The paper proposes to review briefly steps of classical experimental progress towards resistant VUV-XUV coatings. It intends to address some of the new challenges of the VUV-XUV radiation resistant coatings, including material investigations, manufacturing, characterizations and active optical components.

The complete evaluation in the selection of coating designs for production of high performance mirrors must include the scattering losses that are associated with the thin film materials combination. This is especially true for deep ultraviolet [DUV] microlithography applications. Scattering loss data are presented at 193 nm for various coating material designs for operation in argon fluoride laser systems. For overall optimum performance tradeoffs of the spectral reflectance, environmental stability and pulsed laser irradiation lifetime survivability is also discussed.

Noncontact optical profilometry was used to characterize the surface flaws on cavity mirrors used in the IR Upgrade FEL at the Tomas Jefferson National Accelerator Facility (JLab). The FEL exposes the cavity mirrors, which have multilayer dielectric coatings, to a unique pulse format. To date, when lasing at 6 mirons at a PRF of 37.4 MHz, the circulating cw power is in excess of 100 kW, the peak cw irradiance exceeds 30 kW/cm², and the peak irradiance of each pulse is of order 1 GW/cm². While state-of-the-art, these coatings are far from defect-free, yet have survived those operating conditions without damage after hours of use. The use of noncontact profilometry and the latest software allows us to characterize the size, depth, and distribution of defects in the area covered by the beam footprint in a way that is far more useful than a scratch-dig value. These data provide benchmarks for what defects can be tolerated for lasers having similar irradiances.

HfO₂/SiO₂ and ZrO₂/SiO₂ high reflectors at 1.064 microns were deposited by pulsed reactive DC magnetron sputtering. These dielectric thin film high reflectors were deposited with and without the use of an electron source. The electron source greatly decreased arcing of the magnetrons during the deposition process resulting in thin films with fewer defects. The high reflectors were laser damage tested at 1.064 microns. The optical properties of the thin film coatings were characterized prior to laser damage testing. Optical characterization techniques included angle resolved scatter (BRDF), total integrated scatter (TIS), and adiabatic calorimetry. The dependence of the laser damage threshold and optical properties on deposition conditions is reported.

The next generation of high-energy petawatt (HEPW)-class lasers will utilize multilayer dielectric diffraction gratings for pulse compression due to their high efficiency and high damage threshold for picosecond pulses. We have developed a short-pulse damage test station for accurate determination of the damage threshold of the optics used on future HEPW lasers. The design and performance of the damage test laser source, based on a highly stable, high-beam-quality optical parametric chirped-pulse amplifier, is presented. Our short-pulse damage measurement methodology and results are discussed. The damage initiation is attributed to multiphoton-induced avalanche ionization, strongly dependent on the electric field enhancement in the grating groove structure and surface defects. Measurement results of the dependence of damage threshold on the pulse width, angular dependence of damage threshold of diffraction gratings, and an investigation of short-pulse conditioning effects are presented. We report record >4 J/cm² right section surface damage thresholds obtained on multilayer dielectric diffraction gratings at 76.5° incidence angles for 10-ps pulses.

Though light scattering has been extensively studied these last decades, it may still provide new and unique tools to probe optical materials and components, provided that some inverse problems can be solved. A brief summary of advances in this field are here given.

Atomic force microscopy was employed to investigate the morphology of UV, nanosecond-pulsed-laser damage in SiO₂ thin films. Gold nanoparticles, 18.5 nm in diameter and embedded in the film, were used as calibrated absorbing defects. Damage-crater diameter, depth, volume, and cross-sectional profiles were measured as a function of laser fluence and the lodging depth of gold nanoparticles. The results indicate that at laser fluences close to the crater-formation threshold and a lodging depth of a few particle diameters, the dominating regime of the material removal is melting and evaporation. The morphology of craters initiated by deep absorbing defects, with a lodging depth larger than ~10 particle diameters, clearly points to the dominating role of a shock-wave-induced material-removal mechanism. Crater-diameter variation with lodging depth and laser fluence is compared with theoretical predictions.

A major issue in high power lasers for fusion is laser-induced damage on optics and its evolution in time after a large number of shots. Since damage is often characterized by an initial surface crack, its surface usually increases, following an exponential law. Surface scratches have been made on silica samples in order to get calibrated fractures. Then, to test different experimental conditions, we made a variety of scratches in terms of length and depth. The samples are then irradiated by a Nd:YAG laser first at 1064 nm (1w) then at 355 nm (3w). They are successively tested with the scratches facing the laser beam or placed with the scratches on the back surface.

We describe a system to inspect and remove surface debris in-situ from the surfaces of upward-facing mirrors that transport 1053 nm laser light to the target chamber of the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory. Grazing angle (2-5°) illumination with a bar light highlights debris ≈10 mm in size and larger, which is then viewed through windows in the enclosures of selected mirrors. Debris is removed with 1-second bursts of high velocity (76 m/s) clean air delivered across the optic surfaces by a commercially available linear nozzle ("gas knife"). Experiments with aluminum, stainless steel, glass and polystyrene particles of various sizes >30 mm show that particle removal efficiency is near 100% over most of the mirror surfaces for all sizes tested.

Femtosecond ablation has several distinct advantages: the threshold energy fluence for the onset of damage and ablation is orders of magnitude less than for traditional nanosecond laser machining, and by virtue of the rapid material removal of approximately an optical penetration depth per pulse, femtosecond machined cuts can be cleaner and more precise than those made with traditional nanosecond or longer pulse lasers. However, in many materials of interest, especially metals, this limits ablation rates to 10-100 nm/pulse. We present the results of using multiple pulse bursts to significantly increase the per-burst ablation rate compared to a single pulse with the same integrated energy, while keeping the peak intensity of each individual pulse below the air ionization limit. Femtosecond ablation with pulses centered at 800-nm having integrated energy of up to 30 mJ per pulse incident upon thin gold films was measured via resonance frequency shifts in a gold-electrode-coated quartz-crystal oscillator. Measurements were performed using Michelson-interferometer-based burst generators, with up to 2 ns pulse separations, as well as pulse shaping by programmable acousto-optic dispersive filter (Dazzler from FastLite) with up to 2 ps pulse separations.

