JPS63223538A - Interference measuring instrument - Google Patents

Abstract

PURPOSE:To measure an object surface which has a large aperture ratio by making a spherical wave incident on a hologram and also using a diffracted component of (+n)th order. CONSTITUTION:Laser light projected from the final surface 4a of a reference converging lens 4 passes through a filter 5 arranged at the focus position of a lens 4 and also passes through the hologram 6 to impinge on the surface 7 to be measured which is a concave aspherical surface. When a spherical wave is incident, the hologram 6 is a zone plate type hologram where such a pattern that the diffracted component of +1st order has a wave front in an ideal aspherical surface shape at the position of the surface 7 to be measured is drawn. The laser light incident on the surface 7 to be measured is reflected by the surface 7 and diffracted again by the hologram 6 to reach the filter 5. At this time, only diffracted light containing measured surface information, i.e. diffracted component of +1st order in the light reflected by the surface 7 to be measured passes through the filter 5 and the remainder is cut off. Consequently, the contrast of interference fringes is increased and accurate measurement is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an interference measuring device that uses a hologram to create a reference wavefront and inspects an optical element, and particularly to its optical system.

Conventional technology Generally, when measuring the surface shape of an optical element such as a lens or a reflector, a prototype with an ideal surface shape is used, and the light reflected from the surface to be measured and the light reflected from the prototype surface are measured. A commonly used method is to measure the surface shape of an optical element by causing interference and observing the resulting interference fringes. This method is very convenient because it allows you to see at a glance any manufacturing errors in the surface to be measured, but it is difficult to manufacture the prototype surface. In particular, it is extremely difficult to produce a prototype aspheric surface to measure the shape of an aspheric surface such as an aspheric lens that has been frequently used in recent years. As a method to avoid this difficulty, a method has been devised in which a computer ψgram is used instead of a prototype to reproduce a wavefront having the ideal shape of the surface to be measured.

A conventional example of the optical system in this type of interference measurement device is “
Optics Vol. 10 No. 3 (June 1981)” P, 174~
The optical system described in P. 183 is shown in FIG. 7 and will be explained.

In the figure, a laser beam emitted from a laser oscillator (not shown) passes through a lens (lb) and a pinhole (IC) and reaches a lens (20). The lens (2o) images the pinhole (IC) at the center of the surface to be measured (7). The laser beam that has passed through the lens (20) passes through the beam splitter (3) and enters the hologram (6).

The hologram (6) is a zone plate type hologram, and is placed on the optical axis at a location slightly shifted forward or backward from the center of curvature of the surface to be measured (7). FIG. 7 shows an example in which it is placed in the front. In addition, a pattern is drawn on the hologram (6) such that when a spherical wave is incident, its -1st order diffraction component becomes a wavefront with an ideal aspherical shape at the position of the measured surface (7). There is. Lens (20
), the laser beam focused on the center of the surface to be measured (7), that is, the O-order diffraction component due to the hologram (6), is
It is reflected without being affected by the surface precision of the surface to be measured (7) and travels back along the original optical path. Of the components that are diffracted again by the hologram (6), the +11th order diffracted product is reflected by the beam splitter (3).
) and passes through a spatial frequency filter (21) to block unnecessary components and become a reference wavefront.

Wave 1 is reflected by the surface to be measured (7), and the hologram (
6). This becomes the 0th-order wavefront among the components diffracted by the hologram (6). The reference passed through the filter (21)
Both measurement wavefronts interfere with each other and form interference fringes.

The surface to be measured (if the force is manufactured according to the design value, the wavefront of the 1st-order diffracted light reproduced by the hologram (6) is perpendicularly incident on the surface to be measured (7), so the surface to be measured (7)
It is reflected vertically and travels back along the original optical path. Therefore, the reference wavefront and the measurement wavefront that have passed through the filter (21) have the same wavefront shape, and the interference fringes have one color.

