KR20080096979A - Apparatus and method for measuring wavefront aberrations - Google Patents

Abstract

A wavefront aberration measuring apparatus and method are provided to precisely measure the wavefront aberration with the single measurement by measuring the wavefront aberration of the laser beam through at least two interference fringes. A wavefront aberration measuring apparatus includes an interferometer dividing the laser beam to generate at least n interference fringes(n is natural number more than 2), and a wavefront aberration interpretation part determining the size and direction of the wavefront aberration of the laser beam at the same time by using the n interference fringes.

Description

Apparatus and Method for measuring wavefront aberrations

1 is a diagram illustrating an interferometer for measuring wavefront aberration according to the prior art.

2 is a view showing a detailed configuration of the wavefront aberration measurement apparatus according to an embodiment of the present invention.

3 is a block diagram of a wavefront aberration analyzer according to the present invention;

4 is a diagram illustrating a process of determining the direction of wavefront aberration using an interferometer according to the present invention.

Explanation of symbols on the main parts of the drawings

200: illuminator 202: interferometer

204: Image collector 206: Wavefront aberration analyzer

210,212,214,216: beam splitter 220,222: mirror

230: beam reducer 232,234: beam expander

240,242: charge coupled device

The present invention relates to an apparatus and method for measuring wavefront aberration, and more particularly, to an apparatus and method capable of precisely measuring wavefront aberration of an optical component or a laser beam itself by a single measurement.

With the recent rapid development of technology, the application and necessity of optical technology is increasing.

In general, the performance of light (laser beams) used in optical technology can be measured through wavefront aberration. Here, wavefront aberration is the difference between the ideal spherical wave and the actual wavefront, which is caused by the difference in the ray path of the chief and finite rays.

In general, wavefront aberration measurement is performed using a wavefront sensor or an interferometer.

The wavefront meter includes a microlens array on the side from which the laser beam is incident and a CCD (charge coupled device) positioned at a focal length portion thereof. When the parallel laser beam is incident, the positions of the focal points formed by each microlens are designated as a reference focal point, and the focal points formed when the laser beam to be measured are input based on the reference focal point. The deviation from the reference can be used to determine the slope of each wavefront.

However, the measurement of the wavefront aberration using the wavefront measuring instrument has the advantage of measuring the wavefront aberration of light with a single measurement. However, there is a disadvantage in that accuracy and spatial resolution are poor compared to wavefront aberration measurement using an interferometer.

An interferometer is a measuring device that uses an interference phenomenon of light. The interferometer is composed of a beam splitter, a mirror, and a lens to obtain an interference fringe of an incident laser beam, thereby measuring wavefront aberration of the laser beam.

1 is a diagram illustrating an interferometer for measuring wavefront aberration according to the prior art, and illustrates a configuration of a Mach-gender radial radial interferometer.

As shown in FIG. 1, a conventional interferometer includes a first beam splitter BS1, 100, a first mirror M1, 102, a second beam splitter BS2, 104, and a second mirror splitter. M2, 106).

The first beam splitter 100 divides the incident laser beam into an arm1 direction and an arm2 direction.

The laser beam passing through the path of arm1 is reduced by using the beam reducer 108 composed of the convex lens L1 and the concave lens L2, and is reflected by the second beam splitter 104 to form a screen or charge coupled device (CCD). Reach the image acquisition unit 120, such as).

The laser beam passing through the path of arm2 is enlarged through the beam expander 110 composed of the concave lens L3 and the convex lens L4, and passes through the second beam splitter 104 to form a screen or charge coupled device (CCD). To reach.

As described above, two laser beams having different paths (arm1 and arm2) generate an interferogram by wavefront aberration of the laser beam itself, and mathematically interpret an image recorded on a screen or a charge coupled device to laser The wavefront aberration of the beam is calculated.

Such conventional interferometers have better accuracy and spatial resolution than wavefront meters in that they measure wavefront aberration using interference fringes. However, since the wave front aberration of the laser beam is measured through one interference fringe, the size and direction of the wave front aberration of the laser beam cannot be simultaneously measured by a single measurement.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems of the prior art, and an object thereof is to propose a method and apparatus for measuring wavefront aberration, which can simultaneously determine the direction and magnitude of wavefront aberration of an optical component or a laser beam itself by a single measurement. .

According to a preferred embodiment of the present invention to achieve the above object, a wavefront aberration measuring apparatus, comprising: an interferometer for generating at least n (n is a natural number of two or more) interference fringes by splitting the incident laser beam; And a wavefront aberration analyzer for simultaneously determining the magnitude and direction of the laser beam wavefront aberration using the n interference fringes.

