Significance Large-aperture potassium dihydrogen phosphate (KDP) and deuterated potassium dihydrogen phosphate (DKDP) crystals are the only nonlinear optical materials suitable for serving as frequency conversion elements and optical switches in high-power laser facility. However, their anisotropy, soft-brittleness, hygroscopicity, thermal sensitivity, and propensity for cracking impose significant challenges to ultra-precision manufacturing. Conventional grinding and polishing processes are prone to leaving abrasive particles embedded on the surface. These particles serve as precursors to laser-induced damage, significantly diminishing the laser damage resistance of the crystals. Consequently, the simultaneous attainment of full spatial-frequency bandwidth precision and a high laser-induced damage threshold (LIDT) constitutes a pivotal challenge in the advancement of high-power laser facility. Progress To address these challenges, an integrated technical route of "single-point diamond turning (SPDT) + sub-nanosecond laser conditioning + sol-gel coating" has been established. Significant progress has been made in the following areas. Ultra-precision cutting technology and equipment: an anisotropic constitutive model for soft-brittle crystals was developed to reveal the brittle-ductile transition (BDT) behavior during cutting (Fig. 1). Simulations identified the optimal cutting direction along 45 degrees within the (001) plane and a BDT depth of approximately 150 nm. Through process optimization and the innovative design of an integrated vacuum chuck with variable hole density and active temperature control (Fig. 2), surface figure accuracy better than 4 lambda (lambda=632.8 nm) and sub-nanometer roughness [root mean square (RMS)=0.59 nm] were achieved on large-aperture KDP crystals. Surface defects induced by fly-cutting, such as brittle indentations, cracks, protrusion pressure points, ballast, and plastic scratches, were systematically characterized (Fig. 4). Fluorescence microscopy (405 nm) revealed that defects like brittle indentations, cracks, protrusions, and ballast exhibit higher fluorescence intensity than defect-free regions, indicating stronger laser energy absorption and lower LIDT (Fig. 5, Table 1). An explosion simulation model was innovatively proposed to quantify the damage thresholds for different defect types and to elucidate the underlying damage mechanisms (Fig. 8). This model simplifies the complex multi-field coupling problem into a quantifiable explosive process, revealing that local mechanical strength and absorption capability are key factors affecting LIDT. Sub-nanosecond laser conditioning: the mechanisms underlying laser conditioning for the elimination or passivation of both point defects and structural defects were elucidated. A pulse width of 500 ps was identified as the optimal parameter within the 300?800 ps range, as it provides sufficient peak power for electronic excitation while exceeding the lattice heat transfer time necessary for thermal effects. After applying this offline conditioning process to 400 mm aperture DKDP crystals, under ultraviolet laser irradiation, the surface damage density was reduced from 5.02 to 0.55 pp/cm(2), and the bulk damage density decreased from 2?3 to 0.3?0.8 pp/mm(3) (Fig. 10, Table 2), marking a critical step toward engineering application. Sol-gel coatings: to enhance environmental stability and optical performance, multifunctional coatings were developed via sol-gel methods. Moisture barrier coating a novel network-ball embedded structure was created by embedding hexamethyldisilane (HDMS)-modified SiO2 nanoparticles into a siloxane polymer matrix (Fig. 11). This structure yields a tunable refractive index (1.21?1.44), high hydrophobicity (contact angle increased to 109.4 degrees), and exceptional moisture resistance (less than 0.1% transmission loss after 27 weeks at 80% relatively humidity). Antireflective (AR) coating: using methyltriethoxysilane (MTES) to seal surface pores after HMDS modification, an AR coating with low residual reflectance (less than 0.5%@355 nm), high LIDT (more than 20 J/cm(2)), and excellent oil contamination resistance (only 0.097% transmission drop after 20 weeks) was achieved (Fig. 12). Bilayer coating system a precisely designed bilayer system for dual-wavelength (527 nm & 351 nm) antireflection was realized. It combines a high-refractive-index moisture barrier layer and a low-refractive-index AR layer, exhibiting outstanding optical uniformity and environmental stability (0.7% transmission drop after 19 weeks in high humidity) (Fig. 13). Conclusions and Prospects This review comprehensively summarizes recent breakthroughs in ultra-precision manufacturing technology for large-aperture KDP/DKDP crystals achieved through an integrated process route. Significant advances in fly-cutting theory and equipment, defect characterization and suppression, laser conditioning, and functional coating design have collectively and notably enhanced the surface accuracy, laser damage resistance, and environmental stability of these critical optical components. Looking forward, future research should prioritize several key directions: 1) exploring novel processes such as ultra-precision polishing to further suppress mid-spatial-frequency ripples induced during machining; 2) developing multifunctional composite coatings that exhibit lower curing temperatures, higher LIDT, and extended operational lifetimes; and 3) establishing a full-process database that correlates manufacturing defects with damage performance, along with developing efficient, non-destructive online evaluation techniques for comprehensive performance assessment of large-aperture crystal components.

Silicon carbide (SiC) ceramics are widely used as structural materials for various applications. However, the extraordinarily high hardness, brittleness, low material removal rate, and severe tool wear of these materials significantly impact the performance of conventional mechanical processing techniques. In this study, we investigated the influence of different parameters on the material removal rate, surface quality, and surface oxidation during the laser processing of SiC ceramic samples using a high-repetition-frequency femtosecond laser at a wavelength of 1030 nm. Additionally, an experimental investigation was conducted to analyze the effects of a burst mode on the material removal rate. Our results demonstrate that the surface oxidation, which significantly affects the material removal rate, can be effectively reduced by increasing the laser scanning speed and decreasing the laser scanning pitch. The material removal rate and surface quality are mainly affected by laser fluence. The optimal material removal rate is obtained with a laser fluence of 0.4 J/cm2 at a pulse width of 470 fs.

