JPH08338707A - Displacement gage - Google Patents

Abstract

PURPOSE: To obtain an inexpensive displacement gage capable of dealing with a wide range from a low speed to a high speed. CONSTITUTION: Laser light is introduced into a beam splitter 3 through a polarization plate, and divided into measured light and reference light, which are returned to the beam splitter 3 after they are reflected by a reference mirror R1 . The measuring light passes through a 1/8 wave length plate 4, the reference light through a 1/4 wave length plate 5 two times each respectively, they are returned to the beam splitter 3 and interference light C1 is produced. Because the measuring light is returned as circular polarization, the interference light C1 is divided into two polarization components, quantity of respective light is detected, the phase of energy vector of the interference light C1 is investigated and quantity of displacement of an object to be measured M1 is detected from the rotational frequency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an extremely high-precision displacement meter for measuring the displacement of a minute object by utilizing the interference of light.

[0002]

2. Description of the Related Art Heretofore, a heterodyne interferometry method has been known as a method for measuring displacement of a minute object with high accuracy. The heterodyne interferometry method produces two lights having slightly different frequencies, makes them incident on the measurement surface and the reference surface of the object to be measured, respectively, and interferes the reflected light of both with photoelectric conversion to equalize the frequency difference. It obtains an electrical beat signal having a frequency and then obtains the phase difference between the beat signal for reference and the beat signal for reference, which enables highly accurate displacement measurement based on the phase difference of light.

FIG. 9 illustrates a displacement meter according to a conventional example using the heterodyne interferometry. A laser beam emitted from a laser 101 such as a HeNe laser is split into two polarization components by a first beam splitter 130. One of them is converted into light having a slightly different frequency by the frequency shifter 131. If the frequency shifter 131 is, for example, a photoacoustic deflector, it can give a frequency shift of several tens of MHz. The frequency-shifted light is introduced into the second beam splitter 132, superposed on the light transmitted through the first beam splitter 130, and photoelectrically converted by the photodetector element 142 via the analyzer 141. A beat signal having the same frequency as the given frequency can be obtained. This is used as the reference signal A ₀ .

On the other hand, the light beams superposed in the second beam splitter 132 and traveling in another direction are split again by the third beam splitter 133. The light that has passed through the third beam splitter 133 and travels straight is irradiated to the object to be measured M ₀ through the quarter-wave plate 134 placed at the polarization plane and 45 degrees, reflected by the object to be measured M ₀ , and again. Returning to the third beam splitter 133. At this time, the quarter-wave plate 134
Since the polarization direction is rotated by 90 degrees by the action of, the returned light is reflected by the third beam splitter 133 and
Introduced to 39.

Further, the polarized component reflected by the third beam splitter 133 without traveling straight is also the quarter wavelength plate 13
The reference mirror 137 is irradiated through 6 and reflected, and then returns to the third beam splitter 133 again. This light is also 1 /
The polarization direction is rotated by 90 degrees by the function of the four-wave plate 136, and the returned light is transmitted through the third beam splitter 133, goes straight, and is introduced into the photodetection element 139 through the analyzer 138.

The beat signal thus introduced into the photodetector 139 through the analyzer 138 and photoelectrically converted therein is the measurement signal B ₀ , and the phase difference between the measurement signal B ₀ and the reference signal A ₀ is obtained. Is measured to detect the displacement of the measured object M ₀ .

Measurement signal B ₀ of several tens of MHz and reference signal A ₀
The phase difference between and is calculated by the phase meter 144. That is, the phase output of the phase meter 144 becomes an output signal representing the displacement of the measured object M ₀ . When the measured object M ₀ is displaced by ½ of the wavelength of the laser light of the laser 101 in the direction indicated by the arrow P ₀ in FIG. 9, the phase output of the phase meter 144 changes by 360 degrees.
Therefore, when the HeNe laser is used, the phase meter 144
If the resolution of 1 is 1.degree., A resolution of about 2 nm can be obtained.

[0008]

However, according to the above-mentioned conventional technique, as described above, it is based on the heterodyne interferometry method in which the displacement is detected by measuring the phase difference between two beat signals with a phase meter. Although it has an advantage that minute displacements can be measured with extremely high accuracy, generally, the phase meter has a narrow response frequency band and cannot follow an object to be measured moving at high speed.

