JPH11108619A - Scale using laser beam - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ultrahigh-accuracy scale which is not affected by secular changes nor the used environment of the scale. SOLUTION: Scale divisions 12 are marked on a structure 10 in the longitudinal direction, and a light-wave interferometer is provided in the structure 10. The optical path of the interferometer is filled up with a light-transmissive material 11 having a fixed refractive index, and an electric resistor 14 which heats the structure 10 is provided. The length of each scale division 12 marked on the structure 10 is measured as the true value of the division 12 by means of the interferometer provided with a beam splitter 16 and a corner cube 18. When a difference exists between the real value and the nominal length of the division 12, the structure 10 is expanded by heating by supplying an electric current to the resistor 14 by means of a current control section 24 and a controllable power source 22. Therefore, the influence of secular changes and the using environment on the structure 10 can be eliminated by making the true value coincident with the nominal value by controlling the temperature of the structure 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-high-precision scale using a laser beam, which has a structure in which a scale is recorded in the longitudinal direction of a physical medium, and suppresses a change over time and an influence of a use environment.

[0002]

2. Description of the Related Art Conventionally, there are a measuring instrument using a scale and a measuring instrument using a laser beam. The length measurement using a scale enables stable measurement even while moving in the air, but has the following disadvantages. That is, (1) Since the physical medium is affected by the environmental temperature, the length of the portion on which the scale is recorded greatly fluctuates. For example,
In steel system, 11.8 μm / m ° C. accuracy decreases.

[0003] (2) Even if the external medium (gravity) of the physical medium is constant in external force (gravity), the length of the portion on which the scale is recorded becomes non-uniform in the vertical posture, and the accuracy is reduced.

(3) Even if the physical medium is a low-expansion glass (for example, Zerodur (trade name)) which is inextensible with time and insensitive to temperature fluctuation, its length is about 30 to 70 nm / m.year. There is a time-dependent change in the direction, and 0.3 to 0.
This is a large value of about 7 μm / m.

On the other hand, in an interferometer using laser light, the physical characteristics of the laser light whose wavelength is stabilized can be exhibited as it is in a vacuum and high-precision length measurement is possible. It is susceptible to air fluctuations (fluctuations in the refractive index), and there is a limit to its suppression especially during measurement while moving. The normal use environment of measurement equipment is the space where air exists. Under such environment, due to the influence of the fluctuation of air caused by the movement of the object, measurement of about 0.2 μm is possible even with a movement of only 2 μm in the optical path length section of about 1 m. Data fluctuations have been reported.
Therefore, a method of always correcting this air fluctuation using laser light of two wavelengths having greatly different frequencies over the entire optical path section has been put to practical use, but the fluctuation of the measured data is about ± 0.01 μm at rest. Is the limit.

[0006]

As described above, the measurement using a scale and the measurement using a laser beam have disadvantages, respectively, and have a length measurement of the order of 1 m under a normal environment (that is, under an environment where air exists). 1nm, 0.1n in section
It has been extremely difficult to perform stable measurement with a high resolution of about m, especially position measurement of a moving object.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object the same high resolution and accuracy as a laser light wave interferometer provided in a vacuum, and It is an object of the present invention to provide a scale that can be used under a normal environment and that can stably measure a moving object.

[0008]

In order to achieve the above object, a first aspect of the present invention provides a uniform thermal expansion along a longitudinal direction and transmits a laser beam with a uniform refractive index therein. A structure having a substance, a scale provided on an outer wall of the structure, and a light wave for measuring a length of a portion provided with the scale, the light path being provided inside the structure so that an optical path is within the substance. An interferometer and temperature control means for changing the temperature of the structure based on the length measured by the light wave interferometer are provided.

In a second aspect based on the first aspect, the temperature control means is provided along a longitudinal direction of the structure, and generates an electric resistance according to an applied current.
Current supply means for supplying a current to the electric resistor.

In a third aspect based on the second aspect, the electric resistor is connected to a plurality of current-carrying terminals along a longitudinal direction of the structure. A current is supplied to the electric resistor through the plurality of current-carrying terminals to make the expansion and contraction of the structure in the longitudinal direction constant.

