Displacement detection devices and interferometers

Abstract

To provide a displacement detector using an optical interferometer which suppresses change in an interference fringe and enables highly accurate displacement measurement.SOLUTION: A displacement detector includes a first PBS for dividing a beam from a light source part into two, a second PBS that parallelly emits the divided first and second beams, and further divides the two beams into two, first reflection means that is arranged on the side having permeated the second PBS and is movable, beam cross means that is arranged on a side reflected by the second PBS and parallelly emits the first beam and the second beam in a crossed manner, second reflection means capable of being tilt, third reflection means arranged so as to face the second reflection means, light-receiving means for superposing the first beam and the second beam by the first PBS again, and receiving the superposed beam as an interference signal, displacement detection means for converting the received interference signal into displacement information, and a movable part for tilting the second reflection means; and controls the tilt of the second reflection means while following the state of the tilt of the first reflection means.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a displacement detection device for an interferometer. [Background technology]

[0002] In measurements using the principle of the Michelson interferometer, two beams split by a beam splitter, i.e., a reference beam irradiated onto a fixed mirror and an object beam irradiated onto a movable mirror, are then superimposed again by the beam splitter. In this process, changes in the interference fringes caused by changes in the posture of the movable mirror (pitching and yawing) can sometimes cause errors in displacement detection. To suppress this, various methods have been used, such as using a high-precision stage for the moving part of the movable mirror or arranging a corner cube mirror on the movable mirror. However, these methods increase the size of the movable mirror mechanism, which is contrary to the goal of making it smaller and lighter.

[0003] One possible solution to this problem is to offset the error by slightly changing the attitude of the fixed mirror on the reference beam side in accordance with the change in attitude of the movable mirror on the object beam side. However, when this is done using an autocollimator or similar, there are problems such as autocollimator drift, low angular resolution, and low detection speed, making it difficult to provide feedback to the fixed mirror.

[0004] Another proposed method involves sending a second beam into the Michelson interferometer, capturing the beam intensity with a four-segment detector, and detecting the tilt of the movable mirror from the change in intensity, thereby providing feedback to the fixed mirror (see Patent Document 1). However, this method involves a trade-off between the beam diameter and the angle detection resolution. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-42257 Summary of the Invention [Problem to be solved by the invention]

[0006] It is desirable to have a compact and lightweight movable mirror for the object beam, tilt the fixed mirror for the reference beam at high speed so that it follows the tilt of the movable mirror quickly and stably, suppress changes in the interference fringes, and enable high-precision displacement measurement.If this is a Fizeau interferometer, it is preferable to stabilize changes in the interference fringes within the interference plane and increase the precision of shape measurement. [Means for solving the problem]

[0007] A displacement detection device according to one embodiment of the present invention includes a light source unit having a light source and a lens for collimating the emitted beam, a first PBS for splitting the beam from the light source unit into two, and beam splitting means for emitting the split first and second beams in parallel, and further splitting the first and second parallel beams emitted in parallel into two, a movable first reflecting means disposed on the side of the first beam split and transmitted by the second PBS, and a second PBS for crossing the first beam and the second beam, disposed on the side of the second beam reflected by the second PBS, to split the first beam and the second beam. The optical fiber has a beam crossing means for emitting the first beam parallel to the first beam, a tiltable second reflecting means, a third reflecting means disposed opposite the second reflecting means with respect to the second PBS, a light receiving means for receiving an interference signal obtained by superimposing the first and second beams again by the first PBS, a displacement detecting means for converting the received interference signal into displacement information, a servo circuit and servo amplifier for maintaining an arbitrary displacement amount based on the displacement information, and a movable part for tilting the second reflecting means, which controls the tilt of the second reflecting means in accordance with the tilt state of the first reflecting means. Note that the "parallelism of the first and second beams" referred to here may be considered to be included in the concept of "parallel" as long as both beams travel in a straight line next to each other to the extent that displacement detection is possible. [Effects of the Invention]

