JPH1019508A - Light wave interference measuring device and measuring system for variation of reflective index - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure variation of a reflective index of a gas in an optical passage with high accuracy by accurately securing coaxial property of third and fourth lights without influence of variation of an emission direction of a first light. SOLUTION: When an emission direction of a light having a frequency ω2 from a light source 31 is varied with respect to a designed optical axis, lights having frequencies ω2 ', ω3 ' emitted from a frequency modulation section 101 are similarly affected by variation of the emission direction of the light having the frequency ω2 . As a result, since a coaxial property of the lights having the frequencies ω2 ', ω3 ' is attained with high accuracy, a real displacement quantity D of a moving mirror 5 is obtained with high accuracy insusceptibly of variation of the emission direction of the light having the frequency ω2 from the light source 31. Thus, since the coaxial property of the lights having the frequencies ω2 ', ω3 ' is attained with high accuracy insusceptible of variation of the emission direction of the light from the light source 31, it is possible to measure variation of a reflective index of a gas in an optical passage with high accuracy by using a heterodyne interference method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light wave interference measuring apparatus and a refractive index fluctuation measuring system, and more particularly to a light wave interference measuring apparatus and a refractive index fluctuation measuring system for performing highly accurate measurement of a refractive index fluctuation of a gas in an optical path. It is about.

[0002]

2. Description of the Related Art FIG. 15 is a diagram schematically showing a configuration of a conventional light wave interference measuring device. The light wave interference measuring apparatus of FIG. 15 includes a length measuring interferometer for measuring the displacement amount of the movable mirror 5 in the optical axis direction (the direction of the arrow in the figure). In the length measuring interferometer, the length measuring light source 1 emits light of frequency ω ₁ and light of frequency ω ₁ ′ whose frequencies are slightly different from each other and whose polarization directions are orthogonal to each other.

[0003] The two different frequencies of light are incident on a polarizing beam splitter 3 via a beam splitter 2,
The light having the frequency ω ₁ ′ and the light having the frequency ω ₁ are separated. The light of the frequency ω ₁ ′ becomes the reference light, and after being reflected by the fixed mirror 4,
The process returns to the polarization beam splitter 3 again. Also, the frequency ω ₁
Is reflected by the movable mirror 5 and returns to the polarization beam splitter 3 again. Polarizing beam splitter 3
The measurement light and the reference light returned to are emitted from the polarizing beam splitter 3 along the same optical path and interfere via the polarizing plate 6. The interference light generated via the polarizing plate 6 is received by the photoelectric conversion element 7. The measurement beat signal converted by the photoelectric conversion element 7 is input to the phase meter 10.

On the other hand, part of the light of the light and the frequency omega ₁ of the frequency omega ₁ emitted from the long light source 1 measurement 'is reflected by the beam splitter 2, interfering through the polarizing plate 8. Interference light generated via the polarizing plate 8 is detected by the photoelectric conversion element 9 and used as a reference beat signal by a phase meter 10.
Is input to The phase meter 10 obtains the displacement amount D (ω ₁ ) of the movable mirror 5 by measuring the phase change of the measurement beat signal with respect to the reference beat signal, and outputs a signal relating to the displacement amount information to the calculator 11.

[0005] In order to measure the length accurately (highly accurate) by interference of light waves, fluctuations in the refractive index of air (or other gas) in the optical path cannot be ignored. Therefore, a conventional light wave interference measuring apparatus includes a refractive index fluctuation measuring system using a heterodyne interference method for measuring a refractive index fluctuation of air in an optical path. In the light wave interference measuring apparatus equipped with a refractive index fluctuation measuring system, the influence of the refractive index fluctuation of air in the optical path is corrected based on the displacement D (ω ₁ ) of the movable mirror 5 measured by the above-described length measuring interferometer. , The true displacement D of the movable mirror 5 is determined.

[0006] The refractive index fluctuation measuring system is composed of a light of frequency ω ₂ ,
And a light source 151 for emitting in parallel to each other at an interval and a frequency omega ₃ light (ω ₃ = 2ω ₂₎ having twice the frequency omega ₂ of the light. The light having the frequency ω _{2 and} the light having the frequency ω ₃ emitted from the light source 151 enter the acousto-optic elements 154 and 152, respectively. Optical frequency omega ₂ incident on the acousto-optic device 154, 'light (omega _2' frequency omega ₂ which slightly shifted in frequency from the frequency ω ₂ = ω ₂ + Δ
ω ₂ ). Also, the acousto-optic element 152
Light of frequency omega ₃ which is incident on the 'light (omega _3' slightly frequencies omega ₃ shifted frequency from the frequency _{ω 3 = ω 3 + Δω 3} )
Is frequency-modulated.

The light of frequency ω ₃ ′ via the acousto-optic device 152 is incident on the dichroic mirror 155 via the reflector 153, and the light of frequency ω ₂ ′ via the acousto-optic device 154 is directly incident on the dichroic mirror 155. Thus, the light of the frequency ω ₂ ′ and the light of the frequency ω ₃ ′ that are coaxially coupled (along the same optical path) via the dichroic mirror 155 are incident on another dichroic mirror 33. The light having the frequency ω ₂ ′ and the light having the frequency ω ₃ ′ reflected by the dichroic mirror 33 are incident on the polarization beam splitter 3.

The light of frequency ω ₂ ′ and the light of frequency ω ₃ ′
Light reflected by the polarizing beam splitter 3 toward the fixed mirror 4 (reference light) and light transmitted through the movable mirror 5 (measurement light)
And divided into The polarization directions of the reference light and the measurement light are orthogonal to each other, but both include the light having the frequency ω ₂ ′ and the light having the frequency ω ₃ ′. After that, the reference light and the measurement light reflected by the fixed mirror 4 and the movable mirror 5, respectively, enter the polarization beam splitter 3 and are combined,
Emitted along the same optical path. Polarizing beam splitter 3
The reference light and the measurement light combined by are reflected by the dichroic mirror 34 and enter the polarization beam splitter 35. Polarization beam splitter 35, the light of the measuring light reflected by the movable mirror 5 is transmitted through the (light of 'light and the frequency omega _3' of the frequency omega _2), the reference light reflected by the fixed mirror 4 (the frequency omega ₂ ' And light of frequency ω ₃ ′).

The light of the frequency ω ₂ ′ having a smaller frequency among the light of the frequency ω ₂ ′ and the light of the frequency ω ₃ ′ transmitted through the polarizing beam splitter 35 is transmitted to a second harmonic conversion element (hereinafter referred to as “S
The frequency ω ₃ ″ (ω
₃ ″ = 2ω ₂ ′). On the other hand, the light having the frequency ω ₃ ′ passes through the SHG conversion element 36 as it is. As a result, the light converted from the frequency ω ₂ ′ to the frequency ω ₃ ″ by the SHG conversion element 36 and the frequency ω
The light ₃ ′ interferes, and the interference light is detected by the photoelectric conversion element 37. Also, the light of frequency ω ₂ ′ and the light of frequency ω ₃ ′ reflected by the polarization beam splitter 35 are the same as the light converted from frequency ω ₂ ′ to frequency ω ₃ ″ by the action of the SHG conversion element 38. Interference light with the light of the frequency ω ₃ ′ reflected by the fixed mirror 4 is detected by the photoelectric conversion element 39.

The interference signal detected by the photoelectric conversion element 37, ie, the measurement signal, and the interference signal detected by the photoelectric conversion element 39, ie, the reference signal, are input to the phase meter 40, respectively. In the phase meter 40, based on a phase change of the reference signal and the measurement signal, the optical path length variation with respect to the frequency omega ₃ light D
(Ω ₃ ) and optical path length change D for light of frequency ω ₂
(Ω ₂ ), that is, {D (ω ₃ ) −D (ω ₂ )}. ΔD (ω ₃ ) −D (ω obtained by the phase meter 40
₂ ) The signal related to｝ is supplied to the arithmetic unit 11. In the arithmetic unit 11, ΔD (ω ₃ ) −D from the phase meter 40
(Ω ₂ )｝, the displacement amount D of the moving mirror 5 measured by the length measuring interferometer using the length measuring light source 1 based on the signal D
(Ω ₁ ) is corrected, and the true displacement amount (geometric distance) D is obtained.

[0011]

[0007] As described above, in the conventional optical interference measuring apparatus, the light source 151 are emitted in parallel spaced apart from each other and a light of a frequency omega ₂ of the light and the frequency omega _3. Thus, initially by the production error of the light source 151, also over time by the operation of the device, there is a possibility that the emission direction of the frequency omega ₂ of the light emitting direction and the frequency omega ₃ light varies independently of each other. That is, accurate parallelism between the light of the frequency ω _{2 and} the light of the frequency ω ₃ emitted from the light source 151 cannot be ensured. As a result, the light of the frequency ω ₂ ′ via the dichroic mirror 33 is not It is not possible to ensure accurate coaxiality with light of frequency ω ₃ ′.

