This PDF file contains the front matter associated with SPIE Proceedings Volume 8530, including the Title Page, Copyright information, Table of Contents, International Program Committee listing, Symposium Welcome, Summary of Meeting, and the Summary of the Roundtable Discussion.

Laser-induced deformation depends on the atomic structure of the material. In amorphous materials, the deformation is random or isotropic. On the other hand, in single crystals, anisotropic deformations occur in the specific directions, which is because of their regularly-arranged atomic structures. For example, when a fs laser pulse is focused inside rock-salt type crystalline materials (MgO, LiF, etc.) normal to the (001) plane, a void is formed in the photoexcited region and highly concentrated dislocation bands and cleavages are formed in the <110> and <100> directions, respectively. The directions of the dislocation bands and cleavages are often explained by the slip and cleavage planes of the crystals, however, stresses that induce these modifications have not been elucidated. In this study, we observed the dynamics of transient stress distributions after photoexcitation inside various single crystals by a pump-probe polarization microscope. After a femtosecond laser pulse was focused inside a MgO single crystal normal to the (001) plane, a void appeared in the photoexcited region and two stress waves (primary and secondary stress waves) were generated. In the primary stress wave, which propagated faster than the secondary one, the direction of the strain was identical to the propagation direction and the stress direction depended on the direction from the photoexcited region. In the secondary stress wave, there were tensile stresses normal to the (100) planes, which were identical to the cleavage planes.

We investigated formation of defects in four polymers namely Poly (methylmethacrylate) [PMMA], Poly dimethylsiloxane [PDMS], Polystyrene [PS], and Polyvinyl alcohol [PVA] and crystal media such as Lithium Niobate [LiNbO₃]. Spectroscopic studies of the femtosecond (fs) laser modified regions were systematically performed after fabricating several gratings and micro-channels. We observed emission from the fs laser modified regions of these polymers when excited at different wavelengths. Pristine polymers are not paramagnetic, but exhibited paramagnetic behavior upon fs irradiation. LiNbO₃ (LNB) crystal has not shown any defect formation upon laser irradiation. Confocal micro-Raman studies were also performed to establish the formation of defects.

Temperature dependence of laser-induced damage thresholds were measured by Nd:YAG laser (1064-nm wavelength, 4- ns pulse width) and Ti:Sapphire laser (800-nm wavelength, 100-fs, 2-ps, and 200-ps pulse widths) to elucidate the effects of laser-induced damage mechanisms. As experimental samples, SiO₂, MgF₂, Al₂O₃, HfO₂, ZrO₂, and Ta₂O₅ were prepared by electron evaporation. With longer pulses than few picoseconds, laser-induced damage thresholds were increased with decreasing temperature. Temperature dependence was reversed for shorter pulses than a few picoseconds. The effects of temperature at different pulse width to laser-induced damage mechanisms were considered with separated processes. In the conclusions, a temperature effect to free-electron generations by photoionization and multi photon ionization is negligible. However, the temperature affects to electron multiple (electron avalanche) and critical density. Electron multiple decreased at low temperature and the laser-induced damage thresholds increased. On the other hand, critical density decreased at low temperature and the laser damage thresholds decreased. Influence of electron avalanche is much greater than the impact of critical density. Thus, the trend and the strength of the temperature dependence on laser-induced damage threshold will be decided by electron avalanche.

Results of energy deposition measurement in interaction between an ultra-short laser pulse and nanostructured target are described. As a target we used carbon nanotubes and multilayer graphene deposited on a sapphire wafer surface and embedded in a layer of protein. A 25 fs, p-polarised pulses from a 1 kHz-Ti;sapphire laser of energy up to 3 mJ were focused to give intensity up to 2×10¹⁶ W/cm² on a target positioned within an integrating sphere. The absorption measured showed a level in excess of 80 %, increasing with the intensity. The results suggest that the host material (lysozyme) is responsible for the breakdown while the embedded material contributes dominantly to the absorption.

Thin glass sheets (thickness <1 mm) have a great potential in OLED and LCD displays. While the conventional manufacturing methods, such as mechanical scribing and breaking, result in poor edge strength, ultra-short-pulsed laser processing could be a promising solution, offering high-quality cutting edges. However laser precision glass cutting suffers from unwanted material modification and even severe damage (e.g. cracks and chipping). Therefore it is essential to have a deep understanding of the ultra-short-pulsed laser ablation mechanism of transparent dielectrics in order to remedy those drawbacks. In this work, the ablation mechanism of transparent dielectrics irradiated by picosecond laser pulses has been studied. Ultrafast dynamics of free-electrons is analyzed using a rate equation for free-electron density including multi-photon ionization, avalanche ionization and loss terms. Two maps of free-electron density in parameter space are given to discuss the dependence of ablation threshold intensity/fluence on pulse duration. The laser ablation model describing laser beam propagation and energy deposition in transparent dielectrics is presented. Based on our model, simulations and experiments have been performed to study the ablation dynamics. Both simulation and experimental results show good agreement, offering great potential for optimization of laser processing in transparent dielectrics. The effects of recombination coefficient and electron-collision time on our model are investigated.

We studied theoretically the laser-plasma interaction, and performed experiments to investigate the mechanisms giving rise to optical damage in Borosilicate glass using nanosecond laser pulses at wavelength 1064 nm. Our experimental result shows that the optical damage process generated by nanosecond laser pulses is the result of an optically induced plasma. The plasma is initiated when the laser irradiance frees electrons from the glass. Although it may be debated, the electrons are likely freed by multi-photon absorption and the number density grows via impact ionization. Later when the electron gas density reaches the critical density, the electron gas resonantly absorbs the laser beam through collective excitation since the laser frequency is equal to the plasma frequency. The laser energy absorbed through the collective excitation is much larger than the energy absorbed by multi-photon ionization and impact ionization. Our experimental result also shows the plasma survives until the end of the laser pulse and the optical damage occurs after the laser pulse ceases. The plasma decay releases heat to the lattice. This heat causes the glass to be molten and soft. It is only as the glass cools and solidifies that stresses induced by this process cause the glass to fracture and damage. We also show the experimental evidence of the change of the refractive index of the focusing region as the density of the electron gas changes from sub-critical to overcritical, and the reflection of the over-critical plasma. This reflection limits the electron gas density to be not much larger than the critical density.

