Magnetorheological polishing technology is a new type of ultra-precision processing technology. It has many advantages such as no sub-surface defects, easy realization of numerical precision control, high machining accuracy, and high polishing efficiency; and its small removal function size makes it very suitable for processing optical elements with complex geometric structures. It has been widely used in phase optical processing. In order to realize the processing of phase optical elements with a small space periodic structure, a smaller removal function is required. The traditional magnetorheological fluid circulation system structure is difficult to achieve stable control of the magnetorheological fluid flow under small flow conditions. Therefore, it is difficult to obtain a stable small size removal function. This paper analyzes the characteristics of the peristaltic pump, and aiming at the problem of strong pulse in its output flow; uses a pulsation damper and a variable-diameter back pressure pipeline to eliminate the flow pulse, realizes the stable and smooth output of the peristaltic pump flow. On this basis, this paper proposes a new type of magnetorheological fluid circulation system structure, which realizes the stable control of the flow rate under small flow rate conditions. The magnetorheological fluid flow fluctuation error of the magnetorheological fluid circulation system was from more than 25% reduced to less than 2%, realizing high stability control of magnetorheological fluid flow. Use small size removal function to process Continuous Phase Plates (CPP) with large depth and small periodic structure. The CPP is designed with a PV of 5μm and a minimum space period of 3mm. After processing, the residual error RMS of the CPP from 830nm converges to 24.5nm, realize high-precision processing of phase components with large depth and small structure.
Magnetorheological polishing technology is a high-deterministic, high-precision optical ultra-precision processing technology, which is widely used in the polishing of large-aperture flat optical elements. The accuracy of magnetorheological removal function is a key factor that determines the accuracy and efficiency of magnetorheological polishing. In the process of making the removal function, it is easy to introduce spurious fringes when an interferometer is used for surface shape test. The spurious fringes will reduce the extraction accuracy of the removal function, and affect the certainty of magnetorheological polishing. In addition, the introduced spurious fringes contain complex mid- and high-frequency structures, which seriously affect the evaluation of the ability to remove function modification. In order to eliminate the spurious fringes, this paper proposes a method of removing the spurious fringes of the removal function based on the characteristic spectrum band-stop filter. First, the ZooMFFT algorithm is used to amplify the frequency spectrum to realize the effective identification of the spectral characteristics of the spurious fringes; secondly, a specific algorithm is used to eliminate only the frequency spectrum of the spurious fringes, completely retaining the frequency spectrum of the removal function itself, and does not change the valid shape of the removal function, making the spurious fringes are removed. And the Quad Flip operation is used in spectral filtering to improve the filtering accuracy. The magnetorheological making spot experiment uses a fused silica plate element with a diameter of 50mm. A static interferometer is used to measure the component wavefront data, and the tested result data contains obvious spurious fringes. Using the method described in this article, the spurious fringes are completely removed, and the morphological and removal efficiency of the removal function remains unchanged. Compared with the traditional frequency domain band-stop filter, the residual surface RMS of the spurious fringes separation is reduced from 5.45nm to 1.98nm; improve the extraction accuracy of the removal function. It can be effectively used to eliminate the spurious fringes of the magnetorheological removal function.
Traditional optical processing technology is limited by factors such as the geometric size and stability of the removal function of the processing equipment, and it is very difficult to process optical elements with complex structures with large depths and small periods. In particular, the high-efficiency and high-precision processing of optical elements with complex structures whose depth is micron-level and space period is millimeter-level has always been a technical difficulty. This paper proposes a method for processing optical components with complex structures based on magnetorheological polishing technology, focusing on solving the traditional magnetorheological polishing technology small size removal function generation and stable control problems, and obtaining small size removal with stable removal efficiency and morphology function. The magnetorheological fluid circulation system is a key component of the magnetorheological polishing machine. The structure of traditional magnetorheological fluid circulation system is difficult to match the stable transmission of magnetorheological fluid under small flow conditions. This paper proposes a method of dual peristaltic pump & pulsation damper combined with variable-diameter back pressure pipeline to achieve the stability of magnetorheological fluid control. The flow fluctuation error of magnetorheological fluid is reduced from 10-40ml/min to 1-5ml/min. The diameter of the polishing wheel of the machine is 20mm and the flow rate of the magnetorheological fluid is 200ml/min. A two-dimensional sinusoidal surface processing was performed using magnetorheological machine. Sinusoidal surface design PV=1.0λ (λ=632.8nm), RMS=159.5nm, the space period is 3mm, the element size is 50mm×50mm. The width of removal function is 1.33mm, FWHH (Full Width at Half Height) is 0.98mm, and volume removal rate is 0.004mm 3/min. After the element is completed, the residual RMS is reduced from the original 159.5nm to 15.7nm, and its convergence rate is 90.1%, with high convergence accuracy. The experimental results show that the configuration of the magnetorheological fluid circulation system proposed in this paper realizes the generation and stable control of the small size removal function and finally realize the fabricating of phase components with a minimum space period of 3mm.
