JPH11166807A - Frequency separation device and light wave interference measurement device provided with the frequency separation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To cause two light beams of different frequencies that impinge along the same optical path to be separated at a sufficient interval from each other. SOLUTION: A frequency separation device has a deflection means 1 for deflecting and guiding along a first optical path 202 a first light beam of a first frequency which impinges along a predetermined optical path and for deflecting and guiding, along a second optical path 203 different from the first optical path, a second light beam of a second frequency which impinges along the same optical path 201 as the first beam, and has a separation means 11 having a light separating surface 11a for reflecting almost the entire first light beam that impinges thereon via the deflection means 1 and for transmitting almost the entire second beam that impinges thereon via the deflection means 1. The frequency separation device can separate the first and second light beams that impinge coaxially along the common optical axis, and guide them along the two sufficiently spaced optical paths, i.e., the two light beams of different frequencies that impinge thereon along the same optical path can be separated at a sufficient interval from each other.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency separation device and an optical interference measuring device provided with the frequency separation device, and more particularly to a frequency separation device for separating two lights having different frequencies incident on the same optical path. It is about.

[0002]

2. Description of the Related Art For example, JP-A-6-229922 and JP-A-8-226802 disclose a refractive index fluctuation measuring device for measuring a refractive index fluctuation of a gas in an optical path with high accuracy, and a method of measuring the refractive index. A light wave interference measurement device for performing high-precision displacement measurement by incorporating a rate fluctuation measurement device is disclosed. In the devices disclosed in these publications, a second harmonic is generated from a part of the fundamental wave via a nonlinear crystal such as a KTP crystal, and the generated second harmonic and a fundamental wave transmitted directly through the nonlinear crystal. Are propagated along the same optical path. Then, a part of the fundamental wave passing through the predetermined optical path is converted into the second harmonic via the nonlinear crystal, and the converted second harmonic interferes with the original second harmonic passing through the predetermined optical path. In this manner, the refractive index fluctuation of the gas in the optical path is measured with high accuracy based on the interference fringes obtained depending on the dispersion characteristics of the gas in the optical path.

[0003]

As described above, a part of the fundamental wave passing through the predetermined optical path is converted into the second harmonic through the nonlinear crystal, and the remaining fundamental wave which is not converted into the second harmonic is Transmits the nonlinear crystal as it is. When the fundamental wave light transmitted through the nonlinear crystal as it is incident on the photoelectric conversion element together with the second harmonic interference light, it causes a reduction in signal visibility. Therefore, in the related art, in order to remove light of a fundamental wave, an optical filter including a multilayer film is arranged in an optical path between a nonlinear crystal and a photoelectric conversion element.

However, in general, the performance of an optical filter having a multilayer film is not so high, and light of a fundamental wave cannot be sufficiently removed. When a dispersing prism is used, two lights having different frequencies can be completely separated from each other. However, one of the separated lights is separated with a sufficient interval so as not to enter the photoelectric conversion element together with the other light. In this case, the required optical path length becomes large, and the entire apparatus becomes large.

The present invention has been made in view of the above-mentioned problem, and has been made in consideration of the above problem.
It is an object of the present invention to provide a small-sized frequency separation device capable of separating two lights at a sufficient interval, and a light wave interference measurement device provided with the frequency separation device.

[0006]

According to a first aspect of the present invention, a first light having a first frequency incident along a predetermined optical path is converted to the first frequency. A second light having a second frequency different from the first frequency incident along the same optical path as the first light is deflected by a dependent angle and guided along the first optical path. Deflecting means for deflecting by an angle dependent on the frequency of 2 and guiding it along a second optical path different from the first optical path, and the deflecting means incident along the first optical path via the deflecting means. A light separating surface for reflecting substantially all of the first light and transmitting substantially all of the second light incident along the second optical path via the deflecting means. A frequency separation device is provided.

According to a preferred aspect of the first invention, the separating means is configured such that the second light becomes P-polarized light with respect to the light separation surface, and the incident angle of the second light with respect to the light separation surface is Brew. The first light is arranged so as to be substantially equal to the star angle, and the incident angle of the first light to the light separation surface is substantially equal to or larger than the critical angle.

According to a second aspect of the present invention, a length measuring interferometer for measuring an amount of displacement of a movable mirror along a predetermined direction, a first light having a first frequency, and a A light source for supplying a second light having a second frequency different from the first frequency, and a part of the first light output from the light source and passing through a measurement optical path through the movable mirror; Frequency conversion means for converting the light into third light having substantially the same frequency as the frequency of the second light and transmitting the second light having passed through the remaining light of the first light and the measurement light path as it is; An interference signal generation system for generating an interference signal based on interference light between the third light and the second light via the means, wherein the measurement light path is measured based on the interference signal. The interferometer for length measurement according to the refractive index fluctuation due to the gas In the light wave interference measuring apparatus for correcting the measured displacement amount of the movable mirror which is measured by the above, and measuring the geometric displacement amount of the movable mirror, the light wave interferometer may be provided between the frequency conversion means and the interference signal generation system. Item 8 is provided with the frequency separation device according to any one of Items 1 to 7, wherein the frequency separation device includes a remaining portion of the first light and a second light that are incident along the same optical path from the frequency conversion unit. And the third light are separated from each other, and the second light and the third light are guided to the interference signal generation system, and the remaining part of the first light is not caused to reach the interference signal generation system. Provided is a light wave interference measurement device characterized by being removed from an optical path.