The effects of femtosecond laser (wavelength 1035nm, pulse duration 400 fs, pulse repetition rate 1kHz) on the material removal threshold of superalloy was experimentally investigated, specifically the influence of pulse number irradiated on the threshold of superalloy. Irregular bulge structures were found at the bottom of ablation holes. Two sets of material removal thresholds are derived from the value of hole diameter and depth, which are 0.506 J/cm2 , 0.245 J/cm2 , 0.124 J/cm2 , 0.102 J/cm2 , 0.096 J/cm2 and 0.141 J/cm2 , 0.130 J/cm2 , 0.031 J/cm2 , 0.026 J/cm2 , 0.091 J/cm2 for diameter derivation and depth derivation, respectively, for 1, 10, 100, 500 and 1000 pulses. The deduced two sets of thresholds show similar behavior that the threshold sharply decreases with the pulse number and levels out as the pulse number exceeds 100. The phenomenon is due to incubation effects of multi-pulse irradiation on the surface of superalloy and the incubation coefficient for the thresholds calculated by diameter and depth is fitted to be ~0.72 and ~0.77.

实验研究了激光脉冲宽度和脉冲个数对镍基高温合金材料去除阈值的影响,分别在290 fs,1 ps和7 ps脉宽的激光下,使用1,10,50,100,300,500和1000个不同能量的激光脉冲辐照高温合金样品表面.实验结果表明,烧蚀坑尺寸会随脉冲数的增加而增加,而脉冲宽度的增加会加大脉冲个数对烧蚀坑直径的影响.通过烧蚀坑直径的平方值与激光脉冲能量之间存在的对数关系,得到了不同脉冲宽度下镍基高温合金的多脉冲材料阈值.3种不同脉宽下的高温合金多脉冲去除阈值都存在显著的累积效应.根据去除阈值计算得到290 fs,1 ps和7 ps脉宽下的累积效应系数分别为0.88,0.86和0.78.

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.

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.

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.

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.

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.

最小空间周期是连续位相板（CPP）设计和加工过程中的重要特征参数。根据惯性约束聚变大型激光驱动装置的需求,建立不同空间周期的CPP设计与分析方法,研究了不同最小空间周期对磁流变加工和焦斑性能的影响。结果表明,磁流变加工的去除函数尺寸直接与CPP的最小空间周期成线性关系,而加工去除量与最小空间周期的平方根成线性关系,最小周期越大,加工越容易,但加工量越大;焦斑整形性能受最小空间周期的影响小,能量集中度差异小于0.2%,但焦斑顶部均匀性随着最小空间周期变小而变好,5 mm最小周期CPP的焦斑顶部不均匀比15 mm的CPP小3.5%。因此,设计时应尽量减小最小空间周期,但选取的最小空间周期不能大于加工设备的约束条件。

为了提高惯性约束聚变(ICF)激光装置中连续相位板(CPP)的焦斑性能,建立了波前畸变下连续相位板焦斑的理论计算和分析模型,并根据CPP使用条件搭建了三倍频大口径CPP远场离线测试系统.对加工330 mm×330 mm口径的CPP和波前畸变元件进行了理论计算和离线测试实验的对比研究.理论计算和实测的焦斑形貌、参数数值均非常一致,验证了计算模型的正确性和实验系统的可靠性.理论和实验结果一致表明,波前畸变对CPP焦斑性能的影响非常严重,当弥散斑为0.5倍CPP焦斑时,畸变量已对CPP焦斑形貌产生了很大影响,能量利用率下降值大于4％,焦斑半径增大超过20μm,陡边阶数下降1.3阶,不均匀性均方根(RMS)值下降6％,旁瓣份额增长超过0.5％.

This paper analyzes characteristics of wavefront and light intensity of large-aperture continuous phase plates(CPPs) fabricated by magnetorheological finishing(MRF). Three groups of CPPs fabricated by different processing parameters such as scanning intervals and scanning offsets are compared, and the influence of mid-frequency error introduced by MRF on CPPs wavefront and light intensity are analyzed as well. Total analysis indicates that the performance of CPP is poor when scanning interval is 2 mm and scanning offsets are 0.1-0.3 mm; The iteration fabrication efficiency is higher when scanning interval is 2 mm and scanning offsets are 0.4-0.5 mm compared with that when scanning interval is 1 mm and scanning offsets are 0.1-0.3 mm, and the mid-frequency error is improved as well. Further analysis indicates that mid-frequency error introduced by MRF has a greater impact on CPPs wavefront gradient, near-field and side lobe.

在惯性约束聚变（ICF）研究过程中,焦面上聚焦光斑形态要求极为苛刻。基于离线测试平台,从实验上研究了各种应用误差对连续相位板（CPP）远场焦斑能量集中度的影响。得出光束旋转误差、口径误差、平移误差和倾斜误差在可控范围内CPP远场焦斑能量集中度均高于95%,其波动范围小于0.5%,CPP的容忍度较强。而实验畸变波前属于空间频率小于0.02mm-1的低频波前,严重影响了CPP的整形能力,波前畸变是影响能量集中度高于90%的主要因素。

基于全局最小二乘拼接算法和图像融合算法建立了连续相位板(CPP)子孔径拼接检测算法,并根据全局相关匹配原理提出采用面形残差来评价CPP的加工面形.采用高精度动态干涉仪等设备建立了相应的检测系统,并针对430mm×430mm口径CPP开展了数值模拟和检测实验.理论计算结果表明:系统计算误差为0.005nm.实验结果表明,整个检测系统软硬件RMS误差小于5nm,基本满足CPP面形检测要求.从而验证了CPP检测和评价的正确性和可行性.