Fundamental mechanisms of laser induced damage (LID) have been one of the most controversial topics during the forty years of the Boulder Damage Symposium (Ref. 1.) LID is fundamentally a very nonlinear process and sensitive to a variety of parameters including wavelength, pulse width, spot size, focal conditions, material band gap, thermal-mechanical prosperities, and component design considerations. The complex interplay of many of these parameters and sample to sample materials variations combine to make detailed, first principle, models very problematic at best. The phenomenon of self-focusing, the multi spatial and temporal mode structure of most lasers, and the fact that samples are 'consumed' in testing complicate experiential results. This paper presents a retrospective of the work presented at this meeting.

Results of experimental and theoretical studies, carried out in the author's laboratory during past four decades, of fundamental mechanisms of laser induced damage (LID) to transparent solids are reviewed. Major features of LID experimentally observed in optical materials of different classes at various conditions (dependence of damage thresholds on radiation frequency, pulse width, temperature, etc.) are discussed. Theoretical models of both extrinsic (absorbing inclusion-initiated )and intrinsic (impact and multi-photon ionization) damage mechanisms are presented and their predictions for damage features (frequency and pulse-width dependence) are discussed. Peculiarities of LID in an ultrashort (ps-fs) laser pulse duration range are analysed . In this context a relative role of thermo-elastic stress-induced crack formation and ablation processes is considered. Experimentally observed features of LID are compared with theoretically predicted ones and conclusions are outlined on dominating LID mechanisms in real optical materials. Further directions in experimental and theoretical studies are discussed for investigating fundamental LID mechanisms in the ultra-short time domain.

Laser-induced damage threshold determination as a function of the number of incident pulses on a specific optic is a classic problem in laser damage studies. There are several models of the fundamental mechanisms explaining the fatigue laser damage behavior including temperature accumulation and changes of electronic or chemical material structure. Herewith we discuss the effects of unstable laser radiation on S-on-1 laser-induced damage probability. Numerical simulations of S-on-1 measurements for specific cases of defect densities, spot sizes and beam jitters are performed. It is demonstrated that the statistical effects of "pseudo-accumulation" reasoned by unstable laser radiation in transparent dielectrics containing nanometer sized defects leads to accumulation-like behavior. The magnitudes of the random beam walking and the energy fluctuations are directly related to the damage probability. Experimental results are also introduced to illustrate the theoretical results.

Nonlinear losses experienced by the self-focusing femtosecond pulse is shown to have an important effect on the refractive index modifications in fused silica. The region of the maximum induced change is found to coincide with that of the maximum nonlinear losses of the pulse. It is found as well that material densification and the formation of color centers both contribute to the index change in that zone. Experimental results are supported by numerical simulations using model that takes into account accumulation of the permanent refractive index changes and their influence back on the pulse. Both the color center- and compaction-induced changes cause the modification to develop into a waveguide and lead to the narrowing of supercontinuum spectra.

We study the excitation of luminescence, photoionization, and laser-induced breakdown in a multi-component silicate photo-thermo-refractive (PTR) glass, and in fused silica. PTR glass is a high-purity homogeneous photosensitive alkalisilicate glass with intrinsic absorption edge at 5.8 eV (214 nm). Experiments are conducted with ultrashort laser pulses (100 fsec< τ < 1.5 psec) at the wavelengths 780 nm, 1430 nm, and 1550 nm. Filaments are observed inside both glasses and explained by a balance between Kerr self-focusing and free electron defocusing. Keldysh theory is used to model the formation of filaments and values of about 10¹³ W/cm² for laser intensity and 10¹⁹ cm^-3 for free-electron density are estimated. Laser-induced damage by pulses at 1430 nm and 1550 nm is detected in fused silica and PTR glass by third harmonic generation due to the formation of an interface between a damage site and the surrounding glass matrix. It is found that there is an intensity range where luminescence and photoionization in both glasses occurs without laserinduced damage.

Most of the traditional theoretical models of laser-induced ionization were developed under the assumption of constant effective electron mass or weak dependence of the effective mass on electron energy. Those assumptions exclude from consideration all the effects resulting from significant increase of the effective mass with increasing of electron energy in real the conduction band. Promotion of electrons to the states with high effective mass can be done either via laserinduced electron oscillations or via electron-particle collisions. Increase of the effective mass during laser-material interactions can result in specific regimes of ionization. Performing a simple qualitative analysis by comparison of the constant-mass approximation vs realistic dependences of the effective mass on electron energy, we demonstrate that the traditional ionization models provide reliable estimation of the ionization rate in a very limited domain of laser intensity and wavelength. By taking into account increase of the effective mass with electron energy, we demonstrate that special regimes of high-intensity photo-ionization are possible depending on laser and material parameters. Qualitative analysis of the energy dependence of the effective mass also leads to conclusion that the avalanche ionization can be stopped by the effect of electron trapping in the states with large values of the effective mass.

We report the results of theoretical study of damage, induced by Coulomb forces, in (a) solid nanoparticles, and (b) the surface of solid dielectric, ionized by ultrashort laser pulses (USLP). The basic assumption of proposed model is that the damage occurs due to the laser-induced disturbance of charge equilibrium in solid with the further electron emission from irradiated area. When electrons outflow from crystal, the non-compensated positive charge creates a strong electrostatic field, causing the movement of the charged sites and micro- and/or macro- destruction of the condensed matter.

We observe a discontinuity in the scaling between the size of damage and the pulse energy for femtosecond laser pulses tightly focused at a glass surface. This discontinuity corresponds to the threshold for formation and ejection of rings of material surrounding the focus center. The mechanism for the generation of these structures appears distinct from that of the central holes and is ascribed to subsurface absorption leading to thermal expansion and shock wave formation.

