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Ultraviolet Spectrograph Lens
Berlyn Brixner,
Max A. Winkler
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A 700-mm f/4.7 spectrograph camera lens was designed for imaging spectral lines in the 200- to 400-nm region on a 120-mm flat image field. Lens elements of fused silica and crystalline calcium fluoride have so little secondary spectrum that raytracing calculations predict a monochromatic resolution limit of 30 lines/mm without refocusing in the 238- to 365-nm region. Light scattering at the polished calcium-fluoride surfaces is avoided by sandwiching the fluoride elements between fused silica and cementing with silicone fluid. The constructed lens makes good spectrograms.
Improved Aluminum Coatings For The Ultraviolet
David F. Edwards,
Peter LaDelfe,
Ellen Ochoa
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Highly reflective aluminum coatings or aluminum coatings with dielectric overcoats are frequently used in the ultraviolet. The reflectance values published by Hass and his group are generally accepted for this uv region. We have produced evaporated aluminum coatings for a wide range of deposition conditions and none of our coatings exhibit the Hass reflectance characteristics. The reflectance of our coatings appear to be independent of the evaporation pressure and deposition time or rate. Our coatings do not have the characteristic decrease in reflectance with decreasing wavelength. Our main attention has been focused on the origin of a reflectance dip for each of our coatings near 300 nm. This dip has apparently not been reported before and does not appear to be due to adsorbed layers on the film or due to trapped impurities within the film.
Low-Level Losses In Ultraviolet Laser Window Materials
David F. Edwards,
Ellen Ochoa,
Philip Baumeister
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The low-level insertion losses have been measured for uv grade CaF2 and MgF2 using the multipass reflectometer. These losses include surface absorption and surface losses as well as bulk absorption. Comparing our data with absorption measured by calorimetric methods indicates that insertion losses are dominated by surface effects and bulk properties play only a minor role.
Laser Damage Testing Of Coated Reflectors At Excimer Laser Wavelengths
S. R. Foltyn,
B. E. Newnam
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An important parameter in the design of large-scale ultraviolet lasers - such as those envisioned for Inertial Confinement Fusion and Molecular Laser Isotope Separation - is the resistance to optical damage of windows, AR-coatings, and coated reflectors. In addressing the problem of evaluating and optimizing highly reflective dielectric stacks, we have measured the damage thresholds of a variety of 248-nm, 308-nm, and 351-nm reflectors. The coatings were composed of quarterwave stacks of oxide and/or fluoride films deposited on Suprasil 2 substrates. Testing was accomplished at 35 Hz with nominal 10-ns pulses focused to a mean 1/e2 diameter of 0.5 - 0.6 mm. Damage threshold - defined as the highest fluence at which 10/10 sites survived 1000 shots - ranged from 1 - 5 J/cm2, with a strong dependence upon laser wavelength and reflector coating materials.
Thick Optical Films For The Conduction Of Optical And Infrared Radiation
C. N. Bain,
B. A. Gordon,
T. M. Knasel,
et al.
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Thick optical films can be used to direct, channel and conduct optical and infrared radiation, thus making posible new and novel approaches to system design. In this paper, recent research where thick films have been used to concentrate light energy onto photovoltaic cells is described. Incident light is trapped within a thin flat sheet of transparent material by a diffuse reflective surface at the back of the transparent layer. Light is directed in such a way that total internal reflection takes place. Some of this captured light finds its way to photovoltaic cells attached to the back of the layer. Achievable gain depends on the layer thickness, the index of refraction of the trapping material, and the cell shape and size. Since there is no classical solution to the problem, a Monte-Carlo computer model has been used to analyze this system. The model accounts for the effects of Fresnel reflection at all interfaces, optical absorption of light in the trapping layer, and the reflectivity and departures of the diffuser from a purely Lambertian surface. Results indicate that gains of a factor of two in output power may be obtained for sparsely packed arrays of cells and 1.2 to 1.4 for densely packed arrays. The results of tests of experimental solar arrays agree with these calculations.
New Catadioptric Telescope
John L. Richter
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The Acme telescope is a compound telescope that resembles the familiar Cassegrain type except that the main mirror is spherical and the secondary is an achromatic doublet mangin mirror. Three 6-in, aperture f/15 telescope designs are described. With a cemented, all spherical surface achromangin mirror, there is a small amount of coma which can be eliminated by redesigning with an air space between the crown and flint elements of the achromangin mirror, or by cementing them with one of the concave external surfaces of achromangin figured to an hyperboloid. In the examples, the spherical aberration is nil and the chromatic residual is roughly half that of an achromatic objective of the same speed, aperture, and glass types. Readily available crown and flint glasses such as Schott BK-7 and F-2 are entirely satisfactory for the achromangin mirror. Also considered are two examples of Acme-like telescopes with paraboloidal instead of spherical main mirrors.
Interferometry Of Large Diameter Beam Expanders
Bill Mullins
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In laser beam propagation studies, wavefront aberrations introduced by the beam handling optics must be known to truly characterize the atmospheric effects on the laser beam. Hence, careful optical interferometry was performed on a laser beam expander and its asso-ciated relay optics, before it was used for propagation experiments. However, recent studies by Bennett, et al.1, have shown that interferometric measurements of mirrors with enhanced reflection coatings must be performed at the designed wavelength. Bennett's studies show that measuring these enhanced reflecting coatings off-band produces erroneous wavefront errors. An experiment was performed to see if this interferometric measurement problem existed in our system which contained one enhanced reflecting mirror.
Concentration Of A Cassegrain Solar Furnace
M. H. Cobble
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A solar furnace comprising a paraboloidal mirror for tracking the sun and a hyperboloidal reflector having one focus in common with the paraboloid is analyzed to determine the geometric concentration of the system. A numerical ray-trace analysis was carried out to study various geometrical configurations of the two reflectors. In particular, the geometric concentration is calculated for the case when the line joining the foci of the hyperboloid and the axis of revolution of the paraboloid are not coincident. Possible applications of this solar furnace are discussed.
Glancing Incidence Optics For High Power Lasers
P. B. Mumola,
D. C. Jordan
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The design of optical systems for high power lasers is ultimately constrained by the damage of optical surfaces due to the laser fluence. In CO2 laser fusion systems, the damage to copper mirrors in vacuum is attributed to melting produced by the absorbed laser energy. The Fresnel equations indicate that the use of metal mirrors at high angles of incidence (75-85 degrees from normal) results in reduced absorption of properly polarized light. This should lead to a commensurate improvement in damage resistance. Other benefits to be accrued include improved transmitted wavefront quality (for a given mirror figure error) and, of course, lower transmission losses. Application of glancing incidence concepts to advanced CO2 laser fusion systems will be described with emphasis on beam transfer and focusing mirrors. Applications to other systems at shorter wavelengths typical of the rare gas halide lasers will also be discussed.
Out-Of-Focus Intensity Distribution: Effects Of Focal Number And Aberration
George A. Kyrala
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Shock wave experiments contemplated on powerful CO2 lasers require uniform planar illuminations over spot sizes much larger than the diffraction limit focal spot size of the focusing optics. Cost as well as space considerations limit one's choice of optics to parabolic mirrors with focal lengths between 78 and 150 cm, and apertures to 35 cm. Within these constraints one either modifies the incident laser energy distribution to modify the focal spot shape, or works out of the focal plane. I analyze the latter option using a fast Fourier method to calculate the intensity distributions for two focal lengths (78 and 132 cm) and various measured optical distortions (Phase only) for the focusing optics. I found that a 78 cm element produced more uniform spots of diameter 500 pm. The 132 cm element was less sensitive to focusing/positioning error than the 78 cm element.
Mirror Figure-Of-Merit And Material Index-Of-Goodness For High Power Laser Beam Reflectors
Claude A. Klein
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The purpose of this paper is to provide guidelines for assessing the performance of actively cooled high-power/high-energy laser beam (HEL) reflectors. The first segment concerns a highly simplified situation involving steady-state irradiance-mapping distortions only; a comprehensive "mirror figure of merit" may thus be defined for characterizing the performance of HEL reflectors, in the absence of pressure-induced ripples. In the second part, a generalized mirror/heat-exchanger model is used for evaluating the potential impact of coolant-induced distortions, for identifying key properties of mirror materials, and for creating a "material index of goodness" applicable to HEL reflectors; the most promising candidates are silicon carbide and tungsten.
