JPH047447B2 - - Google Patents

Description

[Detailed description of the invention] (Technical field) The present invention relates to a wavefront shape measuring device.

(Prior art) A measurement wavefront whose shape is to be measured and a reference wavefront that is shifted horizontally with respect to the traveling direction of the light are created, and the optical path length of the light that creates one wavefront is changed.
A wavefront shape measurement method is intended in which the phase difference between both wavefronts is measured at each point in their interference region, and the wavefront shape of the measured wavefront is specified by adding the phase differences.

FIG. 1 shows an example of a proposed object shape measuring device using such a wavefront shape measuring method.

Hereinafter, the outline of the wavefront shape measurement method will be briefly explained based on this example of the apparatus, and the problems to be solved by the present invention will also be explained.

In Figure 1, numeral 1 is a laser light source, numeral 1 is a laser light source;
L ₁ and L ₂ are collimator lenses, 2 and 3 are beam splitters, 4 is a plane mirror, L ₃ is an illumination lens, 5 is a piezoelectric element, 6 is a parallel plate, and 7 is a beam splitter ivy, sign
L ₄ is an imaging lens, 9 is an area sensor, and 10 is an object to be measured.

The laser light emitted from the laser light source 1 is
The collimator lenses L ₁ and L ₂ make it a parallel beam of light, and after passing through the beam splitters 2 and 3,
After passing through the lighting lens L ₃ and condensing the light,
The light beam becomes a diverging light beam, enters the shape measurement surface of the object to be measured, and is reflected by the shape measurement surface. This reflected light passes through lens _L3 again in the opposite direction,
The beam splitter 3 separates the beam into two beams.

One of the separated light beams reaches the light receiving area of the area sensor 9 via the beam splitters 2 and 7 and the imaging lens _L4 . The other part of the separated luminous flux is
The light passes through the plane mirror 4, parallel plate 6, beam splitter 7, and imaging lens _L4 , and reaches the light receiving area of the area sensor 9.

Now, when the system of lens _L3 and imaging lens _L4 is used as an imaging system, and the object to be measured 10 and the light-receiving area of the area sensor 9 are connected in an image-forming relationship, each luminous flux in the light-receiving area is The wavefront shape is similar to the shape of the measurement surface of the object to be measured 10. It goes without saying that the magnification of the imaging system gives the ratio of the size of the shape measurement surface and the wavefront shape.

Therefore, by measuring the above wavefront shape,
The shape of the shape measurement surface of the object to be measured 10 can be specified.

Here, some words used in the following explanation will be explained.

Light that contains information about the wavefront shape of the wavefront whose shape is to be measured, and which has not yet been divided into two beams, will be referred to as information light.

The information light is split into two beams to obtain two wavefronts. Any one of these two beams is used as the measurement light,
The other light is called the reference light. The wavefront given by the measurement light is called a measurement wavefront, and the wavefront given by the reference light is called a reference wavefront.

Returning to FIG. 1, the reflected light from the object to be measured 10 is information light. This information light is separated by a beam splitter 3 into two light beams, that is, a measurement light and a reference light. It is completely arbitrary to call either the measurement light or the reference light, but here, for convenience, the light is transmitted from the beam splitter 3 to the beam splitter 2, 7, and the imaging lens _L4 to the area sensor. The light reaching 9 will be called the measurement light, and the light that will reach the area sensor 9 from the beam splitter 3 via the plane mirror 4, the parallel plate 6, the beam splitter 7, and the imaging lens _L4 will be called the reference light.

When the reference light passes through the parallel plate 6, its traveling direction is shifted by a small distance in the lateral direction.
Therefore, as shown in FIG.
Interference fringes 2-3 appear due to interference in the overlapping portions that are shifted from each other and overlapped. The area where the interference fringes 2-3 appear is called an interference area.

Note that the area sensor 9 is a solid-state image sensor in which light-receiving elements are arranged in a two-dimensional array.

