Point diffraction interferometer

Abstract

The invention provides a point diffraction interferometer, comprising: a laser (100) suitable for generating collimated laser; the frequency shifting assembly (200) comprises a first acousto-optic frequency shifter (201) which is suitable for enabling the collimated laser to generate diffraction and obtaining two beams of first sub-laser with a frequency difference; the second acousto-optic frequency shifter (202) is matched with the first acousto-optic frequency shifter (201), so that the two beams of first sub-laser with the frequency difference are diffracted respectively, and two beams of second sub-laser which are parallel and have the frequency difference are obtained; a diffraction assembly (300) configured to diffract the two beams of second sub-laser to generate reference light with a spherical wave front and measurement light with the spherical wave front; wherein the measuring light is reflected by the to-be-measured element after entering the to-be-measured element, and reflected light carrying surface shape information of the to-be-measured element (600) is obtained; and the interference assembly (400) is matched with the diffraction assembly (300) and is suitable for enabling the reference light and the reflected light to generate interference to obtain an interference signal, and the interference signal is used for determining the surface shape information of the element to be detected.

Description

Technical Field

[0001] This invention relates to the field of optical instruments, and in particular to a point diffraction interferometer. Background Technology

[0002] Point diffraction interferometers (PDIs) are a crucial technology in the field of high-precision optical surface shape measurement. Their core idea is to introduce a tiny aperture, close to the working wavelength, into the optical path. Through diffraction within this microaperture, a near-ideal spherical wave is obtained as a reference wavefront, enabling high-precision interferometric measurements. Since the reference wavefront is entirely generated by diffraction, it avoids the manufacturing errors of traditional reference optical elements. Therefore, it is particularly suitable for sub-nanometer and even picometer-level surface shape measurement needs, such as extreme ultraviolet lithography objectives, large-aperture astronomical systems, inertial confinement fusion optical components, and high numerical aperture microscopy. With the continuous improvement of optical manufacturing precision, the demand for surface shape measurement of large-aperture, high numerical aperture, high curvature, or aspherical components continues to increase, placing higher demands on the stability of the PDI structure, the quality of the reference wave, vibration resistance, assembly and adjustment complexity, and system cost.

[0003] Existing point diffraction interferometers are difficult to assemble and adjust, have high assembly costs, and lack long-term stability, which is not conducive to engineering applications. Summary of the Invention

[0004] To address the above problems, the present invention provides a point diffraction interferometer.

[0005] As a first aspect of the present invention, the point diffraction interferometer includes:

[0006] Laser, suitable for generating collimated lasers;

[0007] Frequency shifting components, including:

[0008] The first acousto-optic frequency shifter is suitable for causing the collimated laser to diffract and obtaining two first sub-lasers with a frequency difference;

[0009] The second acousto-optic frequency shifter, in conjunction with the first acousto-optic frequency shifter, causes the two first sub-lasers with a frequency difference to diffract, resulting in two parallel second sub-lasers with a frequency difference.

[0010] The diffraction assembly is configured to diffract two second sub-lasers to generate a reference light with a spherical wavefront and a measurement light with a spherical wavefront; wherein the measurement light is incident on the element under test and reflected by the element under test to obtain reflected light carrying surface shape information of the element under test;

[0011] An interference component, in conjunction with the diffraction component, is suitable for causing the reference light and the reflected light to interfere, thereby obtaining an interference signal, which is used to determine the surface shape information of the element under test.

[0012] According to an embodiment of the present invention, the point diffraction interferometer further includes:

[0013] A focusing component, located between the second acousto-optic frequency shifter and the diffraction component, is adapted to focus two second sub-lasers and transmit the focused two second sub-lasers to the diffraction component.

[0014] According to an embodiment of the present invention, the optical axis of the focusing component is configured to be parallel to the transmission direction of the two second sub-lasers, and the two second sub-lasers are symmetrically distributed on both sides of the optical axis.

[0015] According to an embodiment of the present invention, the focusing assembly includes: an aspherical mirror and a spherical mirror arranged sequentially along the transmission directions of the two second sub-lasers; the spacing between the aspherical mirror and the spherical mirror is configured as follows:

[0016] Under the tolerance conditions of a relative tilt of ±0.05° between the aspherical and spherical mirrors and a relative eccentricity limit of ±0.01 mm between the aspherical and spherical mirrors, the wavefront residual of the second sub-laser after passing through the spherical mirror is reduced to 0.033. , It is the wavelength of the second sub-laser.

[0017] According to an embodiment of the present invention, the point diffraction interferometer further includes:

[0018] The movable component is adapted to adjust the spacing between the first and second acousto-optic frequency shifters and the diffraction angles of the first and second acousto-optic frequency shifters, respectively.

[0019] According to an embodiment of the present invention, the diffraction component is also adapted to reflect the reflected light.

[0020] The interference component includes:

[0021] An imaging mirror is used to combine the reflected light reflected by the diffraction component and the reference light to obtain interference light;

[0022] An imaging mechanism is used to image the interference light to obtain an interference signal.