In the conventional light wave interference measuring apparatus, the difference between the displacement of the movable mirror measured with the light of the frequency ω ₂ and the displacement of the movement measured with the light of the frequency ω ₃ , that is, the light of two different frequencies The influence of the refractive index fluctuation of the air in the optical path is obtained based on the optical path length difference. Therefore, in order to measure the displacement amount of the movable mirror with high accuracy, the light of frequency ω ₂ and the frequency ω
The coaxiality with the light of ₃ must be ensured accurately. If the coaxiality cannot be ensured accurately, errors in the displacement of the movable mirror measured with the light of each frequency are generated independently, and the optical path length difference between the light of two different frequencies cannot be accurately obtained. . As a result, the variation in the refractive index of air in the optical path cannot be measured with high accuracy using the heterodyne interference method, and the measurement accuracy of the light wave interference measurement device is reduced.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to measure a change in the refractive index of a gas in an optical path with higher accuracy without being affected by a change in a direction in which light is emitted from a light source. It is an object of the present invention to provide a light wave interference measurement device and a refractive index fluctuation measurement system using a heterodyne interference method that can be used.

[0014]

In order to solve the above-mentioned problems, according to the present invention, there is provided a length measuring interferometer for measuring a displacement amount of a movable mirror along a predetermined direction, and a method for measuring the displacement amount of the moving mirror. A refractive index fluctuation measuring system for measuring a refractive index fluctuation of a gas in an optical path, wherein the displacement amount of the movable mirror measured by the length measuring interferometer is measured by the refractive index fluctuation measuring system. In the optical interference measurement apparatus for measuring the amount of geometric displacement of the movable mirror by correcting based on the refractive index variation information of the gas, the refractive index variation measuring system includes a first light having a first frequency. And a part of the first light from the light source is converted to a second light having a second frequency different from the first frequency, and the first light and the Frequency conversion means for outputting the second light along the same optical path; The first light and the second light output from the frequency conversion means along the same optical path are frequency-modulated by a predetermined frequency to generate third light and fourth light, respectively. Frequency modulating means for outputting the third light and the fourth light along the same optical path, and the third light and the fourth light via the measuring optical path of the length measuring interferometer. A first interference light generation system for generating a first interference light by heterodyne interference by making a frequency of one of the lights substantially equal to a frequency of the other light; and reference to the interferometer for length measurement. A second interference light for generating a second interference light by heterodyne interference by making a frequency of one of the third light and the fourth light substantially equal to a frequency of the other light through an optical path; A generation system, wherein the first interference light Based on the preliminary second interference light to measure the refractive index variation of the gas in the optical path of the length-measuring interferometer, to provide an optical wave interference measuring apparatus and correcting.

According to a preferred aspect of the present invention, the frequency conversion means converts a part of the first light into a harmonic,
A harmonic conversion element for outputting the harmonic as the second light. Further, the frequency modulating means includes the first
At least one frequency modulation element for frequency-modulating each of the second light and the second light by a predetermined frequency, the third light generated via the at least one frequency modulation element, and the fourth light It is preferable to have a correction optical system for coupling light with the same optical path. In this case, it is more preferable that the frequency modulation element is an acousto-optic element using Bragg diffraction.

Further, according to a preferred aspect of the present invention, the correction optical system separates the third light and the fourth light generated through the frequency modulation element by separating the third light and the fourth light. A refraction optical system for converting the light into parallel light, and a third light and the fourth light which are converted into parallel light through the refraction optical system, along the same optical path. A frequency coupling element. Alternatively, the correction optical system is a diffractive optical system for converting the third light and the fourth light generated via the frequency modulation element into light parallel to each other at an interval by diffracting the third light and the fourth light. And a frequency coupling element for coupling the third light and the fourth light converted in parallel to each other via the diffractive optical system along the same optical path.

According to another aspect of the present invention, a first light having a first frequency is supplied to a refractive index fluctuation measuring system for measuring a refractive index fluctuation of a gas in a predetermined optical path. For converting a part of the first light from the light source into a second light having a second frequency different from the first frequency, and converting the first light and the second light Frequency conversion means for outputting light along the same optical path, and the first light and the second light output along the same optical path from the frequency conversion means are respectively frequency-converted by a predetermined frequency. Frequency modulating means for modulating to generate third light and fourth light and outputting the generated third light and fourth light along the same optical path; The frequency of one of the third light and the fourth light through the other An interference light generation system for generating interference light due to heterodyne interference by substantially matching the frequency of the light, and measuring a refractive index variation of gas in an optical path of the length measuring interferometer based on the interference light. And a refractive index fluctuation measuring system.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a light wave interference measuring apparatus and a refractive index fluctuation measuring system according to the present invention, a first light source emitted from one light source is used.
The second light having a second frequency different from the first frequency is generated based on the first light having the second frequency by the action of a frequency conversion means such as a second harmonic conversion element (SHG conversion element). Light is generated, and the first light and the second light are output along the same optical path. Next, the first light and the second light are frequency-modulated by predetermined frequencies, respectively, by the action of a frequency modulation element such as an acousto-optic element to generate third light and fourth light. 3rd generated
And the fourth light are guided along the same optical path to a predetermined optical path whose refractive index variation is to be measured, by the action of a correction optical system including a refractive optical system and a frequency coupling element, for example.

As described above, in the present invention, the first light source
Even if the emission direction of the light fluctuates with respect to the design optical axis, the third light and the fourth light guided to the predetermined optical path where the refractive index fluctuation is to be measured are the emission directions of the first light from the light source. Are affected in the same way as each other. As a result, the third
Although the traveling directions of the third light and the fourth light may fluctuate with respect to the design optical axis, the third light and the fourth light have predetermined refractive index fluctuations to be measured along the same optical path. Propagation in the optical path. That is, since the coaxiality of the third light and the fourth light can be accurately secured without being affected by the fluctuation of the emission direction of the first light from the light source, the optical path can be adjusted using the heterodyne interference method. Variations in the refractive index of air inside can be measured with high accuracy.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a diagram schematically showing a configuration of a light wave interference measuring apparatus according to an embodiment of the present invention. FIG.
FIG. 2 is a diagram schematically showing an internal configuration of a frequency modulation unit 101 in FIG. The optical interference measurement apparatus of FIG. 1 includes a refractive index fluctuation measurement system for measuring a refractive index fluctuation of air (or other gas) in an optical path. In the refractive index fluctuation measurement system, the light of frequency ω ₂ emitted from the light source 31 is SHG
Part of the light having a frequency of ω ₂ that is incident on the conversion element 32 is SHG
The light is converted into light having a frequency ω ₃ (ω ₃ = 2ω ₂ ) by the conversion element 32, and the remaining light passes through the SHG conversion element 32 as it is. Frequency ω ₂ emitted from SHG conversion element 32
And the light of the frequency ω ₃ enter the frequency modulation unit 101 coaxially along the same optical path.

For the SHG conversion element 32, for example, a nonlinear optical crystal KTiOPO ₄ can be used. The polarization direction of the SHG frequency omega ₃ of light by the SHG element 32 is different from the polarization direction of the frequency omega ₂ of the light directly passes and enters the SHG device 32. Therefore, although not shown in FIG. 1, for example, between the SHG conversion element 32 and the frequency modulation section 101,
Optics for matching the polarization direction of the polarization direction of the frequency omega ₂ of the light and the frequency omega ₃ light (the plurality of wave plates) are arranged.

Referring to FIG. 2, the light of frequency ω _{2 and} the light of frequency ω ₃ which are coaxially incident on the frequency modulation unit 101 along the same optical path enter the acousto-optic device 51 as a frequency modulation device. In the acousto-optic element 51, the ultrasonic wave propagated to the acousto-optic medium causes a periodic change in the refractive index in the acousto-optic medium. As a result, the acousto-optic device 5
Numeral 1 performs the function of a diffraction grating, and incident light undergoes Bragg diffraction at a specific angle. The diffraction angle of Bragg diffraction is
It depends on the frequency of the incident light and the frequency of the ultrasonic wave.
Therefore, different diffraction angles in the different frequency omega ₂ of the light and the frequency omega ₃ optical frequency, travel along different directions, respectively, as shown in FIG.

Here, the ultrasonic wave propagating through the acousto-optic medium is
Let the frequency be Δf and the frequency ω_Two Frequency ω
_Three Of the diffracted light of the acousto-optic element 51
Next-order diffracted light is used. In this case, the frequency ω_Two Light of
Is the frequency ω_Two'(= Ω_Two + Δf) to the light 200
And is diffracted at a predetermined diffraction angle. Also, the frequency ω
_Three The light at the frequency ω_Three'(= Ω_Three + Δf) around the light 300
Wave number modulated and diffracted at a predetermined diffraction angle. Acousto-optics
Frequency ω via element 51_Two'Light 200 and frequency ω
_Three'Light 300 is, for example, a collimator
The light becomes parallel to each other by the lens 52.

Here, light 3 having a small diffraction angle and a frequency ω ₃ ′
If the traveling direction of 00 and the optical axis of the collimating lens 52 are configured to coincide with each other, it is not necessary to use a lens achromatized by the light having the frequency ω ₂ ′ and the light having the frequency ω ₃ ′ as the collimating lens 52. good. Alternatively, the configuration may be such that the traveling direction of the light 200 having a large diffraction angle and the frequency ω ₂ ′ coincides with the optical axis of the collimator lens 52.

