X-ray mirrors are widely utilized in light source devices like synchrotron radiation (SR) and free electron laser (FEL) light source. The Root-Mean-Square (RMS) value of surface figure accuracy required for these mirrors is typically sub-micro radian or sub-nanometer, and their aperture can be hundreds of millimeters or even meters. To achieve this accuracy, deterministic figuring is required and surface figure error is the premise. Interferometric stitching test is commonly utilized. Interferometric test obtains the surface figure of the tested X-ray mirrors mounted on a mounting support. Due to the low stiffness of the X-ray caused by the large ratio of length/width and length/height. The mechanical stress induced by the mounting supports may have a non-negligible influence on the surface figure of the mirror surface. Moreover, this can result in the poor reproductivity when testing the mirror under different mounting situations and with different mounting craft. What kind of aberration will be introduced by the X-ray under different mounting supports and whether it can be neglected is still an issue. To this end, the reproductivity of a typical silicon X-ray mirror with size of 500 mm × 50 mm × 50 mm were tested on different typical supports with a 24” zygo interferometer. Comparison of the introduced aberration type and aberration amplitude were conducted to reveal the deformation of the X-ray mirror on different typical supports. The results of this paper may attract the community that fabricates, tests, or uses X-ray mirror to pay a special attention to the mounting supports when testing the surface figure the X-ray mirrors.
In order to solve the problems of large surface figure errors in large value, high spatial frequency and multi-aberration modes which are beyond the dynamic range of the interferometer, and the problems of slow iterative convergence and high non-convergence rate of existing adaptive compensation optimization algorithm, an adaptive wavefront interferometry utilizing convolutional neural network(CNN) for large surface figure error is proposed, based on the existing adaptive compensation interference detection methods. This paper first introduces the aberration regulation principle of spatial light modulator (SLM), and sets up a convolutional neural network. Then, SLM is controlled to generate Zernike aberrations with different coefficients. Combined with Zygo Verifire interferometer, the corresponding far-field light intensity is collected to compose a labeled data set to train CNN. Finally, a large surface figure error coefficient prediction experiment is carried out with the trained CNN, and the aberration compensation is performed according to the prediction coefficient to verify the effectiveness of the method. The experimental results show that the dense fringes can be transformed into resolvable fringes with this method, and the resolvable probability of full-diameter fringes after compensation is 66.7%. This method is able to greatly improve the performance of adaptive compensation detection, thereby meeting the demand for high dynamic range interference detection technology in the ultra-precision optical surface manufacturing process.
X-ray mirrors play a critical role in transmitting and focusing beams in advanced light source devices like synchrotron radiation (SR) and free electron laser (FEL) light sources. The Root-Mean-Square (RMS) value of surface shape accuracy required for these mirrors is typically sub-micro radian/sub-nanometer, and their aperture can be hundreds of millimeters or even meters. To achieve this accuracy, deterministic figuring is required and surface figure error is the premise. Interferometric stitching test is commonly utilized. However, interferometric test obtains the surface figure error relative to the reference surface. To obtain the absolute surface figure error of X-ray mirrors, it is crucial to obtain the surface figure error of the reference surface before or during stitching measurement and separate it from the stitching result. To achieve this, a reference surface self-calibration interferometric stitching algorithm based on Legendre polynomial fitting is reported. The algorithm fits the reference surface by orthogonal Legendre polynomials in the rectangular domain, and the polynomial coefficients are expressed as interlocking terms in the stitching algorithm. The fitting coefficients are optimized and fitted together with the free compensation terms, i. e. the translation and tilt terms. The paper established the theoretical model of the algorithm and conducted simulations on an X-ray mirror with a length of 300 mm. A two dimensional stitching interferometry device was built. An X-ray mirror with a length of 300 mm was tested utilizing the proposed method. The short-term repeatability was preliminarily analyzed.
Sub-nanometer precision optical components are widely used in optical systems. As the key equipment of sub-nanometer precision calculation, the dynamic performance of ion beam figuring equipment is very important. However, most studies have focused on optimizing ion beam figuring processes; However, the research on the dynamic performance of machine tools with sub-nanometer precision is not sufficient and comprehensive. At present, most of the research is limited to the analysis of plane machining, that is, only the dynamic performance of the scanning axis is analyzed. As the surface shape changes from plane to surface, the accuracy requirement is from nanometer to sub-nanometer, so the influence of the scanning axis cannot be simply considered. The dynamic performance of the target distance axis of three-axis ion beam figuring equipment under different surface shape characteristics and removal function characteristics of the sub-nanometer machining process was analyzed theoretically and simulated. The results of this study can promote the application of three-axis ion beam figuring equipment in the manufacture of complex surfaces and high-precision optical components.
