JPH0454406A - Optical displacement gauge - Google Patents

Abstract

PURPOSE:To obtain high resolution by slanting a surface to be inspected or reference surface by a fine angle to the optical axis of coherent light in constitution wherein the coherent light is made incident on the surface to be inspected and reference surface to obtain interference light between the incident light and reflected light. CONSTITUTION:When the normal of the surface S2 to be inspected and reference surface S1 is slanted by a fine angle theta to the optical axis of the coherent light, pieces of luminous flux reflected by the reference surface S1 and surface S2 to be inspected are processed by wave front composition to form interference fringes because of the mutual optical path difference. Plural mutually out-of- phase positions in the interference fringes are detected by photodetectors 2a and 2b and the intense/weak frequencies of the detection signals are counted to measure the extent of the displacement of the object surface S2, and the direction of the displacement can be decided from the lagging/leading relation of the detection signals, etc.

Description

[Detailed Description of the Invention] [Summary] The present invention is an optical displacement meter that utilizes the interference phenomenon of light. By slightly tilting the normal of at least one of the test surface and the reference surface, which constitute the two reflecting surfaces, with respect to the optical axis of the coherent light, multiple signals with mutually shifted phases can be obtained. The direction of displacement can be determined from the relationship between the delay and advance of the signal.

[Industrial application field]

The present invention relates to an optical displacement meter that measures displacement of a surface to be inspected using light interference phenomena.

This type of optical displacement meter can be used in fields where it is necessary to measure relatively minute displacements, such as precision stages.

[Conventional technology]

FIG. 8 shows the basic configuration of a conventional optical displacement meter using a Fizeau type interferometer.

As shown in the figure, the laser beam reflected by the beam splitter 1 is split between a reference surface S1 and a test surface S, which are arranged parallel to each other.
Irradiate perpendicularly to 2. The respective laser beams reflected by the reference surface S1 and the test surface S2 follow the same path via the beam splitter 1 and interfere with each other. A photodetector 2 detects the intensity of this interference light. Then, as shown in FIG. 9, the output of the photodetector 2 is λ/2(λ
shows a sinusoidal fluctuation with a period of oscillation wavelength of the laser light source, that is, the output of the photodetector 2 is
Since there is a one-cycle change every /2 displacement, the displacement ΔX of the surface to be inspected S2 is measured by, for example, counting the number of cycles of the output of the photodetector 2 and multiplying this count by λ/2. can do.

In addition, if you want to measure not only the displacement ΔX but also the direction of the displacement, use two signals with a phase shift of 90 degrees from each other, and determine the direction from the relationship between the delay and lead of the two-phase signals. I have to.

Specifically, as shown in FIG. 10(a), for example, a λ/8 plate 3 that generates a phase difference of λ/8 between two polarized light components that are orthogonal to each other, and a Polarizing beam splitter 4 that separates the light into transmitted light and refracted light
are inserted in the middle of the optical path in FIG. 8 (note that the λ/8 plate 3 may also serve as the reference surface S as shown in FIG. Two signals (A phase and B phase) having a phase difference of 2 are generated, and these are detected by photodetectors 2a and 2b, respectively. Then, as shown in FIG. 11, the outputs of the photodetectors 2a and 2b corresponding to the A phase and the B phase have a phase difference of 90 degrees from each other, and the outputs of the photodetectors 2a and 2b have a phase difference of 90 degrees, and the outputs correspond to the A phase and B phase depending on the direction of the displacement ΔX. Since the relationship between phase lag and lead is determined, direction can be determined from this relationship.

[Problem to be solved by the invention]

The conventional optical displacement meter described above uses polarizing elements such as the λ/8 plate 3 and the polarizing beam splitter 4 as shown in Figure 1O as a means for determining the direction of displacement, which increases the size of the device. The problem was that it was unavoidable.

Further, in order to achieve high resolution in the above configuration, interpolation of the interference signal, that is, a method of back calculating the displacement from the sine wave of the interference signal is used. For example, the interference signal I obtained from the photodetector can be expressed as 1 = 16 sin (2rλ/2))X, so from this formula, the displacement .

X=((λ/2)/2x)sin-'(1/Io)
According to this method, the resolution can be improved to the range of the S/N ratio of the signal. However, when attempting to interpolate such interference signals, the optical system configuration and circuit configuration become complicated.However, with the above conventional configuration, high resolution can be achieved without complicating the device. It was difficult to achieve this goal.

The present invention has been made in view of the above conventional problems, and its purpose is to enable discrimination of the displacement direction without increasing the size of the device, and to achieve high resolution with a simple configuration. Our objective is to provide an optical displacement meter that can

[Means to solve the problem]

The optical displacement meter of the present invention includes a light source such as a coherent laser, and the coherent light output from the light source is made incident on a test surface and a reference surface, respectively, and the reflected light is wavefront-synthesized. Interfering means configured to obtain interference light, in which the normal to at least one of the test surface and the reference surface is tilted by a minute angle with respect to the optical axis of the coherent light; and interference obtained by the interference means. A plurality of photodetectors convert the light intensity of each of the plurality of interference lights having mutually different phases among the light into an electric signal, and a displacement of the surface to be inspected and a The present invention is characterized by comprising a displacement measuring means for measuring the direction.

