Dynamics of transient absorption in bulk DKDP crystals following laser energy deposition.

The transient changes in the optical properties of bulk DKDP material arising from its exposure to high temperatures and pressures associated with localized laser energy deposition are investigated. Two methods for initiation of laser-induced breakdown are used, intrinsic, involving relatively large energy deposition brought about by focusing of the laser beam to high intensities, and extrinsic, arising from more localized deposition due to the presence of pre-existing absorbing damage initiating defects. Each method leads to a very different volume of material being affected, which provides for different material thermal relaxation times to help better understand the processes involved.

Dynamics of material modifications following laser-breakdown in bulk fused silica.

We report on the material response during the cooling phase in bulk fused silica following localized energy deposition via laser-induced breakdown.We use a time-resolved microscope system to acquire images of the region of energy deposition at delay times covering the entire timeline of events. In addition, this system is configured to perform pump-and-probe damage testing measurements to investigate the evolution of the transient absorption of the modified material. The main features of a damage site are established at approximately 30 ns after the pump pulse, i.e. cracks reach their final size within this time frame. The results reveal that the cracks and melted core exhibit a transient absorption up until about 300 ns and 200 micros delay times, respectively, and suggest that the melted region returns to solid phase at approximately 70 ms delay.

Fracture-induced subbandgap absorption as a precursor to optical damage on fused silica surfaces.

The optical damage threshold of indentation-induced flaws on fused silica surfaces was explored. Mechanical flaws were characterized by laser damage testing, as well as by optical, secondary electron, and photoluminescence microscopy. Localized polishing, chemical leaching, and the control of indentation morphology were used to isolate the structural features that limit optical damage. A thin defect layer on fracture surfaces, including those smaller than the wavelength of visible light, was found to be the dominant source of laser damage initiation during illumination with 355 nm, 3 ns laser pulses. Little evidence was found that either displaced or densified material or fluence intensification plays a significant role in optical damage at fluences >35 J/cm(2). Elimination of the defect layer was shown to increase the overall damage performance of fused silica optics.

Influence of pulse duration on ultrashort laser pulse ablation of biological tissues.

Ablation characteristics of ultrashort laser pulses were investigated for pulse durations in the range of 130 fs-10 ps. Tissue samples used in the study were dental hard tissue (dentin) and water. We observed differences in ablation crater morphology for craters generated with pulse durations in the 130 fs-1 ps and the 5 ps-10 ps range. For the water experiment, the surface ablation and subsequent propagation of stress waves were monitored using Mach-Zehnder interferometry. For 130 fs-1 ps, energy is deposited on the surface while for longer pulses the beam penetrates into the sample. Both studies indicate that a transition occurs between 1 and 5 ps.

Screening of substance abuse in public welfare and child protective service clients: a comparative study of rapid assessment instruments vs. the SASSI.

In light of new welfare reforms, it is essential that public welfare workers move individuals from welfare to work in an expedient fashion. Client alcohol and/or drug use/abuse is one barrier that may inhibit this process. The following study (N = 93) administered both the Hudson Index of Alcohol Involvement, the Hudson Index of Drug Involvement (rapid assessment instruments) and the Miller Substance Abuse Subtly Screening Inventory (SASSI) to public welfare and Child Protection system clients. Results indicated that the two rapid assessment measures strongly correlated with the advanced, commonly used SASSI measure. It is suggested that public welfare workers should institute these rapid assessment instruments at intake to screen clients for alcohol and drug use/abuse. When indicated, the client could then be referred to the appropriate agency for further assessment and treatment, to better prepare them for employment as mandated by new welfare reforms. The rapid assessment instruments were found to be shorter than the SASSI, easily administered, and able to detect alcohol and/or drug use/abuse effectively and efficiently in these populations.

Radiation produced by femtosecond laser-plasma interaction during dielectric breakdown.

Optical breakdown by femtosecond and nanosecond laser pulses in transparent dielectrics produces an ionized region of dense plasma confined within the bulk of the material. This ionized region is responsible for broadband radiation that accompanies the breakdown process. Spectroscopic measurements of the accompanying light have been used to show that, depending on the laser parameters, the spectra may originate from plasma-induced second-harmonic generation, supercontinuum generation, or thermal emission by the plasma. By monitoring the emission from the ionized region, one can ascertain the predominant breakdown mechanism and the morphology of the damage region.

Minimal dispersion refractive index profiles.

