JP2001221609A - Interference fringe analysis method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate the influence of umbilical noise from interference fringe analysis data by performing arithmetic processing on the image data of interfer ence fringes, obtained through a plurality of steps, using a prescribed algorithm by setting the umbilical noise as a higher harmonic signal which is an integer multiple of a data signal to be found. SOLUTION: An optical distance Δ1, between a plane 3 or 4 of a collimator lens 12 and a datum plane 1 or a surface to be measured 2 on the optical axis of the lens 12, is set at a prescribed integral multiple of an optical distance Δ2 between the datum plane 1 and surface 2, and the image of the interference fringes is picked up with a CCD camera 11, whenever the wavelength λ of output light is changed by about λ2/mΔ1 (where m is the number of images picked up). Then the interference fringes are analyzed by subjecting them to arithmetic processings on a plurality of information Ir (x, y) obtained by taking the images, based on equation (1).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interference fringe analyzing method for obtaining phase information of an object to be measured in an interferometer apparatus using a wavelength tunable laser as an observation light source, and more particularly to an optical surface such as a collimator lens and a reference. The present invention relates to an interference fringe analysis method for removing interference fringe noise caused by optical interference of output light from both surfaces.

[0002]

2. Description of the Related Art Conventionally, for example, a Fizeau interferometer has been known as an optical measuring device for measuring a precise optical mirror surface and lens shape.

In such a Fizeau-type interferometer device, the reference surface is brought close to the surface to be measured, the two are illuminated with a monochromatic plane light wave, and the interference fringes generated by the reflected light from both surfaces are converted into C fringes.
An image is taken by an imaging device such as a CD camera, and the obtained interference fringe image is analyzed to observe and measure the shape (phase change) of the surface of the test surface.

On the other hand, as an effective technique for discriminating irregularities when performing interference fringe analysis, a technique of wavelength scanning using a wavelength variable laser as a light source has recently been known. The principle of interference fringe analysis using this tunable laser is as follows.

That is, when the wavelength of the light source is λ, the phase difference between the interference fringes generated by the reflected light from the reference surface and the reflected light from the surface to be measured has an optical path length Δ as well known. The value is divided by the wavelength λ, that is, 2πΔ / λ. If the wavelength changes by a small amount δλ, this phase becomes 2πΔ
/ Λ becomes 2πΔ / (λ + δλ), and α ≒ (2πΔ / λ
² ) It will change by δλ. This phase change amount α
Is proportional to the optical path length Δ between the reflecting surfaces for generating interference fringes. Here, the interference fringe information picked up by the image pickup means is expressed by the following equation unless noise is considered.

[0006]

(Equation 2) Wherein r 1 2, r 2 2 is a reflected light intensity of respective faces.

In this interferometer apparatus, the wavelength change amount δλ is selected so that the phase shift amount α becomes, for example, 0, π / 2, π, 3π / 2, respectively, and the image data I _r (for example, r =
0, 1, 2, 3), and the phase value 2πΔ / λ is obtained by, for example, a four-sample algorithm (described later) or the like to determine the shape of the surface to be measured.

[0008]

As a typical systematic error factor in a wavelength-scanning Fizeau interferometer using a laser light source capable of changing the wavelength, a collimator lens for generating a parallel light beam and a reference surface or a cover are known. Interference fringe noise between the measurement surface and the measurement surface is known. Due to this noise, the measurement result is analyzed as if a convex portion (or concave portion) commonly called “navel”, which does not actually exist, exists at the center of the surface to be measured.

That is, in addition to the light reflected from the reference surface and the light reflected from the surface to be measured, the light reflected from an optical surface such as a collimator lens enters the imaging means such as a CCD camera as noise light, and each of the light is reflected as a reference light. Since interference fringes are formed between the reflected light from the surface and the reflected light from the surface to be measured, extra noise fringes (navels) are added to the interference fringes, which causes the accuracy of the fringe intensity measurement to decrease. Was. In particular, when measuring a highly accurate surface to be measured, this has been a serious problem.

[0010] Such a problem is not limited to the Fizeau-type interferometer, but also occurs in other wavelength-scanning interferometers such as the Michelson type and the Mach-Zehnder type.

The present invention has been made in view of the above circumstances,
In a wavelength scanning interferometer device, interference fringes caused by optical interference between an optical surface such as a collimator lens and output light from a reference surface and / or a surface to be measured can be easily and easily removed from the analysis result. It is an object of the present invention to provide a fringe analysis method.