As the interference means, it is possible to employ a Fizeau type interferometer, a Michelson type interferometer, etc. In particular, in the present invention, one of the test surface and the reference surface, which constitute the two reflecting surfaces of the interferometer, can be used. Another major feature is that both normals are slightly tilted with respect to the optical axis of the coherent light. The angle of inclination can be appropriately set according to the wavelength of the coherent light, the arrangement interval of the photodetectors, etc. within a range in which the photodetector can detect a plurality of interference lights having mutually different phases.

[For production]

When the test surface and the reference surface are small (the normal line of one of them is tilted by a small angle with respect to the optical axis of the coherent light), the light beams reflected by the reference surface and the test surface are wavefront-synthesized, and each other's Interference fringes are generated from the optical path difference.If a photodetector detects multiple locations with different phases among these interference fringes, the displacement of the surface to be detected can be determined by counting the number of cycles of the strength and weakness of the detection signal. The magnitude of the displacement can be measured, and the direction of displacement can be determined based on the delay and lead relationships of the plurality of detection signals.

In this way, the magnitude and direction of displacement can be measured by simply tilting one or both of the test surface and reference surface slightly, without using a conventional polarizing element, making the device more compact. .

In addition, by increasing the number of photodetectors installed and narrowing the installation interval, the detection resolution can be improved by that amount. High resolution can be achieved.

〔Example〕

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

FIG. 1 is a block diagram of a first embodiment of the present invention.

As shown in the figure, this embodiment is constructed using a Fizeau type interference needle, and a laser beam output from a laser light source (not shown) is transmitted via a beam splitter 1 to two reflection points of an interferometer. Reference surface S1 and test surface S2 forming the surface
irradiate. Here, the reference surface S1 is made of a material that transmits part of the laser beam and reflects part of it, and its normal line is inclined by a small angle θ with respect to the optical axis of the laser beam. It is located. On the other hand, the test surface S2 is the reference surface S
1 is placed at a position where the laser beam transmitted through the laser beam enters, so that its normal line is parallel to the optical axis of the laser beam.

Each of the light beams reflected by the reference surface S1 and the test surface S2 travels through the beam splitter 1, and as they travel, wavefronts are combined, and interference fringes are generated at the positions formed. Two photodetectors 2a and 2b are placed at a position where such interference fringes are generated, separated by a predetermined distance l. This interval 1 is set at a position where two points having a phase difference of 90 degrees from each other can be detected among the interference fringes. Here, the inclination angle θ of the reference surface S1
An example of a specific method for setting the arrangement interval l between the photodetectors 2a and 2b will be shown below using FIG.

As shown in the figure, the optical path of each laser beam reaching the two photodetectors 2a and 2b via the reference surface S1 is a
l, a2 and bl, b2, and photodetectors 2a, 2
Let L be the distance corresponding to the optical path al from the arrangement surface b to the reference surface S1. Then, from the geometrical relationship, al, a2
, bl, b2 can be expressed as follows.

at=t.

az ==, i, / cos2θ b, =L −1! −cos2θ−tanθbz =b
+/cos2θ Then, two photodetectors 2a, 2b are detected via the reference plane S1.
The optical path difference ΔL of the laser beam that reaches up to the point ΔL can be expressed as follows.

ΔL= (at 10az) (bt +bz)
= (at −J) (1+1/cos2θ)
=2・cos2θ・tanθ・(cos2θ+1)/c
os2θ=j! -5in2θ Here, the phases differ by π/2 (=90°) when the optical path difference ΔL is λ/4, so the following relationship between θ and l can be obtained.

λ/4= 1-5in2θ, °, θ=(1/2) -5in-'(λ/41)ζ
λ/8f: (λ#!(1) From this relationship, for example, λ=0.78 (μm), !=500 (μm
), then θ= 1.95x 10-' (rad
) is sufficient to tilt the reference plane S1.

By setting θ and l as described above, 2
As shown in FIG. 3, detection signals (A phase, B phase) having a phase difference of 90 degrees can be obtained from the two photodetectors 2a and 2b. Of course, each detection signal shows a sinusoidal variation with a period of λ/2 relative to the displacement ΔX of the surface to be inspected S2. Therefore, based on these detection signals, the magnitude and direction of the displacement ΔX can be determined by an arithmetic circuit (not shown) or the like. For example, the displacement ΔX of the test surface S2 can be measured by counting the number of cycles of the detection signal of the photodetector 2a and multiplying this count by λ/2. In addition, the phase of the detection signal of the photodetector 2a (physiology)
is the phase of the detection signal of the other photodetector 2b (B phase)
The direction of the displacement ΔX can be determined by knowing whether it is behind or ahead of the displacement ΔX. In addition,
As long as pitching or yawing does not occur during movement of the surface to be inspected S2, the phase relationship between the A phase and the B phase remains unchanged.