According to a preferred aspect of the second invention, the frequency conversion means converts a part of the first light into a harmonic, and outputs a harmonic as the third light. It has a conversion element.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a frequency separation device according to the present invention, the action of a deflecting means such as a diffraction grating or a dispersion prism is used.
A first having a first frequency incident along a predetermined optical path;
And deflects the light along the first optical path, and deflects the second light having the second frequency incident along the same optical path along the second optical path different from the first optical path. Lead. Next, for example, a light separating surface of a separating means such as a prism reflects almost all of the first light incident along the first optical path, and reflects the second light incident along the second optical path. Almost all are transmitted. Specifically, the second
Is P-polarized with respect to the light separation surface, the incident angle of the second light with respect to the light separation surface is substantially equal to the Brewster angle, and the incident angle of the first light with respect to the light separation surface is substantially equal to or greater than the critical angle. Set to be.

Thus, in the frequency separation device of the present invention,
The first light and the second light incident coaxially along a common optical axis are separated and guided along two sufficiently spaced optical axes, respectively. In other words, two lights having different frequencies incident on the same optical path can be separated with a sufficient interval. Further, even if the first light and the second light separated by the deflecting means such as the diffraction grating and the dispersion prism partially overlap on the light separation surface, there is no influence on the light separating action. For this reason, the size of the frequency separation device can be easily reduced by shortening the distance between the diffraction grating or dispersing prism as the deflecting means and the prism as the separating means as necessary.

In a light wave interference measuring apparatus incorporating a refractive index fluctuation measuring device for measuring a refractive index fluctuation of a gas in an optical path,
Part of the first light that has passed through the measurement optical path is converted to a second harmonic via frequency conversion means such as a second harmonic converter (SHG converter). On the other hand, the rest of the first light and the second light that has passed through the measurement optical path pass through the SHG conversion element as it is. Here, the first light that has passed through the SHG conversion element as it is, as well as the second harmonic light that has been converted through the SHG conversion element and the second light that has passed through the SHG conversion element as it is, such as a photoelectric conversion element When the light enters the interference signal generation system, it causes a decrease in signal visibility.

In the light wave interference measuring device of the present invention, the frequency separating device according to the present invention is arranged in the optical path between the SHG converting device as the frequency converting means and the photoelectric converting device as the interference signal generating system. . Therefore, the first light that has passed through the SHG conversion element as it is is removed from the optical path without reaching the photoelectric conversion element. As a result, a signal with good visibility can be obtained, so that the refractive index fluctuation due to gas in the optical path can be measured with high precision, and thus the geometric displacement of the movable mirror, that is, the true displacement can be measured with high precision. It becomes possible.

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 frequency separation device according to a first embodiment of the present invention. As shown in FIG. 1, the frequency separation device according to the first embodiment includes a diffraction grating 1 and a light separation element 11 in order from the light incident side (left side in the figure). Two lights having different frequencies enter the diffraction grating 1 coaxially along the same optical path. That is, the light having the frequency ω1 and the light having the frequency ω2 (ω1 <ω2) higher than the frequency ω1 enter the diffraction grating 1 along the common optical axis 201.

In the diffraction grating 1, the light having the frequency ω1 is diffracted at a larger diffraction angle than the light having the frequency ω2 because the diffraction action is performed at an angle inversely proportional to the frequency of the incident light. Thus, the light of frequency ω1 diffracted by the diffraction grating 1 is
Along the optical axis 203, the light of the frequency ω2 diffracted by the diffraction grating 1 is incident on the light separating element 11 along the optical axis 203. As described above, since the diffraction angle of the light having the frequency ω1 is larger than the diffraction angle of the light having the frequency ω2, the optical axis 202 is more inclined than the optical axis 203 with respect to the optical axis 201.

The light separating element 11 converts the light of frequency ω2 into P-polarized light (that is, light having a polarization direction parallel to the plane of FIG. 1) with respect to the light separating surface 11a, and converts the light of frequency ω2 with respect to the light separating surface 11a. They are arranged so that the incident angle becomes equal to the Brewster angle and the incident angle of the light having the frequency ω1 with respect to the light separation surface 11a is equal to or larger than the critical angle.
Therefore, the P-polarized light having a Brewster angle and having a frequency of ω2 that is incident on the light separating surface 11a passes through the light separating surface 11a without being reflected by the light separating surface 11a, and travels straight along the optical axis 203. On the other hand, the light of frequency ω1 incident on the light separating surface 11a at an incident angle equal to or larger than the critical angle is totally reflected by the light separating surface 11a without transmitting through the light separating surface 11a, and the optical axis 2
Guided along 04.

