JP2990309B2 - Optical member for measuring correction coefficient of differential interferometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for splitting a laser beam from a laser light source into a reference beam and a length-measuring beam, and passing the two parallel beams through a reciprocating optical path for returning the beams in the direction in which they came from a returning member. An optical member for correction coefficient measurement (hereinafter simply referred to as a correction member) used for wavelength correction of a differential interferometer that obtains an interference light beam carrying distance information between the return members of the two light beams by combining the two light beams. ).

[0002]

2. Description of the Related Art A differential interferometer as described above can determine a change in the distance between a return beam of a reference beam and a length measurement beam with an accuracy of a length measurement resolution of about nm. However, since this involves a wavelength that changes due to changes in environmental conditions such as temperature and humidity and atmospheric pressure, it is necessary to correct errors based on the change in wavelength. The optical path difference of the reference length that can be known in advance is provided, and the measurement wave number change of the reference length optical path difference due to the change of the wavelength with the change of the environmental condition is obtained for the two environmental conditions. As a result, one of the two environmental conditions is set as a reference condition, and the wave number is, for example, 1 under the condition.
The wavelength of the reference condition, that is, the reference wavelength, is obtained from the amount of movement of the length measuring mirror that only changes, and the wavelength under the other environmental condition that has changed with respect to the reference condition is determined from the reference wavelength, the reference length optical path difference, and the change in the measurement wave number. Calculate and use the obtained wavelength separately for the same environmental conditions and the measured wave number obtained by the length measuring device.
In the method, the reference light beam and the length measuring light beam are passed through the same distance in a vacuum where the environmental conditions do not change and in the changing air, thereby obtaining a wavelength change due to the change in the environmental conditions with respect to the reference wavelength from both the light beams. The wavelength of the environmental condition corresponding to the above-mentioned wavelength change is obtained by the same method as that for obtaining the reference wavelength by the method described above, and the reference wavelength is determined from the wavelength and the wavelength change, and the wavelength corresponding to the reference wavelength and the environmental condition is obtained. A second method of obtaining a wavelength corresponding to the environmental condition from the change, and using the wavelength for the same environmental condition and the measurement wave number obtained by the length meter, temperature, humidity, atmospheric pressure, and CO ₂ concentration of the environmental condition Measure and Edl'en
The third is to determine the refractive index of air by the above relational expressions, then determine the wavelength of the laser beam in the atmosphere by the oscillation frequency of the laser light source, and use that wavelength for the measurement wave number determined under similar environmental conditions. Method, a combination of the third method and the first or second method using the wavelength obtained by the third method as the reference wavelength in the first method or the wavelength of the environmental condition corresponding to the wavelength change in the second method. A fourth method is used. Of these methods, the first and second methods,
In particular, the first method is a simple correction method based on the most direct measurement.

In the first and second methods, a correction member 1 as shown in FIGS. 3 and 4, respectively, is used. The correction member 1 shown in FIG. 3 is made up of a partial back end plate 12 and a front end plate 12
13 in parallel with each other at an interval that can be obtained stably and accurately in advance with respect to changes in environmental conditions. 4 includes a cylindrical wall 14 made of a material having a low linear expansion coefficient such as quartz and a cylindrical wall 14 which is closed at both ends to maintain a vacuum inside the cylindrical wall 14 and at least an opposing wall of the cylindrical wall 14. A light-transmitting end plate 15 on one end side having a surface area extending to both outer sides of the portion, and a reflecting mirror end plate 16 having a reflecting surface 16a formed on a surface facing the light-transmitting end plate 15 on the other end side. .