On the other hand, if the surface to be measured (force is not manufactured according to the designed value), the wavefront reproduced by the hologram (6) will be obliquely incident on the surface to be measured (7), so it will be reflected by the surface to be measured (7). The wavefront does not travel backwards along the original optical path. Therefore, the reference wavefront and the measurement wavefront have different wavefront shapes, and the measured surface (7
) interference fringes are generated that give a shape error. By observing these interference fringes, the shape of the surface to be measured (7) can be measured.

As explained above, in this interference measurement device, the reference wavefront and the measurement wavefront pass through a substantially common optical path, so it has various advantages such as being more resistant to vibrations than the Twyman-Green type or the like interference measurement device.

Problems to be Solved by the Invention By the way, when measuring a surface with a large aperture ratio, it is necessary to increase the numerical aperture of the full cone reproduced by the hologram. As the numerical aperture of the full cone reproduced by the hologram increases, the diffraction angle of the reproduced wavefront increases.

On the other hand, the diffraction angle of the reproduced wavefront is determined by the pitch of the pattern drawn on the hologram, and the finer the pitch of the pattern, the larger the diffraction angle of the reproduced wavefront.

Therefore, the larger the aperture ratio of the surface to be measured, the finer the pitch of the pattern drawn on the hologram must be.

However, the pitch of patterns drawn on holograms cannot be made infinitely finer. In other words, when producing holograms using the photographic reduction method, it is impossible to make the pattern pitch finer than the resolution of the reduction lens, and when producing holograms using the direct writing method, it is impossible to make the pattern pitch smaller than the step width of the drawing device. It is impossible to make it detailed.

In other words, in interference measurement using a hologram, there is a limit to the aperture ratio of the surface to be measured that can be measured, and it is impossible to measure a surface with a large aperture ratio.

SUMMARY OF THE INVENTION An object of the present invention is therefore to provide an interference measurement device that can measure surfaces with large aperture ratios (c%, aspherical surfaces), which have been impossible with conventional devices.

Means for Solving the Problems In order to achieve the above object, the interference side gita of the present invention
- a first exchange means for converting the light emitted from the coherent light source into a parallel plane wave; a semi-transparent mirror disposed obliquely to the traveling direction of the parallel plane wave in the optical path of the parallel plane wave; A transmission prototype whose closest surface is a transmission prototype surface, a second conversion means for exchanging the parallel plane wave to a spherical wave, and between the second conversion means and the surface to be measured. placed,
and a zone plate type hologram manufactured so that when the spherical wave is incident, the +nth order diffraction component (n is a natural number) becomes a wavefront having a predetermined shape at the position where the measured surface is arranged. It is characterized by

Operation By adopting the above configuration, in the interference measuring device of the present invention, the power of the hologram is supplemented by the power of the second conversion means.

An example of an interferometer middle device implementing the present invention will be described with reference to the drawings.

In FIG. 1, laser light is transmitted from a laser oscillator (1a
), a lens (1b), and a pinhole (IC), and is converted into a parallel plane wave by a collimator lens (2), which is the first conversion means. The wave is converted into a parallel plane wave, reaches the converging lens (4), and is converted into a spherical wave. The most measured Cn lens surfaces (4a) (hereinafter referred to as the final surfaces) of the reference balance lens (4) are part of a spherical surface whose center is the focal point of the reference converging lens (4) and whose radius is the puck focus. It was formed in order. It has a transmission type prototype surface, and there is no anti-reflection coating coated on the coach. The laser '+ emitted from the final surface (4a) of the reference convergence lens (4) passes through the filter (5) placed at the focal point of the reference convergence lens (4), passes through the hologram lens (6), and enters the concave shape. The light is incident on the surface to be measured (7) which is an aspherical surface. The hologram (6) is a zone plate type hologram with a pattern drawn such that when a spherical wave is incident, its +1st-order diffraction component becomes a wavefront with an ideal aspherical shape at the position of the measurement surface (7). It is. The laser light incident on the surface to be measured (7) is reflected by the surface (7), diffracted again by the hologram (6), and reaches the filter (5).

