Laser Conditioning Effect Research Articles

Multishot laser damage of multilayer dielectric mirrors in the near-infrared subpicosecond regime

The laser damage resistance of dielectric components of high-power laser facilities to laser irradiation depends significantly on the irradiation sequence. In the short pulse (fs) regime, it is known that continuous irradiation of these components leads to a reduction in the damage threshold, reflecting a laser fatigue effect. Conversely, in the long pulse (ns) regime, progressive irradiation of these components leads to an increase in the damage threshold, reflecting a laser conditioning effect. In this article, we experimentally evaluate the competition between the effects of laser fatigue and laser conditioning for multilayer dielectric components irradiated in the subpicosecond pulse regime in the infrared (∼1µm) through different test sequences. For this purpose, we implemented an original test sequence derived from an S-on-1 type protocol, which consists of irradiating the component until damage. By repeating this sequence at different set points, it was possible to estimate the progressive reduction in damage threshold with the number of laser irradiations and to compare it with that observed during the fluence ramps. Particular attention was also paid to the precise knowledge of the test beam irradiating the component, as a dependence of the beam surface on the test set point was highlighted.

Effect of Laser Conditioning on Surface Modification and Laser Damage Resistance of SiO2 Antireflection Film

SiO2 sol-gel antireflection film coated on fused silica can reduce the reflection loss and improve the transmittance of the optical component, although it is still prone to laser induced damage. Laser conditioning is an effective way to improve the laser induced damage threshold (LIDT) of SiO2 sol-gel antireflection film. In this paper, single-layer SiO2 sol-gel antireflection films pretreated by triple-frequency laser with different parameters are characterized by the macroscopical parameters, such as transmittance, refractive index, and thickness. The law of surface modification and the defect removal mechanism of the SiO2 sol-gel antireflection film by laser conditioning are obtained. It is found that laser conditioning can reduce the thickness of the film and introduce densification. In addition, laser conditioning can eliminate micro-defects, such as vacancies and voids in the preparation of SiO2 sol-gel antireflection films, which is the main reason to improve the laser damage resistance of films. Finally, the laser conditioning process with three step laser energy combinations of (0.2–0.6–1.0) Fth0 (zero damage threshold) is the best one to obtain high transmittance, and excellent effects on structure modification and defect removal of films. The research in this paper provides data support for the engineering application and mechanism research of laser conditioning.

Monitoring laser conditioning effect by real-time thermo-reflectance measurement

An experimental method was proposed and investigated to monitor the effect of laser conditioning (LC) on optical films based on the thermo-reflectance principle. By this means, the initiation and evolution of heat generation around the defects can be determined in situ during pulse laser irradiation. And, the relationship between laser conditioning fluence and thermo-reflectance response are monitored by this real-time control and the laser conditioning effect on surface defects monitored by this method was analyzed by the damage test and weak absorption measurement.

Off-line sub-nanosecond laser conditioning on large aperture deuterated potassium dihydrogen phosphate crystal

The large aperture deuterated potassium dihydrogen phosphate (DKDP) is an important frequency conversion crystal in a large power laser device. There are many defects inside the DKDP bulk material, including the varying element impurities and electronic defects. Comparing with the defect-free material, these bulk defects can easily absorb incident laser energy and pose the risks of initiating damage sites when exposed to high-energy lasers. Beside bulk defects, there are surface defects originating from the DKDP machining process, including cracks, scratches and protuberances. These surface defects affect the damage performance of DKDP crystal by increasing light absorption and weakening local mechanical strength. Due to the defects from both bulk and surface, the actual damage threshold of DKDP crystal is much lower than the expected theoretical value. The lack of its laser damage resistance seriously restricts the laser output power. In this study, the off-line sub-nanosecond laser conditioning technology is proposed to effectively improve the laser damage performance of large aperture DKDP crystal. Its principle is to irradiate DKDP with a mild laser fluence in advance. Although the laser pretreatment cannot directly eliminate the impurities, dislocations, grain boundaries or other macro structural defects in crystals, it indeed changes the distribution and density of intrinsic point defects on a micro-scale. It suggests that the complicated reactions of electron-hole, atom-vacancy and the intrinsic point defect annihilation caused by the microstructural transformation of crystal materials under laser conditioning are the possible reasons for reducing absorption and improving the damage resistance. In this experiment, the result of the damage to high-power laser device shows that the mean surface damage density of DKDP crystal at 9 J/cm<sup>2</sup> decreases from 5.02 pp/cm<sup>2</sup> to 0.55 pp/cm<sup>2</sup> after sub-nanosecond laser conditioning. The laser conditioning can remove the protuberance defects on the surface, thus reducing the surface damage density. In addition, the damage size probably decreases after laser conditioning. There is a leftward shift in the damage size curve after laser conditioning, and its peak decreases from 25 μm to 18 μm–20 μm. In addition, due to the removal effect of laser conditioning on defects, the spatial distribution of damage points after sub-nanosecond laser irradiation turns more uniform. This study provides a foundation for the applications of off-line sub-nanosecond laser conditioning technology in high-power laser facility.