Measurement In Visible Light Of Wavefront Errors Produced By Dielectric-Enhanced Infrared Reflectors: Some Calculations
R. E. Knowlden
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Modern high energy laser mirrors require very high reflectances (greater than 99.9%) for operation. Simple metallic coatings cannot give a high enough reflectance, so further dielectric layers must be added to enhance the reflectance. Such mirrors may be several meters in diameter and strongly curved, making uniform coatings difficult to deposit. The reflectance is not critically dependent on the thicknesses of the layers, but a 1% thickness error for a typical 6-layer design gives a wavefront phase error of .01X. Worse, this error is a function of wavelength; the wavefronts may be very different in visible light than at the infrared design wave-length. Due to a nonlinear relation between coating thickness and the optical phase change on reflection, the wavefront in the visible may look very little like the coating thickness distribution that produces it. A technique for determining coating thickness distributions for an IR coating from interferograms taken at 2 or more visible wavelengths has been developed. The usefulness of the method has been studied by a series of calculations, and the means of obtaining good accuracy are described.
Deformable Mirror As A Phase Aberration Corrector For Fusion Beam Trains
A. B. Budgor
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At Rocketdyne, we have written an interactive 2-D diffraction beam train computer code which is used to predict the corrective capabilities of deformable mirror adaptive optics on lower order aberrations introduced by imperfections in optical components and by slight system misalignment. Operationally, this code utilizes the Fast Fourier Transform algorithm for beam propagation, specifies optical component aberrations in terms of Zernike polynomial series as synthesized from their interferograms or in terms of a multivariate distribution random phase generator, and corrects the accumulated aberration with a continuous deformable mirror whose actuators are modeled by bicubic splines. System performance is described in terms of the Strehl ratio, irradiance, and the encircled energy distribution on the fusion target. This code is being used to conduct a conceptual design and trade-off study on the merits of using deformable mirror adaptive optics for the Helios fusion laser system at the Los Alamos Scientific Laboratory, at infrared and at ultraviolet wavelengths.
Optical Analysis Of Segmented Aperture Averager
Marija S. Scholl
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High energy lasers with a non-uniform beam distribution are used to study materials properties. When it is required that the beam intensity distribution be uniform, beam averaging schemes have to be used, such as a segmented aperture averager. It consists of a set of small mirrors located on the surface described by an equation of second order. In order to determine the optical performance of the segmented aperture averager, a unit amplitude wave is assumed in the analysis. The averaging is accomplished because the beam is spatially multiplexed, resulting in the superposition of beam segments originally at different locations. Two phenomena con-tribute to the final intensity distribution: diffraction and interference. The uniform plane is located in the Fresnel zone, producing a Fresnel diffraction pattern with the intensity distribution dependent primarily on the Fresnel number. Due to the spatial superposition of beam segments, multiple beam interference pattern results because of the interference of light directed from each segment. Multiple beam interference pattern is characterized by high spikes separated by areas of low intensity with many points of zero intensity. The intensity pattern due to the segmented aperture averager is a multiplication of these two effects. The envelope of the intensity distribution is provided by the Fresnel diffraction pattern, while the spike distribution is specified by the interference parameters. The beam uniformity is achieved as an average over the area of several intensity spikes. This beam averaging procedure can only be used if a finite resolution area for intensity uniformity is required.
Design And Fabrication Of Aluminum Mirrors For A Large Aperture Precision Collimator Operating At Cryogenic Temperatures
Joseph B.C. Fuller Jr.,
Paul Forney,
Carl M. Klug
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Contemporary long wavelength infrared (LWIR) sensors, designed for space application, require testing and calibration at cryogenic temperatures for minimization of background noise. Advances in focal plane technology have pushed LWIR sensor optical performance requirements to near diffraction limit thus increasing demands for better test facilities to adequately test and characterize these sensors. It recently became necessary for Lockheed Missiles & Space Company, Inc. (LMSC) to develop a new LWIR sensor test facility. The requirements for this facility were a large unobscured aperture collimator operating with near diffraction limited performance, a mul-tiple source assembly, and two orthogonal scan mirrors (in the collimated beam) all operat-ing at 200 Kelvin in a vacuum tank. A systems engineering approach was used to ensure that the required optical performance would be obtainable at the specified operating temperature. This paper covers the development of the optical system mechanical requirements; the choice of the mirror substrate material; the detail mechanical design and tolerancing of these substrates; their fabrication and testing; and the mounting, alignment, and proposed testing of all the optical elements of the collimator.
Noncontact Alignment And Spacing Of Optics
J. A. Magner,
C. A. Martin
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The performance of individual optics and of optical systems is sensitive to surface figure, to interelement optic spacing and to the precise spatial location of figure defects. However, it is seldom practical during the manufacturing process to make detailed measurements each time one wants to test the optic. In this paper, we describe some simple, yet accurate tests which are appropriate to the constraints of a manufactur-ing environment. For testing surface figure and optical-element spacing, a test using a system of lenses is described. The problem of locating defects in a complicated piece, such as an aspheric, can be solved using an array of corner cubes. Since the intensity of the return beam from the corner cubes is much greater than that from the optic under test, there is no way to be confused by artifacts. Moreover, the accuracy of this method for surface mapping is good. Some problems with the practical implementation of these tests are also discussed.
High-Intensity Transmission Properties Of Conventional And Polymeric Index-Matching Fluids
W. Lee Smith
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Large-aperture harmonic generation is rapidly becoming a standard practice in laser-fusion research. At present, harmonic generation is commonly done with bare nonlinear crystals--that is, crystal plates not protected from atmospheric degradation, and not relieved of Fresnel loss, by anti-reflection windows and index-matching fluid. This situation has been dictated by the recently-observed, aperture-dependent nonlinear loss in harmonic generation cells using layers of conventional index-matching fluids (Cargille 5610 and FC104). While the overall cost of using bare crystals can be tolerated at the small apertures (10-15 cm) in use at present, for the 74-cm harmonic crystal arrays to be used on the Nova laser, loss-free and degradation-free performance is required. In this paper, results will be presented from a study of the 0.53-pm and 1.06-pm high-intensity transmission properties of 10 organic fluids: benzonitrile, benzene, xylene, tetrachloroethylene, acetonitrile, decahydronaphtalene, Cargille 5610, FC104, poly-chlorotrifluoroethylene (Halocarbon 56) and poly-bromotrifluoroethylene (BFC57). We present measurements of the threshold for stimulated Raman scattering (SRS) in 8 of the fluids in a standardized 8-cm, longitudinal geometry. In the two polymeric fluids, SRS was not detected at input intensities up to the superbroadening thresholds (43 and 62 GW/cm2, respectively). Suppression of SRS in these two fluids is achieved by fluorination and polymerization of the parent tetrachloroethylene molecule. Results will also be related from tests in the required transverse geometry, using 8-cm aperture, 0.532-pm pulses of up to 3 GW/cm2. The aforementioned loss mechanism in Cargille 5610 is identified to be transverse stimulated Raman scattering. Finally, we describe a successful demonstration of loss-free, damage-free performance at an aperture-intensity product of 25 GW/cm in Halocarbon 56 fluid.
Optical Quality Of Pulsed Aerodynamic Laser Windows
V. A. Kulkarny,
J. Shwartz,
R. A. Briones
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Aerodynamic windows utilize gas flows to provide pressure and gas species isolation interfaces which are transparent to laser radiation. The feasibility of using a shock tube generated pulsed flow field as a single-shot window for short, high-energy laser pulses was investigated. A primary application of such a window would be for the single pulse gas laser driven inertial confinement fusion test facility currently planned by DoE. Experiments were performed to define operating conditions which cause a minimal degradation in the quality of a laser beam transmitted through the window. In the experiments a scribed dia-phragm shock tube with glass endwalls was used to simulate the window, and a ruby laser pulse was transmitted along the tube axis after the diaphragm burst. The optical quality of the nearly one-dimensional pressure wave field and the turbulent contact interface was recorded holographically at different delay times for a variety of pressures and gas compositions in the driver and driven sections of the shock tube. Interferograms were interpreted using digital techniques to determine beam quality degredations. A beam quality of 1.33, at the ruby laser wavelength, was routinely obtained after transmission through the window flow-field. This performance is more than adequate to meet the laser fusion requirements.
Infrared Polarizer Selection
Thomas A. Leonard
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The selection of a polarizer for use in the 1-12 pm wavelength range usually involves compromises among performance specifications. The University of Dayton Research Institute has investigated many different types of infrared (IR) polarizers to optimize this selection for ellipsometric instrumentation constructed for the Air Force High-Energy Laser program. Performance specifications for polarizers are discussed with particular emphasis on IR polarizers. Types of polarizers suitable for use in the IR are reviewed along with advantages and disadvantages of each. The performance of specific polarizers is also discussed. The information presented should be sufficient for a designer to specify and select the polarizer type best suited to his application.