Thus, in the frequency separation device of the first embodiment, the frequency ω incident coaxially along the common optical axis 201
The light of frequency 1 and the light of frequency ω2 are separated and guided along two optical axes that are sufficiently separated, that is, optical axis 204 and optical axis 203, respectively. In other words, the frequency separation device of the first embodiment can separate two lights having different frequencies incident along the same optical path at a sufficient interval. Further, even if the light of the frequency ω1 incident along the optical axis 202 and the light of the frequency ω2 incident along the optical axis 203 partially overlap on the light separating surface 11a, the light separating effect is not affected. There is no diffraction grating 1 and light separation element 1
The size of the frequency separation device can be easily reduced by shortening the distance between the frequency separation device 1 and the device 1 if necessary.

FIG. 2 is an enlarged view specifically showing the configuration of a light separating element usable in the frequency separating device of the first embodiment. The light separating element 11 shown in FIG. 2 includes a triangular prism 51 and a triangular prism 53 arranged so as to form a medium layer 52 between the triangular prism 51 in this order from the light incident side (left side in FIG. 2). And a triangular prism 5
The boundary surface between 1 and the medium layer 52 forms the light separation surface 11a. Here, it is important that the refractive index of the triangular prism 51 and the refractive index of the triangular prism 53 are set to be larger than the refractive index of the medium layer 52. Therefore, the triangular prisms 51 and 53 may be formed of a suitable optical glass material such as BK7, and an air layer may be formed as the medium layer 52 between the two triangular prisms.

FIG. 3 is a diagram schematically showing the configuration of a frequency separation device according to a second embodiment of the present invention. The second embodiment has a configuration similar to that of the first embodiment. However, the second embodiment is different from the first embodiment only in that a dispersing prism 2 is disposed instead of the diffraction grating 1. In the second embodiment shown in FIG. 3, light having a frequency ω1 and light having a frequency ω2 (ω1 <ω2) higher than the frequency ω1 enter the dispersion prism 2 along a common optical axis 211.

In the dispersing prism 2, the light having the frequency ω2 is deflected at a larger refraction angle (dispersion angle) than the light having the frequency ω1, because the light is refracted at an angle proportional to the frequency of the incident light. Thus, the frequency ω refracted by the dispersion prism 2
The light of No. 1 is incident on the light separating element 11 along the optical axis 212, and the light of frequency ω2 diffracted by the dispersing prism 2 is incident on the optical separation element 11 along the optical axis 213. As described above, since the refraction angle of the light having the frequency ω1 is smaller than the refraction angle of the light having the frequency ω2, the optical axis 213 is more inclined than the optical axis 212 with respect to the optical axis 211. Note that the light separating element 11 is
Like the embodiment, it has the configuration shown in FIG.

The light separating element 11 converts the light having the frequency ω1 into P-polarized light (that is, light having a polarization direction parallel to the paper surface of FIG. 3) with respect to the light separating surface 11a and the light having the frequency ω1 with respect to the light separating surface 11a. They are arranged so that the incident angle becomes equal to the Brewster angle and the incident angle of the light having the frequency ω2 with respect to the light separating surface 11a is equal to or larger than the critical angle.
Therefore, the P-polarized light having the frequency ω1 incident on the light separating surface 11a at the Brewster angle passes through the light separating surface 11a without being reflected by the light separating surface 11a, and travels straight along the optical axis 212. On the other hand, the light of frequency ω2 incident on the light separation surface 11a at an incident angle equal to or greater than the critical angle is totally reflected by the light separation surface 11a without transmitting through the light separation surface 11a, and the optical axis 2
Guided along 14.

Thus, in the frequency separation device of the second embodiment, similarly to the first embodiment, the light of the frequency ω1 and the light of the frequency ω2, which are incident coaxially along the common optical axis 211, are separated from each other. Are guided along two optical axes spaced apart from each other, namely, an optical axis 212 and an optical axis 214. Further, the light having the frequency ω1 incident along the optical axis 212 and the light having the frequency ω2 incident along the optical axis 213 are separated by the light separating surface 1.
1a does not affect the light separating action even if they partially overlap each other.
The size of the frequency separation device can be easily reduced by shortening the interval as necessary.

FIG. 4 is a diagram schematically showing a configuration of a frequency separation device according to a third embodiment of the present invention. The third embodiment has a configuration similar to that of the first embodiment. However, the third embodiment is different from the first embodiment only in that a light separating element is formed by one triangular prism. In the frequency separation device according to the third embodiment, the light having the frequency ω1 and the frequency ω
Light having a frequency ω2 (ω1 <ω2) greater than 1 is incident on the diffraction grating 1 along a common optical axis 221.

The light having the frequency ω 1 diffracted by the diffraction grating 1 travels along the optical axis 222 at the frequency ω
The two lights enter the light separating element 12 along the optical axis 223, respectively. The light separating element 12 is composed of one triangular prism, and an end face 12a facing the incident end face 12b forms a light separating surface. Further, the light separating element 11 converts the light of the frequency ω2 into P-polarized light (that is, light having a polarization direction parallel to the paper surface of FIG. 4) with respect to the light separating surface 12a, and makes the light of the frequency ω2 incident on the light separating surface 12a. The angle is equal to the Brewster angle, and the incident angle of the light having the frequency ω1 with respect to the light separating surface 12a is set to be equal to or larger than the critical angle.

