JP2007064965A - Method of measuring wave aberration of optical element, and method of correcting wave aberration - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To calculate system aberration including also a component not expressed by Zernike expansion. <P>SOLUTION: A method has a space distribution calculation step of wave aberration wherein each wave aberration of an inspection optical element and an auxiliary optical element is treated as a distribution of the wave aberration at each point in a space, and the space distribution of the wave aberration is calculated; a system aberration space distribution calculation step wherein each wave aberration is calculated at every time when the inspection optical element is rotated at a prescribed angle over at least 360 degrees around a measuring optical axis of the inspection optical element, and the mean value of each wave aberration is subjected to the Zernike expansion, and thereby the wave aberration from which only an approximately rotationally symmetric component is subtracted is calculated as a space distribution of the system aberration of the auxiliary optical element; and a characteristic aberration calculation step wherein the wave aberration characteristic to the inspection optical element is acquired by subtracting the system aberration calculated in the system aberration space distribution calculation step from the wave aberration calculated in the space distribution calculation process of the wave aberration. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a method for measuring wavefront aberration of an optical element using interference fringe analysis, and in particular, in an interferometer, in order to cause interference between a reference wavefront reflected from a reference surface and a test wavefront from the test surface, The present invention relates to a wavefront aberration measuring method and a wavefront aberration correcting method capable of correcting a system error caused by an auxiliary optical element or the like for forming a test wavefront from a surface in the same shape as a reference wavefront.

As a method of measuring the shape of the optical element, there is interference fringe analysis using an interferometer system. In the conventional interference fringe analysis, not only the aberration of the optical element as the measurement target but also the aberration (system aberration) due to the interferometer system, particularly the auxiliary optical element optical system that forms the wavefront to be detected in the same shape as the reference wavefront. , Internal aberration) is also measured, so that the shape of the optical element cannot be measured accurately.

On the other hand, before measuring the optical element to be measured, by calculating the system aberration using the original device and the reference surface, and then subtracting the system aberration from the measured aberration of the optical element, A method of correcting aberrations and calculating aberrations specific to the optical element has been proposed (Japanese Patent No. 3327998 (Patent Document 1)).

In this method, in order to calculate the system aberration, the reference light is emitted in a state where the original device or the reference surface is rotated by a certain angle around the optical axis of the incident light. A part of the reference light is reflected by a reference surface formed as a surface having the same shape as the design shape of the surface to be measured of the optical element to be measured. After being reflected by a surface having the same shape as the design shape of the surface to be measured of the element, it passes through the reference surface again. When Zernike expansion is performed on the data obtained by analyzing the interference fringes formed by the reflected light from the reference surface and the light transmitted through the reference surface and reflected by the original device, approximation of various aberration components as system aberrations A value can be obtained as each coefficient of Zernike.

On the other hand, in the measurement of the optical element to be measured, first, it is formed by the reflected light from the reference surface used in the calculation of the system aberration and the light transmitted through the reference surface and reflected by the measured surface of the optical element. The interference fringes that are generated are analyzed. The data obtained by this analysis is also developed in Zernike, and approximate values of various aberration components are calculated as Zernike coefficients. Subsequently, by subtracting each aberration component calculated in advance as a system aberration from each calculated aberration component, it is possible to calculate an aberration specific to the optical element excluding the system aberration.
Japanese Patent No. 3327998 Specification

However, the above-described method cannot correct components that are not expressed as coefficients after Zernike expansion (for example, high-frequency components and local components). In addition, the aberration component obtained by Zernike expansion is only an approximate value. Therefore, the system aberration and the aberration specific to the optical element cannot be separated accurately and easily, and it is difficult to exclude the system aberration from the measurement result.

