Proceedings Volume 3343

High-Power Laser Ablation

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Proceedings Volume 3343

High-Power Laser Ablation

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Volume Details

Date Published: 14 September 1998
Contents: 7 Sessions, 114 Papers, 0 Presentations
Conference: High-Power Laser Ablation 1998
Volume Number: 3343

Table of Contents

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Table of Contents

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  • Keynote Session
  • Plenary Session
  • Physics of High-Power Laser-Surface Interactions
  • Plenary Session
  • Physics of High-Power Laser-Surface Interactions
  • Applications in Coating, Coating Removal, and Material Processing
  • Physics of High-Power Laser-Surface Interactions
  • Lasers for High-Power Photoablation
  • Applications Using Lasers and Other Sources of Propagated Energy to Produce Remote Thrust
  • Applications in Coating, Coating Removal, and Material Processing
  • Beam Propagation, Beamforming, Diagnostics, and Detection Techniques
  • Applications in Coating, Coating Removal, and Material Processing
  • Beam Propagation, Beamforming, Diagnostics, and Detection Techniques
  • Physics of High-Power Laser-Surface Interactions
  • Lasers for High-Power Photoablation
  • Applications Using Lasers and Other Sources of Propagated Energy to Produce Remote Thrust
  • Beam Propagation, Beamforming, Diagnostics, and Detection Techniques
  • Physics of High-Power Laser-Surface Interactions
  • Applications in Coating, Coating Removal, and Material Processing
  • Applications Using Lasers and Other Sources of Propagated Energy to Produce Remote Thrust
  • Physics of High-Power Laser-Surface Interactions
  • Lasers for High-Power Photoablation
  • Applications in Coating, Coating Removal, and Material Processing
  • Physics of High-Power Laser-Surface Interactions
  • Applications in Coating, Coating Removal, and Material Processing
  • Physics of High-Power Laser-Surface Interactions
  • Beam Propagation, Beamforming, Diagnostics, and Detection Techniques
  • Physics of High-Power Laser-Surface Interactions
  • Plenary Session
  • Lasers for High-Power Photoablation
  • Applications in Coating, Coating Removal, and Material Processing
  • Beam Propagation, Beamforming, Diagnostics, and Detection Techniques
  • Lasers for High-Power Photoablation
  • Applications in Coating, Coating Removal, and Material Processing
  • Lasers for High-Power Photoablation
  • Beam Propagation, Beamforming, Diagnostics, and Detection Techniques
  • Applications in Coating, Coating Removal, and Material Processing
Keynote Session
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Eugen Saenger memorial
Dr. Eugen Saenger, The Father of Laser Propulsion, 1905 - 1964.
Then and now
Arthur Kantrowitz
I want to talk today about the enthusiasm for technological progress today and compare it with the situation 30 or 40 years ago. Then most of us looked forward eagerly to advances in technology and to the expansions of humanity's horizons these advances would make possible. We looked forward not only to the advances we could foresee but especially to the surprises which we had come to expect would be the greatest advances. We were full of hope that our children would live better lives.
Laser fusion: the first ten years (1962-1972)
Ray E. Kidder
This account of the beginning of the program on laser fusion at Livermore in 1962, and its subsequent development during the decade ending in 1972, was originally prepared as a contribution to the January 1991 symposium 'Achievements in Physics' honoring Professor Keith Brueckner upon his retirement from the University of San Diego at La Jolla. It is not a review of the international effort in laser fusion research. It is a personal recollection of work at Livermore from my vantage point as its scientific leader, and of events elsewhere that I thought significant. This period was one of rapid growth in which the technology of high-power short-pulse lasers needed to drive the implosion of thermonuclear fuel to the temperature and density needed for ignition was developed, and in which the physics of the interaction of intense light with plasmas was explored both theoretically and experimentally.
Development of the diode-pumped solid state laser and its application to laser fusion and industry
Sadao Nakai, Yasukazu Izawa, Masahiro Nakatsuka, et al.
The progress of implosion physics research and relevant technologies enable us to examine technical and economical feasibility, and to plan the realistic strategy to the commercial power plant. The most important key issue for IFE is the driver technology. The development of the laser fusion driver is opening new industrial technologies based on the photon processes and new fields of high energy physics.
Plenary Session
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Dynamics of femtosecond-laser-induced ablation from solid surfaces
Klaus Sokolowski-Tinten, Joerg Bialkowski, Andrea Cavalleri, et al.
Femtosecond laser induced ablation from solid surfaces has been investigated by means of time resolved microscopy. On transparent materials ablation is initiated by dielectric breakdown and formation of a dense and hot surface plasma. Measurements of the plasma threshold yield values of a few times 1013 W/cm2 with little variation among different materials. This indicates that microscopic surface properties are responsible for surface breakdown. On absorbing semiconductors and metals near-threshold ablation is brought about by hydrodynamic expansion of the laser generated hot and pressurized matter. Upon expansion into vacuum initially metallic materials transform into a transparent state with a high refractive index. The observed behavior is related to general properties of matter in the liquid-gas coexistence regime.
Photophysical ablation
Boris S. Luk'yanchuk, Nikita M. Bityurin, Aleksey Yu. Malyshev, et al.
The photophysical mechanism of laser ablation related to the change in activation energy for desorption (or destruction and further elimination) of electronically excited molecules is discussed. Despite the obvious physical reasons, up to now there is no experimental confirmation that this mechanism plays an important role with nanosecond laser ablation where duration of the laser pulse is longer than the corresponding relaxation time (typically, tens of picoseconds). On the other hand there was no special study of this mechanism for Ultra Short Laser Pulses (USLP) with pulse duration significantly shorter than the electronic energy relaxation time. Here we present the theoretical investigation of the photophysical ablation features with USLP and compare predictions which follow for the photophysical and pure thermal models.
Physical mechanisms of pulsed infrared laser ablation of biological tissues
Andrew D. Yablon, Norman S. Nishioka, B. B. Mikic, et al.
Time-resolved measurement of the stress transients generated by pulsed infrared irradiation and ablation of tissue has demonstrated that these processes are governed primarily by photothermal processes. For ablation of porcine dermis at 2.79 micrometer (Q-sw Er:YSGG) and 10.6 micrometer (CO2), the onset of material removal has been shown to be delayed with respect to irradiation and the magnitude of the generated stress transients are consistent with a model for explosive material removal. Upon consideration of the threshold radiant exposure for ablation, it appears that the mechanism and dynamics of these processes are controlled by explosive boiling as the tissue water is likely to be significantly superheated. To examine this issue further, we employed time- resolved optical interferometry to measure the surface displacement generated by Q-sw Er:YSGG laser irradiation of pure water for radiant exposures below the ablation threshold. This was done to directly measure the dynamic thermal expansion and interphase mass transfer generated by pulsed laser heating. These results are compared to a model which computes the dynamic thermal field within a semi-infinite pool of water undergoing pulsed irradiation while subject to a surface heat flux condition given by kinetic theory. We find that the measured mass fluxes exceed that predicted by simple kinetic theory arguments. The implications of the experimental and model results to pulsed laser ablation of tissue are discussed.
Interaction of high-average-power repetitively pulsed laser radiation with matter
The development of new and more efficient schemes for improving the energy conversion from the optical radiation field into thermal, mechanical and or ionizational energy of target materials is a main concern in numerous laser effect studies, both in fundamental research and industrial applications. The present paper deals with recent investigations, carried out at ISL, by using a novel type of repetitively pulsed CO2-laser. Infrared pulse energies up to 150 J and repetition rates up to one hundred pulses per second provide powers up to 15 kW. Peak powers (close to 100 MW) are depending on pulse lengths, which can be set from about 2 to 10 microseconds. Experiments have been performed to study repetitively pulsed large area, 'out-of-band'-plasma supported processes, induced on optically transparent dielectric materials. Further so-called 'in-band'-studies of semiconductor materials were carried out, by using a frequency doubled Nd-YAG-laser and a dye-laser. Due to the small spot approach, experiments could be performed with rather small pulse energies of 500 to 600 mJ to yield fluences up to several tens of J/cm2. Examples, both concerning the high energy, (out-of-band) and the lower energy (in-band) measurements will be discussed for various types of targets. Mechanisms involved include quasi-explosive thermal ablation and absorption-wave supported impulsive loading due to thermo- mechanical processes.
New processing regime for machining of organic materials
Stuart Astin, Richard D. Pilkington, Stewart W. Williams, et al.
This paper investigates the laser ablation of materials at high intensities. It is known that when drilling organic materials at low and moderate fluences (0.1 - 50 J/cm2) the etch depth per pulse increases exponentially with fluence, essentially following the Beer Law absorption characteristic. For carbon fiber composite the ablation rate reaches a level of about 2 micrometer per pulse for high fluences. However when approaching very high intensities (e.g. greater than 1010 W/cm2) a sharp increase in ablation rate to greater than 30 micrometer per pulse has been observed and used for drilling experiments. Extensive studies of this regime have subsequently been carried out to characterize it. This has included the effect of different focusing lenses and the effect material thickness. For high quality beams the hole quality is good with no heat affected zone and no significant mechanical damage. This new regime may make the large scale excimer laser drilling or cutting of carbon reinforced fiber an economically feasible application due to the increased drilling and cutting rates. Also this method only requires a simple optical system for high beam utilization factors. These issues will be discussed.
Wavelength-dependent modification of insulator surfaces by a picosecond infrared free-electron laser
Ultrashort-pulse lasers are at an increasing rate being used for laser-induced surface modification of insulators, including ablation. Ti:sapphire chirped-pulse amplifier systems, with fundamental wavelengths in the near infrared, can produce efficient ablation and other desirable surface modifications with little collateral damage because the laser energy is deposited on a time scale much shorter than thermal diffusion times. Little is known, however, about how ultrashort pulses interact with insulators at wavelengths in the vibrational infrared. This paper describes surface modifications achieved by picosecond laser irradiation in the 2 - 10 micrometer range. The laser source was a tunable, free- electron laser (FEL) with 1-ps micropulses spaced 350 ps apart in a macropulse lasting up to 4 microseconds, with an average power of up to 3 W. This unusual pulse structure makes possible novel tests of the dependence on fluence and intensity, as well as the effects of resonant vibrational excitation. As model materials systems, we studied calcium carbonate, its isoelectronic cousin sodium nitrate, and fused silica. Particularly intriguing are surface modifications achieved by tuning the laser into vibrational resonances of the target materials, or by tailoring the energy content of the pulse. The mechanisms underlying these effects, and their implications for materials-modification strategies, are discussed.
High-power supersonic chemical oxygen iodine laser
A performance update of DLR's supersonic chemical oxygen iodine laser shows output powers of typically 10 kW CW and a chemical efficiency close to 20%. It is suggested to use a quasi-CW magnetically Q-switched mode of operation with considerably enhanced peak-to-CW power as a new ground based laser candidate for near earth space debris clearing. A preliminary design demonstrates the attractive features of such a laser system as compared to previous approaches. In particular, the chemical oxygen iodine laser promises to be rather compact and to substantially reduce the total debris clearing time.
Tabletop soft x-ray lasers by fast discharge excitation
Jorge J. G. Rocca, Cesar H. Moreno, Brady R. Benware, et al.
We review the progress in the development of ultrashort- wavelength lasers based on discharge excitation. The observation of large soft x-ray amplification in Ne-like Ar at 46.9 nm in a capillary discharge created plasma [Phys. Rev. Lett. 77, 1476, (1996)] have opened a new path for the development of compact and practical soft x-ray lasers. These results have shattered the earlier perception that discharge- created plasmas are insufficiently stable and uniform for soft x-ray lasing. We report results of a detailed characterization of the capillary discharge Ne-like Ar laser, that include near-field and far-field images of the laser output and the measurement of its spatial coherence. The recent demonstration of lasing in Ne-like S at 60.8 nm in material ablated from a solid target by a discharge is also discussed.
Phase locking of 2D structures
Victor V. Apollonov, Sergei I. Derzhavin, Victor I. Kilsov, et al.