Reduction And Elimination Of Surface Texture On Molybdenum Optical Components
D. G. Ewing,
J. W. Bender,
R. McGillicuddy
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Long wavelength, high energy laser systems frequently utilize molybdenum mirror substrates as high flux components. The optically polished surfaces of these substrates normally possess an objectionable texture variously described as "grain boundary relief", "plateau structure", or "dual plane structure". An optical polishing procedure which greatly reduces and, in some cases, eliminates apparent traces of this texture has been identified. Qualitative documentation in the form of phase contrast micrographs are presented. These illustrate the surface textures of a random selection of molybdenum mirrors which are compared with the textures resulting from various Polishing techniques explored at the Developmental Optics Facility (D0F).
Whispering-Mode Waveguide
N. A. Kurnit
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Properties of a relatively new type of waveguide structure of potential use for confining infrared radiation to a small mode volume over long path lengths are reviewed. A single guiding surface with curvature radius p and bend radius R allows propagation of a near-grazing incidence "whispering mode" of transverse width ≈ (λρR/π)1/2 and radial width ≈ 1/2 (λ2R)1/3. For sufficiently large p, the loss per revolution for TE-mode propagation is
π AN, where AN is the normal-incidence reflection loss. Results on a number of prototype structures in general agreement with these considerations are described.
Optical Tooling For Antares
W. Bauke,
A. C. Saxman,
N. O'Kay
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The Antares laser system is a large (40 kJ) CO2 pulse laser system. High energy pulses are transmitted between buildings over path lengths exceeding 90 m. The optical elements are contained within large steel assemblies (power amplifiers, turning chambers, and target chamber) which must be positioned with tolerances of 0.75 mm. The subassblies of optical components must be prepositioned to a precision of 0.25 mm. This precision can easily be obtained by first order surveying techniques and instrumentation.' Although this accuracy is routinely achieved in geodetic network controls and high-precision engineering projects, the Antares optical tooling techniques had to be tailored to the geometry of the system. The basic theoretical optical train centerlines were established throughout the facility. These theoretical references had to be transferred onto solid reference surfaces, often around many physical obstacles. This paper describes the use of a combination of traditional surveying techniques and modern optical tooling methods throughout the integration of building reference planes and the erection of major steel assemblies. The design and measured assembly tolerances are compared.
Small Signal Gain Spectrum Of An 1800 Torr CO2 Amplifier
J. C. Goldstein,
R. F. Haglund Jr.,
J. C. Comly Jr.
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Prominent hot-band effects have been observed in the 9.4 and 10.6 pm gain spectrum of an 1800 Torr electron-beam-controlled-discharge CO2 laser amplifier. The data are in good agreement with theoretical calculations at 53 different frequencies.
In Situ Optical Breakdown-Studies For A Large CO2 Laser Amplifier
C. J. Elliott,
R. G. Bjurstrom,
J. P. Carpenter,
et al.
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We study optical breakdown from target reflected optical pulses in the Gemini laser system. We measure the retropulse fluence (or illuminance) leaving the amplifier in terms of the energy entering the breakdown region and find qualitative agreement but no quantitative agreement with theory. Particulates were observed on nucleopore filters through which gas samples were drawn.
Antares Alignment Gimbal Positioner
R. D. Day,
V. K. Viswanathan,
A. C. Saxman,
et al.
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Antares is a 24-beam 40-TW carbon-dioxide (CO2) laser fusion system currently under construction at the Los Alamos National Laboratory. The Antares alignment gimbal positioner (AGP) is an optomechanical instrument that will be used for target alignment and alignment of the 24 laser beams, as well as beam quality assessments. The AGP will be capable of providing pointing, focusing, and wavefront optical path difference, as well as aberration information at both helium-neon (He-Ne) and CO2 wavelengths. It is designed to allow the laser beams to be aligned to any position within a 1-cm cube to a tolerance of 10 μm.
Design And Construction Of A Short Pulse Front End For A Large CO2 Laser
C. W. Woods,
R. F. Alexander,
C. R. Collins
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A CO2 laser oscillator, switch out, and preamplifier system which produces 1 ns pulses of 10.6 μm light with an energy of about 1 joule was designed and constructed. Commercial CO2 TEA devices were used throughout. Alignment was simplified by using a single He-Ne laser and penta prism reflectors on kinematic mounts. Performance of this system as compared with calculations with the Laser Optical Train Simulation (LOTS) program will be discussed.
Temporal Shaping Of Nanosecond CO2 Laser Pulses In Multiphoton Saturable Absorbers
R. F. Haglund Jr.
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We report experimental observations of temporal pulse-shape distortion in 1.7 ns 10.6-μm laser pulses passing through the multiphoton saturable absorber SF6 and an SF6-based fluorocarbon mixture. The observed effects include a lengthened risetime, delays in peak onset times, and a decrease in the pulse falltime which tends to symmetrize an input triangular pulse. The pulse-shaping effects are seen over a wide range of wavelengths, fluences and gas pressures. Qualitatively similar effects are predicted by a two-level saturable absorber model in an appropriate parameter space.
Passive Retropulse Protection Through Enhanced Optical Breakdown
Jerald V. Parker
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The potential for serious component damage in a laser system due to reflected pulses increases rapidly with output energy. Optical materials for interstage isolators are not available for large 10.6 micron laser sytems. We show that an excellent passive protection technique can be realized using optically induced gas breakdown in the high pressure laser medium. The reflected energy flux is reduced below the component damage threshold by using spatial overlap to increase the strength of the breakdown.
Stimulated Ion-Compton Modulation Of A Laser Beam
Carlos Montes,
Jean Peyraud
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Interaction of strong pulse laser radiation with underdense plasma may show strong reflectivity sometimes modulated with time. Stimulated ion-Compton or Brillouin (in the strong ion damping limit) backscattering explains the red-shifted backscattered spectrum observed. We show that the general nonlinear and spectral dependent problem may also explain the satu-ration of the reflectivity at finite values by pump depletion, as well as time modulation. Space, time and frequency evolutions of the incident and backscattered radiation spectra are governed by nonlinear integrodifferential equations. Numerical computation for a finite homogeneous or inhomogeneous plasma slab are performed and compared to some analytical approaches.
Modeling The CF4 Laser
C. W. Patterson,
R. S. McDowell,
B. J. Krohn,
et al.
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The CO2-pumped CF4 laser is a potentially useful source of line-tunable infrared radiation in the region 605-655 cm 1, and the spectroscopy of CF4 has been carried to the point that the laser frequencies that will result from any given pump line can be calculated to better than 0.01 cm 1. We now report quantitative intensity and line-broadening studies on CF4 and their application to modeling the laser gain. First, absorption measurements on isolated lines in the ν2 + ν4 pump band at a series of pressures yield an effective transition dipole moment for this band of 0.010 Debye. At the same time the transition moment for the (ν2 + ν4) - ν2 laser band has been calculated and agrees well with the results of laser self-absorption measurements. Finally, linewidths determined as a function of pressure yield a pressure-broadening coefficient of ca. 10 MHz/torr, significantly greater than that expected from a hard-sphere gas-kinetic model. From these data the gain of the CF4 laser can be calculated at various pressures and temperatures; the results are in reasonable agreement with measured values.
Kinetics Of A Continuous Wave (CW) Infrared Laser Induced Reaction
R. N. Zitter,
D. F. Koster,
A. Cantoni,
et al.
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Kinetics of the unimolecular decomposition of CH3CF2Cl induced by a CW CO2 laser has been investigated at pressures of 10 to 200 torr and beam intensities of 150 to 1000 W/cm2. Laser induced rate constants k and translational temperatures T have been obtained through several recently developed procedures which include interferometric measurements of T. While in the high pressure limit the laser induced rate constant k is found to be nearly identical to the thermal rate constant kt, we observe that k becomes progressively larger than kt at any T with decreasing pressure, reaching the value of 103 kt at 10 torr. We conclude that the effective vibrational temperature Tv exceeds T as a result of the competition between the rates of photon absorption and deactivation by vibrational-to-translational relaxation. The experiment also reveals some anomalous features of VT transfer at the lower pressures.
Self-Focusing In SF6
J. R. Ackerhalt,
H. W. Galbraith
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We show by explicit calculation of our previously published quasicontinuum model that the molecular susceptibility rapidly approaches zero as higher excited states of the molecule become populated. Hence the overtones of the ν3-pumped mode are totally responsible for the self tocusing effects in SF6. We explicitly calculate the ν3 ladder contribution to the susceptibility. Our vibrational model is a classical triply degenerate anharmonic oscillator in the Cartesian basis with the anharmonicity parameters chosen to be consistent with the latest spectroscopic analysis of the 3ν3 overtone spectrum. The rotational struc-ture is represented by a distribution of these oscillators where the distribution is chosen to correspond to the spectrum of the ν3 fundamental. We find good agreement with the 300K self-focusing data of Nowak and Ham at CO2 P(28), P(20) and P(10) in SF6.