In order to solve the above-described problem, the wavefront aberration measuring method for an optical element of the present invention refers to a test wavefront in order to cause interference between the reference wavefront reflected from the reference surface and the test wavefront from the test surface. A method for measuring wavefront aberration of a test optical element by analyzing an interference fringe between a reference wavefront and a test wavefront using an interferometer having an auxiliary optical element for forming the same shape as the wavefront, The wavefront aberration spatial distribution calculating step for calculating the spatial distribution of the wavefront aberration by using the wavefront aberration of the test optical element and the auxiliary optical element as the distribution of the wavefront aberration at each point in the space, and the test optical element for the test optical The wavefront aberration is calculated every time a predetermined angle is rotated over at least 360 degrees around the measurement optical axis of the element, the average value of the wavefront aberration is Zernike-expanded, and only the approximate rotational symmetry component is subtracted. Auxiliary optical element By subtracting the system aberration calculated in the system aberration spatial distribution calculation step from the wavefront aberration calculated in the system aberration spatial distribution calculation step and the wavefront aberration spatial distribution calculation step, which is calculated as a system aberration spatial distribution. And an intrinsic aberration calculating step for obtaining a wavefront aberration specific to the optical element.

The wavefront aberration correction method of the present invention is an auxiliary optical for forming a test wavefront in the same shape as the reference wavefront in order to cause interference between the reference wavefront reflected from the reference surface and the test wavefront from the test surface. A method for correcting wavefront aberration of a test optical element obtained by analyzing an interference fringe formed by a reference wavefront and a test wavefront using an interferometer having an element, comprising: A wavefront aberration spatial distribution calculating step of calculating the wavefront aberration distribution of the wavefront aberration as the wavefront aberration distribution at each point in the space as the wavefront aberration of the test optical element and the auxiliary optical element, and the test optical element The wavefront aberration is calculated every time a predetermined angle is rotated over at least 360 degrees around the measurement optical axis of the element, the average value of the wavefront aberration is Zernike-expanded, and only the approximate rotational symmetry component is subtracted. And auxiliary optical elements Wave aberration by subtracting the system aberration calculated in the system aberration spatial distribution calculation step from the wave aberration calculated in the system aberration spatial distribution calculation step and the wave aberration calculating in the wave front aberration spatial distribution step. A characteristic aberration calculating step of correcting the wavefront aberration calculated in the spatial distribution calculating step to obtain a wavefront aberration specific to the optical element to be tested.

According to the present invention, it is possible to calculate a system aberration including a component that is not expressed by Zernike expansion by a simple calculation. Thereby, the system aberration can be calculated more accurately, and the aberration specific to the optical element can be calculated with high accuracy. Furthermore, since it is not necessary to prepare the original device and the reference surface built into the design value of the optical element to be measured, the cost for measuring and correcting wavefront aberration can be reduced.

Hereinafter, embodiments according to the present invention will be described in detail with reference to the drawings. In the present embodiment, the test lens 80 is an object of aberration measurement, but the present invention can also be applied to other optical elements.

First, an aberration measuring apparatus used for carrying out the wavefront aberration measuring method and wavefront aberration correcting method of the optical element according to the present invention will be described. This aberration measuring apparatus includes an optical element holding unit 10, an interference fringe forming unit 20, and an interference fringe analyzing unit 42.

An optical element holding unit 10 shown in FIG. 1 includes a rotation holding unit 11 that holds a test lens 80 so as to be rotatable about an optical axis (hereinafter, a measurement optical axis), and a drive unit 12 that rotationally drives the rotation holding unit 11. And comprising. For example, a substantially circular (annular) stage that can be electrically rotated can be used as the rotation holding unit 11, and the measurement optical axis of the test lens 80 is at the rotation center of the disk of the rotation holding unit 11. These are arranged so as to substantially coincide with a line extending perpendicular to the disk. Therefore, when correctly held by the rotation holding unit 11, the axis of rotation coincides with the measurement optical axis.

The rotation holding unit 11 can rotate (turn) at every preset angle α (predetermined angle) with the center of the circle of the disk as the center of rotation. This angle is preferably obtained by dividing 360 degrees by an arbitrary integer. A smaller value is preferable because measurement accuracy increases, but 360 degrees is divided into 18 equal parts in consideration of measurement speed and cost. 20 degrees, 24 degrees divided into 15 parts, and 30 degrees divided into 12 parts are practical. The rotation holding unit 11 repeats the operation of temporarily stopping after rotating at a constant speed for a predetermined angle in accordance with the drive current supplied from the drive unit 12 connected thereto. The drive unit 12 supplies a predetermined drive current to the rotation holding unit 11 based on a control signal output from the control unit 43 connected thereto.

