JP2009115486A - Length measuring method by coincidence method of low coherence interference - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a length measuring method capable of determining accurately a position of an interference fringe without being influenced by amplitude fluctuation of the interference fringe. <P>SOLUTION: A size measuring method utilizing a tandem low coherence interferometer 1 wherein a measuring interferometer 3 is connected to a reference interferometer 4 through an optical fiber 5, which is a high-precision length measuring method by a coincidence method of low coherence interference, is characterized as follows: two wavelengths having each different center wavelength are input into the tandem low coherence interferometer 1 by a low coherence light source; each interference fringe on two points (a gage surface and a base plate surface) of a block gage 2 to be measured is formed relative to the two respective wavelengths; each phase of mutually corresponding interference fringes on the two points is measured; and a distance between the two points of a measuring object is determined by a combination of phases of mutually most coincident values relative to the phases measured respectively with respect to the two respective wavelengths. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for measuring a length with high accuracy by a low coherence interference matching method.

  Precise length measurement is important in production sites or research departments in various industrial technical fields, and particularly precise length measurement is required in precision processing and dimensional measurement in the precision machine industry and semiconductor-related industry.

(Low coherence interference)
Conventionally, length measurement using a two-beam interferometer using light with a low coherence length is known. According to this two-beam interferometer, the interference fringes are formed with the maximum amplitude when the optical path lengths of the interferometers are equal, and the amplitude of the interference fringes gradually decreases as the optical path length difference increases. Therefore, if the spatial position of the interference fringe having the maximum amplitude is determined using the envelope of the interference fringe pattern, precise positioning is possible.

(Tandem low coherence interference)
As shown in Fig. 1, tandem low-coherence interference is used, which consists of two low-coherence interferometers (reference interferometer and measurement interferometer) in tandem as a two-beam interferometer using light of low coherence length. Things are known.

In this tandem low coherence interference, if the optical path length difference (D ₀ −D ₁ ) of the reference interferometer is set longer than the coherence length of the light source used, this interferometer alone will not interfere. In the same way, a measurement interferometer is set up.

When the output light from this reference interferometer is input to the measurement interferometer, these interferometers are integrated, so the optical path length difference (D ₂ -D ₃ ) of the measurement interferometer matches that of the reference interferometer. Only when this is done, an interference fringe of the same kind as that of a normal single interferometer is formed, so that spatial positioning is possible.

(Telemetry)
By the way, if the reference interferometer and the measurement interferometer are connected by an optical fiber using this tandem low coherence interference, the reference interferometer and the measurement interferometer can be separated by a distance of several hundred km. It becomes possible. The present inventors have already proposed such remote measurement (see Patent Document 1).
Japanese Patent No.520327

  The problems of the conventional low coherence interferometer will be described with reference to FIG. FIG. 3A shows the state of interference fringes formed by scanning the interferometer with an electrostrictive device, and is data acquired at different times. Here, symbols such as ETRAC000, ETRAC010, and ETRAC020 indicate data numbers when repeated measurement is performed, and the vertical axis indicates a voltage indicating the amplitude of interference fringes. FIG. 3B is an enlarged view of a part (portion surrounded by a vertical ellipse thick line). Interference fringe amplitude fluctuations may occur due to atmospheric fluctuations, mechanical vibrations, etc., and the maximum and minimum sets of interference fringe amplitudes may be different. For this reason, it is difficult to uniquely determine the interference fringe order.

  In the conventional low coherence interferometry, there is a case where the amplitude of interference fringes fluctuates due to the amplitude fluctuation due to atmospheric fluctuation or mechanical vibration. In particular, when the low coherence interferometer is arranged in tandem, the SN ratio is further deteriorated, so that the amplitude fluctuation of the interference fringes may increase.

  Therefore, depending on the spectral width of the low-coherence light source, the amplitude change may be moderate near the maximum of the interference fringe pattern, and there is a case where the interference fringe with the maximum amplitude is not a zero optical path difference. For this reason, the determination of the position of the interference fringes may not be unique. For this reason, an error of several fringes (integer multiple of λ / 2) may occur.

  The present invention aims to solve the problems of the above-described conventional low-coherence interferometer, in particular, a tandem low-coherence interferometer, and is able to determine the position of the interference fringe without being affected by the amplitude fluctuation of the interference fringe. It is an object of the present invention to realize a length measuring method that can be performed with high accuracy.

