Modeling and optimization of short Er/sup 3+/-Yb/sup 3+/ codoped fiber lasers

A theoretical study of short Er/sup 3+/- Yb/sup 3+/ codoped fiber lasers (EYDFLs) is presented. Examples include centimeter-length EYDFLs pumped at 978 nm, with distributed Bragg reflector (DBR) mirrors at both fiber ends. Approximate quasi-analytical solutions are shown to be in excellent agreement with the exact numerical solution of the rate equations. Using the quasi-analytical model, we demonstrate a design optimization of a short EYDFL. The effects of the Yb/sup 3+/ concentration, laser wavelength, output mirror reflectivity, and pump wavelength are considered and discussed with relation to the optimal laser length and output efficiency.     

Modeling high-power Er/sup 3+/-Yb/sup 3+/ codoped fiber lasers

High-power cladding-pumped Er/sup 3+/-Yb/sup 3+/ codoped fiber lasers (EYDFL) are analyzed with a detailed rate-equations model that takes into account the energy transfer between Yb/sup 3+/ and Er/sup 3+/. Examples of EYDFL with distributed Bragg reflector (DBR) mirrors at both fiber ends are demonstrated. Approximate quasi-analytical solutions to the rate equations show good agreement with the exact numerical solutions for practical operating conditions. An "optimum design" of the laser is discussed in relation to the fiber length that maximizes the output power.     

Fuzzy Logic and Chaos Theory in Time Series Forecasting

This paper presents a method of time series forecasting based on the integration of fuzzy logic and chaos theory. The proposed method has two stages. On the first stage, we consider the time series as a dynamic system and using the methods of mutual information and false nearest neighbors, as a part of applied chaos theory, we reconstruct the phase portrait corresponding to the original time series. On the second stage, we are learning the neuro fuzzy network as a model of time series forecasting using the vectors points of reconstructed phase portrait. We consider all the formalisms necessary for understanding the method and present the results of two computer experiments proving the ability of fuzzy inference accuracy increasing using the selection of optimal parameters of time delay and phase portrait dimension.     

Adsorption and Reactivity of CO2 on Potassium-Covered Re(001)

The interaction of CO₂ with potassium-covered Re(001) has been investigated. This system has been studied by means of work function (Δϕ), optical second harmonic generation (SHG), and temperature-programmed desorption (TPD) measurements. Strong electronic interaction between carbon dioxide and potassium is observed upon adsorption at 90 K. This is indicated by a rapid quenching of the SHG signal of K following postadsorption of CO₂, with a quenching cross section of 70 Ų. Work function change measurements are consistent with such interaction, evidenced by an undepolarization effect, namely, further decrease of the work function upon CO₂ adsorption, below the minimum obtained by pure potassium. In the presence of potassium, the dissociation probability of 0.5 ML adsorbed carbon dioxide increases from 0.5 on the clean metal surface to 0.85 on 1 ML potassium-covered Re(001), information obtained from TPD measurements following heating to 1250 K. It is concluded that a K–CO₂ surface compound is formed upon adsorption at 95 K on the potassium-covered surface.     

Spectrally Resolved Approach for Modeling Short Pulse Amplification in Er$^{3+}$-Doped Fibers

We study pulse propagation in Er³⁺-doped fiber amplifiers (EDFA) within the framework of a spectrally resolved pulse rate-propagation equations model. Our model accounts for the effects of gain dispersion, gain saturation, waveguide and chromatic dispersion, and amplified spontaneous emission. This model allows us to approximate the effects of nonlinear resonant dispersion on short pulse amplification in doped fibers, without reverting to the generalized nonlinear Schroedinger equation. Numerical results of the time-dependent power spectrum of the amplified pulse demonstrate subpicosecond pulse propagation in EDFAs     

Multifocal glioma of the brain. Case report

A case is presented in which two separate concurrent astrocytomas of the brain in the same patient were successfully operated on. The patient has been followed for 3 years and remains well. The authors believe that the diagnosis of multifocal tumors can be established on clinical grounds when the tumors are remote from each other, and when there has been no recurrence of neoplasm between the lesions after a long follow-up period. It is felt that a more optimistic approach to the treatment of multifocal tumors may yield good results.     

Phosphorylation of the WWOX Protein Regulates Its Interaction with p73

We describe a molecular characterization of the interaction between the cancer-related proteins WWOX and p73. This interaction is mediated by the first of two WW domains (WW1) of WWOX and a PPXY-motif-containing region in p73. While phosphorylation of Tyr33 of WWOX and association with p73 are known to affect apoptotic activity, the quantitative effect of phosphorylation on this specific interaction is determined here for the first time. Using ITC and fluorescence anisotropy, we measured the binding affinity between WWOX domains and a p73 derived peptide, and showed that this interaction is regulated by Tyr phosphorylation of WW1. Chemical synthesis of the phosphorylated domains of WWOX revealed that the binding affinity of WWOX to p73 is decreased when WWOX is phosphorylated. This result suggests a fine-tuning of binding affinity in a differential, ligand-specific manner: the decrease in binding affinity of WWOX to p73 can free both partners to form new interactions.     

Ultrashort-Pulse High-Power Yb3+-Doped Fiber Amplifiers

Ultrashort optical pulse propagation in high-power Yb³⁺-doped fiber amplifiers (YDFA) is studied using a spectrally resolved nonlinear power equation for the coupled pulse components. The Yb³⁺ ions transitions are modeled using a rate equation. Examples for high-power YDFAs with normal dispersion show good qualitative agreement with experimental results. We analyze the effects of the incident pulse wavelength, pulse peak power, Yb³⁺ concentration, pump filling factor, fiber length, pumping configuration, pump power and nonlinear index, on the intensity distribution of short amplified pulses. We also demonstrate the spectral compression of an initially negative-chirped pulse.