Polarizing Fizeau interferometer

Abstract

To provide an interferometer, in particular a Fizeau interferometer capable of improving contrast of interferograms.SOLUTION: A Fizeau interferometer 1 is provided, comprising a light source 2, a reference surface 4, a tested surface 5 provided on a support of the Fizeau interferometer, and an image capturing system 3. The Fizeau interferometer uses a polarization reference surface to improve contrast of an interferogram. The present invention also relates to a method of using the Fizeau interferometer provided herein to improve contrast of interferograms obtained by the Fizeau interferometer.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to the field of interferometers, and more particularly to a Fizeau interferometer that improves the contrast of an interference pattern. A Fizeau interferometer includes a light source, a reference surface, a test surface disposed on a support of the Fizeau interferometer, and an imaging system. The present invention further relates to a method of using the Fizeau interferometer of the present invention for improving the contrast of an interference pattern obtained by the Fizeau interferometer.

Background Art

[0002] A Fizeau interferometer system may be used to measure the characteristics of a test surface, for example, disposed on a test surface support. By interfering a reference light beam and a test light beam using a Fizeau interferometer, an interference pattern called an interference image may be formed. The reference light beam is a light beam reflected by the reference surface, and the test light beam is a light beam reflected by the test surface. The characteristics of the test surface affect the interference pattern as compared with the reference surface. Therefore, the characteristics of the test surface can be determined by analyzing the interference pattern. These characteristics may include characteristics related to the height of the test surface.

[0003] In a known Fizeau interferometer, the test surface and the reference surface are arranged opposite to each other.

[0004] A known Fizeau interferometer comprises a light source that emits a light beam traveling along an optical path, the first portion of which extends between the light source and a reflective surface under test placed on a support of the Fizeau interferometer, the reference surface positioned in the first portion of the optical path between the light source and the support of the surface under test, the second portion of which extends from the surface under test to the reference surface and to the imaging system of the Fizeau interferometer that measures the light incident from the second portion of the optical path, the reference surface being partially reflective so as to generate a reference light beam and a test light beam, the reference light beam being emitted from the light source A reference light beam is formed by partial reflection of a light beam incident along the first portion of the optical path, and this reference light beam is reflected towards the imaging system along the second portion of the optical path. A test light beam is formed by passing a light beam that has passed through the reference surface along the first portion of the optical path towards the test surface, and this test light beam is reflected by the test surface towards the reference surface and then towards the imaging system along the second portion of the optical path. The imaging system is configured to measure the interference diagram generated by interfering the reference light beam and the test light beam.

[0005] The light beam under test may pass through a reference surface after being reflected from the surface under test.

[0006] The optimal performance of a Fizeau interferometer is improved by better contrast in the interferogram. This improved optimal performance leads to better measurements with the Fizeau interferometer, for example, enabling tests with improved axial resolution of the surface under test. The interferogram contrast is near maximum when the intensity ratio of the reference beam to the test beam is approximately equal. When the intensity of the reference beam and the test beam are approximately equal, the two beams are said to be in equilibrium.

[0007] The intensity ratio of the two light beams may be uneven, for example, if the reflectivity of the surface under test is low. In this case, the intensity of the reference light beam may be higher than the intensity of the light beam under test. This reduces the contrast of the interferometer and therefore degrades the performance of the Fizeau interferometer.

[0008] Similarly, if the reflectivity of the surface under test is higher than that of the reference surface, the intensity of the test beam may be higher than that of the reference beam. In this case, the contrast of the interferometer will also decrease, and the performance of the Fizeau interferometer may be lower than desired.

[0009] In known Fizeau interferometers, the beam ratio, i.e., the ratio of the reference beam to the test beam in the imaging system, is controlled by changing the reflectivity of the reference plane. The reflectivity of the reference plane may also be controlled by replacing the reference plane with a reference plane having a different reflectivity. Alternatively, the beam ratio may be controlled by inserting a so-called ND (neutral density) filter into the optical path. In known Fizeau interferometers, the ND filter is usually placed between the reference plane and the test plane.

