JPH0484703A - Multiple-wave type interferometer - Google Patents

Abstract

PURPOSE:To make it possible to measure irregular shapes in a specified wavelength region by scanning the Moire fringe of the obscure sum which is generated by the overlapping of interference fringes, making the Moire fringe clear, and obtaining the intensity distribution of the interference fringes based on the equivalent wavelength. CONSTITUTION:Under the state wherein the luminous fluxes from two light sources 1 and 2 are mixed, the luminous fluxes are rediated on a measuring surface 10. Then, the observed interference fringe is the Moire fringe of the sum. The interval between the Moire fringes agree with the interference fringes having the equivalent wavelength lambdae based on lambda1 and lambda2. The Moire fringe of the sum has the intensity distribution such as a beat signal wherein the fine intensity distribuion is present in a large waviness. When a reference plane 11 is moved along the optical axis. the intensity distribution of the interference fringe is changed in response to the moving amount of the reference plane 11. Under the state wherein two interference fringes are overlapped, the fringe scanning for one period is performed for wavelength lambda1 and lambda2. Thus, the clear equivalent interference fringe is obtained. Then, the equivalent wavelength for one period is divided into several parts, and the fringe scanning is performed. Thus, the phase distribution at the equivalent wavelength is obtained. In this way, the irregular shape in the region from several tens of mum to several hundreds of mum can be measured.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a multi-wavelength interferometer that includes a plurality of light sources with different wavelengths and measures the flatness, shape, etc. of a measurement surface.

[Prior art] When measuring the surface shape of an object (plane, spherical, aspherical, etc.), a multi-wavelength interferometer that uses a laser light source with two or more wavelengths produces interference fringes at each wavelength. For example, λ1 and λ
In the case of 2 wavelengths, the interval of the resulting sum of moiré fringes is λ1
and λ2 equivalent wavelength λe (−λ1 ・λ2/1λ1−λ
21). However, these moiré fringes are very unclear and difficult to visually observe, and the methods used to process normal interference fringes cannot be used.For example, λ□−
632.8nm, ^z=670nm, λe≠1
1.4 μm, and the interference fringe interval is approximately 5 μm.

If these blurred interference fringes are made clearer and normal interference fringe processing becomes possible, measurement using the equivalent wavelength generated by mixing two wavelengths of light becomes possible.

[Problems to be Solved by the Invention] However, in the prior art, the dynamic range of a single wavelength laser interferometer was limited to about 5 μm. In addition, the interferometer based on the Moiré method using a grating has a small resolution of 5
0 μm to 100 μm, and it is not possible to measure displacements at the 10 μm to 100 μm level, and there is a demand for the development of an interferometer in this range.

Note that the equivalent wavelength λe≠11.397 μm is approximately equal to the wavelength of the carbon dioxide laser, so it is conceivable to use one carbon dioxide laser instead of two light sources.
In that case, there is a problem in that ordinary optical elements cannot be used and are expensive.

Therefore, the present invention has been made in view of the above-mentioned conventional problems and demands, and an object thereof is to be able to capture Japanese moiré fringes as image data with good clarity, and to be able to capture the Japanese moiré fringes as image data using a conventional interferometer. The number 10 was difficult to measure.
The object of the present invention is to provide a multi-wavelength interferometer capable of measuring uneven shapes in the range from μm to several hundred μm.

[Means for Solving the Problems] The present invention has been made to achieve the above object, and a first aspect of the present invention is to provide at least two light sources having different wavelengths, and to superimpose their respective interference fringes. The moiré fringes that are unclear due to the combination are sharpened by scanning the fringes, and the intensity distribution of the interference fringes according to the equivalent wavelength is obtained.

Further, the second invention provides at least two wavelengths having different wavelengths.
The system uses two light sources, and scans the moiré fringes that are unclear due to the superposition of their respective interference fringes to clarify them, and then scans the obtained equivalent wavelength interference fringes again to detect the phase. , the surface accuracy or shape of the measurement surface is measured.

