JPH0599612A - Laser interferometer - Google Patents

Abstract

PURPOSE:To eliminate the influence of air fluctuation so as to make it unnecessary to correct the wavelengths of laser beams by allowing most part of first and second beams to pass through a sealed container. CONSTITUTION:The S- and P-polarized components of a laser beam B0 from a laser generator 1 are converted by the polarized beam splitter 3 of a sealed container 2 into first and second beams B1, B2, respectively. The beam B1 is 4 circularly polarized, 6 reflected, 2b, 8a transmitted, M1 reflected at the sealed space 8d of a sealed container 8, and converted into Ppolarized light as it 4 passes through an inverse optical path; the Ppolarized light is then 3, 2c, 9 transmitted and 11 focused onto a photodetector 10. The beam B2 is 5 circularly polarized, 7 reflected, 2b, 8a transmitted, M2 reflected, and converted into S-polarized light as it 5 passes through an inverse optical path; the S-polarized light is then 3 reflected and 11 focused onto the detector 10. Then the beams B1, B2 interfere with each other. If in this state the container 8 moves in the (x) direction, the difference in optical path length between the beams B1, B2 is varied and the intensity of light which 10 varies accordingly is detected so as to measure the displacement (x) of the container 8. At this time most part of the beams B1, B2 is within the container 8 and is therefore not affected by the wavering of air.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser interferometer for length measurement, for example.

[0002]

2. Description of the Related Art A laser interferometer for length measurement is used to measure relative displacement between two members with high accuracy in a stepper for semiconductor manufacturing, a highly accurate machine tool, or the like.
FIG. 10 shows the configuration of a conventional laser interferometer.
At, the laser beam B ₀ emitted from the laser oscillator 1 is incident on the polarization beam splitter (PBS) 3. The laser beam B ₀ has a polarization component (P polarization component) parallel to the paper surface of FIG. 10 and a polarization component (S polarization component) perpendicular to the paper surface. Specifically, for example, the oscillation state of the laser oscillator 1 is linearly polarized, and the plane of polarization of the laser beam B ₀ is 45 ° around the optical axis with respect to the polarization beam splitter 3 from a direction perpendicular to the paper surface of FIG. When rotating, or when the oscillation state of the laser oscillator 1 is circularly polarized, the laser beam B ₀ has both polarization components.

In this case, the P-polarized component of the laser beam B ₀ , for example, passes through the joint surface of the polarization beam splitter 3 and becomes the reference beam Br. This reference beam Br
Is transmitted through the quarter-wave plate 5, is reflected by the fixed mirror 15 and is transmitted through the quarter-wave plate 5 again. By making one round trip through the quarter-wave plate 5, the polarization plane is 90 °. It's spinning. Therefore, the polarization state of the reference beam Br directed to the polarization beam splitter 3 becomes S polarization, the reference beam Br is reflected by the joint surface of the polarization beam splitter 3, and then passes through the polarizing plate 9 to pass through the photodetector 1.
Reach 0. The optical axis of the polarizing plate 9 is set to a direction rotated by 45 ° from the direction perpendicular to the paper surface of FIG. 10, and the polarization component of the reference beam Br parallel to the optical axis of the polarizing plate 9 is the photodetector 10. To reach.

On the other hand, the S-polarized component of the laser beam B ₀ emitted from the laser oscillator 1 with respect to the polarization beam splitter 3 is reflected at the joint surface thereof and the length measurement beam Bm is reflected.
Becomes This length measurement beam Bm passes through the quarter-wave plate 4 and then is reflected by the movable mirror 16 and again passes through the quarter-wave plate 4. Causes the plane of polarization to rotate 90 °. Therefore, the polarization state of the length measurement beam Bm traveling toward the polarization beam splitter 3 becomes P-polarized, and the length measurement beam Bm passes through the joint surface of the polarization beam splitter 3 and then passes through the polarizing plate 9 to pass through the photodetector 10. To reach. As for the measurement beam Bm, the polarization component parallel to the optical axis of the polarizing plate 9 reaches the photodetector 10, so that the reference beam Br from the fixed mirror 15 and the measurement beam from the movable mirror 16 are placed on the photodetector 10. Bm interferes.

