High laser damage threshold coatings and damage testing technology

The laser-induced damage threshold(LIDT) of optical coating is a limited factor for development of a high peak power laser. The automatic damage testing facility was built to determine the LIDT of optics at 1 064 nm and 355 nm.. The cleanning and processing procedure of the substrate and coating technique were improved, and the damage resistance of high-reflective coating at 1 064 nm was increased.

Damage Mechanism of Optical Films by High-power Laser

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Corneal damage effects induced by infrared optical parametric oscillator radiation at 3743 nm

The main aim of this paper is to investigate the corneal damage effects induced by mid-infrared optical parametric oscillator(OPO)radiation.Experiments were performed to determine the corneal damage thresholds of New Zealand white rabbit at the wavelength of 3743 nm for ex-posurédurations of 0.1 s,1.0 s and 10.0 s.Through slit-lamp biomicroscope and histopathology,corneal injury characteristics were revealed.The damage thresholds were 3.73 J/cm^(2),7.91 J/cm^(2)and 31.1 J/cm^(2),respectively,for exposure durations of 0.1 s,1.0 s and 10.0 s.The damage data was correlated by an empirical equation:Radiant exposure at the threshold=9.72×exposure duration,^(0.46)where the units of radiant exposure and exposure duration were J/cm^(2)and second.At near-threshold level,corneal injuries at 1 h post-exposure mainly involved the epithelium,and the epithelium damages repaired at 24-h post-exposure.There are sufficient safety margins be-tween the damage thresholds and the maximum permitted exposures from current international laser safety standard IEC 60825-1.

Study on Preparation and Ns-Laser Damage of HfO₂Single Layers

The effects of the main parameters of argon flux, oxygen flux and beam voltage on the surface morphology, transmittance spectrum and laser damage of the HfO2 single layers prepared by ion beam sputtering are studied. The HfO2 amorphous single layers have porous surface morphologies. Different processes will cause differences in coatings absorption and surface morphology, which in turn will cause changes in the spectral transmittance curve. The ion beam sputtering HfO2 single layers have high content of argon (4.5% - 8%). The laser damage of HfO2 single layers is related to argon inclusions and non-stoichiometric defects. The changes of argon flux and beam voltage have a greater impact on argon content and O/Hf ratio. When the argon content in the coatings is lower and the O/Hf ratio is higher, the laser damage thresholds of the HfO2 single layers are higher.

Investigation of high power laser coatings

High laser-induced damage threshold and large aperture were focuses on the studies of high power laser coatings. This paper reports the research activities at our center. Several measures were developed for evaluating characters of laser damage, including determination of laser induced damage threshold and detection of absorption based on surface thermal lensing technique. Defect was deemed to be the initial source of laser damage, and was the main factor restricting the laser damage resistance of optical coatings. The contribution of several kinds of typical defects to laser damage was analyzed, and some deposition measures were adopted to control and eliminate the origin of defect. Furthermore, some post-treatment methods were also employed to alleviate the influence of the defect and to improve the laser damage resistance. Correction mask was introduced to improve the thickness uniformity, and the thickness uniformity can be amended to less than 1% in the range of Φ650 mm. Preliminary investigation related to surface deformation was also conducted.

General design basis for a final optics assembly to decrease filamentary damage

The high-power laser beam in the final optics assembly of high-power laser facilities is often modulated by contamina- tion particles, which may cause local high light intensity, thereby increasing the filamentary damage probability for optical components. To study the general design basis for a final optics assembly to decrease the risk of filamentary damage, different-sized contamination particles deposited on a component surface are simulated to modulate a 351-nm laser beam based on the optical transmission theory, and the corresponding simulation results are analyzed statistically in terms of the propagation characteristic and the light field intensity distribution of the modulated laser beam. The statistical results show that component thickness and distance between components can to some extent be optimized to reduce the appearance of local high light intensity, and the general design basis of component thickness and arrangement are given for different control levels of particle sizes. Moreover, the statistical results can also predict the laser beam quality approximately under the existing optics design and environmental cleanliness. The optimized design for final optics assembly based on environmental cleanliness level is useful to prolong the lifetime of optics and enhance the output power of high-power laser facilities.