[0008] According to the present invention, it is possible to suppress changes in interference fringes in a displacement detection device using an optical interferometer, and to perform highly accurate displacement measurement. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram illustrating a configuration of a displacement detection device according to a first embodiment. [Figure 2] FIG. 1 is a diagram illustrating a specific example of a light source. [Figure 3] FIG. 2 is a diagram illustrating a specific example of a light receiving unit. [Figure 4] FIG. 10 is a diagram illustrating a specific example of a displacement detection unit. [Figure 5] 10A and 10B are diagrams illustrating the operation of the second reflecting means. [Figure 6] FIG. 10 is a diagram illustrating the configuration of a displacement detection device according to a second embodiment. [Figure 7] FIG. 10 is a diagram illustrating the configuration of a displacement detection device according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [First embodiment] Fig. 1 is a diagram illustrating the configuration of a displacement detection device according to the first embodiment, and Fig. 2 is a diagram illustrating a specific example of a light source unit. As shown in Fig. 1, the light source unit 1 is assumed to use a laser as its light source, but it may also use a low-coherence multimode laser, a superluminescence diode, an LED, etc. As shown in Fig. 2, the light source unit 1 includes a light source 2 and a lens 3. The beam emitted from the light source 2 is preferably converted into collimated light by passing through the lens 3.

[0011] Returning to Figure 1, the beam emitted from the light source unit 1 is split into two beams L1 and L2 by PBS1, the "first PBS (Polarizing Beam Splitter)." Beams L1 and L2 are emitted as beams traveling parallel to each other and enter PBS3, the "second PBS (Polarizing Beam Splitter)." PBS3 splits the optical paths of beams L1 and L2.

[0012] The optical path of beam L1 will now be described. After passing through PBS1, beam L1 is polarized as a P-wave component, and is then transmitted through PBS3 and incident on phase plate 11. Here, phase plate 11 is assumed to be a λ / 4 phase plate. As beam L1 passes through phase plate 11, its polarization state changes from a P-wave component to circularly polarized light, and is returned to vertical by mirror 21, which is a movable "first reflecting means." Mirror 21 is positioned at the destination of beam L1 passing through PBS3, and is displaceable by movable part 32.

[0013] This beam L1 is reflected by the mirror 21 and passes through the phase plate 11 again, becoming polarized as an S-wave component. Beam L1, which has become an S-wave component, is reflected by the PBS3 and returned to the PBS3 by the mirrored phase plate 23, which is the "third reflecting means." The mirrored phase plate 23 is placed on the opposite side of the PBS3 from the mirror 22, which will be described later. This mirrored phase plate 23 is assumed to be a λ / 4 phase plate with a mirror attached. Beam L1, which was an S-wave component, becomes a P-wave component after passing through the mirrored phase plate 23, and is transmitted when it re-enters the PBS3.

[0014] The transmitted beam L1 is output as a parallel offset beam by the beam crossing means 30 and enters the phase plate 13. The beam crossing means 30 can be configured, for example, with four plate mirrors (first to fourth mirrors, which will be described later). A λ / 4 phase plate is assumed as the phase plate 13, and the beam L1 is converted from a P-wave component into circularly polarized light. This beam L1 is reflected perpendicularly by the mirror 22, which is the "second reflecting means," and passes through the phase plate 13 again, becoming an S-wave component. The S-wave component beam L1 is reflected by the PBS3 and enters the PBS1 again.

[0015] Next, the optical path of beam L2 will be described. Since the polarization state of beam L2 after passing through PBS1 is an S-wave component, it is reflected by PBS3 and emitted as a parallel beam offset by beam crossing means 30, and then incident on phase plate 14. Beam crossing means 30 is positioned at the reflection destination of beam L2 reflected by PBS3. At this time, the optical paths of beam L1 and beam L2 cross, and the mirrors of beam crossing means 30 are configured so that the optical paths of beam L1 and beam L2 cross, and are emitted as parallel beams that are swapped with each other in the Z direction.

[0016] That is, beam crossing means 30 includes first beam reflecting means 301 and second beam reflecting means 302. First beam reflecting means 301 is configured by providing an L-shaped metal plate with a reflecting surface (first reflecting surface) that constitutes first mirror 311 and a reflecting surface (second reflecting surface) that constitutes second mirror 312. Second beam reflecting means 302 is configured by providing an L-shaped metal plate with a reflecting surface (third reflecting surface) that constitutes third mirror 313 and a reflecting surface (fourth reflecting surface) that constitutes fourth mirror 314. Each reflecting surface is obtained by, for example, evaporating an alloy such as aluminum onto the surface of the metal plate.