[0010] These methods for controlling the beam ratio require numerous different reference planes and / or ND filters, depending on the surface being tested. Each reference plane and ND filter must have appropriate reflectivity and transmittance for the respective surface being tested. The need for various reference planes and / or ND filters can increase the cost of using a Fizeau interferometer.

[0011] Furthermore, to achieve optimal measurements, users of Fizeau interferometers may have to change the configuration of the interferometer each time they measure a different surface, which has the disadvantage of requiring re-positioning of the interferometer. This increases the measurement time. In addition, changing elements within the system increases the risk of contamination of the optical system by dust, fingerprints, etc.

[0012] The object of the present invention is to provide a Fizeau interferometer equipped with alternative means for improving the contrast of the interference diagram generated by the reference light beam and the test light beam.

[0013] Therefore, the Fizeau interferometer according to claim 1 is a Fizeau interferometer comprising a light source that emits a light beam traveling along an optical path, wherein a first portion of the optical path extends between the light source and a reflective surface to be tested placed on a support of the Fizeau interferometer, a reference surface is placed in the first portion of the optical path between the light source and the support of the surface to be tested, a second portion of the optical path extends from the surface to be tested to the reference surface and to an imaging system of the Fizeau interferometer that measures light incident from the second portion of the optical path, the imaging system is configured to measure an interference diagram generated by interfering the reference light beam and the light beam to be tested, and the reference surface is characterized by being a polarizing and partially reflective reference surface, emitted from the light source A light beam incident along the first portion of the optical path is partially reflected and polarized by a reference plane to form a reference light beam. This reference light beam is reflected by the reference plane along the second portion of the optical path toward the imaging system, and the reference light beam has a first polarization angle. The test light beam is formed when a light beam incident along the first portion of the optical path passes through the reference plane toward the test plane, and this test light beam is polarized by the reference plane. This test light beam is reflected by the test plane toward the reference plane and through the second portion of the optical path toward the imaging system, and the test light beam has a second polarization angle. The first and second polarization angles are different, and the Fizeau interferometer... Part 2 of the optical path A Fizeau interferometer further comprising a first polarizer positioned between a reference plane and an imaging system, wherein the first polarizer is configured to allow light with a third polarization angle to pass toward the imaging system.

[0014] The Fizeau interferometer of the present invention can improve the contrast of the interference diagram by polarizing the reference light beam and the test light beam. The reference light beam is formed when a light beam incident on a partially reflective reference plane is partially reflected, and the reference light beam has a first polarization angle, i.e., it is equivalent to the first polarization. The test light beam is formed when a light beam passes through the reference plane, and the test light beam has a second polarization angle, i.e., it is equivalent to the second polarization. The first polarization angle and the second polarization angle are not the same.

[0015] A first polarizer located in front of the imaging system aligns the polarization of the reference beam and the test beam, causing the two beams to interfere. The first polarizer then directs third-polarized light towards the imaging system. Thus, both the reference beam and the test beam are polarized at the third polarization angle by the first polarizer. The intensity ratio of the two beams reaching the imaging system, and therefore the contrast of the interference pattern, depends on the third polarization angle. By appropriately selecting the first polarizer and the corresponding third polarization angle, the contrast of the interference pattern can be improved.

[0016] For example, the polarization of a light beam may be decomposed into x and y components. For instance, the first polarization of a reference light beam may have only an x ​​component, and the second polarization of the test light beam may have only a y component. In the case of Cartesian coordinates, the first and second polarizations are orthogonal. If the third polarization has only an x ​​component, the reference light beam can be completely passed towards the imaging system, but the test light beam is blocked by the polarizer. If the third polarization has both x and y components, for example, if the x and y components are equal, then a portion of the reference light beam and a portion of the test light beam, for example, an equal portion, can be passed towards the imaging system.

