JP3036951B2 - Light wave interference measurement device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical interference measuring apparatus which can be used, for example, for optical heterodyne interferometry.

[0002]

2. Description of the Related Art As one of light wave interferometry methods, there is an optical heterodyne method utilizing a beat signal due to interference between two coherent lights having slightly different frequencies.

In this optical heterodyne method, first, coherent light (hereinafter referred to as orthogonally polarized two-frequency beam) including two linearly polarized lights having a slight frequency difference from each other and having mutually orthogonal polarization planes is generated. Let it. Next, the coherent light is split into two, and one of the coherent lights is guided to a reference interference optical system, and two polarized lights included in the coherent light interfere with each other to obtain a reference beat signal. Further, at least one of the two polarized lights included in the other two separated coherent lights is involved in the measurement object, and a phase change depending on the measurement object is provided between the two polarized lights. Thereafter, these are guided to a measurement interference optical system to similarly obtain a measurement beat signal. Then, the reference beat signal and the measurement beat signal are compared to measure a phase difference between the reference beat signal and the measurement beat signal, and a physical quantity to be measured of the object to be measured is obtained from the phase difference.

Incidentally, the orthogonally polarized two-frequency beam used in the optical heterodyne method needs to have a frequency difference between the two polarized lights equal to or less than a predetermined value. This is because, if the frequency difference between the two polarized lights is greater than a predetermined value, when the beat signal is obtained by interfering the two, the frequency of the beat signal becomes too large, making phase measurement and the like difficult.

Therefore, in order to obtain the orthogonally polarized two-frequency beam, a Zeeman laser device has conventionally been generally used. The orthogonally polarized two-frequency beam obtained from this Zeeman laser device has a small difference in frequency between the two polarized lights, which is suitable for obtaining a beat signal having a frequency that can be easily processed.

[0006] However, this Zeeman laser device has a problem that the frequency difference is appropriate and the overlap of two polarized lights is perfect, but there is frequency crosstalk. That is, each frequency component of the two polarized lights includes the other frequency component.
For this reason, there has been a problem that these beat signals include a noise component due to the crosstalk, which causes a measurement error.

As an apparatus for solving this problem, an apparatus for obtaining an orthogonally polarized two-frequency beam by converting a laser beam of a single wavelength emitted from an ordinary laser apparatus by an orthogonally polarized optical frequency shifter is used. (See, for example, JP-A-63-85301). Here, the orthogonal polarization type optical frequency shifter is a device having a laser beam of a single wavelength incident thereon and having a plane of polarization orthogonal to the laser light therein.
The light is separated into two linearly polarized lights, subjected to different frequency shifts by different frequency shifters, and then multiplexed and output on the same optical path again. Thereby, the crosstalk between the frequencies of the two polarized lights of the orthogonally polarized two-frequency beam obtained can be remarkably reduced.

[0008]

By the way, an apparatus for obtaining an orthogonally polarized two-frequency beam using the above-described orthogonally polarized optical frequency shifter can eliminate an error factor based on frequency crosstalk. There is a problem that beam overlap is deteriorated. In other words, this device separates the laser light emitted from the laser device into two in the orthogonal polarization type optical frequency shifter, and multiplexes them again on the same optical path. It is difficult to completely overlap the light beam on the same optical path again, and it is not always easy to make the polarization directions completely orthogonal. If the two polarized light beams do not completely coincide with each other, the two polarized light beams are involved in another part of the sample when the polarized light is involved (transmitted) to the object to be measured and imparts a phase change. (Transmit through another part). In this case, an error occurs when the states of the two polarized light-related sites are different from each other and a phase difference is given based on a factor unrelated to the physical properties of the object to be measured. If the directions of polarization do not completely coincide with each other, they may cause an error when a beat signal is obtained by interfering them.

As described above, in the conventional apparatus, the apparatus which can remove the frequency crosstalk does not have sufficient beam coincidence, while the apparatus which can sufficiently secure the beam coincidence has sufficient frequency crosstalk. There was a problem that it could not be removed.

