Spatiotemporal mode-locking in multimode fiber lasers

A laser is based on the electromagnetic modes of its resonator, which provides the feedback required for oscillation. Enormous progress has been made in controlling the interactions of longitudinal modes in lasers with a single transverse mode. For example, the field of ultrafast science has been built on lasers that lock many longitudinal modes together to form ultrashort light pulses. However, coherent superposition of many longitudinal and transverse modes in a laser has received little attention. The multitude of disparate frequency spacings, strong dispersions, and complex nonlinear interactions among modes greatly favor decoherence over the emergence of order. Here we report the locking of multiple transverse and longitudinal modes in fiber lasers to generate ultrafast spatiotemporal pulses. We construct multimode fiber cavities using graded-index multimode fiber (GRIN MMF). This causes spatial and longitudinal mode dispersions to be comparable. These dispersions are counteracted by strong intracavity spatial and spectral filtering. Under these conditions, we achieve spatiotemporal, or multimode (MM), mode-locking. A variety of other multimode nonlinear dynamical processes can also be observed. Multimode fiber lasers thus open new directions in studies of three-dimensional nonlinear wave propagation. Lasers that generate controllable spatiotemporal fields, with orders-of-magnitude increases in peak power over existing designs, should be possible. These should increase laser utility in many established applications and facilitate new ones

Cavity Light Bullets: 3D Localized Structures in a Nonlinear Optical Resonator

We consider the paraxial model for a nonlinear resonator with a saturable absorber beyond the mean-field limit and develop a method to study the modulational instabilities leading to pattern formation in all three spatial dimensions. For achievable parametric domains we observe total radiation confinement and the formation of 3D localised bright structures. At difference from freely propagating light bullets, here the self-organization proceeds from the resonator feedback, combined with diffraction and nonlinearity. Such "cavity" light bullets can be independently excited and erased by appropriate pulses, and once created, they endlessly travel the cavity roundtrip. Also, the pulses can shift in the transverse direction, following external field gradients.Comment: 4 pages, 3 figures, simulations files available at http://www.ba.infn.it/~maggipin/PRLmovies.htm, submitted to Physical Review Letters on 24 March 200

Parameter Optimization in Groundwater using Proper Orthogonal Decomposition as a Reduced Modeling Technique

http://www.epsmso.gr/all_conf_index/abstracts/ic-scce_2012_abs054.pdfInternational audienceThis paper deals with different approaches of applying Proper Orthogonal Decomposition in the field of groundwater flow, specifically the Richards equation, which is a convection-diffusion partial differential equation governing the behaviour of unsaturated fluid flow through a porous medium. The motivation for this research is the need to reduce computational complexity in inverse modelling studies, where a significant number of simulations are needed to determine suitable model parameters. Three different methods of implementing Proper Orthogonal Decomposition are explored. The first method is the Petrov-Galerkin method, a method well suited to speeding up linear problems. The second method is a "Hybrid" method, and proposes a linearization of all non-linear functions, building upon the Petrov-Galerkin approach. As such, it is suitable for use in the non-saturated groundwater zone. The third method combines the use of kriging and Proper Orthogonal to create a non-intrusive model for comparison purposes. With these three methods, the suitability of Proper Orthogonal as a reduced modelling method for unsaturated groundwater flow is shown

An Analysis of the Decay B→D∗XℓνˉℓB \rightarrow D^* X \ell \bar\nu_\ell with Predictions from Heavy Quark and Chiral Symmetry

This paper considers the implications of the heavy quark and chiral symmetries for the semi-leptonic decay $B \rightarrow D^* X \ell \bar \nu_\ell$. The general kinematic analysis for decays of the form {\sl pseudoscalar meson $\rightarrow$ vector meson $+$ pseudoscalar meson $+$ lepton $+$ anti-lepton} is presented. This formalism is applied to the above exclusive decay which allows the differential decay rate to be expressed in a form that is ideally suited for the experimental determination of the different form factors for the process through angular distribution measurements. Heavy quark and chiral symmetry predictions for the form factors are presented, and the differential decay rate is calculated in the kinematic region where chiral perturbation theory is valid.Comment: 15 pages, uses jytex.tex and tables.tex; 3 figures not included but available on reques

Unbalanced exchange flow and its implications for the night cooling of buildings by displacement ventilation