Introducing adaptive optics such as spatial light modulators (SLMs) can help troubleshoot the ‘one-to-one’ test mode of interferometric null surface figure tests for aspheric and free-form surfaces. However, the phase controlling accuracy of methods based on birefringence effects and phase-wrapping is too low for applying SLMs in interferometric surface figure tests. To this end, the flexible phase control method based on computer holography for SLMs is presented. The difference of phase control principle between the computer holography-based method and the birefringence effects and phase-wrapping based method is discussed. Phase control accuracy of the proposed method is about λ/30 (λ=632.8 nm) root-mean-square (RMS) according to the theoretical analysis result. The inherent wavefront error of a SLM HoloeyeTM LC 2012 is significantly nulled to from RMS 0.426λ to RMS 0.015λ, which verifies the high phase controlling accuracy of the proposed method. By troubleshooting the phase control accuracy issue, the presented method is promising for promoting the application of SLMs in interferometric surface figure tests.
Jet polishing plays an important role in ultra-precision machining. In this paper, the effects of slotted and slotless nozzles on micro jet machining are investigated. The pressure distribution and magnitude of the two nozzles on the workpiece were first calculated by finite element simulation software. Then a set of comparative tests were conducted on two single crystals of silicon. The results showed that the pressure distribution of the slotless nozzle was more uniform compared to the slotted nozzle, and the pressure value of the slotless nozzle on the surface of the workpiece was greater. In addition, the slotless nozzle is more capable of improving the surface roughness of the workpiece in the same time.
Monocrystalline silicon reflectors are widely used in infrared high energy laser systems. In order to ensure the system to achieve high precision and high stability of beam transmission, the reflector needs to have a good laser load capacity for high power density laser under long time irradiation. However, the evaluation of reflector laser load capacity is influenced by multiple factors, which is difficult to be decoupled one by one, and the multiple index systems are not perfect. In this paper, multi-modal characterization methods such as reflectivity, fluorescence detection and surface roughness detection are proposed to establish the influence model of multiple influencing factors on the laser load capacity of monocrystalline silicon reflector. Through quantitative analysis of these defects with specific types and different properties, the surface cleanliness and integrity of the monocrystalline silicon reflector were analyzed from different angles, and the influence trend of each influencing factor on the laser load capacity of the element was obtained. In this paper, a relatively completed characterization system of monocrystalline silicon and the influence model of the laser load capacity of the monocrystalline silicon reflector have been established effectively. The influencing factors of the laser load capacity on the surface of the monocrystalline silicon reflector are evaluated effectively, which lays a foundation for the efficient acquisition of the monocrystalline silicon reflector with high load capacity.
The fused silica optics are important functional ultraviolet optical elements in the high-power laser system. Contamination plays an extremely important role in laser-induced damage, which will affect the laser damage threshold of the components to varying degrees, and finally affects the output flux of high-power laser system. An organic solvent needs to be used to wipe the optical surface before applying it, and this process will introduce organic pollution. In addition, different optical films and organic solvents have different effects on the laser damage threshold of fused silica, which need to be studied further. In this work, the photo-thermal weak absorption platform was used to test the photothermal weak absorption of three samples before and after the organic solvent treatment. The first sample was tested on the surface of the fused silica substrate; the surface of the second sample was coated with aluminum oxide; and the surface of the third sample was coated with hafnium oxide. The results show that the use of ethanol can increase the photo-thermal weak absorption of the substrate by 29.6%; the photo-thermal weak absorption signal of the fused silica element coated with aluminum oxide is reduced by 20% after the surface is wiped with ethanol; the fused silica element with hafnium oxide coated on the surface reduces the photo-thermal weak absorption signal by 33%. The experimental results verify the feasibility of using ethanol to wipe fused silica components.
Most of the existing defect detectors focus on the size, location, depth and number of defects of the tested components. The instrument is usually large in size and requires high accuracy for the environment and motion devices. In contrast, the direct random bed motion, which aims at finding and locating defects, is highly efficient, low-cost and environmentally practical, while the research on vibration-resistant defect tester is rare.In order to solve this problem, based on the principle of micro-scattering imaging in dark field, a set of optical component surface defect detection device is built, and the influence factors of light intensity, illumination angle, wavelength and other defect detection factors are experimentally studied, in order to provide design basis for the follow-up development of on-line defect detection instrument. The experimental results show that the most important factor affecting the sensitivity is the azimuth angle and pitch angle of the incident light, which is more than 30 degrees between the incident light and the scratch direction. When the pitch angle is between 60 degrees and 70 degrees, the higher detection sensitivity can be obtained. In addition, improving the illumination intensity can help to improve the detection of defects. In the visible range, the wavelength has little effect on the sensitivity.
High-precision cylindrical surfaces are widely utilized in X ray telescope systems, laser systems, synchrotron radiation systems, etc. These systems put forward strict requirements on surface figure errors of cylindrical surfaces. As a supporting technique for fabrication, interferometric null test method for measuring cylindrical surfaces usually requires a Computer Generated-Hologram (CGH) as null. However, Limited by the aperture and F/number of CGHs for cylinders, large convex cylindrical surfaces with small R/number (ratio of the radius of curvature to the aperture) cannot be tested in a single measurement.Subaperture stitching along both axial direction and sagittal direction is required to acquire the full aperture surface figure map. a convex cylindrical mirror with R/3 and 350mm aperture was tested adopting our subaperture stitching test system for cylindrical surfaces. The key challenges are presented here including, subaperture stitching platform, subaperture algorithm, subaperture lattice, and the system alignment.
