Laser-Induced Damage in Optical Materials: 1998

Nomarski interferometer and CCD camera application in the research of optical materials damage processes (Abstract Only)

Shao-Xian Meng, Fei Chen, Haiwu Yu

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We have designed a new detecting system, which consists of Nomarski polarized light interferometer and a digital camera, to investigate the damage of optical materials. The interferometer has an advantage of measuring the transient and residual strain/stress in the optical materials.

Multiharmonic coatings for the YAG laser

Rui-Ying Fan, Zhengxiu Fan, Yanjing Yang, et al.

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We developed several ten sorts of harmonic mirrors, anti- reflector and separator coatings for cavity components, lens, separators to satisfy the needs of YAG harmonic laser system. This paper mainly introduced the optical properties and the damage thresholds of some of these coatings. At a normal or a 45 degree incidence angle, the separators satisfied the requirements such as HR for 1 (omega) , 2 (omega) /HT for 3 (omega) , or HR for 2(omega) /HT for 1 (omega) , 2(omega) , or HR for 2 (omega) , 4(omega) , and the mirrors or anti-reflectors could be simultaneously used for two or three wavelengths among the base wavelength 1053nm and its second, third, fourth harmonic wavelength.

Effects of excimer laser irradiation on the KDP crystal

Hui Gong, Yan Sun, Cheng Fu Li

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The progress in the development of powerful solid state lasers depends to a great extent on the laser damage threshold of KDP single crystal used for harmonics generation of laser radiation. However, the bulk damage threshold of crystal is low.In order to condition KDP crystal, the effects of laser pre-irradiation on the KDP crystals were investigated in this paper. The KDP crystals were irradiated by excimer laser with 308-nm wavelength 35 ns pulsewidth, and we observed that the 215 nm absorption band disappeared, which is correlated with the electronic center charged oxygen vacancy. It is shown that P⁺ and K⁺ ions shifting became slow after excimer laser pre-irradiation by means of the electron probe measurement, which further demonstrated the electronic defects to be repaired. In addition, a single mode Nd:YAG laser with 10 ns pulsewidth was used in the laser damage experiments. Damage thresholds of the KDP crystals increase by a factor of 2-3.

Laser-induced damage of organic-dye-doped ormosils

Hui Gong, Lili Hu, Cheng Fu Li

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Dye doped ormosils have been proposed to create solid-state dye lasers. In this paper we investigated laser performance and laser-induced damage of rhodamine 6G, pyrromethene 567 and pyrromethene 597 organic dyes doped ormosils. A 10 ns, 532nm Q-switched Nd:YAG laser-induced damage thresholds were given in two kinds of ormosil matrices and a variety of dye doped concentration. In addition, laser-induced carbonization, damage mechanism and dye laser performance had been analyzed, and the possible explanation for these experimental results was given. The result indicate that the organic dyes doped ormosils are perspective for highly effective solid-state dye lasers.

CW-laser-induced thermal and mechanical damage in optical materials

Hui Gong, Cheng Fu Li, Zhong Ya Li

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In this paper CW laser induced damage in optical materials were investigated using CW-CO₂ laser. Inhomogeneous absorption will result in thermal stress in optical materials, which will induce crack, fusion of optical materials. These damages are related to microcracks and thermal properties of materials. It is shown that surface microcracks of optical materials is very important factor of CW laser induced damage, and the size of optical materials also has influence on damage thresholds. It is provided that CW laser induced damage mechanisms are thermal-stress accumulation damage and thermal-shock-stress damage. In addition, chemical etching was used to improve the damage resistance of optical materials, and CW laser induced damage thresholds of optical materials largely increased after 5 percent HCl solution etching.

Recent progress in theoretical studies of laser-induced damage (LID) in optical materials: fundamental properties of LID threshold in the wide-pulse-width range from microseconds to femtoseconds

M. F. Koldunov, Alexander A. Manenkov

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We present an overview of our theoretical studies on laser- induced damage (LID) in transparent optical materials including recent results on a pulsewidth dependence of the LID threshold, thermoelastic stress-induced damage criteria, damage morphology features, peculiarities of the LID at ultrashort pulse irradiation. Main attention is focused on inclusion-initiated damage, but some results are related also to intrinsic damage mechanisms. Theoretical results are compared with available experimental data on pulsewidth dependence of the LID threshold in bulk optical materials, on damage feature in thin films and on damage morphology studies. A good agreement of theoretically predicted and observed features is demonstrated, in particular, for the pulsewidth dependence of the LID threshold in a wide pulsewidth range from nanoseconds to femtoseconds.

Contribution to the failure analysis of AlGaAs/GaAs laser diodes

Rodica V. Ghita, D. Cengher, S. Lazanu, et al.

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High power laser diodes are of interest due to their potential use in medicine and military applications. This paper presents a systematic study of rapid degradation on our AlGaAs/GaAs large optical cavity devices. The increase of normalized threshold current vs. time was experimentally studied. An evaluation for normalized threshold current vs. absorption coefficient for different reflectivities is presented. For the optical output vs. time curve an abnormal increase of light characteristics was experimentally observed. The variation of the optical power was correlated to material parameters during operation and this behavior has been proposed as a practical criterion to select devices that are on their route to rapid degradation. The output optical power vs. electron fluence curve was measured for irradiated devices and a dislocation climb motion was assumed.

Rutherford backscattering spectroscopy analysis of Au/Cr/GaAs

D. Pantelica, F. Negoita, Rodica V. Ghita, et al.

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The thermal stability for rapid thermal annealing of Cr/GaAs, Au/GaAs and Au/Cr/GaAs system was investigated using Rutherford backscattering spectroscopy technique. No interdiffusion could be detected in Cr/GaAs and Au/Cr/GaAs systems for low temperature (400°C) rapid thermal annealing. In the Au/GaAs system, Au diffusion and Ga outdiffusion to the surface was observed and a quantitative analysis of the transport of Au in GaAs using RUMP code was performed. A value of D = 1. 1012 cm2/sec was obtained for the diffusion coefficient of Au in GaAs at 400°C. This analysis is related to an attempt of improving the metallization procedure for laser diodes with the active region close to the surface. keywords: rapid thermal annealing, Rutherford backscattering spectroscopy, diffusion, RUMP code.

Characterization of absorption and scatter losses on optical components for ArF excimer lasers

Klaus R. Mann, Oliver Apel, Eric Eva

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The performance of DUV optical components is assessed by measuring both absorption and scatter losses during ArF excimer laser irradiation. Absolute absorptance is determined by employing a high-resolution calorimetric technique which provides greatly enhanced sensitivity compared to transmissive measurements. Thus, the determination of single and two-photon absorption coefficients at energy densities of several 10 mJ/cm² is accomplished. As a result of its sensitivity, UV laser calorimetry can be also employed for fast monitoring of laser induced aging phenomena like color center formation in fused silica or CaF₂. For monitoring of the scatter losses in UV optics, a total scattering setup was recently installed, using an ArF excimer laser as pulsed 193nm light source and a Coblentz hemisphere as integrating element. Results of quantitative absorption and scatter measurements at 193 and 248 nm are presented for coated and uncoated optics, and the contribution of the various loss channels is discussed.

Pointlike destruction in transparent materials caused by single-photon absorption

Vladimir N. Strekalov

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A quantum-kinetic theory is suggested for point-like destructions generation in clear transparent dielectrics. Direct absorption of phonons with an energy exceeding the binding energy of a neural adatom with lattice may become a cause for destruction. The theory contains one phenomenological parameter requiring experimental verification. Nonetheless, the estimates that have been carried out well correlate with the magnitudes of threshold fields observed in experiment. The theory is applicable for ultrashort laser pulses of femtosecond duration. Destruction caused by pico- and nanosecond pulses is construed in terms of the processes occurring on the front edge of a strong laser pulse, in the femtosecond range.

Analysis of the data on the electronic avalanche obtained from numerical solutions of differential-difference and diffusion-type equations (Abstract Only)

Vladimir N. Strekalov, A. V. Voronenko

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Heating of electrons by light and impact ionization of the crystal valence band are described by a complicated differential-difference equation. An analytical solution to this equation is not so far found. To simplify the problem, the unknown solution of the exact equation is presented in a form of Taylor series. Then the initial equation is replaced by an approximate differential equation. Usually, it is a diffusion type of equation. Such approach is the most common for theoretical studies of optical damage. But this step has no real grounds. Consequently we need some possibilities for verification ofthe approximate equations. We suggest using of a numerical method for solving of this equation which is suitable for all equations. It allows us to compare different descriptions of the same phenomenon. If the results are similar then we can suppose that the Taylor's method can be used, and the analytical expressions based on this method are corrected. However we have shown that there is no such similarity of the results (in optical range of frequencies). So, using of Taylor's series and the diffi.ision-like type of equations is not correct. We must look for other methods for exact solving of the initial equation.

Path integrals method and description of a valance band impact ionization under multiphoton heating of electrons

Vladimir N. Strekalov

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The path integrals method is used for description of electrons heating by laser light and for determining the avalanche ionization rate. The limit of ultra-short pulses is considered as well as the stationary ionization regime. It is shown that multiphoton processes of electrons heating are more important than single-photon ones. A new dependence of threshold values on the laser pulse duration is predicted. Estimates are carried out for thresholds in the system 'Ruby laser radiation-silicon'. The thresholds thus found exceed experimental values. The suggested conclusion is that the processes of electron-phonon scattering cannot provide a rate of energy accumulation required for developing an avalanche.

Structural quality of the interface seed crystal in rapidly grown KDP crystals for high-power lasers

Vitaly I. Salo, V. F. Tkachenko, Marina I. Kolybayeva

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Structural quality of the interface 'seed crystal' of rapid grown KDP crystals was studied by the X-ray diffraction method with high resolution. The presence of a transitional zone seed - crystal more than 12 mm in width, with an increased concentration of defects and nonmonotone variation of the crystal lattice parameter was found. Found and determines was the turn angle between two growth sectors which makes approximately 32 arcs. It was shown that formation of defects at growing the crystal in the directions and produces a significant effect on structural quality of the crystal grown in the direction. The main growth defects of structure were found to be impurity striation a low angle quasi-boundaries caused by the mechanisms of layer-by-layer growth of crystals.

Laser damage threshold and optical absorption of KDP crystal dependent on their growth conditions

Igor M. Pritula, Viacheslav M. Puzikov, Vitaly I. Salo, et al.

