Proceedings Volume 1848

24th Annual Boulder Damage Symposium Proceedings -- Laser-Induced Damage in Optical Materials: 1992

Harold E. Bennett, Lloyd L. Chase, Arthur H. Guenther, et al.
cover
Proceedings Volume 1848

24th Annual Boulder Damage Symposium Proceedings -- Laser-Induced Damage in Optical Materials: 1992

Harold E. Bennett, Lloyd L. Chase, Arthur H. Guenther, et al.
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 24 June 1993
Contents: 6 Sessions, 59 Papers, 0 Presentations
Conference: Optical Materials for High Power Lasers 1992
Volume Number: 1848

Table of Contents

icon_mobile_dropdown

Table of Contents

All links to SPIE Proceedings will open in the SPIE Digital Library. external link icon
View Session icon_mobile_dropdown
  • Materials and Measurements
  • Surfaces and Mirrors
  • Mini-Symposium on Damage Issues for Lithographic Optics
  • Thin Films
  • By Title Only
  • Thin Films
  • Fundamental Mechanisms
  • Materials and Measurements
  • Thin Films
  • Surfaces and Mirrors
  • Fundamental Mechanisms
  • Mini-Symposium on Damage Issues for Lithographic Optics
  • Materials and Measurements
  • Fundamental Mechanisms
  • Thin Films
  • Surfaces and Mirrors
  • Thin Films
  • Fundamental Mechanisms
  • Materials and Measurements
  • Thin Films
  • By Title Only
  • Surfaces and Mirrors
  • Fundamental Mechanisms
Materials and Measurements
icon_mobile_dropdown
New challenges in optical materials development
The recent emergence of high performance semiconductor laser diodes and diode arrays emitting at wavelengths between 630 and 2300 nm has enabled the demonstration of several novel diode-pumped solid state laser materials. Narrowly, laser diode arrays may be viewed as simple replacements for conventional flash or arc lamp pumps in solid state lasers. The examples discussed in this paper illustrate that laser diode array pumps are considerably more versatile, enabling novel high performance lasers that are technically not feasible using conventional flashlamps. Additionally, several relatively new nonlinear optical materials, possessing high nonlinearities and excellent phase-matching properties, offer good prospects for the development of efficient, compact, wavelength-diverse all-solid-state laser sources.
Gray-track damage in KTiOPO4 crystals
Patricia A. Morris, August Ferretti, Mark G. Roelofs, et al.
A correlation is observed between the laser and electric-field damage susceptibilities of most KTiOPO4 (KTP) crystals. The observed gray track damage susceptibilities are found to be correlated to the concentrations of defects in the crystals which can stabilize the Ti3+ damage defect sites in KTP. These stabilizing defects are those which can have an effective positive charge, such as oxygen vacancies, protons, and Ba and F impurities. The defects present in KTP crystals depend on the technique (hydrothermal or flux) used for growth. Post-growth processing techniques have been developed which reduce the laser gray track and electric-field damage susceptibilities of both hydrothermal and flux KTP.
Laser damage threshold measurements in Q-switch materials
Madhu A. Acharekar, James L. Montgomery
Materials used for electro-optics (EO) and acousto-optics (AO) Q-switches in lasers include fused silica (SiO2), lithium niobate (LiNbO3), and tellurium oxide (TeO2). The damage threshold measured for these materials is presented in this technical paper. Also, the Q-switch data collected for chromium, thulium, and holmium doped YAG (CTH:YAG) laser operating at 2.1 micrometers are reported.
Laser damage to production- and research-grade KDP crystals
Frank Rainer, L. Jeffrey Atherton, James J. De Yoreo
We present the results of laser damage measurements conducted on potassium dihydrogen phosphate (KDP) and deuterated potassium dihydrogen phosphate (KD*P) crystals that were grown recently for both production and research applications by several sources. We have measured extrinsic damage thresholds that cover wavelengths from 1064 nm to 266 nm at pulse durations in the 3 to 10 ns regime. Many of the samples were extracted from boules grown specifically to yield large-area crystals, up to 32 cm square, for laser fusion applications. These crystals were the result of efforts, both by the Lawrence Livermore National Laboratory (LLNL) and commercial crystal-growth companies, to yield high- threshold KDP. In particular we have established that such crystals can reliably survive fluences exceeding 15 J/cm2 at 355 nm and 20 J/cm2 at 1064 nm when irradiated with 3 ns pulses. We present details of how bulk and surface damage to these crystals scale with pulse duration and wavelength as well as of morphological effects due to laser conditioning.
Transient laser-induced surface deformation of silicon in relation to damage
Chun Chi Ma, Woei-Yun Ho, Rodger M. Walser, et al.
Transient thermal expansion, heat generation and conduction, and nonlinear laser heating of single crystal Si wafers below and above the damage onset has been investigated via a laser pump-probe configuration. The transient photothermal deflection (TPD) technique that we employed consisted of a 10 ns Nd:YAG pump source at 1064 nm with 10 Hz repetition rate. A cw HeNe probe beam was used to probe the localized time dependent slope change of illuminated surface. The deflection of the reflected probe beam, centered at the maximum slope of the irradiated spot, was detected by a fast bicell photodiode. The deformation signals were recorded by a digital camera system in conjunction with a high-speed oscilloscope. The waveforms were later analyzed for surface angular deflection and vertical displacement based on the geometry of the Gaussian irradiation profile. Vertical displacement down to a few nm could be detected and converted into instantaneous peak surface temperature by a first-order, approximate thermal model. Measured displacement and surface temperature were then compared to computer simulations at different fluence levels. They were found to be in excellent agreement to each other. In addition, single and multiple shot experiments were performed to obtain their respective damage onsets and thresholds. Measurement of peak surface deformation at subthreshold fluences gives insight into the thermomechanical processes which may play an important role in multi-pulse damage.
Investigation of the accuracy and precision of the damage-frequency method of measuring laser damage threshold
This paper presents a defect ensemble model based analysis of the accuracy and precision of the damage frequency method (DFM) of laser damage threshold determination. The defect ensemble is used to model an optic under test and thereby gain knowledge of the true threshold. A statistical estimation of the measured damage frequencies is then derived from the binomial distribution, the defect ensemble, and the parameters of the test. The calculated test results are then analyzed using a weighted linear regression to calculate the measured threshold and its uncertainty. The difference between the true threshold and the measured threshold is a direct expression of the accuracy of the DFM. The uncertainty in the measured threshold is a measure of the precision of the DFM. The accuracy and precision of the DFM is evaluated over a range of optic and test parameters. The accuracy and precision results are combined into another figure of merit, called the measurement quality, Q(L). The resulting parametrically described Q(L) is the probability of making an accurate LDT measurement to a precision of L. It is shown that on average under most of the conditions examined, the DFM is an accurate but relatively less precise test method.
