Managing subsurface damage during the shaping process and removing subsurface damage during the polishing process is essential in the production of low damage density optical components, such as those required for use on high peak power lasers. Removal of subsurface damage, during the polishing process, requires polishing to a depth which is greater than the depth of the residual cracks present following the shaping process. To successfully manage, and ultimately remove subsurface damage, understanding the distribution and character of fractures in the subsurface region introduced during fabrication process is important. We have characterized the depth and morphology of subsurface fractures present following fixed abrasive and loose abrasive grinding processes. At shallow depths lateral cracks and an overlapping series of trailing indentation fractures were found to be present. At greater depths, subsurface damage consists of a series of trailing indentation fractures. The area density of trailing fractures changes as a function of depth, however the length and shape of individual cracks remain nearly constant for a given grinding process. We have developed and applied a model to interpret the depth and crack length distributions of subsurface surface damage in terms of key variables including abrasive size and load.

We have developed an experimental technique that combines magnetorheological finishing (MRF) and microscopy to examine fractures and/or artifacts in optical materials. The technique can be readily used to provide access to, and interrogation of, a selected segment of a fracture or object that extends beneath the surface. Depth slicing, or cross-sectioning at selected intervals, further allows the observation and measurement of the three-dimensional nature of the sites and the generation of volumetric representations that can be used to quantify shape and depth, and to understand how they were created, how they interact with surrounding material, and how they may be eliminated or mitigated.

Understanding the behavior of fractures and subsurface damage in the processes used during optic fabrication plays a key role in determining the final quality of the optical surface finish. During the early stages of surface preparation, brittle grinding processes induce fractures at or near an optical surface whose range can extend from depths of a few μm to hundreds of μm depending upon the process and tooling being employed. Controlling the occurrence, structure, and propagation of these sites during subsequent grinding and polishing operations is highly desirable if one wishes to obtain high-quality surfaces that are free of such artifacts. Over the past year, our team has made significant strides in developing a diagnostic technique that combines magnetorheological finishing (MRF) and scanning optical microscopy to measure and characterize subsurface damage in optical materials. The technique takes advantage of the unique nature of MRF to polish a prescribed large-area wedge into the optical surface without propagating existing damage or introducing new damage. The polished wedge is then analyzed to quantify subsurface damage as a function of depth from the original surface. Large-area measurement using scanning optical microscopy provides for improved accuracy and reliability over methods such as the COM ball-dimple technique. Examples of the technique's use will be presented that illustrate the behavior of subsurface damage in fused silica that arises during a variety of intermediate optical fabrication process steps.

The laser damage threshold (LDT) of single crystal zinc germanium phosphide (ZGP), ZnGeP₂, was measured to be 2 J/cm² by the S-on-1 method. This LDT was double the previously measured value of 1 J/cm² and was achieved by improving the polishing technique for ZGP OPO crystals. ZGP is the nonlinear optical crystal of choice for laser frequency conversion in the 2-8 μm spectral range due to properties including its high non-linear coefficient (d₁₄=75 pm/V) and thermal conductivity (0.35 W/cm K). The surface preparation of ZGP parts was determined to be of great importance because laser-induced damage has been observed to always initiate at the surface rather than in the bulk of the material. In this study, the surfaces of ZGP parts fabricated in the same manner apart from the polishing stage were quantitatively examined. Two different polishing techniques were examined, and both uncoated and anti-reflection coated parts were examined for each polishing technique. Surfaces were characterized using scanning white light interferometry (SWLI) in order to determine RMS surface roughness and sample flatness. The photon backscatter technique (PBS) was used to determine the degree of surface and subsurface damage in the sample induced through the fabrication process. Statistical LDT was measured using a high-average-power, repetitively Q-switched Tm,Ho:YLF 2.05-μm pump laser. Laser induced damage was observed after each exposure by examining the site where the laser beam entered the ZGP sample using optical microscopy. On average, lower surface roughness and photon backscatter measurements were a good indicator of ZGP parts exhibiting higher LDT.

In this paper we report several studies of the propagation of the Laser Integration Line (LIL) and Laser Megajoule (LMJ) beams when interactions occur with optical components defects. These studies are mainly achieved on a numerically predictive basis with the CEA MIRO beam propagation code. The flaws that we considered are located at the front or rear surface of the optical components. These surface flaws correspond to engineered defects such as scratches and to surface damage resulting from laser-induced growth process, from mitigation process or from target interactions debris. Results account for the possible downstream impacts of flaws at the rear-surface of the optics and from one component to another along the laser chain. In particular, the influence on the LIL/LMJ end-of-line focal spot intensity and size is predicted.

A major issue in high power lasers for fusion is laser-induced damage on optical components. Since damage is often initiated by a surface crack resulting from polishing, it is important to understand the physics involved in this phenomenon. In this study, calibrated surface scratches have been realized on two silica samples using an indenter-scratcher apparatus. A variety of scratches have been tested by applying different speeds and forces on the scratcher needle. Optical microscope observations show that the scratches made at highest speed create irregular dashed lines. In addition, we have observed, at intermediate speed, an evolution in time of the scratches due to local stresses in silica. One of the samples was irradiated by a Nd:YAG laser beam at 355 nm with the scratches on the exit surface. Microscope observations were made before and after irradiation. Strong dependence on the scratch speed was observed on the local laser damage. Again, temporal evolution of the damage has been observed.

We present a multi-parametric experimental investigation of laser conditioning efficiency and behavior in KDP and DKDP crystals as a function of laser wavelength, fluence, number of pulses, and conditioning protocol. Our results expose complex behaviors associated with damage initiation and conditioning at different wavelengths that provide a major step towards revealing the underlying physics. In addition, we reveal the key parameters for optimal improvement to the damage performance from laser conditioning.

In laser systems using frequency conversion, multiple wavelengths will be present on optical components. We have investigated the growth of laser initiated damage in fused silica in the presence of multiple wavelengths. In particular, we measured growth at 351 nm in the presence of 1053 nm near the threshold of growth for 351 nm alone. The data shows that the sum fluence determines the onset of growth as well as the growth rate. The measured growth coefficient is consistent with all the energy being delivered at 351 nm. Additionally, we measured growth at 527 nm in the presence of 1053 nm near the threshold of growth at 527 nm alone. In this case, the sum fluence also determines the growth coefficient but the rate is consistent with all the energy being delivered at 1053 nm. We present the measurements and discuss possible reasons for the behavior.

Multiple laser irradiations induce a critical issue as regards the time of life of optical components. The problem can appear either in high repetition rate lasers or in high power systems even at low frequency. Two opposite behaviors are commonly observed under repetitive irradiations. A "fatigue effect" of materials under subsequent shots is generally observed and results in a decreasing of laser induced damage threshold (LIDT), but in some cases an improvement of LIDT can be noticed. This second effect linked to the pre-irradiation is well known as "conditioning" of the material. In most cases the LIDT in optical components is specified in 1:1, S:1 or R:1 modes, whatever the application of the real system. The aim of this paper is to show that the LIDT is strongly dependant on the parameters of irradiation such as shot number, shot frequency, wavelength and location in the material (surface or bulk). Therefore in order to approach a "true" value of LIDT it is necessary to test the component in the conditions of use, considering all the influential parameters. To illustrate this purpose the influence of previous parameters is studied for KDP and silica. This study shows that we can define a "functional laser damage threshold" in repetitive shot mode and also that the time of life could be deduced for each component. Furthermore these results can be useful to optimize the parameters involved in the conditioning processes.