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The effect of different crystallization parameters on optical absorption and laser damage threshold of KDP crystals was studied in the present paper. Crystals were grown in the direction of the prespecified angle of synchronism. A significant influence of the solution acidity, supersaturation value, hydrodynamic growth regime on the absorption spectra and bulk laser damage threshold of crystal was established. It was shown that crystals grown at pH equals 2,4 had an impurity - striated structure, the latter resulting in the decrease of bulk laser damage threshold and optical transparency of crystals. An essential improvement of optical transparency and increase of bulk laser damage threshold in crystals grown at pH equals 5 was found. For the first time established was the actually reached level of laser radiation resistance of rapidly grown oriented KDP crystals, grown at a rate of 5-10 mm/day. The obtained mean value of optical break through threshold is about 3 GW/cm². This value is comparable with that typical for KDP crystals grown by a conventional method at a rate of 1 mm/day.

Radiation-stimulated changes of structure and mechanical strength of KDP and DKDP crystals

Vitaly I. Salo, L. V. Atroschenko, Marina I. Kolybayeva, et al.

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Under the effect of ionizing radiation on KDP and DKDP crystals radiation-stimulated defects are formed in them. It was found that crystals with a higher content of cationic impurities are noticeably strengthened under the effect of electron radiation, while in purer, free of dislocations crystal microhardness does not increase. The rise of the microhardness value was found to begin at the same dose as the yellow coloration appears, with this, the input to the radiation surface is colored more intensively than the output one. The radiation strengthening of crystals is supposed to be connected with the formation of aggregations of crystal lattice defects arising under the effect of ionizing radiation. The depth of electrons' penetration calculated theoretically is commensurable with that of radiation-stimulated changes in structure and mechanical strength of crystals. It was shown that in the process of action of electrons and protons there takes place a decoration of microstructure defects that were present in crystals before irradiation.

Effects of layer thickness and number of interfaces on the damage threshold of ultraviolet sol-gel mirrors

Hazel A. McInnes, James E. Andrew, Nicholas J. Bazin, et al.

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Previous results1 have shown that single layers of nitric or acetic acid stabilised zirconia and silica sol-gels have very high laser induced damage thresholds (LIDT). A significant decrease in the LIDT has been found when the materials are combined in layer pairs to form ultra violet mirrors. Investigations of possible causes of the lower LIDT will be reported. The effect of the coherent intensity enhancement at interfaces was tested by measuring the LIDT at 3 55nm on mirrors of different wavelength tuning. Three nine pair mirrors were used. These components are for use at 355nm and the first high reflector was specified at this wavelength. The other two components were made with high reflectivity at 300nm and 400nm and lower reflectivity at 355nm to check how the intensity ofthe internally reflected light would affect the LIDT.

Laser damage study on the first-wall LMJ target chamber

Christelle Dubern, J.-L. Bruneel, Patrick Chadeyron, et al.

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Inertial fusion targets are of two basics types: direct drive and indirect drive. The french Laser MegaJoule (LMJ) will preferentially use indirect drive experiments. However, to contribute to the determination of the best target for inertial fusion energy, both direct and indirect drive will be considered on LMJ-facility. That is why studies on materials for use as first-wall LMJ target chamber and for different applications are of significant interest. In direct drive, targets will be directly heated and imploded by intense 351-nm wavelength laser light and part of this UV energy will be back-scattered off the target surface and consequently deposited onto the target chamber first wall. Such a deposited fluence should average 0.3-J/cm². The aim of this study was to determine both the fluence level at which physical damage occurs on some considered protective materials and the associated damage mechanisms. Investigations have been carried out on boron carbide and carbon-based materials. Basically, experiments consisted of illuminating samples with a 355-nm gaussian pulse of roughly 3-ns duration generated by frequency conversion through KDP- crystals of a YAG-laser light. In our investigations, characterization of ejected species and transient hemispherical directional reflectivity measurement of the illuminated materials were conducted. All these experiments have shown that boron carbide was a better candidate than carbon-based materials as first-wall LMJ target chamber use. Analyzing the defect type, size and density and the composition and physical-state of the considered materials has allowed predicting the impact of induced-ablation of the first-wall on LMJ target chamber use and lifetime. Composition analysis were addressed using micro-Raman spectroscopy. Preliminary experiments have revealed significant changes in damage mechanism versus laser fluence.

Temperature dependences of IR-absorption and CO2-laser damage of ZnSe and CdS single crystals

Yuri A. Zagoruiko, O. A. Fedorenko

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Crystalline materials utilized as elements of constructional and power optics in up-to-date instrument making, undergo the action of rather severe heat loads. Developed in the present paper are earlier investigations of the peculiarities of IR-absorption and damage of transparent crystalline CdS and ZnSe materials under the action of CO₂-laser radiation. Samples with different stoichiometry and crystallographic orientation cut out of crystalline boules grown by the vertical Bridgman method, were studied. The activation energy of free carriers in the samples was determined while measuring the temperature dependences of electrical resistance. The temperature dependences of IR- absorption were measured by the method of laser calorimetry within 285...750 K temperature range. Peculiarities of optical damage were investigated at the same temperatures. The thermal behavior and peculiarities of the damage pattern of the crystal structure are revealed by microscopy method. The obtained experimental data are discussed and compared with those presented by other authors.

Modification of optical properties of ZnSe crystals by means of photothermal treatment

Yuri A. Zagoruiko

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Thermal treatment of ZnSe crystals in a temperature gradient field allows to essentially improve their optical and electrical characteristics. Considered here is the possibility to modify the optical properties of real ZnSe single crystals in the process of photothermal treatment. Such a treatment comprises thermal annealing in a temperature gradient field and simultaneous irradiation of crystalline samples with white light. Investigations were performed on thin ZnSe discs or plates cut out of crystalline boules grown from the metal in graphite crucibles. The samples were annealed while being pulled through 60-70 K/cm temperature gradient field. In the process of annealing larger faces of the samples were irradiated with white light, illuminance reaching 5 10⁴ lx. The described treatment enables to reduce the coefficients of CO₂-laser radiation absorption and scattering in the samples, as well as to raise their laser damage threshold.

Laser resistivity and causes of damage in coating materials for 193 nm by photothermal methods

Holger Blaschke, Roland Thielsch, Joerg Heber, et al.

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The aim of our investigation was to explain the causes and kinds of the destruction of optical coatings during laser radiation at the wavelength of ArF excimer laser. Therefore, HR layer stacks with an increasing number of HL-pairs were deposited on different substrates CaF₂ and fused silica, respectively. SiO₂/Al₂O₃-, LaF₃/MgF₂- and AlF₃/LaF₃-combinations were used as coating materials. While fluoride coatings have been deposited by conventional evaporation, the oxide coatings were deposited by reactive e-beam evaporation with or without plasma ion assistance. The interaction of UV laser radiation with optical coatings as mentioned above was investigated by a pulsed two probe beam photothermal technique as well as optical microscopy, respectively. In the case of fluoride layers the single shot damage threshold increases with higher number of HL-pairs. Additionally, an aging effect could be observed.

Changes in optical interference coatings exposed to 193-nm excimer laser radiation

Joerg Heber, Roland Thielsch, Holger Blaschke, et al.

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The exposure of optical interference coatings to low-fluence DUV-radiation reveals changes of thin layer properties due to interactions between radiation field and thin film structure. An experimental set up for irradiating antireflective as well a high reflective coatings with 193nm excimer laser was used in order to study permanent cumulative changes in optical coatings at fluences ranging from 20mJ/cm² with up to 240 10⁶ laser pulses. The optical ex-situ monitoring of radiation induced modifications enabled the differentiation of coating specific and substrate inherent alteration effects. The identification of conditions as well as degradation processes during the exposure could be achieved for several types of DUV-coating materials. They were deposited with an ultra low loss evaporation process onto calcium fluoride and fused silica substrates. Fluoride coating included LaF₃, Na₃AlF₆, MgF₂, AlF₃ oxide coatings consisted of SiO₂ and Al₂O₃ exclusively.

ArF radiation resistance of optical coatings on CaF2 in relation to the surface finish of the substrate

Roland Thielsch, Joerg Heber, Angela Duparre, et al.

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CaF₂ has received increasing attention as a promising substrate for coatings in the VUV range. Optimization of the optical properties of these optical components requires the study of basic characteristics of the coated and uncoated CaF₂ substrates such as surface roughness, optical performance, absorption and scatter losses, and laser induced damage threshold. The investigations have revealed the influence of different substrate polishing grades on the quality of AR-193nm -and HR-193nm/0 degrees coated samples. LIDT values at the ArF-excimer laser wavelength were measured as high as 5.6 J/cm² and 4.6 J/cm² for the best AR- and HR-coated samples, respectively.

Current status of radiation resistance of dielectric mirrors in the DUV

Helmut Bernitzki, Hans Lauth, Roland Thielsch, et al.

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Dielectric mirrors are key optical components in ArF excimer laser based devices for applications in DUV photolithography as well as in material processing. In all these applications different requirements of laser radiation resistance have to be met in relation to fluence, repetition rate and pulse number lifetime. Investigations have been performed into the radiation resistance of dielectric mirrors consisting of fluorides and oxides with emphasis to the properties of bending point mirrors used in beam delivery systems of wafer steppers. Problems and limitations for the improvements of the laser-induced-damage-thresholds of the coatings are discussed.

Influence of thermal substrate properties on the damage threshold of UV coatings

Holger Blaschke, Sven Martin, Andreas Morak, et al.

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The thermal decay of surface temperature has been calculated and measured on Al₂O₃/SiO₂ coatings by using photothermal technique near damage threshold fluence. It could be shown that under certain conditions the decay times (tau) has been found to be in the order of some microseconds. This fact gives no explanation of the measured decrease of threshold fluence in some cases for higher repetition rates and shot numbers on Al₂O₃/SiO₂ and HfO₂/SiO₂ multilayers for (lambda) equals 248nm. Furthermore, it could not be found any influence of the substrate materials. Thus, other than thermal accumulation is responsible for the lowered damage threshold by increasing repetition rate. Additionally, performed calculations of the thermal decay using 20ns, 248nm laser excitation confirm the experimental results. Even in the case of Al₂O₃/SiO₂ coatings on copper no effect of the substrate as a heat sink could be measured. For HfO₂/SiO₂ coatings the behavior is in accordance with the fact that the thermal conductivity of HfO₂ films is markedly lowered compared to the bulk value.

Laser-induced damage threshold of optical components for high-repetition-rate Nd:YAG lasers

Kai Starke, T. Gross, Detlev Ristau, et al.