Optical windows are important for a wind tunnel to enable observation or imaging of the internal flow-field. In order to reduce the interference to the internal flow and not to modulate the input observation ray field as much as possible, the interior and exterior surfaces of the optical window often adopt complex free-form surface design and must be used in pairs. This requires high accuracy of both the surface form and their relative position. In this paper, Trace-Pro optical software is used for the ray tracing analysis of the wind tunnel observation window defined by discrete points on the inner and outer surfaces. The conclusion is that the collimated beam is also the collimated beam after passing through the single observation window, which means the transmitted wave-front can be well resolved by a standard interferometer. The ray data in Trace-Pro is imported into MATLAB to obtain the modulated wave-front error which is contributed by the difference between the inner and outer surfaces of the monolithic optical window. It hence can be used to guide the corrective machining of the window surface. In addition, the influence of different misalignment on the interferometric test of the window is analyzed. Finally, the method is experimentally demonstrated on an optical free-form window. The surface positioning error is reduced with corrective machining based on the measured transmitted wave-front.
Aspheric surface acts an important role in modern optical systems. The null test utilizing a computergenerated hologram (CGH) is fundamental for precision test of aspheres. Its measurement accuracy mainly depends on the fabrication precision of CGH. However, it is not easy to exactly characterize the fabrication error of the CGH as well as its contribution to measurement uncertainty. In this paper, a new CGH wavefront error evaluation method is presented. The CGH fabrication errors such as duty-cycle error, etching depth inhomogeneity, pattern distortion, etc., are related to the fringe spacing based on elaborate measurement of the CGH microstructures. A scanning white-light interferometer and a high-precision two-axis stage are employed to sample the microstructure at a series of designed locations on the CGH. When the fabrication error is modeled through experiments, it can then help to realize rapid measurement of any other CGHs with significantly reduced number of sampling. The second step is then modeling the contribution of CGH fabrication error to measurement uncertainty according to the scalar diffraction theory. Meanwhile, the wavefront error induced by CGH fabrication error can also be characterized through ray-tracing in lens design software. The fabrication error is incorporated into the discrete phase data of CGH surface and its contribution to the final measurement uncertainty is evaluated through simulations.
Traditional null optics is generally designed for a particular optical surface. It must be redesigned when the test surface is changed no matter the null optic is reflective, transmitted or a CGH. Development of advanced optical machining and testing based on deterministic figuring and null test makes it possible to apply high-order aspheres. This paper presents a plano-concave singlet to realize variable aberration correction for testing different surfaces. The concave surface is an even asphere with high-order terms. By changing the axial distances among the transmission sphere, the null singlet and the test surface, variable aberrations are generated to meet the aberration balance requirement for various surfaces. The residual aberrations are confirmed within the vertical dynamic range of measurement of the interferometer. It enables flexible testing of optical surfaces without dedicated null optics. The optical design verifies that the aspheric singlet can be used to test conic surfaces with different conic constant and radius of curvature ranging from ellipsoid, paraboloid to hyperboloid and an even asphere.
Cylindrical mirrors are key optics of high-end equipment of national defense and scientific research such as high energy laser weapons, synchrotron radiation system, etc. However, its surface error test technology develops slowly. As a result, its optical processing quality can not meet the requirements, and the developing of the associated equipment is hindered. Computer Generated-Hologram (CGH) is commonly utilized as null for testing cylindrical optics. However, since the fabrication process of CGH with large aperture is not sophisticated yet, the null test of cylindrical optics with large aperture is limited by the aperture of the CGH. Hence CGH null test combined with sub-aperture stitching method is proposed to break the limit of the aperture of CGH for testing cylindrical optics, and the design of CGH for testing cylindrical surfaces is analyzed. Besides, the misalignment aberration of cylindrical surfaces is different from that of the rotational symmetric surfaces since the special shape of cylindrical surfaces, and the existing stitching algorithm of rotational symmetric surfaces can not meet the requirements of stitching cylindrical surfaces. We therefore analyze the misalignment aberrations of cylindrical surfaces, and study the stitching algorithm for measuring cylindrical optics with large aperture. Finally we test a cylindrical mirror with large aperture to verify the validity of the proposed method.
Large convex aspheres are difficult to test since larger null optics are required. We propose to test a convex even asphere
by near-null subaperture stitching based on the reconfigurable optical null. This paper starts with an introduction to the
principle of reconfigurable optical null and the near-null stitching algorithm. For the purpose of cross test, alternative
two measurements of the surface are obtained and quantitatively compared with the stitching test. The first one is null
test with a single asphereic lens which is in situ calibrated by an ordinary null. The test beam travels the same path when
used for measuring the convex asphere and calibrated by the ordinary null, which relaxes the tolerance on transmissive
quality and fabrication error of the large aspheric lens. The other alternative is back-through null test with a null lens. A
singlet is designed to balance the positive spherical aberration existing in the back-through test of the even asphere. A
low coherence interferometer is employed to precisely measure or monitor the surface thickness or air clearance in the
null test. The three measurements are finally registered to each other to calculate the difference or similarity
quantitatively.
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