This PDF file contains the front matter associated with SPIE Proceedings Volume 6720, including the Title Page, Copyright information, Table of Contents, Foreward, and Symposium Welcome, Summary of Meeting, and International Program Committee listing.

In the present work phenomena are considered related to the interaction of ultra-short laser pulses, τ_L~0.1 ps, with metallic targets. The absorption of laser pulse results in formation of thin layer of hot electrons strongly superheated (T_e>>T_i) relative to the ion temperature, T_i. Initial thickness of the layer d_heat is small, d_heat~δ, where δ~10 nm is the skin layer thickness. Subsequent developments include the following stages: (1) Propagation of electron thermal wave which expands the hot layer d_heat; (2) Cooling of electrons due to energy transfer to cold ions; (3) Onset of hydrodynamic motion that constitutes the rarefaction wave with positive pressure; (4) Further expansion of target material leading to the appearance of negative pressure; and (5) Long separation process which begins with nucleation of voids and goes on to the total separation of spallation plate. The thickness of the plate is ~10 nm (we call it nanospallation). Theoretical model involves two-temperature hydrodynamic equations with semiempirical EOS for a metal, electron heat conduction and electron-ion energy exchange. The decay of metastable strongly stretched matter is described by molecular dynamics (MD) simulation with extremely large number of atoms. The experimental setup includes femtosecond chromium-forsterite laser operating in the pump-probe regime. The experiments are performed with gold target. Measured ablation threshold for gold is 1.35 J/cm² of incident pump light at inclination 45°, p-polarization. Calorimeter measurements give for the absorbed fluence F_abs=0.3F_inc, therefore the threshold value of F_abs is 0.4 J/cm².

Numerous experimental and theoretical contributions in the past have stressed the detrimental effect of fractures in the generation of surface laser damage sites in fused silica illuminated at 351 nm. However, two very important steps lack for the moment on the way towards a scientific understanding of the role of fractures. 1. a physical model must be developed to predict damage events starting from real defect sites 2. a reproducible measurement must be obtained and compared with calculations. Here we present the theoretical work realized to reach the first goal. Contrary to previous discussions on fractures, the electromagnetic configuration is calculated in the case of a real material, with electronic surface states, bulk defects, and defects dynamics. Due to electromagnetic field enhancement in the fracture, surface defects absorb a sufficient part of laser energy, able to heat silica above the vaporization temperature. This is the initial event that triggers production of more excited states during the pulse, and steep increase of temperature and pressure fields. Comparisons with available experimental results are positive. Calculated fluences of damage initiation are very near those of measured events on engineered fractures, or on real defects in polished samples.

In this paper we analyze the photo-excitation of electron sub-system of a periodic nano-structure by IR laser radiation. The nano-structure is a 1D super-lattice on surface of dielectric or semiconductor material transparent for the incident radiation. Theoretical description of the photo-excitation is based on the recent modification of the Keldysh theory adapted to the 1D case. We show that two specific regimes of the photo-excitation are possible in the super-lattices: photo-excitation suppression corresponding to decrease of the photo-excitation rate with increasing of laser intensity, and singularity regime corresponding to abrupt increase of the photo-ionization rate. Threshold of the singularity regime is calculated as function of laser wavelength and super-lattice period. The obtained results allow to propose a promising application of the super-lattices as intensity limiters for IR optical systems. In particular, we can calculate the period of the super-lattice to provide limiting of input laser intensity at required level due to multi-photon absorption by electrons of the lattice. Temperature of laser-induced heating resulted from total absorption of an incident laser pulse and diffraction distortions induced by the super-lattice are estimated to confirm possibility of utilizing the super-lattices as the intensity limiters.

Currently, most of our thinking about the defects responsible for initiating laser damage considers them as featureless absorbers. However, an increasing body of evidence, particularly involving multi-wavelength irradiation, suggests electronic structure of damage initiators is important in determining both initiation and conditioning behaviors in KDP. The effective absorption coefficient of energy under multi-wavelength irradiation cannot be accounted for by a structureless absorber, but is consistent with an initiator with a multi-level structure. We outline the evidence and assess the ability of such a simple multi-level model to explain these and other experimentally observed behaviors.

By coupling statistics and heat transfer, we investigate numerically laser-induced KDP crystal damage by multi-gigawatt nanosecond pulses. Our model is based on the heating of nanometric absorbing defects that may cooperate when they are sufficiently aggregated. In such a case, they induce locally a strong increase of temperature that may lead to a subsequent damage. Statistics is used to evaluate the initial defect cluster size distribution. When the crystal is illuminated, by considering in addition heat transfer processes, this approach allows to predict damage probabilities and the evolution of the damaged sites density as a function of the laser fluence. We show that the scaling law exponent, linking the critical laser fluence to its pulse duration, takes a value close to 0.3 departing from the standard 0.5 value that is in a good agreement with recents experiments. Furthermore, these results indicate that absorbers involved in KDP damage may be associated with a collection of planar defects.

In this paper, the nature of the crystalline phases observed at the surface damage sites resulting from laser irradiation is investigated by X-ray diffraction. The results are compared against new data on thermal decomposition of KDP salt. The damage sites consist of polycrystalline KDP and partially dehydrated phases. The comparison with the thermal decomposition study allows to assign a temperature range to the overall temperature reached by the surface during the damaging process. Finally, the difference between surface damage and bulk damage is discussed.

