Transverse Patterns in Nonlinear Optical Resonators

The book is devoted to the formation and dynamics of localized structures (vortices, solitons) and extended patterns (stripes, hexagons, tilted waves) in nonlinear optical resonators such as lasers, optical parametric oscillators, and photorefractive oscillators. The theoretical analysis is performed by deriving order parameter equations, and also through numerical integration of microscopic models of the systems under investigation. Experimental observations, and possible technological implementations of transverse optical patterns are also discussed. A comparison with patterns found in other nonlinear systems, i.e. chemical, biological, and hydrodynamical systems, is given. This article contains the table of contents and the introductory chapter of the book.Comment: 37 pages, 14 figures. Table of contents and introductory chapter of the boo

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

Smart control of light in edge-emitting lasers

Tesi en modalitat de compendi de publicacions, amb diferents seccions retallades per drets dels editorsThe invention of the laser triggered the study of light-matter interactions. In turn, the advent of artificial structured materials on micro- and nanometer scales has become a fruitful playground to tailor the propagation and generation of light, even in exotic or counterintuitive ways, uncovering novel physical phenomena. In this thesis, we precisely propose using recently discovered properties of artificial photonic materials, and new schemes, to control the spatiotemporal dynamics of broad area semiconductor lasers and improve their performance. Semiconductor lasers are replacing other laser sources due to their efficiency, compactness and affordable prices, however suffering from a major drawback. The quality of the emitted beam intrinsically deteriorates when power increases, if the aperture of the laser is very broad as compared to wavelength. The highly multimode and unstable emission limits possible applications of these lasers. Although different mechanisms have been proposed to save this obstacle, obtaining a stable and bright emission remains a longstanding open question. This thesis aims at contributing to this goal without compromising their compact design, and to the new field of non-Hermitian Photonics providing new insights into the control of wave dynamics in artificial complex media. Indeed, the physics of open-dissipative, non-Hermitian systems offers new possibilities to utilize the gain and loss for steering optical processes, and is beyond the recent focus on non-Hermitian Photonics. As initially demonstrated in the frame of Quantum Mechanics, systems with gain and losses may still present real eigenvalues of the Hamiltonian (energy) as the purely conservative ones, yet holding other unexpected physical behaviors, derived from an asymmetric coupling between modes. In particular, this was first observed in systems invariant under parity (P-) and time (T-) symmetry ¿ referred as PT-symmetric ¿. Optical systems with complex permittivity are flexible and achievable classical analogs of such quantum systems to realize and explore these effects. As a first step, we propose to use a chirped modulation of the refractive index (chirped photonic crystal) for intracavity filtering the multimode emission of EELs. To numerically assess the filtering performance, we developed a full (2+1)-dimensional spatio-temporal model, including both transverse and longitudinal dimensions plus time, for the evolution of the electric field and carriers. The good agreement between predictions with actual experimental results demonstrates the proposal while validating the model which is used throughout the thesis, with corresponding modifications. We then analyze the effect of intrinsically imposing in phase refractive index and gain modulations within the semiconductor laser, and use the interplay between real and imaginary parts of the non-Hermitic potential to achieve spatial and temporal stabilization. Taking one step further, we propose to divide the EEL cavity into two mirror-symmetric half-spaces, both holding PT-symmetry but with opposite mode coupling. With this geometry, we expect to obtain a two-fold benefit: on the one hand, achieving a spatial-temporal stabilization of the laser, and on the other, localizing the generated field along the symmetry axis. We numerically demonstrate regimes of simultaneous localization and stabilization leading to an enhanced output and improved beam quality. Finally, while thinner lasers show a more stable new temporal and synchronization instabilities arise in EELS arrays (bars) from the coupling between neighboring lasers, leading again to irregular spatiotemporal behaviors. We show that the proposed mirror symmetric non-Hermitian configuration may be extended to couple individual EELs in the array, by a lateral shift between the pump and index profiles. In all cases, the obtained localized and stable output beam may facilitate a direct coupling of the emitted beam to optical fibers.La invenció del làser va representar el tret de sortida per nombrosos estudis