The ultraviolet laser damage resistivity of bulk fluoride optical materials is mostly limited by defects related to the preparation of the surface. Cleavage of crystals produces atomically flat terraces with highest damage thresholds(< 40J/cm2 for ns-pulses at 248nm) while cleavage steps and tips result in a dramatic local reduction of damage resistivity. Conventional polishing introduces contamination, scratches and dislocations yielding damage thresholds of typically 1 O to 20 J/cm2. Advanced preparation techniques like chemo-mechanical polishing and precision grinding provide surfaces with a damage threshold uniform over large areas that is close to that of cleavage terraces. In all cases the damage threshold is determined by light absorption of defect-induced electronic states energetically located in the band gap of the insulator. Band gap states in CaF2 and other fluoride crystals have been investigated by ultraviolet photoelectron spectroscopy, luminescence spectroscopy and photoacoustic absorption measurements. Surface quality was monitored by scanning electron and scanning force microscopy. These techniques allow the detection of minute absorptions and changes in surface structure and it is shown that not only surface preparation but even simple exposure of the surfaces to the atmosphere results in irreversible degradation. Laser damage thresholds obtained for differently prepared surfaces are related to their structural and electronic properties and the primary mechanisms of energy uptake from the laser light and damage due to the thermoelastic response of the crystal are discussed.

A model is suggested for damage of pure transparent dielectrics by ultrashort laser pulses. This model is based on the following concepts. The crystal atoms can absorb light performing, simultaneously, diffusion jumps. After such a jump a defect is generated in the lattice. Accumulation of defects leads to amorphization of the sample. It is believed that amorphization of the focal region of the crystal is the initial stage of the damage. Dissipation of this energy spools the further stages of damage, up to the crack formation. Absorption of light and diffusion jump is a single quantum-mechanical transition. It is inertial-free, and therefore it can be caused by pulses of any duration.It is only important that there should be a sufficient number of diffusion jumps during the pulse for the focal region to get amorphized. This defines a threshold condition for the damage. In the present paper the quantum mechanical probability of photo induced diffusion jumps is found, as well as the critical parameters of the laser field. Estimates for these parameters resulted in realistic values, thus the suggested scenario can play an essential role in optical damage.

A quantum-statistical theory of pure and transparent dielectrics optical damage by ultrashort laser pulses is developed. The cornerstone of the suggested model is the idea of direct absorption of light by ions or atoms forming the solid. The direct photon absorption leads, together with thermal :fluctuations of energy, to a diffusion jump of an ion or atom. After such a jump, a defect appears in the lattice. If the diffusion jumps are sufficiently frequent then, by the end of the laser pulse, accumulation of defects leads to amorphyzation of the crystal. Emergence of the amorphyzated region can be considered the initial stage of optical damage. In amorphous media, the photoinduced diffusion jumps result in additional absorption of energy, and the media becomes super-amorphous. Energy dissipation of this state unfolds the next stages of damage, all the way to formation of cracks. In the present paper, a quantum-statistical method is developed for evaluation of mean rate of the diffusion jumps. The value of this rate determines the threshold of a laser wave field causing amorphyzation or optical damage of the crystal. The threshold values are found in the approximation of a constant binding energy of particles in the lattice, and hence are the upper estimates. However even in this approximation the suggested optical damage mechanism may play a significant role at high temperatures of a sample. Keywords: short pulse, dielectric, optical damage, photo-induced diffusion, amorphyzation.

Multilayer coatings manufactured from metallic hafnium and silica sources by reactive electron beam deposition, are being developed for high fluence optics in a fusion lasers with a wavelength of 1053 nm and a 3 ns pulse length. Damage threshold studies have revealed a correlation between laser damage and nodular defects, but interestingly laser damage is also present in nodule-free regions. Photothermal studies of optical coatings reveal the existence of defects with strong optical absorption in nodule-free regions of the coating. A variety of microscopic techniques were employed to characterize the defects for a better understanding of the thermal properties of nodular defects and role of thermal defects in laser damage. Photothermal microscopy, utilizing the surface thermal lensing technique, was used to map the thermal characteristics of 3 mm X 3 mm areas of the coatings. High resolution subaperture scans, with a 1 micrometers step size and a 3 micrometers pump beam diameter, were conducted on the defects to characterize their photothermal properties. Optical and atomic force microscopy was used to visually identify defects and characterize their topography. The defects were then irradiated to determine the role of nodular and thermal defects in limiting the damage threshold of the multilayer.

We have developed tools and procedures for accurately and economically mapping and inspecting damage and artifacts in the bulk as well as on the surfaces of meter-sized optics intended for use on large-scale lasers such as Beamlet and NIF. Optics are illuminated by white light through the optic edge or externally at grazing incidence using linear fiber-optic arrays. From these coordinates, artifacts are located and digitally recorded with resolutions of less than 5 micrometers using a long-working-distance microscope. Total internal reflection of edge illumination efficiently couples light into the entire optic to inspect for bulk artifacts such as bubbles, inclusions, edge-cladding flaws and laser-induced damage as well as surface scratches and pits which propagate into the substrate. Surface contamination such as dust, fingerprints, coating flaws and cleaning flaws are highlighted by external grazing illumination. The procedures permit accurate recording of the evolution of damage after many laser shots as well as correlation of damage from one optic to the next in a laser chain.

In recent years, the emergence of novel composite films has created considerable interest in development of complex optical coatings for high power laser applications. In the present work, oxygen pressure dependent tunability in optical and structural properties of a mixed- composite thin film material in solid solution has been investigated. It was observed that optical properties of mixed oxide films are greatly influenced by the chemical composition and microstructural changes under various oxygen pressure conditions. Atomic force microscopy is used to study such structural and morphological changes. Energy dispersive x-ray analysis using the feature of a scanning electron microscope made evident the correlation between the oxygen enrichment and tunability in the film refractive index. Since oxygen pressure can be dynamically varied during a deposition process, coatings constructed of suitable mixed- composite thin films can benefit from continuous modulation of the index of refraction eliminating the discrete interfaces. Electric field distributions within several multilayer- equivalent optical coatings developed by a step modulation approach have also been analyzed.

In this paper we analyze features of absorbing inclusion-initiated laser-induced failure of a transparent dielectric surface in a multishop irradiation regime. Ablation and thermoelastic stress-induced damage mechanisms are investigated. The ablation is shown to be the dominant mechanism in the ultrashort pulsewidth domain, whereas the latter is realized at longer pulses. The alteration of damage mechanisms at pulsewidth variation is also characteristic for intrinsic damage mechanisms. Resent experimental results obtained for the ultrashort pulse damage are critically discussed in a context of a theory developed in this paper.

Laser induced damage thresholds, measured using N-on-1 and R-on-1 testing, of single layer high index zirconia and hafnia coatings, derived from nitric or acetic acid stabilized sol-gel processing, are reported. Single layer acetic acid, nitric acid and base catalyzed silica sol-gel coatings have also been tested. The sol-gels were dip coated onto fused silica substrates at a rate of 3mm/s. The refractive index of the materials were measured using transmission spectra of multiple pairs of high and low refractive index materials. Absorption in the UV region was similarly measured with multiple layers of the same material and fitting to a Beer's law behavior. The high index materials used were zirconia or hafnia, the low index material was silica. The compatibility of the various combinations of high and low index materials for physical stacking without cracking, achieving high reflectivity and high damage thresholds are discussed and compared.

Laser damage studies were recently introduced at LOSCM. We first describe our experimental set-up and its performances. The device is equipped with a 7 ns Nd : Y AG pulsed laser. Surface modifications due to laser irradiation are observed in real time through a Nomarski microscope placed behind the sample along the optical axis. The beam is aligned with a He-Ne laser and focused in a spot of 25 μmin diameter, which permits a localized study and the determination of intrinsic and extrinsic damage thresholds. The LIDT is determined by the real time Nomarski observation. To go further into the analysis of laser impact we use an Atomic Force Microscope with a micrometric repositioning system that permits a quantitative study. We detail the first results concerning LIDT and morphology of several materials produced by electron beam deposition at LOSCM (Zns, Na~IF6, Si02, Ta20 5, Hf02 and multidielectric mirrors). The whole characterization is performed in strong correlation with deposition parameters and cleanliness control, which allow us to draw a set of preliminary conclusions.

