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Fundamental mechanisms of optical damage in short-wavelength high-power lasers

Description: Evidence has been accumulating for many years that the physical mechanisms responsible for damage to optical materials in and from high-power, short-wave-length lasers (SWLs) differ in fundamental ways from the thermal processes identified in infrared and visible-wavelength laser damage problems. We propose that this difference stems primarily from the electronic nature of the absorption and excitation processes which occur when SWL photons strike an optical surface, and that electrons, ions and uv photons generated in the laser excitation cycle also contribute to optical damage. In this paper, we present recent experimental results which have pinpointed specific electronic excitation mechanisms which can operate in the high-power laser environment. In many optical materials of interest for SWLs, the deposition of electronic energy creates self-trapped excitons which decay through the energetic expulsion of atoms and molecules from the surface of the material. This erosion process is accompanied by the creation of permanent electronic defects which become nucleation sites for further damage. The relationship between these microscopic mechanisms and observed macroscopic damage phenomenology is discussed, along with evidence for the existence of a surface overlayer which may point the way to radically new techniques for protecting SWL optical elements from laser damage.
Date: October 1, 1985
Creator: Haglund, R.F. Jr.; Tolk, N.H. & York, G.W.
Partner: UNT Libraries Government Documents Department

Damage resistance of coated optics

Description: Successful application of high energy lasers to national needs in strategic defense, inertial confinement fusion and isotope separation is contingent on the development of optical coatings that can withstand very intense radiation. Significant progress has been made in the development of damage resistant coatings over the past 15 years to support the needs of the inertial confinement fusion and isotope separation programs. However, the requirements evolving from the Strategic Defense Initiative (SDI) are much more severe and demand dramatic improvements in the performance of optical coatings beyond the current state of the art. The approach taken at Los Alamos towards solving the problems of optical coatings for SDI is presented.
Date: January 1, 1985
Creator: Wadt, W.R.
Partner: UNT Libraries Government Documents Department

Layered target burnthrough experiments using 50 nsec KrF laser pulses

Description: Experiments have been performed on two types of planar layered targets using the Sprite KrF laser. The targets lused were: (1) 0.25 to 3.0 microns of Al deposited on an SiO/sub 2/ substrate and (2) 0.25 to 3.0 microns of CH (parylene-N) deposited on 0.20 microns of Al on an SiO/sub 2/ substrate. The laser was characterized by a pulse length of 50 nsec FWHM, an intensity of 2 x 10/sup 10/ watts/cm/sup 2/ and a wavelength of 248.5 nm. A filtered photoiodide and a streak camera, each operating in the visible, viewed the rear of the target. We measured the time from the beginning of the laser pulse to the onset of the visible light signal as seen by the photoiodide at the rear of the initially opaque target. This time is referred to as the burnthrough time. We obtain an estimate of the mass ablation by plotting the mass ablation depth (mass density times target thickness in ..mu..gm/cm/sup 2/) versus the burnthrough time. These results are consistent with earlier mass loss measurements and with analytic and hydro-code calculations (LASNEX). The streak camera data shows emission at target positions larger than the laser focal spot, and thus are consistent with 1-D and 2-D calculations which show target surface ablation to be primarily driven by reradiated photons from the hot laser produced plasma.
Date: January 1, 1986
Creator: Kephart, J.F.; Dingus, R.S.; Gitomer, S.J.; Kopp, R.A. & Shaw, M.J.
Partner: UNT Libraries Government Documents Department

Investigation of efficiency optimization in free electron lasers

Description: The dynamics of trapped particles in the presence of the wiggler magnetic field and the electromagnetic radiation field in free electron lasers were investigated. From the results of computer simulations, it was found that, for a given set of parameters, the efficiency of energy extraction from the electron beam maximizes at a particular intensity of the electromagnetic radiation. Based on the physical argument of the dynamics of the trapped particles, a criterion which predicts accurately the optimum intensity of the electromagnetic radiation was obtained. (TFD)
Date: January 1, 1979
Creator: Kwan, T.J.T.
Partner: UNT Libraries Government Documents Department

Aplanatic waxicons

Description: Waxicon mirror components offer many advantages in designing optical systems for transporting high power laser beams. The widespread use of waxicons has been limited because of their high sensitivity to tilt errors. This paper gives the equations of the surfaces of a waxicon that is rigorously corrected for both spherical aberration and coma. Computer ray tracing has confirmed its low sensitivity to tilt errors: if the aplantic waxicon as a whole is tilted by a small angle delta, the RMS wavefront error in the output beam will be proportional to delta/sup 2/.
Date: January 1, 1979
Creator: Minerbo, G.N.
Partner: UNT Libraries Government Documents Department

Far infrared chemical lasers. Technical progress report No. 5, September 1, 1978-August 31, 1979