[0017] The first mirror 311 reflects the beam L1 (first light) from the PBS 3, causing the light to propagate toward the second beam reflecting means 302. The fourth mirror 314 causes the light to propagate toward the phase plate 13 and then toward the mirror 22. The fourth mirror 314 also reflects the light that has been reflected back by the mirror 22, causing the light to propagate toward the first beam reflecting means 301. The first mirror 311 reflects the light, causing it to propagate toward the PBS 3.

[0018] The third mirror 313 reflects the beam L2 (second light) from the PBS3, causing the light to propagate toward the first beam reflecting means 301. The second mirror 312 reflects the light, causing it to propagate toward the phase plate 14 and eventually toward the mirror 22. The second mirror 312 also reflects the light that has been reflected back by the mirror 22, causing the light to propagate toward the second beam reflecting means 302. The third mirror 313 reflects the light, causing it to propagate toward the PBS3.

[0019] A λ / 4 phase plate is assumed as the phase plate 14. Beam L2, which is an S-wave component, becomes circularly polarized light, is reflected vertically by mirror 22, and passes through phase plate 14 again, becoming a P-wave component. Beam L2, which has become a P-wave component, passes through PBS3 and is reflected vertically by mirror-equipped phase plate 24, becoming an S-wave component. This mirror-equipped phase plate 24 may have the same configuration as mirror-equipped phase plate 23. Beam L2, which has become an S-wave component, is reflected by PBS3 and enters phase plate 12. A λ / 4 phase plate is assumed as the phase plate 12, and beam L2 changes from an S-wave component to circularly polarized light, is reflected by mirror 21, and passes through phase plate 12 again, becoming a P-wave component, passes through PBS3, and is superimposed on beam L1 by PBS1.

[0020] In this optical path, it is desirable to adjust the optical path lengths of beams L1 and L2 to be extremely equal using mirrored phase plate 23 or mirrored phase plate 24. In this embodiment, the mirrored phase plate 24 is moved while mirrored phase plate 23 is fixed, and the relationship between the optical path lengths of beams L1 and L2 is appropriately adjusted. The optical path lengths of beams L1 and L2 are set to lengths that allow the two beams to interfere with each other.

[0021] Furthermore, even if the mirror 21 moves significantly in the Z direction, the optical path lengths of the beams L1 and L2 are always kept constant. This not only allows the use of a light source with low coherence, but also cancels out the effects of wavelength changes due to temperature changes in the light source, as well as changes in atmospheric pressure and humidity, enabling consistently stable measurement of the mirror displacement. Furthermore, it also makes it possible to use inexpensive semiconductor lasers, LEDs, etc. as the light source.

[0022] The displacement detection device of this embodiment includes a light receiving means 4 that receives the interference light of beams L1 and L2 that return from PBS3 to PBS1, a displacement detection means 6 that detects the displacement of mirror 21 based on a signal (interference signal) based on the interference light, and a servo circuit 8 and servo amplifier 10 that drive mirror 22 based on the displacement of mirror 21.

[0023] FIG. 3 is a diagram showing a specific example of the light receiving means. Beams L1 and L2 are superimposed by PBS1 and then enter the light-receiving means 4, but because one is a P-wave component and the other is an S-wave component, no interference occurs. Therefore, as shown in Figure 3, the beams are split into two by a non-polarizing beam splitter BS400, one of which is reflected and the other is transmitted. At this time, for one of the reflected beams, the PBS 410 is rotated 45° to extract the interference component of the S-wave component and the P-wave component at a 45° angle. Light-receiving elements 43 and 44 receive the interference light, but the brightness of which is inverted relative to each other, and perform photoelectric conversion.