Accurate characterization of losses in optical multilayers remains critical for laser damage investigation. In particular, due to time deposition and shift of deposition parameters, materials within multilayers have different intrinsic properties according to their vertical location within the stack. For this reason, different extinction coefficients can be found for materials in single layer forms and materials confined in multilayers. Therefore a non- destructive probe is required that should permit to characterize separately each interface or each bulk of a multilayer. This probe is offered by guided wave and photothermal techniques that we present in this paper. Our technique allows us to measure loss anomalies in multilayer waveguides by recording an attenuation coefficient for each propagation mode, as well as a temperature distribution perpendicular to the mode direction. The results are analyzed to determine dissipate properties versus z-location within the stacks. At this step an attempt is made to correlate damage with loss anomalies.

We present new improvements that were achieved at LOSCM Marseilles for a better characterization of optical thin films. Roughness-induced absorption, angular ellipsometry of light scattering and multidielectric resonances are discussed in multilayers. Theoretical and experimental results are given and new applications are emphasized.

The processes of damage in oriented ZnSe, CdS, ZnS, CdTe crystals under the effect of intensive radiation of CO₂ laser were studied, the crystals being grown by the Bridgman method. The dependence of the damage pattern on the crystal structure and orientation was found. Types of cracks, changes in the dislocational structure and chemical composition of films around the damage crater were studied. Experimentally determined were the values of laser damage threshold on different crystallographic planes of A^IIB^VI crystals. Optical and thermomechanical characteristics of optical elements of crystalline zinc selenide with interference coatings of zinc oxide made by the original method of photostimulated oxidizing were investigated. It was found that such optical elements are characterized by high values of laser damage threshold, thermal stability and thermal strength. Revealed and studied was the effect of bulk glow of ZnSe crystals at their local irradiation with intensive radiation of CO₂ laser. This effect precedes the sample damage and is observed in crystals with the concentration of the copper impurity 2 X 10^-5...3 X 10^-4 wt percent.

The results of the study of bulk laser damage threshold and structural characteristics of rapidly grown KDP crystals are presented in this paper. Crystals were grown in the direction of preset angle of synchronism at a rate of 10 mm/day. The developed technique allows to obtain blanks of nonlinear elements right during crystal growth thus decreasing material and power consumption when making products. The grown crystals have bulk laser damage threshold on the level of approximately 5 X 10¹⁰ W/cm² and high structural quality.

The effect of different doses of (gamma) -, proton and electron radiation on optical properties and laser damage threshold of nonlinear optics crystals KH₂PO₄, KH_2xD_2(1-x)PO₄ was studied in the present paper. The samples were irradiated at room temperature along the crystallographic direction. It was found that radiation damage in these crystals has an identical character. Irrespective of the type of radiation the strongest changes occur in the UV part of the absorption spectrum where stripes of radiation - stimulated coloration appear. The originating in crystals radiation - stimulated defects were found to result in the change of laser damage threshold.

Evolution of high-power monochromatic laser field in multilayer optical coatings is simulated and obtained results are presented. Laser-field intensity is supposed to be high enough to induce considerable variations of refraction index. Nonlinear field propagation through single dielectric film and both anti-reflection and high-reflection multilayer coatings is studied. There are considered step and gradient sine-shaped variations of refraction index of the coatings. Formation of field disruption is shown to begin under certain combination of field and film parameters. The disruption can result in super local increasing of field amplitude. Influence of coating parameters upon process of local field increasing is investigated.

Mechanism of superlocal increasing of laser-field amplitude in transparent inclusions with size less that 1/4th of radiation wavelength is proposed and described. It is shown that incident field amplitude must exceed some threshold value for the field increasing to start within the inclusion. Small exceeding of the threshold value results in blow-up increasing of laser field so that the process can be referred to as field blow-up instability. Computer simulation is used to investigate dependence of blow-up instability threshold on inclusion and radiation parameters for 1D propagation of femtosecond pulses through isotropic Kerr-like dielectric. Obtained results are discussed from the viewpoint of laser-induced damage of transparent materials.

There are considered both linear and nonlinear electrodynamic processes resulting, in variation of space distribution of electromagnetic-field amplitude and its local increasing. The processes take place near pre-existing and laser-induced bulk defects in optical media as well as near surfaces. Among linear processes there are considered electromagnetic resonance resulting in great local increasing of field amplitude. The resonances include polaritones, generation of guided modes, evanescent waves and whispering modes. Nonlinear-optical processes together with resonances can result in very effective excitation of eignemodes. To show important role of the mentioned processes there are considered real examples: 1) formation of light-induced nonlinear optical waveguide induced by high-power near-surface electromagnetic field and generation of its resonant guided modes; 2) amplification of field amplitude near spherical and cylinder microdefects induced by formation of resonant whispering models. There is considered influence of laser-induced feedbacks between acting electromagnetic field and additions to polarization of both and surrounding medium on dynamics of the processes. It is shown that it can result in field instability of initial field distribution. There are considered specific regularities of developing laser-induced instability in medium without absorption and possible mechanisms of field stabilization. The general regularities are illustrated with estimations and analysis of conditions of realization of the mentioned processes.

Laser-induced damage threshold (LIDT) of various types of vitreous silica at 1064, 532, 355 and 266 nm was investigated. At 1064 nm no difference of LIDT was observed for all samples. At 1064-355 nm, wavelength-dependence of LIDT of synthetic fused silica (SFS) can be well described by a relation I_th equals 1.45 (lambda) ^0.43, where I_th is LIDT in J/cm² and (lambda) is wavelength in nm. At 266 nm, however, LIDTs were smaller than a half of the calculated value from above relation. This difference can be explained by the damage mechanism; at 266 nm, two-photon absorption-induced defects lowered the LIDT same as a KrF-excimer-laser induced defects, whereas at longer wavelengths two photon process does not occur. LIDTs of fused quartz (FQ) at 532 and 355 nm, and that of a SFS containing about 1000 ppm of Cl and no OH at 355 nm were a little lower than that of the other SFSs. This may be related to the absorption of metallic impurities contained in FQ and dissolved Cl₂ molecule in SFS.

An experimental investigation of laser-induced damage of the Cr:LiSAF crystal under pulses from a Cr:LiSAF laser has been made. For comparison, the same measurements were also made with borosilicate glasses BK7 and K8. The experiments were performed with spot sizes from 0.7 to 50 micrometers . The morphology of the damage area, the spot size dependence, the statistics of the damage thresholds, and the truncation of the transmitted pulses were measured. It was found that for spot sizes of more 10 micrometers , the damage of LiSAF resulted from the presence of local bulk inhomogeneities. For smaller spot sizes the damage resulted from self-focusing. Finally, for spot sizes of less 1 micrometers , the damage resulted form the intrinsic laser-induced damage and its threshold was significantly higher than the threshold for BK7 glass. As a result the spot size dependence for Cr:SAF was found to be stronger than for borosilicate glass.

Photo-ionization of silicate glasses under high-power, 850 nm, femtosecond laser has been studied. It was found that color centers formation and intrinsic luminescence of boro- and alkali-silicate glasses occurred at irradiances well below the threshold for laser-induced damage. Simultaneously a supercontinuum generation in the visible and UV spectral regions was observed in all the glasses studied. From these studies it was deduced that photo- ionization of glass matrix results from linear and two-photon absorption of the short- wavelength component of the supercontinuum. No color center absorption in the visible region was observed in fused silica at irradiances up to the laser damage threshold.

Ultra-fast processes of recombination of free charge carriers optically generated in natural and chemical vapor deposited (CVD) diamond materials; GaAs and Si, were investigated with a 1 nanosecond time resolution by applying a developed microwave-radiation-based technique. Time-dependent responses of reflection and transmission of 2 mm wavelength electromagnetic CW-radiation were recorded when the tested specimens were irradiated by single laser pulses of IR, visible and UV spectral range. A waveguide configuration was used to measure the exponential fall of the reflection/transmission signals corresponding to the recombination times of free carriers. Depending on the material, impurity contamination and laser wavelength, the measured signals were varied from a few to several hundred nanoseconds. The measured recombination times in the bulk of natural and CVD diamond specimens were found to be of 2-4 ns. The distinguished difference between the surface and bulk recombination times was clearly demonstrate in the case of GaAs and Si. Applicability and relevance of the applied technique to non-equilibrium carrier lifetimes measurements and the validity of the result obtained are discussed and confirmed on the base of the analytical analysis of the time- dependent microwave radiation reflection/transmission in a solid excited with high intense laser pulses.

