JPH0619255B2 - Interferometer and interferometer for aspherical surface measurement by optical spatial light modulator using liquid crystal - Google Patents

Description

TECHNICAL FIELD The present invention relates to an interferometer and an interferometer for measuring an aspherical surface such as a paraboloid.

[Prior Art] It is often necessary to measure the optical surface of an object with high accuracy. This measurement has been conventionally performed by various methods, and the most general method among them is the interferometry. This is because the test wavefront reflected by the measurement surface interferes with a "standard" wavefront of known waveform to create interference fringes, which is analyzed to determine the difference in the waveforms of these two wavefronts and thus the shape of the test wavefront. To know.

[Problems to be Solved by the Invention] This measurement technique is relatively easy to use on a flat or spherical surface. This is because it is easy to generate precise plane wavefronts and spherical wavefronts as "standard" wavefronts. However, despite the increasing demand for high precision measurements of aspheric surfaces, such as parabolic measurements, it is extremely difficult to generate precise known "standard" wavefronts in this case. A computer generated hologram (abbreviated as CGH hereinafter) has been applied to this and has achieved some success. Here, the diffraction results in a computer-generated non-linear diffraction grating are used to create a wavefront with the required phase distribution, which is compared with the DUT. However, it is necessary to replace the CGH from the optical system each time the shape of the DUT changes, and for that purpose, it is necessary to replace it with another CGH for generating an appropriate test wavefront for the DUT. . Not only is this inconvenient, it can lead to significant measurement failure under certain circumstances.

Computer-generated large-diameter holograms are also difficult. Since holograms are usually made to operate in a side band mode, high resolution is required to well separate diffracted light and undiffracted light.

In order to achieve the required high resolutions across large apertures, the space-bandwidth CGH has to be very large, which is difficult to meet with currently available recording devices. is there.

The present invention has been made in view of the circumstances as described above, and can be commonly used for measuring surface shapes of various different objects to be measured, and various standard wavefronts required for various tests. Another object is to provide an interferometer for measuring an aspherical surface and an interferometer used for the same, which has flexibility to generate a test wavefront.

[Means for Solving the Problem] To this end, the interferometry method for aspherical surface measurement by the spatial light phase modulation element using the liquid crystal of the present invention causes the standard wavefront and the test wavefront reflected from the test surface to interfere with each other. An interferometric method for measuring a test surface by analyzing interference fringes generated by the method, wherein a liquid crystal layer is arranged in an optical path of an optical system for generating the standard wavefront or the test wavefront, and the liquid crystal layer is traversed. It is characterized in that the spatial distribution of the refractive index of the liquid crystal layer is controlled by controlling the spatial distribution of the electric field, and thereby the spatial phase of the light flux passing through the optical path is modulated.

Further, an aspherical surface measuring interferometer using an optical spatial phase modulation element using the liquid crystal of the present invention has a test wavefront generating optical system for generating a test wavefront and a standard wavefront generating optical system for generating a standard wavefront, An optical spatial phase modulator is arranged in the optical path of the standard wavefront generating optical system, and the optical spatial phase modulator forms a liquid crystal layer and an electric field located on both sides of the liquid crystal layer and across the liquid crystal layer. And at least one electrode of the two electrodes is composed of a plurality of small electrodes distributed along the liquid crystal layer and capable of independently changing the voltage. .

[Operation] In the measurement technique of the present invention, various standard wavefronts are used by using an optical spatial phase modulation element capable of directly modulating the spatial phase distribution of light passing through the standard wavefront generation optical system or the test wavefront generation optical system. Or, since a test wavefront can be generated, various CGHs in the prior art are not required.
The modulation of the optical spatial phase distribution required to generate the standard wavefront or the test wavefront is achieved as a result of the electric field induced optical path length change in the liquid crystal layer. The spatial light phase modulator operates by first-order diffraction.

Furthermore, since the optical spatial phase distribution is modulated by the electric field,
By simply changing the electric field distribution, various known output wavefronts (standard wavefront or test wavefront) can be generated without removing the phase modulation element from the standard wavefront generation optical system or the test wavefront generation optical system. In this way it is possible to measure many different surfaces very conveniently.

The optical spatial phase modulator is energized by the video signal to produce, for example, a known standard wavefront emanating from the interferometer's standard wavefront generation optics, which is measured from the measurement plane in the interferometer's test wavefront generation optics. Interfering with the test wave front of to create interference fringes. The shape of the measurement surface is determined by analyzing this interference fringe.