The overview of the phase-locking technique is presented. Analysis of the parameters that influence on the phase-locking is given. Admissible range of the deviation of the linear laser diode array, external cavity parameters is estimated. Comparison of the external Talbot cavity configuration with the other ones is held. Attention is paid to both investigation and employment of the laser diode arrays consisting of wide aperture lasers that opens an avenue to high power output radiation in a phase-locked mode. Special consideration is given to the cavity supermodes selection. Our experiments both confirm actual feasibility of 1D and 2D phase-locking of LDA's with specified parameters in the external Talbot (Lc equals ZT/4) cavity and illustrate theoretical predictions of the system stability and selectivity, so that: (1) phase-locking in 1D configuration allowed diffractional limited lobes width (delta) (Psi) equals 0.5 mrad; (2) tilting of the output mirror provided 'in-phase' supermode selection; (3) phase-locking in 2D configuration of the two LDA's of N equals 8 lasers, separated by 1600 mkm resulted in diffractional lobes full width at half maximum (delta) (Psi) equals 0.5 mrad in slow axis and (delta) (Psi) equals 0.25 mrad in fast axis.
Nuclear-pumped lasers: problems and perspectives
Eduard P. Magda
The review is devoted to the condition of researches on lasers pumped by nuclear reactors. In the review the brief description of NPL experiments is given, the parameters of perspective laser media and short explanation of processes taking place in gas NPL are also given. Possible schemes of NPL's, LAEL's and concepts of RL's are under consideration. The problems and perspectives of NPL's and RL's are touched upon.
Industrial applications of pulsed lasers to materials microprocessing
The use of pulsed lasers for microprocessing material in several manufacturing industries is presented. Deep-uv photolithography, via hole and ink jet printer nozzle microdrilling, solar panel thin film scribing, texturing of hard disks, annealing amorphous silicon in flat panel displays, fiber Bragg grating production and micro-electro- mechanical system (MEMS) fabrication applications are discussed.
Laser-shock processing of materials and related measurements
Patrice Peyre, Remy Fabbro, Laurent Berthe, et al.
Laser-induced pressure generation and surface modifications induced by laser-shock processing were investigated with the use of different laser conditions including a large range of pulse durations and wavelengths. With the use of several lasers sources, original results could be pointed out concerning the precise influence of laser parameters on the shock generation in water-confined plasma regime. The beneficial influence of short pulse durations (1 to 10 ns) and the deleterious effect of short wavelengths (below 0.532 micrometer) were demonstrated with the use of a VISAR Doppler velocimetry system. Lastly, beneficial effects of LSP were shown on many surface applications such as fatigue and fatigue contact resistance, pitting corrosion and stress corrosion cracking behaviors.
Polychromatic guide star
Renaud Foy, Michel Tallon, Herbert W. Friedman, et al.
We describe the principle of the polychromatic laser guide star (LGS) to recover the tilt information in imaging through the atmosphere. Observations using the AVLIS laser at the Lawrence Livermore National Lab are discussed in terms of returned flux in the ultraviolet. The major items of the program ELP-OA, starting now in France, are briefly reviewed, as well as the organization of the LGS R&D in Europe. Finally the conclusion outlines the possible improvements of the polychromatic LGS to allow us to reasonably implement it at large astronomical telescopes.
Excimer laser beam shaping and material processing using diffractive optics
Stewart W. Williams, Paul J. Marsden, Nick C. Roberts, et al.
Diffractive optical systems have been developed to allow high efficiency material processing using excimer lasers. These systems allow beam homogenizing and shaping for surface treatments or parallel focusing for multiple hole drilling and production of microrelief. Beam utilization factors of 80% have been achieved.
Excimer laser photorefractive surgery of the cornea
Ronald N. Gaster
The 193 nm argon fluoride (ArF) excimer laser can effectively be used to change the radius of curvature of the cornea and thus alter the refractive state of the eye. This change allows myopic (nearsighted) patients to see well with less dependence on glasses or contact lenses. The two major techniques of laser refractive surgery currently in effect in the United States are photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). This paper will discuss these refractive cornea surgical techniques.
Physics of High-Power Laser-Surface Interactions
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Thermomechanical effects in laser-matter interaction
In most of pulsed laser material processing of which drilling, surface treatment, laser cleaning or shock hardening, the incident energy is distributed among several complex phenomena such as reflection, absorption, radiation or hydrodynamic depending on wavelength, polarization, pulse duration, energy end power density, spot size, nature of the material and last but not least the nature and the pressure of the surrounding gas. Experiments were conducted in order to characterize all these phenomena involved in different applications and to contribute in a better understanding of the laser material processing. Experimental data has been obtained from several diagnostic techniques such as integrating sphere, thermocouple probes, ballistic pendulum, pressure gauges, electromagnetic field detectors, spectrographs and high-speed camera over a wide range of energy and power density and gas pressure.
Laser-induced breakdown in water: process for shock wave generation
Jean-Christophe Isselin, Anne-Patricia B. Alloncle, Michel L. Autric
We have characterized laser-induced shock wave in water. Pressure measurements have been carried out using piezoelectric sensors and optical methods (interferometer, high-speed visualization). These experiments give us information on the pressure amplitude, the rise time, the pressure pulse duration, the energy, the shock wave velocity, and the pressure decay. These researches find applications in various fields such as: laser cleaning of sensitive surface, pressure probes calibration, medical treatments (intraocular surgery, lithotripsy).
X-ray emission from spatially modulated copper target surface
Taramati Desai
A spatially modulated copper target surface (groove diameter (lambda) s approximately equals 70 plus or minus 2 micrometer and groove depth dS equals 20 plus or minus 2 micrometer shows an enhanced X-ray fluence and wider angle of emission as compared to optically polished ((lambda) /2), planar copper target when irradiated with laser energy density EL approximately equals 600 J/cm2. Experiments were performed using a 40 ps (FWHM), 1.06 micrometer laser. An array of 5 X-ray detectors at various angle to the laser axis measured the angular distribution of X-ray fluence. Higher X- ray flux and wider angle of emission are attributed to invoking of resonance absorption at the curvature, shape of curvature of the spatially modulated surface groove respectively. In this article we address to the issues of large angle emission of X-rays, their magnitude and scaling laws for both types of targets. Since X-ray emission time depends on laser pulse duration, present experimental results are discussed explicitly on the laser energy density dependance (J/cm2).
Role of electron-phonon coupling in femtosecond laser damage of metals
Sebastian S. Wellershoff, Jens Gudde, Julius Hohlfeld, et al.
Multishot laser damage thresholds of Au-, Ni-, Cr-, and Mo- films on fused silica have been measured with 200 fs/400 nm pulses for a sequence of film thicknesses. We found a pronounced dependence of threshold fluences on film thickness in the ranges below 50 nm for Ni and Cr, below 70 nm for Mo, and extending up to 700 nm for Au. These variations reflect the depth of hot electron diffusion which is governed by the strength of the electron-phonon coupling. The onset of damage is defined as melting for Au-, Ni- and Mo-films. For Cr-films, we could identify the brittle-to ductile phase transition as well as melting. The change of damage thresholds can be described by the two-temperature model, provided the temperature dependence of optical and thermal properties of the material is properly accounted for. By fitting model predictions to the extrapolated single-shot damage thresholds, we find the following values of electron-phonon coupling constants (in units of 1016 W/Km3): g (Au) equals 2 plus or minus 20%, g (Ni equals 36 plus or minus 40%, g (Cr) equals 42 plus or minus 40%, g (Mo) equals 13 plus or minus 40%.
Irradiation of solids with subpicosecond laser pulses: excitation and relaxation dynamics of electrons and phonons
Baerbel Rethfeld, Andreas Kaiser, Martin Vicanek, et al.
Irradiation of metals and dielectrics with subpicosecond laser pulses reveals a variety of different microscopic processes compared to longer pulses. We choose a transport theoretical description of electrons and phonons, which allows us to investigate the contribution of particular collision processes on macroscopic material response, even for highly nonequilibrium subsystems where a hydrodynamic description fails. The absorption of laser energy is described by photon- absorption of free electrons with assistance of phonon collisions. We considered relaxation of the electron gas by electron-electron and electron-phonon collisions. In the case of a dielectric, terms for multiphoton ionization and impact ionization are included. A resulting system of coupled time- dependent Boltzmann equations is solved numerically. For SiO2 as an example of dielectrics we calculate changes in occupation numbers of electrons and phonons, respectively, determining the sequence of excitation and relaxation of the subsystems under ultrashort laser pulse irradiation. It is shown that the essential process of free-electron generation is multiphoton ionization. Additionally, we investigate polarization effects for the case of irradiation of metals. Using parameters for gold we find anisotropic absorption of laser energy, which depends on polarization of the electric field and material parameters as well as on photon energy of irradiating laser light.
Laser ablation and structuring of transparent materials with ultrashort laser pulses
David Ashkenasi, Gerril Herbst, Arkadi Rosenfeld, et al.
We have investigated the material processing of oxides and fluorides using ultrashort laser pulses and have demonstrated a strong improvement when compared to results using longer pulse widths in the nanosecond range. High laser fluences (well above the damage threshold) at 800 nm and 248 nm are used to generate channels with high aspect ratios. Careful beam alignment can eliminate any remaining stress-induced damage outside the channel. At intermediate fluences just above the front surface processing threshold we observe a low ablation rate. In this 'gentle etch' phase it is possible to generate well-defined, smooth pockets and periodic patterns or ripples. The ripples appear when the laser pulse width is shorter than the lifetime of the electrons excited into the conduction band. In the low fluence regime (below the surface damage threshold) the self-focusing of laser pulses in the ps and sub-ps range can be utilized to obtain microstructures inside and on the rear side of the transparent materials.
Plenary Session
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Review of ablation processes at and above the classical threshold
Heinrich Hora, Frederick Osman, Reynaldo Castillo
For the use of laser ablation from solid materials, laser intensities I in the range of few orders of magnitudes above or below 1010 Watts/cm2 are of interest. Though these are of a modest magnitude it is known from the very beginning of laser plasma interaction studies that the ablation process mostly is not following the simple gasdynamic (thermokinetic) heating and hydrodynamic plasma mechanisms but that most complicated phenomena appear which are in the focus of research since their first observations in 1962 by Linlor and Honig. A number of basically new nonlinear mechanisms were discovered including the nonlinear (ponderomotive) force, emission of different groups of plasma where the fast ions show a separation by energy linearly increasing on their ion charge number Z, ponderomotive and relativistic self focusing, generation of higher harmonics in the backscattered radiation, double layer and surface tension effects, parametric instabilities and a stochastic pulsation of the interaction within some ten picoseconds. This all seems to have a threshold of about 1 MW laser power at least if beams of up to 100 wavelength diameter are being used. For the applications of the ablation mechanisms it may well be that further anomalous mechanisms may be discovered with the very wide beams. In order to simulate the awareness of these complexities, this paper is presenting a new and updated synopsis on the anomalous nonlinear mechanisms whose knowledge seems to be settled at present.
Physics of High-Power Laser-Surface Interactions
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Thermal solitons in laser-surface interactions
In spite of soliton was discovered firstly by J. S. Russell as an unusual wave on the water (1834), its rediscovery was made by Zabusky and Kruskal (1965) in connection with the problem of heat transfer in lattice of solids, stated by E. Fermi et. al, which poses the question about nonvalidity of the Fourier law (1822). M. Toda (1979) formulated that in nonlinear lattices the energy is mainly transported by solitons. In 1992 we found experimentally the Wave of Reflection and Conduction (WRC), that can present solitonic mechanism of heat transfer in solids. It has a threshold approximately 10 kW/cm2 and some solitary wave features: (1) the WRC amplitude is all the time of one sign, (2) its velocity UWRC is nearly constant; (3) it reflects from sample surfaces without changing the velocity. WRC-reversible photoinduced phenomenon which appears and could be important in many laser-surface interaction studies. As example, it could be important for the investigation of processes in the materials after pulse laser irradiation, especially in the case of pulse-periodic irradiation. WRC is excited by IR pulse laser and consist from the whole series of more than 20 solitary kind waves, which velocity drop twice starting from speed of sound to less than cm/s. WRC shows an universal character: it can translate the pulse of heat energy through the samples of any of 4 main crystal types and of some amorphous solids (glasses) in the manner different from usual diffusion. WRC characteristics were measured from variation: (1) of the optical constants (by reflection on the surface or absorption in the bulk); (2) of the temperature (by thermovision camera) on the sample surfaces; (3) of the pressure (by pressure pick up); (4) of the conduction on the surface and in the bulk (in the case of conducting samples). Results of investigations are agreeable in spite of experiments were made in 4 laboratories (Russia, France, Japan) by different schemes. A new V.I. Emel'janov's theory of defect recombination wave describes solitons with UWRC approximately cm/sec in solids, preconditioned by the action of powerful short laser pulse and could be considered as a possible physical mechanism of WRC.