A comparison of laser induced damage thresholds (LIDT) of ion assisted deposition (IAD) and standard electron beam deposition dielectric coatings on BK7 glass with different surface roughness was performed. Five types of high reflectance mirrors at 800 nm and two types of high reflectance mirrors at 1064 nm were tested. Mirror coatings were made of ZrO₂ and SiO₂. Automated LIDT measurements were performed according to the requirements of current ISO 11254-2 standard. Two lasers were used for the measurements: Nd:YAG (l = 1064 nm, t = 13 ns) and Ti:Sapphire (l = 800 nm, t = 130 fs ). All measurements were performed at 1-kHz pulse repetition rate (S-on-1 test). A fixed spot size was used for each laser. For 1064 nm it was ~ 70 um and for 800 nm ~ 500 um. The damage morphology and structure of coatings were characterized by an atomic force microscopy (AFM), Nomarski microscopy and X-ray diffraction (XRD).

The Ligne d’Integration Laser (LIL) is a prototype installation at scale 1 of one of the 30 lasers of future Laser Mega Joule. It is intended to validate technological choices made for LMJ and to prepare its exploitation. The facility will contain nearly 10.000 optics and over 4000 m² of mirrors. Cleanliness will be an essential matter in the facility since contamination of optics can reduce their laser damage threshold. Hence, airborne molecular contamination (AMC) has been sampled near optics in strategic places of the LIL. These samplings have shown high levels of organic compounds, notably in the amplifying section, which is expected to be the most sensitive part in the LIL. Suspecting a local source of contamination, outgassing tests of typical materials constituting the amplifying section had been carried out. Among them, one sealing material has been identified as a source of organic contamination near the optics. Effects of this pollution have been investigated by a measurement of laser damage threshold after intentional contamination of optics. This work shows the complexity of the outgassing contamination issue, since several steps are necessary to evaluate the effects of this contamination on optical surfaces: air samplings, identification of sources, outgassing tests, intentional contamination of optics and finally measurement of laser damage threshold.

Several tests were completed to measure the laser-damage threshold of coated optics processed through different cleaning methods. Initial results indicate that the mechanical-scrub cleaning step is critical to high-laser-damage performance.

A technique for localised damage repair of fused silica optical surfaces has been investigated. The study reports the use of a CO₂ laser system at 10.6μm wavelength with 50&mum spot diameter (measured at 1/e2) and pulse duration ranging from 50μs to 200ms. Data of the threshold axial irradiance for the onset of measurable mass loss were produced and compared with heat flow calculations based in "hot" properties of silica, showing a changeover from predominantly 1-d cooling below 300µs to quasi-steady-state 2-d cooling beyond 1ms. Typically, irradiances of about 90% of the threshold for mass loss are then used. Surface melt spots generated with a single laser pulse are found to produce measurable cleaning of the initial polishing swirls and light scratches (~tens of nm deep) at all pulse lengths investigated. A reproducible reference scratch of 1.5μm width and 100-200nm depth made by diamond scribing has been used to simulate smoothing or closing of crack-like features. To fully remove the test scratch requires multiple applications of long pulses. Finally, smoothing of the groups of micron-size surface pits caused by optical damage has been obtained, removing significantly the relative amplitude at high frequencies of the fast Fourier transform with a lower limit of 200 cycles/mm for the 50μm spotsize.

A series of laser optics damaged in the presence of molecular contaminants were analyzed. The properties of the base fused silica was determined to undergo changes with lifetimes of greater than two months. The magnitude of the changes is significantly greater than predicted as the maximum limit for this type of change. The changes in the fused silica provide significant insight into the behavior of silica in high intensity laser optical systems.

The IR Upgrade FEL at the Thomas Jefferson National Accelerator Facility (JLab) was used to make measurements of the absorption in laser cavity mirrors, both high reflectors and outcouplers. Measurements were made at 10, 6, and 3 um, by determining the temperature rise of the cooling water of the FEL cavity mirrors while operating at high average power, and by using a laser vacuum calorimeter and interpreting the data using the ISO 11551 standard.

A new facility has been designed to enhance laser damage resistance at 351 nm of large scale 3w KDP and silica optics by laser treatment. This facility is a prototype, and the process will be industrialized as a means of fabrication of the LMJ optics. The first step of the process is a conditioning/initiation step, which consists of a UV laser raster scan of the whole optics; the second step is a step of detection and analysis of damage possibly initiated during the previous step; the third step is a mitigation step, which consists of a local melting of the detected damage on silica surface, in order to stop their growth. The facility is equipped with a 3w Nd:YAG laser allowing the process of both KDP and silica. A CO₂ laser is used for damage mitigation. Both the lifetime increase and the reduction of the process duration of large scale optics have been taken into account with a view to industrialize the process.

The laser-induced damage threshold of polished fused silica surfaces is much lower than the damage threshod of its bulk. It is well known that contaminations of polished surface are one of the causes of low threshold of laser-induced surface damage. Particularly, polishing contamination such as cerium dioxide (CeO₂) compound used in optical polishing process is embedded inside the surface layer, and cannot be removed by conventional cleaning. For the enhancement of surface damage resistance, various surface treatments have been applied to the removal of embedded polishing compound. In this paper, we propose a new method using slective chemical removal with high-temperature sulfuric acid (H₂SO₄). Sulfuric acid could dissolve only CeO₂ from the fused silica surface. The surface roughness of fused silica treated H₂SO₄ was kept through the treatment process. At the wavelength of 355 nm, the surface damage threshold was drastically improved to the nearly same as bulk quality. However, the effect of our treatment was not observed at the wavelength of 1064 nm. The comparison with our previous results obtained from other surface treatments will be discussed.