[0023] According to an embodiment of the present invention, a diffraction aperture is formed on the diffraction component, and the diffraction aperture is located at the focal point of the focusing component.

[0024] According to an embodiment of the present invention, the point diffraction interferometer further includes:

[0025] The processing component is suitable for performing phase demodulation on the interference signal to obtain the surface shape information.

[0026] According to an embodiment of the present invention, the frequency of the collimated laser is located in the deep ultraviolet band.

[0027] According to an embodiment of the present invention, the two second sub-lasers are, for example, the positive first-order diffracted light of the first acousto-optic frequency shifter diffracted by the second acousto-optic frequency shifter and the zero-order diffracted light of the first acousto-optic frequency shifter diffracted by the second acousto-optic frequency shifter, or the zero-order diffracted light of the first acousto-optic frequency shifter diffracted by the first acousto-optic frequency shifter and the negative first-order diffracted light of the first acousto-optic frequency shifter diffracted by the second acousto-optic frequency shifter, and the positive first-order diffracted light of the first acousto-optic frequency shifter diffracted by the second acousto-optic frequency shifter.

[0028] The point diffraction interferometer provided by the embodiments of the present invention can complete the generation, propagation, and interference processes of reference light and measurement light in a single optical path. Compared with the existing dual-optical-path structure, it simplifies the overall optical path layout of the system, reduces the number of optical components used, lowers the system size and complexity, and improves the system integration. The single-optical-path structure reduces the number of optical component connection nodes, reduces the impact of external environment (such as vibration and temperature changes) on the stability of the optical path, and improves the reliability of the system in long-term operation. On the other hand, it avoids the use of polarization-dependent devices such as beam splitters and waveplates, reducing the introduction of additional wavefront errors. At the same time, it makes the obtained interference signal more stable, and after phase demodulation, it can obtain the surface shape information of the component under test more accurately, thus improving the measurement accuracy.

[0029] Since the two second sub-lasers have parallel and stable frequency difference characteristics after being processed by the frequency shifting component, point diffraction can be achieved at the same diffraction aperture of the diffraction component. There is no need to perform optical path alignment and adjustment for the reference light and the measurement light separately. Only the assembly and adjustment of a single optical path needs to be completed, which significantly reduces the coaxial alignment accuracy requirements of each component, shortens the system setup and maintenance time, improves the assembly and adjustment efficiency, and is more conducive to engineering applications.

[0030] According to embodiments of the present invention, the transmission of optical signals between various devices is carried out using free-space transmission. Compared to the existing technology that mixes fiber optic paths and spatial optical paths, the present invention employs fully free-space transmission, avoiding the introduction of fiber optic devices such as fiber couplers and fiber delay lines. This not only reduces additional interference such as fiber end-face reflection and fiber bending loss, but also eliminates alignment deviations and wavefront distortions that may occur during the conversion between fiber and spatial light, making the optical signal transmission path more stable and providing a high-quality optical signal foundation for subsequent interferometric measurements. Attached Figure Description

[0031] The above and other objects, features and advantages of the present invention will become more apparent from the following description of the invention with reference to the accompanying drawings, in which:

[0032] Figure 1 A schematic diagram of an existing point diffraction interferometer is shown.

[0033] Figure 2 A schematic diagram of a point diffraction interferometer according to an embodiment of the present invention is shown.

[0034] Figure 3 A schematic diagram of a focusing component according to an embodiment of the present invention is shown.

[0035] Explanation of reference numerals in the attached figures:

[0036] 1: Short-coherent laser; 2: 1*2 single-mode fused fiber coupler; 3: Acousto-optic frequency shifter assembly; 4: First objective lens; 5: First quarter-wave plate; 6: Spatial optical delay line; 7: Attenuation stop; 8: Fiber delay line; 9: Second objective lens; 10: Second quarter-wave plate; 11: Imaging element; 12: CCD camera; 13: Pinhole plate; 14: Concave mirror; 100: Laser; 200: Frequency shifter assembly; 201: First acousto-optic frequency shifter; 202: Second acousto-optic frequency shifter; 300: Diffraction assembly; 400: Interference assembly; 401: Imaging mirror; 402: Imaging mechanism; 500: Focusing assembly; 501: Aspherical mirror; 502: Spherical mirror. Detailed Implementation

[0037] The following will combine Figure 1 The existing point diffraction interferometer is described.

[0038] Figure 1 A schematic diagram of an existing point diffraction interferometer is shown.

[0039] like Figure 1 As shown, the existing point diffraction interferometer includes a short coherent laser 1, a 1*2 single-mode fused fiber coupler 2, an acousto-optic frequency shifter 3, a first objective lens 4, a first quarter-wave plate 5, a spatial light delay line 6, an attenuation stop 7, an optical fiber delay line 8, a second objective lens 9, a second quarter-wave plate 10, an imaging element 11, a CCD camera 12, a pinhole plate 13, and a concave mirror 14.

