Scanning interferometer for measuring deviation of optical surfaces shape

FIELD: optics.

SUBSTANCE: device has coherent emission source, first condensing filter, consisting of condensing lens, first and second light-splitting elements, objective, interferometer, consisting of standard and controlled surface, device for measuring optical beam drive length, first projection system, registering block and system for processing interference image, system for projecting auto-collimation images. Also provided is additional condensing filter, mounted in focal plane of objective, device for changing optical length of beam drive is positioned behind the laser, made in form of two transparent diffraction grids, one of which can move in direction, perpendicular to grid rows, first diffraction grid divides emission on two beams, each of which gets on to lenses of condensing filters, while controlled and standard surfaces are deflected relatively to normal line to optical axis.

EFFECT: broader functional capabilities, higher efficiency.

5 dwg

 

The present invention relates to measuring and control technique, namely, devices for measurement of form deviations of polished surfaces of the nominal and can be used to control the shape of the optical parts.

Known interferometer [1], intended for measurement of form deviations of polished surfaces of nominal.

The interferometer consists of a helium-neon laser, a special filter that converts the laser beam into a diverging wave front, splitters, lens, actually the Fizeau interferometer, the system is projected to the camera and pre-configuration of the interferometer.

In the interferometer used method of determining the coordinates of the centers of interference fringes on the interferogram amplitude method, i.e. minimum illumination, further calculating the shape of the wave front reflected the controlled surface and, accordingly, the deviation of the shape of this surface. This method also has disadvantages, namely, that all defects interference pattern arising from defects of the illuminator and the optical system of the interferometer are perceived by the system of registration of interferogram as deformation interferogram caused by the deviation of the shape of the surface of the inspected items from nominal.

Except the amplitude is swesty phase methods of measuring the deflection shape of optical surfaces and devices implement, based on the implementation of scanning the interference pattern. In this case, the defects of the optical system of the interferometer does not change during scanning and do not affect the measurement results.

So in the device [2] scanning interference pattern is carried out by changing the wavelength of the radiation source that is implemented in a known way by changing the optical length of the resonator of the laser. This method requires a special laser that is his undoubted disadvantage.

A device for moving mirror of the interferometer [3], in which the scanning mirror oscillates due to the elasticity of the fastening elements during the rotation of the drive:

The disadvantage of this device is that it can be applicable only for mirrors small size.

The closest in technical essence to the proposed device is an interferometer [4], the concept of which is presented in figure 4.

The interferometer is selected for the prototype, based on the fact that the reference surface makes oscillatory motion along the optical axis. In the end, the entire interference pattern is shifted to one lane while moving the reference surface to 1/2 the wavelength of radiation used in the interferometer source. Measuring three or b is more intensity values of the interference pattern in the points field, corresponding to the position of the pixel matrix, during one cycle of movement of the reference surface, it is possible to calculate the phase of the interferogram in these field points, which corresponds to the phase of the wave front.

The known device [4] contains a source of coherent radiation, located behind the filter condenser mounted in the focal plane of the lens and consisting of a condenser lens and aperture of small diameter, the first and second beam-splitting elements, the lens and the interferometer. The interferometer consists of the controlled and reference surfaces. Both surface perpendicularly the optical axis. The reference surface oscillates along the optical axis of the interferometer, so the length of the beam reflected from this surface, is continuously changing. The device [4] also contains a projection system, which together with the lens projecting an interference pattern on a TV camera and a photodiode matrix, and the projection system autocollimating images intended for pre-setting of the interferometer. With photodiode matrix connected computer, in which the processing of measurement results. The known device [4] can only be used for inspection of parts with a diameter up to 100 mm, so as to provide high frequency oscillatory re edenia details having large dimensions, it is almost impossible.

When applying the known devices for controlling the spherical surfaces are to be paid additional error caused by the shift of the center of curvature of the reference surface relative to the center of curvature of the test surface, which also affects the accuracy of the measurement.

The main task, which is aimed invention is the extension of the operation range of the phase of the interferometer and its application scope.

To achieve the objectives of the proposed device, which, as a prototype, contains a source of coherent radiation, the first filter condenser located in the focal plane of the lens and consisting of a condenser lens in the focal plane where the aperture is of small diameter, the first and second beam-splitting elements, interferometer, consisting of the controlled and reference surfaces, and a device for changing the optical length of the beam, the first projection system, which together with the lens projecting interferential picture on the recording unit and associated with the registering unit processing system interferentsionnoi painting and projection autocollimating images.

Unlike the prototype device for changing the optical length of the stroke of the beam is located in the lighting part of the interferometer, designed as two transparent diffraction gratings, one of which can move in the direction perpendicular to the lines of the grating using piezophiles, the device entered the second filter condenser mounted in the focal plane of the lens, and the reference and controlled surface tilted relative to the normal to the optical axis in different directions at an angle

α=S/4f'about,

where S is the distance between the apertures of the first and second filter-condenser;

f'about- the focal length of the lens.

The essence of the invention consists in that the device for changing the optical length of the beam, made in the form of two transparent diffraction gratings, one of which performs an oscillatory motion perpendicular to the lines of the lattice, creates two beams, the phase difference between which changes with movement of the grid.

After reflection of the reference and the controlled surfaces of these beams interfere with each other forming the interferogram, the phase of which varies with the movement of the grid.

Measuring the intensity of the interference pattern in the locations photodetectors at offset pattern (not less than 3 positions) calculates the phase of the wave front at these points customary methods familiar to the scanning interferometers.

