Dans ces travaux de thèse, nous avons étudié les propriétés dynamiques et spectrales d’une diode laser puissante à 980 nm qui est soumise à un retour optique provenant d’un réseau de Bragg fibré (FBG). Dans cette configuration, le FBG est un élément clé de la thèse d’une part il permet de former une cavité externe par réflexion comme un miroir et d’autre part il est un filtre qui fournit un retour optique sélectif en fréquence (FOF : Filtered Optical Feedback) aux alentours de 980 nm. Nous nous sommes intéressé à étudier cette configuration FOF dans le cadre d’un projet de collaboration entre la société 3S Photonics Group et le Laboratoire de Physique des Lasers. Il s’agit plus particulièrement d’étudier expérimentalement les séries temporelles par un oscilloscope, les spectres RF par un analyseur de spectre RF et les spectres optiques par un analyseur de spectre optique et par un interféromètre Fabry-Perot, afin d’observer, identifier et comprendre ses comportements dynamiques, notamment fluctuations basses fréquences (LFF). Ce système laser FOF est très peu étudié et ces travaux de thèse permettront de nouvelles connaissances techniques ou scientifiques dans un domaine d’application relativement récent, notamment pour l’application dans les systèmes EDFA (Erbium Doped Fiber Amplifier). / In this thesis, we have studied spectrums and the dynamics properties of a high-powerlaser diode emitting at 980 subject to optical feedback via a fibre Bragg grating (FBG). In such a configuration, FBG is a key element of this work because it establishes an ex-ternal cavity with a reflection like a mirror on one hand and it is a filter which provides frequency selective optical feedback (FOF : Filtered Optical Feedback) at around 980 nm on the other hand. We are interested in investigating this FOF configuration for a project in a collaboration between 3S Photonics Group and the Laboratoire de Physique des Lasers.In order to observe, identify and understand its dynamical behaviours, especially low frequency fluctuations (LFF), the experimental studies have been undertaken particularly with time series from an oscilloscope, RF spectra from an RF spectrum analyser and optical spectra from an optical spectrum analyser and Fabry-Perot interferometer. Onlyfew studies have been done with this FOF laser system and our thesis work may allow new technical and scientific understanding for recent application, particularly application to Erbium-doped fibre amplifier.

Nesse trabalho estudamos um laser de estado sólido sujeito a realimentação optica de frequência modificada de um ponto de vista da teoria de bifurcações. Fizemos uma an alise bastante ampla da dinâmica desse laser no espaço de dois parâmetros de injeção (a dessintonização de frequência e a intensidade da injeção) utilizando métodos de integração direta e continuação numérica. Enquanto o método de integração numérica nos possibilitou analisar as dinâmicas mais complexas, incluindo transições para o caos e hipercaos, o método de continuação numérica nos permitiu estudar curvas de bifurcações estáveis e instáveis. A análise foi realizada estudando os efeitos causados pela mudança dos parâmetros que representam o tempo de vida da inversão populacional e a saturação cruzada, responsável pelo acoplamento dos campos dentro do meio ativo. Mostramos que o parâmetro que descreve o tempo de vida da inversão populacional e responsável pelo surgimento de diversas instabilidades no sistema, como o fenômeno de multiestabilidade, surgimento de orbitas periódicas e quase-peri odicas, assim como rotas para o caos via dobramento de período e torus. Para o parâmetro de acoplamento dos campos, mostramos que ele possibilita a presença de hipercaos em nosso sistema, este podendo se apresentar no que denominamos de hipercaos \fraco" e \forte". Dentro da região de hipercaos \forte", mostramos transições determinísticas de dois regimes, em que num deles o laser opera no modo de Q-switching, enquanto que no outro o laser apresenta pequenas oscilações irregulares. Por m, mostramos a existência de uma estatística de eventos extremos dentro do regime hipercaótico. / In this work we studied a solid state laser subjected to frequency-shifted optical feedback from a bifurcation theory point of view. We performed a very broad analysis of the dynamics of this laser in the space of two injection parameters (frequency detuning and injection intensity) using direct integration and numerical continuation methods. While the numerical integration method allowed us to analyze the more complex dynamics, including chaos and hyperchaos transitions, the numerical continuation method allowed us to study stable and unstable bifurcation curves. The analysis was carried out by studying the e ects caused by the change of the parameters that represent the life time of the population inversion and the cross saturation, responsible for the coupling of the elds within the active medium. We show that the parameter that describes the life time of the population inversion is responsible for the appearance of several instabilities in the system, such as the multistability phenomenon, the appearance of periodic and quasi-periodic orbits, as well as routes to chaos via period doubling and torus . For the eld coupling parameter, we show that it allows the presence of hyperchaos in our system, which may present in what we call "weak"and "strong"hyperchaos. Within the "strong"hyperchaos region, we show deterministic transitions of two regimes, in which one laser operates in the Q-switching mode, while in the other the laser presents small irregular oscillations. Finally, we have shown the existence of a extreme events statistic within the hyperchaotic regime.

