JPS6225226A - Method for measuring shape of wave front - Google Patents

Abstract

PURPOSE:To provide a common path for two beams so as to be hardly influenced by disturbance and to reduce size by using a double refractive polarizer for separating measuring light to two beams and making the two beams coherent with each other by using an analyzer. CONSTITUTION:The measuring light is made incident on the double refractive polarizer (Rochon prism) 100 by which normal light LN and abnormal light LA inclined at a slight angle theta0 from the normal light LN are made. The normal light LN and abnormal light LA are passed through the analyzer 200 disposed in such a manner that the crystal directions are made 45 deg. with the polarization direction of the respective light, by which the light are made coherent with each other. The wave fronts WA(x) and WB(x) made by these two beams are interfered with each other on a photodetection surface 300 of an area sensor. The interference fringes are subjected to Fourier transform and after the inclination components are removed therefrom, the fringes are subjected to reverse Fourier transform. The phase difference known from the result thereof is subjected to the prescribed calculation including the integration in the transverse shift direction, by which the wave front shape of the wave front to be measured is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a wavefront shape measuring method, and more particularly to a wavefront shape measuring method using Fourier transform.

(Prior art) It is possible to know the performance of a lens and the surface shape of an object by knowing the wavefront shape of the light flux transmitted through the lens and the light flux reflected on the object surface. As one of the measurement methods.

The measurement light having a 6i11 constant slope plane is separated into two light beams whose traveling optical paths are inclined by a small angle to each other within a predetermined plane, and these two light beams are incident on the area sensor in a state where they are laterally shifted by a small distance from each other within the predetermined plane. Interference fringes measured by an area sensor? Fourier transform is performed, the tilt component is removed, f, then inverse Fourier transform is performed, and for the phase difference known from the first result, the integral in the lateral shift direction? It has been proposed (Japanese Patent Application No. 60-56215) to obtain the wavefront shape by performing predetermined calculations including the above.

Since the present invention relates to the improvement of such a wavefront shape measurement method, the outline of the above wavefront shape measurement method will be briefly explained below, and the problems to be solved by the present invention will also be explained. .

In Figure 6, reference numeral 10 indicates a beam splitter, reference numerals 12 and 14 indicate a plane mirror, and reference numeral 16 indicates an area sensor. Further, in the drawing of the fang 1, the X direction and the X direction are defined as shown in the figure. The mirror surface of the plane mirror 12 is perpendicular to the X direction, but the mirror surface of the plane mirror 14 is tilted by −0 degrees with respect to the X direction.

Now, when measurement light having a measurement wavefront W enters from the beam splinter 101Cx direction as shown in FIG. 6, this measurement light is decomposed into two paths by the beam splitter 10. Among these optical paths, the lf path reflected by the plane mirror 14 is inclined by a small angle θ in the xy plane with respect to the optical path reflected by the plane mirror 12 after passing through the beam splinter 10 .

In other words, beam splitter 10, plane mirror 12.1
The optical system based on 4 Vc splits the measurement light into two light beams whose traveling optical paths are tilted by a small angle θ to each other within a predetermined xy plane. Then, when the area sensor 1 is shifted horizontally by a small distance S,
6. Distance between plane mirror 14 and area sensor 16? Assuming L as shown in the figure, the deviation S is S = L tanθ (1).

The wavefront shape f of the light beam reflected by the plane mirror 12.14 on the area sensor 16 is a function of Xy in the plane,
Let WA(x) and WB(x).

Figures 3 and 4 show enlarged views of WA(x) and WB(x). W B (x) is for WA(x)K,
It moves by S to the square rotation of the X axis, and moves by a minute angle θ to about 1. Considering that the angle θ is the opponent's hand, WB(x
) uses WA(x) (・te, WB(x) = WA(
x+S) + 2rcfox (2). However, fO is fo = -tanθ (3)λ, where λ is the wavelength.