In the field of laser-induced surface damage, it has been shown that localized re-fusion of silica can be used as a mean to mitigate the damage and therefore stop its growth before the use of the optical component is impaired. In this paper, this localized re-fusion was produced using a continuous CO₂ laser. As the damage is reshaped, we observed that a ring of evaporated silica is systematically deposited around the mitigated damage. This evaporated silica is likely to be non-stoechiometric and therefore to present absorption and luminescence properties. Thus we decided to perform photoluminescence measurements in order to analyse the mitigated damages. We performed fluorescence imaging and spectroscopy using 351nm continuous laser excitation. Different experimental conditions were used for the re-fusion process and the consequences on the photoluminescence properties were studied. We also compared these properties to the properties of non-mitigated damages.

To obtain better understanding of particulate contamination, chromium dots (50 x 50 &mu;m²) were deposited on a silica substrate by photolithography. The aim in using this sample is to observe the mechanism of damage initiation that can be attributed to surface contamination of micro-metric size. A Nd:YAG laser irradiated the sample at 1064 nm for different fluences and also different numbers of shots. Several methods were used to characterise the laser effects on the chromium dots and the silica substrate: "Nomarski", "atomic force" and photothermal microscope observations. The laser fluence is found to be the most important parameter for the behaviour of the chromium dots. At low fluence (<1 J/cm²), they become cracked (fractured). At medium fluence (around 1 J/cm²) chromium fusion is reached and chromium oxide appears. Finally at higher fluence (3 J/cm²), although chromium dots are blown off the substrate and small damage to silica occurs on the first shot, the subsequent shots do not lead to a dramatic increase in the damage.

The effective lifetime of a laser optic is limited by both laser-induced damage and the subsequent growth of laser initiated damage sites. We have measured the growth rate of laser-induced damage on polished fused silica surfaces in 10 torr of air at 1053 nm at 10 ns. The data shows exponential growth in the lateral size of the damage site with shot number above a threshold fluence. The size of the initial damage influences the threshold for growth. We will compare the growth rates for input and output surface damage. Possible reasons for the observed growth behavior are discussed.

Progress in the development phases of a new optical material that exhibits very low thermal lensing and robustness against thermal shock will be reported. Material, thermal, and optical properties of the current material formulation, called OFG-04, have been determined on small- and mid-scale samples. Manufacture scale-up to full-scale has been completed and flight windows prepared. Follow-on efforts are now beginning for an optimized formulation that will exhibit even lower bulk absorptance and OPD change with temperature rise.

The recent progresses in optical components manufacturing have permitted to increase strongly the laser-induced damage threshold. However, in high power laser applications, the slightest inhomogeneity of the material can lead to an irreversible breakdown. Considering the difficulty to eliminate the whole defects, it is important to have an accurate tool to exhibit the smallest absorbing centers assumed to be precursors of laser damage. We propose in this paper to describe a non destructive technique based on the photothermal effect induced by local absorbing inhomogeneities in order to detect nano-scale absorbing defects. The purpose will be illustrated by the detection of artificial isolated metallic inclusions of a few ten nanometers in the bulk of transparent substrates and thin-film coatings. The high spatial resolution of detection is obtained thank to a piezolectric 3D stage. Moreover, the photothermal setup coupled with a laser damage facility, permits to follow with high accuracy the evolution of these defects under laser irradiation and determine a pre-damage stage ten times lower than the surface damage.

The new, strong adhesion method has been developed for optical materials to transmit vacuum ultraviolet rays by using silicone oil. Silicone oil (dimethyl siloxane) has the main chain of siloxane bonds like quartz and the side chains of methyl group. By irradiating ultraviolet rays in oxygen atmosphere, the organic silicone oil was photo-oxidized and changed into inorganic glass. The silicone oil was poured into the thin gap between two pieces of silica glass in oxygen atmosphere and irradiated with the Xe₂ excimer lamp. Consequently, the siloxane of the silicone oil was bonded with the O atoms that had been absorbed on the glass surface to form SiO₂. The UV transmittance of the silicone oil was improved by 62%, from 30% before the lamp irradiation to 92% after the 60-minute irradiation. Furthermore, the adhesive strength of the silicone oil was enhanced from 0 kgf/cm² before the irradiation to 180 kgf/cm² after the irradiation. The honeycomb structure board and plane mirrors were adhered with the Xe₂ excimer lamplight and photo-oxidized silicone oil.

In high intensity laser systems, molecular contamination represents a risk to the optics. In most situations, molecular contamination is somewhat of a wild card. It is known that it is not required that the contaminant be condensible to initiate damage within a laser system. It is also known that in many cases materials that pass ASTM E-595, are known to precipitate laser optic damage. What has not been known is why. Methods have been developed for the identification, and potential quantitation of trace material emissions that initiate laser optic damage.

This paper presents data reduction on an experimental set-up that we have recently developed at CESTA, France. It has been implemented to analyze laser-induced damage on optics dedicated to the Megajoule laser project. Our goal is to measure the damage fluence on samples under tests, using a statistical approach on a very large number of sites. The laser-induced damage density is accurately plotted as a function of laser fluence, by measuring the fluence of every single shot during the scan. This improvement of accuracy modifies dramatically the interpretation of the results that can be made, compared to raw data, considering the mean fluence only.

The Optical Sciences Laser (OSL) Upgrade facility, described in last year's proceedings, is a kJ-class, large aperture (100cm²) laser system that can accommodate prototype optical components for large-scale inertial confinement fusion lasers. High-energy operation of such lasers is often limited by damage to the optical components. Recent experiments on the OSL Upgrade facility using fused silica components at 4 J/cm² (351-nm, 3-ns) have created output surface and bulk damage sites that have been correlated to phase objects in the bulk of the material. Optical Path Difference (OPD) measurements of the phase defects indicate the probability of laser-induced damage is strongly dependent on OPD.

We describe a new damage testing approach and instrumentation that provides quantitative measurements of bulk damage versus fluence for several frequencies. A major advantage of this method is that it can simultaneously provide direct information on pinpoint density and size, and beam obscuration. This allows for more accurate evaluation of material performance under operational conditions. Protocols for laser conditioning to improve damage performance can also be easily and rapidly evaluated. This damage testing approach has enabled us to perform complex experiments toward probing the fundamental mechanisms of damage initiation and conditioning.