Surface and bulk effects in silica optics due to high intensity laser light are well known using short pulse and high power laser systems. Surfaces are quickly destroyed mechanically if not properly prepared and thoroughly cleaned. Linear and non-linear absorption of high intensity laser light in the bulk of the optics causes material modifications, like voids, cracks and UV defects. In ablation experiments with very short pulses on wide band-gap dielectrics, periodic surface structures in the form of ripples were found. Surprisingly, we found similar structures on fiber end-faces after long-term irradiation with 405 nm CW laser light. Power densities on the end-face are in the range of 1 MW/cm², three magnitudes of order below the power threshold at which the described damages occur. Nevertheless a ripple structure perpendicular to the polarization direction of the laser was formed and grows with irradiation time. An increased absorption band at 214 nm (E' center) along the fiber was discovered by spectral absorption measurements. E' centers can be generated by 405 nm laser light in the bulk, therefore defects on the surface are possible as well. The generation of defect centers on the silica surface can enhance the formation of an unstable surface layer.

Chemical vapor deposition (CVD) has been used for the production of fused silica optics in high power laser applications. However, relatively little is known about the ultraviolet (UV) laser damage threshold of CVD films and how they relate to intrinsic defects produced during deposition. We present a study relating structural and electronic defects in CVD films to the 355 nm pulsed laser damage threshold as a function of post-deposition annealing temperature (T_HT). Plasma-enhanced CVD, based on SiH₄/N₂O under oxygen-rich conditions, was used to deposit 1.5, 3.1 and 6.4 μm thick films on etched SiO₂ substrates. Rapid annealing was performed using a scanned CO₂ laser beam up to T_HT~2100 K. The films were then characterized using X-ray photoemission spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and photoluminescence (PL). A gradual transition in the damage threshold of annealed films was observed at T_HT up to 1600 K, correlating with a decrease in NB silanol and broadband PL emission. An additional sharp transition in damage threshold also occurs at ~1850 K indicating substrate annealing. Based on our results, a mechanism for damage-related defect annealing is proposed, and the potential of using high-T_HT CVD SiO₂ to mitigate optical damage is also discussed.

To understand laser interaction with dielectrics on a wide time scale we apply different approaches: For a subpicosecond time range we solve complete Boltzmann collision integrals or apply the multiple rate equation (MRE), which focuses on the evolution of the conduction band electron density. The Boltzmann approach includes the valence band dynamics and calculates the transient distribution function for electrons and phonons. It also allows to extract important parameters, like the Auger recombination and impact ionization rate and the electron-phonon coupling parameter, which can be used as input in other models. The multiple rate equation includes density dependent optical parameters and is therefore independent of a critical density criterion to follow dielectric breakdown. The flexibility of the MRE is used to examine, which set of laser parameters causes breakdown, and to convert this knowledge into breakdown maps. It also allows to include a spatial dimension which traces the density evolution in different material depths. This spatial information and the parameters obtained by the Boltzmann approach can be used as input in the density dependent two temperature model (nTTM). The nTTM models heat relaxation and carrier transport on a very wide time scale by using an expanded two temperature model which also includes the transient free electron density. The combination of the individual strengths of our models is capable to simulate a vast range of materials and laser pulses on a timescale of up to several hundred picoseconds and to investigate the effect of transport on the damage threshold.

We have investigated the role of native point defects in the laser damage behavior of amorphous thin films of Sc₂O₃ deposited by ion beam sputtering. Through systematic characterization and detailed modeling we show that native defects influence the 1-on-1 laser induced damage threshold fluence of the Sc₂O₃. This effect, as shown by the model and confirmed by experiments, is pulse duration dependent.

We report on the laser damage resistance of thin films prepared by Ion Beam Sputtering. The samples are fused silica substrates coated with single layer films of pure oxides (SiO₂, Nb₂O₅, ZrO₂, HfO₂, Ta₂O₅, Al₂O₃, Sc₂O₃) and oxide mixtures with various ratios (Nb₂O₅/SiO₂, ZrO₂/SiO₂, HfO₂/SiO_2, Ta₂O₅/SiO₂, Al₂O₃/SiO₂ and Sc₂O_3/SiO₂). For this study the LIDT of more than 60 different samples have measured at 1030nm with pulse durations of 500fs with single pulse irradiation. The results are expressed and compared in terms of LIDT as a function of the measured band gap energy and refractive index. For simple oxide materials a linear evolution of the LIDT with bandgap is observed, with the exception of Sc₂O₃ material where a very high damage threshold is observed, compared to other high index materials. In the case of mixtures, a more complex behavior is evidenced.

Multilayer dielectric gratings (MDGs) are more and more used to compress pulse in the next generation of chirped-pulse amplification (CPA) system for high-energy petawatt (HEPW)-class lasers due to their high efficiency and high damage threshold for picosecond pulses. The damage tests for MDGs were carried out with long pulse (12ns) in air and short pulse (0.66~9.7ps) in vacuum at 1053nm, respectively. The experiment methodologies and results were discussed. For both long and short pulse, the initial damage locates at the grating ridges opposite to the incoming wave, which is consistent with the maximum normalized electric field intensity (NEFI). For long pulse, the damage is characterized by melting and boiling. And for short pulse, the damage is ascribed to multiphoton-induced avalanche ionization because of the electric field enhancement in the grating groove structure. And Measurement results of the dependence of damage threshold on the pulse width are presented. And the damage threshold of MDG in beam normal is 4.4J/cm² at 70° incidence angle for 9.7ps pulse.