[Embodiment] Hereinafter, details of the present invention will be described with reference to the drawings illustrating an embodiment.

In FIG. 1, reference numeral 10 denotes an aspherical surface measuring interferometer, and the aspherical surface measuring interferometer 10 includes a standard wavefront generating optical system 11 and a test wavefront generating optical system 12. The standard wavefront generation optical system 11 includes a mirror M ₁ , lenses L ₃ , L ₂ and L ₄ , a hologram H, a beam splitter BS ₂ , a filter F ₂ , and an optical spatial phase modulation element 1.

The test wavefront generation optical system 12 includes a measurement surface O that is an object to be measured, a lens L ₁ , a beam splitter BS ₁ , and a hologram H. In addition, L ₅ is a lens, F ₁ is a filter, S is a screen, CO is a computer, and ID is an image display device.

The optical spatial phase modulator 1 is arranged in the optical path of the standard wavefront generating optical system 11 in order to modulate the spatial phase distribution of light.

As shown in FIGS. 2 and 3, the spatial light phase modulator 1 comprises a liquid crystal layer 4 (about 6 μm thick) sandwiched between two glass plates 2 and 3. These glass plates 2 and 3 are provided with electrodes 5 and 6, respectively, and the electrode 6 of one glass plate 3 is composed of small electrodes, and the voltage of each small electrode is individually variable. As shown in FIG. 3, many small electrodes 6 are densely arranged in a matrix, and an electric field can be individually applied by the scan electrode driving circuit 7 and the signal electrode driving circuit 8. These small electrodes 6 are arranged in a rectangle 16
Each of the electronic circuits (scan electrode drive circuit 7, signal electrode drive circuit 8) placed in 0 × 120 pixels is
It is arranged so that the electric field distribution across the liquid crystal layer 4 can be modulated according to an external video signal. The inner surface of each of the glass plates 2 and 3 is processed so as to be in a nematic mode in which it is twisted by a twist of 0 to 360 °. The alignment of liquid crystal molecules on the surfaces of the glass plates 2 and 3 is shown in FIG.

This spatial light phase modulator 1 is placed in the optical path of the standard wavefront generating optical system 11 shown in FIG.

The collimated polarized laser beam 20 emitted from the laser light source enters the interferometer and first enters the beam splitter BS ₁ . Wherein a portion of the light is in a spherical wavefront by the lens L ₁ is reflected toward the lens L _1, which is then reflected by the measurement surface O. The returning divergent wavefront is approximately re-collimated by lens L ₁ , part of which passes through beam splitter BS ₁ to hologram H and forms one output beam (test wavefront) of interferometer 10.

Further, another part of the input light that _first enters the beam splitter BS ₁ passes through the beam splitter BS ₁ and reaches the second beam splitter BS ₂ . A portion of that light is now reflected and this portion is used to generate the second output beam (standard wavefront) of interferometer 10. The part transmitted through the beam splitter BS ₂ is lost. Beam splitter B
The reflected light at S ₂ first passes through the spatial light phase modulation element 1 using liquid crystal. This optical spatial phase modulator 1 has 0 to 360
It works in a nematic mode that is twisted by a twist of ゜. Light incident on the front surface of this element is polarized such that its electric field vector is parallel to the long axis of the liquid crystal molecules. The electric field of light traversing the liquid crystal layer 4 is rotated by about 0 to 360 ° between the portion near the entrance of the liquid crystal layer 4 and the portion across the liquid crystal layer, and the polarization of light is also rotated by about 0 to 360 °. The relationship between the alignment of liquid crystal molecules and the electric field vector is shown in FIG.

FIG. 4 is an explanatory view showing an alignment state of polarization of incident light by liquid crystal molecules in the liquid crystal layer 4.

In the figure, reference numeral 21 indicates the direction of alignment of liquid crystal molecules on the front surface of the liquid crystal layer 4.

Reference numeral 22 indicates the orientation direction of liquid crystal molecules on the rear surface of the liquid crystal layer 4.

Reference numeral 23 indicates the orientation direction of the electric field vector of light incident from the front surface of the liquid crystal layer 4.

Reference numeral 24 indicates the orientation direction of the electric field vector of the light emitted from the rear surface of the liquid crystal layer 4 (the electric field crossing the liquid crystal layer 4 is set to a threshold value).

