RU2263279C2 - Method and device for interferometric measuring of shape deviation of optical surfaces - Google Patents

Abstract

FIELD: measuring engineering.

SUBSTANCE: method comprises directing a coherent light beam at the surface to be tested, producing and recording interferogram of the light path difference, and processing the interferogram. The tested and reference surfaces are exposed to the second coherent light beam, and the second interferogram of the light path difference is created. The second interferogram is provided with the additional light path difference with respect to that of the first interferogram, which is equal to the one fourth of the beam wavelength. The light path difference of the first interferogram is determined at specific points of the surface to be tested from the signal of illumination in one of two interferograms. The device comprises source of coherent light, first filter-condenser, first and second light-splitting units, interferometer composed of tested and reference surfaces, unit for measuring optical length of the beam, first projecting unit, recording unit, observing unit, and unit for processing the interferogram. The device also has two light-splitting units between which two pairs of transparent diffraction lattices are interposed. The filter-condenser, the second light-splitting unit, and λ/4 lattice are arranged in series in the direction of the beam.

EFFECT: enhanced precision.

4 cl, 8 dwg

Description

The invention relates to measuring technology, and in particular to methods and devices for measuring deviations of the shape of polished surfaces from the nominal, and can be used, for example, to control the shape of optical parts.

Known interferometric method for measuring the deviation of surface shapes, implemented in the interferometer [1]. The method is based on determining the coordinates of the centers of interference fringes on interferograms by the amplitude method, that is, by minimizing illumination, further calculating the shape of the wavefront reflected by the controlled surface and, accordingly, the deviation of the shape of this surface.

A device [1] is known for measuring deviations of the shape of polished surfaces from the nominal. The device [1] contains a coherent radiation source (gas laser), a condenser with a small aperture that cuts off spurious rays, a lens that creates a parallel beam of rays, the actual interferometer, which is used as a Fizeau plane interferometer, a system for projecting an interferogram onto a television camera, and a preliminary system interferometer settings.

Also known is the amplitude method for measuring surface shape deviations described in the literature [2].

The device [2] for its implementation contains a helium-neon laser, a special filter that converts the laser beam into a diverging spherical wavefront, beam splitters, a lens, the Fizeau interferometer itself, a system for projecting interferograms on a television camera, and a system for projecting self-collimating aperture images on a television camera.

Both known methods [1] and [2], as well as devices for their implementation are amplitude and have disadvantages inherent in the amplitude measurement method. These disadvantages consist in the fact that all defects in the interference pattern that arise due to defects in the illuminator and the optical system of the interferometer are perceived by the interferogram recording system as deformations of the interferogram caused by the deviation of the surface shape of the controlled part from the nominal.

In addition, the use of amplitude methods for determining the wavefront deformation from the coordinates of the points of interference fringes leads to significant errors and does not provide sufficient accuracy for calculating the shape of the surface being monitored.

In addition to amplitude methods, phase methods for measuring the shape deviation of optical surfaces and devices for realizing them are known, based on phase modulation of the interference pattern. So in the patent [3] the phase shift of the interference pattern is carried out by changing the wavelength of the radiation of the source, which is implemented in the known method by changing the optical length of the laser resonator.

In one embodiment of the device [3] implementing this method and described in the literature [3], a gas laser is used as a radiation source. The change in the optical length of the laser cavity is achieved either by moving one mirror, in devices with an external mirror, or by pulling a tube, in devices with an internal mirror. In another embodiment of the known device [3], which uses a semiconductor laser as a radiation source, a change in the wavelength is achieved by changing the excitation current, which in turn changes the temperature of the laser and, therefore, its optical length.

Thus, in the known method [3] for the implementation of phase modulation, as well as in all the proposed device variants, the use of special lasers is necessary, which is an obvious drawback that limits the widespread use of the method and the devices implementing it for monitoring optical products during their mass production.

Closest in its technical essence to the proposed method is a phase method for measuring the deviations of the shape of polished surfaces, described in the source [4].

The schematic diagram of the prototype [4] is presented in Fig. 8.

The prototype method is based on the fact that the reference surface oscillates along the optical axis. As a result, the entire interference pattern is shifted by one band when the reference surface moves 1/2 the radiation wavelength of the source used in the interferometer. By measuring three or more values of the intensity of the interference pattern at the field points corresponding to the position of the pixels of the matrix during one cycle of movement of the reference surface, we can calculate the phase of the interferogram at these points on the field, which corresponds to the phase of the wavefront.

