JPH06167304A - Displacement sensor - Google Patents

Abstract

PURPOSE:To make a length measuring signal highly accurate, highly stable and have multi-wavelength by detecting a reference signal from a flux of modulated laser light and sampling the displacement of a to-be-tested body in accordance with the length measuring signal and a synchronous signal obtained through optical interferences. CONSTITUTION:A part for detecting a length measuring signal is integrally formed in an interference prism. The intensity of the interference fringes is transmitted to a sampling circuit 9 by an optical fiber 8. The circuit 9 photoelectrically converts a reference signal and then detects a zero crossing point by means of a comparator. The length measuring signal is sampled in accordance with a synchronous signal. Since it is impossible to detect only from the value of the length measuring signal sampled by the single-phase synchronous signal whether the initial phase changes due to the displacement or solely the amplitude changes, a multi-phase reference signal is utilized and a multi-phase modulating synchronous signal is obtained. Accordingly, a plurality of length measuring signals are obtained in one modulation cycle, and the initial phase of the length measuring signals is obtained highly accurately in a multi- bucket method without influences of the external disturbances.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interference fringe counting type laser interferometer.

[0002]

2. Description of the Related Art Laser interferometers are widely used as a reference for displacement or angle measurement using displacement in the fields of ultraprecision measurement and ultraprecision machining because of their high resolution and wide dynamic range. Two types of laser interferometers are generally used. One is called a heterodyne method and the other is called an interference fringe counting method.

FIG. 6 shows an embodiment of a heterodyne type laser interferometer. In the heterodyne method, the laser light source 15 is modulated by Zeeman effect or AOM
The main feature of this is the use of a frequency-stabilized laser, and the beat frequency generated by these two slightly different frequencies is Doppler-shifted when the test object shifts in the interferometric measuring optical system 5 that follows. Is used to detect the velocity of the test object. Therefore, in this method, the velocity signal once detected inside the length measuring circuit 19 is integrated and converted into a displacement, and then output. In the heterodyne method, an interference fringe of a light source luminous flux is usually generated in a light source unit, a beat frequency is extracted by a detection head 17,
By using this as a reference signal, the length measurement accuracy is improved.

On the other hand, in the interference fringe counting system, as shown in the embodiment of FIG. 7, a relatively inexpensive single-wavelength stabilized laser can be used as the laser light source 20, and the displacement of the test object can be measured by an interferometer in the latter stage. The long optical system 5 directly detects displacement as a change in intensity of interference fringes. Therefore, this method is characterized in that the counting circuit 24 after photoelectric conversion does not require an integrator, and displacement measurement can be performed at high speed with a relatively simple configuration. For example, the publicly known document Nanotechno
logy 2, (1991) 88-95, laser
interferometric system f
ordisplacement measuremen
t with high precision, SH
With osoe, 1 m / sec with a measurement resolution of 0.6 nm
The high-speed measurement is realized by the interference fringe counting method.

Both methods use the change in the intensity of the interference fringes with respect to the length measurement accuracy, so that there is no difference in principle.

[0006]

However, in the heterodyne type laser interferometer as shown in FIG.
There were the following problems. (1) Since the integrator is required after photoelectric conversion of the detected length measurement signal, it is difficult to increase the speed of the length measurement. (2) In order to eliminate the influence of atmospheric fluctuations from the measurement signal,
It is technically difficult to realize and expensive because the light source becomes complicated when the wavelength of the light source is changed. Further, the embodiment of the interference fringe counting system shown in FIG. 7 has the following problems. (3) Since the system as a whole does not have a reference signal and is in an open loop, noise is likely to be included in the measurement signal due to a change in the intensity of the light flux of the light source, and as a result, the measurement stability is poor. (4) Since the frequency band of the detected length measurement signal includes DC, the influence of DC noise such as external stray light or drift and the influence of amplitude change of interference fringe strength are measured even if a differential optical system is used in the detection unit. Since the signal is directly received, there is a limit to the improvement of measurement stability. The present invention overcomes all of these problems by using an interference fringe counting method, and realizes high precision and stability of a laser interferometer that can measure sub-nanometers and increase in wavelength.