For example, a He light source having a wavelength of 632.8 nm is used.
When frequency is shifted at 100 MHz using a Ne laser and the moving speed of the measured object changes within the range of 0 to 10 m / sec, 10 (m / sec) ÷ (632.8 × 10 ⁻⁹ (m) / 2) ≒
Since it is 32 × 10 ⁶ (Hz), the frequency of the measurement signal (beat signal) input to the phase meter changes in the range of ± 32 MHz centering on 100 MHz. That is, the measurement signal input to the phase meter has a frequency of 68 MHz to 132 MHz, and it is extremely difficult to measure with a phase meter having a narrow response frequency band.

Further, since the frequency shifter and the phase meter, which are indispensable for the heterodyne interferometry, are expensive, it is inevitable to increase the cost of the device.

The present invention has been made in view of the problems of the above-mentioned prior art, and is capable of detecting a displacement of a minute object with high accuracy in a wide speed range from an extremely low speed state to a high speed state. It is an object of the present invention to provide a displacement gauge that does not require expensive measuring parts and is therefore inexpensive.

[0012]

In order to achieve the above object, the displacement meter of the present invention comprises a polarizing means for converting coherent light generated from a light source into a polarized light beam, and the polarized light beam as a reference light and a measurement light. And an interfering means for obtaining interference light by superposing them on the reference surface and the surface of the object to be measured, respectively, a quarter-wave retarder arranged in the optical path of the reference light, A 1/8 wavelength phase shifter arranged in the optical path of the measuring light, a light amount detecting means for dividing the interference light into two polarization components and individually detecting the light amounts of the two polarized light components, and an output of the light amount detecting means. It is characterized by having a calculating means for calculating the displacement amount of the object to be measured based on the above.

Condensing means for converging the measurement light and the reference light on the surface of the object to be measured and the reference surface, respectively, may be provided.

Further, it is preferable that the light receiving surface of the light amount detecting means is disposed at the defocus position of the light collecting means.

Further, it is preferable that a ½ wavelength retarder is provided in the optical path of the interference light.

Further, it is preferable that the calculation means has a calculation table for detecting the rotation of the energy vector of the interference light from the change of the light quantity of each of the two polarization components of the interference light.

[0017]

The measuring light reflected by the surface of the object to be measured and returning to the interference means passes through the ⅛ wavelength retarder twice to cause a 90 ° phase delay in the specific polarization component. For this reason, the measurement light returning to the interference means becomes circularly polarized light whose energy vector rotates as the object to be measured moves.

Therefore, the interference light obtained by combining the measurement light and the reference light is divided into two polarization components having different polarization directions, the respective light amounts are detected, and the combination thereof is examined to determine the energy of the interference light. The forward / reverse rotational speed of the vector is obtained and converted into the displacement amount of the object to be measured.

This displacement meter is an extremely high-precision displacement meter whose measurement unit is 1/2 of the wavelength of the light from the light source, and is inexpensive because it does not require expensive measurement parts such as a frequency shifter or a phase meter. In addition, it has an advantage that it can be applied in a wide range from a case where the object to be measured moves at high speed corresponding to a wide response frequency band to an extremely low speed state.

[0020]

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

FIG. 1 illustrates a displacement meter according to an embodiment, in which a HeNe laser 1 as a light source and a laser beam L ₁ as coherent light emitted from the HeNe laser 1 are polarized with a single polarization direction. A polarizing plate 2 which is a polarization means for converting into a light beam,
A beam splitter 3 which is an interference means for splitting the laser light transmitted through the polarizing plate 2 into a measurement light and a reference light, and a ⅛ wavelength retarder which delays the polarization component of the measurement light in a specific direction by ⅛ wavelength. A certain 1/8 wavelength plate 4, a 1/4 wavelength plate 5 that is a 1/4 wavelength retarder that delays the polarization component of the reference light in a specific direction by 1/4 wavelength, and a 1/8 wavelength plate 4 and Measured object M ₁
Between the objective lens 6 which is a condensing means provided between
It has an objective lens 7 which is a condensing means provided between the four-wave plate 5 and a reference mirror R ₁ which is a reference surface, and the polarization plane of the polarizing plate 2 is set at an angle of 90 degrees with respect to the paper surface. .

