Laser cleaning is widely used to remove surface contaminants and defects due to its low cost, high efficiency, and environmental protection. However, residual thermal stress caused by huge temperature gradient significantly induces thermal distortion and cracks. Due to the lack of understanding of the coupling interaction between laser and fused silica, it is still challenging to reveal the mechanism of thermal stress formation, severely restricting the further development of laser cleaning. In this article, we built a three-dimensional thermo-mechanical model to obtain the stress value and distribution of thermal stress and reveal the evolution mechanism of stress in different times and spaces. Moreover, laser cleaning experiments under different processing parameters were carried out to validate the simulated results, and theoretical simulations fit well with experimental results, which proves the effectiveness of the multi-physics model. This research provides new insights into thermal stress evolution, which can promote further development in laser cleaning technology.
The optical characteristics of KDP/DKDP crystals, including thermal absorption and nonlinear absorption, were investigated through thermal lens method and Z-scan method. It is found that the thermal absorption in KDP crystal behaves anisotropy at 355nm and 532nm and nonlinear absorption anisotropy in KDP/DKDP crystals is also found at 520nm. For KDP and DKDP samples, the thermal absorption coefficient of crystals with different orientations is slightly different. The four-photon absorption coefficient γ of z-cut KDP sample is larger than that of other orientations. The relationship of nonlinear absorption coefficient γ is z<II<I for KDP sample, while II≈z for DKDP sample.
The damage properties of multilayer coatings tested with 1064-nm 30-ps pulses are similar to those tested with 355 nm, nanosecond pulses. A kind of HfO2 / SiO2 high-reflective (HR) coating is prepared by electron beam evaporation. Laser-induced damage of HfO2 / SiO2 HR coatings is tested by 355-nm 7-ns pulses and 1064-nm 30-ps pulses, respectively. Damage morphologies and cross-sectional profiles are characterized using a scanning electron microscope and focused ion beam, respectively. The laser-induced damage thresholds and morphologies in the two tests are compared. The developing processes and damage mechanisms are discussed. Many similarities are found in the two tests: the typical damage morphologies in both tests appeared as micrometer-sized pits when irradiated by low-fluence pulses, while it turned out to be layer delamination when irradiated by high-fluence pulses. Damage onset is nearby the peak of the E-field in the two tests. Damage pits in both tests may be related to thermal stress caused by nanometer-sized isolated absorbers. There are also some differences in the damage properties between two tests: damage pits in 1064-nm 30-ps tests have a much higher density than that in 355-nm 7-ns tests. The detail features and the developing processes of the pits are different.
Laser-induced damage in optical components has always been a key challenge in the development of high-power laser systems. In picosecond regime, the laser-matter interactions are quite complex and the damage mechanism is not yet understood. Therefore, it is necessary to investigate the laser induced damage of optical components in picosecond regime. Our previous study on the laser induced damage in HfO2/SiO2 high-reflective (HR) coatings in 30-ps laser pulses reported the damage morphologies to be high-density micrometer-scale pits, which are similar to the morphologies of HR coatings irradiated by 355-nm pulses in nanosecond regime. Thus, it makes sense to analyze the damage mechanism of HR coatings in picosecond regime by comparing the damage results with those tested with 355-nm pulses. In this study, laser induced damage of HfO2/SiO2 HR coatings are performed by 355-nm, 7-ns pulses and 1064-nm, 30-ps pulses, respectively. Different angles of incidence (AOIs) are operated in the tests, in order to modulate the electric field (E-intensity) distributions in the coating stacks. Damage morphologies and cross-sectional profiles are characterized using scanning electron microscope (SEM) and focused ion beam (FIB), respectively. The laser-induced damage thresholds (LIDTs) and morphologies tested with two different laser pulses are compared. The damage locations are compared with corresponding E-field distributions and the damage reasons are discussed.
The effect of protective layer on the picosecond laser-induced damage behaviors of HfO2/SiO2 high-reflective (HR) coatings are explored. Two kinds of 1064nm HR coatings with and without protective layer are deposited by electron beam evaporation. Laser-induced damage tests are conducted with 1064nm, 30ps S-polarized and P-polarized pulses with different angle of incidence (AOI) to make the electric fields intensity in the HR coatings discrepantly. Damage morphology and cross section of damage sites were characterized by scanning electron microscope (SEM) and focused ion beam (FIB), respectively. It is found that SiO2 protective layer have a certain degree of improvement on laser induced damage threshold (LIDT) for every AOIs. The onset damage initiated very near to the Max peak of e-field, after which forms ripple-like pits. The damage morphology presents as layer delamination at high fluence. The Laser damage resistance is correspond with the maximum E-intensity in the coating stacks.
Transient transmission and scattering changes was observed to study the dynamics during the laser-induced bulk damage
in fused silica at the wavelength of 532 and 355 nm. Within the nanosecond high power laser irradiation, the
transmittance decreased dramatically, indicating an intense absorption. With fluence increasing, the time required to
form the intense absorption was shortened, and at similar fluence, this time interval at the wavelength of 355 nm is
shorter than that at 532 nm. At times, the intense absorption did not cause the macroscopic damage because no variation
in scattering was observed. While the macroscopic damage occurred, the scattering kept on increasing until the end of
pulse.
We measured the absorption of type I doubler KDP crystal at different laser wavelengths (1064, 532 and 355 nm) by
using laser induced deflection (LID) technique. We also performed the absorption measurements in the cases where
beam polarization was parallel to the principal plane (Pd//Pp) and perpendicular to it (Pd⊥Pp). To account for the
experiment results, a model based on crystal dichroism was developed to calculate the absorption coefficients for O
ray (αo) and E ray (αe) at different laser wavelengths. It is found that the dichroism is manifested especially clearly
at 1064nm, but not clearly at 355nm/532nm. It implies that the absorption at 1064nm is mainly due to lattice
absorption, whereas defects absorption is responsible for the absorption at 355nm/532nm.
Laser-induced bulk damage in potassium dihydrogen phosphate (KDP) and its deuterated analog (DKDP) crystals for
nanosecond pulses is caused by light-absorbing precursor defects, which are formed during crystal growth. However,
current chemical analysis and spectroscopy techniques fail to identify the nature of the responsible precursor defects
because of their “invisible” concentration and/or size. In this study, the aim was to explore a novel method for
understanding laser–matter interactions with regard to physical parameters, such as size and concentration, affecting the
ability of damage precursors to initiate damage. Laser-induced damage performance at 1064 nm of KDP crystals grown
using filters of different pore sizes was investigated. By reducing the pore size of filters in continuous filtration growth,
laser damage resistance was improved. Furthermore, a model based on a Gaussian distribution of precursor thresholds
and heat transfer was developed to obtain a concentration and/or size distribution of the precursor defects. The results
revealed that smaller size and/or lower concentration of precursor defects could lead to better damage resistance.
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