[0007]

A polarization state of light from a light source is modulated by using an electro-optical element or a magneto-optical element, and a length measurement signal by an interference optical system in a subsequent stage is converted into an alternating current, and its band is separated from DC. The length measurement signal detected through the interference optical system is obtained by detecting a change in polarization characteristic by a change in intensity of interference fringes. Further, a reference signal is generated from the modulated light source light, the photoelectrically measured length measurement signal is sampled by a synchronization signal having a phase difference of 90 degrees according to the reference signal, and the multi-bucket method is applied to the value. Then, the initial phase of the displacement signal is obtained with high accuracy and the length measurement value is obtained. If the initial phase of the length measurement signal synchronously demodulated by the reference signal changes in this way, it means that the interference optical path length from the light source to the detection unit has changed, that is, the displacement of the test object. Can be measured. A memory is used for the calculation of the multi-bucket. An optical fiber is used to transmit the reference signal. Further, in order to eliminate the influence of atmospheric fluctuations on the measurement signal, in order to make the light source have multiple wavelengths, an SHG element is provided after the electro-optical element or the magneto-optical element described above, and a single wavelength polarization modulation light source is provided. Increase the number of wavelengths. Similar to the case of a single wavelength, the reference signal is detected after the SHG element by the change of the interference fringe intensity. The detection of the length measurement signal that has passed through the interference optical system in the subsequent stage is performed by separating each color by a dichroic filter or the like.

[0008]

In the present invention, unlike the conventional interference fringe counting method,
Since the polarization state of the light source light is modulated, the intensity of the interference fringes detected as a length measurement signal is constantly oscillating. This can be easily converted into DC by passing it through a low-cut filter after photoelectric conversion.
Noise can be removed almost 100%. Since a synchronization signal is generated from the reference signal extracted from the light source light and transmitted through the optical fiber, the length measurement signal accurately synchronized with the modulation signal can be obtained from the AC component having the displacement information that has passed through the previous filter. , The phase can be specified and the length measurement value can be obtained. Further, in the present invention, the synchronization signal is generated every time the phase of the reference signal (equal to the modulation signal) is deviated by 90 ° due to the modulation, and a plurality of length measurement signals are sampled in synchronization with the synchronization signal, thereby implementing the multi-bucket method. The initial phase (displacement) can be obtained with extremely high accuracy by applying it to the calculation of the initial phase, and as a result, highly accurate length measurement can be realized by the interference fringe counting method. Further, according to the present invention, in addition to the disturbance and drift due to the DC component described above, the influence of the change in the amplitude of the interference fringe intensity due to the output fluctuation of the light source, etc. can be almost completely removed. Can be realized at the same time.

A memory is used for the calculation of the multi-bucket, and the calculated value is written in the memory in advance. The calculated value can be obtained at a very high speed in the memory read cycle by simply adding each synchronous measurement signal value or its addition / subtraction value to the memory address, and the following speed of the measurement does not depend on the measurement resolution. , Can be fast. Furthermore, when the light source light is made to have multiple wavelengths in order to remove atmospheric fluctuations, an SHG element is inserted after the polarization modulator. S
Since the HG element cancels the birefringence of the crystal by using two crystals facing each other, the phase is aligned with the polarization-modulated single-wavelength light, and the polarization-modulated harmonic wave is also emitted. To do. However, since the modulation amount is equal to the original fundamental wave and the modulation phase amount is multiplied with respect to the higher harmonic wave, the synchronization reference signal is similarly extracted from the subsequent stage of the SHG element by the change in the interference fringe intensity.

[0010]