[0024] On the other hand, the other beam transmitted by non-polarizing beam splitter BS400 becomes linearly polarized light that rotates depending on the phase state of beams L1 and L2 by passing through phase plate 25, and is incident on PBS 420, where the interference light is photoelectrically converted by light receiving elements 41 and 42. At this time, light receiving elements 41 and 42 receive interference light whose brightness is inverted relative to each other, and the rotation of PBS 420 is finely adjusted so that the phase of the interference light received by light receiving elements 43 and 44 is shifted by 90°. As a result, when the phase of the interference signal at light receiving element 41 is 0°, light receiving element 42 is 180°, light receiving element 43 is 90°, and light receiving element 44 is 270°.

[0025] Next, the mechanism of the displacement detection means 6 will be described. FIG. 4 is a diagram showing a specific example of the displacement detection means 6. As shown in FIG. The signals from photodetectors 41 and 42 are DC-cancelled by differential amplifier 45. This cancels out any DC offset in the electrical signal due to fluctuations in interference intensity. Similarly, the signals from photodetectors 43 and 44 are DC-cancelled by differential amplifier 46. In other words, the outputs of differential amplifiers 45 and 46 have a 90° phase difference from each other, so the interference signals can be detected as sine waves of sine and cosine waves, making it possible to determine the direction of phase progression. Next, each interference signal is converted into a digital signal by A / D converters 47 and 48, and waveform correction processing circuit 49 corrects for any deviations in the amplitude and offset of the sine and cosine waves. The signals are then interpolated by incremental signal generator 50 and output as displacement information.

[0026] This displacement information is not detected as a displacement when the movable part 32 moves precisely in parallel in the Z direction, because no difference in the optical path lengths of beams L1 and L2 occurs. However, in reality, when a slight rotation about the Y-axis direction is applied to mirror 21, a difference in the optical path lengths of beams L1 and L2 occurs. If the wavelength of the light source is λ, and mirror 21 is tilted by ΔZ at the incident position of beams L1 and L2, the difference in optical path lengths of beams L1 and L2 will be 2ΔZ. In other words, the interference signal will be detected as 2ΔZ / λ.

[0027] For example, when λ = 655 nm, ΔZ can be measured as the displacement of an interference period of 327.5 nm, and by interpolating this into approximately 65,000 divisions using the incremental signal generator 41, it becomes very fine displacement information with a resolution of 5 pm. This means that even if the distance between beams L1 and L2 traveling parallel to each other is only 5 mm, the tilt of mirror 21 can be detected with a resolution of 0.0002 arc seconds when converted into an angle.

[0028] To keep this displacement information constant, servo circuit 8 and servo amplifier 10 move movable part 31 to control the angle of mirror 22. In other words, the tilt of mirror 22 is controlled in accordance with the tilt state of mirror 21. This makes it possible to control the tilt of mirror 22 with high resolution to offset changes in the tilt of mirror 21.

[0029] The amount of displacement of this movable part 31 may be very small, and may be configured using a piezoelectric element or the like. In this embodiment, the amount of movement of mirror 21 caused by movable part 32 is about 1 m, while the amount of movement of mirror 22 caused by movable part 31 is about 5 mm. In other words, the amount of movement of mirror 21 can be made larger than the amount of movement of mirror 22. Movable part 32 may be configured using an actuator using a magnet and a coil or the like.

[0030] FIG. 5 is a diagram illustrating the operation of the second reflecting means. In this embodiment, by providing a fulcrum 15 (fixed portion) on the mirror 22 and fixing its position, the mirror 22 (second reflecting means) can be tilted by driving the movable portion 31. For example, as shown in FIG. 5, by tilting the mirror 22 by ΔX at the incident position of the beams L1 and L2, the optical path length difference between the beams L1 and L2 can be changed by 2ΔX. Note that, in this embodiment, a configuration in which the mirror 22 is supported in a cantilevered manner due to the positional relationship between the fulcrum 15 and the movable portion 31 is exemplified, but the supporting form of the mirror 22 is not limited to a cantilevered manner.

[0031] For example, when a beam splitter BS (beam splitting means) is configured on the same surface as the PBS 3, the mirror 21 is used as a mirror for the object light, the mirror 22 is used as a mirror for the reference light, and the beam splitter BS is used in combination with another interferometer (such as a Michelson interferometer or a Fizeau interferometer), a stable wavefront of the interference light can be obtained at all times by controlling the mirror 22 using the displacement detection device of the present invention.