Then, the movable mirror 16 moves its measuring beam Bm.
When moving in the x direction parallel to the optical axis of, the difference in the optical path length between the length measurement beam Bm and the reference beam Br from the laser oscillator 1 to the photodetector 10 changes,
The light intensity on the photo detector 10 changes. This light intensity is I ′, the displacement of the movable mirror 16 is x, and the refractive index of the atmosphere is n.
′, Where the wavelength of the laser beam in vacuum is λ, and I ₀ ′ and φ ₀ ′ are constants, the relational expression between them is
And a signal waveform as shown in FIG. 11 is obtained.

[0006]

I ′ = I ₀ ′ {1 + cos (4πn′x / λ + φ ₀ ′)} Therefore, the displacement x of the movable mirror 16 is conversely known by photoelectrically converting the light intensity I ′ and sequentially measuring it. be able to. In FIG. 11, the cycle Q ′ with respect to the displacement x of the light intensity I ′ is λ / (2n ′).

[0007]

However, in the conventional laser interferometer as described above, when fluctuation occurs in the air in the optical path of the laser beam (in particular, the measuring beam Bm), the optical path is changed due to the change in the refractive index of the air. There is an inconvenience that the length changes and a measurement error occurs. In order to reduce this measurement error, it is possible to arrange a sensor 17 for measuring the temperature, pressure, humidity and the like of the air in the vicinity of the measurement beam Bm as shown in FIG. In this case,
A wavelength correction circuit 18 for processing the photoelectric conversion signal output from the photodetector 10 is provided, and its sensor 17 is provided.
The wavelength λ of the laser beam is corrected and the displacement of the movable mirror 16 is calculated based on the measurement data supplied from

However, this method also has a disadvantage that the local change in the refractive index cannot be completely corrected. That is, since the fluctuation of air is not spatially uniform,
It is not possible to completely correct air fluctuations by measuring the atmospheric conditions at one place. Further, since the movable mirror 16 moves back and forth in the x direction, it is not practical to arrange a large number of sensors in the optical path of the measurement beam Bm.
In view of such a point, the present invention has no influence of air fluctuation,
Moreover, it is an object of the present invention to provide a laser interferometer that does not require wavelength correction of a laser beam.

[0009]

A laser interferometer according to the present invention, for example, as shown in FIG. 1, has a closed space (a side surrounded by a bottom (8a) and two sides (8b, 8c) sandwiching the bottom (8a). 8d) and a closed container (8) in which the first plane mirror M ₁ and the second plane mirror M ₂ are formed on the closed space side of these two sides, and the laser beam B ₀ and the first beam B ₁ And the second
The beam B ₂ is separated into a, first and second beam, respectively first and second laser beam separating optical system to be incident perpendicular to the plane mirror of the sealed container (8) (2-7)
And a photoelectric conversion element (10) for mixing and receiving the first and second beams reflected by the first and second plane mirrors and returned via the laser beam separation optical systems (2 to 7). And a displacement in a direction parallel to the bottom side (8a) of the closed container (8) due to changes in the interference state of the first and second beams reflected by the first and second plane mirrors, respectively. Is to be measured.

In this case, the closed space (8d) of the closed container (8) may be filled with a predetermined gas or may be kept in a vacuum. Further, the bottom side (8a) of the closed container (8) and the two sides (8b, 8c) sandwiching the bottom side may form an isosceles triangle. However, this means that it is substantially an isosceles triangle, and more accurately, the intersection angle between the base (8a) and the side (8b) and the intersection angle between the base (8a) and the side (8c). If they are equal,
It goes without saying that it does not necessarily have to be a triangle. Further, the first beam B ₁ and the second beam B ₂
And may cross each other at or near the bottom (8a) of the closed container (8).

