JPH07174510A - Device and method for measuring length - Google Patents

Abstract

PURPOSE:To perform the highly accurate measurement of displacement by using a conventional length measuring device, and correcting the effect of the fluctuation of refractive index caused by the fluctuation of the air and the like. CONSTITUTION:Laser light 12 is split into a transmitted luminous flux 12a and a reflected luminous flux 12b with a polarization beam splitter 2. The transmitted luminous flux 12a advances toward a detector 6. The reflected luminous flux 12b is transmitted through a 1/4 wavelength plate 7 and a freely expandable vacuum container 3, reflected from a moving mirror 4, transmitted through the polarization beam splitter 2 again, transmitted through a 1/4 wavelength plate 8, reflected from a fixed mirror 9, returned to the original light path, reflected from the polarization beam splitter 2 and directed to the detector 6. At this time, the transmitted luminous fluxes 12a and 12b are overlapped and made to interfere, and the interference fringes are formed. The length between the vacuum container 3 and the moving mirror 4 or the length of the light path in the vacuum container 3 is measured based on the interference fringes. The error caused by the fluctuation of the refractive index in the length measuring environment is removed by correcting the measured length value based on the measured value described above.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a length measuring apparatus and a length measuring method suitable for use in equipment requiring high-accuracy length measurement, such as an aspherical surface processing machine.

[0002]

2. Description of the Related Art With the progress of ultra-precision machining technology, highly accurate displacement measurement using light wave interferometry is widely used. The displacement measurement by the light wave interferometry enables very high resolution measurement, but the measurement value has an error due to the change of the refractive index of the measurement optical path portion due to the environmental change. Therefore, in general, environmental values (temperature, atmospheric pressure, temperature, etc.) are measured, and the refractive index of air is corrected by the formula derived by Edlen.

Further, as a method for reducing the influence of the fluctuation of the refractive index in the measuring environment, the measuring optical path is decompressed, a gas having a low refractive index is filled in the optical path, and the measuring optical path is evacuated. Etc. are taken.

[0004]

However, in the method of measuring the environmental value and correcting the refractive index of air, the fluctuation frequency due to the air fluctuation of the environmental value is high, and the fluctuation of the refractive index is completely corrected. There was a problem that it was difficult to do. Further, the environmental value needs to be measured in the vicinity of the length measuring unit, but in reality, it is difficult to accurately measure the environmental value of the optical path portion due to mechanical restrictions. On the other hand, in the method of filling the gas with low refractive index by vacuuming or decompressing the measuring optical path,
Since it is necessary to bring the vacuum container into contact with the object to be measured, there is a problem that it cannot be used when measuring the position of a two-dimensionally moving object such as an XY stage.

Therefore, the present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to use a conventional length-measuring device to prevent a change in refractive index due to air fluctuations or the like. An object of the present invention is to provide a length measuring device and a length measuring method capable of correcting an influence and performing highly accurate displacement measurement.

[0006]

In order to achieve the above object, a length measuring method according to a first aspect of the present invention divides light emitted from a light source into measuring light and reference light, and measures the optical axis direction of the measuring light. The measurement light reflected by the movable mirror installed on the movable measurement object and the reference light reflected by the reference surface are caused to interfere with each other, and the interference light is detected to measure the distance between the reference point and the movable mirror. In the length measuring method, which has a window made of a transparent material through which the measurement light passes at both ends, the inside is provided with an expandable and contractible vacuum container forming a part of the optical path of the measurement light, and the optical path of the vacuum container By measuring the length or the length between the vacuum container and the movable mirror and correcting the measurement result with this measurement value, the error in the measurement result caused by the fluctuation of the refractive index of the measurement environment is reduced. To remove.

A length measuring apparatus according to a second aspect of the present invention splits light emitted from a light source into measuring light and reference light, and a movable mirror installed on a measuring object movable in the optical axis direction of the measuring light. Interference between the measurement light reflected by and the reference light reflected by the reference surface,
In a length measuring device that measures the distance between a reference point and the movable mirror by detecting the interference light, a window made of a transparent material through which the measurement light passes is provided at both ends, and the inside is one of the optical paths of the measurement light. A stretchable vacuum container forming a part, a measuring means for measuring the length of the optical path in the vacuum container or the length between the vacuum container and the movable mirror, and the measurement result by the measurement value by this measuring means. And an arithmetic unit for removing the error in the length measurement result caused by the fluctuation of the refractive index of the length measurement environment.

