Showing papers by "University of Central Florida published in 1992"

A fast algorithm for active contours and curvature estimation

Abstract: A model for representing image contours in a form that allows interaction with higher level processes has been proposed by Kass et al. (in Proceedings of First International Conference on Computer Vision, London, 1987, pp. 259–269). This active contour model is defined by an energy functional, and a solution is found using techniques of variational calculus. Amini et al. (in Proceedings, Second International Conference on Computer Vision, 1988, pp. 95–99) have pointed out some of the problems with this approach, including numerical instability and a tendency for points to bunch up on strong portions of an edge contour. They proposed an algorithm for the active contour model using dynamic programming. This approach is more stable and allows the inclusion of hard constraints in addition to the soft constraints inherent in the formulation of the functional; however, it is slow, having complexity O(nm3), where n is the number of points in the contour and m is the size of the neighborhood in which a point can move during a single iteration. In this paper we summarize the strengths and weaknesses of the previous approaches and present a greedy algorithm which has performance comparable to the dynamic programming and variational calculus approaches. It retains the improvements of stability, flexibility, and inclusion of hard constraints introduced by dynamic programming but is more than an order of magnitude faster than that approach, being O(nm). A different formulation is used for the continuity term than that of the previous authors so that points in the contour are more evenly spaced. The even spacing also makes the estimation of curvature more accurate. Because the concept of curvature is basic to the formulation of the contour functional, several curvature approximation methods for discrete curves are presented and evaluated as to efficiency of computation, accuracy of the estimation, and presence of anomalies.

Self-focusing and self-defocusing by cascaded second-order effects in KTP.

Abstract: We monitor the induced phase change produced by a cascaded χ(2):χ(2) process in KTP near the phase-matching angle on a picosecond 1.06-μm-wavelength beam using the Z-scan technique. This nonlinear refraction is observed to change sign as the crystal is rotated through the phase-match angle in accordance with theory. This theory predicts the maximum small-signal effective nonlinear refractive index of n2eff≅±2×10−14 cm2/W (±1 × 10−11 esu) for an angle detuning of ±5° from phase match for this 1-mm-thick crystal with a measured deff of 3.1 pm/V For a fixed phase mismatch, this n2eff scales linearly with length and as deff2; however, for the maximum n2eff the nonlinear phase distortion becomes sublinear with irradiance for phase shifts near π/4.

Determination of bound-electronic and free-carrier nonlinearities in ZnSe, GaAs, CdTe, and ZnTe

Abstract: We extend the application of the Z-scan experimental technique to determine free-carrier nonlinearities in the presence of bound electronic refraction and two-photon absorption. We employ this method, using picosecond pulses in CdTe, GaAs, and ZnTe at 1.06 μm and in ZnSe at 1.06 and 0.53 μm, to measure the refractive-index change induced by two-photon-excited free carriers (coefficient σr,), the two-photon absorption coefficient β, and the bound electronic nonlinear refractive index n2. The real and imaginary parts of the third-order susceptibility (i.e., n2 and β, respectively) are determined by Z scans with low inputs, and the refraction from carriers generated by two-photon absorption (an effecitve fifth-order nonlinearity) is determined from Z scans with higher input energies. We compare our experimental results with theoretical models and deduce that the three measured parameters are well predicted by simple two-band models. n2 changes from positive to negative as the photon energy approaches the band edge, in accordance with a recent theory of the dispersion of n2 in solids based on Kramers–Kronig transformations [ Phys. Rev. Lett.65, 96 ( 1990); IEEE J. Quantum Electron.27, 1296 ( 1991)]. We find that the values of σr are in agreement with simple band-filling models.