Pressure And Power Dependence In The Infrared Laser Photolysis Of 1 -Chloro-1 -Fluoroethylene: Experiment And Modeling Calculations
N. S. Nogar,
W. A. Jalenak
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Dissociation of the title compound following infrared multiphoton excitation was monitored via infrared fluorescence of the products, either HC1* or HF*. Total dissociation yield and branching ratio were monitored as functions of, respectively, pressure and photo-lytic intensity. A rate equations model of the excitation and dissociation process is shown to be consistent with both sets of data.
Optical Fiber Probe Using Tunable Laser Absorption Spectroscopy For Combustion Measurements
George Kychakoff,
Ronald K. Hanson
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A tunable laser absorption probe for remote, spatially-defined measurements of species concentration has been developed for application in combustion research. In order to demonstrate the performance of a first generation absorption probe, the non-uniform distribution of Na atoms in a laboratory flat flame has been measured.
Laser-Induced Breakdown Spectroscopy: A Technique For Atomic Detection And Molecular Identification
T. R. Loree,
L. J. Radziemski
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We have been examining the emission spectra from laser-induced breakdown plasmas as a technique for atomic identification. The concept is straightforward and the apparatus simple (see Fig. 1); the plasma is a bright source requiring little subtlety to resolve. By adding time resolution to the detector, one moves from LIBS (Laser-Induced Breakdown Spectroscopy) to TRELIBS (Time-Resolved LIBS) and for most cases gains considerable sensitivity. Our applications have mostly been aimed at the detection of trace constituents. The only previous use of this technique of which we are aware was a measurement of the fuel/air ratios in combustion.
Application Of Dual-Beam Second-Derivative Tunable Diode Laser Infrared Spectroscopy To Trace Gas Measurement At Atmospheric Pressure
R. G. Jungst,
D. R. Tallant
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A dual beam diode laser spectrometer has been constructed using off-axis reflective optics. The spectrometer can be amplitude modulated for direct absorption measurements or frequency modulated to obtain derivative spectra. The spectrometer has high throughput, is easy to operate and align, provides good dual beam compensation, and has no evidence of the interference effects that have been observed in diode laser spectrometers using refractive optics. Unpurged, using second derivative techniques, the instrument has measured 108 parts-per-million CO (10 cm absorption cell, atmospheric pressure-broadened) with good signal/noise. With the replacement of marginal instrumental components, the signal/noise should be substantially increased. This instrument was developed to monitor the evolution of decomposition gases in sealed containers of small volume at atmospheric pressure.
Ultraviolet Phase Conjugation And Its Practical Implications
B. J. Feldman,
Robert A. Fisher,
S. L. Shapiro
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We report the first demonstration of uv phase conjugation. Using a 15 psec, 2660 Å pulse, 0.1% conjugate reflectivities were obtained via degenerate four-wave mixing in 1-mm samples of CS2 mixtures. While pure CS2 did not exhibit the effect, dilution in several uv transmitting solvents opened up a concentration-tunable (2450 Å - 2850 Å) spectral window, allowing the optical Kerr effect to be utilized. Weaker phase conjugation at 2660 Å was also observed in other Kerr media and in saturable absorber media.
Fizeau Wavemeter
James J . Snyder
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The Fizeau Wavemeter is a real-time laser-wavelength measuring instrument intended for use with either pulsed or cw lasers. The instrument contains a static Fizeau interferometer which is illuniinated by the laser. The fringe pattern of the interferometer is sampled by a 1024 element photodiode array and analyzed by a small computer to determine the wavelength of the illuminating laser. An earlier version of the instrumentl demonstrated a resolution of 10 -7 (about 50 MHz in the visible) at a read-out rate of 15 Hz, but suffered from systematic drifts in the calibration. Recent modifications in the software 2 and in the optical system have virtually eliminated. sensitivity to fringe amplitude non-uniformity and to wavefront curvature. Temperature sensitivity has been reduced to a level commensurate with the coefficient of thermal expansion of the interferometer spacer.
Laser-Induced Bioluminescence
G. Daniel Hickman,
Richard V. Lynch, III
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A project has been initiated to determine the feasibility of developing a "complete airborne remote sensing system" for rapidly mapping high concentration patches of bio-luminescent organisms in the world's oceans. Conceptually, this system would be composed of a laser illuminator to induce bioluminescence and a low light level image intensifier for detection of light. Our initial laboratory measurements consisted of using a 2 Joule flash lamp pulsed optical dye laser to excite bioluminescence in the marine dinoflagellate Pyrocystis lunula at ambient temperature using Rhodamine 6G as the lasing dye (585 nm) and a laser pulse width of 1 μ sec. After a latency period of 15-20 msec, the bioluminescence maximum occurred in the blue (480 nm is the wavelength maximum for most dinoflagellate bioluminescence) with the peaking occurring approximately 65 msecs after the laser pulse. Planned experiments will investigate the effect of different excitation wavelengths and energies at various temperatures and salinities of the cultures.
Eight-Chord CO[sub]2[/sub] Interferometer For Plasma Density Measurements On ZT-40
A. R. Jacobson,
L. J. Jolin
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We describe a CO2 laser interferometer which measures the path-integrated density along eight different chords simultaneously in the ZT-40 reversed-field pinch, a toroidal magnetic confinement experiment at Los Alamos. The interferometer system combines several reliable, commercially available components in a package which provides exceptional measurement resolution as well as ease of operation and maintenance.
Far-Infrared Faraday Rotation Measurement Of ZT-40 Poloidal Magnetic Field
M. D. Bausman,
R. W. Carpenter,
K. A. Klare
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An experiment is in progress to measure the poloidal magnetic in ZT-40 (a toroidal reversed-field z-pinch) using far-infrared Faraday rotation at the CH2F2 184.6 micron wavelength.
Multiple-Point Thomson Scattering Measurement For The ZT-40 Reversed Field Pinch
K. B. Freese,
R. S. Massey,
Rita Gribble,
et al.
Show abstract
A multiple-point Thomson scattering device, capable of giving radial electron temperature profiles, has been developed for the ZT-40 reversed field pinch experiment. The measurement is accomplished by observing the Thomson scattered spectrum from along an entire chord of focused ruby-laser light traversing the minor diameter of the plasma. A specially built, nearly diffraction limited laser is focused through the plasma using a long focal length lens. The detector uses specially designed collection optics to focus the entire scattering volume into an image dissecting array of fiber optics, which serve as the entrance slit to an anastigmatic spectrograph. The dispersed spectra from all the spatial locations are detected with a gated microchannel plate image intensifier, and recorded on an optical multichannel analyzer using a SIT tube detector. A computer interface is used for data transmission for analysis. Results from the ZT-40 experiment will be presented.
Use Of A Free-Expansion Jet In Ultrahigh Resolution Raman Spectroscopy
James J. Valentini,
Peter Esherick,
Adelbert Owyoung
Show abstract
Rotationally-resolved, ultra-high-resolution stimulated Raman spectra of CH4, CD4, and SF6 have been obtained in cw and pulsed free-expansion jets, using a simple and inexpensive nozzle-and-vacuum-chamber apparatus. Through variation of the nozzle orifice diameter, nozzle backing pressure, and axial location of the spectroscopic probe region it is possible to nearly independently control the spectral Doppler width, rotational temperature, and molecular density in the jet. Using relatively mild expansion conditions we have been able to cool a pure methane jet to a rotational temperature of 13K. We show here how this con-trol of temperature, independent of density, allows separation of otherwise unresolvable spectral features and unambiguous identification of band-heads, hot-bands, and other spec-tral features.
Stimulated Raman Scattering And Four-Wave Mixing: Steady-State Plane-Wave Analytic Solutions
Jay R. Ackerhalt
Show abstract
The equations for plane-wave steady-state propagation of n fields interacting via four wave processes in a Raman medium have been known since at least 1962. Complete analytic solutions have only been found for the process of harmonic generation. However, solutions have been obtained in the gain regime assuming zero pump depletion. Assuming all fields satisfy the phase matching condition we have found general analytic solutions to these equations. The solutions are complete in that they describe both pump deletion and saturation. Some example problems will be studied.
