JP3374505B2 - Laser interferometer and method for correcting length error - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a highly accurate length measuring device utilizing interference of laser light.

[0002]

2. Description of the Related Art Conventionally, a laser interference type length measuring device has been widely used for high precision position measurement or high precision positioning of a stage or the like used for precision machining or precision measurement. FIG. 3 is a schematic view of a laser length measuring device using a Michelson type interferometer which has been widely used conventionally. The operation will be described below with reference to the drawings.

A laser beam emitted from a light source 1 is split into two beams A and B by a beam splitter 2.
It goes toward a movable mirror 4 provided on the movable stage 3 and a fixed mirror 5 fixed to the beam splitter. Hereinafter, the light flux A will be referred to as length measuring light, and the light flux B will be referred to as reference light.
The measurement light and the reference light, which are respectively folded back by the movable mirror 4 and the fixed mirror 5, overlap again on the beam splitter 2,
Cause interference. The light source 1, the beam splitter 2, the fixed mirror 5, and the detector 6 have a fixed positional relationship with each other. A process display device 13 that performs electrical processing and display is connected to the detector 6 via a cable. The processing and display device 13 calculates the length measurement result based on the detection result of the detector 6,
It is visible to the user. In this example, corner cube prisms are used for the movable mirror 4 and the fixed mirror 5.

Since the interfered laser light repeats bright and dark according to the movement of the movable mirror 4, the detector 6 receives the interference light and counts the bright and dark to measure the moving distance of the movable mirror 4, that is, the stage 3. can do. In the example of FIG. 3, when the movable mirror moves by a distance of ½ of the laser wavelength, the interference light produces bright and dark once.

A method (heterodyne interference method) in which laser light of two frequencies whose frequencies are slightly shifted is emitted from a light source and one of them is folded back by a movable mirror and the other is folded back by a fixed mirror (a heterodyne interference method) is widely used. Furthermore, by electrically dividing the signal obtained by the interference, the measurement resolution can be increased to n.
Techniques for improving to the m-order are also widely used.

In such a conventional laser interferometer, an optical glass such as borosilicate glass or quartz glass is used as a material for an optical element such as a beam splitter, a moving mirror, or a fixed mirror that transmits light. .

[0007]

However, in the laser length measuring apparatus using these optical glasses as the material of the optical element, when the temperature of the optical element changes, the optical path length of the laser passing through the element is increased. However, there is a problem in that the measurement error changes and measurement error occurs. This measurement error occurs when the optical element material is borosilicate glass, the optical path length is 10 m.
The temperature change of 1 ° C. per m reaches several tens of nm.

As a result, when there is a temperature difference between the fixed-side optical element and the moving-side optical element, or when the temperature is different each time the measurement is performed, the measurement can be performed without changing the position of the stage 3 for measuring the length. There were cases where the results were in error. The influence of this error has been a hindrance in realizing a highly accurate laser interferometer for use in ultraprecision machining or ultraprecision measurement.

In view of the above problems, it is an object of the present invention to provide a highly accurate laser interferometer length measuring device and a length measuring error correcting method by reducing the influence of temperature on the length measuring result.

[0010]

In order to achieve the above object, the present invention separates a laser beam into a reference beam and a length measuring beam, and utilizes the interference of the reference beam and the length measuring beam to measure an object to be measured. In the laser interferometric length measuring device for measuring the length, the optical material having a positive coefficient of thermal expansion and a negative temperature coefficient of refractive index, or a negative An optical element made of an optical material having a coefficient of thermal expansion of 1 and a temperature coefficient of positive refractive index was used.

This laser interferometric length measuring apparatus has, for example, a fixed mirror that reflects the reference light and a movable mirror that reflects the length measuring light as a light-transmissive optical element. It is composed of a corner cube prism formed of the optical material.

Alternatively, the laser length measuring device has a beam splitter for separating the reference light and the length measuring light as the light transmissive optical element, and the beam splitter is formed of the optical material.

In the case of having a tube that covers the optical path of the length measuring light and an optical window that seals the openings at both ends of the tube and transmits the length measuring light, it is preferable that the optical window is formed of the optical material. .

The optical material having a positive coefficient of thermal expansion and a negative temperature coefficient of refractive index is, for example, fluorine crown type or phosphate crown type glass.

The optical material having a negative coefficient of thermal expansion and a positive temperature coefficient of refractive index is, for example, partially crystallized glass having a specific composition.

