is measured, but the refractive index fluctuation in the reference optical path can also be measured. Further, in each of the embodiments described above, the refractive index fluctuation measuring system of the heterodyne interference method is used, but a refractive index fluctuation measuring system of the homodyne interference method can be used. In each of the above-described embodiments, the present invention is applied to the length measuring interferometer. However, the present invention is generally applicable to a measuring optical system for optically measuring the displacement of an object. it can.

[0087]

As described above, according to the present invention, a high-precision light wave interference measurement device capable of correcting a measurement error caused by a change in refractive index caused by a change in density of a gas in an optical path and a change in humidity is realized. can do.

[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 a diagram schematically showing an internal configuration of a light source unit 300 of FIG.

FIG. 3 is a diagram schematically showing an internal configuration of a measurement detection unit 320A and a reference detection unit 320B of FIGS. 1 and 7;

FIG. 4 is a diagram schematically showing a configuration of an optical interference measuring apparatus according to a second embodiment of the present invention.

FIG. 5 is a diagram schematically showing an internal configuration of a light source unit 301 of FIG.

FIG. 6 is a diagram illustrating a measurement detection unit 321A and a reference detection unit 3 in FIG. 4;
It is a figure which shows the internal structure of 21B schematically.

FIG. 7 is a diagram schematically showing a configuration of an optical interference measurement apparatus according to a third embodiment of the present invention.

8 is a diagram schematically showing an internal configuration of a light source unit 302 in FIG.

FIG. 9 is a diagram schematically showing a configuration of a conventional light wave interference measurement device.

[Explanation of symbols]

100, 101, 104-106 Beam splitter 110-118 Frequency coupling element 102, 103, 120-128 Frequency separation element 130, 131 Stage 135, 136 Corner cube 140-142 Moving mirror 145, 146 Mirror 150-152 Fixed mirror 160- 169, 440, 401 Polarization beam splitter 170-178 Frequency filter 180-186 SHG conversion element 190-195 Wave plate 200-203 Operation unit 210-219, 430-436 Receiver 220-228 Frequency filter 300-302 Light source unit 310-315 Light source 320, 321 Detector 330 Air tube

Claims (11)

