RU2186336C1 - Interferometer to measure form of surface of optical articles - Google Patents

Abstract

FIELD: holography. SUBSTANCE: interferometer to measure form of surface of optical article incorporates illuminator, beam splitter, synthesized hologram, examined optical surface, diaphragm and recording unit. Synthesized hologram comes in the form of spatial combination of two holograms, one of which is hologram of bearing surface and the other one is hologram of examined surface. Spatial combination of holograms has the form of periodically alternating zone from first and second holograms with alternation period less than T = 2λl/D, where λ is wave length of radiation source; l is distance between diaphragm and hologram; D is diameter of aperture of diaphragm. EFFECT: diminished effect of vibration and optical noise in process of measurement of form of optical surfaces. 2 cl, 4 dwg

Description

 The invention relates to measuring equipment, namely to the field of non-contact optical measurement of the surface shape of optical products, for example, spherical and aspherical mirrors or lenses in the conditions of optical production and laboratory research.

 A well-known interferometer (Russian Patent 2114390, C 01 B 9/02, Interferometer for controlling the surface shape of convex hyperbolic mirrors), comprising a shaper of the working and reference wave fronts, a control spherical mirror, a plate with a hologram and a controlled aspherical mirror with a hole in the center.

 A significant drawback of this interferometer is its low accuracy, which is due to its high sensitivity to vibrations. Sensitivity to vibrations is due to the spatial spacing of the shapers of the reference and measuring wavefronts in the interferometer circuit.

 Also known is an interferometer with a common path of rays (RN Smart. Zone platter interferometer. Arl. Ortix, 1974, vol. 13, No.5, pp. 1093-1099) consisting of a radiation source, a beam splitter, on one side of which a zone plate is installed in series and the object under study, and on the other hand, the diaphragm and the recording device.

 The disadvantage of this interferometer is the low accuracy due to optical noise in the recorded interferogram. Optical noise arises due to diffraction of optical radiation on the axial zone plate and is not completely blocked by the diaphragm, as it propagates along the optical axis.

 The closest technical solution is an interferometer (T. Nonda, Y. Kavamoto, N. Guan, etc. Zoner-plat null interferometer for mesuring aspherisl murror with largé arep. .) consisting of a illuminator, a beam splitter, a synthesized hologram, a studied optical surface, a supporting optical surface, a diaphragm, and a recording device.

 The disadvantage of this device is the low accuracy due to, firstly, the optical noise that appears on the interferogram due to specular reflection of light from a plate with a hologram, light diffraction on the axial structure of the hologram, and secondly, high sensitivity to vibration, which is caused by the use of a flat mirror in the supporting arm of the interferometer.

 The author was tasked with creating an interferometer for measuring the surface shape of precision optical products under the influence of vibrations while increasing the accuracy of measurements.

The problem is solved in that in the interferometer, consisting of a illuminator, a beam splitter, a synthesized hologram. the studied optical surface, the reference optical surface, the diaphragm and the recording device, the synthesized hologram is made in the form of a spatial combination of at least two holograms, one of which is a hologram of the supporting surface, and the other is a hologram of the studied surface. Moreover, the spatial combination of holograms is made in the form of periodically alternating rings with the first and second holograms, and the alternation period is less than T = 2λl / D, where λ is the wavelength of the radiation source, l is the distance between the diaphragm and the hologram. D is the diameter of the diaphragm. In addition, the reflector surface has a spherical shape, and the synthesized hologram, the investigated and supporting optical surfaces, are inclined to the optical axis by an angle φ greater than φ = D / 2l.
The technical result of the invention is to reduce the influence of vibration and optical noise on the measurement of the surface shape of optical products - high aperture spherical and aspherical mirrors in optical production.

New distinctive features of the invention is that the synthesized hologram is made in the form of a spatial combination of at least two holograms, one of which is a hologram of the supporting surface, and the other is a hologram of the investigated surface. The spatial combination of holograms is made in the form of periodic alternating rings with the first and second holograms, and the alternation period is less than T = 2λl / D, where λ is the wavelength of the radiation source, l is the distance between the diaphragm and the hologram. D is the diameter of the diaphragm. The supporting surface has a spherical shape, and the synthesized hologram, the investigated and supporting optical surfaces, are inclined to the optical axis by an angle φ greater than φ = D / 2l.
The proposed invention is illustrated by the following graphic material.

 In FIG. 1 shows a diagram of an interferometer for measuring the surface shape of optical products.

 Figure 2 shows the relative position of the holograms on the optical substrate.

 In FIG. 3 shows an embodiment of an optical system of an interferometer.

 Figure 4 presents a plane view of the recording device when measuring the shape of the optical surface.