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High repetition rate solid-state laser systems are used in many scientific and industrial applications. Especially-Q- switched Nd:YAG lasers are gaining increasing importance as pump sources for frequency conversion processes. These lasers are a potential alternative for several classical gas laser systems. Presently, the development of Nd:YAG systems with higher output power and improved beam parameters suffers from limitations imposed by the power handling capability of the optical components. At the Laser Zentrum Hannover, a facility has been installed for the measurement of the multiple-pulse damage thresholds of optical surfaces according to ISO/DIS 11254-2. For an efficient determination of the threshold values, an on-line algorithm calculating the recommended energy for each site has been elaborated and implemented. The most important coating materials for the NIR spectral range were investigated, in respect to their S- on-1 damage threshold. The results indicate the materials dependent reduction of the damage threshold with an increasing number of pulses per site.

Measurement of optical absorptance according to ISO 11551: parallel round-robin test at 10.6 um

Detlev Ristau, Uwe Willamowski, Herbert Welling

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For an elaborate assessment of the laser-calorimetric standard practice described in the International Standard ISO 11551 'Test methods for absorptance of optical laser components' an extended international round-robin test has been performed. In contrast to a former experiment with a serial concept, in the present parallel test identical commercial sample sets have been sent to the partners after an initial measurement of absorptance at the coordinator, where the experiment was completed by a final inspection and second screening measurement of absorptance. The result of the round-robin test are compared and evaluated on the basis of standard evaluation techniques and alternative statistical methods. With the exception of a few deviations, which can be attributed to specific properties of the individual measurement systems, a good agreement of the absorption values was observed.

Total scatter measurements in the DUV/VUV

Puja Kadkhoda, Detlev Ristau, Ferdinand von Alvensleben

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Light scattering of optical laser components gains in importance for the short wavelength range. A standard procedure for the measurement of 'total scattering' is described in the Draft International Standard ISO/DIS 13696, which is based on integration or collection of the scattered radiation. Meanwhile the measurement concepts have qualified for the VIS- and NIR-spectral region, investigations are necessary for an application of the standard procedure in the DUV/VUV. In this work the optical properties of selected diffuse reflective materials are studied in the DUV/VUV spectral region. For the determination of the angle resolved scattering and the relative spectral reflectance of the samples, a commercial vacuum spectrophotometer is employed. Some of the samples exhibit a cosine angle distribution for the scattered light at 193 nm and 157 nm and can be used as calibration targets. Total scatter values of different thin film samples were measured at 193 nm by using these qualified calibration targets.

Finite element thermal analysis of multispectral coatings for the ABL

Rashmi S. Shah, Jerry Ray Bettis, Alan F. Stewart, et al.

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The thermal response of a coated optical surface is an important consideration in the design of any high average power system. Finite element temperature distribution were calculated for both coating witness samples and calorimetry wafers and were compared to actual measured data under tightly controlled conditions. Coatings for ABL were deposited on various substrates including fused silica, ULE, Zerodur, and silicon. The witness samples were irradiate data high power levels at 1.315micrometers to evaluate laser damage thresholds and study absorption levels. Excellent agreement was obtained between temperature predictions and measured thermal response curves. When measured absorption values were not available, the code was used to predict coating absorption based on the measured temperature rise on the back surface. Using the finite element model, the damaging temperature rise can be predicted for a coating with known absorption based on run time, flux, and substrate material.

Ultraprecision grinding of potassium dihydrogen phosphate crystals for getting optical surfaces (Abstract Only)

Yoshiharu Namba, Masanori Katagiri

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In the NIF research program, potassium dihydrogen phosphate (KDP) crystals have been studied on rapid growth methods and single point diamond turning for getting optical surface. KDP has several weak points such as soft, brittle and deliquescent from the machining point of view. This paper shows a new possibility to get optical surfaces on KDP crystals by ultraprecision grinding. The KDP crystal was ground with a SD5000-75-B diamond wheel under the conditions of wheel peripheral speed vs equals 160m/min, feed per wheel revolution f equals 0.5 micrometers /rev and depth of cut a equals 1 micrometers in a mixture of 10cSt silicon oil and 5 wt percent heptanol as a grinding fluid after grinding of LHG-8 laser glass. The surface roughness of 0.553 nm rms, 0.441 nm Ra or 3.40 nm p-v was obtained by the ultraprecision surface grinding without polishing process. This surface is smoother than single point diamond turned surfaces or optically polished ones ever obtained.

Thermomechanical model of mirror laser damage at 1.06 um: I. Nodule ejection

Jean Dijon, Marc Poulingue, Jean Hue

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A simple tractable model based on a thermomechanical approach of nodule ejection is developed. Our main purpose is to investigate the nodule critically according to the depth of the seed inside of the coating, in order to understand why the deepest nodules located close to the substrate interface are the most critical. A summary of the experimental results concerning the behavior of irradiated nodules is achieved. Then, the model is checked thanks to experimental data. A critical location of the nodules close to the top surface of the coating is found. A supplemental ingredient is added to understand why the nodules close to the substrate interface are critical.

Thermomechanical model of mirror laser damage at 1.06 um: II. Flat bottom pits formation

Jean Dijon, Guillaume Ravel, Bernard Andre

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Other defects than nodules are responsible for laser damage of High Reflection coatings at 1.06 micrometers . Among them, flat bottom pits (FBP) are particularly interesting because they are surface induced damages not related with preexisting cosmetic defects. The damage morphology observed with a Scanning Electron Microscope suggest that damages are related to absorbing nanometric centers. Following this idea, a model of FBP formation is developed in this paper. This model predicts a size dependence of the FBP versus the thickness of the buckled layers; this fact is experimentally confirmed. Two different kinds of FBP have been observed; the second one might be related with the seed of nodular defects.

Combining optical and thermal stresses in multiple-shot laser experiments

Guillaume Ravel, Jean Dijon, Philippe Lyan, et al.

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We report surface damage measurements on single crystal silicon under single and multiple 3 ns pulse laser irradiation at 1.06 micrometers wavelength. First, single shot damage threshold is measured. Then the numbers of pulses required to damage with a probability of 0.5 at various fluences below this 1/1 threshold are obtained at room temperature. These results are fitted with various life models. We look at temperature as a means to accelerate life test. A new feature of the laser test facility in our laboratory is described, allowing surface damage measurements under controlled thermal environment, with testing temperatures ranging from room temperature to 180 Celsius degrees. Experimental demonstration of accelerated failure time for silicon at 1.06 micrometers is provided.

New approach for the critical size of nodular defects: the mechanical connection

Marc Poulingue, Jean Dijon, Michel Ignat, et al.

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The purpose of this paper is to determine the effect induced by the nodular defects in optical thin films under laser irradiation. A strong correlation between the size distribution of defects in optical thin films. The deposition parameters and the laser damage threshold is found. With this approach of the defects in thin films, it confirms that the size of a nodular defect is a critical parameter, while the density of the defects is not. The observations of damaged ares, performed by Scanning Electron MIcroscope (SEM), show that of the smallest ejected nodular defects associated with damaged zones, correspond to a critical mean size around 4 micrometers . A mechanical approach is also used to understand the role of the nodular defects under a mechanical solicitation. During the mechanical experiments it appears that depending on the nodular defects sizes, the initiation of cracking was more or less delayed. From the mechanical experiments, a critical size of about 4 micrometers is deduced. Besides, the analysis of these two different experiments points out that the laser damage induced by nodular defects is strongly related to mechanical fracture parameters.

Beam characterization: application to the laser damage threshold

Jean Hue, Jean Dijon, Pierre Garrec, et al.

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The laser damage threshold determination is linked to many factors as the damage criteria, the test conditions, etc. Among them, the spatial beam characterization as well as the power or the energy in the beam plays a key role. Indeed, an appropriate value has to be assigned to each tested site. This number is important to certify a component or to compare the components. This paper is focused on determining the beam parameters for 2 kinds of laser damage. The first part addresses the peak fluence measurement on YAG laser damage test facilities. The proposed method is applied to stable laser. Stable lasers mean the spatial profile is similar during the experiments. Before the laser damage tests, the spatial beam is visualized with a CCD camera. Them, the spatial beam profile is cautiously processed to extract the relation between the peak fluence and the energy, which is easy to measure in real time. This method is illustrated at 1064 nm. Finally, this procedure is validated on silicon wafer. Indeed, at 1064 nm, the theoretical laser damage threshold of silicon wafer is well known. Furthermore, it is independent of the pulse length in the nanosecond regime. The second part will be shortly devoted to a more specific problem: the CW damage threshold of CO₂ mirrors. It has been previously shown that the 'short' term threshold of these cooled components is well described by a critical temperature. Due to the test geometry, the temperature rise is proportional to the beam power divided by a value homogeneous to the beam radius. In this article, it is sown the knife-edged method allows to literally extract the beam size parameter which appears in the theoretical formula of the critical temperature. The beam just needs to have a symmetry of revolution.

Automatic YAG damage test benches: additional possibilities

Jean Hue, Jean Dijon, Guillaume Ravel, et al.

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The automatic damage test benches allow to obtain more data in less time but also to imagine new laser damage tests. Thereby, new questions can be asked and more answers can be provided. In this paper, the 3 YAG automatic damage test facilities build up at the laboratory will be shown. There are based on well-known principles : an half wave plate to adjust the wished peak fluence and a scattered light measurement in real time to detect the damage appearances. Many tests are developed around these benches as the usual hon-i, N-on-i, R-on-1, S-on-i tests and raster scans previously proposed by other laboratories. On all our test benches, these standard tests are driven by the same software designed at the laboratory : "Wintfl". But new tests and characterizations are also added. Indeed, on these thcilities the aging ofthe optics at various repetition rate under constant fluence (like S-on-i with or without a limit number of shots) are carried out. An automatic defect count has been also implemented. This tool is able to evaluate the number of defects after cleaning or after coating but also between laser irradiation. New tests are also imagined. The so-called HR-on-i test (High Ramp), which is a R-on4 (Ramp) test without beginning at zero has been achieved. It allows to compare 1-on-I and R-on-1 distributions performed with exactly the same damage criterion. Other parameters can be investigated as the speed of the motor ramp during the R-on-1 test, the pulse repetition frequency during the S-on-i test,... In this article, after a short presentation of our test bench facilities, the convenient possibilities of automatic YAG test benches are illustrated with experimental results.

Experimenting with spatial and temporal profiles for the YAG laser damage threshold

Jean Hue, Guillaume Ravel, Jean Dijon, et al.