Normal incidence complex-index refractometry
Robert D. Larrabee
Traditional refractometry of bulk transmissive optical materials is usually restricted to the lossless case where the material can be completely characterized by a real index of refraction. This paper is concerned with the measurement of both the real and the imaginary parts of the index of refraction (i.e., including any loss) using a single prism-shaped specimen of homogeneous material. The normal-incidence configuration is analyzed and discussed in this paper because of its mathematical simplicity compared to the minimum-deviation configuration. The measurement of the imaginary part of the index of refraction (i.e., loss) is practical when the longest and shortest optical paths through the prism differ by an absorption length or more. The measurement of the real part of the index can ignore loss and still be accurate to better than one part per million if the power absorption length in the prism material is longer than 60 wavelengths in that material. An equation to determine the real part of the index of refraction is given when the loss must be taken into consideration.
Surfaces and Mirrors
icon_mobile_dropdown
Test facility for long-focal-length mirrors
Harold E. Bennett, John J. Shaffer
Testing the optical figure and focal length of laser mirrors with radii of curvature in the 10 to 100 m range is difficult. If the mirror is concave, a source can be placed at the center of curvature. Air turbulence over these long path lengths makes interferometry difficult, however, and greatly reduces measurement accuracy. Convex mirrors are even more difficult to measure. A solution is to produce a slightly converging or diverging beam from a virtual source. The actual optical path in which turbulence may develop can then be made very short. A three-element test system consisting of a parabola, a transmission sphere, and a folding flat is described. It is capable of measuring both optical flats and convex or concave mirrors up to 40 cm in diameter with radii of curvature from 10 m to infinity. System accuracy is 1/20th wave rms in optical figure and 0.2% in radius of curvature. A discussion is given of the systematic errors introduced when the parabola is used in other than parallel light.
Thermal behavior of optical mirrors under high-power continuous wave CO2 laser irradiation
Jean Hue, Jean DiJon, Philippe Lyan
From experimental results we characterize the catastrophic damage threshold of mirrors irradiated with cw CO2 laser by a critical temperature. Analytical expressions of the mirror temperature versus beam radius are developed. A comparison between experiments and theory shows that P/R determined in test is the good ratio to compare mirror coatings and to scale test.
Damage thresholds of AR coating on subsurface-damage-removed fused silica glass
Kunio Yoshida, Kanyoshi Ochi, Hidetsugu Yoshida, et al.
We have developed a coating chamber (diameter of 1600 mm(phi )) including ion guns for removal of subsurface which can deposit on the substrate of the maximum diameter of 800 mm (phi ). The damage threshold of AR coating on subsurface removed fused silica glass of 800 mm(phi ) showed 7 J/cm2 at 355 nm, pulsewidth of 0.3 ns. (Abstract only)
Mini-Symposium on Damage Issues for Lithographic Optics
icon_mobile_dropdown
Optical degradation issues for XUV projection lithography systems
We review the degradation susceptibility of the various optical components proposed for use in XUV projection lithography systems at 13 - 20 nm. Contamination by laser plasma target debris and carbonaceous films are two primary hazards that must be essentially nullified. Pertinent experimental results of a number of researchers illustrate the present status.
Laser plasma sources for soft x-ray projection lithography
Martin C. Richardson, William T. Silfvast, Howard Bender III, et al.
Compact laser plasma soft x-ray sources are under serious consideration for projection lithography. Current baseline designs for soft x-ray projection lithography systems place stringent performance and cost requirements on a laser plasma soft x-ray source. The x-ray source must meet specific criteria relating to x-ray fluence in the 13 nm region, the wavelength of preference for projection lithography, to output stability at high (kHz) repetition-rates, to continuous long term operation and to operating costs. Moreover the source must operate in a vacuum enclosure in close proximity to costly x-ray and optical elements which must remain in pristine condition, free from degradation by particulate and plasma emission emanating from the source. We discuss these requirements, and review experiments leading to the optimal design of a laser plasma soft x-ray source.
XUV projection lithography: is optical surface contamination an important limitation?
Marion L. Scott, Brian Emerson Newnam
We report results of an extensive set of oxide and carbon film contamination experiments with Al, Si, Rh, and Ag films and surfaces to quantify the film growth rates and parameter dependencies. These four materials were selected initially because they exhibit total external reflectance at moderate angles of incidence, e.g., > 45 degree(s), as needed for high- reflectance multifacet mirrors. In addition, these materials are candidate films for single- surface and multilayer mirrors at normal incidence as well as transmission filters in XUV projection lithography optical systems. (Abstract only)
Care and feeding of soft x-ray and extreme ultraviolet filters
Forbes R. Powell
Ultra thin metallic foils and organic films have been used as filters for the soft x-ray (SXR) and extreme ultraviolet (EUV) portion of the electromagnetic spectrum, particularly since the advent of space astronomy in the early 1970s. Much of the experience gained from manufacturing, testing, and using these filters is applicable to other applications such as microscopy, lithography, and holography. This review paper attempts to correlate data and experience from a large number of projects. Transmission data is presented along with discussion of causes of filter damage. Lessons learned are presented in the form of general guidelines for preventing filter damage.
Control of debris production of laser plasma sources with high-average XUV power
Fred Bijkerk, Eric Louis, Leonid A. Shmaenok, et al.
Recent investigations on methods to eliminate the contamination of high power laser plasma sources are reviewed. The use of separate methods such as a buffer gas environment or thin tape targets suppresses the debris yield drastically up to a factor of 103. A combination of this with other effective as well as practically convenient methods (use of debris angular characteristics, a rotating shutter, or thin UV filters) provides a truly contamination-free XUV source. As an example of a novel high power source we describe the performance of a 1.5 J KrF laser designed to drive a laser plasma XUV source. The laser was used to generate narrowband XUV radiation at 18 nm. A conversion efficiency of >= 1.4% in a bandwidth of 6% was measured, demonstrating the feasibility of laser plasma sources for applications such as XUV projection lithography, requiring high average power.
Use of Raman spectroscopy in characterizing soft x-ray multilayers: tools in understanding structure and interfaces
Ming Cai, Qi Wang, David D. Allred, et al.