The light 200 at the frequency ω ₂ ′ and the light 300 at the frequency ω ₃ ′ enter the dichroic prism 53 along optical paths parallel to each other. Thus, the light 300 of the frequency omega ₂ 'light 200 of the frequency omega _3' by the action of the dichroic prism 53, is coaxially coupled to, and is emitted from the frequency modulation unit 101 along the same optical path. As described above, the collimating lens 52 serving as a refractive optical system and the dichroic prism 53 serving as a frequency coupling element are coupled with the light 200 having the frequency ω ₂ ′ and the frequency ω
The correction optical system is configured to couple the ₃ ′ light 300 along the same optical path.

Referring again to FIG. 1, the frequency modulation unit 10
1 and the frequency ω emitted coaxially along the same optical path
The light of ₂ ′ and the light of frequency ω ₃ ′ are reflected by the frequency coupling element 33 composed of, for example, a dichroic mirror, and are coupled on the same optical path as the light from the length measuring light source 1 (light near the frequency ω ₁ ). Is done. The frequency coupling element 33 has a characteristic that transmits only light of the frequency omega ₁ near and reflects light at other frequencies.

The frequency ω reflected by the frequency coupling element 33
The light of ₂ ′ and the light of frequency ω ₃ ′ are incident on a polarization separation element 3 such as a polarization beam splitter. The polarization beam splitter 3 is disposed at an angle of 45 ° with respect to the polarization directions of the light having the frequency ω ₂ ′ and the light having the frequency ω ₃ ′. Therefore, the light incident on the polarization beam splitter 3 is light of two different frequencies, namely, reference light reflected by the polarization beam splitter 3 and guided to the fixed mirror 4 and light transmitted through the polarization beam splitter 3 and transmitted to the movable mirror 5. It is split into guided measurement light. As described above, the reference light and the measurement light have polarization directions orthogonal to each other, and both include both light having the frequency ω ₂ ′ and light having the frequency ω ₃ ′.

The reference light (light having the frequency ω ₂ ′ and light having the frequency ω ₃ ′) reflected by the polarization beam splitter 3 is reflected by a fixed mirror 4 composed of a corner cube prism, and then transmitted to the polarization beam splitter 3 again. Return. On the other hand, measurement light (light of frequency ω ₂ ′ and light of frequency ω ₃ ′) transmitted through the polarization beam splitter 3 is also reflected by the movable mirror 5 composed of a corner cube prism attached to a movable table (not shown), and again. The process returns to the polarization beam splitter 3.

Thus, the light having the frequency ω ₂ ′ and the light having the frequency ω ₃ ′ emitted from the polarization beam splitter 3 via the reference optical path, and the polarization beam splitter 3 via the measurement optical path.
The light having the frequency ω ₂ ′ and the light having the frequency ω ₃ ′ emitted from the light source enter the frequency separation element 34 along the same optical path. Similar to the frequency separation element 34 is frequency coupling element 33, for example, a dichroic mirror, has the property of frequency transmits only light of omega ₁ near and reflects light of other frequencies. Therefore, the light of the frequency ω ₂ ′ and the light of the frequency ω ₃ ′ are separated from the light from the length measuring light source 1 (light near the frequency ω ₁ ) by the action of the frequency separating element 34.

The frequency ω reflected by the frequency separating element 34
Light 'light and the frequency omega _3' of _2, i.e., the light of the 'light and the frequency omega _3' of the frequency omega ₂ through the reference optical path, the 'light and the frequency omega _3' of the frequency omega ₂ through the measurement light path The light is incident on the polarization splitting element 35 composed of, for example, a polarization beam splitter along the same optical path. Polarization separating element 35, the light of the measuring light reflected by the movable mirror 5 is transmitted through the (light of 'light and the frequency omega _3' of the frequency omega _2), the reference light reflected by the fixed mirror 4 (the frequency omega ₂ ' And light of frequency ω ₃ ′).

Of the light of frequency ω ₂ ′ and the light of frequency ω ₃ ′ transmitted through the polarization separating element 35, the light of frequency ω ₂ ′ having a smaller frequency is subjected to the frequency ω ₃ ″ by the SHG conversion element 36.
(Ω ₃ ″ = 2ω ₂ ′). On the other hand, the light having the higher frequency ω ₃ ′ passes through the SHG conversion element 36 as it is. As a result, the light converted from the frequency ω ₂ ′ to the frequency ω ₃ ″ by the SHG conversion element 36 and the light having the frequency ω ₃ ′ reflected by the movable mirror 5 interfere with each other, and the interference light is converted into the photoelectric conversion element 37. Is detected by Also, the light of frequency ω ₂ ′ and the light of frequency ω ₃ ′ reflected by the polarization splitting element 35 are the same as the light converted from the frequency ω ₂ ′ to the frequency ω ₃ ″ by the action of the SHG conversion element 38. The interference light with the light of frequency ω ₃ ′ reflected by the fixed mirror 4
Is detected by

The polarization direction of the light having the frequency ω ₃ ″ subjected to the SHG conversion by the SHG conversion elements 36 and 38 is
The polarization azimuth of the light having the frequency ω ₃ ′ transmitted through the SHG conversion elements 36 and 38 as they are is different. SHG conversion element 36
In order to make the interference light between the light having the frequency ω ₃ ″ SHG-converted by the SHG conversion elements 36 and 38 and the light having the frequency ω ₃ ′ transmitted through the SHG conversion elements 36 and 38 as it is,
The polarization directions of the two different frequencies of light must match each other. Therefore, although not shown in FIG. 1, before or after the SHG conversion elements 36 and 38,
It is necessary to arrange an optical system (a plurality of wavelength plates) for adjusting the polarization directions of two different frequencies of light.

The interference signal detected by the photoelectric conversion element 37, ie, the measurement signal, and the interference signal detected by the photoelectric conversion element 39, ie, the reference signal, are input to the phase meter 40, respectively. In the phase meter 40, based on a phase change of the reference signal and the measurement signal, the optical path length variation with respect to the frequency omega ₃ light D
(Ω ₃ ) and optical path length change D for light of frequency ω ₂
(Ω ₂ ), that is, {D (ω ₃ ) −D (ω ₂ )}. ΔD (ω ₃ ) −D (ω obtained by the phase meter 40
₂ ) The signal related to｝ is supplied to the arithmetic unit 11.

The optical interference measuring apparatus shown in FIG. 1 is provided with a length measuring interferometer for measuring the displacement of the movable mirror 5 in the optical axis direction (the direction of the arrow in the figure). In the interferometer for length measurement,
The length measuring light source 1 is composed of two lights whose frequencies are slightly different from each other and whose polarization directions are orthogonal to each other, that is, the frequency ω _1.
And the light of frequency ω ₁ ′ (ω ₁ ′ = ω ₁ + Δω ₁ ) are emitted along the same optical path. Part of the orthogonal two-frequency light emitted from the length measuring light source 1 passes through the beam splitter 2 and then enters the frequency coupling element 33. Incidentally, as described above, frequency coupling element 33 has a characteristic that transmits only light of the frequency omega ₁ near and reflects light at other frequencies.

Accordingly, the light having the frequency ω _{1 and} the light having the frequency ω ₁ ′ transmitted through the frequency coupling element 33 enter the polarization beam splitter 3. The polarization beam splitter 3
Reflect light in the same light polarizing direction of the frequency omega ₁ ', are arranged so as to transmit the frequency omega ₁ of the light and the same light polarizing direction. Accordingly, the light of 'the frequency omega ₁ of the light' polarization beam light and the frequency omega ₁ of the frequency omega ₁ incident on the splitter 3 is reflected by the polarization beam splitter 3 and a corner cube prism as the reference light fixed mirror 4 It is led to. On the other hand, the frequency of light of the frequency omega ₁ of the light of omega ₁ of the light and the frequency omega ₁ 'is transmitted through the polarization beam splitter 3 is directed to the movable mirror 5 composed of a corner cube prism as the measurement light.

After being reflected by the fixed mirror 4, the reference light returns to the polarization beam splitter 3 again. The measurement light transmitted through the polarizing beam splitter 3 is also reflected by the movable mirror 5 and returns to the polarizing beam splitter 3 again. Thus, the frequency ω emitted from the polarization beam splitter 3 via the reference optical path
The light of ₁ ′ and the light of frequency ω ₁ emitted from the polarization beam splitter 3 via the measurement optical path enter the frequency separation element 34 along the same optical path. As described above, the frequency separation element 34 has a characteristic frequency transmits only light of omega ₁ near and reflects light of other frequencies. Therefore, the reference beam and the measuring beam frequency omega ₁ of the frequency omega ₁ 'is
The light passes through the frequency separation element 34. The reference light having the frequency ω ₁ ′ and the measurement light having the frequency ω ₁ transmitted through the frequency separation element 34 interfere with each other via the polarization element 6. The polarizing element 6
Is composed for example of polarizing plates disposed inclined by 45 ° relative to the polarization direction of light with the frequency omega ₁ of the light and the frequency omega ₁ '. The interference light generated via the polarizing element 6 is received by the photoelectric conversion element 7, and the photoelectric conversion element 7 outputs a measurement beat signal (frequency Δω ₁ ) based on the interference light to the phase meter 10.
To supply.