The ISO 11 551 standard for laser calorimetric measurement does not define the temporal regime of the laser irradiation. Comparative experiments were carried out using a cw and pulsed laser beam at 1.06 micrometers wavelength. The pulsed laser parameters are 10 Hz pulse repetition rate and 12 ns pulse length. Nearly the same average power, irradiation time and beam spot size are chosen to ensure the comparability of cw and pulsed irradiation procedure. In this paper the measured temperature curves and calculated absorbencies for different samples are presented.

A set-up for the reflectance measurement of materials or optical system at 1.06 and 10.6 micrometers wavelengths is presented. It is based on the reflection measuring method of the Institut fuer Strahlwerkzeuge in Stuttgart, Germany. In contrast to it, we used an acousto- optical modulator instead of a mechanical chopper mirror. Advantages are the higher maximum chopper frequency of 100 kHz and the absence of mechanically moved parts. In consequence, the signal to noise ratio is improved. Some problems arise form the diffraction efficiency of the modulator and form the generation of higher order diffraction. In this script first results are presented on the calibration of the measuring set-up, the measuring procedure and the calculation of the actual reflectance from the measured data.

An experimental set-up using different pulsed photothermal measuring techniques is presented. The set-up allows investigation of different optical components byu means of the photothermal reflectance microscopy, photothermal transmittance microscopy and the photothermal deflection microscopy both in reflection and transmission. The methods are based on different effects, such as a surface bending caused by thermally induced stress and thermal stress dependence of the refractive index. Therefore and in consequence of different reflectance's of the components, each photothermal techniques has different measuring sensitivity. The combination of several photothermal techniques allows an investigation of all optical component classes. Otherwise, more information could be determined about optical, thermal and mechanical properties of components. The experimental results of various optical components are presented. The absorbance, reflectance, transmittance and thermally conductivity are discussed on the basis of temporally measured signal curves. Thermally induced stress was also detected by means of the photothermal deflection microscopy in transmission.

Assuming the observed scaling of laser damage threshold fluence with the (almost) square root of pulse duration is due to thermal conduction, we develop a formalism for directly comparing pulses of different shapes and durations. We find, for example, that a top hat pulse leads to 15% higher temperature (presumably 15% lower damage threshold) than a Gaussian pulse of the same fluence. We also find that the damage threshold of the expected NIF type pulse should be estimated from a Gaussian pulse with the same peak intensity. We find that the deviation of the scaling of damage threshold from square root of pulse duration has contributions from both the small but finite size of laser energy absorbers and from the temperature dependence of thermal properties. Keywords: laser damage, damage threshold, scaling, thermal conduction

We report the first operation of more than 1000 hours of continuous wave (CW) 100-mW output at 266 nm which was frequency-quadrupled from a Nd:YAG or a Nd:YVO₄ laser. We used a Czochralski (Cz)-grown (beta) -BaB₂O₄ (BBO) crystal device to double a 532-nm wave in an external ring cavity. The round trip cavity loss increasing rate was 7.6 X 10^-5 percent/hour at a 266-nm power density of 270 W/cm². The UV range optical transmission loss of the Cz-grown crystal was evaluated. The optical loss of the Cz-grown crystal for e-ray at 266 nm was improved to 2 percent/cm, which was half of the crystal grown by the top seeded solution growth (TSSG) method. The degradation rate, (delta) _p equals (dP_4w(t)/dt)/P_4w(0), was also evaluated in 30 MW/cm² of 266-nm generation from a Q-switched Nd:YAG laser. The (delta) _p of Cz-crystal was typically 0.1 percent/hour, which was one order of magnitude lower than that of the TSSG- crystals. The fabrication process was also improved. Our system with the crystal device can be put to practical use in the areas of photolithography, material processing and ultra high-density optical disk mastering.

The demonstration of the technical feasibility of very high damage threshold mirrors deposited by an ion beam sputtering process is made. The two chosen materials are Yttrium fluoride for the high index and Lithium fluoride for the low index. The threshold of these two materials at 355nm 3ns is higher than 20J/cm². High reflection stacks have been deposited with a reflection level of 98.5 percent and a threshold which meets the laser megajoule requirements.

Holmium YAG lasers are applied in many modern technology fields. Besides environmental control, especially medical applications because of increasing importance for this laser type because of the advantages for special surgery, involving osteotomy and cutting of strongly vasculated tissue. For an improved efficiency, most of these applications require Ho:YAG- laser systems with increased output power and better beam parameters. A key problem in the development of this new generation of high power Ho:YAG-lasers is the power handling capability of the available optical components. The present investigations are concentrated on the characterization and optimization of optical laser components for the MIR spectral range. A series of partial reflectors, windows and uncoated substrates of different materials has been investigated by laser calorimetry, spectrophotometry and an adapted electron microscopic method. Also, the temperature shift of the spectral characteristics was measured and evaluated in respect to the microstructure of the coating systems. The result are discussed in consideration of the power handling capability of the produced laser components for 2.1 micrometers .

AlGaAs/GaAs high power diode lasers with a nominal output power of 15W were aged at different conditions. At a heatsink temperature of 25 degrees C aging at constant current (CC) and constant power (CP) mode is compared for aging times of 6000 hours. We derived an end-of-life criteria that results in the same lifetime for CC and CP operation assuming identical degradation mechanisms in both cases. The degradation observed differs only significantly beyond 3000-4000 hours of aging with increasing degradation for CP operation. In constant current mode the heatsink temperature is increased resulting in a junction temperature of about 80 degrees C. Assuming an Arrhenius relation the activation energy is estimated. It turns out that different activation energies can be derived either by taking the degradation of the output power at the elevated temperature or at the reference temperature respectively.

The surface thermal lensing technique (STL) successfully resolved and measured the absorptance of transmissive optical components: near-normal angle-of-incidence anti-reflectors and beam splitters. The STL system uses an Ar ion laser to pump the components at 514.5 nm. The absorptance-induced surface deformation diffracts the HeNe probe beam into a photo- detector. The signal intensity was calibrated with a sample of known absorptance. The optical components were designed to function in a copper vapor laser (CVL) transport system, and were previously tested for absorptance with a high power CVL system at 511 nm. To assure proper absorptance data from the STL system, the pump laser power densities were set at the operational level of the coatings, absorptance time trends were monitored, and absorptance area scans were made. Both types of transmissive optics are more stable than the CVL high reflectors that were measured in another study. Parameter studies based on Fresnel diffraction theory were also performed to optimize experimental condition. The STL system was assessed to have 10 ppb sensitivity for absorption measurement given 2 W of pump power.

A study was performed to rank the influence of the parameters that can determine the resistance to laser-induced damage of polished synthetic fused silica at 355 nm. These parameters include the type of fused silica, the polishing process, the cleaning method, the sample thickness, and the sample diameter. The study involved both measurements of the statistics of laser-induced damage thresholds (LIDT) and characterization of the damage morphologies. The samples were irradiated at 20 degrees incidence angle with a 6.5-ns-pulse at 355 nm on an automated damage testing (ADT) system. The LIDT was measured on a minimum of 100 sites for each sample to gather sufficient statistical information and the damage morphology was characterized by Nomarski optical microscopy and SEM. The statistical damage curves were very reproducible. The LIDT was most influenced by the polishing process. Surface micro-pits were found on the surface after irradiation with damaging fluences close to the LIDT. Their density varied form less than 0.1 X 10^-3 to 26 X 10^-3 micrometers ^-2 for the samples polished with the two different processes. The type of fused silica, the cleaning method, and the sample diameter seemed to have secondary effects on the LIDT. Bulk damage was occasionally noticed for thicker samples in areas with very high surface LIDT. Sample cleaning was found to play a role in reducing the number of low LIDT sites, thereby affecting the statistical behavior of the distribution tail, but did not significantly affect the average damage behavior. Finally, input surface (IS) damage occurred statistically less frequently and at higher fluences than output surface (OS) damage. Further work is discussed to understand if this effect is caused by electric field enhancement on the OS, or fundamental differences in crack propagation behavior between IS and OS.