AbstractPassive ventilation of buildings at night forms an essential part of a low-energy cooling strategy, enabling excess heat that has accumulated during the day to self-purge and be replaced with cooler night air. Instrumental to the success of a purge are the locations and areas of ventilation openings, and openings positioned at low and at high levels are a common choice as there is then the expectation that a buoyancy-driven displacement flow will establish and persist. Desirable for their efficiency, displacement flows guide excess heat out through high-level openings and cooler air in through low-level openings. Herein we show that displacement flow cannot be maintained for the full duration of a purge. Instead, the flow must transition to an ‘unbalanced exchange flow’, whereby the cool inflow of air at low level is maintained but there is now a warm outflow and a cool inflow occurring simultaneously at the high-level opening. The internal redistribution of heat caused by this exchange alters the rate at which heat is self-purged and the time thought necessary to complete a purge. We develop a theoretical model that captures and predicts these behaviours. Our approach is distinct from all others which assume that a displacement flow will persist throughout the purge. Based on this enhanced understanding, and specifically that the transition to unbalanced exchange flow changes the rate of cooling and resultant emptying times, we anticipate that practitioners will be better placed to design passive systems that meet their target specifications for cooling.</jats:p

Resonant Absorption in the AGN spectra emerging from photoionized gas: differences between steep and flat ionizing continua

We present photoionization models accounting for both photoelectric and resonant absorption. Resonance absorption lines from C, O, Ne, Mg, Si S and Fe between 0.1 and 10 keV are treated. In particular we consider the complex of almost 60 strong Fe L absorption lines around 1 keV. We calculate profiles, intensities and equivalent widths of each line, considering both Doppler and natural broadening mechanisms. Doppler broadening includes a term accounting for turbulence of the gas along the line of sight. We computed spectra transmitted by gas illuminated by drastically different ionizing continua and compared them to spectra observed in flat X-ray spectrum, broad optical emission line type 1 AGN, and steep X-ray spectrum, narrow optical emission line type 1 AGN. We show that the $\sim 1$ keV absorption feature observed in moderate resolution X-ray spectra of several Narrow Line Seyfert 1 galaxies can be explained by photoionization models, taking into account for resonance absorption, without requiring relativistic outflowing velocities of the gas, if the physical properties of these absorbers are close to those found in flat X-ray spectrum Seyfert 1 galaxies.Comment: 22 pages, 10 figures. Accepted for publication on Ap

QCD Corrections and the Endpoint of the Lepton Spectrum in Semileptonic B Decays

Recently, Neubert has suggested that a certain class of nonperturbative corrections dominates the shape of the electron spectrum in the endpoint region of semileptonic $B$ decay. Perturbative QCD corrections are important in the endpoint region. We study the effects of these corrections on Neubert's proposal. The connection between the endpoint of the electron spectrum in semileptonic $B$ decay and the photon spectrum in $b\rightarrow s\gamma$ is outlined.Comment: 18 pages, uses REVTeX, UCSD/PTH 93-38, CALT-68-1910, JHU-TIPAC-930029 (some changes to the discussion of subleading radiative corrections, and minor typos fixed

The Decay D0→Kˉ∗0π−e+νeD^0\to \bar K^{*0} \pi^- e^+ \nu_e in the Context of Chiral Perturbation Theory

We study the decay $D^0\rightarrow \bar K^{*0} \pi^- e^+ \nu_e$, using $SU(2)_L \otimes SU(2)_R$ chiral perturbation theory for heavy charmed mesons and vector mesons, in the kinematic regime where $p_M \cdot p_\pi/m_M$ (here $M = D^0$ or $\bar K^{*0}$) is much smaller than the chiral symmetry breaking scale, $\Lambda_{\chi SB}$ ( $\Lambda_{\chi SB} \sim$ 1 GeV). We present the leading diagrams and amplitude, and calculate the rate, in the region where, to leading order in our calculations, the $\bar K^{*0}$ is at zero recoil in the $D^0$ rest frame. The rate thus calculated is given in terms of a known form factor and depends on the $DD^* \pi$ coupling constant $g_D$ of the heavy (charmed) meson chiral perturbation theory Lagrangian. A measurement of the above decay, in the aforementioned kinematic regime, can result in the extraction of an experimental value for $g_D$, accurate at the level of our approximations, and give us a measure of the validity of approaches based on chiral perturbation theory in studying similar processes.Comment: 17 pages, Latex, 2 embedded postscript figure

Measurement of the analyzing power in pp elastic scattering in the peak CNI region at RHIC

We report the first measurements of the A_N absolute value and shape in the -t range from 0.0015 to 0.010GeV/c^2 with a precision better than 0.005 for each A_N data point using a polarized atomic hydrogen gas jet target and the 100 GeV RHIC proton beam.Comment: 4 pages, 5 figure