Nanosecond multi-pulse damage investigation of optical coatings in atmosphere and vacuum environments

The cumulative effect of nanosecond multi-pulse irradiation on the dielectric coatings in atmosphere and vacuum environments was investigated. The laser-induced damage thresholds (LIDT) and the life span submitted to multi-pulse irradiation in two environments were compared. For both environments, we observe a decrease in laser-induced damage thresholds (LIDT) when shot number increase. However, the fatigue effect under repetitive shots under atmosphere environments and vacuum cases leads to different speeds of degradation. The correlative theory models were built to explain the experimental results.     

Laser-induced damage of the optical films prepared by electron beam evaporation and ion beam sputtering in vacuum

The laser-induced damage characteristics and adsorption effects of organic contamination molecules of two high reflectors (HR) prepared by electron beam evaporation (EB) and ion beam sputtering (IBS) method at 1064 nm is investigated in vacuum. It is found that EB films show the performance degradation of laser induced damages in vacuum while for IBS film, seems to have no this effect, in comparison with air environment. In addition, EB coatings also have the strong affinity with organic contamination molecules, in contrast of IBS films. The results reveal that ion beam sputtering (IBS) method seem to be one of the favorite film deposition techniques of the optical films used in vacuum and space environments.     

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.     

真空环境下光学薄膜的激光损伤

利用1-on-1损伤测试方法,比较研究了电子束沉积制备的几种薄膜在真空、大气环境下的损伤特性,并对真空环境与大气环境损伤的差异进行了分析。研究结果显示:真空环境下的损伤阈值明显低于大气环境下的损伤阈值,真空环境下的破斑形貌也与大气环境下的破斑形貌有显著差异。气体热传导差异不是真空环境与空气环境下损伤差异的原因。由水的解吸而引起膜层在真空环境中张应力的增大及真空环境中激光辐照过程中产生的非化学计量比缺陷是导致电子束沉积的薄膜真空与大气环境中损伤差异的原因。     

Impact of organic contamination in vacuum on laser-induced damage threshold of TiO2/SiO2 dielectric mirrors

The influence of organic contamination in vacuum on the laser-induced damage threshold (LIDT) of coatings is studied. TiO2/SiO2 dielectric mirrors with high reflection at 1064 nm are deposited by the electronbeam evaporation method and their LIDTs are measured in vacuum and atmosphere, respectively.It is found that the contamination in vacuum is easily attracted to optical surfaces because of the low pressure and becomes the source of damage, O2 molecules in vacuum with contamination can accelerate the laser-induced damage by observing LIDT and damage morphologies. LIDTs of mirrors have a little change in vacuum compared with in atmosphere when the organic contamination is wiped off. The results indicate that organic contamination is a significant reason to decrease the LIDT in vacuum.     

Contamination process and laser-induced damage of HfO2/SiO2 coatings in vacuum

The performances of HfO2/SiO2 single- and multi-layer coatings in vacuum influenced by contamination are studied. The surface morphology, the transmittance spectrum, and the laser-induced damage threshold are investigated. The results show that the contamination in vacuum mainly comes from the vacuum system and the contamination process is different for the HfO2 and SiO2 films. The laser-induced damage experiments at 1064 nm in vacuum show that the damage resistance of the coatings will decrease largely due to the organic contamination.     

Damage investigations of AR coating under atmospheric and vacuum conditions

The damage characteristics of anti-reflection films exposed to 1064 nm nanosecond laser pulses in vacuum and air were investigated. The laser-induced damage thresholds (LIDT) in vacuum and air environments were compared. The damage morphologies were locally analyzed using optical microscopy and scanning electron microscopy (SEM). The depth of the damage sites in the two environments was analyzed by Alpha2Step 500 m. The differences between the damage observed in the two environments were discussed based on the temperature distribution, absorption and vacuum gaseous composition measurements and gas pressure influence analysis. Increased coupling of the plasma shock wave and adsorbing water in the coating layer may be considered as the primary cause of the low LIDT observed in vacuum.     

Impact of organic contamination on laser-induced damage threshold of high reflectance coatings in vacuum

The influence of organic contamination in vacuum on the laser-induced damage threshold (LIDT) of coatings is studied. TiO₂/SiO₂ dielectric mirrors with high reflection at 1064 nm are deposited by the electron beam evaporation method. The LIDTs of mirrors are measured in vacuum and atmosphere, respectively. It is found that the contamination in vacuum is easily attracted to optical surfaces because of the low pressure and becomes the source of damage. LIDTs of mirrors have a little change in vacuum compared with in atmosphere when the organic contamination is wiped off. The results indicate that organic contamination is a significant reason to decrease the LIDT. N₂ molecules in vacuum can reduce the influence of the organic contaminations and prtectect high reflectance coatings.     