High laser intensities are being transmitted through optical fibers in a growing number of applications. Our interest in laser initiation of explosives has led us to examine the transmission of Q-switched, Nd:YAG laser pulses through step-index, multimode, fused-silica fibers for a number of years. A common limiting process is a plasma-forming breakdown occurring at the fiber entrance face. The breakdown threshold at this face depends on the surface characteristics that result from the particular method of end-face preparation. In previous studies we examined entrance-face breakdown thresholds for several different mechanical polishing schedules, and also for several schedules of CO₂-laser conditioning following mechanical polishing. In the present study we examined three end-face preparation methods that were base on the recent availability of exceptionally good cleaved surfaces for our fibers of interest. Using test procedures similar to those in past studies, we examined the cleaved fibers directly, fibers with cleaved surfaces that were subsequently flame polished, and fibers with cleaved surfaces that were subsequently conditioned with a CO₂ laser. All of these preparation methods resulted in fibers that showed a broader range of entrance-face breakdown conditions than found in past studies, together with a susceptibility to subsurface exit-face damage. By introducing additional cleaning steps with the cleaved surfaces, we were able to reduce the variability in breakdown thresholds observed after subsequent CO₂- laser conditioning. A consistent location of exit-face damage sites indicates that subsurface fracturing occurs during the cleaving process, and that the subsequent end-face processing steps were not effective in mitigating damage at these sites. Threshold energies for entrance- face breakdown are also affected by the relation between incident laser energy and the resulting peak local fluence at this surface. Laser characteristics and the design of the laser-to- fiber injection optics determine this relation. The present study utilized an improved injection system consisting of a custom diffractive optical element combined with a lens having a very short focal length. This system produced the lowest value for the ratio of peak-to-average fluence at the entrance face that we have observed, and was very successful in inhibiting internal damage mechanisms along the fiber path.

This paper discusses and formulates expressions for the two main sources of error in the damage frequency method, namely the uncertainty in observed fluence and observed damage probability. Expressions for the two uncertainties are given and examined in the limiting probability cases. The relative sizes of the two sources are compared and it is shown that the uncertainty associated with determining the probability of damage is the dominant term.

This paper introduces an expression for the number of test sites required to complete a damage frequency measurement of the laser damage threshold. This expression is evaluated at many points in the relevant parameter space to produce a contour plot which depicts the implicit trades in designing a DFM measurement.

Thick fluoropolymer films are being evaluated as a potential 'disposable' debris shield to protect high-peak-power laser optics from x-ray and target debris generated in inertial- confinement fusion-ignition experiments. Two obstacles to implementation are optical uniformity and damage threshold. To understand the damage characteristics, transmittance of single 1053- or 351-nm laser pulses has been measured for commercial fluoropolymer films in vacuum. Samples were tested at fluences up to 105 J/cm2 at 351-nm. Both the total transmitted energy for a single shot and the temporal energy transmittance profile during the shot were measured as a function of fluence. In addition, the total focusable transmitted energy was recorded for 351-nm pulses. Results show that transmittance decreases slowly during a single-pulse irradiation, allowing much of the energy to be transmitted at fluences which cause noticeable degradation to the film. The film transmits greater than 90 percent of the 351-nm energy delivered in a beam with spatial average fluence of 8 J/cm2 with modulation up to 15 J/cm2. For 1053-nm laser light, the films do not begin to exhibit noticeable transmittance loss until average fluences exceed 40 J/cm2.

Bulk laser damage in CsLiB₆O₁₀ was measured using a single-shot Q-switched Nd:YAG laser in a transverse and longitudinal single model. The bulk laser damage thresholds of CLBO, with laser irradiation direction (parallel) < 001 > and polarization (parallel) < 001 >, were determined to be 29 GW/cm² at 1.064 micrometers , and 6.4 GW/cm² at 0.266 micrometers . The value at 1.064 micrometers is higher than that of fused quartz, (beta) - BaB₂O₄ and KH₂PO₄. The morphology of bulk damage in CLBO crystal was also observed. THe damage pattern suggests that the < 001 > direction is mechanically weak, which is consistent with the result of the mechanical strength tests.

This paper investigates ion etching process to the surface of CsLiB₆O₁₀ (CLBO) crystal. Laser-induced surface damage was reduced and surface durability of CLBO crystal was improved by removing the subsurface embedded polishing compound. There was no surface degradation as a result of the ion etching. The effects of ion etching on surface damage were measured by a 1-on-1 test at a laser wavelength of 266 nm. Durability of the CLBO crystal was tested by the approximately 7 W fourth harmonic generated by a Nd:YAG laser. The durability of the ion etched surface was improved more than 10 times as compared with the as-polished surface.

On a subject of considerable import to the laser-induced damage community, a two day workshop on the topic, Thin Film Thermal Conductivity Measurement was held as part of the 13th Symposium on Thermophysical Properties at the University of Colorado in Boulder CO, June 25 and 26, 1997. The Workshop consisted of 4 sessions of 17 oral presentations and two discussion sessions. Two related subjects of interest were covered; 1) methods and problems associated with measuring thermal conductivity ((kappa) ) of thin films, and 2) measuring and (kappa) of chemical vapor deposited (CVD) diamond. On the subject of thin film (kappa) measurement, several recently developed imaginative techniques were reviewed. However, several authors disagreed on how much (kappa) in a film differs from (kappa) in a bulk material of the same nominal composition. A subject of controversy was the definition of an interface. In the first discussion session, several questions were addressed, a principal one being, how do we know that the values of (kappa) we obtain are correct and is there a role for standards in thin film (kappa) measurement. The second discussion session was devoted to a round-robin interlaboratory comparison of (kappa) measurements on a set of CVD diamond specimens and several other specimens of lower thermal conductivity. Large interlaboratory differences obtained in an earlier round robin had been attributed to specimen inhomogeneity. Unfortunately, large differences were also observed in the second round robin even though the specimens were more homogenous. There was good consistency among the DC measurements, however, the AC measurements showed much greater variability. There was positive feedback from most of the attenders regarding the Workshop with nearly all respondents recommending another Workshop in three or fewer years. There was general recognition that thin film thermal conductivity measurements are important for predicting the resistance of optical coating materials to laser damage.

Large components typically 620 mm X 440 mm are needed to build up the LMJ (laser megajoule) which belongs to the future generation of the fusion laser. Today, the laboratory laser damage facilities allow to test small samples: generally witnesses with 50 mm in diameter. However, more of the test procedures are manually and small areas are examined. It means that usually just 20 and 50 sites are tested with a small Gaussian beam. Therefore, it is essential to verify if the large components needed for the fusion laser have the threshold above the requirements. It is also important to determine which law exists between the threshold of the witnesses samples and the threshold of the final components. In this paper, these problems are addressed in using the most powerful pulsed Nd:YAG laser in France. With this particular facility it becomes possible to test full scale optics on a few sites with a beam size diameter of 6 cm. Except the test on the real system, these kinds of experiments allow to observe the large beam damage morphologies. A study is at the beginning to try to find the scale law which linked the result obtained on the small witnesses with various methods. It also should lead to a procedure to qualify the full scale optics during the production time.

Size increasing of light absorbing inhomogeneity in condensed media is predicted on a model suggested as a result of under threshold laser impulse influence that generates a plasma cloud about the inhomogeneity. The plasma cloud condenses after pulse ending into particle that is composed of both the sources absorbing inhomogeneity substance and media substance. Theoretical investigation of the optical properties changing the matter of the work. It is shown the carbon soot particle and glass mixing in plasma under repetition of laser pulse could lead to changing in pulse energy absorbed part and thus to changing in light damage threshold regarding the mechanism.

Stoichiometric titanium dioxide films comprised of either amorphous or anatase phases have been deposited onto cleaned silica substrates by means of a low pH sol-gel processing method. Coated optics were then exposed to pulsed 1064 nm light from a Nd:YAG laser. Irradiations were performed on samples heated in an oven held at temperatures between 300 and 450 K. A marked decrease in the threshold for catastrophic damage was observed at increased film temperature for both Q-switched and non-Q-switched pulses which is likely due to a thermally induced increase in film residual stress. However, continued irradiation of damaged regions with non-Q-switched pulses at slightly higher temperatures was found to heal the damaged regions. Subsequent irradiation at somewhat higher temperatures generated additional damage sites which could not be healed upon additional irradiation at still higher temperatures. Films were characterized by means of optical transmission measurements, Raman spectroscopy, and optical microscopy. Results demonstrate the influence of ambient film temperature on irradiation damage phenomena.