In a fourth aspect based on the first to third aspects, the apparatus further comprises a temperature detecting means for detecting a temperature of the structure, wherein the laser beam is supplied when the temperature of the structure falls within a predetermined range. Wherein the difference between the length measured by the light wave interferometer and the nominal value has a phase difference within one wavelength.

In a fifth aspect based on the first to fourth aspects, the light wave interferometer is fixed to one end of the scale and fixed relative to the beam splitter. A reference surface reflector, a test surface reflector fixed corresponding to the other end of the scale, and an optical path difference between reference branch light on the reference surface side and signal branch light path on the test surface side. An optical path difference detector, wherein the temperature control means controls the temperature based on the optical path difference detected by the optical path difference detector.

In a sixth aspect based on the fifth aspect, the light wave interferometer further comprises: a semiconductor light source that emits the laser light; a light source between the light source and the beam splitter; It is characterized by having an isolator arranged between the reflectors to be inspected, and a gas cell arranged between the beam splitter and the optical path difference detecting means.

In a seventh aspect based on the first to fifth aspects, the laser beam is supplied from outside the structure via an optical fiber.

In an eighth aspect based on the first to fifth aspects, the laser beam is supplied through a transparent window formed on an end face of the structure.

According to a ninth aspect, in the first to eighth aspects, the mode of the laser beam is a single mode.

According to a tenth aspect, in the first to ninth aspects, the light wave interferometer uses an optical heterodyne interferometry.

According to an eleventh invention, in the first to tenth inventions, the structure is a single crystal.

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a perspective view of a scale using a laser beam according to the present embodiment. This scale provides uniform thermal expansion along the longitudinal direction, and
A substance 11 having a uniform refractive index through which laser light is transmitted.
And a scale 12 is recorded on the outer wall of the structure 10 along the longitudinal direction. Although the form of the substance that transmits laser light is not limited, a solid is used in the present embodiment.
There are plastics, quartz, quartz, silicon dioxide and the like.

In the inside of the structure 10, an electric resistor 14 which generates heat uniformly in the longitudinal direction in accordance with an applied current is continuously formed, and a scale 12 provided on an outer wall of the structure 10 is formed. Is provided with a light wave interferometer for measuring the length. This light wave interferometer includes a beam splitter 16 that separates an optical path of an incident laser beam into a reference branch optical path 100 and a signal branch optical path 102, a corner cube 18 that reflects the signal branch optical path 102, a reference branch optical path 100, and a signal branch optical path. The apparatus includes an optical path difference detector 20 for detecting an optical path difference of 102, and its optical path is filled with a substance 11 having a uniform refractive index and transmitting a laser beam. Further, a controllable power supply 22 and a current control unit 24 for supplying a current to the electric resistor 14 are provided outside the structure 10.

The structure 10 can be made of, for example, single-crystal silicon or the like, and can be formed by subjecting a long single-crystal silicon body to deep hole processing (such as gun drilling and grinding). Further, the electric resistor 14 can be formed by vapor deposition or sputtering of a metal such as tungsten.

The electric resistor 14 can be formed not on the inside of the structure 10 but on the outer wall in the same manner as the scale 12. The electric resistor 14, the controllable power supply 22, and the current control unit 24 function as temperature control means.

FIG. 2 shows the structure of the light wave interferometer provided inside the structure 10. The incident laser light enters the beam splitter 16 and is separated on one surface into a reference branch optical path 100 and a signal branch optical path 102. The reference branch optical path 100 repeats reflection in the beam splitter 16 and enters the optical path difference detector 20. On the other hand, the signal branch optical path 102 passes through the beam splitter 16 and enters the corner cube 18. Then, the light is reflected by the corner cube 18, enters the beam splitter 16 again, and enters the optical path difference detector 20.

Here, the beam splitter 16 has the structure 1
0 is provided at a position corresponding to one end of the graduation 12 provided on the outer wall of the graduation 12, and the corner cube 18 is provided at a position corresponding to the other end of the graduation 12. Corresponds to the distance from one end of the scale 12 to the other end. The beam splitter 16 and the corner cube 18 are in close contact with the substance 11 that transmits laser light, and a matching oil or the like is applied to the adhered portion so that reflection or attenuation of the laser light does not occur. Accordingly, there is almost no change in the refractive index on the reference branch optical path 100 and the signal branch optical path 102, and the optical path difference detector 20 uses the reference branch optical path 100 and the signal branch optical path 1.
By detecting the optical path difference of 02, the distance from one end to the other end of the scale 12, that is, the true value of the length of the portion where the scale 12 is recorded can be reliably measured.