The present invention has been made in view of the above background, and has as its object to provide a light wave interference measuring apparatus capable of securing the degree of coincidence of beams and eliminating frequency crosstalk. .

[0011]

In order to solve the above-mentioned problems, an optical interference measuring apparatus according to the present invention comprises two linearly polarized light beams having a certain frequency difference from each other and having mutually orthogonal polarization planes. Coherent light generating device for obtaining coherent light including: a first light separating device for separating coherent light emitted from the coherent light generating device into two, and frequency conversion to at least one of the two separated coherent lights And a frequency conversion device that gives a frequency difference to the two coherent lights by performing the above operation, and one of the two coherent lights to which a frequency difference is applied by the frequency conversion device is involved in the measurement target and is included in the coherent light. A measuring optical system that emits light after giving a phase difference to two linearly polarized lights, and a coherent light emitted from the measuring optical system. A second light separating device for separating two linearly polarized lights, and a third light separating device for separating two linearly polarized lights included in the other of the two coherent lights having a frequency difference provided by the frequency converter. One of the two linearly polarized lights separated by the second light separation device, and one of the two linearly polarized lights separated by the third light separation device.
A first interference optical system that multiplexes one of the two linearly polarized lights to obtain a first interference light, the other of the two linearly polarized lights separated by the second light separation device, and the third light A second interference optical system that multiplexes the other of the two linearly polarized lights separated by the separation device to obtain a second interference light, a first photodetector that detects the first interference light, (2) A second photodetector for detecting interference light, and a phase difference measurement for inputting an output signal of the first photodetector and an output signal of the second photodetector and measuring a physical quantity depending on a phase difference between the two. And a light wave interference measuring device. frequency

[0012]

In the above-described configuration, the two coherent light generators having polarization planes orthogonal to each other and emitted from the coherent light generator are now described.
Let the frequencies of the two linearly polarized lights be ν ₀ and ν ₀ ′, respectively. Also, the shift frequencies of the frequency converter are ν ₁ and ν ₂
And Here, it is assumed that the frequency difference between ν ₀ and ν ₀ ′ is significantly larger than the frequency difference between ν ₁ and ν ₂ .
That is, the frequency between ν ₁ and ν ₂ is set to a small value whose difference is suitable for obtaining a beat signal. Then, one of the coherent lights separated into two by the first optical separation device and given a frequency difference by the frequency conversion device has linearly polarized light having a frequency ν ₀ + ν ₁ and a polarization plane orthogonal to the linearly polarized light. This is light containing linearly polarized light having a frequency of ν ₀ ′ + ν ₁ . The other of the coherent lights given the frequency difference by the frequency conversion device has a frequency ν ₀ + ν.
₂ and linearly polarized light having a plane of polarization orthogonal to it and having a frequency of ν ₀ ′ + ν ₂ .

After passing through the measuring optical system, one of these two lights is separated into two linearly polarized lights by the second light separating device, and the other light is directly converted into two linearly polarized lights by the third light separating device. Separated into polarized light.

Then, the first and second optical interference devices combine the two linearly polarized lights separated by the second and third optical separation devices and combine them into first and second interference light having substantially the same frequency | ν ₁ −ν ₂ | beat light can be obtained. Here, since ν ₁ and ν ₂ are obtained by the frequency conversion device, the frequency difference can be easily reduced sufficiently and there is no crosstalk between them, so that a measurement error factor due to the crosstalk occurs. Not even. Therefore, by detecting these beat lights by the first and second photodetectors and measuring the physical quantity dependent on the phase difference between the two by the measuring device, the physical quantity dependent on the phase change of the object to be measured can be accurately obtained. Can be.

In this case, in the measuring optical system, one of the two orthogonally polarized two-frequency beams of the two coherent lights to which a frequency difference is given by the frequency converter is included.
By giving a phase difference depending on the physical properties of the measurement object to these polarized lights so as to participate in the measurement object before separating the two linearly polarized lights, for example, when looking at the birefringence of the measurement object Since the two linearly polarized beams related to the object to be measured are completely coincident with each other, it is possible to eliminate the possibility that a measurement error occurs due to the beam inconsistency.