Waves of reflection and conduction (WRC) was found in 1992 as a new reversible photoinduced phenomenon. Here WRC was studied for IR transparent semiconductors ZnS, ZnSe, Ge in the process of optical damage testing of this materials. It was measured the variations of reflected or transmitted beam intensity of the He-Ne and CO₂ lasers when WRC pulses generated on the front and rear sample surfaces moved through the sample with velocities of order of sound velocity v₁ and approximately 1/4, 1/8, 1/16, 1/32 of v₁ with many reflections. This WRC pulses realize some part of non stationer heat transfer in solids. This WRC component velocities increased up to 2 times with the sample focus region temperature increasing as an effect of the energy pulse growing and different levels of damage occurring. It gives the possibility to develop the technique for the remote nondestructive precatastrophic indications of damage. This work was sponsored by DRET and as project 97- 02-17954 by RFBR.

Pulsed C02 laser damage mechanism of plexiglass (polymethilmethacrylate, PMMA) is controlled by in-depth absorption of the 10.6 mm radiation. The interaction area heating gives rise to thermal or thermal-mechanical damage processes and to generation of Wave of Reflection and Conduction (WRC) . WRC consist of many pulses following each another and differing by velocity about factor of two. He-Ne laser monitoring beam showed the modulation of interaction area reflection by slow components of WRC traveling between front and rear sample surface with velocities - 4.2 and -2,1 cm/s. An IR camera Thermovision 1000 fixed modulations of the temperature and the image of interaction area on the opposite surface of different thickness samples. This modulations correspond to influence of the WRC components with velocities - 0.2; - 0.05; - 0.025 cm/s. This low WRC velocities do not depend in plexiglass upon laser pulse energy in the range 1,10 J/cm2. Relation of WRC to damage process is under study. This work was sponsored by DRET (France) and RFBR (Russie) . Keywords: reflection and conduction wave, slow components of, plexiglass, C02 laser, damage testing.

Experiments on optical damage testing by help of 2.5 microsecond(s) , approximately 0.1 J single pulse of 10.6 (mu) CO₂ laser showed the appearance of cylindrical geometry WRC in BaF₂ single crystal approximately 50 percent transparent for this radiation. It was measured the reflection variations of the He-Ne beam transmitted through the sample at the 30 degree angle to the IR laser axes and crossing it. The nonequilibrium heating of the sample along IR beam axes produced the heat lens and as a consequence the focusing of the He-Ne beam. The cylindrical WRC cross the focused He-Ne beam area and it gives the well pronounced bell shape peak, additional to the analogous reflection variation records made at conditions where He-Ne beam did not cross IR laser axes. The time of focused He-Ne beam crossing by WRC is 40 ms, which correspond to approximately 0.6 mm diameter of focused a beam. One can make conclusions about local optical constants changing inside of solid sample by help of reflection variations in WRC. This work was sponsored by RFBR.

The microstructure of dielectric films is an inherent characteristic of the fabrication process, and imposes distinctive behavior on the electric field distributions in these inhomogeneous media. In this paper, we examine the size dependent problem for cubic defects for an Al₂O₃ dielectric material. Using the LOCALF method to analyze the electric field response, we have developed models for Al₂O₃ dielectric films of varying microstructure size and volume fraction, and compare these results with conventional effective medium methods. Besides the dielectric susceptibility and volume fraction dependencies, the physical size of the heterogeneities creates localized regions of high field intensity.

Multilayer optical thin film coatings are typically based on quarterwave design using alternating high- and low-index materials. The standing wave within these multilayer coatings introduces peaks of electrical field at the interfaces between the high- and low-index materials, which makes the interfaces potentially weaker parts in terms of laser damage resistance. The possible interfacial absorption and thermal resistance will further weaken these interfaces and make them prone to laser induced damage. In this paper a numerical model is developed for analyzing the interfacial effects on the transient temperature rise of multilayer coatings induced by a short pulse laser irradiation. The parameters studied include coating design, thermal physical properties of thin films, and interfacial absorption and thermal resistance. It is found that by choosing an appropriate coating design and eliminating the interfacial effects, the laser induced peak temperature rise of multilayer coatings can be drastically reduced. Examples are presented for both quarterwave and non-quarterwave HfO₂/SiO₂ high reflectivity coatings.

Laser-induced thermal waves are widely used for materials research and other applications. For low absorption optical materials the thermal waves are typically detected by collinear photothermal deflection spectroscopy (CPDS). The available theory of CPDS is based on the approximation of geometric optics, which, while being straightforward and having been extensively used, has obvious shortcomings. For example, to calculate the signal of CPDS, the probe beam size is assumed to be infinitely small. In real applications, however, the probe beam always has a finite dimension, which results in a diffraction effect and leads to unexpected experimental error. In order to overcome such shortcomings, there is a need to develop a more sophisticated theoretical model that can encompass both photothermal deflection and diffraction effects, and give precise predictions of the nature of both effects. In this paper we present a comprehensive diffraction theory suitable for treating the effect of pulsed CPDS. The work is an extension of the theoretical model previously developed for the mirage effect. It provides a detailed theoretical analysis of the laser-induced optical diffraction effect and helps to optimize the experimental parameters. It is a useful tool not only for quantitatively describing CPDS but also for the traditional thermal lens spectroscopy. With the application of the Fresnel diffraction theory, both techniques can be treated in a unified model. A summary of the theoretical modeling and experimental studies will be presented, with an emphasis on the advantages of using a near-field detection scheme for achieving the best sensitivity and stability.

Calorimetric absorption measurements are a valuable supplement for investigation of laser damage processes at substrate materials in the UV region. Our investigations for the influence of the measurement arrangement and irradiation parameters on the absorption and its wavelength dependence were carried out in the frame of standardization project CHOCLAB. The measurement set-up was presented at the Boulder Damage Symposium last year. To investigate the influence of the spectral dependence of absorptance in the UV range comparison measurements at different excimer laser wavelengths 193 nm, 248 nm and 351 nm were carried out. Suprasil and CaF₂ were investigated at different sample thicknesses under variation of irradiation parameters. The measured absorptance of Suprasil at 248 nm and 193 nm increases at growing repetition rates and equal energy density. At a fixed rap rate absorptance is proportional to energy density. Absorptance of CaF₂ under irradiation with 193 nm depends on energy density. For both materials the described effects are enhancing with increasing sample thickness.

Laser irradiation induced damage to several materials of interest for use as 10.6 micrometers laser system windows is investigated in this paper. The irradiation source in these single shot experiments was a pulsed TEA CO₂ laser. Damage initiation in semiconductors has been studied during the interaction by measuring the variation of the transmitted intensity of a He- Ne and a CO₂ cw lasers through the samples. Results show that damages appear at the beginning of the laser-matter interaction process on both surfaces and in the bulk of the materials. The damaged materials have been characterized for various incident fluences by means of optical microscopy and scanning electron microscopy in terms of topography and morphology. The modified surface chemical analysis and the structural analysis have been carried out using energy dispersive x-ray and Raman spectroscopy.

Transverse excited atmospheric pressure CO₂ laser induced damage on BK7 and fused silica has been investigated. Damage processes have been characterized by looking at the transparency changes during irradiation with a helium-neon laser and plasma formation, with a photodiode. Both results are compared with thermal coupling and damage threshold measurements. The two glass qualities show slightly different behavior. BK7 shows surface crazing after irradiation, fused silica shows micro-fractures formation. In both cases, residual tensile stresses induced near the surface on cooling is identified as the most likely cause of cracking. A model was developed to calculate temperature and residual stresses, calculations results gives good agreement with damage thresholds measured.

Laser-induced damages in several reflection coatings that are prepared with variant materials and technologies are investigated. It is found that the damage thresholds of the coatings are very different depending on the impurity absorption. We proved this conclusion by applying Auger Electron Spectroscopy to determine the surface composition of the coatings and measuring of the absorption of these coating. Laser condition of a large size coating is realized through scanning of a smaller laser spot. The condition is achieved through roistering at ten steps increasing from 5 to 80 percent of the zero probability damage threshold of the coating. The effect of the laser condition is directly related to the preparation process and the materials of the coating.

In thin cellulose acetate films, 351-nm laser damage at subnanosecond pulse length can be demonstrated to be driven solely by a slow-buildup-time, transverse stimulated scattering process in which the transverse-scattered wave exhibits neither Raman nor Brillouin spectral shifts. Above threshold, the damage morphology exhibits an orientational selectivity that coincides with the scattered-wave orientation observable down to fluences 20 percent below the damage threshold. The physics of the scattering process remains to be identified.