This article shows the applications of ultrafast light in studying material optical properties and its application for rudimental imaging. Standard methods, when applied to the imaging, can not independently determine the material's thickness and index of refraction. The proposed method is fundamentally different from other imaging such as contrast difference in optical coherent tomography (OCT) or the peak-to-peak intensity ratio as in THz imaging to determine index of refraction and thickness. We show that the application of ultrafast techniques allows simultaneous measurements of material thickness and optical constants in optical precision from transmission measurements. Such finding invites new perspectives in imaging and other applicable disciplines.

The occurrence of filaments in fused silica irradiated by UV laser light is well described by the product of light Intensity by Length of propagation in the material. For a spatially gaussian peak, in the well-known treatment by Marburger et al, this product is predicted to depend upon input power and non linear index. At a wavelength of 355 or 351 nm, the compilation of our past and present measurements give a smaller critical intensity by length product, i.e. a higher non linear index, than previously measured. These values of non linear parameters allow for the prediction of rear surface damage on thick windows. The predictions compare well with damage probability measurements. Even when the intensity is not high enough to generate filaments, self-focusing is still the cause of damage, due to the increase of output intensity and fluence.

An experimental technique has been developed to measure the damage density ρ(Φ) variation with fluence from scatter maps of bulk damage sites in plates of KD₂PO₄ (DKDP) crystals combined with calibrated images of the damaging beam's spatial profile. Unconditioned bulk damage in tripler-cut DKDP crystals has been studied using 351 nm (3ω) light at pulse lengths of 0.055, 0.091, 0.30, 0.86, 2.6, and 10 ns. It is found that there is less scatter due to damage at fixed fluence for longer pulse lengths. The results also show that for all the pulse lengths the scatter due to damage is a strong function of the damaging fluence. It is determined that the pulse length scaling for bulk damage scatter in unconditioned DKDP material varies as τ^0.24±0.05 over two orders of magnitude of pulse lengths. The effectiveness of 3ω laser conditioning at pulse lengths of 0.055, 0.096, 0.30, 0.86, 3.5, and 23 ns is analyzed in term of damage density ρ(Φ) at 3ω, 2.6 ns. The 860 ps conditioning to a peak irradiance of 7 GW/cm² had the best performance under 3ω, 2.6 ns testing. It is shown that the optimal conditioning pulse length appears to lies in the range from 0.3 to 1 ns with a low sensitivity of 0.5 J/cm²/ns to the exact pulse length.

The local structure of KH2PO4 crystals (so-called KDP) at laser-induced damage sites created by irradiation with ~3-ns, 355-nm laser pulses is studied by a combination of Raman scattering and photoluminescence spectroscopies. We compare spectra from pristine material, surface and bulk laser-induced damage sites, as well as from KPO3 references. Results show that irradiation with fluences above the laser-induced breakdown threshold leads to stoichiometric changes at surface damage sites but not at bulk damage sites. New spectroscopic features are attributed to dehydration products. For the laser irradiation conditions used in this study, the decomposed near-surface layer absorbs photons at ~3.4 eV (364 nm). These results may help explain the recently reported observation that surface laser damage sites in KDP crystals tend to grow with subsequent exposure to high-power laser pulses, while bulk damage sites do not.

Since the composite laser media using single crystal such as Nd:YAG and undoped YAG was reported in 1998[1], the composite with various structures provided to experimental or industrial field. However, conventional bonding condition is not enough for laser application, and fabrication process is extremely complex and long delivery owing to necessity of polishing and diffusion bonding. Ceramic composite laser gain media having layer by layer and clad-core structure were fabricated successfully for the first time by advanced ceramic processing. Advanced ceramic technology provides a direct formation of composite having complex structure without polishing and diffusion bonding. The bonding condition of ceramic composite was confirmed to almost optically perfect, so we could oscillate successfully using composite laser media.

Photolithography is a key technolgoy for the production of semiconductor devices. It supports the continuing trend towards higher integration density of microelectronic devices. The material used in the optics of lithography tools has to be of extremely high quality to ensure the high demand of the imaging. Due to its properties CaF2 is a material of choice for the application in lithography systems. Because of the compexity of the lithography tools single lenses or lens system modules cannot be replaced. Therefore the lens material has to last the full lifetime of the tool without major degradation. According to the roadmap for next generation of optical lithography tools, like immersion lithography, the requirements of CaF2 for radiation hardness are increasing considerably. We will present a detailed analysis of the key factors influencing the laser hardness covering the complete production chain. Some aspects of the evaluation methods for testing CaF2 laser durability will be presented.

The ceramic form of yttrium aluminum garnet (YAG) was studied to determine its suitability for high power lasers. The high Nd³⁺ doping, the large material size, and the variable doping level with position in the sample, all achievable in ceramics as opposed to single crystal, may lead to higher power solid state lasers than those currently available. We have compared the optical properties of ceramic YAG doped with 0-9 at% Nd³⁺ to single-crystal, 1 at% Nd:YAG material. Measurements included scattering, thermo-optic behavior, absorption, fluorescence, and laser damage. Measurements of absorption and emission features showed a small but approximately linear increase in line width with increasing Nd3+ concentration. Nd³⁺ fluorescence lifetime was rapidly concentration quenched with the 240-μs lifetime for the 1 at% material decreasing to 30 μs for the 9 at% ceramic material. Bulk and surface laser damage thresholds were measured for undoped and 1% Nd-doped ceramic YAG samples using ns-duration laser pulses at 1.064 μm. Both bulk and surface damage threshold values were found to be at least as high as that of single crystals. Measurements of the refractive index and thermo-optic coefficients showed no difference between the single crystal and ceramic materials at 1% Nd³⁺ doping levels. The scattering in the ceramic material was less than half that of the single crystal. These results suggest that for most optical characteristics, the ceramic material is equal to and in some cases superior to the single crystal material.