Here, in order to express the interference fringes due to wavefronts WA(x) and WB(x), the complex amplitude distribution of WA(x) and WB(x) is expressed as
The intensity distribution of the interference fringes on the rear sensor 16 is 1,!, where g(X) is the intensity distribution. i'(5:)=a(x)+b(x)cos[
WB(X)-WA(X)), which can be written as. Here, a(X) = α(X)+
β2(x), D(X) = 2α(X)β(X)
It is and is. △Wfi-(x) is the wavefront WA
(x) It is the phase difference between the wavefront that has been translated by S in the positive direction of %7.1% and the wavefront WA(X), and this ΔWA(x
) is hereinafter referred to as phase difference.

Perform Fourier transform on both sides of equation (4), and 1. Write the debt on the arrow number 5.

* G(,f) = Aket) + C(,f-fo) +
CCf+fO) Now, to know the wavefront shape, the phase difference △W
A(x)? It's a must to know. Then, in equation (5),
Containing information about ΔWA(X), c(f
-* fo ) and C(f + fO), and it is sufficient to use two of only one of them. Therefore, spear 1 of spear (5) style
By filtering out the term and terms 3-6, we get C(f-fo
) is obtained. Here, fO is related to the tilt angle θ, as is clear from equation (3). Therefore, C
(f - fO) in the frequency space (・tef,
When C(, t) is obtained by shifting the angle, the slope component is removed in CCf).

Therefore, by performing inverse Fourier transform on this c<y), C(x
), but the phase part of this C(x) is the phase difference Δ
WA(x), and we can know the phase difference WA(x)l by.

As stated earlier, the phase difference △WA(x) is ΔWA(x
) = WA (X + S) - WA(x), so S Eix. Therefore, the wavefront shape WA(x) to be determined is the phase)
This can be done by inverting the calculation for the contract ΔWA(x).

That is, it is sufficient to integrate the phase difference ΔWA(X)& in the lateral deviation direction and divide this by the lateral deviation amount S. Next, the entire measurement system is rotated 90 degrees around the incident axis of the measurement light, a wavefront shape of 1 rai/min is obtained by the above process, and on top of this shape, the troughs obtained earlier are By superimposing the wavefront shape of the line, τ and the two-dimensional shape of the wavefront can be determined.

Figure 5 shows the wavefront shape actually measured using the above measurement method.
Give an example.

The problems to be solved by the present invention are as follows.

That is, in the measurement optical system shown in Fig. 6, a beam splinter and two plane mirrors are used to separate the measurement light into two light beams whose traveling optical paths are located at minute corners of each other within a predetermined plane. There is. For this reason, the optical paths of the two beams separated by the beam splitter until they return to the beam splitter are separate optical paths, which are susceptible to disturbances.

That is, the relative positional relationship between the beam splitter and the two plane mirrors is likely to change due to disturbances such as temperature changes and imaging, and errors are likely to occur in the lateral shift amount s and the tilt angle θ.

(Purpose) The present invention has been made in view of the above-mentioned circumstances, and its purpose is to reduce the influence of external disturbances.
The object of the present invention is to provide a wavefront shape measurement method.

(composition〕 The present invention will be explained below.

The invention! The parts that I am disappointed in are listed below.

In other words, a birefringent polarizer is used to separate a constant light of u into two light beams whose traveling optical paths face each other at a small angle within a predetermined plane.
t, the bundles are made coherent with each other by an analyzer.

In this way, the two parts separated by the birefringent polarizer
The optical path of the light beam becomes a so-called common path.

Hereinafter, explanation will be given based on a specific example of the address.

In Fig. 1 showing a single-fruited tube of the present invention, the reference numeral 10 indicates a Longyong prism as a birefringent polarizer, and the reference numeral 200 indicates an analyzer. The reference numeral 300 indicates the light receiving surface of the area sensor.

When the measurement light enters the Ro/Yo/Zrism 100 from the left, the ordinary light LN due to the birefringence of the O-John prism 100
is transmitted straight through this, and the abnormal light LN is transmitted and emitted in a direction tilted by θ with respect to the ordinary light LN.
odor ℃, predetermined minute angle θ.