A length measuring apparatus according to a third aspect of the invention splits light emitted from a light source into measuring light and reference light, and is a movable mirror installed on a measuring object movable in the optical axis direction of the measuring light. Interference between the measurement light reflected by and the reference light reflected by the reference surface,
In a length measuring device that measures the distance between a reference point and the movable mirror by detecting this interference light, a window made of a transparent material through which the measurement light passes is provided at both ends, and the inside is the optical path of the measurement light. An expandable and contractible vacuum container forming a part, a measuring means for measuring the length of the optical path in the vacuum container or the length between the vacuum container and the movable mirror, and the measuring value by the measuring means. Computation means for correcting the length result to remove the error in the measurement result caused by fluctuation of the refractive index of the measurement environment, and expanding and contracting the vacuum container in the optical axis direction of the measurement light based on the calculation result by the calculation means. It is provided with a control device.

A length measuring apparatus according to a fourth aspect of the present invention is the measuring apparatus according to the second or third aspect of the invention, wherein the control unit expands and contracts the vacuum container.

A length measuring apparatus according to a fifth aspect of the present invention splits light emitted from a light source into measuring light and reference light, and a movable mirror installed on a measuring object movable in the optical axis direction of the measuring light. Interference between the measurement light reflected by and the reference light reflected by the reference surface,
A length measuring device for measuring the distance between a reference point and the movable mirror by detecting the interference light, which has first and second windows made of a transparent material through which the measuring light passes, and the inside of which is used for the measurement. An expandable and contractible vacuum container that forms part of the optical path of light; an interferometer that guides the first and second measurement light beams to the vacuum container;
The second window is provided on a moving mirror of the vacuum container, and a reflecting surface is provided on a portion of the surface facing the moving mirror, the portion corresponding to the first measuring light flux. Is formed, and a portion corresponding to the second measurement light beam is a light transmitting portion, and the interferometer divides each of the first and second measurement light beams into a transmission light beam and a reflection light beam. A polarization beam splitter that guides the reflected light flux to the vacuum container, a quarter-wave plate that rotates the polarization plane of the reflected light flux, and each transmitted light flux that has passed through the polarization beam splitter is reflected back to the polarization beam splitter. No. 1 reflecting mirror, the polarizing beam splitter, and the quarter-wave plate, respectively, and one of them impinges on the reflecting surface of the second window of the vacuum container to be reflected and returns to the original optical path to return to the polarizing beam splitter. Transmitted, the second the other of said vacuum vessel
A second reflected light beam that returns through the light transmitting portion of the window and returns from the original light path that is reflected by the movable mirror and that passes through the polarization beam splitter and returns to the polarization beam splitter.
And one of the two reflected light beams reflected by the second reflecting mirror, which is reflected by the second reflecting mirror, again enters the vacuum chamber through the polarizing beam splitter and the quarter wavelength plate and is reflected again by the second window. By passing through the original optical path reflected by the surface and hitting the polarizing beam splitter and reflecting it, one of the two transmitted light beams that first transmitted through the polarizing beam splitter (superposed light beam of the first measuring light beam) is superimposed and interferes. The fringes are formed, and the other passes through the polarization beam splitter, the quarter-wave plate and the vacuum container again and is reflected by the moving mirror again, passes through the original optical path, and strikes the polarization beam splitter to be reflected, thereby reflecting the polarization beam splitter. The interference fringes are formed by being superposed on the other of the two transmitted light fluxes that have been transmitted first (transmission light flux of the second measurement light flux). The means measures the displacement between the interferometer and the movable mirror and the displacement between the interferometer and the second window by the interference fringes of the first and second measuring light beams, and the arithmetic unit is the measuring means. The measurement result is corrected by the displacement of any one of the measurement values according to 1. to eliminate the error in the measurement result caused by the fluctuation of the refractive index of the measurement environment.