The known device [4] contains a coherent radiation source, a filter condenser located behind it, mounted in the focal plane of the lens and consisting of a condenser lens and a small diameter diaphragm, a beam splitting unit, first and second beam splitting elements, a lens and an interferometer. The interferometer consists of controlled and reference surfaces. Both surfaces are perpendicular to the optical axis. The reference surface oscillates along the optical axis of the interferometer, due to which the stroke length of the beam reflected from this surface is continuously changing. The device [4] also contains a projection system, which, together with the lens, projects an interference pattern onto a TV camera and a photodiode array, as well as a system for projecting autocollimation images, intended for presetting the interferometer. A computer is connected to the photodiode array, in which the measurement results are processed.

However, the known method and device [4] have a disadvantage inherent in phase methods and devices that implement them, namely: the intensity of the interferogram at each point of the field varies not only due to the displacement of the reference surface, but also due to the inevitable deformation of the wavefront caused by vibration both reference and controlled surfaces, air fluctuation between the reference and controlled surfaces and other factors. As a result, interferogram phases are determined with significant errors.

In addition, the known device [4] can only be used to control parts with a diameter of up to 100 mm, since it is practically impossible to provide high-frequency oscillatory movement of parts with large dimensions.

When applying the known method and device for monitoring spherical surfaces, an additional error is introduced due to the displacement of the center of curvature of the reference surface relative to the center of curvature of the surface being verified, which also affects the measurement accuracy.

The objective of the invention is to increase the accuracy and reliability of measuring the deviation of the shape of optical surfaces on a phase interferometer by eliminating the influence of vibrations on the measurement results of wavefront deformations on a phase interferometer, as well as expanding the scope of the method and device.

To achieve this technical result, a method for interferometric measurement of the deviation of the shape of the optical surfaces and a system for its implementation.

The method of interferometric measurement of the deviation of the shape of the optical surfaces is based on the fact that a coherent beam of rays focused near the center of curvature of the controlled surface is directed to the controlled surface, a reference surface with the center of curvature located near the center of curvature of the controlled surface is placed into the beam, an interferogram of the path difference is formed and recorded rays reflected from the controlled and from the reference surface, and process it to obtain the values of this different ti turn.

The method differs from the known one in that a second coherent beam of rays is directed to the controlled and to the model surface and a second interferogram of the path difference of rays reflected from the controlled and from the model surface is formed, an additional difference of the path of the rays is introduced into the path difference of the second interferogram compared to the path difference the first interferogram equal to a quarter of the radiation wavelength, and at the given points of the surface to be monitored, the difference in the course of the first interferogram is determined by the signal illuminated in one of the two interferograms for which the condition is true:

,

,

where I is the registered illumination signal at a given point of the interferogram with a measured difference in the path of the rays;

I ₀ - known in advance constant component of the illumination at a given point of the interferogram when changing the difference in the path of the rays;

ΔI - known in advance amplitude of the signal at a given point in the interferogram with changes in the difference in the path of the rays

The proposed method can be implemented in the inventive system of interferometric measurement of the shape of optical surfaces.

The system for interferometric measurement of the shape deviation of optical surfaces contains a coherent radiation source emitting two plane-polarized beams, a beam splitting unit dividing the source beam into two beams, the polarization planes in which are mutually perpendicular, two pairs of transparent diffraction gratings directing the radiation in the corresponding orders, i.e. . at certain angles, a connecting unit that directs radiation with different directions of polarization in one direction, two filter condensers located in the focal plane of the lens and consisting of a condenser lens, in the focal plane of which a small diameter diaphragm is installed, a beam splitting unit that separates radiation with different directions plane of polarization. In addition, the system contains first and second beam splitting elements, an interferometer consisting of a reference and controlled surfaces, a projection system for projecting an interferogram onto a CCD matrix and an associated system for processing the interference pattern and a projection system for autocollimating images.