Description: The intention to use the optically-pumped water vapor laser to study energy absorption and transfer in this triatomic molecule was not realized. The system at this moment is not well-enough understood to be as useful as was originally anticipated. What is known is that laser action in the far infrared is the result of flash-pumping, through suprasil, water vapor in the 10-100 mtorr pressure range, in a large resonator. About ten lines at about 100-200 cm/sup -1/ have been found. Four of them very definitely can be assigned to transitions that evolve from the feeding of the level (001)6/sub 42/. The other lines are of dubious assignment or completely unassignable. All of the hundred or so lines of well-known water discharge laser have been assigned to places in the level scheme where the bend-stretch wave functions are mixed.
Date: January 1, 1979
Creator: Robinson, D. W.
Partner: UNT Libraries Government Documents Department

Calculation of the small scale self-focusing ripple gain spectrum for the CYCLOPS laser system: a status report

Description: The FLAC code (Fourier Laser Amplifier Code) was used to simulate the CYCLOPS laser system up to the third B-module and to calculate the maximum ripple gain spectrum. The model of this portion of CYCLOPS consists of 33 segments that correspond to 20 optical elements (simulation of the cell requires 2 segments and 12 external air spaces). (MHR)
Date: October 1, 1976
Creator: Fleck, J. A. Jr.; Morris, J. R. & Thompson, P. F.
Partner: UNT Libraries Government Documents Department

Progress toward free-electron lasers for applications

Description: Using an electron beam in a magnetic field as the gain medium, free-electron lasers offer a powerful new source of tunable, coherent radiation. Results are just becoming available from several new experiments, but they are confirming the theoretical promise of these devices. Backed up by a strong base of accelerator technology, free-electron lasers are expected to become important in applications requiring wavelengths from the far infrared part of the spectrum to the vacuum ultraviolet.
Date: January 1, 1984
Creator: Brau, C.A.
Partner: UNT Libraries Government Documents Department

Proposed extended tuning range for the Los Alamos mid-infrared adjustable, coherent light experiment (MIRACLE) Facility

Description: The Los Alamos Free-Electron Laser (FEL) Facility has been in operation as an oscillator in the 10..mu..m wavelength regime since 1983. Operations from 10 to 45..mu..m have been recently demonstrated which would provide a new applications capability: A Mid-Infrared Adjustable Coherent Light Experiment (MIRACLE) Facility. We propose to extend this tunability from 3 to 160..mu..m by upgrading the injector, accelerator, and resonator cavity. Potential applications in material science (high-temperature superconductors) and biophysics (DNA spectroscopy) for this wavelength regime are briefly addressed. 7 refs., 2 figs., 2 tabs.
Date: January 1, 1988
Creator: Lumpkin, A. H.; Feldman, D. W.; Warren, R. W.; Stein, W. E.; Watson, J. M.; Sollid, J. E. et al.
Partner: UNT Libraries Government Documents Department

Review of long-pulse laser development

Description: A brief review of some present techniques to obtain long-pulse laser action in excimer discharge devices will be presented. An attempt will be made to point out the strengths and weaknesses of these techniques. 18 refs., 15 figs.
Date: January 1, 1989
Creator: Sze, R.C.
Partner: UNT Libraries Government Documents Department

Integrating sphere photodetector for measurement of continuous-wave and peak laser power

Description: An integrating sphere photodetector utilizing an integrating sphere and a silicon photodiode sensor was developed for measuring both continuous-wave and peak laser power over a spectral wavelength range from 514 to 1064 nm with an estimated average power range from 100 ..mu..W to 50 W and peak power greater than 1 kW. This universal detector simplifies optical coupling, improves the peak-measurement accuracy of pulses having rise-times greater than 10 ns, and increases the accuracy and repeatability of some continuous-wave-measurements applications. Its wide acceptance angle of approximately 30 degrees provides a means of collecting all of the output from such widely diverging sources as laser diodes.
Date: August 1, 1976
Creator: Wonnell, L. D.
Partner: UNT Libraries Government Documents Department

Measurement of peak optical power from laser diodes

Description: An improved method for measuring the peak power output from pulsed-injection laser diodes having risetimes of 10 ns or greater has been developed by the Bendix Kansas City Metrology Laboratory. Based on the combination of an integrating sphere and a silicon photodiode, the system provides the means for collecting all of the widely divergent pulsed output of laser diodes. System accuracy is estimated to be within +-15%. Methods of system calibration are described along with the circuitry used to pulse and measure the diode current.
Date: November 1, 1976
Creator: Wonnell, L. D.
Partner: UNT Libraries Government Documents Department

