By virtue of the characteristics of high machining certainty, fast surface shape convergence and less subsurface damage, magnetorheological finishing (MRF) has been widely applied in ultra-precision machining for optics. However, the convolution of tool influence function (TIF) and the regular movement track make it difficult to avoid the deterioration of mid-spatial frequency (MSF) errors in the MRF process, which will affect the optical properties of optics. In this paper, the main factors affecting the MSF errors in grating path processing are theoretically analyzed. Under the condition of determinate polishing spot, the main influencing factors are the removal depth and processing spacing of a single scan. Through experimental research, the influence of removal depth and processing spacing of a single scan on the MSF errors in the MRF process is acquired. On this basis, the preferred parameters of removal depth and processing spacing of a single scan under specific processing conditions in the MRF process are obtained with the MSF errors of 1 nm as the evaluation index. It provides a theoretical basis and reference value for suppressing the deterioration of MSF errors of optics during MRF process and obtaining high quality optical surfaces.
Mid-spatial-frequency (MSF) error on optical surfaces can do great harm to high-performance laser systems. A non-interferometric way of measuring it is phase retrieval, which has already proved its effectiveness in previous studies. However, the performance of phase retrieval is limited by its long-time iterative process and relies heavily on reliable initial solution. Therefore, in this paper, we put forward a method for fast measurement of MSF error, by introducing advanced deep learning technique into traditional computational imaging methods. Results show that the proposed method simultaneously gains an improvement on convergence speed and a reduction on residual error. The proposed method takes much fewer iterations to converge to the same error level, and has much smaller average residual error than that of the conventional algorithm in the numerical experiments.
In recent years, with the development of new materials, transparent objects are playing an increasingly important role in many fields, from industrial manufacturing to military technology. However, transparent objects sensing still remains a challenging problem in the area of computational imaging and optical engineering. As an indispensable part of 3-D modeling, transparent object sensing is a long-standing research topic, which aims to reconstruct the surface shape of a given transparent object using various kinds of measurement methods. In this paper, we put forward a new method for the sensing of such objects. Specifically, we focus on the sensing of thin transparent objects, including thin films and various kinds of nano-materials. The proposed method consists of two main steps. Firstly, we use a deep convolutional neural network to predict the original distribution of the objects from its recorded intensity pattern. Secondly, the predicted results are used as initial estimates, and the iterative projection phase retrieval algorithm is performed with the enhanced priors to obtain finer reconstruction results. The numerical experiment results turned out that, with the two steps, our method is able to reconstruct the surface shape of a given thin transparent object with a high speed and simple experimental setup. Moreover, the proposed method shows a new path of transparent object sensing with the combination of state-of-art deep learning technique and conventional computational imaging algorithm. It indicates that, following the same framework, the performance of such method can be significantly improved with more advanced hardware and software implementation.
Magnetorheological finishing (MRF) is a deterministic optical element polishing method that achieves material removal by means of the sheared and rheo-logical behavior of magnetorheological fluids. To realize high precision fabrication of large-aperture optical components, MRF technology had been explored in this paper. The main factors affecting the removal efficiency were investigated by orthogonal experiment. It indicated that the influence of the immersion depth and the thickness of ribbon on removal efficiency was more remarkable than the other parameters. The process of MRF machining component is established. Finally, the 590mm × 400mm plane optical element are manufactured using the MRF, and the result are very good.
High-precision inertial accelerometer is mainly used in aviation, aerospace, and military fields. As the core part of high-precision inertial accelerometer, silicon flexible bar has been working in extremely dynamic environment, which would bring in strong impact loads. Hence, the silicon flexible bar always encounters failure due to cracks and fractures caused by the strong impact loads. In this work, we firstly analyzed the dynamic characteristics of silicon flexible bar using Finite Element Method. The main working modes and stress responses of flexible bar under dynamic loads with various frequencies were investigated. Then, the transient impact process of silicon flexible bar was simulated to explore the effect of transient impacting load and period on the stress distribution of silicon part. The stress-strain behavior of silicon flexible bar was analyzed as well. The critical failure acceleration and strength weakness location of silicon flexible bar were finally determined by the impact experiments. The experimental results were compared with simulated ones, which show that: (1) the first-order mode is working mode of flexible bars, which swings up and down around the x-axis. The transient impact load causes bending deformation of flexible bar, which leads to the stress stratification in the z direction and produces a neutral layer where the stress is the smallest. The tensile and compressive stresses are applied in both sides of the neutral layer and the closer to the surface, the greater the stress. (2) The critical failure acceleration of silicon flexible bars is 100g. The root of the flexible bar is the most vulnerable location due to the stress concentration. Under the same impact load, the shorter the loading time, the greater the stress at the root of the bar.
