When potassium dihydrogen phosphate (KDP) crystals are exposed to high-energy laser irradiation, the pre-existing surface defects may act as damage precursors and will reduce the lifespan of the crystal components. Although it has been found that different kinds of surface defects exhibit distinct damage characteristics, the influence of surface defects on the laser-induced damage performance of KDP crystal is not yet clear. In this paper, KDP surface defects have been characterized by multiple measuring methods and classified into five categories according to their structure features. Laser-induced damage tests were then carried out to investigate the laser-induced damage thresholds of different kinds of KDP surface defects as well as the evolution of the morphology of damage sites. The results of the experiment indicate that the damage thresholds of cracks, fracture pits, and surface protuberances are between 6 and $11\mathrm{J}/{\mathrm{cm}}^2$ (355 nm, 3 ns, similarly hereinafter), which are much lower than the thresholds of plastic scratches, discontinuous scratches, and a defect-free KDP surface. In addition, it has been found that fluorescence enhancement is just a necessary condition for reduction of damage thresholds. Finally, reasons for the formation of the most threatening KDP surface defects have been analyzed and corresponding suppression measures have been proposed for increasing the surface damage thresholds of the crystal components.

Continuous phase plate (CPP), as a key diffractive optical element, is difficult to manufacture owing to its random and small features. In this paper, a novel frequency division combined machining (FDCM) method was proposed to improve polishing efficiency of CPP by optimizing the tool influence functions (TIFs) over targeted frequency bands. In addition, the convergence rate of power spectral density (CR-PSD) was proposed to evaluate the correct ability of TIF in different frequency bands, and to determine the division frequency for the combined processing. Through simulation verification, the combined processing with optimized TIFs by FDCM enabled high precision in less total time than that with single TIF processing. The experimental results verified that the method could imprint a 300 × 300 mm CPP with residual root-mean-square 24.7 nm after approximately 6-h bonnet polishing. Comparing the focal spots of designed and fabricated CPPs, the deviation of their energy concentration within 500 microns is only 0.22%. Hence, bonnet polishing using the FDCM is a new technical option for the production of large-aperture CPPs. Furthermore, the FDCM method shows a significant increase in efficiency, and it could be a generic method for CPP processing through other technologies, including magnetorheological and ion beam finishing.

A large-aperture silicon carbide (SiC) aspheric mirror has the advantages of being light weight and having a high specific stiffness, which is the key component of a space optical system. However, SiC has the characteristics of high hardness and multi-component, which makes it difficult to realize efficient, high-precision, and low-defect processing. To solve this problem, a novel process chain combining ultra-precision shaping based on parallel grinding, rapid polishing with central fluid supply, and magnetorheological finishing (MRF) is proposed in this paper. The key technologies include the passivation and life prediction of the wheel in SiC ultra-precision grinding (UPG), the generation and suppression mechanism of pit defects on the SiC surface, deterministic and ultra-smooth polishing by MRF, and compensation interference detection of the high-order aspheric surface by a computer-generated hologram (CGH). The verification experiment was conducted on a Ø460 mm SiC aspheric mirror, whose initial surface shape error was 4.15 μm in peak-to-valley (PV) and a root-mean-square roughness (Rq) of 44.56 nm. After conducting the proposed process chain, a surface error of RMS 7.42 nm and a Rq of 0.33 nm were successfully obtained. Moreover, the whole processing cycle is only about 216 h, which sheds light on the mass production of large-aperture silicon carbide aspheric mirrors.

Under nanosecond pulse irradiation, laser-induced damage of Potassium Dihydrogen Phosphate (KDP) crystal is a multi-physical coupling process which mainly includes energy absorption by precursor defects, temperature and pressure rise in the absorption center, and subsequent micro-explosion event. Till now, related research work mainly focuses on modeling the energy absorption stage and determining the temperature or pressure in the absorption center, but knowledge about the explosion stage is rather limited. In this paper, laser-induced damage of KDP crystal has been investigated through explosion simulation. According to the laser damage test results and morphologies of the damage craters, typical precursor defects inducing KDP surface damage have been determined. Based on the knowledge, equivalent explosion simulation models of the laser damage process have been established to reproduce damage crater formation and shockwave propagation. Finally, laser damage experiments, combined with time resolved techniques, have been utilized to investigate the variation of damage crater size and shockwave speed with laser fluences. Simulation results given by single core explosion models agree well with the experimental results at fluences lower than 60 J/cm², while a multicore explosion model is needed to reliably simulate damage crater formation at higher fluences.

To obtain an economical uniform polishing method, it is effective to combine robot technology with bonnet polishing technology and apply it to the precision processing of large aspheric surfaces. However, the large robotic kinematic error causes significant deterioration to the surface shape of aspheric optics during robotic bonnet polishing (R-BP). To address this problem, research on the evaluation and compensation of the kinematic error was conducted to enhance the prepolishing accuracy on large aspheric surfaces by R-BP. Firstly, a precession control model of R-BP applied to the polishing of large aspheric surfaces was proposed. Then, an evaluation and compensation method of a robot kinematic error was presented based on machining errors. At last, it had been verified by experiments that at the removal depth of 2.5 μm, the variation of surface shape was reduced from 0.42 μm to 0.11 μm after compensation. Meanwhile, the change rate was reduced from 16.8% to 4.4%. Hence, the effectiveness of evaluation and compensation method to improve the prepolishing accuracy was verified, which is beneficial to implement mass production of high-precision large aspheric surfaces with low cost.

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.