Further, the length measurement error correction method according to the present invention is
A method for correcting a length measurement error in a laser interferometer that separates a laser beam into a reference beam and a length measuring beam and measures the length of an object to be measured by utilizing the interference of the reference beam and the length measuring beam,
As the light transmissive optical element through which the reference light and the length measuring light are transmitted, an optical material having a positive thermal expansion coefficient and a negative refractive index temperature coefficient, or a negative thermal expansion coefficient and a positive refractive index temperature coefficient, By using an optical element formed of an optical material having, a light-transmissive optical element through which the reference light and the length measuring term are transmitted, based on the optical path length change based on the thermal expansion coefficient and the refractive index temperature coefficient. It is designed to offset the change in optical path length.

[0017]

In general, when the temperature of the optical element changes, the change ΔL in the optical path length of the laser beam passing through the optical element.
Is calculated by Equation 1.

[0018]

[Equation 1]

Here, L is the optical path length of the laser light passing through the optical element, α is the linear expansion coefficient (thermal expansion coefficient) of the optical element, β is the temperature coefficient of the refractive index of the optical element, and n is n. The refractive index of the optical element, ΔT, is the temperature change of the optical element. Also, the first term of the right-hand side of Equation 1 is L · α · (n−1) · ΔT
Is the change in the optical path length due to the dimensional change of the optical element.
β · ΔT means the change of the optical path length due to the change of the refractive index of the optical element. Α has a positive value in many optical materials including optical glass such as borosilicate glass and quartz glass used in the optical system of the conventional laser length measuring apparatus. At the same time, β of optical glasses such as borosilicate glass and quartz glass also has a positive value.

On the other hand, in the laser interferometer according to the present invention, the optical element constituting the optical system is made of an optical material in which α is positive and β is negative, or α is negative and β is positive. Since it is used, the change in optical path length due to the dimensional change and the change in optical path length due to the change in refractive index cancel each other out, and the resulting change in optical path length can be suppressed to a small level. Furthermore, by using a material in which α and β satisfy equation 2, the change in optical path length can be made almost zero.

[0021]

[Equation 2]

As the length measurement method by laser interference in the present invention, either a DC interference method with a single wavelength (fringe count interference method) or a heterodyne interference method with two frequencies can be used.

[0023]

Embodiments of the present invention will be described in detail with reference to still other drawings.

FIG. 1 is a schematic diagram showing the configuration of an embodiment of the present invention. The configuration of the optical system of the present embodiment (hereinafter referred to as the first embodiment) uses a corner cube prism as the movable mirror and the fixed mirror as in FIG. 3, but the conventional fixed mirror shown in FIG. A fixed mirror 50 is used in place of 5, and a movable mirror 40 is used in place of the conventional movable mirror 4. The other structure is the same as that of the apparatus shown in FIG.

As in the apparatus shown in FIG. 3, the laser light emitted from the light source 1 is separated by the beam splitter 2 and then folded back by the movable mirror 40 and the fixed mirror 50, respectively, and overlaps again on the beam splitter 2 to cause interference. Cause

Now, the optical path lengths of the lasers passing through the movable mirror 40 and the fixed mirror 50 are Lm and Lm in the initial state, respectively.
Lf. When the temperature changes of the movable mirror 40 and the fixed mirror 50 from the initial state are ΔTm and ΔTf, respectively,
From Equation 1, the optical path length changes ΔLm and ΔLf of the laser light at the movable mirror 40 and the fixed mirror 50 are calculated by Equations 3 and 4, respectively.

[0027]

[Equation 3]

[0028]

[Equation 4]

Here, αm and αf are moving mirrors 4, respectively.
0 and the linear expansion coefficient of the fixed mirror 50, nm and nf are the refractive indexes of the movable mirror 40 and the fixed mirror 50, respectively, and βm and βf are the temperature coefficients of the refractive index of the movable mirror 40 and the fixed mirror 50, respectively.

By the above optical path length changes ΔLm and ΔLf,
The change ΔX in the length measurement value that occurs in the laser length measuring apparatus shown in the present embodiment is obtained by Expression 5.

[0031]

[Equation 5]

Further, assuming that the movable mirror 40 and the fixed mirror 50 are made of the same material, ΔX is expressed by the equation 6.

[0033]

[Equation 6]

Here, L = Lm = Lf, α = αm = αf, n
= Nm = nf and β = βm = βf.

Since the change in the length measurement value of the expression 6 cannot be distinguished from the movement of the movable mirror 40, that is, the stage,
It becomes a measurement error as it is.