[0040] A short-coherent laser 1 emits a short-coherent laser with a center wavelength of 633 nm. This short-coherent laser is split into two beams by a 1*2 single-mode fused fiber coupler 2. One beam enters the spatial optical delay line 6 as a reference beam, and the other beam enters the acousto-optic frequency shifter 3 as the measured beam. In the reference optical path, the reference beam passes through the spatial optical delay line 6, the attenuation stop 7, and the fiber delay line 8. After exiting the fiber delay line 8, it passes through the second objective lens 9 and the second quarter-wave plate 10, converges on the pinhole plate 13, and is diffracted into a spherical wave through a small hole in the pinhole plate 13. Finally, it passes through the imaging element. The light to be measured enters the CCD camera 12 through the component 11. In the optical path under test, the light to be measured passes through the acousto-optic frequency shifting component 3, the first objective lens 4, the first quarter wave plate 5, and the pinhole plate 13, and is also diffracted into a spherical wave. The spherical wave is reflected by the concave mirror 14 and the pinhole plate 13, and then enters the CCD camera 12 through the imaging element 11. Finally, the spherical wave of the reference optical path and the spherical wave of the optical path under test carrying the surface shape information of the concave mirror 14 become a plane wave after passing through the imaging element 11, and interfere at the target surface of the CCD camera 12. The surface shape of the concave mirror 14 is then calculated according to the phase shifting algorithm.

[0041] As can be seen from the principle of existing point diffraction interferometers, the reference light and the measured light must be focused onto the point diffraction aperture through independent converging optical paths. Furthermore, since the size of the point diffraction aperture is typically on the sub-micrometer scale, the deviations between the center of the reference light spot and the center of the measured light spot and the center of the aperture must be controlled within an extremely high precision range to meet the accuracy requirements of subsequent interferometric measurements. However, the two beams enter the aperture through different objectives and spatial positions, necessitating the simultaneous high-precision coaxial alignment of multiple optical components (including two sets of objectives, the aperture plate, and the fiber end face). This process places extremely high demands on the mechanical resolution, structural stability, and pose control accuracy of the alignment mechanism, resulting in high system assembly difficulty, high assembly cost, and insufficient long-term stability, thus limiting the practicality and engineering application of point diffraction interferometers. Therefore, this invention proposes a point diffraction interferometer with advantages of low adjustment difficulty, low assembly cost, and high long-term stability.

[0042] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0043] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0044] All terms used herein, including technical and scientific terms, have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0045] Figure 2 A schematic diagram of a point diffraction interferometer according to an embodiment of the present invention is shown.

[0046] like Figure 2 As shown, the point diffraction interferometer includes: a laser 100, a frequency shifting component 200, a diffraction component 300, and an interference component 400.

[0047] Laser 100 is suitable for generating collimated laser. Frequency shifting assembly 200 includes a first acousto-optic frequency shifter 201 and a second acousto-optic frequency shifter 202. The first acousto-optic frequency shifter 201 is suitable for diffracting the collimated laser to obtain two first sub-lasers with a frequency difference. The second acousto-optic frequency shifter 202 cooperates with the first acousto-optic frequency shifter 201 to diffract the two first sub-lasers with a frequency difference to obtain two parallel second sub-lasers with a frequency difference. Diffraction assembly 300 is configured to diffract the two second sub-lasers to generate a reference light with a spherical wavefront and a measurement light with a spherical wavefront; wherein, the measurement light is incident on the device under test 600 and reflected by the device under test to obtain reflected light carrying surface shape information of the device under test 600. Interference assembly 400 cooperates with the diffraction assembly to cause the reference light and the reflected light to interfere to obtain an interference signal, which is used to determine the surface shape information of the device under test.

[0048] The point diffraction interferometer provided in the embodiments of the present invention, with the cooperation of laser 100, frequency shifting component 200 (including first acousto-optic frequency shifter 201 and second acousto-optic frequency shifter 202), diffraction component 300 and interference component 400, can complete the generation, propagation and interference process of reference light and measurement light in a single optical path. Compared with the existing dual-optical-path structure, it simplifies the overall optical path layout of the system, reduces the number of optical devices used, reduces the system size and complexity, and improves the system integration. The single-path structure reduces the number of optical component connection nodes, lowers the impact of external environment (such as vibration and temperature changes) on optical path stability, and improves the reliability of long-term system operation. On the other hand, the frequency shifting component 200 directly realizes beam splitting, frequency shifting, and frequency difference control through diffraction, avoiding the use of polarization-related devices such as beam splitters and waveplates, reducing the introduction of additional wavefront errors. At the same time, the stable and controllable low-frequency heterodyne signal (achieved by the reasonable setting of the frequency shifting frequencies of the first acousto-optic frequency shifter 201 and the second acousto-optic frequency shifter 202) makes the interference signal acquired by the interference component 400 more stable. After phase demodulation, the surface shape information of the component under test can be obtained more accurately, improving the measurement accuracy.

[0049] Since the two second sub-lasers have parallel and stable frequency difference characteristics after being processed by the frequency shifting component 200, point diffraction can be achieved at the same diffraction aperture of the diffraction component 300. There is no need to perform optical path alignment adjustment for the reference light and the measurement light separately. Only a single optical path assembly and adjustment operation needs to be completed, which significantly reduces the coaxial alignment accuracy requirements of each component (laser 100, frequency shifting component 200, diffraction component 300, etc.), shortens the system setup and maintenance time, improves assembly and adjustment efficiency, and is more conducive to engineering applications.