This research investigated the characteristics of single-mode optical pulses generated with orthogonal-polarization optical feedback (OPF) in Fabry-Perot type semiconductor lasers. Single-mode pulse trains with a pulse frequency of 470 MHz and 3.76 GHz were observed. A modified model was proposed to solve the inconsistency between the experimental results and the computer simulations of Otsuka and Chern¡¦s model. These results also solve the problem of a round-trip feedback distance that is too short to enable the feedback system to be implemented, making this technology accomplishable in currently working systems. Furthermore, this investigation recovered and maintained a stable oscillation of every missing longitudinal mode in a hysteresis type mode-hopping gap of a semiconductor laser. A special feature of this method is that both the laser power and spectral purity are preserved during mode recovery and mode switching. The experimental results also reveal that the OPF effectively suppressed mode-hopping in semiconductor lasers and drove the laser into a stable single-mode state. Finally, this research employed OPF to suppress the intensity noise stimulated by coherent optical feedback in a semiconductor laser. At a coherent-feedback level as strong as ¡V14 dB, an OPF ratio of ¡V29 dB could return the laser to its primitive single mode from the multimode, yielding a spectral purity and relative intensity noise (RIN) even better than the solitary values. These discoveries constitute an important contribution to our understanding of applications of semiconductor lasers.

Nesse trabalho estudamos um laser de estado sólido sujeito a realimentação optica de frequência modificada de um ponto de vista da teoria de bifurcações. Fizemos uma an alise bastante ampla da dinâmica desse laser no espaço de dois parâmetros de injeção (a dessintonização de frequência e a intensidade da injeção) utilizando métodos de integração direta e continuação numérica. Enquanto o método de integração numérica nos possibilitou analisar as dinâmicas mais complexas, incluindo transições para o caos e hipercaos, o método de continuação numérica nos permitiu estudar curvas de bifurcações estáveis e instáveis. A análise foi realizada estudando os efeitos causados pela mudança dos parâmetros que representam o tempo de vida da inversão populacional e a saturação cruzada, responsável pelo acoplamento dos campos dentro do meio ativo. Mostramos que o parâmetro que descreve o tempo de vida da inversão populacional e responsável pelo surgimento de diversas instabilidades no sistema, como o fenômeno de multiestabilidade, surgimento de orbitas periódicas e quase-peri odicas, assim como rotas para o caos via dobramento de período e torus. Para o parâmetro de acoplamento dos campos, mostramos que ele possibilita a presença de hipercaos em nosso sistema, este podendo se apresentar no que denominamos de hipercaos \fraco" e \forte". Dentro da região de hipercaos \forte", mostramos transições determinísticas de dois regimes, em que num deles o laser opera no modo de Q-switching, enquanto que no outro o laser apresenta pequenas oscilações irregulares. Por m, mostramos a existência de uma estatística de eventos extremos dentro do regime hipercaótico. / In this work we studied a solid state laser subjected to frequency-shifted optical feedback from a bifurcation theory point of view. We performed a very broad analysis of the dynamics of this laser in the space of two injection parameters (frequency detuning and injection intensity) using direct integration and numerical continuation methods. While the numerical integration method allowed us to analyze the more complex dynamics, including chaos and hyperchaos transitions, the numerical continuation method allowed us to study stable and unstable bifurcation curves. The analysis was carried out by studying the e ects caused by the change of the parameters that represent the life time of the population inversion and the cross saturation, responsible for the coupling of the elds within the active medium. We show that the parameter that describes the life time of the population inversion is responsible for the appearance of several instabilities in the system, such as the multistability phenomenon, the appearance of periodic and quasi-periodic orbits, as well as routes to chaos via period doubling and torus . For the eld coupling parameter, we show that it allows the presence of hyperchaos in our system, which may present in what we call "weak"and "strong"hyperchaos. Within the "strong"hyperchaos region, we show deterministic transitions of two regimes, in which one laser operates in the Q-switching mode, while in the other the laser presents small irregular oscillations. Finally, we have shown the existence of a extreme events statistic within the hyperchaotic regime.

