JPS5953524B2 - Shearing information - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in common path type shearing interferometry.

Shearing interferometry is a method that divides the amplitude of the wavefront to be measured and applies relative displacement to each wavefront to obtain interference fringes.
This method is mainly used for measuring lens aberrations and testing the uniformity of glass materials.

The advantage of this method is that interference fringes of the tested wavefront can be obtained without a reference wavefront for interference. Furthermore, when measuring lens aberration, the lateral aberration of the lens can be directly read from the interference fringes of minute lateral deviations (hereinafter referred to as lateral shears). Conventional shearing interferometry can be roughly divided into methods using optical crystals such as Savart plates and methods using reflective mirrors.

FIG. 1 is a diagram showing an example of a conventional shearing interference method using an optical crystal.

1 is a Savart plate, 2 is a polarizer, and 3 is an analyzer. By this combination, two parallel light beams 4 and 5 having polarization components in the same direction are obtained.

In this method of obtaining the shear amount using an optical crystal, it is possible to use a white light source with a short coherence distance because there is no difference in optical path length between two separate light beams, that is, the reference light beam and the test light beam. Since there is a limit to the size of the crystal, it is not suitable for measuring aberrations of long focal points, large aperture lenses, etc. Furthermore, when measuring aberrations, it is advantageous to select the shear amount according to the wavefront to be measured and to add a small angle between the two wavefronts, but when using an optical crystal, a Wollaston prism is used to create a small angle. etc., and the resulting angle and shear amount are often constant. Shearing interferometry using a reflecting mirror can be roughly divided into those that apply a Hatsender interferometer, those that utilize reflections on the front and back surfaces of a plane-parallel plate, and those that use a common optical path interferometer as in the present invention. Broadly classified.

FIG. 2 shows an example of a conventional shearing interferometer to which a Mach-Zender interferometer is applied.

In the case where the Mach-Zehnder interferometer is applied, the wavefronts whose amplitudes are split by the beam splitter 6 pass through different optical paths, so they are easily affected by the surface accuracy of the reflecting mirrors 7, 8, and 9. θ 3rd
The figure shows an example of a conventional shearing interferometer using a plane parallel plate 10 or an optical wedge.

In this way, when using reflection from the front and back surfaces of a parallel plane plate or an optical wedge, the optical path difference is large, so it is impossible to obtain five interference patterns unless a light source with a long coherence length, such as a laser, is used. In addition, in order to improve measurement accuracy, it is necessary to align the pit of the observation system with the object to be measured or the lens pupil.
In this case, since the optical path difference is large, it is impossible to accurately align the pits, and therefore it is unsuitable for highly accurate measurement. FIG. 4 is a diagram showing an embodiment of a conventional shearing interferometer using a common optical path interferometer. Those using a common optical path shear meter are capable of lateral shearing and angular shearing. As shown in FIG. 4, in the conventional method, in order to give lateral shear to the incident light beam 15, the beam splitter 11 is placed perpendicularly to the X-Y plane perpendicular to the reflecting mirrors 12, 13, and 14. Tilt it so that it forms an angle from the direction. As a result, the light beams 16 and 17 split by the beam splitter 1 had different optical path lengths.

FIG. 5 shows an equivalent development of the optical system formed by the beam splitter 11 and reflecting mirrors 12, 13, and 14 shown in FIG. 4, and the behavior of the light beams 16 and 17 is observed from the front direction. It is a diagram. The light beam 16 passes through the beam splitter 11, is reflected by the reflecting mirrors 12, 13, and 14 in this order, and passes through the beam splitter 11 again.
is not affected by the slope of On the other hand, the beam 17 is reflected by the beam splitter 11, reflected in the order of 4, 13, 12, and reflected by the beam splitter 11 again.
is affected by the inclination of the beam splitter 11. Since the light beam 17 is reflected twice by the beam splitter, the light beams 16 and 17 eventually become parallel light beams. Due to the difference in the optical path lengths of the light beams 16 and 17, a light source with a long coherence distance is required to produce variegated fringes. Furthermore, since the observation system cannot be focused on the surface to be inspected or the pupil plane of the lens, precise measurements are difficult. This phenomenon becomes more pronounced as the lateral shear increases.
The larger the slope of 1 is, the more noticeable it becomes. The purpose of the present invention is to eliminate the difference in the optical path length of the divided light beams, which is a defect of the common optical path type shearing beam meter, and to eliminate the difference in optical path length of the divided light beams, which is a defect of the common optical path type shearing beam meter. The present invention provides a shearing interference device that has a wide measurement range and can be used with a white light source.

The reason why the present invention aims to improve a common optical path type shearing interferometer is that, as is clear from the above, in a common optical path type interferometer, since a reflecting mirror is used, the area of the wavefront to be detected is large. Furthermore, since a minute angle is created by rotating the reflecting mirror, the angle can be set arbitrarily, and the amount of shear can also be made variable.