[Operation] When the interference fringes of two different wavelengths λ1 and λ2 overlap, a sum of moiré fringes is observed, and the fringe spacing is λ1 and λ2.
It matches the interference fringe due to the equivalent wavelength (λe). The sum moiré fringe has an intensity distribution similar to a beat signal with small intensity changes within large undulations, and the large undulations are interference fringes at the equivalent wavelength. This is extremely unclear and is considered impossible with normal interference fringe processing.

Therefore, the blurred equivalent wavelength interference fringes are made clearer by fringe scanning. In other words, when the reference plane is moved along the optical axis, the intensity distribution of the interference fringes changes depending on the amount of movement of the reference plane.For example, when the reference plane is moved parallel to the optical axis at intervals of λ/8. The intensity change at one point on the imaging device is A□, B1, and C.
1, Dl, the phase ω of the interference fringe at one wavelength at each point is expressed as .

Here, the square of the amplitude is TH2-(At Ct)" + (Bt D□)
It is represented by 2.

Therefore, focusing on equation (3), one cycle of fringe scanning is performed for λ or λ2 in a state where the two interference fringes overlap, and the value of TH2 in equation (3) is determined. The value of TH2 at two wavelengths is proportional to the intensity at each point of the sum of moiré fringes. In other words, the value of TH2 is proportional to the change in intensity of the equivalent wavelength interference fringe. When this operation is performed on a two-dimensional interference fringe pattern, clear equislight wavelength interference fringes can be obtained.

In addition, when equivalent wavelength interference fringes are obtained, it is possible to obtain 1
By dividing the period into several parts and scanning the fringe, the phase distribution at the equivalent wavelength can be determined from equation (1).

[Example] Hereinafter, the present invention will be described in detail based on an example shown in the drawings.

FIG. 1 is a diagram of an optical system showing an embodiment of a multiwavelength interferometer according to the present invention. In the figure, the multiwavelength interferometer is
It is equipped with two light sources 1.2 with different wavelengths. The first light source 1 is one that emits visible, infrared, or ultraviolet light, for example, a He-Ne laser (wavelength λ1-=
632.8 nm), and a semiconductor laser (wavelength λ2 = 670 nm) is used as the second light source 2.

3 is a beam mixer, 4 is a condensing lens, 5 is an anamorphic prism, 6 is a diverging lens, 7 is a half mirror 1
8 is a collimator lens, 9 is a main mirror consisting of a half mirror, 10 is a measurement surface, 11 is a reference surface, 12 is a piezo element as a driving device for moving the reference surface 11 in the optical axis direction, 13 is a spatial filter, 14 is the imaging lens,
15 is an imaging device such as a CCD, 16 is an image input device, 17
is a computer that processes images.

The first light source 1, the second light source 2, and the optical element are adjusted so that their optical axes coincide with each other, and the amounts of light from both light sources are adjusted to be equal after the beam mixer 3.

In such a configuration, a laser beam of wavelength λ1 emitted from the first light source 1 passes through the beam mixer 3, becomes a diverging beam by the diverging lens 6, is reflected by the half mirror 7, and is made parallel again by the collimator lens 8. After being turned into light, a part of it hits the main mirror 9 and is reflected and illuminates the measurement surface 10, while the other part passes through the main mirror 9 and illuminates the reference surface 11. Measuring surface 10 and reference surface 11
The reflected light beams that hit and are reflected respectively have shapes corresponding to the shapes of the measurement surface 10 and the reference surface 11. These reflected lights are superimposed on each other by returning along the same optical path, pass through the collimator lens 8, the half mirror 7, and the spatial filter 13, and are imaged by the imaging lens 14 on the imaging surface of the imaging device 15. , interference fringes are formed based on the mutual interference of both reflected lights, and these are transferred to the imaging bag f1.
5.

Now, the difference between the distance 1 from the main mirror 9 to the measurement surface 10 and the distance ρ from the main mirror 9 to the reference surface 11 is δ(
=J, -41,), the measurement light reflected by the measurement surface 10 and the reference light reflected by the reference surface 11 produce a round trip optical path difference of 2δ.

The intensity of the interference pattern generated from this 2δ optical path difference is Icc
cos2 (2πλ1δ+φ) (where φ is an integer).

