298 research outputs found
Energy shedding during nonlinear self-focusing of optical beams
Self-focusing of intense laser beams and pulses of light in real nonlinear media is in general accompanied by material losses that require corrections to the conservative Nonlinear Schrödinger equations describing their propagation. Here we examine loss mechanisms that exist even in lossless media and are caused by shedding of energy away from the self-trapping beam making it to relax to an exact solution of lower energy. Using the conservative NLS equations with absorbing boundary conditions we show that energy shedding not only occurs during the initial reshaping process but also during oscillatory propagation induced by saturation of the nonlinear effect. For pulsed input we also show that, depending on the sign and magnitude of dispersion, pulse splitting, energy shedding, collapse or stable self-focusing may result
Negative Temperature States in the Discrete Nonlinear Schroedinger Equation
We explore the statistical behavior of the discrete nonlinear Schroedinger
equation. We find a parameter region where the system evolves towards a state
characterized by a finite density of breathers and a negative temperature. Such
a state is metastable but the convergence to equilibrium occurs on astronomical
time scales and becomes increasingly slower as a result of a coarsening
processes. Stationary negative-temperature states can be experimentally
generated via boundary dissipation or from free expansions of wave packets
initially at positive temperature equilibrium.Comment: 4 pages, 5 figure
Self-pulsing dynamics in a cavity soliton laser
The dynamics of a broad-area vertical-cavity surface-emitting laser (VCSEL) with frequency-selective feedback supporting bistable spatial solitons is analyzed experimentally and theoretically. The transient dynamics of a switch-on of a soliton induced by an external optical pulse shows strong self-pulsing at the external-cavity round-trip time with at least ten modes excited. The numerical analysis indicates an even broader bandwidth and a transient sweep of the center frequency. It is argued that mode-locking of spatial solitons is an interesting and viable way to achieve three-dimensional, spatio-temporal self-localization and that the transients observed are preliminary indications of a transient cavity light bullet in the dynamics, though on a non negligible background
Temperature dependence of spontaneous switch-on and switch-off of laser cavity solitons in vertical-cavity surface-emitting lasers with frequency-selective feedback
A systematic experimental and numerical investigation of the conditions for the spontaneous formation of laser cavity solitons in broad-area vertical-cavity surface-emitting lasers with frequency-selective feedback by a volume Bragg grating is reported. It is shown that the switching thresholds are controlled by a combination of frequency shifts induced by ambient temperature and Joule heating. The gain level has only a minor influence on the threshold but controls mainly the power of the solitons. At large initial detuning and high threshold gain, the first observed structure can be a high order soliton. In real devices spatial disorder in the cavity length causes a pinning of solitons and a dispersion of thresholds. The experimental observations are in good agreement with numerical simulations taking into account disorder and the coupling of gain and cavity resonance due to Joule heating. In particular, we demonstrate that the existence of the traps explain the spontaneous switch on of the solitons, but do not modify the soliton shape significantly, i.e. the observed solitons are a good approximation of the ones expected in a homogeneous system
Chaotic resonances of a Bose-Einstein condensate in a cavity pumped by a modulated optical field
We present a theoretical analysis of a Bose-Einstein condensate (BEC) enclosed in an optical cavity driven by a modulated external laser beam where the cavity-field variable is adiabatically eliminated. The modulation of the amplitude of the pump laser induces nonlinear resonances and the widespread presence of chaotic oscillations even when repulsive atom-atom interactions are negligible. Close to resonance, varying the modulation amplitude by just a few percent causes abrupt and erratic changes to the output laser intensity with peak power increasing by almost an order of magnitude. We also use a simplified model of the BEC-cavity system that considers only a small number of spatial modes of the BEC to show that, despite the disruptive presence of a modulation in the pump beam, the system can still be considered to be low-dimensional
Frequency and phase locking of laser cavity solitons
Self-localized states or dissipative solitons have the freedom of translation in systems with a homogeneous background. When compared to cavity solitons in coherently driven nonlinear optical systems, laser cavity solitons have the additional freedom of the optical phase. We explore the consequences of this additional Goldstone mode and analyse experimentally and numerically frequency and phase locking of laser cavity solitons in a vertical-cavity surface-emitting laser with frequency-selective feedback. Due to growth-related variations of the cavity resonance, the translational symmetry is usually broken in real devices. Pinning to different defects means that separate laser cavity solitons have different frequencies and are mutually incoherent. If two solitons are close to each other, however, their interaction leads to synchronization due to phase and frequency locking with strong similarities to the Adler-scenario of coupled oscillators
Optical pattern formation with a 2-level nonlinearity
We present an experimental and theoretical investigation of spontaneous
pattern formation in the transverse section of a single retro-reflected laser
beam passing through a cloud of cold Rubidium atoms. In contrast to previously
investigated systems, the nonlinearity at work here is that of a 2-level atom,
which realizes the paradigmatic situation considered in many theoretical
studies of optical pattern formation. In particular, we are able to observe the
disappearance of the patterns at high intensity due to the intrinsic saturable
character of 2-level atomic transitions.Comment: 5 pages, 4 figure
Vector vortex solitons and soliton control in vertical-cavity surface-emitting lasers
The properties of vector vortex beams in vertical-cavity-surface emitting
lasers with frequency-selective feedback is investigated. They are interpreted
as high-order vortex solitons with a spatially non-uniform, but locally linear
polarization state. In contrast to most schemes to obtain vector vortex beams
relying on imprinting the polarization structure, vector vortex solitons form
spontaneously due to the near polarization degeneracy in vertical-cavity
devices. We observe radially, hyperbolic and spiral polarization configurations
depending on small residual anisotropies in the system and multi-stability
between different states. In addition, we demonstrate flip-flop operation of
laser solitons via in principle local electronic nonlinearities. Combining the
two themes might open up a route for a simple device enabling fast switching
between different vector vortex beams for applications. The investigations
connect nicely the fields of nonlinear science, singular optics, structured
light and semiconductor laser technology
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