As laser conditioning ever increases the performance of KDP optics with respect to laser induced bulk damage so that it can meet high-power laser specifications, it is expected that surface damage may become the next threat that must be dealt with. This paper presents new data on surface damage initiation and growth at 3w. A surface damage mitigation process based on the ball-dimpling method is presented along with the first results on the behavior of this process with respect to laser irradiation.

We examine the effect of lattice temperature on the probability of surface damage initiation for 355nm, 7ns laser pulses for surface temperatures below the melting point to temperatures well above the melting point of fused silica. At sufficiently high surface temperatures, damage thresholds are dramatically reduced. Our results indicate a temperature activated absorption and support the idea of a lattice temperature threshold of surface damage. From these measurements, we estimate the temperature dependent absorption coefficient for intrinsic silica.

Mitigation of 351nm laser-induced damage sites on fused silica exit surfaces by selective CO₂ treatment has been shown to effectively arrest the exponential growth responsible for limiting the lifetime of optics in high-fluence laser systems. However, the perturbation to the optical surface profile following the mitigation process introduces phase contrast to the beam, causing some amount of downstream intensification with the potential to damage downstream optics. Control of the laser treatment process and measurement of the associated phase modulation is essential to preventing downstream 'fratricide' in damage-mitigated optical systems. In this work we present measurements of the surface morphology, intensification patterns and damage associated with various CO₂ mitigation treatments on fused silica surfaces. Specifically, two components of intensification pattern, one on-axis and another off-axis can lead to damage of downstream optics and are related to rims around the ablation pit left from the mitigation process. It is shown that control of the rim structure around the edge of typical mitigation sites is crucial in preventing damage to downstream optics.

The effect of irradiated spot size and number of sites exposed for each pulse energy or power density on damage possibility is studied. It is shown that larger irradiating spot size and more sites tested for each pulse energy or power density, more accurate damage data could be obtained. Also the effect of defect distribution should be taken into account and it also affects the accuracy of damage threshold determination. A new method, exponential fitting, is described and it yields more accurate damage onset. And it is derived from but suitable to more than all-degeneration model.

The scaling law of subpicosecond laser induced damage (LID) with respect to pulse duration and band gap for Ti_xSi_1-xO₂ composite films is studied. The band gap in these materials can be changed gradually by varying the composition pa-rameter x. Damage is very deterministic and scaling laws with respect to pulse duration and band gap energy derived previously for pure materials are found to apply to composite films. The scaling can be explained theoretically by using a modified Keldysh theory. The composite materials also show a dependence of the damage threshold as a function of pulse number F(N) (incubation) that is similar to observations in pure dielectric oxides. The measured F(N) is explained with a theoretical model that assumes the formation of an intermediate sample state that increases the absorption of sub-sequent pulses in the train.

A Grazing Incidence Metal Mirror (GIMM) is a chief candidate for beam delivery for Inertial Fusion Energy (IFE). The goal for GIMM survival is greater than 3×10⁸ laser pulses with 5 J/cm² laser fluence normal to the incident beam. Laser-induced damage to metal mirrors is primarily a thermomechanical process. Long-term exposure leads to microstructural evolution analogous to fatigue. We have performed laser-induced damage experiments on high damage threshold aluminum mirrors using commercial KrF excimer (248 nm) lasers. We have studied mirror response to standard, 25 ns long-pulses as well as to IFE prototypic, 5 ns short-pulses achieved using a Pockels Cell. Short-pulse damage fluence was found to be better than predicted using simple thermal diffusion scaling from long-pulse results.

The James Webb Space Telescope (JWST) is a large space based astronomical telescope that will operate at cryogenic temperatures in an Earth-Sun L2 Orbit. The architecture has the telescope exposed to space, with a large sun shield providing thermal isolation and protection from direct illumination from the sun. The instruments will have the capability to observe over a spectral range from 0.6 μm to 28 μm wavelengths. The following paper will present an overview of the architecture and describe some of the features of the optical design for the JWST environment.

Campaigns of laser damage tests at 1w of Nd-YAG laser (1064 nm), 3w and with a combination of these two wavelengths, were conducted to complete previous existing data on damage growth in fused silica output surface. It is known that UV light is very effective in inducing preexisting damage craters to grow. When both wavelengths are present, the effect of 1w beam on damage growth depends on the delay between the IR and the UV beam. When the 1w reaches the sample before the 3w, it has nearly no consequence on growth rate. On the opposite, when the IR beam is delayed and strikes the sample after the 3w pulse, its energy simply adds to the UV in enhancing damage growth. Damage initiation is much more affected by 3w than 1w pulses. However, the number of surface damage craters also increased by the addition of 1w photons to the UV beam.

Previous ultraviolet-pulsed, laser-damage studies using model thin films with gold nanoparticles as artificial absorbing defects revealed damage morphology in a form of submicrometer-scaled craters. It was also demonstrated that for defects smaller than 20 nm, crater formation is preceded by plasma-ball formation around absorbing defects. In this work an attempt is made to verify symmetry of the plasma ball by conducting film irradiation from the side of the air/film or substrate/film interfaces. In each case, crater-formation thresholds are derived and crater morphology is analyzed by means of atomic force microscopy.