EXAMPLE FIG. 1 shows an example of the present invention. The laser light source 1 is generally a single frequency-stabilized He-Ne laser, but may be a semiconductor-excited YAG solid-state laser or another type of laser. The type of light source is not relevant to the invention. In the embodiment of FIG. 1, an isolator 2 that cuts back light is provided immediately after the light source.
Is provided in order to maintain high stability of the light source, and the presence or absence thereof is not related to the configuration of the present invention.
It is not related to the scope of the claims of the present invention to place a phase plate in the latter stage in order to eliminate the polarization direction of the light source luminous flux. The laser beam emitted from the light source passes through an electro-optical element such as LiTaO ₃ to modulate its polarization state. The modulator 3 is not limited to the electro-optical element of this embodiment, and may be a magneto-optical element, or may be mechanically modulated by driving a Babinet-Soleil plate using PZT or the like. . The type of element used for polarization modulation is not relevant to the present invention. In the case of LiTaO ₃ , when a modulation electric field is applied in a direction perpendicular to the direction of travel of the light flux, a phase difference occurs between the electric field vectors of the light flux passing through the interior and the electric field vectors perpendicular to the electric field direction. When the laser light source light is made to enter the crystal by linearly polarized light, it can be made to be circularly polarized light, elliptically polarized light, or linearly polarized light in a direction orthogonal to the time of incidence by the modulating electric field, and then emitted. Now, the mutually perpendicular electric field vector directions that produce a phase difference due to modulation,
When the polarization directions of the reference light flux and the measurement light flux, which are also perpendicular to each other, of the interferometric length measurement optical system 5 in the latter stage are made to coincide with each other, this modulation modulates the phase difference between the reference light flux and the length measurement light flux. become. Therefore, by a known means such as a polarizer, the components of the reference light flux and the length measurement light flux are respectively extracted, and the displacement is detected by the intensity of the interference fringes (length measurement signal),
This interference fringe intensity always changes due to the above-mentioned modulation. If displacement occurs in the test object in the interferometric measuring optical system, if this interference fringe is sampled at the timing precisely synchronized with the modulation, the phase change of the measuring interferometric fringe with respect to the modulation timing will occur due to the change in the interference fringe intensity. You know, and you should know the displacement.

In the embodiment shown in FIG. 1, the portion for detecting the length measurement signal is integrated in the interference prism 6, and the optical fiber 8 is used.
The interference fringe intensity is transmitted to the sampling circuit 9. Of course, this detector may be separate from the interference prism. Further, since the timing of synchronization with the modulation has nothing to do with the displacement of the length-measuring interference optical system 5,
It is possible to easily obtain the interference fringe intensity from the post-stage 4 after modulating the light from the light source by a known method using a polarizer or the like. FIG. 2 shows an embodiment of a reference signal detector for generating the synchronization signal. First, from the left, the modulated light source light enters the prism unit 10. This is taken out by the beam splitter. Since the prism unit 10 is installed at an angle of 45 ° with respect to the mutually perpendicular polarization directions that generate the phase difference of modulation, both components of the mutually perpendicular polarization directions that generate the phase difference are generated by the polarization beam splitter 11. Two interference fringes that can be extracted and are 180 ° out of phase are generated at the same time. Further, in the embodiment, the 90 ° phase plate 13 is passed through one of the interference fringes to finally obtain two-phase reference signals having different 90 ° phases. This is for use in the multi-bucket method described later. Since time resolution is very important for obtaining accurate synchronization timing, it is not good for these reference signals to be transmitted as electric signals which are reflected in the cable or whose waveform is easily deformed by the stray capacitance. As shown in FIGS. 1 and 2, the optical fiber 7 transmits the interference fringe intensity as it is to the sampling circuit 9. In the sampling circuit 9, after photoelectrically converting the reference signal as in the embodiment shown in FIG.
Remove the DC component, detect the zero-cross position with the comparator, pulse it with the differentiating circuit, obtain the synchronization signal,
The length measurement signal is sampled accordingly. Gain adjustment and waveform shaping until the synchronization signal is obtained are appropriately performed in the circuit.

In FIG. 1, the interference measurement optical system 5 detects the displacement of the test object as interference fringe intensity, and the measurement signal transmitted by the optical fiber and photoelectrically converted is processed by the low-cut filter as in the case of the reference signal. Remove the DC component. It is sufficient to set the modulation frequency of the light source light at about twice the tracking speed of the length measurement, and then the modulation band is several M.
Since it will be more than Hz, DC for the length measurement signal after low cut
Almost no ingredients remain. However, if this length measurement signal is directly replaced by a single-phase synchronization signal as a single interference fringe intensity for phase change, a problem occurs. The measurement signal that has passed through the low-cut filter has a voltage value (V) representing the intensity of interference fringes, an amplitude (A), an initial phase (θ), and a modulation angle (ω).
It is represented by t) as follows. V = A · sin (θ + ωt) (1) FIG. 4 shows the equation (1) together with the single-phase synchronizing signal, with the horizontal axis representing time (t) and the vertical axis representing voltage (V). is there. Although the DC component disappears, the voltage value V of the equation (1) fluctuates due to the fluctuation of the amplitude (A), and as shown in FIG. 4, it is only the length measurement signal value sampled by the single-phase synchronization signal. , Did the initial phase change due to displacement,
It is impossible to determine whether the amplitude has simply changed. If the measurement value is output as it is, it is directly affected by the fluctuation of the amplitude of the interference fringes due to the fluctuation of the light source intensity or the fluctuation, and the stability of the measurement becomes low.