In order to increase the laser induced damage threshold of KDP crystal, a well-known solution consists in a laser conditioning process. In our case, the irradiation of the crystal is performed with an excimer laser XeF (λ = 351 nm, 16 ns). The improvements in laser damage thresholds measured at CEA/CESTA laboratory (Lutin, Yag facility 2.5 ns, parallel beam) and at CEA/Ripault laboratory (Excimer facility 16 ns, focused beams) are different. A possible reason to explain this difference is the depth of focus between both facilities. In order to minimize the influence of limited depth of focus, a solution consits in a multi-plane conditioning process. By means of a local study, it is possible to exhibit with a high accuracy the Laser Induced Damage Threshold (LIDT) in different planes along sample irradiation axis (z-axis). The laser damage threshold is measured locally (8 μm) at 355 nm with a Nd:Yag (pulse duration 7 ns) at Fresnel Institute Marseille. Using the local LIDT measurements, the purpose of this paper is to highlight the depth of focus in the excimer conditioning process. We demonstrate that it is possible to exhibit a local increase in the conditioning gain till a maximum value, measured with the excimer laser.

High quality window/plate shaped CaF₂ single crystals have developed by vertical Bridgman method of with diameters of 50, 100, and 200 mm and 210 mm width, 200mm length. The CaF₂ windows of 30mm diameter obtained from big crystals have evaluated the transmission spectra from 120 nm to 220nm by single beam VUV spectrometer. We could obtain bulk transmission spectra by subtracting surface reflection loss between tow kinds of thickness samples. The results show flat spectra without distinct absorption from 130nm to 220nm. In laser-damage tests with the fourth harmonic (4ω) of a Nd:YAG laser below 1NW/cm² at the CaF₂ window, a stable output of 600mW was obtained. ArF laser irradiation of 50mJ/cm² and 6X10⁴ pulses was tried and the degradation in transmission from 130nm to 220nm not observed. These data show that the radiation hardness of our CaF₂ crystals is promising for deep ultra-violet laser applications.

We perform thne conditioning of various KDP crystals with a XeF excimer laser working at 351 nm. We determine the maximum available excimer laser fluence for conditioning without damage initiation within the crystal. We demonstrate enhancement of the damage resistance with the increase of the cumulative excimer laser fluence. Using the conditioning parameters we show that the damage resistance is also dependent on the crystalline orientation of the KDP samples.

We report the measurements of the linear absorption at 1064, 532, and 355 nm and non linear absorption at 355 nm in samples of KDP crystals fabricated with the rapid growth process developed for NIF and LMJ high power lasers.

Progress in building high-energy, short pulse laser systems with peak powers in the 100 to 1000 TW regime and applying them to plasma physics experiments has highlighted the need for debris mitigation solutions compatible with high intensity pulses [1]. Mitigation schemes ideally need to protect focusing optics for a number of laser pulses at reasonable cost without degrading beam quality. In this paper we describe preliminary experiments performed at the VULCAN laser facility to address some of these issues. The short pulse beam was passed through a thin optical shield at intensities up to 4 x 1012 W/cm². The transmission of the shield was measured as a function of intensity along with the near and far field beam quality. Transmission losses occurred at the highest intensities used and these were related to the start of laser damage of the shield. The morphology of the damage features on the surface and in the bulk material was studied by a combination of white light interferometry as well as optical and scanning electron microscopy.

Previously, we reported preliminary results for commercial thin borosilicate glass sheets evaluated for use as a frequently-replaced optic to separate the radiation and contamination produced by the inertial confinement fusion experiments in the National Ignition Facility target chamber from the expensive precision laser optics which focus and shape the 351-nm laser beam. The goal is identification of low cost substrates that can deliver acceptable beam energy and focal spots to the target. The two parameters that dominate the transmitted beam quality are the transmitted wave front error and 351-nm absorption. Commercial materials and fabrication processes have now been identified which meet the beam energy and focus requirements for all of the missions planned for the National Ignition Facility. We present the first data for use of such an optic on the National Ignition Facility laser.

A statistical model for the interpretation of laser damage probability curves is investigated. In a previous study, shapes and slopes of the curves were related to the spot size and to the densities of nanodefects that are responsible for damage. Each kind of precursors was characterized by its damage threshold. We improved this study when considering a Gaussian distribution of precursor thresholds. Accurate probability curves of laser induced damage (1-on-1, 5-ns single shot at 1.064-&mu;m) are then plotted in bulk and at the surfaces of optical components and fitted with the model including Gaussian distribution of precursor thresholds. Threshold mean value, threshold standard deviation and precursor defect densities are extracted for the different kinds of observed precursors. To illustrate our investigation, we present results achieved on different substrates, which establish a better agreement between theory and experiment.

The fluorocarbon thin film and fused silica glass was bonded for an ArF laser light transmittance by using silicon oil. The chemical main structure of the silicon oil has siloxane chains as in the same structure of quartz. This new bonding method was developed with silicone oil and excimer-lamp in an oxygen atmosphere. The silicone oil was put between the fused silica glass and the fluorocarbon (FEP), and an excimer-lamp was irradiated. The silicon oil ((-O-Si(CH3)-O)n) was photo-dissociated and reacted with the oxygen adsorbed on the silica glass surface to produce a SiO2. On the other hand, the H atoms photo-dissociated from the silicon oil pulled out the F atoms of the FEP. As a result, the FEP and the silica glass were combined. The results showed that the silicon oil changed to silica glass by the excited oxygen, which improved the UV rays under 200nm transmittance.

Using a micro channel plate (MCP), we have developed a sensitive method for measuring the onset of laser damage by detecting liberated surface ions and XUV radiation from the laser induced surface plasma. This method is insensitive to optical alignment and therefore assures good repeatability over numerous measurements.