Our group is studying the structure and interfaces of soft x-ray multilayers by various techniques including x-ray diffraction and Raman spectroscopy. Raman spectroscopy is particularly useful since it is sensitive to the identity of individual bonds and thus can potentially characterize the abruptness of interfaces in multilayers. Blocking interfacial mixing is very important in achieving and maintaining high reflectivity. We report our studies of the as-deposited and postannealed structure of Mo/Si and W/C multilayers. A high normal- incidence, peak reflectance is mandatory for imaging applications that involve many reflections. The reported theoretical and achieved reflectances of the Mo/Si system are 80% and 65%, respectively. This loss of 15% can bring about a six-fold loss in system throughput in the eight-reflection system contemplated. The interfaces in the Mo/Si system are thought to play a significant role in the degrading reflectance so characterization techniques which have interfacial sensitivity are particularly important. The Mo/Si multilayer system is susceptible to Raman characterization since both the a-Si spacer layer and the MoSi2 compound which forms at the interface have Raman active modes. In this paper we report the first Raman studies, to the best of our knowledge, of the a-Si layers and their crystallization and the crystallization of the Mo/Si interface of the multilayer brought about by a one-hour 1000 degree(s)C anneal. These changes are apparent in the Raman spectra before they can be unambiguously detected by x-ray diffraction.
Excimer-laser induced degradation in bulk-fused silica
Excimer laser irradiation of fused silica is shown to induce gradual changes in the material, which affect its optical properties. These changes include visible fluorescence, formation of absorption bands at approximately 215 nm (E' centers), and increases in density and index of refraction. The magnitude of these effects varies initially as the square of the laser fluence and linearly with the number of pulses, indicating that they are the result of a two- photon absorption process. Pre-or post-irradiation treatments can be used to reduce the amount of laser induced degradation, especially the formation of color centers.
Thin Films
icon_mobile_dropdown
Laser damage in electron-beam-patterned surface resonant structures
Keith L. Lewis, Colin R. Brewitt-Taylor, Andrew M. Pitt, et al.
Surface resonant structures can be used as spectral filtering elements. Both band-stop and band-pass designs are possible using capacitive and inductive mesh concepts respectively. With the current evolution in electron lithographic technology to allow the realisation of sub-micron features, it has become possible in recent years to produce resonant mesh arrays to high degrees of fidelity and performance over significant areas of optical substrates. These can provide, for example, responses in the infra-red spectral region using elements with a dimensional scale of a few microns. The unit cell design in the mesh can be relatively complex and can be tailored to provide specific spectral responses in the waveband range of interest. Such structures can find a wide variety of applications, especially as dichroic beam splitter elements for the separation of infra-red and RF radiation. The possibility of using such structures for spectral filtering has been known for many years, particularly for the microwave, millimeter and far infra-red wavelength regime. Some of the earlier applications in the far JR have been discussed by UJriCh [1,21 More recently, Byrne et al [31 fabricated capacitive and inductive mesh filters for the infra-red using electron beam lithography and demonstrated the bandstop and bandpass transmissive properties achievable. Byrne and coworkers used a lift-off technique to create capacitive mesh patterns having linewidths of less than O.25j.tm in O.lp.m thick aluminium and gold on calcium fluoride. Crossed capacitive dipoles of aluminium 2.6.tm in length with an aspect ratio of 10:1 produced a broad reflection band at 6.25tm. The measured bandwidth (FWHM) was about 2.5pm. For inductive designs, a two level lift-off approach was adopted, involving pattern transfer from an initial capacitive design using oxygen reactive ion etch techniques. A 1.8pm length slot with aspect ratio of 10:1 produced a resonance at 6.5.tm with peak transmittance of about 80% and bandwidth of 2.75p.m. Low levels of intrinsic absorption in the slot material of the inductive mesh are enhanced by the resonant effect of the mesh and reduce the level of transmittance achievable. It is of interest from the point of view of potential high power laser applications to explore the relationship between the level of optical absorbance and the ensuing laser damage threshold of such meshes. Mohebi [71 explored the response of wire grid polarisers to pulsed C02 laser irradiation at 10.6j.m and found that damage thresholds were largest for the case when the polarisation of the incident radiation was perpendicular to the wires. Antirerfiection coatings reduced the damage threshold. This work assesses the case of crossed dipole meshes and explores the role of absorption by incorporating weakly absorbing films in the meshes and by the addition of dielectric material between the absorber and the antenna plane. It also explores the role of resistive loss in determining the properties of capacitive meshes. The absorber used has optical constants of n = 2.9 - 0.25i which are typical of some transition metal oxides and chalcogenide materials.
Damage threshold study of ion-beam-sputtered coatings for a visible high-repetition laser at LLNL
Ion beam sputtered (IBS) coatings offer some distinct advantages over electron beam deposited (e-beam) coatings, namely environmental stability and extremely low total losses. Initial IBS coatings had excessive stress and low laser damage thresholds. For these reasons, a study was initiated to examine potential laser damage thresholds and stress of IBS coatings. A material study was conducted of IBS coatings produced in industry, and a variety of design techniques were explored to increase laser damage thresholds. A post annealing process and backside coatings were developed to reduce the coating stress.
Photodisplacement studies of the incubation behavior of oxide coatings on fused silica at 248 nm
Zhouling Wu, Eckart Matthias
The photodisplacement technique was applied to investigate incubation effects in 500 nm TiO2, ZrO2, and HfO2 coatings for single laser shots of 248 nm at fluences below the damage threshold. This technique is uniquely suited for incubation studies because of its sensitivity for tracing weak optical absorption with high spatial resolution. For varying fluences the single shot incubation behaves differently in these oxides and seems to correlate with the band gap. In TiO2, where the band gap is smaller compared to the photon energy, the absorption increases exponentially with laser fluence. For ZrO2 and HfO2 with band gaps apparently larger than the photon energy, the absorption saturates at a fluence corresponding to about 30% of the damage threshold. We take this as evidence that the incubation mechanism differs whenever the photon energies are smaller or larger than the band gap.
Correlation of damage threshold and surface geometry of nodular defects on HR coatings as determined by in-situ atomic force microscopy
Michael C. Staggs, Mark R. Kozlowski, Wigbert J. Siekhaus, et al.
Atomic force microscopy (AFM) was used to determine in-situ the correlation between the surface dimensions of defects in dielectric multilayer optical coatings and their susceptibility to damage by pulsed laser illumination. The primary surface defects studied were micrometers -scale domes associated with the classic nodule defect. The optical film studied was a highly reflective dielectric multilayer consisting of pairs of alternating HfO2 and SiO2 layers of quarter wave thickness at 1.06 micrometers . Nodule defect height and width dimensions were measured prior to laser illumination on two different samples. Correlation between these dimensions supported a simple model for the defect geometry. Defects with high nodule heights (> 0.6 micrometers ) were found to be most susceptible to laser damage over a range of fluences between 0 - 35 J/cm2 (1.06 micrometers , 10 ns, and 1/e2 diam. of 1.3 mm). Crater defects, formed by nodules ejected from the coating prior to illumination, were also studied. None of the crater defects damaged when illuminated over the same range of fluences that the nodule defects were subjected to.