Brewster angle plate polarizing beamsplitters play a critical role in splitting and combining beams within high power laser systems. A laser damage competition of polarizer beamsplitter coatings creates the opportunity to survey the current laser resistance these coatings within private industry, governmental institutions, and universities by a direct comparison of samples tested under identical conditions. The requirements of the coatings are a minimum transmission of 95% at “P” polarization and minimum reflection of 99% at “S” polarization at 1064 nm and 56.4 degrees angle of incidence. The choice of coating materials, design, and deposition method were left to the participant. Laser damage testing was performed according to the ISO 11254 standard utilizing a 1064 nm wavelength laser with a 10 ns pulse length operating at a repetition rate of 20 Hz. A double blind test assured sample and submitter anonymity so only a summary of the results are presented. In addition to the laser resistance results, details of cleaning methods, deposition processes, coating materials and layer count, and spectral results are also shared. Because of the large number of samples that were submitted, damage testing was conducted at “P” polarization only with “S” polarization damage testing reserved for next year on these submitted samples. Also the samples were only tested in the forward propagating direction; specifically samples were irradiated from air as the incident medium, through the thin film, and then through the substrate. In summary, a 6:1 difference existed for “P” polarization damage fluences amongst all of the competitors with the dominate variables that impacted the laser resistance being the deposition materials, deposition process, and cleaning method.

Micro- and nano-structured optically functional surface textures continue to exhibit higher performance and longer term survivability than thin-film coatings for an increasing number of materials used within high energy laser (HEL) systems. Anti-reflection (AR) microstructures (ARMs) produce a graded refractive index yielding high transmission over wide spectral ranges along with a chemical, mechanical and laser damage resistance inherited from the bulk optic material. In this study, ARMs were fabricated in the relevant HEL materials sapphire, neodymium-doped YAG, fused silica, BK7 glass, and the magnesium aluminate known as SPINEL. Standardized pulsed laser induced damage threshold (LiDT) measurements were made using commercial testing services to directly compare the damage resistance of ARMs-treated optics to uncoated and thin-film-AR-coated (TFARC) optics at wavelengths of 532nm, 694nm, 800nm, 1064nm, and 1538nm. As found with prior work, the LiDT of ARMs etched in fused silica was typically in the range of 35 J/cm² at a wavelength of 1064nm and a pulse width of 10ns, a level that is comparable to uncoated samples and 3.5 times greater than the level specified by six prominent TFARC providers. The Army Research Laboratory measured the pulsed LiDT at 532nm (10ns) of ARMs in fused silica to be up to 5 times the level of the ion beam sputtered TFARC previously employed in their HEL system, and 2 times higher than a low performance single layer MgF₂ TFARC. This result was repeated and expanded using a commercial LiDT testing service for ARMs in two types of fused silica and for Schott N-BK7 glass. An average damage threshold of 26.5 J/cm² was recorded for the ARMs-treated glass materials, a level 4 times higher than the commercial IBS TFARCs tested.

Power scaling of mid-infrared laser systems based on chromium and iron doped zinc selenide (ZnSe) and zinc sulfide (ZnS) crystals is being advanced through the integration of surface relief anti-reflection microstructures (ARMs) etched directly in the facets of the laser gain media. In this study, a new ARMs texture fabrication process is demonstrated for polycrystalline ZnSe and ZnS material that results in a significant increase in pulsed laser damage resistance combined with an average reflection loss of less than 0.5% over the wavelength range of 1.9-3.0μm. The process was utilized to fabricate ARMs in chromium-doped zinc selenide (Cr₂₊:ZnSe) materials supplied by IPG Photonics and standardized pulsed laser induced damage threshold (LiDT) measurements at a wavelength of 2.09μm were made using the commercial testing services of Spica Technologies. It was found that the pulsed LiDT of ARMs etched in ZnSe and Cr²⁺:ZnSe can match or even exceed the level of a well-polished surface, a survivability that is many times higher than an equivalent performance broad-band thin-film AR coating. The results also indicate that the ARMs plasma etch process may find use as a post-polish damage mitigation technique similar to the chemical immersion used to double the damage resistance of fused silica optics. ARMs etched in Cr²⁺:ZnSe were also evaluated by IPG Photonics for survivability under continuous wave (CW) laser operation at a pump laser wavelength of 1.94μm. Catastrophic damage occurred between power levels of 400-500 kilowatt per square centimeter for both as polished and ARMs textured samples indicating no reduction in CW damage resistance attributable to surface effects.

Experimental and theoretical progress on multiple wavelength laser induced damage of multilayer beam splitters is reviewed. Test method for multiple lasers-induced damage thresholds were proposed based on ISO 11254. Single and multiple laser-induced damage performance of the splitters, including damage probabilities, damage thresholds and damage morphologies, were investigated respectively in order to obtain better understanding of the damage mechanism at 355, 532 and 1064 nm. A judgment criterion for coupling effect of different lasers in inducing damages was proposed and the detailed coupling efficiency was also obtained. The small absorbing particle model and defect statistical model were put forward to explain the experimental results. In addition, damage performance when the lasers arrived at the films with nanosecond-delay in time was researched. Generally, the third harmonics play a key role in the damage initiation.