Growth of laser initiated damage plays a major role in determining optics lifetime in high power laser systems. Previous measurements have established that the lateral diameter grows exponentially. Knowledge of the growth of the site in the propagation direction is also important, especially so when considering techniques designed to mitigate damage growth, where it is required to reach all the subsurface damage. In this work, we present data on both the diameter and the depth of a growing exit surface damage sites in fused silica. Measured growth rates with both 351 nm illumination and with combined 351 nm and 1054 nm illumination are discussed.

Surface and coating technology plays an important role for extending lifetime of fluoride optics for ArF excimer laser applications. Optically finished CaF₂ optics is characterized as top surface and subsurface by means of non-distractive quasi-Brewster angle technique. The subsurface is revealed by removing the top surface via distractive methods. Color centers on plasma ion and laser irradiated CaF₂ optics are discussed. The results suggest that fluorine depletion is associated with laser damage, dense smooth coatings enable one to extend the lifetime of CaF₂ optics.

We observe that by conditioning DKDP using 500 ps laser pulses, the bulk damage threshold becomes essentially equivalent to the surface damage threshold. We report here the findings of our study of laser initiated output surface damage on 500 ps laser conditioned DKDP for test pulses at 351 nm, 3 ns. The relation between surface damage density and damaging fluence (ρ(Φ)) is presented for the first time and the morphologies of the surface sites are discussed. The results of this study suggest a surface conditioning effect resulting from exposure to 500 ps laser pulses.

In the field of optical communication, either fusion splicing of optical fibers or physical contact between optical-fibers using a fiber connector has been utilized as the typical method of optical fiber connection. Optical fiber connectors have been widely employed in optical transmission systems according to their features of easy and quick connection without special apparatus to connect fibers. The power of laser diodes for light sources became more intense and the multiplexing of wavelength (WDM) of a light source was enhanced with increasing traffic data. As a result, intense light transmits through the optical fiber. The high power transmission characteristics of the optical fiber connector are important factors to realize dense wavelength division multiplexing systems (DWDM). In this paper, we present an experimental investigation about the degradation of the transmission properties through the optical fiber connector by introducing the contamination between the end faces of a connector. The metal foils to simulate the contamination at the end of the core were inserted between the optical fibers to cover the core of an optical fiber partially. As metal foils, Nickel, SUS304, and Phosphor Bronze which were typically used as the components of the ferrule and sleeve were selected. The Nd: YAG laser with the wavelength of 1064 nm was used as a high power light source at various output powers. The transmission loss was set by adjusting the insertion of a metal foil into the core region of the fiber and the temperature rising of the connector induced by the absorption of incident light was measured at a sleeve portion. The damage at the end face of the physical contact region was observed using an optical microscope. The temperatures increase of the core of the fiber was estimated for the fiber connector with a zirconia ferrule through the thermal simulation using the MSC Visual Nastran. The damage of the fiber end face was recognized depending on the species of the metal foil and the covering ratio for the core in the case of a high power light transmission, typically, with the average power of 1W. It was estimated that the damage threshold of the fiber end becomes low for the ferrule with lower melting point. The correlation between the damage threshold and various parameters was discussed.

Microstructures built into the surfaces of an optic or window, are an effective replacement for thin-film coatings in anti-reflection (AR) and narrow-band filter applications. AR microstructures exhibit particularly noteworthy performance where an average reflection loss of less than 0.2% over a four-octave range (400-1800nm) has been demonstrated, and a loss of less than 0.03% is routinely achieved for narrow-band applications. Because AR micro-textures provide a gradual change in the refractive index at a material boundary, it is expected that light can propagate through the boundary without material damage at energy levels that are much higher than that found with thin-film interference coatings. Recently, it was shown that the laser induced damage threshold (LIDT) of an inexpensive borosilicate glass window containing AR microstructures was nearly 57 J/cm² at 1064nm (20ns pulse). This LIDT is two to three times greater than the damage threshold of single-layer sol-gel AR coatings on fused silica often reported in the literature. The development of surface relief AR textures for use in high-energy laser applications is presented. Data from scanning electron microscope (SEM) analysis, reflection measurements, and LIDT testing, is shown for high performance AR microstructures fabricated in fused silica, and borosilicate glass. Results of LIDT testing at wavelengths ranging from the near ultraviolet through the near infrared confirm the initial result that AR microstructures can operate at pulsed laser power levels at least two times higher than thin-film coatings. For near infrared applications such as laser weapons and fiber optic communications requiring high performance AR, LIDT levels for AR microstructures in fused silica are found to be at least five times greater than conventional multi-layer thin film coatings. An initial surface absorption test at 1064nm shows that AR microstructures may also exhibit improved lifetimes within continuous wave laser systems.

A revision of a theoretical mechanism proposed last year based upon the known chemical and physical interactions of laser radiation, fused silica, aromatic molecules and environmental factors will be presented, as relates to other proposed mechanisms. This paper specifically addresses the interaction of toluene with 1064nm laser radiation as related to the formation of benzyl radical, and to free radical photochemistry of toluene. This will address specifically, the effects of oxygen and water in the system, the effects of hydroxyl radical in the system, the interpretation of the XPS spectra of laser damaged silica in the presence and absence of aromatic hydrocarbons and the relationship of these points to the photochemistry of silica.