UV damage thresholds are reported together with other optical characterization results for perfluorinated, frame-supported, micron-thickness polymer membranes. These thresholds are among the highest for any material ever measured in the UV. Transmitted wavefront uniformity for pellicles up to 30 cm in clear aperture approaches the wavefront-error-detection limits of state-of-the-art interferometers. Owing to benign fabrication procedures, these foils are highly defect free and lack the birefringence characteristics typical of extruded polymers.

Results from monolayer-film laser-damage studies by various authors have remained difficult to compare, owing to many extrinsic factors having impact on measured damage thresholds and observed damage morphology. Prominent among these factors are the deposition method and conditions during deposition, the choice of starting materials, and the condition of supporting substrates. Here special attention is paid to the film-supporting surface with the goal of eliminating interfacial absorption effects. Fused-silica and float-glass substrates are prepared by various techniques: cleaved, conventionally polished, conventionally polished with added magnetorheological finish, and ion milled after conventional polish. Atomic-force microscopy is employed in determining microroughness and mapping laser-damage morphology features after irradiation at 1054 nm and 351 nm. HfO₂ and SiO₂ monolayers deposited on these surfaces showed large variations in damage threshold and morphology, depending on substrate-finish conditions. In spite of highest microroughness, cleaved-float-glass surfaces yielded the highest damage thresholds in both bare and coated forms. A comparison between HfO₂ and SiO₂ monolayer damage thresholds proved SiO₂ to be generally far superior to HfO₂.

NASA's Goddard Space Flight Center is developing the Geoscience Laser Altimeter System (GLAS) employing a diode pumped, Q-switched, Nd:YAG laser operating at 40 Hz repetition rate. To meet the five-year mission lifetime goal, a single transmitter would accumulate over 6.3 billion shots. Cr⁴⁺:YAG is a promising candidate material for passively Q- switching the laser. Historically, the performance of saturable absorbers has degraded over long-duration usage. To measure the multi-billion shot performance of Cr⁴⁺:YAG, passively Q-switched GLAS-like oscillator was tested at an accelerated repetition rate of 500 Hz. The intracavity fluence was calculated to be approximately 2.5 J/cm². The laser was monitored autonomously for 165 days. There was no evidence of change in the material optical properties during the 7.2 billion shot test. All observed changes in laser operation could be attributed to pump laser diode aging. This is the first demonstration of multi-billion shot exposure testing of Cr⁴⁺:YAG in this pulse energy regime.

NASA's Goddard Space Flight Center is developing technology for satellite-based, high peak power, LIDAR transmitting requiring 3-5 years of reliable operation. Semi-conductor laser diodes provide high efficiency pumping of solid state lasers with the promise of long-lived, reliable operation. 100-watt quasi-CW laser diode bars have been baselined for the next generation laser altimeters. Multi-billion shot lifetimes are required. The authors have monitored the performance of several diodes for billions of shots and investigated operational modes for improving diode lifetime.

The various properties of CsLiB₆O₁₀ crystal are reviewed. CLBO has substantial advantages for growth and UV generation. On the other hand, there have been found some problems on CLBO crystal, i.e. hygroscopic and change of crystal refractive index. The effect of thermal annealing on CLBO will be discussed, which can be a clue to overcome these problems on CLBO.

In this paper, a review of the state-of-the-art in semiconductor laser diode reliability is presented, with a particular regard to the reliability of laser diodes used in long-distance telecommunication system. Remarkable advances in semiconductor laser reliability have been demonstrated over their thirty year history, leading to estimated median device lifetimes in excess of 100 years for most applications.

High power diode lasers are important for a variety of applications including diode-pumped (DP) solid state lasers, DP fiber lasers/amplifiers and a variety of printing and medical applications. Recently, there have been significant advances in the maximum cw powers achieved from both multimode and singlemode lasers in both AlGaAs-based and InGaAsP- based materials. The highest powers from broad area lasers have been achieved with high quality AlGaAs-based materials. For example, cw powers of 11.3 and 16.5 W have been demonstrated at 870 nm from 100 and 200 micrometers wide apertures, respectively. These power levels correspond to a facet loading of 80-110 mW/micrometers and facet power densities of 27 MW/cm² which are approximately 2-fold higher than previously reported results. In this paper we report on the performance, reliability and degradation characteristics of AlGaAs- based lasers.

Experimental data are reviewed on optical damage phenomena occurring underexternal laser irradiation and self-damage phenomena in semiconductor laser materials. Laser irradiation damage threshold is found much higher for monochromatic emission within the transparency range than in absorption range. Different physical models are considered critically to explain experimental data and discuss measures for improvement of the optical 'strength' of laser materials. It is discussed also that calculated thermal balance for semiconductor laser self- damage is not in good agreement with observation of damaging that shows signs of thermal destructive process.

We use various probes to measure local temperature induced by internal or external heating of active or passive devices: Mirage detection can reveal heating of few ppb of the input power, whereas photothermal microscopy provides sub-micron spatial resolution. Temperature distribution is measured through periodic deflection or reflectivity mapping at frequencies high enough to confine heating near the source. Scanning InGaAsP/InP lasers facets, shows the weak influence of nonradiative recombination, in agreement with the high output power of these lasers before degradation. On strained-layer InGaAs quantum well lasers we obtained a drastic temperature rise, that we explain through simple model based on line heating for the laser cavity and point heating located at the facet. On a damaged laser the result, demonstrate clearly the existence of heating zones far from the facet along the laser cavity.

Optical probes are used for the investigation of aging properties of high-power laser diode arrays. Two methods - micro-Raman and laser beam induced current (LBIC) scans are discussed: Micro-Raman spectroscopy - a laboratory based method - was applied in order to investigate facet temperature distributions for fresh and aged high-power laser diode arrays (LDA) under regular operation conditions. Furthermore, facet temperatures were measured for operation close to the catastrophic optical damage (COD) level.For these experimental situation facet temperatures of more than 600 degrees C were obtained. In order to conclude from pure temperature data to the thermal management of the LDA we performed modeling work based on the finite element method. Close to the COD up to 14 percent of the total thermal load of the LDA is concentrated to a very thin region close to the facet. Photocurrent spectra and LBIC were found to indicate aging induced effects such as defect creation as well as strain effects. LBIC - method which also could be used as test method for fabrication processes - allows to map aging induced changes of defect distributions in LDAs. Furthermore, the potential of LBIC as quick strain inspection method is discussed.

A spectral and temperature investigation of fast-grown KDP crystals under high fluence, 355 nm laser irradiation is discussed. Pump-and-probe Raman spectroscopy indicate transient changes of the vibrational spectrum. Photothermal deflection experiments provide information on the temporal behavior of the temperature changes. The presence of emission in the visible and NIR spectral regions is attributed to the presence of impurities and/or defects in the crystal.

Damage studies show that the majority of damage on UV grade fused silica initiates at the front or rear surface. The grinding and polishing processes used to produce the optical surfaces of transparent optics play a key role in the development of defects which can ultimately initiate damage. These defects can be on or breaking through the surface or can be sub-surface and surface defects in transparent materials. Images taken which compare both total internal reflection microscopy and atomic force microscopy show that the observed defects can be less than one micron in size. Total internal reflection microscopy has the added benefit of being able to observe large areas with sub-micron detection. Both off-line and in- situ systems have been applied in the Lawrence Livermore National Laboratory's damage laboratory in order to understand defects in the surface and subsurface of polished fused silica. There is a preliminary indication that TIRM quality can be related to the damage resistance. The in-situ microscope is coupled into a 355 nm, 7.5 ns, 10 Hz Nd:YAG laser system in order to study damage occurring at localized scatter sites revealed with the total internal reflection microscopy method. The tests indicate damage initiating at observed artifacts which have many different morphologies and damage behaviors. Some of the scatter sites and damage morphologies revealed have been related back to the finishing process.

Laser-induced damage on optical surfaces is often associated with absorbing contaminants introduced by the polishing process. This is particularly the case for UV optics. In the present study, secondary ion mass spectroscopy (SIMS) was used to measure depth profiles of finishing-process contamination on fused silica surfaces. Contaminating detected include the major polishing compound components, Al present largely because of the use of Al₂O₃ in the final cleaning process, and other metals incorporated during the polishing step or earlier grinding steps. Depth profile data typically showed an exponential decay of contaminant concentration to a depth of 100-200 nm. This depth is consistent with a polishing redeposition layers formed during the chemo-mechanical polishing of fused silica. Peak contaminant levels are typically in the 10-10 pm range, except for Al which often exceeds 1000 ppm.