On the other hand, the light of the frequency ω ₁ ′ and a part of the light of the frequency ω ₁ emitted from the length measuring light source 1 are reflected by the beam splitter 2 and then enter the polarizing element 8.
The polarization element 8 has a frequency ω ₁ ′, like the polarization element 6.
A polarizing plate for causing interference between light and the frequency omega ₁ of the light. Therefore, the interference light between the light having the frequency ω ₁ ′ and the light having the frequency ω ₁ generated via the polarization element 8 is detected by the photoelectric conversion element 9. The photoelectric conversion element 9 has a frequency ω ₁ ′
The reference beat signal (frequency Δω ₁ ) based on the interference light between the light having the frequency ω ₁ and the light having the frequency ω ₁ is supplied to the phase meter 10. Phase meter 1
In the case of 0, the displacement D (ω ₁ ) of the movable mirror 5 that does not consider the influence of the refractive index fluctuation is obtained by measuring the phase change of the measurement beat signal with respect to the reference beat signal, and this displacement D (ω ₁ ) Is supplied to the arithmetic unit 11.

In the arithmetic unit 11, a signal relating to the displacement D (ω ₁ ) from the phase meter 10 and ΔD from the phase meter 40
Based on the signal related to (ω ₃ ) −D (ω ₂ )}, the true displacement D of the movable mirror 5 in which the measurement error caused by the refractive index fluctuation is corrected is obtained and output. Hereinafter, the displacement amount D of the movable mirror 5
The correction from (ω ₁ ) to the true displacement D will be described.
Optical path length change D for light of frequencies ω ₁ , ω ₂ and ω ₃
(Ω ₁ ), D (ω ₂ ) and D (ω ₃ ) are represented by the following equations (1) to (3), respectively.

[0039]

D (ω ₁ ) = [1 + NF (ω ₁ )] · D (1)

D (ω ₂ ) = [1 + N · F (ω ₂ )] · D (2)

D (ω ₃ ) = [1 + NF (ω ₃ )] · D (3)

Here, D is a geometric distance, and N is the density of air. F (ω) is a function that depends only on the frequency ω of the light without depending on the density of the air if the composition ratio of the air is unchanged. From the above equations (1) to (3), the geometric distance D is given by the following equation (4).

D = D (ω ₁ ) −A [D (ω ₃ ) −D (ω ₂ )] (4) where A = F (ω ₁ ) / [F (ω ₃ ) −F (Ω ₂ )].

D (ω ₃ ) −D of the second term on the right side of equation (4)
(Ω ₂ ) can be obtained by the phase meter 40 as described above. The first term D (ω ₁ ) on the right side can be obtained by the phase meter 10. Therefore, the computing unit 11 calculates the displacement amount D (ω ₁ ) measured by the length measuring interferometer by the operation formula of Expression (4) based on the output signal of the phase meter 40 and the output signal of the phase meter 10. After correction, the true displacement D can be obtained.

In this embodiment, based on the light of frequency ω ₂ emitted from the light source 31,
Frequency omega ₂ of the light and the frequency omega ₃ of the light is generated coaxially along the same optical path. Light of the light and the frequency omega ₃ frequency omega ₂ generated coaxially, by the action of the frequency modulation unit 101, after being frequency modulated in the light of 'light and the frequency omega _3' of the frequency omega _2, respectively, the same optical path Are injected coaxially. Therefore, in this embodiment, even if the emission direction of the light of frequency ω ₂ from the light source 31 changes with respect to the design optical axis, the light of frequency ω ₂ ′ emitted from the frequency modulation unit 101 and the frequency ω ₃ ′ of light will be affected by fluctuations in the emission direction of the frequency omega ₂ of the light to one another as well.

As a result, the light having the frequency ω ₂ ′ and the frequency ω
_Although the traveling direction of the ₃ ′ light may fluctuate with respect to the design optical axis, the light having the frequency ω ₂ ′ and the light having the frequency ω ₃ ′ have the same optical path whose refractive index is to be measured along the same optical path. Propagate inside. That is, since the coaxiality between the light having the frequency ω ₂ ′ and the light having the frequency ω ₃ ′ can be accurately ensured, the light emitted from the light source 31 is not affected by the change in the emission direction of the light having the frequency ω ₂ ,
(D (ω ₃ ) −D (ω ₂ )) can be obtained with high accuracy. Thus, in the present embodiment, it is possible to accurately secure the coaxiality between the light of the frequency ω ₂ ′ and the light of the frequency ω ₃ ′ without being affected by the fluctuation of the emission direction of the light from the light source. Using the interference method, the refractive index fluctuation of air in the optical path can be measured with high accuracy, and the measurement accuracy of the light wave interference measurement device can be improved.

FIG. 3 shows a first example of the frequency modulation unit 101 of FIG.
It is a figure which shows the structure of a modification roughly. The configuration of the first modification of FIG. 3 is similar to the configuration of the embodiment of FIG.
The only difference is that the collimating lens 52 is replaced by a diffraction grating 54. Therefore, in the first modified example, the frequency modulating unit 101 is coaxial along the same optical path.
The light having the frequency ω _{2 and} the light having the frequency ω ₃ incident on the diffraction grating 54 enter the diffraction grating 54 as the light 201 having the frequency ω ₂ ′ and the light 301 having the frequency ω ₃ ′ via the acousto-optic element 51. The diffraction angle of the diffraction grating 54 with respect to the light having the frequency ω ₂ ′ is about twice as large as the diffraction angle of the same order with respect to the light having the frequency ω ₃ ′ having a frequency twice as high.

Therefore, in the first modification, the acousto-optic device 5
1 so that the light 301 'light 201 and the frequency omega _3' of the diffracted frequency omega ₂ are parallel to each other at a distance through a diffraction grating 54, the diffraction in accordance with the diffraction angle of the acousto-optic element 51 The pitch of the grating 54 is defined. Thus, the light 201 having the frequency ω ₂ ′ parallel to each other via the acousto-optic element 51
The light 301 with the frequency ω ₃ ′ is coaxially coupled via a dichroic prism 55 which is a frequency coupling element. Note that the diffraction grating 54 may be an amplitude type or a phase type. In the first modification, it is advantageous that the required eccentricity accuracy of the diffraction grating 54 with respect to the design optical axis is relatively loose.

FIG. 4 shows a second example of the frequency modulation unit 101 of FIG.
It is a figure which shows the structure of a modification roughly. The configuration of the second modification of FIG. 4 is similar to the configuration of the embodiment of FIG.
The only difference is that a beam expander (56, 57) is used in place of the collimating lens 52 of FIG. In the embodiment of FIG. 2, only the collimating lens 52 may not be able to separate the light 300 'light 200 and the frequency omega _3' of the frequency omega ₂ at a sufficient distance. Therefore, in the second modification, a pair of positive lenses 56 and 5
7 is used.

According to the configuration of the second modification, the acousto-optic device 5
Light 30 'light 202 and the frequency omega _3' of the frequency omega ₂ through 1
2 can be separated sufficiently from each other. The light 202 at the frequency ω ₂ ′ and the light 302 at the frequency ω ₃ ′ via the beam expander are coaxially coupled via the dichroic prism 59 as a frequency coupling element. further,
By inserting the stop 58 in the optical path between the pair of positive lenses 56 and 57, background light (unnecessary light) such as zero-order light generated by the acousto-optic element 51 can be blocked. Further, for example, 'by the optical axis of the traveling direction and the beam expander optical 302 is configured to match the frequency omega ₂ as a pair of positive lenses 56 and 57' small frequency omega ₃ at diffraction angles and light It is not necessary to use a lens achromatized with light of the frequency ω ₃ ′.

FIG. 5 shows a third example of the frequency modulation section 101 of FIG.
It is a figure which shows the structure of a modification roughly. The configuration of the third modification of FIG. 5 is similar to the configuration of the embodiment of FIG.
The only difference is that a dispersing prism (wedge-shaped prism) 60 is used instead of the collimating lens 52. In the third modified example, the light 203 with the frequency ω ₂ ′ and the light 303 with the frequency ω ₃ ′ from the acousto-optic element 51
Become parallel and mutually separated light beams by the action of the dispersing prism 60. Frequency ω ₂ ′ via dispersive prism 60
Light 203 of frequency ω ₃ ′ and light 303 of frequency ω ₃ ′
Are coupled coaxially by a dichroic prism 61 which is a frequency coupling element that transmits light of the frequency ω ₃ ′ and reflects light of the frequency ω ₃ ′. It should be noted that the dispersion prism 60, which is a refracting optical system, is used.
Is set in accordance with the diffraction angle of the acousto-optic element 51.

