JP2595050B2 - Small angle measuring device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical small angle measuring device.

(Prior Art) The applicant of the present invention has previously filed a patent application for a minute angle measuring apparatus capable of simultaneously and accurately measuring minute angles in two orthogonal directions by interference fringe analysis. Japanese Patent Application No. 62-
This is the invention according to Japanese Patent No. 234117, which is a beam expander for expanding a laser beam from a laser light source into a parallel light beam, a beam splitter for amplitude-dividing the parallel light beam into two light beams, and reflecting one of the split light beams. And a reference reflecting mirror for fringe scanning, and an area sensor for imaging an interference fringe generated by superimposing the reflected light flux of the other divided light flux on the reflection surface to be measured and the reflected light flux of the reference reflecting mirror with the beam splitter. And analyzing the interference fringe imaged by the area sensor and measuring the angle formed by the wavefront of the light beam to measure the angle of the measurement target.

According to the minute angle measuring device according to the above application, the angle is measured as a surface using an area sensor, and the angle is measured by analyzing an interference fringe pattern using a fringe scanning method. There is an effect that the inclination angles with respect to two orthogonal axes can be measured simultaneously with high accuracy and high resolution.

(Problem to be Solved by the Invention) In the minute angle measuring device according to the above-mentioned application, the angular resolution is about 0.5 × 10 ⁻⁶ rad (0.1 second), and it is not possible to meet the demand for higher sensitivity. . Further, the measurement angle range is fixed, and measurement cannot be performed over a wide angle range.

The present invention is a further refinement of the minute angle measuring device according to the above-mentioned application, and further improves the resolution by increasing the sensitivity, and a small angle measuring device capable of expanding the measurable angle range. The purpose is to provide.

(Means for Solving the Problems) The present invention provides a beam expander for expanding a laser beam from a laser light source into a parallel light beam, a beam splitter for amplitude-dividing the parallel light beam into two light beams, and one of the split light beams. Imaging a fringe-scanning reference mirror that reflects light and an interference fringe caused by superimposing the reflected light flux of the other split light beam on the reflection surface to be measured and the light beam reflected by the reference reflector with the beam splitter. A small angle measuring device that has an area sensor to measure the angle of the object to be measured by analyzing the interference fringes captured by the area sensor and measuring the angle formed by the wavefront of the light beam. An afocal lens is provided in an optical path of a light beam that is split by the beam splitter and that is directed to the surface to be measured.

The magnification M of the afocal lens may be M> 1.

(Operation) If there is an angle between the light beam reflected by the measurement target surface and the light beam reflected by the reference reflecting mirror, an interference fringe corresponding to this angle is generated on the area sensor. By performing the analysis, the angle of the measurement target can be measured. The resolution of the angle measurement is determined by the magnification M of the afocal lens. The higher the magnification M, the higher the resolution. If the magnification M is set to M> 1, the resolution is not so expected, but the measurable angle range can be expanded.

(Example) Hereinafter, an example of a minute angle measuring device according to the present invention will be described with reference to the drawings.

In FIG. 1, reference numeral 11 denotes a laser light source such as a He-Ne laser. A laser beam emitted from the laser light source 11 is converted into a parallel light beam expanded by a beam expander 12 and then converted by a beam splitter 13. The light beam is amplitude-divided. The parallel luminous flux split in the reflection direction by the beam splitter 13 is reflected by the reference reflecting mirror 14 and is reflected by the beam splitter.
The light passes through 13 and enters a CCD camera 15 which is an area sensor. On the other hand, the other parallel light beam split in the transmission direction by the beam splitter 13 is converted into an expanded parallel light beam by the afocal lens 30, and then reflected by the plane reflecting mirror 16 to be measured, and is reflected by the afocal lens 30. On the other hand, the light beam propagates in the opposite direction to become a reduced parallel light beam, is further reflected by the beam splitter 13, and enters the CCD camera 15. Therefore, the reference reflector 14
Reflector 16 to be measured with one parallel light beam reflected by
The parallel luminous flux reflected by is superimposed by the beam splitter 13 to generate an interference fringe pattern.
The image is captured by the CCD camera 15. The captured interference fringe pattern can be observed by the monitor 24.