[0011]

According to the present invention, the length between the vacuum container and the movable mirror or the length of the optical path in the vacuum container is measured, and the measured value is corrected by the measured value to adjust the refractive index variation in the measuring environment. Since the error is removed, highly accurate displacement measurement is possible.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the embodiments shown in the drawings. First, the basic principle of the present invention will be described with reference to FIG. 1A and 1B are schematic configuration diagrams of a Michelson-type laser interferometric length measuring device, which measures the displacement of a movable mirror 4 attached to an XY stage (measurement target) 5. . 1 is a laser light source, 2 is a polarization beam splitter, 3 is a vacuum container, and 6 is a detector. Of the two surfaces of the polarization beam splitter 2 facing each other, the quarter-wave plate 7 is fixed to one surface facing the movable mirror 4, and the quarter-wave plate 8 and the fixed mirror 9 are fixed to the other surface. It is fixed. The vacuum container 3 has a fixed cylinder 3A and a movable cylinder 3B movable in the axial direction of the fixed cylinder 3A, and the internal air pressure is reduced to 1 KPa or less.
The closed end surface of B has a first and second transparent material, respectively.
Windows 10 and 11 are respectively provided. Moving mirror 4
Are arranged on the XY stage 5 so as to face the second window 11 on the extension of the axis of the vacuum container 3. The detector 6 is arranged on the opposite side of the laser light source 1 with the polarization beam splitter 2 interposed therebetween.

In such a configuration, when the laser light 12 emitted from the laser light source 1 is incident on the polarization beam splitter 2, the dielectric multi-layer film 2A causes two measuring light beams (transmission linearly polarized light) whose polarization directions are orthogonal to each other. And reflected linearly polarized light) 12a and 12b, one measuring light beam 12a goes straight through the polarization beam splitter 2 toward the detector 6, and the other measuring light beam 12b is reflected to 1
/ 4 wavelength plate 7 and vacuum container 3 are reflected, and are reflected by the moving mirror 4 to return to the same optical path, and by passing through the ¼ wavelength plate 7, the polarization plane is rotated by 90 ° with respect to the incident light. Therefore, the light passes through the polarization beam splitter 2. And
This measurement light beam 12b passes through the quarter-wave plate 8 and is reflected by the fixed mirror 9, and again passes through the quarter-wave plate 8 to rotate the polarization direction by 90 °. It is reflected and enters the detector 6. For this reason,
The two measuring light beams 12a and 12b are superposed on each other and interfere with each other, and form an image on the image sensor of the detector 6 to form interference fringes. Then, the distance (Lv +) from the first window 10 to the moving mirror 4 is measured by measuring the change in brightness of the interference fringes.
La) can be measured.

As shown in (a), the length of the vacuum container 3 when the interference fringe measuring device of the length measuring device is reset is Lv,
The distance between the second window 11 of the vacuum container 3 and the movable mirror 4 is La. Consider a case where the length of the vacuum container 3 changes by dv and the distance between the second window 11 and the movable mirror 4 changes by da as the XY stage 5 moves, as shown in FIG. At this time, the displacement Xd obtained by measuring the interference fringes with the length measuring optical path being entirely vacuum
And the difference Dx of the actual displacement (dv + da) of the movable mirror 4 can be calculated by the following equation. Dx = {(Lv + dv) / λv + (La + da) / λa} · λv− (Lv + La + dv + da) = (La + da) · (na−1) ··· (1) However, Dx: all measuring optical paths are obtained as a vacuum. Difference between displacement and actual displacement of movable mirror 4 (correction value) Lv: Vacuum container 3 when the interference fringe measuring device is reset
La: Distance between the second window 11 and the moving mirror 4 when the interference fringe measuring device is reset dv: Change in length of the vacuum container 3 da: Second window 11 and the moving mirror 4 Variation of the distance between λv: wavelength of laser light in vacuum λa: wavelength of laser light in air na: refractive index of air at the wavelength of laser light

Therefore, the distance La between the second window 11 and the movable mirror 4 when the interference fringe measuring device is reset, the second window 1
The correction value Dx is obtained from the amount of change da in the distance between the movable mirror 4 and the moving mirror 4 and the value of the refractive index na of the air at the wavelength of the laser light, and this value is obtained by the displacement (dv + da ± D
The correct displacement (dv + da) of the movable mirror 4 can be obtained by subtracting from x).