Therefore, the P-polarized light having a frequency of ω2 incident on the light separating surface 12a at the Brewster angle passes through the light separating surface 12a without being reflected by the light separating surface 12a,
It travels straight along the optical axis 223. On the other hand, light having a frequency ω1 incident on the light separating surface 12a at an incident angle equal to or larger than the critical angle is totally reflected by the light separating surface 12a without transmitting through the light separating surface 12a, and is guided along the optical axis 224. Thus, also in the frequency separation device of the third embodiment, the light having the frequency ω1 and the light having the frequency ω2 which are coaxially incident along the common optical axis 221 are separated, and two optical axes which are sufficiently separated, that is, The light is guided along the optical axis 224 and the optical axis 223, respectively.

Further, even if the light of frequency ω1 incident along the optical axis 222 and the light of frequency ω2 incident along the optical axis 223 partially overlap on the light separating surface 12a, the light is separated. Therefore, the size of the frequency separation device can be easily reduced by shortening the distance between the diffraction grating 1 and the light separation element 12 as necessary. In the third embodiment, it goes without saying that a dispersion prism can be used instead of the diffraction grating 1.

FIG. 5 is a diagram schematically showing a configuration of an optical interference measuring apparatus according to a fourth embodiment of the present invention. FIG. 6 is a diagram schematically showing an internal configuration of frequency shift section 170 in FIG. The optical interference measuring apparatus according to the fourth embodiment includes a pair of frequency separating devices, and each frequency separating device has the internal configuration shown in the first embodiment. The light wave interference measurement apparatus shown in FIG. 5 supplies light for measuring the change in the refractive index of the gas in the optical path and light for measuring the displacement of the movable mirror 105 in the optical axis direction (the direction of the arrow in the figure). Are provided. The light source 101 has a frequency ω
₂ and its second harmonic, frequency ω ₃ (ω ₃ = 2 ×
ω ₂ ) is emitted along the same optical path. Light source 101
The light of frequency ω _{2 and} the light of frequency ω ₃ emitted from
The light enters the frequency shift unit 170 along the same optical path.

Referring to FIG. 6, frequency shift section 170
Light having a frequency of ω ₃ incident on the dichroic mirror 1
The light passes through 71 and enters the polarization beam splitter 172. Frequency ω reflected by polarization beam splitter 172
₃ of the light after passing through the reflecting mirror 175, for example, enters the frequency shifter 176 consisting of acousto-optic device, the frequency omega ₃ different slightly in frequency from the frequency omega ₃ by the frequency shifter _{176 '(= ω 3 + Δω} a ) Is frequency-shifted to light. On the other hand, light of frequency omega ₃ transmitted through the polarizing beam splitter 172, a frequency shifter 173, frequency omega ₃
Is slightly different from the frequency ω ₃ ″ (= ω ₃ + Δ
ω _a ′).

The light of the frequency ω ₃ ′ passing through the frequency shifter 176 is reflected by the reflecting mirror 177,
The light having the frequency ω ₃ ″ passing through each of the polarization beam splitters ₃ directly enters the polarization beam splitter 174. Thus, the light having the frequency ω ₃ ′ and the light having the frequency ω ₃ ″, which are coupled along the same optical path again by the polarization beam splitter 174, enter the dichroic mirror 184.

On the other hand, the light of frequency ω ₂ from the light source 101 is reflected by the dichroic mirror 171 and then enters the polarization beam splitter 178. Light of frequency omega ₂ is reflected by the polarizing beam splitter 178, is frequency shifted to the optical frequency omega ₂ different slightly in frequency from the frequency _{ω 2 '(= ω 2 +} Δω b) by the frequency shifter 179. Further, the light of frequency ω ₂ transmitted through the polarization beam splitter 178 is transmitted through the reflection mirror 181 to the frequency shifter 18.
Incident on 2, the frequency omega ₂ of different slightly in frequency from the frequency omega ₂ by the frequency shifter _{182 '' (= ω 2 +} Δ
ω _b ′). The frequency shift amounts Δω _a , Δω _a ′, Δω _b, and Δω _b ′ in the frequency shifters 176, 173, 179, and 182 are different from each other.

The light of frequency ω ₂ ′ via frequency shifter 179 is directly transmitted to frequency ω ₂ ″ via frequency shifter 182.
Are reflected by the reflecting mirror 183 and then enter the polarization beam splitter 180. Thus, the light having the frequency ω ₂ ′ and the light having the frequency ω ₂ ″, which are coupled along the same optical path again by the polarization beam splitter 180, enter the dichroic mirror 184. Light at frequency ω ₃ ′ and frequency ω combined by dichroic mirror 184
The light of ₃ ″, the light of frequency ω ₂ ′, and the light of frequency ω ₂ ″ are transmitted through the beam splitter 185 to the frequency shift unit 1.
Emitted from 70 along the same optical path.

Returning to FIG. 5, the frequency shift unit 170
The light having the frequency ω ₃ ′, the light having the frequency ω ₃ ″, the light having the frequency ω ₂ ′, and the light having the frequency ω ₂ ″, which are emitted from the polarizing beam splitter 103, are reflected by the reflecting mirror 154. Incident. Polarization beam splitter 103 is arranged to transmit light of 'light and the frequency omega _3' of 'frequency omega ₂ and the frequency omega ₂ reflects the light of' light and the frequency omega ₃ of ''. Therefore, the light of the frequency ω ₂ ′ and the light of the frequency ω ₃ ′ reflected by the polarization beam splitter 103 become reference light and pass through the reference optical path. That is, the light is emitted from the polarizing beam splitter 103 after reciprocating twice in the optical path between the polarizing beam splitter 103 and the corner cube 104 as a fixed mirror by the action of the corner cube 106.