Now, as shown in the lower part of FIG. 2, the wavefront of the measurement light, that is, the measurement wavefront, is expressed as W(x), and the wavefront of the reference light, that is, the reference wavefront, is expressed as W(X+S). S is the amount of lateral deviation between both wavefronts, and in accordance with FIG. 1, it is determined by the amount of lateral deviation of the reference beam caused by the parallel plate 6.

Note that the wavefronts W(X) and W(X+S) should originally correspond to the shape of the shape measurement surface of the object to be measured 10 in a similar manner, but in FIG. , the general shape is shown.

Now, the measurement wavefront W(X) and the reference wavefront W(X+S)
are out of phase with each other due to the amount of shift S.

By the way, the plane mirror 4 can be displaced in a direction perpendicular to the mirror surface by the action of the piezoelectric element 5. This allows the optical path length of the reference light to be adjusted.

When the plane mirror 4 is displaced in this way, the part of the reference light reflected by the plane mirror 4 is shifted in the lateral direction, but as will be described later, the amount of displacement of the plane mirror 4 is a minute distance on the order of the wavelength of the laser beam. , therefore,
The influence of the lateral shift of the reference light caused by the displacement of the plane mirror on the above-mentioned shift amount S can be ignored.

Now, measurement wavefront W(X), reference wavefront W(X+S)
The phase difference between
It is expressed as (X). Of course, this phase difference has meaning only in the interference region.

By the way, this phase difference ΔW(X) is given by ΔW(X)=W(X+S)-W(X), and when the amount of shift S is small, the second or higher order minute term of S is cut off, It is given as ΔW(x)=∂W/∂X・S (1). Therefore, if the amount of deviation S is set to a size that satisfies equation (1), the measured wavefront W
(X) can be determined by adding ΔW(x)/S, ie by performing the integration 1/S∫ΔW(x)dx (2). After all, the shape of the shape measurement surface of the object to be measured can be specified by adding the phase difference ΔW(x).

To obtain the phase difference ΔW(X), proceed as follows.

If the optical path difference between the measurement light and the reference light is the measurement light 2 on the light receiving area 91 (Fig. 2) of the area sensor 9,
-1 is given as A(x)=a exp[i2k・W(X)] (3) where a is the amplitude, i is the imaginary unit, and wave number k=2π/λ (λ is the wavelength), and the reference light 2-2 is given as B(X+S)=bexp[i2k·(W(X+S)+l)](4) where b is the amplitude.

From this, the light intensity distribution I ₀ (Xl) of the interference fringes 2-3 in the interference region is, as is well known, I ₀ (Xl) = a ² + b ² + 2abcos2k [W (X) - W (X + S) - l] (5) is given. As it is, it is difficult to handle it as it is, so we standardize it by dividing both sides of equation (5) by (a ² + b ² ).

I(Xl)=1+γcos2k [W(X)-W(X+S)-l] (6) Here, γ=2ab/a ² +b ² When formula (6) is Fourier transformed with respect to l, ξ=
2kl, I(Xl)=1/2a ₀ + _∞ 〓 ⁿ⁼¹ a _o cosnξ+ _∞ 〓 ⁿ⁼¹ b _o sinnξ (7) a _o =∫I(Xl)cosnξdξ (8) b _o =∫I( Xl) sinnξdξ (9).

Comparing equations (6) and (7), equation (6) has n=2
Since it does not include the above vibrational components, for n > 1, a _o = b _o = 0 ₀ Therefore, equation (7) is: I (Xl) = 1/2a ₀ + a ₁ cosξ + b ₁ sinξ = 1/2a ₀ +a ₁ cos2kl+b ₁ sin2kl (7').

On the other hand, in formula (6), paying attention to the fact that ΔW=W(X)−W(X+S), it becomes I(Xl)=1+γcos2kΔW.cos2kl +γsin2kΔW.sin2kl (6′). From this, a ₀ =2, a ₁ =γcos2kΔW, b ₁ =γsin2kΔW are obtained. Therefore, tan2kΔW=b ₁ /a ₁ , and from this, the phase difference ΔW is given by ΔW=1/2ktan ⁻¹ b ₁ /a ₁ (10).