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Since the value and the shape of the pulse widths and also the spatial profile vary from one test bench to another, it appears interesting to see how the damage threshold evolves with these two parameters. The scaling laws have 2 main impacts: the possibility to compare easily the results between the laboratories and to help the understanding of the damage phenomena. But it can be also a great help on the laser damage threshold specifications of the optical components. Moreover, the spatial law versus the component area could become a request in the future specifications. The first part of this paper is devoted to experimental damage result performed with YAG laser at 1.064 micrometers in R- on-1 mode and in raster scan. 2 pulse widths close to a Gaussian profile is used: around 3 ns and 12.5 ns. Furthermore, 3 laser spot sizes with a shape close to a Gaussian beam are used. Their impact on the laser damage threshold is studied. The results are analyzed with the purpose of finding a tool to extrapolate the result at larger components. To test full large optics is time consuming. A primary automatic test is suggested too quickly eliminate the components below the specifications.

1.06-um laser irradiation on high-reflection coatings inside a scanning electron microscope

Marc Poulingue, Jean Dijon, Pierre Garrec, et al.

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The output fluence of a high peak power laser like the French laser project 'Laser Megajoule' is strongly related to the damage threshold of mirrors and polarizers consisting of multilayer dielectric coatings. It has been recognized for a long time that growth defects as nodules are playing a significant role in 1.06 micrometers laser damage of these components. In order to determine the critical parameters of the nodule and the associated mechanisms during the laser damage process, we have performed laser irradiation of samples inside the vacuum chamber of a scanning electron microscope (SEM). The SEM allows us to choose a particular area of the sample and to observe it before and after irradiation at a micro size scale. The damage evolution ca be analyzed shot by shot. In this paper, the experimental method is described. SEM pictures of irradiated laser shots are showed. Data about damage mechanisms are derived from these very high resolution morphological observations.

Observation of photoexcited emission clusters in the bulk of KDP and laser conditioning under 355-nm irradiation

Stavros G. Demos, Michael C. Staggs, Ming Yan, et al.

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Defect clusters in the bulk of large KDP crystals are revealed using a microscopic fluorescence imaging system and CW laser illumination. Exposure of the crystal to high power 355-nm, 3-ns laser irradiation leads to a significant reduction of the number of observed optically active centers. The initially observed defect cluster concentration is approximately 10⁴-10⁶ per mm³ depending on the crystal growth method and sector of the crystal. The number of defect clusters can be reduced by a factor of 10² or more under exposure to 355-nm laser irradiation while their average intensities also decreases. Spectroscopic measurements provide information on the electronic structure of the defects.

Laser damage in multispectral optical coatings for the ABL

Alan F. Stewart, Lynn Bonsall, Jerry Ray Bettis, et al.

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Coatings designed for use in the Airborne Laser (ABL) have stringent requirements for reflectance over several spectral bands in addition to extremely low absorption and high damage threshold at the 1315nm output of the chemical oxygen iodine laser. The complexity of these coatings leads to difficulty in design and fabrication particularly on curved optical surfaces with large apertures. A series of witness samples were fabricated to evaluate the state-of-the-art for this type of coating and provide appropriate design criteria for the ABL optical train. Damage testing at 1315nm under CW conditions was performed at the RADICL laser facility at Kirtland AFB. Limited optical characterization before and after the test was performed at the OCEL facility to evaluate the quality of the samples and to identify damage. The results of these test and characterization will be discussed.

Improvement of laser-induced surface damage in UV optics by ion beam etching (CsLiB6O10 and fused silica)

Tomosumi Kamimura, Kazuhito Nakai, Yusuke Mori, et al.

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The laser-damage resistance of CLBO and fused silica surfaces was successfully improved after removing polishing compound by ion beam etching. The polishing compound embedded in the CLBO and fused silica surfaces was to a depth of less than 100 nm. We were able to remove polishing compound without degrading the surface condition when the applied ion beam voltage was less than 200 V. After surface etching, the effects of polishing compound removal on surface damage were characterized for the surface laser- induced damage threshold at 355 nm and surface lifetime at 266 nm as a function of etching depth. In the fused silica surface, we found improvement of the surface LIDT up to 15 J/cm² as compared with that of the as-polished surface of 7.5 J/cm². For the irradiation of a 266 nm high- intensity and high-repetition laser light, the exit surface lifetime of CLBO and fused silica could be more doubled compared with that of the as-polished surface.

Laser-induced damage of photo-thermo-refractive glasses for optical holographic element writing

Oleg M. Efimov, Leonid B. Glebov, Semyon Papernov, et al.

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Bulk and surface-damage thresholds of photo-thermo- refractive glasses used for hologram writing were measured after different stages of development. Values obtained are compared with threshold from widely used glasses, such as BK7 or fused silica. Polished glass samples were exposed to 325-nm He-Cd laser radiation and thermodeveloped at 520 degrees C. Nd-glass, nanosecond laser pulses were used to irradiate the bulk and surfaces of glasses. It was found that laser-induced damage thresholds of photo-thermo- refractive glasses are within a factor of 2-4 of those for fused silica or BK7 glass. This indicates that phase element made from these glasses are promising candidates for use in high-power laser systems. Possible mechanisms of laser- induced damage are discussed.

How small stresses affect 351-nm damage onset in fused silica

Faiz Dahmani, John C. Lambropoulos, Stephen J. Burns, et al.

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This paper presents an innovative technique for designing high-strength optical glasses in which slow crack growth is arrested and the laser-induced damage threshold is significantly higher. The technique consists simply of applying compressive stresses in the surfaces of a glass. An increase of up to 70 percent in fused-silica exit-surface as well as front-surface threshold has been obtained.

Theoretical modeling of laser-matter interaction in spatial filter pinholes for high-energy pulsed lasers

Florian Bonneau, Patrick Combis

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A major concern in Megajoules and NIF laser studies is the spatial filter pinhole closure. The answer to this problem is a new pinhole architecture and suitable materials. The conical pinhole design at LLNL is the best candidate tested on the NIF prototype: Beamlet. We have investigated the material dependence in terms of laser-solid and laser-plasma interaction. We have demonstrated that a proper choice of material delays the plasma build-up to the end of the ICF drive pulse. To model the pre-plasma step, it is necessary to take into account optical and thermal parameters of the material, and processes of transition, fusion and vaporization. Preliminary calculations predicted the observed material dependence.

Polishing of fiber optics in the manufacturing environment

David Tracy, Stephen C. Pyke

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Lasers for industrial material processing, medicine, and military applications are widely used with fiberoptic cable assemblies for flexible beam delivery. Current methods for preparation of fiberoptic end faces employ traditional polishing techniques and CO₂ laser polishing. As higher powers become commonplace in laser applications, more attention is being focused on providing surfaces with a high damage threshold. Our experience with various abrasive media has suggested that final polishing with colloidal silica yields fibers with the longest life under high power conditions. We describe techniques for obtaining a final polish on connected fiberoptic cables using conventional polishing techniques. Results are presented comparing several different formations of colloidal silica for applying the final polish. Optical inspection is used to grade surfaces. Resulting surface finishes will be discussed in terms of the mechanical and chemical effects that combine to produce a high quality fiberoptic endface preparation.

National round-robin test on laser induced damage at 1.064 um: revised data reduction and correlation analysis

Jonathan W. Arenberg, Wolfgang Riede, Ulrike Broulik, et al.

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This paper presents a second look at analyzing the results from the recently held German National damage test round robin experiment. This experiment was performed under the auspices of CHOCLAB to validate the method in ISO/DIS 11254- 1.2. The result of the experiment are re-examined in terms of an improved threshold determination procedure, and the result are correlated for each sample type. Differences in detailed experimental set-up are examined and their effect on the outcome evaluated. The measured threshold are shown to agree well with one another intramurally. Results from two of the three laboratories involved show good extramural correlation.

Effects of accelerated aging on fiber damage thresholds

Robert E. Setchell

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Laser-induced damage mechanisms that can occur during high- intensity fiber transmission have been under study for a number of years. Our particular interest in laser initiation of explosives has led us to examine damage processes associated with the transmission of Q-switched, Nd:YAG pulses at 1.06 micrometers through step-index, multimode, fused silica fiber. Laser breakdown at the fiber entrance face is often the first process to limit fiber transmission, but catastrophic damage can also occur at either fiber end face, within the initial 'entry' segment of the fiber, and at other internal sites along the fiber path. Past studies have examined how these various damage mechanisms depend upon fiber end-face preparation, fiber fixturing and routing, laser characteristics, and laser-to-fiber injection optics. In some applications of interest, however, a fiber transmission system may spend years in storage before it is used. Consequently, an important additional issue for these applications is whether or not there are aging processes that can result in lower damage thresholds over time. Fiber end-face contamination would certainly lower breakdown and damage thresholds at these surfaces, but careful design of hermetic seals in connectors and other end-face fixtures can minimize this possibility. A more subtle possibility would be a process for the slow growth of internal defects that could lead to lower thresholds for internal damage. In the current study, two approaches to stimulating the growth of internal defects were used in an attempt to produce observable changes in internal damage threshold. In the first approach, test fibers were subjected to a very high tensile stress for a time sufficient for some fraction to fail from static fatigue. In the second approach, test fibers were subjected to a combination of high tensile stress and large, cyclic temperature variations. Both of these approaches were rather arbitrary due to the lack of an established growth mechanism for internal defects. Damage characteristics obtained from fibers subjected to each of these aging environments were compared to results from fresh fibers tested under identical conditions. A surprising result was that internal damage was not observed in any of the tested fibers. Only breakdown at the fiber entrance face and catastrophic damage at both end faces were observed. Fiber end faces were not sealed during the accelerated aging environments, and thresholds at these faces were significantly lower in the aged fibers. However, most fibers transmitted relatively high pulse energies before damaging, and a large fraction never damaged before we reached the limits of our test laser. The absence of any observable affect on internal damage thresholds is encouraging, but the current results do not rule out the possibility that some other approach to accelerated aging could reveal a growth mechanism for internal defects.

Effects of wet etch processing on laser-induced damage of fused silica surfaces

Colin L. Battersby, Lynn Matthew Sheehan, Mark R. Kozlowski

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Laser-induced damage of transparent fused silica optical components by 355 nm illumination occurs primarily at surface defects produced during the grinding and polishing processes. These defects can either be surface defects or sub-surface damage. Wet etch processing in a buffered hydrogen fluoride solution has been examined as a tool for characterizing such defects. A study was conducted to understand the effects of etch depth on the damage threshold of fused silica substrates. The study used a 355 nm, 7.5 ns, 10 Hz Nd:YAG laser to damage test fused silica optics through various wet etch processing steps. Inspection of the surface quality was performed with Nomarski microscopy and Total Internal Reflection Microscopy. The damage test data and inspection results were correlated with polishing process specifics. The result show that a wet etch exposes sub-surface damage while maintaining or improving the laser damage performance. The benefits of a wet etch must be evaluated for each polishing process.

Extrapolation of damage test data to predict performance of large-area NIF optics at 355 nm

Michael D. Feit, Alexander M. Rubenchik, Mark R. Kozlowski, et al.