Now, consider a case where the correct displacement of the movable mirror 4 is measured by measuring the distance between the second window 11 and the movable mirror 4 using another measuring means to obtain the correction value Dx. (La + d
Since the measurement error in a) and the value of the refractive index na of the air at the wavelength of the laser light include an error, Dx also includes an error. The measurement error of (La + da) is δa, and the error of na is δ
The error Ex included in Dx is represented by the following equation, where n is n. Ex = Dx− (La + da + δa) · (na + δn−1) = − (La + da) · δn−δa · (na−1) −δa · δn (2) However, Ex: measurement error δa included in the correction value Dx : Error in measurement of distance between second window 11 and movable mirror 4 δn: Error in refractive index na of air at wavelength of laser light In the above two equations, the first term is determined by δn (La + d
The allowable value of a) is shown. The second term represents the allowable value of the measurement error δa of da determined by na. The third term represents the product of the measurement error δa of da and the error δn of the refractive index.

As an example, the measurement error Ex of the correction value Dx is 1n.
δa is calculated when m is less than or equal to (La + da). δn is caused by air fluctuations and environmental changes, but in general, when a temperature change of 1 ° C, a pressure change of 2.5 mmHg, and a relative humidity change of 80% occur, the refractive index changes around na × 10 ^-6. Occurs. Therefore, assuming that the laser beam is HeNe and the length is measured in standard air, δn and na are set to na = 1.0002767, δn = 1.0002767 × 10 ⁻⁶ (3), respectively. When the maximum value of La + da and the allowable value of the measurement error δa of da are calculated, La + da <1 mm, δa <3.6 μm (4) The product of δa and δn in the second term of the above two equations is δa · δn = 3.6 × 10 ⁻³ nm (5), and this value can be sufficiently ignored.

It is easy to expand and contract the vacuum container 3 so that the distance between the second window 11 and the movable mirror 4 becomes 1 mm or less according to the movement of the XY stage 5. In addition, the measurement error of the change in the distance between the second window 11 and the movable mirror 4 is 3.6 μm.
It is also sufficiently feasible to set m. In this example, the calculation error Ex was set to 1 nm or less, but the measurement error Ex
If the permissible value of is increased, the permissible values of La and δa are further increased.

The difference D between the displacement obtained when the length-measuring optical path is entirely exposed to the air and the actual displacement of the movable mirror 4.
The equation for obtaining x (correction value) is shown by the following equation. Dx = {(Lv + dv) / λv + (La + da) / λa} · λa− (Lv + La + dv + da) = (Lv + dv) · {(1 / na) -1} ··· (1 ′)

Next, an embodiment of a length measuring device for realizing the principle of the present invention will be described. 2 is a cross-sectional view showing an embodiment of the length measuring device, FIG. 3 is a perspective view of an interferometer and an optical element, FIG. 4 is a view showing a beam splitter and a measuring light beam, and FIG. 5 is a configuration of the interferometer and a measuring beam. FIG. 6 is a diagram showing a luminous flux, and FIG. 6 is a perspective view of the second window. In the figure, the same components as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted. This embodiment uses a double-pass interferometer to measure the displacement (L) of the interferometer 22 and the movable mirror 4 when the length measurement optical path is regarded as a vacuum.
1) and the displacement (L2) between the interferometer 22 and the second window 11 of the vacuum container 3 are simultaneously measured.

In FIG. 2, reference numeral 20 denotes an arithmetic unit which simultaneously measures the displacements L1 and L2 by the interference fringes of the laser light and corrects the length measurement value by using one of these measured values. By doing so, the error due to the fluctuation of the refractive index in the measurement environment is removed, and the correct measurement value is calculated. Reference numeral 21 is a control device that is driven by an output signal from the arithmetic device 20 to expand and contract the vacuum container 3, 23 is a beam splitter, 24 and 25 are reflecting mirrors, and 26 is a control device 2.
It is a hollow motor that is driven by the output signal from 1 to expand and contract the vacuum container 3.

The interferometer 22 is arranged in the optical path between the laser light source 1 and the vacuum chamber 3, and has a quarter wavelength plate 7, a polarization beam splitter 2, and first and second reflecting mirrors (corner cubes). It has 28 and 29. In addition, the interferometer 22
On the surface facing the laser light source 1, the four transparent parts 30 (3
0a to 30d), two of which are light incident portions 30a and 30b, and the other two are light emitting portions 30c and 30d.
Is formed. The first and second reflecting mirrors 28 and 29 are composed of two plane mirrors orthogonal to each other as shown in FIG.

Of the two surfaces of the second window 11 of the vacuum container 3, the surface facing the movable mirror 4 is the light entrance portion 30b of the interferometer 22.
And only the portion corresponding to the light emitting portion 30d is the transparent portion 32a,
32b is formed, and the other portions are coated with the reflection coat 33 to form a reflection surface.

