A fizeau interferometer adapted to detect reflectivity of a surface to achieve high interference contrast

Abstract

The present application relates to the field of optical technology, a Fizeau interferometer with high interference contrast by adapting the reflectivity of detection surface, comprising laser light source, 1 / 2 wave plate, converging lens group, illumination diaphragm, polarization beam splitter, collimating lens group, Faraday rotator and polarization reference mirror group arranged in sequence along the first light path direction; further comprising imaging diaphragm, imaging lens group and ccd camera arranged in sequence along the second light path direction; the 1 / 2 wave plate is installed on a rotating mechanism for adjusting the fast axis angle; the first light path direction is the same as the transmission optical axis direction of the polarization beam splitter, and the second light path direction is the same as the reflection optical axis direction of the polarization beam splitter. The maximum light intensity of the interference signal is jointly adjusted by the output power of the laser light source and the 1 / 2 wave plate; the contrast of the interference signal is adjusted by the Faraday rotator.

Description

Technical Field

[0001] This invention relates to the field of optical technology, and more specifically to the Fizeau interferometer. Background Technology

[0002] Interferometer measurement, as a high-precision device for modern optical inspection, has a wide range of applications.

[0003] The Fizeau interferometer, through its unique optical path structure, ensures that the reference and measurement beams interfering during detection share a common optical path. This effectively cancels out most system detection errors introduced by optical component fabrication and instrument assembly, thereby improving detection accuracy. However, according to the principle of interference, the interference contrast of two beams is the ratio of the difference between the maximum and minimum intensities of the interference fringes to the sum of the maximum and minimum intensities. For the Fizeau interferometer, due to its common optical path characteristic, matching the intensity of the reference and measurement beams is difficult. When the reflectivity of the detection surface is too high or too low (e.g., after coating), low interference contrast often results. Although the increased dynamic range of CCD cameras and the development of modern phase resolution technology can mitigate the impact of low interference contrast on interferometric measurement accuracy to some extent, interference contrast remains a significant factor inherent in the principle of the Fizeau interferometer, often limiting its measurement accuracy for detection surfaces with high and low reflectivity. Based on the principle of interference, when the reference light and the measurement light are linearly polarized with the same polarization direction and the polarization amplitude (light intensity) is also the same, the two beams can form complete coherence, and their interference contrast is the highest. In principle, the accuracy of the interference phase information obtained by resolution is also the highest (without the measurement and resolution error in principle).

[0004] To address the interference contrast issue, the conventional approach in Fizeau interferometers is to introduce an additional energy attenuation device into the external detection light path or to coat the reference surface of the reference mirror with an anti-reflection coating. However, this approach introduces additional system detection errors, and the energy ratio between the reference light and the measurement light cannot be continuously adjusted. It can only bring the energies of the reference light and the measurement light closer within a certain range, which cannot achieve the highest interference contrast in principle, nor can it achieve the most ideal interference detection accuracy. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention provides a Fizeau interferometer that adapts to the reflectivity of the detection surface to achieve high interference contrast, thus solving at least one of the aforementioned technical problems.

[0006] The technical solution of the present invention is: a Fizeau interferometer that adapts to the reflectivity of the detection surface to achieve high interference contrast, characterized in that it includes a laser source, a half-wave plate, a converging mirror group, an illumination aperture, a polarizing beam splitter, a collimating mirror group, a Faraday rotator, and a polarizing reference mirror group arranged sequentially along the first optical path direction;

[0007] It also includes an imaging aperture, an imaging lens group, and a CCD camera arranged sequentially along the second optical path;

[0008] The first optical path direction is the same as the transmission optical axis direction of the polarization beam splitter, and the second optical path direction is the same as the reflection optical axis direction of the polarization beam splitter;

[0009] The half-wave plate is mounted on a rotating mechanism for adjusting the fast axis angle; by rotating the half-wave plate, the polarization direction of the transmitted beam is changed, and finally, in conjunction with the polarization beam splitter, the energy of the transmitted beam is attenuated and the attenuation ratio is continuously adjustable.