Therefore, according to this embodiment, as described above, the reference surface S1
By simply tilting the lens at a slight angle θ, polarizing elements such as conventional polarizing beam splitters and λ/8 plates can be used (see Figure 10).
Since it is possible to measure the magnitude and direction of the displacement ΔX without using the device, it is possible to downsize the device.

Furthermore, if the purpose is simply to determine the direction of displacement,
The phase difference does not necessarily need to be 90°, and the phase difference can be appropriately set within a range that allows the A phase and the B phase to be clearly distinguished. However, if the phase difference is set to 90°, the rising and falling intervals of the A phase and B phase will be at equal intervals, so the above-mentioned number of cycles can be counted with higher precision. There is an advantage. Of course, instead of setting the phase difference to π/2 (=90°), π/2+2nπ
(where n is a natural number), similar advantages can be obtained.

Next, FIG. 4 is a block diagram of a second embodiment of the present invention.

In this embodiment, a photodetector array 12 is installed such that the interval l between the two photodetectors 2a and 2b in the first embodiment shown in FIG. 1 is divided into 12 equal parts. The configuration is the same as that of the first embodiment.

According to this embodiment, as shown in FIG. 5, 12 detection signals with a pitch of λ7/24 are obtained, so the resolution is
This is 12 times higher than in the example. In this way, by increasing the number of photodetectors installed and narrowing the installation interval,
Since the resolution can be improved by that amount, the resolution can be easily increased without interpolating the interference signal as in the conventional method.

Next, FIG. 6 is a block diagram of a third embodiment of the present invention.

In this example, a Michelson type interferometer is used instead of a Fizeau type interferometer, and similarly to the first example, the normal to the reference surface S1 is set at a small angle θ with respect to the laser optical axis. It is placed at an angle.

Even when a Michelson type interferometer is used in this way, as in the case where a Fizeau type interferometer is used, the two photodetectors 2a and 2b have a 90° angle to each other as shown in FIG.
It is possible to obtain detection signals (phase A, phase B) whose phases are shifted by .degree., and it is possible to measure the magnitude and direction of the displacement Δχ of the surface to be inspected S2 based on this detection signal. Of course, there is no need for a conventional polarizing element, and high resolution can be easily achieved by increasing the number of photodetectors as shown in FIG.

In each of the above-described embodiments, the reference surface SI is tilted, but the test surface S2 may be tilted instead, or both the reference surface S1 and the test surface S2 may be tilted. good.

〔Effect of the invention〕

According to the present invention, the magnitude and direction of displacement can be measured without using a conventional polarizing element, so it is possible to downsize the apparatus.

Furthermore, high resolution can be easily achieved by simply increasing the number of photodetectors without interpolating interference signals as in the prior art.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of the first embodiment of the present invention, and FIG. 2 is a configuration diagram of the first embodiment of the present invention.
The inclination angle θ of the reference plane and the arrangement interval of the photodetectors in this example! FIG. 3 is a diagram showing the relationship between the photodetector output and displacement in the first embodiment. FIG. 4 is a configuration diagram of the second embodiment of the present invention. , Figure 5 is the second
Figure 6 is a diagram showing the relationship between the photodetector output and displacement in the third embodiment of the present invention, and Figure 7 is a diagram showing the configuration of the third embodiment of the present invention.
Figure 8 is a diagram showing the basic configuration of a conventional optical displacement meter using a Fizeau interferometer, Figure 9 is a diagram showing the relationship between the photodetector output and displacement in the example of Figure 8 A diagram showing the relationship between output and displacement, Figures 10 (a) and (b) are both concrete configuration diagrams of a conventional optical displacement meter using a Fizeau type interferometer, Figure 11
The figure is a diagram showing the relationship between the photodetector output and displacement in FIG. 10. DESCRIPTION OF SYMBOLS 1... Beam splitter, 2a, 2b... Photodetector, 12... Photodetector array, SI... Reference surface, S2... Test surface.

Claims (1)

[Claims] A configuration comprising: a coherent light source; coherent light outputted from the light source is made incident on a test surface and a reference surface, and the reflected light is wavefront-synthesized to obtain interference light. and an interference means formed by tilting the normal of at least one of the test surface and the reference surface by a minute angle with respect to the optical axis of the coherent light, and interference light obtained by the interference means,
a plurality of photodetectors that convert the light intensity of each of the plurality of interference lights having mutually different phases into electrical signals; and measuring the displacement and direction of the test surface based on the respective output signals of the plurality of photodetectors. An optical displacement meter comprising a displacement measuring means.