FIG. 6 shows a fourth example of the frequency modulation section 101 of FIG.
It is a figure which shows the structure of a modification roughly. In the fourth modification, a pair of diffraction gratings 62 and 6 having the same pitch
An acousto-optic element 51 is provided in the optical path between the optical element 3 and the optical element 3. Therefore, the light of the frequency ω _{2 and} the light of the frequency ω ₃ that are coaxially incident on the diffraction grating 62 are diffracted at different diffraction angles, and the light 204 of the frequency ω ₂ and the light 30 of the frequency ω ₃
It becomes 4 and it is injected. The diffraction angle of the frequency omega ₂ of the light 204 is approximately twice the diffraction angle of the same order with respect to the light 304 of the frequency omega _3. Thus, the light 304 of the light 204 and the frequency omega ₃ frequency omega ₂ incident on the acousto-optic device 51 at different angles from each other, after being diffracted while being frequency modulated by the acoustic optical element 51, the light of the frequency omega ₂ '205
And light 305 of frequency ω ₃ ′ is emitted.

[0050] Incidentally, 'equal to the diffraction angle of the light 205, incident angle and frequency omega ₃ frequencies omega ₃ of the light 304' incident angle and frequency omega ₂ of the frequency omega ₂ of the light 204 and the diffraction angle of the light 305 The pitch between the pair of diffraction gratings 62 and 63 is set depending on the diffraction angle of the acousto-optic element 51 so that the two are equal. As a result, the diffraction grating 6
By the action of the diffraction grating 63 having the same pitch as 2 and and diffraction grating 62 are placed symmetrically, coupled coaxially to the light 305 'light 205 and the frequency omega _3' of the frequency omega _2.
Thus, in the fourth modification, the entire configuration of the frequency modulation unit 101 can be simplified without using a frequency coupling element such as a dichroic prism shown in the first, second, and third modifications. . In FIG. 6, the pair of diffraction gratings 62 and 63 and the acousto-optic element 51 are arranged at an interval, but the pair of diffraction gratings 62 and 63 are provided on both sides of a holder (not shown) of the acousto-optic element 51. It is also possible to paste directly.

FIG. 7 shows the fifth example of the frequency modulation section 101 of FIG.
It is a figure which shows the structure of a modification roughly. The configuration of the fifth modification of FIG. 7 is similar to the configuration of the fourth modification of FIG.
The only difference is that a pair of dispersion prisms 64 and 65 are used instead of the pair of diffraction gratings 62 and 63 in FIG. Therefore, the light of the frequency ω _{2 and} the light of the frequency ω ₃ that are coaxially incident on the dispersing prism 64 are refracted at different refraction angles, and then the light 20 of the frequency ω ₂ is refracted.
6 and light 306 of frequency ω ₃ are emitted. Thus, the frequency omega ₂ of the light 206 and the frequency omega ₃ light 306 incident on the acousto-optic device 51 at different angles from each other
Is frequency-modulated and diffracted by the acousto-optic device 51, and then emitted as light 207 at the frequency ω ₂ ′ and light 307 at the frequency ω ₃ ′.

[0052] Incidentally, 'equal to the diffraction angle of light 207, incident angle and frequency omega ₃ frequencies omega ₃ of the light 306' incident angle and frequency omega ₂ of the frequency omega ₂ of the light 206 and the diffraction angle of the light 307 Are equal, a pair of dispersing prisms 64 and 65
Is set depending on the diffraction angle of the acousto-optic element 51. As a result, the dispersion prism 64 is disposed symmetrically with respect to the acousto-optic element 51 and the dispersion prism 64.
Same by the action of the dispersion prism 65 having an apex angle, is coupled coaxially to the light 307 'light 207 and the frequency omega _3' of the frequency omega ₂ and. In FIG. 7, the pair of dispersion prisms 64 and 65 and the acousto-optic element 51 are arranged at an interval, but a pair of dispersion prisms 64 and 65 are provided on both sides of a holder (not shown) of the acousto-optic element 51. It is also possible to paste directly.

FIG. 8 shows a sixth example of the frequency modulation unit 101 of FIG.
It is a figure which shows the structure of a modification roughly. The configuration of the sixth modification of FIG. 8 is similar to the configuration of the fourth modification of FIG.
It is basically different only in that two frequency modulators shown in FIG. 6 are arranged in series. In the sixth modification, the heterodyne frequency can be reduced by using the two acousto-optical elements 51a and 51b.

As an example, assume that the modulation frequency Δf ₁ of the acousto-optic element 51a is 80 MHz and the modulation frequency Δf ₂ of the acousto-optic element 51b is 79.9 MHz. Then, for any of the frequency omega ₂ of the light and the frequency omega ₃ light, utilizing a first order diffracted light in the acousto-optic element 51a, it utilizes the -1st-order diffracted light in the acousto-optic device 51b. In this case, the light of the frequency ω _{2 and} the light of the frequency ω ₃ that are coaxially incident on the frequency modulation unit 101 become light 208 of the frequency ω ₂ and frequency ω ₃ 308 via the diffraction grating 66, respectively. The light 208 having the frequency ω ₂ and the frequency ω ₃ 308 are frequency-modulated by the acousto-optic element 51a, and the frequency ω ₂ ′ (= ω ₂ +
Δf ₁ ) light 209 and frequency ω ₃ ′ (= ω ₃ + Δf)
The light 309 of ₁ ) is incident on the diffraction grating 67.

The light 209 having the frequency ω ₂ ′ and the light 309 having the frequency ω ₃ ′ which are coaxially coupled via the diffraction grating 67 are converted into the light 210 having the frequency ω ₂ ′ and the frequency ω ₃ ′ via the diffraction grating 69, respectively. 310. The light 210 having the frequency ω ₂ ′ and the frequency ω ₃ ′ 310 are frequency-modulated by the acousto-optic device 51b, and the frequency ω ₂ ″ (= ω ₂ ′ −Δf ₂ = ω ₂ + Δf _1).
The light 211 of −Δf ₂ = ω ₂ +0.1 MHz and the frequency ω ₃ ″ (= ω ₃ ′ −Δf ₂ = ω ₃ + Δf ₁ −Δf ₂ = ω)
₍₃ + 0.1 MHz) and enters the diffraction grating 70. Light 211 of frequency ω ₂ ″ and frequency ω ₃ ″
Of light 311 are coaxially coupled via the diffraction grating 70,
The light is emitted from the frequency modulation unit 101 along the same optical path.

Thus, the frequency modulating unit 10 of the sixth modification is
In 1, the light frequency omega ₂ of the light and the frequency omega ₃ incident is subjected to only the frequency modulation 0.1MHz, respectively. The light 211 of the frequency ω ₂ ″ and the light 311 of the frequency ω ₃ ″ that are coaxially emitted from the frequency modulation unit 101 are guided to the reference optical path and the measurement optical path as described above with reference to FIG. Light at frequency ω ₂ ″ and frequency ω via the measurement optical path
The light of ω ₂ ″ having a small frequency among the light of ₃ ″ is SHG
The frequency ω ₃ ′ ″ (= 2ω ₂ ″) by the operation of the conversion element 36.
Interfering the conversion into light, and light converted frequency omega ₃ '''frequency omega ₃ through the light and the measurement optical path of''is. Here, actually, the frequency ω ₃ ′ ″ (= 2 (ω ₂ + Δf ₁ −Δf ₂ ))
Is subjected to frequency modulation of 0.2 MHz, and light having a frequency ω ₃ ″ (= ω ₃ + Δf ₁ −Δf ₂ ) is 0.1 MHz.
Frequency modulation. Accordingly, the beat frequency (Heterondain frequency) of the interference light of the light converted frequency omega ₃ '''frequency omega ₃ through the light and the measurement optical path of''becomes 0.1 MHz.

Meanwhile, the frequency omega ₂ via a reference light path '' of the light and the frequency omega ₃ '' of ₂ Do omega small frequency of the light ''
Is converted into light having a frequency ω ₃ ′ ″ (= 2ω ₂ ″) by the action of the SHG conversion element 38. Then, the light interferes of converted frequency omega ₃ '''frequency omega ₃ through the light and the reference light path of'', the beat frequency (Heterondain frequency) becomes 0.1 MHz. Thus, in the sixth modification, the heterodyne frequency can be reduced by using a pair of acousto-optic elements, so that the resolution of the apparatus, that is, the measurement accuracy is improved. It goes without saying that the pitch of the diffraction gratings 66, 67, 69 and 70 is set in accordance with the diffraction angles of the acousto-optic devices 51a and 51b. Also, a pair of diffraction gratings 67 and 69
By inserting the stop 68 in the optical path between the light source and the background light (unnecessary light) such as zero-order light generated in the acousto-optic element 51a.
Can be shut off.

FIG. 9 shows the seventh example of the frequency modulation section 101 of FIG.
It is a figure which shows the structure of a modification roughly. The configuration of the seventh modification of FIG. 9 is similar to the configuration of the sixth modification of FIG.
The only difference is that the pair of diffraction gratings 67 and 69 and the stop 68 in FIG. 8 are omitted and a dichroic prism 73 is provided. Therefore, in the seventh modification, the light of the frequency ω _{2 and} the light of the frequency ω ₃ that are coaxially incident on the frequency modulation unit 101 become the light 212 and the frequency ω ₃ 312 of the frequency ω ₂ via the diffraction grating 71, respectively. . The light 212 at frequency ω ₂ and the frequency ω ₃ 312
The frequency is modulated by the acousto-optic element 51a to become light 213 having a frequency ω ₂ ′ and light 313 having a frequency ω ₃ ′.