A feature of the proposed system is that 4 sources of interferometer radiation are formed, of which two are the actual diaphragms of the condenser and two of their imaginary images. These 4 sources, when the radiation is reflected from the reference and controlled surfaces, form two interference patterns that are projected onto a single CCD. In this case, the alignment of the interferometer is performed in such a way that the second interference pattern is phase shifted by π / 2 relative to the first, and the first or second diffraction gratings can oscillate perpendicular to the gratings of the gratings to change the phases of the interfering rays. In this case, the first and second interference patterns shift by 1 band when the phase difference changes by 2π.

The proposed system for interferometric measurement of the shape of optical surfaces that implements the claimed method makes it possible to use it for high-precision control of flat and spherical parts of any size, and in the presence of vibration of the controlled part and the device itself, which was made possible by the following.

In the proposed intensity system by which the phases are calculated, interferograms are recorded simultaneously at all points of the picture; in the proposed system using beam splitting devices formed four measuring channel, in contrast to the one in the prototype. Thanks to the additional channels, it became possible to obtain two interferograms shifted in phase relative to each other by π / 2. In addition, the device for changing the optical path length of the beam, located in the prototype directly in the interferometer, is installed in the proposed device in the lighting system and is made in the form of two diffraction gratings, one of which can move perpendicular to the grating strokes. This arrangement of the device for changing the optical path length of the beam, as well as its implementation allows you to measure large parts with high accuracy. The introduction of a second projection system, a separation and connecting prism into the system, allows the projection of two interferograms onto the registration unit. Blocks for dividing the laser beam into two, with different directions of the polarization planes, and then converging them into each of the two filter capacitors will make it possible to obtain two identical interference patterns, phase shifted by π / 2. The inclination of the reference and controlled surfaces relative to the normal to the axis of the device is determined by the formula

α = s / 4f ′ _rev ,

where s is the distance between the diaphragms of the first and second filter-condensers;

f ′ _about - focal distance of the lens, provides the formation of interference patterns.

Thus, the combination of the above features allows us to solve the tasks.

Obtaining two interferograms shifted relative to each other by π / 2 and simultaneously taking information from these interferograms allows you to get the correct results even with interferogram displacements caused by random factors, such as vibration. The displacement of the interference pattern due to a change in the path length of the rays makes it possible to apply this method in interferometers to control the plane of parts of any size provided by the dimensions of the parts of the interferometer, since the unit creating the phase incursion is located in the illuminating part of the interferometer and does not depend on the size of the controlled part.

The method and the system implementing it are also suitable for monitoring spherical parts, since the alignment of the parts during the scanning process is not disturbed.

The proposed method and system for measuring the deviation of the shape of the optical surfaces are illustrated by drawings.

Fig.6 and Fig.7 shows a diagram of the path of the rays forming the interference patterns, explaining the inventive method.

In Fig.6 and Fig.7 indicated:

A is the reference surface;

B - controlled surface;

α is the angle of inclination of the reference and controlled surface;

O is the plane passing through the main point of the lens;

D and E - diaphragms of filter-condensers;

D ₁ and E ₁ - imaginary images of the diaphragms of the filter-condensers;

O ₁ - the center of the interference pattern obtained as a result of interference of rays I and II;

O ₂ - rays III and IV, respectively.

Figure 2, 3, 4 presents a schematic diagram of one of specific examples of a system for interferometric measurement of the shape deviation of optical surfaces in accordance with the invention.

The system contains a coherent radiation source 1, for example, an LGN-303 laser emitter, a mirror 2 behind it, and a beam splitter containing a beam splitter 3, which divides the laser radiation into two beams with mutually perpendicular planes of polarization, each of the rays passes through a transparent diffraction grating 4, which decomposes the beam so that the maximum energy is directed to ± I order. Next, the rays pass through the second transparent diffraction grating 5 and become parallel to each other. Behind the diffraction gratings, a light-connecting element of block 6 is installed, which directs the rays with mutually perpendicular directions of the plane of polarization to one of two filter-condensers 7 and 8, consisting of a condenser lens, with a diaphragm 9 and 10 installed in its focal plane. A second filter is installed a beam splitting unit 11 (FIG. 4 and FIG. 5), consisting of two beam splitting plates 12 and 13, reflecting rays of the same direction of polarization and transmitting rays with a perpendicular direction of polarization and. Mirrors 14 change the direction of radiation. The plates 15, mounted between the beam splitter plate 12 and the mirrors 14, shift the imaginary image of the condenser diaphragm so that the distance between the center of the condenser diaphragm and its imaginary image is ≈1.6 mm (diameter of the diameter of the condenser lens). After the beam splitting unit, a λ / 4 plate is installed, which converts plane-polarized light into circularly polarized light, the first 17 and second 18 beam-splitting elements, the lens 19, and the flat interferometer 20, made according to the Fizeau scheme and consisting of a reference 21 and 22 controlled surfaces.