At this time, some of the light reflected by the surface to be measured (7) passes through the measurement target, and other unnecessary light is blocked. This increases the contrast of interference fringes, which will be explained later, and enables more accurate measurement. The measurement wavefront that has passed through the filter (5) enters the reference converging lens (4) from its final surface (4a), is converted into a parallel plane wave or an approximately parallel plane wave, and is then reflected by a semi-transparent mirror. The light enters the imaging lens (8).

By the way, the final surface (4a) of the reference converging lens (4) is
Since it is part of a spherical surface with the focal point of the reference converging lens (4) as the center and the back focus as the radius, the parallel plane wave incident on the reference converging lens (4) is directed to this final surface (
4a). Moreover, since this surface (4a) is not coated with an antireflection film, about 4 inches of light is reflected by this surface (4a).

That is, a part of the parallel plane wave incident on the reference converging lens (4) is vertically reflected by the final surface (4a), travels back along the original optical path, and becomes a parallel plane wave again. This parallel plane wave is then used as a reference wavefront. Thereafter, this reference wavefront is reflected by the semi-transparent mirror 0) and enters the imaging lens (8).

The measurement wavefront and the reference wavefront incident on the imaging lens (8) interfere with each other to produce interference fringes, which are imaged by the imaging lens (8) onto the imaging element (9).

If the surface to be measured (7) is manufactured according to the design value, the wavefront reproduced by the hologram (6) will be the same as the surface to be measured (7).
7), it travels backward along the original optical path and returns to a parallel plane wave through the hologram (6), filter (5), and reference converging lens (4). That is, the measured wavefront is converted into a parallel plane wave. Therefore, the interference fringes produced by the measurement wavefront and the reference wavefront will be one color.

If the surface to be measured (7) is not manufactured according to the design value,
The wavefront reproduced by the hologram (6) is the surface to be measured (
7] and reflected by the surface to be measured (7), the wavefront is:
A deviation that is twice the difference between the surface to be measured (7) and the design value occurs.

Therefore, this reflected light passes through the hologram (6), filter (5), and reference converging lens (4) without completely retracing its original optical path, and is then converted into an approximately parallel plane wave. At this time, the surface to be measured (7) is filtered by the filter (5).
Of the reflected light from ), non-convertible light that does not contain information on the surface to be measured is blocked. The measurement wavefront converted into an approximate parallel plane wave and the reference wavefront converted into a parallel plane wave interfere with each other to produce interference fringes, and these interference fringes are focused on the imaging element (9) by the imaging lens. By observing these interference fringes, the shape of the surface to be measured (7) can be measured.

Note that, according to the method described above, it is not possible to measure errors smaller than half the wavelength of the laser beam used, but smaller errors can be measured by arranging the surface to be measured (7) with a slight inclination. be able to. In other words, if the surface to be measured (7) is placed with a slight inclination, the surface to be measured (7)
If the wavefront is manufactured according to the designed value, the interference fringes generated by the measurement wavefront and the reference wavefront will be straight parallel fringes.

However, the interference fringes do not form, and the amount of deviation included in the measured wavefront (the amount twice the difference between the surface to be measured (7) and the design value) appears as the amount of distortion of the interference fringes. Therefore, by measuring this amount of distortion, it is possible to measure an error smaller than a half wavelength of the laser beam used.

As explained above, in the interference measurement device of this embodiment,
Since a spherical wave is incident on the hologram (6) and +11th-order diffraction formation by the hologram (6) is used, as described below, even if the diffraction angle is made smaller compared to the case where a parallel plane wave is incident. A wavefront of the same shape can be reproduced, and the pitch of the pattern of the hologram (6) can be made coarser.

That is, as shown in FIG. 2, compared to (a) when a parallel plane wave is incident on the hologram (6) and the incident light is diffracted at a diffraction angle of 01 to reproduce the wavefront, the hologram (6)
A spherical wave is incident on the hologram (6), and +
When using 11th order diffraction (b), simply convert the spherical wave to a spherical wave close to the radius of curvature of the incident spherical wave, or to an aspheric wave whose radius of curvature is approximately the radius of curvature of the incident spherical wave. Therefore, the diffraction angle θ2 can be set to θ2 minus θ1. Therefore, the optical system (b) can make the pitch of the pattern of the hologram (6) coarser than the optical system (a).