Atomic force microscopy of laser-conditioned and laser-damaged amorphous TiO2 sol-gel thin films
Nancy J. Hess, Gregory J. Exarhos, Martin J. Iedema
Amorphous TiO2 films are spin-cast from highly acidic ethanol precursor solutions of titanium ethoxides onto fused silica and single crystal silicon substrates at 2000 to 3000 rpm. Immediately after spinning, the resulting homogeneous film is approximately 350 nm thick and has a refractive index of 1.6 as determined from UV-VIS-NIR transmission measurements. After 20 hours at room temperature, the refractive index increases to 1.8 and is accompanied by a decrease in film thickness. Mild heat-treatment of the film results in a further increase of the refractive index to 2.0. Thicker, homogenous films can be prepared by depositing subsequent layers. Raman spectra of these films indicate the presence of the amorphous phase of titania. Prior to mild heat-treatment, the amorphous titania films are observed to crystallize to the anatase phase upon exposure to cw radiation from an Argon-ion laser at a fluence of 2.5 Mw cm-2. Crystallization is initiated approximately 1.2 seconds after exposure and the illuminated spot becomes fully crystalline after 3.6 seconds. However, the laser fluence can be increased to 6.0 Mw cm-2 without resulting in crystallization if the amorphous film is conditioned by exposure to progressively greater laser fluence beginning below the damage threshold. Similarly, films that have experienced a mild heat-treatment prior to irradiation have a damage threshold greater than 6.0 Mw cm-2. In this study, atomic force microscopy is used to evaluate the microstructure of the laser-conditioned and laser-damaged titania films at the micron to submicron scale.
By Title Only
icon_mobile_dropdown
Effect of laser irradiation conditions on the optical damage resistance of dielectric coatings
A. V. Belotserkovets, A. V. Bessarab, Gennadi A. Kirillov, et al.
We report the measured damage thresholds of dielectric coatings of optical elements in the 120 TW iodine laser system ISKRA-5 when irradiated by pulses with approximately 0.5 ns duration at 1315 nm. The main part of high-reflection coatings is characterized by the damage thresholds from 10 to 30 J/cm2. The damage thresholds of antireflection coatings for most components are significantly lower, namely: from 3 to 8 J/cm2. It has been shown that the damage resistance of high-reflection coatings is greatly dependent on whether the coating is on the entrance or exit surface to radiation, whereas for antireflection coatings and quarter-wave SiO2 films such dependence is practically absent. The damage threshold of polarization coatings depends on radiation polarization. The obtained results are explained by a mechanism connected with a preferential concentration of absorbing defects, causing damage at the boundary of a coating and substrate.
Thin Films
icon_mobile_dropdown
Effect of binders on the damage threshold and refractive index of coatings prepared from colloidal suspensions
Ian M. Thomas
The addition of polyvinyl alcohol (PVA) to aqueous suspensions of AlOOH, ZrO2, and HfO2 has been found to increase the damage threshold and refractive index of coatings prepared from these suspensions. The increase in damage threshold may be due to the increase in coating strength brought about by the enhanced particle-to-particle bond strength due to the polymer binder. The binder also increases the refractive index by decreasing the porosity. This leads to a reduction in the number of layer pairs required for high reflection in a typical HR coating. A mirror consisting of only ten layer pairs of HfO2-SiO2 with PVA binder in the HfO2 had a reflectivity of over 99% at 1.06 micrometers and a damage threshold of 25 - 28 J/cm2 at 1.06 micrometers with a 10 ns pulse.
New room-temperature deposition technique for optical coatings
Philippe F. Belleville, Herve G. Floch
We describe a new coating method `laminar flow coating' (LFC) technique developed to obtain highly reflective (HR) laser damage resistant sol-gel multidielectric coatings. Such coatings are used in high-power lasers for inertial confinement fusion experiments (ICF). This technique uses substrates in an upside-down position and a travelling wave of coating solution is laminary transported under the substrate surface with a tubular dispense unit. This creates a thin-film coating by the solvent evaporation. Satisfactory results have been obtained onto 20 cm square glass substrates regarding the optical performances, the thickness uniformity, the edge-effects and the laser damage resistance. This deposition technique combines the advantages of both classical techniques: the substrate non-exclusive geometry such as in dip- coating and the small solution consumption such as in spin-coating. The association of sol-gel colloidal suspensions and LFC coating process has been demonstrated as a promising way to produce cheap specific optical coatings.
Surface morphology, phase characterization, and stability of solution-deposited zirconia films
Gregory J. Exarhos, Nancy J. Hess, Susanne Wood
Deposition of zirconia films and multilayers from aqueous solution involves complexation of the precursor metal nitrate with a water soluble organic complexant. Following spin casting and associated water evaporation, an amorphous organic film is formed. Heat treatment initiates oxidation of the organic phase by nitrate, leaving behind a high index stoichiometric oxide film (n equals 2.01). Small grain sizes (approximately equals 50 nm) observed by AFM analysis stabilize the cubic and tetragonal crystalline phases of zirconia which are identified using XRD and Raman spectroscopy. Annealing above 700 degree(s)C causes recrystallization of these films to the monoclinic phase. A fluorescence method based upon introduction of a Sm+3 dopant into the film is shown to be a sensitive probe of the inherent crystalline phase and phase homogeneity and is used to evaluate crystalline phases in laser damaged coatings. These films exhibit visible damage areas upon irradiation at 532 nm with pulse energies in excess of 10 J/cm2. An increase in grain size accompanied by the presence of a disordered phase of zirconia is observed. Results are compared with damage regions induced in similarly irradiated PVD zirconia films.
Effect of hydrogen concentration in conventional and IAD coatings on the absorption and laser-induced damage at 10.6 um
Manfred Rahe, Detlev Ristau, Holger Schmidt
In this paper, data of single layers of YbF3, BaF2, YF3, and NaF and multilayer coatings produced by conventional thermal evaporation (boat, e-beam) and ion assisted deposition (IAD) are compared. Hydrogen concentration depth profiling was performed using nuclear reaction analysis based on the reaction 1H(15N, (alpha) (gamma) )12C. Absorption was measured with the aid of a laser calorimeter and a cw CO2 laser. A computer-controlled test facility with a TEA CO2 laser was used for determining the 1-on-1 damage thresholds of the coatings. The results point out that the absorption and damage behavior of coatings for the CO2 laser wavelength are related to the total amount of species containing hydrogen. Most of the IAD coatings exhibit a lower hydrogen contamination than conventional thin films.
Absorption and damage thresholds of low-defect-density hafnia deposited with activated oxygen
Robert Chow, Steven Falabella, Gary E. Loomis, et al.