[0016]

EXAMPLES First Embodiment FIG. 1 is a diagram showing the configuration of an optical interference measuring apparatus according to a first embodiment of the present invention. Hereinafter, referring to FIG.
Examples will be described in detail.

In FIG. 1, reference numeral 1 denotes a He—Ne laser device. The He-Ne laser device 1 is driven by a driving device 1a and emits a laser beam L of an orthogonally polarized two-frequency beam, which is two longitudinal mode laser beams. That is, from the He over Ne laser device 1, the frequency [nu ₀ of P
A laser beam (coherent light) L including the polarized light LaV and the S-polarized light LbH having the frequency ν ₀ ′ is emitted. Where ν
The frequency difference between ₀ and ν ₀ ′ is as large as 685 MHz.

The laser beam L is applied to the unpolarized beam splitter 2
Is separated into two. One of the two separated laser beams is emitted with its frequency shifted by ν ₁ by the acousto-optic device 4 (frequency conversion device). That is, the P-polarized light L having the frequency ν ₀ + ν ₁ is output from the acousto-optic element 4.
Laser light containing a1V and S-polarized light Lb1H having a frequency ν ₀ ′ + ν ₁ is emitted. The other laser beam is reflected by the reflecting mirror 3 and guided to the acousto-optic device 5,
The frequency is shifted by ν ₂ by the acousto-optic element 5 and emitted. That is, from the acousto-optic element 5, the P-polarized light La2V having the frequency ν ₀ + ν ₂ and the frequency ν ₀ ′ + ν
A laser beam containing the S-polarized light Lb2H of ₂ is emitted. Note that the acousto-optical elements 4 and 5 are driven by a drive circuit 45. Ν ₁ is 80 MHz, ν ₂ is 8
0.025 MHz, the difference between the two is 0.025 MHz
And small. Here, an optical system including the He—Ne laser device 1, the polarization beam splitter 2, the reflecting mirror 3, and the acousto-optic devices 4 and 5 is referred to as an orthogonal polarization two-frequency two-beam optical system 100.

The laser beam emitted from the acousto-optic device 4 is reflected by the polarization beam splitter 6 on the P-polarized light La1V passing straight through and the S-polarized light Lb1H whose course is changed by 90 °.
And separated. The transmitted P-polarized light La1V goes straight through the reflecting mirror 7
Is reflected by the non-polarizing beam splitter 8, and is reflected and redirected by 90 ° for the S-polarized light Lb 1.
H is guided to the non-polarizing beam splitter 9.

On the other hand, the laser beam emitted from the acousto-optic device 5 passes through the wave plate 10, the Faraday medium 11, and the wave plate 12, and then enters the polarization beam splitter 13. Here, by passing through the wave plate 10, the P-polarized light La2v and the S-polarized light Lb2H become circularly polarized lights which are opposite to each other, and are converted again into the P-polarized light and the S-polarized light by the wave plate 12. In this case, when passing through the Faraday medium 11, a phase difference is given to the counter-circularly polarized light. Therefore, the P-polarized light and the S-polarized light emitted from the wave plate 12 include the phase difference. This phase difference depends on the current flowing through the coil 11a wound on the Faraday medium 11. Therefore, if this phase difference is obtained, the value of the current flowing through the coil 11a can be obtained. Here, the Faraday medium 11 is an object to be measured, and a current value flowing through the coil 11a is a physical quantity to be measured.

The polarization beam splitter reflects the P-polarized light La2v and the S-polarized light Lb2H emitted from the wave plate 12, and reflects the P-polarized light La2V that is transmitted straight and changes the course by 90 °.
And polarized light Lb2H. P-polarized light La2V transmitted straight
Is guided to the non-polarizing beam splitter 8, and the above-described reflecting mirror 7 is
Is combined with the P-polarized light La1V reflected and guided by
The light becomes interference light and is detected by the light detector 14.