Kramers-Kronig relations in nonlinear optics

Abstract: We review dispersion relations, which relate the real part of the optical susceptibility (refraction) to the imaginary part (absorption). We derive and discuss these relations as applied to nonlinear optical systems. It is shown that in the nonlinear case, for self-action effects the correct form for such dispersion relations is nondegenerate, i.e. it is necessary to use multiple frequency arguments. Nonlinear dispersion relations have been shown to be very useful as they usually only require integration over a limited frequency range (corresponding to frequencies at which the absorption changes), unlike the conventional linear Kramers-Kronig relation which requires integration over all absorbing frequencies. Furthermore, calculation of refractive index changes using dispersion relations is easier than a direct calculation of the susceptibility, as transition rates (which give absorption coefficients) are, in general, far easier to calculate than the expectation value of the optical polarization. Both resonant (generation of some excitation that is long lived compared with an optical period) and nonresonant ‘instantaneous’ optical nonlinearities are discussed, and it is shown that the nonlinear dispersion relation has a common form and can be understood in terms of the linear Kramers-Kronig relation applied to a new system consisting of the material plus some ‘perturbation’. We present several examples of the form of this external perturbation, which can be viewed as the pump in a pump-probe experiment. We discuss the two-level saturated atom model and bandfilling in semiconductors among others for the resonant case. For the nonresonant case some recent work is included where the electronic nonlinear refractive coefficient,n2, is determined from the nonlinear absorption processes of two-photon absorption, Raman transitions and the a.c. Stark effect. We also review how the dispersion relations can be extended to give alternative forms for frequency summation which, for example, allows the real and imaginary parts ofχ(2) to be related.

Direct measurements of nonlinear absorption and refraction in solutions of phthalocyanines

Abstract: Direct measurements are reported of the excited singlet-state absorption cross section and the associated nonlinear refractive cross section using picosecond pulses at 532 nm in solutions of phthalocyanine and naphthalocyanine dyes. By monitoring the transmittance and far-field spatial beam distortion for different pulsewidths in the picosecond regime, it is shown that both the nonlinear absorption and refraction are fluence (energy-per-unit-area) rather than irradiance dependent. Thus, excited-state absorption is the dominant nonlinear absorption process, and the observed nonlinear refraction is also due to real population excitation.

Nonlinear optical properties of carbon-black suspensions (ink)

Abstract: We performed a series of experiments on suspensions of carbon particles in liquids (ink) and carbon particles deposited on glass to determine the mechanisms for the observed optical-limiting behavior. Both materials show reduced transmittance for increasing fluence (energy per unit area). We found that nonlinear scattering dominates the transmissive losses and that the limiting is fluence dependent, so that limiters based on black ink are effective for nanosecond pulses but not for picosecond pulses. Additionally, the nonlinear scattering and the limiting behavior cease after repeated irradiation. For the liquid, flowing eliminates this effect. All the data obtained are consistent with a model of direct heating of the microscopic-sized carbon particles by linear absorption with subsequent optical breakdown initiated by thermally ionized carriers. A simple calculation gives temperatures higher than the sublimation temperature at the onset of limiting. Emission spectra measurements show singly ionized carbon emission lines with a hot blackbody background emission consistent with temperatures of ≃4000 K. A rapid expansion of the microscopic plasmas generated by the breakdown will effectively scatter further input light. Indeed, in time-resolved experiments the trailing portion of the pulse is most heavily scattered. The time-resolved transmittance of a weak cw probe beam also follows the temporal dependence of the singly ionized carbon emission (≃102 ns). We directly monitored the expansion of the scattering centers by angularly resolving the scattered light for different input fluences and fitting to Mie scattering theory. Since the carbon is black and the microplasmas are initiated by linear absorption, the limiting is extremely broadband. Within the context of this model we discuss the limitations and optimization of ink-based optical limiters.

Measurement of nondegenerate nonlinearities using a two-color Z scan.

Abstract: A simple dual-wavelength (two-color) Z-scan geometry is demonstrated for measuring nonlinearities at frequency ωp owing to the presence of light at ωe. This technique gives the nondegenerate two-photon absorption (2PA) coefficient β(ωp; ωe) and the nondegenerate nonlinear refractive index n2(ωp; ωe), i.e., cross-phase modulation. We demonstrate this technique on CS2 for n2 and on ZnSe where 2PA and n2 are present simultaneously.