The analysis of modifications induced by laser damage in optical materials is important for understanding the damage process. In this work, we study the morphological and structural modifications induced by a nano-second pulsed laser (355 and 1064nm) on fused silica samples (Suprasil, Herasil) and silica thin films (deposited by IP, IAD, EBD, IAD). The morphological changes are analyzed using optical microscopy, optical 3D surface profiler. The absorption modifications are measured by photothermal microscopy with a micronic resolution. Luminescence (excited at 244nm) mappings are also performed on damage sites and the luminescence spectra analyzed. Based on these measurements, we study the different laser damage step that we have identified: initiation by absorbing nanoscale defects, heating of the defect and the surrounding matrix, modification of the surrounding material that becomes absorbing, and macroscopic damage at final. We identify and spatially resolve several kinds of defects induced by catastrophic breakdown and we also discuss the observed structural changes on damage sites, taking into account the physical processes involved.

At the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL), mitigation of laser surface damage growth on fused silica using single and multiple CO₂ laser pulses has been consistently successful for damage sites whose lateral dimensions are less than 100 μm, but has not been for larger sites. Cracks would often radiate outward from the damage when a CO₂ pulse was applied to the larger sites. An investigation was conducted to mitigate large surface damage sites using galvanometer scanning of a tightly focused CO₂ laser spot over an area encompassing the laser damage. It was thought that by initially scanning the CO₂ spot outside the damage site, radiating crack propagation would be inhibited. Scan patterns were typically inward moving spirals starting at radii somewhat larger than that of the damage site. The duration of the mitigation spiral pattern was ~110 ms during which a total of ~1.3 J of energy was delivered to the sample. The CO₂ laser spot had a 1/e²-diameter of ~200 μm. Thus, there was general heating of a large area around the damage site while rapid evaporation occurred locally at the laser spot position in the spiral. A 30 to 40 μm deep crater was typically generated by this spiral with a diameter of ~600 μm. The spiral would be repeated until there was no evidence of the original damage in microscope images. Using this technique, damage sites as large as 300 μm in size did not display new damage after mitigation when exposed to fluences exceeding 22 J/cm² at 355 nm, 7.5 ns. It was found necessary to use a vacuum nozzle during the mitigation process to reduce the amount of re-deposited fused silica. In addition, curing spiral patterns at lower laser powers were used to presumably "re-melt" any re-deposited fused silica. A compact, shearing interferometer microscope was developed to permit in situ measurement of the depth of mitigation sites.

For the determination of the scatter behavior of materials with different aggregate states such as solid components, liquids and particles an Angle Resolved Scatter (ARS) measurement set-up has been developed at LZH. The set-up can be operated in conjunction with different laser sources and arc lamps. Spectral investigations in the range from the DUV to the NIR can be performed by using a monochromator and filters. Mappings are possible by moving of the test specimens in two spatial directions. In this paper, we present result on efficiency measurements for holographic gratings in the visible spectral range. The spatial transfer functions of transmittive gratings are measured very precisely at 633nm. Also, scatter characteristic of nano and micro particles in liquids as well as the scatter distribution of some functional dielectric coated samples are discussed.

Molecular contamination in laser systems presents a significant risk to laser operation. The principle reason for molecular contamination being a significant risk is the lack of knowledge concerning the interactions of the contaminants, optics, laser radiation, and intra-laser environment. A long term instrumented vacuum operation test was carried out to investigate the potential contamination effects in a laser. The test provided information concerning the behavior of the laser operation of a two wavelength 532 and 1064nm q-switched Nd:YAG, the behavior of the residual contamination and residual gases in the vacuum environment. The interactions of the contaminants and the residual gases in the system in the presence of laser radiation are discussed.

In the midst of the Mega Joule Laser project, a study of the impact of organic contamination on optical surfaces has been launched. Last year, we presented results on intentionally contaminated optics by outgassing products of a typical material of the LIL (Ligne d'Integration Laser, the prototype laser line of the future LMJ). A small quantity of organic contamination deposited on high reflective mirrors decreased their R/1 laser induced damage threshold. As the LIDT R/1 test procedure may "condition" the optical component, further raster scan tests have been implemented on new intentionally contaminated samples to assess the test procedure impact on the LIDT results for different contaminations. The aim of this work is double: -First, the impact of organic contamination deposited on optical surfaces by outgassing will be evaluated by laser induced damage threshold measurement, after a laser shot at nominal fluence ; -The second objective is to evaluate the real effects of "conditioning", notably towards organic contamination deposited on optics.

This paper presents recent studies of the propagation of high-power laser beams like Laser Integration Line (LIL) and Laser Megajoule (LMJ) beams when interactions occur with environmental pollution particles. The studies are mainly achieved with the CEA-DAM MIRO beam propagation code. The highest intensifications in the downstream propagation are obtained for phase objects such as dielectric particles rather than for amplitude objects such as metallic particles. Dramatic amplifications of Kerr nonlinear effects both inside the component and at its rear-surface can occur depending on the particle size.

We investigated laser-induced deposition processes on BK7 substrates under the influence of pulsed Q-switched Nd:YAG laser radiation, starting from small toluene partial pressures in a background vacuum environment. The composition and structure of the deposit was analyzed using microscopic methods like Nomarski DIC, dark-field and white-light interference microscopy, TEM, EDX and XPS. We found a distinct threshold for deposition built-up dependant on the partial pressure of toluene (0.2 J/cm² at 0.1 mbar, 0.8 J/cm² at 0.01 mbar toluene). The deposits strictly followed the spherical geometry of the laser spot. No deposit accumulated on MgF₂ AR coated BK7 samples even at high toluene partial pressures. The onset of deposit was accompanied by periodic surface ripples formation. EDX and XPS analysis showed a carbon-like layer which strongly absorbed the 1 μm laser radiation. The typical number of shots applied was 50 000. In addition, long term lifetime tests of more than 5 Mio. shots per site were run.

Detailed surface chemical analysis of laser induced damage to a Nd:YAG total internal reflection (TIR) slab, in conjunction with knowledge of optical behaviors can provide the identification of not only the damage that occurred, but how it was initiated. Careful evaluation of surface analytical results can provide a great deal more information. Laser damage occurring in high quality optical components at laser intensities well below expected damage thresholds are anomalous. Detailed chemical surface analysis combined with fundamental knowledge of optics provided not only identification of the precipitating mechanism but, identification of issues in optical component manufacture and complex chemical reactions within the affected region. Laser-contaminant, contaminant-contaminant, and contaminant-optic reactions were detected.

The theoretical treatment of laser-induced damage to optical materials has in the past been largely based upon phenomenological observations, empirical treatments and the non-linear effective medium approximation. In some instances such as intrinsic damage thresholds, these approaches show merit. In many other cases, such as those related to contamination, laser optical damage, specific treatment of both matter and energy is required. The base assumptions of some of the more common theories of laser material interactions are discussed and their effects upon the predicted behavior identified. While this paper does not provide a quantitative solution to the issue of laser damage thresholds, it provides physically sound descriptions of interactions and points the way to potential solutions.

In this study, the environment inside an operational laser system was monitored over a period of three months using a surface acoustic wave sensor. The environment experienced by the sensor was subject to repeated vacuum pumpdown, nitrogen purge and chemical flow processes. The data collected during this period demonstrated the fact that this type of sensor is subject to both accumulation and desorption mechanisms. Surface conditions were clearly active and changing over time. By tailoring the sensor surface to be equivalent to that of the optical coatings in the system, it was believed that the sensor provided an excellent view of the condition of the surface of those optical coatings. Monitoring a system using a device of this type may, in the near term provide some knowledge of readiness. In the long term, this type of monitoring may assist in the selection of compatible materials and effective design for control of contamination.