Furthermore, since the optical path is common, if there is a defect in the surface precision of the reflecting mirror, the divided luminous flux will be affected in the same way, so it has the advantage of being less affected by the surface precision and precision of the reflecting mirror. be. The principle of the present invention is to use one beam splitter and a plurality of reflecting mirrors so that the wavefront reflected by the beam splitter and the wavefront transmitted through the beam splitter travel in opposite directions along a common optical path. The lateral shear can be reduced by configuring an optical path in the same direction and giving at least two of the plurality of reflecting mirrors an angle of inclination in substantially the same direction and at substantially the same angle with respect to the direction perpendicular to the plane containing the common optical path. This is to prevent a difference in optical path length from occurring when one occurs.

Hereinafter, the present invention will be explained in detail using Examples. FIG. 6 is a perspective view of an embodiment of a shearing interferometer according to the present invention, in which a common optical path is constructed by one beam splitter and three reflecting mirrors.

7'' is a plan view of the optical system shown in FIG. If the test object is an object, the test lens itself is an objective lens that has a focal point on the test object.For example, if the test object is a lens with a large diameter, the common optical path described below is used. Since the width of the parallel light beam incident on the beam increases, the common optical path becomes larger.
In order to prevent this increase in size, an afocal lens system consisting of a pair of convex lenses (
A collimator lens) is provided. Therefore, in such a case, the above-mentioned 18 becomes a part of the collimator lens. Therefore, hereinafter, 18 will be simply referred to as a lens, including this meaning. 1
9 is a beam splitter 1, 20, 21, 22 are reflecting mirrors, and the beam splitter 19 and the reflecting mirrors 20, 21.
, 22 constitute a common optical path.

The beam R entering the beam splitter 19 is parallel to the X-Y plane, and the beam splitter 19 and the reflecting mirror 21 are perpendicular to the X-Y plane. Reflector 20
and 22 are rotated by approximately the same angle in the same direction with respect to the z-axis.

That is, the reflecting mirror 20 has a rotation angle in the x direction as shown by arrow 23, and the reflecting mirror 22 also has a rotation angle in the x direction by approximately the same angle as shown by arrow 24. A light beam R emitted from a light source (not shown) and made parallel by a lens 18 enters a beam splitter 19 in a state parallel to the XY plane.

The light beam A reflected by the beam splitter 19 is reflected by the reflecting mirror 2.
When reflected at 0, an angle is given to the X-Y plane and the beam enters the reflecting mirror 21. When the light beam A from the reflector 21 is reflected by the reflector 22, it is returned to a light beam parallel to the X-Y plane, and is reflected by the beam splitter 19, which then passes through the imaging lens 25.
incident on . At this time, a position P1 of the light beam R entering the beam splitter 19 and a position P2 of the light flux R entering the beam splitter 1 again from the reflecting mirror 22 are shifted in the Z-axis direction. On the other hand, the beam B which has been amplitude-divided and transmitted by the beam splitter 19 is given an angle in the opposite direction to the beam A with respect to the X-Y plane by the reflecting mirror 22, and then enters the reflecting mirror 21. When the light beam from the reflecting mirror 21 is reflected by the reflecting mirror 20, it is returned to a light beam parallel to the X-Y plane, passes through point P3 of the beam splitter, and heads toward the imaging lens 25. The amount of displacement that the light beam A receives in the Z-axis direction and the amount of displacement PlP3 that the light beam B receives in the Z-axis direction are equal.

FIG. 8 shows a simulated development of the common optical path consisting of the beam splitter 19 and the reflecting mirrors 20, 21, and 22, and a front view of the states of the luminous flux A and the luminous flux B in the optical path. is shown.

In FIG. 8, in order to easily compare the optical path lengths of the light beams A and B, the position P1 where the light beam R enters the beam splitter is set differently for the light beams A and B. As shown in FIG. 8, since the light beams A and B pass through a common optical path as parallel light beams, they simply overlap with a lateral shear in the Z-axis direction without having a relative optical path difference or relative angle.

Therefore, interference fringes can be obtained even when a white light source with an extremely short waving distance is used. In addition, when accurately measuring or photographing interference fringes, it is necessary to use the imaging lens 25 to accurately align the pits with the object to be measured or the pupil of the test lens provided in front of the beam splitter 19. In the invention, since no optical path difference occurs before the two wavefronts divided as described above reach the imaging lens, it is possible to accurately align the pits. Further, since the amount of lateral shear depends only on the rotation angles of the reflecting mirrors 20 and 22, it can be made arbitrary, that is, variable. Furthermore, when measuring lens aberrations, it is advantageous to add a small angle between the two wavefronts in a direction perpendicular to the lateral shear in addition to the lateral shear; in this case, the beam splitter 19 or Reflector 20
, 21, 22 is rotated within the X-Y plane. In the embodiment shown in FIG. 6, it is considered most advantageous to rotate the reflector 2. This is because the influence on the luminous flux A and the luminous flux B is almost equal, and the reflecting mirror 21
This is because they can be moved independently. In the above example, the reflecting mirrors 20 and 22 are angled by the same amount with respect to the Z axis in order to perform shearing.

