JP5421677B2 - Displacement measuring device using optical interferometer - Google Patents

Description

  The present invention relates to a displacement measuring apparatus using an optical interferometer, and more particularly to a displacement measuring apparatus capable of measuring minute displacement at the nm level.

  Laser interferometers are widely used to measure distance or displacement. As such an interferometer, as shown in FIG. 8, a light source 34 such as a laser beam, and the light irradiated from the light source 34 is divided into reference light and measurement light, and the reflected reference light and measurement light are reflected. , A reference reflector 36 that reflects the reference light from the beam splitter 35, a measurement reflector 37 that reflects the measurement light from the beam splitter 35, the reference light and the measurement light And a photodetector 39 for detecting the interference light.

As shown in FIG. 8, the displacement measuring device 31 using such a laser interferometer includes a light source 34, beam splitters 35 and 35 ′, a photodetector 39, a reference reflector 36, and a fixed portion 32. Is provided, and the measurement reflector 37 is provided on the movable portion 33. For example, the fixed part 32 and the movable part 33 are installed and used on an object to be measured such as a plate material. When the object to be measured is deformed in the direction of the block arrow, the measurement reflecting plate is displaced together with the movable portion 33, the wavefront of the interference light is changed as shown in FIG. 9, and the interference light detected by the photodetector is changed. The light intensity changes. Assuming that the wavelength of light used for measurement is λ, when the measurement reflector is displaced by one wavelength λ, light-dark changes such as light → dark → light or dark → light → dark occur twice. Therefore, by detecting the number n of changes in brightness,
Displacement = n × λ / 2 (1)
The amount of displacement can be measured using the following equation.

JP 2007-271624 A

  However, such a displacement measuring device 31 has the following problems. As shown in FIG. 10, when the movable portion 33 is tilted, the measurement reflecting plate 37 is tilted, and as shown in FIG. 11, between the wavefront of the reflected light of the measurement light and the reflected light of the reference light. Inclination occurs. Fine interference fringes are generated by the inclination of the wavefront. When this interference fringe is generated, the displacement amount cannot be measured by the above equation (1).

  Therefore, the present invention proposes a displacement measuring device that can prevent the generation of fine interference fringes even when the movable portion is inclined.

  In the present invention, as a first solving means, in a displacement measuring apparatus using an optical interferometer, the first base and a second base that is movably installed with respect to the first base, On the first base, a light source, a beam splitter for branching light emitted from the light source into reference light and measurement light, and interference fringes caused by interference between the reference light and the measurement light are detected. On the second base, a first reflecting plate that reflects the reference light and a second reflecting plate that reflects the measurement light are provided on the second base. The reflecting surface of the first reflecting plate and the reflecting surface of the second reflecting plate are installed so as to be parallel to each other, and either one of the first reflecting plate or the second reflecting plate Through the third reflector provided on the first base, the beam splitter We propose a displacement measurement apparatus al light is irradiated.

  The optical axes of the reference light and measurement light branched by the beam splitter are perpendicular to each other. Here, by irradiating either the reference light or the measurement light to the third reflecting plate, the optical axes of the reference light and the measurement light become parallel to each other. Thus, the reflecting surface of the first reflecting plate and the reflecting surface of the second reflecting plate can be installed so as to be parallel to each other. Since the first reflector and the second reflector are both provided on the second base and the reflecting surfaces are provided in parallel to each other, the wavefront of the reference light and the wavefront of the measuring light Are always parallel to each other. Thus, the first solving means can prevent the occurrence of fine interference fringes due to the inclination of the wavefront.

  According to the present invention, it is possible to prevent the occurrence of fine interference fringes even when the movable portion is inclined. Thereby, it is possible to obtain a displacement measuring device capable of measuring minute displacement at the nm level.