The implementation of CPA on one of the HELEN laser beams has prompted the need for investigations into the survivability of reflective and refractive focussing optics during laser-target interaction experiments. These experiments generate debris that has the potential to contaminate the optics cumulatively over time, and in some cases produce high-energy shrapnel, which may damage the focussing optics on an individual shot. Inserting a thin (2mm) coated glass substrate into the beam path allows for its use as a mitigation shield by obstructing the line of sight for debris and shrapnel to travel directly from the target to the focussing optic. A series of experiments have been conducted using such debris shields on the CPA beam line at the HELEN laser facility AWE, Aldermaston. The influence of the debris shield on focal spot behaviour will be described. Directionality and contamination density of target debris has been characterised by the use of thin glass witness plates and post-shot inspection by optical microscopy.

It is still assumed that optical components submitted to laser fluences orders of magnitude below their laser induced damage threshold (LIDT) will last for ever. However, depending upon environmental conditions, the accumulation of low fluence laser pulses leads to a progressive contamination and eventually to a damage of the optical components. In order to study the physics of the laser induceded contamination, a laser test bench has been developed. The experimental cell is dry-pumped and a mass spectrometer controls the environment around the optical component. An infrared camera diagnosis follows the sample surface temperature. This paper contains preliminary results obtained on anti-reflective coatings on fused silica tested at 532 nm with a pulse repetition rate of 10 kHz and a pulse width of 100 ns.

Pulsed laser cleaning of sub- and micron-sized (0.3-10 μm) monodispersed model spherical polystyrene particles and fused silica particles from fused silica and glass optical surfaces was performed by means of ns TEA CO₂ laser. Efficient removal of these absorbing particles has been demonstrated in certain laser fluence range, which is below the threshold for ablative damage of the fused silica and glass substrate. Removal mechanisms of dry and steam laser cleaning of various critical optical surfaces are discussed.

In operational laser systems, it is often difficult to keep optical components completely free of foreign material. We have investigated the performance of high damage threshold 1.053 μm high reflectors in the presence of surface contaminants. We have looked at the impact of stainless steel, aluminum, Azurlite^(R), dust, cotton fibers and polyester fibers on the performance of the mirrors under laser irradiation. The first four contaminants were deposited in sizes ranging from 30 microns to 150 microns. The fibers included lengths ranging to several millimeters. The testing was done at either a single fluence in the range of 6 J/cm² to 24 J/cm², or a ramped sequence of shots starting at 1 J/cm². We will present data showing the onset of damage, the type of damage, and the propensity to damage growth in the fluence range studied.

A key lesson learned from the earliest optics installed in the National Ignition Facility (NIF) was that the traditional approach for maintaining cleanliness, such as the use of cleanrooms and associated garments and protocols, is inadequate. Assembly activities often negate the benefits provided by cleanrooms, and in fact generate contamination with high damage potential. As a result, NIF introduced "clean assembly protocols" and related practices to supplement the traditional clean room protocols. These new protocols included "clean-as-you-go" activities and regular bright light inspections. Introduction of these new protocols has greatly reduced the particle contamination found on more recently installed optics. In this paper we will describe the contamination mechanisms we have observed and the details of the clean assembly protocols we have successfully introduced to mitigate them.

One of the major issues met in the operating of high power lasers concerns the cleanliness of laser components. In this context, in order to assess laser-induced damage in presence of metallic particulate contamination, we study the behaviour of aluminum on a silica substrate. Model samples containing calibrated aluminum square dots of 50 x 50 μ² have been deposited by photolithography on a silica substrate. The sample was irradiated by a Nd:YAG laser at 1064 nm with different fluences and also different numbers of shots on each dot. Then the initial aluminum dot zone and the surrounding silica were analyzed using Nomarski microscopy, profilometry and photothermal microscopy. Laser fluence is revealed to be a very important parameter for the behaviour of aluminum dots. For example, it is possible to find a fluence of irradiation where aluminum dots are blown off the substrate and only small modifications occur to silica. In this case, increasing the number of shots doesn't significantly affect the silica surface.

Recently, a new glass composition - oxyfluoride glass (OFG^TM) - has been promoted as an ideal solution of the Airborne Laser (ABL) window problem in the sense that it will allow obtaining large "athermal" windows for high-energy lasers (HEL) operating at the chemical oxygen-iodine laser (COIL) wavelength [K. Billman, et al., Proc. SPIE 5647, 207 (2005)]. In this context, we emphazise that the performance of a laser-window material candidate must be assessed not only in terms of its ability to transmit high-power beams without generating undue optical distortion but also in terms of the constraints imposed by stress-related failure modes. Here, we provide the tools to carry out an analysis of both pressure- and beam-induced stresses, in addition to thermally induced aberrations, and illustrate the procedure through model windows made of (111)-oriented CaF₂, fusion-cast CaF₂, and OFG^TM glass. Regarding OFG, we find that (a) this material will be vulnerable to surface compressive stresses on account of its poor thermal conductivity; (b) the stress-birefringence contribution to optical distortions cannot be ignored, which rules out creating a zero-distortion ABL window; and (c) based on Strehl ratios, and in the absence of stress-driven failure modes, OFG outperforms Si0₂ but does not match the performance of fusion-cast CaF₂. Regarding CaF₂, we find that (a) fusion-cast CaF₂ exhibits substantial stress-induced birefringence, which prohibits using this material if depolarization is an issue; (b) highly-oriented (111)CaF₂ exhibits no stress-birefringence at the COIL wavelength, in accord with previous investigations at HF/DF frequencies; and (c) in principle, (111) CaF₂ windows may transmit aberration-free beam fluences in the 1-MJ/cm² range but will require improvements in strength to achieve reliable designs.

Dominating ionization mechanisms are revealed for bulk dielectrics irradiated by ultrashort (femtosecond) laser pulses, analyzing reported in literature well-established and consistent experimental data dealing with dependences of electronhole plasma density on laser intensity, damage thresholds and bulk damage (surface crater) size on dielectric bandgaps. Multi-photon ionization was found to be predominant at low laser intensities (I_las<I_break≈10-50 TW/cm²), while at higher intensities - I_las>I_break - ionization processes are strongly damped by unknown non-linear mechanisms, one of those could be Auger recombination, and are followed by microscopic damage of dielectric materials. In the latter range of I_las the most accurate source of experimental information on ionization processes is scaling relationships between damage thresholds for corresponding dielectrics and their dielectric bandgaps, fs-laser pulsewidth, wavelength and polarization, as well as spatial size of damage (surface crater) on bandgap width or laser pulse energy. Theoretical analysis of these experimental data and some experimental scaling relationships provides an important insight into interplay of various ionization mechanisms and Auger recombination, supported by results of our numerical calculations of EHP density vs. I_las, and enables to explain other separate important experimental facts on high-intensity ionization in bulk dielectrics.

Recently we have discovered a new regime of ionization taking place in wide band-gap crystalline dielectrics under action of high-intensity laser radiation. Its characteristic feature is occurring of a singularity on dependence of ionization rate on laser intensity what corresponds to blow-up increasing of electron density in conduction band of irradiated material. Assuming that process referred to as collective ionization to be the starting point of initiating intrinsic bulk single-shot damage, we associate damage threshold with the singularity one. Then we consider some consequences resulting from that assumption. In particular, we analyze dependence of the threshold on laser and material parameters in order to find possibilities of experimental checking of our theoretical predictions.

Using our previously reported model of femtosecond laser ionization in bulk solid dielectrics, density of point defects in a focal volume of arbitrary solid dielectric is predicted as a function of laser intensity I_las. This dependence shows a nonlinear increase with an exponent 3S (where S is the minimum number of laser photons to excite a carrier in a dielectric material over its bandgap) below a certain threshold intensity I_break and with an exponent S above the threshold. Assuming that these point defects coalesce within the focal volume and the volume of the resulting spherical void is proportional to the total number of point defects generated (the point defect density times the focal volume), theoretical curves of void diameter vs. I_las are obtained, exhibiting non-linear increase with the exponents S for I_las<I_break and S/3 for I_las>I_break, respectively. Comparison of these theoretical predictions with experimental data on femtosecond laser microstructuring of diamond demonstrates their good agreement and gives a new insight into damage mechanisms of bulk solid dielectrics irradiated by ultrashort laser pulses.

