Interferometer

Abstract

To provide an interferometer allowing simultaneous measurement of displacement amount along an optical axis direction and angular variation amount about the optical axis of a measurement object in a non-contact manner.SOLUTION: Light flux from a laser oscillator 12 transmits through a 1 / 4 wavelength plate 16 provided on a stage 14 and is reflected by a ball lens 18. A first beam splitter 22 arranged between the laser oscillator 12 and the stage 14 splits the light flux and a second beam splitter 24 splits the reflected light flux into two. The two split light fluxes are detected by two photodetectors 30, 32 via polarizers 26, 28 which are differently angled. On the other hand, the light flux transmitting through the first beam splitter 22 again passes between the laser oscillator 12 and the stage 14 and is reflected by the first beam splitter 22. This light flux is further divided into two by the second beam splitter 24 and finally detected by the two photodetectors 30, 32.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an interferometer capable of simultaneously measuring, in a non-contact manner, the amount of displacement along the optical axis direction of a light source and the angle of rotation about the optical axis of a measurement object, which is an object in three-dimensional space. [Background technology]

[0002] Interferometers are used to measure the position and orientation of an object, as well as the distance to the object. Laser interferometers have been used in recent years to measure the position and orientation of an object with high precision. In this laser interferometer, a retroreflector such as a corner cube or cat's eye is fixed as a reflecting mirror corresponding to the measurement point, and the reflected light from these is used for measurement.

[0003] On the other hand, the following Patent Documents 1 to 3 are known as conventional techniques for such interferometers. Patent Document 1 discloses a technology in which a light beam from a laser light source is irradiated onto an object to be measured, and the reflected light is captured by an interferometer, thereby calculating the rolling angle of the object to be measured from the displacement stored on a light-receiving element in the interferometer.

[0004] Moreover, Patent Document 2 discloses a technique relating to a laser interferometer that measures distance by receiving a laser beam emitted from a light source and reflected by an object to be measured. Furthermore, Patent Document 3 discloses a technique relating to an interferometer measurement system that uses three interferometers to simultaneously measure the linear displacement and roll angle of a moving object. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP2010-66090A [Patent Document 2] JP2011-64610A [Patent Document 3] Japanese Patent Application Publication No. 2-290502 Summary of the Invention [Problem to be solved by the invention]

[0006] However, even in Patent Documents 1 to 3, when a single interferometer is used, it is only possible to measure the displacement of the object along the optical axis direction or the amount of rotation of the object around the optical axis. On the other hand, if it is limited to measuring rotational motion, devices capable of high-precision measurement such as a rotary encoder are known apart from the above Patent Documents, but it is difficult to detect rotational motion without contact. The present invention has been made in view of the above-mentioned background, and has an object to provide an interferometer that enables simultaneous, non-contact measurement of the amount of displacement along the optical axis direction and the amount of angular change around the optical axis of an object to be measured. [Means for solving the problem]

[0007] The invention described in claim 1 that solves the above problem comprises a light source that generates a linearly polarized light beam; an object to be measured including a quarter-wave plate disposed on the optical axis of a light beam from a light source and a reflecting mirror that reflects the light beam along the optical axis; a first beam splitter that is disposed on an optical axis between the light source and the object to be measured and that is capable of transmitting the light beam and reflecting a part of the light beam to separate the light beam; a second beam splitter that splits the light beam reflected by the first beam splitter into two; two photodetectors that receive the two light beams split by the second beam splitter through polarizing plates having mutually different angles; It is an interferometer including:

[0008] According to the interferometer of the invention of claim 1, a linearly polarized light beam generated by a light source passes through a first beam splitter and is incident on an object to be measured, and passes through a quarter-wave plate provided on the object to be measured, where the light beam becomes circularly polarized. Furthermore, the light beam is reflected along its optical axis by a reflecting mirror and passes through the quarter-wave plate again, returning to linearly polarized light. As a result, the light beam that has returned to linear polarization from the object to be measured is incident on the first beam splitter.

[0009] After that, part of the light beam that has made one round trip with the object to be measured and entered the first beam splitter as described above is reflected and sent to the second beam splitter, where it is further split into two.The two split light beams are then received by two photodetectors via polarizing plates with mutually different angles.

[0010] On the other hand, the light beam that entered the first beam splitter but transmitted through it is reflected along the optical axis at the light source, and passes through the first beam splitter again in the same manner as above and is again incident on the object to be measured. Accordingly, the light beam that has passed through the quarter-wave plate and become circularly polarized is reflected along the optical axis by the reflecting mirror and passes through the quarter-wave plate again. As a result, the light beam that has returned to linear polarization from the object to be measured is incident on the first beam splitter.