[0012]

An interference fringe analyzing method according to the present invention comprises a laser light source capable of changing a wavelength λ of output light with time, a light beam from the laser light source being converted into a parallel light beam, and An optical system for guiding on the surface to be measured, the reference surface and / or
Alternatively, in an interferometer apparatus including an imaging unit that captures interference fringe information obtained by optical interference of a light beam from a measured surface, a predetermined optical surface of the optical system, the reference surface or the measured surface optical distance delta _1, wherein the set to a predetermined integer multiple of the optical path length delta ₂ between the surface to be measured and the reference plane, substantially λ ^{2 /} mΔ the wavelength lambda of the output light on the optical axis between ₁ (m
(The number of images to be captured) each time, the interference fringe image is imaged by the imaging means, and a plurality of interference fringe image information I _r (x, y) obtained by the imaging is _expressed by the following equation ( The method is characterized in that arithmetic processing based on 1) is performed to perform interference fringe analysis.

[0013]

(Equation 3)

Further, the predetermined optical surface is a lens surface on a divergent light beam side and / or a parallel light beam side of a collimator lens. Further, the interferometer device is a Fizeau type.

Further, when the plurality of predetermined optical surfaces are provided, image information obtained by performing the interference fringe analysis for each optical surface and from which noise based on each optical surface has been removed is combined. It is also possible to configure so as to obtain the final interference fringe image information.

Further, the reference surface or the measured surface can be moved in a direction in which the optical axis extends, and the reference surface or the measured surface is related to the movement at a position where noise based on each of the optical surfaces is minimized. Is preferably stopped.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an interference fringe analysis method according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a Fizeau-type interferometer apparatus for performing an interference fringe analysis method according to the present embodiment. In the present embodiment, a case where the above-described predetermined optical surface is a lens surface of a collimator lens will be described as an example.

In this Fizeau-type interferometer, laser light emitted from a monochromatic wavelength-variable laser light source 11 such as a laser diode capable of changing the wavelength λ of output light is collimated by a collimator lens 12 (having an incident surface of 3 and an exit surface of 3). Are indicated by 4), and are incident on the reference surface 1 of the reference plate 13 and the measurement surface 2 of the measurement object 14. The light beam reflected by the reference surface 1 and the light beam reflected by the measured surface 2 go back in the optical path while interfering with each other, are reflected by the semi-transparent mirror 15, and
An interference fringe having phase information of the measured surface 2 is formed on the imaging surface of the D camera 16.

The obtained interference fringe image information is subjected to predetermined arithmetic processing in an arithmetic unit 17 so that effective and highly accurate interference fringe analysis is performed.

In the interferometer apparatus using such a wavelength tunable laser light source 11, interference fringe noise is generated between the reference surface 1 and the entrance surface 3 or the exit surface 4 of the collimator lens 12 that generates a parallel light beam. The measurement results are analyzed as if a convex portion (or concave portion) commonly called “navel”, which does not actually exist, exists in the center of the surface to be measured.

This is because, in the system, in addition to the reflected light from the reference surface 1 and the reflected light from the measured surface 2, reflected light from the entrance surface 3 or the exit surface 4 of the collimator lens 12 is noise. The light enters the CCD camera 16 as light, and forms interference fringes between the light reflected from the reference surface 1 and the light reflected from the surface 2 to be measured. Since the surface 4 is almost flat near the optical axis, an extra noise fringe (navel noise) 20 is added near the position corresponding to the optical axis of the interference fringe as shown in FIG. Is a cause of the decrease.

Therefore, in the present embodiment, the arrangement positions of the reference surface 1, the measured surface 2, and the entrance surface 3 or the exit surface 4 of the collimator lens 12 are set at predetermined positions, and the obtained interference fringe image is obtained. The data is subjected to arithmetic processing described later in the arithmetic unit 17 so as to remove the "navel noise".

Hereinafter, the arrangement positions of the optical surfaces 1, 2, 3, and 4 and the arithmetic processing performed by the arithmetic unit 17 will be described.

Here, the distance between the reference surface 1 and the surface to be measured 2 is d ₁ , the thickness of the reference plate 13 is d ₂ , its refractive index is n _2, and the thickness of the collimator lens 12 is D
₃ , the refractive index is n _3, and the distance between the collimator lens 12 and the reference plate 13 is d ₄ . Incidentally, the refractive index of air and n _1. The amount of deviation of the measured surface 2 from a perfect plane is δ (x, y).

Further, the optical path length between the reference surface 1 and the measured surface 2 is Δ (= n ₁ d ₁ ). The optical path length between the reference surface 1 and the lens surface 4 is Δ ₁₄ (= n ₂ d ₂ + n).
₁ d ₄ ) The optical path length between the reference surface 1 and the lens surface 3 is Δ ₁₃ (= n ₂ d ₂ + n
₁ d ₄ + n ₃ d ₃ ) The optical path length between the measured surface 2 and the lens surface 4 is Δ ₂₄ = Δ ₁₄ + Δ The optical path length between the measured surface 2 and the lens surface 3 is Δ ₂₃ = Δ ₁₃ + Δ. Define.