That is, when the optical path difference 2δ is an odd multiple of λ1/2, 2δ=
λ1/2・(2n−1) (where n is an integer), which cancel each other out and make the light darker.When it is an even multiple of λ1/2, it becomes 2δ−λ1/2・(2n), which strengthens each other and makes it brighter. Become. As a result, an interference image consisting of interference fringes based on mutual interference is formed.

The laser beam of wavelength λ2 emitted from the second light source 2 is
After passing through the condenser lens 4 and the anamorphic prism 5 and being reflected by the beam mixer 3, the light passes through the same optical path as the light beam of the first light source 1 described above and reaches the measurement surface 10.
The reference surface 11 is irradiated, and the reflected light is imaged by the imaging lens 14 on the imaging surface of the imaging device 15, thereby forming interference fringes.

The interference fringe due to the wavelength λ2 is the wavelength λ! Similarly, 2δ=
When λz/2 (2n-1), it becomes dark and 2δ=λz/2
(2n), it becomes brighter.

When the intensity distribution of the interference fringe with the wavelength λl and the intensity distribution of the interference fringe with the wavelength λ2 overlap on the imaging surface of the imaging bag f15, the intensity distribution becomes a moire fringe which is the sum of both interference fringes.

In other words, when the measurement surface IO is irradiated with a mixture of the light beams from the two light sources 1.2, the observed interference fringes are the sum of moiré fringes. It is clear in principle that the interval between these moire fringes matches the interval between interference fringes due to equal wavelengths (λ) due to λl and λ2, and hereinafter, these moire fringes will be referred to as equivalent wavelength interference fringes.

FIG. 2(a) is a diagram showing interference fringes due to wavelength λ.

The interval between the interference fringes is λ,/2 0.3 μm. Same (b)
The figure shows the sum of moiré fringes resulting from the superposition of interference fringes with wavelength λl and interference fringes with wavelength λ2.

FIG. 3(a) is a diagram showing interference fringes of two wavelengths λ1 and λ2, where the lower part of the wave is dark and the higher part is bright. Figure (b) shows the "undulation" of the wave caused by the superposition of two interference fringes.As is clear from this figure, when two interference fringes overlap, there are areas where brightness and darkness change drastically and areas where brightness and darkness change. For example, if there is no change in brightness and the areas with intermediate brightness are connected, this is the moiré pattern MO.
(It can be said that this corresponds to the white part in Fig. 2(b)).

As is clear from FIG. 3(b), this moiré fringe MO has an intensity distribution like a beat signal with small intensity changes within large undulations, so the contrast is extremely poor.
It is difficult to observe visually, and normal interference fringe processing is impossible.

■ Clarification of blurred sum moire fringes Therefore, only the intensity distribution due to sum moiré fringes is extracted from the blurred interference fringes by fringe scanning to obtain a clear intensity distribution of equivalent wavelength interference fringes. That is, when the reference surface 11 is moved along the optical axis by the piezo element 12, ρ changes and δ
changes. Therefore, the intensity distribution of the interference fringes is
It changes depending on the moving scenery. This is called fringe scanning.

Figure 4 is a plot of the intensity change at one point on the imaging plane of interference fringes caused by light with only wavelength λ1 (or A2), where the vertical axis is the scene and the horizontal axis is the voltage applied to the piezo element 12. It is voltage. As shown in Fig. 4, when the reference surface 11 is changed in the optical axis direction four times by A1/8 (by A1/4 with respect to the optical path length) using the piezo element 12, the difference at one point on the imaging surface is If the intensity changes are A□, B, , C,, D, then the phase ω of the interference fringe at one wavelength at each point is...
・ ・ (1) Represented by Al.

Here, the square of its amplitude is TH2= (A,, -C1)2 + (B, -D,)
It is represented by 2.

Focusing on equation (3), perform one cycle of fringe scanning for wavelength λl in a state where two interference fringes overlap as shown in the photo in Figure 2(b), and find the value of TH2 in equation (3). .