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For the aggressive fluence requirements of the NIF laser, some level of laser-induced damage to the large 351 nm final optics is inevitable. Planning and utilization of NIF therefore requires reliable prediction of the functional degradation of the final optics. Laser damage test are typically carried out with Gaussian beams on relatively small test ares. The test yield a damage probability vs. energy fluence relation. These damage probabilities are shown to depend on both the beam fluence distribution and the size of area tested. Thus, some analysis is necessary in order to use these test results to determine expected damage levels for large aperture optics. We present a statistical approach which interprets the damage probability in terms of an underlying intrinsic surface density of damaging defects. This allows extrapolation of test results to different sized areas and different beam shapes. The defect density is found to vary as a power of the fluence.

Laser damage performance of fused silica optical components measured on the beamlet laser at 351 nm

Mark R. Kozlowski, Ron P. Mouser, Stephen M. Maricle, et al.

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A statistics-based model is being developed to predict the laser-damage-limited lifetime of UV optical components on the NIF laser. in order to provide data for the model, laser damage experiments were performed on the Beamlet laser system at LLNL. Three prototype NIF focus lenses were exposed to 351 nm pulses during four experimental campaigns, each consisting of 23 to 38 pulses at NIF relevant fluences. Each lens was sol-gel AR coated and all laser exposures were performed in a vacuum environment. Through inspections of the lens before, during and after the campaigns, pulse-to- pulse damage growth rates were measured for damage initiating both on the surfaces and at bulk inclusions. Radial growth rates measured for rear surface damage was typically 10x higher than that measured in the bulk or at the front surface. No significant correlation of growth rate to precursor type was indicated. For 5 J/cm², 3 ns pulses the typical radial growth rate was nominally 20 micrometers /pulse. Average growth rates measured on three lenses made by two manufacturers were in good agreement. While the growth rate clearly increased with fluence, the data obtained was insufficient to quantify the dependence. The growth rates reported here were 20x-50x higher than values predicted from off-line studies of bare surfaces in air.

Crack propagation in fused silica during UV and IR nanosecond-laser illumination

Alberto Salleo, R. Chinsio, Francois Y. Genin

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The functional lifetime of large-aperture optical components used on a laser such as the National Ignition Facility is an important engineering parameter. To predict the lifetime of fused silica transmissive optics, it is necessary to measure the rate of damage propagation as a function of fluence and understand the effects of the laser parameters. In order to begin such predictions without a large-area flat-top laser beam, damage growth experiments were conducted using a small Gaussian beam. Damage was initiated by producing mechanical flaws on the surface of the optic. Since output surface damage in transmissive topics can propagate at least two orders of magnitude faster than input surface damage, the experiments were focused on damage initiated at the output surface. The experiments showed that if damage was initiated, it could not grow at fluences below a threshold of about 5 and 8 J/cm² at 355 and 1064 nm, respectively. When damage was able to propagate under laser irradiation, the phenomenon occurred in two stages. Initially, the damage grew both laterally and along the optical axis at a rate varying linearly with fluence. Lateral growth stopped in areas where the fluence was lower than the growth threshold. At this point, the area of damage typically filed the size of the beam and the rate of axial damage propagation significantly decreased. During this stage, laser irradiation drilled at constant rate a channel through the window. During stage I, the damage area grew much faster at 355 nm than at 1064 nm. During stage II, 355 nm light drilled four to five times faster than 1064 nm light. At given fluence at 1064 nm, the drilling rate did not change between 3 ns and 10 ns. Finally, drilling at 1064 nm produced a well-defined cylindrical damaged region while 355 nm light generated less regular clusters of cracks.

Current 3-omega large optic test procedures and data analysis for the quality assurance of National Ignition Facility optics

Sheldon Schwartz, Michael D. Feit, Mark R. Kozlowski, et al.

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A reliable metric is required to describe the damage resistance of large aperture 3(omega) transmissive optics for the NIF laser. The trend from single site testing to the more statistically valid Gaussian scanning test requires a well modeled experimental procedure, accurate monitoring of the test parameters, and careful interpretation of the resulting volumes of data. The methods described here provide reliable quality assurance data, as well as intrinsic damage concentration information used to predict the performance expected under use conditions. This paper describe the equipment, test procedure, and data analysis used to evaluate large aperture 3(omega) optics for the NIF laser.

Defect characterization on superpolished fused silica surfaces polished for high-power-laser applications at 355 nm (Abstract Only)

Lynn Matthew Sheehan, Zhouling Wu

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Laser damage threshold of super-polished fused silica at 355-nm is believed to be limited by localized defects associated with surface/substrate damage and contamination caused by the polishing process. Characterization of both the defects and their correlation with laser damage present a technical challenge: most of the existing characterization methods do not directly address absorption and thermo- mechanical response issues relevant to laser damage. In this work a dark-field photothermal microscopy (DPTM) with micron-level resolution is used for localization and identification of damage precursors for various fused silica surfaces. The experimental system uses the multiple UV lines from an Argon-ion laser. The samples studied are witness fused silica samples from various polishing vendors. In addition to DPTM, some of the samples are also studied using high-resolution optical microscopy and total internal reflection microscopy. The characterization result are then compared with laser damage testing data at 355-nm with a 7.5-ns pulse width. It is shown that while the DPTM data ca be correlated between damaged sites and DPTM defects is not obvious.

Laser conditioning methods of hafnia-silica multilayer mirrors

Christopher J. Stolz, Lynn Matthew Sheehan, Stephen M. Maricle, et al.

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A variety of 1064-nm laser conditioning methods were investigated to find the optimum method for production of large aperture (0.25 m2) multilayer coatings for the National Ignition Facility and Laser MegaJoule. Two conditioning methods were explored, multi-step and single-step on two different laser systems. Off-line conditioning was done on PLATO, a small beam diameter ('-1 mm) raster scanning system. On-line conditioning was done on Beamlet, a single beam prototype of the National Ignition Facility with a large rectangular beam (35 cm x 35 cm). Concurrent with this work, coating process development for low-defect density high damage threshold coatings was realized by switching from hafnia to hafnium starting materials. The results of this study suggest that single-step raster-scan off-line laser conditioning is an effective method to raise the damage threshold of multilayer mirrors deposited from hafnium and silica by reactive electron beam deposition.

Role of starting material composition in interfacial damage morphology of hafnia-silica multilayer coatings

S. C. Weakley, Christopher J. Stolz, Zhouling Wu, et al.

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Flat bottom pits, previously shown to be unstable above a critical fluence, may be impacted by interfacial characteristics of the multilayer. TEM cross-sections reveal multilayer coatings deposited from hafnium have fewer interfacial voids than those deposited from hafnia. To correlate this reduction with the occurrence of flat bottom pits, multilayer high reflectors deposited by reactive electron-beam evaporation form both metallic and oxide sources were damage tested with 3-ns, 1064-nm pulses. To shift the electric-field peaks to the adjacent interface, half of the sample included an additional buried half-wave of silica in their all quarter-wave design. All quarter-wave reflectors deposited from hafnia had flat bottom pit damage in the outer six layers at fluences of 20 J/cm². Interfacial damage also occurred in hafnium deposited coatings at fluences as low as 20 J/cm², but at significantly different depths. The only interfacial damage observed on the all quarter-wave coatings was at the substrate multilayer interface. Flat bottom pits were observed in the buried half-wave coatings with a correlation to electric-field position and preference to interface type.

Laser-induced damage of absorbing and diffusing glass surfaces under IR and UV irradiation

Pamela K. Whitman, K. Bletzer, James L. Hendrix, et al.

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In high peak power lasers used for inertial confinement fusion experiments, scattered and reflected light can carry sufficient energy to ablate metal structures or even damage other optics. Absorbing and diffuse scattering materials are required to manage the 'ghosts', stimulated Raman scattering (SRS) and unconverted light in the laser chain and target chamber. Absorbing and diffuse scattering glasses were investigated for use in the NIF target chamber to safely dissipate up 60-80 J/cm² 1053-nm light while also withstanding up to 2 J/cm² of soft x-ray. In addition these glasses were evaluated for use at 1053-nm and 351-nm to dissipate stray light and to absorb stimulated Raman scattering from the conversion crystals. Glass samples with surfaces ranging from specular to highly scattering were evaluated. The morphologies of laser damage at 1064 nm and 355 nm were characterized by Nomarski optical microscopy. Laser damage was quantified by measuring mass loss. Surface treatment and bulk absorption coefficient were the two material properties most strongly correlated to laser damage. Etched and sandblasted surfaces always had lower damage threshold than their specular counterparts. Reducing rear surface fluence either by bulk absorption or scattering at the input surface delayed the onset of catastrophic failure under extreme conditions.

Laser modulated scattering as a nondestructive evaluation tool for optical surfaces and thin film coatings

Zhouling Wu, Michael D. Feit, Mark R. Kozlowski, et al.

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Laser modulated scattering (LMS) is introduced as a non- destruction evaluation tool for defect inspection and characterization of optical surface sand thin film coatings. It allows simultaneous measurement of the DC and AC scattering signals of a probe laser beam from an optical surface. by comparison between the DC and AC scattering signals, one can differentiate absorptive defects from non- absorptive ones. This paper describes the principle of the LMS technique and the experimental setup, and illustrates examples on using LMS as a tool for nondestructive evaluation of high quality optics.

Single-beam photothermal microscopy: a new diagnostic tool for optical materials

Zhouling Wu, Michael D. Feit, Mark R. Kozlowski, et al.

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A novel photothermal microscopy (PTM) is developed which uses only one laser beam, working as both the pump and the probe. The principle of this single-beam PTM is based on the detection of the second harmonic component of the laser modulated scattering (LMS) signal. This component has a linear dependence on the optical absorptance of the tested area and a quadratic dependence on the pump laser power. Using a pump laser at the wavelengths of 514.5- and 532-nm high-resolution photothermal scans are performed for polished fused silica surfaces and a HfO₂/SiO₂ multilayer coatings. The results are compared with those from the traditional two-beam PTM mapping. It is demonstrated that the single-beam PTM is more user-friendly than conventional two-beam PTM and, offers a higher spatial resolution for defect detection.

Characterization of surface and subsurface defects in optical materials using near-field evanescent wave (Abstract Only)

Ming Yan, Stan Oberhelman, Wigbert J. Siekhaus, et al.

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Optical properties of sub-micron defects is of interest in many application, including laser induced damage in optical materials. In-situ scanning atomic force microscopy has been used previously to establish a direct correlation between a particular structure surface inhomogeneity and the initiation of local laser damage at that inhomogeneity, as in the case of nodular defects in coatings. Recent development in near field scanning optical microscopy shows that this technique can also provide information on both morphology and optical properties of localized defects.