 The proposed interferometer (Fig. 1) consists of a illuminator 1, a beam splitter 2, a synthesized hologram 3, a studied optical surface 4, in the center of which a reference optical reflective surface 5, aperture 6 and a recording device 7 are installed. The illuminator 1 is made, for example, in the form of an optical coupled laser 8, a focusing lens 9 and a micro aperture 10. The recording device 7 consists, for example, of a lens 11 and a video camera 12. The synthesized hologram 3 (figure 2) is made in the form of a combination of two holograms 13 and 14. Out of the zone the main hologram has an additional zone with a reflecting synthesized hologram 15. In the embodiment of the interferometer in figure 3, a collimating lens 16 is installed in front of the hologram 3.

 The interferometer (figure 1) works as follows. The luminous flux from the illuminator 1, made in the form of a laser 8 installed in series, a micro lens 9 and a micro-diaphragm 10, enters the beam splitter 2. The reflected luminous flux enters the synthesized hologram 3. When the hologram 3 is illuminated, several diffraction orders are formed at its output.

где с₁= 0.8-1.2 - постоянный коэффициент, зависящий от положения и размеров отражателя, l₁ - расстояние вдоль оптической оси от осветителя 1 до голограммы 3, d- расстояние от голограммы до вершины исследуемой отражающей поверхности 4. Если центр кривизны отражателя совпадает с вершиной исследуемой поверхности, то с₁=1.The hologram is made in the form of a spatial combination of two independent holograms, so that the first one forms a wave front (curve u ₁ ) of a spherical shape centered at point s ₁ , and the second one forms a wave front (curve U ₂ ) that repeats the shape of the studied optical surface 4. The supporting optical surface (reflector) has a spherical shape. The wave front u ₁ focuses exactly in the center (point s ₁ ) of the curvature of the supporting surface 5 (reflector), which is installed near the top of the investigated surface 4, for example, an aspherical mirror with a hole in the center. In this diffraction order, the hologram acts as a lens with a paraxial focal length

where c ₁ = 0.8-1.2 is a constant coefficient depending on the position and size of the reflector, l ₁ is the distance along the optical axis from the illuminator 1 to the hologram 3, d is the distance from the hologram to the top of the investigated reflective surface 4. If the center of curvature of the reflector coincides with vertex of the investigated surface, then with ₁ = 1.

The light flux reflected from the reflector surface enters the hologram 3 in the same way, diffracts again on its structure, passes through a beam splitter 2 and focuses into the aperture (point s ₂ ) of the diaphragm 6. The diaphragm is a spatial filter and passes this light flux, but blocks optical radiation propagating outside the optical axis. The diameter of the aperture D is selected from the conditions for the convenience of setting up and filtering the scattered radiation as
D = (10-50) • λl / a, (2)
where λ is the wavelength of the radiation source, l is the distance from the diaphragm to the hologram 3, and a is the light diameter of the hologram 3. At l = 450 mm and a = 30 mm, λ = 633 nm, the diameter of the diaphragm is D = 0.1-0.5 mm. After passing through the diaphragm, the luminous flux enters the recording device 7, consisting of a lens 11 and a video camera 12. This luminous flux has a uniform distribution of phases and amplitudes and is a reference wavefront in the interferometer (plane P).

где d₁= ±(d-R), R - параксиальный радиус кривизны исследуемой поверхности, знак (-) соответствует мнимому (этот вариант представлен на фиг.1), а (+) действительному положению фокальной точки s₃. Если поверхность 4 имеет выгнутую форму, то
d₂=d+R.Another wavefront U ₂ at the output of the hologram is a measuring wavefront. The hologram is made in such a way that the shape of the measuring wavefront exactly repeats the shape of the investigated surface 4, if it is made perfectly. In this case, all the rays emerging from the hologram fall onto the surface under investigation at normal angles. In the paraxial approximation (or in the case when the investigated optical surface is a spherical mirror), the focal length of the hologram (distance Os ₃ ) in this diffraction order is

where d ₁ = ± (dR), R is the paraxial radius of curvature of the investigated surface, the sign (-) corresponds to the imaginary (this option is shown in figure 1), and (+) the actual position of the focal point s ₃ . If surface 4 has a curved shape, then
d ₂ = d + R.

After reflection, the light flux passes through hologram a second time. After diffraction, a diffraction order arises on the hologram structure, which focuses in the region of point s ₂ lying in the diaphragm 6. Other diffraction orders are blocked by the diaphragm and do not pass to the recording device 7. This light flux is used as measuring and interferes with the reference beam in the plane R.