Laser induced damage of optical components is a concern in many applications in the commercial, scientific and military market sectors. Numerous component manufacturers supply “high laser damage threshold” (HLDT) optics to meet the needs of this market, and consumers pay a premium price for these products. While there’s no question that HLDT optics are manufactured to more rigorous standards (and are therefore inherently more expensive) than conventional products, it is not clear how this added expense translates directly into better performance. This is because the standard methods for evaluating laser damage, and the underlying assumptions about the validity of traditional laser damage testing, are flawed. In particular, the surface and coating defects that generally lead to laser damage (in many laserparameter regimes of interest) are widely distributed over the component surface with large spaces in between them. As a result, laser damage testing typically doesn’t include enough of these defects to achieve the sample sizes necessary to make its results statistically meaningful. The result is a poor correlation between defect characteristics and damage events. This paper establishes specifically why this is the case, and provides some indication of what might be done to remedy the problem.

An oxidizing agent is needed for silicone oil to be photo-oxidized with Xe2 excimer-lamp. However, the lamp light did not reach the silicone oil on the surface of the substrate satisfactorily, and the photo-oxidation reaction of the silicone oil layer was hard to take place properly. In order to find the appropriate conditions for supplying the proper amount of an oxidizing agent to silicone oil, the vacuum ultraviolet light that passed the silicone oil layer was made fluoresce in the phosphor to monitor the progress of the photo-oxidation reaction. As the vitrification by photo-oxidation reaction of the silicone oil layer improved, the fluorescence intensity of the phosphor increased. While monitoring the change of the fluorescence intensity, the supply of the oxidizing agent and the irradiation time of the vacuum ultraviolet light were controlled; as a result, the new method to efficiently form a transparent, photo-oxidized thin film has been established.

In the past decades, efforts have been concentrated on reaching more laser resistant multilayers optical components in the Infrared (IR) range. New designs and materials have been investigated and among them binary or ternary oxide mixtures have revealed to be very profitable to improve the laser damage resistance of the coatings in the IR. The physical characteristics of such mixed materials are indeed tunable and the deposition process associated allows to obtain multilayers with smoother interfaces, which reduces considerably the damage threshold of the component. The present work is focused on the study of pure materials and their binary oxide mixtures in the UV range, using S-on-1 testing, for two different laser beam sizes. Samples resistance to multipulse irradiation is then compared for both beam sizes, extracting the data with a thermal model assuming nanometric inclusions. The fatigue effects of the set of sample have also been investigated, showing no clear trend of fatigue for all tested components.

The generation of third harmonic radiation (THG) is required for many pulsed solid-state laser applications in industry and science. In this contribution, the coatings for two necessary optical components, dichroic mirrors and nonlinear optical (NLO) crystals are in the focus of investigation. Because of the high bulk damage threshold lithium triborate (LBO) crystals are applied for this study. HfO₂/SiO₂ mixtures are employed as high refractive index material to improve the power handling capability of the multilayers. All coatings are produced by ion beam sputtering (IBS) using a zone target assembly for the deposition of material mixtures. The atomic composition and the oxidation ratio of different HfO₂/SiO₂ mixtures are analyzed by X-ray photoelectron spectroscopy (XPS). The influence of different deposition temperatures and post annealing on the optical properties and the amorphous micro structure of the films is investigated by UV/Vis/NIR spectroscopy and X-ray diffraction (XRD). The laser induced damage thresholds at 355 nm wavelength for nanosecond pulse durations are measured in a 10,000on1 experiment according with the standard ISO21254. Furthermore, the optical components are tested under real application conditions.

Mixed metal oxide coatings of ZrO₂ / SiO₂ , and Nb₂ O₅ / SiO₂ as well as films of pure SiO₂ , ZrO₂ and Nb₂ O₅ have been prepared by the Ion Beam Sputtering (IBS) technique and characterized on their physical properties. The Laser-Induced Damage Thresholds (LIDT) of these samples have been measured at 1064 nm with the 1-on-1 mode in the nanosecond regime. The optical resistance results obtained from laser damage probability curves indicate a decrease of the LIDT in both sets of the mixtures when the content of the high index material is increased. By comparing the LIDT to the bandgap values (Eg) that have been measured, a dependence of LIDT to Eg is evidenced. Following these results, comparisons are made with the case of 500fs LIDT that have been measured on the same samples. It is found that the same behavior is observed on both cases (ie LIDT dependence with Eg). Discussions are then conducted on the possible physical mechanisms to explain the results and it is found that in the case of the nanosecond regime, the LIDT dependence on Eg can be explained by a critical temperature reached during the laser damage process that exhibit a dependence on the band-gap of the material. The influence of the film thermal conductivity on the values of the critical temperature is studied.

Group III nitrides are wide band-gap semiconductors which are commonly used in high power and high frequency electronics and optoelectronics. A rapid development of GaN/InGaN devices is in progress however many technological improvements are still demanded. One of them is a convenient formation of electrical contacts attached to appropriate layers. Currently a selective etching step of GaN and InGaN layers is performed by using quite expensive methods such as plasma, chemical-lithographic or electron beam exposure. However, very little research has been done towards investigation of an alternative selective laser etching possibility. Therefore in this work we study optical resistance and damage morphology of thin film GaN and In_xGa_1-xN layers grown on sapphire substrates in the femtosecond regime. Laser induced damage threshold (LIDT) tests were carried out in both S-on-1 and 1-on-1 regimes by exposing samples from front (deposited) and rear (substrate) sides. For optical resistance testing a femtosecond Yb:KGW laser combined with harmonic generator covering near IR spectrum to visible and UV was used. Experimental results of optical resistance dependence on band-gap in In_xGa_1-xN layers with different indium concentration (X up to 22%) are presented. Also detailed morphology study for different laser wavelengths is performed and discussed.