Comparative study of laser-assisted removal and transfer in the deposition of Al2O3 and ZrO2 thin films for different pulse duration
Ernst-Wolfgang Kreutz, Mirka Alunovic, Holger Stamm
Investigations of material transfer comparatively has been carried out using micro- to nano second laser-assisted removal of ceramics (Al2O3, ZrO2) onto different substrates (steel, Si) for various laser parameters (wavelength, pulse energy, repetition rate, fluence) and processing variables (processing gas properties, target- substrate-distance). The transfer of the material was examined by high-speed photography, beam deflection, and optical spectroscopy. The experimental results, which are related to theoretical ones, are discussed in view of applications.
Applications in Coating, Coating Removal, and Material Processing
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Laser modification of iron under atmospheric conditions: a study on target surface composition and laser-induced plasma
Volkmar Nejedl, Thomas Witke, Andreas Lenk, et al.
Pure iron samples were irradiated with short single laser pulses (6 ns) under atmospheric conditions. For the irradiation a Q-switched Nd:YAG laser (wavelength 1064 nm) was used. The second and third harmonics (532 nm and 355 nm) were used for investigations at different wavelengths. The composition and expansion of the laser induced plasma were investigated using Optical Emission Spectroscopy (OES) and a High Speed Framing Camera (HSFC), respectively. The composition and morphology of the target surfaces were investigated by means of Auger Electron Spectroscopy (AES) and Scanning Electron Microscopy (SEM). AES measurements indicate that the near surface region of all laser spots is enriched with nitrogen (about 15 at-%) and covered by a thin oxidized layer. This surface composition was found to be similar for all wavelengths. It was established that the density (1019. . . 1023 cm-3), the composition (Fe+, N+ and O+), the temperature (30,000. . . 35,000 K) and the expansion of the laser induced plasma are similar for all wavelengths as well. Because of the high density the plasma absorbs the laser irradiation completely and the wavelength becomes less important for the laser target interaction. We assume that the cooling process leads to a formation of compounds of iron and nitrogen which are deposited on the surface.
Physics of High-Power Laser-Surface Interactions
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Optimization of high-power excimer laser for TFT annealing
Heinrich Endert, Bruno Becker-de Mos, Uwe Stamm, et al.
The flat panel display market is presently experiencing an annual growth rate of over 20%, and shows no signs of slowing down. Polycrystalline-silicon (poly-Si) thin film transistor (TFT) devices offer both improved quality and decreased cost over amorphous silicon displays, and will undoubtedly capture an ever increasing share of this market. However, producing poly-Si TFTs in high volume has presented a number of practical challenges. One of these challenges is transforming an amorphous silicon layer into poly-Si. For several years, researchers have investigated using excimer lasers for this purpose. In excimer laser annealing, a high-power excimer laser beam is scanned across the surface of a substrate coated with amorphous silicon. The intense UV light causes the silicon to melt, leading to subsequent recrystallization in polycrystalline form. This process is very non-linear, and places tight tolerances on laser beam parameters such as energy stability and beam uniformity. Furthermore, for this technique to be successfully integrated into a high volume production line, the laser must exhibit extremely high reliability, minimum downtime and low cost of ownership. In the past few years, excimer laser manufacturers have made significant technical improvements, leading to products that can successfully meet the rigorous demands of industrial TFT production. As a result, major display manufacturers in Japan, Korea and Europe have now adopted this technology for mass production.
Laser ablation of Al2O3-TiC ceramics: influence of laser fluence
Victor Oliveira, Rui M. Vilar, O. Conde
This paper presents a study of laser ablation of Al2O3-TiC ceramics. The dependence of the ablation rate and of the surface topography and constitution after ablation as a function of laser fluence and number of pulses was investigated. At low laser fluences, the surface develops a globular topography which is responsible for the decrease of the ablation rate and the increase of roughness. At higher laser fluences, the removal rate and the surface topography slightly depend on the number of pulses and on the laser fluence. The roughness also increases with the number of pulses but smoother surfaces can be achieved.
Two-dimensional simulation of high-power laser-surface interaction
S. Robert Goldman, Mark D. Wilke, Ray E.L. Green, et al.
For laser intensities in the range of 108 - 109 W/cm2, and pulse lengths of order 10 microseconds or longer, we have modified the inertial confinement fusion code Lasnex to simulate gaseous and some dense material aspects of the laser-matter interaction. The unique aspect of our treatment consists of an ablation model which defines a dense material-vapor interface and then calculates the mass flow across this interface. The model treats the dense material as a rigid two-dimensional mass and heat reservoir suppressing all hydrodynamic motion in the dense material. The computer simulations and additional post-processors provide predictions for measurements including impulse given to the target, pressures at the target interface, electron temperatures and densities in the vapor-plasma plume region, and emission of radiation from the target. We will present an analysis of some relatively well diagnosed experiments which have been useful in developing our modeling. The simulations match experimentally obtained target impulses, pressures at the target surface inside the laser spot, and radiation emission from the target to within about 20%. Hence our simulational technique appears to form a useful basis for further investigation of laser-surface interaction in this intensity, pulse-width range.
Processing of diamond and ceramics by picosecond/nanosecond laser pulses in IR-UV spectral range
Taras V. Kononenko, Serge V. Garnov, Sergej M. Pimenov, et al.
Laser processing of hard materials, in particular, diamond and ceramics, demands an optimal laser system for a particular application: cutting, drilling, patterning, etc. A comparative experimental study of laser drilling of CVD diamond and 3D microstructuring of ceramics has been performed varying the main laser radiation parameters -- wavelength, pulsewidth and pulse energy. Ablation of CVD diamond films by nanosecond (5 - 9 ns) and picosecond (150 - 300 ps) pulses at three different wavelengths of 1078, 539 and 270 nm in the range of laser fluence 1 divided by 103 J/cm2 was investigated. Ablation rates at different stages of laser drilling -- for shallow craters, deep holes with aspect ratio of 3 divided by 5 and ultradeep channels (aspect ratio up to 30 divided by 40) -- were measured and compared. The effect of waveguide propagation of laser radiation in deep channel was revealed. The role of laser-plasma interaction as well as the modification of a surface layer are discussed. The results of 3D laser microstructuring of ceramics -- AlN, SiC, Al2O3, Si3N4 -- are demonstrated. The rectangular in plane structures (pockets) with typical size of 200 X 200 X 100 micrometer3 with steep walls and flat bottom were produced. The influence of radiation parameters on the quality of the tested structures, mainly, on the bottom roughness and walls steepness has been analyzed.
Lasers for High-Power Photoablation
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High-power waveguide lasers
During the past few years high power lasers which incorporate intracavity optical waveguiding have been demonstrated in a number of different geometric formats. These include rectangular planar waveguide structures, two-dimensional multi-element waveguide array lasers and annular waveguide devices, all of which depend crucially on the operational flexibility of the transverse radiofrequency excitation technique. Here, we review the fundamental issues which underlie the attractions of the use of waveguiding structures in the design and construction of ultracompact, diffusion- cooled lasers which are efficient and operate at high average power levels. In particular, we review the properties of large area discharge planar waveguide CO2/CO lasers, where multi-kilowatt cw power levels have been demonstrated with excellent beam quality and efficiency. It is shown that similar concepts may also be applied to solid state lasers. In addition, the use of the multi-element array concept for high power scaling will be examined, and the operating characteristics of an ultra-compact 64 (4 X 16) element array laser operating at 2 kW cw output power will be described.
Kinetics of pulsed CO electric discharge gas mixtures
James P. Reilly, James D. Strohschein
In this paper, we summarize the electron pumping and molecular transfer kinetics of pulsed CO lasers, as to their influence, as well as the influence of gas pressure and temperature, on pulsed laser emission line structure, pulse duration and energy output per unit gain volume. Correspondence with existing experimental laser data is shown and compared to these factors. Implications for pulsed laser applications are explored.
High-average-power industrial laser for precision machining
David A. Burchman, Dave Hoffmaster, Jason P. Machan, et al.
High brightness, high average power, diode pumped Nd:YAG solid state lasers (DPSSL) are being developed by TRW as part of the Precision Laser Machining Technology Reinvestment Program. The use of diode pump arrays in place of flashlamps, and zig-zag slab geometry, allow lasers to be scaled to power beyond the current generation of lamp-pumped rod lasers while providing excellent beam quality. The efficiency is 3 - 4 times better using diode arrays in place of flashlamps resulting in less waste heat in the laser medium and reduced optical aberrations. The corresponding beam quality provides more than an order of magnitude increase in the average intensity available at the workpiece, thus enabling new machining capabilities.
High-energy-per-pulse excimer laser for silicon annealing
Bruno Godard, Dorian Zahorski
Forty-five joule per pulse XeCl laser with good homogeneity (plus or minus 1%) and repeatability (plus or minus 1%) allow to increase the quality and decrease the cost of annealing of amorphous Silicon compared to Solid Phase Crystallization and Scanning Excimer Laser Annealing.
Heat capacity disk laser
George F. Albrecht, Steven B. Sutton, E. Victor George, et al.
In this paper we describe the concept, and the basic scaling relationships of solid state heat capacity lasers. Intermediate between single shot and average power systems, the heat capacity concept scales solid state lasers to MW levels of burst power.
5-kW lamp-pumped Nd:YAG lasers and their applications
Hirotaka Kanazawa, Akihiro Nishimi, Minoru Uehara, et al.
Design and performance details of a 5-kW cw arc-lamp-pumped Nd:YAG laser are given. Two types of 5-kW Nd:YAG lasers have been developed, which have enough laser power and beam intensity necessary to realize deep penetration and high speed welding of metals. Experiments of welding and thick plate cutting were performed in order to investigate the ability of 5-kW Nd:YAG lasers. Maximum penetration of 15 mm is achieved with stainless steel and that of 6 mm with aluminum alloys. Moreover high speed welding up to 8 m/min is realized.
700-W diffusion-cooled large-area 40.68-MHz excited CO2 laser employing split-wave hybrid confocal resonator
Peter Vitruk, James Schemmer, Stan Byron
A novel non-waveguide, non-free-space CO2 laser resonator cavity, referred to as the split-wave hybrid (SWH) resonator, is described. Traditional resonator mirrors combined with two specially designed light reflecting electrode walls, which enclose the active medium, form the SWH resonator cavity. Light reflecting walls in the split-wave resonator act as wave-front-splitting mirrors in an interferometer, similar to a Fresnel double mirror or Lloyd mirror interferometer. Wave- front of the intra-cavity laser beam is significantly tilted with respect to the resonator walls, which facilitates lowest order mode selection in this resonator. Additionally, electrode wall surfaces contain discontinuities, which further enhances non-waveguide mode discrimination in the SWH resonator.
1-kW industrial excimer laser (308-nm)
John C.M. Timmermans
The short wavelength of excimer lasers offers advantages for industrial applications such as cutting, drilling surface treatment and annealing. At NCLR a 1 kW (1 J, 1 kHz) XeCl- laser is under development. This laser has several advantages above commercial excimer lasers that enables it to go to the high average power of 1 kW combined with an excellent beam quality.
Nuclear pumping of the iodine laser revisited
The O2-I2 transfer laser is gaining interest as a high-power laser for a variety of applications. There remains an interest in alternate pumping techniques for some situations, however. Here we review earlier work on the potential use of nuclear pumping, either for direct pumping of O2 or for pumping of an excimer flashlamp in a photolytic iodine laser system.
Considerations in short-wavelength (extreme-UV and x-ray) free-electron laser using quantum interference
Sher Alam, Cleo Bentley Jr.
Ordinary Free-Electron Lasers (FELs) can be found in successful operation in the spectral range from millimeters to ultraviolet wavelengths. However the operation of the common FELs in the extreme ultraviolet and X-ray wavelength regimes faces certain adverse effects. Some of the main obstacles in the way of the realization of X-ray FEL are electron momentum spread and angular divergence. Another point to keep in mind is that ordinary FELs work on the principle of 'momentum population inversion.' By this one means that electrons with momenta larger than the resonant value contribute to the gain whereas electrons with momenta smaller than the resonant value contribute to the loss. Thus to ensure a net gain we need more electrons with momenta lying in the upper momentum domain than in the lower one i.e. a 'momentum population inversion.' Keeping these points in mind Scully and co-workers have proposed using the ideas of Lasing Without Inversion (LWI) to achieve the successful operation of short-wavelength (extreme UV and X-ray) FELs. The purpose of this work is to, as a first step, critically analyze the theoretical and the practical feasibility of the proposals by Scully and co-workers. In particular we take a look at the following issues: Can the LWI FEL's operate efficiently even with a strongly inhomogeneous, broad electron momentum distribution? How practically feasible is the two-section Cherenkov Transition Radiation (TR) FEL? Does this Cherenkov TR FEL allow a complete absorption cancellation and LWI operation even in the case of a very broad electron momentum distribution compared to the homogeneous width? Is the gain of LWI FEL greater than the gain of the usual FEL by a factor of 100 (i.e. by two orders of magnitude)? How realistic is the proposed 'classical selective interference?' Moreover is the 'classical selective interference' free of the angular spread limitation? How valid is the small-signal gain calculation? The saturation would invalidate the small-signal analysis: when precisely does the saturation set in? The saturation is expected to affect the phase coherence required for the interference: what is the form of dependence of phase coherence on the saturation?