Nesse trabalho estudamos um laser de estado sólido sujeito a realimentação optica de frequência modificada de um ponto de vista da teoria de bifurcações. Fizemos uma an alise bastante ampla da dinâmica desse laser no espaço de dois parâmetros de injeção (a dessintonização de frequência e a intensidade da injeção) utilizando métodos de integração direta e continuação numérica. Enquanto o método de integração numérica nos possibilitou analisar as dinâmicas mais complexas, incluindo transições para o caos e hipercaos, o método de continuação numérica nos permitiu estudar curvas de bifurcações estáveis e instáveis. A análise foi realizada estudando os efeitos causados pela mudança dos parâmetros que representam o tempo de vida da inversão populacional e a saturação cruzada, responsável pelo acoplamento dos campos dentro do meio ativo. Mostramos que o parâmetro que descreve o tempo de vida da inversão populacional e responsável pelo surgimento de diversas instabilidades no sistema, como o fenômeno de multiestabilidade, surgimento de orbitas periódicas e quase-peri odicas, assim como rotas para o caos via dobramento de período e torus. Para o parâmetro de acoplamento dos campos, mostramos que ele possibilita a presença de hipercaos em nosso sistema, este podendo se apresentar no que denominamos de hipercaos \fraco" e \forte". Dentro da região de hipercaos \forte", mostramos transições determinísticas de dois regimes, em que num deles o laser opera no modo de Q-switching, enquanto que no outro o laser apresenta pequenas oscilações irregulares. Por m, mostramos a existência de uma estatística de eventos extremos dentro do regime hipercaótico. / In this work we studied a solid state laser subjected to frequency-shifted optical feedback from a bifurcation theory point of view. We performed a very broad analysis of the dynamics of this laser in the space of two injection parameters (frequency detuning and injection intensity) using direct integration and numerical continuation methods. While the numerical integration method allowed us to analyze the more complex dynamics, including chaos and hyperchaos transitions, the numerical continuation method allowed us to study stable and unstable bifurcation curves. The analysis was carried out by studying the e ects caused by the change of the parameters that represent the life time of the population inversion and the cross saturation, responsible for the coupling of the elds within the active medium. We show that the parameter that describes the life time of the population inversion is responsible for the appearance of several instabilities in the system, such as the multistability phenomenon, the appearance of periodic and quasi-periodic orbits, as well as routes to chaos via period doubling and torus . For the eld coupling parameter, we show that it allows the presence of hyperchaos in our system, which may present in what we call "weak"and "strong"hyperchaos. Within the "strong"hyperchaos region, we show deterministic transitions of two regimes, in which one laser operates in the Q-switching mode, while in the other the laser presents small irregular oscillations. Finally, we have shown the existence of a extreme events statistic within the hyperchaotic regime.