When the area sensor's light-receiving surface 5UO shifts horizontally by an amount of So, that is, tanOo.L.

The branching point between Kuzuko L and the abnormal Motomi and Uke 5 are 600
is the distance between However, the ordinary light and the extraordinary light do not interfere with each other as they are, so the analyzer 200 is arranged so that the crystal direction of the analyzer 200 forms an angle of 45° with respect to the polarization direction of the ordinary light and the extraordinary light, and the polarization direction of each is adjusted. Together, these two beams of light can interfere with each other.

In Fig. 2 showing another positive example of the present invention, reference numeral 110
indicates a Wollaston prism as a birefringent polarizer, and numerals 200 and 30q indicate an analyzer and an area sensor light-receiving surface, as in Figure 1.

The measurement light is provided by a Wollaston prism 110.

They are separated from each other by tilting by a small angle θ1, are made coherent with each other by the analyzer 200, and interfere on the light receiving surface 600.

At this time, the wave surface WA1. (x), the lateral shift amount S1 of WBI(x) is determined by the branch point of ordinary light and extraordinary light and the light receiving surface 30.
As the distance between tJ and Ll.

It is given by θ1 S4 = 2 jan-. In addition, the wave surface W
A1(X) is the original wave surface WA(x) to be measured.
(Fig.) is tilted by -θ1 with respect to the original axis, but this tilt can be easily corrected in the compensation process.

It is possible to obtain the desired wavefront shape WA(x)Y.

In addition, to measure the five-dimensional shape of the wavefront WA,
]・Is it horizontal in Figure 1 and Figure 2? , in the direction perpendicular to the drawing (・), but for this purpose α
(The birefringent polarizer is a 7-yon prism 1.)
00 or only the Wollaston prism 110 can be rotated 90 degrees around the optical axis. of course,
You can also rotate the entire optical system (・. In addition, the separation angle is 0° (
(Figure 1) and θ1 (Figures J and 2) are the prism angles φ. (1st
By adjusting φ1 (Fig. 2) and θ), the angle can be set to a perfect angle.

Furthermore, in order to equalize the optical power of the ordinary light and the extraordinary light, it is necessary to use a birefringent polarizer (τ), which is the angle of the incident measurement light, or to It is sufficient to use linearly polarized light with .

(Effects) According to the present invention as described above, a novel wavefront shape measuring method can be provided. In this method, the two feet are separated and served.
Since the luminous flux becomes a common path, is it affected by disturbance? Extremely difficult to accept. Furthermore, the measurement optical system can be made more compact.

[Brief explanation of drawings]

Figure 3.1 is a diagram for explaining one example of winding of the present invention, Figure 2 is a diagram for explaining another example of winding of the present invention, 1.6
Figures 5 through 5 are diagrams for explaining the prior art. 100... Mouth/Yon prism, 110... Wollaston prism, 200... Analyzer, light receiving surface of 600 mountain area sensor, WA(x)... Wave surface Atsushi 4

Claims (1)

[Claims] A measurement light beam having a measurement wavefront is separated into two light beams whose traveling optical paths are inclined at a small angle to each other within a predetermined plane, and these two light beams are
The interference fringes measured by the area sensor are made incident on the area sensor in a state in which they are shifted laterally by a small distance from each other within the predetermined plane, and the interference fringes measured by the area sensor are Fourier transformed, the tilt component is removed, and then the inverse Fourier transform is performed. In the wavefront shape measurement method, which obtains the wavefront shape of the measurement wavefront by performing predetermined calculations, including integration in the lateral shift direction, on the phase difference, a birefringent polarizer is used to direct the measurement light so that the traveling optical path A method for measuring a wavefront shape, which is characterized in that the two light beams are separated into two light beams tilted by a small angle to each other within the beam, and the thus separated two light beams are made coherent with each other by an analyzer.