From equation 6, it can be seen that even if temperature changes occur in each of the movable mirror 40 and the fixed mirror 50, the length measurement values do not change if the temperature changes are exactly the same. Moving mirror 4
Since 0 and the fixed mirror 50 are in the same environment, it is generally considered that there is no significant difference in temperature change between them. However, a difference in temperature change may occur depending on the size of the device, the place of contact with the ground, the use environment, and the like. In particular, in ultra-high precision length measurement, even a minute temperature change difference causes a non-negligible error.

In the first embodiment, Lm = Lf≈12 mm. If the movable mirror 40 and the fixed mirror 50 are conventionally made of borosilicate glass, the absolute value of {α · (n−1) + β} is about 5 × 10 −6 (1 / ° C). Therefore,
When ΔTm-ΔTf = + 1 ° C (or -1 ° C), the measurement error ΔX is as large as 30 nm.

On the other hand, in the first embodiment, the moving mirror 40
The fixed mirror 50 is expressed by the formula 2, and the linear expansion coefficient α is 16.5 × 10 −6 (1 / °)
C), the refractive index temperature coefficient β is -7.4 × 10 -6 (1 / °
C), made of glass having a refractive index n of 1.446. In this case, the absolute value of {α · (n−1) + β} is about 0.04 × 10 −6 (1 / ° C), so ΔTm−Δ
Even when Tf = + 1 ° C (or -1 ° C), the length measurement error ΔX is only about 0.25 nm, which is 1/10 of the conventional value.
It is possible to suppress it to 0 or less.

In the first embodiment, since the optical path lengths of the length measuring light and the reference light passing through the beam splitter 2 are the same, the temperature change of the beam splitter 2 does not cause a length measuring error. Therefore, the beam splitter 2 uses a conventional optical element.

FIG. 2 is a schematic diagram showing the configuration of another embodiment of the present invention. In the present embodiment (hereinafter, referred to as the second embodiment), a quadrature dual frequency heterodyne system is adopted as the length measurement system. Hereinafter, the effects of the present invention will be described with reference to the drawings.

Two linearly polarized light beams which are simultaneously emitted from the light source 1 and are orthogonal to each other are separated by the deflecting beam splitter 20 having the quarter wavelength plate 12, and one polarized light beam (reference light beam) is separated.
Passes through the polarization beam splitter 20 as it is, and goes toward the detector 6. The other polarized light (measurement light) is reflected by the reflection surface of the polarization beam splitter 20 and travels toward the mirror 7. The measurement light reflected by the mirror 7 is reflected by the beam splitter 2
After passing through 0, it is reflected by a mirror 8 fixed to a moving stage (not shown) and then reflected by a beam splitter 20 again to overlap with the reference light.

Further, in the second embodiment, the tube 9 covers the laser optical path between the polarization beam splitter 20 and the mirror 8 in order to reduce the influence of air fluctuations. This tube 9 is an optical window 1 that transmits laser light.
Both ends are sealed by 0 and 11. In this embodiment,
A change in the distance between the mirror 7 and the mirror 8 is obtained as a length measurement value.

In the laser interferometer of the construction shown in the second embodiment, when temperature changes occur in the optical windows 10 and 11, the laser light (length measuring light) of the portion passing through both optical windows 10 and 11 is measured. Because the optical path length changes. Measurement error occurs. Further, in the second embodiment, since the optical path lengths of the length measuring light and the reference light passing through the polarization beam splitter 20 are different, the temperature change of the polarization beam splitter 20 also causes a length measuring error. With this configuration, in the second example, unlike the case of the first example, even if the temperature change of each optical element is the same, the temperature change leads to an error in the length measurement value.

Now, in the initial state, the optical path lengths of the laser beams passing through the optical windows 10 and 11 are adjusted to Lw.
Then, assuming that the temperature change of the optical windows 10 and 11 from the initial state is ΔTw, the change ΔLw of the optical path length of the laser in the optical window portion is expressed by Formula 7 from Formula 1.

[0045]

[Equation 7]

Here, αw, nw and βw are the coefficient of linear expansion of the optical window, the refractive index and the temperature coefficient of the refractive index, respectively. Further, it is assumed that the optical windows 10 and 11 are made of the same material and have the same temperature change.

Similarly, assuming that the difference between the optical paths of the length measuring light and the reference light passing through the polarization beam splitter in the initial state is Lbs, and the temperature change of the polarization beam splitter from the initial state is ΔTbs, the polarization from Equation 1 is obtained. The change ΔLbs in the optical path length of the laser in the beam splitter section is expressed by Equation 8.

[0048]

[Equation 8]

Here, αbs, nbs and βbs are the linear expansion coefficient, the refractive index and the temperature coefficient of the refractive index of the polarization beam splitter, respectively.