A model for wide bandgap materials was developed to study the breakdown behavior under multiple subpicosecond laser pulse illumination. While this model has been applied to the study oxide materials, it is general enough to be used with any wide bandgap material. The model distinguishes two types of midgap trapping states - shallow and deep traps (defects), which can be native or laser induced. Excitation of these midgap states enhances the seed for the avalanche ionization process that causes breakdown, lowering the damage fluence for pulses later in the train. A set of rate equations for the conduction band electron density and population dynamics of the trap states was solved numerically to predict the damage threshold as a function of pulse number F(M). The effect of trap level parameters such as density, absorption cross-section, and the initial population on the shape of F(M) is discussed. Comparison is made to experimental data for oxide thin films.

The lifetime of silica optics in high power laser facility as the Laser MégaJoule (LMJ) is typically limited by the initiation of surface damages and their subsequent growth. To prevent this problem, a mitigation technique is used: it consists in a local melting of silica by CO₂ laser irradiation on the damage site. Because of the difficulty to produce efficient mitigated sites with large depth, the characterization of damage site to mitigate is very important. In this context, confocal microscopy appears to be an efficient solution to detect precisely cracks present under the damage site.

A laser damage competition was held at the 2008 Boulder Damage Symposium in order to determine the current status of thin film laser resistance within the private, academic, and government sectors. This damage competition allows a direct comparison of the current state-of-the-art of high laser resistance coatings since they are all tested using the same damage test setup and the same protocol. A normal incidence high reflector multilayer coating was selected at a wavelength of 1064 nm. The substrates were provided by the submitters. A double blind test assured sample and submitter anonymity so only a summary of the results are presented here. In addition to the laser resistance results, details of deposition processes, coating materials, and layer count will also be shared.

A Petawatt facility called PETAL (PETawatt Aquitaine Laser) is under development near the LIL (Ligne d'Integration Laser) at CEA Cesta, France. PETAL facility uses chirped pulse amplification (CPA) technique. We herein review various studies made to develop pulse compression gratings for CPA application with high laser induced damaged threshold. Different multilayer dielectric (MLD) gratings have been manufactured to exhibit different electric field maximum values in the pillars of the grating. A damage testing facility operating at 1.053μm, 500fs pulse duration is used to damage test the parts manufactured from these designs. It is evidenced that for fixed incidence and materials the damage of the grating is directly related to the electric field intensity maximum in the material, which depends on the groove profile. Laser induced damage thresholds of 5 J/ cm² is experimentally reached with very high and uniform efficiencies. New structures are currently under study, gratings with mixed metal/dielectric layers MLD or more exotic 2D and 3D photonic crystals devices. For each case, we detail the design and expected performances. We also give some diffraction efficiency and laser damage measurements when samples were manufactured.

Large aperture laser pulse compressor designs use several diffraction gratings in series and sometimes tiled together to compress an amplified 1 to 10 ns pulse to 0.1 to 10 ps. The wavefront of the compressed pulse must be well controlled to allow focusing to a small spot on a target. Traditionally, multilayer dielectric gratings (MLDG) have been fabricated onto high thermal expansion substrates such as BK7 glass to prevent crazing and excessive bending due to tensile coating stress when operated in high vacuum. However, the high CTE of the BK7 can cause wavefront distortion and changes in the period of the grating. This work uses ion-assisted deposition of HfO₂/SiO₂ films to increase the compressive stress in MLD layers to allow use of silica substrates in the compressor vacuum environment. Stress, coating uniformity, and damage results are reported. The process was scaled to full size (91cm × 42cm) MLD gratings for use in the Osaka University LFEX laser system. Diffracted wavefront results from the full scale gratings is presented.

As a consequence of the ongoing interest for deployment of laser systems into space, optical coatings have to be developed which allow for reliable long term operation under vacuum conditions. Extensive laser damage tests for space qualification of laser optics have recently been performed at the DLR and LZH laser damage test facilities in the IR, VIS, and UV spectral range within the ESA-ALADIN (Atmospheric Laser Doppler Instrument) test campaign. These tests have consistently revealed the degradation of the LIDT values for e-beam evaporated dielectric coatings under vacuum environment, which occurred independently of wavelength and type of coating (HR or AR) and other parameters. Dense coatings like IAD-based coatings, on the other hand, did not show this effect. Water desorption and diffusion processes seem to mediate the degradation under vacuum exposure.

Longterm damage mechanism investigations have identified effects responsible for laser induced damage. Particularly, the fundamental wavelength of Q-switched solid state lasers as well as their second and third harmonics have been analyzed in the context of the corresponding damage mechanisms. As a consequence of the immense progress in the production of coatings with lowest optical losses, the damage behavior of state of the art coating systems can typically be traced back to the contribution of microscopic defects and inclusions in the coatings for the VIS- and NIR-spectral range. In contrast, the influence of the intrinsic and surface absorptance can generally not be neglected in coatings for the DUV/VUV spectral region. This aspect gains of importance in the course of an increasing interest in the fourth harmonic for applications in research and industry. Therefore, the present paper is dedicated to investigations in oxide optical coatings for 266nm. This work has been performed to establish a database on the correlation of contamination and respective cleaning procedures to the damage threshold in the UV spectral region. Absorptance and degradation effects are identified by means of ISO certified laser calorimetry.

Fluoride materials are typically used as optical coatings for Deep UV applications such as semiconductor lithography steppers/scanners equipped ArF excimer laser (193nm). To extend its lifetime, laser durability of fluoride optical coatings deposited by conventional thermal evaporation method were investigated. From relation between coating defects amount and LIDT, it was apparent the laser durability of fluoride multilayer coatings are strongly spoiled by their coating defects. These defects are observed by naked eyes, Nomarski microscopy, AFM and SEM for more details. Finally it was found that defects broke the coating structures.

