Angular dependence of the thermal-lens effect on LiSrAlF6 and LiSrGaF6 single crystals

We apply thermal-lens (TL) spectrometry to measure the angular dependence of the TL effect on colquiriite single crystals. The experiments were performed with LiSrAlF(6) and LiSrGaF(6) using a two-beam mode-mismatched configuration. The results show that it is possible to minimize the TL effect by selecting the appropriate crystal orientation. Our data also show that the anisotropy of the linear thermal expansion coefficient drives the amplitude of the TL effect, including the inversion from focusing to defocusing as the crystal orientation angle tends to the c-axis direction. The results may be useful for those working to develop a high-power laser using LiSrAlF(6)(:Cr) and LiSrGaF(6)(:Cr) single crystals, allowing for optimization of the designed laser cavity.     

Diode-pumped Yb3+:KYF4 femtosecond laser

Passive mode locking of a diode-pumped Yb(3+):KYF(4) laser is demonstrated using a semiconductor saturable absorber mirror. A high-stability, transform-limited pulse train with a repetition rate of 57 MHz is generated. Solitonlike pulses with maximum average output power of 250 mW, minimum pulse duration of 170 fs, and rms time jitter of 360 fs were obtained.     

Top-hat cw-laser-induced time-resolved mode-mismatched thermal lens spectroscopy for quantitative analysis of low-absorption materials

Thermal lens spectroscopy is a highly sensitive and versatile photothermal technique for material analysis, providing optical and thermal properties. To use less expensive multimode non-Gaussian lasers for quantitative analysis of low-absorption materials, this Letter presents a theoretical model for time-resolved mode-mismatched thermal lens spectroscopy induced by a cw laser with a top-hat profile. The temperature profile in a sample was calculated, and the intensity of the probe beam center at the detector plane was also derived using the Fresnel diffraction theory. Experimental validation was performed with glass samples, and the results were found well consistent with literature values of the thermo-optical properties of the samples.     

Numerical Modeling of PMD

A procedure for evaluating system performance considering all orders of polarization mode dispersion (PMD) is presented. The method provides engineering rules for computing system penalty introduced by PMD. Computer simulation of PMD induced penalty indicates that for generating a realistic system budget, effect of higher order PMD in presence of chromatic dispersion is essential. Considering all of these penalties, in order to obtain a system-reach of 3200 km at 40 Gb/s, a fiber PMD of less than 0.04 ps/km^1/2 is required when at the signal wavelength chromatic dispersion is 0. In presence of 18 ps/nm chromatic dispersion for the same system reach this PMD value reduces to about 0.02 ps/km^1/2.     

Ultrasound-assisted oxidative process for sulfur removal from petroleum product feedstock

A procedure using ultrasonic irradiation is proposed for sulfur removal of a petroleum product feedstock. The procedure involves the combination of a peroxyacid and ultrasound-assisted treatment in order to comply with the required sulfur content recommended by the current regulations for fuels. The ultrasound-assisted oxidative desulfurization (UAOD) process was applied to a petroleum product feedstock using dibenzothiophene as a model sulfur compound. The influence of ultrasonic irradiation time, oxidizing reagents amount, kind of solvent for the extraction step and kind of organic acid were investigated. The use of ultrasonic irradiation allowed higher efficiency for sulfur removal in comparison to experiments performed without its application, under the same reactional conditions. Using the optimized conditions for UAOD, the sulfur removal was about 95% after 9 min of ultrasonic irradiation (20 kHz, 750 W, run at 40%), using hydrogen peroxide and acetic acid, followed by extraction with methanol.     

Quantum Walks,Weyl Equation and the Lorentz Group

Quantum cellular automata and quantum walks provide a framework for the foundations of quantum field theory, since the equations of motion of free relativistic quantum fields can be derived as the small wave-vector limit of quantum automata and walks starting from very general principles. The intrinsic discreteness of this framework is reconciled with the continuous Lorentz symmetry by reformulating the notion of inertial reference frame in terms of the constants of motion of the quantum walk dynamics. In particular, among the symmetries of the quantum walk which recovers the Weyl equation—the so called Weyl walk—one finds a non linear realisation of the Poincaré group, which recovers the usual linear representation in the small wave-vector limit. In this paper we characterise the full symmetry group of the Weyl walk which is shown to be a non linear realization of a group which is the semidirect product of the Poincaré group and the group of dilations.     

The revival of the Baldwin effect

The idea that a genetically fixed behavior evolved from the once differential learning ability of individuals that performed the behavior is known as the Baldwin effect. A highly influential paper [G.E. Hinton, S.J. Nowlan, Complex Syst. 1, 495 (1987)] claimed that this effect can be observed in silico, but here we argue that what was actually shown is that the learning ability is easily selected for. Then we demonstrate the Baldwin effect to happen in the in silico scenario by estimating the probability and waiting times for the learned behavior to become innate. Depending on parameter values, we find that learning can increase the chance of fixation of the learned behavior by several orders of magnitude compared with the non-learning situation.     

Comparison of Value-at-Risk models using the MCS approach

This paper compares the Value-at-Risk (VaR) forecasts delivered by alternative model specifications using the Model Confidence Set (MCS) procedure recently developed by Hansen et al. (Econometrica 79(2):453–497, 2011). The direct VaR estimate provided by the Conditional Autoregressive Value-at-Risk (CAViaR) models of Engle and Manganelli (J Bus Econ Stat 22(4):367–381, 2004) are compared to those obtained by the popular Autoregressive Conditional Heteroskedasticity (ARCH) models of Engle (Econometrica 50(4):987–1007, 1982) and to the Generalised Autoregressive Score (GAS) models recently introduced by Creal et al. (J Appl Econom 28(5):777–795, 2013) and Harvey (Dynamic models for volatility and heavy tails: with applications to financial and economic time series. Cambridge University Press, Cambridge, 2013). The MCS procedure consists in a sequence of tests which permits to construct a set of “superior” models, where the null hypothesis of Equal Predictive Ability (EPA) is not rejected at a certain confidence level. Our empirical results, suggest that, during the European Sovereign Debt crisis of 2009–2010, highly non-linear volatility models deliver better VaR forecasts for the European countries as opposed to other regional indexes. Model comparisons have been performed using the \(\textsf {R}\) package MCS developed by the authors and freely available at the CRAN website.     

Self-dual sets of type (m, n) in projective spaces

In this paper we introduce and analyze the notion of self-dual k-sets of type (m, n). We show that in a non-square order projective space such sets exist only if the dimension is odd. We prove that, in a projective space of odd dimension and order q, self-dual k-sets of type (m, n), with , are of elliptic and hyperbolic type, respectively. As a corollary we obtain a new characterization of the non-singular elliptic and hyperbolic quadrics.     

Gap Functions and Lyapunov Functions

Equilibrium problems play a central role in the study of complex and competitive systems. Many variational formulations of these problems have been presented in these years. So, variational inequalities are very useful tools for the study of equilibrium solutions and their stability. More recently a dynamical model of equilibrium problems based on projection operators was proposed. It is designated as globally projected dynamical system (GPDS). The equilibrium points of this system are the solutions to the associated variational inequality (VI) problem. A very popular approach for finding solution of these VI and for studying its stability consists in introducing the so-called "gap-functions", while stability analysis of an equilibrium point of dynamical systems can be made by means of Lyapunov functions. In this paper we show strict relationships between gap functions and Lyapunov functions.