The interference fringe forming unit 20 can use a known interferometer system. In this embodiment, as shown in FIG. 1, a semiconductor laser (LD) (light source) 21 and an interference fringe observation CCD (charge coupled device) are used. 41 and an optical system 30 are used. The optical system 30 includes a collimator lens 31, a mirror 32, a half mirror 34, a condenser lens 35, a reference plane plate 36, a pinhole 37, and a reflection reference concave mirror 38. In this embodiment, the target auxiliary optical element having the system aberration to be corrected is the reflection reference concave mirror 38.

The semiconductor laser 21 is driven by a driver 22 and emits a predetermined laser beam as reference light. The driver 22 supplies a drive current to the semiconductor laser 21 based on a control signal output from the control unit 43 connected thereto. Semiconductor race. A collimator lens 31 and a mirror 32 are disposed in order from the semiconductor laser 21 side in the traveling direction of the laser light emitted from the 21. Laser light emitted from the semiconductor laser 21 is collimated by a collimator lens 31 and reflected by a mirror 32 disposed at an inclination of 45 degrees with respect to the optical path, whereby the traveling direction is bent by 90 degrees.

On the optical path of the reflected light by the mirror 32, a half mirror 34, a reference plane plate 36, a lens 80 to be tested, and a reflection reference concave mirror 38 are arranged in this order from the mirror 32 side. The light reflected by the mirror 32 is transmitted through a half mirror 34 disposed at an inclination of 45 degrees with respect to the optical path, and is incident on a reference plane plate 36.

The reference flat plate 36 is a flat glass plate whose surface is polished with high accuracy, and a reference surface 36 a is provided on a surface far from the half mirror 34. The reference surface 36a has a property of transmitting part of the light incident on the reference flat plate 36 and reflecting the rest. Using this property, it is possible to obtain interference fringes between the light reflected by the reference surface 36a and the light that has passed through the reference surface 36a and then passed through the lens to be examined.

The reflected light from the reference surface 36 a is reflected by the half mirror 34, passes through the condenser lens 35, and then enters the CCD 41 through the pinhole 37 disposed on the optical path. Instead of the CCD 41, other imaging devices (for example, CMOS (Complementary Metal Oxide Semiconductor)) can be used.

On the other hand, the light that has passed through the reference surface 36 a passes through the lens 80 to be examined, is reflected by the reflective reference concave mirror 38, and passes through the reference flat plate 36 again. The transmitted light is reflected by the half mirror 34 and then enters the CCD 41 through the condenser lens 35 and the pinhole 37. This light and the reflected light from the reference surface 36a interfere to form interference fringes. The formed interference fringes are converted into electrical signals by the CCD 41 and input to the interference fringe analysis unit 42.

The reference flat plate 36 is held by a support device (for example, an electric θ stage) 51. The support device 51 is connected to the control unit 43 via a reference plane plate driving unit 52. The reference flat plate drive unit 52 that has received the drive signal output from the control unit 43 supplies a predetermined drive current to the support device 51, whereby the support device 51 uses the optical axis of the reference flat plate 36 as a reference. It can be inclined by an arbitrary angle (inclination angle) θ with respect to the traveling direction of incident light on the flat plate 36. By inclining the reference plane plate 36 (reference surface 36a) in this way, interference fringes (space carriers, tilt fringes) corresponding to the inclination angle are generated, and the reference plane plate 36 is not inclined. By analyzing the interference fringes obtained by superimposing the interference fringes representing the shape of the lens 80 to be inspected at times, the wavefront aberration of the lens 80 to be examined can be obtained.

As shown in FIG. 2, the interference fringe analysis unit 42 includes an A / D converter 421, a frame memory 422, a D / A converter 423, and a calculation unit 424 therein. For example, a personal computer can be used as the interference fringe analysis unit 42, but the A / D converter 421, the frame memory 422, the D / A converter 423, and the calculation unit 424 are configured as independent devices. Also good.

Signal charges (interference fringe image signals) accumulated in each pixel are sequentially input from the CCD 41 to the A / D converter 421. In the A / D converter 421, each input analog signal is converted into a digital signal (interference fringe image data). The interference fringe image data is stored at an address corresponding to the pixel of the CCD 41 in the frame memory 422 connected to the A / D converter 421.