[0050] According to embodiments of the present invention, the transmission of optical signals between various devices is carried out using free-space transmission. Compared to the existing technology that mixes fiber optic paths and spatial optical paths, the present invention employs fully free-space transmission, avoiding the introduction of fiber optic devices such as fiber couplers and fiber delay lines. This not only reduces additional interference such as fiber end-face reflection and fiber bending loss, but also eliminates alignment deviations and wavefront distortions that may occur during the conversion between fiber and spatial light, making the optical signal transmission path more stable and providing a high-quality optical signal foundation for subsequent interferometric measurements.

[0051] According to an embodiment of the present invention, the frequency of the collimated laser is located in the deep ultraviolet band. For example, the wavelength of the collimated laser is 193 nm.

[0052] According to embodiments of the present invention, existing technologies employ a hybrid structure of optical fiber and spatial light, relying on devices such as polarization beam splitters and quarter-wave plates. However, in the deep ultraviolet band, these devices suffer from limitations in material selection, high manufacturing costs, short lifespans, and the tendency to introduce additional wavefront errors. The present invention utilizes a design that combines dual spatial optical-acoustic-optical frequency-shifting components in series for pure free-space transmission. This eliminates the need for the aforementioned polarization and beam-splitting devices. Furthermore, collimated lasers in the deep ultraviolet band can be stably transmitted directly in free space, avoiding the problems of high coupling loss and end-face reflection interference that easily occur with optical fiber devices such as fiber couplers and fiber delay lines in the deep ultraviolet band. This improves the system's adaptability and reliability in the deep ultraviolet band.

[0053] An acousto-optic frequency shifter is an optical device that utilizes the acousto-optic effect to diffract incident light by introducing propagating ultrasonic waves into an optical medium, thereby obtaining a specific frequency shift at different diffraction orders. When high-frequency sound waves propagate in a transparent optical medium (such as quartz, TeO2, etc.), they cause periodic density changes in the optical medium, resulting in periodic modulation of the refractive index.

[0054] (1);

[0055] in, The static refractive index represents the inherent refractive index of the optical medium when it is not affected by sound waves. Δn represents the refractive index modulation depth, which is the maximum amplitude of the refractive index change and determines the diffraction efficiency. Ω represents the angular frequency of the sound wave, where Ω = 2πf. a , where f a This is the ultrasonic driving frequency. K represents the sound wave number. Λ represents the wavelength of the sound wave in the crystal. X represents the spatial coordinates, i.e., the displacement along the direction of sound wave propagation. T represents the time variable, reflecting the translation of the refractive index field over time (i.e., the moving grating). This periodic refractive index modulation is optically equivalent to a time-shifting phase grating. Incident light in the acousto-optic crystal satisfies momentum and energy conservation, exchanging momentum and energy with the sound wave; therefore, the angles and frequencies of different diffraction orders change accordingly.

[0056] According to an embodiment of the present invention, the two second sub-lasers can be, for example, the positive first-order diffracted light of the first acousto-optic frequency shifter diffracted by the second acousto-optic frequency shifter and the zero-order diffracted light of the first acousto-optic frequency shifter diffracted by the second positive first-order diffracted light of the second acousto-optic frequency shifter. Alternatively, the two second sub-lasers can also be, for example, the zero-order diffracted light of the first acousto-optic frequency shifter diffracted by the second acousto-optic frequency shifter and the negative first-order diffracted light of the first acousto-optic frequency shifter diffracted by the second acousto-optic frequency shifter, and the positive first-order diffracted light of the first acousto-optic frequency shifter diffracted by the second acousto-optic frequency shifter.

[0057] According to an embodiment of the present invention, the above-mentioned point diffraction interferometer further includes: a focusing component 500, located between the second acousto-optic frequency shifter 202 and the diffraction component 300, which is suitable for focusing two second sub-lasers and transmitting the focused two second sub-lasers to the diffraction component 300.

[0058] According to an embodiment of the present invention, the focusing component 500 can precisely focus two parallel second sub-lasers with a stable frequency difference output from the second acousto-optic frequency shifter 202, so that the focused beam can be accurately transmitted to a single diffraction aperture of the diffraction component 300. Compared with the prior art where the two beams need to be focused by separate converging mirrors, the single-path focusing design of the focusing component 500 in this embodiment of the present invention avoids the accumulation of alignment deviations caused by multiple converging mirrors, ensuring the focusing overlap of the two second sub-lasers at the diffraction aperture. This provides a key guarantee for the subsequent generation of high-quality spherical wave reference light and measurement light, thereby improving the basic accuracy of interferometric measurements.

[0059] Figure 3 A schematic diagram of a focusing component according to an embodiment of the present invention is shown.