[0011] After that, the light beam that has made two round trips between the object to be measured and entered the first beam splitter as described above is reflected and sent to the second beam splitter, where it is further split into two.Then, the two split light beams are received by two photodetectors via polarizing plates with mutually different angles in the same manner as described above.

[0012] As described above, according to the interferometer of this claim, the light beam reflected once and making one round trip on the reflecting mirror of the measured object, and the light beam reflected twice and making two round trips are received by two photodetectors via polarizing plates with different angles. In other words, the difference in the optical path length between the light beam reflected once and making one round trip on the reflecting mirror and the light beam reflected twice and making two round trips corresponds to twice the displacement of the measured object, so the displacement amount along the optical axis direction of the measured object becomes clear.

[0013] On the other hand, the linearly polarized light beam becomes circularly polarized after passing through the quarter-wave plate of the object to be measured once, and returns to linearly polarized light after passing through it again, but the polarization angle at which it returns to linear polarization is determined by the relationship between the polarization angle of the initial linear polarization and the position of the quarter-wave plate. As a result, there are two types of polarization angles, as there are light beams that pass through the quarter-wave plate twice after reflecting once on the reflecting mirror of the object to be measured, and light beams that pass through the quarter-wave plate four times after reflecting twice, and so the difference between these two types of polarization angles exists, and the amount of angular change around the optical axis becomes clear when a photodetector detects this difference.

[0014] Therefore, the interferometer of this claim enables simultaneous non-contact measurement of the amount of displacement along the optical axis direction and the amount of angular change around the optical axis of the object to be measured. In other words, these measurements are possible from the light beam that makes one round trip and two round trips between the object to be measured and the first beam splitter, but the light beam that makes three or more round trips does not attenuate and impede the measurement.

[0015] According to the interferometer of the invention of claim 2, the light source is a laser light source that generates a light beam, and a reflective material is provided on the surface of the light source that generates the light beam. As a result, the light source is a laser light source that can reliably generate linearly polarized light, and the reflective material on the surface of the light source that generates the light beam makes it possible to send the light beam again to the object to be measured.

[0016] According to an interferometer such as that of the invention of claim 3, there are two polarizing plates, one corresponding to each of the two photodetectors, and one polarizing plate is oriented at 45 degrees and the other polarizing plate is oriented at 135 degrees. Therefore, the light beam that has passed through the quarter-wave plate twice and the light beam that has passed through the quarter-wave plate four times are each separated by the polarizing plate and can be detected by the two photodetectors.

[0017] According to an interferometer such as that of the invention of claim 4, since the object to be measured is capable of being displaced in a direction along the optical axis and of undergoing an angular change around the optical axis, the amount of such displacement and angle change can be reliably detected by the interferometer of the present invention. Effect of the Invention

[0018] The interferometer according to the present invention makes it possible to simultaneously measure the amount of displacement along the optical axis direction and the amount of angular change around the optical axis of an object to be measured in a non-contact manner. Accordingly, when three interferometers according to the present invention are used in combination, it becomes possible to measure any movement of the object to be measured in three-dimensional space, expanding the range of measurement applications. [Brief description of the drawings]

[0019] [Figure 1] 1 is a schematic diagram of a laser interferometer according to an embodiment of the present invention; [Diagram 2] FIG. 2 is an explanatory diagram illustrating an optical path length of a laser interferometer according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, an embodiment of an interferometer according to the present invention will be described in detail with reference to the drawings. 1, in a laser interferometer 10 of this embodiment, a laser oscillator 12 that generates linearly polarized laser light, which is a light beam F, along an optical axis C1 is used as the light source of this embodiment, and a mirror 13 that is a reflector perpendicular to the optical axis C1 is provided on an emission surface 12A of this laser oscillator 12 that generates the light beam F. Here, the direction perpendicular to the paper surface in Fig. 1 is the Y direction, the emission direction of the laser light from the laser oscillator 12 is the Z direction, and the direction perpendicular to these Y and Z directions is the X direction.

