Nonlinear refractive index in erbium-doped optical amplifiers

Nonlinear effects in erbium-doped fibre amplifiers are analysed by a numerical approach based on the combined use of the finite element method and the Runge-Kutta algorithm. In particular, the variation of the refractive index due to the perturbation in the gain coefficient is evaluated through the nonlinear Kramers-Kronig relations. For the first time the nonlinear coefficients of the polynomial expansion of the refractive index variation have been calculated. Several kinds of perturbation, produced by signal and pump intensity, by amplifier length and by dopant concentration have been considered to investigate the refractive index change behaviour. The use of the finite element method allows the local evaluation of the pump and signal intensities without introducing any simplifying hypothesis on the electromagnetic field distribution.     

Optical parametric amplification in all-silica triangular-core photonic crystal fibers

The present analysis demonstrates that photonic crystal fibers can be designed to enhance their properties for optical parametric amplification. In particular, it is possible to achieve high nonlinear coefficient values and flattened dispersion curves around 1550 nm to fully exploit the four-wave-mixing effect. These results have been obtained for triangular-core photonic crystal fibers without the need of modifying the refractive-index profile by adding dopants in the fiber cross section.     

Inner cladding influence on mode interaction in symmetry-free photonic crystal fibers under heat load

A further output power scaling of Yb-doped double-cladding photonic crystal fiber lasers, required for practical applications in many industrial fields, is currently prevented by thermal effects, which significantly worsen the beam quality. In this work the mode interaction and, consequently, the single-mode regime of Yb-doped air-silica symmetry-free photonic crystal fibers have been numerically investigated in different heat load conditions using simulation tools based on the finite element method. The effect of the main inner cladding geometric parameters, in particular of the inner cladding dimension, on the guided-mode delocalization has been analyzed, with the aim to identify a design of the large mode area active fiber providing single-mode operation despite the presence of thermal effects. Simulation results have demonstrated that the inner cladding enlargement causes a stronger coupling between core-guided and cladding modes, as well as an increase of the differential overlap between fundamental and first higher-order mode for moderate heat load values. However, the inner cladding influence becomes less important under severe heat load, being the fiber behaviour mainly affected by the thermally-induced refractive index change in the doped core.     

Synthesis of cns potentially active bicyclic and tricyclic derivatives of pyrazolo[1,5-a][1,3]diazepines

Reaction of 5-ethylamino-3-phenylpyrazole with dicarboxylic acid anhydrides in tetrahydrofuran in the presence of 1,3-dicyclohexylcarbodiimide, afforded bicyclic and tricyclic derivatives of pyrazolo[1,5-a][1,3]-diazepines. The tricyclic derivatives constitute a new class of compounds. Characterisation of these products was effected with ir, pmr, and ¹³C nmr spectral data.     

Sensitive observables of quantum mechanics

We prove that there are quantum mechanical observables which are sensitive to the type of state-vector (first type or second type) describing two correlated physical systems, in the sense that the expectation value of these sensitive observables is measurably different in the two cases. The proof centers around Bell's inequality since we show that in quantum mechanics forall state-vectors of the second type (and only for them) sensitive observables exist in the absence of super-selection rules. Experimental verification of the existence of sensitive observables rules out local hidden variables.     

The International Atomic Time and the Velocity of Light

Atomic clocks distributed around the world communicate with one another by means of radio signals. The synchronization signals sent by a transmitting station always reach the receiving station on time, at any hour of the day and in any season, despite the motion of the Earth. For some authors this means that these signals propagate isotropically (with one way velocity c), even with respect to the Earth's surface. In fact this may not be so; we show that the proper working of the network says nothing about the one-way velocity, as it is consistent with another theory, empirically (almost) equivalent to special relativity, in which the one-way speed of light has a directional dependence in moving frames.     

Proof of a quantum mechanical nonlocal influence

First it is proved that, in a deterministic theory, Malus' law requires that, if a photon is successively transmitted by two polarizers with appropriately chosen settings, the first transmission influences a hidden variable (co-) determining the second one. We derive from this that in an ideal EPR experiment (giving the result predicted by quantum mechanics for two correlated photons transmitted by two polarizers with suitably chosen settings) there has to be a nonlocal influence from the first transmission interaction to the second. Subsequently we argue that we can abandon determinism as an assumption so that the locality hypothesis is in any case untenable if the predictions of quantum mechanics are all correct.     

The relativity principle and the nature of time

Old and recent ideas concerning the nature of time are reviewed, starting from Mach's refusal of Newton's absolute time. Many experiments show that the slowing down of moving clocks is a real phenomenon. Such must then also be the so-called “twin paradox,” which owes its name to its evident incompatibility with the philosophy of relativism (not to be confused with the theory of relativity). The Lorentz reformulation of special relativity started by postulating physical effects of the ether, but accepted Einstein's clock synchronization. Only because of this Lorentz could not understand the advantages of an easily deducible different theory. As stressed by Popper, one of the main problems of the usual approach is the introduction of a superdeterministic universe. Recent results obtained by the author show that a theory is possible, based on relative time but on absolute simultaneity, in which the conceptual difficulties of relativity are avoided.     

Noninvariant one-way velocity of light and particle collisions

Recently published space-time transformations between inertial systems (different from the Lorentz transformations) are reviewed. Energy and momentum are defined consistently with these new transformation laws. Formally they equal the usual relativistic expressions only in the privileged frame, but numerically they do so in all inertial frames. All the precise experimental data concerning thresholds for inelastic processes, particle masses, and so on, can thus be explained also within this new theoretical framework.