Use Of Matrix Methods In The Calculation Of The Normal Modes Of Molecular Vibrations And Refinement Of Molecular Force Fields
Vasudevan Lakshminarayanan
Show abstract
In this article the various matrices and matrix operations used in the analysis of vibrational spectra of polyatomic molecules are described. This is applied to the quantitative refinement of molecular force fields for Wilson G-F calculations and criteria are developed which must be satisfied by an acceptable force field. This method permits an unambiguous choice between different force fields which predict the observed frequencies but give different potential energy distributions amongst the various internal coordinates.
Sub-Doppler Optoacoustic Spectroscopy
M. I. Buchwald,
Michael S. Sorem
Show abstract
Sub-Doppler resolution at 10 μm in SF6 has been achieved using the optoacoustic effect. A single-frequency cw CO2 laser was used and audio frequency intermodulation was measured using lock-in detection. The 10P16 line was used to resolve a portion of the SF6 ν3 Q-branch.
Absorption Spectra Of Agl At Pressures To 136 kbar
D. H. Liebenberg,
J. Hudson
Show abstract
Spectral absorption measurements in Agl are reported at pressures up to 136 kbar using a diamond anvil cell. In the NaC1 phase between 5 and 70 kbar the absorption edge shift is found to be nearly linear with pressure. Broadening of the low energy tail of the absorption increases with pressure. No indication of a sudden jump into a CsCl phase is found near 100 kbar and the possible influence of larger pressure gradients in earlier measurements is discussed.
Solid-State Phase Transitions In CuCl Under Hydrostatic Pressures To 12.8 GPa
D. H. Liebenberg,
R. L. Mills,
C. Y. Huang,
et al.
Show abstract
We examine the phase transitions in solid CuCl under hydrostatic conditions at pressures to 12.8 GPa. The transition at 4.4 GPa from zinc-blende to tetragonal is observed. Our negative observations for the upper transition at 8.2 GPa and for the formation of an opaque phase due to the disproportionation reaction support the contention that pressure gradients are important in affecting the behavior of pure CuCl.
Signal Enhancement By Spectral Equalization Of High-Frequency Broadband Signals Transmitted Through Optical Fibers
J. W. Ogle,
P. B. Lyons,
M. A. Holzman
Show abstract
A new technique is discussed for signal strength enhancement of high-frequency (>1 GHz), spectrally-broad, analog signals through 1 km of optical fiber. The conventional method for optical measurement of high-frequency gamma ray signals is to use an optical fiber placed in the gamma radiation beam as a Cerenkov transducer. The radiation-induced optical signal is degraded by material dispersion in transmission through the fiber. To reconstruct the signal (which is carried by each wavelength) and preserve the high-frequency components, a narrow-band filter selects an appropriate bandwidth (1 nm). The filtered signal is detected using a high-speed, microchannel-plate (MCP) photomultiplier detector and recorded on high-speed oscilloscopes. This new technique replaces the narrow-band filter with a wavelength multiplexer crevice. The broadband signal input is dispersed by a grating, and narrow spectral components are collected into an array of fibers used as optical delay lines to compensate for the material dispersion in the kilometer of fiber. The delay fibers, each cut to an appropriate length, couple the spectrally equalized light onto the detector. This technique improves the frequency response by eliminating the wing contributions of narrow-band filters, and increases the signal magnitude by utilizing more of the available broadband spectrum. The new technique is applicable to any optical system wherein material dispersion limits system band-width.
New Liquid Scintillators For Fiber Optic Applications
S. S. Lutz,
L. A. Franks,
J. M. Flournoy,
et al.
Show abstract
New long-wavelength-emitting, high-speed, liquid scintillators have been developed and tailored specifically for plasma diagnostic experiments employing fiber optics. These scintillators offer significant advantages over commercially available plastic scintillators in terms of sensitivity and bandwidth. FWHM response times as fast as 350 ps have been measured. Emission spectra, time response data, and relative sensitivity information are presented.
Modes Of Unstable Ring Resonator With Self-Imaging Aperture
Carl M. Wiggins
Show abstract
In this paper we compare the converged solutions of infinite Fresnel number, negative branch unstable ring resonators of circular mirrors and apertures to the low-order symmetric and nonsymmetric geometric eigenmodes that one would expect in the limit where diffraction effects can be ignored. We use the fact that the equivalent collimated round-trip propagation length of a negative branch ring resonator can be zero under conditions that allow an aperture of the optical system to be imaged onto itself, i.e., to be self-imaged. Using a numerical resonator model, we show that for certain size spatial filters placed in the focal plane, the dominant ℓ = 0 and ℓ = 1 modes are given by the lowest order symmetric and nonsymmetric geometric modes, thus verifying the self-imaging aperture concept for resonators employing round optics. (Here, ℓ is the azimuthal mode index.) We present detailed results which show that a spatial filter may be used to stabilize modes over a wide range of eigenvalues in addition to those for which it is self-imaging.
Analytic Software For The Testing Of Annular Resonator Concepts
Michael F. Abernathy,
Salvatore J. Cusamano
Show abstract
A software system used for comprehensive control of annular laser-resonator alignment testing is described. The system permits soft-ware control of alignment mirrors, acquisition of raw experimental data, two- and three-dimensional curve-fitting of experimental data to a user-defined model, and two- and three-dimensional computer graphic display of raw data and model output. Hardware techniques used in conducting these experiments are discussed and examples of computer analytic output are presented.
Optical System Design And Performance Of The Lockheed Sensor Test Facility (LSTF)
J. B.C. Fuller Jr.,
J. R. Grammer
Show abstract
The advancing state-of-the-art for long-wavelength infrared (LWIR) sensors has created a need for increasingly sophisticated low-background sensor test facilities. A new low-temperature, low-background, optical target collimator system has been designed and built to meet these new sensor requirements. The design incorporates a Ritchey-Chretien off-axis optical system with an effective focal length of 500 in. and an aperture of 25 in. This optical system provides excellent high-resolution, diffraction-limited performance for wavelengths of 0.76 μm and longer, both on-axis and off-axis. It has a useful field of view in excess of ±0.5 deg. Two scan mirrors in the collimated beam provide a beam steering capability of 6 deg.
Optical Method For Determining The State-Of-Charge Of A Lead-Acid Battery
C. R. Derouin,
R. E. Bobbett,
J. B. McCormick,
et al.
Show abstract
An optic device utilizes the index of refraction and critical angle to determine the state of charge of a lead-acid battery.
Effects Of Propagation, Transverse Mode Coupling, Diffraction, And Fluctuations On Superfluorescence Evolution
Farres P. Mattar
Show abstract
Using proven computational methods developed to efficiently treat transverse and longitudinal dynamic reshaping associated with single-stream propagation effects in cooperative light-matter interactions, a realistic superfluorescence (SF) theory was constructed in close collaboration with experimentalists. A semi-classical model based on the Maxwell-Bloch equa-tions (which rigorously encompasses diffraction, transverse density variations and inhomoge-neous broadening) is used. Furthermore, the medium initiation is stimulated by a coherent pulse of an area θ0 which varies radially, propagates along the rod axis and tips the indi-vidual Bloch vectors over an angle θ0 from its upright position. This effective initiation is treated in using either (a) an homogeneous average tipping angle or (b) instantaneous longitudinal and transverse fluctuations. The Cs datas are correctly simulated for the first time.* Important remark At this time, I wish to express my appreciation and give credit to Gibbs, McCall and Feld for their many contributions in the form of numerous relevant discussions, preparatory ana-lytical work and help in selecting details of realistic models based on their close contact with laboratory results. In addition, Dr Gibbs' participation in carrying the calculations accelerated the rate of progress in my research. Let me take this occasion to thank Dr. Gibbs, Dr. McCall and Dr. Feld for their energetic and enthousiastic collaboration.
Transverse Effects In Swept-Gain Superradiance: Evolution From The Superradiant State
C. M. Bowden,
F. P. Mattar
Show abstract
Results of numerical calculations are presented and analyzed for pulse generation and subsequent stabilization in large propagation distance z , for a collection of two-level absorbers which are swept-excited by an impulse inversion along the z-direction at the speed of light in the medium. The calculation is performed using the coupled Maxwell-Bloch formalism and for the conditions that T2 = T1 , T2 > ιc, g/κ > > 1, where T2 is macroscopic dipole moment dephasing time, T1 is the longitudinal relaxation time for the absorber, ιc is the characteristic superradiant cooperation time among the absorbers and g/κ is the linear gain, g , to diffraction loss, κ, ratio. Results of the calculation for nonlinear pulse evolution and propagation for one spacial dimension (planar case) is compared with the results for the comparable case where transverse mode coupling is included.