With respect to laser-based applications below 200 nm, fluoride materials used as layer and substrate materials are most prominent in optical components for beam shaping, steering and focussing. High band gaps and comparatively low extinction coefficients are the outstanding parameters of these components. However, fluoride coatings are exceedingly sensitive concerning surface contamination, handling, ambient atmosphere, humidity and total energy load. A set of fluoride layer stacks from different coating plants has been investigated by spectroscopic methods measuring the optical performance, laser calorimetry detecting the absorptive losses and LIDT testing the radiation resistivity of the specimens. Another set of samples was applied to an UV treatment system in nitrogen atmosphere before testing the optical performance by the procedures listed above.

A systematic study was undertaken to improve the laser-damage resistance of multilayer high-reflector coatings for use at 351 nm on the OMEGA EP Laser System. A series of hafnium dioxide monolayer films deposited by electron-beam evaporation with varying deposition rates and oxygen backfill pressures were studied using transmission electron microscopy (TEM), x-ray diffraction (XRD), and refractive index modeling. These exhibit microstructural changes for sufficiently slow deposition rates and high oxygen backfill pressures, resulting in an absence of crystalline inclusions and a lower refractive index. Hafnia monolayers exhibited laser-damage resistance as high as 12 J/cm² at 351 nm with a 0.5-ns pulse. This process was utilized in the fabrication of reduced electric-field-type multilayer high-reflector coatings. Measured laser-damage thresholds as high as 16.63 J/cm² were achieved under identical test conditions, an exceptional improvement relative to historical damage thresholds of the order of 3 to 5 J/cm².

The organic silicone oil applied over the surface of a fused silica glass or Kaliumdihydrogenphosphat (KDP) nonlinear optical crystal was changed to an inorganic glass by the photochemical oxidization using a Xe₂ excimer lamp in the air. As a result, the thin film acquired a characteristic of high power laser tolerance equivalent to quartz. Dimethylsiloxane silicone oil was spin-coated on the surfaces of a fused silica substrate and KDP to form a film of 100-nm thickness; which were irradiated with the Xe₂ excimer lamp light (wavelength 172 nm, power density 10 mW/cm²) for 60 minutes in oxygen atmosphere. The films were further irradiated with the Nd: YAG laser of ω (1.06 μm) or 2ω (0.503 μm), and the laser damage test (J/cm²/10 ns) was conducted. The laser damage threshold of the photo-oxidized 100 nm thick film formed on the fused silica substrate was 72 J/cm² in ω and 107 J/cm² in 2ω. On the KDP substrate, the laser damage threshold of the thin film was 32.4J/cm² in ω and 32.6 J/cm² in 2ω.

We present a complete systematic study on the effect of assist beam energy on SiO₂/HfO₂ quarter wave stacks deposited by dual ion beam sputter (DIBS) deposition. Increasing assist beam energy results in lower surface roughness and reduced micro-crystallinity. The coatings also show reduced mechanical stress. The improvements in the structural properties are accompanied by a reduction in the absorption loss and an increase in the laser resistance to damage at 1 μm.

In this work we use electron spin resonance (ESR) spectroscopy to investigate defects in dual ion beam sputtered HfO₂ and SiO₂ films. "As-grown" SiO₂ films exhibit an ESR feature consistent with an E' center associated with an oxygen vacancy previously reported. A similar feature with axial symmetry is seen in HfO₂ films. The defect giving rise to the HfO₂ ESR feature is distributed throughout the film. In addition, post process annealing of HfO₂ and SiO₂ films greatly reduces these defects.

Subpicosecond laser induced breakdown of dielectric films has gained a great deal of attention in laser nano- and micromachining and in the development of optical coatings for the next generation of high-power ultrafast laser system. The understanding of the fundamental processes affecting the breakdown behavior and how they depend on the material properties and the film deposition is highly desirable for improving the coating performance. In the present work we compare the single and multiple pulse damage behavior of as-grown and annealed HfO₂ films. Annealing can reduce the film absorption near the band edge but its impact on the single and multiple femtosecond pulse damage behavior remained open. Damage measurements with pairs of pulses of variable subpicosecond delay in bulk fused silica revealed a partial recovery toward single pulse behavior on a few hundred fs time scale. We investigate if such behavior also occurs in hafnia films and identify the time scale for a full recovery. Our experimental results are compared with existing theoretical models[1], which allows us to suggest microscopic changes that occur during the annealing process.

The session Materials and Measurements deals with laser-induced damage to the bulk of transparent optical media, in relation with the material fabrication and its structure. Damage measurements are reported together with the characterization of all the material properties connected to the damage process (optical losses, luminescence, linear and non linear optical properties, thermal properties, elastooptic coefficients... and defects). Moreover are included the new diagnostic tools developed for measuring these quantities, which presents a continuing challenge as materials are improved in quality and diversity. The whole results serve as a foundation in modeling works for the understanding of fundamental mechanisms. The studied materials and their characterization are required by numerous applications. This wide and multi-faceted field generated more than thirty per cent of the whole conference papers over the last ten years. Among the important topics of this period are DUV materials and measurements, characterization of non linear materials and effects, the non destructive detection of nanoprecursors, damage at short pulses and the novel materials and geometries: micro and nano materials and structures. An overview of the session is given, mainly focused on the last ten years, some important achievements and trends for the future are presented.

