1,721,028 research outputs found
Space inversion symmetry breaking and pattern selection in nonlinear optics
No full textPattern formation in nonlinear optical cavities, when an advection-like term is present, is analysed. This term breaks the space inversion symmetry causing the existence of a regime of convective instabilities, where noise-sustained structures can be found, and changing the pattern orientation and the selected wavevector. The concepts of convective and absolute instability, noise-sustained structures and the selection mechanisms in two dimensions are discussed in the case of optical parametric oscillators and a Kerr resonator. In the latter case, in which hexagons are the selected structure, we predict and observe that stripes are the most unstable structures in the initial linear transient. In the nonlinear regime of the absolute instability these stripes destabilize and hexagons form. Their orientation is dictated by that of the transient stripes and therefore by the advection term. In the convective regime we predict and observe disordered noise-sustained hexagons, preceded in space by noise-sustained stripes
Noise-sustained convective structures in nonlinear optics
No full textEvidence of noise-sustained patterns in nonlinear optical systems is given. They are found in passive optical cavities, filled by Kerr type nonlinear media, when the angle of incidence of the pump beam is not zero, in a regime of convective instability. These patterns arise as a macroscopic manifestation of dynamically amplified noise, with amplification factors of up to 10(5). We characterize the difference between noise-sustained and deterministic patterns in terms of statistical properties of the field spectral intensity
Pattern formation in presence of walk-off for a Type-II optical parametric oscillator
No full textWe show the relevance of walk-off effects in pattern formation in a type II optical parametric oscillator at frequency degeneracy. With walk-off neglected only phase patterns are formed, and the intensity distribution is homogeneous. Walk-off changes the instability from absolute to convective for some parameter range. In the absolutely unstable regime it induces for each polarization component of light a competition between two phase stripe patterns (traveling waves) of different wavelength. Phase stripe patterns at each of the wavelengths are equally likely to be selected, and, after a transient regime of coexistence, one of them takes over. In the convectively unstable regime the existence of intensity patterns sustained by noise is shown. The patterns arise from the interference between traveling waves that are generated by the dynamical amplification of noise
Two-dimensional noise-sustained structures in optical parametric oscillators
No full textThe problem of two-dimensional (2D), transverse, noise-sustained pattern formation is theoretically and numerically studied, in the case of an optical parametric oscillator, for negative signal detuning. This gives a complete analysis of a 2D, convective, pattern forming system which is also relevant to more general 2D physical systems. For the optical parametric oscillator, the transversal walk-off due to the nonlinear crystal birefringence, exploited to phase match the frequency down-conversion process, turns the instability to convective up to a certain threshold. In this regime, noise-sustained patterns can be observed. These structures are a macroscopic manifestation of amplified microscopic noise which, in the context of optics, can be of quantum nature. Directly observable properties of the near and far field as well as statistical properties of the spectral intensity help to distinguish noise- from dynamics-sustained structures. Moreover, the analysis indicates that the walk-off term breaks the rotational symmetry of the 2D model. This causes a preferential selection of the stripe orientation, which would be otherwise random, the modulus of the wave vector being the only restricted value. At the convective threshold an entire set of spatial modes becomes unstable, whereas the threshold of absolute instability depends on the relative orientation of the mode. Beyond the threshold for absolute instability, this causes the coexistence, in the linear regime of evolution, of modes that are absolutely unstable, and others that are only convectively unstable. The numerical solutions of the dynamical equations of the system under study confirm the analytical predictions for the value of the instability thresholds and the kind of pattern selected. Moreover, they allow us to investigate the nonlinear regime showing qualitatively the coexistence of modes with different types of instability and giving a quantitative characterization of the transition from noise-sustained to dynamics-sustained structures
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