[0032] In this case, as shown in FIG. 1, a PBS3 may be provided in a partial region of the substrate where the beam splitter BS is provided. That is, the PBS3 can be laminated on the substrate by vapor deposition or the like. The area (length) of the PBS3 is smaller than that of the beam splitter BS. In this laminated structure, the beams L1 and L2 of the displacement detection device are transmitted through or reflected by the region of the PBS3 (also referred to as the "PBS region"). The object light and reference light of the interferometer are transmitted through or reflected by the region of the beam splitter BS (also referred to as the "BS region") outside the PBS region.

[0033] [Second embodiment] 6 and 7 are diagrams showing the configuration of a displacement detection device according to the second embodiment. In the first embodiment shown in FIG. 1, when mirror 21, which is the "first reflecting means," rotates around the Y-axis direction as its rotation axis, mirror 22 is also rotated around the Y-axis direction as its rotation axis to offset the angular deviation of the optical axis. However, in reality, it is quite conceivable that rotation around the X-axis direction will also be added.

[0034] Therefore, the second embodiment shown in FIGS. 6 and 7 has a configuration characterized by including both a first displacement detection device that controls the second reflecting means in accordance with the rotation state about the Y-axis direction of the first reflecting means (mirror 21) as the rotation axis and the rotation state about the X-axis direction of the first reflecting means, and a second displacement detection device that controls the second reflecting means in accordance with the tilt state about the X-axis direction of the first reflecting means.

[0035] That is, the rotation axes of the first and second displacement detection devices, which take tilt into consideration, are perpendicular to the Z axis, and four beams L1, L2, L3, and L4 are emitted from PBS1 and PBS2 so as to be parallel. The optical paths of beams L3 and L4 are based on the same principle as those of beams L1 and L2, so a detailed explanation will be omitted.

[0036] The first displacement detection device has the same configuration as the displacement detection device of the first embodiment. The second displacement detection device includes a light source unit 201, a light receiving means 204, a mirror-equipped phase plate 25, and a mirror-equipped phase plate 26. The second displacement detection device also includes a phase plate 211 corresponding to phase plate 11, a phase plate 212 corresponding to phase plate 12, a phase plate 213 corresponding to phase plate 13, and a phase plate 214 corresponding to phase plate 14. The displacement detection device of this embodiment also includes a beam crossing means 230, a displacement detection means 206, a servo circuit 208, and a servo amplifier 210.

[0037] The beam crossing means 230 has a mirror configuration in which beams L1 and L2 cross and switch places on the XZ plane, and beams L3 and L4 cross and switch places on the YX plane, and are emitted parallel to each other. The beam crossing means 230 may have a first beam crossing means and a second beam crossing means. The first beam crossing means may have a configuration similar to that of the beam crossing means 30 of the first embodiment. The second beam crossing means may have a configuration similar to that of the beam crossing means 30, although the arrangement angle is different from that of the first beam crossing means.

[0038] As with the first displacement detection device, it is desirable to adjust the optical path lengths of beams L3 and L4 of the second displacement detection device so that they are extremely equal using a mirrored phase plate 25 or a mirrored phase plate 26. A movable part 33 is added to mirror 22, and by adding control over the rotation of mirror 22 about the Z-axis direction as its rotation axis, it is possible to cancel out the rotation of mirror 21 about the X-axis direction as its rotation axis.

[0039] The displacement detection device of the first or second embodiment may be installed in an optical interferometer (interference analysis device) such as a Michelson interferometer or a Fizeau interferometer. This interferometer includes a first mirror having a first reflecting means on the same surface, a second mirror having a second reflecting means on the same surface, and a beam splitter having a second PBS on the same surface.

[0040] Specifically, referring to FIG. 1, this interferometer includes a light source unit separate from light source unit 1, mirror 21 as a "first mirror," mirror 22 as a "second mirror," beam splitter BS, and a photodetector. In this interferometer, light emitted from the light source unit is split into object light that passes through beam splitter BS and reference light that is reflected by beam splitter BS. The object light is reflected by a measurement target placed on mirror 21 and returns to beam splitter BS. The reference light is reflected by mirror 22 and returns to beam splitter BS. The object light and reference light are combined (superimposed) by beam splitter BS, and the resulting interference light is detected by the photodetector. In this interferometer, mirror 21 constitutes an object arm, and mirror 22 constitutes a reference arm.