[0010] The incident light of the polarization reference mirror group is linearly polarized light, the returned reference light is linearly polarized light with the same polarization direction as the incident light, and the returned measurement light is linearly polarized light with the polarization direction perpendicular to the incident light.

[0011] The returning reference light and measurement light re-enter the Faraday rotator and rotate synchronously in polarization direction, then pass through the polarization beam splitter again; the reference light and measurement light entering the polarization beam splitter have different polarization directions.

[0012] The imaging lens group images the reference light and the measurement light onto the focal plane of the CCD camera and forms an interference signal. The reference light and the measurement light at the focal plane of the CCD camera are linearly polarized light with the same polarization direction. At this time, the contrast of the interference signal is determined by the ratio of the intensity of the reference light to the intensity of the measurement light.

[0013] The maximum light intensity of the interference signal of the interferometer of the present invention is jointly adjusted by the output power of the laser light source and the half-wave plate, and can be continuously adjusted arbitrarily within a range to ensure that the interference signal intensity is within the dynamic range of the CCD camera;

[0014] The contrast of the interferometer signal is adjusted by the Faraday rotator. The contrast can be adjusted arbitrarily from 0 to 1, and in particular, it can achieve a theoretical maximum contrast of 1.

[0015] More preferably, the rotating mechanism is an electric rotating mechanism.

[0016] Easy to control electrically.

[0017] More preferably, the illumination stop is located at the focal point of the converging lens group, and the aperture of the illumination stop is smaller than the theoretical Airy disk diameter of the converging lens group. The illumination stop and the converging lens group work together to produce an approximately ideal aberration-free point as the light source for the subsequent optical path.

[0018] More preferably, the polarizing reference mirror assembly includes a first medium, a reference surface, a second medium, and an antireflection coating arranged sequentially along the first optical path direction;

[0019] The first medium is an amorphous optical medium;

[0020] The second medium is a birefringent crystal optical medium.

[0021] The antireflection coating eliminates stray light reflected from the second medium. The reflectivity of the reference light at the reference surface is determined by the difference in refractive index between the first and second media.

[0022] More preferably, the second medium is a quarter-wave plate.

[0023] More preferably, the first medium is fused silica optical glass.

[0024] More preferably, the second medium is a magnesium fluoride birefringent crystal.

[0025] More preferably, the antireflective coating is a multilayer antireflective coating. A four-layer antireflective coating is preferred.

[0026] More preferably, the first medium and the second medium are connected by optical adhesive.

[0027] More preferably, the polarization beam splitter transmits P-polarized light and reflects S-polarized light.

[0028] More preferably, the polarization beam splitter includes any one of a PBS optical thin film, a polarization diffraction device, and a micro polarization element array.

[0029] The beneficial effect is that it can perfectly solve the energy matching problem between the common optical path reference light and the measurement light in the Fizeau interferometer. It can achieve arbitrary adjustment of the intensity ratio of the reference light and the measurement light under any reflectivity of the detection surface without the need for external devices or replacement of interferometer components, thereby achieving the theoretically highest interference contrast and improving the measurement range and measurement accuracy of the Fizeau interferometer in principle. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the present invention;

[0031] Figure 2 This is a schematic diagram of the polarization reference mirror assembly of the present invention;

[0032] Figure 3 This is a schematic diagram illustrating the adjustment of the intensity ratio of the reference light and the measurement light when the reflectivity of the detection surface is high, according to the present invention.

[0033] Figure 4 This is a schematic diagram illustrating the adjustment of the intensity ratio of reference light and measurement light when the reflectivity of the detection surface is low, according to the present invention.

[0034] In the diagram: 1-1 is the laser source; 1-2 is the half-wave plate; 1-3 is the converging mirror group; 1-4 is the illumination stop; 1-5 is the polarization beam splitter; 1-6 is the collimating mirror group; 1-7 is the Faraday rotator; 1-8 is the polarization reference mirror group; 1-9 is the imaging stop; 1-10 is the imaging mirror group; 1-11 is the CCD camera; 1-12 is the detection surface; 2-1 is the first medium; 2-2 is the second medium; 2-3 is the reference surface; and 2-4 is the antireflection coating. Detailed Implementation

[0035] The present invention will now be further described with reference to the accompanying drawings.