Now, assuming that two light beams that have been amplitude-divided and reflected by the beam splitter 13 are incident on the imaging surface of the CCD camera 15 at the same angle, the entire incident light beams have the same phase and become one color. If there is an angle 間 に between the two light beams, an interference fringe corresponding to the angle Θ is generated on the imaging surface of the CCD camera 15. As shown in FIG. 2, when the pitch of the interference fringes is P and the wavelength of the light source is λ, PtanΘ = λ. When measuring in a small angle range such as an autocollimator, it can be approximated as tanΘ = Θ. Therefore, Θ = λ / P.

In FIG. 1, an interference fringe generated on the imaging surface of the CCD camera 15 is photoelectrically converted and A / D converted, and
Stocked in. The stocked signal is input to the personal computer 22. The personal computer 22 analyzes the angle formed by the wavefronts of the two light beams, and calculates 1/1 of the angle formed by the wavefronts of the two light beams in two orthogonal directions (2 °).
The angle (Θ) of 2 is displayed on the display 23.

However, simply reading the pitch of the interference fringes
Noise is generated due to the influence of optical contrast, diffraction by dust and dirt, and the measurement accuracy is degraded. Therefore, the first
In the illustrated embodiment, the reference reflecting mirror 14 is fixed to the piezo element 18, and the piezo element 18 is controlled by the personal computer 22 to cause the reference reflecting mirror 14 to perform fringe scanning, thereby performing high-accuracy measurement of interference fringes.

Fringe scanning method is a method of photoelectrically measuring the intensity of interference fringes by changing the phase of the reference light stepwise, and calculating the Fourier series from the measured value to obtain the phase of the object light,
Based on the following principle.

The intensity of the interference pattern can be expressed as follows for an arbitrary point x.

Regarded as ^{_{I (x) = u 0 2}} (x) + u γ 2 + 2u 0 (x) u γ × cos [[Phi _gamma - [Phi] ₀ (x)] (1) cosine function of the phase [Phi _gamma reference light so Can be expressed in the form of a Fourier series of only the DC component and the fundamental component as follows.

I (x, Φ) = a 0 (x) + a 1 (x) cosΦ + b 1 (x) sinΦ
(2) However, Φ _γ = Φ is set for simplicity. Assuming that Φ is changed by 1 / n of the period 2π over the P period, Φ _j = j = (2π / n) j = 1,2,3..., Np (3) Let the intensity of the interference pattern corresponding to Φ _j be I (x,
Φ), the coefficients in equation (2) are given by the following equations, respectively.

From this, Φ ₀ (x) = tan ⁻¹ [b ₁ (x) / a ₁ (x)] mod 2π (7), and the phase of the object light is obtained. When scanning the entire interference pattern to determine the phase distribution, the uncertainty of 2π can be removed by considering the continuity of the phase.

As shown in FIG. 3, the afocal lens 30 includes a lens 31 having a focal length f and a lens 31 having a focal length Mf, and the magnification of the afocal lens 30 is M times. When the light beam reflected by the plane reflecting mirror 16 to be measured enters the lens 32 at an angle of Θ, the lens 31
The light exits at an angle of Θ ′. Where Θ ′ = M
It becomes Θ. Therefore, the interference fringe pattern generated by the light beam reflected by the reference reflecting mirror 14 and the light beam reflected by the plane reflecting mirror 16 are superimposed by the beam splitter 13 as compared with the case where the afocal lens 30 is not used. M
It can be read as a double interference fringe, that is, an M-fold angle, and an M-fold high sensitivity can be achieved.

According to the fringe scanning method, a measurement accuracy of λ / 100 can be obtained, and the magnification of the afocal lens 30 can be obtained even when a 2/3 inch size, that is, 8.8 mm × 6.6 mm is used as the imaging body of the CCD camera 15. When M is 5 times, the angle analysis of (λ / 100) / (6.6 × M) = (λ / 100) / (6.6 × 5) 100.2 × 10 ^-6 (rad) is possible. As a result, a measurement accuracy of 0.1 × 10 ⁻⁶ (rad) (0.02 seconds) can be obtained.