In the interference test process of a parallel flat optical element, the light beam after multiple reflections on the front and rear surfaces of the element with the wavefront to be measured often cause multi-surface interference, forming spurious fringes on the interferogram, which will introduce large errors to the wavefront measurement. Spurious fringes mainly have a great impact on the parameters such as wavefront gradient root mean square (GRMS) and mid-spatial-frequency power spectral density (PSD). The RMS value of the wavefront containing spurious fringes is generally significantly larger than the true value, which will affect the accurate measurement and evaluation of the wavefront quality of optical components. Existing spurious fringes suppression methods often have the disadvantages of multiple adjustment steps in the test process or high hardware requirements, and it is difficult to match the requirements for rapid and high-efficiency test of high-precision optical flat components. This paper proposes a method for removing spurious fringes in interference test based on characteristic spectrum band-stop filter, which can achieve accurate removal of spurious fringes. First, by using the ZoomFFT algorithm to zoom up the spectrum of the wavefront data points, the effective identification of the spectral characteristics of the spurious fringe is realized; then the band-stop filter in a specific area is used according to the spectral characteristics of the spurious fringes, only the frequency spectrum of the spurious fringes is removed, and the wavefront data of the component to be tested is completely retained without changing its own shape; the Quad-Flip operation and error function filter window are used for spectrum filtering, which effectively reduces Gibb's noise in the frequency domain due to the sudden truncation of the input data edge during FFT transformation. The transmitted wavefront of a fused silica element with a diameter of Φ100mm was tested on the ZYGO static interferometer, and the test results contained a large number of spurious fringes. After processing by this method, the spurious fringes were removed. The mid-spatial-frequency wavefront RMS of the component is reduced from 5.365nm to 3.678nm. The method does not need to add additional hardware and tedious measurement and adjustment operations, the calculation is fast, and the spurious fringes removal is accurate.
Due to the characteristics of large depth, small period and high steepness, phase optical elements with complex structure need to use a small size removal function in magnetorheological processing, and use very small line spacing and step size values when planning the polishing path,so the dwell time matrix is very large, and the dwell time calculation speed is slow; besides, because of the complicated phase optical profile, it is difficult to achieve high-precision convergence of the dwell time. This paper proposes a fast and high-precision numerical iterative dwell time algorithm for complex structure phase optical elements. this paper proposes the concept of the dwell point matrix, which realizes the methods of the FFT convolution multi-core parallel algorithm to calculate the dwell time in the entire iterative calculation process. Also, to achieve high-precision convergence of the dwell time calculation, this paper proposes a calculation rule based on machine dynamic performance matching, when calculating the dwell time, the speed, acceleration, and speed smoothness of the machine were matched with the performance of the magnetorheological machine, which improves the stability of the machine. A large-diameter Continuous Phase Plate (CPP) is processed on a magnetorheological machine. The shape of the CPP contains a random structure of various periods. The initial RMS = 228.07nm, the CPP data matrix size is 2424 × 2424, and the line spacing is 0.6mm, the dwell time is calculated using the algorithm described in this article, the entire calculation process takes only 4.2 seconds, the calculation speed is about 3 times faster than the traditional iterative methods, the CPP residual error RMS converges to 10.2nm; After the CPP processing is completed, the CPP actual residual error RMS is reduced from the original 228.07nm to 15.6nm, and its convergence rate is 93.1%, which shows that the algorithm has high calculation efficiency and convergence accuracy.
Spiral phase plate (SPP) for generating vortex hollow beams has high efficiency in various applications. However, it is difficult to obtain an ideal spiral phase plate because of its continuous-varying helical phase and discontinued phase step. This paper describes the demonstration of continuous spiral phase plate using filter methods. The numerical simulations indicate that different filter method including spatial domain filter, frequency domain filter has unique impact on surface topography of SPP and optical vortex characteristics. The experimental results reveal that the spatial Gaussian filter method for smoothing SPP is suitable for Computer Controlled Optical Surfacing (CCOS) technique and obtains good optical properties.
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