WO2003098194A1

WO2003098194A1 - Point diffraction interferometer using inclined-section optical fiber light source and its measuring method

Info

Publication number: WO2003098194A1
Application number: PCT/KR2002/001332
Authority: WO
Inventors: Seung-Woo Kim; Hag-Yong Kim
Original assignee: Korea Advanced Institute of Science and Technology KAIST
Current assignee: Korea Advanced Institute of Science and Technology KAIST
Priority date: 2002-05-03
Filing date: 2002-07-15
Publication date: 2003-11-27
Anticipated expiration: 2004-11-03
Also published as: KR100465784B1; KR20030086020A; AU2002319926A1

Abstract

Disclosed is a point diffraction interferometer for analyzing the surface profile of an object in a predetermined shape. The present invention relates to a phase-shifting point diffraction interferometer using inclined-section optical fiber light source, which is capable of minimizing a system error by directly using spherical waves emitted from a point beam source as a reference wavefront to move an error caused by a reference surface.

Description

POINT DIFFRACTION INTERFEROMETER USING INCLINED-

SECTION OPTICAL FIBER LIGHT SOURCE AND ITS

MEASURING METHOD

Technical Field The present invention relates to a point diffraction interferometer for analyzing a surface profile of an object having any shape, and more particularly, to a phase-shifting point diffraction interferometer using an inclined-section optical fiber light source, which is capable of minimizing a system error by directly using spherical waves emitted from a point light source as a reference wavefront to remove an error caused by a reference surface. Background Art

Since most of interferometers obtain information on a profile of an obj ect to be measured through interference with a reference surface , measurement results depend on how well the reference surface is trimmed . hile a reliable reference surface is necessary to obtain accurate measurement results , it is very difficult to well trim the reference surface . To solve such a problem, there is a point diffraction interferometer, as illustrated in FIG . 1 , using spherical waves emitted from a point light source such as a pinhole as a reference wavefront of the interferometer . A measurement wavefront 440 arriving from a target object to be measured is concentrated on an image surface where a spatial filter 420 exists, with a distorted form due to a profile of the object as indicated by reference numeral 430. The spatial filter 420 has a small pinhole 400 for creating a spherical wave 450. A part of a concentrated measurement wavefront 460 passing through the pinhole 400 becomes the spherical wave 450. The concentrated measurement wavefront 460 passes through the spatial filter 420 and interferes with the spherical wave 450 created by a pinhole 410, thereby generating an interference pattern. Information on a profile of the target object can be obtained by analyzing the interference pattern. However, the above point diffraction interferometer does not coincide in starting points of the reference wavefront and measurement wavefront and they are apart from each other. This error of the measuring system must be removed in analyzing the interference pattern. Further, since an error in trimming the pinhole has an effect on the reference wavefront, it is necessary to check the pinhole before using it. Furthermore, the transmissivity of the spatial filter and the size of the pinhole should be adjusted in order to control the visibility of the interference pattern, and it is very complicated to correct the measuring system in measuring various objects having different reflectivities.

Disclosure of the Invention

Accordingly, the present invention has been made in view of the above-mentioned problems of the point diffraction interferometer using a pinhole, and it is an object of the present invention to provide a point diffraction interferometer using an inclined-section optical fiber light source.

To accomplish the object of the present invention, there is provided a variable optical split phase shifting apparatus for increasing measurement accuracy and easily controlling the visibility of an interference pattern. There is also provided a compensation principle which can remove an error of a point diffraction interferometer using an inclined-section optical fiber light source.

It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

Brief Description of the Drawings Further objects and advantages of the invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 illustrates a principle of a point diffraction interferometer using a pinhole; FIG. 2 illustrates a point diffraction interferometer using an inclined- section optical fiber light source according to a first embodiment of the present invention; FIG. 3A is a front view of an optical fiber chuck member illustrated in FIG. 2;

FIG. 3B illustrates optical fiber fixing tubes of the optical fiber chuck member of FIG. 3A; FIG. 3C illustrates paths of beams traveling through the optical fiber chuck member of FIG. 3A;

FIG. 4 illustrates a path of a beam traveling into the air from an optical fiber whose end is inclined;

FIG. 5 illustrates a point diffraction interferometer using an inclined- section optical fiber light source according to a second embodiment of the present invention;