The light having the frequency ω ₂ ″ and the light having the frequency ω ₃ ″ transmitted through the polarization beam splitter 103 become measurement light and pass through the measurement optical path. That is, the light is emitted from the polarizing beam splitter 103 after reciprocating two times in the optical path between the polarizing beam splitter 103 and the corner cube 105 as a movable mirror by the action of the corner cube 106. Measurement light emitted from the polarizing beam splitter 103 (that is, light of frequency ω ₂ ″ and light of frequency ω ₃ ″)
Enters the beam splitter 155 after passing through the reflecting mirrors 107 and 108. Similarly, the reference light (that is, the light of the frequency ω ₂ ′ and the light of the frequency ω ₃ ′) emitted from the polarization beam splitter 103 is directly transmitted to the beam splitter 15.
5 is incident.

[0034] In the beam splitter 155, part of the light of the 'light and the frequency omega ₂ of the' reference light and the frequency omega ₂ of the measuring light 'is transmitted, light' light and the frequency omega ₂ of the 'frequency omega ₂ the light of 'light and the frequency omega _3' of 'the balance and the frequency omega ₃ is reflected. Part of the light of the low frequency ω ₂ ″ of the measurement light reflected by the beam splitter 155 is S
The HG conversion element 112 converts the second harmonic into light having a frequency of 2ω ₂ ″, and the rest of the light having a frequency of ω ₂ ″ is S
The light passes through the HG replacement element 112 as it is. Further, of the measurement light reflected by the beam splitter 155, light having a high frequency ω ₃ ″ also passes through the SHG conversion element 112 as it is. In addition, as the SHG conversion element 112, for example, a KTiOPO ₄ crystal can be used.

The light of frequency 2ω ₂ ″ converted via the SHG conversion element 112, the light of frequency ω ₂ ″ and the light of frequency ω ₃ ″ transmitted through the SHG conversion element 112 as they are
The light enters the frequency separation device 117 along the same optical path. Note that, as described above, the frequency separation device 117 has the internal configuration shown in FIG. Further, the light having the frequency 2ω ₂ ″ and the light having the frequency ω ₃ ″ have substantially the same frequency, and the frequency is higher than the frequency of the light having the frequency ω ₂ ″.

Therefore, as described in the first embodiment, the light having the frequency ω ₂ ″ (the light having the smaller frequency: the light having the smaller frequency: the first light) diffracted by the diffraction grating 1 is provided inside the frequency separation device 117.
In the embodiment, the light having the frequency 2ω ₂ ″ and the light having the frequency ω ₃ ″ (the light having the higher frequency: first light) diffracted by the diffraction grating 1 along the optical axis 202 along the optical axis 202. (Corresponding to the light of the frequency ω2 in the embodiment) respectively, enters the light separating element 11 along the optical axis 203. The light separating element 11 converts the light having the frequency 2ω ₂ ″ and the light having the frequency ω ₃ ″ into the light separating surface 1.
1a becomes P-polarized light and the incident angle of the light having the frequency 2ω ₂ ″ and the light having the frequency ω ₃ ″ with respect to the light separating surface 11 a becomes equal to the Brewster angle.
They are arranged so that the angle of incidence of light of frequency ω ₂ ″ with respect to a is equal to or greater than the critical angle.

Accordingly, the P-polarized light having a frequency of 2ω ₂ ″ and the light having a frequency of ω ₃ ″ incident on the light separating surface 11 a at the Brewster angle are not reflected by the light separating surface 11 a and are not reflected by the light separating surface 11 a. , The light travels straight along the optical axis 203 and reaches the photoelectric conversion element 114. On the other hand, the light separation surface 1
Light having a frequency of ω ₂ ″ incident on 1 a at an incident angle equal to or greater than the critical angle is totally reflected by the light separating surface 11 a, guided along the optical axis 204, and from the optical path without reaching the photoelectric conversion element 114. Removed. Thus, the light of frequency 2ω ₂ ″ converted by the SHG conversion element 112 via the measurement optical path interferes with the light of frequency ω ₃ ″ transmitted directly through the SHG conversion element 112 via the measurement optical path. Photoelectric conversion element 11
4 is detected. The signal from the photoelectric conversion element 114 is supplied to a phase meter 115.

Similarly, of the reference light reflected by the beam splitter 155, a part of the light having the small frequency ω ₂ ′ is converted by the SHG conversion element 111 into the second harmonic, that is, the light having the frequency 2ω ₂ ′. , The rest of the light at frequency ω ₂ ′ is S
The light passes through the HG conversion element 111 as it is. Further, of the reference light reflected by the beam splitter 155, light having a high frequency ω ₃ ′ also passes through the SHG conversion element 111 as it is. The light having the frequency 2ω ₂ ′ converted through the SHG conversion element 111 and the light having the frequency ω ₂ ′ and the light having the frequency ω ₃ ′ transmitted through the SHG conversion element 111 as they are are separated along the same optical path. It is incident on 116. Note that, as described above, the frequency separation device 116 has the internal configuration shown in FIG. Further, the light having the frequency 2ω ₂ ′ and the light having the frequency ω ₃ ′ have substantially the same frequency, and the frequency is higher than the frequency of the light having the frequency ω ₂ ′.