Effect of Laser Conditioning and Different Adhesive Strategies on Bond Strength of a Bulk Fill and a Microhybrid Resin Composite

Purpose: To evaluate the effect of laser conditioning and adhesive strategies on bond strength of a bulk fill and a microhybrid resin composite. Methods: 96 human posterior teeth’s buccal and lingual surfaces were ground flat to expose dentin surfaces with standard smear layer (N=192). The specimens were divided into 16 experimental groups according to (1) Laser-conditioning (Laser-etched group/control group); (2) Adhesive strategies (etch&rinse, Adper Single Bond2(SB2), two step self-etch ClearfilSE Bond(CSE), multi-mode adhesive All-Bond Universal with self-etching technique(ABU-SE), or etch&rinse technique(ABU-ER); (3) Restorative materials (Z250/SDR). All the restorative applications were applied according to the manufacturer’s directions. The specimens were stored in 100% humidity for 24 hours. The specimens were placed on a shear bond test machine at a crosshead speed of 1 mm/min. The values were recorded at the time of failure of composite resins. The bond strength values for shear forces were reported in MPa and derived by dividing the imposed force at the time of fracture by the bonded area. The failed samples were examined under a streomicroscope at X20 magnification to determine the mode of failure. The data were statistically analyzed with three-way ANOVA and multiple comparisons were made by Bon Ferroni test at 5% significance level (pResults: SDR and Z250 yielded statistically significant difference when bonded with ABU-ER without laser conditioning (p=0.008). The predominant mode of failure was mixed type. Conclusion: Laser conditioning did not affect shear bond strength values of a bulk fill and a microhybrid resin composites regardless of adhesive system.

Effect of laser conditioning on nanoprecursor for improving laser damage resistance of HfO2 coatings

An experimental and analytical approach is described to investigate the effect of laser conditioning on the nanoprecursor of the Hf cluster for improving the damage resistance of HfO2 coatings. Two test methods of 1-on-1 and R-on-1 (usually, R-on-1 is referred to as laser conditioning) were employed to measure the damage probabilities of samples of HfO2 coating. In addition, the measured damage probabilities of 1-on-1 and R-on-1 were compared with the simulations based on Mie scatter theory, heat transfer and damage probability calculation to determine the size distribution of the nanoprecursor of the Hf cluster, which indicates the decrease in their sizes. X-ray photoelectron spectroscopy analysis reveals that laser conditioning has increased the relative oxygen content, which implies that a portion of Hf atoms in the nanoprecursors are oxidized to HfO2 due to O atoms from the decomposed water and air, with a resulting decrease in their size and absorption and an increase in their laser damage threshold. The experimental and simulative results indicate that the size decrease and distribution variation of the Hf clusters may be another one of the mechanisms for laser conditioning to increase the damage resistance of HfO2 coatings.

Effect of Laser-arc Hybrid Welding Energy Parameters on Welding Stability

The high-speed camera was used to collect the droplet transfer pattern and arc pattern of the laser-arc hybrid welding process. Using the methods of image processing and mathematical statistics, the effects of different laser and arc power conditions on the welding stability were studied. The results show that the melting width depends on the welding current, the depth of penetration depends on the laser power, the droplet transition pattern, the actual filament spacing and the arc length determine the welding stability of the laser arc hybrid welding.

Modeling the effect of nanosecond laser conditioning on the femtosecond laser-induced damage of optical films.