Laser conditioning, i.e. pre-exposure to less than damaging laser fluence, has been shown to improve the damage resistance of KDP/DKDP frequency conversion crystals. We have extended our damage model, small absorbing precursors with a distribution of sizes, to describe various damage related properties such as damage density and effects of laser conditioning in crystals. The model assumes the rate limiting process for both initiation and conditioning depends on temperature and that separate threshold temperatures exist for either conditioning or damage initiation to occur. This is reasonable in KDP/DKDP since the melting temperature is far below the temperatures associated with plasma formation and damage events. This model is capable of accounting for some recently observed damage-conditioning behaviors.

The Feit-Rubenchik size-selection damage model has been extended in a number of ways. More realistic thermal deposition profiles have been added. Non-spherical shapes (rods and plates) have been considered, with allowance for their orientation dependence. Random variations have been taken into account. An explicit form for the change of absorptivity with precursor size has been added. A simulation tool called GIDGET has been built to allow adjustment of the many possible parameters in order to fit experimental data of initiation density as a function of fluence and pulse duration. The result is a set of constraints on the possible properties of initiation precursors.

Coherent band spectrum renormalization in dielectrics by a strong anisotropic electric field of intense laser pulses is considered in terms of dynamic Franz-Keldysh effect (DFKE). Comparing to permanent DFKE shifts of center-zone (Γ-point) non-degenerate band extrema, DFKE band spectrum renormalization of degenerate center-zone and zone-edge band extrema occurs as their simultaneous shifts and splitting. Theory-group analysis is invoked to construct the DFKE Hamiltonian for typical dispersion curves of valence and conduction bands in dielectric materials.

Nonlinear absorption measurements at 800 nm and 400 nm in single wavelength high reflection (HR) dielectric mirrors were performed, according to the ISO 11551 standard by pulse, gradient and exponential absorption evaluation methods, using pulsed, diode pumped femtosecond laser system with pulse duration ~130 fs. Pulsed laser output at 1kHz repetition rate had 1 W and 0.36 W average power at 800 nm and 400 nm, respectively. The HR mirrors were made of ZrO₂ and SiO₂ layers. The beam was focused into the mirror, and changing the beam power by step attenuator, it was possible to evaluate nonlinear absorption at different intensities up to intensity close to damage threshold. The nonlinear absorptance for 400 nm pulses at the femtosecond pulse intensity 0.8 TW/cm² was 0.48 % and ~20 times exceeded the nonlinear absorptance for the 800 nm pulses.

It was found that high purity soda lime glass shows a markedly different induced absorption spectra when exposed to different types of ionizing radiation such as UV lamp or femtosecond and nanosecond laser pulses. The following irradiation was used in the experiments: nanosecond pulses at the fundamental and harmonics of a Nd:YAG laser (λ = 1064, 532, 355, and 266 nm), femtosecond pulses of a Ti:sapphire laser operating at λ = 780 nm, ultraviolet rays from a high pressure Xe lamp, X-rays, and Gamma rays. Features of radiation defect generations are discussed.

N-on-1 LIDT measurements were performed on ytterbium doped preforms used to make high peak power fiber amplifiers. Damage measurements were complicated by large index of refraction changes across the preforms. These difficulties were overcome by monitoring the beam profile before and after the samples and by only taking data where the transmitted beam was not significantly distorted. Single and 1000 shot data suggest slight laser conditioning of the preforms and rule out laser fatigue in the doped cores and surrounding fused silica. At 1064 nm, inside the emission spectra, there seemed to be little influence of the Yb dopant concentration on the measured LIDT.

Many applications are emerging that require optical coatings with the ability to withstand high fluence and that at the same time possess very low scatter and absorption loss while demonstrating resistance to environmental influences. Optical coatings fabricated with the ion beam sputtering (IBS) process possess these characteristics. This paper provides an overview of the optical and mechanical characteristics of IBS coatings and compares their laser damage threshold characteristics to coating fabricated with electron beam (e-beam) and ion assisted electron beam (IAD) techniques.

Laser conditioning effects of the HfO₂/SiO₂ High-reflective (HR) and Anti-reflective (AR) coatings at 1064 nm and the accumulation effects of multi-shot laser radiation were investigated. The HfO₂/SiO₂ HR and AR coatings were prepared by E-beam evaporation (EBE). The single-shot and multi-shot laser induced damage threshold was detected following ISO standard 11254-1.2, and the laser conditioning was conducted by three-step raster scanning method. It was found that laser damage resistance of the HR coatings for both single-shot and multi-shot laser radiation was enhanced after laser conditioning, and the single-shot laser induced damage threshold (LIDT) got the greatest enhancement. There was obvious shot to shot laser damage accumulations for HR samples and the LIDT decreased as the shot number increased. Laser conditioning can also improve the multi-shot laser damage accumulations behaviors of the HR samples. However, the AR coatings had no laser conditioning enhancement and multi-shot laser damage accumulation effects. A Nomarski microscope was employed to map the damage morphology, and it found that the damage behavior was defect-initiated for both samples.

Using photo-excited silicone oil developed a new protective hard coating method for high power laser to present the tolerance in water. The silicone oil was spin-coated onto the surface of an optical material and then irradiated with a xenon excimer lamp in the air, which transformed the organic silicone oil into inorganic glass. This technique has enabled an optical thin film capable of transmitting ultraviolet rays of wavelengths under 200 nm and possessing the characteristics of homogeneity, high density, resistance to environmental effects and to water, anti-reflective in water, and Mohs scale value of 5.

Thin-film polarizers are essential components of large laser systems such as OMEGA EP and the NIF because of the need to switch the beam out of the primary laser cavity (in conjunction with a plasma-electrode Pockels cell) as well as providing a well-defined linear polarization for frequency conversion and protecting the system from back-reflected light. The design and fabrication of polarizers for pulse-compressed laser systems is especially challenging because of the spectral bandwidth necessary for chirped-pulse amplification. The design requirements for a polarizer on the OMEGA EP Laser System include a T_p greater than 98% over a spectral range of 1053±4 nm while maintaining a contrast ratio (T_p/T_s) of greater than 200:1 (500:1 goal) over the same range. An allowance must be made for the uniformity of the film deposition such that the specifications are met over the aperture of the component while allowing for some tolerance of angular misalignment. Production results for hafnia/silica designs will be shown, illustrating high transmission and contrast over an extended wavelength/angular range suitable for the 8 nm spectral bandwidth of OMEGA EP. Difficulties in production will also be illustrated, as well as the methods being implemented to overcome these challenges. A key challenge continues to be the fabrication of such a coating suitable for use on fused-silica substrates in a dry environment. Laser-damage thresholds for 1-ns and 10-ps pulse widths will be discussed.

Multilayer dielectric (MLD) diffraction gratings are a key component for the construction of high-peak-power, pulse-compressed laser systems. While a great deal of effort has been devoted to the design of optimal grating structures and the etching of these structures into the MLD coating, there has not been the same effort put into the optimization of the MLD coating itself. The primary characteristics of the multilayer that must be considered during design include minimization of the standing wave created in the photoresist because of the reflectivity of the coated optical surface, creation of a sufficiently high reflectivity at the use wavelength and incidence angle in a dry environment, proper balance of the individual layer materials to yield a coating with an overall neutral or slightly compressive stress, and a high laser-damage threshold for the wavelength and pulse duration of use. This work focuses on the modification of a standard MLD mirror, while considering these characteristics, to allow the fabrication of a diffraction grating with higher efficiency and laser-damage threshold than is typically achieved. Scanning electron microscopy (SEM) images of the grating structures demonstrate smoother shapes with lower roughness due to the holographic exposure. Damage testing performed at 1053 nm with a pulse width of 10 ps demonstrates the MLD coating has a sufficiently high laser-damage threshold to form the basis of reflection gratings that survive in high-fluence applications.