A critical component for the OMEGA EP short-pulse petawatt laser system is the grating compressor chamber (GCC). This large (12,375 ft³) vacuum chamber contains critical optics where laser-pulse compression is performed at the output of the system on two 40-cm-sq-aperture, IR (1054-nm) laser beams. Critical to this compression, within the GCC, are four sets of tiled multilayer-dielectric- (MLD) diffraction gratings that provide the capability for producing 2.6-kJ output IR energy per beam at 10 ps. The primary requirements for these large-aperture (43-cm × 47-cm) gratings are diffraction efficiencies greater than 95%, peak-to-valley wavefront quality of less than λ/10 waves, and laser-induced-damage thresholds greater than 2.7 J/cm² at 10-ps measured beam normal. Degradation of the grating laser-damage threshold due to adsorption of contaminants from the manufacturing process must be prevented to maintain system performance. In this paper we discuss an optimized cleaning process to achieve the OMEGA EP requirements. The fabrication of MLD gratings involves processes that utilize a wide variety of both organic materials (photoresist processes) and inorganic materials (metals and metal oxides) that can affect the final cleaning process. A number of these materials have significant optical absorbance; therefore, incomplete cleaning of these residues may result in the MLD gratings experiencing laser damage.

In this paper, we present the continued joint effort of ESA/ESTEC and DLR laser laboratories of improving the fluorescence monitoring technique towards a quantitative means for analysis of UV laser-induced deposit formation on optical samples in vacuum. In addition, a separate low power UV fluorescence excitation light source was implemented into the system allowing the investigation of laser-induced deposition occurring during irradiation of optics with IR and VIS light beams.

In this paper we review contamination and damage that has occurred since the HELEN laser was converted for use as a chirped pulse amplification (CPA) system. We concentrate on the largest components in the facility that operated in a vacuum environment in the short pulse (500fs) parts of the system. This experience will be valuable for planning commissioning and operations on the successor facility, ORION that is currently being constructed. The optical components located in the vacuum sections will be described. We have an interest in laser-induced damage and contamination from the long pulse only (~1ns), short pulse only (500fs) and combined regimes. For most of the operations the CPA system has operated at a wavelength of 1053nm (1ω). Some experiments have also been conducted with 527nm (2ω) CPA light derived by the use of a KDP doubling crystal. Damage to the infra red pulse compression gratings has occurred infrequently with a minority of high-energy shots. Contamination of multilayer dielectric plane turning mirrors has arisen from target disassembly. Focussing systems using off axis parabolic mirrors have sustained contamination from debris and a dielectric protected silver reflector used for green light suffered laser induced damage. Debris shields placed between the target and the parabolas have been used on selected experiments. The shields were anti-reflection coated with single layer, sol gel silica. These shields became contaminated on the target facing sides and in the case of 2ω operation also from blow off of the damaged mirror coating that caused a two-pass transmission loss in the system. A number of characterisation methods were used to evaluate and quantify the damage and contamination. These include macroscopic photography, microscopy, reflectometry and transmission spectroscopy.

Optical properties of various thin films such as Nitrocellulose, Mylar, and Polyimide were investigated with respect to their application as laser debris shields. Studies on optical and spectral transmission quality, absorption, stress induced birefringence, and damage threshold have been performed. Scalability to large apertures was also considered. Studies were performed of how focusing geometry, target alignment, and mechanical components can help mitigate target debris traveling back to the focusing optic.

Laser damage testing of ion beam sputtered (IBS) films for 1064 nm, Q-switched, low repetition rate operation typically involves the measurement of small physical defects that can be detected visually or photographically. For this kind of damage the small absorber model adequately describes most of the observations. We discuss why the damage threshold for IBS anti-reflection (AR) coatings is lower than for high reflectors and polarizers. We report on a new process technique that eliminates the physical damage to IBS AR coatings for fluences up to 21 J/cm², possibly higher. The success of this process and the lower damage for AR's indicates that the small absorbers are at the substrate/coating interface or are in the subsurface region of the substrate. We also discuss ongoing experiments to look for subtle forms of laser damage involving changes of the optical parameters of the thin film materials, under long-term exposure to Q-switched 1064 nm radiation and continuous 355 nm radiation.

A comparative study is made on the laser damage resistance of monolayers coatings made with different technologies. HfO₂ and SiO₂ thin films have been deposited on fused silica substrates with Dual Ion Beam Sputtering, Electron Beam Deposition (with and without Ion Assistance) and Reactive Low Voltage Ion Plating technologies. The laser damage thresholds of these coatings have been determined at 1064nm and 355nm using a nanosecond pulsed YAG laser, and a 1-on-1 test procedure.

The utilization of oxide mixtures as layer material for coating design has been transferred to the ion beam sputtering technology and was applied to high-reflecting as well as anti-reflecting components at the wavelength of 193nm. Exclusively, the oxides SiO₂ and Al₂O₃ are candidates for appropriate thin film designs below 200nm. Experimental data received from laser-calorimetric measurements, spectroscopic investigations and laser-induced damage tests are presented for several specimens. With respect to state-of-the-art thin film deposition in the DUV spectral range, conventional quarterwave designs have also been characterized and will be compared to the sputtered mixed oxide coatings.