The interference fringe image data stored in the frame memory 422 is converted into an analog signal by the D / A converter 423 connected to the frame memory 422. The converted analog signal is displayed on an output unit 44 (for example, a monitor, a CRT display, a liquid crystal display, or a printer) provided outside the interference fringe analysis unit 42. Conventional interference fringe analysis can be performed by the display on the output unit 44.

On the other hand, a calculation unit 424 is also connected to the frame memory 422. The arithmetic unit 424 can determine the wavefront aberration of the lens 80 or the like by analyzing the interference fringe image data output from the frame memory 422 using the spatial phase shift method described below.

A control unit 43 is connected to the calculation unit 424, and an input unit 45 (for example, a keyboard and a mouse), a reference plane plate driving unit 52, and the driving unit 12 are connected to the control unit 43. In the control unit 43, the attitude of the reference plane plate 36 is changed with respect to the reference plane plate drive unit 52 or the drive unit 12 based on a signal input from the calculation unit 424 (interference fringe analysis unit 42) or the input unit 45. Output necessary control signals. Based on this control signal, the reference plane plate driving unit 52 supplies a driving current necessary for changing the attitude of the reference plane plate 36 to the support device 51. The drive unit 12 supplies a drive current necessary for rotating the rotation holding unit 11 with respect to the measurement optical axis of the lens 80 based on a control signal from the control unit 43. In addition, the wavefront aberration of the lens 80 to be obtained obtained by analysis using the spatial phase shift method is stored in the storage unit 46 connected in the control unit 43.

と表せる。ここで、ＷＡは被測定光学素子の波面収差、ＷＢは補助光学素子の波面収差、添字のａはｚｅｒｎｉｋｅ展開で表すことができるコマ収差、非点収差、高次のコマ収差、高次の非点収差等非回転対称成分（以下非回転対称成分）、ｓはｚｅｒｎｉｋｅ展開で表すことができるデフォーカス、球面収差、高次の球面収差等の回転対称成分、（以下近似回転対称成分）、ｉｒはｚｅｒｎｉｋｅ展開で表すことができないイレギュラーな成分（高周波成分や局所的な成分）を示す。被検レンズ８０と反射基準凹面鏡３８の間に、十分な面精度且つ平行度を有する適切な平行平板を入れると、ＷＢｓを容易に打ち消すことが出来るので、本実施形態ではこれを補正対象外とし、ここで得られる波面収差の空間分布を、

とする。
保存された３６０度分の波面収差の空間分布は、演算部４２４により、すべて加算された後に測定されたデータの数で除することにより、平均値が算出される。この平均化された波面収差の空間分布をＷＳ（ｘ，ｙ）とすると、

と表される。
ここで、被検レンズ８０の非回転対称成分は、回転に伴い、加算平均処理しているため、その成分は打ち消し合し、その値は０と見なせる。また、ｉｒの成分は加算平均処理しているため、打ち消しあい、その値は０と見なせる。また、回転対称成分は、回転に伴い、不変である。故に加算平均処理で、単なる平均化となる。よって各項は以下のように表せる。

一方、反射基準凹面鏡３８は被検レンズ８０を回転に対して、不動であるので、加算平均により打ち消しあうことはなく、単なる平均化となる。

よって、この平均化された波面収差の空間分布は、以下のように表せる。

前記の平均化された波面収差の空間分布ＷＳ（ｘ，ｙ）をＺｅｒｎｉｋｅ展開し、これを分解する。

Ｚｅｒｎｉｋｅ多項式は直交多項式であるため、ここから近似回転対象成分のみを減算することができ、これを減算したシステム収差の空間分布ＷＳ’（ｘ，ｙ）は

と表せる。
ｎ＝０の位置における被検レンズ８０固有の収差ＷＴはＷ０−ＷＳ’で表すことができる。

したがって、３６０度に渡って算出した波面収差の空間分布より簡単な演算で、被検レンズ８０固有の収差を求めることができる。 In the present embodiment, in order to calculate the system aberration, the above-described interference fringe analysis is performed each time the lens 80 to be measured is rotated by a predetermined angle over 360 degrees around the measurement optical axis. it can. This interference fringe analysis may be performed over 360 degrees. Thus, the wavefront aberration is obtained n times for each rotation of 360 / n degrees, and the wavefront aberration thus obtained is stored in the storage unit 46. When the spatial distribution of this wavefront aberration when n = 0 is W0 (x, y),