The ⅛ wave plate 4 is set at an angle of 45 ° with respect to the polarization direction of the incident light, and the measurement light transmitted through the beam splitter 3 passes through the ⅛ wave plate 4. It is condensed on the surface of the object to be measured M ₁ by the objective lens 6, is reflected by the surface, passes through the 1/8 wavelength plate 4 again, and returns to the beam splitter 3. Thus, since the measurement light passes through the 1/8 wavelength plate 4 twice, when returning to the beam splitter 3, a 1/4 wavelength delay is obtained in the polarization component in a specific direction. Further, the 1/8 wavelength plate 4 is set to 45 degrees with respect to the polarization direction of the incident light as described above, and therefore,
The measurement light returning to the beam splitter 3 becomes circularly polarized light.

On the other hand, the reference light, which is the reflected light of the beam splitter 3, passes through the quarter-wave plate 5 and is condensed on the surface of the reference mirror R ₁ by the objective lens 7, and is reflected by the surface and again the quarter-wavelength. It passes through the plate 5 and returns to the beam splitter 3. In this way, the reference light passes through the quarter-wave plate 5 twice, so that when it returns to the beam splitter 3, a polarization component of a specific direction is delayed by a half wavelength. 1 /
When the set angle of the four-wave plate 5 is 45 degrees with respect to the polarization direction of the incident light, the plane of polarization of the reference light rotates by 90 degrees.

When the beam splitter 3 has the polarization characteristic, the reflection characteristic of the reference light at the beam splitter 3 changes due to the change of the polarization direction, and the light utilization efficiency is improved. That is, by providing the quarter-wave plate 5, the light utilization efficiency is improved and the intensity of the detection signal is improved.

The beam splitter 3 superimposes the measurement light and the reference light returned from the DUT M ₁ and the reference mirror R ₁ again, and as a coherent light C ₁ , a light detecting section 8 which is a light quantity detecting means.
Introduce to. Since the measurement light returns as circularly polarized light, the energy vector of the interference light C ₁ immediately after being superposed is 1 each time the object to be measured M ₁ moves (displaces) by 1/2 wavelength.
It rotates, and the rotation direction is clockwise or counterclockwise depending on the moving direction of the object to be measured.

That is, when the reference light and the measurement light are divided into a polarization component of 45 degrees and a polarization component of 135 degrees with respect to the paper surface, the reference light is linearly polarized light parallel to the paper surface.
Although there is no phase difference between the 45-degree component and the 135-degree component, since the measurement light is circularly polarized light, there is a 90-degree phase difference between the 45-degree component and the 135-degree component. Therefore, the amount of light of the 45-degree component and the amount of light of the 135-degree component of the interference light C ₁ obtained by superimposing these changes with the phase difference of 90 degrees due to the displacement of the DUT M ₁ , respectively. The amount of displacement of the object to be measured M ₁ can be known by measuring the changes and obtaining the normal and reverse rotation speeds of the energy vector of the interference light C ₁ .

The photodetector 8 is a 1/2 wavelength retarder 1 /
A two-wave plate 81 is provided, and the plane of polarization of the interference light C ₁ is rotated by 45 ° by setting this at an angle of 22.5 ° with respect to the paper surface. As a result, the polarization component of the interference light C ₁ at 45 degrees and 1
The polarization planes of the polarization component of 35 degrees are 0 degree and 90 degrees, and the photodetector beam splitter 83 and the photodetector 8 which will be described later are provided.
It is possible to prevent the arrangement of the optical system including 4,85 from becoming complicated. If the half-wave plate 81 is omitted, the photodetector beam splitter 83 should be set at an angle of 45 degrees with respect to the paper surface, and the photodetector element 85 should be set on the optical axis of the reflected light of the photodetector beam splitter 83. I have to.

Interfering light C ₁ which has passed through the half-wave plate 81
Passes through the field stop 82 and the photodetector beam splitter 83.
Incident on. The field stop 82 is arranged on the image plane of each of the objective lenses 6 and 7, so that the interference light C ₁ is imaged on the field stop 82. The diaphragm diameter of the field diaphragm 82 is, for example, 50 mm in diameter.
a [mu] m, if the objective lens 6 and 7 10 × objective lens, a measurement spot diameter region of 5μm on the surface of the object to be measured M _1. This measurement spot size can be changed by changing the size of the field stop 82. If the measurement spot size is 5 μm in diameter in this way, the elevation of the liquid level in the hole having a diameter of about 20 μm can be measured with high accuracy.