Therefore, inside the frequency separation device 116, as described in the first embodiment, the light having the frequency ω ₂ ′ (light having a smaller frequency: the frequency in the first embodiment) The light of frequency ω ₂ ′ and the light of frequency ω ₃ ′ (light having a higher frequency: light of the frequency ω _{2 in} the first embodiment) corresponding to the light of ω 1 along the optical axis 202 are diffracted by the diffraction grating 1. (Corresponding to light) enter the light separating element 11 along the optical axis 203. Then, the light separating element 11
Is that the light of frequency 2ω ₂ ′ and the light of frequency ω ₃ ′ are P-polarized with respect to the light separation surface 11 a, and the incident angles of the light of frequency ₂ ω ₂ ′ and the light of frequency ω ₃ ′ with respect to the light separation surface 11 a are Brew. It becomes equal to the star angle and the light separation surface 11a
Are arranged such that the angle of incidence of light of frequency ω ₂ ′ with respect to

Therefore, the P-polarized light having a frequency of 2ω ₂ ′ and the light having a frequency of ω ₃ ′ incident on the light separating surface 11a at the Brewster angle pass through the light separating surface 11a without being reflected by the light separating surface 11a. After that, the light goes straight along the optical axis 203 and reaches the photoelectric conversion element 113. On the other hand, the light separation surface 11
The light of frequency ω ₂ ′ incident on a at an incident angle equal to or greater than the critical angle is
After being totally reflected by the light separating surface 11 a, it is guided along the optical axis 204 and is removed from the optical path without reaching the photoelectric conversion element 113.

In this manner, the SHG conversion element 1 passes through the reference optical path.
After passing through the light of frequency 2ω ₂ ′ converted at 11 and the reference light path, S
The light having the frequency ω ₃ ′ transmitted through the HG conversion element 111 as it is interferes with the light, and the interference light is detected by the photoelectric conversion element 113. The signal from the photoelectric conversion element 113 is supplied to the phase meter 115. In the phase meter 115, based on the signal of the photoelectric conversion element 113 and the signal of the photoelectric conversion element 114, {ΔD (ω ₃ ) −ΔD (ω ₂ )} as refractive index fluctuation information of the gas in the optical path.
And outputs it to the arithmetic unit 160. Since the phase meter 115 compares the interference signals corresponding to the two interference lights, the heterodyne frequency (2Δω) in each interference signal is compared.
_{b -Δω a)} and (2Δω _{b '-Δω a')} and it is necessary to equal.

On the other hand, the measurement light (light of frequency ω ₂ ″) transmitted through the beam splitter 155 is reflected by the reflecting mirror 120, and the reference light (light of frequency ω ₂ ′) transmitted through the beam splitter 155 is Directly, polarizing beam splitter 1
It is incident on 56. Thus, the polarization beam splitter 15
The measurement light and the reference light combined via the light 6 are coupled to the polarizing plate 15.
7 via. Interference light between the measurement light and the reference light is detected by the photoelectric conversion element 158, and a signal from the photoelectric conversion element 158 is supplied to the phase meter 159.

[0043] The beam splitter 185 of the frequency shift portion 170 reflects a portion of light of 'light and the frequency omega ₂ of the' frequency omega ₂ incident, the frequency omega ₂ 'of the light and the frequency omega _2' ' The light of the frequency ω ₃ ′ and the light of the frequency ω ₃ ″ are transmitted. Light 'light and the frequency omega ₂ of the''reflected frequency omega ₂ in the beam splitter 185 interferes through the polarizing plate 186. The interference light is detected by the photoelectric conversion element 187, and the photoelectric conversion element 187 supplies the detected signal to the phase meter 159.

The signal from the photoelectric conversion element 158 for detecting the interference light of the light in the phase meter 159, 'frequency omega ₂ having passed through the light and the measurement optical path of' the reference frequency omega ₂ that has passed through the optical path ', the photoelectric conversion Based on the signal from the element 187, the displacement amount ΔD of the movable mirror 105 without considering the influence of the refractive index fluctuation.
(Ω ₂ ) is obtained, and a signal related to the displacement amount ΔD (ω ₂ ) is supplied to the arithmetic unit 160. In the arithmetic unit 160, the refractive index fluctuation information ｛ΔD (ω ₃ ) −ΔD (ω
₂ ) Signal related to｝ and measured displacement Δ from phase meter 159
Moving mirror 1 in which a measurement error caused by a change in refractive index is corrected based on a signal related to D (ω ₂ ) and Expression (4) described later.
A geometric displacement amount (true displacement amount) ΔD is obtained and output.