  In order to solve the above-mentioned problems, the present invention performs phase determination of interference fringes using low-coherence light sources having two wavelengths having different center wavelengths in a dimension measurement method using low-coherence interference, and uses measured values of these phases. And providing a high-accuracy length measurement method using a low-coherence interference matching method, wherein the distance between two points is determined by matching.

  In order to solve the above-described problem, the present invention provides a dimensional measurement method using a tandem low-coherence interferometer in which a measurement interferometer and a reference interferometer are coupled via an optical fiber. Two different wavelengths are input to a tandem low coherence interferometer, and for each of the two wavelengths, interference fringes are formed at two points of the object to be measured, and the phases of the corresponding interference fringes at the two points are measured. In addition, for the above-described phases measured for two wavelengths, the distance between two points of the object to be measured is determined by the combination of the phases that best match each other. Provide a length measurement method.

According to the present invention, the following effects are produced.
(1) In the present invention, precise measurement is possible without depending on the degree of coherence of the light source used. That is, even if a broad spectrum light source is not used (in general, it is difficult to determine the order of interference fringes with the maximum amplitude), it is only necessary to obtain the phase of an arbitrary maximum interference fringe. is there.

(2) In a light source with a high degree of coherence, an interference phenomenon occurs even with a single tandem interferometer, and a periodic error peculiar to length measurement occurs. In the present invention, the optical path length difference of each interferometer Can be made longer than the coherence length of the light source to be used.

  That is, in the light source having a wavelength of 1.55 μm and a spectrum width of about 500 MHz with a good coherence degree as shown in FIG. 9, the coherence length is 60 cm. However, in the present invention, a tandem interferometer is used according to the coherence degree of the light source used. By changing the optical path difference of each of the low-coherence interferometers (see FIG. 8), it is possible to solve the problem of periodic error peculiar to length measurement.

  Embodiments of a length measurement method using a low coherence interference matching method according to the present invention will be described below with reference to the drawings based on examples.

  The length measurement method according to the present invention will be described in an example in which the block gauge 2 to be measured is measured by the tandem low coherence interferometer 1 shown in FIG. In this tandem low coherence interferometer 1, a measurement interferometer 3 and a reference interferometer 4 are coupled via an optical fiber 5 (in this example, an optical fiber for general communication is used so that measurement at a remote place is possible). Has been.

  Low coherence light from a low coherence light source (ASE, SLD, etc.) enters the measurement interferometer 3 through the optical fiber circulator 6 and the general communication optical fiber 5. In the measurement interferometer 3, the light enters the beam splitter 8 through the collimator 7, and the low low coherence light 10 travels from the beam splitter 8 toward the base plate 9 and the second low coherence light travels toward the block gauge 2 to be measured. The light is divided into coherence light 11 and reference light 13 directed to the reference mirror 12.

  The first low-coherence light 10 is reflected from the base plate 9 and enters the reference interferometer 4 through the beam splitter 8 and the collimator 7 together with the reference light 13.

  The first low-coherence light 10 from the measurement interferometer 3 enters the beam splitter 15 through the collimator 14 in the reference interferometer 4, and is directed toward the reference reflector 16 and the corner reflector 18 in the stage 17 by the beam splitter 15. Divided. The corner reflector 18 is configured to be movable in the optical path direction by the electrostrictive element 19 in the stage 17.

  The first low-coherence light 10 reflected by the reference mirror 16 enters the detector 20 through the beam splitter 15. The first low-coherence light 10 reflected by the corner reflector 18 is further reflected by the reflecting mirror 21 and returns to the corner reflector 18 and enters the detector 20 through the beam splitter 15.

  When the optical path length difference is adjusted by the electrostrictive element 19 or the like in the stage 17 so that the reference interferometer 4 coincides with the position of the surface of the base plate 9, a low coherence interference fringe is formed for the first low coherence light 10. Therefore, spatial positioning of the surface of the base plate 9 is possible by this interference fringe measurement.

  On the other hand, the second low coherence light 11 in the measurement interferometer 3 is reflected by the block gauge 2 to be measured and enters the reference interferometer 4 along with the reference light 13 along the same path as the first low coherence light 10. In the interferometer 4, similarly to the first low-coherence light 10, the beam is split by the beam splitter 15 toward the reference mirror 16 and the corner reflector 18 in the stage 17, and reflected and detected by the reference mirror 16 and the corner reflector 18, respectively. Enter the vessel 20.