FIG. 5 shows an embodiment of the present invention which solves this problem. In FIG. 5, a multiphase modulation synchronization signal is obtained by using a multiphase reference signal, and the modulation 1
Obtaining multiple length measurement signal values during a cycle, and obtaining the initial phase of the length measurement signal with high accuracy without the influence of disturbance by the multi-bucket method,
The above problems are overcome. In the multi-bucket method, a periodic signal represented by a single trigonometric function is accurately sampled at 90 ° intervals, and the initial phase value, amplitude value, and DC bias are calculated by simple calculation using the orthogonal relationship of trigonometric functions. This is a method of obtaining values and the like. When the multi-bucket method is applied to the equation (1), since the unknowns are only the amplitude (A) and the initial phase (θ), the length measurement signal may be sampled at two points of ωt = 0 ° and 90 °. Letting the values at that time be V ₁ and V ₂ , respectively, the following equation is obtained. V ₁ = A · sin (θ) (2) V ₂ = A · sin (θ + 90 °) = A · cos (θ) (3) Therefore, the initial phase (θ) and amplitude (A) are θ = tan ⁻¹ (V ₁ / V ₂ ) (4) A = squart (V ₁ ² + V ₂ ² ) (5) This is the 2-bucket method, and according to the present invention, the above-mentioned 9
Since V ₁ and V ₂ can be accurately obtained by the two-phase reference signals having different 0 ° phases, the initial phase value can be obtained with high accuracy according to the equation (4) without the influence of the amplitude fluctuation. In the embodiment of FIG. 5, the length measurement signal from which the DC component is removed is digitized by the AD converter, and ωt =
Values of 0 ° and 90 ° are connected to addresses in the memory.
Since the initial phase value is uniquely determined by the values of V ₁ and V ₂ ,
If the values of all the combinations are written in the memory in advance, the operation of the equation (4) can be executed in the read cycle of the memory, and the initial phase value can be obtained extremely quickly. The amplitude value A is similarly obtained at high speed. The obtained amplitude value A is fed back to the gain adjustment of the length measurement signal as shown in FIG. 5, and the AGC is operated so as to always use the maximum dynamic range of the AD converter.
As a result, the quantization error that accompanies the digitization of the length measurement signal can be minimized, so that the accuracy of the initial phase value can be maintained high.

As described above, the DC component of the length measurement signal is almost completely removed by the low-cut filter. However, if there is an offset error in the comparator or OP amplifier in the subsequent stage, the DC component is slightly generated. Remain.
Since this DC component causes a drift due to a temperature change, etc., when increasing the resolution of the initial phase (length measurement),
Cause real harm. To apply the multi-bucket method at this time, the equation (1) is not sufficient, and the DC term (D) must be added as in the following equation (6). V = A · sin (θ + ωt) + D (6) Since there are three unknowns, ωt = 0 °, 90 °, 180
It suffices to sample the length measurement signal at three points. Letting the values at that time be V ₁ , V ₂ , and V ₃ , respectively, the following equation is obtained. V ₁ = A · sin (θ) + D (7) V ₂ = A · sin (θ + 90 °) + D = A · cos (θ) + D (8) V ₃ = A · sin (θ + 180 °) + D = −A · sin (θ) + D (9) Therefore, initial phase (θ), amplitude (A), DC component (D)
Is θ = tan ⁻¹ ((V ₁ −V ₃ ) / (2 · V ₂ −V ₁ −V ₃ )) (10) A = ½ · squart ((V ₁ −V ₃ ) ² + ( V ₁ + V ₂ −2 · V ₂ ) ² ) (11) D = (V ₁ + V ₃ ) / 2 (12) This is the 3-bucket method, and since the symmetrical operation is performed with respect to V ₂ , the initial phase value Is less affected by the error of the sync signal than the two-bucket method, but it is a little complicated,
In particular, the calculation of the amplitude A is too complicated to use the arithmetic memory. In this case, the following 4-bucket method is advantageous in terms of easy calculation and following speed.