Hereinafter, the measured displacement amount ΔD (ω
The correction from ₂ ) to the geometric displacement ΔD will be described. Optical path length D for light at frequencies ω ₂ and ω ₃
(Ω ₂ ) and D (ω ₃ ) are represented by the following equations (1) and (2), respectively. D (ω ₂ ) = {1 + NF (ω ₂ )} · D (1) D (ω ₃ ) = {1 + NF (ω ₃ )} · D (2)

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) and (2), the geometric distance D is given by the following equation (3). D = D (ω ₂ ) −A ′ {D (ω ₃ ) −D (ω ₂ )} (3) where A ′ = F (ω ₂ ) / {F (ω ₃ ) −F (ω ₂ )}
It is. Therefore, referring to equation (3), the geometric displacement ΔD is given by the following equation (4). ΔD = ΔD (ω ₂ ) −A ′ {ΔD (ω ₃ ) −ΔD (ω ₂ )} (4)

As described above, in the lightwave interference measuring apparatus of the fourth embodiment, a part of the light having the frequency ω ₂ ″ passing through the measurement optical path is S
The second harmonic (frequency 2
ω ₂ ″), and the rest of the light at the frequency ω ₂ ″ passes through the SHG conversion element 112 without being converted to the second harmonic. Here, the light of the frequency ω ₂ ″ that has passed through the SHG conversion element 112 as it is
12 and the light of frequency 2ω ₂ ″
When incident on the photoelectric conversion element 114 together with the light of the frequency ω ₃ ″ transmitted through the G conversion element 112, the photoelectric conversion element 114
Causes a decrease in the visibility of the output signal.

However, in the optical interference measuring apparatus of the fourth embodiment, the SHG conversion element 112 and the photoelectric conversion element 114
Since the frequency separation device 117 according to the first embodiment is arranged in the optical path between the optical conversion element 114 and the light having the frequency ω ₂ ″ that has passed through the SHG conversion element 112 as it is.
Is removed from the light path without reaching. Similarly, SHG
Since the frequency separation device 116 is arranged in the optical path between the conversion element 111 and the photoelectric conversion element 113, the light having the frequency ω ₂ ′ transmitted through the SHG conversion element 111 without Removed from As a result, in the fourth embodiment, the photoelectric conversion elements 113 and 1
Since it is possible to obtain a signal with good visibility as the output signal of 14, it is necessary to measure the refractive index fluctuation due to gas in the optical path with high accuracy, and thus to measure the geometric displacement ΔD of the movable mirror 105 with high accuracy. Becomes possible.

In order to make the polarization direction of the light of frequency 2ω ₂ ′ coincide with the polarization direction of the light of frequency ω ₃ ′ behind the SHG conversion element 111, the For example, 1 for light at frequency ω ₂ '
It is preferable to insert a / 2 wavelength plate. Similarly, SHG
In order to make the polarization direction of the light having the frequency 2ω ₂ ″ coincide with the polarization direction of the light having the frequency ω ₃ ″ behind the conversion element 112, for example, the frequency ω _{2 ″} may be provided immediately before the SHG conversion element 112 as necessary. It is preferable to insert a half-wave plate for '' light.

When the polarization direction of the light of frequency 2ω ₂ ′ and the polarization direction of the light of frequency ω ₃ ′ do not match behind the SHG conversion element 111, the light of frequency 2ω ₂ ′ and the light of frequency ω ₃ ′ Cannot be set to the P-polarized light with respect to the light separation surface 11a, and at least a part of at least one of the light having the frequency 2ω ₂ ′ and the light having the frequency ω ₃ ′ is reflected by the light separation surface 11a. And will be removed from the optical path.
Similarly, the frequency 2 behind the SHG conversion element 112
If the polarization direction of the light of ω ₂ ″ does not match the polarization direction of the light of frequency ω ₃ ″, at least a part of the light is reflected by the light separating surface 11 a and removed from the optical path. As a result, the signal intensity obtained via the polarizer inserted immediately before the photoelectric conversion elements 113 and 114 decreases. Therefore, it is preferable to insert a half-wave plate immediately before the SHG conversion elements 111 and 112 as described above. Particularly, in an extreme case where the polarization direction of the light of frequency 2ω ₂ ′ and the polarization direction of the light of frequency ω ₃ ′ are orthogonal to each other behind the SHG conversion element 111,
It is essential to insert a half-wave plate immediately before and 112.

In the fourth embodiment, the frequency separation device according to the first embodiment is arranged in the optical path between the SHG conversion device and the photoelectric conversion device. However, the frequency separation device according to the first embodiment is used. Alternatively, a frequency separation device according to the second or third embodiment can be used. Also, the fourth
In the embodiment, the present invention is described based on an example in which an SHG conversion element is used as a frequency conversion unit in a refractive index fluctuation measurement system. However, a part of the first light having passed through the measurement optical path is converted into light having substantially the same frequency as the frequency of the second light having passed through the measurement optical path, and the remainder of the first light and the second light have been converted.
The present invention can be applied to a light wave interference measurement device having a general frequency conversion means that transmits the light as it is.

[0052]

As described above, according to the frequency separation apparatus of the present invention, the first light and the second light which are incident coaxially along the common optical axis are separated from each other, and a sufficient distance is provided between them. It can be guided along two optical axes separated from each other, that is, two lights having different frequencies incident along the same optical path can be separated at a sufficient distance. Even if the first light and the second light partially overlap on the light separation surface, there is no effect on the light separation action. Therefore, the distance between the deflecting means and the separation means should be shortened as necessary. Accordingly, the size of the frequency separation device can be easily reduced.