The light 213 having the frequency ω ₂ ′ and the light 313 having the frequency ω ₃ ′ via the acousto-optic element 51 a are frequency-modulated by another acousto-optic element 51 b, and the light 214 having the frequency ω ₂ ″ and the frequency ω ₃ ″ light 314 is incident on the diffraction grating 72. The light 214 having the frequency ω ₂ ″ and the light 314 having the frequency ω ₃ ″ become parallel to each other at an interval via the diffraction grating 72 and are coaxially coupled via a dichroic prism 73 which is a frequency coupling element. Is done.

FIG. 10 is a diagram schematically showing a configuration of an eighth modification of frequency modulating section 101 in FIG. Eighth of FIG.
The configuration of the modified example is similar to the configuration of the sixth modified example in FIG. 8, but is basically different only in that the pair of diffraction gratings 67 and 69 in FIG. 8 are omitted. Therefore, in the eighth modified example, the frequency ω ₂
The light of frequency ω ₃ and the light of frequency ω ₃ become light 215 of frequency ω ₂ and frequency ω ₃ 315 via the diffraction grating 74, respectively. Light 215 at frequency ω ₂ and frequency ω ₃ 315
Is frequency-modulated by the acousto-optic element 51a, and the frequency ω ₂ ′
Of light 216 and light 316 of frequency ω ₃ ′.

The light 216 having the frequency ω ₂ ′ and the light 316 having the frequency ω ₃ ′ via the acousto-optic element 51 a are frequency-modulated by another acousto-optic element 51 b, and the light 217 having the frequency ω ₂ ″ and the frequency ω ₃ ″ light 317 is incident on the diffraction grating 76. The light 217 having the frequency ω ₂ ″ and the light 317 having the frequency ω ₃ ″ are coaxially coupled via the diffraction grating 72. As compared with the seventh modified example of FIG. 9, in the eighth modified example, the distance between the pair of acousto-optical elements 51a and 51b is increased. However, the diaphragm 75 blocks unnecessary diffracted light generated in the acousto-optical element 51a. Can be. In the eighth modification, the dichroic prism 73 can be omitted.

FIG. 11 is a diagram schematically showing a configuration of a ninth modified example of frequency modulating section 101 in FIG. Ninth of FIG.
The configuration of the modification is similar to the configuration of the seventh modification of FIG. 9 except that a pair of dispersion prisms 77 and 78 are used instead of the pair of diffraction gratings 71 and 72 of FIG. only that it uses a different diffraction light of sign and order which each is basically differs with respect to frequency omega ₂ of the light and the frequency omega ₃ of light in the acousto-optic elements 51a and 51b.

In FIG. 11, the light having the frequency ω _{2 and} the light having the frequency ω ₃ which are coaxially incident on the frequency modulation unit 101 are:
After being refracted by the dispersion prism 77, the light 318 of the light 218 and the frequency omega ₃ frequency omega _2. Frequency ω ₂
The light 218 frequency omega ₂ by the acoustic optical element 51a '(-
(First-order diffracted light) light 219 is frequency-modulated, and the frequency ω ₃
Of the light 318 by the acousto-optic element 51a has a frequency ω ₃ ′ (+
It is frequency-modulated to light 319 (third-order diffracted light). afterwards,
Frequency omega ₂ 'light 219 of frequency omega ₂ by the acoustic optical element 51b' is frequency-modulated in the 'light 220 (+ 1st-order diffracted light),' frequency omega ₃ by the acoustic optical element 51b is a light 319 'frequency omega _3' ( The light is frequency-modulated to light 320 of (-3rd-order diffracted light).

The light 220 having the frequency ω ₂ ″ and the light 320 having the frequency ω ₃ ″ become parallel and spaced apart from each other by the dispersion prism 78, and are coaxially coupled by the dichroic prism 79 as a frequency coupling element. Are emitted from the frequency modulation unit 101 along the same optical path. Here, it goes without saying that the apex angles of the dispersing prisms 77 and 78 are set according to the diffraction angles of the acousto-optic elements 51a and 51b.

FIG. 12 is a diagram schematically showing a configuration of a tenth modification of frequency modulating section 101 in FIG. The configuration of the tenth modification of FIG. 12 is similar to the configuration of the ninth modification of FIG. 11, but a pair of acousto-optic elements 51a and 51b of FIG.
The only difference is that the stop 81 is widened and the diaphragm 81 is interposed, and the dichroic prism 79 is omitted.

In FIG. 12, the light having the frequency ω _{2 and} the light having the frequency ω ₃ which are coaxially incident on the frequency modulation unit 101 are:
The light 22 having the frequency ω ₂ refracted by the dispersing prism 80
1 and light 321 of frequency ω ₃ . Light 221 of the frequency omega ₂ is 'is a frequency modulated light 222 (-1-order diffracted light), frequency omega ₃ light 321 of frequency omega ₃ by acousto-optic element 51a' frequency omega ₂ by the acoustic optical element 51a (+3 order diffracted light ) Is frequency-modulated to the light 322. After that, the light 222 having the frequency ω ₂ ′ passes through the diaphragm 81, and is then transmitted by the acousto-optic device 51 b to the light 22 having the frequency ω ₂ ″ (+ 1st-order diffracted light).
3 is frequency-modulated.

On the other hand, the light 322 having the frequency ω ₃ ′ is
, The frequency ω ₃ ″
It is frequency-modulated to light (323-order diffracted light) 323. Thus, the light 223 having the frequency ω ₂ ″ and the light 323 having the frequency ω ₃ ″ are coaxially coupled by the dispersing prism 82 and emitted from the frequency modulation unit 101 along the same optical path.
As described above, in the tenth modification, the interval between the pair of acousto-optic elements 51a and 51b is widened as compared with the ninth modification, but the stop 81 blocks unnecessary diffracted light generated in the acousto-optic element 51a. be able to. In the tenth modification, the dichroic prism 79 can be omitted.

FIG. 13 is a diagram schematically showing a configuration of an eleventh modification of frequency modulating section 101 in FIG. Eleventh configuration modification of FIG. 13 is similar to configuration of the tenth modification of FIG. 12, the light with the frequency omega ₂ of the light and the frequency omega ₃ in a pair of acousto-optic elements 51a and 51b in FIG. 13 The only difference is that diffracted lights having the same sign and the same order are used.

In FIG. 13, the light having the frequency ω _{2 and} the light having the frequency ω ₃ which are coaxially incident on the frequency modulation unit 101 are:
After being refracted by the dispersion prism 83, the light 324 of the light 224 and the frequency omega ₃ frequency omega _2. Frequency ω ₂
The light 224 of the frequency ω ₂ ′ (−
(First-order diffracted light) 225 is frequency-modulated to a frequency ω ₃
The light 324 of the frequency ω ₃ ′ (−
It is frequency-modulated to light 325 (first-order diffracted light). afterwards,
Frequency omega ₂ 'light 225, after via a throttle 84, the frequency omega ₂ by the acoustic optical element 51b' is frequency modulated light 226 '(+ 1st-order diffracted light).

On the other hand, the light 325 having the frequency ω ₃ ′ is
, The frequency ω ₃ ″
(+ 1st-order diffracted light) is frequency-modulated to light 326. Thus, the light 226 having the frequency ω ₂ ″ and the light 326 having the frequency ω ₃ ″ are coaxially coupled by the dispersing prism 85 and emitted from the frequency modulation unit 101 along the same optical path.

In the frequency modulators of the above-described embodiments and the modifications, the order of the diffracted light selected by each acousto-optic element for each frequency light is selected without being limited to the above-described example. be able to. Further, the modulation frequency for the light of each frequency can be selected without being limited to the specific modulation frequency shown in the above-described embodiment and each modification. In the fourth to eleventh modifications, a diffraction grating or a dispersion prism is used as the correction optical system. However, a lens system may be used instead of the diffraction grating or the dispersion prism. In this case, the lens system is a lens group including a plurality of lenses, and the lens group needs to have chromatic aberration according to the diffraction angle of the acousto-optic element with respect to light of each frequency. When configuring a correction optical system with lens groups,
Although the configuration of the frequency modulation unit becomes complicated, the loss of the light amount can be reduced. In the case where the light emitted from the light source has a large light beam diameter or divergent light is emitted, light condensed on the acousto-optic element or light with a reduced light beam diameter can be made incident. Thereby, the diffraction efficiency in the acousto-optic device can be improved.

FIG. 14 is a diagram schematically showing a configuration of a refractive index fluctuation measuring system according to an embodiment of the present invention. In the refractive index variation measurement system of Figure 14, the light of the frequency omega ₂ emitted from the light source 86 enters the SHG device 87, the frequency omega ₂
A part of the light of the frequency ω ₃ (ω
₃ = 2ω ₂ ), and the rest is SHG light.
The light passes through the conversion element 87 as it is. SHG conversion element 87
Light of the light and the frequency omega ₃ injection frequency omega ₂ is incident to the frequency modulation unit 101 coaxially along the same optical path from.

