17 research outputs found
Physics and Applications of Laser Diode Chaos
An overview of chaos in laser diodes is provided which surveys experimental
achievements in the area and explains the theory behind the phenomenon. The
fundamental physics underpinning this behaviour and also the opportunities for
harnessing laser diode chaos for potential applications are discussed. The
availability and ease of operation of laser diodes, in a wide range of
configurations, make them a convenient test-bed for exploring basic aspects of
nonlinear and chaotic dynamics. It also makes them attractive for practical
tasks, such as chaos-based secure communications and random number generation.
Avenues for future research and development of chaotic laser diodes are also
identified.Comment: Published in Nature Photonic
Réponse excitable et propriétés neuromimétiques de micropiliers lasers à absorbant saturable
Excitability is a well known property of biological neurons. In excitable systems, the response to a perturbation above the excitable threshold is of all-or-none type. Other properties exist in neurons such as the refractory periods and temporal or spatial summation of input stimuli.Excitability has been demonstrated in many III-V semiconductor material devices. Thanks to their nonlinear properties it could be possible to realize neuromimetic and all-optical signal processing with high speed and low energy consumption. Thanks to progress in fabrication techniques it is possible to fabricate high quality micropillar laser with saturable absorber. Thus, using micropillars it could be possible to realize neural photonic networks analog to neural networks.In this thesis work, I studied neuron-like properties of a micropillar laser with a saturable absorber. My main results are : 1) fabrication of micropillars has been improved leading to an increase of their robustness and a reduction of the laser threshold. 2) well known properties of biological neurons, such as excitability, existence of refractory periods, temporal summation, have been demonstrated experimentally and have been numerically analyzed with the Yamada model. 3) propagation effects of excitations have been demonstrated in one-dimensional structures : wire lasers and chains of coupled micropillars.The demonstration of neuromimetic properties in micropillar lasers with saturable absorber and the evidence of propagation of excitations pave the way to neuromorphic networks based on coupled micropillars for neuromimetic signal processing like information encoding with excitable pulses and realization of optical memories.L'excitabilité est une propriété bien connue des neurones biologiques. Il s'agit d'une réponse de type tout-ou-rien à une perturbation au delà d'un seuil caractéristique appelé seuil excitable. D'autres propriétés importantes existent dans les neurones comme les périodes réfractaires et la sommation temporelle ou spatiale de stimuli d'entrée.L'excitabilité a été étudiée dans certains composants actifs à semiconducteur et notamment les composants à semiconducteurs III-V. Leurs propriétés neuro-mimétiques pourraient permettre de traiter l'information de façon tout-optique avec une grande bande passante et une faible consommation.Grâce aux nouvelles techniques de micro-nano fabrication, il est devenu possible de fabriquer des micropiliers lasers à absorbant saturable. Ces micropiliers pourraient permettre la réalisation de réseaux de micropiliers couplés excitables analogues à des réseaux de neurones photoniques.Dans cette thèse j'ai étudié les propriétés neuro-mimétiques de micropiliers lasers à absorbant saturable intégré. Les principaux résultats de cette thèse sont les suivants : 1) la technique de fabrication des micropiliers a été améliorée conduisant à une augmentation de leur durée de vie et une diminution du seuil laser. 2) des propriétés de base des neurones biologiques, comme l'excitabilité, l'existence des périodes réfractaires, la sommation temporelle, ont été mises en évidence expérimentalement et analysées à l'aide du modèle de Yamada. 3) des effets de propagation d'excitations ont été démontrés dans des structures unidimensionnelles : des lasers ligne et des chaînes de micropiliers couplés.La démonstration des propriétés neuromimétiques de micropiliers lasers à absorbant saturable et la mise en évidence de la propagation d'excitations ouvrent la voie à la réalisation de réseaux de micropiliers couplés pour les traitements neuromimétiques des signaux qui pourront être exploités pour de la logique codée à l'aide de pics excitables ainsi que pour du stockage d'information dans des circuits mémoires tout-optiques
Spatio-temporal dynamics of lasers and photorefractive oscillators under rocking: phase-bistable patterns and localized structures
El objectiu de aquesta tesi es l’estudi teòric, analĂtic i numèric, de
la dinà mica espaciotemporal d’oscil·ladors òptics no lineals sotmesos a un
forçament bicromà tic (rocking). Aquest tipus d’injecció té
la caracterĂstica principal de trencar la invariĂ ncia de fase (qualsevol fase del
camp complex) del sistema lliure (sense forçament) i genera un sistema que és
biestable en fase, ja que Ăşnicament dues fases (separades per ÂĽ) sĂłn permeses
per a les solucions estacionà ries homogènies.
Aquest canvi en la naturalesa del sistema provoca l’aparició d’una nova
dinà mica caracteritzada per la presència d’un nou tipus d’estructures espacials
en el pla transversal bidimensional: patrons biestables de fase en els quals
dominis d’ambdues fases conviuen separades per parets de domini (Ising si
la intensitat s’anul·la en elles o Bloch, en cas contrari). Aquests dominis poden
evolucionar a patrons homogenis (d’una de les dues fases) o uns altres, més
complexos, que els efectes de curvatura condueixen a la creaciĂł de patrons
laberĂntics segons els valors dels parĂ metres del sistema. A mĂ©s, poden existir
estructures localitzades (dominis de grandĂ ria mĂnima estables) en la forma de
solitons de cavitat d’anell fosc.