Frequency Response Of The Thermal Lens
N. J. Dovichi,
J. M. Harris
Show abstract
In order to maximize the analytical performance of thermal lens calorimetry, it is useful to know the system frequency response which is formed from the product of the Fourier transform of the thermal lens impulse response with the frequency components of the excitation waveform. This result is used to describe the variation in the small signal response of double beam thermal lens calorimetry with chopping frequency. This information allows the evaluation of the trade-off between signal averaging rate and frequency roll-off to determine an optimal chopping frequency.
Ozone Air Broadening Coefficients In The 9 Am Region
J. M. HoeII,
C. N. Harward,
C. H. Bair,
et al.
Show abstract
Air broadening coefficients and line strengths are reported for selected ozone lines in the 8.8 to 10μm spectral region. These parameters were obtained from a least squares fit of the Voigt line shape to experimental ozone absorption spectra obtained with a tunable diode laser. Analysis of 11 lines in the νl and 18 lines in the ν3 band suggest a transition dependent coefficient although no definitive relationship was identified., The average air broadening coefficient for the νl and ν3 ozone lines is 0.077 and 0.083cm-1/atm, respectively. The measured line strengths are typically 10% to a factor of 2 larger than those given in the AFGL compilation.
Meteorological Lidar Developments
Thomas D. Wilkerson,
Geary K. Schwemmer
Show abstract
We summarize a research program on remote optical measurements of humidity and temperature and give new results of such measurements as well as new data on the near IR absorption spectrum of H2O. The basic atmospheric technique is differential absorption lidar (DIAL) using narrow band, tunable dye lasers. Approximate wavelengths are 720,820, 940 nm (H2O) and 690, 760 nm (02; temperature). The requisite absorption line strengths and widths are measured in a controlled environment by means of grating spectroscopy and tunable lasers. Prospects for routine lidar meteorology, even from airplane and satellite platforms, appear excellent in several important applications, the conditions and the optical properties of the atmosphere can be obtained remotely by these means.
High-Precision Multipass Reflectometer
David F. Edwards,
Philip Baumeister
Show abstract
The multipass reflectometer has been shown to be a convenient and precise instrument for the measurement of spectral reflectances in excess of 0.99. This report gives a brief sketch of the initial setup of the reflectometer, its operation, optimization of para-meters, and some limitations to the expected precision. The instrumental precision is set by the uncertainty in the computer fit of a straight line to the measured data. Systematic errors due to nonuniform photosurfaces and the effects of astigmatism have been minimized. We have used this reflectrometer to measure the absolute reflectance of evaporated aluminum films in the uv and visible regions. It has also been used to measure the low-level insertion losses of laser window materials for this same spectral region.
Scanning Microellipsometer For The Spatial Characterization Of Thin Films
D. J. Dunlavy,
R. B . Hammond,
R. K. Ahrenkiel
Show abstract
A polarization-modulated ellipsometer was constructed to investigate the optical properties of surfaces and transparent thin films. In the latter case, the measurement gives a unique determination of the index of refraction n and film thickness t. Using a HeNe laser light source, the beam was focused to a spot size of 50 μm. By stepping the sample across the focal point of the laser beam in both x and y directions, the spatial uniformity could be measured. This apparatus was particularly useful for optical profiling laser-annealed oxide films grown on GaAs. A new technique for laser annealing native oxides on GaAs produced the need for observing spatial structure with spatial resolution of less than 100 μm. Here the laser pulse produced a "crater" in the oxide due to localized healing and subsequent densification of the film (Figs. 1 & 2). This technique allows profiling of film index of refraction and thickness across the laser irradiated area--about 2 to 3 mm in our case. A number of applications in microelectronics are suggested.
Design And Performance Of A New Emission-Line Coronagraph
R. N. Smartt,
R. B. Dunn,
R. R. Fisher
Show abstract
A new emission-line solar coronagraph is described that photographically records coro-nal emission of the Fe XIV (5303 Å) and Fe X (6374 Å) lines, and prominences and the solar disk in Hα (6563 Å). The basic optical system consists of a 20-cm aperture, f/11 aspheric singlet objective and four secondary optical systems--one for each image channel--that are sequentially switched into the beam. Interference filters and a specially-designed Lyot birefringent filter isolate spectral bands ≈ 1 Å at the three wavelengths. The efficiency of this filtering and special constructional features result in an exceptionally small amount of scattered light reaching the film plane. Coronal images can be recorded in skies some 2.5 times brighter than is typical for earlier designs of emission-line corona-graphs. Features of the data obtained so far, including high contrast and high spatial resolution images and the detection of extremely rapid coronal changes, are discussed.
High-Speed Photodetector For Plasma Diagnostics
P. B. Lyons,
L. D. Looney,
J. Ogle,
et al.
Show abstract
Recent progress in four different classes of high-speed photodetectors are reviewed with emphasis on parameters of importance in plasma diagnostics. Two types of biplanar vacuum photodiodes are compared, the ITT F4014 and the Hamamatsu 1328. A conventional photo-multiplier, the nine-stage Hamamatsu R928, has been modified to improve its bandwidth. Microchannel plate photomultipliers from two vendors have been studied. Data for the ITT F4126X, ITT F4141, Varian VPM-173, VPM-221, and VPM-225 are summarized. A hybrid photomultiplier using a PIN diode for a gain element has also been investigated. The various tubes and types provide very different limitations on system performance.
Design And Fabrication Of A High-Damage Threshold Infrared Smartt Interferometer
R. B. Hammond,
A. J. Gibbs
Show abstract
It has been shown that a Smartt interferometer may be used as a very precise alignment tool for infrared lasers.(1) This interferometer may also be used effectively to investi-gate the phase front of a laser pulse. To use this tool for applications to high-power, short-pulse laser systems such as Helios and Antares, however, it has been necessary to fabricate a structure with the unique optical characteristics of the Smartt interferometer com-bined with a very high optical-damage threshold. We have been successful in this effort by utilizing the high technology, process control, and unique properties of semiconductor-grade, single-crystal Si.
Optical Analysis And Alignment Applications Using The Infrared Smartt Interferometer
V. K. Viswanathan,
P. D. Bolen,
I. Liberman,
et al.
Show abstract
The possiblility of using the infrared Smartt interferometer for optical analysis and alignment of infrared laser systems has been discussed previously. In this paper, optical analysis of the Gigawatt Test Facility at Los Alamos, as well as a deformable mirror manufactured by Rocketdyne, are discussed as examples of the technique. The possibility of optically characterizing, as well as aligning, pulsed high energy laser systems like Helios and Antares is discussed in some detail.
Large Area, High Speed Detectors For The Ultraviolet
S. C. Stotlar,
E. J. McLellan,
C. R. Jones
Show abstract
Although there are several types of detectors useful in the ultraviolet (uv), few have subnanosecond response time with large detector area. Krypton fluoride and other pulsed uv lasers have pulse capabilities ranging from less than 100 ps to 30 ns. For these lasers, the energy/pulse can range from nanojoules (in 100 ps) to joules (in 10 ns). Strontium barium niobate (SBN) pyroelectric detectors have been developed for infrared (ir) usage under similar pulse conditions. This material is absorptive below 400 nm. Design of appropriate detectors for the uv requires (primarily) characterization of the uv and vacuum ul-traviolet (vuv) pulse damage threshold, and examination of the surface preparation tech-niques. Large-area detectors are desirable because uv radiation can be difficult to focus and uv beams can be nonuniform. Los Alamos has developed sub-one-hundred-ps detectors up to 0.1 cm2 and subnanosecond detectors 1 cm2 in the area for the ir. These designs are being adapted to the uv. Results of this program are reported.
Nanosecond Gating Properties Of Proximity-Focused Microchannel-Plate Image Intensifiers
N. S.P. King,
G. J. Yates,
S. A. Jaramillo,
et al.
Show abstract
Some fundamental properties of 18-mm-diam gated proximity-focussed microchannel-plate (MCP) image intensifiers used as fast image shutters in the 1 to 10 ns range have been identified and studied. Light pulses (≈ 5-ps-wide) from a modelocked dye laser optically sample the gated MCP. Shuttering is achieved by applying a forward-biasing electrical gate pulse to the quiescently reverse-biased photocathode-MCP interface. Variable delay (≈ 30-ps jitter) between the gate pulse and the laser pulse permit tracing the MCP's optical response. Gating speeds, turn-on and turn-off patterns, the asymmetric spatial dependence of the MCP optical response, and resolution effects as functions of gate pulse width and photocathode-MCP bias have been characterized. Shutter times of ≥ 750 ps and ≤ 5 1p/mm resolution with the MCP fully on were observed. Variations in the intensity profiles of the phosphorl.s spatial response for uniform photocathode illumination are measured with a calibrated silicon-intensified-target (SIT) focus projection, scan (FPS) television camera and a high-speed video digitizer while photomultipliers (PMTs) monitor the laser pulse and the phosphor's spatially integrated output intensities. The characterization system, gating and biasing circuits, and experimental results will be presented.