It is a schematic diagram which shows the 1st example of the displacement measuring device of this invention. FIG. 2 is a schematic diagram showing a state when the movable part is inclined in the displacement measuring apparatus of FIG. 1. It is a schematic view showing a displacement measuring equipment according to a reference example. It is a schematic view showing a displacement measuring equipment according to another reference example. It is the figure which observed the signal observed with the photodetector of FIG. 4 with the oscilloscope. Is a schematic diagram showing still displacement measuring equipment according to another reference example. Is a schematic diagram showing still displacement measuring equipment according to another reference example. It is a schematic diagram which shows the conventional displacement measuring device. It is a schematic diagram which shows the mode of interference of reference light and measurement light. It is a schematic diagram which shows the problem of the conventional displacement measuring device. It is a schematic diagram which shows the mode of interference of the reference light and measurement light in the state shown in FIG.

  An embodiment according to a displacement measuring apparatus of the present invention will be described based on the drawings. First, FIG. 1 shows a first example of the displacement measuring apparatus of the present invention.

  The displacement measuring device 1 includes a light source 4, a beam splitter 5 that branches light emitted from the light source 4 into measurement light and reference light, a second reflector 7 that reflects measurement light from the beam splitter 5, A first reflecting plate 6 having a reflecting surface arranged in parallel with the reflecting surface of the second reflecting plate 7 and a third reflecting the reference light from the beam splitter 5 toward the first reflecting plate 6. A light detector 9 for detecting interference light between the reference light reflected by the first reflector 6 and the measurement light reflected by the second reflector 7, and the light from the light source 4. A beam splitter 5 ′ that transmits and reflects the interference light toward the photodetector 9. Further, a λ / 4 wavelength plate 10 may be provided as necessary.

  The displacement measuring device 1 includes a first base 2 and a second base 3 that is movably installed with respect to the first base 2. The first base 2 is provided with a light source 4, beam splitters 5, 5 ′, a third reflecting plate 8, a photodetector 9 and a λ / 4 wavelength plate 10. The second base 3 is provided with a first reflecting plate 6 and a second reflecting plate 7. Hereinafter, although the first base 2 is described as a fixed part and the second base is a movable part, the first base 2 may be a movable part and the second base 3 may be a fixed part.

  Laser light is used as the light source 4. The light source 4 includes lenses such as a collimator lens (not shown). The light source 4 is controlled by control means (not shown).

  A prism or the like is used for the beam splitters 5 and 5 '. In the present embodiment, there are two beam splitters: a beam splitter 5 that branches light from the light source 4 into reference light and measurement light, and a beam splitter 5 ′ that reflects interference light toward the photodetector 9. Although used, both actions may be combined into a single beam splitter.

  As the photodetector 9, a photoelectric conversion element that converts light intensity into an electric signal, such as a CCD or a CMOS sensor, is used. The photodetector 9 is connected to signal processing means (not shown). The electric signal generated by the photodetector 9 is processed by a signal processing means (not shown), converted into the number of changes in brightness, and converted into a displacement amount of the second base. The photodetector 9 may be one in which the sensor part is divided into two or more parts.

  As the first reflecting plate 6, the second reflecting plate 7, and the third reflecting plate 8, in addition to the reflecting mirror, a highly reflective metal plate, a resin plate, or the like is used. The first reflecting plate 6 and the second reflecting plate 7 are provided on the second base 3 and installed so that the reflecting surfaces are parallel to each other. Since the measurement light and the reference light branched by the beam splitter 5 are branched in a direction perpendicular to each other, one of them needs to change the direction of light by the third reflector 8. In FIG. 1, a third reflecting plate 8 is provided between the first reflecting plate 6 and the beam splitter 5 to change the direction of the reference light. In some cases, a third reflecting plate 8 may be provided between the second reflecting plate 7 and the beam splitter 5 to change the direction of the measurement light. Since the third reflector 8 is installed on the first base 2, the positional relationship with the beam splitter 5 and the reflection angle are constant.