An influence of substrate temperature and working gas in coating plant during evaporation process on the laser-induced damage threshold (LIDT) of high reflection dielectric coatings was experimentally investigated. Also a LIDT comparison of ion assisted deposition (IAD) and conventional electron-beam evaporation (non-IAD) coatings fabricated under the same substrate temperature (300 °C) was performed. A set of different type high reflection mirrors were tested for LIDT at 532 nm for 3.4 ns pulses: one type of non-IAD and six types of IAD evaporated at different substrate temperatures and different working gases. All coatings were made on BK7 glass substrates from ZrO₂ and SiO₂. The computer controlled test station for LIDT measurements according to the requirements of current ISO 11254-2 standard was used. All measurements were performed at 10 Hz pulse repetition rate (S-on-1 test). The tests were performed at fixed spot size. Strong LIDT dependence on substrate temperature of was observed.

Lithium triborate, LiB₃O₅ (LBO) is a popular nonlinear optical crystal typically used for frequency conversion. For power scaling of laser radiation in transparency range of LBO crystals with maintaining low reflection losses is very important to reach high optical resistance of anti-reflective dielectric coatings. The measurements of laser-induced damage thresholds (LIDT) of AR coated LBO used for second and third harmonic generation of Nd:YAG lasers were performed at 1064, 532, and 355 nm wavelengths for ~4 ns pulses. Two types of coatings where tested; a dual peak anti-reflection at 1064 nm and 532 nm, three peak anti-reflection at 1064 nm, 532 nm, and 355 nm. Ion beam sputtering and magnetron sputtering technologies were used for coatings deposition. Automated LIDT measurements were performed according to the requirements of current ISO 11254-2 standard. The obtained LIDT were in range of 5-21 J/cm².

Hafnia is one of the most utilized high-index materials in thin-film multilayer coatings for high-power lasers. It is well established that in the HfO₂/SiO₂ multilayers for 351 nm, nanosecond-pulse applications the damage is driven by the nanoscale absorbers localized in the hafnia layers. In this work, damage-crater formation thresholds and morphology are investigated for the undoped HfO₂ monolayer films and films containing isolated gold nanoparticles of 2- and 5-nm average diameter. Atomic force microscopy is used for characterization of the damage craters produced by 351-nm, 0.5 ns laser pulses. This comparative study of crater-geometry variation with laser fluence for doped and undoped films allowed the estimation of properties of the intrinsic absorbing defects in the hafnia films.

High density, improved adhesion and environmental stability are the main features of dielectric optical coatings produced using ion-assisted deposition (IAD) technology. However, investigations of resistance of IAD coatings to intensive laser radiation show controversial results. A series of experiments were done to examine the influence of ion gun operation on the transmittance of fused silica substrates. It was shown that operation of ion source introduced extinction in UV spectral range. Optical properties of single hafnia layers and multilayer dielectric mirrors deposited using conventional e-beam evaporation and different modes of IAD were investigated. Microstructural analysis using X-ray diffraction (XRD) measurements and AFM scanning of coated areas was carried out. Single hafnia layers deposited using high energy ion assistance had more amorphous structure with smaller crystallites of monoclinic phase. High reflection UV mirrors deposited using high energy ion assistance had slightly higher mean refractive indices of hafnia, higher extinction than conventional e-beam deposition, but demonstrated slightly higher laser induced damage threshold (LIDT) values measured at 355 nm. Deposition using the lowest energy ions produced the most porous coatings with the best LIDT of 7.7 J/cm².

The OMEGA EP Facility includes two high-energy, short-pulse laser beams that will be focused to high intensity in the OMEGA target chamber, providing backlighting of compressed fusion targets and investigating the fast-ignition concept. To produce 2.6-kJ output energy per beam, developments in grating compressor technology are required. Gold-coated diffraction gratings limit on-target energy because of their low damage fluence. Multilayer dielectric (MLD) gratings have shown promise as high-damage-threshold, high-efficiency diffraction gratings suitable for use in high-energy chirped-pulse amplification [ B. W. Shore et al., J. Opt. Soc. Am. A 14, 1124 (1997).] Binary 100-mm-diam MLD gratings have been produced at the Laboratory for Laser Energetics (LLE) using large-aperture, holographic exposure and reactive ion-beam etching systems. A diffraction efficiency of greater than 99.5% at 1053 nm has been achieved for gratings with 1740 grooves/mm, with a 1:1 damage threshold of 5.49 J/cm² diffracted beam fluence at 10 ps. To demonstrate the ability to scale up to larger substrates, several 100-mm substrates have been distributed over an aperture of 47 × 43 cm and successfully etched, resulting in high efficiency over the full aperture. This paper details the manufacture and development of these gratings, including the specifics of the MLD coating, holographic lithography, reactive ion etching, reactive ion-beam cleaning, and wet chemical cleaning.

Antireflection (AR) coatings typically damage at the interface between the substrate and coating. Therefore the substrate finishing technology can have an impact on the laser resistance of the coating. For this study, AR coatings were deposited on Yb:S-FAP [Yb³⁺:Sr₅(PO₄)₃F] crystals that received a final polish by both conventional pitch lap finishing as well as magnetorheological finishing (MRF). SEM images of the damage morphology reveals laser damage originates at scratches and at substrate coating interfacial absorbing defects. Previous damage stability tests on multilayer mirror coatings and bare surfaces revealed damage growth can occur at fluences below the initiation fluence. The results from this study suggest the opposite trend for AR coatings. Investigation of unstable HR and uncoated surface damage morphologies reveals significant radial cracking that is not apparent with AR damage due to AR delamination from the coated surface with few apparent cracks at the damage boundary. Damage stability tests show that coated Yb:S-FAP crystals can operate at 1057 nm at fluences around 20 J/cm² at 10 ns; almost twice the initiation damage threshold.

For high intensity lasers it is very important to choose appropriate optical elements. Since invention of high power lasers laser-induced damage of optical coatings was subject of extensive investigations. At high laser intensities the self-focusing in optical elements appears and intensity at rear optics surface can be much higher than at the front surface. Due to this damage of rear-surface can be reached much faster than damage of the front surface. We investigated the influence of self-focusing on damage threshold in fused-silica windows with anti-reflective coatings on both sides. In our experiments we used titanium-sapphire chirped pulse amplification system (130 fs, 2 mJ, 1 kHz repetition rate pulses at 800 nm). We have tested 1 mm, 3 mm and 6 mm thickness fused-silica windows with identical anti-reflective coatings. The front surface of the samples was placed in the waist of focused beam. The experiments were performed for effective spot diameters on the front 145 μm, 95 μm and 43 μm respectively. The experiments showed the self-focusing of beam inside the fused silica window and self-focusing dependence on initial beam diameter. The damage behavior was dependent on irradiation history. Also we found quite strong nonlinear absorption in fused silica.

AWE has operated lasers for studying the properties of materials at high temperatures and pressures since the early 1970s [1]. A brief review of those facilities will be given along with a description of the changes to HELEN in the period 2003 - 2005 to enable it to provide both chirped pulse amplification [CPA] pulses and nanosecond pulses to laser targets simultaneously. The CPA beam is now giving laser pulses of typically 70 Joules in 500 femtoseconds. As a necessary part of operating such a system it has been necessary to study how laser-target debris is distributed from both long and short pulse interactions so that the contamination or degradation of focussing optics during operational activities is minimised. The short pulse experiments and the characteristics of debris fields emanating from laser targets will be described. Damage thresholds were established for coated and uncoated debris shields with millimetre to centimetre size beams. Future developments of the laser facilities at AWE in this decade will also be described as well as possible debris mitigation options that may be employed.

