Search

In the traditional machining method, the surface quality will get worse during the surface figuring process while the surface figure will be destroyed after ultra-smooth polishing process. To obtain an ultra-precision optical surface, these two processes are conducted iteratively which is time-consuming and unstable. A new fabrication method combined with the magnetorheological finishing (MRF) and hydrodynamic effect polishing (HEP) technology was presented. The surface figure error can be quickly depressed by MRF due to its high deterministic figuring ability, while the surface quality will get worse due to the introduction of MRF polishing marks. The material removal of HEP depends on the chemical interaction between the nanoparticles and workpiece in elastic mode, and an ultrasmooth surface with sub-nanometer level can been easily achieved with the surface figure well maintained. The experiment results show that the MRF polishing marks are removed clearly and the surface figure is well maintained after HEP process. Combined with MRF, a quartz glass with initial figure accuracy of 0.415 λ RMS (λ = 632.8 nm), mid-spatial frequency and high spatial frequency roughness of 0.407 nm RMS and 0.525 nm RMS has been improved to figure accuracy of 0.05 λ RMS, mid-spatial frequency and high spatial frequency roughness of 0.268 nm RMS and 0.163 nm RMS. Power spectral density (PSD) analysis indicated that the surface spatial frequency error has all been greatly depressed. It demonstrates HEP is an effective ultra-smooth polishing method with good ability to maintain the initial surface figure. [ABSTRACT FROM AUTHOR]

Abstract: We present a method for optical wavefront measurement using phase retrieval with subaperture stitching method in situations where detected intensity patterns are undersampled or where the limit of aberrations is determined by the largest spot size overfilling the detector array. In this technique we use a smaller moving subaperture to produce a number of adequately sampled intensity patterns captured by the detector located near the focus plane. These patterns are then used to retrieve the full-aperture wavefront through an optimization phase retrieval algorithm. The relationship between the optimization parameters of moving subaperture and the support information is given. Then expressions for the gradient of an error metric with respect to the optimization parameters are presented. An experimental setup used to measure the reflected wavefront of a concave mirror by employing this method is arranged. The experimental results indicate that the phase retrieval results are in excellent agreement with that obtained with interferometer, which confirms the validity of this method. [Copyright &y& Elsevier]

Abstract: We present a method for calibrating the position error of phase retrieval with transverse translation diversity in optical wavefront in situation where the position error sharply influences algorithm precision. This method involves testifying that the essence of the position error in phase retrieval is the translation in frequency domain and the minimum of iterative error against position error reaches when there is no position error. Then the least square method is used for fitting the relationship between the iterative error and the assumed position error in polynomial function. The computer simulations used to prove the validity of this method are described. The results indicated that this method can calibrate the position error to approximately 1/500mm which nearly has no influence on the phase retrieval. [Copyright &y& Elsevier]

Abstract: Phase retrieval technology for off-axis ellipsoid surface measurement with a high dynamic range algorithm is proposed in this paper. A high dynamic range algorithm (HDRA) for phase retrieval is proposed. We proved the validity of it in simulation. Then the HDRA is used to measure an off-axis ellipsoid. The measurement setup was established and the HDRA was applied to retrieve the surface shape of the off-axis ellipsoid. The experimental results show that the phase retrieval results are good agreement with that obtained by interferometer, which confirms the validity of this method. [Copyright &y& Elsevier]

Aspheric optics has been widely used in the space telescope system. The large amount of material removal and mid-frequency errors district the high-precision and high-efficiency fabrication of large-scale aspheric optics. On the basis of parsing the figuring ability of tool influence functions (TIFs) and principles of smoothing polishing technology, the multi-TIFs process is proposed in this paper. Then the selection principle and replacement time of TIFs have been discussed. Furthermore, the multi-TIFs process is optimized with frequency filtering method. This filtering method with multi-TIFs can obtain better surface accuracy in a faster way compared to the traditional method. Finally, a Φ540mm SiC off-axis aspheric mirror is polished to verify the proposed method. [ABSTRACT FROM AUTHOR]

Hydrodynamic effect polishing (HEP), in which the material is removed in elastic region by chemical impact reaction, can effectively remove the surface/subsurface damage to get an ultrasmooth surface. However subsurface damage contains scratches and machining residual stress are hidden below the surface and can’t be easily detected. The subsurface/surface properties such as hardness and elastic modulus have direct relationship with the subsurface damage. Therefore, a nano-imprint method analysis of the optical subsurface quality was presented to evaluate the hydrodynamic effect polishing process effect. Polishing experiment was conducted on a conventional pad pre-polished silica glass. The surface before and after polishing process was firstly observed by atomic force microscope and ZYGO white-light interferometer. The observation shows the surface damage such as the scratches and bump structures are completely removed by HEP process, and the surface roughness rms value decreased to near 0.3 nm. To evaluate the subsurface damage, nano-imprint was conducted on the processed and unprocessed area under different load with different impressed depth. The imprint results show the processed surface become soft and easily be impressed, while the unprocessed surface becomes hard. The processed and unprocessed surface shows the same impress depth when impressed depth exceeded 200 nm. Analysis shows the surface becomes hard under the effect of the residual stress in subsurface damage which is generated in the traditional machining process with material removal in brittle or plastic mode. Material is removed in elastic mode by chemical impact reaction in HEP, and the surface/subsurface damage can be effectively removed. It demonstrates that HEP processed surface properties are much close to the bulk material. [ABSTRACT FROM AUTHOR]