The effect of nanosecond laser conditioning on the femtosecond laser-induced damage behaviors of Al2O3, HfO2, SiO2 single layers and Al2O3/SiO2 high reflectors (HR) are explored. During femtosecond laser damage test, negative effects on enhancing the femtosecond laser-induced damage threshold (LIDT) of optical films after the nanosecond laser conditioning is found, which is opposite to the LIDT improvement in the nanosecond range. To explain the mechanism after nanosecond laser conditioning, a theoretical model including multiphoton ionization (MPI), avalanche ionization (AI) and decays of electrons with one defect state is built to simulate the evolution of electron density in the conduction band. A permanent mid-gap defect state resulting from the process of laser conditioning is introduced in our model, which is found to contribute seed electrons to conduction band and hence accelerate the final breakdown. Both the experimental result and theoretical calculation agree very well with each other.

532 nm HfO2/SiO2高反膜的激光预处理效应

532 nm HfO₂/SiO₂高反膜的激光预处理效应

Effect of UV Laser Conditioning on the Structure of KDP Crystal

Multiparametric raster scanning experiments for KDP crystals are carried out to study the laser conditioning efficiency as a function of laser fluence, fluence step, and pulse sequence by using ultraviolet (UV) laser irradiation with pulse duration of approximately 7 ns. It indicates that damage resistance of KDP can be enhanced after conditioning process. And laser conditioning efficiency depends on the maximal fluence which is below the damage threshold. Raman spectra and photothermal absorption have also been studied on KDP crystals before and after multiparametric laser conditioning. Photothermal absorption data reveal that absorbance of conditioned KDP crystal decreases with the increase of laser fluence and the damage threshold of low absorption area is higher. Raman analysis reveals that the effectiveness of laser conditioning relies mainly on the individual mode of PO4molecule.

Improvement of the laser-induced damage resistance of optical coatings in vacuum environments

Two measures were presented to improve the laser-induced damage resistance of the optical films used in vacuum environments. Firstly, the impact of a few O2, N2 and He2 introduced in vacuum on the laser-induced damage threshold of the dielectric mirror coatings were investigated. Then, laser conditioning effects in atmosphere environments on the laser-induced damage threshold of the dielectric mirror coatings used in vacuum environments were also investigated. It was found that a few O2, N2 introduced in vacuum can effectively improve the laser damage resistance of the optical films in vacuum environment due to the resistance to air-vacuum effect. However, a few He2 have no this effect on laser induced damage threshold (LIDT) of the films. In addition, laser conditioning in atmosphere can also effectively improve the LIDT of the optical thin films used in vacuum environment.

Improving the performance of high-laser-damage-threshold, multilayer dielectric pulse-compression gratings through low-temperature chemical cleaning

A low-temperature chemical cleaning approach has been developed to improve the performance of multilayer dielectric pulse-compressor gratings for use in the OMEGA EP laser system. X-ray photoelectron spectroscopy results guided the selection of targeted cleaning steps to strip specific families of manufacturing residues without damaging the grating's fragile 3D profile. Grating coupons that were cleaned using the optimized method consistently met OMEGA EP requirements on diffraction efficiency and 1054 nm laser-damage resistance at 10 ps. The disappearance of laser-conditioning effects for the highest-damage-threshold samples suggests a transition from a contamination-driven laser-damage mechanism to defect-driven damage for well-cleaned components.

CO2 laser conditioning of porcelain surfaces for bonding metal orthodontic brackets

Bonding to porcelain remains to be a challenge in orthodontic treatments. The objective of this study was to evaluate the effect of CO2 laser conditioning of porcelain surfaces on shear bond strength (SBS) of orthodontic brackets. Eighty feldspathic porcelain specimens were divided into four groups of 20. In each group, half of the porcelain surfaces were deglazed, while the others remained glazed. The specimens in groups 1 to 3 were treated with a fractional CO2 laser for 10s using 10mJ of energy, frequency of 200Hz and powers of 10W (group 1), 15W (group 2) and 20W (group 3). In group 4, a 9.6% hydrofluoric (HF) acid gel was used for 2min. A silane coupling agent was applied before bracket bonding, and the SBS was measured with a universal testing machine after 24h. Deglazing caused significant increase in SBS of laser treated porcelain surfaces (p < 0.05), but had no significant effect on SBS when HF acid was used for etching (p = 0.137). ANOVA revealed no significant difference in SBS values of the study groups when glazed surfaces were compared (p = 0.269). However, a significant between group difference was found among the deglazed specimens (p < 0.001). Tukey test revealed that the bond strengths of 10W and 15W laser groups were significantly higher than that of the HF acid group (p < 0.05). Laser conditioning with a fractional CO2 laser can be recommended as a suitable alternative to hydrofluoric acid for deglazed feldspathic porcelain.