An Analytical Model for the Refractive Index Power Spectrum and Its Application to Optical Scintillations in the Atmosphere

Abstract: A spectral model for refractive index fluctuations, showing the characteristic ‘bump’ at high wavenumbers, is proposed for theoretical studies involving optical propagation. Using this spectrum, analytical expressions are developed for the variance of log-amplitude that show good agreement with numerical results based on the Hill spectrum.

Nondegenerate two-photon absorption in zinc blende semiconductors

Abstract: An algorithm is presented for the calculation of the nondegenerate two-photon absorption coefficient by using second-order perturbation theory and a Kane band-structure model, including the effects of nonparabolicity and nonzone-center wave functions. The polarization dependence is included by correctly accounting for the symmetry of the electronic wave functions. A comparison is made with degenerate two-photon absorption data in various zinc blende semiconductors, and excellent agreement is found without the use of fitting parameters. Comparisons are also made with nondegenerate two-photon absorption spectra measured in ZnSe and ZnS by using a picosecond continuum and with some polarization-dependent measurements obtained by a two-color Z-scan measurement.

Tracking control of rigid-link electrically – driven robot manipulators

Abstract: This paper illustrates a simple, hand-crafted approach which can be used to design tracking controllers for rigid-link electrically-driven (RLED) robot manipulators. The control methodology is intuitively simple since it is based on concepts readily identified by most control engineers. To illustrate the approach, we develop a corrective tracking controller for the RLED robot dynamics which yields global exponential stability for the link tracking error under the assumption of exact model knowledge. To compensate for the uncertainties in the rigid-link electrically-driven robot model, we then design a corrective robust tracking controller which yields global uniform ultimate bounded stability of the link tracking error. The proposed controller is robust with regard to parametric uncertainties and additive bounded disturbances while correcting for the typically ignored electrical actuator dynamics.

On methods for continuous systems with quadratic and cubic nonlinearities

Abstract: Methods for determining the response of continuous systems with quadratic and cubic nonlinearities are discussed We show by means of a simple example that perturbation and computational methods based on first discretizing the systems may lead to erroncous results whereas perturbation methods that attack directly the nonlinear partial-differential equations and boundary conditions avoid the pitfalls associated with the analysis of the discretized systems We describe a perturbation technique that applies either the method of multiple scales or the method of averaging to the Lagrangian of the system rather than the partial-differential equations and boundary conditions

Observational data collection methods for services marketing: An overview

Abstract: Applications of observational data collection methods to services marketing research are explored. Three key dimensions that distinguish the various forms of observational methods are presented. General applications of observational research to services marketing phenomena are posited. Specific applications of observational research to services quality are suggested. Issues concerning the implementation of direct human observation are considered.

Hydromagnetic flow of a dusty fluid over a stretching sheet

Abstract: Analysis of hydromagnetic flow of a dusty fluid over a stretching sheet is carried out with a view to throw adequate light on the effects of fluid-particle interaction, particle loading, and suction on the flow characteristics. The equations of motion are reduced to coupled non-linear ordinary differential equations by similarity transformations. These coupled non-linear ordinary differential equations are solved numerically on an IBM 4381 with double precession, using a variable order, variable step-size finite-difference method. The numerical solutions are compared with their approximate solutions, obtained by a perturbation technique. For small values of β the exact (numerical) solution is in close agreement with that of the analytical (approximate) solution. It is observed that, even in the presence of a transverse magnetic field and suction, the transverse velocity of both the fluid and particle G phases decreases with an increase in the fluid-particle interaction parameter, β, or the particle-loading parameter, k . Moreover, the particle density is maximum at the surface of the stretching sheet, and the shearing stress increases with an increase in β or k .