[0060] like Figure 3As shown, according to an embodiment of the present invention, the focusing assembly includes an aspherical mirror 501 and a spherical mirror 502 arranged sequentially along the transmission directions of the two second sub-lasers; the spacing between the aspherical mirror 501 and the spherical mirror 502 is configured such that, under the tolerance conditions of a relative tilt of ±0.05° between the spherical mirror and the aspherical mirror and a relative eccentricity limit of ±0.01mm between the spherical mirror and the aspherical mirror, the wavefront residual of the second sub-laser after passing through the spherical mirror 502 is reduced to 0.033. , It is the wavelength of the second sub-laser.

[0061] The following is a detailed explanation of the design principles of the focusing component:

[0062] The focusing assembly 500 focuses the straight beam (two second sub-lasers). To meet the quality requirements of subsequent pinhole diffraction, the converging beam needs to have a high numerical aperture and wavefront accuracy, as well as a small spot diameter. However, since the beam is incident off-axis, using conventional spherical mirrors in this scenario will produce significant off-axis aberrations such as coma, astigmatism, and significant higher-order spherical aberrations. This will cause the converging spot to diffuse, reduce the wavefront quality, and affect the wavefront quality and energy utilization of pinhole diffraction. In addition, conventional mirror groups often require multiple spherical mirrors, which increases the system size and complexity.

[0063] The focusing component 500 in this embodiment of the invention adopts a two-piece structure combining an aspherical lens and a spherical lens, which occupies less space. The surface shape equation of the optical surface of the spherical lens can be written as equation (2).

[0064] (2);

[0065] Where c is the paraxial curvature of the spherical mirror surface, h is the beam height, and k is the conic coefficient. For a conventional sphere, k=0, and the optical surface can be considered as... The sum of the nth order aberrations. Clearly, as h increases, higher order aberrations become increasingly significant, ultimately leading to an increase in optical aberrations. For aspherical surfaces, k is not equal to 0, offering greater degrees of freedom and making them more effective at compensating for off-axis aberrations and higher-order aberrations generated when off-axis beams are incident.

[0066] To design the focusing component that best meets the requirements, a composite evaluation index combining wavefront quality assessment and physical optics analysis was adopted, comprehensively considering the geometric optical aberrations of the focusing component and the size of the diffraction spot in physical optics. Furthermore, based on the principles of finite element analysis, the energy utilization, uniformity, and far-field wavefront quality of the far-field diffraction wavefront after the converging spot passes through the pinhole were comprehensively analyzed and optimized before and after obtaining the physical optics diffraction wavefront. After obtaining the initial optimization results, Monte Carlo analysis was used to analyze the tolerance errors of the structure to ensure that the tolerance requirements of the focusing component do not exceed the actual standards.

[0067] Analysis revealed that placing the aspherical surface on the back surface of the spherical mirror can significantly reduce the curvature of the subsequent spherical mirror and significantly reduce the sensitivity of component tilt and decentering, thereby lowering the tolerance standards and making it easier for the system to meet design specifications in actual production.

[0068] In the final optimized structure, the parameters of the aspherical and spherical mirrors are as follows: The front surface of the aspherical mirror is a standard sphere with a radius of curvature of 800 mm, a center thickness of 3.5 mm, and an effective aperture of 7.6 mm. The rear surface uses a hyperbolic aspherical mirror with a conicity k=-2.66 and a radius of curvature of -6.18 mm. The front surface curvature of the spherical mirror is 6.55 mm, the center thickness is 3.4 mm, and the rear surface curvature is 32.02 mm. The front-to-back spacing between the two mirrors is 0.7 mm, and both are made of C79-80 material. The use of a hyperbolic aspherical mirror with a conicity of -2.66 provides stronger compensation for higher-order spherical aberrations and off-axis aberrations, ensuring that the laser can achieve near-diffraction-limited focusing quality even at short focal lengths. This reduces the wavefront residual to 0.033 under the tolerance conditions of element tilt ±0.05° and element eccentricity limit ±0.01 mm. .

[0069] At the 193nm wavelength, this structure can compress the focusing spot diameter to the micrometer level to match the pinhole size of the point diffraction plate, ensuring that the energy coupling efficiency and diffraction wavefront quality meet the interferometer requirements. The focusing component uses a high-refractive-index glass material, enabling short focal length incident light (f=6.7mm), which allows for a more compact structure.

[0070] Continue to refer to Figure 2 According to an embodiment of the present invention, a diffraction aperture is formed on the diffraction component 300, and the diffraction aperture is located at the focal point of the focusing component. A metal film is deposited on the diffraction component 300 to reflect the reflected light again. The interference component 400 includes an imaging mirror 401 and an imaging mechanism 402. The imaging mirror 401 is adapted to combine the reflected light reflected by the diffraction component and the reference light to obtain interference light. The imaging mechanism 402 is adapted to image the interference light to obtain an interference signal.

[0071] According to an embodiment of the present invention, the diffraction component 300 has the dual functions of generating spherical waves through diffraction and reflecting reflected light, achieving the refracted transmission of reflected light without the need for additional reflectors or other devices. The imaging mirror 401 of the interference component 400 directly combines the reference light and reflected light, eliminating the complex optical path refracting and combining structures of traditional schemes, and reducing the number of optical components and optical path adjustment nodes. This design not only shortens the optical signal transmission path and reduces the impact of external vibrations and airflow disturbances on the consistency of the two beams' propagation, but also avoids phase deviations introduced by the cooperation of multiple devices, making the combined interference light more stable and laying the foundation for the imaging mechanism to obtain high-quality interference signals.