This thesis is a theoretical and experimental study of the dynamics of an edge-emitting laser diode (850 nm) with phase-conjugate feedback. The experimental device is designed to see the dynamical range of the laser through the temporal and spectral properties while the feedback rate varies. Phase-conjugate feedback is performed through four-wave mixing in a photorefractive crystal. The propagation time of the laser beam in the external cavity is termed external time delay. Under the effect of the feedback, the system shows a wide dynamical range including chaos and self-pulsing states which characteristic properties are determined by the length of the external cavity. For the first time self-pulsing states at frequencies multiples of the fundamental frequency of the external cavity are evidenced. Simulations carried out based on the commonly-used Lang-Kobayashi laser rate equations provide theoretical confirmations to the experimental observations. The main topics tackled here are chaos crisis and bistability of pulsing solutions, self-pulsing regimes (through their stabilization and destabilization) and the transitions between them, characterization of extreme events of two kinds along with their statistical distribution and delay-induced deterministic coherence resonance of low frequency fluctuations. Beyond the fundamental interest of these results and the many comparisons that can be made with other laser systems, applications in the field of all-optical signal generation and control of chaos are direct consequences of this study.

La spectroscopie d'absorption moléculaire est un outil incontournable non seulement pour la physique fondamentale et la métrolgie mais aussi pour des domaines aussi divers que les sciences environnementales, la planétologie ou l'astrophysique. Ces dernières années, des techniques spectroscopiques qui exploitent l'amplification résonnante d'interaction entre lumière laser et molécules dans une cavité optique ont fourni des détectivités exceptionnelles sur l'axe d'absorption, tandis que l'axe de fréquence des spectromètres n'atteignait généralement pas le même niveau de précision.Dans cette thèse, nous avons répondu à ce défi en développant la spectroscopie en cavité par temps de déclin stabilisée en fréquence par rétroaction optique (OFFS-CRDS en anglais). Cette nouvelle technique présente une combinaison unique de stabilité et résolution fréquentielles sub-kHz, d'un niveau d'intensité lumineuse intra-cavité de l'ordre du kW/cm^2, d'une detectivite de 2 x 10^(−13) cm^(−1)Hz^(-1/2) limitée par le bruit de photons, et d'une limite de détection de 8.4 x 10^(−14) cm^(−1) sur une plage spectrale étroite. Ces performances inédites sont dues à l'asservissement de la cavité spectroscopique à un laser balayé en fréquence par modulation à bande latérale unique et stabilisé par rétroaction optique avec une cavité en V de réference ultrastable. Pour transférer la cohérence de ce laser sub-kHz à des lasers plus bruiteux dans d'autres gammes spectrales à travers un peigne de fréquence optique, nous avons exploré une nouvelle méthode de clonage de phase par une correction anticipative à large bande passante et démontré une erreur résiduelle de phase de 113 mrad. En appliquant l'OFFS-CRDS à la spectroscopie du CO2 à 1.6 μm, nous avons obtenu un spectre large bande avec une dynamique de 8 x 10^5, et nous avons déterminé douze fréquences de transition absolues avec une exactitude de l'ordre du kHz en mesurant des Lamb dips sub-Doppler en absorption saturée avec un dispositif équipé d'un peigne de fréquence. Par ailleurs, nous avons procédé à une analyse détaillée des sources d'erreurs systematiques en CRDS et nous avons déduit une formule analytique pour le déclin de cavité non-exponentiel dans un régime faiblement saturé qui est susceptible de contribuer à de futures mesures de moments de transition dipolaire indépendantes de la concentration. Nos résultats ouvrent des perspectives prometteuses pour des applications métrologiques de l'OFFS-CRDS, comme par exemple l'étude de profils de raie poussés, la mesures de rapports isotopiques et la spectroscopie d'absorption saturée extensive dans le proche infrarouge. / High-precision molecular absorption spectroscopy is a powerful tool for fundamental physics and metrology, as well as for a broad range of applications in fields such as environmental sciences, planetology and astrophysics. In recent years, spectroscopic techniques based on the enhanced interaction of laser light with molecular