We present the results of experimental studies of formation and evolution of multiply ionized (multicharged) laser micro-size plasma produced in gases (air, nitrogen, argon and helium) and inside the transparent solids (fused silica) with high intensity (up to ~ 10¹⁷ W/cm²), ultrashort (τ ~100 fs), 800nm/400nm laser pulses tightly focused in a region of ~1.5 μm in diameter. The measuring techniques and experimental setups for generation and precise optical diagnostics of laser-induced plasma - pump-probe microinterferometry and ultrafast spectroscopy are described. The measured spatiotemporal distributions of plasma refractive index/electron density and plasma spectra are demonstrated. In the experiments, the main attention was paid to the most intriguing initial stage of ultrafast plasma formation and evolution characterized by strong laser-matter and laser-plasma coupling resulting in efficient photoionization of material and plasma heating. We found out that the almost complete ionization (down to nuclei) of the initial gas occurs even at the initial stage of plasma formation. Besides, it was observed, for the first time, that a characteristic time of laser plasma formation considerably (in times) exceeds the duration of the pump laser pulse. This postionization process is attributed to impact ionization of plasma by hot electrons heated due to inverse bremsstrahlung. A theoretical model describing the mechanism of plasma postionization by hot photoelectrons was proposed. We compare the results of the experiments with what the theory predicts - the results of electron density calculations are in good agreement with the experimental data. The dynamics of plasma emission (spectral continuum and spectral line formation) in UV-visible spectral range was investigated with a picosecond time resolution applying the developed ultrafast streak-camera-based spectrometer. The spatiotemporal distributions of refractive index of laser irradiated fused silica were recorded with pump-probe microinterferometry. It was demonstrated that the induced refractive index of laser-matter interaction area changes its sign from the positive to the negative during the laser irradiation and again to the positive one after the laser pulse ends.

We report on time resolved imaging of the dynamic events taking place during laser-induced damage in the bulk of fused silica samples with nanosecond temporal resolution and one micron spatial resolution. These events include: shock/pressure wave formation and propagation, transient absorption, crack propagation and formation of residual stress fields. The work has been performed using a time-resolved microscope system that utilizes a probe pulse to acquire images at delay times covering the entire timeline of a damage event. Image information is enhanced using polarized illumination and simultaneously recording the two orthogonal polarization image components. For the case of fused silica, an electronic excitation is first observed accompanied by the onset of a pressure wave generation and propagation. Cracks are seen to form early in the process and reach their final size at about 25 ns into the damage event. In addition, changes that in part are attributed to transient absorption in the modified material are observed for delays up to about 200 microseconds.

Thermal lens effects are one of the major problems in using optics for high power laser applications such as optical lithography or material processing. The thermal lens results from the combination of the absorption in the bulk material or the optical coatings, the thermal conductivity and the temperature change of the refractive index (dn/dT). We present how the laser induced deflection (LID) technique allows the direct and absolute measurement of the bulk and surface or coating absorption. The LID measurement signal, comprising of absorption, thermal conductivity and dn/dT, is directly used to compare the tendency to built thermal lenses in different optical materials. Furthermore, it is shown how the LID measurement signal in principle can be used to determine the thermo-optical material constants thermal conductivity and dn/dT. Regarding direct absorption measurements, a new experimental strategy is introduced to separate bulk from surface or coating absorption. Hereby, the closest attention is paid to measure directly the residual absorption of transparent optical coatings, e.g. single layers or AR coatings, with negligible contribution from the substrate absorption. In addition, numerical simulations of the thermal lens induced probe beam deflection are introduced, which allow to design optimized strategies for particular measurement problems.

We report on a new experimental technique for monitoring laser-induced shock waves and thermal waves above the sample surface called total internal reflection based photothermal or photoacoustic deflection (TIR based PTD/PAD deflection). It is based on the changes in transmissivity of a prism which is operated near the condition of total internal reflection for a HeNe laser beam propagating parallel to the sample surface at a small distance. The HeNe laser beam is probing photoacoustic or photothermal waves originating from a sample surface due to interaction with a pulsed Nd:YAG laser beam. The method is compared with standard online detection techniques like scatter probe monitoring and plasma detection, and found to be a very sensitive and practical tool. It also showed its suitability for selectively monitoring several surfaces (e. g. front and rear surface) of optical components, and attributing the damage starting point. Therefore, the method might be used for monitoring of surface damage on laser crystals or valuable components. Keywords: photothermal deflection, photoacoustic deflection, laser damage, total internal reflection.

When a modulated laser beam irradiates an optical component, the laser-induced surface deformation has both direct current (DC) and alternating current (AC) portions. Explicit surface deformation and surface thermal lens (STL) theory models are developed to describe the DC and AC portions of the surface deformation and corresponding STL signals. Experimentally, a setup combining laser calorimetry (LC) and STL technique is developed to measure the absolute absorptance and laser-induced surface deformation of optical components. The absorptance measurement is implemented by LC with excellent stability and repeatability. The surface deformation measurement is realized with STL amplitude by defining an approximately linear relationship between the AC (or DC) STL amplitude and the maximum AC (or DC) deformation. As an example, the deformation value of a BK7 substrate coated with a TiO₂/SiO₂ film stack of absolute absorptance 1.32×10^-3, irradiated by a 1064nm laser with 3.8W power is determined to be 34.3 nm with the experimental STL amplitude, in good agreement with the theoretical value of 35.8 nm calculated by the explicit surface deformation model. An indirect approach is proposed to determine accurately the irradiation beam radius by fitting the experimental data of the radial AC intensity change at the detection plane to the explicit STL model. By performing a theoretical fit to the experimental frequency dependence of the STL amplitude, the thermal properties of the optical component (i.e. the thermal diffusivity) can also be determined.