According to another aspect of the present invention, there is provided an apparatus for generating an interference fringe for wavefront aberration measurement, the apparatus comprising: a plurality of beam splitters for dividing a path of an incident laser beam into at least k (k is a natural number of 3 or more); And at least one lens for enlarging or reducing the laser beam moving along the k paths, wherein the plurality of beamsplitters form at least n interference fringes, where n is a natural number of two or more. An interference fringe generating apparatus for measurement is provided.

According to another aspect of the present invention, there is provided a wavefront aberration measuring method, comprising: incident a laser beam; Dividing the incident laser beam to form at least n (n is a natural number of two or more) interference fringes; And simultaneously determining the magnitude and direction of the laser beam wave front aberration using the n interference fringes.

As the present invention allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description. However, this is not intended to limit the present invention to specific embodiments, it should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention. In describing the drawings, similar reference numerals are used for similar elements.

Terms such as first, second, A, and B may be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the first component may be referred to as the second component, and similarly, the second component may also be referred to as the first component. The term and / or includes a combination of a plurality of related items or any item of a plurality of related items.

When a component is referred to as being "connected" or "connected" to another component, it may be directly connected to or connected to that other component, but it may be understood that other components may be present in between. Should be. On the other hand, when a component is said to be "directly connected" or "directly connected" to another component, it should be understood that there is no other component in between.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "comprise" or "have" are intended to indicate that there is a feature, number, step, operation, component, part, or combination thereof described in the specification, and one or more other features. It is to be understood that the present invention does not exclude the possibility of the presence or the addition of numbers, steps, operations, components, components, or a combination thereof.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art. Terms such as those defined in the commonly used dictionaries should be construed as having meanings consistent with the meanings in the context of the related art, and shall not be construed in ideal or excessively formal meanings unless expressly defined in this application. Do not.

 Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In describing the present invention, in order to facilitate the overall understanding, the same reference numerals will be used for the same means regardless of the drawing numbers.

2 is a block diagram of a wavefront aberration measuring apparatus according to an embodiment of the present invention.

As shown in FIG. 2, the wavefront aberration measuring apparatus according to the present invention may include an illumination system 200, an interferometer 202, an image collector 204, and a wavefront aberration analyzer 206.

The illumination system 200 enters a laser beam having an arbitrary wavelength and which is the object of wavefront aberration measurement. The laser beam may be a pulsed laser beam.

The interferometer 202 divides and reflects an incident laser beam to generate an interference fringe, and may preferably be a radial shear interferometer.

As shown in FIG. 2, the interferometer 202 according to the present invention includes a plurality of beam splitters (a light splitter, 210 to 218) and a plurality of mirrors (220, 222) that transmit part of the laser beam and reflect part of the laser beam. And it may include a plurality of lenses (L1 to L7).

In FIG. 2, P1 is a plane of the wavefront to be measured, and P2 to P4 are conjugate planes of P1.

According to one preferred embodiment of the present invention, the interferometer 202 includes at least k (k is at least 3 natural numbers) laser beam paths, thereby generating at least n (n is at least 2 natural numbers) interference fringes. do.

The following description will focus on the generation of two interference fringes through three laser beam paths, but the present invention is not limited thereto, and the generation of more interference fringes through more paths may be included in the scope of the present invention. Should be understood.

Referring to FIG. 2, the first beam splitter 210 splits an incident laser beam.

The laser beam split through the first beam splitter BS1 and 210 moves along the first path arm1 and the second path arm2.

At this time, the first laser beam traveling along the first path is reduced to a predetermined size in the beam reducer 230 which is reflected by the first mirrors M1 and 220 and is composed of the convex lens L1 and the concave lens L2. .

A portion of the laser beam reduced as described above is reflected by the second beam splitter BS2 212 and reaches the image collector 204.

The image collecting unit 204 is a device capable of collecting images, and may be a screen or a charge coupled device that captures the generated interference fringe. Since the interferometer 202 according to the present invention generates a plurality of interference fringes, the image collecting unit 204 may include a screen or a charge coupling device corresponding to the number of generated interference fringes.

Hereinafter, the image collecting unit 204 will be described as including first and second charge coupling elements 240 and 242 corresponding to two interference fringes.

Meanwhile, some of the laser beams along the first path are divided by the first beam splitter 210 and move along the second path. Here, the second path is a path passing through the first beam splitter 210 and reflected through the third beam splitter BS3 and 214.

