Interferometer for measuring a surface shape of optical products

 

(57) Abstract:

Interferometer for measuring a surface shape of optical products consists of the light source, beam splitter, synthesized holograms, investigated the optical surface, the reference optical surface, the aperture and the recording device. The synthesized hologram made in the form of a spatial combination of the two holograms, one of which is a hologram of the reference surface, and the other is a hologram of the surface. The spatial combination of holograms made in the form of periodically alternating zones with the first and second holograms, with the period of rotation is smaller than T = 2l/D, where is the wavelength of the radiation source, l is the distance between the diaphragm and the hologram, D is the diameter of the hole of the diaphragm. The technical result - the reduction of the influence of vibrations and optical noise in the measurement of the shape of optical surfaces. 2 C.p. f-crystals, 4 Il.

The invention relates to measurement devices, and specifically to the field of contactless optical measurement of the surface shape of optical products, for example, spherical and aspherical mirrors or lenses in terms of optical engineering and laboratory research.

A significant drawback of this interferometer is its low accuracy, which is due to the high sensitivity to vibration. Sensitivity to vibrations due to spatial separation shapers reference and measurement wavefronts in the scheme of the interferometer.

Also known interferometer with the General course of rays (R. N. Smrt. Zn lt interferometer. l. tis, 1974, vl. 13, NR.5, PP. 1093-1099) consisting of a radiation source, beam splitter, one side of which has consistently zone plate and the object under examination, and on the other hand aperture and the recording device.

The disadvantage of this interferometer is the low accuracy due to optical noise registered in the interferogram. Optical noises occur due to the diffraction of optical radiation in the axial zone plate and not completely blocked by the aperture, as propagated along the optical axis.

The closest technical solution AOS. of SPI, 1992, vl. 1720, PP. 305-310. ) composed of the light source, beam splitter, synthesized holograms, investigated the optical surface, the reference optical surface, the aperture and the recording device.

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

The author undertook the task of creating an interferometer for measuring a surface shape precision optical products exposed to vibration while increasing accuracy.

The problem is solved in that in the interferometer, consisting of light, the beam splitter, the synthesized hologram. the analyzed optical surface, the reference optical surface, the aperture and the recording device, synthesized hologram made in the form of spatial combinations of at least two holograms, one of which is the result of the combination of holograms made in the form of periodically alternating rings of the first and second holograms, moreover, the period of rotation is less than T = 2l/D, where is the wavelength of the radiation source, l is the distance between the diaphragm and the hologram. D is the diameter of the aperture. In addition, the surface of the reflector has a spherical shape, and the synthesized hologram, studied and reference optical surface inclined to the optical axis at an angle greater than = D/2l.

The technical result of the invention is to reduce the influence of vibrations and optical noise on the measurement of the surface shape of optical products - vysokoaperturnykh spherical and aspherical mirrors in the optical conditions of production.

New distinctive features of the invention is that the synthesized hologram made in the form of spatial combinations of at least two holograms, one of which is a hologram of the reference surface, and the other is a hologram of the surface. The spatial combination of holograms made in the form of periodic alternating rings of the first and second holograms, and the period of rotation is less than T = 2l/D, where is the wavelength of the radiation source, l is the distance between the diaphragm and the hologram. D is the diameter of the aperture. Bearing surface is spherical in f is at an angle greater than = D/2l.

The proposed invention is illustrated in the following graphic material.

In Fig. 1 presents a diagram of the interferometer for measuring a surface shape of optical products.

In Fig.2 shows the relative positions of the holograms on an optical substrate.

In Fig. 3 presents an embodiment of the optical system of the interferometer.

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

The proposed interferometer (Fig.1) consists of the light source 1, the beam splitter 2, the synthesized hologram 3, the analyzed optical surface 4, in the centre of which there is reference optical reflecting surface 5, the diaphragm 6 and the recording device 7. The illuminator 1 is made, for example, in the form of optically coupled laser 8, a focusing lens 9 and microdamage 10. Recording device 7 is, for example, of the lens 11 and the camera 12. The synthesized hologram 3 (Fig.2) made in the form of combinations of two holograms 13 and 14. Outside the main hologram caused additional area with reflective synthetic hologram 15. In the embodiment of the interferometer of Fig.3 before the hologram 3 mouth is from the illuminator 1, made in the form prescribed sequentially laser 8, microobjective 9 and microdamage 10, enters the beam splitter 2. The reflected luminous flux is supplied to the synthesized hologram 3. When the illumination of the hologram 3 its output has formed several diffraction orders.

The hologram is made in the form of a spatial combination of two independent holograms, so that the first of them forms a wave front curve (u1spherical shape with a center point of s1and the second forms a wave front curve (U2), the shape of the investigated optical surface 4. The reference optical surface (reflector) has a spherical shape. Wave front u1focuses in the exact center point (s1) curvature of the reference surface 5 (reflector), which is installed near the top of the sample surface 4, for example, aspherical mirrors with a hole in the center. In this diffraction order hologram performs the function of the lens paraxial focal length

< / BR>
where C1= 0.8-1.2 - constant coefficient depending on the position and size of the reflector, l1- the distance along the optical axis of the illuminator 1 to the hologram 3, d is the distance between the Noah and the sample surface, the1=1.