As a method of detecting the optical path difference, the phase difference between the two optical paths may be detected as the intensity of the interference light using, for example, the homodyne method.

FIG. 3 shows an example of a laser light source for supplying laser light used in a light wave interferometer.
As a light source, a laser diode (LD) can be used, which is provided in an LD thermostat 30 and its temperature is kept constant. The laser light emitted from the laser diode enters the beam splitter 33 via the isolator 32. The isolator 32 prevents return light from being incident on the laser diode and stably operates the laser diode. The laser light incident on the beam splitter 33 is split into two, and the laser light transmitted through the beam splitter 33 is incident on the beam splitter 16 and the corner cube 18 via the isolator 34.
Note that the isolator 34 is also for preventing the return light and operating the laser diode stably, similarly to the isolator 32.

On the other hand, the laser light reflected by the beam splitter 33 enters the gas cell. The gas cell 36 is a cell filled with a gas having a specific absorption line, and attenuates only light of a specific frequency. The output of the gas cell 36 is supplied to a photodiode (PD) 38, which converts the output of the gas cell into an electric signal. Therefore, only the specific frequency attenuated by the gas cell 36 is extracted as a slightly attenuated signal. The signal from the photodiode 38 is supplied to the lock-in amplifier 40.

The lock-in amplifier 40 extracts only an output of a specific frequency by synchronous detection and supplies it to a compensator 42.

The compensator 42 is for improving the characteristics (response, stability, etc.) of the closed loop, like the PID controller and the like used in the feedback loop.
The output from 2 is supplied to the LD driver 44.

The LD driver 44 generates an LD drive signal modulated at a constant frequency and supplies it to the laser diode in the LD thermostat 30. Thereby, the emission frequency of the laser diode is adjusted to be constant.

The LD thermostat 30, the isolator 3
2. The beam splitter 33, the isolator 34, the gas cell 36, and the photodiode 38 can be arranged in the structure 10.

The scale of the present embodiment has the above-described configuration, and functions as follows. That is, first, the optical path difference detector 20 measures the length of the portion where the scale 12 is recorded by the light wave interferometer provided inside the structure 10. Needless to say, in order to measure the optical path difference, it is necessary to use laser light having a wavelength such that the phase difference detected by the optical path difference detector 20 is within one wavelength. Then, the measured value (specifically, the optical path difference data between the reference branch optical path 100 and the signal branch optical path 102) is output to the optical path difference detector 20.
Is supplied to the current controller 24, and the difference from the nominal value of the scale is calculated. This difference is due to aging and the effect of the use environment (expansion and contraction of the medium due to temperature fluctuation).
In order to eliminate the difference between the nominal value and the true value, the current control unit 24
Supplies a control signal to the controllable power supply 22, supplies a current corresponding to the difference to the electric resistor 14, and heats the structure 10 uniformly in the longitudinal direction.

As described above, even if the structure 10 expands and contracts due to a change with time and the use environment, the degree of the expansion and contraction is accurately measured by the light wave interferometer, and the length of the scale 12 portion of the structure 10 is measured. Can be matched with the nominal value, and as a result, highly accurate length measurement is possible.

In the above description, the case where the current is supplied to the electric resistor 14 to expand the structure 10 by heating to make the true value equal to the nominal value has been described. Since the true value and the nominal value are made to match by controlling the temperature of the structure 10, not only the structure 10 is heated but also the structure 10 is cooled by cold water, low-temperature gas, or the like, so that the true value and the nominal value are reduced. Can also be matched. Specifically, when the true value is smaller than the nominal value, heating and expansion are performed, and when the true value is greater than the nominal value, cooling and contraction are performed.

In the above embodiment, the electric resistor 14 is formed uniformly (continuously) in the structure 10, but the electric resistor 14 is formed along the longitudinal direction of the structure 10. It is also possible to provide a plurality of current-carrying terminals, thereby appropriately selecting a region to which a current is to be applied and limiting a heating region.

