Showing papers on "Coherence (physics) published in 1989"

Coherence effects in deep inelastic scattering

Abstract: We present a framework for deep inelastic scattering, with bound state properties in accordance with a QCD force field acting like a vortex line in a colour superconducting vacuum, which implies some simple coherence effects. Within this scheme one may describe the results of present energies very well, but one obtains an appreciable depletion of gluon radiation in the HERA energy regime.

Theoretical noise performance of coherence-multiplexed interferometric sensors

Abstract: If a number of fiber-optic interferometric sensors are arranged so that their outputs are returned to the user via a common optical bus, then some method of distinguishing the returns from different sensors must be used to recover individual signals. One such method involves using light with a short coherence length, so that returns from different sensors will be mutually incoherent. The interferometric signal associated with each sensor can then be recovered via appropriate optical processing. The author considers sensors multiplexed using this technique and calculates their noise performance. It is found that for systems with only a few sensors, the minimum detectable phase is limited by the noise associated with incoherent interference; this can be minimized by using light with as short a coherence length as is practical. The maximum number of sensors that can be multiplexed is limited by optical power loss. A ladder topology is tentatively found to give the best performance. >

Chrono-coherent imaging for medicine

Abstract: A method for imaging with visible and near-visible light inside media, such as tissues, which have strong light scattering is described. The chrono-coherent imaging (CCI) method is demonstrated for a transmission geometry where an absorbing object is completely hidden from normal visual observation by scattered light. The resultant images are most similar to X-rays, with cumulative transmission showing absorption features and refractive index differences in the media. Laser coherence properties, coherence measurements, the relation of CCI to light-in-flight holography, holographic film properties relevant to CCI, a particular optical setup for CCI, the results of a demonstration experiment imaging an absorbing object hidden by light scattering, and an experiment to estimate the clinical applicability of CCI are discussed. >

Computing oriented texture fields

Abstract: A novel algorithm for computing the orientation field for a flowlike texture is presented. The basic idea behind the algorithm is to use an oriented filter, namely the gradient of the Gaussian, and perform manipulations on the resulting gradient vector field. The most important aspect of the algorithm is that it is provably optimal in estimating the local orientations of an oriented texture. An added strength of the algorithm is that it is simpler and has a better signal-to-noise ratio than previous approaches, because it utilizes fewer derivative operations. Also proposed is a measure of coherence which is based on the use of the angle and coherence images as intrinsic images. An analysis of oriented textures requires the computation of these intrinsic images as a first step. In this sense, the computation of the orientation field, resulting in the intrinsic images, is indispensable in the analysis of oriented textures. Results from a number of experiments indicate the usefulness of the angle and coherence intrinsic images. >

Intermittency in the coherence collapse of a semiconductor laser with external feedback.

Abstract: We report the first experimental observation of type-II intermittency in an optical system. The type-II intermittency is observed in the light intensity of a GaAs/GaAlAs semiconductor laser with external feedback in the regime of the coherence collapse. We conclude the occurrence of a time-inverted type-II intermittency from the time distribution of the observed intensity breakdowns, with the injection current as control parameter. This interpretation is confirmed by a reconstructed Poincar\'e plot exhibiting the spiraling behavior of the type-II intermittency.

Spin coherence and Humpty-Dumpty. III. The effects of observation.

Abstract: In recent work a Stern-Gerlach interferometer (SGI) was considered in which a polarized beam of spin-(1/2 particles is split by a Stern-Gerlach apparatus into two partial beams, and then subsequent Stern-Gerlach deflecting magnets are used to reconstitute these two beams into one. In these studies it was shown that when such a coherent polarized beam passed through a SGI, some spin coherence is inevitably lost. In this regard, folk wisdom concerning irreversibility provides something of a guide to the present problem, since we all know that when Humpty-Dumpty had his great fall nobody could put him together again. In the present paper we consider the fate of our spin-(1/2 Humpty-Dumpty when a detector is present that is sensitive to the passage of particles along one trajectory, but not the other. It is not surprising that coherence is destroyed as soon as one is able to tell along which path the atom traveled. However, there seems to be no general agreement about the mechanism of coherence loss. Our conclusion is that the loss of coherence in measurements on quantum systems can always be traced to the dynamics of correlations between the measuring apparatus and the system being observed.