[0072] According to an embodiment of the present invention, the optical axis of the focusing component 500 is configured to be parallel to the transmission direction of the two second sub-lasers, and the two second sub-lasers are symmetrically distributed on both sides of the optical axis.

[0073] According to an embodiment of the present invention, the optical axis of the focusing component 500 is parallel to the transmission directions of the two second sub-lasers, and the two second sub-lasers are symmetrically distributed on both sides of the optical axis, so that the two beams can enter the focusing component 500 at the same incident angle. This design ensures that the focusing component has consistent focusing ability for the two beams, which can be converged at the center of the diffraction aperture of the diffraction component 300, avoiding focusing shift or spot distortion caused by incident angle deviation, laying the foundation for generating two high-quality spherical waves (reference light and measurement light), and ensuring the accuracy of subsequent interferometric measurements. The ability of the reflected light and the reference light to be combined at the imaging mirror is based on the symmetrical distribution of the second sub-lasers on both sides of the optical axis. If the two sub-lasers are not distributed on both sides of the optical axis, but are biased to the same side of the optical axis or have no symmetrical relationship, their propagation paths after passing through the focusing component will lose symmetry. The path of the reference light directly transmitted to the imaging mirror cannot be aligned with the path of the measurement light after reflection by the mirror under test and refracted by the diffraction component, resulting in a significant spatial offset, and they cannot overlap in the beam combining area of ​​the imaging mirror.

[0074] The point diffraction interferometer of this invention performs surface shape measurement based on the principle of heterodyne interferometry. Heterodyne interferometry is an interferometric measurement technique that artificially introduces a known, stable, and minute frequency difference between a reference beam and a measurement beam, causing the interference fringes to change periodically in the time dimension. Unlike traditional in-phase interferometry, heterodyne interferometry does not obtain phase information through mechanical phase shifting or spatial scanning, but rather achieves wavefront or displacement measurement through phase demodulation of the time-domain signal. Let the complex electric fields of the reference beam and the measurement beam be... They are respectively:

[0075] (3);

[0076] (4).

[0077] in These represent the complex electric field amplitudes of the reference light and the measurement light, respectively. To reference the frequency of the complex electric field of light and measure the frequency of the complex electric field of light, The initial phase of the complex electric field of the reference light is the same as the initial phase of the frequency of the complex electric field of the measurement light.

[0078] The detected light intensity is superimposed on the detector. for:

[0079] (5);

[0080] After unfolding, the interference light intensity is obtained. :

[0081] (6);

[0082] in The frequency difference between the reference light and the measurement light. The phase difference is given. Note that the AC quantity of the interference signal is a low-frequency signal that varies with time; therefore, heterodyne interference phase shifting can be performed on it to obtain the corresponding phase information.

[0083] Interference contrast V is defined as maximum value and minimum value expression

[0084] (7)

[0085] Equation (8) can be obtained by calculating according to equation (7).

[0086] (8).

[0087] in To determine the intensity of the reference light and the measurement light, we can find from the above formula that when... The maximum contrast can be obtained when and The greater the relative deviation, the smaller the contrast, the less obvious the interference phenomenon, and the lower the measurement accuracy.

[0088] The following is a detailed analysis of the light intensity of the reference light and the measurement light in this scheme.

[0089] Measuring the intensity of light It is represented as follows.

[0090] (9).

[0091] This represents the zero-order diffraction efficiency of the first acoustic-optical frequency shifter. The zero-order diffraction efficiency of the second acousto-optic frequency shifter (3) is indicated. This indicates the diffraction efficiency of the diffraction component. This indicates the energy reflectivity of the component. This indicates the energy reflectivity of the reflective film coated on the diffraction component. This indicates other common losses in the system.

[0092] Reference light intensity It is represented as follows.

[0093] (10).

[0094] in, This represents the positive first-order diffraction efficiency of the first acousto-optic frequency shifter. This represents the negative first-order diffraction efficiency of the second acousto-optic frequency shifter.

[0095] According to an embodiment of the present invention, it should be noted that the interval between the first and second acousto-optic frequency shifters affects the distance between the two emitted parallel beams, which in turn affects the angle formed by the two beams after passing through the converging mirror. Let the distance between the two acousto-optic frequency shifters be d, and the angle between the zero-order beam and the first-order beam of any acousto-optic frequency shifter be d. The focal length of the focusing component is f, and the angle between the converging beams is... The following relationship exists.

[0096] (11).

[0097] Therefore, the same components can be used to arrange structures with different angles, increasing the adaptability of the instrument.

[0098] Since different test components have different energy reflectivities, the light intensity of the measurement optical path will change with the test component while other parameters remain unchanged. This will lead to changes in stripe contrast and thus affect the measurement accuracy.