Influence of Vacuum Organic Contaminations on Laser-Induced Damage of 1064 nm Anti-Reflective Coatings

We investigate the influence of vacuum organic contaminations on laser-induced damage threshold （LIDT） of optical coatings. Anti-reflective （AIR） coatings at 1064 nm made by Ta2 O5/SiO2 are deposited by the ion beam sputtering method. The LIDTs of AR coatings are measured in vacuum and in atmosphere, respectively. It is exhibited that contaminations in vacuum are easily to be absorbed onto optical surface because of lower pressure, and they become origins of damage, resulting in the decrease of LIDT from 24.5J/cm^2 in air to 15.TJ/cm^2 in vacuum. The LIDT of coatings in vacuum has is slightly changed compared with the value in atmosphere after the organic contaminations are wiped off. These results indicate that organic contaminations are the main reason of the LIDT decrease in vacuum. Additionally, damage morphologies have distinct changes from vacuum to atmosphere because of the differences between the residual stress and thermal decomposability of filmy materials.     

氧化物薄膜抗1064nm脉冲激光损伤的特性研究

对向种氧化物介质膜料的单层膜、增透膜及高反膜的激光损伤特性进行了实验研究,分别测试了单脉冲和多脉冲１０６４ｎｍ激光的损伤阈值,分析和讨论了实验的物理现象及其结果。研究表明ＳｉＯ２、Ｇｄ２Ｏ３、Ｅｒ２Ｏ３、Ｙ２Ｏ３具有激光损伤阈值高、损伤程度轻的特点,特别对１０Ｈｚ激光而言,更是明显;增透膜及高反膜加λ／２层ＳｉＯ２对其激光损伤程度和损伤几率。     

Effect of Process Parameters on Laser Damage Threshold of TiO2 Coatings

We investigate the laser damage behaviour of an electron-beam-deposited TiO2 monolayer at different process parameters. The optical properties, chemical composition, surface defects, absorption and laser-induced damage threshold （LIDT） of films are measured. It is found that TiO2 films with the minimum absorption and the highest LIDT can be fabricated using a TiO2 starting material after annealing. LIDT is mainly related to absorption and is influenced by the non-stoichiometric defects for TiO2 films. Surface defects show no evident effects on LIDT in this experiment.     

Influence of negative ion element impurities on laser induced damage threshold of HfO2 thin film

Negative ion element impurities breakdown model in HfO₂ thin film was reported in this paper. The content of negative ion elements were detected by glow discharge mass spectrum analysis (GDMS); HfO₂ thin films were deposited by the electron-beam evaporation method. The weak absorption and laser induced damage threshold (LIDT) of HfO₂ thin films were measured to testify the negative ion element impurity breakdown model. It was found that the LIDT would decrease and the absorption would increase with increasing the content of negative ion element. These results indicated that negative ion elements were harmful impurities and would speed up the damage of thin film.     

Comparison of laser-induced damage in Ta2O5 and Nb2O5 single-layer films and high reflectors

Ta2O5 and Nb2O5 films are deposited on BK7 glass substrates using an electron beam evaporation method and are annealed at 673 K in the air.In this letter,comparative studies of the optical transmittance,microstructure,chemical composition,optical absorption,and laser-induced damage threshold(LIDT) of the two films are conducted.Findings indicate that the substoichiometric defect is very harmful to the laser damage resistance of Ta2O5 and Nb2O5 films.The decrease of absorption improves the LIDT in films deposited by the same material.However,although the absorption of the Ta2O5 single layer is less than that of the Nb2O5 single layer,the LIDT of the former is lower than that of the latter.High-reflective(HR) coatings have a higher LIDT than single layers due to the thermal dissipation of the SiO2 layers and the decreased electric field intensity(EFI).In addition,the Nb2O5 HR coating achieves the highest LIDT at 25.6 J/cm 2 in both single layers and HR coatings.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.     