By the way, in the interference measurement device shown in Fig. 7, a spherical wave is incident on the hologram (6);
) is used to diffract the hologram (6) to the same side as the incident light with respect to the normal line (see FIG. 2(C)). As is clear from FIG. 2 (al + (c+), the diffraction angle θ3 of the hologram (6) used in the interference measurement device shown in FIG. 7 is θ3>01, and when a parallel plane wave is incident (al Compared to hologram (
6) The pitch of the pattern must be made finer.

Therefore, as in this embodiment, when a spherical wave is made incident on the hologram (6), and the +11
In the case (b) using the next order folding, that is, the bologram (
6] is supplemented by the power of the second conversion means (4), the diffraction angle of the hologram (6) can be minimized among the three cases, and as a result, the diffraction angle drawn on the hologram (6) The pitch of the pattern can be made coarsest.

Conversely, if a hologram (6) with the same pitch as the pattern being drawn is used, the angle between the normal of the hologram (6) and the light beam reproduced by the hologram (6) is best found in (b). This makes it possible to measure the surface with the largest aperture ratio. That is, by using the present invention, it becomes possible to measure surfaces that were impossible to measure in the cases of (a) and (C) because the aperture ratio was too large.

Furthermore, since the bologram (6) used in this example is a zone plate type, there is no need to introduce a carrier frequency, and the pitch of the pattern can be made coarser than in other holograms that require the introduction of a carrier frequency. Can be done.

Needless to say, it is possible to use the +2nd order, +3rd order, etc. diffraction components of the hologram (6).

Next, with reference to FIG. 3, an embodiment will be described in which the present invention is implemented in an interference apparatus for measuring a convex surface. In addition, in Fig. 3, parts common to Fig. 1 are omitted.
Description of that part will also be omitted. In the same figure,
The parallel plane wave that has passed through the semi-transparent mirror has its beam diameter expanded by the beam expander (1o), reaches the reference converging lens (4), and is converted into a spherical wave. The final surface (4a) of the reference converging lens (4) is a transmission type standard surface and is not coated with an antireflection film. The laser beam converted into a spherical wave passes through the hologram (6) and enters the convex surface to be measured (7).
6) Similar to what is shown in Figure 1, when a spherical wave is incident, a pattern is created in which the +1st-order diffraction component becomes a wavefront with an ideal aspherical shape at the position of the measured surface (7). This is a drawn zone plate type hologram. And the first
Similar to the embodiment shown in the figure, the measurement wavefront reflected from the surface to be measured (7) and the reference wavefront reflected from the final surface (4a) of the reference converging lens (4) are caused to interfere, and the interference fringes are observed. By doing this, the shape of the surface to be measured (7) can be measured.In this case, a filter that blocks unnecessary light reflected by the non-measurement surface (7) is placed at the focal plane of the imaging lens (8). be done.

FIG. 4 is a diagram showing another embodiment of the present invention. In this example, two holograms (6A), (6B
) is used. In other respects, there is no essential difference from the embodiments shown in FIGS. 1 and 3. As shown in Figure 5,
Compared to the case (a) where the incident light is diffracted by two holograms (6A3° (6B)) and the case where the incident light is diffracted by one hologram (a), each hologram (6A
), (6B) diffraction angle θ4. θ5 can be made small, and as a result, the pattern pitch drawn in each hologram (6A) and (6B) can be made coarse. In other words, when diffracting light at a certain angle θ, if the light is diffracted by two holograms, each hologram (6A) and (6B)
The diffraction angle θ4. Since θ5 satisfies the relationship θ=04+05, θ4. The diffraction angle θ is smaller than the diffraction angle θ when light is diffracted by one hologram.

As shown in FIG. 4, the convex shape was also measured in this example, but since the diameter of the convex surface was small, the beam expander (1) used in the example shown in FIG.
0) is unnecessary.