A modified IBS-process was used to create mixtures of oxide coating materials. The process allows to manufacture new designs, whereas the important optical and electronic properties of the material can be varied in a wide range. Especially for ultra short pulse applications, higher damage thresholds can be achieved. In this paper, LIDT measurements of mixed and pure single layers are presented. The coatings were investigated at different wavelengths and in a wide pulse duration range. The results of the measurements confirm the empirical law of the linear LIDT dependency on the absorption gap. Based on this empirical law, the Refractive Index StEps Down (RISED) concept was developed. From the data of the single layer measurements, an optimization of RISED optical components in the fs-regime will lead to even higher damage thresholds. Particularly, for high reflecting mirrors the damage threshold could be doubled for different dielectric coating materials. Additionally, the paper presents a theoretical analysis of the stack LIDT on the basis of the single layer properties.

The effect of electric field distribution on the laser damage probability curves of multilayer coatings is investigated. The interpretation of laser damage probability curves uses a statistical model, where shapes and slopes of the curves are related to the spot size and to the densities of nanodefects that are responsible for damage and where each kind of precursors is characterized by its damage threshold. This statistical model is improved by considering value and shape of electric field in each layer of the coating. Typical multilayer coating constituted by alternate materials of high and low index are considered. For each kind of nanoprecursors and for each fluence, the value of electric field which leads to damage is calculated. This relative threshold value permits to know with accuracy the ratio of the coating thickness where the irradiation fluence is greater than the nanoprecursor threshold and to finally estimate the appropriate laser damage probability curves.

In order to study the effect of material properties on the laser induced damage of dielectric coatings at a wavelength of 248 nm, multilayer coatings were deposited by electron beam reactive evaporation technique onto fused silica substrates with the materials of hafnium oxide, aluminum oxide and silicon dioxide. Laser-induced damage thresholds (LIDTs), morphologies and profiles of damage sites of multilayer thin films were measured to investigate the damage mechanism. Besides, with our programmed software, the temperature rise in the multilayers was calculated to better understand the relationship between damage morphology, electric field peak location and depth of damage sites. The results indicate that the absorption of defect and the electric field distribution of thin film greatly contribute to LIDTs of thin films, and the control of defect, especially defect with strong absorption, is still the only way to improve the laser radiation resistivity of coatings in the UV spectral region.

We investigate the variations that occur with changes in the number of layers and with the use of the assist beam main and assist beam energy on the morphology of HfO₂/SiO₂ quarter wave stacks deposited by dual ion beam sputtering. We show how the addition of sequential HfO₂/SiO₂ bilayers, up to eight, affects the surface roughness and micro-crystallinity of the top HfO₂ layer. We also show that use of the assist source significantly smooths the surface while simultaneously reducing microcrystallinity. The HfO₂/SiO₂ structures are very robust and can withstand fluences in excess of 3 J/cm² generated by 1ps pulses from a chirped amplified Ti:Sapphire laser.

Various investigations show that damage threshold of optical coatings by intense ultrashort laser pulses is closely related to the intensity of electric field at layer interfaces. LIDT measurements of high reflectance optical coatings using femtosecond pulses at 800 nm wavelength are presented. ZrO₂, HfO₂ and Ta₂O₅ as high refractive index materials for two sets of experiments were chosen. Two different coating designs were investigated: standard quarter-wavelength design with SiO₂ overcoat and modified "E-field" non quarter-wavelength design with suppressed electric field. Damage sites were studied using optical and AFM microscopes. Relation between electric field distribution and damage morphology was observed. The results demonstrate, that suppressing electric field at layer interfaces enables to increase LIDT for high reflectance coatings almost twice if compared to standard quarter-wavelength design when using ultrashort laser pulses. However electric field distribution is sensitive to variations in thicknesses of outer layers, so deposition process should be precisely controlled to get improvement in LIDT of coatings.

An optical bench was developed to measure laser induced damage thresholds in 1:1 and S:1 modes on mirror and grating samples. The laser based on Ti. Sapphire technology delivers Gaussian pulse of 500fs with a maximal energy of 3mJ at 1057nm. The experimental setup can deliver on the sample a peak fluence of 9J/cm² in right section of the beam with a spot size of 200μm (diameter at 1/e²). Laser induced damage thresholds have been measured on several multidielectric samples produced with different processes. We present in this paper the damage testing setup in details and give some of the obtained results.

The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL) will routinely fire high energy shots (approaching 10 kJ per beamline) through the final optics, located on the target chamber. After a high fluence shot, exceeding 4J/cm² at 351 nm wavelength, the final optics will be inspected for laser-induced damage. The FODI (Final Optics Damage Inspection) system has been developed for this purpose, with requirements to detect laser-induced damage initiation and to track and size it's the growth to the point at which the optic is removed and the site mitigated. The FODI system is the "corner stone" of the NIF optic recycle strategy. We will describe the FODI system and discuss the challenges to make optics inspection a routine part of NIF operations.