FIG.6A illustrates a phase shifting unit applied to the point diffraction interferometer of FIG. 5;

FIG. 6B illustrates phase shifting in the phase shifting unit of FIG 6A; FIG. 7 illustrates a point diffraction interferometer using an inclined- section optical fiber light source according to a third embodiment of the present invention;

FIG. 8 illustrates an image taken from a spatial filter, for describing a setup observer shown in FIG. 7; and FIG. 9 is a block diagram of a point diffraction interferometer measuring system. Best Mode for Carrying Out the Invention

The present invention will now be described in detail in connection with preferred embodiments with reference to the accompanying drawings. For reference, like reference characters designate corresponding parts throughout several views.

Prior to a description of preferred embodiments of the present invention, a process of obtaining information on a measurement object from an interference pattern generated from an interferometer will be described hereinbelow in brief. A reference beam traveling into the air from an optical fiber creates an interference pattern by interfering with a measurement beam reflected from a target object to be measured (hereinafter, referred to as measurement object" ). Although the interference pattern includes profile information of the measurement object, it is difficult to obtain accurate profile information of the measurement object by only the interference pattern. In order to obtain the accurate profile information of the measurement object through operations, a variety of interference patterns should be acquired by varying the phase of a measurement beam or a reference beam. Many studies have already been made on analysis algorithms for obtaining the profile information of the measurement object using the various interference patterns. Since the published many algorithms have advantages and disadvantages according to conditions used and properties of the measurement object, a user should select an algorithm favorable for measurement. After selecting the algorithm, the user obtains the interference patterns as many as the number necessary for the selected algorithm. The algorithm processing procedure is disclosed in many papers, and thus a detailed description thereof is omitted. Hereinafter, the preferred, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 2 illustrates a point diffraction interferometer using an inclined- section optical fiber light source according to a first embodiment of the present invention. Referring to FIG. 2, if a beam is irradiated from a light source unit 10, the beam is incident upon an optical splitting unit 16 via an optical quantity controlling unit 12, and a mirror 14 which is an optical path changing unit. A typical square beam splitter or a half mirror is used as the optical splitting unit 16. The optical splitting unit 16 splits the incident beam into a measurement beam and a reference beam. The measurement beam is incident upon a measurement beam optical fiber 26 through a measurement beam optical connector 18. The reference beam is incident upon a reference beam optical fiber 28 through an optical quantity controlling unit 22 and a reference beam optical connector 20. The reference beam optical fiber 28 has a phase shifting unit 24 on a given section to control an optical path, and a cylindrical piezoelectric element is used as the phase shifting unit 24. The measurement beam optical fiber 26 and the reference beam optical fiber 28 are fixed at an optical fiber chuck member 30. A beam traveling into the air through the measurement beam optical fiber 26 is used as a measurement beam 46, and a beam traveling into the air through the reference beam optical fiber 28 is used as a reference beam 50. A measurement beam 48 incident upon a measurement object 34 is reflected with the surface profile information of the measurement object 34 and further reflected by a reflective surface 31 installed at an end of the optical fiber chuck member 30. In this case, it is preferable that a large quantity of beam out of the measurement beam incident upon the measurement object 34 is reflected by a measurement surface and returns back to the reflective surface 31. Therefore, an optical path compensating member 32 is installed between the measurement object 34 and the optical fiber chuck member 30, according to a profile or location of the measurement object 34. The measurement beam reflected from the reflective surface 31 is combined with the reference beam. The combined beam passes through a lens 36 and a spatial filter 38, and arrives at an image acquiring unit 42 via an image lens 40. In the preferred embodiment of the present invention, a CCD (Charge-Coupled Device) camera is used as the image acquiring unit 42.

In this measuring system, two optical quantity controlling units 12 and 22 are used. The optical quantity controlling unit 12 in front of the light source unit 10 is used to control the strength of the entire light incident upon the measuring system, whereas the optical quantity controlling unit 22 installed at the rear of the beam splitting unit 16 is used to control the strength of the reference beam according to reflectivity of the measurement object. In general, the strength of light reflected after being incident upon the measurement surface is not uniform because the measurement object does not have the same reflectivity. Therefore, the measurement beam and the reference beam are not equal to each other in strength of light. If a difference in strength between the measurement beam and the reference beam is large, measurement accuracy may be deteriorated. However, the difference can be compensated by the optical quantity controlling unit 22. In the drawing, although the optical quantity controlling unit 22 is installed on an optical path of the reference beam to control the strength of the reference beam, it may be installed on an optical path of the measurement beam to control the strength of the measurement beam.