FIG. 4 shows a configuration in such a case. The electric resistors 14 are formed in a ring shape at predetermined intervals in the structure 10, and each of the electric resistors 14 is connected to an energizing terminal k 1, k 2,... Kn. .. Are provided in the controllable power supply 22. When the detection signal from the optical path difference detector 20 is supplied to the current control unit 24, the current control unit 24 selects one of the plurality of energizing terminals (for example, k1) so that the true value and the nominal value match. ON only) and k2) to partially heat the structure 10. Thereby, even if the longitudinal expansion and contraction of the structure is not uniform due to the gravity acting on the structure 10 or the external force acting from the structure support mechanism, the longitudinal expansion and contraction is corrected almost uniformly. The true value and the nominal value can be matched.

The shape of the electric resistor 14 is arbitrary. The electric resistor 14 is formed uniformly in the structure 10 similarly to FIG. 1, and a plurality of energizing terminals are connected to an arbitrary portion thereof to partially energize. And then heat it.

In the above embodiment, the structure 10
It is also possible to further provide a temperature detector for detecting the temperature of the electric resistor 14 and to control the current supplied to the electric resistor 14 based on the detected temperature.

FIG. 5 shows a configuration in which the true value and the nominal value are matched using the temperature detector.
The temperature detector 50 is provided in the structure 10 and detects and outputs the temperature. Then, a current is supplied from the controllable power supply 22 to the electric resistor 14 so that the detected temperature is set to a value within a predetermined range. In this state, the detection signal of the light wave interferometer becomes 1
A laser beam having a wavelength having a phase difference within the wavelength is supplied so that the difference between the true value and the nominal value of the length of the scale 12 is within one wavelength.

More specifically, in the configuration shown in FIG. 5, the difference between the indicated temperature value supporting a predetermined range of temperature and the detected temperature value is calculated by the subtractor 51, and after amplification, the threshold value detection circuit 54
To supply. The threshold value detection circuit 54 determines whether or not the difference between the indicated temperature value and the detected temperature value is equal to or greater than an allowable threshold value. If not, the switch 52 is switched to the contact a side, and the difference between the two temperature values is set to the controllable power supply 2.
Feed to 2. The controllable power supply 22 supplies a current corresponding to the difference between the two temperature values to the electric resistor 14 and heats the electric resistance 14 until the two temperature values substantially match.

When the difference between the two temperature values is equal to or smaller than the allowable threshold value, that is, when the temperature of the structure 10 falls within a predetermined range, the threshold value detection circuit 54 sets the nominal value as a true value. A laser beam having a wavelength such that the value difference is within one wavelength is introduced into the lightwave interferometer, and the switch 52 is set to b
The electrical resistor 14 is heated by switching to a contact point so that the true value and the nominal value match. As a result, the true value and the nominal value can be matched with higher accuracy, and highly accurate length measurement can be performed regardless of the use environment.

In the above embodiment, a laser diode provided in the structure 10 is used as a laser light source. However, the present invention is not limited to this, and a laser diode provided outside the structure 10 may be used. Laser light can also be used.

FIG. 6 shows a laser beam emitted from a laser light source 60 provided outside the structure 10 to an optical fiber 62.
Is shown to guide to the lightwave interferometer in the structure 10 using. The laser light introduced into the structure 10 via the optical fiber 62 enters the collimator light source 64, is converted into parallel light, and then enters the beam splitter 16 of the light wave interferometer.

On the other hand, FIG. 7 shows another example in which an external light source is used. Laser light is introduced from the transparent window 66 into the light wave interferometer in the structure 10.

Although the embodiments of the present invention have been described above, in each of the above cases, it is preferable that the laser beam used is in a single mode from the viewpoint of the length measurement accuracy of the light wave interferometer.

Further, as a method of measuring the length of the scale in the light wave interferometer, it is preferable to use an optical heterodyne interferometry capable of detecting with higher accuracy, instead of the homodyne method described above. In optical heterodyne interferometry, a temporal carrier is generated by changing the phase of a reference light at a constant rate of change with respect to a signal light, and the frequency of light introduced into a reference branch and a signal branch is changed. And the phase difference between the two branches is a function of time in units of beat angular frequency. As a result, the phase difference measurement can be performed not by detecting the intensity as in the homodyne method but by detecting the time difference, so that a general-purpose measuring instrument such as a time counter can be used, and the scale 12 of the structure 10 can be more accurately measured. Can be measured, and the amount of expansion and contraction can be measured.