It can be expressed. Here, WA is the wavefront aberration of the optical element to be measured, WB is the wavefront aberration of the auxiliary optical element, the subscript a is coma aberration, astigmatism, high-order coma aberration, Non-rotationally symmetric components such as point aberration (hereinafter referred to as non-rotationally symmetric components), s is a rotationally symmetric component such as defocus, spherical aberration, higher order spherical aberration, etc. Indicates irregular components (high-frequency components and local components) that cannot be represented by zenike expansion. If an appropriate parallel plate having sufficient surface accuracy and parallelism is inserted between the test lens 80 and the reflective reference concave mirror 38, WBs can be easily canceled out. In the present embodiment, this is excluded from correction. , The spatial distribution of wavefront aberration obtained here,

And
An average value is calculated by dividing the spatial distribution of the stored wavefront aberration for 360 degrees by the number of data measured after all are added by the calculation unit 424. If the spatial distribution of the averaged wavefront aberration is WS (x, y),

It is expressed.
Here, since the non-rotationally symmetric component of the test lens 80 is subjected to addition averaging processing with rotation, the components cancel each other and the value can be regarded as zero. Further, since the ir component is subjected to the averaging process, it is canceled out and the value can be regarded as zero. The rotationally symmetric component is invariant with rotation. Therefore, the averaging process is simply an averaging process. Therefore, each term can be expressed as follows.

On the other hand, the reflection reference concave mirror 38 does not move with respect to the rotation of the lens 80 to be examined, and therefore does not cancel each other out by addition averaging and is simply averaged.

Therefore, the spatial distribution of the averaged wavefront aberration can be expressed as follows.

The averaged wavefront aberration spatial distribution WS (x, y) is Zernike-expanded and decomposed.

Since the Zernike polynomial is an orthogonal polynomial, only the approximate rotation target component can be subtracted from this, and the system aberration spatial distribution WS ′ (x, y) obtained by subtracting this is

It can be expressed.
The aberration WT inherent to the test lens 80 at the position of n = 0 can be represented by W0−WS ′.

Therefore, the aberration inherent to the lens 80 can be obtained by a simple calculation from the spatial distribution of the wavefront aberration calculated over 360 degrees.

Therefore, the wavefront aberration calculated over 360 degrees is averaged, and after Zernike expansion, the approximate rotational symmetry component is subtracted, and a simple calculation of subtracting the approximate rotational symmetry component eliminates most of the aberration component specific to the lens 80 to be measured. It can be obtained as a spatial distribution. The spatial distribution of the system aberration is subtracted from the spatial distribution of wavefront aberration (data including both the reflection reference concave mirror 38 and the aberration specific to the test lens 80) obtained as a result of interference fringe analysis of the test lens 80 at a certain angle. By doing so, the aberration specific to the lens 80 can be calculated more accurately.

Further, since the spatial distribution of the system aberration is calculated using the spatial distribution of the wavefront aberration, it is possible to remove high frequency components that cannot be expressed by the Zernike expansion. Zernike expansion is performed to subtract the rotationally symmetric component, but the rotationally symmetric component to be subtracted is only an approximate value, and the spatial distribution of system aberrations holds high-frequency components that cannot be expressed by the Zernike expansion. . Further, by obtaining a spatial distribution of wavefront aberration over at least 360 degrees, it is possible to calculate a local component that appears in the measurement in a specific angle range, and to remove this to obtain a more accurate aberration specific to the lens 80 to be measured. it can. Zernike expansion is performed to subtract the rotationally symmetric component, but the rotationally symmetric component to be subtracted is only an approximate value, and local components that are not expressed in the Zernike expansion are retained in the spatial distribution of system aberrations. ing.

When the aberration measurement is performed on a plurality of test lenses 80 having the same design value, the interference fringe analysis for calculating the system aberration may be performed only on the test lens 80 to be measured first. You may perform about the lens 80.