A portion of the second laser beam traveling along the second path is reflected by the third beam splitter 214 and then in the first beam expander 232 composed of the concave lens L3 and the convex lens L4. It is enlarged and passes through the second beam splitter 212 to reach the first charge coupling device 240.

In this case, the second laser beam moved along the second path forms the first laser beam moved along the first path and a first interference fringe 1.

According to the present invention, it may include a third path formed by the third beamsplitter 214.

As shown in FIG. 2, a portion of the laser beam that reaches the third beamsplitter 214 travels along the third path through the third beamsplitter 214.

Here, the third path is a path that passes through the third beam splitter 214 and is reflected by the second mirror 222.

The third laser beam moving along the third path is reflected through the second mirror 222 after passing through the convex lens L7, and the second beam expander composed of the concave lens L5 and the convex lens L6. It passes through 234 to reach the second charge coupling device 242.

At this time, the third laser beam passing through the second beam expander 234 passes through the fourth beam splitter 216 to reach the second charge coupling device 242.

Meanwhile, a portion of the first laser beam moving along the first path passes through the second beam splitter 212 and moves to the fourth beam splitter 216, and is reflected by the fourth beam splitter 216 to reflect the second charge. A coupling element 242 is reached.

The first laser beam thus reached forms the second interference fringe with the third laser beam.

According to the present invention, the first charge coupling device 240 of the image collecting unit 204 captures the first interference fringe formed by the first and second laser beams, and the second charge coupling device 242 is formed of the first charge coupling device 240. Capture the second interference fringe formed by the first and third laser beams.

The plurality of interference fringes thus captured are provided to the wavefront aberration analyzer 206.

The wavefront aberration analyzer 206 measures the magnitude and direction of the wavefront aberration of the laser beam at once through a plurality of interference fringes using a predetermined program.

3 is a block diagram of a wavefront aberration analyzer according to the present invention.

As shown in FIG. 3, the wavefront aberration analyzer 206 according to the present invention includes an interference fringe receiver 300, a wavefront aberration magnitude calculator 302, a defocus aberration calculator 304, and a wavefront aberration direction determiner. 306 may include.

The interference fringe receiving unit 300 receives a plurality of interference fringes captured by the image collecting unit 204.

According to the present invention, in order to simultaneously measure the wavefront aberration magnitude and direction of the laser beam, the interference fringe receiving unit 300 may receive at least two or more interference fringes 400 and 402 as shown in FIG. 4.

The wavefront aberration magnitude calculator 302 determines the magnitude of the wavefront aberration for each of the plurality of interference fringes. The wavefront aberration sizing method will be described in detail below.

Thus, when calculating the wavefront numerical magnitude, there may be two solutions with different signs. Here, the two solutions are directions of wavefront aberration, as shown in FIG. 4, as two solutions 410 and 412 for the first interference fringe 400 and two solutions 414 and 416 for the second interference fringe 402, respectively. This means that they are different from each other.

In order to determine one of wavefront aberrations different from each other, the defocus aberration calculator 304 according to the present invention calculates defocus aberrations for a plurality of interference fringes, and the wavefront aberration direction determiner 306 calculates the By comparing the focus aberration, the wavefront aberration direction 420 of the laser beam to be measured is determined.

When calculating wavefront aberration through at least two interference fringes obtained by providing an additional laser beam travel path, a single measurement may determine the direction and magnitude of the wavefront aberration of the laser beam.

Hereinafter, a process of measuring the wave front aberration through the plurality of interference fringes in the wave front aberration analyzer 206 will be described in more detail.

First, a method of determining the magnitude of the wave front aberration of the laser beam with respect to the first interference fringe is described.

Referring to FIG. 2, when the electric field of the first laser beam traveling along the first path is referred to as the electric field of the second laser beam traveling along the second path, each electric field strength of the first charge coupling device is It is expressed as Equation 1 below.

는 측정하고자 하는 레이저 빔의 파면 수차이고,

는 축소된 레이저 빔 크기에서 확대된 레이저 빔이 파면 수차이다. here,

Is the wave front aberration of the laser beam to be measured,

Is the wave front aberration of the enlarged laser beam at the reduced laser beam size.