Destruction of quantum coherence in a nonlinear oscillator via attenuation and amplification.

Abstract: The exact solution to the master equation of a nonlinear oscillator, subject to damping or amplification, is presented. The effect of such incoherent processes on the quantum-coherence properties and recurrences is obtained. It is shown that amplification destroys quantum coherence more rapidly than does attenuation.

Transverse laser-mode structure determination from spatial coherence measurements: Experimental results

Abstract: We demonstrate experimentally the feasibility of a certain coherence-theoretic algorithm for determining the relative modal weights of a laser beam that consists of multiple Hermite-Gaussian transverse modes. A computer-controlled Young's two-pinhole interferometer with a linear CCD array is used for the coherence measurements. The required scaling parameter (beam width) is obtained iteratively by library routines. Numerical simulations on the stability of the algorithm in the presence of experimental noise are also presented.

Synthesis of Polychromatic Light Sources with Arbitrary Degrees of Coherence: Some Experiments

Abstract: A simple optical system for synthesizing two-dimensional planar sources having a wide variety of one-dimensional spectral degrees of coherence is analysed. A pupil-mask acting on the dispersed image of a polychromatic incoherent slit is used to control the spectral degree of coherence of the source. Examples of sources exhibiting interesting spectral shifts and modulations in the far field are shown.

Picosecond time-scale phase-related optical pulses: measurement of sodium optical coherence decay by observation of incoherent fluorescence

Abstract: A technique for producing phase-related picosecond optical pulses is described. A sequence of two phase-related pulses, well separated in time, is applied to the D1 line in Na vapor. When the phase between the two pulses is scanned, oscillations in the incoherent fluorescence are observed. This demonstrates the well-defined phase relationship between the pulses. Measuring the amplitude of the oscillations as a function of pulse separation yields the Na optical coherence decay. A beat arising from the ground-state hyperfine splitting is observed, and the results agree well with calculation.

Fourth-order interference technique for determining the coherence time of a light beam

Abstract: A new experimental procedure for determining the (second-order) coherence time of a light beam is described that is based on a combination of conventional interferometry with an intensity-correlation technique. It permits measurements of coherence times that are several orders of magnitude shorter than the resolving times of the detectors, which ordinarily limit correlation measurements. The validity of the method is demonstrated by an experiment in which the transverse fluorescent light from a dye laser is measured.

Size dependence, coherence, and scaling in turbulent coupled-map lattices.

Abstract: In this paper the crossover from ``small'' to ``large'' chaotic systems is studied. The behavior of the largest Lyapunov exponent in a system of coupled chaotic maps shows that this crossover is remarkably sharp, and allows us to define a coherence length beyond which the system is effectively large. Between the coherent chaos of the small system and the incoherent chaos (turbulence) of the large one there is a stable window starting at the linear instability point for the uniform chaotic state in which the lattice becomes effectively one dimensional. The scaling of the coherence length close to the onset of turbulence is investigated and compared to recent predictions.

Quasiperiodic behavior in beam-driven strong Langmuir turbulence

Abstract: The evolution of unmagnetized beam‐driven strong Langmuir turbulence is studied in two dimensions by numerically integrating the Zakharov equations for systems pumped by monochromatic and broadband negative‐damping drivers with nonzero central wavenumber. Long‐time statistically steady states are reached for which the dependence of the evolution on the driver wavenumber, growth rate, and bandwidth is examined in detail. For monochromatic drivers, a quasiperiodic cycle is found to develop if the driver wavenumber is sufficiently large. In this cycle, energy from the driven mode undergoes a sequence of weak‐turbulence backscatter decays, which transfer energy to an approximately isotropic long‐wavelength condensate. During this phase, beam‐aligned chains of propagating beat waves develop and perpendicular density waves are also excited. Subsequently, nucleation of waves in density cavities causes a series of wave collapses (involving coherent wave–wave interactions) to occur, during which short‐wavelength damping reduces the system energy in discrete steps. Finally, the cycle restarts. The characteristic frequency of the quasiperiodic cycle and the average system energy are both approximately proportional to the growth rate. Broadening of the driver in wavenumber tends to degrade the system‐wide coherence of the cycle, but its main features appear to survive on the scale of the coherence length of the driver.