[0011]

According to the present invention, for example, as shown in FIG. 1, the hermetically sealed container (8) is oriented in the direction of the base (8a) (x direction) with respect to the laser beam separating optical system (2 to 7). Upon moving, the first beam B reflected from the first mirror M _{1
The difference} between the optical path length of _{1 and} the optical path length of the second beam B ₂ reflected from the second mirror M ₂ gradually changes. Therefore, by detecting the change in the interference state between the _first beam B ₁ and the second beam B ₂ with the photoelectric conversion element (10),
The displacement of the closed container (8) in the direction parallel to the bottom side (8a) can be measured.

In this case, most of the first and second beams pass through the closed space (8d) of the closed container (8), and there is no gas fluctuation inside the closed space (8d). .. Further, since the distance between the laser beam separation optical system (2 to 7) and the closed container (8) can be shortened and does not change much, the influence of air fluctuations in the distance can be almost ignored. Therefore, even if air fluctuations occur around the laser interferometer, there is no effect on the fluctuations and no error occurs in the measurement results.

The bottom side (8a) of the closed container (8)
When the two sides (8b, 8c) sandwiching this bottom form an isosceles triangle, the closed container (8) has its bottom (8) with respect to the laser beam separation optical system (2-7).
Even if the relative displacement in the direction perpendicular to a) occurs, the first beam B ₁
The optical path length of the second beam B ₂ and the optical path length of the second beam B ₂ do not change. Therefore, since the relative displacement in the direction perpendicular to the base (8a) is not measured, only the displacement in the direction (x direction) parallel to the base (8a) can be accurately measured.

Further, the first beam B ₁ and the second beam B _{1
If 2} and ₂ are made to intersect with each other in the vicinity of the bottom side (8a) of the closed container (8), the bottom side (8a) is temporarily assumed.
Even if there is unevenness in thickness depending on the location, the optical path lengths of the _first beam B ₁ and the second beam B ₂ change by the same amount. Therefore, an error does not occur in the measurement result due to the unevenness of the thickness of the bottom side (8a).

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Various embodiments of the laser interferometer according to the present invention will be described below with reference to FIGS. In these embodiments, the parts corresponding to those in FIG. 10 are designated by the same reference numerals, and detailed description thereof will be omitted.

[First Embodiment] FIG. 1 is a plan sectional view of the first embodiment. In FIG. 1, reference numeral 2 designates a hollow first closed container as a whole. The side surface of the hollow portion of the first closed container 2 is surrounded by a light-transmitting laser beam incident surface 2a, a light-transmitting laser beam input / output surface 2b, a light-transmitting laser beam emitting surface 2c, and other surfaces. A polarizing beam splitter 3, two quarter-wave plates 4,5, and two plane mirrors 6, 7 are installed in the hollow portion. Then, the hollow portion of the first closed container 2 is filled with a predetermined gas or kept in vacuum. Further, the laser oscillator 1 is fixed to the outside of the laser beam incident surface 2a of the first closed container 2, and the polarizing plate 9, the condenser lens 11 and the photodetector 10 are arranged outside the laser beam emitting surface 2c.

Reference numeral 8 denotes a second closed container having a closed space 8d. The side surface of the closed space 8d of the second closed container 8 is a light-transmissive bottom 8a, and the bottom 8a intersects at a vertical angle α. One side 8b and its bottom side 8a are covered with a second side 8c intersecting at an apex angle β. On the surfaces of the first side 8b and the second side 8c on the side of the closed space 8d, a flat first reflecting surface M ₁ and a second reflecting surface M ₂ having extremely high surface accuracy are formed. Then, the closed space 8d is also filled with a predetermined gas or kept in a vacuum. Further, the laser beam input / output surface 2b of the first closed container 2 and the bottom side 8a of the second closed container 8 are arranged so as to face each other, and the second closed container 8 is parallel to the bottom side 8a. It shall be movable in the x direction. This means that the second closed container 8 is attached to, for example, a moving table that slides in the x direction, whereby the displacement of the moving table in the x direction can be measured.