[0017] Thus, the third polarization angle determines the beam ratio reaching the imaging system, which allows for improved contrast in the interferogram.

[0018] One embodiment of the Fizeau interferometer is a first variable-angle polarizer such that the third polarization angle is a variable third polarization angle. In this embodiment, by changing the polarization of the first polarizer, it is possible to change the third polarization angle, and thus the beam ratio and contrast of the interference diagram. Advantageously, it is not necessary to replace the first polarizer of the Fizeau interferometer with another first polarizer having a different third polarization angle in order to change the contrast of the interference diagram. Preferably, the first variable-angle polarizer is a dichroic polarizer.

[0019] In another embodiment of the Fizeau interferometer, a second polarizer is provided between the light source and the reference plane in the first portion of the optical path, and the second polarizer is a second variable-angle polarizer configured to polarize light traveling in the first portion of the optical path between the light source and the reference plane, the polarized light having a variable fourth polarization angle.

[0020] The second polarizer controls the beam ratio and, consequently, the contrast of the interference diagram by polarizing the first portion of the optical path of the light beam traveling toward the reference plane. When the light beam traveling toward the reference plane has a fourth polarization angle close to the first polarization angle, most of the light beam is reflected by the reference plane, forming the reference light beam. In contrast, when the light beam traveling toward the reference plane has a fourth polarization angle close to the second polarization angle, only a small portion of the light beam is reflected by the reference plane, forming the reference light beam. In both cases, the remaining portion of the light beam passes through the reference plane, forming the test light beam. By controlling the fourth polarization angle in this way, the beam ratio can be controlled, and the contrast of the interference diagram can be improved. More advantageously, this embodiment allows control of the intensity of the test light beam when it is incident on the test plane by controlling the fourth polarization angle. Preferably, the second polarizer is a variable-angle half-wave plate.

[0021] In embodiments having a first and / or second variable-angle polarizer, the first and / or second variable-angle polarizer may be a variable-angle half-wave plate or a variable-angle dichroic polarizer. Regardless of the preferred embodiment, either the first and / or second polarizer may be embodied by any suitable, optionally variable-angle polarizer.

[0022] In one embodiment of a Fizeau interferometer, the imaging system is further configured to measure the contrast of the interference diagram produced by interfering a reference light beam with a test light beam. The Fizeau interferometer can improve the contrast of the interference diagram. In this embodiment, the imaging system is advantageously configured to enable the measurement of the contrast of the interference diagram, thereby enabling the quantification of the effect of changing a third or fourth polarization angle, or to improve the contrast of the interference diagram more effectively. The imaging system may be configured to measure the contrast by comparing dark and bright regions of the interference diagram. The imaging system may be configured to measure the contrast by measuring how clearly the bright and dark regions are distinguished, for example, by considering the size of regions having intermediate brightness in the interference diagram.

[0023] In one embodiment of the Fizeau interferometer, the polarization reference plane is preferably a reflective polarizer, and is a wire grid polarizer. Wire grid polarizers are advantageously used and, experimentally, efficiently reflect and transmit light under the incident light conditions typically seen in Fizeau interferometers. Other polarization reference planes may also be used in the present invention.

[0024] In one embodiment, the light source of the Fizeau interferometer is preferably configured to emit monochromatic light, and the light source is a monochromatic laser. In the embodiment, the wavelength of the light source may be configured according to the surface under test. For example, if the surface under test to be measured is small, a light source emitting light of a smaller wavelength may be required.

[0025] In one embodiment of a Fizeau interferometer, the reference beam and the test beam have orthogonal polarization.

[0026] The present invention further relates to a method for determining the characteristics of a surface under test, and uses the Fizeau interferometer of the present invention.