Highly reflective mirrors have been widely used in high power lasers, laser gyros, and gravitational-wave detection, etc. However, reliable measurement of high reflectivity (R>99.99%) is difficult. In this paper a novel optical feedback cavity ring-down technique (OF-CRD) by re-injecting the strong optical feedback from the ring-down cavity (RDC) into the oscillator cavity of a Fabry-Perot diode laser is developed for the ultra-high reflectivity measurement. The laser line is narrowed and occasionally in resonance with one or more ring-down cavity modes. The amplitude of the RDC output signal is enhanced by a factor of over two orders of magnitude, compared with the conventional phase-shift CRD technique. Four pairs of cavity mirrors with different reflectivity are used to investigate the sensitivity and reproducibility of the OF-CRD technique. The accuracy is greatly enhanced from about 0.003% to 0.00003% as the reflectivity of cavity mirrors increases from about 99.8% to 99.996%. A folded RDC with cavity length of 70cm is constructed by inserting a planar test mirror into the linear RDC and the reflectivity of the test mirror is statistically determined to be 99.9526±0.0004%. The OF-CRD is simple, reliable, highly-sensitive and cost efficient.

In many high energy laser systems, optics with HMDS sol gel antireflective coatings are placed in close proximity to each other making them particularly susceptible to certain types of strong optical interactions. During the coating process, halo shaped coating flaws develop around surface digs and particles. Depending on the shape and size of the flaw, the extent of laser light intensity modulation and consequent probability of damaging downstream optics may increase significantly. To prevent these defects from causing damage, a coating flaw removal tool was developed that deploys a spot of decane with a syringe and dissolves away the coating flaw. The residual liquid is evacuated leaving an uncoated circular spot approximately 1mm in diameter. The resulting uncoated region causes little light intensity modulation and thus has a low probability of causing damage in optics downstream from the mitigated flaw site.

The presence of defects in optical materials can lead to bulk damage or downstream modulation and subsequent surface damage in high fluence laser systems. An inclusion detection system has been developed by the National Ignition Facility Optics Metrology Group. The system detects small inclusions in optical materials with increased sensitivity and speed over previous methods. The system has detected all known inclusions and defects and has detected previously undetected defects smaller than 5 microns.

A rasterscan test procedure [L. Lamaignère et al, Rev. Sci. Instrumen. 78, 103105 (2007)] has been implemented in order to determine low laser damage density of large aperture UV fused silica optics. This procedure was improved in terms of accuracy and repeatability and is now used for the determination of bulk damage density for KDP crystals. The large area (volume) scanned during tests permits to measure very low damage density. On small samples, small area are tested using the normalized 1/1 test procedure consisting on the irradiation of few sites at several fluences. The classical damage probability plot is converted in terms of damage density. The two testing procedures are complementary: the 1/1 mode is practical to test a wide fluence range while the rasterscan mode allows exploring low damage densities with higher accuracy. Tests have been carried out on several facilities using several pulse durations and spatial distributions. We describe the equipment, test procedure and data analysis to perform this damage test with small beams (Gaussian beams, about 1mm @ 1/e, and top hat beams). Then, beam overlap and beam shape are the two key parameters which are taken into account in order to determine damage density. After data analysis and treatment, a repeatable metrology has been obtained. Moreover, the consideration of error bars on defects distributions permits to compare data between these installations. This allows us to reach reproducibility, a necessary condition in order to share results and to make reliable predictions of laser damage resistance. Other tests are realized with larger beams (centimeter sized) and with a single shot. Due to a large beam contrast, a large fluence range is then covered. Then after data treatment, we find a good correlation between tests realised with small and large beams. This allows us to make tests with different laser characteristics (spectral modulations, pulse duration, laser polarisation) and then to study their influences on laser damage.

Multipulse laser induced damage optical materials is an important topic for many applications of nonlinear crystals. We studied multi pulse damage in X-cut KTiOPO₄. A 6ns Nd:YAG laser has been used with a weakly focused beam. A fatigue phenomenon has been observed and we try to clarify the question whether or not this phenomenon necessarily implies material modifications. Two possible models have been checked, both of them predicting increasing damage probability with increasing pulse number while all material properties are kept constant: (i) Pulse energy fluctuations and depointing increase the probed volume during multiple pulse experiments. The probability to cause damage thus increases with increasing pulse number. However, this effect turned out to be too small to explain the observed fatigue. (ii) Assuming a constant single shot damage probability p1 a multipulse experiment can be described by statistically independent resampling of the material. Very good agreement has been found between the 2000-on-1 volume damage data and the statistical multipulse model. Additionaly the spot size dependency of the damage probability is well described by a precursor presence model. Supposing that laser damage precursors are either transient or, if they are permanent, irradiation of the precursor above its threshold only causes damage with a small probability, the presented data can be interpreted without supposing material modifications.

The use of the error bar is a critical means of communicating the quality of individual data points and a processed result. Understanding the error bar for a processed measurement depends on the measurement technique being used and is the subject of many recent works, as such, the paper will confine its scope to the determination of the error bar on a single data point. Many investigators either ignore the error bar altogether or use a "one size error fits all" method, both of these approaches are poor procedure and misleading. It is the goal of this work to lift the veil of mysticism surrounding error bars for damage observations and make their description, calculation and use, easy and commonplace. This paper will rigorously derive the error bar size as a function of the experimental parameters and observed data and will concentrate on the dependent variable, the cumulative probability of damage. The paper will begin with a discussion of the error bar as a measure of data quality or reliability. The expression for the variance in the parameters is derived via standard methods and converted to a standard deviation. The concept of the coverage factor is introduced to scale the error bar to the desired confidence level, completing the derivation

Laser-induced damage growth on the surface of fused silica optics has been extensively studied and has been found to depend on a number of factors including fluence and the surface on which the damage site resides. It has been demonstrated that damage sites as small as a few tens of microns can be detected and tracked on optics installed a fusion-class laser, however, determining the surface of an optic on which a damage site resides in situ can be a significant challenge. In this work demonstrate that a machine-learning algorithm can successfully predict the surface location of the damage site using an expanded set of characteristics for each damage site, some of which are not historically associated with growth rate.