[0099] To maintain a consistently high stripe contrast (stripe contrast V close to 1, reference light intensity in the optical path) With measurement of light intensity in the optical path (Approximately equal), we can change the fringe contrast by adjusting the diffraction efficiency of the acousto-optic frequency shifter. Based on the working principle of the acousto-optic frequency shifter, we can express its diffraction efficiency as the input RF power of the frequency shifter, resulting in the following formula.

[0100] (12);

[0101] (13).

[0102] This indicates the input RF power of the first acoustic-optical frequency shifter. This indicates the input radio frequency power of the second acoustic-optical frequency shifter (3). This represents the input RF power of any acoustic-optical frequency shifter when the first-order diffraction efficiency is 100%.

[0103] The ratio of the intensity of the reference light to the intensity of the measuring light can be written as equation (14).

[0104] (14)

[0105] Adjustment It can satisfy

[0106] (15).

[0107] At this point, the intensity of the measuring light is equal to the intensity of the reference light, and the ideal interference contrast V=1. The above process illustrates that the point diffraction interferometer of this invention has the function of flexibly adjusting the contrast of the interference fringes to adapt to test elements with different surface reflectivities, thus enhancing the instrument's versatility and engineering application capabilities.

[0108] According to an embodiment of the present invention, the point diffraction interferometer further includes a moving component. The moving component is suitable for adjusting the distance between the first and second acousto-optic frequency shifters, as well as the diffraction angles of the first and second acousto-optic frequency shifters. By adjusting the distance between the frequency shifting mechanism and the second acousto-optic frequency shifter, and their respective diffraction angles, the interval between the two second sub-lasers can be precisely controlled. Combined with the focusing effect of the focusing component 500, the emission angles of the measurement light and the reference light can be flexibly adjusted, adapting not only to the diffraction requirements of different aperture specifications of the diffraction pinhole plate 5, but also meeting the diverse measurement scenarios of the components under test (such as plane mirrors, convex mirrors, concave mirrors, large-aperture optical components, high numerical aperture components, etc.), thus expanding the application range of the point diffraction interferometer.

[0109] According to embodiments of the present invention, the processing component is suitable for performing phase demodulation on the interference signal to obtain surface shape information. Specifically, the surface shape information can be determined using the interference signal based on a multi-step phase-shifting orthogonal phase demodulation algorithm.

[0110] The following combination Figure 2 The working process of the point diffraction interferometer in this embodiment of the invention will be described in detail.

[0111] Laser 100 emits a collimated laser with a narrow linewidth and a center wavelength of 193 nm. This collimated laser is first incident on a first acousto-optic frequency shifter 201, where it is split into zero-order diffracted light and positive first-order diffracted light. Subsequently, these diffracted lights enter a second acousto-optic frequency shifter 202, where each beam is diffracted again. The zero-order diffracted light from the first acousto-optic frequency shifter 201, and the negative first-order diffracted light from the second acousto-optic frequency shifter 201, are spatially parallel and selected for subsequent optical paths. The remaining diffracted beams are effectively excluded from measurement because their propagation direction deviates from the measurement range. As can be seen from the characteristics of the acousto-optic frequency shifter, the frequency of the zero-order diffracted light is the same as the incident light frequency. The frequency of the positive first-order diffracted light is increased by a set frequency shift amount compared to the incident light frequency, while the negative first-order diffracted light is the opposite, decreasing by a set frequency shift amount compared to the incident light frequency. By reasonably setting the frequency shifting frequencies of the two acousto-optic frequency shifters, for example, to 200MHz and 200MHz+5Hz respectively, the optical frequency difference between the two parallel beams is exactly 5Hz. Thus, a stable and controllable low-frequency heterodyne signal is introduced into the system, satisfying the basic requirements of heterodyne interference for frequency difference.

[0112] Two heterodyne parallel beams are symmetrically incident on the focusing component 500 and converged onto the diffraction aperture of the coated point diffraction component 300 (e.g., a diffraction pinhole plate). After passing through the diffraction aperture, the beams form two high-quality spherical waves. The reference beam is incident on the camera via the imaging mirror 401, while the measurement beam is incident on the mirror under test and reflected back to the point diffraction plate. The metal film coated on the point diffraction plate is reflected by the imaging mirror and forms time-varying heterodyne interference fringes with the reference beam on the camera. By performing time-domain sampling and phase demodulation of the interference signal, high-precision heterodyne phase-shifting measurement of the wavefront information of the mirror under test can be achieved.

[0113] It should be noted that by controlling the distance and diffraction angle of the frequency shifter, the lateral spacing between the two parallel beams can be flexibly controlled, thereby controlling the emission angle of the measurement beam and the spherical beam to meet the measurement needs of various scenarios.

[0114] The point diffraction interferometer provided by the embodiments of the present invention is based on an heterodyne point diffraction interferometric measurement scheme with a dual spatial optical-acoustic-optic frequency shifter cascade structure. By performing multi-stage acousto-optic diffraction and frequency modulation on the same laser beam, two spatially parallel, frequency-controllable, and stable frequency-difference measurement and reference beams are simultaneously generated in a single optical path. This achieves the point diffraction process of two beams using only one focusing component and a single diffraction component. The embodiments of the present invention complete the generation, propagation, and interference of the reference and measurement beams within the same spatial path. This method eliminates the technical limitations of traditional heterodyne interferometric systems, which require two independent optical paths and two sets of converging optical systems, simplifying the system's optical path structure, reducing assembly complexity, and significantly improving the system's stability and repeatability.