  The λ / 4 wavelength plate 10 may be provided as necessary. The light emitted from the light source 4 passes through the λ / 4 wavelength plate 10 and becomes circularly polarized light having a phase of λ / 4. The light reflected by the first reflecting plate 6 and the second reflecting plate 7 passes through the λ / 4 wavelength plate 10 and changes from circularly polarized light to linearly polarized light. When the polarizing prism 5 ′ is inserted, it is preferable to insert λ / 4 because the loss of light can be suppressed. The light returned from the reflecting mirror as linearly polarized light is incident on the polarizing prism 5 ′ at a polarization angle that is 90 degrees different from the linearly polarized light angle of the light source, so that almost all light quantity is reflected by the polarizing prism. Then, the light enters the photodetector 9.

  The operation of the displacement measuring apparatus 1 will be described with reference to FIG. As shown in FIG. 2, when the second base 3 is inclined, the reflecting surface of the second reflecting plate 7 is also inclined. However, since the first reflecting plate 6 is also installed on the second base 3, the reflecting surface of the first reflecting plate 6 is inclined. Since the first reflecting plate 6 and the second reflecting plate 7 have the same inclination, the reflecting surfaces of the first reflecting plate 6 and the second reflecting plate 7 are always kept parallel. Therefore, no inclination occurs between the wavefront of the reflected light of the reference light and the wavefront of the reflected light of the measurement light.

  The direction of the reference light reflected by the first reflecting plate 6 is changed by the third reflecting plate 8, passes through the beam splitter 5, and is irradiated toward the photodetector 9 by the beam splitter 5 '. . The measurement light reflected by the second reflecting plate 7 is changed in direction by the beam splitter 5 and is irradiated toward the photodetector 9 by the beam splitter 5 ′. Since the third reflector 8 and the beam splitter 5 are installed on the first base and the reflection angle is constant, the measurement light reflected by the first reflector 6 and reflected by the third reflector 8 is used. The reflected light and the measurement light reflected by the second reflector and reflected by the beam splitter 5 are not inclined with respect to each other.

  Since no tilt occurs between the wavefront of the reference light and the wavefront of the measurement light, no fine interference fringes are generated. Thereby, even if the second base 3 is inclined, the displacement can be measured by the above equation (1).

Next, a description will be given of the displacement measurement equipment according to the reference example. It shows the structure of a displacement measurement equipment according to the reference example in FIG.

The displacement measuring device 11 is installed such that the reflecting surface of the first reflecting plate 16 provided on the second base 13 and the reflecting surface of the second reflecting plate 17 are in substantially the same direction. Thus, it is different from the displacement measuring apparatus 1 of the first example.

In such a configuration, the first reflecting plate 16 and the second reflecting plate 17 can be installed side by side. Therefore, the reference example is more than the second base 2 of the displacement measuring apparatus 1 of the first example. The second base 13 of the displacement measuring device 11 can be made smaller. As a result, the entire displacement measuring device can be reduced in size.

  In addition, although you may install so that the reflective surface of the 1st reflective plate 16 and the reflective surface of the 2nd reflective plate 17 may be on the same surface, as shown in FIG. A step 16 a may be provided on either the reflecting surface of the reflecting plate 16 or the reflecting surface of the second reflecting plate 17.

  For example, as shown in FIG. 4, the first reflector 16 is divided into two parts by the step 16a, and a step of λ / 8 is provided on the other half side with respect to the half side. Thereby, as shown in FIG. 5, two interference lights A and B having a phase difference of λ / 4 are generated. FIG. 5 is an observation of the signal read by the photodetector 19 divided into two with an oscilloscope. In FIG. 4, when the second base 13 is displaced in the vertical direction of the drawing, when the second base 13 is displaced in one direction, the signal in which the interference light A precedes the interference light B by λ / 4 as shown in FIG. Is detected by the photodetector 19. Further, when displaced in the other direction, as shown in FIG. 5B, a signal in which the interference light A is delayed by λ / 4 from the interference light B is detected by the photodetector 19. Thus, the direction of displacement of the second base 13 can be determined by detecting the phase difference generated in each sensor of the photodetector 19 divided into two.