Ultraviolet and infrared lasers with high efficiency
Victor F. Tarasenko, Evgenii H. Baksht, Andrei V. Fedenev, et al.
The results of the experimental study of UV and IR lasers pumped by various methods are presented. The accelerators with radially converging or planar e-beams pumping gas mixtures at pressures up to 3 atm and self-sustained discharges were used. The highest laser radiation energies in the UV up to 2 kJ have been obtained at (lambda) equals 308 nm. Output of 100 J at (lambda) equals 1.73 micrometer in Ar-Xe mixture and 50 J at (lambda) equals 2.03 micrometer in He-Ar-Xe mixture was obtained from the e-beam laser with active volume of 600 l. Output energies of 110 J at (lambda) equals 308 nm and 90 J at (lambda) approximately 250 nm, respectively, were achieved in compact high-power e-beam laser with chamber of 20 cm in diameter and 30 l active volume. This e-beam geometry was shown to be very promising for excitation of non-chain HF- laser media. HF-laser efficiency with respect to deposited energy as high as approximately 10% and energy up to 200 J at (lambda) approximately 2.8 micrometer were demonstrated. Amplification of XeCl-laser beam from master oscillators and amplifiers under conditions of strong amplified spontaneous emission is considered. Investigations of CO2 laser excited by e-beam controlled discharge and e-beam ignited discharge were performed. The highest laser output in the IR of 3 kJ have been obtained at (lambda) equals 10.6 micrometer.
Intracavity phase conjugation of pulsed CO2 laser radiation
Phase conjugation process at intracavity degenerate four wave mixing of long pulse CO2 laser radiation in its active medium has been studied experimentally. Parametric dependencies of energetic and temporal characteristics of phase conjugation signal are analyzed. Phase conjugation reflectivity comes up to 7.5 - 10%. A periodical dependency of the reflectivity on optical delay with a period of approximately 10-1 and 10-3 cm has been observed.
Wide-aperture picosecond CO2 laser system
Victor V. Apollonov, K. Kh. Kazakov, N. V. Pletnyev, et al.
The construction of CO2 laser system generating a train of subnanosecond laser pulses with total train energy up to 5 J is reported. A record level of laser energy was obtained due to utilization of unique 5 X 5 cm2 aperture, 6 atm X- ray preionized CO2 amplifier. The estimations of individual pulses durations in the train are given. The prospective of upgrading of present configuration of laser system towards shortening of laser pulses duration and increasing the pressure of working gas mixture of the amplifier and its efficiency are discussed.
Scaling of a Q-switch CO2 laser for pulsed laser deposition
Andreas Husmann, Markus Niessen, Frank Gruembel, et al.
In Pulsed Laser Deposition the laser parameters pulse duration, repetition rate, and average output power are important parameters for control of the resulting film properties. In order to obtain high deposition and coverage rates the use of powerful laser radiation sources is required. A new approach is the use of Q-switch CO2 lasers, making accessible the range of kW average output powers for PLD. A common industrial fast axial gas flow rf-excited CO2-laser is equipped with a mechanical Q-switch assembly. The characteristic properties of this laser radiation source are studied by measurements of the average output power and of the spatial and temporal intensity distribution of the laser radiation. Results show, that the laser operates in multimode or in TEM10 mode depending on the use of apertures in the resonator. Variation of the electrical rf-excitation (in synchronization with the Q-switch) influences the pulse duration and shape, repetition rate, and average output power. Based on the experimental findings model calculations are carried out, in order to predict the pulse energy and average output power of the laser radiation source, when operating with different Q-switch parameters, e.g. higher repetition rates.
Applications Using Lasers and Other Sources of Propagated Energy to Produce Remote Thrust
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Flight experiments and evolutionary development of a laser-propelled transatmospheric vehicle
Franklin B. Mead Jr., Leik N. Myrabo, Donald G. Messitt
In a series of spectacular experiments conducted at the High Energy Laser Systems Test Facility (HELSTF), White Sands Missile Range (WSMR), NM, using 13- to 15-cm diameter, 40- to 60-g vehicles designed to fly on the 10 kW PLVTS pulsed carbon dioxide laser (1 kJ pulses for 30 microsecond duration at 10 Hz), Prof. Leik Myrabo of Rensselaer Polytechnic Institute (RPI) and Dr. Franklin Mead of the Air Force Research Laboratory's (AFRL) Propulsion Directorate, have been successfully flying laser propelled Lightcraft under a joint Air Force/NASA flight demonstration program. The axisymmetric Lightcraft vehicles are propelled by airbreathing, pulsed- detonation engines with an infinite fuel specific impulse. Impulse coupling coefficients have been measured with ballistic pendulums as well as a piezoelectric load cell and fall in the range of 100 to 200 N/MW. Horizontal wire-guided flights up to 400 ft, using a unique laser beam pointing and tracking guidance system, have demonstrated up to 2.0 G's acceleration measured by a photo-optic array. Spin-stabilized free-flights with active tracking/beam control have been accomplished to altitudes of 15.25 meters. This paper will summarize the progress made to date on the Lightcraft Technology Demonstration flight test program, since the first 12 - 14 July 1996, experiments at HELSTF.
Removal of small space debris with orbiting lasers
Wolfgang O. Schall
Space debris at low Earth orbits (LEO) in the size range of 1 to 10 cm in diameter poses a severe threat on the International Space Station and other valuable space assets. High-power laser radiation may be the most feasible means to mitigate this problem. Under the irradiation of a high-power laser beam part of the debris material is ablated and provides an impulse to the debris fragment. Proper direction of the impulse vector allows either to deflect the object trajectory to miss the station (defense option) or to reduce the orbital energy of the debris and force it on a trajectory through the upper atmosphere. There the debris burns up instantaneously or after a few revolutions (cleaning option for LEO). A space based deployment of the laser is favored for several reasons: The lack of laser transmission through the atmosphere reduces the total the total system substantially, laser range and detection requirements are inferior and the laser can be used against an immediate threat. Peculiarities of the geometrical situation in the orbital plane are described. Based on a 100 kW average power laser, aluminum as a typical material, and some other assumptions, the capability and limitations with respect to the debris velocity and mass are calculated for both options of the laser utilization.
Orion: challenges and benefits
ORION is a practical proposal for removing the 150,000 pieces of manmade space debris in the 1- to 10-cm size range now orbiting the Earth below 1500 km altitude which threaten large space systems in low Earth orbit. It is based on using the thrust produced by pulsed laser ablation of a thin layer on the debris surface to drop its perigee sufficiently for reentry and burnup. Applied when the object is rising between about 45 and 15-degree zenith angle, the necessary (Delta) v is of order 100 m/s. A laser of 30 kW average power at 10-ns pulsewidth and a 6-m mirror with adaptive optics can clear near-Earth space of these debris in 2 years of operation. Technical challenges faced by such a system include: heavy demands on detection, tracking and adaptive optics arising from the tiny optical cross section of the smallest debris and the required pointing accuracy and steering rate, stimulated Raman conversion and nonlinear refraction of the laser beam in the atmosphere, uncertainty of momentum coupling coefficients (Cm) for some materials, and high-average-power laser development. It is crucial that the system we propose be developed under international aegis, to insure that its installation does not increase international tensions. It should be viewed as a single-pay lifetime insurance policy for the World's space assets whose premium is less than 1% of the protected asset value, an excellent rate for such contracts.
Power beaming for orbital debris removal
Orbital debris in low-Earth orbit ranging in size from 1 to 10 cm in diameter can be detected but not tracked reliably enough to be easily avoided by spacecraft. In addition, shielding protection is extremely difficult and costly to accomplish for sizes above 1 - 2 cm. Debris in this size regime traveling at mean velocities on the order of 20000 miles per hour may cause catastrophic damage. Using adaptive optics technologies, a ground-based pulsed laser of sufficient power ablating the debris particle's surface to produce small momentum changes may, in several hundred pulses, lower a target debris particle's perigee sufficiently for atmospheric capture. A single laser facility could remove all of the 1 - 10 cm debris below 1500 km in altitude in approximately three years. A technology demonstration of ground based laser removal is proposed which would pave the way for the implementation of such a debris removal system. The cost of the proposed demonstration is comparable with the estimated annual cost of spacecraft operations in the present orbital debris environment.
Ablation scaling laws for laser space debris clearing
This paper analyzes laser induced impulse and associated scaling laws appropriate to the intensity range of interest for laser space debris clearing (106 - 1010 W/cm2). A simple radiative fluence model is used augmented by empirical values from some recent experiments.
Applications in Coating, Coating Removal, and Material Processing
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Planar laser and nonlinear optical waveguides fabricated by pulsed laser deposition
Minoru Obara, Kyoichi Adachi, Tetsu Hasegawa, et al.
Using KrF laser ablation the fabrication of Yb:YAG, and Cr:YAG planar waveguides as well as nonlinear Co:BaTiO3 waveguide has been carried out. The waveguide structure was fabricated by deposition of a rare-earth-doped YAG layer on an undoped substrate. The optical waveguide was characterized by optical absorption spectra, photoluminescence spectral intensity, optical propagation loss, and valence state by XPS. Co:BaTiO3 nonlinear optical, thin films have been fabricated with good quality using a new hybrid process of two-target pulsed laser deposition and in-situ pulsed laser annealing. Epitaxial Co:BaTiO3 films with Co/Ti concentrations of up to 5% were achieved by this method. Particles on the growing film were almost eliminated by in-situ laser annealing. Cobalt dopant concentrations were easily adjusted and controlled.
Predicting laser coating removal rates
Our measurements of laser coating removal rate for 14- micrometer-thick black organic paint coatings on a refractory substrate showed the rather astonishing result that total exposure time (Delta) t required to ablate the coating (for these particular thin organic coatings, in air) is described by (Delta) t equals C X I-3/2 sec where x is the coating thickness, I the incident pulse intensity (W/cm2) and the constant C equals 1.4 X 107 W3/2-s/cm4. This was true for wavelength 350 nm less than (lambda) less than 10.6 micrometer and incident intensity 81 W/cm2 less than I less than 424 MW/cm2, covering a 7-order-of- magnitude. Such a simple result is phenomenal because of the range of different physical processes known to be involved (simple boiling, thermal conduction and differential thermal expansion for the long pulses, but the combined effects of shock spallation, large excursions within the solid equation of state, and acceleration, compression, shocking, thermal conduction, radiation, phase explosion and even some ionization for the shortest pulses). If extensible to other coatings and substrates, it is also very valuable for the design of expensive laser coating removal facilities, which are now receiving strong interest for very large-scale decontamination and aircraft repainting. Our first-order parametric model fits the experimental results to within better than a factor-of-3 in laser intensity throughout the above range.
Industrial laser-based coatings removal systems
David A. Freiwald, Henry C. Peebles, Roger P. Case
Industrial-cleaning-rate laser systems have been built and tested for removing various types of coatings, such as rad- contaminated coatings, non-rad but hazmat-contaminated coatings (e.g., Pb-based paint), and non-hazardous coatings from various types of substrates such as concrete, metals, and composite materials.
Ablation of concrete surfaces using a compact high-power CO2 waveguide array laser with beam shaping
The composite beam from a 64-element carbon dioxide array laser, which produces an average power output of 2 kW, has been used in a series of surface processing experiments. In one set of experiments related to nuclear power station decommissioning, the array beam has been used to process the surface of a range of different density concrete materials. Scabbling trials have achieved concrete removal rates of approximately 1200 cm3hr-1kW-1 and average scabble depths of approximately 5 mm with incident power densities as low as 80 Wcm-2.