Since the beginning of the 90's the generation of high-intense laser pulses has known an unprecedented evolution thanks to the conjunction of the possibility of the Chirped Pulse Technique and the availability of spectrally broad-band laser media. Lasers capable of producing petawatt pulses can now be built on few optical tables in a small laboratory. We review the generation and the amplification of ultra-short pulses by the Chirped Pulse Amplification technique.

To enable high-energy petawatt laser operation we have developed the processing methods and tooling that produced both the world's largest multilayer dielectric reflection grating and the world's highest laser damage resistant gratings. We have successfully delivered the first ever 80 cm aperture multilayer dielectric grating to LLNL's Titan Intense Short Pulse Laser Facility. We report on the design, fabrication and characterization of multilayer dielectric diffraction gratings.

We report on the design and construction of the Texas Petawatt Laser. This research facility will consist of two, synchronized laser systems that will be used for a wide variety of high intensity laser and high energy density science experiments. The first laser is a novel, high energy (200 J), short pulse (150 fs) petawatt-class laser that is based on hybrid, broadband optical parametric chirped pulse amplification (OPCPA) and mixed silicate and phosphate Nd:glass amplification. The second laser will provide 500 J at 527 nm (>1 kJ @1053 nm) with pulse widths selectable from 2-20 ns. Design and construction began in early 2003 and is scheduled to complete in 2007. In this report we will briefly discuss some of the important applications of this system, present the design of the laser and review some of the technology used to achieve pulse durations approaching 100 fs. Currently, the facility has been renovated for laser construction. The oscillator and stretcher are operational with the first stage of gain measured at 2×10⁶. Output energies of 500μJ have been achieved with good near field image quality. Delivery has been taken for Nova components that will compose the main amplifier chain of the laser system.

We present a summary view of the DARPA Super High Efficiency Diode Sources (SHEDS) and Architecture for Diode High Energy Laser Systems (ADHELS) programs. The goal of these programs is development of technology of a future compact, field-deployable high energy laser (HEL) system.

Calcium fluoride and fused silica are low absorbing key optical bulk materials for pulsed DUV laser application. Due to their large band gap fluoride thin films play a key role for applications in the DUV spectral region. For their main use in laser microlithography, these materials are commonly characterized by in situ transmission measurements. A differentiation is not possible between absorption and scattering. Therefore, experimental techniques are highly attractive which selectively characterize the absorption process using small samples. A direct absorption measurement method using laser induced deflection of a probe beam (LID) was introduced and applied to bulk and thin film materials. Laser induced fluorescence (LIF) in bulk materials and coatings is investigated to correlate the absorption with microscopic properties like intrinsic and extrinsic defects.

We summarize recent investigations of the density and morphology of bulk damage in KDP crystals as a function of pulse duration, temporal profile, wavelength, and energy fluence. As previously reported by Runkel et al.¹, we also find that the size of bulk damage sites varies roughly linearly with pulse duration for pulses between 1 ns and 9 ns. However this trend no longer applies at pulse durations below 1 ns. Experiments measuring the damage density and size distribution as a function of wavelength confirm many previous works which indicated a strong dependence of damage density with wavelength. However, we also find that the size of damage sites is relatively insensitive to wavelength. Further we see damage due to Flat-In-Time (FIT) pulses has different pulse length and fluence dependence than Gaussian pulses. We demonstrate that a simple thermal diffusion model can account for observed differences in damage densities due to square and Gaussian temporally shaped pulses of equal fluence. Moreover, we show that the key laser parameter governing size of the bulk damage sites is the length of time the pulse remains above a specific intensity. The different dependences of damage density and damage site size on laser parameters suggest different absorption mechanisms early and late in the damaging pulse.

An experimental technique has been utilized to measure the variation of bulk damage scatter with damaging fluence in plates of KH₂PO₄ (KDP) crystals. Bulk damage in unconditioned and laser-conditioned doubler-cut KDP crystals has been studied using 527 nm (2ω) light at pulselengths of 0.3 - 10 ns. It is found that there is less scatter due to damage at fixed fluence for longer pulselengths. In particular, there is ~4X increase in fluence for equivalent scatter for damage at 2ω, 10 ns as compared to 0.30 ns in unconditioned KDP. The results for the unconditioned and conditioned KDP show that for all the pulselengths the scatter due to the bulk damage is a strong function of the damaging fluence (θ^~5). It is determined that the 2ω fluence pulselength-scaling for equivalent bulk damage scatter in unconditioned KDP varies as τ^0.30±0.11 and in 3ω, 3ns ramp-conditioned KDP varies as τ^0.27±0.14. The effectiveness of 2ω and 3ω laser conditioning at pulselengths in the range of 0.30-23 ns for damage induced 2ω, 3 ns is analyzed in terms of scatter. For the protocols tested (i.e. peak conditioning irradiance, etc.), the 3ω, 300 ps conditioning to a peak fluence of 3 J/cm² had the best performance under 2ω, 3 ns testing. The general trend in the performance of the conditioning protocols was shorter wavelength and shorter pulselength appear to produce better conditioning for testing at 2ω, 3 ns.

While KDP and DKDP crystals remain the only viable solution for frequency conversion in large aperture laser systems in the foreseeable future, our understanding of damage behavior in the presence of multiple colors is very limited. Such conditions exist during normal operation where, for third harmonic generation, 1w, 2w and 3w components are present with different energy ratios as they propagate inside the crystal. The objective of this work is to shed light into the damage behavior of frequency conversion crystals during operational conditions as well as probe the fundamental mechanisms of damage initiation. We have performed a series of experiments to quantify the damage performance of pristine (unconditioned) DKDP material under simultaneous exposure to 2w and 3w laser pulses from a 3-ns Nd:YAG laser system as a function of the laser fluences at each frequency. Results show that simultaneous dual wavelength exposure leads to a much larger damage density as compared to the total damage resulting from separate exposure at each wavelength. Furthermore, under such excitation conditions, the damage performance is directly related to and can be predicted from the damage behavior of the crystal at each wavelength separately while the mechanism and type of defects responsible for damage initiation are shown to be the same at both 2w and 3w excitation.

At very high powers the energy for a single shot in the LIL/LMJ laser is today limited among others by the robustness of the KDP-based components used for frequency conversion. Subsequently it is vitally important to improve as much as possible the Laser Induced Damage Threshold (LIDT) of these components to make possible even more powerful shots. The exceptionally large aperture of such lasers (40*40 cm²) required the development of rapid growth methods. Investigations are under way to improve the damage resistance of such materials by implementing more efficient conditioning procedures. In this work we focus on composition heterogeneities induced by the rapid growth method in KDP crystals and we examine the impact on the laser-damage resistance. Two LIDT measurement facilities are used to investigate KDP triplers robustness. Spatially resolved LIDT measurements at 355 nm show that the LID resistance is significantly lower in some regions. The efficiency of the excimer conditioning in the different regions is also addressed.

Experimental results are presented on using one or more additional windows to reduce the distortion from existing windows transmitting high average power laser beams. A concept is presented for a compound window that will neither distort nor depolarize a high power beam.

KDP crystals are currently used for frequency conversion and Pockels cells in large aperture laser systems such as the LMJ and NIF. These different functions are obtained by cutting the KDP crystals with different orientations. We show by measuring the LIDT with three different facilities, that the cut angle plays a key role in the damage mechanism. Consistently with the three measurement set-ups, we demonstrate that the doublers have a weaker LIDT value than the triplers. The z-cut KDP samples have a LIDT higher than both the doublers and the triplers. These results are analyzed in terms of probed volumes and pulse duration.