The afocal lens may be a convex-convex lens as in the embodiment of FIG. 1, or may be a concave-convex lens or a zoom afocal lens.

Also, in the angle display as a measurement result, it is natural that the angle formed by the wavefront of the light beam is corrected by the magnification M of the afocal lens, and the reversal of the angle direction by the afocal lens is also corrected. (See FIG. 3).

By the way, as shown in FIG. 4, in a normal angle measuring device, when the inclination angle of the reflection surface of the measuring object 16 is large or the angle variation is large, as shown in FIG. The overlapping portion 38 between the reflected light beam 36 and the reflected light beam 34 from the reference reflecting mirror 14 becomes smaller, and the spread range of the two light beams 34 and 36 becomes wider, so that the CCD camera 15
The light flux protrudes from the image pickup surface of (1). In such a case, all the two light beams 34 and 36 should be C
It needs to be captured by the CD camera 15. Therefore,
In such a case, the magnification M of the afocal lens 30 in the above embodiment is set to M <1, and the displacement of the two light beams due to the inclination of the measurement target 16 is reduced. FIG. 6 shows a case where the magnification M of the afocal lens 30 is set to M <1. When the incident angle to the afocal lens 30 is Θ and the emission angle from the afocal lens 30 is Θ ′, From Θ ′ = MΘ, Θ ′ <Θ, and as shown in FIG. 7, it is possible to make the overlapping portion 38 of the two light beams 34 and 36 larger than in the case of FIG. Expansion can be achieved.

When comparing FIG. 4 with FIG. 6, the distance from the beam splitter 13 to the reflection surface of the object 16 to be measured should be equal, but the distance is different due to the visibility in drawing.

(Effect of the Invention) According to the present invention, an angle is measured as a surface using an area sensor, and an angle is measured by analyzing an interference fringe pattern using a fringe scanning method. The tilt angle with respect to two orthogonal axes can be measured simultaneously with high accuracy and high resolution.
Since the afocal lens is provided in the optical path of the light beam that is split by the beam splitter and travels toward the object to be measured, by setting the magnification M of the afocal lens to M> 1, the measurement sensitivity can be increased to further increase the resolution. It is possible, and by setting the magnification M to M <1, the angle range that can be measured can be expanded. By appropriately setting the magnification of the afocal lens in this manner, the measurement angle range can be set freely.

[Brief description of the drawings]

FIG. 1 is an optical layout diagram showing one embodiment of a minute angle measuring device according to the present invention, FIG. 2 is an explanatory diagram showing the relationship between the wavelength and angle of two light beams and the pitch of interference fringes in the above embodiment, FIG.
FIG. 4 is an optical arrangement diagram showing the operation of the afocal lens in the above embodiment, FIG. 4 is an optical arrangement diagram generally showing the operation when the tilt angle of the object to be measured is large, and FIG. FIG. 6 is a light beam cross-sectional view showing a state in which two light beams are superposed on each other, FIG. 6 is an optical arrangement diagram showing an example in which the magnification of the afocal lens in the above embodiment is reduced, and FIG. It is a light beam sectional view which shows the state of two light beams overlapping. 11 ... Laser light source, 12 ... Beam expander, 13 ...
Beam splitter, 14… Reference mirror, 15… Area sensor, 16… Measurement object, 30… Afocal lens

Claims (2)

(57) [Claims] 1. A beam expander for expanding a laser beam from a laser light source into a parallel light beam, a beam splitter for amplitude-dividing the parallel light beam into two light beams, and a reference for reflecting and fringe scanning one of the split light beams. A reflecting mirror, and an area sensor for imaging an interference fringe generated by superimposing the reflected light beam on the reflection surface to be measured of the other split light beam and the reflected light beam by the reference reflecting mirror by the beam splitter. In a minute angle measuring apparatus configured to measure an angle of a measurement target by analyzing an interference fringe imaged by the area sensor and measuring an angle formed by a wavefront of a light beam, the beam is split by the beam splitter and measured. A minute angle measuring device comprising an afocal lens provided in a light beam path toward a target surface. 2. The minute angle measuring apparatus according to claim 1, wherein the magnification M of the afocal lens is M> 1.