The Gaussian profile beam has been de rigeur for damage threshold measurements for decades. This paper formulates the cumulative probability of damage (CPD) curve for the Gaussian and an arbitrary distribution of defects in fluence (intensity) space. It is seen that the CPD for the Gaussian is relatively insensitive to the underlying distribution of defects. The CPD is reformulated for a flat top distribution and shown to be far more influenced by the underlying defect distribution. The paper concludes with a discussion of the relationship between the defect distribution, sample size, threshold and measured threshold, for both the Gaussian and flat top profiles. It will be shown than the CPD for Gaussian beams increases with increasing fluence regardless of the distributions of the defects in fluence. The Flat Top CPD will only increase with increasing defect density.

S-on-1 laser-induced damage threshold (LIDT) dependence on the pulse duration at two different wavelengths was experimentally investigated in metallic and dielectric laser mirrors. LIDT's of high-reflective dielectric coatings made of alternating λ/4 layers of TiO₂/SiO₂ and Ta₂O₅/SiO₂ and those of protected metallic Au and Ag coatings were tested at 800 nm and 400 nm wavelengths with Ti:Sapphire laser pulses of 46 fs, 130 fs and 1.8 ps duration. S-on-1 measurements were performed according to international ISO 11254-2 standard using 10000 pulses/per site and compared with 1-on-1 measurements.

The ISO 11254 standard for LIDT tests suggests two possible spatial beam profiles for damage testing. Accordingly, an equal set of samples was tested with a Gaussian TEM₀₀ as well as with a top-hat beam profile at different beam diameters. It was found that for the investigated HfO₂/SiO₂ high reflectors there was no threshold dependence on the beam diameter at 355nm. The damage threshold values measured with the Gaussian and the top-hat beam were in good correlation.

We present S/1 and R/1 test results on unconditioned and 355 nm (3ω), 500 ps laser conditioned DKDP. We find up to ~2.5X improvement in fluence in the S/1 performance after 3ω, 500 ps conditioning to 5 J/cm². For the first time, we observe a shift to higher fluences in the R/1 results for DKDP at 3ω, 7 ns due to 500 ps laser conditioning. The S/1 results are compared to ρ(Φ) results previously measured on the same DKDP crystal [1]. A consistent behavior in fluence was found between the S/1 and ρ(Φ) results for unconditioned and 500 ps conditioned DKDP. We were successful at using Poisson statistics to derive a connection between the S/1 and ρ(Φ) results that could be tested with our data sets by trying to predict the shape of the ρ(Φ) curve. The value for the power dependence on fluence of ρ(Φ) derived from the S/1 data was ~11 ± 50%. The results presented and discussed here imply a strong correlation between the damage probability (S/1) test and ρ(Φ). We find a consistent description of the two test types in terms of a power law ρ(Φ) and that this basic shape held for all cases, i.e. the shape was invariant between unconditioned and conditioned results.

The laser damage probability of nonlinear optical crystals depends on many different factors. In addition to the fabrication process (crystal growth, cutting, polishing, coating) the damage threshold of non-linear crystals is influenced by the usage of the crystal. The anisotropy of material properties like the complex refractive index, the mechanical yield strength and the polarizability may cause anisotropy of the Laser Induced Damage Threshold (LIDT). The LIDT may depend on the propagation direction or the polarization of the light. Anisotropy in the LIDT has in fact been observed in different crystals. The dependency of the KDP-LIDT on the propagation direction and its independence on the polarization direction reported by Burnham et al. is an example that is not yet fully understood. For KTP it has been evidenced by Hu et al. that the grey-tracking threshold is polarization dependent. In this contribution we discuss the bulk laser damage resistance of two isomorphous non-linear crystals: KTiOPO₄ (KTP) and RbTiOPO₄ (RTP). All tests are performed using a nanosecond laser at 1064nm wavelength. For both crystals all polarization and propagation directions parallel to the principal axes have been tested. In addition we investigated two typical devices: two types of Pockels cells in RTP and a SHG-cut crystal in KTP. The results are analyzed on the basis of an anisotropic sensitivity of the crystal to the electric field and the efficiency of second harmonic generation.

The laser resistance of large optical components remains an important limitation for the performances and the maintenance costs in LMJ or NIF projects. For practical reasons LIDT studies are commonly performed with small samples and table top lasers whose characteristics change from one to another. In these conditions, it is necessary to know exactly the influence of the different experimental parameters (wavelength, spot size, ...) on the final data. These considerations are particularly true in heterogeneous materials as KDP crystals. Indeed the use of different laser beam sizes (from μm to hundreds μm) to plot laser damage probability curves had clearly shown that at 355nm in KDP, it is possible to exhibit a limit of irradiated area which permit to distinguish two different LIDT associated with two laser damage precursors densities. This prior result has put in evidence the influence of irradiated beam size in the discrimination of different kinds of defects in KDP. We present in this paper a systematic study of beam size effect in KDP for three different wavelengths: 355nm, 532nm and 1064nm. This study performed in 1:1 and R:1 mode will reveal precursors for each wavelength and their respective evolution under repetitive shots for small and large beams. This multi-parameters study will help us to highlight mechanisms involved in laser-induced damage in KDP crystal.