We have investigated the ion beam sputtering (IBS) of Y₂O₃ as an alternative to HfO₂ for high index layers in interference coatings for high power lasers. We present results on structural and optical properties of Y₂O₃ deposited from an oxide target. The IBS Y₂O₃ films are amorphous with a surface roughness of <6 Å for λ/4 thickness at 1 μm. The Y₂O₃ films are transparent with extremely low absorption loss at 1 μm and laser damage is in the range 10 J/cm² when tested at a wavelength of 1.03 μm and pulse duration of 375 ps.

Atomic Layer Deposition (ALD) allows for the deposition of homogeneous and conformal coatings with superior microstructural properties and well controllable thickness. As a consequence, ALD-processes have moved into the focus of optical thin film research during the last decade. In contrast to this, only a relatively small number of investigations in the power handling capability of ALD-coatings have been reported until now. The present contribution summarizes results of a study dedicated to the optical properties of single layers and high reflecting coating systems of TiO₂ and Al₂O₃ deposited by ALD. Besides Laser Induced Damage Threshold (LIDT) values, the spectral characteristics as well the absorption and scatter losses are discussed.

During the last decade, coating processes have been extended to the reproducible deposition of composite materials on the basis of simultaneous evaporation or sputtering. Especially, ion beam sputtering from a zone target in conjunction with sophisticated optical broadband monitoring offers several advantages for the production of oxide coatings with defined mixture ratios and even Rugate filter systems with a continuous variation of the composition ratio in the depth of the layer structure. With only two materials on the zone targets, a large dynamic range of refractive index values covering the indices of the pure materials can be achieved. Recent studies on the properties of the produced oxide composites indicate a variety of interesting aspects opened by this new class of material. Among others, a blue shift of the absorption characteristic was observed for ternary oxides, and an increased LIDT, particularly for sub-picosecond coatings, has been reported. Also a number of investigations of fundamental damage mechanisms could be carried out by considering the tunable band gap energy of the coating material. In this endeavor, a group of international collaborators joined in modeling, testing and evaluating the properties of a variety of ternary oxide systems. A verification of principal material qualities was transferred to applicable multilayer coatings in a combined effort. In this paper, an overview on the achievements of these current studies is presented before the background of high power laser applications.

The fact that chromophores doped into a polymer self heal after photodegradation seems to contradict the common understanding that molecular damage is a thermodynamically irreversible process. We have proposed a model that takes into account all observations, including the kinetics of photodegradation and recovery as a function of concentration, temperature, intensity, and sample thermal/intensity history. Correlations between chromophores, perhaps mediated through van der Waals forces or hydrogen bonding with the polymer, appear to actively favor the undamaged species by inducing healing in analogy to Bose-Einstein condensation. This model is shown to predict the behavior of photo-induced decay and recovery experiments as measured with amplified spontaneous emission and absorption spectroscopy.

Direct Optical Initiation (DOI), uses a moderate energy laser to shock initiate secondary explosives, via either a flyer plate or exploding metal foil. DOI offers significant performance and safety advantages over conventional electrical initiation. Optical fibres are used to transport the optical energy from the laser to the explosive device. A DOI system comprises of a laser, one or more optical fibres, and one or more laser detonators. Realisation of a DOI system is greatly eased by the use of fibre-to-fibre connections, allowing for easy integration into bulkheads or other interfaces, such as firing tanks and environmental test chambers. Fibres to fibre connectors capable of transmitting the required energy densities are not commercially available. Energy densities in the region of 35 J cm^-2 are required for initiation, above the damage threshold of typical optical fibres. Laser-induced damage is typically caused by laser absorption at the input face due to imperfections in the surface polishing. To successfully transmit energy densities for DOI, a high quality fibre end face finish is required. A fibre-to-fibre connection utilizing micro-lens array injection into a large-core, tapered optical fibre, a hermetic fibre bulkhead feedthrough, and a disposable test fibre has been developed. This permits easy connection of test detonators or components, with the complex free-space to fibre injection simplified to a single operation. The damage threshold and transmission losses of the fibre-to-fibre connection have been established for each interface.

High energy laser pulses were fired into a 365μm diameter fiber optic cable constrained in small radii of curvature bends, resulting in a catastrophic failure. Q-switched laser pulses from a flashlamp pumped, Nd:YAG laser were injected into the cables, and the spatial intensity profile at the exit face of the fiber was observed using an infrared camera. The transmission of the radiation through the tight radii resulted in an asymmetric intensity profile with one half of the fiber core having a higher peak-to-average energy distribution. Prior to testing, the cables were thermally conditioned while constrained in the small radii of curvature bends. Single-bend, double-bend, and U-shaped geometries were tested to characterize various cable routing scenarios.

A selection of commercially available high-power optical fibres have been characterised for radiation susceptibility in Sandia’s Annular Core Research Reactor (ACRR). The fibres were subjected to a total gamma and neutron dose >2 Mrad(Si) in a 7 ms pulse. The neutron fluence was >10¹⁵ n/cm². Changes in the transmission characteristics of optical fibres carrying high energy, short duration laser pulses (power densities of around 1.5 GW/cm²) were measured. All fibres survived at least two consecutive radiation exposures, showing typical transient transmission losses of around 20%. Post radiation exposure, the transmission characteristics returned to those of pristine fibres within one minute.

Laser-induced damage of optical surfaces, thin film coatings, and materials is greatly influenced by imperfections such as surface and interface roughness and surface or subsurface defects. All these imperfections give also rise to light scattering. Light scattering techniques are thus well suited to identify and characterize damage-relevant features. Additionally, they are non-contact, highly sensitive, and enable large sample areas to be investigated. Conventional characterization techniques are usually confined to small sample areas. A light scattering method will be presented that provides roughness and defect maps even of large and curved surfaces. Subsurface defects also play a critical role as damage precursors. Many detection methods are still based on wedging and/or etching the sample surface. A new non-contact approach to detect subsurface damage using polarized light scattering will be presented. In addition, a method will be discussed that provides information about the structural and optical properties of multilayer coatings by analyzing the scattered light distribution. Even small deviations of the illumination and the design wavelengths or angles can lead to substantial field enhancements inside the coating which can be clearly observed as resonant scattering wings.