The analogy between optics and quantum mechanics is exploited by considering a 2-D quantum system whose Schroedinger equation is closely related to the wave equation for light propagation in an optical fiber. From this viewpoint, Marcatili's condition for minimal-dispersion-refractive-index profiles, and the Olshansky- Keck formula for rms pulse spreading in an alpha-profile fiber may be derived without recourse to the WKB approximation. Besides affording physical insight into these results, the present approach points out a possible limitation in their application to real fibers.

Spatial coherence of laboratory soft-x-ray lasers.

Employing a time-dependent model with counterpropagating beams and saturable gain, we calculate the degree of coherence for a range of x-ray lasers. From these results we infer a scaling law that can be used to estimate the degree of coherence of a general class of x-ray lasers.

Mode properties of optical fibers with lossy components by the propagating beam method.

The theory of optical fibers has been concerned mainly with propagation in lossless waveguides. Although real fibers are composed of core materials with extremely small absorption coefficients, they may have claddings and jackets with substantially higher losses. These components can in turn selectively attenuate certain higher-order guided modes. The propagating beam method utilizes configuration space solutions to a scalar wave equation to generate the mode propagation constants and eigenfunctions for a general class of weakly guiding fibers. This method has now been generalized to treat fibers with absorbing components such as claddings and jackets. It is now possible to compute with this generalized method both the mode attenuation coefficients and mode eigenfunctions for such fibers. Results are given for planar and circularly symmetric waveguides with lossy claddings and jackets.

Spectral approach to optical resonator theory.

A new computational method for unloaded optical resonators is developed based on the discrete Fourier analysis of information generated by repeated iterations of the optical field corresponding to transits between reflectors. The method is a straightforward extension of the propagating beam method developed earlier for optical fibers for extracting modal properties from numerical solutions to the paraxial scalar wave equation. The method requires computation of a field correlation function, whose Fourier transform reveals the eigenmodes as resonant peaks. Analysis of the location and breadth of these peaks determines the resonator eigenvalues. When the eigenvalues are known, additional discrete Fourier transforms of the field are used to generate the mode eigenfunctions. This new method makes possible the unambiguous identification and accurate characterization of the entire spectrum of transverse resonator modes.

Computation of mode properties in optical fiber waveguides by a propagating beam method.

Propagating beam solutions for optical waveguides can be made to generate such mode-related properties as propagation constants, relative mode powers, and group delays with high precision and considerable flexibility. These quantities are needed in the analysis of optical fiber dispersion. The technique requires the generation of correlation functions from the numerical solutions of a wave equation. These correlation functions are in turn Fourier-transformed with respect to axial distance z. The resulting spectra display sharp resonances corresponding to mode groups, and the positions and heights of these resonances determine the previously mentioned mode properties. The spectral analysis is made highly accurate by the use of line-shape fitting techniques. With this method, mode group delays can be determined to a precision of +/-0.12 psec/km using a computation covering a 5-cm propagation path.

Computation of mode eigenfunctions in graded-index optical fibers by the propagating beam method.

The propagating beam method utilizes discrete Fourier transforms for generating configuration-space solutions to optical waveguide problems without reference to modes. The propagating beam method can also give a complete description of the field in terms of modes by a Fourier analysis with respect to axial distance of the computed fields. Earlier work dealt with the accurate determination of mode propagation constants and group delays. In this paper the method is extended to the computation of mode eigenfunctions. The method is efficient, allowing generation of a large number of eigenfunctions from a single propagation run. Computations for parabolic-index profiles show excellent agreement between analytic and numerically generated eigenfunctions.

Mode properties and dispersion for two optical fiber-index profiles by the propagating beam method.

The propagating beam method generates solutions for the electric field in a graded-index optical fiber that emphasize beam characteristics rather than modal properties. Through Fourier analysis with respect to axial distance z, these solutions can be made to yield such mode properties as the propagation constants beta(n), the mode group delays partial differentialbeta(n)/ partial differentialomega, and the mode eigenfunctions. The propagating beam method has been applied to a detailed study of two index profiles with finite thickness cladding: an axisymmetric power-law (alpha = 1.85) profile both without and with an on-axis dip. In nine successive computer runs, eighty-five and eighty-four bound or guided modes were excited and characterized for the two respective profiles. The mode group delays near cutoff for both profiles show large deviation from those derived with the WKB method. In addition, sets of almost degenerate modes near cutoff show large differences in group delay. Modes with low azimuthal quantum number are strongly perturbed by the central dip. It is found that rms pulse dispersion is quite sensitive to the inclusion or exclusion of modes near cutoff, but that frequency response bandwidth is not. This leads to the conclusion that fiber bandwidth cannot be accurately inferred from rms pulse dispersion and may explain why broadband multimode fibers exist dispite strong perturbation of the modes near cutoff.