The SHG conversion element 87 can be, for example, a nonlinear optical crystal KTiOPO ₄ . The polarization direction of the light having the frequency ω ₃ subjected to the SHG conversion by the SHG conversion element 87 is different from the polarization direction of the light having the frequency ω ₂ which is incident on the SHG conversion element 87 and transmitted as it is. Therefore, although not shown in FIG. 14, for example, between the SHG conversion element 87 and the frequency modulation unit 101, the polarization direction of the light of frequency ω ₂ and the polarization direction of the light of frequency ω ₃ are matched. Optical system (a plurality of wavelength plates) is arranged.

The frequency modulating unit 1 is coaxial along the same optical path.
Light with the frequency omega ₂ of the light and the frequency omega ₃ incident on 01 is incident on the acousto-optic device 88 as a frequency modulating device. In the acousto-optic element 88, the light having the frequency ω ₂
'Is a frequency modulated light 227, the frequency omega ₃ optical frequency omega _{3' 2} are frequency modulated light 327. Light 2 at frequency ω ₂ '
27 and the light 327 of the frequency ω ₃ ′ become parallel to each other by the collimating lens 89 and enter the dichroic prism 90. Thus, the light 227 having the frequency ω ₂ ′ and the frequency ω ₃ ′ are generated by the action of the dichroic prism 90.
The light 327 is coupled coaxially and emitted from the frequency modulation unit 101 along the same optical path.

The light of frequency ω ₂ ′ and the light of frequency ω ₃ ′ which are emitted coaxially along the same optical path via frequency modulation section 101
Is incident on a movable mirror 91 composed of a corner cube prism via an optical path whose refractive index fluctuation is to be measured. The light of the frequency ω ₂ ′ and the light of the frequency ω ₃ ′ reflected by the corner cube prism 91 enter the SHG conversion element 92 via the optical path whose refractive index variation is to be measured. In the SHG conversion element 92, of the light of the frequency ω ₂ ′ and the light of the frequency ω ₃ ′, the light of the frequency ω ₂ ′ having a smaller frequency is converted to the frequency ω ₃ ″ (ω
₃ ″ = 2ω ₂ ′) is subjected to SHG conversion, and light having a large frequency ω ₃ ′ passes through the SHG conversion element 92 as it is. As a result, the frequency ω ₂ ′
An optical frequency omega ₃ '' converted light and corner cube prism (movement mirror) to the frequency omega ₃ which is reflected by 91 'will interfere from the interference light is detected by the photoelectric conversion element 93.

The interference signal detected by the photoelectric conversion element 93 is input to the phase meter 94. In the phase meter 94, based on the phase of the interference signal, the optical path length variation with respect to the optical path length variation D (omega ₃₎ and the frequency omega ₂ of the light with respect to the frequency omega ₃ light D
(Ω ₂ ), that is, {D (ω ₃ ) −D (ω ₂ )}. ΔD (ω ₃ ) −D (ω obtained by the phase meter 94
₂ ) The signal related to｝ is supplied to the computing unit 95. As described above, the signal related to {D (ω ₃ ) −D (ω ₂ )} is a signal including information on the refractive index fluctuation. Therefore, the computing unit 95 can relatively determine the change in the refractive index at the time of measurement based on {D (ω ₃ ) −D (ω ₂ )}.

In the embodiment of FIG. 14, even if the emission direction of the light of frequency ω ₂ from the light source 86 fluctuates with respect to the design optical axis, the light of the frequency ω ₂ ′ The light of ω ₃ ′ is similarly affected by the variation of the emission direction of the light of frequency ω ₂ . As a result, the traveling directions of the light of frequency ω ₂ ′ and the light of frequency ω ₃ ′ may vary with respect to the design optical axis, but the frequency ω ₂ ′
_{The 2} ′ light and the frequency ω ₃ ′ light propagate along the same optical path. That is, the coaxiality between the light having the frequency ω ₂ ′ and the light having the frequency ω ₃ ′ can be accurately secured without being affected by the fluctuation of the emission direction of the light having the frequency ω ₂ from the light source 86. D ([omega] ₃ ) -D ([omega] ₂ )) can be determined with high precision, and thus the refractive index fluctuation of air in the optical path can be measured with high precision.

The embodiment of FIG. 14 shows an example in which the frequency modulation section 101 according to the embodiment of FIG. 2 is used. However, in the embodiment of FIG. 14, the frequency modulation unit 101 according to each of the modifications of FIGS. 3 to 13 may be used, or the frequency modulation unit 101 according to another appropriate configuration example of the present invention may be used. Good. In the embodiments and modifications described above, when measuring the refractive index variation, are used with optical frequencies omega ₃ with twice the frequency omega ₂ of the light and omega _2. However, in the present invention, it is important that two lights having different frequencies from each other passing through the optical path interfere with each other by making the frequency of one light substantially equal to the frequency of the other light. Therefore, for example, the refractive index fluctuation may be measured using a third harmonic or higher order light using a harmonic conversion element.

[0079]

As described above, according to the present invention, it is possible to accurately maintain the coaxiality of two lights without being affected by the fluctuation of the emission direction of the light from the light source. It can be used to measure the refractive index fluctuation of air in the optical path with high accuracy. As a result, with a simple configuration that does not require the conventional means for compensating for the variation in the direction of emission of light from the light source, the geometric displacement of the movable mirror, and thus the true displacement of the movable table, etc., can be measured with high accuracy. It becomes possible.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration of a light wave interference measuring apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram schematically showing an internal configuration of a frequency modulation unit 101 of FIG.

FIG. 3 is a diagram schematically showing a configuration of a first modification of the frequency modulation unit 101 of FIG. 1;

FIG. 4 is a diagram schematically showing a configuration of a second modification of the frequency modulation unit 101 in FIG. 1;

FIG. 5 is a diagram schematically showing a configuration of a third modification of the frequency modulation unit 101 in FIG. 1;

FIG. 6 is a diagram schematically showing a configuration of a fourth modification of the frequency modulation unit 101 in FIG. 1;

FIG. 7 is a diagram schematically showing a configuration of a fifth modification of the frequency modulation unit 101 in FIG. 1;

FIG. 8 is a diagram schematically showing a configuration of a sixth modification of the frequency modulation unit 101 in FIG. 1;

FIG. 9 is a diagram schematically showing a configuration of a seventh modification of the frequency modulation unit 101 in FIG. 1;

FIG. 10 is a diagram schematically showing a configuration of an eighth modification of the frequency modulation unit 101 in FIG. 1;

11 is a diagram schematically showing a configuration of a ninth modification example of the frequency modulation section 101 in FIG. 1. FIG.

FIG. 12 is a diagram schematically showing a configuration of a tenth modified example of the frequency modulation unit 101 in FIG. 1;

FIG. 13 is a diagram schematically showing a configuration of an eleventh modification of the frequency modulation unit 101 of FIG. 1;

FIG. 14 is a diagram schematically showing a configuration of a refractive index fluctuation measurement system according to an example of the present invention.

FIG. 15 is a diagram schematically showing a configuration of a conventional light wave interference measurement device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Measurement light source 2 Beam splitter 3 Polarization separation element 4, 5 Corner cube prism 6, 8 Polarizer 7, 9, 37, 39 Photoelectric conversion element 10, 40 Phase meter 11 Computing device 31 Light source 32, 36, 38 SHG conversion Element 33 Frequency coupling element 34 Frequency separation element 51 Acousto-optic element 52 Collimating lens 53 Dichroic prism 54 Diffraction grating 56, 57 Beam expander 58 Stop 60 Dispersion prism 101 Frequency modulator

Claims (19)