X-ray diffraction is a non destructive technique used in order to characterize defects in the single crystal. Unfortunately, this analysis can not be performed throughout the whole volume on thick KH₂PO₄ (KDP) crystals used in the high power lasers systems like NIF and LMJ, these crystals having a thickness close to 10 mm. Considering the usual energy range radiation used for X-ray diffraction and topography (20-30 keV), the beam is rapidly absorbed by the material. However, this problem can be solved by the use of high energy X-ray radiation in order to analyse the complete volume of crystal. The principle of this device will be exposed and preliminary results are shown along with corresponding optical measurements.

We present experimental results aiming to reveal the relationship between damage initiating defect populations in KDP and DKDP crystals under irradiation at different wavelengths. Our results indicate that there is more than one type of defects leading to damage initiation, each defect acting as damage initiators over a different wavelength range. Results showing disparities in the morphology of damage sites from exposure at different wavelengths provides additional evidence for the presence of multiple types of defects responsible for damage initiation.

Hollow core photonic crystal fibres (HC-PCFs) show significant improvement over standard solid-core single-mode fibres and although short pulses (around 60 ns pulse width) and energies greater than 0.5 mJ were delivered in a single spatial mode through the hollow-core fibre, providing the pulse energy and high beam quality required for micro-machining of metals, the predicted performance (10's of mJ's) has not yet been achieved. The damage threshold limitations of the HC-PCF were investigated, both by coupling the laser into the fibre core, and by focusing the laser spot directly onto the photonic cladding structure surrounding the hollow core to elucidate the fundamental damage mechanism of this 'web-like' structure. For 1064nm delivery damage occurs exclusively at the launch end face with either partial or complete ablation of the photonic crystal cladding around the core. The pulse energies at which this occurs have been identified using Q-switched Nd:YAG lasers either pulsed from 10 Hz to 100 kHz (10 ns and 60 ns pulse width) or in single-shot mode to isolate the initial damage event. It is proposed that a contributing factor to the damage is the mode-mismatch between the gaussian profile of the incident laser beam and the fundamental mode of the HC-PCF (which is unlike that of conventional fibre). Pulse delivery and damage thresholds for HC-PCF designed for 532 nm operation are also reported. These fibres have noticeably lower damage thresholds compared with the 1064 nm fibre and in this instance damage occurs exclusively along the length of the fibre, yet appears to be independent of bend radius. It is proposed that these fibres may be failing at imperfections within the fibre introduced during the fabrication process.

We studied filamentation, front surface damage and rear surface damage at 1064 nm and 351 nm with nanosecond pulses on a fused silica optical window. With temporally single-mode pulses, self-focusing occurs together with front surface damage, which is attributed to a Stimulated Brillouin Back Scattering (SBS) wave. The use of temporally multi-mode pulses suppresses the occurrence of front surface damage, and increases self-focusing. With single-mode pulses, the observation of filaments seems coherent with standard Kerr self-focusing effect, and can be understood according to the numerical treatment by Marburger et al, using non linear index values measured in other experiments. However, when multi-mode pulses were used, filaments occurred for much smaller peak intensities, by about a factor of 2. In this case, the non linear index causing self-focusing appears to be twice bigger. This second case is relevant to the situation of vacuum windows in high power laser installations, where the spectrum of light is widened to get rid of SBS. We discuss the physical effects that could be causing the enhancement of self-focusing.

Dynamics of damage formation by focusing intense femtosecond pulses inside the fused silica glass is studied in wide energy range. Damage usually is initiated in the zone near geometrical focus, which is preceded by the zone where beam propagates in the form of multiple filaments. For high repetition rate pulses damage appears as an extended narrow track along the beam path, which forms due to the propagation of the initial damage zone toward the laser source. For low repetition rate pulses extended damage tracks don't form.

We report the first demonstration of polycrystalline Nd-doped YAG ceramics with almost perfect pore-free structure and Nd-doped YAG single crystal by advanced ceramic processing. The laser conversion efficiency of pore-free polycrystalline Nd:YAG ceramics is extremely high and its optical quality is comparable to that of commercial high quality Nd:YAG single crystal. Moreover, we have succeeded also in fabrication of Nd:YAG single crystal, which enables laser oscillation, by solid-state reaction method. Laser oscillation efficiency was very low when the pores were remained inside single crystal, however the laser oscillation efficiency of pore-free Nd:YAG single crystal was slightly higher than that of polycrystalline Nd:YAG ceramics having grain boundaries. From this fact, it was found that the optical scattering inside the Nd:YAG ceramics occurs mainly at the residual pores and the scattering at the grain boundary is very little. In addition, we confirmed that high concentration Nd:YAG single crystal can be fabricated by sintering method. Applying the above single crystallization technology by sintering method, we have demonstrated the fabrication of layer-by-layer single crystal composite and micron size spherical single crystal.

The major bottleneck for the development of robust and cost-effective femtosecond amplification systems is the uncertainty concerning the damage threshold of Ti: Sapphire crystals. Up to now, Ti: Sapphire is the only material that supports the generation of temporally short pulses (few femtosecond) at high repetition rates, and overcoming this bottleneck will represent a major advance in laser performance for all the femtosecond community. Currently, when pumped at 532nm, the uncertainty on Ti:Sapphire damage threshold, is about a factor of ten. The empirically estimated threshold is 10J/cm² but for safety reasons the femtosecond laser community (especially the companies producing the lasers) uses the conservative value of 1J/cm². Such a low pumping fluency means low extraction efficiency during the amplification process and a great waste of pumping energy, the most expensive part of a Ti:Sapphire amplifier. In order to remove this bottleneck, we launch a complete analysis of all the factors that influence the damage threshold in Ti:Sapphire Crystals. Our program is to first measure the bulk threshold to define the upper threshold limit, and the influence of Ti ion concentration in the crystal garnet. Then, we will analyze all the surface effects that influence the value of the threshold. These effects depend on the polishing, on the cleaning process, as well as the type of anti-reflective coating. Only a complete understanding of all the mechanisms involved in threshold limitation will allow us to produce Ti:Sa crystals with the best performances. The study of the characteristics of the Ti:Sapphire damage threshold will not be complete and reliable without a complete characterization of the pump beams (temporal and spatial modulations), and this analysis will be done with nanosecond and picosecond pulses at 532nm. Finally, to complete the exploration of the the behavior of the titanium doped sapphire crystal, we will characterize the damage threshold with femtosecond pulses, at 800nm to reach the deterministic dielectric threshold and validate fundamentals models and simulation results. To our knowledges this is the first time that such a complete characterization is done for Ti:Sapphire laser crystals. We will present the first conclusions about the experiments as well as the methods we will employ in our systematic analysis.

We have applied z-scan technique for measuring absorption coefficients of highly transparent optical materials (glass) and have used those for characterizing the power and power density of high power near IR laser beams. Self-phase modulation due to thermal indexing is the process underlying this technique. Glass plates with appreciable absorption coefficients are used for calibration purposes and for verifying the results of measurements. Fundamentally, as low absorption coefficients as 10^-5 cm^-1 can be measured by scanning a plate of a transparent optical material in the focal region of a lens. The sensitivity of this technique proved to be high enough to reveal strong variation between the absorption coefficients of optical windows made of the same material BK7 but obtained from different sources. We have suggested and used a novel procedure, scanning nonlinear lens profiling, for characterization of homogeneity of optical glasses and other transparent optical materials. Most importantly, the technique can be used for fast and high precision measurement of power of high power laser beams without evoking large temperature increase and related problems.

This paper presents a direct comparison of the two main techniques for the determination of laser damage threshold. The two techniques are compared directly to understand the intrinsic biases of each test and what is causing these biases. The comparison is done in terms of accuracy and repeatability of the resulting measurements. It will be shown that under the vast majority of cases the damage frequency method underestimates the true value and has poor repeatability and that the binary search method over estimates the threshold, but is far more repeatable.