Laser light 12 emitted from the laser light source 1
Is split into two light beams by the beam splitter 23, that is, first and second measuring light beams 12a and 12b,
These measuring luminous fluxes 12a and 12b are transmitted to the respective light entering portions 30.
The light enters the interferometer 22 from a and 30b. In addition, the first and second measurement light beams 20 a and 20 that have entered the interferometer 22.
As shown in FIG. 5, b is further divided into two by the polarization beam splitter 2, so that the transmitted light fluxes 12a-1 and 12b- that pass through the polarization beam splitter 2 and travel toward the first reflecting mirror (reference surface) 28. 1 and reflected light beams 12a-2, 12b-2 reflected by the polarization beam splitter 2, transmitted through the quarter-wave plate 7 and directed to the vacuum container 3 are split. Each transmitted light beam 1 transmitted through the polarization beam splitter 2
2a-1 and 12b-1 impinge on and reflect the first reflecting mirror 28, pass through the polarization beam splitter 2, and exit the light.
It is emitted from each of c and 30d, is reflected by the reflecting mirrors 24 and 25 (FIG. 3), and is incident on each of the detectors 6A and 6B.

On the other hand, the two reflected light beams 12a-2 and 12b-2 which are reflected by the polarization beam splitter 2 are 1
The light enters the vacuum chamber 3 via the quarter wave plate 7.
One of the reflected light beams 12a-2 returns to the original optical path after being reflected by the reflection coat 33 of the second window 11 and passes through the quarter wavelength plate 7 so that the polarization plane is 90 degrees with respect to the incident direction.
Since it rotates by 90 degrees, it passes through the polarization beam splitter 2, is reflected by the second reflecting mirror 29, and is again reflected by the polarization beam splitter 2.
And again enters the vacuum chamber 3 through the quarter-wave plate 7, is reflected by the reflection coat 33 of the second window 11 and returns to the original optical path, and passes through the quarter-wave plate 7 to make the polarization plane. Is rotated 90 °. Therefore, this reflected light flux 12a
-2 is emitted to the outside of the interferometer 22 from the light emitting unit 30c by hitting and reflecting the polarization beam splitter 2.
The light is reflected by the reflecting mirror 24 and enters the detector 6A. As a result, the transmitted light beam 12a-1 and the reflected light beam 12a-2 described above are superposed on each other and interfere with each other to form an interference fringe. In this way, the reflected light flux 12a-2 is reflected twice by the second window 11, emitted from the interferometer 22, and enters the detector 6A.

Of the two reflected light beams 12a-2, 12b-2 reflected by the polarization beam splitter 2, the other reflected light beam 12b-2 enters the vacuum chamber 3 through the quarter wavelength plate 7 and 2 is transmitted through the transparent portion 32a (FIG. 6) of the window 11 and is reflected by the movable mirror 4 to return to the original optical path,
Since the plane of polarization is rotated by 90 ° with respect to the incident direction by passing through the quarter-wave plate 7, it passes through the polarization beam splitter 2, is reflected by the second reflecting mirror 29, and is again reflected by the polarization beam splitters 2 and 1 /. The light again enters the vacuum chamber 3 via the four-wave plate 7, passes through the transparent portion 32b of the second window 11 and is reflected by the movable mirror 4 to return to the original optical path, and passes through the quarter-wave plate 7. By doing so, the plane of polarization is rotated by 90 °. Therefore, the reflected light beam 12b-2 is reflected by the polarization beam splitter 2
The light is emitted from the light emitting section 30d to the outside of the interferometer 22 by being reflected on the detector 6 and reflected by the reflecting mirror 25.
It is incident on B. As a result, the reflected light beam 12b-2 and the transmitted light beam 12b-1 are superposed on each other and interfere with each other to form an interference fringe. Thus, the reflected light flux 12b-
2 is reflected by the movable mirror 4 twice and emitted from the interferometer 22.
It is incident on the detector 6B.