[0036] See Figures 1 to 4 Specific embodiment 1: A Fizeau interferometer for achieving high interference contrast by adapting to the reflectivity of the detection surface, comprising a laser source 1-1, a half-wave plate 1-2, a converging mirror group 1-3, an illumination aperture 1-4, a polarizing beam splitter 1-5, a collimating mirror group, a Faraday rotator 1-8, and a polarizing reference mirror group arranged sequentially along a first optical path. The polarizing reference mirror group is located between the Faraday rotator 1-8 and the detection surface 1-12.

[0037] It also includes an imaging aperture 1-9, an imaging lens group 1-10, and a CCD camera 1-11 arranged sequentially along the second optical path; a half-wave plate 1-2 is mounted on a rotating mechanism for adjusting the fast axis angle; the incident light of the polarization reference lens group is linearly polarized light, the returned reference light is linearly polarized light with the same polarization direction as the incident light, and the returned measurement light is linearly polarized light with the polarization direction perpendicular to the polarization direction of the incident light; the first optical path direction is the same as the transmission optical axis direction of the polarization beam splitter 1-5, and the second optical path direction is the same as the reflection optical axis direction of the polarization beam splitter 1-5.

[0038] Laser source 1-1 is a laser that outputs linearly polarized light, serving as the light source for the optical path described later. Here, laser source 1-1 includes lasers that directly output linearly polarized beams, as well as lasers that output linearly polarized beams through an external polarization device.

[0039] The half-wave plate 1-2 is rotated along the optical axis by an electric drive device. The rotation of the half-wave plate 1-2 by the electric drive device adjusts the direction of the fast axis deflection, thereby adjusting the polarization direction of the transmitted beam. The rotation of the half-wave plate 1-2, in conjunction with the polarization beam splitter 1-5 (described later), achieves a variable energy attenuation function.

[0040] The converging lens assembly and illumination stop 1-4 are used to converge parallel beams and form an aberration-free ideal point source as an interference source. Illumination stop 1-4 is located at the focal point of the converging lens assembly, and its aperture is smaller than or equal to the theoretical Airy disk diameter of the converging lens assembly. The illumination stop and the converging lens assembly work together to produce an approximately ideal aberration-free point source as the light source for subsequent optical paths.

[0041] Polarizing beamsplitters 1-5 are PBS optical thin-film beamsplitters. Polarizing beamsplitters 1-5 function through the PBS optical thin film, transmitting polarized light whose polarization direction is parallel to the PBS film surface and reflecting polarized light whose polarization direction is not parallel to the PBS film surface. Optionally, polarizing beamsplitters 1-5 can be implemented using polarization diffraction devices, micro-polarization element arrays, etc.

[0042] The focal point of the collimating lens group transmitted through polarizing beam splitter 1-5 coincides with the position of illumination stop 1-4, and the focal point of the reflection through polarizing beam splitter 1-5 coincides with the position of imaging stop 1-9. Here, the collimating lens group collimates the beam propagating along the first optical path direction into a parallel beam and converges the beam propagating against the first optical path direction. The collimating lens group and converging lens group 1-3 have different focal lengths, working together to achieve beam expansion. The focal point of the collimating lens group coincides with that of imaging lens group 1-10, achieving interference imaging.

[0043] The Faraday rotator 1-8 is an adjustable electromagnetic rotator 1-8 capable of continuously adjusting the rotation of the polarization angle. The Faraday rotator 1-8 achieves rotation of the beam's polarization direction electromagnetically, and the rotation angle is adjusted by regulating the current intensity. Optionally, the Faraday rotator 1-8 can achieve rotation angle adjustment through a variable Wilder constant or a variable magnetic field optical path length. Adjusting the rotation angle of the Faraday rotator 1-8 controls the polarization direction of the beam entering the reference mirror group.