[0027] The present invention further relates to a method for improving the contrast of an interference pattern in a Fizeau interferometer, which uses a Fizeau interferometer including a first variable-angle polarizer. The method comprises: - illuminating a reference surface and a test surface with a light source; - determining the contrast of the interference pattern measured by an imaging system; - changing a third polarization angle to change the contrast of the interference pattern; - improving the contrast of the interference pattern by changing the third polarization angle. The method steps may be executed simultaneously. For example, the third polarization angle may be changed while the reference surface is illuminated and the contrast of the interference pattern is being determined. The method steps may also be executed iteratively. For example, the third polarization angle may be changed a number of times to determine the third polarization angle that results in an interference pattern having a desired contrast.

[0028] By improving the contrast of the interference pattern, the optical performance of the Fizeau interferometer can be improved. An interference pattern with improved contrast may be an interference pattern having a higher contrast ratio.

[0029]

[0030] The present invention further relates to a method for improving the contrast of an interference pattern in a Fizeau interferometer, which uses a Fizeau interferometer including a second variable-angle polarizer. The method comprises: - illuminating a reference surface and a test surface with a light source; - determining the contrast of the interference pattern measured by an imaging system; - changing a fourth polarization angle to change the contrast of the interference pattern; - improving the contrast of the interference pattern by changing the fourth polarization angle. The method steps may be executed simultaneously. For example, the fourth polarization angle may be changed while the reference surface is illuminated and the contrast of the interference pattern is being determined. The method steps may also be executed iteratively. For example, the fourth polarization angle may be changed a number of times to determine the fourth polarization angle that results in an interference pattern having a desired contrast.

[0031] The present invention further relates to a Fizeau interferometer comprising a light source that emits a light beam traveling along an optical path, wherein a first portion of the optical path extends between the light source and a reflective surface under test placed on a support of the Fizeau interferometer, a reference surface is positioned in the first portion of the optical path between the light source and the support of the surface under test, a second portion of the optical path extends from the surface under test to the reference surface and to an imaging system of the Fizeau interferometer that measures light incident from the second portion of the optical path, the imaging system is configured to measure an interference diagram generated by interfering the reference light beam and the light beam under test, and the reference surface is characterized by being a polarizing and partially reflective reference surface, emitted from the light source and traveling along the first portion of the optical path The incident light beam is partially reflected and polarized by the reference plane to form a reference light beam, which is then reflected by the reference plane towards the imaging system along the second portion of the optical path, and the reference light beam has a first polarization angle. The test light beam is formed when the incident light beam along the first portion of the optical path passes through the reference plane towards the test plane along the first portion of the optical path, and this test light beam is polarized by the reference plane, and this test light beam is directed by the test plane towards the reference plane and then through the second portion of the optical path towards the imaging system, and the test light beam has a second polarization angle, and the first and second polarization angles are different, and the Fizeau interferometer, Part 2 of the optical path A Fizeau interferometer further comprising a first polarizer positioned between a reference plane and an imaging system, wherein the first polarizer is configured to allow light with a third polarization angle to pass toward the imaging system.

[0032] The first polarizer is a first variable-angle polarizer such that the third polarization angle is a variable third polarization angle, and the Fizeau interferometer further includes a second polarizer provided between the light source and a reference plane in a first portion of the optical path, the second polarizer being a second variable-angle polarizer configured to polarize light traveling through the first portion of the optical path between the light source and the reference plane, the polarized light having a variable fourth polarization angle.