As long as this angle is approximately the same amount, there is no practical problem. In the above-mentioned embodiment, the two reflecting mirrors are provided with inclination angles so that the inclinations of the light beams generated on these two surfaces toward the X-Y plane are canceled out as a whole. As is clear from this, in a common optical path consisting of one beam splitter and a plurality of reflecting mirrors, at least two of the plurality of reflecting mirrors are aligned in the X-Y plane. When a lateral shear is produced in a wavefront-split light beam by applying an inclination angle of substantially the same angle from a direction perpendicular to the same direction to the same direction, the light beam entering the common optical path and the light beam also exiting the common optical path are aligned in parallel planes. As long as the two wavefront-split beams exiting the common optical path are parallel, the direction and angle of each reflective surface tilted within the common optical path may be arbitrary. The spacing between the surfaces may also be arbitrary. FIG. 9 is a perspective view showing an embodiment using one beam splitter 26 and two reflecting mirrors 27 and 28, and FIG. 10 is a plan view of the same figure.

In this case, the lateral shear is caused by the mirror 27 tilted in the direction of arrow 29 and the mirror 28 tilted also in the direction of arrow 30. The light flux R from the lens 18 and the common optical path 26,
The light beams A'' and B'' emitted from the light beams 2, 7, and 28 are in a plane parallel to the X-Y plane, and the light beams A'' and B'' are parallel to each other.

FIG. 11 is a perspective view showing an example of a case in which one beam splitter 31 and four reflecting mirrors 32, 33, 34, and 35 are used, and FIG. 12 is a plan view of the same figure. .

In this embodiment, in order to produce lateral shear, the reflecting mirror 32 is inclined in the direction of arrow 36, and the reflecting mirror 35 is inclined in the direction of arrow 37 by substantially the same angle. Of course, any three of the four reflecting mirrors 32, 33, 34, 35, or all of the reflecting mirrors can be inclined. Even in that case, the light flux R entering the beam splitter 31 from the lens 18 and the common optical path 31, 32,
The common optical path must be formed so that the light beams A'' and B'' emitted from the light beams 33, 34, and 35 are in parallel planes, and the light beams A'' and B'' are parallel to each other.
As described above, in the common optical path type shearing interferometry according to the present invention, in a common optical path constituted by a beam splitter and a plurality of reflecting mirrors, a light beam incident on the common optical path and a beam exiting from the common optical path are separated. By tilting at least two or more of the reflecting mirrors in the common optical path, while maintaining the divided beams to be in parallel planes and parallel to each other, It separates the incident light beam into two light beams having a shear in the lateral direction.

Therefore, in the common optical path type shearing interferometer of the present invention, since no optical path difference occurs in the divided light beams, more precise measurements can be made than with the conventional common optical path type shearing interferometer. It is possible to use a white light source with a short wading distance, etc.
It is a simple method with excellent effects.

[Brief explanation of the drawing]

1, 2, 3, and 4 are diagrams showing each device of a conventional shearing interferometer, and FIG. 5 is an equivalent expanded view of a part of the optical system shown in FIG. 4, FIG. 6 is a perspective view showing an embodiment of the shearing angle meter of the present invention, FIG. 7 is a plan view of the optical system shown in FIG. 6, and FIG. 8 is an example of the optical system shown in FIG. 6. FIG. 9 is a perspective view showing another embodiment of the shearing interferometer of the present invention, and FIG.
FIG. 11 is a plan view of the illustrated optical system, FIG. 11 is a perspective view showing another embodiment of the shearing interferometer of the present invention, and FIG.
FIG. 3 is a plan view of the illustrated optical system. 18... Luns, 19... Beam splitter, 20, 21, 22... Reflector, 25.
...Imaging lens, A, B, R...Light flux.

Claims (1)

[Claims] 1 In shearing interferometry, in which a wavefront to be measured is divided into two wavefronts, a lateral displacement is given between the two wavefronts, and then the two wavefronts are superimposed, the beam is separated by a single beam splitter and multiple reflecting mirrors. The wavefront reflected by the splitter and the wavefront transmitted through the splitter constitute an optical path in which the wavefront propagates in opposite directions along a common optical path, and at least two of the plurality of reflecting mirrors are provided with an optical path perpendicular to the plane containing the common optical path. With respect to the direction
Shearing interferometry is characterized by giving almost the same angle of inclination in almost the same direction.