In the 4-bucket method, ωt = 2 is added to the 3-bucket method.
A length measurement signal V ₄ of 70 ° is added and the following is performed. V ₄ = A · sin (θ + 270 °) + D = −A · c0s (θ) + D (13) Therefore, initial phase (θ), amplitude (A), DC component (D)
Is θ = tan ⁻¹ ((V ₁ −V ₃ ) / (V ₂ −V ₄ )) (14) A = 1/2 · squart ((V ₁ −V ₃ ) ² + (V ₂ −V ₄ ) ² ) (15) D = (V ₁ + V ₂ + V ₃ + V ₄ ) / 4 (16) In this case, the address of the arithmetic memory is (V ₁ −
V ₃₎ and giving a value of _(V 2 -V _4). Therefore, it is necessary to perform the subtraction after sampling the length measurement signal, but the circuit does not require much load. As a result, the amplitude value A can be calculated in the arithmetic memory in the same manner, and the length measurement signal can be converted into AGC.

To further improve the accuracy of the initial phase (length measurement), the 5-bucket method is used. According to the present invention, the primary component of the time error included in the synchronization signal can be canceled by this, so that higher accuracy can be achieved. V ₁ = A · sin (θ−180 °) + D = −A · sin (θ) + D (17) V ₂ = A · sin (θ−90 °) + D = −A · cos (θ) + D (18) V ₃ = A · sin (θ) + D (19} V ₄ = A · sin (θ + 90 °) + D = A · cos (θ) + D (20) V ₅ = A · sin (θ + 180 °) + D = −A · sin (θ) + D (21) Therefore, initial phase (θ), amplitude (A), DC component (D)
Is θ = tan ⁻¹ ((2 · V ₃ −V ₁ −V ₅ ) / 2 · (V ₄ −V ₂ )) (22) A = squart (4 · (V ₂ −V ₄ ) ² + ( V ₁ + V ₅ −2 · V ₃ ) ² ) / 4 (23) D = (2 · V ₃ + V ₁ + V ₂ + V ₄ + V ₅ ) / 6 (24) Somewhat complicated, but symmetrical with respect to V ₃ Since addition and subtraction are carried out dynamically, even if there is an error in the phase difference between the two-phase reference signals in the present invention, the primary component is canceled by the effect of averaging.

The multi-bucket method described above is a means for obtaining the initial phase of the length measurement signal with high accuracy and easily in the present invention. However, even if such a method is used, an accurate initial phase value cannot be obtained by polarization modulation based on the phase difference. This is because, since the modulation is performed in the traveling direction of the light flux, the frequency in two orthogonal polarization directions with a phase difference, that is, the wavelength, changes due to the Doppler shift. This variation amount is, for example, 10 at the modulation frequency.
MHz, and the modulation amount for one wavelength is 2 × 10 ⁻⁵
It becomes a degree and cannot be ignored. The reference signal according to the present invention only has information on the relative phase difference between two polarizations orthogonal to each other, and such an absolute wavelength variation cannot be detected.
As a means for solving this, in the present invention, the length measurement signal value or the initial phase when the modulation is performed in the direction in which the light flux relatively advances and the phase difference increases, and when the modulation is performed in the opposite direction and the phase difference decreases The solution was achieved by averaging the values. The averaging may be performed on the length measurement signal or after calculating the initial phase, but the length measurement signal is measured at the timing when the absolute values of the changes in the phase difference due to the modulation become almost equal in both directions. It is important to sample. This timing can be easily realized by the above-described embodiment of the present invention. For example, when the synchronization signal is generated from the two-phase reference signal, the length measurement signal is sequentially sampled when the phase difference of the light flux is modulated, and the sampling is sequentially performed in the opposite direction. In this case, the arrangement of the measured values is simply reversed, so that 3 buckets or 5 buckets can be treated as equivalent and averaged, and in the case of 2 buckets or 4 buckets, one of the arrangements can be used. By reversing, it is possible to average without changing anything arithmetically. Since this series of operations can be performed in one cycle of modulation, even if such an operation is performed, the length measurement value can be obtained at least at the modulation frequency. By adding such an operation of the present invention to the multi-bucket method, it has become possible to perform extremely highly accurate length measurement at high speed with a simple configuration.