[0033] The present invention further relates to a Fizeau interferometer including a light source that emits a light beam traveling along an optical path, wherein a first portion of the optical path extends between the light source and a reflective surface under test placed on a support of the Fizeau interferometer, a reference plane is placed in the first portion of the optical path between the light source and the support of the surface under test, a second portion of the optical path extends from the surface under test to the reference plane and to an imaging system of the Fizeau interferometer that measures light incident from the second portion of the optical path, the imaging system is configured to measure an interference diagram generated by interfering the reference light beam and the light beam under test, and the reference plane is a polarizing and partially reflective reference plane with a variable polarization angle, and is emitted from the light source and along the optical path A light beam incident along the first part is partially reflected and polarized by the reference plane to form a reference light beam, which is then reflected by the reference plane towards the imaging system along the second part of the optical path, and the reference light beam has a first polarization angle. The test light beam is formed when a light beam incident along the first part of the optical path passes through the reference plane towards the test plane along the first part of the optical path, and this test light beam is polarized by the reference plane, and this test light beam is reflected by the test plane towards the reference plane and then through the second part of the optical path to the imaging system, and the test light beam has a second polarization angle, and the first and second polarization angles are different, and the Fizeau interferometer, Part 2 of the optical path A Fizeau interferometer further comprising a first polarizer positioned between a reference plane and an imaging system, wherein the first polarizer is configured to allow light with a third polarization angle to pass toward the imaging system.

[0034] In this embodiment of the present invention, the reference plane is configured to polarize the reference beam and the test beam at a variable polarization angle. As with other embodiments described herein, this allows control of the beam ratio between the reference beam and the test beam, and consequently, the contrast of the interference pattern. [Brief explanation of the drawing]

[0035] Herein, embodiments of the present invention will be described only illustratively with reference to the accompanying schematic diagrams, where corresponding reference numerals indicate corresponding parts.

[0036] [Figure 1] This shows a Fizeau interferometer comprising a polarization reference plane and a first polarizer. [Figure 2] This shows a Fizeau interferometer comprising a polarization reference plane and a first polarizer with a variable polarization angle. [Figure 3] This shows a Fizeau interferometer comprising a polarization reference plane, a first polarizer, and a second polarizer. [Figure 4A] The polarization of the reference beam and the test beam, and the effect of the first polarizer on their polarization are shown. [Figure 4B] The polarization of the reference beam and the test beam, and the effect of the first polarizer on their polarization are shown. [Figure 4C] The polarization of the reference beam and the test beam, and the effect of the first polarizer on their polarization are shown. [Figure 4D] The polarization of the reference beam and the test beam, and the effect of the first polarizer on their polarization are shown.

[0037] Figure 1 shows a Fizeau interferometer 1 including a polarization reference plane 4 and a first polarizer 8. In this embodiment, the first polarizer 8 is configured to allow light with a third polarization angle to pass towards the imaging system 3.

[0038] The Fizeau interferometer 1 includes a light source 2 that emits a light beam traveling along an optical path, which includes a first section 6 and a second section 7. The light source 2 may be a monochromatic light source such as a monochromatic laser. The first section 6 of the optical path extends between the light source 2 and the surface under test 5 when the surface under test 5 is placed on the support of the Fizeau interferometer 1. A polarization reference plane 4 is positioned between the light source 2 and the surface under test 5 in the first section 6 of the optical path. The reference plane 4 and the surface under test 5 are positioned facing each other.

[0039] The second portion 7 of the optical path extends from the surface under test 5 to the reference surface 4, and through the reference surface 4 to the imaging system 3. The imaging system 3 is configured to measure the light incident from the second portion 7 of the optical path. Multiple lenses 11 are arranged in the optical path to guide the light along the path. The second portion 7 of the optical path partially overlaps with the first portion 6 of the optical path. In this embodiment, a beam splitter 10 is positioned in the optical path to direct the light reflected by the reference surface 4 and the surface under test 5 toward the imaging system 3, and to direct the light from the light source 2 toward the reference surface 4 and the surface under test 5. The beam splitter 10 separates the first portion 6 of the optical path from the second portion 7 of the optical path.

[0040] The imaging system 3 is configured to measure an interference diagram obtained by interfering a reference light beam 32 with the test light beam 31. For example, the imaging system 3 may be connected to a computing means for further processing the interference diagram, such as extracting information about the test surface 5 from the interference diagram or displaying the interference diagram on a display device. In embodiments of the present invention, the imaging system 3 may also be configured to directly measure the contrast of the interference diagram.