The photodetector beam splitter 83 receives the interference light C
₁ to the sheet surface is divided into 0 degree polarization component A ₁ with 90 ° polarization component B _1. As described above, the changes in the amounts of light of the two polarized components due to the movement of the object to be measured M ₁ have a phase difference of 90 degrees with each other.
4,85.

By disposing the field stop 82 on the image forming plane of the objective lenses 6 and 7 and disposing both the photo-detecting elements 84 and 85 at the de-focus position, precise alignment of these is not possible. It has the advantage of being required.

Both polarization components A ₁ and B ₁ are converted into electric signals by photodetectors 84 and 85, respectively, and amplifiers 86 and 87 are used.
Are amplified by the first and second detection signals S ₁ and T ₁ , respectively.
Becomes When the DUT M ₁ reciprocates in the direction indicated by the arrow P ₁ in FIG. 1, both detection signals S ₁ and T ₁ are shown in FIG.
The moving waveform of the device under test M ₁ shown in FIG.
As shown in (c), they change substantially periodically with a phase difference of 90 degrees.

When the DUT M ₁ moves by ½ wavelength, the energy vector of the interference light C ₁ makes one rotation, and the phases of both detection signals S ₁ and T ₁ change by 360 degrees.
When it is desired to measure the object M _{1 to} be moved at a high speed of 10 m / sec, the laser light L of the HeNe laser 1 is used.
_Since the wavelength of ₁ is 632.8 nm, the detection signal S ₁ ,
The frequency of T ₁ is 10 (m / sec) ÷ (632.8 × 10 ^-9 (m) / 2) ≈
It becomes 32 × 10 ⁶ (Hz). Therefore, the photodetector elements 84 and 85, the amplifier 86,
For 87, one having a frequency characteristic higher than this is used. An analog-to-digital converter 88, which will be described later, is one that performs digital conversion at a sampling rate of 130 MHz or more, which is about four times this frequency.

Both detection signals S ₁ and T ₁ are converted into digital signals by a 2-channel analog-digital converter 88,
This signal is input to the signal processing unit 89, which is a calculation means, and the signal processing unit 89 calculates the displacement amount of the object to be measured M ₁ by the procedure shown in the flowchart of FIG. 3, and displays the result on the display unit 9.

The calculation procedure in the signal processing unit 89 is as follows. First, the data of the _first detection signal S ₁ input from the channel 1 of the signal processing unit 89 is converted into X ₀ (i),
The data of the second detection signal T ₁ input from the channel 2 will be called Y ₀ (i).

In step S1, variables are initialized. That is, the data number i, the initial value ph0 of the phase, and the initial value Z (0) of the measured object position are set to 0, respectively. X ₀ (i),
Y ₀ (i) is a signal that oscillates around a certain offset value as shown in (b) and (c) of FIG. 2 due to the influence of the amount of light or stray light. The offset values Xofs and Yofs are subtracted from the signal X ₀ (i) and the signal Y ₀ (i) of the channel 2 to obtain data X and Y oscillating around 0.

By determining in step S3 which of the following eight combinations the combination of X and Y is, the phase ph of the energy vector of the coherent light C ₁ has eight regions ph = 0 shown in FIG. Know which of ~ 7.

When X ≧ 0 and Y ≧ 0 and | X | ≧ | Y | ph = 0, when X ≧ 0 and Y ≧ 0 and | X | <| Y |, ph = 1 X <0 and Y ≧ 0 And when | X | <| Y | ph = 2 X <0 and Y ≧ 0 and | X | ≧ | Y | ph = 3 X <0 and Y <0 and | X | ≧ | Y | When ph = 4 X <0 and Y <0 and | X | <| Y | ph = 5 X ≧ 0 and Y <0 and | X | <| Y | ph = 6 X ≧ 0 and Y <0 When | X | ≧ | Y |, ph = 7. The function that performs this operation is represented by phase (X, Y).