In order to obtain information on a measurement surface by a measurement algorithm, the phase of the reference beam or the measurement beam should be shifted. For this purpose, a certain section of the reference beam optical fiber 28 is wound on the outer circumference of the cylindrical piezoelectric element to be used as the phase shifting unit. If current is applied, the cylindrical piezoelectric element is extended in a radius direction, and the reference optical fiber 28 is also lengthened, thereby shifting the phase of the reference beam. Even though the cylindrical piezoelectric element installed on the reference beam optical fiber is installed on the measurement beam optical fiber, the same effect is obtained. FIGs. 3A to 3C illustrate a detailed construction of the optical fiber chuck member 30.

FIG. 3A is a view in the direction of an arrow K in FIG. 2, illustrating the optical fiber chuck member 30. Two optical fibers 80 and 84 are fixed at the interior of optical fiber fixing tubes 82 and 86. The optical fiber fixing tubes 82 and 86 has a structure that ends of the optical fibers 80 and 84 are conveniently installed. The optical fiber fixing tubes 82 and 86 have commercially been manufactured and have come onto the market. In the measuring system, a part of each of the optical fiber fixing tubes 82 and 86 is trimmed to form a united shape so that the ends of the two optical fibers 80 and 84 can be arranged as near as possible (refer to FIG. 3B).

FIG. 3C illustrates paths of beams traveling into the air through the optical fibers 80 and 84. A beam transmitted through the optical fiber 84 travels into the air with one direction 85 and a beam transmitted through the optical fiber 80 travels into the air with another direction 81. Of the beams traveling in the two directions, one is used as the measurement beam and the other is used as the reference beam.

FIG. 4 illustrates a path of a beam traveling into the air from an optical fiber whose end is inclined. A traveling direction of a beam emitted from an optical fiber whose end is inclined follows from Snelf s law on the assumption that a refractive index n_co of a core constituting the optical fiber is approximately identical to a refractive index n_cι of a cladding constituting the optical fiber. If an end of the optical fiber is tilted at an angle θ_t with a normal 69 of an inclined

reflective surface 64 as shown in FIG. 4, a beam 66 traveling into the air

forms an angle 6>₀ with the normal 69. If the refractive index of the optical

fiber is 1.45 and #, is 29.19°, then #₀ is 45° from equation n x sin θ_t = sin θ_o . If

the beam 66 traveling into the air returns back from the measurement object, the returning beam 67 is reflected by the inclined reflective surface 64. The beam 66 traveling into the air from the optical fiber is at right angles with a beam 68 reflected from the inclined reflective surface 64 after returning back from the measurement object. Therefore, the overall arrangement of the measuring system can be minimized and the interference pattern can easily be obtained by the CCD camera.

FIG. 5 illustrates a point diffraction interferometer using an inclined- section optical fiber light source according to a second embodiment of the present invention. The measuring system of FIG. 5 is similar to that of FIG. 2 but proposes a new phase shifting unit combined with a beam splitter phase shifting unit as the phase shifting unit for shifting the phase of a reference beam or a measurement beam.

The overall construction of the measuring system of FIG. 5 will now be described in brief. If a beam is irradiated from a light source unit 110, the beam is incident upon an optical splitting unit 113 via a mirror 1 1 1 which is an optical path changing unit. The beam is split into a measurement beam and a reference beam by the optical splitting unit 113. The measurement beam is incident upon a measurement beam optical fiber 126 via an optical quantity controlling unit 122 and an optical connector 118, and the reference beam is incident upon a reference beam optical fiber 128 via an optical connector 120. An end of the measurement beam optical fiber 126 and an end of the reference beam optical fiber 128 are fixed at an optical fiber chuck member 130. A beam 146 traveling into the air from the end of the measurement beam optical fiber 126 is reflected from the surface of a measurement object 134 and further reflected by a reflective surface 131 installed at the end of the optical fiber chuck member 130. A reference beam traveling into the air from the end of the reference beam optical fiber 128 forms a beam 150 along with a measurement beam reflected from the reflective surface 131. The beam 10 passes through a lens 136 and a spatial filter 138, and arrives at an image acquiring unit 142 via an image lens 140. In the measuring system of FIG. 5, an optical path compensating member is not used between the measurement object 134 and the optical fiber chuck member 130 on the assumption that most of the measurement beam reflected from the measurement object 134 returns back to the optical fiber chuck member 130 due to the curvature of the measurement object 134. A phase shifting unit 1 14 applied to the measuring system of FIG. 5 will be illustrated in detail in FIGs. 6A and 6B.