Here, an analysis method using the spatial phase shift method will be described.
When the optical axis of the reference plane plate 36 is tilted by an angle θ (an angle depending on the wavelength λ of the semiconductor laser 21) with respect to the traveling direction of the incident light on the reference plane plate 36, the frame memory is calculated by the calculation unit 424. Based on the interference fringe image data output from 422, if attention is paid to one period with interference fringes, a spatial signal intensity distribution I (x) represented by the following expression A can be calculated. In Formula A, y is omitted for one-dimensional display.
<Formula A>
I (x) = a + b · cos [φ + 2πνx]
Here, a is the average intensity distribution, b is the amplitude, ν is the spatial frequency of the spatial carrier, and φ is the initial phase at a certain point.

この式により被検レンズ８０の各点における位相を求めることができ、２次元的にこれを繰り返せば、被検レンズ８０等の波面収差を定量的に求めることができる。 Regarding the formula (A), assuming that a and b have constant values, the initial phase φ is expressed by the following formula B where the signal strengths of the four pixels described above are I ₀ , I ₁ , I ₂ , and I ₃ , respectively. It is represented by
<Formula B>

With this equation, the phase at each point of the test lens 80 can be obtained, and by repeating this two-dimensionally, the wavefront aberration of the test lens 80 and the like can be obtained quantitatively.

As a method for calculating the initial phase, a temporal phase shift method (fringe scan method) can be used instead of the spatial phase shift method. In the temporal phase shift method, the distance between the reference plane plate 36 and the test lens 80 is moved in the direction of the optical axis by the amount scanned by one period (2π) of the interference fringes, and at the same time, the interference fringes are 2π / N. Every time (N is an integer) is scanned, an image is captured a total of N times, and the initial phase is calculated from the change in brightness of each image.

The spatial distribution of system aberrations can be expanded by Zernike in the calculation unit 424, and aberration components represented by the respective coefficients can be used. When there is noise in the spatial distribution of system aberrations, it is preferable because noise can be removed by performing Zernike expansion.

Next, the flow of the system aberration calculation step and the wavefront aberration spatial distribution calculation step of the lens 80 to be examined will be described. In the following description, after the system aberration spatial distribution calculating step is executed, the wavefront aberration of the test lens 80 is measured, and then the wavefront aberration specific to the test lens 80 is calculated. The wavefront aberration may be measured, and then the system aberration spatial distribution calculating step may be executed, and then the wavefront aberration specific to the lens 80 to be measured may be calculated.

(1) System aberration spatial distribution calculation step (FIG. 3)
First, of the light vertically incident on the reference plane plate 36, the light that has passed through the reference plane plate 36, reflected by the reflective reference concave mirror 38, and again transmitted through the reference plane plate 36, and the light reflected by the reference plane plate 36 The components of the semiconductor laser 21, the CCD 41, and the optical system 30 are arranged so as to follow the same optical path and enter the CCD 41, and the operation of interference fringe analysis for system aberration calculation is started (step S 100). ). Since it is confirmed by a well-known method whether or not it has entered along the same optical path, the description thereof is omitted here.

Next, an angle α (predetermined angle) for rotating (rotating) the lens 80 to be measured is set for each interference fringe formation around the optical axis of the incident light (step S101). The operator operates the input unit 45 to set a desired angle α (predetermined angle) (for example, 30 degrees) or a desired division number D (for example, when the predetermined angle is 30 degrees, 360 degrees is set to 30 degrees). Enter divided 12). The input numerical value is stored in the storage unit 46. Here, zero (n = 0) is stored in the storage unit 46 as an initial value of the number n of rotations by a predetermined angle. n increases by 1 every time the test lens 80 is rotated by a predetermined angle, and when n = D, the test lens 80 rotates 360 degrees and returns to the initial position.

Subsequently, when the operator operates the input unit 45 to input the tilt angle value θ, the control unit 43 outputs a control signal to the reference plane plate drive unit 52, and the reference plane plate drive unit 52 that receives the control signal supports it. A drive current necessary for the reference flat plate 36 to be inclined by an angle θ with respect to the optical path of the incident light is supplied to the device 51 (step S102). As a result, the reference flat plate 36 is inclined at an angle θ with respect to the optical path of the incident light.