Correlation between Coating Adhesion and Damage Threshold: Simple Method of Reliability Assessment for Optoelectronic Applications

It is demonstrated that inherent coating adhesion and damage threshold are correlated for Ta2O5 and HfO2 coatings widely used in optoelectronic devices. By utilizing a newly proposed 1-h boiling water test combined with the optical aging under high-power laser irradiation, we show that an optical coating that survives the 1-h boiling water test withstands the damage threshold, ensuring the field service life even in harsh environments. Besides the standard evaluation methods, which may have limitations for applications required in harsh environments, the 1-h boiling water test can serve as an alternative method of reliability assessment for optical coatings. A heuristics herein can be used as a gating item for qualification of optical coatings for various applications.

卫星用高能激光防护技术发展现状

高能激光技术不断发展,逐渐成为卫星在轨安全的威胁。介绍了高能连续激光和脉冲激光武器对卫星损伤机制的研究进展。针对卫星太阳电池阵、相机、星敏感器等光学系统以及热控多层、复合材料推进剂储箱等非光学关键结构防护特点,总结分析了线性光学防护薄膜、非线性光学防护薄膜、相变材料、机械快门、高损伤阈值薄膜和防护罩等各种高能激光防护技术的主要原理、发展现状和技术特点。

Postprocessing treatments to improve the laser damage resistance of fused silica optical surfaces and SiO_2 coatings

The combination of deep wet etching and a magneto-rheological finishing （MRF） process is investigated to simultaneously improve laser damage resistance of a fused-silica surface at 355 nm. The subsequently deposited SiO2 coatings are researched to clarify the impact of substrate finishing technology on the coatings. It is revealed that a deep removal proceeding from the single side or double side had a significant impact on the laser-induced damage threshold （LIDT） of the fused silica, especially for the rear surface. After the deep etching, the MRF process that followed does not actually increase the LIDT, but it does ameliorate the surface qualities without additional LIDT degradation. The combination guarantee both the integrity of the surface＇s finish and the laser damage resistance of the fused silica and subsequent SiO2 coatings.

PEG含量对多孔膜光学带隙及激光损伤特性的影响

采用溶胶凝胶技术结合旋涂法,在石英基底及Si基底上分别制备了孔隙率不同的TiO_(2)和SiO_(2)薄膜,并对两者的光学及激光损伤特性进行了研究。根据透射光谱曲线计算出薄膜的光学带隙,发现光学带隙随着孔隙率的增大而增大,TiO_(2)薄膜的光学带隙大小为3.75 eV~3.97 eV,SiO_(2)薄膜的光学带隙大小为3.52 eV~3.78 eV。椭偏测量结果表明,当聚乙二醇(polyethylene glycol,PEG)质量分数从0、0.8%、4.0%提高至8.0%时,在波长1064 nm处,TiO_(2)薄膜的孔隙率从11.5%、14.1%、30.9%增大至38.7%,折射率从2.0635、2.0165、1.7481降低至1.6409;SiO_(2)薄膜的孔隙率从4.04%、4.6%、5.7%提升至13.9%,折射率从1.4386、1.4358、1.4204降低至1.3879。除PEG质量分数为0.8%的TiO_(2)薄膜外,所有TiO_(2)和SiO_(2)薄膜样品的消光系数均优于10-3,说明薄膜的吸收较小。薄膜的激光损伤阈值(laser induced damage threshold,LIDT)受孔隙率的影响较大,孔隙率越大,TiO_(2)薄膜的激光损伤阈值越高,其值最高可达16.7 J/cm~2;而SiO_(2)薄膜的激光损伤阈值较未添加PEG的激光损伤阈值并未提高,当PEG质量分数从0.8%增加到8.0%时,SiO_(2)薄膜的激光损伤阈值提高了1.9 J/cm~2。总而言之,提高孔隙率有助于提高薄膜的抗激光损伤性能。