The application of laser-induced damage spot size effect for laser conditioning mechanism

We propose the spot-size effect in laser-induced damage to optical thin films as a method to investigate the defect removal model which is one of the laser conditioning mechanisms, and the theoretical and experimental studies are presented. A standard deviation σ 2 is set up for fitting the damage thresholds which are obtained by multi-spot size test. The laser conditioning effect on ZrO 2 /SiO 2 multilayer high reflective films is analyzed through this application. It is found that the mean distance of two initiating defects is increased by laser conditioning, which means that the defect density is decreased. Thus, the initiating defects are removed partially by laser conditioning. Furthermore, the intrinsic damage threshold of the film is little affected through the laser conditioning process.

Laser conditioning of high-reflective and anti-reflective coatings in vacuum environments

Laser conditioning effects of the dielectric mirror coatings in vacuum environments were investigated. The laser-induced damage thresholds (LIDT) in vacuum environments before and after laser conditioning were compared. It is found that laser conditioning in vacuum environments decrease the LIDT of the component. Laser conditioning effects in vacuum and atmosphere environments were also compared and investigated. The negative effects of laser conditioning in vacuum environments were discussed and analyzed with defect statistical model, energy dispersive X-ray analysis and absorption measurements.

Laser-conditioning mechanism of ZrO2/SiO2 HR films with fitting damage probability curves of laser-induced damage

The method of fitting damage probability curves of laser-induced damage is introduced to investigate the laser-conditioning mechanism of ZrO2/SiO2 high re°ection (HR) films. The laser-induced damage thresholds (LIDTs) of the sample are tested before and after the laser-conditioning scanning process. The parameters of the defects are obtained through the fitting process of the damage probability curve. It can be concluded that the roles of laser conditioning include two aspects: removing defects with lower threshold and producing new defects with higher threshold. The effect of laser conditioning is dependent on the competition of these two aspects.

Effect of UV laser conditioning on fused silica in vacuum

The effect of laser conditioning on the fused silica was investigated by laser-induced damage threshold (LIDT) tests with Nd:YLF laser. The fused silica was pre-irradiated with pulses or raster scans methods by 351 nm and 8–10 ns laser prior to S-on-1 damage testing in vacuum. The damage threshold of fused silica after UV laser conditioning increased by a factor of 1.4. The laser conditioning mechanism was discussed.

电子束蒸发制备HfO2/SiO2高反膜的1064 nm激光预处理效应

电子束蒸发制备HfO2/SiO2高反膜的1064 nm激光预处理效应

The effect of material, thermal and laser conditioning on the damage threshold of type II tripler‐cut DKDP crystals

AbstractBy using the traditional temperature‐reduction method, KD2PO4 (DKDP) crystals were grown from 85%‐deuterated aqueous solution synthesized by different kind of KH2PO4 (KDP) raw materials. An experimental technique was employed to measure the variation of bulk damage threshold in plates of DKDP crystals. Bulk laser damage in unconditioned, thermal‐conditioned and laser‐conditioned type II tripler‐cut DKDP crystals has been studied using 1064 nm (ω) and 355 nm (3ω) laser at pulse lengths of 5ns, 5ns respectively. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

电子束蒸发制备三倍频分离膜的0.532μm激光预处理效应

The 0.532-\mum laser conditioning of HfO2/SiO2 third harmonic separator fabricated by electron-beam evaporation (EBE) was studied. The laser induced damage threshold (LIDT) of the separator determined by 1-on-1 test is 9.1 J/cm2 and it is 15.2 J/cm2 after laser conditioning determined by raster scanning. Two kinds of damage morphologies, taper pits and flat bottom pits, are found on the sample surface and they show different damage behaviors. The damage onset of taper pits does not change obviously and the laser conditioning effect is contributed to the flat bottom pits, which limits the application of laser conditioning.