Next, the reference light is emitted from the semiconductor laser 21 by operating the driver 22 (step S103). The emitted light is reflected by the mirror 32 through the collimator lens 31, passes through the half mirror 34, and enters the reference flat plate 36. A part of the incident light is reflected by the reference surface 36 a of the reference plane plate 36, reflected by the half mirror 34, and then enters the CCD 41 through the condenser lens 35 and the pinhole 37. The light that has not been reflected by the reference surface 36a passes through the test lens 80 and then enters the reflection reference concave mirror 38. If the test lens 80 is correctly arranged, the light is reflected so as to follow the same optical path as the incident light. The reflected light from the reflective reference concave mirror 38 passes through the test lens 80 and the reference flat plate 36 again, is reflected by the half mirror 34, and then enters the CCD 41 through the condenser lens 35 and the pinhole 37 (step S104). ). As described above, the CCD 41 receives the reflected light from the reference surface 36a and the light that has been transmitted through the reference plane plate 36 and then transmitted through the lens 80 and reflected by the reflective reference concave mirror 38. Interference fringes are formed by light.

The formed interference fringes can be analyzed by display on the output unit 44, and detailed and quantitative analysis can be performed by calculating the phase in the interference fringe analysis unit 42 (step S105). The wavefront aberration obtained as an analysis result is stored in the storage unit 46.

After analyzing the interference fringes, the control unit 43 determines whether or not the test lens 80 is rotated 360 degrees. This determination is made based on whether or not the number of rotations n stored in the storage unit 46 is equal to the division number D (step S106). When it is not rotated 360 degrees, that is, when n is smaller than D (NO in step S106), the test lens 80 is further rotated by a predetermined angle, and n is incremented by 1 and stored in the storage unit 46. (Step S107). Thereafter, formation of interference fringes (step S103), incidence on the CCD (step S104), and interference fringe analysis (step S105) are performed again.

When the control unit 43 determines that the test lens 80 has rotated 360 degrees, that is, when n is equal to D (YES in step S106), the average of the wavefront aberration stored for each rotation of a predetermined angle A value obtained by subtracting the approximate rotational symmetry component from the value (a spatial distribution of system aberration) is calculated (step S108). The calculated value is stored in the storage unit 46, and the operation ends (step S109).

(2) Wavefront aberration spatial distribution calculation step (wavefront aberration spatial distribution step, intrinsic aberration calculation step) of the test lens 80 (FIG. 4)
After the system aberration calculation step is completed, the test lens 80 to be measured is fixed on the rotation holding unit 11, and the wavefront aberration space distribution calculation step of the test lens 80 is started (step S200).

First, the reference light is emitted from the semiconductor laser 21 by operating the driver 22 (step S201). The emitted light is reflected by the mirror 32 through the collimator lens 31, passes through the half mirror 34, and enters the reference flat plate 36. A part of the incident light is reflected by the reference surface 36 a of the reference plane plate 36, reflected by the half mirror 34, and then enters the CCD 41 through the condenser lens 35 and the pinhole 37. The light that has not been reflected by the reference surface 36a passes through the test lens 80 and then enters the reflection reference concave mirror 38. If the test lens 80 is correctly arranged, the light is reflected so as to follow the same optical path as the incident light. The reflected light from the reflective reference concave mirror 38 passes through the test lens 80 and the reference flat plate 36 again, is reflected by the half mirror 34, and then enters the CCD 41 through the condenser lens 35 and the pinhole 37 (step S202). ). As described above, the CCD 41 receives the reflected light from the reference surface 36a and the light that has been transmitted through the reference plane plate 36 and then transmitted through the lens 80 and reflected by the reflective reference concave mirror 38. Interference fringes are formed by light.

The formed interference fringes can be analyzed by display on the output unit 44, and detailed and quantitative analysis can be performed by calculating the phase in the interference fringe analysis unit 42 (step S203). The spatial distribution of wavefront aberration obtained as an analysis result is stored in the storage unit 46 (wavefront aberration spatial distribution calculation step).

After analyzing the interference fringes, the arithmetic unit 424 saves the average value of the wavefront aberration from the spatial distribution of the wavefront aberration obtained in this step (the step of calculating the wavefront aberration spatial distribution of the lens 80 to be tested) for each rotation of a predetermined angle. Then, the value obtained by subtracting the approximate rotational symmetry component (system aberration spatial distribution) is subtracted (step S204). As a result, the wavefront aberration specific to the lens 80 can be calculated (inherent aberration calculation step). The result of the subtraction is stored in the storage unit 46, and the wavefront aberration spatial distribution calculation step ends (step S205).

