Diffraction interferometer (options)

 

Diffraction interferometer to measure the shape of concave surfaces and mirrors of the second order contains consistently located at the main optical axis controlled surface, the phase plate/4, a zone plate, the center of which coincides with the center of curvature of the test surface, a beam splitter made in the form of a polarizing cube, the output aperture of the projection lens, the recording device. Also contains the illuminator, the optical axis which is perpendicular to the main optical axis, and consisting of a light source and lens, forming a converging beam. Axes of the phase plates/4 is oriented at an angle of 45° to the polarization vector of the beam-splitting cube. The technical result is an increase in accuracy, reliability and noise immunity measurements in the control errors of the production of surfaces of the second order. 2 N. p. F.-ly, 5 Il.

The invention relates to measuring technique and can be used for precise non-contact monitoring form concave surfaces (uncoated and mirror) of the second order is related surface will be called mirrors.

Known diffraction interferometer with the General progress of the rays, which combines the reference and measurement shoulders [1]. The interferometer consists of sequentially located on the main optical axis controlled surface, a zone plate, a beam splitter, aperture, recording device, illuminator, located on the optical axis, perpendicular to the main optical axis, and consisting of a light source and lens, forming a converging beam. The scheme of the interferometer principle will not change if the illuminator is placed on the main optical axis and the recording device on the normal to that axis. The input beam is split by a zone plate into several diffraction orders. One of them focused in the center of the test surface, forms a reference beam, and the other filling the aperture of this surface, and measuring the beam. After reflection from the mirror these beams pass through a zone plate and form an interference pattern that is perceived by the registering device. In the form of bands in the interference pattern can be judged about the aberrations of the surface. Schema interferometers are classified in [1] depending on the position of the con is similar to the surface in the center of the zone plate, 2-I and 3-I modification to the provisions of the center of curvature in the-1st and +1st-magic zone plate, respectively.

Diffraction interferometers with the General progress of the rays, in contrast to other interferometers used to control the shape of the surfaces, do not require reference surfaces comparison, expensive and bulky lenses and beam splitters; little resistant to temperature fluctuations, environmental and mechanical vibrations. These characteristics make these systems attractive for laboratory and production control parts with surfaces of the second order, which are widespread in optical instrumentation. Known industrial interferometer similar purpose (type Fizeau firm "Zygo" [2]) has a large size, requires a stable temperature conditions and isolation from mechanical vibrations. This device is used in the laboratory as a model. Diffraction interferometer and the interferometer type Fizeau to control the shape of the surfaces of the domestic industry produces not.

Diffraction interferometer, in which the center of curvature of the test surface coincides with the center of the zone plate (1-I modi is ntre field interference observed maximum intensity (light bar), reducing the impact of outside diffraction orders of magnitude compared with other schemes (2-I and 3-I modifications), in which the Central strip of the dark. Scheme 1 modification does not require a high quality of manufacturing optical elements of the interferometer and can work with coherent and incoherent light sources. The latter is explained by the fact that they interfere noninverted wavefronts. In schemes 2 and 3 modifications, on the contrary, interfere inverted fronts, i.e., an inverted one relative to another by 180°, so to the optical components and light sources must meet more stringent requirements.

If there are advantages inherent in the schemes of the 1st modification, they have several disadvantages. Here the focus point of the measuring beam does not coincide with the center of curvature of the surface, i.e., testing is not the center of curvature. The latter circumstance leads to instrumental error. In addition, unwanted diffraction orders formed by a zone plate in reflected and transmitted light, making the interference field interference and distortion. This reduces the contrast of interference fringes and difficult rassist be attributed to the result of measurement of a specific point of the surface.