Calculation of dispersion in graded-index multimode fibers by a propagating-beam method.

Methods are developed for extracting from a numerical propagating-beam solution of a scalar wave equation the information necessary to compute the impulse-response function and the pulse dispersion for a multimode graded-index fiber. It is shown that the scalar Helmholtz equation and the parabolic wave equation have the same set of eigenfunctions in common and that the eigenvalues for the two equations are simply related. Thus one can work exclusively with the simpler parabolic equation. Both the mode eigenvalues (propagation constants) and mode weights, which are necessary for determining the impulse response, can be obtained with high accuracy from a numerical Fourier transform of the complex field-correlation function by the use of digital-filtering techniques. It is shown how a solution obtained in the absence of profile dispersion can be simply corrected for the presence of profile dispersion. In an illustrative example a gradedindex fiber with a central dip in its profile is considered.

Light propagation in graded-index optical fibers.

An accurate numerical method is described for solving the Helmholtz equation for a general class of optical fibers. The method yields detailed information about the spatial and angular properties of the propagating beam as well as the modal propagation constants for the fiber. The method is applied to a practical graded-index fiber under the assumptions of both coherent and incoherent illumination. A spectral analysis of the calculated field shows that leaky modes are lost and steady-state propagating conditions are established over a propagation distance of a fraction of a meter.

A life cycle model of public policy issues in health care: the importance of strategic issues management.

Public policy affects health and social services organizations. Senior management has a responsibility to prevent inappropriate demands of stakeholders from predominating and to influence the outcome of public policy to the benefit of their organization through the strategic issues management process. This article presents a public policy issue life cycle model, life-cycle stages and suggested strategies, paths issues can take in the life cycle, and factors that affect issue paths. An understanding of these dynamics can aid senior managers in shaping and changing public policy issues and lessening external environment threats to their organization.

High-power narrow-band pulses with wavelengths tunable about 1.053 µm from a synchronously pumped optical parametric oscillator.

We have constructed an optical parametric oscillator to generate 75-ps near-transform-limited pulses with wavelengths tunable about 1.053 µm for use in pump-probe studies of self-focusing. The singly resonant oscillator uses a Brewster-cut LiB(3)O(5) crystal that is oriented for type-II phase matching and synchronously pumped by the amplified and frequency-tripled pulse trains from a mode-locked and Q-switched Nd:YLP laser. An intracavity Pockels cell is used to switch out single 0.5-MW pulses at rates of 1 to 10 Hz. The design, construction, and performance of the oscillator are discussed. Measured performance is compared with design predictions and with detailed numerical simulations.

An analysis of federal narcotic detoxification policy: implications for rehabilitation.

The current federal narcotic detoxification policy, limiting such treatment to 21 d, is analyzed with respect to its impact on the success rate for complete withdrawal. Data are drawn from an historical review of American Medical Association statements on narcotic addiction and a review of empirical data on short- and long-term detoxification. The 3-week limit is shown to interfere with achieving successful detoxification rates, although it continues to guide federal detoxification policy. Empirical evidence supports long-term detoxification that recognizes other factors as also having a therapeutic effect. It is time to revise the current federal policy to more accurately reflect the progress made in long-term care.

Simple spectral method for solving propagation problems in cylindrical geometry with fast Fourier transforms.

We describe a spectral method for solving the paraxial wave equation in cylindrical geometry that is based on expansion of the exponential evolution operator in a Taylor series and use of fast Fourier transforms to evaluate derivatives. A fourth-order expansion gives excellent agreement with a two-transverse-dimensional split-operator calculation at a fraction of the cost in computation time per z step and at a considerable savings in storage.

Accurate description of ultrawide-angle beam propagation in homogeneous media by Lanczos orthogonalization.

We demonstrate accurate modeling of ultrawide-angle beam propagation in homogeneous media by Lanczos orthogonalization for acceptable longitudinal step sizes. Accuracy is determined by comparison with solutions to the Helmholtz equation obtained by a band-limited Fourier series method.