The arithmetic unit 20 includes the detectors 6A and 6B.
From the interference fringes obtained in step 2, the displacement L1 of the interferometer 22 and the movable mirror 4 and the displacement L2 between the interferometer 22 and the second window 11 of the vacuum container 3 when the length measurement optical path is regarded as a vacuum are measured. These values show the displacement Xd and the variation dv of the length of the vacuum container 3 which are obtained by measuring interference fringes in which the length-measuring optical paths shown in the above principle are all vacuum. Therefore, the displacement Xm of the movable mirror 4 can be obtained by performing the calculation shown in the above equation (1) in the arithmetic unit 20 as follows. Xm = L1-Dx = L1- {La + (L1-L2)} (na-1) ... (6)

At this time, in (L1-L2), since the measuring positions of the measuring light beam 12a and the measuring light beam 12b are on the same axis, no Abbe error occurs. Also, the measurement light flux 1
Since the position of the surface of the second window 11 of the vacuum container 3 on the side of the movable mirror 4 is measured by 2a, the length of the vacuum container 3 can be accurately changed without being affected by the deformation of the second window 11. The quantity L2 can be measured.

Next, even if the XY stage 5 moves, L
An example for making a + da 1 mm or less will be described.
The arithmetic unit 20 continuously calculates the difference between L1 and L2 in addition to the arithmetic operation of the above equation 6, and sends the value to the control unit 21 as an analog signal or a digital signal. Control device 21
Controls the hollow motor 26 by an output signal proportional to the difference between L1 and L2. When the hollow motor 26 rotates, a force in the optical axis direction is generated by the feed screw provided on the outer circumference of the movable barrel 3B of the vacuum container 3, and the movable barrel 3B is expanded and contracted.
As a result, a servo loop is formed in which L1 is the target position and the deviation between the current position and the target position is L1-L2, and control is performed so that the distance between the second window 11 of the vacuum container 3 and the movable mirror 4 becomes constant. To be done. At this time, the second window 1 of the vacuum container 3
Since the accuracy of maintaining the distance between 1 and the movable mirror 4 is 1 mm or less, it can be easily realized. Further, the motion accuracy (straightness, inclination) required when the vacuum container 3 expands and contracts may be extremely low because L1 and L2 are arranged coaxially.

Although the above embodiment uses the laser beam, the present invention is not necessarily limited to this.

[0031]

As described above, according to the length measuring apparatus and the length measuring method of the present invention, the length between the vacuum container and the movable mirror or the length of the optical path in the vacuum container is measured, By correcting the length measurement value with the measured value, the error due to the fluctuation of the refractive index in the length measurement environment was removed, so the length measurement on the order of nanometers, which could not be measured due to the influence of the length measurement environment, This can be easily achieved while maintaining the measurement flexibility of the length measuring device.

[Brief description of drawings]

1A and 1B are configuration diagrams showing a basic configuration of the present invention.

FIG. 2 is a cross-sectional view showing an embodiment of a length measuring device.

FIG. 3 is a perspective view of an interferometer and an optical element.

FIG. 4 is a diagram showing a beam splitter and a measuring light beam.

FIG. 5 is a diagram showing a configuration of an interferometer and a measurement light beam.

FIG. 6 is a perspective view of a second window of the vacuum container.

[Explanation of symbols]

 1 Laser Light Source 2 Polarization Beam Splitter 3 Vacuum Container 4 Moving Mirror 5 XY Stage (Measurement Object) 6 Detector 7 1/4 Wave Plate 8 1/4 Wave Plate 9 Fixed Mirror 10 First Window 11 Second Window 20 Arithmetic device 21 Control device 22 Interferometer 23 Beam splitter 24 Reflecting mirror 25 Reflecting mirror 26 Hollow motor

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Akira Ishida 3 2-3 Marunouchi, Chiyoda-ku, Tokyo Inside Nikon Corporation

Claims (5)