We report the optimisation of a periodically poled lithium niobate (PPLN) optical parametric oscillator (OPO) pumped by a diode-pumped, Q-switched multiaxial TEM00 mode Nd:YAG laser operated at 1064 nm. Total conversion efficiency exceeding 66 % was achieved. This OPO is used in a coherent spectrophotometer for optical component characterisation.

We present the results of Z-scan studies on a new setup in the sub-picosecond regime (central wavelength 800nm) carried out on solid and liquid materials such as pure water and silica. These measurements are made possible thanks to a high sensitivity setting up of our Z-scan method and in-situ characterizations of the spatio-temporal parameters of the beam. Besides, with the use of a newly adapted numerical simulation, only calibration errors of measurement devices are significant. These measurements are then used to separate the different contributions to the nonlinear refractive index from nanosecond scale mechanisms like electrostriction and/or thermal relaxation.

An edge illumination technique has been designed using a monochromatic light source that improves the identification of surface flaws on optics. The system uses a high-resolution CCD camera to capture images of the optics. Conventional edge illumination methods using white light sources have been plagued by light leaking around the optics causing high background levels. The background combined with lower resolution cameras has made it difficult to determine size and intensity characteristics of the flaws. Thus photographs taken of the optics are difficult to analyze quantitatively and do not allow for the detection of small, faintly illuminated sites. Infrared diodes have been utilized to illuminate large-scale (43 cm x 43 cm) fused silica optics, and a two-dimensional array CCD camera has been used to collect the image data. Flaw sizes as small as ~10 μm have been detected. A set of frames has been built to support the infrared sources where one diode array per side is magnetically attached to the frame. The diodes inject light into the optic causing the sites to illuminate, which can be detected by the camera. A customized mounting design has been implemented to secure the frames to the stage, or base, for image acquisition. The design uses a dual bracket assembly to support the frames. With this design for optical illumination, quantitative data has been obtained of the surface flaws. A comparison of the peak intensity, total integrated intensity and size of the flaws measured in these images and the size of the flaws as measured using a microscope will be presented.

The paper presents results of application of the free particles method for computer modelling of a phenomenon of laser interaction with matter. Alterations made to the method relies on a procedure for modelling of laser radiation absorption and determination of boundary conditions. Computation results for some initial-boundary conditions are presented.

Our paper is devoted to theoretical analysis of mechanisms of laser-induced dmaage of transparent solids by femtosecond laser pulses in single-shot regime. The duration of the pulses is so small that the phonon sub-system practically does not take part in the processes occurring during the direct action of laser pulse. It means that the process of direct damage starts with a certain delay after the laser pulse. We have come to conclusion that it is reasonable to separate out three main stages of the process of macroscopic damage: 1) the direct laser-solid interaction during pulse action including mulitphoton absorption, excitation of the electron subsystem near the material surface and fast leaving of the irradiated area by electrons (e.g., through photoelectron emission); 2) fast after-action including breaking of electrical neutrality in thin near-surface layer and acceleration of ions; 3) slow or delayed after-action including moving of fast ions into bulk accompanied by heating up of the material through collisions resulting in macroscopic thermal damage. In this presentation we focus on the first two stages, i.e., excitation of the electron sub-system, electron emission and development of electrostatic instability often referred to as Coulomb explosion. Estimations performed on the basis of the Keldysh formula show possibility to reach extremely high density of electrons in conduction band (up to 50% of total number of valence electrons) at laser intensity slightly above 10 TW per sq. cm. The electrons can leave the irradiated area before the laser pulse ends. We utilize Keldysh formula to estimate the total number of electrons lost through emission and show the number to be high enough for significant breaking of electrical neutrality and fomration of relatively large positive charge localized in the irradiated area. Assuming the multiphoton ionization to give the dominant contribution to absorption, we estimate the total number of electrons lost through emission and show the number to be high enough for significant breaking of electrical neutrality and formation of relatively large positive charge localized in the irradiated area. Assuming the mulitphoton ionization to give the dominant contribution to absorption, we estimate the characteristic thickness of the ionized layer and show the positive charge to be localized in the layer which is approximately 1 micrometer thick. Then we estimate velocities and energies of ions accelerated by the laser-induced charge and show possibility of appearing ions with MeV energies. The penetration depth characteristic of those ions is an order of 10 micrometers what implies possibility of heating and thermal damage of the material with formation of deep craters.

The results of experimental studies on shock wave propagation in plexiglass are discussed. The wave was generated by a single laser pulse of 3 ns and 30 ns duration and of energy from 2 to 30 J. The characteristics of velocity variations of shock wave front propagation are presented in this paper.

We investigated the crystals with different non-destructive optical diagnostics during the conditioning of KDP with an excimer laser at 351 nm. We measured in the same time the luminescence, the absorption and bulk scattering of the material. These observations pointed out the defects within the crystal. We demonstrated a correlation of the optical signal intensity with the laser damage threshold of KDP.

Pulsed-laser induced shock wave development in fused silica is analyzed using nonlinear wave mechanics and applied to thin-film spallation experiments. Due to the negative nonlinear elasticity of fused silica, a laser-induced Gaussian stress pulse evolves into a shock after travelling a certain distance in a fused silica substrate. Experimental observations confirm theoretical predictions of shock development. A decompression shock forms and greatly enhances interfacial failure of a thin film deposited on the substrate. The effectiveness of this wave mechanism is further demonstrated by the successful application in testing ultra-thin low dielectric film/Si substrate interfaces.

We report experimental observation of modification and damage in bulk of fused silica induced by intense multiple femtosecond pulses. At power levels several times exceeding critical power for self-focusing the modification is in the form of some permanent changes inside the material and these effects are closely correlated with the beam filamentation process. Longer distances of propagation and much higher pulse energies produce bulk damage in the form of scattering zones. The formation of damage in anti-reflective-coated fused silica windows is also reported. Numerical simulations involving self-focusing, multiphoton absorption and permanent change of the refractive index of the bulk material were found to be in agreement with the experimental results.

