Optics in high-power solid-state laser devices will be damaged and adhere to contaminates on the surface during manufacturing and operation, which reduces the service life. To facilitate the positioning and recognition of the flaw (damage and contaminates) in the process of automatic repair, the dark-field flaw size calibration algorithm was studied. In this paper, a flaw size calibration model based on convolutional neural network was established. The size, exposure time, and loss function of damage and contaminate were optimized respectively. The training parameter of the damage regression model is determined to be size of less than 500μm, 30ms of exposure time and SmoothL1 loss function, and the parameter of contaminate is 0-100μm, 20ms of exposure and SmoothL1 loss function. After data balancing, the calibration precision was improved. To improve the calibration consistency of the full-size segment, a comprehensive size calibration strategy is proposed. The CNN regression algorithm is used for small size, and the pixel-level calibration algorithm is used for large size. Compared with the single regression calibration algorithm, the relative error and absolute error of the calibration results of small damage (<340μm) are reduced by 41.4% and 44.13% respectively. The relative error and absolute error of small contaminate (<85μm) are reduced by 40.26% and 30.63%.
Surface defects on optics seriously affect their service life in high-power laser system. To address this issue, repair technologies for large-aperture optics are extensively employed. However, the operation of high-power laser system generates a large number of optics requiring repair. Currently, microscopic inspection is widely used for target points classification due to its high accuracy, but it has limitations such as small field of view and low efficiency. Utilizing the OTSU algorithm and Convolutional Neural Networks (CNN), a dark-field classification model for optical surface target points has been developed. Upon conducting experiments with several network models, the ResNet model, a probability threshold of 0.998 were determined. In conclusion, the classification accuracy for defects was 99.18% and for contaminants was 90.34%. The outcomes show that target points on the surfaces of optics can be accurately classified using this method.
Micron-level surface damages can seriously affect the optical and mechanical properties of large-aperture optics, so it is of great significance to perform high-precision inspection and mitigation of surface damages. Surface micro-damages are not only multi-category but also randomly distributed on the optics. Manual inspection and mitigation are not only time-consuming but also difficult to ensure the accuracy and reliability of mitigation, so it cannot meet the engineering requirements for large-volume mitigation of optics. In order to solve this problem, an intelligent inspection system framework for optics surface damage mitigation is proposed in this paper. The inspection system consists of an optics surface ranging fitting module, a dark-field rapid inspection module, and a multi-illumination micro-inspection module. Based on the above modules, a damage inspection process flow is established, including automatic alignment of optics surface, rapid dark-field inspection of flaws with large field of view, microscopic precision inspection of damages with small field of view, automatic configuration and execution of damage mitigation strategies, and quality control of mitigation structure. In order to realize the above flow, a predictive model library related to damage inspection is constructed, including: a dark field classification and size calibration model, a microscopy multi-classification model, and a mitigation quality inspection model. The automation and intelligence of the process flow is achieved by replacing the manual decision-making process with predictions from the model library. Based on the proposed framework, the working principle and workflow of the damage intelligent inspection system are explained, and the inspection efficiency and automation level of the system are evaluated in this paper. The intelligent inspection system framework proposed in this paper can provide technical support for future high-volume mitigation of large-aperture optics.
CO2 laser polishing process can significantly improve the surface quality and the Laser-Induced Damage Threshold (LIDT) of fused silica optics. However, due to the thermal history of the laser polishing process, the increment of the fictive temperature inside the modification layer would cause densification and residual stress, which critically affect the surface accuracy and the service life of fused silica optics. In this work, a 3D multi-physical coupling model including temperature, fluid flow and fictive temperature was established. Based on the fictive temperature distribution of the fused silica polished by CO2 lasers, the mechanism of laser annealing on the modified layer was revealed. The annealing results of fused silica were defined as three states including incomplete annealing, perfect annealing and over annealing. Based on the simulation results, the fictive temperature inside the modified layer was completely reduced with no increment of modified layer depth under the perfect annealing state. Additionally, the residual stress and the fictive temperature after the laser annealing were characterized by the Raman spectrum. The fictive temperature was reduced by 16.8 % and the residual stress was effectively reduced. This work can provide theoretical and experimental guidance for the control of surface modification and residual stress of fused silica optics polished by CO2 lasers.
Potassium dihydrogen phosphate (KDP) crystal has been regarded as the solely irreplaceable component in laser-driven
inertial confinement fusion (ICF) facilities. Nevertheless, the laser-induced damage on KDP crystal surfaces under highenergy
laser irradiation considerably restricts the output power of ICF facilities. The laser damage event on KDP surface
is an extremely complex process, among which the non-heat initial energy deposition is regarded as the major absorbed
energy source, determining the subsequent thermal damage process and final damage morphology. The initial energy
deposition process is a non-heat stage, where the plasmas are generated from ionization processes under intense laser
irradiation. However, there is still no available model that can well reproduce the dynamic interaction behaviors between
the high-energy laser and plasmas in the initial energy deposition process, resulting in the laser-induced damage
mechanisms on KDP crystal surface still not fully revealed. In this work, a Particle-In-Cell (PIC) model is established to
investigate the initial dynamic damage behaviors of KDP crystals under intense laser irradiation. On basis of this model,
the crater formation process and the particle ejection dynamics involved in the laser damage event are reproduced. The
reproduced characteristic parameters of laser damage craters on KDP input and output surfaces, and the obtained particle
ejection angles are consistent with the previously reported laser damage morphology, which verifies the effectiveness of
the established PIC model. This work could provide theoretical means for investigating the initial energy deposition
process and also offer further insights in understanding the laser-induced damage mechanisms of KDP crystal
components.