Т. е. если ρ₁ = ρ₂, то b=T/2, а если ρ₁=0.9 и ρ₂=0.05 (соответственно, алюминиевое покрытие и оптическое стекло), то b~Т/3.To form two independent wavefronts U ₁ and U _{2, the} synthesized hologram 3 is made in the form of periodically alternating rings with the first 13 and second 14 holograms (figure 2). The period of the alternation of holograms is chosen less than
T <2λl / D. (5)
In this case, diffraction orders due to the ring structure are blocked by the diaphragm. The diffraction efficiency of the hologram in the wavefronts U ₁ and U ₂ is determined, respectively, as
η ₁ = (b / T) ² • η (6a)
and
η ₂ = (1-b / T) ² • η, (6b)
where b is the width of the rings with the structure of the first hologram, η is the diffraction efficiency of the structure itself. If the hologram structure is made in the form of a binary relief structure with a rectangular profile and depth λ / (n-1), where n is the refractive index of the hologram substrate material 3, then η = 0.4. Thus, at b = 772, the diffraction efficiency for each of the wave fronts will be η ₁ = η ₂ = 0.1. From the expressions (6) it follows that in order to obtain the highest possible contrast of interference fringes (the condition of equality of the intensities of the reference and measuring beams), it is necessary to choose the width of the ring structure from the relation
η $\genfrac{}{}{0ex}{}{\text{2}}{\text{1}}$ • ρ ₁ = η $\genfrac{}{}{0ex}{}{\text{2}}{\text{2}}$ • ρ ₂ , (7)
where ρ ₁ and ρ ₂ , respectively, are the reflection coefficients of the surface of the reflector 5 and the object under study 4. Equality (7) is satisfied when the width of the rings of the first hologram is equal

That is, if ρ ₁ = ρ ₂ , then b = T / 2, and if ρ ₁ = 0.9 and ρ ₂ = 0.05 (aluminum coating and optical glass, respectively), then b ~ T / 3.

To eliminate light entering the recording device that is specularly reflected from the hologram, the hologram substrate and the investigated reflective surface are tilted with respect to the optical axis by an angle φ> D / l. The angle φ is selected in the range from 0.5 ^o to 1 ^o . In this case, specular reflections from the optical plate with the hologram, as well as from the paraxial region of the investigated optical surface, are blocked by the diaphragm and do not pass to the recording device.

Other embodiments of the interferometer are also possible. Figure 3 shows an embodiment of the interferometer with an additional collimating lens 16 installed in front of the synthesized hologram 16. In this case, the hologram 3 is mounted in a parallel light beam, which makes it possible to somewhat simplify the alignment of the interferometer and change the distance between the hologram 3 and the illuminator 1. When illuminating the hologram with a plane wave front, the hologram has a focal length f ₁ = c ₁ d.

For accurate installation of the hologram 3 in space at an angle φ outside the zone of the holograms 13 and 14, an additional reflective hologram 15 is applied in the form of an annular zone (figure 2). The period of the diffraction structure is chosen in such a way that when the plate is tilted to the desired angle φ, the light flux is reflected and diffracted exactly backward, passes through the diaphragm and enters the recording device. With proper adjustment of the angular position of the plate, a bright ring 17 is observed in the recording device (plane P), located around the interference pattern 18 of the investigated surface, as shown in FIG. 4. When using an optical system with a collimating lens 16 (illumination of the hologram with a flat beam of light, as shown in Fig. 3), the additional hologram has the form of a linear diffraction grating with a period b = λsin (2φ).
In the proposed interferometer, both beams (reference and measuring) pass simultaneously all the optical components and have the same optical path length, therefore, the influence of disturbing factors (vibrations, air flows, coherence length, etc.) is significantly reduced. Since in the proposed device the supporting surface has a spherical shape and the reference beam is focused on this surface (cat-eye reflector), the influence of tuning errors is minimized, and the dimensions of the reflector (several millimeters) are significantly smaller than the light diameter of the hologram and, accordingly, the dimensions of a flat mirror in famous devices. This allows you to explore objects (mirrors) with a small central hole, as well as without it.

 The main advantages of the proposed technical solution are as follows. The general course of the measuring and reference light beams in the proposed interferometer makes it possible to measure the surface shape of optical products under the influence of vibrations and air flows while increasing accuracy by eliminating optical noise. This makes it possible to accurately measure the surface shape of high-aperture spherical and aspherical mirrors under conditions of optical production.

Claims (3)

 1. An interferometer for measuring the surface shape of optical products, consisting of a illuminator, a beam splitter, a synthesized hologram, a studied optical surface, a supporting optical surface, aperture and a recording device, characterized in that the synthesized hologram is made in the form of a spatial combination of at least two holograms, one of which is a hologram of the supporting surface, and the other is a hologram of the surface under study, and the spatial combination of holograms is made in ide of periodically alternating zones with the first and second holograms, with an alternation period shorter than T = 2λl / D, where λ is the wavelength of the radiation source, l is the distance between the diaphragm and the hologram, D is the diameter of the aperture.  2. The interferometer according to claim 1, characterized in that the supporting surface has a spherical shape.  3. The interferometer according to claim 1, characterized in that the synthesized hologram, the investigated and reference optical surfaces are inclined to the optical axis by an angle φ greater than φ = D / l.

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