In other words, the value of TH2 is proportional to the change in intensity of the equivalent wavelength interference fringe. When this operation is performed on a two-dimensional interference fringe pattern, clear equivalent interference fringes can be obtained from the superposition of two interference fringes as shown in FIG. 2(c).

FIG. 3(C) is a diagram showing the intensity distribution of the equal wavelength interference fringes at this time, and shows that the fine fringes have been cleared and the moiré fringes have become clearer.

■ Shape measurement of measurement surface using equivalent wavelength interference fringes Figure 3 (c
) Once the intensity distribution of the equivalent wavelength interference fringes is obtained, the phase distribution at the equivalent wavelength can be obtained from equation (1) by dividing one cycle of this equivalent wavelength into several parts and scanning the fringes. I can do it.

Here, a method of equally dividing one period of the intensity distribution of the equivalent wavelength interference fringes in FIG. 4 and determining the phase will be explained using four divisions as an example.

As mentioned above, in FIG. 4, the intensity distribution of equivalent wavelength interference fringes can be determined from the intensity changes at points Al, Bl, C1, and DI using equation (1).

Next, divide this equivalent wavelength into 4 parts to obtain points A, , A2A3,
The phase amount can be calculated from the intensity change of A4 using equation (2).

Equivalent wavelength interference fringes found from (Al , Bl , c, , DI ) using equation (1) and (A2 , B2 , Cz
,D21, and (A, ,B, ,C,,D3
), (total 4 found from A4, B4, Ca, D41)
The shape of the measurement surface can be determined from the equivalent wavelength interference fringes.

In other words, one equivalent wavelength interference fringe can be obtained from four interference fringe patterns, and by repeating this operation four times, four equivalent wavelength interference fringes can be obtained. Then, the phase distribution at the equivalent wavelength can be calculated from equation (2) from these four equidominant wavelength interference fringes, and the shape can be measured (if one period is divided into four, a total of 16 interference fringe patterns are used) ).

However, in order to realize this operation, (A I , B engineering, C1, Dt l, (Az , B2 , C2, D21 (
A3, B3, C3, Ds) each divides one cycle of A1 or A2 into four, and furthermore, (AI, A2
, A3, and A41 must divide one period of the equivalent wavelength λ8 by A1 and A2 into four.

■“Fringe scanning method for “clarification of interference fringes” and “shape measurement J” Next, we will explain the method for repeating the fringe scanning explained in ■ above and detecting the phase for shape measurement. .

As described above, FIG. 4 is a graph showing the intensity change of the interference fringes depending on the wavelength λ1, and shows the intensity change at one point on the imaging surface when the reference surface 11 is moved in the optical axis direction. A point on the graph (
Al, Bl, CI, D () is the 0 point (Az, B2, 'C2, D21, (AS, B
3, C3, D3), +A4, B4- C4, D41
The only difference is the initial position of one period, and similarly one period is divided into 4
This is the divided position.

Reference plane 1 is placed at these (A+, B+, C+, D+) positions.
By moving 1, the interference fringes change by 1/4 period. The initial position (A+, A2, A3, A4) is
This is the position where one period of the equivalent wavelength interference fringe is equally divided into four.

In FIG. 5, the horizontal axis is the peak position (maximum value, minimum value position) of the intensity change in FIG. 4, and the vertical axis is the drive voltage to the piezo element 12.

When λ, = 0.6328 μm and A2 = 0.670 μm, the equivalent wavelength λ. becomes 11.397zzm.

As is clear from FIG. 3, about 18 interference fringes due to λ□ are formed within one cycle of the intensity change of the interference fringes due to A8. In other words, if we consider the interference fringes due to the wavelength of Al as a reference, the positions of (A1, A2, A3, A4) will be (0, 4, 5, 9, 13, 51) with the number of interference fringes due to the wavelength of A1. .

Note that the points plotted in Figure 4 + AI, A2, A, ,
The positions of A and l are determined by the number of interference fringes based on the position of the interference fringes of A1.

Points (A,, B,, C,, D, ), (A2, B 2,
Another method for determining the positions of C2, Dz), etc. is to perform fringe scanning with both A1 and A2 lights mixed, and directly detect the intensity change of the sum of the moiré fringes. From the change in intensity, the point (A1, Bl −Ct +
I) i l, +A2, B2, C2, D21...
・You can decide the position of etc.