Surface engineering using excimer lasers
Anthony J. Pedraza, Douglas H. Lowndes
Excimer laser irradiation of insulators produces structural and chemical modifications in the near-surface region of these materials. These changes have lead to the usage of excimer lasers to engineer the surface of insulators for various applications, as illustrated in four examples presented here: (1) Laser-enhanced bonding of deposited metallic films. A very strong bonding between metallic films and Al2O3 can be achieved if the substrates are pulsed-laser treated prior to deposition. AES reveals that strong bonding occurs when an intermediate interfacial compound forms as a metallic film is deposited on a laser-irradiated substrate. (2) Laser encapsulation of metallic particles in silica. Thin films of gold, copper and iron deposited on silica can become encapsulated as small particles upon pulsed laser irradiation XTEM indicates two distinctive stages in the encapsulation process, during one laser pulse. In the first stage, the film melts and clusters into small particles, and in the second stage, the particles are driven into the substrate. (3) Laser- induced surface activation for electroless deposition. In this process, a pattern is imprinted on a substrate by laser irradiating its surface through a mask. Upon immersion of the substrate in an electroless solution, a metallic film is deposited only on the laser-exposed area. Auger emission spectroscopy (AES) and cross sectional transmission electron microscopy (XTEM) indicate that electroless deposition is promoted by the presence of metallic aluminum in AlN and in Al2O3, and of substoichiometric oxide in Al2O3, as well. Other laser irradiation effects that also could induce activation are analyzed. (4) Laser-induced deactivation of a previously activated area.
Machining with high-brightness lasers
Xiangli Chen
Drilling and cutting results using high brightness Nd:YAG lasers are presented and compare with that of conventional Nd:YAG lasers. Materials tested include superalloys, intermetallics, and composites. The master-oscillator-power- amplifier (MOPA) laser system has an average power of 100 W and about 1.2 diffraction-limited beam quality. It is Q- switched and/or modelocked. Hence pulse lengths are in the hundreds of picoseconds to hundreds of nanoseconds range, resulting in beam intensities of 108 - 109 W/cm2. It is shown that the shortening of pulse length and the subsequent increase in beam intensity result in much thinner recast layers and heat affected zones (HAZ), less microcracking or delamination of materials, and much better geometry stability. Both the fundamental 1064 nm and the second harmonic wavelength of 532 nm are tested, showing substantial additional advantage for the shorter wavelength.
Laser cleaning of metal surfaces
Craig T. Walters, Bernard E. Campbell, Robert J. Hull
There is a critical need to replace ozone-depleting substances and hazardous chemicals that, in the past, have been used routinely in aerospace maintenance operations such as precision cleaning of metal surfaces. Lasers now offer the potential for removal of many organic materials from metals without the use of any solvent or aqueous cleaning agents. This paper presents quantitative results of laser-cleaning process-development research with a pulsed Nd:YAG laser and several common metals and organic contaminants. Metal coupons of Stainless Steel 304, Aluminum 5052, and Titanium were contaminated with known amounts of organic oils and greases at contamination levels in the 5 to 200 (mu) g/cm2 range. A fiber-optic-delivered 1064-nm pulsed laser beam (20-Hz repetition rate) was scanned over the coupons with different overlap and pulse fluence conditions. Measurements of mass loss revealed that all levels of initial contamination could be removed to final cleanliness levels less than 3 (mu) g/cm2, at which point the mass loss measurements became uncertain. Pulse fluence thresholds for initial cleaning effects and practical cleaning rates for several metal and contaminant combinations are reported. From the totality of the results, an overall picture of the contaminant removal mechanism is emerging. For semi-transparent films, it is conjectured that a thermo-mechanical effect occurs wherein the laser energy is absorbed predominantly in the metal substrate which expands on the nanosecond time scale. This rapid expansion, in combination with some material evaporation at the film/metal interface, is believed to eject the contaminant film directly into aerosol droplets/particles which can be swept away and collected for recycle or cost- effective disposal in a compact form. Evidence for this mechanism will be presented.
Mechanisms of laser surface cleaning in liquid media
Anne-Patricia B. Alloncle, Jean-Christophe Isselin, Marjorie Provost, et al.
This paper deals with an experimental study on the mechanisms of laser surface cleaning process in a liquid medium. Experiments have been performed with oxides of different compositions irradiated with two different wavelengths: 1064 nm, 532 nm. Interpretations have been inferred from high speed visualizations, pressure recordings and surface samples analysis. Interesting results on the dynamics of the cavity resulting from ablation and its potential contribution to the cleaning effects have been carried out.
Pulsed laser deposition of metal oxide films
Quanxi Jia
Pulsed laser deposition (PLD), which provides very unique features compared to the conventional physical vapor deposition technique such as sputtering, is one of the most powerful techniques to deposit conductive oxide thin films. Over the past several years, we have optimized the processing conditions to deposit high quality conductive RuO2 and SrRuO3 thin films by PLD. We show that the substrate temperature during the deposition process plays an important role in determining the structural and electrical properties of these films. Epitaxial RuO2 and SrRuO3 thin films with a room-temperature resistivity of 35 (mu) (Omega) -cm and 280 (mu) (omega) -cm, respectively, have been successfully deposited by PLD.
Pulsed laser deposition: prospects for commercial deposition of epitaxial thin films
Ross E. Muenchausen
In this paper we briefly define pulsed laser deposition, current applications, research directed at gaining a better understanding of the pulsed laser deposition process, and suggest some future directions to enable commercial applications.
Characterization of amorphous carbon films grown by pulsed laser deposition
Michael P. Siegal, Luz J. Martinez-Miranda, N. John DiNardo, et al.
Amorphous carbon (a-C) films grow via energetic processes such as pulsed-laser deposition (PLD). The cold-cathode electron emission properties of a-C are promising for flat-panel display and vacuum microelectronics technologies. These ultrahard films consist of a mixture of 3-fold and 4-fold coordinated carbon atoms, resulting in an amorphous material with 'diamond-like' properties. We study the structures of a-C films grown at room temperature as a function of PLD energetics using x-ray reflectivity, Raman spectroscopy, high- resolution transmission electron microscopy, and Rutherford backscattering spectrometry. While an understanding of the electron emission mechanism in a-C films remains elusive, the onset of emission is typically preceded by 'conditioning' where the material is stressed by an applied electric field. To simulate conditioning assess its effect, we use the spatially-localized field and current of a scanning tunneling microscope tip. Scanning force microscopy shows that conditioning alters surface morphology and electronic structure. Spatially-resolved electron energy loss spectroscopy indicates that the predominant bonding configuration changes from predominantly 4-fold to 3-fold coordination.
Q-switched and mode-locked solid state laser for precision machining of transparent materials
Andreas Lenk, Thomas Witke, Ulrich Franz
A cw-pumped and mode-locked solid state laser emits an endless pulse sequence with pulse duration of about 50 - 100 ps and pulse intervals of about 10 ns. A pulse sequence of 300 ns is formed by the optional use of Q-switching. The largest pulse of this pulse sequence reaches a peak power of 2 MW. Power densities in the range of 1010 . . 1012 W/cm2 are available by suitable focusing. Such a laser system has been first used for precision machining. Because of the high power density non-linear effects allow the machining of transparent materials. The temporal and spatial formation of a laser induced plasma in the case of surface and sub-surface focusing has been investigated by a special high speed framing microscope. It consists of four electronic high speed cameras coupled with a long distance microscope via image splitter. The sub-surface focusing regime has been applied to anti- counterfeiting or decorative 2d and 3d-marking. The surface focusing regime (micro laser plasma sputtering) has been applied to pixel based micro marking of glass surfaces, creating of special scattering surfaces and non-slip finishing of glass flooring. Repetition rates up to 500 Hz guarantee an efficient production because of with processing time.
Theoretical and experimental investigations on the role of oxidation and gas flow in the machining of steel molds due to melt removal with high-power CO2 lasers
Andreas Penz, Stephan Fazeny, Dieter Schuoecker
For the machining of metal cavities and reliefs with high power lasers, both the gas mixture and the actual formation of the flow is essential. High ablation rates with an appropriate surface quality can be reached only with an optimized gas flow and a sound process understanding. The impinging gas flow results in a shear force and a pressure gradient over the melt pool. These values also depend on the actual geometry of the erosion front and can not be predicted in advance. To investigate the gas flow with different nozzle arrangements numeric simulations are carried out. Calculations and experiments show that working with an inclined gas flow -- the so called laser planing process -- improves the material removal rate significantly. An optimum angle for the gas- nozzle can be shown. By using a mixture of oxygen and nitrogen as working gas, additional heat can be supplied to the process. Knowing the fraction of material which can be oxidized per time unit is important to calculate the additional heat from the chemical oxidation. Furthermore, the thickness of the formed oxide layer influences the absorption of the laser power effectively. In this paper it will be shown that the change of the absorptivity is the key parameter for a high ablation rate. The chemical reaction also changes the properties of the melt pool. These effects influence the process significantly and they are of fundamental importance to achieve a high material removal rate. This will be shown in various experiments. A simplified process model for melt removal will be presented and compared with experimental results. The model includes two important oxidation effects, which are improved absorptivity and the additional heat transfer. Furthermore the shear force and the pressure gradient over the melt pool will be taken into account to calculate the thickness of the melt film.
Ablation processes induced by UV lasers in metals and ceramics
Jose Luis Ocana, Gines Nicolas, Michel L. Autric, et al.
The interaction in air of high intensity excimer lasers (KrF) with metals (Aluminum and aluminum alloys) and ceramics (Al2O3, ZrO2, AlN, SiC) has been investigated. Results concern the dynamics of the generated plasma and include the visualization of the luminous plasma front and the developed shock waves by means of an ICCD camera. At the same time, a shadowgraphy optical device has allowed to observe simultaneously the formation and expansion of plasma and shock wave fronts propagating into the surrounding gas during and after the irradiation pulse (20 ns). Complex structures inside the plasma plume have been observed inducing turbulence phenomena after irradiation that could be detrimental for high repetition rates and need further study. From a theoretical point of view, numerical simulations of the described irradiation experiments have been attempted trying to predict the observed plasma dynamics and, at the same time, to provide a macroscopic estimate of the mechanical transformations induced in the treated material. A 1D thermofluiddynamic code with detailed atomic and EOS parameters has been used for the simulation of the plasma dynamics and a full 3D finite element code provided with temperature dependent material data has been used for the macroscopic assessment.
Beam Propagation, Beamforming, Diagnostics, and Detection Techniques
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High-power hollow fibers
Hollow glass fibers for delivery of high-powered infrared lasers. The hollow fibers are consisted of a glass capillary tube and metal and polymer layer on the inside of the glass tube. As the dielectric layer, polymers that are transparent at the desired wavelength are utilized. The fibers are fabricated by a simple, liquid phase technique. Firstly, a silver film is formed inside the glass tubing by a mirror plating technique. Then a polymer film is formed on the silver by flowing a polymer solution and following dry and cure process. The delivery test using Nd:YAG ((lambda) equals 1.06 micrometer), CO (5 micrometer), and CO2 (10.6 micrometer) lasers as the light source show that the hollow fiber is a promising candidate as a transmission media of high-peak power, high-energy, infrared lasers for ablation.
Limitations and applications of the colliding shock lens
Max M. Michaelis, M. Kuppen, G. R. Turner, et al.
Gas lenses are indestructible and capable of focusing two orders of magnitude higher laser intensities than solid lenses. Conventional gas lenses such as the Spinning Pipe Gas Lens (SPGL) are however long, bulky and optically weak. The recently patented Colliding Shock Lens (CSL) is a shorter more powerful gas lens. It also offers the possibility of greater apertures than the 3 cm currently achievable with other gas lenses. We present recent experimental work on the CSL and propose two new applications apart from the CSL Q-Switch and other applications described elsewhere. They are the Colliding Shock Laser and the Colliding Shock Light Guide. The latter could find application in laser accelerator research.
Thermal blooming in NPLs
Maria Petra, George H. Miley
Nuclear-pumped lasers (NPLs) potentially offer an attractive method for high power laser applications such as a space power beaming. However, thermal gradients created by the pumping of the ion medium are producing a dynamic thermal blooming which must be understood and controlled for accurate beam control and focusing. In this paper, basic experimental studies of blooming under a variety of conditions are presented. It is shown that one promising approach for reducing the effect is to employ a combined volume-wall pumping technique.
Experimental realization of a laser telescope with overall compensation for distortions via phase conjugation
Rostislav B. Andreev, Valery M. Irtuganov, Alexey Leshchev, et al.
For the first time was realized the laser telescope (beam director), in which the phase conjugation provides the compensation for the distortions, caused by the primary and secondary mirrors. Nearly diffraction limited laser beam divergence was realized under the significant disadjustments and disalignments of the segmented mirror.
Numerical simulation of observational telescope with dynamic holographic correction
Nataly A. Bezina, Alexey Leshchev, Michael V. Vasil'ev, et al.