[0041] The reference surface 4 is partially reflective so that the reference light beam 32 and the test light beam 31 are formed. The reference light beam 32 is formed by the partial reflection of the light beam emitted by the light source 2. The test light beam 31 is formed when the light beam passes through the reference surface 4 along the first portion 6 of the optical path and then passes towards the test surface 5. The reference light beam 32 is reflected by the reference surface 4 along the second portion 7 of the optical path toward the imaging system 3. The test light beam 31 passes through the reference surface 4 along the first portion 6 of the optical path toward the test surface 5. Then, the test light beam 31 is reflected by the test surface 5 along the second portion 7 of the light beam, returns toward the reference surface 4, and proceeds toward the imaging system 3.

[0042] The reference surface 4 is a polarization reference surface configured to reflect light at a first polarization angle and allow light to pass through at a second polarization angle. The first and second polarization angles are different. The reference light beam 32 is polarized at the first polarization angle by the polarization reference surface, and the test light beam 31 is polarized at the second polarization angle by the polarization reference surface.

[0043] In the embodiment, the polarization reference plane 4 is embodied as a reflective polarizer, and preferably, the polarization reference plane 4 is embodied as a wire grid polarizer. The advantage of the wire grid polarizer is that it allows the wire grid polarizer to polarize the reference beam and the test beam under the incident light conditions typically seen in a Fizeau interferometer.

[0044] The Fizeau interferometer 1 further includes a first polarizer 8 positioned between the reference plane 4 and the imaging system 3 in a second portion 7 of the optical path. The first polarizer 8 is configured to allow third-polarized light to pass toward the imaging system 3. In embodiments, the first polarizer 8 is embodied as a half-wave plate or a dichroic polarizer. Other embodiments of the first polarizer are also possible.

[0045] By polarizing the light using the polarization reference plane 4, the reference light beam 32 and the test light beam 31 have different polarizations. It is known that light beams with different polarizations do not interfere optimally. The first polarizer 8 aligns the polarizations of the reference light beam 32 and the test light beam 31, enabling interference of the light beams.

[0046] Furthermore, in this embodiment, the first polarizer 8 can select the relative proportion of the reference light beam 32 compared to the test light beam 31. This will be explained in detail below with reference to Figures 4A to 4D.

[0047] By selecting the relative ratio of the two light beams, the beam ratio between the intensity of the reference light beam 32 and the intensity of the test light beam 31 is controlled. As a result, by selecting a suitable first polarizer 8 with an appropriate third polarization angle, the contrast of the interference diagram obtained by interfering the reference light beam 32 and the test light beam 31 can be improved. This is particularly important when the reflectance of the test surface 5 differs significantly from that of the reference surface 4. When the reflectances of the two surfaces 4 and 5 differ significantly, the contrast of the resulting interference diagram decreases, negatively impacting the measurement performed by the Fizeau interferometer 1. This embodiment of the present invention enables improved measurement, particularly when the reflectances of the reference surface 4 and the test surface 5 differ.

[0048] Figure 2 shows a second embodiment of the Fizeau interferometer 1. The Fizeau interferometer 1 in Figure 2 includes a polarization reference plane 4 and a first polarizer 28. In this embodiment, the first polarizer 28 has a variable polarization angle. The embodiment of the Fizeau interferometer 1 shown in Figure 2 may be the same as the embodiment shown in Figure 1, except that it has a first variable-angle polarizer 28.

[0049] In the embodiment shown in Figure 2, the first polarizer 28 has a variable third polarization angle, which allows control of the beam ratio between the reference light beam 32 and the test light beam 31 in the imaging system 3. By controlling the beam ratio, the contrast of the interference diagram can be improved, for example, by increasing the contrast. The first variable-angle polarizer 28 may be a dichroic polarizer 28.