Nowadays, more powerful and challenging laser systems are built to meet the need of evolving technology. In this context, the aim of the HiLASE project [1] is to develop a multi-joule picosecond laser system working in kHz repetition rate regime. The outputs of the project will provide not only unique source for both scientific and industrial applications, but also great challenge for supporting technologies. The key parameter of all optical components in laser and beam delivery structure is the laser induced damage threshold, which limits intensities manageable by the system. The following paper presents results of LIDT test of mirrors intended to use in laser system built within the HiLASE project as well as advanced LIDT test station design, which will use HiLASE laser as source.

The comparison of laser-damage-densities (LDD) measurements performed with pulsed laser radiation at different facilities is tricky due to numerous parameters involved. These parameters, namely pulse length, profile, and frequency, beam size, as well as method of damage detection have significant impact on final result. Previous methods and suitable data processing developed to determine with accuracy and repeatability the LDD allow us to achieve this comparison, e.g. the reproducibility. Since such studies are related to the life-time predictions for large aperture optical materials used in high-power lasers, the question that is addressed in this presentation concerns the representativeness of such results as regards of laser damage with large and real beams. Tests with large beams of centimetric size on a high power laser facility have beam performed according to a parametric study and are compared to small beam laboratory tests. More the emphasis is on the optical component thickness that may affect both damage initiation and damage growth due to the occurrence of non-linear effects that intensify damage issue.

Anthraquinones are a class of organic dyes with a wide range of uses in optical devices that require intensities above the damage threshold for operation. It has been demonstrated that some anthraquinones doped into (poly)methyl methacrylate(PMMA) demonstrate the novel effect of self healing. One theory of decay and self healing is photocharge ejection and recombination. Using digital imaging we probe the electric field dependent decay and recovery of the anthraquinone disperse orange 11(DO11) doped into PMMA. We find that the electric field works to mitigate decay and improves recovery, as well as find that for large fields the sample appears to recover beyond 100%.

The role of the Hf nanoclusters as near-UV, nanosecond-pulse laser-damage initiators in HfO₂ and SiO₂-pair–based multilayer coatings remains speculative. In this work we use photothermal heterodyne imaging (PHI) to investigate absorption in HfO₂ and SiO₂ monolayers containing embedded nanometer-sized Hf clusters produced by backsidethrough- thin-film ablation. Hf cluster size and areal-density distributions were characterized using transmission electron microscopy. PHI measurements were taken for cluster-containing samples and for similarly prepared HfO₂ and SiO₂ film samples of the same thickness without clusters. These data allow us to evaluate a possible role in the damageprocess initiation of two hypothetical sources of the localized absorption—Hf clusters and high-density areas of electronic defects.

A photo-thermal measurement device for quantitative determination of absorptance in DUV optics was developed. It is based upon a Hartmann-Shack wavefront sensor with extreme sensitivity, accomplishing spatially resolved monitoring of thermally induced wavefront distortions. Caused by the temperature dependence of the refractive index as well as thermal expansion, the initially plane wavefront of a test laser is distorted into a convex or concave lens, depending on sign and magnitude of index change and expansion. Since the extent of deformation is directly proportional to the absorption loss, the parallelized photo-thermal technique can be employed for a rapid assessment of the material quality. Monitoring the fluence dependence of the thermal lens effect accomplishes evaluation of both single- and two-photon absorption coefficients. Moreover, a separation of surface and bulk absorptance can be achieved. Along with a description of the technique we present results from absorption measurements on fused silica and CaF₂ under 193nm irradiation. The data are compared with theoretical results obtained from a solution of the heat diffusion equation.

Laser damage measurements share similarities with testing of explosives, namely the sample or sample site is damaged or modified during the measurement and cannot be retested. An extensive literature exists for techniques of measurement of the “all fire” and “no fire” levels for explosives. These levels hold direct analogy to the “all damage” or 100% probability of damage or the “all safe” or 0% probability of damage. The Maximum Likelihood Estimate method, which is the foundation of this technique, is introduced. These methods are applied to an archetypal damage probability model and the results shown to be accurate and unbiased.

In this paper new laser-induced damage threshold testing system operating in broad range of pulse repetition rates (from 0.02 Hz up to 200 kHz) is introduced. The system is capable to test either bare or coated optical components, used for high average and peak power femtosecond laser applications. Pulses of tunable duration (300 - 5000 fs) from diode pumped Yb:KGW solid state laser are employed at fundamental wavelength (1030 nm) and its II-IV harmonics (515 nm, 343 nm and 258 nm). Thanks to advanced adaptive damage detection technique so called S-on-1 tests are performed with single shot resolution. The capabilities of the system were characterized and demonstrated on niobia and zirconia - single layer dielectric coatings at different repetition rates.

In this paper, we give the third installment of our ongoing investigation into the nature of the laser survivability curve (LSC). In this year’s report, We examine a set of identically polished samples coated with the same AR coating design. One set coated using IAD process and the other e-beam. In the samples investigated show similar asymptotic behavior within manufacturing methods, but each technique behaves differently.

The automated laser damage threshold test systems of different test modes are developed recently with micron-scale damage events automated detection, location and re-inspection. The system is carried out using a 10 ns pulsed Nd:YAG laser with a repetition rate of 10 Hz. In view of the requirements of weak site identification and growth test for initial damage sites, we pay more attention to the raster scan protocol. The automated test system is enabled by the pulsed stage movement method. A one pulse to one image correspondence have been set up during scans, which is available for the later confirmation of the automated damage detection results and the growth study at specified test sites. The new and grown defects are decided by comparing the pre-image and current image at the same place during different scans. Currently, the defect comparison rules and tolerance are being optimized to improve the accuracy of test systems.