The length measuring device shown in FIG. 3 is a laser beam of two wavelengths of linearly polarized light orthogonal to each other in the heterodyne system or a single beam in the interference fringe system, as indicated by a solid line L with an arrow from a laser light source (not shown). A circularly polarized laser beam of wavelength
The polarization beam splitter 2 splits the reference light beam R indicated by an alternate long and short dash line indicated by an arrow and the length measuring light beam M indicated by an alternate long and two short dash line indicated by an arrow. The light beam M passes through the reciprocating optical path twice, and the length-measuring light beam M passes through the reciprocating light path twice reflected by the front end plate 13 of the correcting member 1. Thereafter, the two light beams M and R are associated by the polarizing beam splitter 2. Partial back plate 12 and front plate 13
And the interference light flux I indicated by a dotted line with an arrow carrying information on the distance between the reflection surfaces is input to the detection means (not shown in FIG. 3), and the wavelength changes with environmental conditions. The change in the measurement length between the reflection surfaces is obtained as a change in the beat frequency of two wavelengths in the heterodyne method, and as a change in the intensity of the interference fringe in the interference fringe coefficient method. The first method uses the measurement length change obtained by the length measuring device shown in FIG. 3, the reference length separately obtained as the length of the spacer 11, and the other similar differential interferometers shown in FIG. It corresponds to the other environmental condition when the measurement length change is obtained from the reference wavelength obtained as the movement amount for one wavelength of the length measuring mirror under the same condition as the one environmental condition when the measurement length change is obtained by the length meter. In this method, a wavelength is determined, and this wavelength is used as a wavelength of a measurement wave number determined by a similar environmental condition and a length measuring instrument. The fourth method is a method using a wavelength corresponding to the same environmental condition obtained by the above-described third method, instead of the above-described reference wavelength. Further, the reference beam R and the measuring beam M shown in FIG.
The result does not change even if is replaced. Reference numeral 3 denotes a corner cube prism, 4 denotes a λ / 4 phase shift plate, and 5 denotes a reflecting mirror.

In the length measuring device shown in FIG. 4, a laser beam L similar to that shown in FIG. 3 has an optical system similar to that shown in FIG. 3, an optical system using a conventionally known polarization sharing plate or the like, or has been invented by the present inventors. The reference light beam R and the length measuring light beam M are branched by a branching and associating optical system 6 for interference such as an optical system using an interference prism filed by Japanese Patent Application No. 3-44890.
The reference light beam R passes through the vacuum inside the cylindrical wall 14 and passes through the reflector end plate.
The measurement light flux M passes through the air outside the cylindrical wall 14 twice and passes through the reciprocating optical path reflected by the reflecting surface 16a of the reflecting mirror end plate 16 twice. After passing through, the reference light beam R and the measuring light beam M are associated by the branching association optical system 6 and are incident on the detection means 7 as an interference light beam I carrying wavelength information changed with respect to the reference wavelength due to a change in environmental conditions. The detection means 7 obtains a wavelength relatively changed with respect to the reference wavelength. Also in this case, the reference wavelength is set to the first.
In the same way as the method described above, it is separately obtained by a length measuring device or the third method,
A wavelength changed according to environmental conditions is obtained from the reference wavelength and the wavelength of the above-mentioned relative change, and the wavelength is multiplied by the same environmental conditions and a measurement wave number obtained by a length measuring instrument.

[0006]

As described above, the conventional correction member 1 shown in FIG. 3 has at least a spacer member.
11 and a partial back end plate 12 in which an anti-reflection coat is provided on the center of one side of the light-transmitting plate and reflection coats are provided on both sides thereof,
Since it has a structure in which it is bonded to the front end plate 13, there is a problem that it takes time and effort to make it with high accuracy and the cost is high. In addition, the correction member 1 shown in FIG. 4 requires the inside of the cylindrical wall 14 to be evacuated even if the translucent end plate 15 is simpler than the partial back end plate 12 in FIG. However, there is a problem that the production is more troublesome and the cost is increased.

The present invention has been made in order to solve the above-mentioned problems, and has a simple structure, can be obtained easily, accurately and inexpensively, has excellent robustness, and has functions similar to those of the correction member 1 shown in FIG. An object of the present invention is to provide a correction member that does not change.

[0008]

According to the present invention, a laser beam from a laser light source is split into a reference beam and a length-measuring beam, and the two parallel beams are passed through a reciprocating optical path for returning the beams in the direction in which they came from a return member. After that, in a correction member used for wavelength correction of a differential interferometer that obtains an interference light beam carrying distance information between the return members of the two light beams by combining the two light beams, both end surfaces are polished to a smooth surface. And a rod-shaped fixed-length reflecting member having one end surface as a reflecting surface, and one side of the fixed-length reflecting member, the length of which protrudes laterally from the other end surface, is polished to a smooth surface and the central portion thereof is polished. A bar-shaped girder that is combined with the fixed-length reflecting member in a T-shape in accordance with the other end face of the fixed-length reflecting member, and the smooth surface of the one side that protrudes to both sides from the fixed-length reflecting member is a reflecting surface. For interferometers of differential type characterized by comprising a reflective member Located wood, to achieve the above object by this configuration.