이 아니게 되고 다른 파면 수차

로 표현해야 한다. 이 때, 빔 확대기 및 빔 축소기를 통과한 빔들(제 1경로를 따라서 진행한 빔과 제 2 경로를 따라서 진행한 빔)의 크기비가 하기에서와 같이 S로 주어지게 된다.Here, referring to FIG. 2, the size of the first interference fringe (on the screen) has the same size as the beam traveling along the first path. This size becomes smaller than the size of the beam traveling along the second path, and part of the laser beam traveling along the second path, i.e., part of the wavefront, is used to form the first interference fringe. The wavefront aberration of this beam is no longer

This will not be and other wavefront aberration

Should be expressed as In this case, the size ratio of the beams (the beam traveling along the first path and the beam traveling along the second path) passing through the beam expander and the beam reducer is given as S as follows.

The intensity of the interference fringe formed by the electric fields is expressed by Equation 2 below.

는 제1 및 제2 레이저 빔의 파면 수차로부터 아래의 수학식 3과 같이 표현된다. Where the phase difference

Is expressed by Equation 3 below from wavefront aberration of the first and second laser beams.

는 제1 전하결합소자(240)에서 캡쳐된 간섭무늬로부터 계산되는 파면 수차이다.here,

Is a wavefront aberration calculated from the interference fringe captured by the first charge coupling device 240.

을 구하는 방법은 아래와 같다. Wavefront aberration of the actual laser beam from this calculated wavefront aberration

How to obtain is as follows.

,

,

을 Zernike polynomial

로 표현하면 아래의 수학식 4와 같다.Each wavefront aberration

,

,

Zernike polynomial

When expressed as shown in Equation 4 below.

Substituting Equation 4 into Equation 3 above, the wavefront aberration calculated from the interference fringe is expressed by Equation 5 below.

라면(여기서

=1/S S: magnification) 축소된 레이저 빔의 크기에서 레이저 빔의 파면 수차

는 아래의 수학식 6과 같다. At this time, the size ratio of the beam traveling along the first path and the second path (please briefly explain what the size ratio of the beam is)

Ramen (where

= 1 / SS: magnification) Wavefront aberration of the laser beam at the size of the reduced laser beam

Is the same as Equation 6 below.

는 Zernike polynomial의 radial 성분이고

는 azimuthal 성분이다. here,

Is the radial component of Zernike polynomial

Is an azimuthal component.

는 Zernike polynomial 전개를 통해서 아래의 수학식 7과 같이 정리할 수 있다. At this time,

Can be summarized as in Equation 7 through the Zernike polynomial expansion.

Equation (7) is summarized by grouping Zernike polynomials having the same radial order n and azimuthal order m and expressed by using a determinant, Equation 8 below.

Substituting the determinant into Equation 5 and reexpressing Equation 5 as a determinant is shown in Equation 9 below.

의 각각은 얻어진 제1 간섭무늬 1(interferogram1)로부터 계산된 Zernike 계수들이다. here

Each of are Zernike coefficients calculated from the obtained first interferogram 1.

According to Equation 9, Zernike coefficients are calculated from the obtained first interference fringe 1, and with the calculated Zernike coefficients, Equation 9 can be used to calculate Zernike coefficients of the original laser beam wavefront aberration, that is, the magnitude of the wavefront aberration. have.

는 양과 음의 부호를 갖는 두 가지 해가 존재한다는 것을 의미한다. 양과 음의 부호는 파면 수차의 방향이 다름을 의미한다. The magnitude of the wavefront aberration can be calculated from the interference fringe.

Means that there are two solutions with positive and negative signs. Positive and negative signs mean that the direction of the wavefront aberration is different.

According to the present invention, the second interference obtained by adding one path arm3 as described in FIG. 2 to find out which solution is the solution of the actual laser beam wavefront aberration in two different directions of the first interference fringe Use patterns.

As shown in Fig. 2, the second interference fringe is generated from the third laser beam to which arbitrarily known aberration is added using the convex lens L7.

Using this method, the newly calculated defocus (defocus) aberration using the second interference fringe (interferogram2) can be obtained through Equation 10 below.

The defocus aberration is an aberration related to the focus of the lens or the focusing mirror. When a wavefront without any aberration (that is, a plane with a constant phase of the laser beam) passes through the lens or focusing mirror, the spatial phase (that is, the wavefront) is transformed into a quadratic function, which is called defocus aberration. do.

By comparing the defocus aberration value of the second interference fringe obtained through Equation 10 with the value of the defocus aberration of the first interference fringe calculated by Equation 9, the wavefront aberration direction of the laser beam can be determined.