When measuring the shape deviation of spherical surfaces, a nozzle is used that converts a flat wave front into a spherical one, converging in the center of curvature of the last nozzle surface, which is a reference. When combining the centers of curvature of the reference and controlled surfaces, a Fizeau interferometer is formed, equivalent to a flat interferometer. Controlled 22 and reference 21 surfaces are inclined to the normal of the optical axis of the interferometer by an angle

α = s / 4f ′ _rev ,

where s is the distance between the diaphragms of the filter capacitors;

f ′ _about - the focal length of the lenses.

The system comprises a dividing prism 23 mounted behind the first beam splitting element 17 in the backward ray path, the first projection system 24 projecting together with the lens the first interference pattern into the intermediate image plane. The second projection system 25 projects the second interference pattern also into the intermediate plane in which the connecting prism 26 is located, behind which the third projection system 27 is located, projecting the intermediate image of the interference patterns onto the CCD matrix of the camera 28 of the recording unit associated with the interference pattern processing system. The third projection system consists of two lenses 29 and 30, moreover, the lens 29 has a variable focal length to maintain a constant image size of the interference pattern on the CCD matrix when changing the dimensions of the monitored part.

To maintain a constant distance between interference patterns on the CCD matrix, the prism 26 together with the lens 30 move along the axis in proportion to the change in the focal length. To preset the interferometer, there is a system for projecting autocollimation images 31.

The system (figures 1, 2, 3, 4) works as follows.

Laser radiation from a beam splitting unit is divided into two components with mutually perpendicular planes of polarization. Each of the rays passes through two transparent diffraction gratings and diffracts on them. Using the ± I diffraction orders, we obtain two rays parallel to each other at the exit from the grating. When moving one of the gratings in a direction perpendicular to the direction of the strokes, the phase of one beam changes relative to the other by 2π when moving the grating by

l = d / 2,

where d is the lattice pitch.

Further, the rays pass through the beam splitting unit and fall on the lens of the filter-condenser. Rays having mutually perpendicular planes of polarization pass through each capacitor. From the diaphragms of the respective filter-condensers, 4 spherical wave fronts emerge, which become flat after the lens 19.

When a beam with a beam splitting block hits a plate 12, a beam with one plane of polarization is reflected from it, and passes with a perpendicular one. The mirrors change the direction of the rays and they fall on the beam splitter plate, which also reflects the rays reflected by the plate 12 and passes the rays passing through the plate 12.

Plane-parallel plates 15 shift imaginary images of the diaphragms 9 and 10 relative to themselves by 1.6 mm (diameter of the condenser lens).

All four rays I, II, III, and IV pass through a λ / 4 plate, which converts plane-polarized light into circularly polarized light.

The rays pass through the translucent elements 17 and 18 and exit the lens 14 in the form of a plane light wave. The reference surface 21, mounted at an angle α relative to the normal to the optical axis, reflects the rays incident on it, and an image of the diaphragm 9 is formed in the focal plane of the lens 19, which can be observed using the autocollimation image projection system 31. The slopes of the controlled surface 11 combines the image of the diaphragm 9 with the image of the diaphragm 10 reflected from the controlled surface 22. In this case, the autocollimation images of the imaginary diaphragms 8 and 10 will also be combined. The angle between the reference 21 and the controlled 22 surfaces provides parallelism of the rays reflected from them and obtaining interference patterns that the lens 19 through the beam splitting elements 17 and 18, the separation 23 and the connecting 26 prisms, projection systems 24 or 25 and the projection system 27, are designed on a CCD matrix 28. Since the optical path length of rays I and II changes when the diffraction gratings 4 are shifted, the interference pattern will shift by one band when the path length changes by one wavelength. When the interference pattern is shifted, the maximum and minimum illumination values at each pixel of the CCD matrix are measured, which are used in further calculations of the wavefront phase. Moving one of the diffraction gratings 5 when aligning the interferometer, we achieve that the second interference pattern is shifted relative to the first by π / 2. After shifting the interference patterns by one band by shifting the lattice 5, the movement of the lattice 5 stops and the intensities at each pixel of the CCD matrix are measured for both interferograms, from which the wavefront phases are calculated. Knowing the phase of the wavefront reflected from the controlled surface, it is possible to calculate all the parameters of this surface, namely the radius, the standard deviation from the sphere at each point.