Thermal quenching of fluorescence in chromium-doped fluoride laser crystals

Abstract: The temperature dependence of the fluorescence lifetime of chromium ions in the fluorides LiBaAlF6, LiCaAlF6, LiSrAlF6, and LiSr0.8Ca0.2AlF6 is reported. In each crystal the lifetime at low temperatures is nearly constant. Above a temperature that depends on the host the lifetime drops rapidly with increasing temperature. This strong decrease is attributed to nonradiative decay processes. From the experimental data the radiative decay rate, the thermal quenching activation energy, and the nonradiative decay rate are determined. Excellent agreement with the Mott model is then demonstrated by using these parameters.

Heat transfer in an electrically conducting fluid over a stretching surface

Abstract: An analysis is carried out to study the heat transfer characteristics in an electrically conducting fluid over a stretching sheet with variable wall temperature and internal heat generation or absorption. Two eases are studied, namely, (i) the sheet with prescribed surface temperature (PST case) and (ii) the sheet with prescribed wall heat flux (PHF case). The solutions for the temperature, the heat transfer characteristics and their asymptotic limits for large Prandtl number (σ) are obtained in terms of Kummer's and parabolic cylinder functions. It is shown that asymptotic limits are not possible for small Prandtl number; this is due to the solution changing by O(1) on length scale of 1/σ. For large Prandtl number, a boundary layer of width 1/σ at η = 0 and an internal layer of width ∝1/σ near the turning point are noticed.

An application of functional dependencies to the topological analysis of protection schemes

Abstract: The concept of functional dependency is applied to the problem of relay coordination in protection systems. An algorithm is developed for the identification of a minimal break point set (BPS) of relays of a protection topology. This algorithm is an improvement over existing algorithms in that it identifies a minimal BPS within a time period that is a polynomial function of the number of relays, while the earlier algorithms had exponential time behavior. In the case of large protection schemes, the saving in computation costs is considerable. An algorithm is developed for the selection of a relative sequence matrix; this algorithm also has polynomial time complexity. The functional dependency approach is seen to be more flexible and more powerful than the graph theoretic schemes, and holds out much promise for the development of efficient computer-aided design tools for the protection engineer. >

Predictive modeling techniques of software quality from software measures

Abstract: The objective in the construction of models of software quality is to use measures that may be obtained relatively early in the software development life cycle to provide reasonable initial estimates of the quality of an evolving software system. Measures of software quality and software complexity to be used in this modeling process exhibit systematic departures of the normality assumptions of regression modeling. Two new estimation procedures are introduced, and their performances in the modeling of software quality from software complexity in terms of the predictive quality and the quality of fit are compared with those of the more traditional least squares and least absolute value estimation techniques. The two new estimation techniques did produce regression models with better quality of fit and predictive quality when applied to data obtained from two software development projects. >

Games teams play: A method for investigating team coordination and performance

Abstract: Teams are playing an increasingly important role in the workplace. However, reviews of the team performance literature have suggested that there are serious deficiencies in our understanding of team processes and performance (e.g., Dyer, 1984). These difficulties may be attributable, in part, to the lack of laboratory methodologies to investigate team performance. This paper describes the use of low-fidelity simulations as a potentially useful paradigm for researching team coordination and performance. This paradigm is advantageous in that it offers relatively high levels of experimental control and task representation at a low cost.

A case-based reasoning approach to real estate property appraisal

Abstract: Case-Based Reasoning (CBR) has emerged as an alternative to rule-based reasoning techniques for the design of expert systems. This paper concentrates on the issues involved in the application of the case-based reasoning techniques to a specific domain, property appraisal. Case-based reasoning has been recently favored because it seems to resemble more closely the psychological process humans follow when trying to apply their knowledge to the solution of problems: adapting solutions of similar problems handled in past experiences to address present situations. Property appraisal or valuation is a domain characterized by having a single parameter in its solution—the value of the property being appraised. This makes it different from most of other domains to which case-based reasoning has been applied. Those other domains require the satisfaction of multiple goals, which are related to one another in some type of explanation or plan. Because of the fact that properly appraisal has a single goal, it is particularly important to find the best possible answer for that solution. In addition to this, the achievement of consistency is also essential in this domain in which different experts may reach different answers even having the same data at their disposition. By modelling the market data approach of appraisal, using adaptations of case-based reasoning techniques, such as the similarity links and the critics, and integrating other techniques, (i.e., the use of comfort factors), a case-based reasoner for property appraisal is implemented addressing the issues just mentioned above.