In FIG. 1, the laser beam B ₀ emitted from the laser oscillator 1 is a laser beam incident surface 2a.
To the polarization beam splitter 3 in the hollow portion of the first closed container 2, the S-polarized component with respect to the joint surface of the beam splitter 3 is reflected to become the first beam B ₁ , and the P-polarized component is transmitted. And becomes the second beam B ₂ .
When the oscillation state of the laser oscillator 1 is linearly polarized light, its polarization plane is set to 4 around the optical axis with respect to the polarization beam splitter 3.
Place it at an angle of 5 °. In this case, the first beam B ₁ is 1
After passing through the quarter-wave plate 4 to become circularly polarized light and reflected by the plane mirror 6, the first reflection surface M of the closed space 8d of the second closed container 8 is transmitted through the laser beam input / output surface 2b and the bottom side 8a.
Incident vertically on ₁ .

The first reflected perpendicularly by the first reflecting surface M ₁
Beam B ₁ of the
After being reflected by the plane mirror 6 through b, it passes through the quarter-wave plate 4. As a result, the first beam B ₁ has passed through the quarter-wave plate 4 twice, and becomes P-polarized with respect to the joint surface of the polarization beam splitter 3. Therefore, the first beam B ₁ passes through the joining surface, then passes through the laser beam emitting surface 2c and the polarizing plate 9, and is focused on the photodetector 10 by the condenser lens 11. The polarizing plate 9
The polarization axis of is tilted by 45 ° around the optical axis with respect to the joint surface of the polarization beam splitter 3.

On the other hand, the second beam B ₂ passes through the quarter-wave plate 5, becomes circularly polarized light, is reflected by the plane mirror 7, and then passes through the laser beam input / output surface 2b and the bottom side 8a to form the second beam B 2. The light is vertically incident on the _second reflecting surface M ₂ of the closed space 8d of the closed container 8. Second beam B ₂ is reflected perpendicularly by the second reflecting surface M _2, bottom 8a and the laser beam output surface 2b
After being reflected by the plane mirror 7, the light passes through the quarter-wave plate 5. As a result, the second beam B ₂ is emitted by the quarter wave plate 5
Has passed twice, and the polarization beam splitter 3
It becomes S-polarized with respect to the junction surface of Therefore, the second beam B ₂ is reflected by the joining surface, then passes through the laser beam emitting surface 2 c and the polarizing plate 9, and is focused on the photodetector 10 by the condenser lens 11. On this photodetector 10, the first beam B ₁ and the second beam B ₂ interfere with each other.

When the second closed container 8 moves in the x direction parallel to the bottom side 8a in this state, the optical path length of the _first beam B _{1 and} the optical path length of the second beam B ₂ respectively change. Since the positive and negative of these changes are opposite, the difference in optical path length between the two changes, and the light intensity on the photodetector 10 changes in a sinusoidal shape accordingly. This principle will be described in detail with reference to FIG.

In FIG. 2, the second closed container 8 of FIG.
Is schematically represented by a triangular figure. The closed container 8 that was initially in the position indicated by the solid line is x, in the direction parallel to the bottom side.
It is assumed that a translational movement of y is made in a direction perpendicular to the base to reach a position 8T indicated by a dotted line. In this case, the reflective surface M
_The angles formed by ₁ and the reflecting surface M ₂ with the base are α and β, respectively.
Therefore, if the increments in the lengths of the one-way paths through which the first beam B ₁ and the second beam B ₂ pass are L ₁ and L ₂ , respectively, these L ₁ and L ₂ are functions of x and y. It can be expressed by the following equation.