Preliminary study of the recycling of spent gases from the iodine laser

Description: A process is proposed for recycling the spent gases from the iodine laser. Using a simplified form of this process, a preliminary estimate of the energy cost to recycle the perfluoroisopropyliodide-argon gas mixture used as the laser medium in the iodine laser was made. If r is defined as the ratio of the recycling energy to the output energy of the laser, a figure of 70 is arrived at for the most optimistic conditions and 160 for the most probable conditions. A major portion of the energy necessary for recycling is consumed in the synthesis of the perfluoroiodide compound. An estimate of the cost to replace the lost perfluoroiodide, if purchased, is from $2.4 to 10 million per year, based on a continuous operation at the 100-kJ laser power level.
Date: September 10, 1976
Creator: Pearson, R. K. (ed.)
Partner: UNT Libraries Government Documents Department

Gas laser cells: stress analysis of cylindrical cells with radial injection of electrons

Description: The problem is to develop a design procedure for pressurized cylindrical excimer laser cells. Excitation of the pressurized laser gas is by a short pulse of high energy electrons injected through the cell wall. In traversing the wall, the electrons lose energy and heat the wall. Because of rapid gas heating, a shock wave is generated which produces an additional load on the wall. Selected for analysis is a simple, conservative model of a cylindrical cell pressurized to double the shock pressure. The hoop stress is given by the product of a coefficient, which accounts for the shock over pressure, and the classical hoop stress relation. The shock over pressure coefficient is linearly related to the volumetric energy deposition with slope inversely proportional to the product of initial gas density, specific heat, and temperature. The derived equation is applied to a design study of a selected range of laser parameters. The applicability of this simple analysis is discussed. Electron energy absorption curves for several metals are also presented.
Date: February 14, 1975
Creator: Lai, W.
Partner: UNT Libraries Government Documents Department

Scalable nanosecond CO/sub 2/ laser technology. Progress report for period November 7, 1976--January 7, 1977

Description: This second bi-monthly progress report on the Scalable Nanosecond CO/sub 2/ Laser Technology Program covers work performed from November 8, 1976 to January 7, 1977. Under Task I - Saturable Absorber Studies, small signal absorption measurements over the 9 and 10 ..mu.. bands were carried out on various molecules to determine their potential as gain isolators. Saturation and relaxation experiments are being set-up to further evaluate some of these absorber media. Continued modeling and theoretical analyses of saturable absorber media under Task II - Theoretical Studies are providing the necessary inputs in the selection of various molecules. Under Task III - Flowing Saturable Absorber Device, the flow-field structure of gain and absorber regions was studied. A species concentration probe is being built to determine the amount of contamination of the gain medium by saturable absorber medium due to mass diffusion in the mixing layer. The nanosecond CO/sub 2/ oscillator is being readied under Task IV - Nanosecond CO/sub 2/ Oscillator to conduct saturation experiments of candidate saturable absorber media under both single and multi-line conditions.
Date: February 1, 1977
Partner: UNT Libraries Government Documents Department

Stable 1 --> 0 carbon monoxide laser

Description: A stable carbon monoxide laser capable of oscillation on transitions in the fundamental 1 ..-->.. 0 band is described. This laser is also capable of a quasi-continuous tunability in the range of 4.7 to 8 microns.
Date: July 1, 1978
Creator: Gerlach, R. & Amer, N.M.
Partner: UNT Libraries Government Documents Department

Accurate machining

Description: Very minor test part experience of the all-rotary machine has been obtained. Results at this point appear to verify that the machines (both in the three axis and in the phonograph configuration) are inherently more accurate than machines with slides. Much more work is needed to define the desirable operating parameters and the size capacity of the all rotary machine. A second generation three axis machine is in the design stage which abandons mounting the vertical axes from the top, and instead has three vertical axes at various pin points of link points on a complex tool arm. The part then spins on a fixed axis. This machine is conceived to produce large diameter contoured laser mirrors with great accuracy. Going to a complex set of tool arm linkages greatly reduces the weight and forces on components to be moved during machining, reducing further the very minute deflection in the machining process.
Date: January 1, 1976
Creator: Burnham, M. W.
Partner: UNT Libraries Government Documents Department

Mechanics of micromachining

Description: The current state of machining theory is examined for relevance to micromachining. Of particular interest are those features of the theory which are important to optical surface finishes and surface characteristics. The relation of transverse strain or side flow to the nature of the machining marks is one example of interest. Correlation with measurements of machining parameters and surface finishes is given.
Date: January 1, 1976
Creator: Burnham, M. W.
Partner: UNT Libraries Government Documents Department

Metal vapor excimer laser. Quarterly progress report, November 1, 1977--January 31, 1978. [Cd/Hg]