Effect of laser-induced damage on the National Ignition Facility optical design (Abstract Only)

Jeffrey A. Paisner

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The National Ignition Facility for Inertial Confinement Fusion (NIF), now under construction at LLNL, will contain a neodymium glass laser system with more than 7000 large optical components capable of irradiating fusion targets with shaped laser pulses of up to 500TW and 1.8 MJ at 351 nm. The laser must operate at the highest feasible fluence to control the cost of the facility, so laser-induced damage and the mitigation of damage are major drivers in the optical design. Cleanliness is essential to reduce damage and obscurations, so optical components for the facility will be transferred from clean assembly areas to the laser bays in sealed clean containers that will insert these optics into the laser without exposing them to contamination from the building environment. 'Ghost' or stray light beams can be a source of damage, and the identification and analysis of both linear and nonlinear ghosts and their interactions with other laser components has been a major part of the optical design effort. Damage and nonlinear effects at 351 nm are much more challenging than at the 1053 nm laser fundamental frequency, and this has had a major effect on the design of the final optics at the target chamber. The vacuum barrier at the target chamber will be at 1053 nm to improve laser performance and increase safety, and the 351-nm components most likely to damage will be easily removable for servicing. The presentation will cover these and other features of the design, and our strategy for procuring these topics with high damage thresholds.

Comparative test of bulk laser-induced damage of experimental crystals at 0.85 um

Oleg M. Efimov, Leonid B. Glebov, Dennis A. Hammons, et al.

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Comparative tests of bulk laser-induced damage of some experimental crystals having prospects for laser applications were performed for evaluation of exploitation reliability of elements made from these crystals. A number of fluoride and oxide crystals doped with ions of Ce, Cr, Ti, and Zr were studied in these measurements. BK7 glass was tested under the same experimental conditions as a comparison. Pulses from a single transverse mode, Cr: LiSAF laser (? = 0.85 tm, t 60 ns) were used in these experiments. The laser radiation was focused to a small spot size (2 tm) in the bulk of these crystals to exclude the influence of inclusions and inhomogeneities on the results. Absorption spectra of these crystals were measured to be sure that there is no strong absorption in the region of the irradiating wavelength. The possible effects of dopant ions on the laser-induced damage thresholds of the crystals studied is discussed.

Brief history of laser-induced breakdown (Abstract Only)

Nicolaas Bloembergen

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Breakdown of air by a focused pulse from a Q-switched ruby laser was first observed in 1962. This marked the beginning of studies on laser-generated plasmas and of laser-induced breakdown in solids, liquids and gases. A historical overview will be presented of major developments in the basic physical processes involved, including multiphoton ionization and avalanche breakdown. Recent experiments on plasma creation and damage induced by femtosecond pluses have thrown new light on this subject, which continues to be of great technical interest.

New materials design for laser hardening applications (Abstract Only)

Ruth Pachter

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New materials with fast non-linear optical (NLO) response over broad spectral bandwidths, macromolecular structures for optical switching and advanced absorbing dyes, which are critical for laser eye and sensor protection, are explored. The design of such novel materials requires that material properties are carefully optimized, for example, to achieve large NLO coefficients, high conjugation, appropriate ground and excited state absorption, as well as exhibit defined structural motifs, folding patterns and properties, especially when derivatized with NLO chromophores. A comparative study of porphyrins with meso-acetylene substituents carried out with the local density functional approximation, the gradient-corrected DFT, and HF calculations, is discussed in detail. The calculated electronic structures clearly show that the acetylene group contributes to the it-electron conjugation along the porphyrin ring, being consistent with experimental results. Recent calculations on variety of porphyrins are also reported. For reliable properties predictions, a fundamental understanding of solvent effects is important, and large molecules have to be modeled from first principles. We report calculations of interactions with the effective-fragment potential (EFP) method, where solvent molecules are placed around a solute to generate correctly the first solvation cell within an ab initio framework, using the parallelized GAMESS code, providing insight and good agreement with experiment. For example, we correctly predict the change in the most stable isomer for glutamic acid from the neutral form (gas-phase) to the zwitterionic form (in solution), by using the EFP method with a larger number of solvent fragments. A recent implementation of simulated annealing within EFP to optimize the positions of the fragment molecular systems is discussed. The development of global optimization techniques for determining energy minima of complex macromolecules is described. In addition, simulations to evaluate the properties of a polymer dispersed liquid crystal system of interest are discussed. The utilization of this approach to the study of liquid crystalline materials is described. These challenge applications in computational chemistry and materials science enable us to derive crucial structure-to-property relationships for candidate materials and enhance the capability for 'real materials' design and atomic-scale control, all of which utilize extensively scalable high-performance computing.

Quality of optical components and systems for laser applications

Rainer G. Schuhmann

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The requirements for optical components and systems which are used for laser applications are different and normally stronger compared to standard applications. First of all, the quality is determined by the optical design of components and systems are used for different applications from DUV to NIR. Furthermore, especially in high power systems the properties of the optical elements limit their general use and their life time. For the industrial fabrication of optics in large quantities special production procedure and additional measurements for the characterization of the quality have to be performed. As is well known, the short term as well as the long term stability is limited by the material, the polishing quality and the coating. Therefore toady new standards for specification and testing of absorption, scattering and laser damage are applied in the industrial fabrication. Standardized laser components and typical data of surface quality and damage threshold will be discussed.

Is it opportune to study laser-induced damage of coatings by optical characterization?

Emile P. Pelletier, Francois Flory, Josep Massaneda, et al.

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High quality thin films completely free of defects is the goal for higher damage threshold but it remains a dream. Specific means of characterization to measure absorption losses by photothermal deflection and scattering losses are currently used. The analysis of experimental results permits to understand how the real behavior of thin films can be affected by localized defects. With the development of the mapping technique, the comparison between absorption and scattering intensity maps of the surface of coatings appears to be a good tool for this study. Moreover we need a more basic information concerning the multilayer under study and characterization by guided wave appears to be really fruitful. It permits the determination of the refractive index and the thickness of the different layers. Additional measurements of attenuation of guided waves can be used for the location of the absorption sites in the depth of the coating.

Marathon testing of optical materials for 193-nm lithographic applications

Vladimir Liberman, Mordechai Rothschild, Jan H. C. Sedlacek, et al.

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We have undertaken a systematic evaluation of both bulk material sand optical coatings designed for 193-nm lithographic applications. These studies are performed at realistic fluences and pulse counts in excess of 10⁹. Measurements of absorption is fused silica show a large variation in performance for different samples in both initial and laser-induced absorption. Calcium fluorides samples show less variation in laser-induced absorption and appear to be more stable under irradiation of 0.2-1 billion pulses. Laser-induced densification of fused silica appears to follow an empirical power law; however, an order of magnitude spread in densification is observed among grades. For optical antireflectance coatings, we have characterized the initial 'laser-cleaning' phenomenon for various coatings. We have observed that laser-cleaned coatings deposited on CaF₂ substrates exhibit higher initial optical losses at 193 nm than their counterparts on SiO₂ substrates. However, the losses for coatings on CaF₂ substrates are reduced over irradiation times of 0.2-1 billion pulses to final values comparable to their SiO₂ counterparts. Finally, we have characterized various catastrophic failures of coating material, such as induced losses, adhesion failure and laser-induced thinning.

Measurement and analysis of compaction in fused silica

Douglas C. Allan, Charlene M. Smith, Nicholas F. Borrelli

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Laser irradiation of fused silica produces compaction. Irradiation at photolithographic wavelengths of 248nm or 193nm produces density increase of order parts per millon. While these changes are small, the accompanying increase in index of refraction may be large enough to degrade the performance of photolithographic exposure optics. This paper reports experimental of compaction by interferometry and theoretical analysis of the elastic response of the glass samples to extract the sample-independent unconstrained compaction. The power law behavior of compaction vs. exposure dose is presented, along with a Monte Carlo analysis of error bars on this curve.

Dependence of compaction in fused silica on laser pulsewidth at 248 nm (Abstract Only)

Richard L. Sandstrom, Richard G. Morton, Thomas P. Duffey

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Designers of DUV lithographic lenses are faced with serious materials problems relating to compaction and color-center formation in fused silica, especially at 193 nm. However, these problems, while less sever, are not negligible at 248 nm. Compaction appears to be the more serious, since it degrades imaging performance and effectively sets the lifetime limit for the lens. Previous damage studies have clearly shown that fused silica compacts as a function of the parameter grouping (NI²/(tau) ), where (tau) is the pulsewidth. This fact has strongly influenced the design of the excimer laser light source by stressing repetition rate over pulse energy as a way of achieving high average power, and by driving the optical pulsewidth to be as long as possible. These studies, however, have emphasized the dependence of damage rates on the energy density I(mJ/cm²), whereas the optical pulsewidth (tau) has been given only cursory attention and has not been well controlled during the damage experiments. In this paper we report the results of an experiment to more clearly establish the functional dependency of compaction on laser pulsewidth.

1-kHz 5-W ArF excimer laser for microlithography with highly narrow 0.7-pm bandwidth and issues on durability related to optical damage

Tatsuo Enami, Takeshi Ohta, Hirokazu Tanaka, et al.

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We present the result of billion level durability test in the newest model of ArF excimer laser, KLES-G5A, for microlithography and durability issues relate to optical damage. The developed ArF laser achieves 5 W of output power with integrated energy stability less than +/- 0.3 percent at 1 kHz repetition rate. The spectral characteristic by FWHM of the deconvolved spectrum is less than 0.7 pm and the 95 percent energy of the total energy is within 2.0 pm band, which is designed to be suitable for partially achromatized refractive projection optics. Durability test of 1 billion pulses has already finished, and we have observed very slight changes in the performance. Prior to the durability test, irradiation examinations have been done. It demonstrates that CaF₂ material has superior stability to SiO₂ even as the high fluence DUV optical parts of line-narrowed ArF laser.