In the present invention, in order to prevent the measurement value from fluctuating due to atmospheric fluctuations, in order to make the light source multi-colored, an SHG element may be arranged after the polarization modulator. Further down,
An optical system similar to the embodiment shown in FIG. 3 for detecting the reference signal is provided. Since the crystal used for SHG has birefringence, two crystals are usually made to face each other to cancel them. Therefore, since the polarization state of the incident light is maintained even at the time of emission, the harmonic wave is also emitted as a single polarization state. That is, if the incident light is polarization-modulated, the harmonics are also polarization-modulated. The polarization-modulated synchronization signal can be obtained by a reference signal detection optical system placed at a subsequent stage. Further, an interference optical system for length measurement is arranged in the subsequent stage, and a detection optical system for each wavelength is arranged after that. The principle of length measurement at each wavelength is the same as in the embodiments described above. With the displacement gauge of the present invention having such a configuration, since a length measurement value can be obtained with high accuracy and high speed for each wavelength, a final measurement without fluctuation that is guided according to the dispersion formula of the atmosphere based on this is obtained. The long value can be obtained with high accuracy and at high speed.

In the above embodiments of the present invention, the phase difference between the reference light and the length measuring light by the electro-optical element was used as the polarization modulation method, but the scope of the present invention is not limited to this. For example, a magneto-optical element may be used as the modulation element. In addition, even if it is a modulation method in which the Faraday effect or the like is used as the modulation method to rotate (rotate) the azimuth of polarization by the coil magnetic field, all the above-described embodiments can be applied,
It is possible to realize an interference fringe counting type displacement meter that is highly accurate and highly stable, and that can easily be multicolored. In this case, since the modulation direction is orthogonal to the traveling direction of the light beam, the Doppler shift due to the modulation does not occur, and therefore it is not necessary to average a plurality of initial phase values to cancel this. Therefore, as compared with the phase difference modulation, the system becomes simpler, which facilitates speeding up and higher accuracy.

[0020]

With the simple structure and method according to the present invention, disturbance and noise can be removed and the initial phase of the interference fringes for length measurement can be obtained with high accuracy and at high speed. Therefore, stable measurement with sub-nanometer accuracy is possible. A laser interferometer can be easily obtained at low cost. Further, it is easy to make the light from the light source multi-colored, and it is possible to stably and accurately correct the influence of atmospheric fluctuations at high speed.

[Brief description of drawings]

FIG. 1 is an optical path layout diagram of a first embodiment.

FIG. 2 is an optical path layout diagram of a second embodiment.

FIG. 3 is a circuit block diagram of a third embodiment.

FIG. 4 is a timing chart of the fourth embodiment.

FIG. 5 is a circuit block diagram of a fifth embodiment.

FIG. 6 is a schematic diagram of a conventional laser fringe measuring system using an interference fringe counting method.

FIG. 7 is a schematic diagram of a conventional heterodyne type laser interferometer.

[Explanation of sign]

1 ... Laser light source, 2 ... Isolator, 3 ... Modulation element, 4 ... Reference signal detection unit, 5 ... Interferometric length measurement optical system, 6 ...
Interference prism, 7 Optical fiber,
8: optical fiber, 9: sampling circuit,
10 Prism unit, 11 Beam splitter, 12 Polarizing beam splitter, 13
... λ / 4 phase plate, 14 ... condensing lens, 1
5 ... 2 frequency stabilized laser light source, 16 ... beam splitter, 17 ... detection head, 18 ...
Transmission cable, 19 Length measuring circuit,
20: Single frequency stabilized laser light source, 21: λ /
4 phase plate, 22 ... Detection head, 23 ...
Optical fiber, 24 ... Counting circuit.

Claims (15)