  For the second low-coherence light 11 as well, when the optical path length difference of the reference interferometer 4 is adjusted to the position of the surface of the block gauge 2 to be measured of the measurement interferometer 3 by the electrostrictive element 19 or the like in the stage 17, Since the similar interference fringes are formed, the spatial positioning of the surface of the block gauge 2 to be measured becomes possible.

  In this way, since the spatial positions of the surface of the block gauge 2 to be measured and the base plate 9 are determined, the dimensions of the block gauge 2 to be measured can be measured. In this example, remote measurement of the block gauge 2 to be measured is realized.

  As described above, in the tandem low coherence interferometer 1 shown in FIG. 2, the light from the low coherence light source (ASE) 22 enters the measurement interferometer 3 through the optical fiber circulator 6 and the general communication optical fiber 5, Reflected by the measurement block gauge 2 and returned to the original state.

  When this light is incident on the reference interferometer 4 and the optical path length difference is adjusted by the electrostrictive element 19 or the like in the stage 17 so that the reference interferometer 4 coincides with the position of the surface of the gauge to be measured, a low coherence interference fringe is obtained. Therefore, spatial positioning is possible by this interference fringe measurement.

  Next, when the optical path length difference of the reference interferometer 4 is matched with the position of the base plate 9 of the measurement interferometer 3, the same interference fringe interference fringes as described above are formed, so that spatial positioning becomes possible and the measured object is measured. The dimension measurement of the block gauge 2 is realized remotely.

  In such a tandem low-coherence interferometer 1, conventionally, the order of interference fringes has been determined by using the amplitude of the interference fringes. However, the interference fringes that are formed generally contain more noise due to the optical system, mechanical vibrations of the mechanical system, electrical signal-to-noise ratio, atmospheric fluctuations, etc., and therefore there are fluctuations in the amplitude of the interference fringes. For this purpose, the false fringe order may be determined.

  A feature of the present invention is that such a low coherence interference fringe employs a means called “matching method” in order to solve the problem of measurement error due to amplitude fluctuation of the interference fringe due to amplitude fluctuation of light.

In the coincidence method, the wavelengths λ ₁ and λ ₂ are respectively incident on the interferometers from light sources having different wavelengths, and the phases φ ₁ and φ _{2 of the} interference fringes (actually multiplied by λ / π) are measured. In order to uniquely determine the order of interference fringes from these phase measurements, a set shifted by one fringe (half wavelength) is prepared as follows.
...
λ ₁ + φ ₁ λ ₂ + φ _{2
λ 1/2 + φ 1 λ} 2/2 + φ 2
φ ₁ φ _{2
-Λ 1/2 + φ 1 -λ} 2/2 + φ 2
−λ ₁ + φ ₁ −λ ₂ + φ ₂
...
When a combination that matches the values of the wavelengths λ ₁ and λ ₂ is found, the order of the interference fringes is obtained, and the value at this time becomes a true decision value.

As shown in FIG. 4, a wavelength λ ₁ (center wavelength λ ₁ = 1.30 μm in the example of FIG. 4) and a wavelength λ ₂ (center wavelength λ ₂ = 1 in the example of FIG. 4) having different center wavelengths. .50 μm) is input to the interferometer, an interference fringe pattern signal (light intensity signal) as shown in FIG. 4 is formed and detected. As shown in FIG. 4, a location where two wavelengths having different wavelengths match is a location where the amplitude of the interference fringe is maximized.

Therefore, when the principle of the matching method is applied to the present invention and two light sources are used in the tandem low coherence interferometer 1 shown in FIG. 2 and two different wavelengths λ ₁ and λ ₂ are input, FIG. As shown, for each of the two wavelengths λ ₁ and λ ₂ , an interference signal on the measurement gauge surface by the first low-coherence light 10 and an interference fringe pattern signal on the base plate 9 by the second low-coherence light 11 (of the light intensity). Signal).

For the two wavelengths λ ₁ and λ ₂ , instead of finding the interference pattern having the maximum amplitude in the pattern signal of the interference pattern shown in FIG. Measure the phase. Of the phases obtained by measuring the two wavelengths λ ₁ and λ ₂ , the combination of the phases closest to each other is close to the true value of the length of the block gauge 2 to be measured.

FIG. 6 is a diagram illustrating a measurement example of the phase of each interference fringe for each of the two wavelengths λ ₁ and λ ₂ . From these measurements, various dimensions (lengths) are assumed at each wavelength. However, it can be seen that these measurements are an integral multiple of one-half of the center wavelength of the light source used.