Literature

1. Interferometer IKD-110. Technical description and instruction manual. Part I. S - 30.60.025 TO 1991

2. US patent No. 4201473, CL. G 01 B 9/02, op. 6.05.80.

3. European patent No. 0144510, class. G 01 B 9/02, op. 06/19/85.

4. Interferometer. Mark III model, technical description is a prototype.

Claims (4)

1. The method of interferometric measurement of the deviation of the shape of optical surfaces, which consists in sending a coherent beam of rays focused near the center of curvature of the surface to be monitored, placing a sample surface with the center of curvature located near the center of curvature of the surface to be inspected, forming and registering interferogram of the difference in the path of the rays reflected from the controlled and from the reference surfaces, and process it to obtain the values of et a difference in the path, characterized in that a second coherent beam of rays is directed to the controlled and sample surface and a second interferogram of the difference in the path of the rays reflected from the controlled and from the reference surfaces is formed, an additional difference in the path of the rays is introduced into the difference in the path of the rays of the second interferogram compared to the difference the path of the rays of the first interferogram equal to a quarter of the radiation wavelength, and at the given points of the surface to be monitored, the difference in the path of the first interferogram is determined by the signal broadcasts in that of two interferograms for which the condition | II ₀ | ≤ΔI / √2, where I is the registered illumination signal at a given point of the interferogram with a measured difference in the path of the rays; I ₀ - known in advance constant component of the illumination at a given point of the interferogram when changing the difference in the path of the rays; ΔI is the amplitude of the change in the signal at a given point in the interferogram when the signal changes at a given point in the interferogram with changes in the difference in the path of the rays. 2. A system for interferometric measurement of the shape deviation of optical surfaces, comprising a coherent radiation source, a first filter capacitor located in the focal plane of the lens and consisting of a condenser lens, in the focal plane of which a small diameter aperture is installed, the first and second beam splitting elements, an interferometer consisting from controlled and reference surfaces mounted perpendicular to the optical axis, as well as a device for changing the optical path length of the beam, the first pr a projection system, which, together with the lenses, projects the first interference pattern onto the recording unit and a monitoring device, an interference pattern processing system and a self-collimating image projection system associated with the recording unit, characterized in that two beam splitting units are introduced into the system, the first beam splitting unit is located behind the coherent source radiation and consists of a beam-splitting and light-connecting elements, between which there are two pairs of transparent gratings, behind the first beam splitting unit there are two filter-condensers, the second beam splitting unit dividing the rays in each channel into two, having mutually perpendicular planes of polarization and offset relative to each other so that the distance between the imaginary images of the diaphragms of the condenser is equal to the distance between the diaphragms themselves, located behind the filter-condensers, a λ / 4 plate is installed behind the second beam splitting unit, which converts linearly polarized light into circularly polarized light, at the same time, a dividing prism is installed in the system, installed behind the first beam-splitting element in the return path, a second projection system projecting the image of the second interferogram into the intermediate plane, and a connecting prism installed in the intermediate plane of the image of the interferogram, as well as a third projection system, projecting the image two interference patterns on the CCD matrix of the registration unit, the reference and controlled surfaces of the interferometer inclined to the optical axis of the interferometer 90 at an angle ± α, where α = s _1/4 f _'of where s ₁ - distance between the diaphragms of filter condensers, f' _of - lens focal distance. 3. The system according to claim 2, characterized in that the second beam splitting unit located behind the filter-condensers is made in the form of two beam splitting plates with a coating that transmits one direction of the plane of polarization and reflects mutually perpendicular, two mirrors located between the plates and two inclined plane-parallel glass plates displacing the rays and providing the required displacement of the rays relative to each other. 4. The system according to claim 2, characterized in that the third projection system projecting the intermediate plane of the interferogram image onto the CCD matrix of the recording unit consists of two lenses, one of which has a variable focal length, and the other is connected with the connecting prism with the possibility of movement in proportion to the focal length of the first lens.

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