[0021] A box-shaped stage 14 with one open end, which is the object to be measured, is positioned on this optical axis C1, and a housing 14A of this stage 14 can rotate around the optical axis C1, whose rotation angle is represented by Δα, and can at least linearly displace along the optical axis C1, represented by ΔZ. Furthermore, a quarter-wave plate 16 is installed on the optical axis C1 in this stage 14, and a ball lens 18, which is a reflecting mirror that can reflect the light beam F transmitted through the quarter-wave plate 16 along the optical axis C1, is also installed and held by a bracket 20 that constitutes a part of the stage 14. Therefore, as the stage 14 moves and rotates, the quarter-wave plate 16 and the ball lens 18 also move and rotate together.

[0022] Note that ball lens 18 here is a lens made of high refractive index glass precision-polished into a spherical shape, and has the function of reflecting incident light in a direction parallel to and opposite to the direction of incidence. Bracket 20 has a recess for holding ball lens 18, and the surface of this recess facing ball lens 18 is coated to make it a reflective surface.

[0023] On the other hand, a first beam splitter 22 is disposed on the optical axis C1 between the laser oscillator 12 and the stage 14 at an angle of 45 degrees to the optical axis C1, which transmits the light beam F as it is and reflects a part of the light beam F to separate the light beam F. The light beam F transmitted through the first beam splitter 22 as it is is once reflected by the mirror 13 of the laser oscillator 12 and follows the same optical path as the initial light beam F from the laser oscillator 12, but the light beam F reflected by the first beam splitter 22 at this time is also sent out as reflected light R along the reflected optical axis C2.

[0024] The second beam splitter 24, which splits the reflected light R reflected by the first beam splitter 22 into two, is disposed at a position in the X direction on the reflected light axis C2 of the first beam splitter 22, at an angle of 45° with respect to the reflected light axis C2, thereby transmitting the reflected light R as is and reflecting a portion of the reflected light R.

[0025] Accordingly, the reflected light R, which is two light beams separated by the second beam splitter 24, is received by two photodetectors 30, 32 via polarizing plates 26, 28 having mutually different angles. Specifically, a first polarizing plate 26 oriented at 45 degrees is disposed immediately before the first photodetector 30 that receives the light reflected by the second beam splitter 24. Also, a second polarizing plate 28 oriented at 135 degrees is disposed immediately before the second photodetector 32 that receives the light transmitted through the second beam splitter 24. Since the angle of the linearly polarized light from the laser oscillator 12 is 45 degrees, the inclination of the photodetectors 30, 32 is set based on this angle.

[0026] Next, a specific operation of the laser interferometer 10 according to this embodiment will be described. According to laser interferometer 10 of the present embodiment, laser oscillator 12 having mirror 13 disposed on laser light emission surface 12A generates linearly polarized light beam F, which passes through first beam splitter 22 and enters stage 14. Accordingly, light beam F passes through quarter-wave plate 16 and becomes circularly polarized light, and is reflected by ball lens 18 along optical axis C1, which is coaxial with the quarter-wave plate, so that it passes through quarter-wave plate 16 again in the opposite direction and becomes linearly polarized light.

[0027] As a result, the linearly polarized light beam F returns from the stage 14 and enters the first beam splitter 22. A part of the linearly polarized light beam F that enters the first beam splitter 22 is reflected and sent to the second beam splitter 24, which follows the optical path K1 shown by the solid line in Fig. 1. The reflected light R is then further split into two by the second beam splitter 24. Thereafter, the two split reflected lights R are received by two photodetectors 30, 32, respectively, via polarizing plates 26, 28 that are angled differently from each other, one oriented at 45 degrees and the other oriented at 135 degrees.

[0028] On the other hand, light beam F that is incident on first beam splitter 22 but passes through it as it is is reflected along optical axis C1 by mirror 13 in laser oscillator 12, passes through first beam splitter 22 again, and is again incident on stage 14. Accordingly, similarly to the above, light beam F passes through quarter-wave plate 16 to become circularly polarized light, and ball lens 18 reflects light beam F along optical axis C1, and passes through quarter-wave plate 16 again, causing light beam F to return to linearly polarized light again, and then enters first beam splitter 22 from stage 14.

[0029] The light beam F incident on the first beam splitter 22 is reflected and sent to the second beam splitter 24, which follows the optical path K2 shown by the dotted line in Fig. 1. Then, as described above, it is further split into two by the second beam splitter 24. Thereafter, the two reflected light beams R split in the same manner as above are received by two photodetectors 30 and 32, respectively, via polarizing plates 26 and 28 which are angled differently from each other.