基于损伤程度量化评估的光学薄膜元件激光损伤阈值测量方法

光学薄膜元件是高功率激光器中的关键器件,其抗激光损伤的能力对整个激光系统的运行至关重要。精确测量薄膜元件激光诱导损伤阈值对提升激光器使用寿命与出光效率有着重要意义。提出一种新型激光损伤程度量化评估法,该方法对薄膜元件在不同能量密度下的激光损伤程度量化分析,通过损伤趋势拟合,评估激光损伤阈值。对激光损伤区域的量化采用图像超分辨白光显微干涉测量法,可实现纳米量级测量精度。通过仿真验证该测量方法可实现纳米量级损伤结构的三维重构,重构损伤区域误差小于0.01%。在实验部分,以激光谐振腔镜及窗口片为测试样品,无需大量重复性激光损伤实验,基于单片样品在一组不同能量密度激光束照射下产生的单次损伤结果实现测量,结果数据与S-on-1方法吻合,偏差小于0.5 J/cm^(2),两个样品多次测量结果的标准差分别为0.361 J/cm^(2)和0.064 J/cm^(2)。

Contamination process and laser-induced damage of HfO_2/SiO_2 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.

Research on quantum well intermixing of 680 nm AlGaInP/GaInP semiconductor lasers induced by composited Si-Si_(3)N_(4) dielectric layer

The optical catastrophic damage that usually occurs at the cavity surface of semiconductor lasers has become the main bottleneck affecting the improvement of laser output power and long-term reliability.To improve the output power of 680 nm AlGaInP/GaInP quantum well red semiconductor lasers,Si-Si_(3)N_(4)composited dielectric layers are used to induce its quantum wells to be intermixed at the cavity surface to make a non-absorption window.Si with a thickness of 100 nm and Si_(3)N_(4)with a thickness of 100 nm were grown on the surface of the epitaxial wafer by magnetron sputtering and PECVD as diffusion source and driving source,respectively.Compared with traditional Si impurity induced quantum well intermixing,this paper realizes the blue shift of 54.8 nm in the nonabsorbent window region at a lower annealing temperature of 600 ℃ and annealing time of 10 min.Under this annealing condition,the wavelength of the gain luminescence region basically does not shift to short wavelength,and the surface morphology of the whole epitaxial wafer remains fine after annealing.The application of this process condition can reduce the difficulty of production and save cost,which provides an effective method for upcoming fabrication.

Modified porous silica antireflective coatings with laser damage resistance for Ti:sapphire

Porous SiO2 antireflective (AR) coatings are prepared from the colloidal silica solution modified with methyltriethoxysilane (MTES) based on the sol-gel route. The viscosity of modified silica suspensions changes but their stability keeps when MTES is introduced. The refractive indices of modified coatings vary little after bake treatment from 100 to 150℃. The modified silica coatings on Ti:sapphire crystal, owning good homogeneity, display prominent antireflective effect within the laser output waveband (750-850 nm) of Ti:sapphire lasers, with average transmission above 98.6%, and own laser induced damage thresholds (LIDTs) of more than 2.2 J/cm2 at 800 nm with the pulse duration of 300 ps.

Influence of SiO_3^(2-)Impurity on the Optical Properties of KDP Crystal

In this paper,potassium dihydrogen phosphate（KDP） crystals were grown in the presence of a series of silicate（SiO32-） impurity concentrations via conventional temperature cooling and rapid growth methods,respectively.It revealed that the SiO32-impurity could lead to the decrease of transmittance at the region of ultraviolet band for pyramidal sectors and slightly increased the transmittance for prismatic sectors.SiO32-could enter into the crystal lattice and create lattice defects,which consequently increased the density of light scatter.The decrease of laser damage threshold was attributed to the lattice defects and the redundant electrons brought by the replacement of SiO32-at the PO43-position.