As shown, the phase shifting unit 114 is combined with the optical splitting unit 1 13. The combined structure is similar to a general square beam splitter. However, a sliding member 115 along which the phase shifting unit 114 can slide is diagonally installed. The sliding member 1 15 adheres to the optical splitting unit 113. A push unit 1 12 for moving the phase shifting unit 114 is installed at one side of the phase shifting unit 1 14 and a restoring unit 94 for restoring the phase shifting unit 1 14 is installed at the other side of the phase shifting unit 114, so that the phase shifting unit 114 can slide along with the sliding member 115.

If a beam is irradiated from a Z direction, a part of the beam is reflected in an A direction and the other part of the beam is transmitted in a B direction, by the optical splitting unit 113. In this case, the beam transmitted in the B direction passes through the phase shifting unit 114 by a traveling distance x. If the phase shifting unit 114 is pushed by the push unit 112 as illustrated in FIG. 6B, the phase shifting unit 1 14 moves along the sliding member and thus a traveling distance y of the beam passing through the phase shifting unit 114 is loner than x by d. That is, since the beam should pass through the phase shifting unit 114 having a larger refractive index than air, the traveling distance of the beam is increased and thus a phase is shifted.

FIG. 7 illustrates a point diffraction interferometer using an inclined- section optical fiber light source according to a third embodiment of the present invention.

In FIG. 7, a compensation beam for compensating for an error of a measuring system is proposed and a setup observer for easily generating interference patterns is additionally provided. This measuring system is almost equal to that shown in FIG. 5 and thus a description will be made of only the compensation beam and the setup observer. First, the compensation beam is described. A beam is split into a reference beam and a redundant beam by an optical splitting unit 213 and a phase shifting unit 214. The reference beam is incident upon a reference beam optical fiber 225 via an optical quantity controlling unit 222 and an optical connector 118. The redundant beam is connected to a compensation beam optical fiber 224 or a measurement beam optical fiber 228 by an optical switch 220 to serve as a compensation beam or a measurement beam. The compensation beam optical fiber 224 and the measurement beam optical fiber 228 are fixed at an optical fiber chuck member 230. The compensation beam traveling into the air from an end of the compensation beam optical fiber 224 is arranged to move toward a CCD camera 242, and the measurement beam traveling into the air from an end of the measurement beam optical fiber 228 is arranged to move toward a measurement object 234.

The compensation beam for compensating for the measuring system is used as follows. The redundant beam is connected to the compensation beam optical fiber 224 by the optical switch 220. The compensation beam traveling into the air from the end of the compensation beam optical fiber 224 moves to the CCD camera 242, and the reference beam traveling into the air from the end of the reference beam optical fiber 225 is incident upon the CCD camera 242 through a half mirror 260, thereby forming an interference pattern. Since the interference pattern is formed by the compensation beam and the reference beam both passing through only the measuring system, its information includes an error of the measuring system. Therefore, an error component of the measuring system can be extracted by analyzing the interference pattern. Next, a process of measuring a surface profile of a measurement object is described. The optical switch 220 connects the redundant beam to the measurement beam optical fiber 228 to use the redundant beam as the measurement beam. The measurement beam traveling into the air from the end of the measurement beam optical fiber 228 is incident upon the surface of the measurement object 234 and then reflected. The reflected measurement beam is further reflected by a reflective surface 231 installed at the end of the optical fiber chuck member 230 and directs toward the CCD camera 242 through the half mirror 260. The measurement beam is added to the reference beam to form an interference pattern. The interference pattern includes an error of the measuring system together with information on the measuring surface. Since the error of the measuring system is previously extracted from the interference pattern obtained by the compensation beam and the reference beam, compensation for the error of the measuring system can be performed. A process of compensating for the error of the measuring system can be summarized by the following steps of: ( 1) acquiring the interference pattern including the error of only the measuring system through the reference beam and the compensation beam, and extracting the error of the measuring system;

