JPH047446B2 - - 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 at each point in their interference region is measured and the phase differences are added to specify the shape of the measured wavefront.

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 chip. Splitter, code L ₄ is imaging lens, code 9 is area sensor,
Reference numeral 10 indicates 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 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 the lens _L3 again in the opposite direction, and is separated into two beams by the beam splitter 3.

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 L ₃ and imaging lens L ₄ 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 imaging relationship, each luminous flux in the above-mentioned 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, we will explain how the beam passes from the beam splitter 3, through the beam splitters 2 and 7, and through 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.
At the top, they are shifted and overlapped with each other, and interference fringes 2-3 appear due to interference in the overlapped portions. The area where the interference fringes 2-3 appear is called an interference area.

Note that the area sensor 9 is a solid-state imaging device 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 the two wavefronts, and according to 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. 2, the explanation is given in a general manner. Therefore, 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)
As shown in the bottom diagram of Figure 2, 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 is given by △W(X) △W(X) = W(X+S) - W(X), and when the amount of shift S is small, the second or higher order of S is cut off. , △W(X)=dW/dX・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 l, the measurement light 2 on the light receiving area 91 (FIG. 2) of the area sensor 9
-1 is given as A(x)=aexp[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, where b is the amplitude, B(X+s)=bexp[i2k.(W(x+s)+l)](4
) is given.

From this, the light intensity distribution Io(xl) of the interference fringes 2-3 in the interference region is, as is well known, Io(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=
As 2・kl, I(x,l)=1/2a _p + _∞ 〓 ⁿ⁼¹ a _o cos nξ+ _∞ 〓 ⁿ⁼¹ b _o sinnξ (7) a _o =∫I(xl)cos nξdξ (8) b _o =∫I(xl)sin nξdξ (9).

Comparing equations (6) and (7), equation (6) has n=2
Since the above vibration components are not included, for n > 1, a _o = b _o = o _p Therefore, equation (7) is: I(xl) = 1/2a _p + a ₁ cosξ + b ₁ sinξ = 1/2a _p +a ₁ cos2kl+b ₁ sin2kl (7)

On the other hand, when paying attention to the fact that ΔW=W(x)−W(x+S), equation (6) becomes I(xl)=1+γ cos2kΔW. cos2kl+γsin2k.ΔWsin2kl (6)′. From this, a ₀ =2. a ₁ =γcos2k△W, b ₁ =γsin2k△W are obtained. Therefore, tan2k△W=b ₁ /a ₁ , and from this, the phase difference △W is given by △W=1/2k ₂ tan ^-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 (xl) sinξdξ (9′).

This integration is performed approximately as follows.
In other words, the optical path difference l between the measurement light and the reference light is, as mentioned earlier, when the piezoelectric element 5 causes the plane mirror 4 to
It can be changed by displacement. Therefore, the plane mirror 4 is displaced in N stages by the piezoelectric element 5, with 11/2N, which is the maximum wave input, 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″), so in the end, the phase difference △W is is given by

Therefore, in order to obtain ΔW(x), the following procedure may be used.

That is, the light intensity I (x.lj) at each point X in the interference area is measured by the area sensor 9,
Multiply this measured value by cos2klj and 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) 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 an axis parallel to the optical axis, the measurement wavefront and the reference wavefront are shifted in the y direction perpendicular to the X axis, and the same measurement in the y direction is performed. By specifying W and y, the shape of the shape measurement surface of the object to be measured 10 can be specified from W(x) and W(y).

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 is designed to reach the area sensor 9 via the imaging lens L ₄ , 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 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 measurement position 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 includes a condensing lens;
It has a half mirror or beam splitter and first and second plane mirrors.

The condensing lens focuses the information light having information on the measurement wavefront.

Half mirror or beam splitter is
The high-speed focusing by the condensing lens is divided into measurement light and reference light during focusing.

The first plane mirror is arranged so as to be orthogonal to the optical axis of the light beam at one of the focusing positions of the light beam divided into the measurement light and the reference light. The optical axis of the light flux refers to an optical axis that coincides with the light beam passing through the optical axis of the condensing lens.

The second plane mirror is arranged at an angle with respect to the optical axis of the light beam at the other focusing position of the two divided light beams.