The aim of our consideration is to clarify the influence of band structure of crystalline solids on ionization rate described in the framework of the Keldysh model. In this connection we compare the Keldysh formula with its analogs obtained for several models of band structure. We clearly demonstrate strong dependence of the ionization rate on band model especially in case of the transitional (from multiphoton to tunneling) and the tunneling regimes. An expression for the adiabatic parameter also depends on particular band model, and we present its new definition relating it to lattice constant, electric-field strength and laser frequency. We demonstrate possibility of a new regime of ionization referred to as collective ionization in which conditions for an effective jump of almost all valence electrons over the forbidden band within one period of field oscillations are provided. We present rigorous estimation of the threshold for that effect and show it to be close to 10 TW per square cm for most transparent materials. The influence of band model on multiphoton ionization rate depends on ratio of band gap to photon energy and is the weakest if the ration is slightly below an integer. In this connection we demonstrate that a proper choice of a band model can explain the well-known difference between calculated and measured values of ionization rate.

Spectral emission from optical breakdown in the bulk of a transparent dielectric contains information about the nature of the breakdown medium. We have made time resolved measurements of the breakdown induced emission caused by nanosecond and femtosecond infrared laser pulses. We previously demonstrated that the emission due to ns pulses is blackbody in nature allowing determination of the fireball temperature and pressure during and after the damage event. The emission due to femtosecond pulse breakdown is not blackbody in nature; two different spectral distributions being noted. In one case, the peak spectral distribution occurs at the second harmonic of the incident radiation, in the other the distribution is broader and flatter and presumably due to continuum generation. The differences between ns and fs breakdown emission can be explained by the differing breakdown region geometries for the two pulse durations. The possibility to use spectral emission as a diagnostic of the emission region morphology will be discussed.

The high instantaneous powers associated with femtosecond lasers can color many nominally transparent materials. Although the excitations responsible for this defect formation occur on subpicosecond time scales, subsequent interactions between the resulting electronic and lattice defects complicate the evolution of color center formation and decay. These interactions must be understood in order to account for the long term behavior of coloration. In this work, we probe the evolution of color centers produced by femtosecond laser radiation in soda lime glass and single crystal sodium chloride on time scales from microseconds to hundreds of seconds. By using an appropriately chosen probe laser focused through the femtosecond laser spot, we can follow the changes in coloration due to individual or multiple femtosecond pulses, and follow the evolution of that coloration for long times after femtosecond laser radiation is terminated. For the soda lime glass, the decay of color centers is well described in terms of bimolecular annihilation reactions between electron and hole centers. Similar processes are also occurring in single crystal sodium chloride. Finally, we report fabrication of permanent periodic patterns in soda lime glass by two time coincident femtosecond laser pulses.

Bulk damage sites in frequency conversion crystals scatter and/or absorb laser light leading to interference and downstream intensification .We find that laser induced bulk damage sites in DKDP exhibit a "shell" of structurally and/or chemically modified material surrounding a central core as indicated by SEM and optical micrographs and micro Raman spectral maps. We hypothesize that the modified material has been shock wave densified and estimate the amount of densification and its effect on scattering. A simple model indicates that densification of several percent is likely and that the scattering cross section may be larger than the geometric area of the inner core by an order of magnitude.

Phenomenological model of photo-elastic effect in bulk dielectrics damaged by femtosecond laser radiation in the regime of direct laser-writing of waveguides is proposed. Positive and negative changes of refraction index due to densification and stretching in laser-written damage tracks inside silica glasses and fluorite, respectively, are explained in terms of laser-induced point defect generation. This model predicts spatial narrowing or broadening of such tracks in silica glass and fluorite bulk samples, respectively, related to spatial point defect distribution due to interactions of carriers with long-wavelength optical and acoustic vibrations as well as due to interactions of point defects with longwavelength acoustic vibrations. Direct laser writing of perspective different contrast microstructures with lower-index boundaries in fluorite and other dielectrics is discussed.

Single-mode (SM) fiber lasers and amplifiers are constrained to low output powers by fundamental physical limitations of the fiber, specifically, by low energy storage and by the onset of nonlinear processes in the fiber. The simplest way to overcome both limiting factors is to increase the core size, but maintaining SM operation imposes an upper limit. Further power scaling is possible with multimode (MM) fiber, but the poor beam quality generally associated MM fiber is unacceptable for most applications. We have developed a technique (bend-loss-induced mode filtering) that allows the core size to be increased significantly beyond the SM limit while maintaining diffraction-limited beam quality and high efficiency. In this method, coiling of the fiber is used as a form of distributed spatial filtering to suppress all but the fundamental mode of a highly MM fiber amplifier. Unlike conventional spatial filtering, in which high-order modes are discarded, the mode-filtering technique does not result is substantial loss of efficiency because high-order modes are suppressed along the entire length of the amplifier and are thus prevented from building up significant intensity. We will review this method and recent experimental results for both cw and pulsed fiber sources, including nonlinear frequency conversion of mode-filtered fiber lasers. Optical damage issues will also be discussed.

The last two years has seen rapid development in the field of High Power Fiber Lasers. Reported output powers have dramatically increased by several orders of magnitude, from 10W to 1KW. While the generic benefits of fiber lasers are well known (e.g. efficiency and compactness) the long term reliability of system components needs to be proven before widespread deployment can occur. In this contribution, we address issues related to high power handling capabilities of fibre optical branching components. Monolithic, fused fiber devices are clearly the component of choice to enable fully integrated, reliable fiber lasers. The required components can be classed in two distinct groups. The first group are single mode devices whose function relies on cladding coupling based on fused viconical tapers and these include tap couplers, wavelength combiners/splitters, and filters. The second group includes multi-mode devices, primarily used for the power combining of multi-mode pump diodes. We describe the latest developments in state-of-the-art fused fiber components for fiber laser systems, including the use of low ratio tap couplers for optical feedback and control in fully integrated fiber laser systems of up to 100W, as well as polarization maintaining (PM) tap couplers and PM wavelength combination/splitting. We further describe the main issues relating to the practical manufacture and reliability performance of those devices for use in high power systems.