(2) acquiring the interference pattern including surface information of the measurement object and the error of the measuring system through the measurement beam and the reference beam, and obtaining result information including the surface information of the measurement object and the error of the measuring system;

(3) compensating for the result information obtained from the second process by using the error of the measuring system obtained from the first process to acquire only the surface information of the measurement object without the error of the measuring system.

A detailed description of the first to third steps is omitted.

The setup observer will now be explained with reference to FIGs. 7 and 8.

FIG. 8 illustrates an image of the bottom of a spatial filter 238, observed through the half mirror 260, an inclined mirror surface 233 installed at an end of an optical fiber chuck member 232, and a mirror 254.

The interference pattern in which the reference beam and the compensation beam are combined or the reference beam and the measurement beam are combined passes through a central hole 439 of the spatial filter 238 and is captured by the CCD camera 242 through an image lens 240. However, the reference beam, compensation beam or measurement beam may not pass through the central hole 439 according to locations of the measurement object 234, half mirror 260 and optical fiber chuck members 230 and 232, and may be blocked at any portion 440 of the spatial filter 238, thus not to generate the interference pattern. A location of the beam on the spatial filter 238, which does not pass through the central hole 439 of the spatial filter 238, can be observed by a setup observer 256. The location of the beam can be shifted toward the central hole 439 by slightly modifying an angle of the measurement object or the optical fiber chuck members. If the measuring system is not equipped with the setup observer, it will be difficult to move the beam toward the central hole of the spatial filter.

A measuring process of the measuring system will now be described with reference to FIG. 9.

FIG. 9 is a block diagram illustrating the role of a central controller in the measuring system of FIG. 7.

A central controller 344 is in charge of the overall measuring process and controls a phase shifting controller 350, an optical switch controller 348, and an operation and image processor 346. A beam generated from a light source 310 is incident upon an optical splitter and phase shifter 314. A part of the beam passes through an optical quantity controller 322 and an optical connector 318 so as to be used as a reference beam 333. The other part of beam passes through an optical switch 320 and an optical connector 321 so as to be used as a measurement beam 330 and a compensation beam 331. As described in conjunction with FIG. 7, if the interference pattern is generated by the measurement beam and the reference beam or the compensation beam and the reference beam, the interference pattern is captured by an image acquiring unit 342, passes through an image acquiring unit controller 343, and is subject to an operation process. A detailed description of the other parts of the measuring system is omitted.

While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments but only by the appended claimed. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.

Claims

What is claimed is:

1. A phase-shifting point diffraction interferometer using an inclined-section optical fiber light source, comprising: an optical splitting unit for splitting a beam irradiated from a light source into a measurement beam and a reference beam; a measurement beam optical fiber for transmitting the measurement beam; a reference beam optical fiber for transmitting the reference beam; an optical fiber chuck member for fixing ends of the reference beam optical fiber and the measurement beam optical fiber; a phase shifting unit for phase-shifting one of the reference beam and measurement beam; and an image acquiring unit for acquiring an image of an interference pattern generated by the reference beam and the measurement beam; wherein the measurement beam optical fiber is installed on the optical fiber chuck member to be toward a measurement object, the reference beam optical fiber is installed on the optical fiber chuck member to be toward the image acquiring unit, the measurement beam is incident upon a measuring surface of the object and then reflected, and the reflected beam is reflected by a reflective surface installed at an end of the optical fiber chuck member and interferes with the reference beam traveling into the air from the reference beam optical fiber, to travel toward the image acquiring unit.

2. The phase-shifting point diffraction interferometer as claimed in claim 1 , wherein the phase shifting unit is formed by winding a certain section of the reference beam optical fiber on a cylindrical piezoelectric element.

3. The phase-shifting point diffraction interferometer as claimed in claim 1, wherein the phase shifting unit is combined with the optical splitting unit and has a diagonally installed sliding member fixed at the optical splitting unit, for a sliding movement.