Note that a ₁ and b ₁ are all 0 where n>1, and from equations (8) and (9), a ₁ = ∫I (Xl) cosξdξ (8') b ₁ = ∫I It is given by (Xl) sinξdξ (9′).

The integration is performed approximately as follows. That is, the optical path difference l between the measurement light and the reference light can be changed by displacing the plane mirror 4 using the piezoelectric element 5, as described above. Therefore, the plane mirror 4 is displaced in N steps by the piezoelectric element 5, with 1/2N of the wavelength λ as one step.

As a result, l changes by λ/2 in steps of λ/2N. If l at each step is expressed as lj = λ/2Nj, equations (8') and (9') are respectively a ₁ = kλ/N _N 〓 ^j=1 I(X.lj) cos2klj (8″ ) b ₁ = kλ/N _N 〓 ^j=1 I(X.lj)sin2klj (9″) Therefore, in the end, the phase difference ΔW is is given by

Therefore, to obtain ΔW(X), you can do as follows.

That is, the light intensity I (X.lj) at each point X in the interference region is measured by the area sensor 9,
Multiply this measured value by cos2klj.sin2klj to get I(X.lj)
Calculate cos2klj, I(X.lj)sin2klj. This is repeated for each step of the N-step displacement of the plane mirror 4 by the piezoelectric element 5, and each calculated value is sequentially added to obtain _N 〓 ^j=1 I(X.lj)cos2klj, _N 〓 ^j=1 I( X.lj) Obtain sin2klj, divide the latter by the former to obtain its arctangent function value, and multiply this by 1/2k to obtain the value of equation (11).

After that, by using this phase difference ΔW(X) and adding the phase differences according to equation (2), the measurement wavefront W(X) can be specified. Next, the parallel plate 6 is rotated 90 degrees around the optical axis, the measurement wavefront and the reference wavefront are shifted in the Y direction perpendicular to the X axis, and W(Y) is obtained by the above process. From W(Y), the shape of the shape measurement surface of the object to be measured 10 can be specified.

The above is an overview of the wavefront shape measurement method.

For example, if the phase difference ΔW(X) is as shown in the upper part of FIG. 3, the shape of the wavefront obtained by adding these values will be as shown in the lower part of FIG. 3.

FIG. 4 shows an example of actual measurement results.

Now, in the wavefront shape measuring device shown in Fig. 1, the measurement light is transmitted through the beam splitters 2 and 7 and the imaging lens _L
The reference light reaches the area sensor 9 via the plane mirror 4, parallel plate 6, beam splitter 7,
Since it reaches the area sensor 9 via the imaging lens _L4 , it is necessary to take thorough anti-vibration measures for the measuring device during measurement. That is, if the relative positional relationship of the optical systems constituting the respective optical paths of the measurement light and the reference light is distorted due to vibration, measurement accuracy is immediately adversely affected.

(Objective) Therefore, an object of the present invention is to provide a wavefront shape measuring device that has excellent vibration resistance and is compact.

(composition) The present invention will be explained below.

The wavefront shape measuring device of the present invention is characterized by a half mirror and a plane mirror.

The half mirror and the plane mirror are arranged close to each other and opposite each other, and their respective mirror surfaces are parallel to each other. A plane mirror can be displaced by a piezoelectric element in a direction perpendicular to the mirror surface.

The information light enters the half mirror and is split into measurement light and reference light. The light that has passed through the half mirror is reflected by the plane mirror, and by passing through the half mirror again, the light that has been reflected by the half mirror is shifted laterally with respect to the direction in which the light travels. This realizes a lateral shift between the measurement light and the reference light. Further, by displacing the plane mirror using the piezoelectric element, the optical path length change necessary for determining the phase difference ΔW(X) is realized.

Hereinafter, a detailed description will be given with reference to the drawings.

FIG. 5 shows one embodiment of the invention. In order to avoid complication, the same reference numerals as in FIG. 1 are given to those items that are considered to have no risk of confusion.