After passing through the liquid crystal layer 4, the light is collected by the lens L ₂ ,
It passes through the spatial filter F ₁ to the lens L ₃ and then is reflected by the mirror M ₁ and passes through the liquid crystal layer 4 of the spatial light phase modulation element 1 again to the lens L ₃ , the filter F ₁ and the lens L 3.
It returns to the liquid crystal layer 4 via ₂ . On the way back through the liquid crystal layer 4, the polarization of the light is turned back by the same angle, and the polarization of the light emitted from the front surface of the liquid crystal layer 4 becomes almost the same as when the light enters the liquid crystal layer, and the state of the liquid crystal molecules changes. Irrelevant.

The light emitted from the front of the liquid crystal layer 4 passes through the beam splitter BS ₂ toward the hologram H. It is very difficult to keep the optical path length passing through the liquid crystal layer 4 constant and to manufacture such a liquid crystal modulator at low cost. Therefore, since the aberration introduced by this modulator tends to be large, the purpose of the hologram is to correct these aberrations and output a high quality output light beam from the standard wavefront generating optical system 11 of the interferometer 10. That is. This hologram H
Also acts as a beam combiner, merging the two beams of the interferometer. These beams are then lens L
₅ , an interference fringe pattern is formed on the screen S. The spatial filter F ₂ is used to remove unnecessary-order diffracted light from the lattice structure of the pixels in the spatial light phase modulation element, unnecessary diffracted light, and scattered light from the hologram H.

By analyzing the interference fringe pattern on the screen S, it is possible to know the distance from the spherical surface of the surface to be measured, which is an aspherical surface, and the spatial light phase modulation distribution by the spatial light phase modulation element using liquid crystal.

[Effects of the Invention] As described above, according to the present invention, it can be commonly used for measuring the surface shape of various different objects to be measured, and it is necessary for various kinds of tests only by changing the command from the computer. It is possible to obtain an interferometer for measuring an aspherical surface and an interferometer used for the aspherical surface, which has flexibility to generate various standard wavefronts.

Other Embodiments The embodiments described above are examples in which the present invention is applied to an interferometer using a hologram, but the present invention can also be applied to other types of interferometers. That is, FIG. 5 is an example in which the present invention is applied to a Twyman Green interferometer, FIG. 6 is an example in which the present invention is applied to a Fizeau interferometer, and FIG. 7 is a Machtz. Ender (Mach
Zender) This is an example applied to an interferometer.

In the Twyman-Green interferometer 30 shown in FIG. 5, the laser light source 13, the microscope objective lens MO, and the collimators C are used.
L, beam splitter BS, converging lens CVL ₁ , reference spherical surface R, aspherical measuring surface O, converging lens CVL ₂ , optical spatial phase modulator 1, optical planes OF ₁ , OF ₂ , imaging lens IL, image sensor IS Is equipped with.

In this Twyman-Green interferometer, one optical system has a precise converging lens CVL ₂ and a perfect reference spherical surface R,
The same converging lens CVL ₁ , the measurement surface O, and the optical spatial phase modulation element 1 are arranged in the other optical system. In this embodiment, the aberration of the spatial light phase modulation element 1 is corrected by the optical planes OF ₁ and OF ₂ in close contact with it. This correction method can also be used for the aberration correction of the spatial light phase modulation element in another embodiment described later. If the 2PI method phase modulation can be applied to the liquid crystal as well, the self-aberration of the spatial light phase modulation element 1 can be corrected with it.

Since the same lens is used for both optical systems in the Twyman-Green interferometer, the aberrations of these lenses are cancelled.

Also, the number of sources of aberration is one less. However, using the same lens is not essential. The optical spatial phase modulator can basically correct aberrations in the system by appropriate modulation like a variable mirror.

The imaging lens IL forms an image of the plane of the curved mirror surface on the image sensor IS. As the image sensor IS, for example, a CCD camera that does not use a lens can be used. This image shows an aspherical measurement surface by interference fringes.

In the Fizeau interferometer 40 shown in FIG. 6, the laser light source 13,
Microscope objective lens MO, beam splitter BS, collimators CL, reference spherical surface RF, optical spatial phase modulator 1,
It includes a converging lens CVL, a measurement surface O, a filter F, an imaging lens IL, an image sensor IS, an image display device ID that drives the optical spatial phase modulation element 1, and a computer CO that controls the image display device ID.

This Fizeau interferometer 40 is a conventional Fizeau interferometer in which the laser light source 13 is used as a light source and the second reference surface is replaced with the optical spatial phase modulation element 1.