On the other hand, the reflected S has changed its course by 90 °.
The polarized light Lb2H is reflected by the reflecting mirror 16 and guided to the non-polarized beam splitter 9, and is multiplexed with the S-polarized light Lb1H reflected and guided by the above-described polarized beam splitter 6, and becomes an interference light by the photodetector 15. Is detected.

Since the interference light detected by the photodetector 14 is light having a frequency ν ₀ + ν ₁ and interference light having a frequency ν ₀ + ν ₂ , the output of the photodetector 14 has a frequency | ν
₁ −ν ₂ |. Similarly, the photodetector 15
Output signal includes a phase change due to the Faraday medium 11, but the frequency of the beat signal is also approximately | ν ₁ −ν ₂ |. These signals are filtered through filters 18 and 19 to remove noise components at frequencies other than the vicinity of | ν ₁ −ν ₂ |, and then guided to a phase measurement device 20 where the phase difference is measured.
The result is guided to an arithmetic processing circuit (CPU) 21. C
The PU 21 performs a phase difference analysis process, and calculates a current value flowing through the coil 11a by comparing with a calibration curve or the like stored in advance.

In the above embodiment, since the frequencies ν ₁ and ν ₂ for obtaining the beat signal are obtained by the frequency converter, the frequency difference is sufficiently small and there is almost no crosstalk between them. No measurement error due to crosstalk occurs. Since the P-polarized light and the S-polarized light passing through the Faraday medium 11 are emitted from the same laser device, the beams are completely coincident. Therefore, there is no possibility of causing a measurement error due to beam mismatch. Moreover, if the polarization beam splitter 13
Separation is incomplete, a part of the S polarized light Lb2H be reflected by crosstalk caused by transmitting with P-polarized light La2V, the frequency of the beat signal by the crosstalk | ν ₀ + ν ₁ -ν ₀ ´-ν ₂ | and | ν
₀₋ ν ₀ ′ |, and the frequency of the beat signal to be measured |
ν ₁ −ν ₂ |, they can be easily removed by the filters 18 and 19 and do not cause an error.

Second Embodiment FIG. 2 is a diagram showing a configuration of an optical interference measuring apparatus according to a second embodiment of the present invention.

This embodiment has portions common to the above-described first embodiment. Therefore, common portions are denoted by the same reference numerals, and detailed description thereof is omitted. In the following, different points from the first embodiment will be described. This will be mainly described. This embodiment is common to the first embodiment in the orthogonally polarized two-frequency two-beam optical system 100 and the measurement system from the photodetector 14 and the photodetector 17.

In FIG. 2, one of the two lights emitted from the orthogonal polarization two-frequency two-beam optical system 100, that is, the P-polarized light La1V of the frequency ν ₀ + ν ₁ and the frequency ν ₀ ′.
The light containing the S-polarized light Lb1H of + ν ₁ is inverted in the polarization directions of P and S by the λλ plate 22, and the frequency ν ₀ + ν
₁ of S-polarized light La1H frequency ν ₀ '+ ν ₁ of the P-polarized light Lb1V
Is incident on the reflecting mirror 23 and contains the reflecting mirror 23
And is guided to the polarizing beam splitter 24.

The orthogonally polarized two-frequency two-beam optical system 10
0 Two other of the light emitted from, i.e., the light including the S-polarized light Lb2H frequency [nu ₀ + [nu ₂ of the P-polarized light La2V frequency [nu ₀ '+ [nu ₂ passes through the birefringent material 25 The light enters the polarization beam splitter 24. In this case, when passing through the birefringent substance 25, the P-polarized light La2V and the S-polarized light L2
A phase difference depending on the amount of birefringence of the birefringent substance 25 is provided between b2H and b2H. Here, the object to be measured is the birefringent substance 25, and the physical quantity to be measured is the amount of birefringence.