[0030] As described above, in this embodiment, the amount of displacement represented by the displacement ΔZ along the optical axis C1 direction of the stage 14 is measured as follows. First, the movement of ball lens 18 accompanying linear displacement ΔZ of stage 14 along optical axis C1 generates a phase difference between optical paths K1 and K2, causing interference, but the difference in optical path length between optical paths K1 and K2 corresponds to twice the displacement ΔZ of stage 14. For this reason, the amount of displacement of stage 14 is equal to half the difference in optical path length of light beams F received by two photodetectors 30, 32, respectively, and as a result, laser interferometer 10 of this embodiment can easily detect the amount of displacement along the optical axis C1.

[0031] On the other hand, when measuring the amount of change in angle represented by the rotation angle Δα of the stage 14 about the optical axis C1, the following occurs. As described above, the linearly polarized light beam F from the laser oscillator 12 becomes circularly polarized light when it passes through the quarter-wave plate 16 on the stage 14 once, and becomes linearly polarized light again when it passes through the quarter-wave plate 16 a second time. At this time, due to the relationship between the polarization angle of the initial linearly polarized light from the laser oscillator 12 and the position of the quarter-wave plate 16, the polarization angle of the linearly polarized light changes depending on the number of times it passes through the stage 14.

[0032] Accordingly, in the optical path K1, the light beam F makes one round trip through the quarter-wave plate 16, and in the optical path K2, the light beam F makes two round trips through the quarter-wave plate 16, so that the polarization angles of the linearly polarized light are different between the light beam F that has passed through the optical path K1 and the light beam F that has passed through the optical path K2. Specifically, the light beam F of the optical path K1 that has made one round trip through the quarter-wave plate 16 and the light beam F of the optical path K2 that has made two round trips through the quarter-wave plate 16 are each reflected by the first beam splitter 22 and sent to the second beam splitter 24, where they are split.

[0033] One of the reflected lights R is detected by a first polarizing plate 26 inclined at 45 degrees, and the other is detected by a second polarizing plate 28 inclined at 135 degrees, and these detected signals are detected by two photodetectors 30, 32. In other words, in the laser interferometer 10 of this embodiment, the polarization angle of the linearly polarized light differs based on the difference in the number of times the light has passed through the quarter-wave plate 16, so that it is possible to detect the rotational motion of the stage 14 around the optical axis C1 as well.

[0034] As described above, the laser interferometer 10 of this embodiment enables simultaneous non-contact measurement of the displacement ΔZ, which is the amount of displacement of the stage 14 along the optical axis C1 direction, and the rotation angle Δα, which is the amount of angular change around the optical axis C1. At this time, the mirror 13 on the exit surface of the laser oscillator 12 enables the light beam F to be sent again to the stage 14, making it possible to measure these from the light beam F that has made one round trip between the stage 14 and the first beam splitter 22 and the light beam F that has made two round trips. Accordingly, it is possible to generate not only the light beam F that makes one round trip but also the light beam F that makes two round trips, and three or more round trips do not attenuate the light beam F and impede the measurement.

[0035] On the other hand, in this embodiment, there are two polarizing plates 26, 28, and the orientation of the first polarizing plate 26 is 45 degrees, while the orientation of the other, second polarizing plate 28 is 135 degrees. Therefore, the light beam F that has passed through the 1 / 4 wavelength plate 16 twice and made one round trip, and the light beam F that has passed through the 1 / 4 wavelength plate 16 four times and made two round trips are separated by the polarizing plates 26, 28, respectively, and can be reliably detected by the two photodetectors 30, 32.

[0036] Here, the principle of simultaneously measuring the displacement and the angle will be explained using the following mathematical expressions. First, the general formula for polarization is shown below. The Jones matrix R(θ) for the polarization rotator is given by the following formula (1).

[0037]

number

[0038] Furthermore, assuming that the phase is E, the Jones matrix T(θ,E) when the polarizing element is rotated by θ is expressed by the following equation (2).

[0039]

number

[0040] The Jones matrix L4(α) of the λ / 4 wave plate that rotates by α is expressed by the following equation (3), and the Jones matrix M of the mirror is expressed by the following equation (4).

[0041]

number

[0042] Moreover, the Jones vector of linearly polarized light tilted by t degrees is expressed by the following (5).

[0043]

number

[0044] The Jones matrix I45 when the first polarizing plate is tilted at 45° is expressed by the following formula (6), and the Jones matrix I135 when the second polarizing plate is tilted at 135° is expressed by the following formula (7).