Laser damage measurements in nonlinear optical crystals, in particular in biaxial crystals, may be influenced by several effects proper to these materials or greatly enhanced in these materials. Before discussion of these effects, we address the topic of error bar determination for probability measurements. Error bars for the damage probabilities are important because nonlinear crystals are often small and expensive, thus only few sites are used for a single damage probability measurement. We present the mathematical basics and a flow diagram for the numerical calculation of error bars for probability measurements that correspond to a chosen confidence level. Effects that possibly modify the maximum intensity in a biaxial nonlinear crystal are: focusing aberration, walk-off and self-focusing. Depending on focusing conditions, propagation direction, polarization of the light and the position of the focus point in the crystal, strong aberrations may change the beam profile and drastically decrease the maximum intensity in the crystal. A correction factor for this effect is proposed, but quantitative corrections are not possible without taking into account the experimental beam profile after the focusing lens. The characteristics of walk-off and self-focusing have quickly been reviewed for the sake of completeness of this article. Finally, parasitic second harmonic generation may influence the laser damage behavior of crystals. The important point for laser damage measurements is that the amount of externally observed SHG after the crystal does not correspond to the maximum amount of second harmonic light inside the crystal.

This is a short review of the main reasons of the laser induced damage space scaling, when damage is caused by the first laser shoot (1-on-1) and by the sequence of N laser pulses to one and the same place (N-on-1). It is shown that the laser pulse shape is important to laser damage and scaling in some cases. The accumulation of material property changes and LID threshold for N-on-1 process has strong dependence on the front and on the back pulse slopes: the material property remnant changes become les and eventually accumulative damage disappears when the pulse slopes are increased. When damage scaling is defined by competition of the self-focusing and SBS processes, the long front slope is favored to SBS. The theoretical model explains damage from the first laser shot (1-on-1), damage from sequence of laser pulses (N-on-1 LID), accumulation and damage scaling. The volume damage of transparent dielectrics by laser radiation, when photon energy is more then twice less material ionization (hν<E_i/2) is the main point of interest here.

Laser-induced breakdown in the bulk of transparent dielectric materials is associated with the generation of extreme localized conditions of temperatures and pressures. In this work, we perform pump and probe damage testing experiments to investigate the evolution of transient absorption by the host material arising from modifications following confined laser energy deposition in fused silica and DKDP materials. Specifically, we measure the size of the damage sites observed in the region of spatial overlap between the pump and probe pulses versus probe time delay and energy. Results of this proof-of-principle experimental work confirm that material modifications under extreme conditions created during a damage event include transient optical absorption. In addition, we found that the relaxation times of the induced absorption are very distinct for DKDP and SiO₂ even under identical excitation conditions, on the order of 100 ns and 100 μs, respectively.

The ongoing development in microlithography towards further miniaturization of structures creates a strong demand for lens material with nearly ideal optical properties. Beside the highly demanding requirements on homogeneity and stress induced birefringence (SIB), low absorption is a key factor. Even a small absorption is associated with a temperature increase and results in thermally induced local variations of refractive index and SIB. This could affect the achievable resolution of the lithographic process. The total absorption of the material is composed of initial absorption and of absorption induced during irradiation. Thus, the optimization of both improves the lifetime of the material. In principal, it is possible to measure transmission and scattering with a suitable spectrometer assembly and calculate absorption from them. However, owing to the influence of sample surfaces and errors of measurement, these methods usually do not provide satisfactory results for highly light-transmissive fused silica. Therefore, it is most desirable to find a technique that is capable of directly measuring absorption coefficients in the range of (1...10)•10^-4 cm^-1 (base 10) directly. We report our first results for fused silica achieved with the LID technique. Besides a fused silica grade designed for 193 nm applications, grades with higher absorption at 193 nm were measured to test the LID technique. A special focus was set on the possibility of measuring initial absorption without the influence of degradation effects.

For an assessment of the optical quality of DUV optics, a high-sensitivity wavefront analyzer system based on the Hartmann-Shack principle is employed. The device accomplishes precise on-line monitoring of wavefront deformations of a collimated test beam transmitted through the laser-irradiated site of a sample. Due to the achieved sub-nm resolution, it can be used as an alternative to calorimetric and interferometric measurements for 'at wavelength' testing of optics, e.g. for on-line registration of thermal lensing effects or compaction in fused silica. By recording wavefront distortions of fused silica samples of different thickness and at different fluences the contribution of bulk and surface to the total absorption as well as one- and two-photon effects can be separated.

So far, laser calorimetry (LCA) as absorptance measurement procedure according to ISO 11551 has been commonly performed at a variety of selected laser wavelengths. Thus, this procedure has been a valuable tool for optimization processes of dielectric coatings. For an even more comprehensive detection of any absorbing contribution in a dielectric layer stack a free selectable test wavelength gives more detailed insights in optics characterization. According to this approach an OPO system was implemented in a laser calorimetric test bench. By this, a tunable laser source with sufficient laser power is available to conduct calorimetric absorptance measurements. Results of detailed investigations of the contribution of the material constituents of a dielectric optical component are presented in this paper.