In order to characterize the effect of thermal annealing on laser damage resistance of KDP, several combinations of laser conditioning and thermal annealing were applied to two SHG KDP samples. One sample was tested at 3ω, 16ns and the other one at 3ω, 2.5ns. Results show that whereas thermal annealing improves laser damage for a 16ns pulse, no effect can be measured at a pulse length of 2.5ns. Combining laser conditioning and thermal annealing has a stronger effect on laser damage resistance than laser conditioning alone, even for a 2.5ns pulse length for which thermal annealing was found to have little or no influence. It was also found that for a short pulse length maximum gain was obtained when thermal annealing was applied after laser conditioning.

Large-aperture laser systems, currently designed to achieve high energy densities at the target location (exceeding ~ 1011 J/m³), will enable studies of the physics of matter and radiation under extreme conditions. As a result, their optical components, such as the frequency conversion crystals (KDP/DKDP), may be exposed to X-rays and other ionizing radiation. This in turn may lead to a change in the damage performance of these materials as they may be affected by radiation-induced effects by either forming new damage initiation centers or interacting with the pre-existing damage initiating defects (so-called damage precursors). We present an experimental study on the laser-induced bulk damage performance at 355-nm of DKDP crystals following X-ray irradiation at room temperature. Results indicate that the damage performance of the material is affected by exposure to X-rays. We attribute this behavior to a change in the physical properties of the precursors which, in turn, affect their individual damage threshold.

A number of optical limiting effects of organic compounds related to two-photon absorption (TPA) processes have been reported recent years. Actually, very large TPA cross-sections (σ) above thousands GM are obtained. However, the anti-damage property of organic compounds is still a problem, which limits their application on high power lasers. Thus, to increase the anti-damage property of organic compounds is important. Dicyanomethylene (DCM) is a strong electron acceptor. Some reported DCM derivatives show good stability and nonlinear optical activity. In this work, we investigate the optical limiting and anti-damage properties of two novel DCM derivatives in different solvents by nonlinear transmission method using femtosecond 1064 nm laser as excitation source. The results show that solvents have distinct effects on both damage threshold and TPA capability of compounds, indicating the solvent selection is very important. Moreover, an interesting phenomenon is observed that a linear absorption appears after damage threshold for both compounds in all solvents, which is supposed due to TPA induced excited absorption. More detailed discussions are presented.

We report on a comparative study of the damage threshold of ytterbium-doped laser materials which are important for diode-pumped, high-energy class short pulse lasers. Both surface and bulk damage thresholds at the lasing wavelength of 1030 nm were investigated. A pulse duration of 6.4 ns was chosen which allows a scaling of the damage threshold for gain media in q-switched lasers as well as chirped-pulse amplifiers. In order to achieve comparability and repeatability of the damage measurements the surface preparation of the used samples was kept constant. Furthermore, the correlation of the bulk damage threshold and the UV absorption spectra was analyzed.

We have investigated a relationship among the bulk laser-induced damage threshold (LIDT) and YAG ceramics with various structural defects. The correlation of scattering defect density and laser damage resistance was clearly observed. A high-quality YAG ceramic having a low-scattering density showed a higher LIDT than that of a low-quality YAG ceramic. Laser damage threshold (LIDT) of high-quality YAG ceramic was almost the same as that of a single crystal. In addition, the high-quality Nd:YAG ceramics with low-defect density showed an excellent oscillation efficiency which was comparable to that of a single crystal. Thus, high-quality YAG ceramic with low-defect density is more reliable as a material which is highly resistant to laser damage.

We have observed and characterized wavelength-dependent laser damage thresholds in crystalline germanium induced by trains of high-power infrared picosecond laser pulses at wavelengths ranging from 2.8 μm to 5.2 μm, using the Vanderbilt Free-Electron Laser. In this wavelength range, photon energies are well below the band-gap energy. As the wavelength is increased, threshold fluences are observed to increase by a factor of five over the studied wavelength range. Two- and three- photon absorption is the predominant photon energy absorption mechanism up to 4.4 μm. At wavelengths above 4.8 μm tunnel absorption appears to be the primary absorption mechanism. Wavelength and fluence dependent transmission and reflection measurements provide valuable insight into the nature of the damage mechanisms.

Multi-stripe laser diodes are important for a wide range of applications including pump sources for high power fiber amplifiers and industrial applications. It is now believed that multimode single stripe laser diodes with high reliability have been designed and fabricated by several manufacturers. We have data which show FIT (failure in 109 hours) rates of ~ 12 FIT for 100 μm wide multimode emitters at power levels 2.5 W for a 15 year operation at 20 C. We have developed a method for calculating the survival probability of such multimode lasers when they are assembled in the form of a multi-stripe array. For a demanding application, a multi-stripe array can be considered a failure if one emitter in the array fails whereas for some other applications higher number of emitter failures is acceptable. The survival probability of the entire ensemble of lasers in the array as a function of number of stripes, number of failures, operating power level, and, near neighbor thermal interaction has been studied.