[0045]

number

[0046] Next, the basic equations for the laser interferometer 10 of this embodiment are shown below. The light emitted from the laser oscillator 12, passing through the first beam splitter 22 and the quarter-wave plate 16, reflected by the ball lens 18, passing through the quarter-wave plate 16 again and reflected by the first beam splitter 22 is designated as optical path K1, as described above, whereas the light passing through the first beam splitter 22, being reflected by the mirror 13 on the surface of the laser oscillator 12, and traveling in the same manner as optical path K1 is designated as optical path K2, as described above.

[0047] In this embodiment, the Jones matrix q1 of the optical path K1 at t=π / 4 is expressed by the following equation (8), and the Jones matrix q2 of the optical path K2 is expressed by the following equation (9). q1=M*L4(-α) * M*L4(α) *v(π / 4) ···(8) Formula q2=M*L4(-α) * M*L4(α) *M*L4(-α) * M*L4(α) *v(π / 4)...Equation (9)

[0048] In this case, as shown in FIG. 2, if the optical path length from laser oscillator 12 to first beam splitter 22 is l1, the optical path length from first beam splitter 22 to ball lens 18 is l2, and the optical path length from first beam splitter 22 to photodetectors 30, 32 (in the figure, the distance to second photodetector 32 is shown) is l0, then the total optical path length p1 of optical path K1 is shown in (10) below, and the total optical path length p2 of optical path K2 is shown in (11) below, and the phase difference between optical paths K1 and K2 can be found from these equations. p1=l0+l1+2l2...(10) formula p2=l0+3l1+4l2...(11) formula

[0049] In the above embodiment, the mirror 13 is disposed as a reflecting material on the surface of the laser oscillator 12, but other members such as lenses may be disposed as reflecting materials instead. The size of the ball lens used in the above embodiment can be selected from a variety of sizes, and a diameter of about 2 mm to 5 mm is considered, for example. However, instead of the ball lens, a retroreflector such as a corner cube or a cat's eye may be used, or a flat reflecting mirror may be used.

[0050] A light receiving element such as a general photodiode can be used as the light detectors 30, 32. Furthermore, the data changed and output by the light detectors 30, 32 can be input to a processing device such as a computer and converted into a form that can be viewed by an output device such as a display. In addition, the distance from the laser oscillator 12 to the stage 14 in this embodiment can be several tens of centimeters or several meters, for example.

[0051] On the other hand, the laser interferometer 10 of this embodiment has only one laser oscillator 12 serving as a light source, but by further arranging one light source in each of two directions relative to the stage 14 and arranging two sets each of a first beam splitter, a second beam splitter, two polarizing plates and two photodetectors, a total of three interferometers can be installed, which makes it possible to detect the amount of displacement and rotation of the stage 14 from three directions, and therefore makes it possible to measure the overall movement of an object in three-dimensional space.

[0052] Although the embodiment of the present invention has been described above, the present invention is not limited to the embodiment, and various modifications can be made without departing from the spirit of the present invention. [Industrial Applicability]

[0053] The present invention can be applied to various industrial fields in addition to measuring the position and orientation of a measurement object. [Explanation of symbols]

[0054] 10 Laser interferometer 12 Laser oscillator (light source) 13 Mirror (reflective material) 14 Stage (measurement object) 16 1 / 4 wave plate 18 Ball lens (reflector) 22 First beam splitter 24 Second beam splitter 26 First polarizing plate 28 Second polarizing plate 30 First photodetector 32 Second photodetector F luminous flux R Reflected light K1 light path K2 light path

Claims

1. A light source that generates a linearly polarized light beam; an object to be measured including a quarter-wave plate disposed on an optical axis of a light beam from a light source and a reflecting mirror that reflects the light beam along the optical axis; a first beam splitter that is disposed on an optical axis between the light source and the object to be measured and that is capable of transmitting the light beam and reflecting a part of the light beam to thereby separate the light beam; a second beam splitter that splits the light beam reflected by the first beam splitter into two beams; two photodetectors that receive the two light beams split by the second beam splitter through polarizing plates having mutually different angles; An interferometer including:

2. 2. The interferometer according to claim 1, wherein the light source is a laser light source that generates a light beam, and the light source that generates the light beam has a reflector on its surface.

3. 3. An interferometer as described in claim 1 or claim 2, comprising two polarizing plates, one for each of the two photodetectors, one polarizing plate oriented at 45 degrees and the other polarizing plate oriented at 135 degrees.

4. 2. The interferometer according to claim 1, wherein the object to be measured is capable of being displaced in a direction along the optical axis and of being changed in angle about the optical axis.

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