A simple and sensitive photothermal technique-photothermal detuning (PTDT), which is based on the absorption-induced shift of reflectance or transmission spectrum of an optical coating, is developed to measure the absorption of coated optical components. A PTDT theory is developed to describe the signal's dependence on the structural parameters of the optical coatings and on the geometric parameters of the experimental configuration. An experiment is performed to measure the PTDT signal of a highly reflective multilayer coating used in 532nm by using a probe beam with a wavelength of 632.8nm. By optimizing the incident angle of the probe beam, the measurement sensitivity is maximized. Good agreements between the theoretical predictions and experimental results are obtained.

Exponential-decay based cavity ring-down (CRD) techniques, such as the pulsed-CRD and continuous-wave (cw) CRD employing a fast switch to shut down the laser beam, are widely used for high reflectivity measurement. In this paper the influence of the response time of the experimental apparatus on the high reflectivity measurement is investigated theoretically and experimentally. Theoretical expressions taking into account the instrumental response time are given for both pulsed- and cw-CRD techniques, respectively. By establishing a simple cw-CRD setup employing detectors with different response time, the influence of the instrumental response time on the high reflectivity measurement is experimentally investigated. By applying a multi-parameter estimation technique to determine simultaneously the cavity decay time and the overall response time of the experimental apparatus via fitting the experimental CRD signal to the corresponding theoretical model, the influence of a long instrumental response time on the reflectivity determination is eliminated. The reflectivities of the cavity mirror measured with detectors with different rise time are in excellent agreement. On the other hand, the error of high reflectivity measurement increases with the increasing rise/fall time of the apparatus in cases that the CRD signals obtained by detectors with relatively slow rise time are simply treated with a single exponential decay fitting procedure.

In this work, we present the first successful demonstration of a non-contact technique to precisely measure the 3D spatial characteristics of laser induced surface damage sites in fused silica for large aperture laser systems by employing Optical Coherence Tomography (OCT). What makes OCT particularly interesting in the characterization of optical materials for large aperture laser systems is that its axial resolution can be maintained with working distances greater than 5 cm, whether viewing through air or through the bulk of thick optics. Specifically, when mitigating surface damage sites against further growth by CO₂ laser evaporation of the damage, it is important to know the depth of subsurface cracks below the damage site. These cracks are typically obscured by the damage rubble when imaged from above the surface. The results to date clearly demonstrate that OCT is a unique and valuable tool for characterizing damage sites before and after the mitigation process. We also demonstrated its utility as an in-situ diagnostic to guide and optimize our process when mitigating surface damage sites on large, high-value optics.

A recent optical technique is reviewed to identify the scattering origins (surface roughness or bulk heterogeneities) and eliminate scattering sources in a selective way. Applications concern the field of optical interference coatings, remote sensing and imaging in random media.

Previous work concluded that plasma scalds on laser-conditioned multilayer dielectric mirror coatings are a stable, benign damage morphology. Recent large-aperture measurements indicate that plasma scalds may lead to fratricide of down-stream optics by increasing beam contrast. This paper describes the results of measurements performed to examine the effect of quasi-periodic plasma scalds covering the entire clear aperture on downstream beam modulation. A collimated, linearly-polarized 1053-nm beamline was constructed that irradiated approximately 5 cm² of the plasma scalded region. This beam was propagated ~8 meters and sampled with a 10-bit, megapixel CCD camera and analyzed for contrast (peak/average intensity). A lineout across the sample was built up by translating the optic across the beam. The contrast results were compared to a baseline wedged flat with surface figure of λ/100 and a contrast adder for the plasma scalds calculated. This was defined by. In all, optics with average plasma scald fractions of 0.9, 2.3, 4 and 14% were measured. Preliminary results indicate that plasma scald fractions of 4% and below contribute a contrast adder of less than 2.5%.

Identification and spatial registration of laser-induced damage relative to incident fluence profiles is often required to characterize the damage properties of laser optics near damage threshold. Of particular interest in inertial confinement laser systems are large aperture beam damage tests (>1cm²) where the number of initiated damage sites for Φ>14J/cm² can approach 10⁵-10⁶, requiring automatic microscopy counting to locate and register individual damage sites. However, as was shown for the case of bacteria counting in biology decades ago, random overlapping or 'clumping' prevents accurate counting of Poisson-distributed objects at high densities, and must be accounted for if the underlying statistics are to be understood. In this work we analyze the effect of random clumping on damage initiation density estimates at fluences above damage threshold. The parameter ψ=aρ= ρ/ρ₀, where a=1/ ρ₀ is the mean damage site area and ρ is the mean number density, is used to characterize the onset of clumping, and approximations based on a simple model are used to derive an expression for clumped damage density vs. fluence and damage site size. The influence of the uncorrected ρ vs. Φ curve on damage initiation probability predictions is also discussed.

Transmission, absorption and laser induced fluorescence (LIF) measurements were performed to reveal the applicability of different grade CaF₂ for 248 nm laser applications. No emission from self-trapped excitions could be found in LIF measurements after irradiation with 100k pulses for all grades. Therefore, three-photon excitation could be excluded up to 1 J/cm². Whereas emission at 420 nm and partially the double-peak at 313/333 nm could be found in LIF measurements. UV-VIS difference spectra did not show any absorption bands after 248 nm irradiation of the samples. Optical elements from CaF₂ promise high life expectancy at 248 nm if a standard laser polish is used and hot spots are avoided.