[0009]

In other words, the correcting member of the present invention has a rod-shaped constant-length reflecting member whose both end surfaces are polished to a smooth surface and one end surface of which is a reflecting surface, and a length protruding from the end surface of the constant-length reflecting member. A simple structure in which one side is polished to a smooth surface and a bar-shaped girder reflective member whose reflective surface has a portion that protrudes to both sides from the end face of the fixed-length reflective member is formed in a T-shape. , Which can be easily and accurately manufactured at low cost, has excellent robustness, and has a reflecting surface for reciprocating the reference light beam and the length measuring light beam between the reflecting surface of the fixed-length reflecting member and the reflecting surface of the girder-shaped reflecting member, respectively. By doing so, it can be used to correct the effect of the wavelength that changes depending on the environmental conditions as described with reference to FIG.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the embodiments shown in FIGS.

FIG. 1 is a perspective view showing an example of a correction member of the present invention, and FIG. 2 is a schematic plan view showing an example of a length measuring instrument for measuring a correction coefficient using the correction member of the present invention.

The correction member 1 shown in FIG. 1 is made of a material having a low linear expansion coefficient such as quartz, preferably a material having a linear expansion coefficient of 10 ^-7 or less. Is provided with a reflective coat, and is a rectangular rod-shaped fixed-length reflecting member 17 formed on the reflecting surface 17a, and one side of a length formed of the same material and protruding from the end surface of the fixed-length reflecting member 17 is a smooth surface. A structure in which a portion which is polished and protrudes from the end face of the fixed-length reflecting member 17 to both sides thereof is provided with a reflecting coat and a rectangular bar-shaped girder reflecting member 18 formed on a reflecting surface 18a in a T-shape. This coupling may be by ringing of the smooth end surface of the fixed-length reflecting member 17 and the center of the smooth side surface of the girder-shaped reflecting member 18, or may be further formed by the smooth side surface of the girder-shaped reflecting member 18. Through the connecting screw or friction pin into the fixed length reflective member 17 from the opposite side. It may be what was done.

The length measuring instrument shown in FIG. 2 branches the laser beam L into a reference beam R and a length measuring beam M, and associates the two beams R and M to emit an interference beam I to the detecting means 7. 6
The method uses the interference prism disclosed in Japanese Patent Application No. 3-44890 and uses the correction member 1 shown in FIG. 1 to determine the change in the measured wavenumber of the optical path difference due to the change in the wavelength due to the change in the environmental conditions.
The wavelength according to the environmental conditions is obtained as follows.

The geometrical optical path length difference D between the reference light beam R and the length measuring light beam M is determined by the fact that both light beams R and M take twice the reciprocating light path on which the correction member 1 reflects. Is four times the reference length L. The optical path length difference D at the wavelength lambda ₀ of the environmental conditions that are determined as wavenumber M ₀ is the relationship D = 4L = M _{0 λ 0} obtained, the wavelength of different environmental conditions the optical path length difference D lambda ₁
In relation D = 4L = M ₁ λ ₁ obtained as wavenumber M ₁ is obtained. However, what is obtained by the length measuring device of FIG. 2 is M ₁ −M ₀ = ΔM of a change in wave number due to a change in environmental conditions. Therefore, lambda _{0 0} a a lambda ₁ as a reference wavelength from both relational expression described above lambda,
When calculated as a function of L, ΔM, λ ₁ = λ ₀ / (1 + λ ₀ · Δ
M / 4L).

At λ ₀ , the optical path length difference between the reference light beam and the measured light beam is 4 by the same differential interferometer under a certain environmental condition described above.
It can be obtained by a method of obtaining the moving distance of the length measuring mirror that changes by the wavelength or a method using the above-described relational expression of the third method. Therefore, the wavelength λ ₁ according to the environmental conditions
Is obtained from λ ₀ , L, ΔM. By multiplying the wavelength λ ₁ by the same environmental condition and the measurement wave number obtained by the length measuring instrument, it is possible to obtain a measurement length in which the influence of the wavelength change due to the change of the environmental condition is corrected.