Spectral properties of the coherence collapsed state of a semiconductor laser with delayed optical feedback

Abstract: A theoretical analysis of the coherence collapsed state for a single-mode semiconductor laser with a moderate amount of delayed optical feedback that is operating well above threshold is presented. Both phase fluctuations and the amplitude fluctuations satisfy a bivariate Gaussian distribution. The autocorrelation properties are determined self-consistently from the rate equations. The asymmetry of the optical power spectrum is discussed. >

Reflectometry by means of optical-coherence modulation

Abstract: Novel reflectometry with millimetre or submillimetre spatial resolution is proposed to evaluate optical components or circuits. The optical coherence is modulated to have a periodic delta-function shape along the optical path, and the backscattering intensity is obtained directly. The experiment demonstrates a resolution of about 10 mm in air with a Fabry-Perot laser diode.

Symmetry and isotropic coherence transfer

Abstract: A general treatment of spin dynamics during coherence transfer is presented as an example of a many-body problem under strong coupling. In a previous paper, coherence transfer by isotropic mixing (IM) between two coupled spins was examined. However, the greatest experimental interest in the technique of IM centers on the transfer of coherence between spins that are not directly coupled. The dynamics of coherence transfer in multiple spin systems are cast in a superoperator formulation, in which diagonalization of the mixing Liouvilian matrix allows derivation of the coherence transfer frequencies from the eigenvalues. Development of a particular basis of the spin density operator space facilitates diagonalization of the strong coupling Liouvillian. This basis is completely reduced with respect to rotation, permutation, and particle number. Treatment of the many-body aspect is based on an approach using the Young tableaux. The exact calculation of the dynamics of coherence transfer in a three spin system i...

Mode selection and stability of a semiconductor laser with weak optical feedback

Abstract: The authors present measurements that show that a semiconductor laser with weak optical feedback oscillates in the external-cavity mode with the narrowest linewidth rather than the mode with minimum threshold gain. A frequency fluctuation model that explains this narrowest linewidth operation is proposed. This model also correctly predicts mode-hopping frequencies. It is shown that the frequency span of the external-cavity modes determines the critical feedback level that causes coherence collapse. >

Effects of Atomic Coherence on Collapses and Revivals in the Binomial State of the Field

Abstract: The effects of the coherence between the states of a two-level atom on the phenomenon of collapses and revivals in an undamped binomial state of the electromagnetic field are investigated. It is found that the Rabi oscillations exhibit qualitatively different behaviour for different phases of coherence between the levels. This behaviour in the binomial state of the field is in contrast with that in a coherent state field, in which case the Rabi oscillations are qualitatively the same for all values of the coherence between the two atomic levels.

A simulation for spaceborne SAR imagery of a distributed, moving scene

Abstract: A computer simulation that is designed to represent aspects of spaceborn synthetic-aperture radar (SAR) imagery of the ocean surface is presented. The simulation is unique in that a scatterer density (per resolution cell) is explicitly included, thus allowing the incorporation of various scattering natures, from purely specular to purely diffuse. The simulation may be applied to ocean surface wave imaging, since velocity bunching and scene coherence times are also included. Certain assumptions inherent in the velocity bunching formulation limit the applicability of the simulation in its present form to spaceborn SAR systems only. Two experiments based on this simulation are considered: (1) the effect of varying the target density; and (2) the effect of the mean scene coherence time in the imaging of a ocean swell system. >

Bath‐induced vibronic coherence transfer effects on femtosecond time‐resolved resonant light scattering spectra from molecules

Abstract: Effects of vibronic coherence transfer induced by the heat bath on ultrafast time‐resolved resonant light scattering (RLS) spectra are theoretically investigated within the master equation approach. The vibronic coherence initially created by a coherent optical excitation transfers to other vibronic coherent states due to inelastic interactions between the vibronic system concerned (the relevant system) and the heat bath. The vibronic coherence transfer results in the quantum beats in the time‐resolved RLS spectra. The bath‐induced vibronic transition operator is derived in the double space representation of the density matrix theory. Model calculations of the femtosecond (fs) time‐resolved RLS spectra are performed to demonstrate the effects of the bath‐induced vibronic coherence transfer.