Particle Image Velocimetry (PIV) is a well known measurement protocol for analyzing the dynamic behavior of fluids in liquid or gaseous phases (granulate analysis is also possible). With respect to the demands of the measurement accuracy, a high fluence at the observation zone is required. Presently, this can only be realized by using very precisely aligned equipment and high power laser pulses. For industrial applications a simpler set-up is needed. Thus the research project is aimed at the development of a portable endoscopic-based solution which requires the guidance of laser light through optical fibers. The realization of such an optical fiber system is a technical challenge since the high instantaneous energies, which exist in the laser pulse, can cause irreversible damage to the optical fiber. Consequently, the main goal is the determination of the maximum fluences, that different fiber core bulk materials can tolerate, and the comparison of these results with the maximum achievable fluence when transmitting light through optical fibers. A simple theoretical modeling tool for the approximation of the power handling capability was developed. Based on this theoretical analysis, Laser Zentrum Hannover examined the impacts that laser pulses and fiber materials have on light incoupling and guidance. An experimental set-up was developed to investigate the laser light resistance of different fiber bulk materials as well as the fibers themselves. This paper introduces the measurement set-up and the results of LIDT measurements of several fiber core materials. Furthermore, the fiber measurement set-up, achievable fluences, transmission efficiencies as well as the typical fiber damage behavior are presented.

In many applications of ArF - excimer lasers, a specific degradation effect is observed for the CaF₂ outcoupling windows which starts assumedly at the rear surface and results in a characteristic damage morphology. In the present study, this degradation mechanism is examined in a measurement series involving a variety of window samples and irradiation sequences in an excimer laser with typical numbers of up to 2×10⁸ pulses for each component. The irradiated samples were inspected by scanning spectrophotometry, TOF-SIMS, electron microscopy and other analytical techniques in order to clarify the underlying degradation mechanisms. On the basis of the experimental findings, coating strategies will be outlined to improve the lifetime of CaF₂ - output couplers in 193nm excimer lasers.

This paper deals with the relation between fracture mechanics and 355 nm laser damage at the surface of fused silica. It is organized in 3 parts. First, we discuss about the link between cracks and laser initiation of surface damage. A 1D model was proposed last year to explain how a nanometer wide, clean, uncontaminated crack could trigger a macroscopic damage event. Here, using the model, we try to express a damage criterion able to reproduce experimental features. In a second part, we consider the relationship between laser damage and mechanical damage by indents or impacts. From Auerbach's law, it is straightforward to derive an energy density threshold for Hertzian crack initiation. With the laser fracture interaction model, a laser fluence threshold of cone crack formation can be calculated. When cone cracks are present, a series of shot at moderate fluence will increase their length exponentially. This is a possible explanation for exponential damage growth at the exit surface of fused silica.

Laser-induced damage initiation in silica has been shown to follow a power-law behavior with respect to pulse-length. Models based on thermal diffusion physics can successfully predict this scaling and the effect of pulse shape for pulses between about 3ns and 10ns. In this work we use sophisticated new measurement techniques and novel pulse shape experiments to test the limits of this scaling. We show that simple pulse length scaling fails for pulses below about 3ns. Furthermore, double pulse initiation experiments suggest that energy absorbed by the first pulse is lost on time scales much shorter than would be predicted for thermal diffusion. This time scale for energy loss can be strongly modulated by maintaining a small but non-zero intensity between the pulses. By producing damage with various pulse shapes and pulse trains it is demonstrated that the properties of any hypothetical thermal absorber become highly constrained.

We are interested in maximizing the performance of fiber lasers and amplifiers, particularly for amplification of ps-ns pulses. The observed pulse energies from large mode area fiber amplifiers routinely exceed the reported bulk damage threshold of silica. We have undertaken a program to establish the intrinsic damage thresholds of silica that are relevant for fiber applications. We use a single transverse / single longitudinal mode Q-switched Nd:YAG laser focused to an 8-µm spot several Rayleigh ranges deep in silica windows for the nanosecond measurement, and a Q-switched, mode locked Nd:YAG laser for the picoseconds measurements. Our key findings include: 1. The damage threshold is deterministic rather than statistical for both ns and ps pulses. The threshold varies less than 1% from location to location. 2. The intrinsic damage threshold of silica is 475±25 GW/cm² (fluence = 3850 J/cm²) for 8 ns pulses and approximately 3 times higher for 14 ps pulses. 3. There is no difference in damage thresholds among Corning's A0, B1, C1, D1, D2, and D5 grades of silica. 4. A tight focus is required to avoid large self focusing corrections and to avoid SBS for the 8-ns pulses. 5. Damage morphologies are reproducible from pulse to pulse but change with focal spot size and pulse duration. In all cases, damage appears to begin exactly at the focus and then move upstream approximately one Rayleigh range. 6. The dependence of the damage threshold fluence on pulse duration is nearly linear for pulse durations longer than 50 ps. The square root of duration dependence reported by several investigators for the 50 ps to 10 ns range is refuted. 7. The variation of damage fluence with pulse duration from 20 fs to 20 ns and beyond is well described by a single electron avalanche rate equation with three fixed rates for the avalanche, multiphoton ionization, and electron recombination terms. 8. Our damage threshold is consistent with the most reliable DC field breakdown threshold. 9. We verified in detail the self focusing corrections and the SBS thresholds for our measurement conditions. 10. The damage threshold is affected little by mechanical strain at levels similar to those in polarization-preserving fiber.

'Thermal lenses' in fused silica due to absorbed UV laser radiation can diminish the achievable spatial resolution of the lithographic process in semiconductor wafer steppers. We developed a measurement system for spatially resolved registration of induced wavefront deformations, utilizing a Hartmann-Shack wavefront sensor with extreme sensitivity (λ/10,000). The photo-thermal technique can be employed for a rapid assessment of the material quality, since the wavefront deformation is directly proportional to the absorption losses. Along with a description of this new technique, we present results from photo-thermal measurements on fused silica under 193nm irradiation, as well as a comparison with thermal theory.