Radial Pulse Propagation And Impedance Characteristics Of Optically Shuttered Channel Intensifier Tubes
J. L. Detch Jr.,
B. W. Noel
Show abstract
Electrically gated proximity-focused channel intensifier tubes are often used as optical shutters. Optimum nanosecond shuttering requires both understanding the electrical pulse propagation across the device structure and proper impedance matching. A distributed-transmission-line model is developed that describes analytically the voltage-and current-wave propagation characteristics as functions of time for any point on the surface. The optical gain's spatial uniformity and shutter-open times are shown to depend on the electrical pulse width and amplitude, and on the applied bias. The driving-point impedance is derived from the model and is expressed as a function of an infinite sum of terms in the complex frequency. The synthesis in terms of lumped-constant network elements is realized in first-and second-Foster equivalent circuits. Experimental impedance data are compared with the model's predictions and deviations from the ideal model are discussed.
An Image Intensified Linear Self-Scanned Array Instrument
C. B. Johnson,
R. E. Blank
Show abstract
With the advent of the linear solid-state self-scanned arrays (SSAs), a new era in spectroscopic instrumentation began. Problems relating to photographic image measurements could be circumvented by using the electronic readout afforded by the SSA. Although film readout does have some advantages, the relative advantages of SSA readout are compelling: excellent metric properties of the SSA pixels; high quantum efficiency; wide spectral range of sensitivity; electronic readout permits direct data processing of signal; high sensitivity and wide dynamic range can be achieved by using state-of-the-art electronic circuits and SSAs operated under low dark current conditions and long exposures.
Development Of Two Infrared Position Sensors
Prescott C. Grey,
Brian E. Player,
Peter Kafalas
Show abstract
Reliable detectors for locating the position of a focal spot of visible radiation have been commercially available, but comparable infrared position sensors have not. This paper describes the development of two new detectors for infrared position sensing: (1) a room-temperature PbSe sensor, and (2) a liquid-nitrogen-cooled InSb sensor. With suitable electronics signal processing, each of these large sensor elements shows a linear response with x-and y-position displacement in the central region of the detector. These sensors are superior to previously reported detectors in threshold sensitivity, temporal frequency bandwidth, and drift. Detector characteristics and applications are discussed.
Multigigahertz Beam Diagnostics For Laser Fusion
R. C. Smith,
E. K. Hodson,
R. L. Carlson
Show abstract
A system to make ultra wideband measurements of fast laser pulses and their induced target interactions at a distance of approximately 38 m from the target location is discussed. The system has demonstrated an overall bandwidth of 3 GHz with projected unfolding to 4 GHz. This system allows high resolution teporal history diagnostics in a remote location providing high EMI and radiation immunity.
Continuous Wave (CW) Chemical Laser Outcoupled Beam Diagnostic System
G. Golnik,
B. D. Seery,
R. W. Steffen,
et al.
Show abstract
Few optical technology issues are unique to the chemical laser. However, many characteristics associated with gain medium phenomena contribute to nondiffraction-limited performance of a low pressure chemical laser and thus present interesting challenges to the optical technologist. Among these are: resonant dispersion; spatial and temporal variations in both gain and refractive index; population cascade and competition effects; polarization; high op-tical gain; complicated, nonequilibrium excitation and deexcitation kinetic processes; and large equivalent Fresnel numbers. The properties of a CW chemical laser medium and output beam are determined by a complex and interrelated series of physical processes (e.g., reac-tion chemistry, flow dynamics, mode-medium interaction, resonator mirror stability, etc.). Knowledge of these processes can be gained by utilizing the characteristic frequencies at which each occurs to decouple the effects of the various processes. Correlations between beam centroid motion (jitter) and mirror acceleration is an example. In order to make such correlations, all diagnostic measurements should be made with as much temporal bandwidth as available. This paper describes the experiments, the plan for processing the data, and the expected results from the data processing.
Optical Diagnostics Of CO2 Laser Fusion Targets Using Backscattered Light
D. E. Casperson
Show abstract
With the f/2.4 focusing optics on one of the eight Helios CO2 laser beam lines, direct backscattered light from a variety of glass microballoon targets has been observed. The quantities. that have been measured include: (1) the total backscattered energy; (2) relative amplitudes of the backscattered fundamental and low harmonics (n=1, 2, 3) of the 10.6 μm incident light; (3) the 3/2 harmonic emission from a double pulse backscatter experiment; (4) the temporally resolved 10.6 pm light using a fast pyroelectric detector and a Los Alamos 5-GHz oscilloscope; and (5) the time-integrated spectrally resolved fundamental using a 3/40meter spectrometer and a high resolution pyroelectric detector array (resolution ≈ 40 Å at 10.6 μm). The suitability of these diagnostics for evaluating the CO2 laser plasma in terms of stimulated scattering processes, plasma density gradients, velocity of the critical surface, etc., is discussed.
Calorimetry At 10.6 /Am Using Organic Volume Absorbers
J. E. Sollid,
C. W. Bjork
Show abstract
Copper-Kapton laminate foils will be utilized as isoperibol calorimeters for pulsed energy measurements at Antares with apertures up to 1.5 m. These are volume absorbing calorimeters in which the radiation is absorbed in a layer tens of wavelengths thick, thereby reducing surface temperature and increasing damage threshold as compared to surface absorbing calorimeters. Kapton was chosen for its high absorption coefficient, 330 cm-1 at P(20) in the 10 pm band of CO2, and its useful short pulse damage threshold, 2.8 J/cm2. The copper backing integrates the absorbed heat pulse. The resultant temperature pulse is sensed by Type E thermocouples (chrome-constantan) soldered to the copper. Time vs voltage traces of the thermocouples can then be analyzed to obtain the energy absorbed. Calorimeters can be fabricated to virtually any size or shape, using photo-lithographic techniques. Utilizing single elements or several in a thermopile further expands the versatility of this technique.
Opto-Mechanical Devices For The Antares Automatic Beam Alignment System
T. Swann,
C. Combs,
J. Witt
Show abstract
Antares is a 24-beam CO2 laser system for controlled fusion research, under construction at Los Alamos National Laboratory. Rapid automatic alignment of this system is required prior to each experimental shot. Unique opto-mechanical alignment devices, which have been developed specifically for this automatic alignment system, are discussed. A variable focus alignment telescope views point light sources. A beam expander/spatial filter processes both a visible krypton-ion and a 10.6 μm CO2 alignment laser. A periscope/carousel device provides the means by which the alignment telescope can sequentially view each of 12 optical trains in each power amplifier. The polyhedron alignment device projects a point-light source for both centering and pointing alignment at the polyhedron mirror. A rotating wedge alignment device provides a sequencing point-light source and also compensates for dispersion between visible and 10.6 μm radiation. A back reflector "flip-in" remotely positions point-light sources at the back-reflector mirrors. A light source box illuminates optic fibers with high-intensity white light which is distributed to the various point-light sources in the system.
Position Diagnostics For A Magnetic-Pinch Imploding Cylinder X-Ray Generator
J. D. Seagrave,
C. W. Woods,
L. A. Jones
Show abstract
Two diagnostic systems have been developed to assess the position and shape of a cylindrical imploding plasma sheath x-ray source. Both systems employ batteries of inexpensive nitrogen lasers. For measurement of the cylinder radius, eight 75-μJ, 1.2 -ns (FWHM) nitrogen lasers will form a series of narrow beams displaced parallel to the cylinder diameter. As the plasma implodes over a 2-μs period, these position-sampling lasers are fired in sequence at times expected to result in partial shadowing. The 2-cm wide beams strike a position-sensitive detector made up of a helical fiber-optic delay line of 2-mm pitch and 4-ns delay/turn. The clad plastic fiber is doped with Rhodamine B at the points where the 337.1-nm laser beams strike, and the fluorescent red flashes propagate along the optical delay line in both directions to avalanche photodiodes. One signal is displayed on a 5-MHz vertical raster; the other is displayed on four turns of a 2-MHz spiral sweep. The envelopes of both pulse trains are displayed on a conventional dual-beam scope. The nitrogen lasers are aligned axially and displaced radially by a system of mirrors. To assess the symmetry of the implosion, another battery of four nitrogen lasers is used to provide paraxially displaced multiple-image pinhole schlieren pictures.