Scattering of electromagnetic fields of any state of coherence from space-time fluctuations

Abstract: Developpement d'une theorie, valable a la precision de l'approximation de Born du 1er ordre, de la diffusion de champs electromagnetiques, applicable dans des conditions beaucoup plus generales que les theories courantes. L'analyse n'est pas restreinte a un milieu en equilibre thermique et s'applique dans les conditions de resonance et de non resonance du milieu

k-photon Jaynes-Cummings model with coherent atomic preparation: Squeezing and coherence.

Abstract: The k-photon Jaynes-Cummings model is studied with respect to the properties of the radiation mode. In particular, the generation of coherence and squeezing in the one- and two-photon model is examined for the case that the field is initially in the vacuum state and the atom is prepared in a coherent superposition of states. Coherence is found in the one-photon model, and squeezing appears in both the one- and the two-photon model, the maximum noise reduction being 25% and 45%, respectively. In the one-photon model coherent squeezed radiation is generated, whereas the two-photon model yields a squeezed vacuum. For an initial atomic preparation close to the ground state, this squeezed vacuum may be interpreted as the result of two-photon interaction between the quantized cavity field and a classical atomic current.

Noise in nearly-single-mode semiconductor lasers.

Abstract: We report the results of a numerical study of nearly-single-mode semiconductor lasers. The stochastic nonlinear dynamical equations of the field and population inversion are integrated and the intensity fluctuations and line shapes of the laser are investigated. The second mode affects the coherence of the light output appreciably, particularly near the laser threshold regime. The accuracy of the linearized theory for a single-mode laser is tested from far below to far above threshold. It is found to be very accurate both far below and far above threshold, but significant discrepancies are seen in the threshold region. We automatically include the coupling of the intensity and phase fluctuations in the line-shape calculations, resulting in the experimentally observed asymmetry of the relaxation oscillation sidebands. The numerical technique developed here is easily applicable to multimode lasers and is shown to be a convenient and powerful probe of the coherence properties of semiconductor lasers.

A theoretical study of coherence transfer by isotropic mixing

Abstract: Computer simulations of coherence transfer functions under the isotropic mixing hamiltonian are presented for amino acids and the deoxyribose moiety in DNA. They allow the determination of the optimum mixing time in a two-dimensional TOCSY experiment. The effects of experimental parameters and pulse imperfections are discussed for practical pulse sequences.

Demonstration of the phase coherence of the superconducting wavefunctions between conventional and high- Tc superconductors

Abstract: IF phase coherence of the quantum-mechanical wavefunctions between conventional (BCS) and high-transition-temperature (high-Tc) superconductors were shown to exist, this would place powerful constraints on the as yet unknown mechanism in the latter materials. Here we demonstrate the quantum coherence of the macroscopic superconducting wavefunctions of a conventional and a high-Tc superconductor by the observation of persistent supercurrents and the quantization of magnetic flux in units of h/2e within a composite ring of niobium and YBa2Cu3O7 (YBCO). This shows unambiguously that the macroscopic wavefunctions are coupled at the interface between the two superconductors, regardless of the mechanism and nature of electronic pairing in high-Tc superconductors. The experiment is a variant of an earlier measurement in which the quantization of flux was measured in an YBCO superconductor ring1. In our experiment, an adjustable niobium point was used to bridge the gap in a superconducting circuit of YBCO. In addition to observing persistent supercurrents around the ring with discrete flux states separated by the flux quantum, h/2e, the critical current across the junctions also exhibits the expected flux-quantum periodicity. This provides the first demonstration of a d.c. superconducting quantum interference device (SQUID) formed from a composite high-Tc/BCS superconducting circuit.