Motivation for this work included observations at Lawrence Livermore National Laboratory and elsewhere of a correlation between increasing laser damage thresholds (DT) and both decreasing nodular-defect density and absorption of coatings. We reduced the nodular-defect densities by a factor of over 4x in hafnia (HfO2) coatings deposited by reactive e-beam evaporation from a Hf target source. In order to increase the metal oxidation kinetics at the coated surface, Hf was e-beam deposited reactively with O2 activated by a (mu) -wave discharge. The effect of using activated O2 during the evaporation of a HfO2 target source was also evaluated. A series of HfO2 layers were made with various conditions; we alternated between two (mu) -wave configurations, Hf and HfO2 targets and two reactive O2 pressures. Laser DTs (1064 nm - 10 ns pulses), absorption (at 511 nm), and nodular- defect densities from these coatings are reported. The DT correlated inversely with the coating absorption.
Thermal transport studies of nanometric layer stacks by mirage detection
Zhouling Wu, Lanhua Wei, Pao-Kuang Kuo
Resonator mirrors for x-ray and XUV lasers are commonly made of nanometric layer stacks of two different materials. In these applications the crucial nondestructive evaluation tasks are the determination of thermal transport properties and thermal deformation of the mirrors. In this paper we report our recent progress in thermal diffusivity determination of Ag/W and Al/W multilayer stacks by mirage detection of laser induced thermal waves. The results are compared with those of single layer samples of component materials determined by the same method. A noteworthy advantage of the mirage method is that it measures primarily the in- plane heat flow in the metal film to obtain its thermal properties and it does not need the ultra- high modulation frequencies when applied to ultra-thin films as do other photothermal techniques.
Fundamental Mechanisms
icon_mobile_dropdown
GOI's investigations of the interaction of high-power radiation with matter
Alexey M. Bonch-Bruevich, Mikhail N. Libenson
Interaction of laser radiation with matter ,so called high power optics (in Russian "Force optics "), is one of the leading areas of quantum electronics, laser and optical physics. Force optics (the term introduced for the first time by A.M.Bonch-Bruevich) came to being at the beginning of sixties years after lasers invention and is aimed to investigate mechanisms and regularities of photophysical processes occurring under the action of intense laser radiation in various substances (solids, gases, plasmas, etc.). GO! (S.I.Vavilov State Optical Institute) is one of the main scientific centers of the USSR in power optics from the advent. Beginning with 1962 fundamental and applied theoretical and experimental investigations on the most part of principal and actual trends in high power optics were carried. Independently significant works were made in other scientific centers of the USSR (in the first place in the Physical Institute of the Academy of Sciences (FIAN, afterwards IOFAN)). Pioneering results in power optics were obtained in the Photophysical department of GOl. During 30 years of investigations there were published more than 200 articles by co—workers of the department, prepared more than 100 reports at various international, all-union conferences symposiums and seminars. All-union conferences on light interaction with matter have high authority among specialists. These conferences, with up to 500 specialists taking part and up to 250 reports, are traditionally organized by GOl once every three year, beginning with 1969. In the present article there are enlisted several results obtained in the photophysical department in various years and having principal meaning for developing the physical notions about laser radiation interaction with solids. Concrete results that does ifiustrate these notions are also given.
Pulse-width dependence of laser damage in optical materials: critical analysis of available data and recent results for nanopicosecond region
Serge V. Garnov, Alexander S. Epifanov, Sergei M. Klimentov, et al.
A critical analysis of available theoretical and experimental data for a pulse-width dependence of laser induced damage thresholds (LIDT) in various optical materials is given along with new results of our recent measurements obtained in nano-picosecond range at 1064 nm wavelength for alkali halide crystals (NaCL, KCl, KBr). Three YAG:Nd laser oscillator- amplifier systems operating in Q-switch and mode-locking modes provided highly stable light pulses at 2 ns, 15 ns, and 50 ps with Gaussian spatial profile of beams were used in the experiments. Special attention was paid to the comparability of the LIDT test conditions for various pulse widths, implying the carefully controlled similarity of beam intensity spatial distributions and temporal profiles. The experimental data are analyzed on the basis of theoretical predictions for different damage mechanisms. It is concluded that pulse-width scaling of LIDT is still a problem, and an adequate approach to its solution is formulated.
Theory and modeling of laser-induced free-electron heating in wide-gap solids
Douglas Arnold, Eduard Cartier
A microscopic theory for the interaction of intense laser radiation at visible and near-infrared wavelengths with free electrons in a wide-band-gap solid is presented. We calculate the free- electron mediated energy transfer from the laser field to the solid and the electron- multiplication rate due to band-to-band ionization as a function of laser intensity at wavelengths in the range 250 nm (lambda) < 10 micrometers , using SiO2 as an example. The formalism is based on a Monte Carlo integration of the Boltzmann transport equation. The interaction of the laser radiation with the free electrons is treated both within the standard classical approximation and quantum mechanically using second-order perturbation theory. Steady state Monte Carlo results are used in rate equations to make a direct comparison to experimental laser-induced heating data.
Modeling of electric-field enhancement at nodular defects in dielectric mirror coatings
J. F. DeFord, Mark R. Kozlowski
In dielectric multilayer optical coatings, laser induced damage is often associated with micrometers -scale surface defects such as the well known nodule defect. The interaction mechanism of the laser light with the coating defects is not understood, however. Historically, laser damage has been associated with peaks in the standing-wave electric-field distribution within the multilayer films. In the present work we use a finite-difference time-domain electromagnetic modeling code to study the influence of 3-D nodule defects on the E-field distribution. The coating studied is a dielectric multilayer HR consisting of alternating quarter- wave layers of HfO2 and SiO2 at 1.06 micrometers . The nodule is modeled as a parabolic defect initiated at a spherical seed. The modeling results show that E-field enhancements as large as a factor of 4 can be present at the defects. The enhancement shows a complex dependence on the size, depth, and dielectric constant of the seed material. In general, defects initiated by large, shallow seeds produce the largest E-fields. Voids at the nodule boundary influence the E-field distribution, but have a small effect on the peak field.
Materials and Measurements
icon_mobile_dropdown
Research at the SUMMA International Research Institute radiation test facility
Richard D. Nelson, Royal G. Albridge, Alan V. Barnes, et al.
The SUMMA International Research Institute has been established to develop advanced technology in the civilian and military sectors. An initial program of the Institute is a radiation test facility operating in conjunction with the Vanderbilt University Free Electron Laser (FEL).
International standard test method for laser-induced damage threshold of optical surfaces
Steven C. Seitel, Adolf Giesen, J. Becker
The International Organization for Standardization (ISO) has developed a draft international standard entitled `Test Method for Laser Induced Damage Threshold of Optical Surfaces' (ISO/DIS 11254). It is based on the familiar single-pulse-per-site damage frequency procedure, and provides a consistent method for obtaining and comparing damage threshold results between different laboratories. A brief review of the draft standard's contents and its current status are given. (Abstract only)
Thin Films
icon_mobile_dropdown
Laser damage of HR, AR-coatings, monolayers, and bare surfaces at 1064 nm
Serge V. Garnov, Sergei M. Klimentov, Ali A. Said, et al.