Reflected from the surface of the reflector luminous flux enters the same path back to the hologram 3, again dirigeret on its structure, passes the beam splitter 2 and is focused into the hole (point s2) diaphragm 6. Aperture is a spatial filter and transmits this light flow but blocks the optical radiation propagating outside the optical axis. The diameter of the aperture D is selected from terms of ease of configuration and filtering of the scattered radiation as

D = (10-50)l/a (2)

where is the wavelength of the radiation source, l is the distance from the aperture to the hologram 3 and the light diameter of the hologram 3. When l=450mm and a=30mm, =nm, the aperture diameter of D = 0.1-0.5 mm Passing through the aperture of the luminous flux supplied to the recording device 7 consisting of a lens 11 and the camera 12. This luminous flux is uniformly distributed phases and amplitudes and is the reference wave front interferometer (plane P).

Another wave front U2the output of the hologram is measuring the wavefront. The hologram is made so that the shape of the measurement wavefront exactly reproduces the shape of the examined surface 4, if it is made Itami. In the paraxial approximation (or in the case when the analyzed optical surface is a spherical mirror, the focal length of the hologram (the distance OS3this diffraction order equal to

< / BR>
where d1= (d-R), R is a paraxial radius of curvature of the surface being examined, the minus sign (-) corresponds to an imaginary (this option is shown in Fig.1), and (+) to the actual location of the focal point s3. If the surface 4 has a curved shape,

d2=d+R.

After reflection luminous flux again passes through the hologram 3. After diffraction on the structure of the hologram occurs diffraction order that is focused in the area of point s2lying in the diaphragm 6. Other diffraction orders are blocked by the aperture and do not pass to the register 7. This luminous flux is used as a measuring and interferes with the reference beam in the plane of the R.

For the formation of two independent wavefronts U1and U2the synthesized hologram 3 is made in the form of periodically alternating rings of the first 13 and second 14 holograms (Fig.2). The period D of the rotation of the hologram is chosen less than

T<21= (b/T)2(6a)

and1=2= 0.1. From expressions (6) it follows that to obtain the highest contrast of the interference fringes (the condition of equality of the intensities of the reference and measuring beams), you need to choose the width of the ring structure of the formula:

211=222, (7)

where1and2accordingly, the reflectivity of the surface of the reflector 5 and the investigated object 4. Equality (7) is performed with the width of the rings of the first hologram is equal to

< / BR>
I.e. if1=2b=T/2, and if1=0.9 and 2=0.05 (respectively, the aluminum coating and the optical glass), then b~T/3.

To eliminate the ingress to the registering device is specularly reflected from the hologram light, the substrate with the hologram and analyzed reflecting surface tilted with respect to the optical axis at an angle >D/l. The angle selected in the range of 0.5oto 1o. In this case the tion of the surface are blocked by the aperture and do not pass to the register.

Alternative embodiments of the interferometer. In Fig.3 presents an embodiment of the interferometer is installed in front of the synthesized hologram additional collimating lens 16. In this case, the hologram 3 is installed in the parallel beam of light, which allows to simplify the alignment of the interferometer and to change the distance between the hologram 3 and the illuminator 1. When the illumination of the hologram plane wavefront, the hologram has a focal length f1= C1d.

For precise installation of the hologram 3 in space at an angle outside of the hologram 13 and 14 caused additional reflective hologram 15 in the form of an annular zone (Fig.2). The period of the diffraction patterns is selected in such a way that the tilting of the plate at a desired angle , luminous flux is reflected and dirigeret right back, passes the aperture and is supplied to a recording device. When properly configured, the angular position of the plate, in the recording device (plane P) there is a bright ring 17 located around the interference pattern 18 and the sample surface, as shown in Fig. 4. When using an optical system with a collimating lens 16 (lit klonoa lattice with period b = sin(2).

The proposed interferometer both beams (reference and measurement) simultaneously pass through all the optical components and have the same optical path length, so the influence of disturbing factors (vibration, air flow, length, coherence, etc.,) is significantly reduced. Since the proposed device, the support surface has a spherical shape and the reference beam is focused on this surface (reflector type "cat's eye"), the impact of configuration errors is minimized, and the size of the reflector (a few millimeters) is substantially less than the length of the diameter of the hologram and, accordingly, sizes of flat mirrors in known devices. This allows you to capture objects (mirrors) with a small Central hole, and also without it.

The main advantages of the proposed solution are as follows. The General course of the measurement and reference light beams in the proposed interferometer allows to measure the surface shape of optical products in terms of vibrations and air flow while increasing the accuracy by eliminating the optical noise. This enables high-precision measurement of the surface shape of vysokopitatelny the surface shape of optical products, consisting of light, the beam splitter synthesized holograms, investigated the optical surface, the reference optical surface, the aperture and the recording device, characterized in that the synthesized hologram made in the form of spatial combinations of at least two holograms, one of which is a hologram of the reference surface, and the other is a hologram of the sample surface, and the spatial combination of holograms made in the form of periodically alternating zones with the first and second holograms, with the period of rotation smaller than T = 2l/D, where is the wavelength of the radiation source, l is the distance between the aperture and the hologram D is the diameter of the hole of the diaphragm.

2. The interferometer under item 1, characterized in that the bearing surface has a spherical shape.

3. The interferometer under item 1, characterized in that the synthesized hologram, studied and reference optical surface inclined to the optical axis at an angle greater than = D/l.

 

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