[0044] The polarization reference mirror group receives linearly polarized incident light. The returning reference light is linearly polarized with the same polarization direction as the incident light, while the returning measurement light is linearly polarized with the polarization direction perpendicular to the incident light. Both the returning reference and measurement lights re-enter Faraday rotators 1-8, where their polarization directions rotate synchronously. They then pass through polarization beamsplitter 1-5. Polarization beamsplitter 1-5 transmits the beam component with its polarization direction parallel to the PBS film surface, reflecting only the beam component with its polarization direction non-parallel to the PBS film surface, which then enters the subsequent imaging mirror group 1-10, achieving energy attenuation of the interference beam. Since the reference and measurement lights entering polarization beamsplitter 1-5 have different (orthogonal) polarization directions, the reference and measurement lights reaching the interference focal plane have different energy attenuation ratios. Adjusting the rotation angle of Faraday rotators 1-8 allows for arbitrary intensity ratios of the reference and measurement lights.

[0045] Imaging lens group 1-10 images the reference light and measurement light onto the focal plane of CCD camera 1-11 and forms an interference signal.

[0046] The novel Fizeau interferometer uses wavelength phase shifting for interferometric measurement. Alternatively, the novel Fizeau interferometer uses mechanical phase shifting for interferometric measurement.

[0047] The main functions implemented are described as follows: 1) The maximum light intensity of the interferometer interference signal is jointly adjusted by the output power of the laser source 1-1 and the half-wave plate 1-2, and can be continuously adjusted arbitrarily within the range to ensure that the interference signal intensity is within the dynamic range of the CCD camera 1-11; 2) The contrast of the interferometer interference signal is adjusted by the Faraday rotator 1-8, and can be arbitrarily adjusted from 0 to 1, especially to achieve a theoretical maximum contrast of 1.

[0048] Figure 2 As shown, the polarization reference mirror assembly includes a first medium 2-1, a reference surface 2-3, a second medium 2-2, and an antireflective coating 2-4 arranged sequentially along the optical path. The first medium 2-1 is an amorphous optical medium with a refractive index of N. The second medium 2-2 is a birefringent crystalline optical medium with an o-ray refractive index of No and an e-ray refractive index of Ne. No can be equal to or different from N; Ne can be equal to or different from N; but No and Ne are always different. Furthermore, the second medium 2-2 has a specific thickness characteristic, causing a 45° phase delay in polarized light passing through it, thus exhibiting the characteristics of a quarter-wave plate. When the beam passes through the second medium 2-2 twice, the polarized light experiences a 90° phase delay, meaning the polarization direction of the linearly polarized light rotates by 90°. The reference surface 2-3 is located at the interface between the first medium 2-1 and the second medium 2-2. The reflectivity of reference surface 2-3 is determined by the difference in refractive index between the first medium 2-1 and the second medium 2-2 according to the Fresnel reflectivity formula. The reflectivity of reference surface 2-3 varies within a range when the polarization direction of the incident beam changes.

[0049] The main functions of the polarization reference mirror group are described as follows: 1) When the incident beam is linearly polarized, the reference light returned by the polarization reference mirror group is linearly polarized, and the polarization direction of the reference light is consistent with that of the incident light; 2) When the incident beam is linearly polarized, the detection light returned by the polarization reference mirror group is linearly polarized, and the polarization direction of the detection light is perpendicular to that of the incident light; 3) When the polarization direction of the incident linearly polarized beam changes, the polarization directions of the returned reference light and the measurement light change synchronously and remain orthogonal to each other.

[0050] In this embodiment

[0051] The first medium 2-1 is fused silica optical glass.

[0052] The second medium 2-2 is a magnesium fluoride birefringent crystal, with the fast axis of the magnesium fluoride crystal parallel to the reference plane and perpendicular to the optical axis of the reference mirror. The magnesium fluoride birefringent crystal has a specific thickness, causing a 45° phase delay in the light beam passing through it, thus exhibiting the characteristics of a quarter-wave plate.

[0053] Reference plane 2-3 is located at the interface between fused silica optical glass and magnesium fluoride birefringent crystal, and its reflectivity and transmittance are determined by the Fresnel refractive index formula. In this example, the reference plane is a plane.