Laser induced damage thresholds and morphologies have been investigated in a variety of uncoated and coated surfaces including monolayers and multi-layers of different chemical compositions. Both antireflective (AR) and highly reflective (HR) were tested. Testing was done at 1064 nm with 25 picosecond and 8 nanosecond YAG:Nd laser single pulses. Spot diameter in the experiments varied from 0.09 to 0.22 mm. Laser damage measurement procedure consisted of 1-on-1 (single laser pulse in the selected site) and N-on-1 experiments including repeated irradiation by pulses of the same fluence and subsequently raised from pulse to pulse fluence until damage occurred. The highest picosecond damage thresholds of commercially available coatings averaged 12 - 14 J/cm2, 50% less than thresholds obtained in bare fused silica. Some coatings and bare surfaces revealed a palpable preconditioning effect (an increase in threshold of 1.2 to 1.8 times). Picosecond and nanosecond date were compared to draw conclusions about pulsewidth dependence. An attempt was made to classify damage morphologies according to the type of coating, class of irradiating, and damage level.
Surfaces and Mirrors
icon_mobile_dropdown
Unoccupied surface states on GaP(111) surfaces
Alex V. Hamza
Unoccupied surface electronic states have been observed on GaP(111) surfaces by momentum- resolved inverse photoemission spectroscopy (IPES). On the GaP(111):P surface an unoccupied state is detected 1.2 eV above the Fermi level. This state is associated with Ga surface adatoms, since prolonged exposure to the incident electron beam removes this IPES feature and reduces the Ga/P ratio in Auger electron spectrum. Preferential photon absorption at 2.27 eV by this surface can be explained by promotion of an electron from the valence band maximum to the unoccupied surface state detected in this work.
Fundamental Mechanisms
icon_mobile_dropdown
Nonequilibrium behavior and defect diffusion in laser heating of semiconductors
Woei-Yun Ho, Chun Chi Ma, Rodger M. Walser, et al.
A new theoretical framework for modeling the nonlinear laser heating of semiconductors is presented by incorporating the dynamical behavior of semiconductors; the temperature-carrier coupling, the generation and recombination of defects, the diffusion of defects, the diffusion of impurities by defect-dopant pair mechanism, and chemical reaction between species. In this study, we apply our model to n-type silicon irradiated by a nanosecond pulsed Nd:YAG laser. The dynamical evolution of laser-semiconductor interaction process is examined by calculation of carrier, defect, and impurity concentration profiles.
Mini-Symposium on Damage Issues for Lithographic Optics
icon_mobile_dropdown
Effects of KrF laser radiation on fused-silica glass: a comparison of samples exposed in air vs vacuum
Iraj K. Pour, Douglas J. Krajnovich, Andrew C. Tam, et al.
High-purity synthetic fused silica glass is known to undergo changes in optical properties during high repetition rate KrF laser exposures. (Results of studies on six glass types irradiated in air are summarized elsewhere at this symposium.) We have also irradiated several samples in vacuum (248 nm, 300 Hz, (Phi) equals 400 mJ/cm2). Transmission at 248 nm, transmission at 210 nm, fluorescence at 650 nm, and vacuum cell pressure were monitored in real time. Although bulk outgassing is expected to be very slow at room temperature, our results indicate that the vacuum environment does affect the interior of the sample. Furthermore, a curious irradiation effect on the transmission of the CaF2 windows used on the vacuum cell is discussed.
248-nm lens materials: performance and durability issues in an industrial environment
Douglas J. Krajnovich, Iraj K. Pour, Andrew C. Tam, et al.
The short wavelengths and high peak powers of rare gas-halide excimer lasers suggest many uses in industry. To exploit these opportunities, lasers must be developed that can operate reliably and economically at high repetition rates and high pulse energies, and optics must be fabricated that can withstand hundreds of millions or billions of laser shots without degradation. On the laser front, rapid progress is being made by several manufacturers. This talk focuses on the optics, in particular, transmissive optical materials for KrF laser applications. A variety of synthetic fused silicas were tested in front of an industrial KrF laser at high repetition rate (300 Hz) and moderate fluence (500 mJ/cm2). All samples develop a characteristic absorption band at 210 nm in the early stages of the exposure. The absorption relaxes gradually after the laser is turned off. In almost every sample, this gradual behavior is followed by a large and sudden increase in the 248 nm absorption (up to 12% per cm). The nature, consequences, and possible causes of this `strong absorption transition' are discussed. Some data on high-purity single-crystal CaF2 is included for comparison.
Materials and Measurements
icon_mobile_dropdown
Laser-induced damage in step-index multimode fibers
Ellen B. Stechel
In the present study, fiber damage experiments were conducted using a new approach for injecting the laser beam into fiber samples. The purpose of this approach was to achieve a test configuration that was independent of the mode structure of the particular laser being used. The method developed was successful in eliminating internal damage within the first few hundred fiber diameters, but resulted in the fiber entrance face being the most likely damage site. This feature, however, proved quite useful for re-examining the effects of CO2-laser polishing on damage thresholds at the entrance face. Using a test sequence similar to that used previously, a particular laser-polishing schedule was found that significantly improved damage resistance. In addition, preliminary damage experiments were performed during which fiber samples were subjected to much higher bending stresses under carefully controlled conditions.
Fundamental Mechanisms
icon_mobile_dropdown
Intrinsic optical breakdown of glasses
O. N. Bosyi, Leonid B. Glebov, Oleg M. Efimov, et al.
In this paper the influence of multiphoton absorption, self-focusing, absorbing impurity, mode composition, and focusing conditions of laser radiation on the regularities and the absolute values of breakdown threshold of glasses have been studied. The conditions necessary for the realization of intrinsic optical breakdown have been determined. It is shown that under focusing of the neodymium laser radiation in spots of 0.4 - 0.8 micrometers diameters statistical spread in breakdown threshold values, temporal and size dependence are absent. Breakdown thresholds for alkali silicate glass K8 and fused quartz KY1 are similar and are almost 1013 W/cm2. On the basis of the obtained results a new model of intrinsic optical breakdown mechanism has been proposed.
Thin Films
icon_mobile_dropdown
Damage thresholds of fluoride multilayers at 355 nm
Robert Chow, Mark R. Kozlowski, Gary E. Loomis, et al.
Fluoride multilayer coatings were evaluated for use in 355 nm high reflector applications. The LaF3/Na3AlF6, NdF3/Na3AlF6, and GdF3/Na3AlF6 multilayers had laser damage thresholds of 20 J/cm2, 17.9 J/cm2, and 7.4 J/cm2 (measured at 10 ns pulsewidths), respectively. High tensile stresses in the coatings restricted this evaluation to only 5-layer-pair partial reflectors (49 - 52%). The LaF3/Na3AlF6, NdF3/Na3AlF6, and GdF3/Na3AlF6 multilayers had tensile stresses of approximately equals 1.1 X 109, 1.3 X 109, and 9.3 X 108 dynes/cm2, respectively. Substrate material and glow-discharge processing of the substrates were found to influence the density of stress-induced coating fractures and damage thresholds in some cases. If stress fracturing and scatter can be controlled, these fluoride material combinations are suited for 3(omega) applications.