Figure 6 shows a flat transmission type prototype (4A) as a transmission type prototype.
As a second conversion means for converting a parallel plane wave into a spherical wave, when a parallel plane wave is incident, a spherical wave is generated. 2 shows an optical system in which two converting means (4B) are provided separately. The flat transmission type prototype (4A) has a surface to be measured (
7) The side surface (4a) is the prototype surface, and the surface (4a)
In the case of the anti-reflection film, the light enters the core from the left and reaches the prototype surface (4a), where part of the light is reflected by that surface (4a) and the rest is transmitted through that surface (4a). The parallel plane wave transmitted through the prototype surface (4a) is a hologram (
4B), and a spherical wave is reproduced by the hologram (4B). The laser light converted into a spherical wave passes through the hologram (6) and enters the surface to be measured (7). Similar to the one shown in Figure 1, the hologram (6) is such that when a spherical wave is incident, its +1st-order diffraction component becomes a wavefront with an ideal aspherical shape at the position of the surface to be measured (7). It is a zone plate type hologram with a pattern drawn on it. As in the embodiment shown in FIG. The shape of the surface to be measured (7) can be measured.

In this example, the reference converging lens (4) in the example shown in FIG. Since this hologram has a more compact optical system than the embodiment shown in FIG. 1 using a converging lens (4), it is easier to manufacture than a hologram that reproduces an aspherical wave.

Effects of the Invention As explained above, in the interferometric measuring device of the present invention, since a spherical wave is incident on the hologram and +nn order acid component is used, the diffraction angle of the hologram can be made smaller compared to the conventional interferometric measuring device. Even if the wavefront has the same shape, it is possible to reproduce a wavefront of the same shape. Therefore, it is possible to make the pitch of the pattern drawn on the hologram coarser, and using a hologram with patterns drawn at the same pitch, it is possible to measure surfaces with a large aperture ratio, which was impossible with conventional measuring devices. becomes possible.

Furthermore, when measuring the same surface to be measured, the pitch of the hologram pattern can be made coarser, so the hologram can be manufactured easily and at low cost.

Furthermore, since the hologram used in the present invention is of the zone plate type, there is no need to introduce a carrier frequency, and the pitch of the pattern can be made coarser by that amount.

According to the embodiment, since there are a plurality of holograms, the diffraction angle of each hologram can be further reduced, and the pitch of the pattern drawn on each hologram can be made even coarser.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of the optical system of an interferometric measurement device embodying the present invention, and FIG. 2 is a diagram showing that the diffraction angle in a hologram can be made small by the present invention ((b) (present invention), FIG. 3 is a diagram showing an optical system in which the present invention is applied to measurement of convex surfaces, FIG. 4 is a diagram showing a modification example using a plurality of holograms in the present invention, and FIG. is a diagram showing that the diffraction angle of the porogram can be further reduced by the modification (Φ) is a modification), and FIG. FIG. 7 is a diagram showing a conventional example of an optical system of an interference measuring device. 1...Coherent light source 2...First conversion means 3...Semi-transparent mirror 4a...Transmission type prototype surface 4.4A...Transmission type prototype 4.4B...Second Conversion means 6...Hologram 7...Surface to be measured Applicant: Minolta Camera Co., Ltd. Figure 3 Figure 4a Figure 5 @O・θ4suθ5

Claims (1)

[Claims] 1. A first conversion means for converting light emitted from a coherent light source into a parallel plane wave; and a semi-transparent mirror disposed obliquely with respect to the traveling direction of the parallel plane wave in the optical path of the parallel plane wave. , a transmission prototype whose surface closest to the surface to be measured is a transmission prototype surface; a second conversion means for converting the parallel plane wave into a spherical wave; the second conversion means and the measurement target. placed between the
and a zone plate type hologram manufactured so that when the spherical wave is incident, the 1+nth order diffraction component (n is a natural number) becomes a wavefront having a predetermined shape at the position where the measured surface is arranged. An interference measurement device featuring: 2. The interference measurement device according to claim 1, wherein the hologram is composed of a plurality of holograms.