The invention solved the problem of increasing the accuracy, reliability and noise immunity measurements in the control form concave surfaces of the second order. To solve the problem in diffraction interferometer containing consistently located at the main optical axis controlled surface, a zone plate, the center of which coincides with the center of curvature of the test surface, a beam splitter, aperture, recording device (for example, photometric), the illuminator, the optical axis which is perpendicular to the main optical axis, and consisting of a light source and lens, forming a converging beam, introduces a phase plate/4 between the zone plate and controlled by the surface and set the projection lens between the output aperture and the recording device. In the 1st version as a beam splitter is used a polarizing cube. In the 2nd embodiment, the beam splitter is in the form of a semitransparent mirror, tilted at an angle of 45° to the optical axis, or depolarization cube, and between the projection lens and the recording device is introduced Polaroid. In both options is selected from the relation, proposed by the authors,

where- valid instrumental error, D, and R is the diameter and the radius of curvature of the test surface.

Distinctive features of the proposed interferometers, in comparison with the known interferometer [1], the closest to him, are the introduction of the phase plate/4 between the testing surface and the zone plate, the establishment of the projection lens between the output aperture and the recording device and the choice of the focal length f of the zone plate according to the offered valuewhere- valid instrumental error, D, and R is the diameter and the radius of curvature of the test surface.

In addition, for the 1-St variant of the interferometer distinctive features are the implementation of the beam splitter in the form of a polarizing cube that can be used for interferometer as sources of polarised light (laser) and unpolarized (incandescent lamp), and the orientation of the axes of the phase plate at an angle of 45° to the polarization vector speedliting is sustained fashion features are the introduction of the Polaroid between the lens and the recording device, performing beam-splitting element in the form of a semitransparent mirror or depolarization cube and the orientation of the axes of the phase plate at an angle of 45° to the polarization vector of the incoming radiation.

With the introduction of the phase plate/4 eliminates reflected diffraction orders, creating unwanted intensity modulation in the interference pattern. The lens for aperture allows you to combine the image of the interference pattern with the image surface, which leads to increase the contrast of the fringes and to improve the accuracy of measurement of aberrations in specific points on the surface. The use of zone plates, focal length which is consistent with the parameters of the test surface, i.e., performed in accordance with a ratio ofleads to reduce the instrumental error to an acceptable level.

The invention is illustrated by the following materials.

Fig.1 and Fig.2 - optical system, respectively the 1st and 2nd versions of the proposed diffraction interferometer.

Fig.3 (POS. 1, 2) - interferogram characterizing plenipot. 2 - f=25 mm).

Fig. 4 - interferogram obtained with the interferometer firm "Zygo" (POS. 1, 3, 5) and diffraction interferometer (POS. 2, 4, 6).

Fig. 5 is a photograph of the device.

In the scheme of the 1-St variant (Fig.1) 1 - light source, 2 - lens, 3 - polarizing beam-splitting cube, 4 - zone plate, 5 - controlled surface (mirror), 6 - phase plate/4, 7 - output aperture, 8 - projection lens, 9 - photometric.

The proposed device operates as follows. In the interferometer receives convergent pencil of rays formed by the lens 2 from the light source 1 and reflected by the beam splitter cube 3. Part of the beam forming a reference wave, passes through the zone plate 4 without deviation (0-th order diffraction [0]) and focuses at the center (point A1) surface (mirror) 5. The other part forming the measuring wave, dirigeret in the +1st order [+1], is focused at the intermediate point of A2and then diverging beam fills the aperture of the mirror. After reflection from the mirror both beams return to the zone plate, while the reference beam dirigeret in the +1st order in [0, +1], and the measurement takes place without deviation [+1, 0]. If distance is cuirous in the same point a3and fringe in the interference pattern have infinite width (uniform bright field). For small displacements of the mirrors across or along the optical axis exhibits bands of finite width or ring, respectively. Interference field is localized in the mirror plane, depicted in photometric 9 by the lens 8. At the same time on photometric projected image of the surface of the mirror that allows you to accurately compare the measurement result with a specific location on the surface of the mirror. In the output plane of the pupil, i.e., focused reference [0, +1] and measurement [+1, 0] beams, the aperture 7, the shielding partially outside of the diffraction orders in transmitted light[0, 0], [+1, -1] and [-1, +1]. These rules significantly attenuated by the aperture and practically do not distort the measurement result, since the projected light on the interference band in the center of the picture. Reflected 0th and +1st orders are eliminated as follows. After reflection from the polarizing cube 3 light beam becomes polarized with the polarization vector perpendicular to the plane of the drawing (Fig.1). Inside the interferometer working beams (reference and measurement) double-pass phase allows workers beams, past the interferometer, and delays reflected from the zone plate diffraction orders, in which the polarization vector is not changed.