KDP晶体本征中性点缺陷的第一性研究

 用第一性原理研究了KH2PO4(KDP)晶体中性本征点缺陷的形成能并计算了常温下点缺陷的浓度。计算得到中性填隙氢原子的形成能为2.05 eV,进而得到298 K下的浓度约为1.21×10^-17 mol/L。由于填隙氢原子在带隙中形成缺陷能级,并使能隙降低了2.6 eV, 因此消除填隙氢原子有利于提高晶体在355 nm附近的激光损伤阈值。计算得到的氧间隙、氧空位、钾空位和氢空位的形成能分别为0.60、5.25、6.50 和6.58 eV,常温下它们在晶体中也以较高的浓度存在。钾空位使晶胞体积增大约3.2%,并可能提高晶体电导率,从而降低光损伤阈值。P取代K的反位结构缺陷形成能尽管较低(4.1 eV), 但由于晶体生长溶液中P是以PO4四面体的形式存在,故此点缺陷的存在几率很小。     

Formation energies and electronic structures of native point defects in potassium dihydrogen phosphate

The formation energies and the equilibrium concentration of vacancies,interstitial H,K,P,O and antisite structural defects with P and K in KH 2 PO 4 (KDP) crystals are investigated by ab initio total-energy calculations.The formation energy of interstitial H is calculated to be about 2.06 eV and we suggest that it may be the dominant defect in KDP crystal.The formation energy of an O vacancy (5.25 eV) is much higher than that of interstitial O (0.60 eV).Optical absorption centres can be induced by defects of O vacancies,interstitial O and interstitial H.We suggest that these defects may be responsible for the lowering of the damage threshold of the KDP.A K vacancy defect may increase the ionic conductivity and therefore the laser-induced damage threshold decreases.     

Effects of water vapor in high vacuum chamber on the properties of HfO2 films

The influence of water vapor content in high vacuum chamber during the coating process on physical properties of HfO2 films was investigated. Coatings were deposited on BK7 substrates by electron beam evaporation and photoelectric maximum control method. An in situ residual gas analyzer (RGA) was used to monitor the residual gas composition in the vacuum chamber. The optical properties, microstructure,absorption and laser-induced damage threshold (LIDT) of the samples were characterized by Lambda 900 spectrophotometer, X-ray diffraction (XRD), surface thermal lensing (STL) technique and 1064-nm Qswitched pulsed laser at a pulse duration of 12 ns respectively. It was found that a cold trap is an effective equipment to suppress water vapor in the vacuum chamber during the pumping process, and the coatings deposited in the vacuum atmosphere with relatively low water vapor composition show higher refractive index and smaller grain size. Meanwhile, the higher LIDT value is corresponding to lower absorbance.     

HfO_2单层膜的吸收和激光损伤阈值测试

薄膜吸收是降低膜层激光损伤阈值的重要原因,为了研究薄膜吸收对激光损伤阈值的影响,对HfO2单层膜在1 064 nm处的吸收及其在不同波长激光辐照下的损伤阈值进行了测试和分析。研究结果表明:薄膜的激光损伤阈值由薄膜吸收平均值(决定于薄膜中缺陷的种类和数量)和吸收均匀性(决定于薄膜中缺陷的分布)共同决定;根据HfO2单层膜在1064 nm波长处的吸收值,不但可以定性判断薄膜在1 064 nm波长,而且还可以判断在其它波长激光辐照下的抗激光损伤能力。     

Influence of Annealing Temperature on Structure, Optical Loss and Laser-Induced Damage Threshold of TiO2 Thin Films

TiO2 thin films are prepared on fused silica with conventional electron beam evaporation deposition. After annealed at different temperatures for 4h, the spectra and XRD patterns of the TiO2 thin film are obtained. Weak absorption of coatings is measured by the surface thermal lensing technique, and laser-induced damage threshold （LIDT） is determined. It is found that with the increasing annealing temperature, the transmittance of TiO2 films decreases. Especially when coatings are annealed at high temperature over 1173K, the optical loss is very serious. Weak absorption detection indicates that the absorption of coatings decreases firstly and then increases, and the absorption and defects play major roles in the LIDT of TiO2 thin films.     

He-Ne散射光检测光学薄膜激光损伤阈值

准确测定光学薄膜的激光损伤阈值可以衡量光学薄膜的抗激光损伤能力,测定损伤阈值的关键是准确地判定损伤的发生与否。建立了He-Ne散射光检测光学薄膜激光损伤阈值系统。通过测量同一样品点的He-Ne散射光能量变化来判断薄膜表面发生的损伤,并对制备的类金刚石薄膜与HfO₂/SiO₂反射膜进行了阈值测试。与等离子体闪光法的阈值测试结果进行比较,具有较好的一致性。分析表明：He-Ne散射光测试系统能有效地判断出激光诱导损伤,易于实现在线检测。