Surface defects in fused silica have been characterized using Near Field Scanning Optical Microscopy (NSOM). Using total internal reflection of a p- or s-polarized laser beam, optical scattering from defects located on the surface itself as well as in the subsurface layer of polished fused silica has been measured by NSOM. The local scattering intensity has been compared with simultaneously measured surface topography. In addition, surface defects intentionally created on a fused silica surface by nano-indentation have been used to establish a correlation between optical scattering of s- and p-polarized light, surface morphology and the well known subsurface stress-field associated with nano-indentation.

Interest in producing high-damage-threshold KH₂PO₄ (KDP) and DKDP for frequency conversion and optical switching applications is driven by the requirements of the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL). At present only the best crystals meet the NIF system requirements at the third harmonic and only after a laser conditioning process. Neither the mechanism for damage in bulk DKP nor the mechanism for conditioning is understood. As part of a development effort to increase the damage thresholds of DKP and DKDP, we have been developing techniques to pinpoint the locations where damage will initiate in the bulk material. After successfully developing a diagnostic tool that will find these locations, we will use other measurement techniques to determine how these locations differ form the surrounding materials and why they cause damage. This will allow crystal growers to focus their efforts during the growth process in improving damage thresholds. Previously we reported that there was a low correlation between defects in crystals located using light scatter and the sites where damage would initiate. Damage, when it occurs, is almost certainly associated with a localized heating of the crystal which results in mechanical damage to or chemical decomposition of the crystal lattice. If this heating occurs at fluences below the damage threshold, we should be able to measure this heating and predict the locations where damage will initiate prior to damaging the crystal. As a result, we are developing photothermal techniques to probe the bulk material to look for heating due to localized absorption and to determine the correlation between localized heating and the initiation sites for damage.

Considerable attention has been paid over the years to the problem of growing high purity KDP and KD*P to meet damage threshold requirements on succeeding generations of inertial confinement fusion lasers at LLNL. While damage thresholds for these materials have increased over time, the current National Ignition Facility (NIP) maximum fluence requirement (redline) for KD*P frequency triplers of 14.3 J/cm2 at 351 nm, 3 ns has not been reached without laser (pre)conditioning. It is reasonable to assume that. despite the rapid increase in damage thresholds for rapidly grown crystals, ·a program of large scale conditioning of the 192 NIF triplers will be required. Small area ramp (R/1) tests on single sites indicate that KDP damage thresholds can be raised on average up to 1.5X the unconditioned values. Unpublished LLNL 3m raster conditioning studies on KDP, however, have not conclusively shown that off-line conditioning is feasible for KD*P. Consequently, investigating the feasibility of on-line conditioning of NIF triplers at 3m has become a high priority for the KDP damage group at LLNL. To investigate the feasibility of on-line conditioning we performed a series of experiments using the Optical Sciences Laser (OSL) on numerous samples of conventional and rapid growth KDP and KD*P. The experiment entailed exposing sites on each sample to a range of ramped shot (Nil) sequences starting at average fluences of -2 J/cm2 (in a 7 mm "top hat" beam @ 351 nm, 3 ns) up to peak fluences of approximately 13 J/cm2• Test results indicated that the most effective conditioning procedure entailed a 7-8 shot ramp starting at 2 J/cm2 and ending at 12-13 J/cm2• The pinpoint onset fluence for the 8/1 tests was 1.4 times that of the unconditioned site. Damage evolution appears to be exponential as a function of increasing fluence. When damage occurs after conditioning however, pinpoint density evolution exhibits a greater slope than less conditioned sites. The overall reduction in the total pinpoint number can be as high as 300X. Despite laser conditioning , the pinpoint onset for the samples considered is below the NIF redline fluence of 14.3 J/cm2• In addition, the exponential pinpoint evolution curves indicate that damage levels at NIF redline fluences will be on the order of 1 Q4 pinpoints/mm3• 1bis suggests that there will be significant damage in NIF triplers, however, substantial damage has not been observed in the large Beamlet tripler (conventionally grown KD*P) under similar exposure conditions. By applying the OSL damage evolution curves to model NIF THG output spatial profiles it is possible to show damage in NIF triplers will be slight, consisting of isolated clusters with a few pinpoints at high fluence portions of the beam. This prediction has been verified by scatter mapping the 37 cm Beamlet tripler crystal. These results will be discussed in a future memo. These results indicate the feasibility of on-line conditioning for the NIF laser. Keywords: KDP, DKDP, KD*P, bulk laser damage, laser conditioning

Starting from the absorption of laser energy at a subsurface nanoparticle in fused silica, we simulate the consequent buildup of stresses and resulting mechanical material damage. The simulation indicates the formation of micropits with size comparable to a wavelength, similar to experimental observation. Possible mechanism for enhanced local light absorption are discussed.

A morphology study was conducted on fused silica surfaces damaged by single pulse laser irradiation at 355 nm. The physical characterization of the surface showed that cracks initiate at ellipsoidal pits on both input and output surfaces. The size of the pit increases with laser pulse-length and the orientation of the ellipse is perpendicular to the electric field. The pits are less than 300 nm deep. Cracks initiate along the main axis of the ellipse. The morphology of the cracks is different for input vs. output surfaces. The output surface crack exhibits a shell- like morphology typical of localized compressive stress. The input surface crack shows, on the other hand, a star-like pattern. These differences seem to be caused by differences in plasma propagation on the surface. The extent of damage and the number of shells or branches increases with increasing fluence. A molten morphology can be observed at the center of the cracks for fluences above the pit formation threshold. This indicates that local temperatures can be in excess of several thousand degrees C.

A set of fused silica windows were artificially contaminated by sputtering various thin film materials onto the surface to simulate target chamber contamination of high fluence optical components. The samples were then tested 1/1 and N/1 at 355 nm in air and vacuum with a 3- ns Nd:YAG laser to measure the damage threshold as a function of film thickness and film composition. The damage morphologies were characterized by Nomarski optical microscopy and SEM. The results show that thin film contamination leads to a decrease in damage threshold. The test also show a difference in damage behavior between air and vacuum. The results were compared to a model of the absorption of laser light in the film, shockwave generation and reflection, and heating dependence on film composition and thickness.

Scratches of measured width were produced on the surface of a IV grade fused silica window using a diamond tip. Two scratch morphologies were observed: plastic and brittle. The scratches were irradiated with a 355 nm laser pulse. The laser-induced damage threshold (LIDT) of the unscratched output surface was 15 J/cm² at 3-ns. The LIDT of the scratched surface as a function of scratch width was then measured for both input and output surface scratches. Input surface scratches of width smaller than 10 micrometers did not influence the LIDT of the silica window. On the output surface, 7 $mUm wide scratches lowered the LIDT by a factor of two. For larger scratches, the LIDT reached an asymptotic value of 5 J/cm² on both input and output surface. Possible reasons for this LIDT drop could be electric field enhancement in the cracks below the scratch, the presence of contamination particles in the scratch, or the weakening of the material because of existing mechanical flaws.

The technique of chirped pulse amplification makes possible the investigation of dielectric breakdown as a function of pulse duration over a continuous regime of pulse duration from the 100 ps to the tens of fs and for material with bandgap varying from the single eV to several eV. This study leads to the surprising result that even for 100 fs pulses the dielectric breakdown is dominated by impact ionization even in the case of single photon absorption. For larger bandgap the seeding of the avalanche is produced by multiphoton ionization. This makes possible for the first time the direct determination of the electron ionization rate in important electronic materials like silicon as a function of the electric field up to 2 10⁸ V/an which is impossible by conventional electronic means.