In the scheme of the 2nd version of the interferometer (Fig.2) is applied to the light source (12) with linear polarization, the vector which is perpendicular to the plane of the drawing (Fig.2), and a beam splitter is in the form of depolarization cube or semi-transparent mirror 10. In this scheme, in addition to the items listed in the 1st version and having the same designations, between the lens 8 and photometrical 9 additionally introduced the Polaroid 11, the axis of polarization which is perpendicular to the polarization vector of the input radiation. Inside the interferometer, as well as in the 1st embodiment, the plane of polarization of the workers (the reference and measuring beams are rotated by 90°. In the opposite direction a beam splitter 10 flows in the direction of photometric 9 all outgoing beams: working and reflected from the zone plate, but through the Polaroid 11 are only the first, and reflected, in which the polarization vector is the same as the incoming beam is delayed.

In both variants of the interferometer instrumental error is reduced to an acceptable level by agreeing F. the economic mirrors. You know, if the subject (point source) is not in the center of curvature of the mirror, having spherical aberration. In our case (Fig.1 and Fig.2) the object and image to the mirror is a2and a3located away from him at distances S1=R-S2and S1=R+S3, respectively. Based on [3] and the expression for S1, spherical aberration can be represented as follows:

where u is the radial coordinate on the surface of the mirror, R is the radius of curvature of the mirror.

For a zone plate as depicting elements (lenses), the object and image are points A1and A2located away from her at distances R and S2. Based on the formulas of geometric optics, linking these distances to the focal length f of the zone plate, have S2=fR/(R+f). Substituting the last relation in (1), we obtain

Aberrations introduced itself by a zone plate, depend only on its position in the scheme of the interferometer. In the measuring and reference beams zone plate works as a positive lens, but in the first case, converging, and the second in divergent PU proportional to the square roots of integers. In the latter case, the zone plate makes the measuring beam aberrationptand in reference toprequal to [4]

whereis the radial coordinate in the zone plate, R is the distance from the zone plate to the substantive points equal to the radius of curvature of the mirror. Taking into account that=uf/R, from (3) and (4) it follows that the difference aberrationspmade by a zone plate in the measuring and reference beams equal to

On the basis of (2) and (5) the total wave aberration diagrams (full instrumental error) will be expressed by the ratio

From (6) it follows that in the center of the mirror (u=0) aberration is zero, and at the endpoints, i.e., when u=ua=D/2 (D, uathe diameter and the radius of the mirror surface), reaches the maximum value. By small defocus, namely the displacement of the mirror along the axis, you can enter additional difference in interferiruyushchei the value of the aberration is minimized instrumental errormax) is observed at u = 0,707(D/2) and equals

From the comparison of (6) with u=uaand (7) shows that the sign of the maximum error is changed to reverse, but the value is reduced by 4 times. The defocus operation is similar to the transition from a comparison of the test surface with a "peak" area for comparison with the nearest field, adopted in conventional methods of controlling the shape of the surfaces on a test glass.

From (7) it follows that the instrumental errormaxthe absolute value will not exceed the allowable errorif the focal length f of the zone plate corresponds to value

When control of a parabolic surface instrumental errorand maximummaxare determined from the relations

If the focal length of a zone plate corresponds to (8), the second part of equation (10), which minimized the instrumental error of the interferometer when �948.gif" border="0">maxin fact coincides with the expression for the maximum deviation of the ideal parabolic surface from the sphere (the first part of equation (10)), which is used for normal control of the parabolas on a spherical test glass. The difference between the measured value deviations and calculated based on (10), will characterize the accuracy of surface preparation. Similar consideration can be spend on the possibility of application of diffraction interferometer for monitoring other types of surfaces of the second order.