[0041] That is, in such an interferometer, it is possible to perform measurements using interference light while suppressing the influence of the tilt of the first mirror using the displacement detection device. [Explanation of symbols]

[0042] 1 light source unit, 2 light source, 3 lens, 4 light receiving means, 6 displacement detection means, 8 servo circuit, 10 servo amplifier, 11 phase plate, 12 phase plate, 13 phase plate, 14 phase plate, 15 fulcrum, 21 mirror, 22 mirror, 23 phase plate with mirror, 24 phase plate with mirror, 25 phase plate, 25 phase plate with mirror, 26 phase plate with mirror, 30 beam crossing means, 31 movable part, 32 movable part, 33 movable part, 41 light receiving element, 42 light receiving element, 43 light receiving element, 44 light receiving element, 45 differential amplifier, 46 differential amplifier, 201 light source unit, 204 light receiving means, 206 displacement detection means, 208 servo circuit, 210 servo amplifier, 211 phase plate, 212 phase plate, 213 phase plate, 214 Phase plate, 230 beam crossing means, 301 first beam reflecting means, 302 second beam reflecting means, 311 first mirror, 312 second mirror, 313 third mirror, 314 fourth mirror, BS beam splitter, L1 beam, L2 beam, L3 beam, L4 beam.

Claims

1. a light source unit including a light source and a lens for collimating the emitted beam; a first PBS that splits the beam from the light source unit into two beams; a second PBS including a beam splitting means for emitting the first and second split beams in parallel, and further splitting the first and second beams emitted in parallel into two beams; a movable first reflecting means disposed on the side of the first beam split and transmitted by the second PBS; a beam crossing means disposed on the side of the second beam reflected by the second PBS, for crossing the first beam and the second beam and emitting the first beam and the second beam in parallel; a tiltable second reflecting means; a third reflecting means disposed opposite the second reflecting means with respect to the second PBS; a light receiving means for receiving the first beam and the second beam superimposed again by the first PBS as an interference signal; a displacement detection means for converting the received interference signal into displacement information; a servo circuit and a servo amplifier for maintaining the second reflecting means at a predetermined position based on the displacement information; a movable portion that tilts the second reflecting means; Equipped with a displacement detection device that controls the tilt of the second reflecting means in accordance with the tilt state of the first reflecting means;

2. 2. The displacement detection device according to claim 1, further comprising: a first displacement detection device that controls the second reflecting means by following the rotation state about the X-axis direction as the rotation axis, in accordance with the rotation state of the first reflecting means disposed on an XY plane, about an X-axis direction as the rotation axis and about a Y-axis direction as the rotation axis, and a second displacement detection device that controls the second reflecting means by following the rotation state about the Y-axis direction as the rotation axis.

3. 3. The displacement detection device according to claim 1, wherein the second PBS, the first reflecting means, and the second reflecting means share the same surface as the beam splitting means and reflecting means of another Michelson interferometer.

4. the optical path lengths of the first beam and the second beam split by the first PBS are equal; 4. The displacement detection device according to claim 1, wherein the light source is a low-coherence light source.

5. the light source comprises a low coherence light source; 4. The displacement detection device according to claim 1, wherein optical path lengths of the first beam and the second beam split by the first PBS are set to lengths that allow interference between the two beams.

6. An interferometer including the second PBS, the first reflecting means, and the second reflecting means constituting the displacement detection device of any one of claims 1 to 5, a second light source unit separate from the light source unit; a first mirror having the first reflecting means on the same surface; a second mirror having the second reflecting means on the same surface; a beam splitter having the second PBS on the same plane; a photodetector that detects interference light obtained when, of the light emitted from the second light source unit and split by the beam splitter, object light that has passed through the beam splitter and then reflected by a measurement target placed on the first mirror and reference light that has been reflected by the beam splitter and then reflected by the second mirror are returned to the beam splitter and combined; An interferometer comprising:

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