[0050] Figure 3 shows a third embodiment of the Fizeau interferometer 1. The Fizeau interferometer 1 in Figure 3 includes a second polarizer 9 provided between the light source 2 and the reference plane 4 in the first portion 6 of the optical path, the second polarizer 9 being a second variable-angle polarizer 9 configured to polarize light traveling through the first portion 6 of the optical path between the light source 2 and the reference plane 4, and the polarized light having a variable fourth polarization angle.

[0051] The second polarizer 9 polarizes the light beam traveling toward the reference plane 4 in the first portion 6 of the optical path, thereby enabling control of the beam ratio and, consequently, the contrast of the interference diagram. For example, if the light beam traveling toward the reference plane 4 has a fourth polarization angle close to the first polarization angle, most of the light beam is reflected by the reference plane 4, forming the reference light beam 32. In contrast, if the light beam traveling toward the reference plane 4 has a fourth polarization angle close to the second polarization angle, only a small portion of the light beam is reflected by the reference plane 4, forming the reference light beam 32. In both cases, the remaining portion of the light beam passes through the reference plane 4, forming the test light beam 31. By controlling the fourth polarization angle, the beam ratio can be controlled, thereby improving the contrast of the interference diagram. This embodiment is even more advantageous in that by controlling the fourth polarization angle and, consequently, the proportion of light passing through the reference plane 4, the intensity of the test light beam 31 when incident on the test surface 5 can be controlled.

[0052] Preferably, the second polarizer 9 is implemented as an angle-variable half-wave plate.

[0053] Figures 4A, 4B, 4C, and 4D show the polarization of the reference light beam 32 and the test light beam 31, and the effects of the first polarizers 8 and 28 on them. The reference light beam 32 and the test light beam 31 are indicated by arrows in the figures, where the length of the arrow indicates the amplitude of the light beam, and the direction of the arrow in the xy plane indicates the polarization of the light beam. The third polarization angle 33 is indicated by a thinner arrow in the figure.

[0054] Figures 4A and 4C show the reference light beam 32 and the test light beam 31 before passing through polarizers 8 and 28 under two different conditions.

[0055] Figure 4A shows the situation where the reference light beam 32 and the test light beam 31 are orthogonally polarized while having similar amplitude and intensity. The reference light beam 32 is polarized along the y-axis, and the test light beam 31 is polarized along the x-axis. The projection of the light beams onto the third polarization angle 33 of the first polarizers 8, 28 is shown by the dashed lines.

[0056] As seen in Figure 4B, both the reference beam 32 and the test beam 31 align to the third polarization angle 33 and have the same amplitude after the light beams in Figure 4A pass through polarizers 8 and 28. In this figure, arrow 34 indicates both the reference beam 32 and the test beam 31. Figures 4A and 4B show the situation where the reference beam 32 and the test beam 31 have orthogonal polarization with the same amplitude. In this case, if the third polarization angle 33 is equal to 45 degrees, which is the polarization angle of both of those light beams, then the light beams after the first polarizer have the same intensity.

[0057] A different situation is shown in Figures 4C and 4D. As seen in Figure 4C, the polarizations of the reference light beam 32 and the test light beam 31 are orthogonal in this example as well. However, the test light beam 31 has a larger amplitude than the reference light beam, probably as a result of the highly reflective test surface 5.

[0058] As seen in Figure 4C, the third polarization angle 33 is no longer 45 degrees for both light beams. This effect can be seen in Figure 4D. The resulting reference light beam 32 and test light beam 31 are again indicated by arrows 34. Also, in this example, the reference light beam 32 and test light beam 31 had different amplitudes before passing through the first polarizers 8 and 28, but after passing through the first polarizers 8 and 28, their amplitudes and polarizations became the same. Thus, the beam ratio was different from 1 before the first polarizers 8 and 28, but after the first polarizers 8 and 28, the beam ratio approached 1. The effect of the third polarization angle 33 on the amplitudes of the reference light beam 31 and test light beam 32 can be clearly confirmed.