First, the polarization beam splitter 24 reflects the S-polarized light La1H having the frequency ν ₀ + ν _{1 in} the direction of 90 ° out of the light guided by the reflecting mirror 23 and the frequency ν ₀ ′ + ν.
_{The 1} P-polarized light Lb1V is transmitted to separate them. Further, of the light that has passed through the birefringent material 25, the frequency ν ₀ + ν ₂ of the P-polarized light La2V directly transmitted to S-polarized light La1H and multiplexes the above frequency ν ₀ + ν _1, whereas, the frequency [nu ₀ '+ Ν ₂ S-polarized light Lb2H is reflected in the 90 ° direction to be separated from P-polarized light La2V, and the frequency ν
₀ '+ [nu and multiplexes the _first P-polarized light Lb1V. These multiplexed lights pass through the analyzers 26 and 27, respectively, and become interference lights, thereby becoming photodetectors 14 and 17 respectively.
Is detected by As in the first embodiment, the frequencies of the beat signals sent from the photodetectors 14 and 17 are both | ν ₁ −ν ₂ |.

In the second embodiment, as in the first embodiment, it is possible to eliminate the influence of crosstalk while maintaining the perfect coincidence of the beams, and to obtain an accurate birefringence amount.

Third Embodiment FIG. 3 is a diagram showing a configuration of an optical interference measuring apparatus according to a third embodiment of the present invention.

This embodiment has portions common to the above-described second embodiment, and therefore, the following description will focus on the differences from the second embodiment. This embodiment is common to the second embodiment in the orthogonally polarized two-frequency two-beam optical system 100 and the measurement system from the analyzer 26 and the analyzer 27.

In FIG. 3, one of the two lights emitted from the orthogonal polarization two-frequency two-beam optical system 100, that is, the P-polarized light La1V of the frequency ν ₀ + ν ₁ and the frequency ν ₀ ′.
The light including the S polarized light Lb1H of + ν ₁ is reflected by the polarizing beam splitter 31 in the 90 ° direction.
H and P-polarized light La1V that is transmitted as it is.

The S-polarized light Lb1H reflected in the 90 ° direction is
After passing through the ４λ plate 32, the light is reflected by the movable reflector 33, passes through the １ ／ λ plate 32 again, and reaches the polarization beam splitter 31 again. This S-polarized light Lb1H is 1/4
Since the light passes through the λ plate 32 twice, the frequency is converted into the P-polarized light Lb1V while the frequency is maintained, and the light passes through the polarizing beam splitter 31 and reaches the polarizing beam splitter 36 this time. Here, the P-polarized light Lb1V is provided with a phase change depending on the moving amount of the moving reflector 33. Therefore, by calculating this phase change, the moving amount of the moving reflector 33 can be obtained. Therefore, the object to be measured here is the moving reflector 32.
And the physical quantity to be measured is the amount of movement of the movable reflector 32.

The P-polarized light La1V transmitted as it is is:
After passing through the ４λ plate 34, the light is reflected by the fixed reflecting mirror 35, passes through the ４λ plate 34 again, and reaches the polarization beam splitter 31 again. This P-polarized light La1V is 1/4
Since the light passes through the λ plate 34 twice, it is converted to the S-polarized light La1H while maintaining the frequency, and is reflected in the 90 ° direction by the polarizing beam splitter 31 this time.
And reaches the polarization beam splitter 36.

The polarizing beam splitter 36 reflects the S-polarized light La1H guided through the polarizing beam splitter 31 in the direction of 90 °, transmits the P-polarized light Lb1V as it is, and separates them.

Here, the other of the two lights emitted from the orthogonal polarization two-frequency two-beam optical system 100, that is, the P-polarized light La2V of the frequency ν ₀ + ν ₂ and the frequency ν ₀ ′ +
The light including the ν ₂ S-polarized light Lb2H is also introduced into the polarizing beam splitter 36, and is separated by the polarizing beam splitter 36 into P-polarized light La2V that is transmitted as it is and S-polarized light Lb2H that is reflected in the 90 ° direction. At the same time, they are combined with the light guided and separated through the polarizing beam splitter 31. That is, the frequency ν ₀ + ν ₂
After the S-polarized light La1H and the P-polarized light La2V having the frequency ν ₀ + ν ₂ are multiplexed, the light becomes an interference light through the analyzer 26 and is detected by the photodetector 14. Also, the frequency ν ₀ ′ + ν ₂
The S-polarized light Lb2H and the P-polarized light Lb1V having the frequency ν ₀ ′ + ν ₁ are multiplexed, similarly, become an interference light through the analyzer 27 and detected by the photodetector 17. These photodetectors 14, 1
7 are | ν ₁ −
ν ₂ |.