In this study the applicability of commonly used Damage Frequency Method (DFM) is addressed in the context of Laser-Induced Damage Threshold (LIDT) testing. A simplified computer model representing the statistical interaction between laser irradiation and randomly distributed damage precursors is applied for Monte Carlo experiments. The reproducibility of LIDT predicted from DFM is examined under both idealized and realistic laser irradiation conditions by performing numerical 1-on-1 tests. A widely accepted linear fitting resulted in systematic errors when estimating LIDT and its error bars. For the same purpose a Bayesian approach was proposed. A novel concept of parametric regression based on varying kernel and maximum likelihood fitting technique is introduced and studied. Such approach exhibited clear advantages over conventional linear fitting and led to more reproducible LIDT evaluation. Furthermore LIDT error bars are obtained as a natural outcome of parametric fitting which exhibit realistic values. The proposed improvements are of practical importance in LIDT metrology.

In this contribution we present a technology for deposition and testing of interference coatings for optical components designed to operate in power pulsed lasers. The aim of the technology is to prepare components for high power laser facilities such as ELI (Extreme Light Infrastructure) or HiLASE. ELI is a part of the European plan to build a new generation of large research facilities selected by the European Strategy Forum for Research Infrastructures (ESFRI). These facilities rely on the use of diode pumped solid state lasers (DPSSL). The choice of the material for the lasers' optical components is critical. Some of the most important properties include the ability to be antireflection and high reflection coated to reduce the energy losses and increase the overall efficiency. As large amounts of heat need to be dissipated during laser operation, cryogenic cooling is necessary. The conducted experiments served as preliminary tests of laser damage threshold measurement methodology that we plan to use in the future. We designed a special apparatus consisting of a vacuum chamber and a cooling system. The samples were placed into the vacuum chamber which was evacuated and then the samples were cooled down to approximately 120K and illuminated by a pulsed laser. Pulse duration was in the nanosecond region. Multiple test sites on the sample's surface were used for different laser pulse energies. We used optical and electron microscopy and spectrophotometer measurements for coating investigation after the conducted experiments.

Positive-tone diazonaphthoquinone / novolak (DNQ / novolak) resist was stripped from Si wafer by using a pulsed laser beam from visible to near infrared. Silicon wafer with resist was sunk in water to utilize irradiated laser energy effectively. When the resist was irradiated with the fundamental wavelength of the Nd:YAG laser, the resist was stripped from the Si wafer. No damage could be detected from the processed silicon wafer surface. The resist stripping effect in water condition was improved due to both the thermal expansion of the Si wafer and pressure from water. And also, laser irradiation of wavelength 532 nm, having large photon energy, was found to have a higher resist stripping effect than that of wavelength 1064 nm.

The new sandwich concept for absolute photo-thermal absorption measurements using the laser induced deflection (LID) technique is introduced and tested in comparison to the standard LID concept. The sandwich concept’s idea is the decoupling of the optical materials for the pump and probe beams by placing a sample of investigation in between two optical (sandwich) plates. The pump beam is guided through the sample whereas the probe beams are deflected within the sandwich plates by the thermal lens that is generated by heat transfer from the irradiated sample. Electrical simulation and laser experiments reveal that using appropriate optical materials for the sandwich plates, the absorption detection limit for photo-thermally insensitive materials can be lowered by up to two orders of magnitude. Another advantage of the sandwich concept, the shrinking of the currently required minimum sample size, was used to investigate the laser induced absorption change in a Nd:YVO₄ crystal at 1030nm. It was found that the absorption in Nd:YVO₄ lowers due to the laser irradiation but partially recovers during irradiation breaks. Furthermore, absorption spectroscopy has been performed at two LBO crystals in the wavelength range 410...600nm to study the absorption structure around the SHG wavelengths of common high power lasers based on Neodymium doped laser crystals.

An automated test station to measure the laser-induced damage threshold (LIDT) according to ISO 21254-1,2,3,4:2011 standards is presented. The laser is a single longitudinal mode, 500 mJ, 6 ns, Q-switched, 10 Hz, linearly polarized, 1064 nm laser, with 2-nd and 3-rd harmonic capabilities. The machine is able to operate the S-on-1 test (S = 500), or the Type 2 endurance (durability) test. The main blocks of the station are described, emphasizing some original solutions. Preliminary results of LIDT measurements using the S-on-1 test on several coatings and on uncoated fused silica substrates with various degrees of roughness are also presented.

The reliable online detection of damage events is a very important part in the measurement of multi-pulse laser-induced damage thresholds (LIDT) of optical components. In many measurement protocols, a certain threshold for radiation scattered by the test site is defined as damage criterion. Nevertheless, for example in the measurement of uncoated substrates or antireflective coatings online damage detection on the basis of scattering is often observed to be of less consistency. In such cases accumulation effects over a large number of pulses may appear and eventually result in a catastrophic failure of the optic before a significant indication of the scatter detector. As a consequence, valuable information on the nature of the damage may be lost. In this paper the use of online reflectance- and transmittance-measurements as a different kind of switch-off criterion for LIDT-measurements is considered. The measurements are performed at 266nm using a Q-switched ns-laser-source. Advantages and disadvantages of the detection system are discussed and compared to a typical online scatter detection system. Slight changes in reflectance and transmittance might be used as a pre-indicator of a catastrophic failure of the optic and may allow for an interpretation of LIDT-measurements in the context of lifetime testing.