Aperture-averaging factor for optical scintillations of plane and spherical waves in the atmosphere

Abstract: Previous analyses of aperture averaging of optical scintillations in the turbulent atmosphere have generally involved either numerical integrations or approximation formulas based on asymptotic results for large apertures. Here I develop the exact expressions for the aperture-averaging factor in the weak-turbulence regime for both plane and spherical waves. For computational ease, accurate approximation or interpolation formulas that can be applied in most cases of interest with greater accuracy than previous approximations are also developed.

Flux Chamber Design and Operation for the Measurement of Municipal Solid Waste Landfill Gas Emission Rates

Abstract: This paper describes a hybrid Flux Chamber-Soil Gas Probe methodology for measuring municipal solid waste (MSW) gas emission rates. Following the design of the flux chamber, the chamber was laboratory tested to define its mixing characteristics and optimum operating parameter values. Flux chamber operating parameters included: chamber pressure, sweep air flow rate, landfill surface insertion depth, and sweep air velocity. Optimum operating parameter values were determined by operating the flux chamber on a simulated subsurface emission source and varying the operating parameters. The laboratory tests indicated that the flux chamber could be operated to provide zero biasing of gas emission rates, resulting in accurate measurement of gas emission rates.

Parallel heap: an optimal parallel priority queue

Abstract: We describe a new parallel data structure, namely parallel heap, for exclusive-read exclusive-write parallel random access machines. To our knowledge, it is the first such data structure to efficiently implement a truly parallel priority queue based on a heap structure. Employing p processors, the parallel heap allows deletions of θ(p) highest priority items and insertions of θ(p) new items, each in O(log n) time, where n is the size of the parallel heap. Furthermore, it can efficiently utilize processors in the range 1 through n.

The impact of issue framing on public opinion: Toward affirmative action programs

Abstract: This article elaborates on Myrdal's question regarding the systematic subordination of minorities in an egalitarian society, and keys on the methodology used to assess attitudes of whites in regard to equality for blacks. Specifically, the article addresses questions associated with reactions to equal opportunity programs and the framing of survey questions designed to measure attitudes. Evidence is elaborated which supports the contention that public reactions to equal opportunity programs are affected by how these programs are presented to the public.

Nonlinear optical response of C60 and C70.

Abstract: Second-order hyperpolarizibilities γ of the buckminsterfullerenes C70 at three different wavelengths were determined by third-harmonic generation in a toluene solution and compared with those of C60. Strong effects with γ exceeding 5 × 10−32 esu were observed in the three-photon resonant regime. The nonresonant measurements indicate a negative real hyperpolarizibility in C70 and a positive γ for C60, both of the order of 10−33 esu.

A periodic second harmonic spatial power combining oscillator

Abstract: The authors present a periodic second-harmonic spatial power combining oscillator. The power combining is achieved by phase locking the oscillators at the fundamental frequency and combining the second-harmonic power in space through an array of microstrip patch antennas. The effect of moding and multiple device-circuit interaction is investigated. This circuit is planar, and therefore simplifies the design of monolithic circuits, X-band Gunn diodes are used for the purpose of demonstration. >

Generation of 150-fs tunable pulses in Cr:LiSrAlF6.

Abstract: We report ultrashort-pulse generation in a Cr:LiSrAlF6 solid-state laser. The 15-mm crystal was pumped by 1 W of red light from a cw krypton laser and actively mode locked at an 82-MHz repetition frequency using an acousto-optic modulator. Wavelength tuning was demonstrated over the range 800–880 nm, limited by the bandwidth of the optical components. By including intracavity dispersion compensation, pulse widths of 150 fs were obtained with evidence that self-phase modulation contributes to spectral broadening and pulse shortening.