[0054] Antireflective coating 2-4 is located on the back surface of magnesium fluoride birefringent crystal. It is a common 4-layer AR film used to reduce stray light.

[0055] The preferred embodiment of the present invention is described below:

[0056] The laser source 1-1 outputs a linearly polarized beam with a horizontal polarization direction, a beam cross-section diameter of 5 mm, and a coherence length of >100 m.

[0057] Half-wave plate 1-2 is a 0th-order half-wave plate 1-2 with a phase delay deviation of less than 1°, and the initial optical axis of the wave plate is horizontal. The focal length of the converging lens group 1-3 is 23mm, and the entrance pupil diameter is 6mm.

[0058] The illumination apertures 1-4 are metal pinholes with a diameter of 8 μm.

[0059] Polarizing beam splitters 1-5 transmit horizontally polarized beams with a transmittance Tp>99.9% and reflect vertically polarized beams with a reflectance Rs>99.9%.

[0060] The collimating lens group has a focal length of 600mm and an exit pupil diameter of 105mm.

[0061] The aperture of Faraday rotators 1-8 is 105mm, and the rotation angle adjustment range is 0±45°.

[0062] The polarization reference mirror group is a plane reference mirror. The surface shape error accuracy PV of reference plane 2-3 is <60nm (including curvature error). The polarization degree of the returned reference light is >95%, the polarization degree of the measured light is >95%, and the angle between the polarization directions of the reference light and the measured light is 90°±1°.

[0063] The reflectivity of the reference light directly returned by the polarization reference mirror group is 0.5%, and the transmittance of the measurement light directly emitted by the polarization reference mirror group is >99%.

[0064] Imaging stops 1-9 are stray light elimination aperture stops with a light transmission diameter of 9.5 mm.

[0065] The focal length of imaging lens groups 1-10 is 43mm.

[0066] The detection surface size of CCD camera 1-11 is 3.584mm × 3.584mm.

[0067] When measuring a detection surface with a reflectivity >90% using the novel Fizeau interferometer of the preferred embodiment, the schematic diagram shows the ratio of reference light to measurement light intensity achieved by adjusting the half-wave plate 1-2 and the Faraday rotator 1-8, as shown in the figure. Figure 3 As shown.

[0068] Figure 3 In the left region of the image, point a shows the polarization of the reference light after the Faraday rotator; point b shows the polarization of the measured light after the Faraday rotator; point c shows the polarization of the reference light after the polarization beam splitter; and point d shows the polarization of the measured light after the polarization beam splitter. Figure 3 In the right region of the diagram, a1 shows the polarization of the reference light after the Faraday rotator (after adjusting the rotation angle of the Faraday rotator); b1 shows the polarization of the measurement light after the Faraday rotator (after adjusting the rotation angle of the Faraday rotator); c1 shows the polarization of the reference light after the polarization beam splitter (after adjusting the rotation angle of the Faraday rotator); and d1 shows the polarization of the measurement light after the polarization beam splitter (after adjusting the rotation angle of the Faraday rotator).

[0069] The diagram illustrates the ratio of reference and measurement light intensities achieved by adjusting the half-wave plate 1-2 and the Faraday rotator 1-8 when measuring a detection surface with a reflectivity <0.1% using the novel Fizeau interferometer of the preferred embodiment. Figure 4 As shown.

[0070] Figure 4 In the left region of the diagram, point e shows the polarization of the reference light after the Faraday rotator; point f shows the polarization of the measured light after the Faraday rotator; point g shows the polarization of the reference light after the polarization beam splitter; and point h shows the polarization of the measured light after the polarization beam splitter.

[0071] Figure 4 In the right region of the diagram, e1 shows the polarization of the reference light after the Faraday rotator (after adjusting the rotation angle of the Faraday rotator); f1 shows the polarization of the measurement light after the Faraday rotator (after adjusting the rotation angle of the Faraday rotator); g1 shows the polarization of the reference light after the polarization beam splitter (after adjusting the rotation angle of the Faraday rotator); and h1 shows the polarization of the measurement light after the polarization beam splitter (after adjusting the rotation angle of the Faraday rotator).