Optical Characterization Method For Concentrating Solar Collectors Using Reverse Illumination
Keith Masterson,
Harry Gaul
Show abstract
This paper presents an optical method for checking overall alignment, aligning elements, identifying areas of extreme distor-tion, and determining receiver intercept factors of solar concentrators. The reverse illumination method is performed by viewing the concentrator on its optical axis and through a telescope located several kilometres away. A high visibility target or source placed at the focus is visible in the aperture of the concentrator. Portions of the aperture not filled by the image of the target are ineffective in concentrating the sun's energy on a receiver with the same dimensions as the target. The ob-server may instruct an assistant by radio to adjust the concentrator to optimize illumination and, hence, alignment. A fluorescent orange target works well during the day. At night, an illuminated, light-colored target enables one to obtain high contrast photographs. Image analysis of these records gives a quantitative measure of reflector alignment and intercept factor for the collector. This method is illustrated using results from two different parabolic dishes.
Annular Apertures Focused In The Fresnel Region
Y. P. Kathuria,
G. Herziger
Show abstract
This paper investigates the Fresnel diffraction of a monochromatic light beam due to annular square and circular apertures, when the system is focused in the Fresnel region. Perspective plots of the intensity distribution diagrams for modified Fresnel number(FMOD)=1, 2, . . . ; Lense focusing factor(FLNS)=1 and obstruction ratio(EPS)=0.01 are generated with IBM-370/168 computer. A comparison is done with the correspon-ding diagrams for unobstructed apertures. The results indicates that the central obstruction ratio(EPS=0.01) yields a systematic change in the shape of the intensity pattern over the image radius of a circular aperture. On the other hand the shape of the diffraction pattern remains unchanged for the square and the annular square apertures. It appears that the ring pattern generated with the annular circular aperture for FMOD=1, FLNS=1 and EPS=0.01 can effectively be used to drill good quality holes by using high power laser beam.
Distinctions Between Undersampling And Aliasing Errors In Numerical Evaluation Of Diffraction Patterns
George N. Lawrence,
Calvin C. Barnard
Show abstract
Calculation of small angle, near field Fresnel diffraction patterns of a propagating light beam is a common task in the system analysis of various high energy laser systems. The basic mathematical principles which cause spatial domain aliasing and undersampling errors are well understood. However, the work to develop these fundamental principles into concise expressions of intensity errors has not been addressed previously . Based on new intensity error estimates of aliasing and rough estimates of undersampling errors, we attempt to clarify the different nature of these two phenomena by illustrating their different functional dependencies. Further, we argue that in most cases of practical interest, aliasing errors completely dominate undersampling errors.
Lasers As A Tool For Plasma Diagnostics
Franz C. Jahoda
Show abstract
Lasers can be used as non-perturbative probes to measure many plasma parameters. Plasma refractivity is primarily a function of electron density, and interferometric measurements of phase changes with either pulsed or CW lasers can determine this parameter with spatial or temporal resolution over several orders of magnitude sensitivity by using laser wavelengths from the near UV to the far infrared. Sub-categories include density gradient and/or turbulence determinations from the laser beam deflection and magnetic field magnitudes from polarization rotation by birefringence. Laser scattering from free electrons yields the most fundamental electron temperature measurements in the plasma parameter range where individual scattering events are uncorrelated in phase and ion temperature or plasma wave and turbulence structure in the opposite limit. The smallness of the scattering cross section generally limits these experiments to single points in space and time using powerful Q-switched lasers. Extensions in both spatial and temporal domains are current research topics. Laser scattering from bound electrons can be many orders of magnitude larger if the laser is matched to appropriate resonance frequencies and can be used in specialized circumstances for measuring low-ionized impurity or dominant species neutral concentrations and velocities. The mini-course will emphasize the fundamental physics underlying the many techniques and draw on particular applications examples based largely on the author's experience in magnetic confinement fusion research.
Surface Characterization Of Diamond-Turned Metal Optics
J. M. Bennett,
D. L. Decker
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Single-point diamond machining is a versatile technique for producing metal optics in conventional as well as unusual shapes. The diamond-turning process is now well enough understood and controlled that specular, low-scatter surfaces can be produced. However, the grooved nature of the surfaces makes it difficult to relate some of the surface characterization parameters used for polished optics to the performance of the diamond-turned parts in an optical system. For example, surface roughness derived from total integrated scattering measurements may be much different from the measured profile roughness. Thus it is important to understand the physical mechanisms involved in the various surface character-ization parameters such as total and angular scattering, reflectance, and absorption and their relation to the microtopography and metallurgical structure of the diamond-turned surfaces. The significance of the surface characterization will be discussed and examples of characterization techniques useful for diamond-turned optics will be described.
Modernizing Michelson's Stellar Interferometer
R. Hanbury Brown
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Since the completion of the program of stellar angular diameter measurements with the Narrabri intensity interferometer, the Chatterton Astronomy Department of the University of Sydney has been engaged in an investigation to determine the type of interferometer best suited for future high angular resolution studies. As part of this investigation the Department is currently exploring the feasibility of building a Michelson stellar interferometer using modern techniques and detectors to surmount the atmospheric and stability problems that limited the development of the original Michelson interferometers. The main part of this program is the design and construction of an 11 m fixed baseline prototype instrument to establish whether fringe visibility can be measured with an accuracy of better than ± 2% over a range of atmospheric seeing conditions and instrumental parameters. The design of the prototype instrument, its status, and prospects are described.
Coherent Propagation Effects In Multiple-Photon Molecular Excitation
C. D. Cantrell,
F. Rebentrost,
W. H. Louisell
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We discuss coherent propagation effects in vapors of polyatomic molecules under conditions of multiple-photon excitation, with emphasis on the generation of new frequencies in the opposite limits of rapidly-switched-on versus adiabatically-switched-on pulses.
Figures Of Merit For Short Wavelength Lasers
Alexander J. Glass
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In developing an appropriate figure of merit for short wavelength, high power lasers, due consideration must be given to the application of the laser. For most high-power applications, energy or power delivered into a given solid angle in the far field is a suitable criterion. Performance measures appropriate to imaging systems are too stringent for energy delivery systems, and place too much emphasis on phase perturbations. Using the concepts of intensity-weighted average quantities, developed by Talanov, we derive a formalism to describe aberrated beams. Three types of aberrations are identified: Seidel, periodic, and stochastic. We find that stochastic aberrations mimic diffraction in their effect, and are more important than diffraction at short wavelengths. Materials limitations on short wave-length laser performance are discussed.
Geometrical Optics In A Laser Laboratory
Robert E. Hopkins
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About four years ago I joined the University of Rochester Laboratory for Laser Energetics; being apprehensive that my extensive experience in classical geometrical optics would be of little use to a laser fusion program. The Laboratory was in the early phase of building the 24 beam OMEGA laser system and it soon became evident that once again basic geometrical optics could make significant contributions to a modern new development. In this talk I will describe some of the basic optical concepts encountered during the engineering phase of building a large laser system. The optical concepts are not new but often forgotten. It illustrates again that geometrical optics takes one far down the road to solving optical problems and is used in almost every new advance in science and engineering.
Performance-Limiting Factors In Optical Coatings
H. A. Macleod
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Some of the features of optical thin films that contribute to their performance limits are surveyed. Emphasis is placed on differences between thin film materials and their bulk form.
Uncertainties Associated With Inertial Fusion Ignition
Gene H. McCall
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At present the energy required to drive an inertial fusion implosion is not well-defined. In the past the energy predicted to produce a breakeven yield, where the thermonuclear energy release is equal to the driver energy, has varied from 103 to 106 joules. The reason for this large variation is partly that experiment and theory have now defined previously poorly understood physical effects and partly that more conservative estimates are now used for those effects which are not yet understood.
A Coherent Approach To Optical Engineering
R. R. Shannon
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Carrying through the layout, design and engineering of an optical system is a major task of the optical engineer. Successful application of the techniques involved requires an understanding of why some of the simple formulae that are used for describing image, location, size, quality, and so on, apply as well as they do to the very complex nonlinear problem of optical imagery. This paper reviews some of the approaches used in optical engineering and indicates how they form a consistent model for the analysis of optical systems.
The Role Of Shear In Interferometry
W. H. Steel
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The shear in an interferometer system depends not only on the type of interferometer and its adjustment but also on how the interferometer is used. It is the important parameter that enters into measurements of spatial coherence, from which such quantities as stellar diameters can be derived, and of wavefronts by common-path interferometry. In association with the tilt, the shear determines where the fringes are localized, and its variation can give a scan of this localization.