4. The phase-shifting point diffraction interferometer as claimed in claim 3, wherein the phase shifting unit has one side installed at a push unit for moving the phase shifting unit and the other side installed at a restoring unit for restoring the phase shifting unit, so as to slide along the sliding member.

5. The phase-shifting point diffraction interferometer as claimed in claim 1, wherein an optical path compensation member is installed between the optical fiber chuck member and the measurement object.

6. The phase-shifting point diffraction interferometer as claimed in claim 1 , wherein a spatial filter for increasing definition of the interference pattern is installed between the optical fiber chuck member and the image acquiring unit.

7. The phase-shifting point diffraction interferometer as claimed in any one of claims 1 to 6, wherein the measurement beam optical fiber is arranged in such a manner that the measurement beam traveling into the air from the measurement beam optical fiber forms an angle of 45° with a normal of an inclined reflective surface, and the reference beam optical fiber is arranged in such a manner that the reference beam traveling into the air from the reference beam optical fiber forms an angle of 90° with the measurement beam.

8. A phase-shifting point diffraction interferometer using an inclined-section optical fiber light source, comprising: an optical splitting unit for splitting a beam irradiated from a light source into a reference beam and a redundant beam; a phase shifting unit for phase-shifting one of the reference beam and the redundant beam; an optical switch for connecting the redundant beam to a measurement beam optical fiber or a compensation beam optical fiber; the measurement beam optical fiber for transmitting the redundant beam determined as the measurement beam when the redundant beam is connected to the measurement beam optical fiber; the compensation beam optical fiber for transmitting the redundant beam determined as the compensation beam when the redundant beam is connected to the compensation beam optical fiber; a reference beam optical fiber for transmitting the reference beam; a reference beam optical fiber chuck member for fixing an end of the reference beam optical fiber; an optical fiber chuck member for fixing ends of the measurement beam optical fiber and the compensation beam optical fiber; an image acquiring unit for acquiring an image of an interference pattern created by the combination of the reference beam and the measurement beam, or the reference beam and the compensation beam; and a half mirror for moving the reference beam traveling into the air from the reference beam optical fiber toward the image acquiring unit; wherein the measurement beam optical fiber installed on the optical fiber chuck member to be toward a measurement object and the compensation beam optical fiber installed on the optical fiber chuck member to be toward the image acquiring unit.

9. The phase-shifting point diffraction interferometer as claimed in claim 8, wherein the measurement beam is reflected from a measurement surface of a measurement object, and the reflected beam is reflected by a reflective surface installed at an end of the optical fiber chuck member and interferes with the reference beam, thus to travel to the image acquiring unit.

10. The phase-shifting point diffraction interferometer as claimed in claim 8, wherein the phase shifting unit is combined with the optical splitting unit and has a diagonally installed sliding member fixed at the optical splitting unit, for a sliding movement.

1 1. The phase-shifting point diffraction interferometer as claimed in claim 8, wherein a spatial filter for increasing definition of the interference pattern is installed between the optical fiber chuck member and the image acquiring unit.

12. The phase-shifting point diffraction interferometer as claimed in any one of claims 8 to 11, wherein the measurement beam optical fiber is arranged in such a manner that the measurement beam traveling into the air from the measurement beam optical fiber forms an angle of 45° with a normal of an inclined reflective surface, and the compensation beam optical fiber is arranged in such a manner that the compensation beam traveling into the air from the compensation beam optical fiber forms an angle of 90° with the measurement beam.

13. The phase-shifting point diffraction interferometer as claimed in any one of claims 8 to 1 1 , further comprising an inclined mirror surface installed at an end of the optical fiber chuck member, for observing an image of the bottom of the spatial filter, and a mirror arranged so as to observe the image from a setup observer.

14. A measuring method for a phase-shifting point diffraction interferometer using an inclined-section optical fiber light source, comprising the steps of: splitting a beam irradiated from one light source into a reference beam and a redundant beam through an optical splitting unit; determining the redundant beam as a compensation beam or a measurement beam by an optical switch; acquiring an interference pattern including an error of only a measuring system by using the reference beam and the compensation beam; acquiring an interference pattern including an error of the measuring system and information on a measuring surface by using the reference beam and the measurement beam; and removing the error of the measuring system from the acquired two interference patterns to analyze the measuring surface.