Actually, when the measured values at each wavelength are shown as combinations of values close to each other, what is shown in FIG. 7 is obtained. Since the combination of +2.50 of the wavelength λ ₁ and +2.52 of the wavelength λ ₂ has a matching degree of −0.02 and is a combination of values that are closest to each other, it is the arithmetic average value thereof. 51 was determined.

  Thus, in the present invention, instead of focusing on the interference fringe having the largest amplitude, for each of the two wavelengths, pay attention to the appropriately large interference fringe, measure the phase thereof, and approximate phases to each other. By measuring the length using, the final measurement accuracy can be greatly improved even in a noisy signal.

  In the present invention, precise measurement is possible without depending on the degree of coherence of the light source used. That is, even if a broad spectrum light source is not used (in general, it is difficult to determine the order of interference fringes with the maximum amplitude), it is only necessary to obtain the phase of an arbitrary maximum interference fringe. is there.

  However, in a light source having a high degree of coherence, an interference phenomenon occurs even with a single tandem interferometer, and a periodic error peculiar to length measurement occurs. In order to solve this problem, the optical path length difference of each interferometer may be set longer than the coherence length of the light source to be used. For example, as shown in FIG. 8, the optical path length difference between the first low-coherence light 10 traveling toward the base plate 9 and the second low-coherence light 11 traveling toward the block gauge 2 to be measured is increased.

  That is, when a light source with a good coherence degree as shown in FIG. 9 is used, the optical path difference of each low-coherence interferometer of the tandem interferometer can be changed according to the coherence degree of the light source used. This significance is very great.

  The best mode of the length measurement method using the low coherence interference matching method according to the present invention has been described above based on the embodiments. However, the present invention is not limited to such embodiments, and It goes without saying that there are various embodiments within the scope of the technical matter described.

  Since the length measurement method according to the low coherence interference matching method according to the present invention has the above-described configuration, it is connected to each home and is connected to a general communication light source (having a narrow spectrum width, but in each wavelength field, L , S, etc.) can be used for precise length measurement, and in the future it will be possible to collaborate with businesses in the communications field, and future development is highly expected. Since not only the measurement optical fiber network but also the optical communication network can be used, a great future development can be expected.

It is a figure explaining the principle of a tandem type low coherence interferometer. It is a figure explaining the tandem type low coherence interferometer to which the method of the present invention is applied. It is a figure explaining the problem of the determination of the interference fringe order by the amplitude fluctuation of the low coherence interference fringe formed. It is a figure which shows the mode of the low coherence interference fringe at the time of measuring with the light source of a different wavelength. It is a figure which shows the mode of the interference fringe of a tandem type | mold low coherence interferometer at the time of measuring with the light source of a different wavelength. It is a figure explaining the analysis of the experiment example which applied the coincidence method to the tandem-type low coherence interferometer. It is a figure explaining the analysis result of the example of experiment which applied the coincidence method to the tandem type low coherence interferometer. It is a figure explaining the tandem type | mold low coherence interferometer with a large optical path difference with which the method of this invention is applied. It is a figure which shows the mode of the interference fringe of a low-coherence interferometer at the time of using a light source with good coherence.

Explanation of symbols

1 Tandem low coherence interferometer
2 Block gauge to be measured
3 Measurement interferometer
4 Reference interferometer
5 Optical fiber
6 Optical fiber circulator
7 Collimator
8 Beam splitter
9 Base plate
10 First low coherence light
11 Second low-coherence light
12 Reference mirror
13 Reference light
14 Collimator
15 Beam splitter
16 Reference mirror
17 stages
18 Corner reflector
19 Electrostrictive element
20 Detector
21 Reflector
22 Low coherence light source (ASE)

Claims (2)

In the dimension measurement method using low coherence interference,
Low coherence, characterized in that the phase of interference fringes is determined by a low-coherence light source of two wavelengths having different central wavelengths, and the distance between the two points is determined by matching the measured values of these phases. High-precision length measurement method by interference matching method. In a dimensional measurement method using a tandem low coherence interferometer in which a measurement interferometer and a reference interferometer are coupled via an optical fiber,
Two low-coherence light sources are used to input two wavelengths having different center wavelengths to a tandem low-coherence interferometer, and for each of the two wavelengths, interference fringes are formed for two points of the object to be measured. Measure the phase of the corresponding interference fringes,
For the above-mentioned phases measured for two wavelengths, the distance between two points of the object to be measured is determined by the combination of the phases that best match each other. Measuring method.

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