[Claims] 1. A light emitted from a light source is split into a measuring light and a reference light, and the measuring light and the reference surface are reflected by a movable mirror installed on a measuring object movable in the optical axis direction of the measuring light. In a length measuring method of interfering with a reflected reference light and detecting the interference light to measure the distance between a reference point and the movable mirror, a window made of a transparent material through which the measurement light passes is provided at both ends. The inside of the vacuum container is provided with an expandable and contractible vacuum container that forms part of the optical path of the measurement light, and the length of the optical path in the vacuum container or the length between the vacuum container and the movable mirror is measured. By correcting the length measurement result with a value, an error in the length measurement result caused by a change in the refractive index of the length measurement environment is removed. 2. The light emitted from the light source is divided into measurement light and reference light, and the measurement light and the reference surface reflected by a movable mirror installed on a measurement target movable in the optical axis direction of the measurement light In a length measuring device that interferes with a reflected reference light and detects the interference light to measure the distance between a reference point and the movable mirror, a window made of a transparent material through which the measurement light passes is provided at both ends. An expandable vacuum container whose inside forms a part of the optical path of the measuring light, and a measuring means for measuring the length of the optical path in the vacuum container or the length between the vacuum container and the movable mirror, A length measuring device, comprising: a calculation device that corrects the measurement result with a measurement value obtained by a measuring unit to remove an error in the length measurement result caused by a change in a refractive index of a length measuring environment. 3. The light emitted from the light source is split into a measuring light and a reference light, and the measuring light and the reference surface are reflected by a moving mirror installed on a measuring object movable in the optical axis direction of the measuring light. In a length measuring device that interferes with a reflected reference light and detects the interference light to measure the distance between a reference point and the movable mirror, a window made of a transparent material through which the measurement light passes is provided at both ends. An expandable and contractable vacuum container whose inside forms a part of the optical path of the measuring light, and a measuring means for measuring the length of the optical path in the vacuum container or the length between the vacuum container and the movable mirror,
Computation means for correcting the length measurement result by the measurement value by the measurement means to remove an error in the length measurement result caused by the refractive index variation of the length measurement environment, and the vacuum container based on the calculation result by the calculation means. A length measuring device comprising a control device for expanding and contracting the length measuring light in the optical axis direction. 4. The length measuring device according to claim 2 or 3, wherein the control device expands and contracts the vacuum container. 5. The light emitted from the light source is split into a measuring light and a reference light, and the measuring light reflected by a movable mirror installed on a measuring object movable in the optical axis direction of the measuring light and the reference surface. In a length measuring device that interferes with a reflected reference light and detects the interference light to measure the distance between a reference point and the movable mirror, a first and second measuring device made of a transparent material through which the measuring light passes. An expandable and contractible vacuum container having a window and forming a part of the optical path of the measuring light; an interferometer for guiding the first and second measuring light beams to the vacuum container; measuring means; The second window is provided on the moving mirror of the vacuum container, and a reflecting surface is formed on a portion of the surface facing the moving mirror, which corresponds to the first measuring light beam, A portion corresponding to the second measuring light beam is a light transmitting portion, and the interferometer includes the first and second portions. The polarization beam splitter that splits the measurement light flux into a transmitted light flux and a reflected light flux and guides each reflected light flux to the vacuum container, a quarter-wave plate that rotates the polarization plane of the reflected light flux, and the polarization beam splitter. A first reflecting mirror that reflects each transmitted luminous flux and returns to the polarizing beam splitter again, and one that passes through the polarizing beam splitter and the quarter-wave plate, one of which is the reflecting surface of the second window of the vacuum container. The reflected beam returns to the original optical path and returns through the polarized beam splitter, and the other returns through the transparent portion of the second window of the vacuum chamber and reflected by the moving mirror to return to the original optical path. A second reflecting mirror that reflects the two reflected light beams that pass through the splitter and returns the reflected light beams to the polarization beam splitter; and two reflecting mirrors that strike and reflect the second reflecting mirror. One of the emitted light beams is again converted into the polarization beam splitter and the 1/4.
Of the two transmitted light fluxes that first pass through the polarizing beam splitter by being incident on the inside of the vacuum vessel via the wave plate and then again reflected by the reflecting surface of the second window and passing through the original optical path and then being reflected by the polarizing beam splitter. One is superposed on one (the transmitted luminous flux of the first measuring luminous flux) to form an interference fringe, and the other is transmitted again through the polarizing beam splitter, the quarter-wave plate and the vacuum container and is reflected again by the movable mirror. By passing through the optical path and hitting the polarizing beam splitter to be reflected, the other of the two transmitted light beams that first transmitted through the polarized beam splitter is superimposed on the other (the transmitted light beam of the second measuring light beam) to form an interference fringe, The measuring means measures the displacement between the interferometer and the movable mirror and the displacement between the interferometer and the second window by the interference fringes of the first and second measuring light beams. However, the arithmetic unit corrects the length measurement result by displacement of any one of the measured values by the measuring means, and removes an error in the length measurement result caused by a refractive index variation of the length measuring environment. And measuring device.