Furthermore, in the present embodiment, as a method of controlling the temperature of the structure 10, heating by an electric resistor and cooling by a refrigerant have been described, but any method of changing the temperature of the structure 10 may be used. It is possible, and they are all included in the technical scope of the present invention.

[0049]

As described above, according to the present invention,
Under an environment with a constant refractive index, the true value of the scale is measured with high precision using an optical interferometer, and the scale is expanded and contracted to match the nominal value with the true value based on the results. The influence is removed, and ultra-high-precision length measurement is possible even under a normal environment.

[Brief description of the drawings]

FIG. 1 is a perspective view of a scale according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of a light wave interferometer of the embodiment.

FIG. 3 is a configuration diagram of a laser light source according to the embodiment.

FIG. 4 is an explanatory view of an arrangement of an electric resistor and a current-carrying terminal according to another embodiment.

FIG. 5 is a configuration diagram of a scale according to another embodiment.

FIG. 6 is an explanatory view of an arrangement of a laser light source in another embodiment.

FIG. 7 is an explanatory view of an arrangement of a laser light source in another embodiment.

[Explanation of symbols]

Reference Signs List 10 structure, 11 laser light transmitting material, 12 scales, 14 electric resistor, 16 beam splitter, 18
Corner cube, 20 optical path difference detector, 22 controllable power supply, 24 current control unit, 30 LD constant temperature bath, 32 isolator, 33 beam splitter, 34 isolator, 36 gas cell, 38 photodiode, 40
Lock-in amplifier, 42 compensator, 44 LD driver, 50 temperature detector, 54 threshold detection circuit, 62
Optical fiber, 64 collimated light sources, 100 reference branch light path, 102 signal branch light path.

Claims (11)

[Claims] 1. A structure having a substance that provides uniform thermal expansion along a longitudinal direction and transmits laser light with a uniform refractive index therein, and a scale provided on an outer wall of the structure; An optical wave interferometer for measuring the length of a portion where the scale is provided is provided such that an optical path thereof is inside the substance inside the structure, and based on the length measured by the optical wave interferometer, A scale using laser light, comprising: temperature control means for changing the temperature of the structure. 2. The scale according to claim 1, wherein the temperature control unit is provided along a longitudinal direction of the structure, and generates an electric resistance according to an applied current, and supplies a current to the electric resistance. A scale using laser light, comprising: a current supply unit for supplying; 3. The scale according to claim 2, wherein a plurality of current-carrying terminals are connected to the electric resistor along a longitudinal direction of the structure, and the current supply means includes a plurality of current-carrying terminals. A scale using laser light, characterized in that a current is supplied to the electric resistor through a terminal for use to make the expansion and contraction of the structure in the longitudinal direction constant. 4. The scale according to claim 1, further comprising a temperature detecting unit configured to detect a temperature of the structure, wherein the laser light is provided such that the temperature of the structure is within a predetermined range. In some cases, the scale using laser light has a wavelength such that a difference between the length measured by the light wave interferometer and a nominal value is a phase difference within one wavelength. 5. The scale according to claim 1, wherein the light wave interferometer is fixed to one end of the scale and fixed relative to the beam splitter. A reference surface reflector, a test surface reflector fixed corresponding to the other end of the scale, and an optical path for detecting an optical path difference between the reference branch light on the reference surface side and the signal branch light path on the test surface side. And a difference detector, wherein the temperature control means controls the temperature based on the optical path difference detected by the optical path difference detector. 6. The scale according to claim 5, wherein the light wave interferometer further includes: a semiconductor light source that emits the laser light; and between the light source and the beam splitter, and between the beam splitter and the surface to be measured. A scale using a laser beam, comprising: an isolator disposed between devices, and a gas cell disposed between the beam splitter and the optical path difference detecting means. 7. The scale according to claim 1, wherein the laser light is supplied from outside the structure via an optical fiber. 8. The scale according to claim 1, wherein the laser light is supplied through a transparent window formed on an end face of the structure. Scale. 9. The scale according to claim 1, wherein the mode of the laser light is a single mode. 10. The scale according to claim 1, wherein the light wave interferometer uses an optical heterodyne interferometry. 11. The scale according to claim 1, wherein the structure is a single crystal.