This paper presents a revised procedure for the international standard procedure for the measurement of laser damage threshold, ISO 11254-1. This paper presents a suggested revision of the published international standard. This paper gives a step by step recipe for the calculation of the threshold using this revised technique and gives expression of the estimation of the uncertainty in that estimate.

The current version of ISO 11254-1 the laser damage standard, calls for the use of the so called damage frequency method. This method uses a linear regression to find the estimate of the damage threshold. Using a similar but physically motivated choice for the regression function will be shown to hold great promise for amelioration of many of the well known foibles of the damage frequency method and there by ISO 11254-1. The efficacy of the new fitting function is shown via a Monte Carlo examination and application to experimental data.

This work summarizes the results from an extensive test campaign in which space-based laser optics were qualified for the upcoming ESA ADM-Aeolus mission. 14 different types of optical components from different suppliers were tested at the Nd:YAG laser wavelength according to the ISO standard 11 254 - 2 for multiple pulse testing. A new technique based on transient pressure sensing was developed to monitor the occurrence of damage on a sample surface exposed to a vacuum environment. Parallel testing of reference samples showed a distinct degradation under vacuum compared to atmospheric or pressurized environment. For all samples tested we found a typical behavior in the characteristic damage curves attained: A sharp drop in LIDT for small pulse numbers followed by a smooth decrease for larger pulse numbers (laser fatigue effect).

An automatic test facility has been developed with an infrared nanosecond laser, based on frequency mixing of YAG and Dye laser, tunable in the range 2 to 4.5 microns, in order to study the laser-induced damage mechanisms in Infrared optical components. Determination of an accurate LIDT is obtained thanks to a perfect knowledge of experimental characteristics (spatial and temporal beam profile, damage detection mode, energy measurement). Metrology and procedures are detailed and some damage probability curves obtained for bare infrared substrates (ZnSe) are presented. Particularly, we compare surface behaviour for different procedures of test. Damage probability curves are then interpreted in terms of density and damage threshold of precursor centers thanks to a statistical model.

The laser damage test for qualifying a coating run of anti-reflection coated optics consists of scanning a pulsed 1064 nm laser to illuminate approximately 2400 sites over a 1 cm x 1 cm area on a test sample. Scans are repeated at 3 J/cm² increments until the fluence specification for the optic is reached. In the past, initiation of 1 or more damage sites was classified as a failed coating run, requiring the production optics in the corresponding coating lot be reworked and recoated. Recent laser damage growth tests of 300 repetitive pulses performed on numerous damage sites revealed that all were stable up to 20 J/cm². Therefore the acceptance criteria has been modified to allow a moderate number of damage sites, as long as they are smaller than the allowed dig size and are stable (do not grow). Consequently many coating runs that previously would have been rejected are now accepted, resulting in higher yield, lower cost, and improved delivery schedule. The new test also provides assurance that initiated damage sites are stable during long term operation.

Research Electro-Optics Inc. (REO) has recently developed a new laser damage testing facility for the purpose of optimizing process parameters for fabrication and coating of high-damage optics. It also enables full or sample qualification of optics with laser damage specifications. The fully automated laser damage testing system uses microscope photography for detection of damage and a 3 ns pulse length 1064 nm laser for irradiation of the sample. It can test and statistically analyze damage events from a large number of shots, enabling large area testing for low probability events. The system measures and maps sizes and locations of damage sites down to a few microns in diameter. The results are not subject to variations due to the human operator. For coatings deposited by ion beam sputtering, small defects (less than 20 microns) are found to be most prevalent at the fluences specified for small optics for the National Ignition Facility. The ability to measure and characterize small defects has improved REO's ability to optimize their processes for making coated optics with high damage thresholds. In addition to qualifying particular parts, the periodic testing also assures that equipment and processes remain optimized.

Optical fiber networks are being developed that require higher optical power levels. Examples include long haul communication with Raman amplification and fiber to the premises. Previous studies indicate that tightly bent optical fiber can mechanically fail when exposed to high optical power levels. In an extreme case where fiber is sharply bent and subjected to a power level of 1 to 2 W in the near-infrared wavelength window, optical fiber can fail in minutes. It also has been shown that time to failure decreases with increasing bend stress and optical power. This study is a further investigation of the physical events leading to failure. Previously we demonstrated that the optical signal that escapes the core of bent fiber passes into the coating, where a small amount is absorbed and converted to heat. As a result the coating can be heated to significant temperatures resulting in degradation over time. This paper focuses on several key aspects of the failure kinetics associated with bent fiber under high power. As a result of bending, optical power leaked from the core is distributed in the glass cladding and polymer coating. We have modeled this power distribution and compared it with measured temperature profiles in the coating. The results show that this redistribution of the power is key to establishing the distribution of temperature in the coating and ensuing degradation. This understanding is used to design glass and coating solutions for inhibiting this potential failure mode.

Apparently ground based lasers (GBLs) can be used to induce short circuits between solar cells on some satellite arrays. The vulnerability does not exist in the arrays used on U.S. Milsats (military satellites).

Absorption is one of the main factors which cause damage to optical coatings, under the radiation of high power lasers. Surface thermal lensing (STL) technique was developed into a practical high-sensitivity apparatus for the weak absorption analysis of optical coatings. A 20 W continuous-wave 1064 nm Nd:YAG laser and a 30 mW He-Ne laser were employed as pump source and probe source, respectively. Low noise photoelectrical components and an SR830 DSP lock-in amplifier were used for photo-thermal deformation signal detection. In order to improve sensitivity, the configuration of the apparatus was optimized through choosing appropriate parameters, that including pump beam spot size, chopper frequency, detection distance, waist radius and position of probe beam. Coating samples were mounted on an x-y stage which was driven by high precision stepper motors. Different processes of absorption measurements, including single spot, linear scan and 2-dimension area scan, could be performed manually or automatically under the control of PC program. Various optical coatings were prepared by both electron beam evaporation and ion beam sputtering deposition. High sensitivity was obtained and low to 10 ppb absorption could be measured by surface thermal lensing technique. And a spatial resolution of 25 micron was proved according to the area scanning which traced out the profile of photo-thermal defects inside optical coatings. The system was employed in the analyses of optical absorption, absorption uniformity and defect distribution, and revealed the relationship between laser-induced damage and absorption of optical coatings.

The chirp of an ultrashort laser pulse can be extracted accurately in real-time using a simple modified autointerferometric correlation (MOSAIC) technique. Our newly developed time-domain algorithm is well suited for low signal-to-noise conditions. We display results revealing high sensitivity to chirp with signal-to-noise levels approaching the noise floor. Correction algorithms have been developed to accommodate signal distortions arising from bandwidth limitations, interferometer misalignment, and non-quadratic detector response.

A multi-wavelength laser based system has been constructed to measure defect induced beam modulation (diffraction) from ICF class laser optics. The Nd:YLF-based modulation measurement system (MMS) uses simple beam collimation and imaging to capture diffraction patterns from optical defects onto an 8-bit digital camera at 1053, 527 and 351 nm. The imaging system has a field of view of 4.5 x 2.8 mm² and is capable of imaging any plane from 0 to 30 cm downstream from the defect. The system is calibrated using a 477 micron chromium dot on glass for which the downstream diffraction patterns were calculated numerically. Under nominal conditions the system can measure maximum peak modulations of approximately 7:1. An image division algorithm is used to calculate the peak modulation from the diffracted and empty field images after the baseline residual light background is subtracted from both. The peak modulation can then be plotted versus downstream position. The system includes a stage capable of holding optics up to 50 pounds with x and y translation of 40 cm and has been used to measure beam modulation due to solgel coating defects, surface digs on KDP crystals, lenslets in bulk fused silica and laser damage sites mitigated with CO₂ lasers.