With this definition, in the present embodiment, the collimator lens 12 that causes navel noise is used.
As the first condition for eliminating the influence of the reflected light (scattered light) from each of the lens surfaces 3 and 4, the surfaces 1, 2, and
The arrangement positions of these surfaces 1, 2, 3, and 4 are set so that the predetermined optical path length between 3 and 4 becomes an integral multiple of Δ as shown below.

The optical path length between the reference surface 1 and the lens surface 4 is Δ ₁₄ = (mp ₂ +2) Δ The optical path length between the reference surface 1 and the lens surface 3 is Δ ₁₃ = Δ ₁₄ + Δ ₃₄ = {m
(p ₁ + p ₂ ) +2} Δ The optical path length between the surface 2 to be measured and the lens surface 4 is Δ ₂₄ = Δ + Δ ₁₄ = (m
p ₂ +3) Δ The optical path length between the surface 2 to be measured and the lens surface 3 is Δ ₂₃ = Δ + Δ ₁₃ = {m
_{(p 1 + p 2) +3} } Δ The optical path length delta is _{Δ = Δ 34 / mp 1,} Δ 34 is an optical path length ₌ n 3 _{d 3} between the collimator lens front and back surfaces, m
Is the number of images, and p ₁ and p ₂ are natural numbers.

Here, the interference fringes generated by the reflected light from the reference surface 1 and the reflected light from the lens surface 4 have a phase of 2π.
Because the delta ₁₄ / lambda, when the arrangement position of these surfaces 1,4 set as above, the phase change amount when the output wavelength of the light source 11 has just changed δλ is proportional to the optical path length delta _14, (2
πΔ ₁₄ / λ ² ) δλ = (mp ₂ +2) α.

Accordingly, the image data I (x, y) obtained from the CCD camera 16 and taking into account the interference fringes generated by the reflected light from the surfaces 1, 4 is represented by the following equation (2).

[0030]

(Equation 4)

It should be _noted, it is a reflected light intensity of ^{_{r 1 2, r 2 2,}} r 3 2 Each face 1,2,3. Similarly, the reference surface 1 and the lens surface 3, the measured surface 2 and the lens surface 4, and the measured surface 2
Considering the interference fringes on the lens surface 3 and the CCD camera 16
Is approximately expressed by the following equation (3).

[0032]

(Equation 5)

In the above equation (3), the first term on the right side is a term representing signal data to be obtained, and the second and subsequent terms are terms representing noise. As shown by the above equation (3), noise is represented by setting the optical path length between the lens surfaces 3 and 4 and each of the reference surface 1 and the measured surface 2 to be an integral multiple of the distance Δ. All terms are harmonics of the integer order of the phase shift amount α.

Here, the order of the noise terms is mp ₂ +2, mp ₂ +3, and m (p ₁ +
p ₂₎ +2 _{order, m (p 1 + p 2} ) a + 3rd. As is generally known, a phase shift algorithm (shown by the following equation (1)) of m images (m is an integer of 5 or more) is mk + 2
Since the phase detection can be performed while suppressing the influence of the harmonic components of the 3rd and mk + 3rd order (k is an integer of 0 or more), this algorithm is applied to the camera input image signal (C
When applied to the image data signal obtained from the CD camera 16, the phase α of the signal term of the first term on the right side can be detected without being affected by noise.

[0035]

(Equation 6)

Here, specific numerical values in the above method are shown. Note that the minimum number of images m required for this method is 5.
When the optical thickness Δ ₃₄ of the collimator lens 12 is 150 mm, for example, m = 5, p ₁ = 1, and p ₂ = 1, the air gap Δ = 30 mm, and the optical distance Δ between the collimator lens 12 and the reference surface 1 ₁₄ = 210 mm. Also, the noises are the seventh and
8, 12, and 13 harmonics.

The data of m images used in the phase shift algorithm expressed by the above equation (1) is as follows.
Specifically, the output wavelength of the tunable laser light source 11 is shifted by an amplitude δλ (λ ² / mn ₁ d ₁ ; usually smaller than 1 nm), and m is sequentially imaged by the CCD camera 16.
The interference fringe image data I _r (x, y) is used.

When there are a plurality (three or more) of optical surfaces that cause navel noise, it is generally difficult to set all of these optical surfaces to the ideal positions as described above. Therefore, the reference plane 1 or the measurement target plane 2 is allowed to move in the vertical direction in the drawing, and the movement is stopped at a position where the navel noise to be removed most seems to have disappeared. It is preferable to configure so as to capture the image of (1).