Annealing Effects on Structural,Optical Properties and Laser-Induced Damage Threshold of MgF2 Thin Films

The chemical structures, optical properties and laser-induced damage thresholds of magnesium fluoride films annealed at different temperatures were investigated. The results showed that the stoichiometry of MgF2 film changed a little with the increase in annealing temperature. Analysis of the optical properties indicated that excellent antireflection behavior of the film in the range of 200-400 nm can be obtained by the samples coated with MgF2 film. The refractive index increased and the extinction coefficient decreased with increasing annealing temperature. Compared with the asdeposited films, the laser-induced damage threshold was improved after annealing process and decreased with the increase in annealing temperature, which was probably due to the denser film and more absorption centers under higher annealing temperature.

Standardization in optical coating characterisation

In the rapid development course of laser technology and modern optics, optical metrology continuously gains importance for the quality management in the industrial production environment and also for research in optical coatings. Besides absorption and scatter losses, the spectral characteristics and laser induced damage thresholds are considered as common quality factors for coated optical components and often define the optimization targets for new products and applications. Also, these quality parameters are the basis for the comparison of commercial optics and can be found in the product catalogues of most manufacturers of optical components. As a consequence, standardization of characterisation procedures for these fundamental properties evolved to a crucial point for the optics industry. During the last decade, adapted standard measurement techniques have been elaborated and discussed by representatives from many industrial companies and research institutes within working groups of the International Organisation for Standardization (ISO). In this contribution, the current state of standardized characterisation techniques for optical coatings is summarised. Selected standards for the measurement of absorption (ISO 11551), scattering (ISO 13696) and laser induced damage thresholds (ISO 11254, Parts 1 and 2) will be described and discussed in view of their applicability and reproducibility. The report will be concluded by an outlook on the current projects and future tasks of standardization in optics characterisation.

Study on the laser-induced damage threshold of sol-gel ZO_2-PVP coatings

In order to improve the laser-induced damage threshold （LIDT） of the ZrO2 film, the effect of polyvinyl pyrrolidone （PVP） with different average molecular weight on coatings is studied. The ZrO2 sol is prepared by the hydrolysis of zirconium n-propoxide with acetylacetone as the chelating agent. The PVP （K16-18） is more suitable to obtain high LIDT coatings than other PVPs. The LIDT of the ZrO2-PVP coatings is improved with the increase of the PVP （K16-18） content. When the PVP （K16-18） content is 2.4 wt.-~0, the LIDT of the ZrO2-PVP coatings is 43.5 J/cm2 （1064 nm, 12 ns）. The ZrO2 sol and ZrO2-PVP （K16- 18） sol have the good stability in 3 months. The refractive indices of the ZrO2-PVP coatings decrease with the increase of the PVP （K16-18） content.

Accumulation effect of SiO_2 protective layer on multi-shot laser-induced damage in high-reflectivity HfO_2/SiO_2 coatings

The accumulation effects in high-reflectivity （HR） HfO2/SiO2 coatings under laser irradiation are investigated. The HR HfO2/SiO2 coatings are prepared by electron beam evaporation at 1 064 nm. The laser-induced damage threshold （LIDT） are measured at 1 064 nm and at a pulse duration of 12 ns, in 1-on-1 and S-on-1 modes. Multi-shot LIDT is lower than single-shot LIDT. The laser-induced and native defects play an important role in the multi-shot mode. A correlative theory model based on critical conduction band electron density is constructed to elucidate the experimental phenomena.

Research progress on mid-IR nonlinear optical crystals with high laser damage threshold in China

Nonlinear optical(NLO)crystals have been playing an increasingly important role in laser science and technology.The NLO crystals used in the middle infrared(mid-IR)region,compared with the NLO crystals in the other wavelength regions,are still not good enough for the application of high-energy laser.The main defect is that their laser damage thresholds(LDT)are low.Chinese scientists have made a lot of important contributions to the UV and visible NLO crystals.In the last decade,they also did a lot of work on the mid-IR NLO materials.The main purpose of these researches is to increase the LDT and simultaneously balance the other properties.This paper presents a brief summary of their research progress in this topic on three types of materials:chalcogenides,oxides,and halides.The emphasis is put on the design strategy and quality control of the crystals.