On the basis of the 1-St variant of the proposed optical scheme (Fig.(1) Institute of automation and Electrometry, Siberian branch of the Russian Academy of Sciences developed the diffraction interferometer for monitoring concave surfaces and mirrors of the second order in laboratory and industrial conditions (Fig.5). Test interferometer showed that it is easily adaptable and can work reliably with coherent (He-Ne or semiconductor laser) and incoherent (incandescent) light sources. The interference fringes have a high contrast, not distorted reflected glare and are not susceptible to mechanical vibrations. When the focal length of a zone plate f, is equal to 25 mm, instructor mirrors D/R=1/10-1/5, respectively, which satisfies the allowable errors of optical engineering.

In Fig.3 shows the interferogram (pictures interference fields), illustrating the influence of the choice of the focal length f of the zone plate on the instrumental errormax. Interferogram obtained for an ideal spherical surface with parameters: R=781,3 mm and D=120 mm as a light source was used semiconductor laser with a wavelength=0.65 micron. In the first case (Ref.1) f=50 mm and there is a bend in ~0.2 bands, indicating the presence of an instrumental error of the corresponding value (7). In the second case (Ref. 2) f corresponds to the relation (8), the lines are straight, as it should be for an ideal (betaversion) surface. In Fig.4 shows interferograms of two spherical (POS. 1, 2 and 3, 4) and one parabolic (POS. 5, 6) surfaces, tolerances of manufacture are controlled by the industrial interferometer firm "Zygo" (POS. 1, 3, 5) and on the developed diffraction interferometer (POS. 2, 4, 6). From the comparison of interferograms obtained on two interferometers for the same surfaces, widn is reliable.

Thus, in the proposed diffraction interferometer reduced instrumental error by applying the zone plate focal length, the value is consistent with the parameters of the test surface, reached a high contrast interference pattern due to the application of the projection lens, focusing on photometric simultaneously interference picture and controlled surface, and eliminated the reflected diffraction orders and bright glare using polarization optics. These technical solutions allow to improve the accuracy, reliability and immunity measurements in the control error of making concave surfaces and mirrors of the second order in laboratory and industrial conditions of the optical instrument.

Sources of information

1. R. N. Smartt, "Zone Plate Interferometer", Appl. Opt. 13, 1093-1099 (1974).

2. Optical production control./Ed. by D. Malacara. M.: Mashinostroenie, 1985.

3. N. N. Michelson. Optical telescopes. Theory and design. M.: Nauka, 1976.

4. G. A. Lenkova. "To the question about betaversion diffractive lenses", avtometriya, 3, 126-131 (2000).

Claims

1. Diffraction iately location on the main optical axis of the test surface, phase plates/4, zone plate, the center of which coincides with the center of curvature of the test surface, the beam splitter made in the form of a polarizing cube, the output aperture of the projection lens, the recording device, the presence of the illuminator, the optical axis which is perpendicular to the main optical axis, and consisting of a light source and lens, forming a converging beam, the axes of the phase plates/4 is oriented at an angle of 45° to the polarization vector of the beam-splitting cube, and the focal length f of the zone plate is selected from a ratio

where- valid instrumental error;

D and R are the diameter and the radius of curvature of the test surface.

2. Diffraction interferometer to measure the shape of concave surfaces and mirrors of the second order, which is characterized by a consistent location on the main optical axis of the test surface, the phase plates/4, zone plate, the center of which coincides with the center of curvature of the test surface, sweetely is#176; to the optical axis, the output aperture of the projection lens, Polaroid, recording device, the presence of the illuminator, the optical axis which is perpendicular to the main optical axis, and consisting of a source of linearly polarized light with the polarization vector oriented perpendicular to the principal optical axis, and lens, forming a converging beam, the axes of the phase plates/4 is oriented at an angle of 45° to the polarization vector of the radiation of the light source and the focal length f of the zone plate is selected from a ratio

where- valid instrumental error;

D and R are the diameter and the radius of curvature of the test surface.



 

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