Laser damage thresholds of 10 and 20 micron-core diameter solid-core photonic crystal fibres (PCF) and hollow-core photonic band gap (PBG) fibres have been measured. The studies were carried out using a Nd:Yag laser (30nsec pulses at 10Hz), which is optimally coupled into the fibres by careful mode matching, providing a coupling efficiency greater than 90%. It has been shown that the damage threshold of the 10-micron PBG fibre occurs for pulse energies close to 1 mJ, equivalent to a fluence well in excess of 1kJ/cm² propagating down the fibre. This is a factor of 4 larger than the damage threshold of the 10-micron diameter solid-core PCF. However, the damage threshold of the large-core PBG is smaller than that of the PCF. Theoretical modelling based only on the optical modal properties of the single-mode PBG fibre shows that an enhancement by a factor 25 should be obtainable. Thus there are different mechanisms potentially responsible for the fragility of larger core PBG fibres. In an experimental study of bend losses it ahs been found that it is possible to bend the 10-micron PBG fibre up to the breaking point bend radius (less than 1mm). The critical bend radius for the 20-micron PCF. A summary will be presented of the results of the experimental and theoretical studies, highlighting possible reasons for the observed trends for the two different forms of fibre.

In this paper we review the damage mechanisms that need to be considered when building high power fibre lasers. More specifically we look at thermal issues, optically induced coating damage, bulk and surface damage thresholds of the host glass. We also discuss the reliability of tapered fibre bundles and Bragg gratings at these power densities.

Optics damage under high-intensity illumination may be the direct result of laser light interaction with a contaminant on the surface. Contaminants of interest are small particles of the materials of construction of large laser systems and include aluminum, various absorbing glasses, and fused silica. In addition, once a damage site occurs and begins to grow, the ejecta from the growing damage site create contamination on nearby optic surfaces and may initiate damage on these surfaces via a process we call "fratricide." We report on a number of experiments that we have performed on fused silica optics that were deliberately contaminated with materials of interest. The experiments were done using 527-nm light as well as 351-nm light. We have found that many of the contaminant particles are removed by the interaction with the laser and the likelihood of removal and/or damage is a function of both fluence and contaminant size. We have developed an empirical model for damage initiation in the presence of contaminants.

In comparison to studies at longer pulse regimes, investigations of laser induced damage threshold indicate a contrary behavior on the fs-scale for the dielectric coatings. In general, experiments reveal an electronic cause of the damage. The strong correlation of theoretical calculations with experimental data of laser induced damage thresholds for quartz verifies this assumption. Consequently, the characteristic function of the wavelength dependence of the damage threshold differs in this range from the classical behavior. The quantized structure of the electronic transition leads to a typical step function of the LIDT in dependence on the band gap energy of the materials. Hence, the step should be observed between energy levels from n to n+1 electron ionization. In detail, the probability for the transition of the electron from the valence band to the conduction band changes abruptly. In an international cooperation with the University of Vilnius the wavelength dependence of the LIDT was investigated for dielectric coatings of Ti_xSi_1-xO₂ as a function of the stoichiometry. The measurements were performed for a wavelength range from 600 to 800 nm and at a pulse duration of 130 fs by using an OPA laser system. The step from two photon to three photon ionization was measured. The assumption of the mentioned behavior of the fs-damage was proven. For different concentrations of silicon and titanium in the oxide, the electronic structure of the material changes. The experiments have shown an increasing gap energy and LIDT for a high content of silica.

Ultra-short pulse laser systems with high peak power densities are increasingly applied in fundamental and industrial research. Furthermore, these radiation sources are also considered as promising tools for innovative applications in the fields of precise micro-machining and medicine applications. For an improvement of production throughput and economic efficiency, the development of femtosecond laser systems with output powers beyond the actual level of about a few Watts is highly demanded. Further progresses in performance are mainly inhibited by the damage handling capability of laser optical components. A promising strategy for an improvement of present fs-optics is the utilization of high band-gap coating materials. Several investigations in modeling of the damage mechanisms in dielectrics were performed recently. Typically, damage occurs if a critical conduction band population was generated by multi-photon and avalanche-ionization during the initial stage of the ultrashort pulse. Nevertheless, the influence of multi-photon excitation and electron donators (color centers) in the band-gap as sources of initial electrons is still unclear. For studying non-linear absorption effects of dielectric coating materials near the transition wavelengths between two orders of multi-photon absorption, a femtosecond laser system equipped with an optical parametric amplifier was utilized providing ultrashort pulses over a wide wavelength range. The laser-calorimetric measurements indicate a drastic change in the non-linear absorptance behavior for the investigated dielectrics. The results underline the dominant role of multi-photon excitation compared to intra-band electron donators for the generation of conduction band electrons in the case of high performance coatings manufactured by ion beam sputtering.

This research demonstrates that the presence of trace levels of contaminants to an otherwise evacuated system, leads to rapid onset of damage to optical elements in the presence of 1064 nm laser radiation. This onset is observed as a lifetime reduction. Specifically, 1064 nm radiation from a pulsed laser having approximately 800 mJ/cm² average fluence (<1.5 J/cm² peak fluence) illuminated fused silica windows used to seal a vacuum chamber, with a pressure <1.3x10^-1 Pa (<1.0x10^-3 torr). In the absence of any contamination the windows were demonstrated to show no signs of damage up to 2.3x10⁶ laser pulses. When gas phase toluene was introduced into the system at varying concentrations (<1.3 x10^-1 - 41Pa) (<1.0x10^-3 - 3.1x10^-1 torr), the onset of damage was seen to be a function of the toluene concentration, and damage was seen to occur as rapidly as 30,000 laser pulses. This phenomenon was also observed when the windows had a commercially applied coating of MgF₂ on the surface in the vacuum system. Similar experiments using acetone as the contaminant led to no observed damage for either type of optic, even at high concentrations. However, experiments using commercial adhesives, commonly used in spacecraft construction, do provide evidence of lifetime reduction. A discussion of possible mechanisms leading to damage is also included but none has been established from this work.