A damage morphology study was performed with a 355 nm, 8-ns Nd:YAG laser on synthetic UV-grade fused silica to determine the effects of post-polish chemical etching on laser-induced damage, compare damage morphologies of cleaved and polished surfaces, and understand the effects of the hydrolyzed surface layer and water-crack interactions. The samples were polished, then chemically etched in a buffered HF solution to remove 45, 90, 135, and 180 nm of surface material. Another set of sample was cleaved and soaked in boiling distilled water for 1 second and 1 hour. All the samples were irradiated at damaging fluences and characterized by Normarski optical microscopy and scanning electron microscope. Damage was initiated at micro-pits on both input and output surface of the polished fused silica sample. At higher fluences, the micro-pits generated cracks on the surface. Laser damage of the etched fused silica surface shoed that the real density of micro-pits decreased with etched thickness. SIMS analysis of the polished surface showed significant trace contamination levels within a 50 nm surface layer. Micro-pits formation also appeared after irradiating cleaved fused silica surfaces at damaging fluences. Linear damage tracks corresponding cleaving cracks were often observed on cleaved surfaces. Soaking cleaved samples in water produced wide laser damage tracks.

Grinding is considered to be a rough machining process in the field of optics; a polishing process must follow the grinding process for getting optical-quality surfaces. An ultraprecision surface grinder with hydrostatic oil bearings and a glass-ceramic spindle of extremely low thermal expansion was developed to get smooth optical surfaces without any polishing process. Various optical materials such as NbF1, BK7, LHG08 fused silica, KTP, KDP and CLBO were ground into optical surfaces after empirically determining the conditions required to attain ductile-mode grinding. An extremely smooth surface less than 0.1 nm rms was obtained on BK7 glass by the ultraprecision grinding process. The laser-induced damage threshold was measured on variously finished LHG-8 laser glass at (lambda) equals 1.053 micrometers and 1-ns pulse width. The damage threshold was measured at 22.2 J/cm² on a ground surface with the polarization parallel to the grinding direction. This number is higher than that obtained by optical polish. The damage threshold of 293 J/cm² was also obtained on a ground LHG-8 glass surface at (lambda) equals 1.053 micrometers and 30-ns pulse width.

KDP single crystals grown from aqueous solutions by the recirculation method from nominally 'pure' raw material and added impurities Ca, Si, Pb, Cr, hydrogen peroxide were studied by high resolution x-ray method. It was found that additional introduction of impurities leads to a deterioration of structural quality of crystals. The main growth defects of crystals at dislocation density approximately 10² cm^-2 at the absence of zonarity and sectionality is the impurity striation and presence of low angle quasi-boundaries. One can observe a correlation between the concentration of low angle quasi-boundaries, dispersion of structure sensitive parameters and the value of bulk laser damage threshold.

Subsurface damage, namely grinding and polishing fractures or scratches hidden by the polishing redeposition layer, may lower laser damage threshold by reducing fracture strength, providing sites for absorbing contaminants, and modulating the electromagnetic field. Polishing compound may affect 355-nm laser damage by absorption and scratching. Fused silica surfaces were finished with different grinding and polishing removal depths to leave different extents of subsurface damage from grinding. Three surfaces were polished with ceria and three with zirconia. The surfaces were 355-nm laser damage tested using an automated 'R- on-1' protocol, and also characterized for surface roughness, subsurface damage as revealed by etching, and total internal reflectance microscopy (TIRM). The zirconia polishing produced the highest laser thresholds, especially when the surface was free of polishing and grinding damage. The zirconia polishing was prone to damaging the surface, which significantly reduced laser thresholds at some sites. the ceria polishing left very little polishing damage, and showed uniform damage thresholds when there was no subsurface damage from grinding present. Residual Blanchard fractures lowered thresholds dramatically. Subsurface fractures from fine loose grinding sometimes reduced threshold dramatically and sometimes very little. TIRM images correlate fairly well with microphotographs of subsurface damage revealed by etching.

The measurement of intrinsic laser induced damage thresholds (LIDT) in optical components for continuous wave (CW) CO₂ radiation has been investigated. A combination of analytical and numerical models showed that the temperature rise is mainly determined by the surface absorption in transmissive as well as reflective components, and is proportional to the ratio of power to linear dimension (P/d) of the irradiated spot rather than to the conventional power/area (P/d²) parameter. The former ratio therefore represents the correct power scaling law for LIDT measurement in CW laser systems. The precise time domain within which this law holds is a function of spot diameter. This prediction has been confirmed by experimental LIDT tests on well characterized uncoated ZnSe substrates and copper mirrors, and on coated ZnSe windows and copper mirrors. Measured P/d values, though lower than predicted by modelling are considerably higher than those inferred from the technical literature, and show that transmissive components may be used at much higher powers than are at present believed. The results indicate that surface absorption occurs primarily in the sub-surface processing layer. This has been shown by transmission electron microscopy and spectroscopic ellipsometry to be a few hundred nm in depth.

Development of high damage threshold, 50 cm, rapidly grown KD*P frequency triplers for operation on the National Ignition Facility (NIF) in the 14 J/cm², 351 nm, 3 ns regime requires a thorough understanding of how the crystal growth parameters and technologies affect laser induced damage. Of particular importance is determining the effect of ionic impurities which may be introduced in widely varying concentrations via the starting salts. In addition, organic particulates can contaminate the solution as leachants from growth platforms or via mechanical ablation. Mechanical stresses in the crystals may also play a strong role in the laser-induced damage distribution (LIDD), particularly in the case of large boules where hydrodynamic forces in the growth tank may be quite high. In order to investigate the effects of various impurities and stresses on laser damage we have developed a dedicated, automated damage test system with diagnostic capabilities specifically designed or measuring time resolved bulk damage onset and evolution. The data obtained makes it possible to construct characteristic damage threshold distributions for each samples. Test results obtained for a variety of DKP samples grown form high purity starting salts and individually doped with Lucite and Teflon, iron, chromium and aluminum show that the LIDD drops with increasing contamination content. The results also show that solution filtration leads to increased damage performance for undoped crystals but is not solely responsible for producing the high LIDDs required by the NIF. The highest LIDD measured on a rapidly grown sample indicate that it is possible to produce high damage threshold material using ultrahigh purity, recrystallized starting salts, continuous filtration and a platform designed to minimize internal stress during growth.

The standardization of laser damage testing is an important prerequisite for development and quality control of optical components exposed to high-power laser beams. The ISO working group ISO/TC 172/SC9/WG6 is currently engaged with the preparation of a standard for laser damage testing. The first part of this standard which is concerned with 1-on-1 damage testing has reached the status of a draft international standard: ISO/DIS 11254-1. To test for the practicability of this characterization method, a round robin experiment was performed within the framework of the EUROLASER-CHOCLAB project. Identical sets of typical Nd:YAG laser components were distributed to the above-mentioned laboratories for LIDT measurements in conformance with the standard. The results are summarized in this paper. Additionally, damage morphologies were examined by microscopic techniques like Nomarksi differential interference contrast and scanning force microscopy.

The laser damage facility at DLR, Institut fuer Technische Physik, is based on the small-spot technique for 1/1 or S/1 investigations according to the ISO standards 11254-1.2 and 11254-2, respectively. A primary task is to measure out-of-band LIDTs of optical components used in the IR spectral range. The laser source is a pulsed, 10 Hz repetition frequency Nd:YAG laser providing a Gaussian intensity distribution on the sample. In this work, coated and uncoated samples of ZnSe were exposed to 1.06 micrometers radiation, and single and multiple pulse damage thresholds were measured. Time-resolved laser damage assessment based on scatter probe techniques was used to monitor the temporal development of the damage under multipulse irradiation. Up to a certain fluence limit above the multipulse threshold, a slight but stable surface modification irradiation. Up to a certain fluence limit above the multipulse threshold, a slight but stable surface modification was found. When raising the fluence above this value, severe damage occurred. The damage morphology was revealed with the help of microscopic techniques like Nomarski DIC and scanning force microscopy.

In this paper we report an indirect method based on photomultiplier response calibration to measure the radiant power of the internal second harmonic generation (ISHG) from InGaAs/GaAs/AlGaAs strained single quantum well buried heterostructure laser diodes. We observed enhanced ISHG radiant power, of the order of magnitude of 10^-8 W. This phenomenon represents a signature of the beginning of the process of catastrophic optical degradation of the LD mirror facet layers, where the nonlinear optical interaction occurs.

The nature of surface optical breakdown is generally related to the presence of pre-existing and laser-induced defects. Techniques and devices of near-field optical microscopy (NFOM) are proposed to be used to identify the defects and investigate their evolution under action of high-power laser fields. NFOM provide detailed high-resolved information about local surface optical properties and surface microstructure as well as about space distribution of laser fields at the surface. General principles of NFOM and some ways to apply it to investigation of surface optical damage of transparent materials are considered in this paper.