As described above, in the present embodiment, the system aberration spatial distribution obtained by subtracting the approximate rotational symmetry component from the average value calculated in the wavefront aberration spatial distribution calculation step and the system aberration spatial distribution calculation step of the lens 80 to be tested is used. System aberrations are excluded. Therefore, components (for example, high frequency components and local components) that cannot be expressed by Zernike expansion can be corrected, and therefore the wavefront aberration inherent to the lens 80 can be calculated easily and with high accuracy.

Although the present invention has been described with reference to the above embodiment, the present invention is not limited to the above embodiment, and can be improved or changed within the scope of the purpose of the improvement or the idea of the present invention.

It is a figure which shows the structure of the aberration measuring apparatus used for the aberration measuring method and aberration correction method which concern on embodiment of this invention. It is a figure which shows the structure of the interference analysis part which concerns on embodiment of this invention. It is a flowchart which shows the flow of the system aberration calculation step which concerns on embodiment of this invention. It is a flowchart which shows the flow of the wavefront aberration calculation step which concerns on embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical element holding | maintenance part 11 Rotation holding | maintenance part 12 Adjustment drive part 20 Interference fringe formation part 21 Semiconductor laser 30 Optical system 36 Reference plane board 36a Reference surface 38 Reflection reference concave mirror 41 CCD
42 Interference fringe analysis unit 43 Control unit 44 Output unit 45 Input unit 46 Storage unit 80 Test lens (optical element)
422 Frame memory 424 arithmetic unit α rotation angle (predetermined angle)
θ Inclination angle

Claims (2)

In order to cause interference between the reference wavefront reflected from the reference surface and the test wavefront from the test surface, an interferometer having an auxiliary optical element for forming the test wavefront in the same shape as the reference wavefront is used. A method for measuring wavefront aberration of the optical element to be detected by analyzing interference fringes between the reference wavefront and the wavefront to be detected,
A wavefront aberration spatial distribution calculating step for calculating the wavefront aberration of the test optical element and the auxiliary optical element as a distribution of wavefront aberration at each point in space;
Every time the test optical element is rotated by a predetermined angle over at least 360 degrees around the measurement optical axis of the test optical element, a wavefront aberration is calculated, and an average value of the wavefront aberrations is calculated as follows: Zernike expansion, subtracting only the approximate rotational symmetry component, and calculating a system aberration spatial distribution calculating step as a system aberration spatial distribution of the auxiliary optical element;
An intrinsic aberration calculating step for obtaining a wavefront aberration specific to the optical element to be measured by subtracting the system aberration calculated in the system aberration spatial distribution calculating step from the wavefront aberration calculated in the wavefront aberration spatial distribution calculating step; ,
A wavefront aberration measuring method for an optical element, comprising: In order to cause interference between the reference wavefront reflected from the reference surface and the test wavefront from the test surface, an interferometer having an auxiliary optical element for forming the test wavefront in the same shape as the reference wavefront is used. A method for correcting wavefront aberration of a test optical element obtained by analyzing interference fringes formed by the reference wavefront and the test wavefront,
A wavefront aberration spatial distribution calculating step of calculating the wavefront aberration of the test optical element and the auxiliary optical element as a distribution of wavefront aberration at each point in space;
Each time the test optical element is rotated by a predetermined angle about at least 360 degrees around the measurement optical axis of the test optical element, a wavefront aberration is calculated, and an average value of the wavefront aberrations is calculated. Zernike expansion, subtracting only the approximate rotational symmetry component, and calculating a system aberration spatial distribution calculating step as a system aberration spatial distribution of the auxiliary optical element;
The wavefront aberration calculated in the wavefront aberration spatial distribution calculating step is corrected by subtracting the system aberration calculated in the system aberration spatial distribution calculating step from the wavefront aberration calculated in the wavefront aberration spatial distribution calculating step. An intrinsic aberration calculating step for obtaining a wavefront aberration specific to the optical element to be tested;
A wavefront aberration correction method for an optical element, comprising:

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