Then, any one of the first and second plane mirrors is displaced by the piezoelectric element to change the optical path length difference l between the measurement light and the reference light.

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, the symbol _L5 is a condenser lens, the symbol 8,112 is a beam splitter, the symbol 11 is a first plane mirror,
Reference numeral 12 indicates a second plane mirror.

The light from laser light source 1 is passed through a collimating lens.
The light beams are collimated by L ₂ and L ₃ , and then passed through the beam splitter 112 and lens L ₃ to the object to be measured 1.
irradiated to 0. The reflected light from the object to be measured 10, that is, the information light, enters the condenser lens _L5 via the lens _L3 and the beam splitter 112, and is condensed by the condenser lens _L5 into a beam splitter 8. The beam enters the beam and is split into two beams by the beam splitter 8 on the way to being focused. One of the split beams is focused on a plane mirror 11 and the other is focused on a plane mirror 12.

The plane mirror 11 is perpendicular to the optical axis of the convergent light beam incident thereon. Therefore, the light reflected by the plane mirror 11 travels in the direction of incidence and reaches the area sensor 9 via the beam splitter 8 and the imaging lens _L4 .

On the other hand, the plane mirror 12 is tilted by a small angle θ with respect to the optical axis of the light that is convergently incident thereon, and therefore, the optical axis direction of the light reflected by the plane mirror 12 is as follows with respect to the incident optical axis. Tilt by 2θ. This reflected light reaches the area sensor 9 via the beam splitter 8 and the imaging lens _L4 . Note that the plane mirror 12
Since the reflection point at is the focal point of the imaging lens _L4 , the optical axes of both the measurement light and the reference light incident on the area sensor 9 are parallel.

As can be seen from the above description, in the apparatus of the present invention, the information light is divided into the measurement light and the reference light by a half mirror or a beam splitter, in this embodiment the beam splitter 8. Further, the measurement wavefront and the reference wavefront are shifted laterally with respect to the traveling direction of the light by the second plane mirror.

The plane mirror 11 is transformed by the piezoelectric element 5 in a direction perpendicular to the mirror surface.

To measure the wavefront shape, 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, and the measured value is subjected to a predetermined calculation to calculate the phase difference ΔW. This is done by obtaining (x) (Equation (11)) and adding it according to Equation (2). Measurement in the Y direction is performed using a plane mirror 12.
Rotate 90° around the incident optical axis to obtain △W
Obtain (y) and add this.

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 by the imaging relationship,
By measuring this wavefront shape, the shape of the object to be measured was identified. 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. Note that the change in optical path length is
This can also be done by displacing a plane mirror. Furthermore, it goes without saying that a half mirror can be used instead of the beam splitter.

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

In this wavefront shape measuring device, most of the optical systems through which the measurement light and the reference light pass are common to each other, and therefore have excellent vibration resistance and can be made compact as a whole.

[Brief explanation of drawings]

Figures 1 to 4 are diagrams for explaining the wavefront shape measurement method, Figure 5 is a diagram showing one embodiment of the present invention, and Figure 6 is a diagram showing another embodiment of the present invention. . 1...Laser light source, _L2 , _L3 ...Collimator lens, 8, 12...Beam splitter,
L ₃ ...Lens for illumination, L ₄ ...Imaging lens, L ₅
...Condensing lens, 11...First plane mirror, 12...
...Second plane mirror, 5...Piezoelectric element, L...Test lens.

Claims (1)

[Claims] 1. A measurement wavefront whose shape is to be measured, and this wavefront,
A reference wavefront is created that is shifted horizontally with respect to the direction in which the light travels, and the optical path length of the light that makes up one wavefront is changed, and the phase difference between the two wavefronts is measured at each point in their interference region. In the wavefront shape measurement method, which specifies the shape of the measurement wavefront by adding phase differences, there is a condenser lens that converges information light having information about the measurement wavefront, and a condensed light beam by this condenser lens. a half mirror or beam splitter that splits the beam; a first plane mirror disposed perpendicular to the optical axis of the beam at one focusing position of the beam split into two by the half mirror or beam splitter; and a second plane mirror arranged at an angle with respect to the optical axis of the light beam at the other focusing position of the light beam, and any one of the first and second plane mirrors,
A wavefront shape measuring device characterized in that the optical path length is changed by displacement using a piezoelectric element.