[Formula 2] L ₁ = −sin α · x + cos α · y L ₂ = sin β · x + cos β · y

Therefore, the refractive index of the gas in the closed container 8 is n
Then, the reciprocal optical path length difference ΔP between the first beam B ₁ and the second beam B ₂ can be expressed by the following (Equation 3).

ΔP = 2n (L ₂ −L ₁ ) = 2n {(sin β + sin α) x + (cos β−cos α) y}

If the closed container 8 is an isosceles triangle, then α = β holds, the second term of (Equation 3) disappears, and (Equation 3) becomes the next (Equation 4).

## EQU00004 ## .DELTA.P = 4 nsin .alpha.x Since this (Equation 4) does not include y, it means that even if there is a displacement y in the direction perpendicular to the base, it does not affect the measurement result. Therefore, when the second closed container 8 can be regarded as an isosceles triangle, even if the guide mechanism of the closed container 8 in the x direction has a swing in a direction perpendicular to the bottom side, the measurement result is accurate only in the x direction. The displacement is indicated, and the measurement accuracy is improved.

If the light intensity on the photodetector 10 in this case is I, and I ₀ and φ ₀ are constants, respectively,
The relationship between the light intensity I and the displacement x is as in the following (Equation 5), and its waveform is as shown in FIG.

(5) I = I ₀ {1 + cos (8πnsinα · x / λ + φ ₀ )} Therefore, the displacement x of the second hermetically sealed container 8 is to be known by photoelectrically converting the light intensity I and sequentially measuring it. You can In FIG. 3, the period Q with respect to the displacement x of the light intensity I
Is λ / (4nsinα).

In this example, most of the optical paths of the first beam B ₁ and the second beam B ₂ are inside the second hermetically sealed container 8, so that most of the optical paths are affected by air fluctuations. Never receive. In addition, the first closed container 2 and the second
The first beam B ₁ and the second beam B ₂ both pass through the atmosphere and are affected by the fluctuation of air between the closed container 8 and the closed container 8 of FIG. You can almost ignore it. Also, beam B ₁ and beam B
₂ is ejected from the first closed container 2 and the photodetector 1
It passes through the atmosphere until it reaches 0, but beam B
_{Since 1} and the beam B ₂ pass through the same optical path, there is no influence of air fluctuations. Of course, it goes without saying that the polarizing plate 9, the condenser lens 11, and the photodetector 10 may be placed in the first closed container 2. Therefore, according to the laser interferometer of this example, it is possible to almost completely eliminate the influence of air fluctuations.

Further, as shown in FIG. 1, in this example, the first beam B ₁ and the second beam B ₂ intersect in the bottom side 8a of the second closed container 8 or in the vicinity of the bottom side 8a. ing.
Therefore, even if the thickness of the base 8a varies in the x direction, the change amount of the optical path length of the first beam B ₁ and the change of the optical path length of the second beam B ₂ due to the thickness of the base 8a. The quantity is almost the same, and there is an advantage that the influence of the variation in the thickness can be eliminated.

Furthermore, if the change in the internal volume due to thermal expansion of the second closed container 8 is small, the refractive index of the gas (or vacuum) inside the closed container 8 is always constant. Therefore, even if the temperature, pressure, humidity, etc. of the atmosphere change, stable measurement can be performed without being affected by them.
That is, if the refractive index n when the gas is first sealed in the closed container 8 is accurately known, it is not necessary to perform wavelength correction at all.

In the above embodiment, the homodyne detection method is used in which the displacement is detected as a change in the intensity of two laser beams that interfere with each other on the photodetector 10. However, in order to determine the forward or backward movement of the closed container 8, The photodetector 10 may be composed of a plurality of light receiving elements, and a phase plate or the like for partially changing the phase may be arranged. Alternatively, a heterodyne detection method using mutually coherent laser beams having slightly different frequencies as the first beam B ₁ and the second beam B ₂ may be used. In this case,
The photoelectric conversion signal of the light intensity I becomes a phase modulation signal, and the forward or backward information can be obtained from the signal of one phase.

