368 research outputs found
Polarization dependent Brillouin gain in randomly birefringent fibers
An extensive study of the alignment between the pump, the signal and the
polarization dependent gain (PDG) vectors in stimulated Brillouin amplification
in randomly birefringent fibers is realized by numerically integrating the
equations governing the propagation. At the fiber output, the signal tends to
align to the PDG vector for large pump power because of the nonlinear
polarization pulling effect. The PDG vector, for large random birefringence,
aligns to a state that has the same linear component of the pump but opposite
circular component.Comment: 3 pages submitted to IEEE Photonics Technology Letter
Phase-Sensitive Mode Conversion and Equalization in a Few Mode Fiber Through Parametric Interactions
The parametric interaction in few mode fibers is theoretically and numerically studied in the particular case in which the signal and the idler waves are frequency degenerate but mode nondegenerate. Under simplifying hypotheses, we derive analytical formulas for the phase-insensitive and phase-sensitive amplification gain and conversion efficiency. The analytical formulas are in very good agreement with the numerical solutions of a full vectorial model that takes into account losses, mode coupling, and all possible four-wave mixing interactions. In the phase-sensitive regime, we predict that for small input pump powers, a large and tunable phase-sensitive extinction ratio can be achieved on one mode, whereas the other mode power remains essentially unaffected. Finally, in the high-gain regime, the self-equalization of the output power on different modes can be also achieved
Polarization control for slow and fast light in fiber optical, Raman-assisted, parametric amplification
Efficient slow and fast light fiber devices based on narrow band optical
parametric amplification require a strict polarization control of the waves
involved in the interaction. The use of high birefringence and spun fibers is
studied theoretically, possible impairments evaluated, and design parameters
determined.Comment: 20 pages, 7 figures, preprint submitted to Comptes Rendue
Single pump, parametric amplification in randomly-birefringent, unidirectionally spun fibers
A systematic study of the effects of polarization mode dispersion on
broad-band and narrow-band, single pump, fiber parametric amplifiers is
realized through numerical solutions of the equations governing the
interaction. The nonlinear polarization rotation is shown to be a relevant
effect that can increase gain randomness when it mixes with polarization mode
dispersion. In unidirectionally spun fibers the signal-pump alignment can be
highly increased and the gain enhanced. However, in spite of the enhanced
alignment, large polarization mode dispersion phase-mismatches the interaction
and the gain decreases to zero.Comment: 3 pages, 4 pictures, submitted to Photonics Technology Letter
The architecture of partisan debates: The online controversy on the no-deal Brexit
We propose a framework to analyse partisan debates that involves extracting, classifying and exploring the latent argumentation structure and dynamics of online societal controversies. In this paper, the focus is placed on causal arguments, and the proposed framework is applied to the Twitter debate on the consequences of a hard Brexit scenario. Regular expressions based on causative verbs, structural topic modelling, and dynamic time warping techniques were used to identify partisan faction arguments, as well as their relations, and to infer agenda-setting dynamics. The results highlight that the arguments employed by partisan factions are mostly constructed around constellations of effect-classes based on polarised verb groups. These constellations show that the no-deal debate hinges on structurally balanced building blocks. Brexiteers focus more on arguments related to greenfield trading opportunities and increased autonomy, whereas Remainers argue more about what a no-deal Brexit could destroy, focusing on hard border issues, social tensions in Ireland and Scotland and other economy- and healthcare-related problems. More notably, inferred debate leadership dynamics show that, despite their different usage of terms and arguments, the two factions’ argumentation dynamics are strongly intertwined. Moreover, the identified periods in which agenda-setting roles change are linked to major events, such as extensions, elections and the Yellowhammer plan leak, and to new issues that emerged in relation to these events
Convective instability induced by nonlocality in nonlinear diffusive systems
We consider a large class of nonlinear diffusive systems with nonlocal
coupling. By using a non-perturbative analytical approach we are able to
determine the convective and absolute instabilities of all the uniform states
of these systems. We find a huge window of convective instability that should
provide a great opportunity to study experimentally and theoretically noise
sustained patterns.Comment: 5 pages, accepted for publication in PR
Macroscopic quantum fluctuations in noise-sustained optical patterns
We investigate quantum effects in pattern formation for a degenerate optical parametric oscillator with walk-off. This device has a convective regime in which macroscopic patterns are both initiated and sustained by quantum noise. Familiar methods based on linearization about a pseudoclassical field fail in this regime and new approaches are required. We employ a method in which the pump field is treated as a c-number variable but is driven by the c-number representation of the quantum subharmonic signal field. This allows us to include the effects of the fluctuations in the signal on the pump, which in turn act back on the signal. We find that the nonclassical effects, in the form of squeezing, survive just above the threshold of the convective regime. Further, above threshold, the macroscopic quantum noise suppresses these effects
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