Described are the methods of numerical design and simulation of observational telescopes, using the thin (plain) dynamic hologram as the correct for image distortions, imposed by the primary mirror surface defects.
Large-numerical-aperture imaging bypass system with dynamic holographic correction for primary mirror distortions
Michael V. Vasil'ev, Vladimir A. Berenberg, Alexey Leshchev, et al.
Large numerical aperture telescope with nonlinear optical correction for distortions, designed for the remote self- luminous object imaging, was realized in experiment and investigated. Dynamic hologram, recorded in optically addressed liquid crystal spatial light modulator, was used as the corrector. Nearly diffraction limited performance of the system was demonstrated.
Polychromatic correction for aberration in the lenses of telescopic systems using liquid crystal optically addressed spatial light modulator
Vladimir A. Berenberg, Alexey Leshchev, Michael V. Vasil'ev, et al.
Given are the results of experimental study on the quasi real time holographic correction for the lens distortions in the passive observational telescope in the visible range of spectrum, using the liquid crystal optically addressed spatial light modulator.
Laser-induced ultrafast phase transitions in solids using optical anisotropy
Michail B. Agranat, Sergei I. Anisimov, Sergei I. Ashitkov, et al.
A new method of crystalline order detection in highly absorbing anisotropic crystals is worked out. The method is based on partial transformation of incident p-polarized electromagnetic wave into s-polarized reflected wave due to optical anisotropy. The method makes possible to follow changes of crystalline structure in thin (10-6 - 10-5 cm) surface layers of solids. Using picosecond laser pulses and streak camera 'Agat,' surface melting and evaporation of Zn and C (graphite) are studied. Direct observation of the melting of graphite subjected to picosecond laser pulse is performed. The kinetics of solidification of fused surface layers are studied.
Applications in Coating, Coating Removal, and Material Processing
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Production of amorphous graphite in picosecond laser interaction experiments
Michail B. Agranat, Sergei I. Anisimov, Sergei I. Ashitkov, et al.
Formation of a liquid phase with subsequent transition to a uniform amorphous state of surface layer upon solidification is observed under action of picosecond laser pulses on microcrystalline graphite. This phenomenon is registered on a definite type of graphite and with the radiation incident on a plane parallel to the sixfold symmetry axis, and only for certain parameters of laser pulse. A study of melting and solidification of graphite is performed using a new method based on partial transmission of incident p-polarized wave into s-polarized reflected wave due to optical anisotropy. A structural analysis of the amorphous phase is performed by electron microscopy and Raman scattering spectroscopy. Periodic surface structures with a period of the order of the wavelength of the heating pulse was detected on amorphous graphite region. The orientation of the structures correlates with polarization of incident laser pulse. The instability to formation of these structures is assumed to be connected with surface electromagnetic wave excitation. The characteristic time of existing of liquid phase and of solidification processes is determined to be approximately 10-10 s.
Beam Propagation, Beamforming, Diagnostics, and Detection Techniques
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Computer simulation of light propagation through optical coatings with inhomogeneous microstructure
Vitali E. Gruzdev, Anastasia S. Gruzdeva
There are presented initial results of investigation of influence of local optical and thermal parameters of thin dielectric films and optical coatings with inhomogeneous micromorphology on light propagation. Linear and nonlinear light propagation accompanied by laser-induced heating have been considered. Modeling is used to investigate the process described by coupled nonlinear equations. Obtained results show critical role of nonlinear phenomena resulting in formation of local maxima of laser intensity inside the films which is followed by highly inhomogeneous local heating of the films.
Computer simulation of nonlinear dynamics of high-power laser field in low-Q cylindrical defect embedded into transparent dielectric
Vitali E. Gruzdev, Anastasia S. Gruzdeva, Mikhail N. Libenson
The aim of our paper is to investigate possibility of local increasing of amplitude of high-power laser radiation in transparent microinclusions in dielectric media. There is presented theoretical approach to the problem allowing to show possibility of formation of unstable field structure in microsphere and microcylinder with near-resonant parameters. The instability is shown to develop as a result of laser- induced formation of sphere's or cylinder's eigenmode accompanied by positive feedback through laser-induced variation of refractive index. The instability is shown to have threshold depending on radiation and inclusion parameters. Computer modeling is used to get more detailed information about the described processes in dielectric cylinder. The presented results of modeling show dynamics of nonlinear evolution of high-power laser field in isotropic microcylinder. Several parametric dependences of instability threshold are measured using results of modeling. The presented results are discussed from the viewpoint of laser ablation and laser-induced damage of low-absorbing optical materials.
Physics of High-Power Laser-Surface Interactions
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Evolution of rough dielectric surface in high-power laser field
Vitali E. Gruzdev, Anastasia S. Gruzdeva, Mikhail N. Libenson
This paper is devoted to investigation of some aspects of laser-driven evolution dielectric surface under action of high-power laser radiation. Main problems to be considered are (1) electrodynamic processes at the surface initiating laser- induced variations of surface relief; (2) possible mechanisms of formation of feedbacks between scattering and heating; (3) possible waveguide mechanisms of formation of surface ripple patterns. In the first case we investigate initiating of laser-induced deformation of dielectric surface through light scattering by both single relief defects and periodic relief modulation. Our goal is to study possibility of formation of local maximums of light intensity resulting in local heating of the surface. To consider the third problem we investigate positive feedbacks accompanying laser-surface interaction through mutual influence of electrodynamic and thermal processes. Obtained results show how the mentioned processes and feedbacks can influence laser-induced surface modification. Computer modeling is widely used to investigate all the mentioned problems. The presented results are discussed from the viewpoint of laser ablation and surface laser-induced damage of transparent optical materials.
Lasers for High-Power Photoablation
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Concept of the power-reactor-pumped laser for technology applications
Andrey V. Gulevich, Peter P. Dyachenko, Victor N. Kononov, et al.
Conception of a high-power pulsed reactor-pumped laser system (RPLS) based on new physical principles (direct nuclear-to- optical energy conversion) for the technology and space application is discussed. The development of an energy model of RPLS consisting of the ignition two-core fast-burst reactor reactor module and a thermal subcritical laser module filled with an Ar-Xe laser active medium is reported. Some of the experimental results are also presented.
Applications Using Lasers and Other Sources of Propagated Energy to Produce Remote Thrust
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Explosive initiation using radiation from a shock wave-pumped solid state laser
Anatoly S. Boreisho, Alexander F. Leonov, Leonid Yu Frolov-Bagreev
Results of theoretical simulation and field tests of a shock wave pumped solid state laser are presented. The laser has demonstrated real capabilities to initiate explosives. Operating without external sources of electrical power, the laser is pumped with the radiation from converging axially symmetric shock waves in argon.
Beam Propagation, Beamforming, Diagnostics, and Detection Techniques
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Negative optical feedback correction for the extended range of distortions
Proposed in the novel method of dynamic nonlinear-optical correction for distortions in wide spectral band. The method is based on combining of the negative optical feedback correction and dynamic holography correction in the system, using optically addressed phase modulators.
Physics of High-Power Laser-Surface Interactions
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Bound-to-delocalized-state transition in a strong nonresonance laser field
Vladimir K. Mukhomorov
By using Kramers-Henneberger unitary transformation, the rapidly oscillating perturbation is transposed to the argument of a potential function. It is shown that the depth of the potential-energy well in which the electron is localized decreases monotonically with an increase in the strength of the external field. The analytic technique of a shifted 1/N expansion was used to express the position of the ground energy level as a function of the external-source intensity. The critical value of a parameter characterizing the external source is found for which the ground bound quasi-discrete electron level vanishes. The localized-to-delocalized-state transition is identifying as a diffuse nonmetal-metal phase transition.
Modeling of nanosecond-laser ablation: calculations based on a nonstationary averaging technique (spatial moments)
N. D. Arnold, Boris S. Luk'yanchuk, Nikita M. Bityurin, et al.
Semi-analytical approach to a quantitative analysis of thermal ns laser ablation is presented. It permits one to take into account: (1) Arbitrary temperature dependences of material parameters, such as the specific heat, thermal conductivity, absorptivity, absorption coefficient, etc. (2) Arbitrary temporal profiles of the laser pulse. (3) Strong (Arrhenius- type) dependence of the ablation velocity on the temperature of the ablation front, which leads to a non-steady movement of the ablation boundary during the (single) pulse. (4) Screening of the incoming radiation by the ablated products. (5) Influence of the ablation (vaporization) enthalpy on the heating process. (6) Influence of melting and/or other phase transformations. The nonlinear heat conduction equation is reduced to three ordinary differential equations which describe the evolution of the surface temperature, spatial width of the enthalpy distribution, and the ablated depth. Due to its speed and flexibility, the method provides powerful tool for the fast analysis of the experimental data. The influence of different factors onto ablation curves (ablated depth h vs. fluence (phi) ) is studied. Analytical formulas for (phi) th and h((phi) ) dependences are derived and discussed. The ablation curves reveal three regions of fluence: Arrhenius region, linear region, and screening region. Threshold fluence (phi) th and Arrhenius tails at (phi) less than (phi) th, are affected heavily by the temperature dependences in material parameters, surface evaporation rate, and pulse duration and shape. In contrast, the slope of the ablation curves at (phi) greater than (phi) th, is determined almost exclusively by the latent heat of vaporization, high temperature dependence of absorptivity, and, in the case of screening, by the absorption coefficient of the plume (alpha) g. In the screening region ablated depth increases logarithmically with fluence and its qualitative behavior is weakly affected by the temperature dependence in (alpha) g (T). Small vaporization enthalpy results in a sub-linear h((phi) ) dependence, which, nevertheless, remains faster than logarithmic. With weakly absorbing materials ablation may proceed in two significantly different regimes -- without or with ablation of the heated subsurface layer. The latter occurs at higher fluences and reveals significantly higher ablation temperatures, but is weakly reflected on the ablation curves. Calculations are performed in order to study the: (1) Influence of the duration and temporal profile of the laser pulse on the threshold fluence, (phi) th. This is particularly important for strong absorbers were the heat conduction determines the temperature distribution. (2) Influence of the temperature dependences in material parameters on the ablation curves (ablated depth versus laser fluence) for regimes (phi) approximately equals (phi) th and (phi) very much greater than (phi) th. (3) Consequences of shielding of the incoming radiation at high fluences. (4) Differences in ablation curves for materials with big and small ablation enthalpy (e.g., metals and polymers which ablate differences in ablation curves for materials with big and small ablation enthalpy (e.g., metals and polymers which ablate thermally). Nanosecond laser ablation has been studied for a large variety of different materials and laser wavelengths. As an illustrative example, the method is applied to the quantitative anlaysis of the single pulse ablation of polyimide Kapton TM H.
Applications in Coating, Coating Removal, and Material Processing
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Nanocrystal film deposition by laser ablation of diamond compact
Alexander F. Pal, Yevgeny A. Ekimov, Dmitry A. Mazalov, et al.
Laser ablation of diamond compacts made of ultradispersed diamond powders was used for producing thin diamond films with thickness of about 1 micrometer. X-ray and Raman analyzing methods verified the fact of diamond phase transfer upon the substrate during the PLD process. The obtained films reveal electron field emission with the current density up to 10 mA/cm2 at reduced electric field approximately 20 V/micrometer, and that may make it possible to produce flat cold emission cathodes of high area.
Applications Using Lasers and Other Sources of Propagated Energy to Produce Remote Thrust
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Thermomechanical fracture on pressurized cylindrical vessels
Robert D. Y. Tzou, Kwong Shing Chiu, Jorge E. Beraun, et al.
This work studies the rapid fracture developed on the surface of a pressurized cylindrical vessel when heated by an intensified energy source. The primary concerns are the interactions between the rapid thermal expansion and the internal pressure that exerts on the interior surface. From a mechanical point of view, the thermal loading tends to develop a crack along the circumferential direction of the cylindrical vessel. The excessive internal pressure established within the cylindrical vessel, on the other hand, tends to develop a crack in the axial direction. Combination of the two mechanisms results in a capricious pattern of rapid fracture that needs to be fully understood in thermal processing. Special features in this work include the dynamics plasticity induced by the combined thermomechanical loading at short times, as well as the temperature-dependent thermomechanical properties that evolve in the load-time history.
Physics of High-Power Laser-Surface Interactions
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Model of laser-induced material removal of polymers and gas dynamics
Mirko Aden, David A. Wesner, Ernst-Wolfgang Kreutz, et al.