As described above, in the first embodiment, when the second closed container 8 is an isosceles triangle, even if the closed container 8 translates, no measurement error occurs. However, if the closed container 8 rotates, a measurement error may occur. FIG. 4 shows a state in which the closed container 8 is rotated in a plane parallel to the paper surface of FIG. 4, and in FIG. 4, the closed container 8 at the position initially shown by the solid line is rotated and is shown by a chain line. Position 8R shall be reached. Further, the incident beam is shown by a solid line, and the reflected beam after the closed container 8 is rotated is shown by a dotted line. However, in FIG. 4, for simplification, the first closed container 2, the quarter wave plates 4 and 5, the polarizing plate 9, the photodetector 10 and the like in FIG. 1 are omitted.

As can be understood from FIG. 4, the incident first beam B ₁ and second beam B ₂ are reflected by the reflecting surface in the closed container 8 and are indicated by dotted lines in the first beam B ₁ ′.
And the second beam B ₂ ′, the first beam B ₁
′ And the second beam B ₂ ′ become two beams having a divergence angle when entering the photodetector 10. Therefore, there is a disadvantage that the interference state on the photodetector 10 is changed and a measurement error occurs or the interference does not occur. The second embodiment shown below has improved this inconvenience.

[Second Embodiment] FIG. 5 is a sectional plan view of the optical system of this embodiment. In FIG. 5, parts corresponding to those in FIG. 1 are designated by the same reference numerals and detailed description thereof will be omitted. However, unlike the case of FIG. 1, the first closed container 2 uses the laser beam incident surface 2a also as the laser beam emitting surface, and the shape of the side surface of the hollow portion is slightly different, but the same reference numerals are used. ing.

In FIG. 5, reference numeral 12 denotes a non-polarizing beam splitter, which is placed outside the laser beam incident surface 2a of the first closed container 5 and is symmetrical with respect to the joint surface of the beam splitter 12. The laser oscillator 1 and the optical elements 9, 10 and 11 are arranged in the. Further, the corner cube 13 is arranged on one surface side of the polarization beam splitter 3 in the hollow portion of the first closed container 2. The corner cube 13 is an optical component that reflects the incoming laser beam in the same direction. Other configurations are shown in FIG.
Is the same as.

In this case, the laser beam B ₀ emitted from the laser oscillator 1 passes through the non-polarization beam splitter 12 and the polarization beam splitter 3 in the first closed container 2.
Then, it is separated into a _first beam B 1 of reflected light and a second beam B ₂ of transmitted light. The first beam B ₁ and the second beam B ₂ are reflected by the first reflecting surface M ₁ and the second reflecting surface M _{2 in} the second closed container 8 and returned to the polarization beam splitter 3 again. Come on. At this time, both beams have passed through the quarter-wave plate twice, so that the first beam B ₁ passes through the beam splitter 3 and the second beam B ₂ is reflected by the beam splitter 3, Both beams enter the corner cube 13.

The first beam B ₁ reflected by the corner cube 13 is transmitted through the polarization beam splitter 3, and the second beam B ₂ reflected by the corner cube 13 is reflected by the polarization beam splitter 3 and then again. The light is reflected by the first reflecting surface M ₁ and the second reflecting surface M _{2 in} the second closed container 8 and returns to the polarization beam splitter 3. At this time, since both beams have passed through the quarter-wave plate twice more, the first beam B ₁ is reflected by the beam splitter 3 and the second beam B ₂ is transmitted through the beam splitter 3. Then, both beams return to the non-polarizing beam splitter 12 in the opposite direction on the laser beam incident surface 2a. Both beams reflected by the non-polarizing beam splitter 12 are a polarizing plate 9 and a condenser lens 1.
1 and interferes on the photodetector 10, and a displacement signal is output from the photodetector 10.