[Claims] 1. A measuring optical system for optically measuring an amount of displacement of an object, and a refractive index fluctuation for measuring a refractive index fluctuation due to a density fluctuation and a humidity fluctuation of a gas in an optical path of the measuring optical system. A measurement system, wherein the displacement amount of the object measured by the measurement optical system is corrected based on the refractive index fluctuation information of the gas in the optical path measured by the refractive index fluctuation measurement system, whereby the geometry of the object is corrected. A light wave interference measurement device for measuring a target displacement. 2. The measuring optical system is a length measuring interferometer for measuring a displacement amount of a movable mirror along a predetermined direction, wherein the refractive index fluctuation measuring system has a first frequency having different frequencies from each other. A first light for outputting the light, the second light and the third light along substantially the same optical path;
A light source unit, and a frequency of the first light among the first light and the second light passing through a predetermined optical path of the length measuring interferometer is set to the second light.
A first interference light generation system for generating a first interference light by making the frequency substantially equal to the frequency of the light, and the second light and the third light passing through the predetermined optical path of the length measuring interferometer. A second interference light generation system for generating a second interference light by causing a frequency of the second light of the light to substantially match a frequency of the third light, wherein the first interference light and the second Based on the second interference light, the refractive index fluctuation due to the density fluctuation and humidity fluctuation of the gas in the predetermined optical path of the length measuring interferometer is measured, and the displacement of the movable mirror measured by the length measuring interferometer is measured. The optical interference measuring apparatus according to claim 1, wherein the amount is corrected. 3. The interferometer for length measurement, based on any one of the first light, the second light, and the third light emitted from the first light source unit, The optical interference measuring apparatus according to claim 2, wherein the displacement amount of the movable mirror is measured. 4. The first light source unit includes: a light source that supplies a fourth light and a fifth light that is a harmonic of the fourth light; and a part of the fourth light from the light source to a harmonic. Converted,
A first harmonic conversion element for emitting the harmonic as sixth light, a first frequency modulation element for slightly modulating the frequency of the fourth light and emitting the first light, A second frequency modulation element for slightly modulating the frequency of the fifth light and emitting the second light; and a second frequency modulation element for slightly modulating the frequency of the sixth light and emitting the third light. The first interference light generation system has a second harmonic conversion element for converting a part of the first light into a harmonic, and the second interference light generation system has a second harmonic conversion element. The first interference light is generated by heterodyne interference between a harmonic obtained through a wave conversion element and the second light, and the second interference light generation system harmonically modulates a part of the second light. Having a third harmonic conversion element for converting into a wave, and via the third harmonic conversion element Optical interference measuring apparatus according to claim 2 or 3, characterized in that to generate the second interference light obtained harmonic by heterodyne interference between the third light. 5. The fifth light is a second harmonic of the fourth light, and the first harmonic converter converts a part of the fourth light into a second harmonic.
The second harmonic conversion element converts a part of the first light into a second harmonic.
The third harmonic conversion element converts a part of the second light into a second harmonic.
The light wave interference measurement device according to claim 4, wherein the light wave interference measurement device converts the light into harmonics. 6. The first light source unit includes: a light source that supplies the first light and the second light that is a harmonic of the first light; and a part of the second light from the light source that is a harmonic. Converted into waves,
A fourth harmonic conversion element for emitting the harmonic as the third light, wherein the first interference light generation system is configured to convert a part of the first light into a harmonic. A fifth harmonic conversion element, wherein the first interference light is generated by homodyne interference between a harmonic obtained through the fifth harmonic conversion element and the second light; The generation system has a sixth harmonic conversion element for converting a part of the second light into a harmonic, and the harmonic obtained through the sixth harmonic conversion element and the third harmonic The optical interference measuring apparatus according to claim 2, wherein the second interference light is generated by homodyne interference with light. 7. The second light is a second harmonic of the first light, and the fourth harmonic converter converts a part of the second light into a second harmonic.
The fifth harmonic conversion element converts a part of the first light into a second harmonic.
The sixth harmonic conversion element converts a part of the second light into a second harmonic.
7. The light wave interference measurement device according to claim 6, wherein the light wave interference measurement device converts the light into harmonics. 8. The measuring optical system is a length measuring interferometer for measuring a displacement amount of a movable mirror along a predetermined direction, and the refractive index variation measuring system has a first frequency having different frequencies from each other. Light, second light and third light, fourth light slightly different in frequency from the first light, fifth light slightly different in frequency from the second light, and third light 6th with slightly different frequency from light
And a second light source unit for outputting the first light and the first light via a reference optical path of the length measuring interferometer through a substantially same optical path. A first interference light generation system for generating a first interference light with the fourth light, and a second interference light generation system for transmitting the second light and the length measurement interferometer through a reference optical path of the length measurement interferometer. A second interference light generation system for generating a second interference light with the fifth light via a measurement optical path; and the third light and the length measurement via a reference optical path of the length measuring interferometer A third interference light generation system for generating a third interference light with the sixth light via the measurement optical path of the interferometer for use in the first, the second, and the third interference light. Based on the interference light, measure the refractive index fluctuation due to density fluctuation and humidity fluctuation of the gas in the reference light path and the measurement light path of the length measuring interferometer, 2. The optical interference measuring apparatus according to claim 1, wherein the displacement amount of the movable mirror measured by an interferometer is corrected. 9. The second light source unit includes: a light source that supplies a seventh light and an eighth light that is a harmonic thereof; and a part of the seventh light from the light source to a harmonic. Converted,
A first harmonic conversion element for emitting the harmonic as a ninth light; and a first harmonic conversion element for slightly modulating a part of the seventh light by a first modulation frequency and emitting the first light. A first frequency modulation element, and a second frequency modulation element for slightly modulating the frequency of the remainder of the seventh light by a second modulation frequency different from the first modulation frequency and emitting the fourth light as the fourth light A third frequency modulating element for slightly modulating a part of the eighth light by a third modulation frequency and emitting the second light, and A fourth frequency modulation element for slightly modulating the frequency by a fourth modulation frequency different from the modulation frequency and emitting the fifth light, and a part of the ninth light is slightly frequency-modulated by a fifth modulation frequency. Modulated and emitted as the third light And a sixth frequency for slightly modulating the remainder of the ninth light by a sixth modulation frequency different from the fifth modulation frequency and emitting the sixth light as the sixth light. The optical interference measuring apparatus according to claim 8, further comprising a modulation element. 10. The eighth light is a second light of the seventh light.
The first harmonic conversion element converts a part of the second light into a second harmonic.
The light wave interference measurement device according to claim 9, wherein the light wave interference measurement device converts the light into harmonics. 11. The apparatus according to claim 1, further comprising an optical path fluctuation detecting means for detecting an optical path fluctuation of at least one of the plurality of lights.
The light wave interference measurement device according to the item.