The potassium dihydrogen phosphate (KDP) crystals suffer from nanosecond pulse laser irradiation and are susceptible to damage during the operation of ICF system. In particular, the microcracks on the surface of KDP crystals caused by the single-point diamond fly-cutting (SPDF) process are more likely to cause serious damage under the subsequent laser irradiation. However, the mechanism of laser damage is still unclear. A model that can well represents the laser damage response is very important to reveal the mechanism of laser-induced damage. In this work, the electromagnetic field, stress field and temperature field are coupled, the mechanical characteristics of KDP material are considered, and the reasonable strength equation is applied to model the laser damage response of KDP crystal. Then, the conical crack is taken as an example to explore the laser damage response process of KDP crystal caused by surface defects under laser irradiation. It is found that the surface conical cracks have a great influence on the response process and the morphological characteristics of the laser damage. The existence of surface conical crack defects would lead to the extension of the longitudinal cracks beneath the damage crater, which has great disadvantages for the repairing of the laser damage sites. This work is of great guidance for avoiding the defects-induced damage and improving the service life of the crystal applied in ICF systems.
During the grinding and polishing processes of hard-brittle fused silica optics, the defects would be inevitably formed on the finished surface. Fused silica has a high absorption coefficient for far-infrared lasers, which makes the CO2 laser processing to be the potential repairing technology for machining-induced defects on fused silica surfaces. In this work, using a low-power CO2 laser, a new repairing method to heal the machining-induced micro-defects on the surface of fused silica is proposed. Then, based on the nonlinear thermodynamic parameters of fused silica material, a thermal transfer model under CO2 laser irradiation and a dynamic defect healing model were established. On basis of that, the influence of CO2 laser parameters on the maximum surface temperature and the temperature distribution inside the silica material was investigated. It is found that, under the low-power and near-continuous CO2 laser irradiation, the maximum melting depth can be obtained under the non-evaporative condition. The defect healing process under various laser powers was explored as well. It is found that the defects would be more difficult to be healed under a laser with higher-power, smaller beam size or shorter pulse width. This work can provide theoretical guidance for the determination of the optimal parameters in the laser healing process and the optical manufacturing strategies of fused silica optics.
The issues of laser-induced damage of transparent dielectric optics severely limit the development of large laser systems. In order to explore the mechanism of nanosecond laser damage on KDP surface, a multi-physics coupling dynamics model and a time resolved detection system were developed to obtain the transient dynamic behaviors of laser damage. The behaviors of laser energy transmission, thermal field distribution and damage morphology during nanosecond laser irradiation on KDP surface were simulated. It is found that the enhancement of light intensity caused by surface defect plays an important role in the initial energy deposition and damage initiation of the laser irradiation area. The evolution of the temperature field and fluid flow during subsequent laser irradiation contributes to the laser damage process. The simulated evolution of heat absorption source is verified by the transient images of local defect-induced laser damage captured by the ultra-fast experimental detection system. This work provides further insights in explaining the laserinduced damage by surface defects on KDP crystals.
KEYWORDS: Modulation, Crystals, Diffraction, Laser induced damage, Optical components, Laser crystals, Micro cutting, Micromachining, Near field diffraction, High power lasers
Micro-machining has been proved the most effective method to mitigate the laser-induced surface damage growth on potassium dihydrogen phosphate (KDP) crystal in high power laser systems. However, the phase contrast of outgoing laser beam, introduced by the mitigated KDP surface, would cause light propagating turbulence and downstream intensification with the potential to damage downstream optics. In this work, a Gaussian mitigation pit with width of 800μm and depth of 10μm is fabricated on KDP rear surface by micro-milling. The effect of the mitigation pit on downstream light intensification is analyzed through propagation calculations based on Fresnel diffraction integral theory. The light intensity modulations reach a peak value at the position of 10mm downstream from the rear surface, decrease sharply subsequently and get stable eventually. The results indicate that the modulations induced by Gaussian mitigation pits would change with various downstream locations. It is essential to notice the unacceptable downstream intensification and reduce the risk of laser damage on other optics by choosing an appropriate installation location.
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.
Micro-machining has been regarded as the most promising method to mitigate the laser damage growth on KDP/DKDP crystal surfaces. In this work, the near-field and far-field light modulations caused by three kinds of typical mitigation contours (spherical, Gaussian and conical) were theoretically investigated and compared to determine the optimal contours for achieving the minimum light intensification. Then, based on Computer Aided Manufacturing (CAM), a specific machining flow combining layer milling (rough repairing) and spiral milling (fine repairing) was developed to repair the surface damage with high efficiency and surface quality. Finally, the morphology, transmittance and laser damage resistance of the repaired KDP surfaces were tested. The theoretical and experimental results indicate that the conical mitigation contours mostly possess the best repaired surface quality and optical performance. The developed combined rough and fine machining flow could be applied as a practical repairing flow to mitigate the laser-induced surface damage growth of KDP crystal optics.
Laser induced periodic surface structures (LIPSS) were generated via interaction of multiple 90
femtosecond 1900 - 3600 nm mid IR laser pulses (3 -10,000) on single crystal Ge targets. For specific
laser parameters, both low and high frequency LIPSS are found together, which are oriented
perpendicular to each other. Study of polarization dependence of LIPSS revealed that orientation and
symmetry of interaction could be controlled by rotating polarization of laser pulses. Low frequency
LIPSS formation was consistent with surface plasmon coupling of laser pulses with excited Ge.
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