A model is built up to describe the effects of the laser- induced material removal of polymers and the dynamics of the volatile species. The heating and removal are described by the volume-heating of the irradiated polymer due to the absorption of laser radiation within the optical penetration depth and a phase transition from the condensed to the gas state of the polymer. The delivered energy due to the absorption of laser radiation is distributed to the different channels of decomposition in accordance to the temperature. The dynamics of the volatile species is calculated by the use of the Euler equations (non-dissipative continuum mechanical equations of the conservation laws of mass, momentum and energy). The flow field pattern of the gas phase (density, velocity, and pressure) during the material removal of PE, PP and PMMA and the depth of material removal per pulse are calculated. The maximum pressure for PMMA exceeds the one for PE, which exceeds the one of PP. This is attributed to the optical penetration depth of the three polymers. The depth of material removal increases with increasing fluence this effect is strongest for PP followed by PE and smallest for PMMA.
Lasers for High-Power Photoablation
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High-power CO2 gas-dynamic lasers for applications in technology
Yuri B. Konev, Yurii V. Lipukhin, Leonid I. Danilov, et al.
The performance of high power gas dynamic laser facilities is described. The key features of design of these installations and their applications in technology are discussed. Results of gas dynamic laser test runs are provided. The GDL using heat exchangers for laser gas heating, transportable by automobile and rail road platforms, are described.
Applications in Coating, Coating Removal, and Material Processing
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Shock wave generated during laser ablation
Christian Stauter, Philippe Gerard, Joel Fontaine
In this paper, we present a theoretical and experimental study of pulse laser ablation of metals and ceramics. We develop a model to calculate the energy deposited in the generated shock wave. This model is limited to the regime of low irradiance in which the vapor remains transparent to the laser light. Calculations are based on classical models of vaporization and lead to the evaluation of the mass of ablated material and vapor kinetic energy. Experimental investigation of the generated shock wave has been made using KrF excimer and Q- switched Nd:YAG lasers. We show that it is possible to use an opto-acoustical deflection technique of a probe laser beam to obtain shock wave energy. For the 2 classes of irradiated samples, the same profile for the dependence of shock wave energy versus laser irradiance is observed. A good correlation between calculated values and experimental data is obtained for shock wave energy and ablation rate variation with laser irradiance.
Physics of High-Power Laser-Surface Interactions
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Observation of self-channeling and modification in optical fibers using a high-intensity femtosecond laser
Sung-Hak Cho, Hiroshi Kumagai, Isao Yokota, et al.
We report the first experimental observation of laser-induced self-channeled plasma formation and bulk modification in multimode optical fibers during excitation by a high-intensity (above 1011 W/cm2) femtosecond (110fs) Ti:sapphire laser. The solid-density plasma formation at various input intensities and the corresponding variation of anti-Stokes spectra were observed simultaneously. The threshold for plasma formation in the optical fiber was found to be 8 X 1011 W/cm2. When the input intensity exceeded 1.5 X 1012 W/cm2, self-channeled plasma formation was observed with a length of 9 - 10 mm from the input end of an optical fiber. It was observed that the bulk modification, after 5 minutes of irradiation at 1.7 X 1012 W/cm2, resulted in a 5 micrometer diameter and approximately 6 mm length, in two types of multimode optical fiber with core/cladding diameters of 100/110 micrometer and 200/220 micrometer.
Applications in Coating, Coating Removal, and Material Processing
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Picosecond-laser-pulse-induced heat and mass transfer
Vadim Kostrykin, Markus Niessen, Juergen Jandeleit, et al.
The interaction of picosecond and sub-picosecond laser pulses with metals is investigated both, theoretically and experimentally. Analyzing the Boltzmann equation for electrons and phonons the hyperbolic two-temperature model of heat conduction in metals is obtained. In particular the parameter range for which the hyperbolic effects are significant is analyzed. For calculations a numerical algorithm based on the method of lines is developed. Experimentally laser pulses with a duration of 40 ps are used to remove thin metal films. The removal process is analyzed by pump and probe measurements with the time resolution of 40 ps. The single-shot removal threshold and the removal rate per pulse are determined for copper. By this technique the existence and the propagation of shock waves in the ambient atmosphere induced by the removal process are detected. Theoretical calculations are compared with experiments and the results from the literature.
Reflectance of laser-damaged spacecraft thermal control materials
Robert K. Freeman, Fred A. Rigby, Steve E. Doerr, et al.
Damage to materials by continuous wave (CW) irradiation of an infrared (IR) laser is dependent on thermal transfer, resulting from absorption of laser energy into the material. Under laser illumination conditions, the thermal radiative properties of a material are often not constant. Reflectance, for example, can vary dramatically as a result of thermal decomposition of paints and coatings. The Air Force Research Laboratory Laser Effects Branch designed and fabricated unique reflectometers for making accurate measurements of surface reflectance under conditions of continuous heating. A number of structural materials, paints, and films were tested to determine their temperature dependent reflectance characteristics in vacuum and laboratory ambient environments. The experiment series produced temperature-dependent reflectance data needed for accurate computation of laser coupling for thermal control surface degradation analyses. Previous prediction algorithms relied solely upon ambient, room temperature reflectance data to determine the thermal response of paints and films, oftentimes leading to inaccurate results. Much was learned about the breakdown of typical aerospace thermal control materials in vacuum and ambient conditions. Temperature-dependent 1.31 micrometer laser reflectance models, developed from the in-vacuo data, are presented for nine coatings (paints and films) on aluminum and titanium substrates.
Excimer laser shock hardening
Stephane Branly, Bruno Godard
An excimer XeCl MOPA has been developed by SOPRA for applications such as X-ray generation and shock hardening of metals and alloys. The chain delivers 3.3 Joules with 42 ns pulse width.
Two-step ablation for CVD SiO2 film by ArF excimer laser
Yuko Seki
In LSI restructuring systems which use lasers, via-hole formations are conducted by using laser ablation. In transparent films it is difficult to make a hole without causing damages on underlying interconnections, because laser ablation using visible wavelengths overheats the underlying metals. This paper propose two-step ablation method for transparent films, that is consist of absorption layer formation process and ablation process. Ablations of SiO2 on aluminum interconnections were achieved using this two-step ablation process, without damage appeared as thermal expansion or resistance increase.
Destruction of polytetrafluoroethylene under the action of carbon dioxide laser radiation at low pressure
Eugene M. Tolstopyatov, Leonid F. Ivanov, Peter N. Grakovich, et al.
Anomalous, in comparison to other polymers, behavior of PTFE under the action of continuous CO2-laser radiation in vacuum is analyzed. Under the above conditions the polymer is found to decompose into TFE and solid particles in the form of fibers measuring 5 ... 20 micrometer in diameter or powder with micrometer particle size. Formation of two zones of polymer degradation differing in structure and viscosity is established. The temperature of PTFE surface during degradation reaches 800 K. The bulk of the layer subjected to radiation is found to contain gaseous bubbles. The qualitative model of PTFE degradation process is discussed.
Paint removal from aluminum and composite substrate of aircraft by laser ablation using TEA CO2 lasers
Akira Tsunemi, Akira Endo, Daiji Ichishima
A high power TEA CO2 laser was applied to strip paints from the surface of aircraft. For our experimental samples, aluminum and fiber-reinforced composite substrate were painted as the completely same way as normal aircraft. As a result of delicate control of the irradiation parameters, the surfaces of not only aluminum substrate but also composite substrate were clearly exposed without any damages. Removed materials were found out to be effectively collected by a combination of a micro filter and activated carbon powder.
Physics of High-Power Laser-Surface Interactions
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Improvement of luminescent properties of thin-film phosphors by excimer laser processing
Joanna M. McKittrick, C. F. Bacalski, Gustavo A. Hirata, et al.
Thin-films of europium doped yttrium oxide, (Y1-xEux)2O3, were deposited on sapphire substrates by metallorganic chemical vapor deposition. The films, -400 nm thick, were weakly luminescent in the as-deposited condition. A KrF laser was pulsed once on the surface of the films at a fluence level between 0.9 - 2.3 J/cm2. One pulse was sufficient to melt the film, which increased the photoluminescent emission intensity. Melting of a rough surface resulted in smoothing of the surface. The highest energy pulse resulted in a decrease in luminous intensity, presumably due to material removal. Computational modeling of the laser melting and ablation process predicted that a significant fraction of the film is removed by ablation at the highest fluence levels.
Beam Propagation, Beamforming, Diagnostics, and Detection Techniques
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Time-of-flight mass spectroscopy of femtosecond laser ablation of solid surfaces
Andrea Cavalleri, Klaus Sokolowski-Tinten, Joerg Bialkowski, et al.
We have investigated femtosecond laser-induced ablation of gallium arsenide and silicon using time-of-flight mass spectroscopy. Below the ablation threshold we observe free flight desorption of atoms from the laster heated surface. The absence of collisions between particles leaving the solid allows to obtain the maximum surface temperature during laser irradiation of Gallium Arsenide. We estimated maximum surface temperatures of the order of 3500 K at the ablation threshold, where we observed a step-like increase in the number of detected particles. In the case of Silicon the existence of molecules of up to 6 atoms does not allow to measure the surface temperature. With increasing fluence free flight desorption transforms into a collisional expansion process. The behavior of Gallium particles can be quantitatively described through Knudsen-layer theory, indicating that Gallium particles expand as a non-ideal gas close to the ablation threshold ((gamma) equals Cv/Cp less than 5/3). Above fluences of approximately 2.5 Fth (gamma) approaches 5/3 indicating an ideal gas behavior for the expanding material. Dilution into the two phase regime of a superheated liquid characterizes ablation close to threshold.
Physics of High-Power Laser-Surface Interactions
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Temperature and residual stress evolution in CO2-laser-irradiated glass
Franck Guignard, Michel L. Autric, Vincent Baudinaud
CO2 laser induced damage mechanisms on Borosilicate BK7 glass and fused silica have been investigated in order to get their damage thresholds and consequently, the transparence changes in the visible range. The absorption of the radiation leads to the heating of a thin layer close to the surface which induces thermal stresses. As long as temperature remains below the glassy temperature Tg, the glass behaves as an elastic material. At higher fluence, temperature increases over Tg, the glass starts to behave as a viscous liquid and stress relaxation occurs. During cooling to ambient temperature, this relaxation process leads to residual tensile stresses and if they exceed the tensile strength, cracks may appear. Residual stresses formation strongly depends on the laser pulse shape, the absorption coefficient related to the sample and the thermo-mechanical properties of glass. A 1- Dimensional numerical model has been developed to calculate the time evolution of temperature and stresses profiles inside the irradiated samples.
Pulsed-CO2-laser-induced damage mechanisms in semiconductors
Sebastian Lefranc, Michel L. Autric
Laser irradiation induced damage to several materials of interest for use as 10.6 micrometer laser system windows and lenses is investigated in this paper. The irradiation source in these single shot experiments was a pulsed TEA CO2 laser (lambda equals 10.6 micrometer, (tau) pulse equals 3.5 microsecond, I equals 1 - 100 MW/cm2 onto the sample). A time resolved study of the damage process in semiconductors (Ge, ZnSe, ZnS) has been carried out during the interaction by measuring the variation of the transmitted and reflected intensity of a CO2 cw laser through the samples. An analysis of the pulse shape dependence on the damage parameters has been investigated. Results show that damages are initiated by the high power peak of the laser pulse 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 morphology.
Plenary Session
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AMT GDL: power-supply-independent mobile gas-dynamic laser for industrial application
Victor V. Apollonov, P. A. Drosdov, O. N. Favorsky, et al.
A brief comparison of various types of lasers of 50 - 100 kW power range for industrial use is presented, taking into account the most important technical and economic details. Listed is consumption of fuel, gas components, water, atmospheric air, also electric power required for some of lasers described. Its emphasized that the most prospective is high power laser of gas-dynamic type. It is featured by the outstanding weight-dimensions and specific characteristics. Essential advantage of the proposed gas-dynamic mobile laser is independence of stationary supply of electric power generally required for other types. Combined with independence of electric power plant, the totality of its technical properties, reliability and relatively low operation expenses makes it especially attractive solution of wide range of technological problems like worn reactors utilization, heavy- gauge metal cutting, thin oil films water pollution, etc., namely by means of autonomous mobile technological gas-dynamic laser (AMT GDL).
Lasers for High-Power Photoablation
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High-power SSD-based pulse nonchain HF(DF) laser
Victor V. Apollonov, Konstantin N. Firsov, Sergey Yu. Kazantsev, et al.