As is the scanning device is installed in the lighting part of the interferometer, it is equally suitable for flat and spherical surfaces of any size, certain parts of the interferometer.

Thus, the set of the above signs allowed to obtain a specific technical result, which is expressed in the extension of the operation range of the phase of the interferometer and applications, due to the fact that obtaining the phase of the picture by changing the optical length of the two rays have the opportunity to inspect the flatness of the details of any size, provided the dimensions of the parts of the interferometer, because the node that creates the phase shift does not depend on the size of the controlled items. The system is also suitable for control of spherical components, so as not broken centriole parts in the scanning process.

The invention is illustrated by drawings, where figure 1-shows a diagram of the formation of the phase difference between two rays passing through the movable diffraction grating; figure 2 presents a schematic diagram of the device; Fig; 3 is the circuit of one of the concrete examples of execution of the node changes the phase of the interfering beams, 4 - presents a section along a-A; figure 5 is presented a schematic diagram of the device, taken as a prototype.

Scanning interferometer for measuring the deflection shape of the optical surfaces of the soda which provides a source of coherent radiation 1, for example, a laser emitter of LGN-303, located behind it, the node changes the phase of the interfering beams 2, consisting of two diffraction gratings 3 and 4, one of which performs oscillatory motion and two filter-condenser 5, located in the focal plane of the lens 6 and consisting of a lens, the focal planes which are located aperture 7 and 8. The device also includes first and second beam-splitting elements 9 and 10, the interferometer 11, consisting of reference 12 and 13 controlled surfaces, inclined to the optical axis of the device so that the rays from the aperture 7, reflected by the reference surface and light from the aperture 8, reflected a controlled surface were parallel and were interferonbeta among themselves.

The device also includes a projection system 14, which together with the lens 5 projects the interference pattern on the TV camera 15, which is connected to the computer, engaged in the processing of measurement results, and the projection system autocollimating images 16.

Reference 12 and 13 controlled surface tilted relative to the normal to the optical axis in different directions at an angle

α=S/4 f'about,

where S is the distance between the apertures of the first and second filter-condenser;

f'about- the focal length of the lens.

In one of the concrete examples of how to perform the evil of changing the phase of the interfering beams of the device node includes a first diffraction grating 3, which through the elastic hinge suspension 17 and piezophiles 18 may oscillate perpendicular to the lines of the grating 3. For moving grate 3 is fixed grating 4, which deflects the incident rays and makes them parallel.

The device operates as follows.

The formation of the phase difference between two rays passing through the movable diffraction grating 3 shown in figure 1, where the lattice are indicated as follows:

A - movable diffraction grating 3;

B - stationary diffraction grating 4;

l is the displacement of the diffraction grating.

If you take the point About the reference point, the phase of the wave in the plane will be:

FC1=F0-2π/λ·Δ1,

and in the plane With2

FC2=F0+2π/λ·Δ2,

as Δ12=lsin α, a sin α=λ/d

where d is the grating pitch, the phase difference of the beams I and II will be

Δf=fc2-FC1=2π/λ. 2Δ=4π·l/d

Changing the phase difference of the beams I and II 2π will be at offset grid at d/2.

The laser beam 1 of the first diffraction grating 3 is divided into two beams, each of which falls on the lens filter-condenser 5, in the focal plane which includes the diaphragm 7 and 8, which transmits only the main thread and cutting all pairs of the attributes rays. From the diaphragm out two spherical wavefront, after which the lens is flat.

The reference surface 12, mounted at an angle relative to the normal to the optical axis, reflects the incident rays and the focal plane of the lens forms the image of the diaphragm, which can be observed through the projection system autocollimating images 15. The tilt of the test surface 13 is superimposed upon the image of the aperture 8 with the image of the aperture 7, reflected from the reference surface 12.

When reflected from the reference and the controlled surfaces the rays parallel to each other and form an interference pattern. When the movement of the diffraction grating of the phase fronts coming from the diaphragms 7 and 8 are changed relative to each other and the interference pattern is shifted to one lane when changing the phase difference of 2π.

SOURCES of INFORMATION

1. U.S. patent N 4201473, IPC G 01 In 9/02, 1980

2. European patent N 0144510, IPC G 01 In 9/02, 1985

3. USSR author's certificate N 1337651, IPC G 01 B 9/02, 1985

4. The interferometer. Model MARK III. Technical description of the prototype.

Scanning interferometer for measuring the deviation of the shape of optical surfaces containing a source of coherent radiation, the first filter condenser located in the focal plane of the volume of ctiva and consisting of a condenser lens, in the focal plane where the aperture is of small diameter, the first and second beam-splitting elements, the lens, the interferometer comprising a reference and controlled surfaces, as well as a device for changing the optical length of the beam, the first projection system, which together with the lens projecting an interference pattern on the recording unit connected to the processing system interference pattern, the projection system autocollimating images, which together with the lens projects the image of the aperture on the recording unit, characterized in that the interferometer is introduced an additional filter condenser mounted in the focal plane of the lens, the device for changing the optical length of the beam is for laser, made in the form of two transparent diffraction gratings, one of which can move in the direction perpendicular to the lines of the lattice, using piezophiles, the first diffraction grating splits the radiation into two beams, each of which falls on the lens filter-condenser and controlled and the reference surface is inclined relative to the normal to the optical axis in different directions at an angle of α=S/4 f'aboutwhere S is the distance between the apertures of the first and second filter-condenser; f'about- focal distance the lens.



 

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