In the second embodiment, the two beams B ₁ and B ₂ are reflected twice by the reflecting surfaces M ₁ and M ₂ , respectively, so that the sensitivity is doubled as compared with the first embodiment. , Second
The relation between the displacement x in the direction parallel to the bottom of the closed container 8 and the light intensity I on the photodetector 10 is as follows.
However, the second closed container 8 is an isosceles triangle, and the two base angles of the triangle are α.

## EQU6 ## I = I ₀ {1 + cos (16πnsinα · x / λ + φ ₀ )}

Next, referring to FIG. 6, the second closed container 8
Describes the case of rotating in a plane parallel to the paper surface of FIG.
To do. FIG. 6 shows that the closed container 8 has a surface parallel to the paper surface of FIG.
It shows the state of rotating inside, and in this FIG.
The closed container 8 at the position indicated by the line rotates and
It shall reach the position 8R shown. Also a laser oscillator
Incident beam B from 1 ₀ Within the polarizing beam splitter
The second beam B, which is the component that has transmitted 3₂ Is shown by a solid line
And is the component reflected by the polarization beam splitter 3.
First beam B₁Is indicated by a dotted line. However, in this Figure 4
Laser oscillator 1 in FIG. 5, unpolarized beam for simplicity
Splitter 12, first closed container 2, quarter wave plate
4, 5, polarizing plate 9, condensing lens 11 and photodetection
The data 10 and the like are omitted.

As can be understood from FIG. 4, the incident first beam B ₁ and second beam B ₂ are at the position 8R after rotation.
The first beam B ₁ ′ shown by the dotted line and the second beam B ₂ ′ shown by the solid line are reflected by the reflecting surface in the closed container 8 present in the above, and these first beam B ₁ ′ and second beam B ₂ 'becomes mutually parallel beams after exiting the non-polarization beam splitter 12. Therefore, in this second embodiment, even if the second hermetically sealed container 8 is rotated, it is reflected by the second sealed container 8.
Since the main beam is incident on the condenser lens 11 in parallel, a good interference state is always maintained.

[Third Embodiment] This third embodiment is also a structural example for improving the inconvenience caused by the rotation of the first embodiment. FIG. 7 shows a sectional plan view of the optical system of the third embodiment,
In FIG. 7, parts corresponding to those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted. In this third embodiment, FIG.
One of the plane mirrors 6 in the first embodiment is replaced with the penta prism 14. Other configurations are similar to those in FIG.

Next, the case where the second closed container 8 rotates in a plane parallel to the paper surface of FIG. 7 will be described. FIG. 8 shows a state in which the closed container 8 is rotated in a plane parallel to the paper surface of FIG. 8, and in FIG. 8, the closed container 8 at the position initially shown by the solid line is rotated and shown by a chain line. Position 8R shall be reached. Further, the first beam B ₁ and the second beam B ₂ incident on the _second closed container 8 are shown by solid lines, and the reflected beam after the closed container 8 is rotated is shown by dotted lines.
However, in FIG. 8, for simplification, the laser oscillator 1, the first closed container 2, the quarter wave plates 4 and 5, the polarizing plate 9, the condenser lens 11, the photodetector 10 and the like in FIG. 7 are omitted. It can be understood from FIG. 8 that the two reflected beams are incident on the condenser lens 11 in parallel to each other even if the closed container 8 is rotated.

Next, the measurement error (so-called Abbe error) when the second closed container 8 rotates in the above embodiment will be described with reference to FIG. FIG. 9 shows a regular state of the closed container 8 by a solid line, and the closed container 8 is rotated leftward by an angle θ in a plane parallel to the paper surface of FIG. 9 around an arbitrary point P on the bottom side to a position 8R. The reached state is indicated by a one-dot chain line. In this case, the first beam B ₁ and the second beam B ₂
In each one way, the optical path changes by S _L and S _r . Further, the shape of the side surface of the closed container 8 is an isosceles triangle with an apex angle of δ, the distance from the point P to the midpoint of the bottom is r, and from the midpoint to the point where the two beams enter the closed container 8. Let be the distance of d. In addition to these conditions, if θ≈0,
The distances S _L and S _r are respectively given by the following equations.