According to the light wave interference measuring apparatus of the present invention, the frequency separating apparatus according to the present invention is provided in the optical path between the frequency converting means such as the SHG converting element and the interference signal generating system such as the photoelectric converting element. Is disposed, the harmful light transmitted through the SHG conversion element as it is is removed from the optical path without reaching the photoelectric conversion element. As a result, a signal with good visibility can be obtained, so that the refractive index fluctuation due to gas in the optical path can be measured with high precision, and thus the geometric displacement of the movable mirror, that is, the true displacement can be measured with high precision. It becomes possible.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration of a frequency separation device according to a first embodiment of the present invention.

FIG. 2 is an enlarged view specifically showing a configuration of a light separating element that can be used in the frequency separating device of the first embodiment.

FIG. 3 is a diagram schematically showing a configuration of a frequency separation device according to a second embodiment of the present invention.

FIG. 4 is a diagram schematically showing a configuration of a frequency separation device according to a third embodiment of the present invention.

FIG. 5 is a diagram schematically showing a configuration of an optical interference measurement apparatus according to a fourth embodiment of the present invention.

FIG. 6 is a diagram schematically illustrating an internal configuration of a frequency shift unit 170 of FIG. 5;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Diffraction grating 2 Dispersion prism 11, 12 Light separation element 11a, 12a Light separation surface 51, 53 Prism 52 Air layer 101 Light source 103 Polarization beam splitter 104-106 Corner cube 111, 112 SHG conversion element 113, 114 Photoelectric conversion element 115, 159 Phase meter 116, 117 Frequency separation device 155, 156 Beam splitter 160 Operation unit 171, 184 Dichroic mirror 172, 174 Polarization beam splitter 178, 180 Polarization beam splitter 173, 176 Frequency shifter 179, 182 Frequency shifter

Claims (9)

[Claims] 1. A first light having a first frequency incident along a predetermined optical path is deflected by an angle dependent on the first frequency and guided along the first optical path, and the first light is guided by the first optical path. A second light having a second frequency different from the first frequency incident along the same optical path as that of the second light is deflected by an angle dependent on the second frequency, and a second light different from the first optical path is deflected. Deflecting means for guiding along the optical path, and reflecting substantially all of the first light incident along the first optical path via the deflecting means, and reflecting the second light via the deflecting means. The second incident along the optical path of
And a separating means having a light separating surface for transmitting substantially all of the light. 2. The separation means, wherein the second light is P-polarized with respect to the light separation surface, and an incident angle of the second light with respect to the light separation surface is substantially equal to a Brewster angle; 2. The frequency separation device according to claim 1, wherein the first light is incident on the light separation surface so that an incident angle of the first light is substantially equal to or larger than a critical angle. 3. The frequency separation device according to claim 1, wherein the deflecting unit has a diffraction grating for diffracting incident light at an angle depending on a frequency. 4. The frequency separation device according to claim 1, wherein the deflecting unit has a dispersion prism for refracting incident light at an angle depending on a frequency. 5. The separation means includes:
It has a first prism and a second prism arranged so as to form a predetermined medium layer between the first prism and the first prism. The refractive index of the first prism and the second prism Is set to be larger than the refractive index of the medium layer,
The frequency separation device according to any one of claims 1 to 4, wherein a boundary surface between the first prism and the medium layer forms the light separation surface. 6. The frequency separation device according to claim 5, wherein the medium layer is an air layer. 7. The light separating device according to claim 1, wherein the separating unit includes a single prism, and an end surface of the prism facing an incident end surface forms the light separating surface. Item 8. The frequency separation device according to item 1. 8. A length measuring interferometer for measuring an amount of displacement of a movable mirror along a predetermined direction, a first light having a first frequency and a second frequency different from the first frequency. A light source for supplying the second light having a second light having a frequency substantially the same as the second frequency, wherein a part of the first light output from the light source and passing through the measurement optical path through the movable mirror is provided. 3 light and the first light
Frequency conversion means for transmitting the rest of the light and the second light passing through the measurement light path as it is, based on interference light between the third light and the second light via the frequency conversion means. And an interference signal generation system for generating an interference signal, according to the refractive index fluctuation due to gas in the measurement optical path measured based on the interference signal, the measured by the length measuring interferometer In a light wave interference measurement device that corrects a measured displacement amount of a moving mirror and measures a geometric displacement amount of the moving mirror, between the frequency conversion unit and the interference signal generation system,
The frequency separation device according to any one of claims 1 to 7 is provided, wherein the frequency separation device is configured to control the remaining portion of the first light incident along the same optical path from the frequency conversion unit and the second light. Separating the light and the third light, guiding the second light and the third light to the interference signal generation system, and causing the remainder of the first light to reach the interference signal generation system. A light wave interference measurement apparatus characterized in that the light wave interference is removed from the optical path without any change. 9. The apparatus according to claim 1, wherein the frequency conversion unit includes a harmonic conversion element for converting a part of the first light into a harmonic and outputting the harmonic as the third light. The optical interference measurement apparatus according to claim 8.