In order to set the length of the fixed length reflecting member 17 to the reference length L, it is assumed that the reflecting surface 17a of the fixed length reflecting member 17 and the reflecting surface 18a of the girder shaped reflecting member 18 have the same thickness. It is preferable that a reflection coating is not provided on the central portion of the smooth side surface of the girder-shaped reflection member 18 with which the smooth end surface of the fixed-length reflection member 17 contacts.
It is also preferable to obtain a strong adhesive force when the joint is made by ringing of a smooth surface. If a reflective coat is provided between the joining surfaces or an adhesive is used,
After joining, it is necessary to measure the distance between the reflecting surfaces 17a and 18a and to consider the thermal expansion and contraction of the adhesive.

If the linear expansion coefficient of the fixed-length reflecting member 17 is α, the length of 0 ° C. of the fixed-length reflecting member 17 is defined as L ₀ and t ° C.
Is changed by L ₀ (1 + αt), λ = 4L ₀ (1 + α) where λ and M are the wavelength and wave number for which the optical path length difference is obtained.
Since t) / M is obtained, this λ may be used as the wavelength. However, when a material having α of 10 ⁻⁷ or less is used for the constant-length reflecting member 17, the measurement accuracy of a general laser interferometer is 10 ⁻⁷ to 10 ^−7.
Since it is about ^−8, it is sufficient to make the length of the fixed-length reflecting member 17 constant.

Although the linear expansion coefficient of the girder-shaped reflecting member 18 is not as important as that of the fixed-length reflecting member 17, the girder-shaped reflecting member 18 is also fixed-length reflecting member 18 so that thermal stress and distortion do not occur at the joint between them. It is preferable to use a material having a linear expansion coefficient similar to that of the member 17, and more preferably a material having a linear expansion coefficient of 10 ⁻⁷ or less. The fixed-length reflecting member 17 and the girder-shaped reflecting member 18 are preferably formed in the shape of a square bar as shown in the drawing, from the viewpoint of productivity and the like, but are not limited thereto.

[0019]

The correcting member of the present invention has a simple structure,
It can be made easily, accurately and inexpensively, and has excellent robustness.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an example of a correction member according to the present invention.

FIG. 2 is a schematic plan view showing an example of a length measuring instrument for measuring a correction coefficient using the correction member of the present invention.

FIG. 3 is a schematic configuration diagram showing an example of a length measurement device for obtaining a correction coefficient for the influence of a wavelength change.

FIG. 4 is a schematic configuration diagram showing an example of a length measurement device for obtaining a correction coefficient for the influence of a wavelength change.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Correction member 17 Constant-length reflective member 18 Digit-type reflective member 17a Reflective surface 18a Reflective surface 6 Branching association optical system 7 Detecting means L Laser beam R Reference beam M Measuring beam I Interference beam

Claims (3)

(57) [Claims] 1. A laser beam from a laser light source is split into a reference beam and a length-measuring beam, and the two parallel beams are passed through a reciprocating optical path for returning in a direction coming from a return member. In a correction coefficient measuring optical member used for wavelength correction of a differential interferometer that obtains an interference light beam carrying distance information between the return members of the light beam, both end surfaces are polished to a smooth surface and one of them is polished. A rod-shaped fixed-length reflecting member having an end surface as a reflecting surface, and one side of a length protruding laterally from the other end surface of the fixed-length reflecting member is polished to a smooth surface, and the central portion thereof is fixed-length reflecting member. From a rod-shaped girder reflector having a smooth surface on one side which is combined with the fixed length reflection member in a T-shape in accordance with the other end surface of the member and which protrudes to both sides from the fixed length reflection member and is a reflection surface. Optical section for measuring correction coefficient of differential interferometer Wood. 2. An optical member for measuring a correction coefficient of a differential interferometer according to claim 1, wherein a material having a linear expansion coefficient of 10 ⁻⁷ or less is used for at least the constant-length reflecting member. 3. The fixed-length reflecting member and the girder-shaped reflecting member are combined in a T-shape by ringing on a smooth surface.
Optical member for measuring the correction coefficient of the differential interferometer.