[0072] Based on the experimental results of the preferred embodiment, the present invention can adapt to extremely high or extremely low reflectivity of the reflecting surface, adjust the reference light and the measurement light in any proportion, achieve the theoretically highest interference contrast, and does not introduce any additional error in principle, and is easy to operate.

[0073] The above are merely preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A Fizeau interferometer that adapts to the reflectivity of the detection surface to achieve high interference contrast, characterized in that, It includes a laser source, a half-wave plate, a converging mirror group, an illumination stop, a polarizing beam splitter, a collimating mirror group, a Faraday rotator, and a polarizing reference mirror group arranged sequentially along the first optical path. It also includes an imaging aperture, an imaging lens group, and a CCD camera arranged sequentially along the second optical path; The first optical path direction is the same as the transmission optical axis direction of the polarization beam splitter, and the second optical path direction is the same as the reflection optical axis direction of the polarization beam splitter; The half-wave plate is mounted on a rotating mechanism for adjusting the fast axis angle. By rotating the half-wave plate, the polarization direction of the transmitted beam is changed, and finally, in conjunction with the polarization beam splitter, the energy of the transmitted beam is attenuated and the attenuation ratio is continuously adjustable. The incident light of the polarization reference mirror group is linearly polarized light, the returned reference light is linearly polarized light with the same polarization direction as the incident light, and the returned measurement light is linearly polarized light with the polarization direction perpendicular to the incident light. The polarization reference mirror assembly includes a first medium, a reference surface, a second medium, and an antireflection coating arranged sequentially along the first optical path direction; The first medium is an amorphous optical medium; The second medium is a birefringent crystal optical medium; The returning reference light and measurement light re-enter the Faraday rotator and rotate synchronously in polarization direction, then pass through the polarization beam splitter again; the reference light and measurement light entering the polarization beam splitter have different polarization directions. The imaging lens group images the reference light and the measurement light onto the focal plane of the CCD camera and forms an interference signal. The reference light and the measurement light at the focal plane of the CCD camera are linearly polarized light with the same polarization direction. At this time, the contrast of the interference signal is determined by the ratio of the intensity of the reference light to the intensity of the measurement light.

2. The Fizeau interferometer according to claim 1, which achieves high interference contrast by adapting the reflectivity of the detection surface, is characterized in that: The rotating mechanism is an electrically operated rotating mechanism.

3. The Fizeau interferometer according to claim 1, which achieves high interference contrast by adapting the reflectivity of the detection surface, is characterized in that: The second medium is a quarter-wave plate.

4. The Fizeau interferometer according to claim 1, which achieves high interference contrast by adapting the reflectivity of the detection surface, is characterized in that: The first medium is fused silica optical glass; The second medium is a magnesium fluoride birefringent crystal.

5. A Fizeau interferometer for achieving high interference contrast by adapting to the reflectivity of the detection surface according to claim 1, characterized in that: The illumination stop is located at the focal point of the converging lens group, and the aperture of the illumination stop is smaller than the theoretical Airy disk diameter of the converging lens group.

6. The Fizeau interferometer according to claim 1, which achieves high interference contrast by adapting the reflectivity of the detection surface, is characterized in that: Antireflective coatings are multilayer antireflective coatings.

7. The Fizeau interferometer according to claim 1, which achieves high interference contrast by adapting the reflectivity of the detection surface, is characterized in that: The first medium and the second medium are connected by optical adhesive.

8. The Fizeau interferometer according to claim 1, which achieves high interference contrast by adapting the reflectivity of the detection surface, is characterized in that: The polarization beam splitter transmits P-polarized light and reflects S-polarized light.

9. A Fizeau interferometer for achieving high interference contrast by adapting to the reflectivity of the detection surface according to claim 1, characterized in that: The polarization beam splitter includes any one of PBS optical thin films, polarization diffraction devices, and micro polarization element arrays.

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