In step S4, the moving distance (displacement amount) Z (i) of the object to be measured M ₁ is integrated. When the DUT M ₁ approaches, the phase p of the energy vector of the interference light C ₁
h rotates in the positive direction (counterclockwise), and conversely rotates in the negative direction (clockwise) when moving away. Therefore, Z (i) is incremented by 1 each time the phase ph makes one rotation in the positive direction, and conversely, the object M ₁ moves away, and Z (i) is decremented by 1 each time the phase ph makes one rotation in the negative direction. The distance traveled is required. In the present embodiment, this is +1 when the positive side of the X axis in FIG. 4 is crossed in the positive direction as indicated by arrow E when changing from the previous phase ph0 to the next phase ph in the direction of arrow F. As shown in the figure, it is realized by -1 when crossing in the negative direction. FIG. 5 shows a table, which is a calculation table, for increasing / decreasing Z (i) depending on the combination of the positions ph0 and ph. By referring to this table, increase / decrease with respect to Z (i) can be immediately determined.

In step S5, a signal abnormality is detected. The cause of the abnormality is obtained by the logical sum of two, that is, the abnormality due to insufficient received light amount and the abnormality in phase rotation. The amount of received light changes depending on the surface condition of the object to be measured M _{1. If} the amount of received light is insufficient, the noise component of the signal becomes relatively large, and correct measurement cannot be performed. The abnormal phase rotation is when the direction of phase rotation cannot be determined. This may be because the moving speed of the object to be measured M ₁ is too fast and the phase rotates 180 degrees or more during one sample period, or the noise is mixed. Also in this case, correct measurement cannot be performed.

First, whether the received light amount is insufficient is judged from the received light amount Pow which is the root mean square of the X and Y vibration components after subtracting the offset component. That is, when Pow> (X ² + Y ² ) ^1/2 is smaller than the lower limit value PowLimit, it is determined to be abnormal. A function for calculating the amount of received light is Power (X,
It is represented by Y).

On the other hand, the abnormal phase rotation is judged to be abnormal when it cannot be determined whether the phase has rotated in the positive direction or the negative direction. That is, the change from the previous phase ph0 to the next phase ph is as follows: ph0 = 0 → ph = 4 ph0 = 1 → ph = 5 ph0 = 2 → ph = 6 ph0 = 3 → ph = 7 ph0 = 4 → ph = 0 ph0 = 5 → ph = 1 ph0 = 6 → ph = 2 ph0 = 7 → ph = 3, the rotation direction becomes unknown. FIG. 6 shows a table as a second calculation table in which the unknown rotation direction is set to 1. By referring to this table, an abnormal combination can be immediately obtained. The operation of drawing this table is expressed as error (ph, ph0).

In step S6, ph is prepared for repetition.
The value of 0 is updated and i is incremented by 1.

In step S7, repeated determination is made.
If there is more data to be processed, the process returns to point A and repeats from step S2. If the data to be processed is over, this routine ends.

At the end of the routine, the moving distance of the object to be measured M ₁ is stored in Z (0) to Z (i), and err (0) to e
The abnormality detection information is stored in rr (i-1). The unit of movement distance is 1 / wavelength of the laser light used.
2 is a unit. HeN with a wavelength of 632.8 nm
If the e-laser is used, Z (0) to Z (i) may be multiplied by 0.3164 to express the unit of distance in μm.

The measurement result is displayed on the screen of the display unit 9.

According to the present embodiment, the measuring light reflected by the object to be measured is converted into circularly polarized light and superposed on the linearly polarized reference light, thereby creating interference light whose energy vector is rotated by the movement of the object to be measured. , The amount of rotation of the energy vector and the number of rotations of the energy vector are detected to detect the amount of light of each of the two polarization components having a phase difference of 90 degrees, thereby measuring the moving direction and the moving amount of the measured object.

Therefore, when the object to be measured moves at a high speed, this can be dealt with by using a photodetector or an amplifier having a high response frequency. Since a phase meter having a narrow response frequency band is not required unlike the conventional example, from the measurement of the displacement of the measured object moving at an extremely low speed to the measurement of the displacement of the measured object moving at a high speed of 10 m / sec, for example. , It has the advantage that it can measure extremely wide range with high accuracy.

Further, since it is possible to measure up to a unit of 1/2 of the wavelength of the laser light used and the measurement spot size can be made extremely small, it is suitable for measuring a minute displacement of a minute object.

Furthermore, since the object to be measured is not contacted by irradiating the object to be measured, there is an advantage that it has little influence on the object to be measured. In addition, an expensive frequency shifter or phase meter as in the conventional example is required. Since it does not, it can greatly contribute to the price reduction of the displacement gauge.