We report on two approaches to strongly shorten life time testing of fused silica's absoption degradation upon 193 nm laser irradiation. Both approaches are based on enhancing the two photon absorption (TPA) induced generation of E' and NBOH defects centers in fused silica compared to common marathon test irradiation parameters. For the first approach the irradiation fluence is increased from typical values H<1 mJ/cm² to H=10 mJ/cm², therefore increasing the peak laser power for a more efficient TPA process. To avoid microchannel formation in the samples, being a common break-down criterion in marathon tests based on transmission measurements, a small sample of 10 mm length is irradiated and the absorption is measured directly by the laser induced deflection (LID) technique. For comparing the experimental results with a real marathon test at H=1.3 mJ/cm², an experimental grade sample with very low hydrogen content, i.e. fast absorption changes due to reduced defect annealing, is choosen. During the fluence dependent absorption measurements after the prolonged irradiation at H=10 mJ/cm² it is found, that both experiments reveal very comparable absorption data for H=1.3 mJ/cm². For investigating standard material with high hydrogen content, i.e. slow absorption increase due to effective defect annealing, a sample is cooled down to -180 °C in a special designed experimental setup and irradiated at a laser fluence H=10 mJ/cm². To control the increase of the defect density and to determine the end of the TPA induced defect generation, the fluorescence at 650 nm of the generated NBOH centers is monitored. Before and after the low temperature experiment, the absorption coefficient is measured directly by LID technique. By applying both, elevated laser fluence and low temperature, the ArF laser induced generation of E' and NBOH centers in the investigated sample is terminated after about 1.2*10⁷ laser pulses. Therefore, a strong reduction of irradiation time is achieved in comparison to about 10¹⁰ pulses required in common marathon test applications.

Laser-induced growth of optical damage often determines the useful lifetime of an optic in a high power laser system. We have extended our previous work on growth of laser damage in fused silica with simultaneous 351 nm and 1053 nm laser irradiation by measuring the threshold for growth with various ratios of 351 nm and 1053 nm fluence. Previously we reported that when growth occurs, the growth rate is determined by the total fluence. We now find that the threshold for growth is dependent on both the magnitude of the 351 nm fluence as well as the ratio of the 351 nm fluence to the 1053 nm fluence. Furthermore, the data suggests that under certain conditions the 1053 nm fluence does not contribute to the growth.

During the development of the laser megajoule (LMJ), a high power laser facility dedicated to DT fusion, CEA has made important efforts to understand and improve laser induced damage threshold of fused silica optics at the wavelength of 351 nm. For several years, with various industrials and academics partners, we have focused on optimizing the grinding, lapping and polishing processes to increase materials performance. In this paper, we describe our efforts in various fields: subsurface damage characterization, lapping process simulation, diamond grinding and lapping machine instrumentations, ... Our concern is to control and manage the material removal at each step of the process in order to reduce the cracks region extension and thus to diminish the damage density.

Susceptibility to laser damage of optical-material surfaces originates from the nature of the surface as a transitional structure between optical-material bulk and its surroundings. As such, it requires technological processing to satisfy figure and roughness requirements and is also permanently subjected to environmental exposure. Consequently, enhanced absorption caused by mechanical structural damage or incorporation and sorption of microscale absorbing defects, and even layers of organic materials, is always characteristic for optical-material surfaces. In this review physics of interaction of pulsed-laser radiation with surface imperfections for different types of optical materials (metals, semiconductors, dielectrics, etc.), mechanisms of damage initiation, damage morphology, and damage-site growth under repetitive pulse irradiation are discussed. Consideration is also given here to the surface treatments leading to the reduction of damage initiation sites, such as laser cleaning and conditioning, removal of the surface layers affected by the grinding/polishing process, and mitigation of the damage growth at already formed damage sites.

In 2007, the pulsed laser induced damage threshold (LIDT) of anti-reflecting (AR) microstructures built in fused silica and glass was shown to be up to three times greater than the LIDT of single-layer thin-film AR coatings, and at least five times greater than multiple-layer thin-film AR coatings. This result suggested that microstructure-based wavelength selective mirrors might also exhibit high LIDT. Efficient light reflection over a narrow spectral range can be produced by an array of sub-wavelength sized surface relief microstructures built in a waveguide configuration. Such surface structure resonant (SSR) filters typically achieve a reflectivity exceeding 99% over a 1-10nm range about the filter center wavelength, making SSR filters useful as laser high reflectors (HR). SSR laser mirrors consist of microstructures that are first etched in the surface of fused silica and borosilicate glass windows and subsequently coated with a thin layer of a non-absorbing high refractive index dielectric material such as tantalum pent-oxide or zinc sulfide. Results of an initial investigation into the LIDT of single layer SSR laser mirrors operating at 532nm, 1064nm and 1573nm are described along with data from SEM analysis of the microstructures, and spectral reflection measurements. None of the twelve samples tested exhibited damage thresholds above 3 J/cm² when illuminated at the resonant wavelength, indicating that the simple single layer, first order design will need further development to be suitable for high power laser applications. Samples of SSR high reflectors entered in the Thin Film Damage Competition also exhibited low damage thresholds of less than 1 J/cm² for the ZnS coated SSR, and just over 4 J/cm² for the Ta2O5 coated SSR.

We have investigated the growth mechanisms for laser induced contamination of space optics in vacuum, particularly during the early stages of the deposit formation. Experiments have been performed in vacuum to study the influence of the environmental conditions and the condition of the optical surface, using a variety of physical and chemical techniques. In particular, different methods of conditioning the surface prior to irradiation and cleaning the surface after irradiation have been tested.