Random replicators with high-order interactions

We use tools of the equilibrium statistical mechanics of disordered systems to study analytically the statistical properties of an ecosystem composed of N species interacting via random mutual interactions, as well as via deterministic self-interactions of order p>/=2. We show that the main effect of increasing the order of the interactions among the species is to make the system less competitive, in the sense that the fraction of extinct species is greatly reduced. In addition, we find that for p>2 there is a threshold value which gives a lower bound to the concentration of the surviving species, preventing then the existence of rare species and, consequently, increasing the robustness of the ecosystem to external perturbations.     

Microfluorescence analysis of nanostructuring inhomogeneity in optical fibers with embedded gallium oxide nanocrystals

A spectroscopic protocol is proposed to implement confocal microfluorescence imaging to the analysis of microinhomogeneity in the nanocrystallization of the core of fibers belonging to a new kind of broadband fiber amplifier based on glass with embedded nanocrystals. Nanocrystallization, crucial for achieving an adequate light emission efficiency of transition metal ions in these materials, has to be as homogeneous as possible in the fiber to assure optical amplification. This requirement calls for a sensitive method for monitoring nanostructuring in oxide glasses. Here we show that mapping microfluorescence excited at 633 nm by a He-Ne laser may give a useful tool in this regard, thanks to quasi-resonant excitation of coordination defects typical of germanosilicate materials, such as nonbridging oxygens and charged Ge-O-Ge sites, whose fluorescence are shown to undergo spectral modifications when nanocrystals form into the glass. The method has been positively checked on prototypes of optical fibers--preventively characterized by means of scanning electron microscopy and energy dispersive spectroscopy--fabricated from preforms of Ni-doped Li?O-Na?O-Sb?O?-Ga?O?-GeO?-SiO? glass in silica cladding and subjected to heat treatment to activate gallium oxide nanocrystal growth. The method indeed enables not only the mapping of the crystallization degree but also the identification of drawing-induced defects in the fiber cladding.     

Ultraviolet radiation- and γ radiation-induced color centers in germanium-doped silica glass and fibers

Induced optical losses and paramagnetic Ge(n) centers were investigated in germanium-doped silica glass and optical fibers after γ and ultraviolet (UV) irradiation. It was found that both types of irradiation created similar effects. By means of selective UV irradiation, Ge (1 and 3) centers were identified in optical absorption spectra, presumably as induced bands centered at 4.4 eV and 6.2 eV, respectively. Moreover, photobleaching of Ge(1) centers took place under 266-nm wavelength excitation. In optical fibers no difference was observed between γ- and UV-induced loss spectra in the wavelength range from 480 nm to 1,900 nm. Partial reversibility of the photocoloration was observed. For comparison, the coloration effects were studied in glass prepared by means of modified chemical vapor deposition (MCVD) vapor-phase axial deposition (VAD), and plasma-activated chemical vapor deposition (PCVD) techniques.     

Germania-glass-core silica-glass-cladding modified chemical-vapor deposition optical fibers: optical losses, photorefractivity, and Raman amplification

Germania-glass-core silica-glass-cladding single-mode fibers (deltan as great as 0.143) with a minimum loss of 20 dB/km at 1.85 microm were fabricated by modified chemical-vapor deposition. The fibers exhibit strong photorefractivity, with type IIa index modulation of 2 x 10(-3). A Raman gain of 300 dB/(kmW) was determined at 1.12 microm. Only 3 m of such fibers is sufficient for constructing the 10-W Raman laser at 1.12 microm with a 13-W pump at 1.07 microm.     

Yb-Bi pulsed fiber lasers

A new type of pulsed fiber laser is suggested and developed - Yb-Bi lasers. In such lasers the Yb fiber laser is Q-switched by use of a saturable absorber, a Bi-doped fiber placed in its own resonator, and pulsed lasing is obtained in both fiber lasers. Continous-wave diode-clad pumping of the Yb-Bi lasers at a 975 nm wavelength with power up to 16.5 W results in pulsed laser action in a spectral diapason of 1050-1200 nm with a maximum pulse energy of up to 100 microJ, an average power up to 7.5 W, and a repetition rate up to 100 kHz.     