DUV laser light sources (Abstract Only)

Sergei V. Govorkov, Robert Willard

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High power laser sources of deep UV radiation play an important role in numerous applications such as microlithography, micromachining and material modification. Usefulness of deep UV lasers for each specific application arises from their unique characteristics, most importantly wavelength, temporal and spatial coherence and intensity distribution of the beam. Recent progress in the areas of DUV optical materials, laser resonators and electronics made possible dramatic improvements in power levels, brightness and intensity uniformity at these extremely short wavelengths, combined with long term stability and reliability. We report on recent advances in development of high power excimer lasers at the wavelengths of 248 nm, 1 93 nm and 1 57 nm for applications requiring high spectral brightness, such as microlithography, and high spatial and temporal coherence, such as writing of fiber Bragg gratings. We also present new data on high power all-solid-state laser sources of 266 nm, 213 nm and tunable UV radiation which are becoming a real alternative to excimer lasers in some DUV applications. We discuss impact of the long-term degradation of optical elements on the performance, reliability and running costs of recently developed DUV lasers

Characteristics of deep-UV optics at 193 nm and 157 nm

Glen P. Callahan, Bruce K. Flint

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The increasing use of Argon Fluoride (ArF) 193 nm excimer lasers in microlithography, surgical procedures and other applications have created the requirement for an increasingly larger number of specialized optical components and thin film coatings. At 193nm these include coated components that can withstand long duration exposure to UV radiation without significant change in performance. Similar coatings and components for Fluorine lasers operating at 157 nm are under development. At these deep UV wavelengths, potential coating materials as well as substrates are very limited due to absorption and impurities. Characteristics of various types of thin film coatings including reflective, anti-reflective, beam-splitting, beam attenuating and optical bandpass filters, at both 193nm and 157nm, are measured using a specially designed vacuum spectrophotometer system. Results of these measurements as well as plans for further coating development are presented.

Laser damage of optical coatings from UV to deep UV at 193 nm

Jean Dijon, Etienne Quesnel, Catherine Pelle, et al.

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Scanning electron microscope observations off damaged coatings from 355nm to 193nm are reported. From these results it appears that oxide based coating are strongly limited at 193nm while they performed very well at the other wavelength. Damages occurs at the coating surface for oxides while they concern both surface and substrate interfaces with fluoride base coatings. While fluorides have a better threshold than oxide at 193nm they are limited by localized defects. The ability to overcome this limitation with ion beam sputtering fluoride material at 193nm starting from results at 355nm.

Coated optics for DUV excimer laser applications (Abstract Only)

Jens Ullmann, Michael Mertin, Hans Lauth, et al.

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Recent developments of DUV-excimer laser applications have gained in demands for radiation resistant coated components at interesting wavelengths. To meet the requirements of long term reliability and high pulse number throughput a superior performance of the optical components with lowest absorption and scattering losses is necessary. Within the framework of the German Joint Research Project "OPUS II" efforts are made to investigate the optical properties, the radiation resistance, and long term stability of single layers and layer systems of interest in the DUV. The evaluation of optical coatings and coating systems (AR and H R) on different substrate materials was carried out by scattering experiments, atomic force microscopy, infrared spectroscopy, calorimetric absorption measurements, determination of laser induced damage threshold (1-on-i, 1 000-on-i) and scratch tests. Additionally from spectrophotometric measurements the optical behaviour of the films was examined.

Laser-induced instability in transparent materials induced by low-absorbing microinclusion

Anastasia S. Gruzdeva, Vitali E. Gruzdev, Vladimir L. Komolov, et al.

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There are reviewed and summarized several theoretical models describing various mechanisms of developing of laser-induced temperature and field instabilities in both absorbing and nonabsorbing microinclusions. The key points are different mechanisms of positive feedbacks between laser-field and matter parameters connected with laser-induced variations of both refraction and absorption. Most attention is paid to application of the models to investigation of laser-induced damage of transparent materials, in particular, evaluation of damage-threshold fluence. General criterion for evaluation of damage threshold is deduced from the presented models are discussed.

Field instability in high-quality dielectric sphere and disk: implication to the problem of laser-induced damage of microsphere and microdisk resonators

Vitali E. Gruzdev, Mikhail N. Libenson

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The problem of initiating of laser-induced damage (LID) of microspherical and microdisk resonators and nature of processes limiting ultimate pump intensity of the resonators are considered. Electrodynamic processes rather than absorption-induced heating are shown to play major role in initiating of LID of ideal passive resonators. Theoretical model to describe local increasing of high-power electromagnetic field in the microresonator is presented to study the processes. It is based on idea of formation of unstable field structure in resonant dielectric microcavity connected with formation of positive feedbacks resulting in light-induced variation of quality factor and its spectrum.

Time-evolved optical properties of liquid crystalline molecules

Kim F. Ferris, Gregory J. Exarhos, Steven M. Risser

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While many studies have focused on the computation of static properties of small organic molecules, dynamic fluctuations can introduce important perturbations to the electronic structure and subsequent optical properties. In this paper, we present results from combined molecular dynamics/quantum mechanical calculations on the linear and nonlinear optical properties of 5-alkyl-cyanobiphenyl molecule. As opposed to the static structure commonly used in computations of optical properties, the dynamic picture requires a number of configurations with unique optical properties. As an example, the torsion angle between the phenyl rings in the optimal static picture is 40.1 degrees, but is better described as a probability distribution in the time dependent problem. Simulations of the nematic phase using periodic boundary conditions further emphasize the importance of these fluctuations on the electronic structure of this system.

Damage in fused-silica spatial-filter lenses on the OMEGA laser system

Amy L. Rigatti, Douglas J. Smith, Ansgar W. Schmid, et al.

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Vacuum surface damage to fused-silica, spatial-filter lenses is the most prevalent laser-damage problem occurring on the OMEGA laser system. Approximately one-half of the stage C- input and output, D-input, E-input, and F-input spatial- filter lenses are currently damaged with millimeter-scale fracture sites. With the establishment of safe operational damage criteria, laser operation has not been impeded. These sol-gel-coated lenses see an average fluence of 2 to 4 J/cm² at 1053 nm/1 ns. Sol-gel coatings on fused-silica glass have small-spot damage thresholds at least a factor of 2 higher than this peak operational fluence. It is now known that the vacuum surface of OMEGA's spatial-filter lenses are contaminated with vacuum pump oils and machine oils used in the manufacture of the tubes; however, development-phase damage tests were conducted on uncontaminated witness samples. Possible explanations for the damage include absorbing defects originating form ablated pinhole materials, contamination nucleated at surface defects on the coating, or subsurface defects from the polishing process. The damage does not correlate with hot spots in the beam, and the possibility of damage from ghost reflections has been eliminated. Experiments have been initiated to investigate the long-term benefits of ion etching to remove subsurface damage and to replace sol-gel layers by dielectric oxide coatings, which do not degrade with oil contamination.

Subpicosecond studies of carrier dynamics in laser-induced breakdown

Fabien Quere, Stephane Guizard, Philippe Martin, et al.

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In this paper, we present measurements of the excited carrier density in various wide band gap oxides irradiated by short laser pulses, at intensities below and above breakdown threshold. This is achieved with the help of time resolved interferometry in the frequency domain, a technique which was successfully used to study the dynamics of photoexcited carriers in insulators. The result obtained in different experimental conditions, distance from the surface, pump intensities and duration, during or after the pump pulse, are discussed and compared to the models recently developed to explain optical breakdown.

Automated damage test facilities for materials development and production optic quality assurance at Lawrence Livermore National Laboratory

Lynn Matthew Sheehan, Sheldon Schwartz, Colin L. Battersby, et al.

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The Laser Program at LLNL has developed automated facilities for damage testing optics up to 1 meter in diameter. The system were developed to characterize the statistical distribution of localized damage performance across large- aperture National Ignition Facility optics. Full aperture testing is a key component of the quality assurance program for several of the optical components. The primary damage testing methods used are R:1 mapping and raster scanning. Automation of these test methods was required to meet the optics manufacturing schedule. The automated activities include control and diagnosis of the damage-test laser beam as well as detection and characterization of damage events.

Production of distributed phase plates using an energetic ion process

Douglas J. Smith, Joy A. Warner, Nelson E. LeBarron, et al.

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Laser-driven implosion experiments require optical phase conversion to create a uniformly irradiated target. Distributed phase plates provide a quasi-random phase front that aids in beam smoothing on the target; however, the DPP must survive the high fluences of the tripled OMEGA beam at 351 nm. The continuous DPP produces higher efficiency and less risk of damage to opposing optics than the previous binary design. DPPs are created by exposing a gray scale pattern in photoresist and then etching the pattern in to silica. Several problems were solved during the development stage of ion etching DPPs. The etch uniformity was limited to less than 6 percent across a 28-cm clear aperture by modeling the 16-cm ion source and erosion characteristics of the photoresist and silica. Surface texturing was linked to overheating of the photoresist by the ion source and was solved by radiant cooling. Near-field defects capable of focusing damage in levels of fluence on downstream optics were created in the photoresist exposure process and were removed after etching. The damage thresholds of the silica surface generally increase after etching is fare is taken to avoid re-sputtering of tooling onto the optics surface. Sixty ion-etched DPPs were installed in December 1997 and, currently, damage has not been observed on the optics.

Spectroscopic studies of pulsed-laser-induced damage sites in heated CaF2 crystals

Brian J. Bozlee, Gregory J. Exarhos, Randy W. Teel

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Based upon previous studies of temperature dependent defect state formation in high energy alpha particle irradiated CaF₂ analogous temperature dependent effects were expected to occur when heated crystals were exposed to Q- switching emission from a pulsed Nd:YAG laser. It was shown in the (alpha) -irradiation work that optimum clustering of calcium atoms to form colloids would occur at a critical temperature. In recent work irradiation of isothermally heated crystals at fluences on the order of 1 J/cm² produced both catastrophic mechanical damage to the crystal as well as localized defect states within the lattice. Below 200 degrees C the mechanical damage was severe, as evidenced by extensive fractures radiating from the damage center along the crystal cleavage planes. However, above 200 degrees C, these fracture lines revealed the presence of CaF₂ lattice forbidden bands associated with calcium colloids in addition to a blue shift of the principal T_2g CaF₂ mode frequency. The T_2g line shift signifies the presence of compressive stress in the CaF₂ lattice induced by the presence of the calcium colloids. Forbidden phonon line intensities varied with Raman probe wavelength, and the associated Raman excitation profile followed the wavelength response of the optical absorption band at 580 nm, which is ascribed to colloid resonance absorption. The presence of colloid was detected at all temperatures, and the extent of colloid formation was found to be only weakly dependent upon the crystal temperature. This lack of sensitivity may be ascribed to highly located heating of the crystal surface during the laser pulse, which could supersede the effect of ambient temperature. Pulsed Raman studies of thin CaF₂ section sat laser fluences just below the damage thresholds did indeed indicate temperature excursions within the laser footprint. For samples held at temperatures above 200 degrees C, the color centers were found to absorb more strongly in the 400-500 nm and NIR regions, Probably indicating greater aggregation of F centers in the crystal.

Dependence of the HfO2 thin film structure on the momentum transfer in ion-beam-assisted deposition

Salvatore Scaglione, Francesca Sarto, Antonella Rizzo, et al.