In the third embodiment, the influence of crosstalk can be eliminated, and an accurate movement amount can be obtained. Note that, in this embodiment, unlike the first and second embodiments, due to the nature of the measurement object and the physical quantity to be measured,
The coincidence of the beams in the sense as the problem in the first and second embodiments does not matter.

In each of the above embodiments, the acousto-optic device 4 in the orthogonally polarized two-frequency two-beam optical system 100 is used.
Although the example of spatially propagating the light emitted from and 5 has been described, this can also be propagated using a polarization-maintaining optical fiber. Further, another optical path may be constituted by an optical waveguide.

Further, as the laser device for generating the orthogonally polarized two-frequency beam, two semiconductor laser devices can be used instead of the He-Ne laser device.

Further, if a polarization independent optical isolator is inserted in each optical path, the effect of the return light can be eliminated.

Furthermore, although an example using an acousto-optic device as the frequency conversion device has been described, an optical frequency shifter utilizing the electro-optic effect may be used. Also, any one of the acousto-optic elements need not necessarily be inserted.

[0043]

As described above in detail, the present invention separates coherent light including two linearly polarized lights having a frequency difference and mutually orthogonal polarization planes into two, and performs different frequency conversion on each of them. , One of which is related to the measurement object and 2
The two linearly polarized lights are separated after giving a phase difference, and each of the two separated polarized lights is obtained by separating the two polarized lights included in the other coherent light of the two separated light. A phase change caused by an object to be measured is obtained by obtaining two beat signals of a measurement beat signal and a reference beat signal by multiplexing and interfering with each of the polarized lights and comparing the two beat signals. Thus, errors due to frequency crosstalk and beam mismatch do not occur.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an optical interference measuring apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a configuration of an optical interference measurement apparatus according to a second embodiment of the present invention.

FIG. 3 is a diagram illustrating a configuration of an optical interference measurement apparatus according to a third embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... He-Ne laser apparatus, 2, 8, 9 ... Non-polarization beam splitter, 4, 5 ... Acousto-optic element, 6, 13, 24,
31, 36: polarization beam splitter, 11: Faraday medium, 14, 17: photodetector, 20: phase detector, 25
... birefringent substance, 33 ... moving reflector, 100 ... orthogonal polarization two-frequency two-beam optical system.

Claims (1)

(57) [Claims] 1. A coherent light generator for obtaining coherent light including two linearly polarized lights having a certain frequency difference and having mutually orthogonal polarization planes, and a coherent light emitted from the coherent light generator. A first light separation device that separates the two into two, a frequency conversion device that performs frequency conversion on at least one of the two separated coherent lights to give a frequency difference between the two coherent lights, A measurement optical system that emits one of the two coherent lights provided with the difference after the two linearly polarized lights included in the coherent light participate in one of the coherent lights and emits a phase difference; and A second light separation device for separating two linearly polarized lights included in the emitted coherent light, and a frequency conversion device. A third light separation device that separates two linearly polarized lights included in the other of the two coherent lights to which a frequency difference has been given; and one of the two linearly polarized lights separated by the second light separation device. A first interference optical system that multiplexes one of the two linearly polarized lights separated by the third light separation device to obtain a first interference light, and two light beams separated by the second light separation device. A second interference optical system that combines the other of the linearly polarized light and the other of the two linearly polarized light separated by the third light separation device to obtain a second interference light; and the first interference light. A first photodetector for detecting the second interference light, a second photodetector for detecting the second interference light, and an output signal of the first photodetector and an output signal of the second photodetector. Phase difference measuring device for measuring a physical quantity dependent on phase difference Interference measurement device.