[0059] Figures 4A to 4D illustrate the present invention, which utilizes first polarizers 8 and 28 to control beam ratio and contrast, but other embodiments of the present invention, such as those including a second polarizer 9, operate on the same principle.

Claims

1. Equipped with a light source that emits a light beam that travels along the optical path, The first portion of the optical path extends between the light source and the surface under test placed on the support of the Fizeau interferometer, The reference surface is positioned in the first portion of the optical path between the light source and the support of the surface under test, The second portion of the optical path extends from the surface under test to the reference surface, and to the imaging system of the Fizeau interferometer that measures the light incident from the second portion of the optical path. The imaging system is configured to measure an interference diagram generated by interfering a reference light beam with a test light beam, and the reference surface is characterized by being a polarizing and partially reflective reference surface, and the light beam emitted from the light source and incident along the first portion of the optical path is partially reflected and polarized by the reference surface to form the reference light beam, and this reference light beam is reflected by the reference surface along the second portion of the optical path toward the imaging system. The reference light beam has a first polarization angle, and the test light beam is formed when the light beam incident along the first portion of the optical path passes along the first portion of the optical path through the reference surface toward the test surface, this test light beam is polarized by the reference surface, and this test light beam is reflected by the test surface toward the reference surface and through the second portion of the optical path toward the imaging system. The light beam under test has a second polarization angle, The first polarization angle and the second polarization angle are different, and the Fizeau interferometer further comprises a first polarizer positioned between the reference plane and the imaging system in the second portion of the optical path. The first polarizer is configured to allow light with the third polarization angle to pass through by changing the polarization angle of the reference light beam and the test light beam toward the imaging system to a third polarization angle. Fizeau interferometer.

2. The first polarizer is a first variable-angle polarizer such that the third polarization angle is a variable third polarization angle. The Fizeau interferometer according to claim 1.

3. The optical path further comprises a second polarizer provided between the light source and the reference plane in the first portion of the optical path, The second polarizer is a second variable-angle polarizer configured to polarize light traveling through the first portion of the optical path between the light source and the reference plane, The polarized light has a variable fourth polarization angle. The Fizeau interferometer according to claim 1.

4. The first variable-angle polarizer is a variable-angle half-wave plate or a variable-angle dichroic polarizer. The Fizeau interferometer according to claim 2.

5. The imaging system is further configured to measure the contrast of the interference diagram generated by interfering the reference light beam and the test light beam. The Fizeau interferometer according to claim 1.

6. The reference plane is a reflective polarizer, The aforementioned reference plane is a wire grid polarizer. The Fizeau interferometer according to claim 1.

7. The light source is configured to emit monochromatic light, The light source is a monochromatic laser. The Fizeau interferometer according to claim 1.

8. The reference light beam and the test light beam have orthogonal polarizations. The Fizeau interferometer according to claim 1.

9. A method for determining the characteristics of the surface under test, The Fizeau interferometer described in claim 1 is used. method.

10. A method for improving the contrast of an interferometer's interference pattern, At least the Fizeau interferometer described in claim 2 is used, The aforementioned method, - Illuminating the reference surface and the surface under test with the light source, - Determining the contrast of the interferogram measured by the imaging system, - Changing the contrast of the interference diagram by changing the third polarization angle, -By changing the third polarization angle, the contrast of the interference diagram is improved, Equipped with, method.

11. A method for improving the contrast of an interferometer's interference pattern, At least the Fizeau interferometer described in claim 3 is used, The aforementioned method, - Illuminating the reference surface and the surface under test with the light source, - Determining the contrast of the interferogram measured by the imaging system, - Changing the fourth polarization angle to change the contrast of the interference diagram, -By changing the fourth polarization angle, the contrast of the interference diagram is improved, Equipped with, method.

Images