Developments in ion-assisted multilayer coatings
Thomas R. Culver, Walter T. Pawlewicz, John H. Zachistal, et al.
The advantages of high current, low voltage ion assisted deposition (IAD) previously demonstrated for single layer films were extended to complex multilayer oxide coatings for medium to large scale applications. Optimization of the IAD parameters led to low absorptance coatings from the IR to the UV. Stress adjustment and matching of layers provided greater cohesive strength and mechanical integrity throughout the stacks. In addition near bulk indices gave enhanced optical performance. Examples of multilayer high reflector stacks for UV, visible, and near IR laser application are given with emphasis on absorption, optical performance in band, elimination of physisorbed water, and at the waterband, environmental stability, and coating durability.
Relaxation of dielectric thin films under Ar+ laser irradiation
Zhouling Wu, C. Z. Tan, J. Arndt, et al.
Ar+ laser irradiation can produce change, most of the time decrease, in optical absorption of various electron beam deposited (EBD) dielectric thin films. Similar phenomena were also found in bulk materials prepared under high pressure and high temperature conditions. In this paper we report our recent progress in understanding the problem by employing a relaxation function which can fit the in-situ measured time dependent change in absorption of various dielectric thin films. Results of SiO2, TiO2, Ta2O5, ZrO2, HfO2 films and SiO2 bulk samples were reported and compared.
Photothermal comparison of reactive low-voltage ion-plated and electron-beam-deposited TiO2 thin films
Zhouling Wu, Eckart Matthias, A. Messinger, et al.
TiO2 thin films of different thicknesses were prepared by conventional electron beam deposition (EBD) and reactive low-voltage ion-plating (RLVIP) techniques. These samples were designed to investigate the thickness dependent optical and thermal properties of the TiO2 coatings and the corresponding damage thresholds. In this paper we present a detailed comparison of the two groups of samples by using various photothermal techniques. The data reported include thermal conductivity, defect density, optical absorption, laser damage threshold, as well as laser conditioning effect by using Ar+ laser irradiation. The general trend shown by the data is that the ion-plated samples have higher absorption, lower damage threshold, yet better thermal conductivity, lower defect density, and almost perfect stability under Ar+ laser irradiation.
Surfaces and Mirrors
icon_mobile_dropdown
Damage to copper mirrors by high-power CO2 laser radiation
A. Angenent, C. R. Haas, Joachim Koch, et al.
A focused, cw CO2 laser is used to irradiate polished Cu mirrors at power densities near 40 kW/cm2. Uncoated mirrors and mirrors having a reflection-enhancing Au coating with a protective Y2O3 layer are tested. During irradiation in-situ measurements of the temperature distribution in the surface are made and the morphology is monitored by interferometry. Chemical changes are determined by ex-situ x-ray photoemission and scanning Auger measurements. Damage to uncoated mirrors is connected with irreversible deformation, an increase in thermal gradients under irradiation (attributable to increased absorption), and oxidation of the surface layer, with formation of Cu2O and CuO. While the whole surface becomes oxidized, the effect is strongest in the laser focus, where an enhanced carbon content is also found. Similar thermal and morphological changes occur in coated mirrors under analogous irradiation conditions, but the relatively inert Y2O3 protective layer is not chemically changed. Structural changes are present in the surface and may contribute to the changed absorption.
Thin Films
icon_mobile_dropdown
Laser damage studies of silicon oxy-nitride narrowband reflectors
Jonathan R. Milward, Keith L. Lewis, K. Sheach, et al.
A series of sinusoidally modulated, plasma deposited, silicon oxy-nitride, narrow band reflectors have been examined with a view to understanding the relative roles of electric field effects, defect type, surface roughness, thickness, and coating absorption on the laser damage threshold. The damage threshold measurements were made at 0.532 micrometers with a range of spot sizes, a pulse length of 15 ns (full width at half maximum intensity), and each site was tested with 100 shots at a 10 Hz repetition rate. The damage threshold was essentially constant at around 2 J/cm2 for all the samples, and was defect dominated. Three types of topological defects were discovered using a WYKO three dimensional surface profiler, and one of the defect types was responsible for a large fraction of the damage events. It is postulated that this 5 micrometers hemispherical defect may behave either as a microlens which enhances the peak fluence that the underlying coating is subjected to, or as a center for enhanced electric field effects.
Fundamental Mechanisms
icon_mobile_dropdown
Self-focusing and optical damage in Cr:LiSAF and Cr:LiCAF
Martin C. Richardson, M. J. Soileau, P. Beaud, et al.
The Cr-doped Colquiriites, Cr:LiSAF, and Cr:LiCAF, are new solid-state laser materials with high gain cross-sections, very large gain bandwidths, and relatively long fluorescence lifetimes (approximately 100 microsecond(s) ) making them amenable to flashlamp pumping. These laser materials are capable of providing tunable high power laser radiation in the near IR and are very suitable for ultrashort pulse generation and amplification. For these high power laser applications, however, a knowledge of the optical damage thresholds and mechanisms in these materials is essential. We present results of measurements of bulk damage thresholds, of basic material properties, and of the nonlinear refractive indices of these materials. We have also observed optical damage due to self-focusing in a femtosecond chirped pulse Cr:LiSAF regenerative amplifier. We discuss these observations and give an estimate of the critical power of self-focusing for these materials.
Defect-induced variations in the local electric fields in inhomogeneous media
Using a finite element representation of a dielectric film, we have examined the effects of surface defects on the local electric field and its intensity. Surface defects possess the potential to affect the local electric fields in a dielectric film in a manner similar to those introduced in the bulk solid. Both raised and removed regions tend to focus the electric field vector about the defect, and the interaction of two defects can lead to an enhanced local electric field intensity. In these cases, effective medium methods would approximate the film by an idealized defect-free structure, and thus would not note the sensitivity of the electric field to surface structure.
Materials and Measurements
icon_mobile_dropdown
OPO lasers: high-power reflectors and couplers
Donny Maladji A. Aminou, Kenneth E. Reilly, C. Schmidt
Optical parametric oscillator (OPO) based laser sources are wide bandwidth tunable solid-state lasers which are presently suffering from the lack of optical components capable of handling high power fluxes with broad bandwidth. This paper discusses some progress we made in producing broadband reflectors, input and output couplers that can withstand high power lasers, and that are broad band enough to allow for the use of fewer sets of optics. In a previous paper, we described thin film design and production that showed beneficial results as OPO optical components. In the present paper, we address progress we made. We introduce some new designs and discuss power damage thresholds.