To explain the newly appearing symbols in the figure, numeral 8 indicates a half mirror, and numeral 11 indicates a plane mirror.

These half mirror 8 and plane mirror 11 constitute the characteristic part of the present invention.

As shown in the figure, the half mirror 8 is fixedly arranged at an angle of 45 degrees with respect to the information light, that is, the light from the side of the object to be measured 10, and the surface facing the plane mirror 11 is half-shaped. The surface is transparent.

The plane mirror 11 is arranged close to and parallel to the half mirror 8, and is adapted to be displaced by the piezoelectric element 5 in a direction perpendicular to the mirror surface. Further, the surface of the half mirror 8 opposite to the semi-transparent surface is subjected to anti-reflection treatment.

Therefore, as shown in FIG. 5, when information light is incident on the half mirror 8, part of it is transmitted through the semi-transparent surface of the half mirror 8, and the other part is reflected by the semi-transparent surface. Thus, the information light is split into measurement light and reference light. Light transmitted through half mirror 8 (indicated by broken line)
is reflected by the plane mirror 11, passes through the half mirror 8 again, and becomes light that is parallel to the light reflected by the half mirror 8 (shown by a solid line) and shifted laterally with respect to the traveling direction. Thereafter, the measurement light and reference light are passed through the imaging lens _L4 to the area sensor 9.
It comes to.

By displacing the plane mirror 11 with the piezoelectric element 5, it is possible to change the optical path length shown by the broken line.

Therefore, while changing the optical path length using the piezoelectric element 5, the area sensor 9 measures the light intensity at each point in the interference region, performs a predetermined calculation on the measured value, and calculates the positions of both the measurement and reference wavefronts. By obtaining the phase difference ΔW(X) (Equation (11)) and adding this according to Equation (2), the shape W(X) of the measurement wavefront is obtained, and the measured object 10 is moved around the incident optical axis. Rotate 90 degrees to obtain W(Y) using the same process, then these W(X), W
The shape of the shape measurement surface of the object to be measured 10 can be specified from (Y).

FIG. 6 shows another embodiment of the invention. In the embodiment shown in FIG. 5, the shape of the object to be measured 10 is similarly reproduced as a wavefront shape using an imaging relationship, and the shape of the object to be measured is identified by measuring this wavefront shape. In the embodiment shown in FIG. 6, the shape of the wavefront produced by the lens L to be tested itself is measured. Thereby, the lens function of the lens L to be tested can be easily checked.

(Effects) As described above, according to the present invention, a novel wavefront shape measuring device can be provided.

In this device, since most of the optical systems constituting the optical paths of the measurement light and the reference light are common, the device has excellent resistance to vibration and can be made compact.

[Brief explanation of drawings]

1 to 4 are diagrams for explaining a wavefront shape measurement method, FIG. 5 is a diagram showing one embodiment of the present invention, and FIG. 6 is a diagram showing another embodiment of the present invention. 1...Laser light source, _L1 , _L2 ...Collimator lens, _L4 ...Imaging lens, 8...Half mirror, 11...Plane mirror, 5...Piezoelectric element, 10...
...Object to be measured.

Claims (1)

[Claims] 1. A measurement wavefront whose shape is to be measured and a reference wavefront that is shifted horizontally from the wavefront with respect to the traveling direction of the light are created, and the optical path length of the light forming one of the wavefronts is changed,
In the wavefront shape measurement method, which measures the phase difference between both wavefronts at each point in their interference area and adds this phase difference to determine the wavefront shape of the measured wavefront, the method uses a half mirror and a mirror close to the half mirror. A plane mirror is disposed opposite to the half mirror so that its mirror surface is parallel to the half mirror, and information light having measurement wavefront information is made incident on the half mirror to form the measurement light and the reference light. The measurement light and the reference light are shifted laterally with respect to the traveling direction of the light by dividing the light, which has passed through the half mirror, is reflected by the plane mirror, and is transmitted through the half mirror again, and A wavefront shape measuring device characterized in that the optical path length is changed by displacing the plane mirror in a direction perpendicular to the mirror surface using a piezoelectric element.