As described above, in the case of using the interferometer according to the present invention, at least two interference fringes are obtained, thereby calculating the magnitude of the wavefront aberration, and at the same time comparing the defocus aberrations obtained for each interference fringe, Because the direction can be determined, the wave front aberration of the laser beam can be measured at one time.

Preferred embodiments of the present invention described above are disclosed for purposes of illustration, and those skilled in the art will be able to make various modifications, changes, and additions within the spirit and scope of the present invention. Additions should be considered to be within the scope of the following claims.

As described above, according to the present invention, at least two or more interference fringes are obtained by adding one optical path to the interferometer, and the wavefront aberration is calculated for the obtained interference fringes.

In addition, according to the present invention, since two or more interference fringes are obtained, it is possible to determine not only the wavefront aberration magnitude but also the direction of the laser beam at a time through a single measurement.

In addition, according to the present invention, it is possible to accurately measure the wave front aberration of the laser beam with a single measurement, which is applied to an apparatus for measuring and correcting the wave front of the pulsed laser beam, thereby improving the quality of the pulsed laser beam. .

Claims (15)

In wavefront aberration measuring apparatus, An interferometer for dividing an incident laser beam to generate at least n (n is a natural number of two or more) interference fringes; And And a wavefront aberration analyzer configured to simultaneously determine the magnitude and direction of the laser beam wavefront aberration using the n interference fringes. The method of claim 1, And the interferometer divides the laser beam into at least k paths (k is a natural number of 3 or more). The method of claim 1, And the interferometer includes a plurality of beamsplitters, mirrors, and lenses for generating first and second interference fringes. The method of claim 1, The interferometer, A first beamsplitter for dividing the incident laser beam into first and second paths; And a second beam splitter for reflecting a portion of the laser beam moving along the first path and allowing a portion of the laser beam moving along the second path to form the first interference fringe. Wavefront aberration measurement device. The method of claim 4, wherein The interferometer, A third beam splitter configured to pass a portion of the laser beam moving along the second path through a third path; And And a fourth beam splitter for passing a laser beam moving along the third path and reflecting the laser beam passed through the second beam splitter to form a second interference fringe. Device. The method of claim 5, The interferometer, A lens for adding a predetermined aberration of the laser beam passing through the third beamsplitter; And Waveform aberration measuring apparatus further comprises a beam enlarger for enlarging the laser beam passing through the lens. The method of claim 5, The interferometer, A first mirror reflecting a laser beam passing through the first beamsplitter; And And a second mirror for reflecting the laser beam passing through the third beamsplitter. The method of claim 1, Waveform aberration measurement apparatus further comprises an image acquisition unit having n imaging elements for capturing the n interference fringes. The method of claim 8, The wavefront aberration analysis unit, An interference fringe receiving unit receiving n interference fringes from the image collecting unit; A wavefront aberration magnitude calculator for calculating a wavefront aberration magnitude for each of the n interference fringes; A defocus aberration calculator for calculating a defocus aberration for each of the n interference fringes; And a wavefront aberration direction determiner for determining a direction of wavefront aberration by comparing defocus aberrations for each of the n interference fringes. An apparatus for generating an interference fringe for measuring wave front aberration, A plurality of beam splitters for dividing a path of an incident laser beam into at least k (k is a natural number of 3 or more); At least one lens for enlarging or reducing the laser beam moving along the k paths, And the plurality of beamsplitters form at least n interference patterns (n is a natural number of two or more). The method of claim 10, The plurality of beam splitters, A first beamsplitter for dividing the incident laser beam into first and second paths; And a second beam splitter for reflecting a portion of the laser beam moving along the first path and allowing a portion of the laser beam moving along the second path to form the first interference fringe. Interference pattern generating device for measuring wavefront aberration. The method of claim 11, The plurality of beam splitters, A third beam splitter configured to pass a portion of the laser beam moving along the second path through a third path; And And a fourth beam splitter for passing a laser beam moving along the third path and reflecting the laser beam passing through the second beam splitter to form a second interference fringe. Interference pattern generating device for. The method of claim 12, A lens for adding a predetermined aberration of the laser beam passing through the third beamsplitter; And And a beam enlarger for enlarging the laser beam passing through the lens. The method of claim 12, A first mirror reflecting a laser beam passing through the first beamsplitter; And And a second mirror for reflecting the laser beam passing through the third beamsplitter. In the wavefront aberration measurement method, Incident a laser beam; Dividing the incident laser beam to form at least n (n is a natural number of two or more) interference fringes; And And determining the magnitude and direction of the laser beam wave front aberration simultaneously using the n interference fringes.

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