Thin Films
icon_mobile_dropdown
Antireflective coatings on optical fibers for high-power solid state lasers
Manfred Dieckmann, Uwe Willamowski, Detlev Ristau, et al.
This paper presents investigations to improve the quality of high power antireflective coatings for optical fibers used in material processing with Nd:YAG lasers. The coatings were produced by electron-beam deposition and ion-beam-sputtering on step-index, multimode fibers with a core diameter of 1000 micrometers . The reflection losses, the absorption, and the laser-induced damage thresholds of these coatings were determined. Hafnia, tantala, and silica were selected as deposition materials for the AR-coatings. For measuring the reflection losses of the coated fiber surfaces, a diode-pumped Nd:YAG laser was used. Absorption measurements were performed by photothermal displacement spectroscopy, a surface sensitive technique which makes scanning of the fiber surface possible. These data were compared to calorimetric values determined from quartz substrates which were deposited in the same coating run. The R-on-1 damage thresholds of the coated fibers were measured using a two- stage single-mode Nd:YAG laser with a pulse duration of 12 ns. A four-stage cw system with 1.2 kW output power and a pulsed Nd:YAG slab laser were used to evaluate the performance of the coated fibers.
Damage threshold of fluoride HR coatings at 352 nm
Takao Izawa, N. Yamamura, R. Uchimura, et al.
Laser damage thresholds of fluoride HR coating were investigated at 352 nm, 10 ns pulses. Additionally, an effective method to reduce the crazing in fluoride HR coatings is studied. A high threshold greater than 30 J/cm2 was achieved in LaF3/AlF3 coating when the laser conditioning procedure was used. The IAD process or double stack design with a combination of fluoride and HfO2SiO2 coating was effective in reducing the crazing.
High-threshold fluoride AR coatings for KDP crystal at 352 nm
Takao Izawa, N. Yamamura, R. Uchimura, et al.
The laser damage thresholds and other properties of 352 nm fluoride AR coatings for KDP substrate were investigated. The HoF3 (or LaF3)/AlF3 AR coating deposited by IAD process had a high threshold of 24 J/cm2 (10 ns) when the laser conditioning procedure was used. These fluoride ARs were durable for wiping and also did not degrade during more than one year under room atmosphere.
By Title Only
icon_mobile_dropdown
Repetitively pulsed laser damage in optical thin films
The theory of laser-induced damage in dielectric films, containing highly absorbing impurities, developed previously for single-shot damage is extended to the case of multiple shots. Calculations are carried out for micro-, nano-, and pico-second pulses, with the time interval between pulses varying from equal to the pulse duration to two-orders of magnitude greater than the pulse duration. It is found that in the case of spherical impurities the size of the easiest to damage particle is in general a complicated function of the laser intensity, pulse duration, and time interval between pulses. However, for sufficiently large number of pulses, the dependence is much simpler, characterized by constant values of the following: (1) pulse duration multiplied by the number of shots divided by the square of the radius of the easiest to damage impurity; (2) the minimum intensity needed to cause damage multiplied by the radius of the easiest to damage impurity.
Investigation of nonlinear absorption and laser damage on lithium niobate single crystals
Yang Sun, Cheng Fu Li, Zhong Ya Li, et al.
The thresholds of surface and bulk damage by 10 ns Nd:YAG laser in LN single crystals are measured. The accumulation effect produced by multipulse laser irradiation is also investigated. Temporal transmission measurement is employed to monitor the nonlinear absorption in the crystals during YAG laser irradiation. It turns out that the multiphoton absorption is the main damage mechanism in both surface and bulk damage processes. The direct reasons leading to visible damage are the accompanied stress explosion in the bulk and the thermal melting and etching on the surface. The multipulse damage is mainly the effect of the accumulation of micro-damages.
Laser treatment to improve the ability to resist laser damage in potassium dihydrogen phosphate crystals
Yang Sun, Cheng Fu Li, Huixing Gong
Excimer, Nd:YAG, and CO2 laser are used to pre-irradiate KDP crystal samples at subthreshold energies. The change of damage threshold produced by laser treatment is investigated. Excimer laser treatment can effectively raise the bulk damage threshold of KDP crystals, YAG laser pre-irradiation can raise it a little, but cw CO2 laser treatment makes it descend. The improvement of laser treatment differs with the laser wavelength, the laser power, and the total deposited energy. Laser treatment can also improve the surface characteristics of KDP crystals, but the effect is not as good as that in the bulk. It is deduced that laser treatment can restore the stress field remains, the micro-cracks, and the dislocations derived from crystal growth and polish processing. However, it has little effect on the defects caused by impurities. These produce the different effects under various treatment conditions and the difference between interior and surface in improving the characteristics of materials by laser treatment.
Surfaces and Mirrors
icon_mobile_dropdown
Degradation of vacuum-exposed SiO2 laser windows
Thomas M. Stephan, B. Van Zyl, Robert C. Amme
Observations of a damage phenomenon at the surface of fused silica and crystalline quartz windows are presented. Uncoated windows were mounted at Brewster's angle to facilitate the introduction of a vacuum chamber directly into the cavity of an Ar-ion laser (488 - 514 nm). The transmission of these windows, prior to evacuating the chamber to less than 1 Torr, approaches the theoretical value of > 99.9%, remaining constant indefinitely. However in our normal usage, the chamber is evacuated (P < 10-7 Torr), exposing the windows to high vacuum as well as UV borelight from the laser discharge. After several hours of operation, the intracavity power is observed to decrease monotonically (by approximately 15% per hour) accompanied by the development of a red fluorescence on the inside window surface where exposed to the visible laser radiation. Partial rejuvenation of the windows can be accomplished by reintroduction of gas into the vacuum chamber. Possible damage mechanisms are presented.
Fundamental Mechanisms
icon_mobile_dropdown
Damage threshold measurements using femtosecond excimer laser
Klaus R. Mann, G. Pfeifer, Guenter Reisse
In the UV spectral range commonly excimer lasers with a pulse length of 20 - 30 ns are used for damage testing of optical components. In this paper we present for the first time damage threshold data measured with a KrF excimer laser (248 nm) of 560 fs pulse duration. The tested materials were several UV transmitting crystals, fused silica, and dielectric coatings. The comparison of the threshold data with the respective nanosecond laser results at the same wavelength reveals that in all cases lower damage thresholds are obtained. However, the decrease in threshold was not as strong as could be expected from the more than 4 orders of magnitude higher pulse power density at the same fluence level. Transmission measurements indicate a strong decrease of the transmitted radiation with increasing pulse power below the onset level of damage, which can be interpreted by enhanced multiphoton absorption.