Maturation of auditory temporal integration and inhibition assessed with event-related potentials (ERPs)

Background

We examined development of auditory temporal integration and inhibition by assessing electrophysiological responses to tone pairs separated by interstimulus intervals (ISIs) of 25, 50, 100, 200, 400, and 800 ms in 28 children aged 7 to 9 years, and 15 adults.

Results

In adults a distinct neural response was elicited to tones presented at ISIs of 25 ms or longer, whereas in children this was only seen in response to tones presented at ISIs above 100 ms. In adults, late N1 amplitude was larger for the second tone of the tone pair when separated by ISIs as short as 100 ms, consistent with the perceptual integration of successive stimuli within the temporal window of integration. In contrast, children showed enhanced negativity only when tone pairs were separated by ISIs of 200 ms. In children, the amplitude of the P1 component was attenuated at ISIs below 200 ms, consistent with a refractory process.

Conclusions

These results indicate that adults integrate sequential auditory information into smaller temporal segments than children. These results suggest that there are marked maturational changes from childhood to adulthood in the perceptual processes underpinning the grouping of incoming auditory sensory information, and that electrophysiological measures provide a sensitive, non-invasive method allowing further examination of these changes.     

Absorption and scattering in bismuth-doped optical fibers

The optical-loss spectra of bismuth-doped aluminosilicate glass core optical fibers have been investigated in the spectral range 200–1700 nm. Absorption (active and inactive) and scattering losses have been separated. The optical fibers under study were fabricated by methods of modified chemical vapor deposition and surface-plasma chemical vapor deposition.     

Luminescence and selective heat radiation of Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> upon the resonant and thermal laser excitation

Yb₂O₃ polycrystals with a size of up to 10 mm are synthesized using the sintering and melting of the ultrapure Yb₂O₃ powders by the CO₂-laser radiation with the power P _L ≤ 100 W at the wavelength λ = 10.6 μm at the melting point T _m = 2703 K, forming due to surface tension in melt, and crystallization in air. The analysis of the polycrystal microstructure using the methods of optical and electron microscopy and X- ray diffractometry shows that perfect oxide crystallites are formed in the course of crystallization after melting-through. The transformation of the luminescence and selective heat radiation (SHR) spectra of the Yb₂O₃ polycrystals is studied under the resonant excitation at λ ≈ 975 nm using a laser diode and the laser heating at the wavelength λ = 10.6 μm. When the resonant excitation power of the Yb³⁺ ions increases from 0.15 to 4.5 W, the Stokes luminescence of the Yb₂O₃ polycrystals is sequentially transformed into SHR and the thermal radiation of the crystal lattice. The transformation of the emission spectra of the Yb₂O₃ polycrystals with an increase in the laser heating intensity by about four orders of magnitude can be represented as the low-temperature heat radiation, spectral burst of the thermodynamically nonequilibrium SHR of the Yb³⁺ ions, and the high-temperature radiation of the crystal lattice. The temperature dependence of the luminescence spectra and SHR of the Yb₂O₃ polycrystals on the intensity of the laser and laser-thermal excitation and the concentration quenching of the Yb³⁺ luminescence in oxides indicate the key role of the interaction of the f-electron shell of the Yb³⁺ ions with the natural oscillations of the crystal lattice in the processes of the multiphonon excitation and nonradiative (multiphonon) and radiative (vibronic) relaxation.     

Furnace chemical vapor deposition bismuth-doped silica-core holey fiber

A bismuth-doped-pure-silica holey fiber is fabricated using a fiber preform made by the furnace chemical vapor deposition method. The spectroscopic properties of the fiber are studied, and laser action at λ=1450 nm with an efficiency of 12% is demonstrated.     

Bismuth-doped-glass optical fibers--a new active medium for lasers and amplifiers

Optical fibers with bismuth-doped silicate and germanate glass cores were fabricated by the modified chemical vapor deposition technique (solution and vapor-phase Bi incorporation). The fibers revealed an efficient luminescence with a maximum in the 1050-1200 nm spectral range, FWHM up to 200 nm, and a lifetime of the order of 1 ms.