The wave front gradient is a key parameter to the low frequency wave front of the aspherical components, which ultimately affects the focusing performance of the optical system. CCOS（Computer Controlled Optical Surfacing）is a technique widely used in precision polishing of aspherical lens. First of all, the reasons for the deterioration of the gradient are analyzed. Secondly, the control technology is designed for various main factors. This paper also found a technological scheme that could inhibit the error of the wave pattern in theory. Finally, combined with the actual conditions of engineering processing, the results of control process methods are verified. The results show that GRMS value of a large aperture（400mm × 400mm）aspheric element is reduced from 0.013 λ/cm to 0.008λ/cm.
Subsurface damage (SSD) caused during machining process can decrease the mechanical strength of BK7 glass optical elements, which would shorten the lifetime. Rotary ultrasonic face milling (RUFM) is interest of many engineering applications, especially for machining optical glass which need the high surface quality and less SSD depth. In this paper, the effects of several critical factors (i.e. spindle speed, feed rate, and cutting depth) on machining quality and efficiency in RUFM were investigated experimentally and further analyzed. The relationships between the process parameters and the machining quality were obtained. Moreover, an optimized method was proposed for the further applications of RUFM, taking care of both machining quality and efficiency.
Magnetorheological finishing (MRF) is a deterministic optical element polishing method that achieves material removal by means of the sheared and rheo-logical behavior of magnetorheological fluids. For the magnetorheological processing optics, an experimental study of the magnetorheological processing force was carried out. It was obtained that the relationship between the process parameters and element forces in magnetorheological polishing .The process parameters in MRF are the rotation speed of the polishing wheel, the magnetic field strength, the liquid flow rate, and the immersion depth. For MRF-600 magnetorheological polishing machine with polishing wheel Φ300mm, the normal forces range from 2N to 32N.According to the fitting curve between the magnetic field and the normal forces, the force is 6 times larger than that under the zero magnetic field condition. At the same time, the polished spots were collected under different magnetic field intensity. The law of the removal rate of polishing spots was obtained.
Based on the theoretical removal function model, the internal relation between the fluid dynamic pressure parameters and the processing parameters of the magnetorheological removal function is analyzed. The main technological parameters affecting the removal function are clarified. The single-factor influence experiment was carried out for four technological parameters (liquid flow, concentration of polishing liquid, immersion depth and thickness of ribbon).The relationship curve between the single factor parameter and the removal function volume and the single factor parameter and the removal area is given. The influence of different single-factor parameters on magnetorheological removal function is obtained. The processing of a concave mirror is guided by this rule. High quality optical elements are obtained.
Large size of YCa<sub>4</sub>O(BO<sub>3</sub>)<sub>3</sub>(YCOB) crystals were grown both by Czochralski and Bridgman methods. Large size elements as large as 60 mm clear aperture were cut and polished with surface flatness of 1/5 wavelength. Optical
homogeneity of YCOB crystal was found in the order of 10<sup>-6</sup>. Laser damage thresholds of several YCOB crystal
elements were tested using different laser facilities with different pulse widths or wavelengths, with thresholds varied
from 0.8 GW/cm<sup>2</sup> to more than 1 TW/cm<sup>2</sup>. One SHG and two optical parametric chirped-pulse amplification (OPCPA)
experiments were executed to characterize the nonlinear optical properties of YCOB crystals and the quality of the
crystals. The results shown that YCOB had good performance in OPCPA application, especially with low content of
parameter florescence. Combined with good NLO performance and possibility to grow large size crystals, YCOB crystal
was a good choice for high power OPCPA applications.