FIG. 7 shows a modification of this embodiment.
This is the laser beam L ₁ emitted from the HeNe laser _1.
Is split into two by a beam splitter 11, one of which is introduced into a television camera 12 for monitoring, the position of the measurement light with respect to the object M ₁ to be measured, that is, the position of the measurement spot is monitored by a screen 13, and the analog-digital converter 8
Instead of 8, binarization circuits 14 and 15 for digitally converting the _first and second detection signals S ₁ and T ₁ into two values, a pulse converter 16, and a 16-bit up / down counter 17. The digital-analog converter 18 is used.

[0051] When the DUT M ₁ is moved as shown in (a) of FIG. 8, the first based on two polarization components of the interference light C _1, the second detection signal S _1, T ₁ (FIG. 8 (B), (c)
(Shown in FIG. 8) are digitally converted in the binarization circuits 14 and 15, respectively, to obtain pulse waveforms shown in (d) and (e) of FIG. The threshold for binarization is variably set by the variable resistor, and this corresponds to the change in the offset voltage due to the change in the light amount of the reflected light of the device under test M ₁ .

Although it is impossible to measure up to an extremely low speed (DC), even if there is a change in the amount of reflected light, AC coupling can be used with the threshold fixed at 0V.

The outputs of the binarization circuits 14 and 15 are output by the up / down counter 17 from (f) and (g) of FIG.
It is converted into an up pulse and a down pulse shown in. A circuit for a general two-phase rotary encoder can be used as it is for the up / down counter 17.

By converting the output of the up / down counter 17 from digital to analog, the graph shown in (h) of FIG. 8 is obtained. This modification has the advantage that the signal processing is simple and the measurement result can be obtained in real time by a general-purpose pen recorder, oscilloscope, or the like.

[0055]

Since the present invention is configured as described above, it has the following effects.

The displacement of a minute object can be detected with high precision in a wide speed range, and expensive measuring parts are not required. Therefore, an inexpensive displacement meter can be realized.

[Brief description of drawings]

FIG. 1 is an explanatory diagram illustrating a displacement meter according to an embodiment.

FIG. 2 is a graph showing a position of an object to be measured and changes in two detection signals.

FIG. 3 is a flowchart showing a calculation procedure of the displacement meter of FIG.

FIG. 4 is a diagram illustrating a phase of an output of a light detection unit.

FIG. 5 shows a table for calculating the displacement amount of the object to be measured based on the phase of the output of the light detection unit.

FIG. 6 shows a table for generating an error signal based on the phase of the displacement gauge output.

FIG. 7 is an explanatory diagram illustrating a displacement meter according to a modification.

FIG. 8 is a graph showing a position of an object to be measured, changes in two detection signals, binarization of these, output of an up-down counter and measurement results.

FIG. 9 is an explanatory diagram illustrating a displacement meter according to a conventional example.

[Explanation of symbols]

 1 HeNe laser 3 Beam splitter 4 1/8 wavelength plate 5 1/4 wavelength plate 6,7 Objective lens 8 Photodetector 9 Display 12 TV camera 81 1/2 wavelength plate 83 Photodetector beam splitter

Claims (6)

[Claims] 1. A polarizing means for converting coherent light generated from a light source into a polarized light beam, and dividing the polarized light beam into a reference light and a measurement light, which are respectively reflected by the reference surface and the surface of the object to be measured. Interference means to obtain the interference light by overlapping with
The 1/4 wavelength phase shifter arranged in the optical path of the reference light, the 1/8 wavelength phase shifter arranged in the optical path of the measurement light, and the interference light are divided into two polarization components. A displacement meter having a light amount detecting means for individually detecting the light amounts of both and a calculating means for calculating the displacement amount of the object to be measured based on the output of the light amount detecting means. 2. The displacement meter according to claim 1, further comprising a condensing unit that condenses the measurement light and the reference light on the surface of the object to be measured and the reference surface, respectively. 3. The light receiving surface of the light amount detecting means is a defocusing means.
The displacement meter according to claim 2, wherein the displacement meter is arranged at a focus position. 4. The displacement meter according to claim 1, wherein a 1/2 wavelength phase shifter is arranged in the optical path of the interference light. 5. The calculation means has a calculation table for detecting the rotation of the energy vector of the interference light based on changes in the light amounts of the two polarization components of the interference light. Displacement meter described in paragraph. 6. The calculation means has a second calculation table for detecting an abnormality of the interference light based on a change in the amount of light of each of the two polarization components of the interference light. Displacement meter described.