samples in high-finesse optical cavities have provided outstanding detection sensitivities on the absorption axis, while the spectrometer frequency axis rarely met as high precision standards.In this thesis, we addressed this challenge by the development of Optical Feedback Frequency-Stabilized Cavity Ring-Down Spectroscopy (OFFS-CRDS). This novel technique features a unique combination of sub-kHz frequency resolution and stability, kW/cm^2-level intracavity light intensity, a shot-noise limited absorption detectivity down to 2 x 10^(−13) cm^(−1)Hz^(-1/2), as well as a detection limit of 8.4 x 10^(−14) cm^(−1) on a narrow spectral interval. This unprecedented performance is based on the tight Pound-Drever-Hall lock of the ring-down cavity to a single-sideband-tuned distributed-feedback diode laser which is optical-feedback-stabilized to a highly stable V-shaped reference cavity. To transfer the coherence of this sub-kHz laser source to noisier lasers in other spectral regions through an optical frequency comb, we have explored a novel high-bandwidth feed-forward phase cloning scheme and demonstrated a residual phase error as low as 113 mrad. Applying OFFS-CRDS to the spectroscopy of CO_2 near 1.6 μm, we obtained a broadband spectrum with a dynamic range of 8 x 10^5 and retrieved twelve absolute transition frequencies with kHz-accuracy by measuring sub-Doppler saturated absorption Lamb dips with a comb-assisted setup. Furthermore, we have performed a comprehensive analysis of systematic error sources in CRDS and derived an analytic formula for the non-exponential ring-down signal in a weakly saturated regime, which may contribute towards future concentration-independent transition dipole moment measurements. Our results open up promising perspectives for metrological applications of OFFS-CRDS, such as advanced absorption lineshape studies, isotopic ratio measurements and extensive saturated absorption spectroscopy in the near infrared.

L'objectif de cette thèse est d'étudier la faisabilité de l'interférométrie par rétro-injection optique (IRO) pour la mesure de vitesse en dynamique des matériaux. La dynamique des matériaux est l'étude du comportement des matériaux soumis à des chocs (chocs laser, chocs plan,compression isentropique, projectiles, etc.). Dans ces conditions, nous cherchons à mesurer des vitesses pouvant aller jusqu'à 10 km/s. La technique IRO est couramment utilisée comme capteur embarqué pour mesurer de faibles vitesses dans divers domaines. Cependant, très peu d'études ont été réalisées sur les limites hautes vitesses de ce type de capteur. La rétro-injection optique provoque des effets non linéaires dans la cavité laser : elle perturbe la puissance d'émission optique. Suivant le taux de rétro-injection optique, le laser peut présenter un comportement chaotique et il n'est alors plus possible de récupérer l'information de la vitesse et/ou du déplacement de la cible à partir des signaux. Nous avons pour cela développé des modèles mathématiques et réalisé un grand nombre de simulations numériques afin d'évaluer les performances et les limitations de ce système. Nous avons notamment étudié l'influence de la réflectivité de la cible, de la longueur et de la fréquence de la modulation de la cavité externe. / The purpose of this thesis is to study the feasibility of optical feedback interferometry (OFI) for measuring velocities for dynamical material's behaviour studies. Dynamical material's behaviour studies permit to analyse the shocked material when subjects to shocks (laser shocks, isentropic compression, projectiles, etc.). In these conditions, we seek to measure velocities up to 10 km/s. The OFI technique is regularly used as an embedded system to measure slow velocities in various fields. However, very few studies have been performed for determining velocities measurement limits for this system. As a matter of fact, the optical feedback induces nonlinear effects into the laser's cavity : it disrupts the laser's emitted optical power. Depending on the optical feedback strength, the laser can show chaotic behaviour, then it is no longer possible to get the information for the target's velocity or displacement regarding the signal. In this study, we have been developing mathematical models and performing a wide range of numerical simulations to study the performances and the limits of the OFI technique. We have been also studying the influence of the targets reflectivity, the length and the modulation frequency of the external cavity.