  In addition, when the reflecting surface of the first reflecting plate 16 and the reflecting surface of the second reflecting plate 17 are installed on the same plane, as shown in FIG. A reflecting plate 16 ′ integrated with two reflecting plates may be used. When the first reflector and the second reflector are divided and installed, it is necessary to match the angles of each other. As the wavelength of the light used becomes shorter, the angle adjustment becomes very strict. However, the integrated reflector 16 'makes it easy to adjust the angle, which is effective for a displacement measuring apparatus that uses light having a short wavelength that is difficult to adjust.

Another reference example is shown in FIG. The displacement measuring device 21 of the reference example shown in FIG. 7 changes the direction of the measurement light branched by the beam splitter 25 by the third reflecting plate 28 and irradiates the second reflecting plate 27, and the first reflection. The plate 26 and the second reflecting plate 27 are different from the displacement measuring device 11 of the above reference example in that the plate 26 and the second reflecting plate 27 are installed perpendicular to the object to be measured. In such a configuration, it is possible to measure the displacement angle when the object to be measured is deformed in the direction of the block arrow.

  When the object to be measured is deformed in the direction of the block arrow, an inclination is generated between the second base 23 and the first base 22. In this case, since the first reflecting plate 26 and the second reflecting plate 27 are installed so that the reflecting surfaces are substantially the same surface, the wave front of the reflected light is not inclined. On the other hand, since the second reflecting plate 27 is farther from the object to be measured than the first reflecting plate 26, the movement amount due to the deformation of the object to be measured is the second reflecting plate rather than the first reflecting plate 26. 27 is larger.

  The displacement measuring device 21 can detect a difference in moving distance between the first reflecting plate 26 that reflects the reference light and the second reflecting plate 27 that reflects the measuring light. The difference in moving distance is obtained from the number of changes in brightness detected by the photodetector 29. The displacement angle can be calculated by obtaining the distance between the reference light spot irradiated on the first reflector 26 and the measurement light spot irradiated on the second reflector 27.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made within the technical scope of the present invention.

1, 11, 21, 31 Displacement measuring device 2, 12, 22, 32 First base (fixed portion)
3, 13, 23, 33 Second base (movable part)
4, 14, 24, 34 Light source
5, 5 ', 15, 15', 25, 25 ', 35, 35' Beam splitter 6, 16, 26, 36 First reflector (reference reflector)
16 'Reflector 16a Step 7, 7, 27, 37 Second reflector (measurement reflector)
8, 18, 28 Third reflector 9, 19, 29, 39 Photo detector 10, 20, 30, 40 λ / 4 wavelength plate

Claims (3)

In a displacement measuring device using an optical interferometer,
The first base,
A second base movably installed relative to the first base;
On the first base, a light source, a first beam splitter for branching light emitted from the light source into reference light and measurement light, and transmitting the light emitted from the light source, the reference light is transmitted. And a second beam splitter that reflects the interference light generated by the interference with the measurement light, and a detector that detects interference fringes due to the interference light, and
On the second base, a first reflecting plate that reflects the reference light and a second reflecting plate that reflects the measurement light are provided,
On the first base, a third reflector for reflecting the reference light from the first beam splitter is provided,
The light source, the second beam splitter, the first beam splitter, and the third reflector are arranged in a straight line in this order,
The measurement light is reflected at right angles to the straight line by the first beam splitter and is incident on the second reflecting plate, and the reference light is perpendicular to the straight line by the third reflecting plate. The reflecting surface of the first reflecting plate and the reflecting surface of the second reflecting plate are arranged in parallel and opposite to each other so as to be reflected and incident on the first reflecting plate. Displacement measuring device. The first base has a convex portion on which the first beam splitter and the third reflecting plate are arranged,
The displacement measuring apparatus according to claim 1, wherein the second base has a concave portion surrounding an outer shape of the convex portion of the first base. The displacement measuring device according to claim 1, wherein a step is provided on one of the reflecting surface of the first reflecting plate and the reflecting surface of the second reflecting plate.

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