[Claims] 1. A length measuring interferometer for measuring a displacement amount of a movable mirror along a predetermined direction, and a refractive index variation for measuring a refractive index variation of a gas in an optical path of the length measuring interferometer. A measuring system, wherein the displacement is corrected by correcting the displacement amount of the movable mirror measured by the length measuring interferometer based on the refractive index fluctuation information of the gas in the optical path measured by the refractive index fluctuation measuring system. In a light wave interferometer for measuring a geometric displacement of a mirror, the refractive index fluctuation measuring system includes: a light source for supplying a first light having a first frequency; and a first light source from the light source. For converting a part of the light into second light having a second frequency different from the first frequency, and outputting the first light and the second light along the same optical path. Frequency conversion means, before being output from the frequency conversion means along the same optical path The third light and the fourth light are generated by frequency-modulating the first light and the second light respectively by a predetermined frequency, and the generated third light and the fourth light are the same. Frequency modulating means for outputting along the optical path of the third light and the fourth light via the measurement optical path of the length measuring interferometer, the frequency of one of the third light and the fourth light of the other light A first interference light generation system for generating a first interference light by heterodyne interference by making the frequency substantially coincide with the frequency, and the third light and the fourth light via a reference optical path of the length measuring interferometer A second interference light generation system for generating a second interference light by heterodyne interference by making the frequency of one of the lights substantially equal to the frequency of the other light, wherein the first interference light and the second In the optical path of the length measuring interferometer based on the interference light And measuring and correcting the refractive index fluctuation of the gas. 2. The apparatus according to claim 1, wherein the frequency conversion unit is a harmonic conversion element for converting a part of the first light into a harmonic and outputting the harmonic as the second light. The optical interference measurement apparatus according to claim 1. 3. The apparatus according to claim 1, wherein the frequency modulating means includes at least one frequency modulating element for frequency modulating each of the first light and the second light by a predetermined frequency, and via the at least one frequency modulating element. 3. The optical interference measurement apparatus according to claim 1, further comprising a correction optical system that couples the third light and the fourth light generated along the same optical path. 4. . 4. The frequency modulation device according to claim 3, wherein the frequency modulation device is an acousto-optic device using Bragg diffraction.
The light wave interference measurement device according to item 1. 5. The correction optical system for converting the third light and the fourth light generated through the frequency modulation element into light parallel to each other at an interval by refracting the third light and the fourth light. It has a refractive optical system and a frequency coupling element for coupling the third light and the fourth light, which have been converted into parallel with each other via the refractive optical system, along the same optical path. The optical interference measuring apparatus according to claim 3 or 4, wherein 6. The correction optical system for converting the third light and the fourth light generated through the frequency modulation element into light parallel to each other at an interval by diffracting the third light and the fourth light. A diffractive optical system, and a frequency coupling element for coupling the third light and the fourth light, which have been converted to be parallel to each other via the diffractive optical system, along the same optical path. The optical interference measuring apparatus according to claim 3 or 4, wherein 7. The correction optical system is provided on a first correction optical system provided closer to the frequency conversion unit than the frequency modulation element, and is provided on the first interference light generation system side more than the frequency modulation element. A second correction optical system, wherein the first correction optical system diffracts the first light and the second light incident on the frequency modulation means along the same optical path. Wherein the second correction optical system diffracts the third light and the fourth light generated through the frequency modulation element to generate the third light and the fourth light. The optical interference measuring apparatus according to claim 3 or 4, wherein the optical interference measuring apparatus is a diffractive optical system for coupling the light beam along the same optical path. 8. The correction optical system is provided on a first correction optical system provided closer to the frequency conversion unit than the frequency modulation element, and is provided on the first interference light generation system side more than the frequency modulation element. A second correction optical system, wherein the first correction optical system is a refraction optic for refracting the first light and the second light incident on the frequency modulation means along the same optical path. Wherein the second correction optical system refracts the third light and the fourth light generated through the frequency modulation element, thereby forming the third light and the fourth light. The optical interference measuring apparatus according to claim 3 or 4, wherein the optical interference measuring apparatus is a refraction optical system for coupling the light beams along the same optical path. 9. The frequency modulation unit includes: a first frequency modulation element for frequency-modulating each of the first light and the second light by a first frequency; and a first frequency modulation element. The two lights passing through the first
And a second frequency modulation element for frequency-modulating only a second frequency having a sign different from that of the first frequency and having an absolute value slightly different from the absolute value of the first frequency. The light wave interference measurement device according to 3 or 4. 10. The correction optical system according to claim 1, wherein the correction optical system is a first frequency modulation element provided on a side closer to the frequency conversion unit than the first frequency modulation element.
A correction optical system, a second correction optical system provided closer to the first interference light generation system than the first frequency modulation device, and a correction optical system provided closer to the frequency conversion unit than the second frequency modulation device. A third correction optical system, and a fourth correction optical system provided closer to the first interference light generation system than the second frequency modulation element, wherein the first correction optical system has the same optical path. Is a diffractive optical system for diffracting the first light and the second light incident on the frequency modulation means along the axis, and the second correction optical system is provided via the first frequency modulation element. By diffracting the two generated lights,
A diffractive optical system for combining two lights along the same optical path, wherein the third correction optical system diffracts the two lights incident along the same optical path via the second correction optical system. The fourth correction optical system is configured to diffract the third light and the fourth light generated through the second frequency modulation element so as to diffract the third light. The optical interference measuring apparatus according to claim 9, wherein the optical interference measuring apparatus is a diffractive optical system for coupling the light and the fourth light along the same optical path. 11. The correction optical system according to claim 1, further comprising: a first optical modulator provided on a frequency conversion unit side with respect to the first frequency modulation element.
A correction optical system, a second correction optical system provided closer to the first interference light generation system than the first frequency modulation device, and a correction optical system provided closer to the frequency conversion unit than the second frequency modulation device. A third correction optical system, and a fourth correction optical system provided closer to the first interference light generation system than the second frequency modulation element, wherein the first correction optical system has the same optical path. Is a refraction optical system for refracting the first light and the second light incident on the frequency modulation means along the axis, and the second correction optical system is provided via the first frequency modulation element. By refracting the two generated lights,
A third correcting optical system for refracting the two lights incident along the same optical path via the second correcting optical system. The fourth correction optical system is configured to refract the third light and the fourth light generated through the second frequency modulation element, thereby refracting the third light. 10. The optical interference measuring apparatus according to claim 9, wherein the optical interference measuring apparatus is a refractive optical system for coupling the light and the fourth light along the same optical path. 12. The correction optical system according to claim 1, wherein said correction optical system is provided on a first frequency conversion element side of said first frequency modulation element.
A correction optical system, and a second correction optical system provided closer to the first interference light generation system than the second frequency modulation element, wherein the first correction optical system is arranged along the same optical path. A diffractive optical system for diffracting the first light and the second light incident on the frequency modulating means, wherein the second correction optical system includes the first frequency modulating element and the second frequency A diffractive optical system for diffracting the third light and the fourth light generated through a modulation element to combine the third light and the fourth light along the same optical path. The optical interference measuring apparatus according to claim 9, wherein: 13. The correction optical system according to claim 1, wherein the first optical modulator is provided on a side of the frequency conversion unit with respect to the first frequency modulation element.
A correction optical system, and a second correction optical system provided closer to the first interference light generation system than the second frequency modulation element, wherein the first correction optical system is arranged along the same optical path. A refraction optical system for refracting the first light and the second light incident on the frequency modulation unit, wherein the second correction optical system includes the first frequency modulation element and the second frequency. A refraction optical system for refracting the third light and the fourth light generated through a modulation element to couple the third light and the fourth light along the same optical path; The optical interference measuring apparatus according to claim 9, wherein: 14. The correction optical system according to claim 1, wherein the correction optical system is provided on a first frequency conversion element side of the first frequency modulation element.
A correction optical system, and a second correction optical system provided closer to the first interference light generation system than the second frequency modulation element, wherein the first correction optical system is arranged along the same optical path. A diffractive optical system for diffracting the first light and the second light incident on the frequency modulating means, wherein the second correction optical system includes the first frequency modulating element and the second frequency A diffractive optical system for diffracting the third light and the fourth light generated through a modulation element to convert the third light and the fourth light into light parallel to each other at an interval, and The light wave interference measuring apparatus according to claim 9, further comprising a frequency coupling element for coupling the third light and the fourth light along the same optical path. 15. The correction optical system according to claim 1, further comprising: a first optical modulator provided closer to the frequency converter than the first frequency modulator.
A correction optical system, and a second correction optical system provided closer to the first interference light generation system than the second frequency modulation element, wherein the first correction optical system is arranged along the same optical path. A refraction optical system for refracting the first light and the second light incident on the frequency modulation unit, wherein the second correction optical system includes the first frequency modulation element and the second frequency. A refractive optical system for converting the third light and the fourth light generated through a modulation element into light parallel to each other at an interval by refracting the third light and the fourth light; The light wave interference measuring apparatus according to claim 9, further comprising a frequency coupling element for coupling the third light and the fourth light along the same optical path. 16. A refractive index fluctuation measuring system for measuring a refractive index fluctuation of a gas in a predetermined optical path, comprising: a light source for supplying a first light having a first frequency; A part of the first light is converted to a second light having a second frequency different from the first frequency, and the first light and the second light are output along the same optical path. Frequency conversion means for converting the first light and the second light output from the frequency conversion means along the same optical path by a predetermined frequency, respectively. Frequency modulating means for generating the third light and the fourth light generated along the same optical path; and the third light and the third light passing through the optical path. Make the frequency of one of the four lights substantially equal to the frequency of the other light And an interference light generation system for generating interference light due to heterodyne interference, wherein a refractive index variation of a gas in an optical path of the length measuring interferometer is measured based on the interference light. Refractive index fluctuation measurement system. 17. A frequency conversion device for converting a part of the first light into a higher harmonic, and outputting the higher harmonic as the second light. The refractive index fluctuation measurement system according to claim 16. 18. The apparatus according to claim 18, wherein the frequency modulating means includes at least one frequency modulating element for frequency modulating each of the first light and the second light by a predetermined frequency, and via the at least one frequency modulating element. 18. The refractive index fluctuation measurement according to claim 16, further comprising a correction optical system for coupling the generated third light and the fourth light along the same optical path. system. 19. The refractive index fluctuation measurement system according to claim 18, wherein the frequency modulation element is an acousto-optic element using Bragg diffraction.