Thus, in such a multi-wavelength interferometer, conventionally, the intensity distribution of the sum of moiré fringes produced by the superposition of interference fringes of wavelength λ and A2 has poor clarity and is difficult to use for shape measurement. However, by calculating the value of equation (3) using the computer 17, it has become possible to obtain a clear intensity distribution of equivalent wavelength interference fringes from the sum of moiré fringes. In addition, by repeating the fringe scanning method again so as to divide one period of the intensity change of the obtained equivalent wavelength interference fringes into four, and calculating the value of equation (2) this time, it is possible to detect the phase at the equivalent wavelength. I can do it. Therefore, by connecting these phase distributions, the shape of the measurement surface 10 can be determined.

When considering the interference fringes due to the wavelength of A1 as a reference, if one period of intensity change is divided into four equal parts, the position of the reference plane 11 is changed 4 x 4 times, a total of 16 times, and the intensity distribution and phase distribution of the interference fringe pattern are calculated. By determining , the shape of the measurement surface 10 can be determined in a non-contact manner with high precision.

FIG. 2(d) is a diagram showing the sum of moiré fringes obtained as a result of phase calculation.

When λ+ = 632.8 nm, λz = 670 nm, λe
= 11.397 μm, and the interval of interference fringes due to A0 is λe/2 = 5.699 μn1. This is λ! This is about 18 times the interference fringe spacing λ,/2'=0.3 μm. Even when considered simply, it can be seen that the dynamic range is about 18 times that of the single wavelength interferometer using A1.

Note that in the device manufactured by the present inventors, the wavelength λ
At □2632.8 nm, A2 = 670 nm, a dynamic range of about 200 μm was confirmed for a luminous flux with an effective diameter of 60 mmφ.

[Effects of the Invention] As explained above, the multi-wavelength interferometer according to the present invention scans the blurred sum moiré fringes caused by the superposition of interference fringes, so that the multi-wavelength interferometer according to the present invention scans the blurred sum moiré fringes resulting from the superposition of interference fringes. By repeating fringe scanning on the obtained equivalent wavelength interference fringes, phase detection is performed, which improves the surface accuracy and shape of the measurement surface. can be measured.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an optical system showing an embodiment of a multiwavelength interferometer according to the present invention, and FIG. 2(a) shows interference fringes at wavelength 1,
(b) is a sum of moiré fringes resulting from the superposition of interference fringes with wavelength λ and interference fringes with wavelength λ2, (c) is a sharpened sum of moiré fringes, and (d) is a diagram showing the results of phase calculation. Figure 3 (a) is a diagram showing the superposition of interference fringes and equivalent wavelength interference fringes, (b) is a diagram showing the undulation J of the wave due to the superposition of interference fringes, and (c) is the intensity of the equivalent wavelength interference fringes. Figure 4 is a graph showing the intensity change of interference fringes depending on the wavelength λl of the He-Ne laser. Figure 5 shows the peak position, maximum value, and minimum value of the intensity change in Figure 4 on the horizontal axis. ), and the driving voltage to the piezo element 12 is plotted on the vertical axis.1.2...Light source, 3...-Beam mixer 7...Half mirror 18.Z, 9...Main mirror. II... Reference plane, 12... 15... Imaging device, 16, 17... Computer.

Claims (2)

[Claims] (1) At least two light sources with different wavelengths are provided, and the blurred sum of moiré fringes caused by the superposition of their respective interference fringes is scanned to clarify them, and the intensity distribution of the interference fringes according to the equivalent wavelength is obtained. A multi-wavelength interferometer characterized by: (2) Equipped with at least two light sources having different wavelengths, performs fringe scanning to clarify the blurred sum of moiré fringes caused by the superposition of their respective interference fringes, and further performs fringe scanning on the equivalent wavelength interference fringes. By performing phase detection,
A multi-wavelength interferometer characterized by measuring the surface accuracy or shape of a measurement surface.