The stop position may be determined by calculation. That is, in the above example, the reference surface 1 optical distance between the other surfaces, including the distance between the surface to be measured 2, the following in relation to the optical path length delta ₃₄ between the front and back surfaces of the collimator lens 12 The setting is made so that the expression (4) is satisfied. Δ = Δ ₃₄ / mp ₁ (4)

Further, in this case, an image for interference fringe analysis is taken in each position where each navel noise seems to have disappeared, a portion where the navel noise has disappeared is cut out from each image, and these are synthesized by image processing. By doing so, it is possible to obtain interference fringe image data from which all navel noise has been removed. FIG. 3 conceptually illustrates this. For example, as shown in FIG.
When A to C are present, the reference surface 1 or the measured surface 2 is moved in the vertical direction in the drawing, and at each position where the navel noises 20A to 20C appear to disappear, the interference fringe analysis is performed. Import image (Fig. 3 (B) ~
(D)) Thereafter, the image data shown in FIGS. 3B to 3D are synthesized by image processing to obtain interference fringe image data from which all navel noise has been removed.

The interference fringe analysis method of the present invention is not limited to the above-described embodiment, and various other modes can be changed. For example, not only the collimator lens surface but also other navel noise The present invention can be applied to all optical surfaces (for example, a half mirror surface) that cause a problem, and when a plurality of collimator lenses are used, all or some of the lens surfaces can be applied.

The method of the present invention is applicable not only to the case of using a Fizeau-type interferometer, but also to other interferometers, for example, Michelson-type or Mach-Zehnder-type interferometers.

[0043]

As described above, according to the interference fringe analysis method of the present invention, in the interferometer apparatus using the wavelength variable laser as the observation light source, the optical surface causing the navel noise and the reference surface or optical distance delta ₁ between the surface to be measured, is set to a predetermined integer multiple of the optical path length delta ₂ between the reference surface and the measurement surface, the phase change amount is surface measured noise fringes representing the navel noise Is an integral multiple of the phase change amount of the data signal. As a result, the noise fringe of the navel noise becomes an integer multiple harmonic signal of the data signal to be obtained, so that a predetermined discrete Fourier transform algorithm is applied to a plurality of interference fringe image data obtained each time the output light wavelength is shifted by a predetermined amount. , The influence of navel noise can be eliminated from the interference fringe analysis data finally obtained. As a result, a highly accurate and highly reliable measured surface shape can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an interferometer apparatus for performing an interference fringe analysis method of the present invention.

FIG. 2 is a diagram for explaining navel noise removed by the interference fringe analysis method of the present invention.

FIG. 3 is a diagram for explaining an example of an interference fringe analysis method according to the embodiment of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 reference surface 2 measured surface 3 lens surface (incident surface) 4 lens surface (exit surface) 11 tunable laser light source 12 collimator lens 13 reference plate 14 measured object 15 half mirror 16 CCD camera 17 arithmetic unit 20, 20A to 20C Navel noise

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Nobuaki Ueki 1-324 Uetake-cho, Omiya-shi, Saitama F-term in Fuji Photo Optical Co., Ltd. (reference) 2F064 AA09 AA15 CC03 CC04 EE05 FF01 FF08 GG22 GG44 HH03 HH08 JJ01

Claims (5)

[Claims] 1. A laser light source capable of changing a wavelength λ of output light with time, an optical system for converting a light beam from the laser light source into a parallel light beam and guiding the light beam onto a reference surface and a surface to be measured, and the reference surface. And an imaging means for imaging interference fringe information obtained by optical interference of a light beam from the measured surface, wherein: a predetermined optical surface of the optical system; the reference surface or the measured surface; the optical distance delta ₁ on the optical axis between the surface, the said reference plane is set to a predetermined integer multiple of the optical path length delta ₂ between the surface to be measured, substantially the wavelength lambda of the output light lambda ² / Each time the image is changed by mΔ ₁ (m is the number of images to be captured), an interference fringe image is captured by the imaging unit, and a plurality of interference fringe image information I _r (x,
An interference fringe analysis method, wherein y) is subjected to arithmetic processing based on the following equation (1) to perform interference fringe analysis. (Equation 1) 2. The interference fringe analyzing method according to claim 1, wherein the predetermined optical surface is a lens surface on a divergent light beam side and / or a parallel light beam side of a collimator lens. 3. The method according to claim 1, wherein the interferometer is a Fizeau type. 4. When there are a plurality of said predetermined optical surfaces, image information obtained by performing said interference fringe analysis on each optical surface and from which noise based on each optical surface has been removed is combined. The interference fringe analysis method according to claim 1, wherein final interference fringe image information is obtained. 5. The reference surface or the measured surface at a position where the reference surface or the measured surface is movable in a direction in which the optical axis extends, and the noise based on each of the optical surfaces is minimized. The interference fringe analysis method according to claim 1, wherein the method is stopped.