The change ΔX in the length-measuring value that occurs in the laser length-measuring device shown in the present embodiment due to the changes ΔLw and ΔLbs in the optical path length is obtained by the equation 9.

[0051]

[Equation 9]

This change ΔX in the length measurement value is indistinguishable from the change in the distance between the mirrors 7 and 8, and thus becomes a length measurement error as it is. In this embodiment, Lw = 20 mm, Lbs =
It is 10 mm. If the polarizing beam splitter and optical window were conventionally made of borosilicate glass, then ΔTw = ΔTbs = + 1 ° C (or -1 ° C).
At this time, the measurement error ΔX becomes 75 nm.

On the other hand, in the second embodiment, as in the first embodiment, the polarization beam splitter 20 and the optical windows 10 and 11 are made of a fluorine crown type glass and have a linear expansion coefficient α of 16.5 × 10. It is made of glass having -6 (1 / ° C), temperature coefficient of refractive index β of -7.4 × 10 -6 (1 / ° C), and refractive index n of 1.446. for that reason,
Even when ΔTw = ΔTbs = + 1 ° C. (or −1 ° C.), the measurement error ΔX can be suppressed to about 0.6 nm, which is 1/100 or less of the conventional value.

[0054]

According to the present invention, in the laser interferometric length measuring apparatus, since the length measuring error caused by the temperature change of the optical element can be greatly reduced, the length measuring accuracy can be remarkably improved. . As a result, it is possible to perform highly accurate precision measurement on the order of nm, which is required for ultraprecision machining or ultraprecision measurement.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a first embodiment of the present invention.

FIG. 2 is a conceptual diagram showing a second embodiment of the present invention.

FIG. 3 is a conceptual diagram showing a conventional technique.

[Explanation of symbols]

1 ... Laser light source 2, 20 ... Beam splitter 3 ... Movement stage 4, 40 ... Movable mirror 5, 50 ... Fixed mirror 6 ... Detector 7, 8 ... Mirror 9 ... tube 10, 11 ... Optical window 12 ... Quarter wave plate 13: Processing display device

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01B 9/02 G01B 11/00

Claims (7)

(57) [Claims] 1. A laser beam is separated into a reference beam and a length measuring beam,
In a laser interferometric length measuring apparatus for measuring the length of an object to be measured by utilizing the interference of the reference light and the length measuring light, a positive thermal expansion coefficient is used as a light transmitting optical element through which the reference light and the length measuring light are transmitted. And an optical element having a negative temperature coefficient of refractive index, or an optical element formed of an optical material having a negative coefficient of thermal expansion and a positive temperature coefficient of refractive index, is used for laser interference measurement. apparatus. 2. The light transmissive optical element includes a fixed mirror that reflects the reference light and a movable mirror that reflects the length measuring light, and the fixed mirror and the movable mirror are corners formed of the optical material. The laser interferometer according to claim 1, wherein the laser interferometer is formed of a cube prism. 3. A beam splitter that separates the reference light and the length measuring light as the light transmissive optical element, and the beam splitter is formed of the optical material. Laser interferometer. 4. A tube that covers the optical path of the length measuring light, and an optical window that seals the openings at both ends of the tube and transmits the length measuring light, the optical window being formed of the optical material. Claims 1 and 2 characterized in that
Alternatively, the laser interferometer length measuring apparatus according to the item 3. 5. The optical material having a positive coefficient of thermal expansion and a negative temperature coefficient of refractive index is a glass of a fluorine crown type or a phosphate crown type.
The laser interferometer length measuring device according to any one of to 4. 6. The laser interference according to claim 1, wherein the optical material having a negative coefficient of thermal expansion and a positive temperature coefficient of refractive index is partially crystallized glass having a specific composition. Length measuring device. 7. A laser beam is separated into a reference beam and a length measuring beam,
A method for correcting a length measurement error in a laser interferometer length measuring apparatus that measures the length of an object to be measured by utilizing the interference between the reference light and the length measurement light, wherein the reference light and the length measurement light are transparent As the element, use of an optical material having a positive coefficient of thermal expansion and a negative temperature coefficient of refractive index, or an optical element formed of an optical material having a negative coefficient of thermal expansion and a positive temperature coefficient of refractive index With respect to the light transmissive optical element through which the reference light and the length measurement term are transmitted, the laser interference characterized by canceling out the change in optical path length based on the thermal expansion coefficient and the change in optical path length based on the temperature coefficient of refractive index. Measuring error correction method in measuring device.