[0042]

[Formula 7] S _L = (r + d) sin (δ / 2) ・ θ S _r = (r + d) sin (δ / 2) ・ θ Therefore, S _L = S _{r is} always satisfied, and the measurement error due to rotation does not occur. .. That is, the Abbe error is 0 at any point on the bottom of the closed container 8.

The present invention is not limited to the above-mentioned embodiments, and it goes without saying that various configurations can be made without departing from the gist of the present invention.

[0044]

According to the present invention, the first beam and the second beam
Since most of the optical path of the beam is present in the closed space of the closed container, there is an advantage that there is no influence of air fluctuation and the wavelength correction of the laser beam is unnecessary. Further, when the bottom side of the closed container and the two sides sandwiching the bottom side form an isosceles triangle, there is an advantage that a measurement error does not occur even if the closed container shakes in a direction perpendicular to the bottom side. Further, when the first beam and the second beam are made to intersect each other at the bottom side or in the vicinity of the bottom side, there is an advantage that a measurement error due to the variation in the thickness of the bottom side does not occur.

[Brief description of drawings]

FIG. 1 is a sectional view showing an optical system of a first embodiment of a laser interferometer according to the present invention.

FIG. 2 is a conceptual diagram showing a case where a second closed container is translated in the first embodiment.

FIG. 3 is a waveform diagram showing the light intensity detected in the first embodiment.

FIG. 4 is a conceptual diagram showing a case where a second closed container is rotated in the first embodiment.

FIG. 5 is a sectional view showing an optical system according to a second embodiment of the present invention.

FIG. 6 is a conceptual diagram showing a case where the second closed container is rotated in the second embodiment.

FIG. 7 is a sectional view showing an optical system of Example 3 of the present invention.

FIG. 8 is a conceptual diagram showing a case where the second closed container is rotated in the third embodiment.

FIG. 9 is a diagram for explaining an Abbe error caused by rotation of a second closed container in the example of the present invention.

FIG. 10 is a configuration diagram showing a conventional laser interferometer.

FIG. 11 is a waveform diagram showing the light intensity detected by a conventional laser interferometer.

[Explanation of symbols]

 1 Laser Oscillator 2 First Sealed Container 3 Polarizing Beam Splitter 4,5 5 1/4 Wave Plate 6,7 Plane Mirror 8 Second Sealed Container 9 Polarizing Plate 10 Photodetector 11 Condensing Lens 12 Non-Polarizing Beam Splitter 13 Corner Cube 14 Penta prism

Claims (3)

[Claims] 1. A closed space having a side surface surrounded by a bottom side and two sides sandwiching the bottom side, the first side being on the side of the closed space of the two sides.
A closed container having a flat mirror and a second flat mirror formed therein, a laser beam is separated into a first beam and a second beam, and the first and second beams are respectively separated into the first and second beams of the closed container. A laser beam splitting optical system which is vertically incident on the second plane mirror is mixed with the first and second beams reflected by the first and second plane mirrors and returned through the laser beam splitting optical system. A photoelectric conversion element for receiving light is provided, and displacement in a direction parallel to the bottom side of the hermetic container is caused by a change in interference state of the first and second beams reflected by the first and second plane mirrors, respectively. A laser interferometer characterized by being measured. 2. The laser interferometer according to claim 1, wherein a bottom side of the closed container and two sides sandwiching the bottom side form an isosceles triangle. 3. The first beam and the second beam are:
The laser interferometer according to claim 1, wherein the laser interferometers intersect with each other at the bottom of the closed container or in the vicinity of the bottom.