Thesis (MSc (Physics))--University of Stellenbosch, 2006. / External cavity diode lasers are used increasingly as sources of light in applications ranging from industrial photonic systems to basic laboratory research on the interaction of light and atoms. External cavity diode lasers offer more stable output frequency and narrow spectral bandwidth than the typical free-running diode lasers. These characteristics are achieved by exploiting the sensitivity of diode lasers to external optical feedback. In this study the design and development of an external cavity diode laser system for future applications in spectroscopy and laser cooling of rubidium atoms is presented. The external cavity diode laser including mechanical components and control electronics of the system is developed from basic components. The system uses frequency selective optical feedback from a diffraction grating in a Littrow configuration to provide collimated, narrow-band, frequency tunable light near 780 nm. The external cavity diode laser is designed to increase the mode-hop-free frequency tuning range, and allow accurate frequency tuning and stabilisation. A low-noise current source and a temperature controller for thermal stability were developed as part of the system since the output frequency changes with temperature and current. The temperature controller is optimised experimentally for the thermal characteristics of the external cavity. An electronic sidelock servo circuit for frequency locking of the external cavity diode laser to an external reference for long term frequency stabilisation is proposed and discussed. The servo circuit electrically controls the grating tilt and the current through the diode laser in order to lock the frequency of the diode laser. The external cavity diode laser is optimised and characterised near 780 nm. Results obtained in this study indicate that the external cavity diode laser is suitable for future applications in spectroscopy and laser cooling of neutral rubidium atoms.

L'interférométrie à rétro-injection optique, également connu sous le nom de Self-Mixing, permet de concevoir des capteurs qui sont compacts, auto-alignés et sans contact. Dans ce phénomène, une partie du faisceau laser de retour réfléchi par la cible rentre dans la cavité active de laser et fait varier ses propriétés spectrales. La diode laser agit alors comme une source de lumière, un microinterféromètre ainsi qu'un détecteur de lumière. Dans cette thèse, un capteur de déplacement, basé sur la rétro-injection optique, a été optimisé de sorte que des mesures précises peuvent être obtenues en temps réel. Le capteur est robuste à la disparition des franges de self-mixing pour des vibrations harmoniques. Il est également capable de s'adapter à un changement dans le régime de feedback optique et peut donc extraire le déplacement dans les cas les plus répandus expérimentalement, à savoir un feedback faible puis modéré. L'utilisation de l'optique adaptative, sous la forme d'une lentille liquide, a également été démontrée pour ce capteur, ce qui nous a permis de maintenir le capteur dans un régime de feedback favorable. L'influence du speckle a également été réduite de telle sorte que le capteur mesure jusqu'à la gamme centimétrique pour des cibles non- oopératives. Une nouvelle technique est également présentée, elle permet de rendre le capteur insensible aux vibrations mécaniques parasites qui fausseraient la mesure pour des conditions industrielles. / Optical Feedback Interferometry, also known as Self-Mixing, results in compact, selfaligned and contact-less sensors. In this phenomenon, a portion of the laser beam is back reflected from the target and enters the active laser cavity to vary its spectral properties. The laser diode then simultaneously acts as a light source, a micro- nterferometer as well as a light detector. In this thesis, a self-mixing displacement sensor has been optimised so that precise measurement can be obtained in real-time. The sensor is robust to the disappearance of self-mixing fringes for harmonic vibrations. It is also able to auto-adapt itself to a change in the optical feedback regime and so can extract displacement from the weak as well as moderate feedback regime signals. The use of adaptive optics, in the form of a liquid lens, has also been demonstrated for this sensor, which has allowed us to maintain the sensor in a fringe-loss less regime. The influence of speckle has also been reduced so that the sensor can now measure up to the centimetric range for non-cooperative targets. A novel technique has also been presented that makes the sensor insensitive to parasitic mechanical vibrations that would falsify the measurement under industrial conditions.