It is established that higher stability of a self-sustained volume discharge (SVD) without pre-ionization in mixtures of SF6 with hydrocarbons (deuterocarbons) is explained by higher surface density of cathode spots and the corresponding reduction of the current across a spot. It is shown that SVD may occur in mixtures of SF6 with hydrocarbons (deuterocarbons) without special systems for gas pre- ionization in extremely compact system of electrodes. Wide- aperture non-chain HF (DF) lasers with high radiation energy can be created on this basis. Such laser with the aperture approximately 20 cm provided the generation energy 190 J in the case of HF and 152 J in the case of DF and the electric efficiency 3 and 2.4 %, respectively.
Applications in Coating, Coating Removal, and Material Processing
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Surface properties modifications obtained on ceramics and metals resulting from excimer laser processing technique
The unique properties of a UV laser beam (high energy, short pulse duration) allow to transform the surface of ultrahard materials such as ceramics. In this way, a KrF excimer laser was used in this study in order to modify in selected zones, the surface of metals (aluminum alloy, titanium alloy and stainless steel) and oxides (Al2O3, ZrO2), carbide (SiC) and nitride (AlN). These ceramics possess good mechanical and thermal properties but exhibit a brittle behavior due to the granular structure. In a suitable range where the irradiated zone is melted and defects are removed, initial properties are modified (roughness, porosity, hardness, chemical composition). A cleaned and smoothed surface can be obtained without pores and cracks. These sites where corrosion attack starts are minimized and can lead to improve functions in potential industrial applications. The results presented in this work have been obtained by different analysis techniques such as scanning electron microscopy (SEM) to examine morphology, Auger spectroscopy (AES) to give chemical composition and depths profiles, mechanical tests to show roughness and hardness, grazing X-ray diffraction (XRD) to find structure.
Application of CO laser for frequency-selective surface heat treatment of polymer materials
Andrei A. Ionin, Yurii M. Klimachev, Henry Kobsa, et al.
Frequency selected Q-switched e-beam controlled-discharge CO- laser has been researched and developed for surface heat treatment of polymeric materials [poly(ethyleneterephthalate) and nylon] having strong absorption bands near wavelength of approximately 6 micrometer. The laser generates pulses (including short ones with duration (tau) 0.1 approximately 1 - 10 microsecond(s) ) having different spectral content within 4.9 - 6.5 micrometer spectral range. Different geometry and methods of irradiation were used to process samples with the laser radiation of different temporal, spectral and energy density characteristics. Different types of microstructure were formed on the surfaces of the samples. Experimental conditions corresponding to each type of microstructure were defined. Visual (macro) changes of polymeric material properties (if any) and their correlation with formed microstructures were analyzed.
Laser-induced back-ablation of aluminum thin films using picosecond laser pulses
Anthony B. Bullock, Paul Robert Bolton
A study of Laser-Induced Back-Ablation of Aluminum thin film targets with picosecond laser pulses is reported. Ablated plume edge velocities are studied as a function of film thickness, laser pulsewidth, and incident laser fluence. Edge velocity results are compared to a model of total transmitted fluence incident at the substrate/film interface. A model including laser-induced avalanche ionization and multi-photon ionization mechanisms in the substrate shows a transmitted fluence limit which coincides with observed edge velocity limits.
Laser ablation efficiency of pure metals with femtosecond, picosecond, and nanosecond pulses
Alexandre F. Semerok, Catherine Chaleard, Vincent Detalle, et al.
Laser ablation of pure metals by femtosecond, picosecond and nanosecond pulses is studied experimentally in air at atmospheric pressure. Craters created by interaction of visible and UV laser pulses with the targets are investigated. The dependence of the ablation efficiency in terms of ablated volume per unit of energy on the pulse duration and wavelength is discussed.
Beam Propagation, Beamforming, Diagnostics, and Detection Techniques
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First observation of laser-triggered lightning
Tatsuhiko Yamanaka, Shigeaki Uchida, Yoshinori Shimada, et al.
Laboratory and field experiments for laser triggered lightning were performed in order to induce so called triggered lightning, in which an electric leader initiates from tall objects on the ground to thunderclouds, using a plasma channel produced by CO2 lasers and Nd lasers. Significant milestone was achieved in technological developments and verification of a scientific feasibility of the techniques such as determination of laser trigger timing which lead to laser triggered natural lightning. In the laboratory experiments technologies to produced long plasma channels and characteristics of discharge process induced by plasma channels were extensively investigated. In the field experiments developed are an automatic laser trigger system using an RF burst emission by a preliminary breakdown, methods to measure lightning path, thundercloud movement and field strength, and to make a continuous plasma channel at the top of the lightning tower which acts effectively for leader initiation.
Laser plasma as an effective ion source
Karel Masek, Josef Krasa, Leos Laska, et al.
Ions in different charge state and with different energy distribution are generated in the process of interaction of intense laser radiation with solid targets. Multiply charged ions of medium- and high-Z elements (Al, Co, Ni, Cu, Sn, Ta, W, Pt, Au, Pb, Bi), produced by photodissociation iodine laser system PERUN ((lambda) equals 1.315 micrometer, EL approximately 40 J, (tau) approximately 500 ps) are reported. Corpuscular diagnostics based on time-of-flight method (ion collectors and a cylindrical electrostatic ion energy analyzer) as well as Thomson parabola spectrometer were used in the experiments. The ions in maximum charge state up to about 55+ and with energies of several MeV were registered at a distance of about 2 m from the plasma plume. Measured ion current densities higher than 10 mA/cm2 in about 1 m from the target demonstrate the performance of laser ion source. A theoretical interpretation of ion spectra is attempted.
Lasers for High-Power Photoablation
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Performance of the high-power PROGRESS Nd:glass laser facility
Vladimir A. Malinov, Alexander V. Charukchev, Victor N. Chernov, et al.
We present performance of PROGRESS Nd:glass laser facility which consists of a six beam PROGRESS-M phosphate Nd:glass laser, 30 TW PROGRESS-P picosecond YLF:Nd glass laser, which uses chirped pulse amplification (CPA) technique and target chamber. Laser is capable to focus simultaneously at 1,054 micrometer the energy of 1.5 kJ in 1.5 ns and power of 3.5 TW in 200 ps on the fusion target. We report performance of a single beam 0.5 kJ PROGRESS-1M laser. This laser with output rod amplifier 14 cm is the prolongation of one of the beam of the multi-beam laser. PROGRESS-P CPA laser uses YLF:Nd oscillator, single mode optical fiber, Nd:glass rod amplifiers with output diameter of 85 mm. At the output, the chirped pulse with energy about 45 J is compressed up to 1.4 ps in the single-pass compressor on two holographic gratings, which produces power of 22 TW.
Applications in Coating, Coating Removal, and Material Processing
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Ultrafast ablation with high-pulse-repetition-rate lasers: I. Theoretical considerations
Eugene G. Gamaly, Andrei V. Rode
The novel technique for deposition of a high quality thin films of different materials by the use of picosecond laser pulses delivered on a target with the repetition rate of several tens of megahertz is proposed. The differences of the proposed method from the conventional pulse laser deposition is due to shorter laser pulses (picosecond instead of nanosecond) and higher repetition rate which is tens of megahertz (MHz) instead of tens of hertz (Hz). The method allow us to significantly improve the quality of a film due to a decrease by nine orders of magnitude in a number of particles evaporating during a single laser pulse in comparison to that produced by nanosecond, 30 Hz lasers, thus removing a major disadvantage of laser deposition method which is the formation of particulates on the film. The use of a very high repetition rate laser also leads to a qualitatively new mode of vapor-substrate interaction. Due to the short time (10-7 - 10-8 sec) between the laser pulses a quasi-continuous laser plume is formed. The particles evaporated by the previous laser pulse and just deposited on the substrate do not cool down significantly by the time when the particles from the following laser pulse arrive, and therefore their chemical bonds are still reactive. As a result, the high repetition rate regime of evaporation allows for formation of structures on the substrate such as, for example, carbon nanotubes, polymer chains, etc. Another advantage is the opportunity of scanning the laser focal spot over different targets of different materials which allows for a deposition of multilayered films containing mono-atomic layers of different atoms. The results on ultrafast laser ablation and deposition are presented in two joint papers. In this paper (Part I) we present the theoretical justification of the ultrafast laser deposition method, calculating the vapor characteristics, evaporation and deposition rates for the optimal evaporation regime for different modes of laser- target interaction. In the second paper (Part II) the experimental results on evaporation of a graphite target, deposition of high quality diamond-like (DLC) films and the comparison of the laser plume characteristics to theory are presented.
Ultrafast ablation with high-pulse-rate Nd:YAG lasers: II. Experiments on deposition of diamondlike carbon films
Andrei V. Rode, Barry Luther-Davies, Eugene G. Gamaly
The novel technique of ultrafast pulsed laser deposition has been experimentally demonstrated by depositing high quality diamond-like carbon films using high repetition rate Nd:YAG lasers. A very effective evaporation regime was achieved by keeping the laser intensity on the target surface close to the optimum values determined in Part 1 of this paper. Evaporation of the target by low energy laser pulses at an intensity of 109 W/cm2 allows the elimination of particles from the vapor and results in films with very high surface quality, while the very high repetition rate increases the overall deposition rate. Results are presented on the evaporation of carbon using either a 10 kHz, 120 ns Q-switched Nd:YAG laser, or a 76 MHz 60 ps mode-locked Nd:YAG laser. The number of particles visible in optical microscope on the DLC film deposited using the mode-locked laser was less than one particle per mm2. SEM images demonstrated that the deposited film had a very fine surface texture of with nanoscale irregularities on the surface. AFM surface microroughness measurements revealed a saturation-like behavior of the RMS roughness at the level 12 nm over the whole deposited surface area for 10 kHz Q-switched laser evaporation and almost an atomic level (less than 1 nm) roughness for the 76 MHz mode-locked laser evaporation. Raman spectroscopy of the deposited films indicated that they were a mixture of sp3 and sp2 bonded amorphous carbon. The thickness of the diamond-like carbon film deposited simultaneously on two 4 inch silicon wafers varied by only plus or minus 5% over an area of approximately 250 cm2 and the deposition rate was approximately 2 - 6 angstrom/s at a distance of approximately 150 mm from the target.
Lasers for High-Power Photoablation
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Improvement of power and spatially temporal characteristics of iodine laser radiation
I. N. Voronich, V. A. Eroshenko, A. I. Zaretskii, et al.
The results of experimental investigations on a shortening degree of pulse radiation in the output amplifier of the high- power subnanosecond photodissociation laser are submitted depending on density of input energy, steep of forward front and input pulse duration. As the result of the more optimal distribution of the inversion density and input radiation intensity on a radius of a beam the homogeneous spatially- temporary structure of radiation (STSR) at the output of the facility is obtained.
UV and VUV efficient excilamps
Operating parameters of powerful excilamps with different discharge geometry pumped by glow discharges, high-pressure volume discharge with UV-preionization and barrier discharge are presented. Intense radiation of Ar2*, Kr2*, Xe2*, ArF*, KrBr*, Cl2, KrCl*, KrF*, XeI*, XeBr*, XeCl*, XeF*, I2 and IBR molecules was obtained in rare gas or in rare gas -- F2 (CH3Br, Cl2, HCl, I2, NF3) mixtures. Excilamps with high spatial uniformity of the output, narrow emission line and high gas life-time were developed. It was shown that efficiency of luminescence of exciplex molecules KrCl* of about 30% can be obtained in high- voltage glow discharge and positive column of glow discharge. Output at (lambda) approximately 222 and 308 nm up to 200 W from single excilamp and 500 W from three excilamps operating in parallel was demonstrated. It was shown that the efficiency of barrier discharge excilamps pumped by sinusoidal pulses several tens microseconds in duration can be sufficiently improved.
Beam Propagation, Beamforming, Diagnostics, and Detection Techniques
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Development of the nonlinear optical element for light beam apodization and large-aperture laser amplifier decoupling
Vitaly M. Mizin, Yuri V. Senatsky, Serge K. Sobolev, et al.
The subject of the presented work is to develop a fundamentally novel highaperture optical decoupling element with a practically nonlimited time of operation ensuring simultaneously decoupling of the amplifying cascades of high - power laser facilities and an improvement of the beam spatial structure
Applications in Coating, Coating Removal, and Material Processing
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Formation of periodic structures under pulsed UV-laser ablation of stretched polymers
Vladimir I. Emel'yanov, K. I. Eriomin
The theory of non-coherent periodic structures observed in stretched polymers after UV-laser ablation is developed. The geometries, periods, and times of formation of structures are calculated as a function of laser-light intensity, laser- induced temperature, and external or frozen in stresses. The theoretical results are in good agreement with experimental data obtained with nanosecond and subpicosecond laser pulses.