Also in this embodiment, the aberration correction of the spatial light phase modulation element 1 is performed on the optical plane, or the self spatial aberration correction is performed using the spatial light phase modulation element 1 or the high quality glass is used. The converging lens CVL constitutes a spherical wave reflected from the measurement surface O. If this measurement surface O is not a spherical surface, the reflected wave surface will also be different from a spherical surface. The spatial light phase modulation element 1 forms a phase distribution in which the phase difference between the measurement surface O and the perfect spherical surface is doubled. By measuring this phase modulation, the difference between the measurement surface O and the spherical surface is detected. The image forming lens IL uses an image sensor (for example, CCD
Formed as interference fringes on the array.

In the Mach-Zehnder interferometer 50 shown in FIG. 7, the laser light source 13, the microscope objective lens MO, the collimators C
L, beam splitter BS ₁ , mirror M ₁ , measuring lens O (object to be measured), beam splitter BS ₂ , imaging lens IL, filter F, image sensor IS, optical spatial phase modulation element 1, converging lens CVL, mirror M _2, the image display apparatus ID for driving the optical spatial phase modulator 1 includes a computer CO for controlling the image display apparatus ID.

In this embodiment, an optical spatial phase modulator 1 is incorporated in a conventional Mach-Zehnder interferometer. A twisted nematic type liquid crystal is not suitable as the liquid crystal in the spatial light phase modulation element 1.

The measurement lens O, which is the object to be measured, the optical spatial phase modulator 1, and the high-quality converging lens CVL are equidistant from the output beam splitter BS ₂ . The imaging lens IL forms the image of the measuring lens O on the image sensor IS. The surface of the measuring lens O can be observed as interference fringes.

[Brief description of drawings]

FIG. 1 is a structural explanatory view of an interferometer according to an embodiment of the present invention, FIG. 2 is a schematic sectional explanatory view of an optical spatial phase modulation element, and FIG. 3 is a schematic planar explanatory view of the optical spatial phase modulation element. FIG. 4 is an explanatory view showing the orientation of polarization of incident light incident on the spatial light phase modulation element, FIG. 5 is a structural explanatory view showing an example in which the present invention is applied to a Twyman-Green interferometer, and FIG. 6 is the present invention. FIG. 7 is a structural explanatory view showing an example in which is applied to a Fizeau interferometer, and FIG. 7 is a structural explanatory view showing an example in which the present invention is applied to a Mach-Zehnder interferometer. 1 ... Optical spatial phase modulator 2, 3 ... Glass plate 4 ... Liquid crystal layer 5 ... Electrode 6 ... Electrode 7 ... Scan electrode drive circuit 8 ... Signal electrode drive circuit 10 ... Interference for aspherical surface measurement 11: Standard wavefront generating optical system 12: Test wavefront generating optical system 13: Laser light source 20: Polarized laser beam 30: Twyman Green interferometer 40: Fizeau interferometer 50: Mach-Zehnder interferometer M ₁ ... Mirrors L ₁ , L ₂ , L ₃ , L ₄ , L ₅ ... Lens H ... Holograms BS, BS ₁ , BS ₂ ... Beam splitters F, F ₁ , F ₂ ... Filter S ... Screen MO ... Microscope objective lens CL ... Collimators CVL, CVL ₁ , CVL ₂ ... Converging lens O ... Measuring surface OF ₁ , OF ₂ ... Optical plane IL ... Imaging lens IS ... Image RF: Reference plane CO: Computer ID: Image display device

Claims (2)

[Claims] 1. An aspherical measurement interferometry method for measuring a test surface by analyzing interference fringes generated by interfering a standard wave surface and a test wave surface reflected from the test surface, the standard wave surface or the test wave surface being generated. A liquid crystal layer is arranged in the optical path of the optical system, and the spatial distribution of the refractive index of the liquid crystal layer is controlled by controlling the spatial distribution of the electric field that crosses the liquid crystal layer. Interferometry for aspherical surface measurement by optical spatial phase modulator using liquid crystal characterized by modulating light 2. A test wavefront generation optical system for generating a test wavefront and a standard wavefront generation optical system for generating a standard wavefront,
An optical spatial phase modulator is disposed in the optical path of the standard wavefront generating optical system, and the optical spatial phase modulator forms a liquid crystal layer and an electric field located on both sides of the liquid crystal layer and crossing the liquid crystal layer. At least one of the two electrodes is composed of a plurality of small electrodes distributed along the liquid crystal layer and capable of independently changing the voltage. Interferometer for aspherical surface measurement using optical spatial light modulator with rotating liquid crystal