[Claims] 1. A means for modulating the polarization state of a light beam emitted from a laser light source, detecting a reference signal from the modulated light beam by the intensity of interference fringes, and generating a synchronizing signal based on the reference signal. By further using the modulated light flux, and having a means for detecting the displacement of the test object by optical interference, by sampling the length measurement signal obtained by this means according to the synchronization signal described above. An interference fringe counting type displacement meter characterized by measuring the displacement of a test object. 2. A means for modulating the polarization state of a light beam emitted from a laser light source, wherein the modulated light beam is SHG.
By passing through the element, after obtaining the light flux of a plurality of wavelengths coaxially, the reference signal is detected from the light flux by the interference fringe intensity,
Having means for generating a synchronization signal based on it, further using the modulated light flux, having a means for detecting the displacement of the test object by a plurality of wavelengths by optical interference,
An interference fringe counting type displacement meter, characterized in that the displacement of a test object is measured by sampling the length measurement signals for a plurality of wavelengths obtained by this means in accordance with the aforementioned synchronization signal. 3. The displacement meter according to claim 1 or 2, wherein, when modulating the polarization state of the light source luminous flux, the phase of the intensity change of the reference interference fringes at a single wavelength or a plurality of wavelengths is shifted by 90 degrees due to the modulation. An interference fringe counting type displacement meter characterized by having a means for generating a synchronization signal for each. 4. The interference fringe counting system according to any one of claims 1 to 3, wherein the polarization direction of the light source luminous flux is modulated by rotating the light flux to the left or right around the traveling direction as an axis. Displacement gauge. 5. The displacement gauge according to any one of claims 1 to 3, when the light source light is divided into vibration components of electric field or magnetic field vectors which are perpendicular to each other, one direction of the components is relatively advanced relative to the other phase. Characterized by performing polarization phase modulation by delaying or delaying, and making each azimuth coincide with each polarization direction of the reference light and the length measuring light in the optical system for detecting the displacement of the test object by interference. , Interference fringe counting type displacement meter. 6. The displacement meter according to any one of claims 1 to 5, wherein the length measurement signal based on the intensity of the interference fringes is photoelectrically converted, the DC component is removed, and further sampling is performed in accordance with the synchronization signal based on the reference signal, and then the length measurement is performed. Displacement meter of interference fringe counting method, which is characterized by obtaining an initial phase of a signal. 7. The displacement meter according to any one of claims 1 to 6, wherein the length measurement signal based on the intensity of interference fringes is sampled in accordance with a synchronization signal based on a reference signal, and the plurality of sampling values are measured by the 2-bucket method. Displacement meter of interference fringes counting method, characterized in that the initial phase of is calculated. 8. The displacement meter according to any one of claims 1 to 6, wherein the length measurement signal based on the intensity of interference fringes is sampled in accordance with a synchronization signal based on a reference signal, and the three-bucket method is used for a plurality of sampling values to measure the length measurement signal. Displacement meter of interference fringes counting method, characterized in that the initial phase of is calculated. 9. The displacement meter according to any one of claims 1 to 6, wherein the length measurement signal based on the intensity of interference fringes is sampled in accordance with a synchronizing signal based on a reference signal, and the length measurement signal is obtained by using the 4-bucket method for a plurality of sampling values. Displacement meter of interference fringes counting method, characterized in that the initial phase of is calculated. 10. The displacement meter according to any one of claims 1 to 6, wherein a length measurement signal based on the intensity of interference fringes is sampled in accordance with a synchronization signal based on a reference signal, and the plurality of sampling values are set to 5
An interference fringe counting type displacement meter characterized in that the initial phase of a length measurement signal is obtained using the bucket method. 11. The method according to any one of claims 7 to 10, wherein, when obtaining the initial phase of the length measurement signal, the interference fringe intensity values of a plurality of length measurement signals sampled in accordance with the synchronization signal based on the reference signal are digitized and By connecting the result of addition or subtraction with the interference fringe intensity value directly or to the address of the memory and uniquely determined by the value of those addresses, the calculation result stored in the memory in advance is read from the data of the memory, A displacement meter characterized by obtaining an initial phase of a length measurement signal. 12. When obtaining the amplitude of a length measurement signal, the interference fringe intensity values of a plurality of length measurement signals sampled in accordance with a synchronization signal based on a reference signal are digitized and the values are directly measured. Alternatively, the result of addition and subtraction with other interference fringe intensity values is connected to the addresses of the memory, and the calculation result stored in advance in the memory, which is uniquely determined by the values of those addresses, is read out from the data of the memory. Displacement meter characterized by obtaining the amplitude of a long signal. 13. The optical fiber according to claim 1, wherein the detected reference signal is transmitted to an electric circuit for generating a synchronizing signal or an electric circuit for determining an initial phase of a length measuring signal. Let's say a displacement meter. 14. The measurement signal according to any one of claims 1 to 13, wherein the polarization phase modulation is periodically performed, and a synchronization signal based on a reference signal is used in each of a region where a phase difference of a light beam increases and a region where the phase difference decreases due to the modulation. A displacement meter characterized in that the initial phase is obtained after sampling, averaging it, and averaging it. 15. The length measurement signal according to any one of claims 1 to 13, wherein the polarization phase modulation is periodically performed, and a synchronization signal based on a reference signal is used in each of a region where a phase difference of a light beam increases and a region where the phase difference decreases due to the modulation. Displacement meter characterized by sampling and measuring the initial phase, and averaging it to obtain a length measurement value.