[0115] In existing external difference diffraction interferometry systems, in addition to the commonly used fiber-space light hybrid structure, additional optical devices (such as polarization beamsplitters and non-polarization beamsplitters) are typically relied upon to achieve beam splitting, phase shifting, or interference condition control. While the introduction of these devices enables optical path function allocation, it also increases the number of optical devices, complicates the system structure, and increases the degrees of freedom in assembly and adjustment. In the deep ultraviolet band, the selection of materials for waveplates and beamsplitters is limited, resulting in high cost and short lifespan. Multiple beam splitting and polarization modulation can easily introduce additional wavefront errors, reducing measurement accuracy. This invention proposes an external difference diffraction interference method based entirely on a spatial optical-acoustic-optical frequency shifter. The method directly achieves beam splitting, frequency shifting, and frequency difference control through the spatial optical-acoustic-optical frequency shifter, avoiding the use of fiber beam splitters, fiber frequency shifters, or fiber delay structures, as well as spatial beam splitting elements. It realizes frequency modulation and heterodyne generation under a pure spatial optical path, avoiding the use of polarization and beam splitting devices such as waveplates and beam splitters, reducing the number of system components, and reducing the sources of system error. While ensuring the heterodyne interferometry measurement function, it reduces or even eliminates the dependence on polarization-related optical devices such as waveplates and beam splitters.

Claims

1. A point-diffraction interferometer, comprising: a laser adapted to generate a collimated laser beam; a frequency shifting assembly, comprising: a first acousto-optic frequency shifter adapted to diffract the collimated laser beam to obtain two first sub-laser beams having a frequency difference; a second acousto-optic frequency shifter cooperating with the first acousto-optic frequency shifter to diffract the two first sub-laser beams to obtain two second sub-laser beams parallel and having a frequency difference; a diffraction assembly configured to diffract the two second sub-laser beams to generate a reference light having a spherical wave front and a measurement light having a spherical wave front; wherein the measurement light is reflected by a to-be-measured element after being incident to the to-be-measured element to obtain a reflected light carrying surface profile information of the to-be-measured element; an interference assembly cooperating with the diffraction assembly and adapted to interfere the reference light and the reflected light to obtain an interference signal used to determine the surface profile information of the to-be-measured element.

2. The point diffraction interferometer of claim 1, wherein, further comprising: a focusing assembly located between the second acousto-optic frequency shifter and the diffraction assembly and adapted to focus the two second sub-laser beams and transmit the focused two second sub-laser beams to the diffraction assembly.

3. The point diffraction interferometer of claim 2, wherein, An optical axis of the focusing assembly is configured to be parallel to a transmission direction of the two second sub-laser beams, and the two second sub-laser beams are symmetrically distributed on two sides of the optical axis.

4. The point diffraction interferometer of claim 2, wherein, The focusing assembly comprises, in sequence along the transmission direction of the two second sub-laser beams, an aspheric mirror and a spherical mirror; a spacing between the aspheric mirror and the spherical mirror is configured to: Under the tolerance conditions that the relative inclination of the aspheric mirror and the spherical mirror is ±0.05° and the relative eccentricity limit of the aspheric mirror and the spherical mirror is ±0.01mm, the wavefront residual error of the second sub-laser after passing through the spherical mirror is reduced to 0.033 , is the wavelength of the second sub-laser.

5. The point diffraction interferometer of claim 1, wherein, further comprising: a moving assembly adapted to adjust a spacing between the first acousto-optic frequency shifter and the second acousto-optic frequency shifter and a diffraction angle of each of the first acousto-optic frequency shifter and the second acousto-optic frequency shifter.

6. The point diffraction interferometer of claim 1, wherein, The diffraction assembly is further adapted to reflect the reflected light, The interference assembly comprises: an imaging mirror adapted to combine the reflected light reflected by the diffraction assembly and the reference light to obtain interference light; an imaging mechanism adapted to image the interference light to obtain the interference signal.

7. The point diffraction interferometer of claim 2, wherein, A diffraction hole is formed on the diffraction assembly, and the diffraction hole is located at a focal point of the focusing assembly.

8. The point diffraction interferometer of claim 1, wherein, further comprising: a processing assembly adapted to perform phase demodulation on the interference signal to obtain the surface profile information.

9. The point diffraction interferometer of claim 1, wherein, A frequency of the collimated laser beam is in a deep ultraviolet band.

10. The point diffraction interferometer of claim 1, wherein, The two second sub-laser beams are first-order positive diffraction light of the first acousto-optic frequency shifter, zero-order diffraction light of the first acousto-optic frequency shifter, or zero-order diffraction light of the second acousto-optic frequency shifter and negative first-order diffraction light of the second acousto-optic frequency shifter.

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