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Evaporated Afnia thin film suffer of high porosity that causes the variation of the optical parameters when the film is exposed to the atmosphere. The ion beam assisted deposition is a useful method to obtain dense and adherent thin film. In this work, the effect of the (Xe) low energy ion beam assistance on the optical properties of HfO₂ thin film is investigated. The deposition parameters are expressed in terms of momentum transfer per arrival atoms. Dense films were obtained increasing the P value. The Sigmund's model was applied to describe the collision cascade mechanism and to calculate the threshold energy at which no sputtering occurs. The experimental measurements of sputtering yield were compared with the calculated values. Assisting the growing film by Xi ions at energy lower than the energy threshold, a high value of refractive index has been obtained, as well as for the sample assisted at high P values.

Status of the LMJ program in France: effect of increased damage resistant coatings on megajoule-class laser performances (Abstract Only)

Michel L. Andre

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The performance of high-power and high-energy solid state lasers have been, since the very beginning, limited by laser damage on optics and particular antireflection and mirror coatings. In 1972 damage threshold of 1 to 2 J/cm² at 1 nanosecond were usual numbers; in 1986 with high-energy lasers like Nova at LLNL and Phebus at CEA, pick powers reached 6 to 8 J/cm² and now, for new projects like NIF in the US and LMJ in France, the goal is in the range of 20 to 25 J/cm².

Comparison of production techniques for silicon oxynitride rugates and their effect on laser damage thresholds

Keith L. Lewis, Richard Blacker, M. Corbett, et al.

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A great deal of interest has been shown in the potential of rugate designs for the realization of dielectric mirrors with enhanced resistance to laser induced damage. Some of the benefits are arguably a result of the fact that the mirrors are essentially composed of inhomogeneous materials, with the effective refractive index continuously varying between the two limits defined by the particular design selected. A study has been carried out of the potential of three different processes for the fabrication of mirrors using a 40 period silicon oxynitride mirror design for 532nm, with design goals of R equals 99.9 percent and bandwidth close to 10 percent. The study has compared reactive magnetron sputtering, ion assisted deposition and microwave plasma CVD. Conventional quarter wave stacks were also produced for use at the same wavelength, with the same number of periods to act as a reference. Laser damage thresholds were measured using a Q-switched YAG laser at both the resonance band of the stack and off-resonance at 1064nm. The results highlight the role of mirror design, process characteristics and film morphology.

High-index oxides for UV applications (Abstract Only)

John Simpson, Andrew M. Pitt

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Hafnia is the high index material of choice for UV optical coatings. However not all UV devices need the full window of transparency of hafnia and some filter and mirror designs could benefit from the availability of higher index materials. One example is the demanding requirement for bandpass filters operating in the 260-300nm region of the UV where devices are required with out of band rejection levels to greater than OD 10. Alloying hafnia with a higher index material would be expected to produce a thin film of some intermediate index, where the refractive index would primarily be determined by the relative content of the two oxides whether or not the two components of the alloy are miscible. In the former case a homogeneous material would result while in the latter case the product would be a nanocomposite material with a precipitate of one phase in a host largely comprising the other component. In the case of immiscible mixtures, the absorption edge of the composite will be governed by the high index component in substantially pure form. On the other hand, when solid solutions are formed, the bandgap will depend on composition.

Laser megajoule optical components damage threshold measurements with a large-size beamlet laser (Abstract Only)

Anne Fornier, Philippe Feru, B. Pinot, et al.

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Large components are needed to build up the Laser Megajoule which belongs to the future generation of fusion laser. It is essential to determine the laser damage threshold of full-size components with a large-size laser beam, and to compare this value to the threshold obtained on small samples with a small gaussian laser beam. So, we have, in the first place, tested some small samples from the same coating run on REOSC, CEA and LLNL facilities and, in a second series of experiments, we have tested three full-size optical components on the LLNL beamlet laser. Two types of optical components have been tested on the beamlet: - one polarizer: 710 X 400 nm; - two HfO₂/SiO₂ mirrors, from the same coating run: 620 X 440 mm. Before beamlet irradiation these components have been continued at 10 ns on the LLNL Plato facility. Concerning the mirrors, two different conditioning procedures have been applied: - on the first mirror a 5-step conditioning procedure; - on the second mirror a 2-step conditioning procedure. Then, each mirror has undergone the same test on the beamlet. In this paper we report the result obtained on the polarizer and the mirrors after beamlet irradiation.

Predicting bulk damage in NIF triple harmonic generators

Michael J. Runkel, W. H. Williams, James J. De Yoreo

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Recently reported experiments have investigated the statistics of laser damage in KDP and KD*P. Automated damage tests have allowed cumulative failure and damage probability distributions to be constructed. Large area tests have investigated the feasibility of on-line laser conditioning and damage evolution for tripler harmonic generation crystal on the NIF. These test have shown that there is a nonzero probability of damage at NIF redline fluence and that the damage pinpoint density evolves exponentially with fluence.

Monte Carlo simulation of the R/1 automated damage test

Michael J. Runkel

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In this paper, a Monte Carlo computer analysis of the R/1 automated damage test procedure currently in use at LLNL is presented. This study was undertaken to quantify the intrinsic sampling errors of the R/1 ADT method for various types of optical materials, particularly KDP and fused silica, and to provide a recommended minimum number of test sites. A gaussian/normal distribution of 10 J/cm² average fluence was used as a damage distribution model. The standard deviation of the distribution was varied to control its shape. Distributions were simulated which correspond to the damage distribution of KDP and fused silica. A measure of the variability in test results was obtained by random sampling of these distributions and construction of the cumulative failure probability 'S' curves. The random samplings were performed in runs of 100 'tests' with the number of samples per test ranging from 2 to 500. For distributions with (mu) /(sigma) equals 5-10, the study found an intrinsic error of 3 to 5 percent in the maximum deviation from the distribution average when using 100 sites test. The computations also showed substantial variation in the form of the CFD for any given test.

Configuration optimization and sensitivity comparison among thermal lens, photothermal deflection, and interference detection techniques

Bincheng Li, Eberhard Welsch

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Photothermal techniques, such as probe beam deflection and thermal lens detection, have been widely used for low absorption measurement, thermal characterization, and laser- induced damage detection of optical coatings. In specially configured photothermal techniques, the probe beam either detects the photothermally induced refractive index change inside the sample via propagation through the interacting region in the measured sample, or detects the surface displacement via reflection from the deformed surface. Usually, due to the very low absorption of the sample or/and the short interaction length, a very high sensitivity is required for such applications. It is therefore of importance to maximize the sensitivity for each measurement, by selecting appropriate detection scheme and optimizing the performance of the selected scheme. In this paper, we first maximize the sensitivity of these photothermal techniques by configuration optimization, then compare their maximum sensitivity. The applicability of the pulsed photothermal techniques to optical coating characterization is also discussed.

Ultraviolet laser damage and optical properties of thin films deposited by reactive-rf-magnetron sputtering (Abstract Only)

Kunio Yoshida, Takeshi Kamiya, Noriaki Tochio, et al.

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Optical coatings deposited by conventional evaporation processes have a number of shortcomings including weak adhesion which depends upon the subsurface condition of substrate and the change of spectral characteristics in the relative humidity. Thin films deposited by reactive-rf- magnetron sputtering improve these problems. Laser-induced damage thresholds at 355nm, measured by 1-on-1 and R-on-1 methods, of substrate and single high index zirconia, hafnia and aluminum oxide, and low-index silicon dioxide coated by magnetron sputtering, are reported.

Laser-induced damage measurement: problems of scaling

R. M. Wood

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Laser Induced damage threshold measurements have been made over the last 30 years. However it is still not easy to predict the LIDTs ofmaterials and components from measurements made at different wavelengths, spot sizes and pulse lengths. In practice this leads to unwanted damage, overspecified components and/or under utilised capacity. In certain circumstances this can also lead to safety hazards. This paper will summarise the understanding gained from the research done over the years. It will emphasise that it is necessary to ascertain the mechanisms leading to damage before scaling can be applied. Suggestions as to the direction of thture research will be made.

Manufacture, optical performance, and laser damage characteristics of diffractive optics for the National Ignition Facility

Jerald A. Britten, Steve M. Herman, Leslie J. Summers, et al.

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We have fabricated demonstration diffractive optic plates at full scale for the NIF laser. These include an off-axis focusing beam sampling grating, a color separation grating, and a kinoform phase plate for spatial beam smoothing. Fabrication methods and optical performance of these DOPs are discussed. IT was discovered that the sol-gel antireflective coating normally applied to high-power transmissive optics partially planarizes the diffractive structures, particularly on the color separation grating used for color management at target, to the extent that optical performance and laser damage threshold are negatively impacted. The effect of sol-gel coatings on grating performance, the feasibility of placing all diffractive structures on a single surface, and future work in this area are discussed.

Protected silver coatings for flashlamp-pumped Nd:glass amplifiers

Norman L. Thomas, Alvin C. Erlandson, Joseph C. Farmer, et al.

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A durable protected silver coating was designed and fabricated for possible use on flashlamp reflectors in the National Ignition Facility to avoid tarnishing under corrosive conditions and intense visible light. This coating provides a valuable alternative for mirror coatings where high reflectance and durability are important requirements. This paper describes a protected silver coating having high reflectance from 400 nm to 10,000 nm. The specular reflectance is between 95 percent and 98 percent in the visible region and 98 percent or better in the IR region.

What is laser conditioning: a review focused on dielectric multilayers

Herve Bercegol

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Laser conditioning is a phenomenon which has been much explore during the last 15 years. Many optical components for high power laser have shown enhancement of their laser- induced threshold after under-threshold pre-irradiation. The improvement brought by laser conditioning is though to be necessary in the building of new NIF and LMF lasers, especially for dielectric multilayers and KDP crystals. However, we lack a complete physical explanation for the observations of conditioning. In particular, we do not know the effect of most physical parameters like pulselength. wavelength, repetition frequency of the laser shot. In this tentative review, we first gather results relevant to the general problem of laser conditioning, especially on dielectric multilayers irradiated with nanosecond pulselength. We find necessary to come back to the basic notion of damage and threshold. As a definition basis, we establish that laser conditioning is a damage with minor optical consequence and no continuing damaging with successive testing. This way of presenting the phenomenon contains a non-threshold description of optical damage. We finally explore some specific questions, that are technologically important for LMJ and NIF construction. Particularly, is laser conditioning necessary once we admit the occurrence of small damages, that is with a functional definition of damage threshold.

Laser-Induced Damage in Optical Materials: 1998

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