Altres mètodes de trencament de la simetria de fase han sigut usats per
a controlar la dinà mica de molts sistemes. Un dels més populars és la
ressonà ncia paramètrica, i.e. injectar un camp la freqüència del qual és
aproximadament el doble de la freqüència natural de oscil·lació del sistema. No
obstant això, aquests mètodes són menys versà tils que el rocking, el qual pot
aplicar-se a una Ă mplia gamma de sistemes com el lĂ ser, que sĂłn insensibles
a la ressonà ncia paramètrica. De fet, s’han fet múltiples propostes teòriques i
experimentals d’aplicació del rocking a diferents sistemes (òptics i no òptics).
En el domini d’aquesta tesi, ens centrarem en la influència del rocking en dos
sistemes que han sigut estudiats profusament en la literatura, donat el seu gran
interès tant des del punt de vista fonamental comprà ctic:là sers i oscil·ladors fotorrefractius.
Al llarg d’aquesta tesi, estudiarem detalladament la influència del
rocking en aquests sistemes. Com és usual en el camp de la ciència no
lineal, Ă©s convenient deduir equacions que descriguen el comportament
d’aquests sistemes prop dels punts (punts crĂtics) on emergeixen les solucions
estacionĂ ries del sistema. Aquestes equacions (anomenades de parĂ metre
d’ordre) tenen una forma aparentment simple i són capaces de descriure
multitud de sistemes no lineals, fĂsics, quĂmics, biològics.. (l’única diferència
és el significat dels diferents parà metres, però l’estructura matemà tica és la
mateixa), per la qual cosa posseeixen un carĂ cter universal. AixĂ mateix,
analitzarem l’estabilitat de les solucions trobades i realitzarem simulacions
numèriques dels diferents models teòrics. Es presentaran els següents resultats:
A partir de les equacions de MB amb injecciĂł rocking, es deduirĂ una
equació de parà metre d’ordre per a là sers de classe C amb desintonia positiva
de la cavitat i s’estudiaran numèricament els patrons del sistema.
Per a là sers de classe B, s’obtindrà un model reduït de dues equacions
i s’analitzarà la seua dinà mica temporal i la influència de la desintonia de la
injecció rocking. També esmostraran patrons espacials obtinguts a partir de la
simulaciĂł de les equacions de MB.Es desenvoluparĂ un model unificat (vĂ lid per a desintonies de la cavitat
positives i negatives) per a là sers de dos nivells (classe C i A) i oscil·ladors
fotrorefractius, proporcionant els dominis d’estabilitat dels estats biestables en
fase i estudiant numèricament els patrons espacials que apareixen. S’analitzarà la dinà mica temporal d’un là ser bidireccional amb injecció rocking i es presentaran alguns resultats preliminars de patrons espacialsThe objective of this thesis is the theoretical, analytical and numerical, study of the spatio-temporal dynamics of optical oscillators under bichromatic forcing (rocking). This kind of injection possesses the feature of breaking the phase invariance (any phase of the complex field is possible) of the free-running system and generates a phase-bistable system in which two only phases are allowed for the homogeneous stationary solutions.
This change in the nature of the system enables a new dynamics characterized by the presence of a new kind of spatial structures in the bidimensional transverse plane: bistable phase patterns in which both phases coexist separated by domain walls (Ising if they have null intensity or Bloch if it is different from zero). These domains can evolve either to homogeneous patterns (in which only one phase is present) or to more complex ones, in which curvature effects lead to the emergence of labyrinthic patterns depending on the value of the parameters of the system. Moreover, localized structures (stable minimum-size domains) as dark-ring cavity solitons can exist. In the scope of this thesis, we have focused on the influence of rocking in two systems which have been studied profusely in the literature, as they are very interesting both from a fundamental and a practical point of views: lasers and photorefractive oscillators.
Along this thesis, we will study the influence of rocking in those systems in detail. As it is usual in nonlinear science, is convenient to derive equations describing the behaviour of those systems close to (critical) points where the stationary solutions emerge. These equations (called order parameter equations) are relatively simple and are able to describe a large number of nonlinear systems: physical, chemical, biological.. (the meaning ot the parameters being the only difference , but the mathematical structure is the same). Moreover, we will analyze the stability of the solutions and we will perform numerical simulations of the theoretical models. The following results will be presented:
Starting from the MB equations under rocking injection, an order parameter equation will be derived for class C lasers with positive cavity detuning and the patterns of the system will be studied numerically. A reduced model of two equations will be obtained for class B lasers and its temporal dynamics and the influence of the detuning of rocking injection will be studied. We will also show spatial patterns obtained from simulations of the MB equations. A unified model (valid for positive and negative cavity detunings) for two level lasers (class C and A) and photorefractive oscillators will be developed, providing the stability domains of the phase bistable states and studying numerically the spatial patterns that arise from the system.
The temporal dynamics of a bidirectional laser under rocking injection will be analyzed and some preliminary results regarding spatial patterns will be given
Intermittency and Self-Organisation in Turbulence and Statistical Mechanics
There is overwhelming evidence, from laboratory experiments, observations, and computational studies, that coherent structures can cause intermittent transport, dramatically enhancing transport. A proper description of this intermittent phenomenon, however, is extremely difficult, requiring a new non-perturbative theory, such as statistical description. Furthermore, multi-scale interactions are responsible for inevitably complex dynamics in strongly non-equilibrium systems, a proper understanding of which remains a main challenge in classical physics. As a remarkable consequence of multi-scale interaction, a quasi-equilibrium state (the so-called self-organisation) can however be maintained. This special issue aims to present different theories of statistical mechanics to understand this challenging multiscale problem in turbulence. The 14 contributions to this Special issue focus on the various aspects of intermittency, coherent structures, self-organisation, bifurcation and nonlocality. Given the ubiquity of turbulence, the contributions cover a broad range of systems covering laboratory fluids (channel flow, the Von Kármán flow), plasmas (magnetic fusion), laser cavity, wind turbine, air flow around a high-speed train, solar wind and industrial application