Description: During the reporting period the final construction stages of the Cd: Hg experiment was reached. The major accomplishments were in the experimental areas where the vapor supply oven was completed, the guide magnet assembly tested, and tests on the electron beam energy deposition with the Febetron 736 were started. In terms of discharge/kinetic modeling, the matter of recombination ''smoothing'' discussed in the last Quarterly was further investigated. The degree of axial electron density ''smoothing'' arising from recombination was found to depend on both the electron-ion recombination and molecular ion formation rates: the slower the molecular ion formation rate relative to the electron-ion recombination rate, the lesser the degree of smoothing. Recombination ''smoothing''can still be significant in the CD: Hg experiment, although a precise estimate is difficult to achieve since the effect depends strongly on the rates (and their temperature dependence) which are not well known.
Date: March 1, 1978
Creator: Kovacs, M.A. & Jacob, J.H.
Partner: UNT Libraries Government Documents Department

Normalization of the scattered light from an isolated defect illuminated by a Gaussian beam

Description: The intensity distribution of the beam from a laser operated in the zero order configuration for the transverse electromagnetic field (TEM/sub 00/ mode) is Gaussian in a plane perpendicular to the axis of the beam. In some applications it is desirable to have a uniform intensity over a certain region in space. For example, when a Gaussian beam is incident on a smooth surface containing small isolated defects, the light scattered by a defect will depend on the position of the defect relative to the center of the beam. In the past, several techniques have been devised to convert a Gaussian intensity profile into a uniform intensity over a specified region in space. In the present work a different approach is taken. A method of normalization is described which makes direct use of the Gaussian intensity distribution of the TEM/sub 00/ mode. By this method the amount of light scattered by a defect can be normalized to the value which would be observed if the defect were located at the center of the beam, for a defect small in size compared with the 1/e/sup 2/ diameter of the Gaussian profile. The normalization requires that three measurements be made of the scattering by the defect for an arbitrary location of the defect relative to the center of the Gaussian beam, and that the 1/e/sup 2/ radius of the Gaussian beam and the interval between adjacent illuminated spots on the surface are known. Experimental data verifying the theory were obtained from isolated defects on a superfinished spherical surface.
Date: July 1, 1980
Creator: Klingsporn, P.E.
Partner: UNT Libraries Government Documents Department

Narrow band tuning with small long pulse excimer lasers

Description: We discuss frequency narrowing and tuning with simple dispersion elements with small long-pulse excimer lasers. The improved performance over short-pulse lasers is discussed and attributed to the increased number of round trips. A physical model of the dynamics of line narrowing is presented.
Date: December 1, 1985
Creator: Sze, R.C.; Kurnit, N.; Watkins, D. & Bigio, I.
Partner: UNT Libraries Government Documents Department

Long-pulse excimer lasers for materials processing and medical applications

Description: Present day commercial excimer lasers with the exception of the Helionetics device are inherently short pulse ultraviolet sources. The pulses are short due to the inherent instability of the avalanche discharge with electronegative elements in the gas mix. The utilization of an initial fast voltage risetime allows one to overcome this difficulty and a number of long-pulse lasers are built using this technique. While the initial lasers of this form used a series rail-gap to achieve the fast voltage risetime, subsequent schemes using magnetic pulse compression and a fast prepulse have greatly improved the viability of the technique as a commercial device. Presently, however, these devices are generally rather bulky. This report discusses a passive stabilization technique that not only allows long pulse laser operation, but allows for a much slower charging circuit as well as a factor of 10 gain in pulse repetition rate per given gas flow compared with more conventional excimer laser systems. The slower charging circuit greatly eases the stress on the pulse power components such as thyratrons and capacitors and should be an important factor contributing to dependable industrial excimer laser systems. The gain of a factor of 10 in pulse repetition rate per unit gas flow results in extremely high pulse repetition rate laser systems in a very compact package; such lasers should find a place in a number of industrial and medical applications. 10 figs.
Date: January 1, 1986
Creator: Sze, R.C.
Partner: UNT Libraries Government Documents Department

Spotsize effects in laser damage testing

Description: For the past three years a program of testing and evaluation of HR coatings at excimer laser wavelengths has been in progress. During this period, an unconventional method has been employed to plot damage data: ten sites are irradiated at each test fluence and the fraction of damaging sites is plotted as a function of peak fluence. The resulting curves slope from a fluence at which no damage was produced to a higher fluence at which all ten sites damaged. Attempts to explain wide slope variations among a variety of tested samples have led to a simple model which correlates slope, defect density, and Gaussian spotsize of the test beam. Specifically, the model predicts steep slopes if the beam is large compared to the mean defect spacing. In contract to conventional notation, threshold is defined as the highest fluence at which the sample could not be damaged. Implicit in the model is that threshold thus defined is independent of spotsize while conventional thresholds have a calculable dependence. In the limit of large spotsize, the two thresholds are identical. Test results for various spotsizes are reported and analyses of previously published experimental data are presented in support of this model.
Date: January 1, 1982
Creator: Foltyn, S.R.
Partner: UNT Libraries Government Documents Department