Laser machining of optics to mitigate surface defects has greatly enhanced the ability to process large optics such as those found in fusion-class lasers. Recently, the use of assist reactive gases has shown promise in enhancing manifold etching rates relative to ambient conditions for CW-laser exposures. However, these methods still require significant heating of the substrate that induce residual stress, redeposit coverage, material flow, and compromise the final surface finish and damage threshold. While very reactive fluorinated gases are capable to reduce treatment temperatures even further, they are also inherently toxic and not readily transferable to large processing facilities. In this report, we look at whether a short-lived gas plasma could provide the safe and effective etchant sought, while still reducing the thermal load on the surface. We test this approach using a YAG laserinduced gas plasma to act as a source of the etchant for fused silica, a common optical material. The configuration and orientation of the beam and optical apparatus with respect to the surface was critical in preventing surface damage while etching the surface. Results with N₂ and air gas plasmas are shown, along with a description of the various experimental implementations attempted.

All laser targets subjected to heating by focused energetic lasers for the study of high temperature plasmas produce shrapnel, debris or radiation directly or indirectly that impact and coat the optics, instruments and surfaces used in the vacuum chambers employed for such studies. We describe the spatial distributions of the target ejecta arising from various configurations of thin walled metal cylinder targets and their mechanical mounting systems that have been deployed on the Helen and ZBeamlet lasers. We also demonstrate how this data can be used to evaluate threats to optical surfaces and ancillary instruments in high energy, high power laser systems. The methods used for characterizing and quantifying material plumes will also be described.

Laser-induced damage threshold (LIDT) in optical coating is very sensitive to organic contaminations accumulated in coating layers during storage and using condition. The sources of contamination are commonly exists, and optical coatings are easily contaminated regardless to the environment pressure, LIDT at ns region decreased largely by contamination, but LIDT at ps seems insensitive. In this study, we have investigated the influence of contamination of optical coating on LIDT and other optical properties. We examined several kinds of coating to clarify the sensitivity to the contamination. Degradations of LIDT were commonly observed in e-beam deposition, IAD and IBS. Some coatings changed spectral characteristics by contamination, and other coatings did not change. Some samples were contaminated as received condition, and some were very clean. Furthermore, we have investigated the characteristics of LIDT in dielectric coatings under the controlled contamination. LIDT of coating drops to 1/2 in the saturated toluene vapor at room temperature.

The lifetime of optical components submitted to high laser fluences is decreased under organic contaminated environment. Our previous studies have shown that chemical species outgassed from materials present in the laser environment of the Ligne d’Intégration Laser (LIL) and in the optics packaging (phthalates, silicones, and aromatic compounds) are potential contaminants for optics. In order to avoid the presence of such molecules in the Megajoule Laser (LMJ) environment, a new comprehensive program is started up using a qualified Micro-chamber/Thermal Extractor (M-CTE250 Markes International) for controlled contaminations of optics. The final target is the development of a qualification procedure to determine the compatibility of materials used for the building of the LMJ with the LMJ optics. First results of this program will be presented.

Various scratches on fused silica optics after polishing have been characterized with confocal microscopy and then tested with nanosecond UV laser. Scratches are identified as a major contributor to laser damage even if they are only a few micrometers wide. We propose a process in order to remove these scratches whose depth ranges from 2 to 16 μm. We use a CO₂ laser to heat the scratched areas at high temperature which will heal fractures due to viscous flow. The characterizations were completed by laser damage tests that finally prove the effectiveness of the repair. We conclude also that this repair process proves to be fast, localized to the scratch and clean because no debris are generated.

Despite the growing improvement in optical polishing and deposition technologies optical resistance of the laser components used for high-power UV applications remains insufficient in many cases. In this study influence of different fused silica substrate preparation, post treatment processing and deposition techniques are examined in terms of surface roughness, optical scattering and laser damage performance. The conventional techniques of polishing, etching, and finally surface cleaning of substrates have been investigated. Further, a part of samples were also coated with SiO₂ monolayer by Ion Beam Sputtering (IBS) technique. Surface quality was characterized prior to and after the treatment and deposition processes by the means of total integrated scattering (TIS) and atomic force microscopy (AFM). The experimental results of surface roughness measurements exhibited a good correlation between AFM and TIS methods. Further optical resistance was characterized with 10 ns duration pulses for 355 nm wavelength laser radiation performing 1-on-1 sample exposure test with high resolution micro-focusing approach. A dominating damage precursor ensembles produced during manufacturing processes were identified and directly compared. Finally, the conclusions about the quality influencing factors of investigated processes were drawn.

The laser damage resistance (LDR) is a measure of the laser fluence that a coating can withstand without damaging when exposed to a large number of pulses. The LDR of UV coatings has been studied at 266 nm on two common substrate materials. Significantly higher values for the LDR have been measured for the same coating deposited on CaF₂ substrate compared to fused silica substrates. Various parameters such as the surface roughness, the absorption and the subsurface damage of these quite different materials were measured in an effort to explain the performance difference. The laser damage morphologies of the coatings were also studied.

This paper describes a novel approach for the suppression of contamination enhanced laser damage to optical components by the use of fluorinated coatings that repel organic contaminates. In prior work we studied laser damage thresholds induced by ppm levels of toluene under nanosecond 1.064 μm irradiation of fused silica optics. That work showed that moderate vapor-phase concentrations (< 15%) of water and alcohols dramatically increased the laser damage threshold. The data are consistent with the hypothesis that water and alcohols interact more favorably with the hydroxylated silica surface thereby displacing toluene from the surface. In this work, preliminary results show that fluorinated self assembled monolayer coatings can be used to accomplish the same effect. Optics coated with fluorinated films have much higher survival rates compared with uncoated optics under the same conditions. In addition to enhancing survival of laser optics, these coatings have implications for protecting spacecraft imaging optics from organic contamination.