de la interacció entre la llum i la matèria. A banda, el desenvolupament de nous materials fotònics artificials en escales micro i nanomètriques ha esdevingut un camp fructífer per al control de la propagació i generació de la llum, fins i tot de maneres exòtiques o contra intuïtives, revelant nous fenòmens físics. En aquesta tesi proposem, precisament, utilitzar nous materials fotònics artificials i nous esquemes per controlar la dinàmica espai-temporal dels làsers de semiconductor d'apertura ampla, per millorar-ne les propietats. Els làsers de semiconductor estan reemplaçant altres fonts de llum làser gràcies a la seva eficiència, format compacte i preu assequible. Malgrat tot, pateixen un gran inconvenient: el deteriorament del feix emès en augmentar la potència, especialment si l'amplada del làser és molt gran respecte la longitud d'ona. Quan l'emissió esdevé altament multimode i inestable en limita les possibles aplicacions. Encara que s'han proposat diferents mecanismes per superar aquest problema, aconseguir una emissió estable sense comprometre'n el format compacte, és encara una qüestió oberta. Aquesta tesi té com a objectiu contribuir a la millora dels làsers de semiconductor i a l'estudi del control de la dinàmica dels làsers mitjançant el nou camp de la fotònica no hermítica. De fet, la física dels sistemes oberts no Hermítics ofereix noves possibilitats per utilitzar la permitivitat complexa per dominar processos òptics i és la causa del recent interès en la fotònica no hermitiana. Primer, es va demostrar en el marc de la Mecànica Quàntica que els sistemes oberts o no Hermítics, tot i tenir guanys o pèrdues poden presentar autovalors reals del Hamiltonià (valors constants de l'energia) i altres comportaments físics inesperats, derivats d'acoblaments asimètrics entre modes. Efecte observat inicialment en sistemes invariants sota la paritat (P-) i la simetria de temps (T-), anomenats PT-simètrics. Els sistemes òptics amb permitivitat complexa són anàlegs clàssics, flexibles i assequibles d’aquests sistemes quàntics per realitzar i explorar aquests nous efectes. Primer, proposem fer servir una modulació de l’índex de refracció per al filtrat, intacavitat, de l'emissió multimode d'amplificadors i làsers de semiconductor. Per l'anàlisi numèrica desenvolupem un model espai-temporal complet, que inclou dues dimensions espacials, transversal i longitudinal, més l'evolució temporal del camp elèctric i dels portadors. Aquest model s'utilitza al llarg de tota la tesis amb les modificacions corresponents i és contrastat també experimentalment. A continuació, analitzem l'efecte d'imposar modulacions intrínseques de l’índex de refracció i el guany, en fase i, dins del làser de semiconductor. Gràcies a la interacció entre parts reals i imaginàries del potencial no hermític s’aconsegueix una estabilització espacial i temporal. Fent un pas més, dividim la cavitat làser en dos espais PT-simètrics (simetria de mirall) amb un acoblament en sentit oposat. Amb aquesta geometria, esperem obtenir un doble benefici: d'una banda, aconseguir una estabilització espacial-temporal del làser, i per una altra banda, localitzar el camp generat en l'eix de simetria. Es demostra numèricament règims de localització i d'estabilització simultànies, augmentant la potència emesa tot millorant la qualitat dels feix. Finalment, tot i que els làsers més estrets mostren una emissió més estable, els làsers propers s’acoblen quan formen part d'una matriu. Demostrem que l'acoblament asimètric també pot ser utilitzat en barres de làsers de semiconductor per estabilitzar-los temporalment i concentrar-ne l’emissió. L'acoblament asimètric es produeix mitjançant un desplaçament lateral entre el bombeig i l'índex de refracció. En tots els casos, el feix de sortida localitzat i estable obtingut pot facilitar un acoblament directe del feix emès a les fibres òptiques.Postprint (published version

Specialty Fiber Lasers and Novel Fiber Devices

At the Dawn of the 21st century, the field of specialty optical fibers experienced a scientific revolution with the introduction of the stack-and-draw technique, a multi-steps and advanced fiber fabrication method, which enabled the creation of well-controlled micro-structured designs. Since then, an extremely wide variety of finely tuned fiber structures have been demonstrated including novel materials and novel designs. As the complexity of the fiber design increased, highly-controlled fabrication processes became critical. To determine the ability of a novel fiber design to deliver light with properties tailored according to a specific application, several mode analysis techniques were reported, addressing the recurring needs for in-depth fiber characterization. The first part of this dissertation details a novel experiment that was demonstrated to achieve modal decomposition with extended capabilities, reaching beyond the limits set by the existing mode analysis techniques. As a result, individual transverse modes carrying between ~0.01% and ~30% of the total light were resolved with unmatched accuracy. Furthermore, this approach was employed to decompose the light guided in Large-Mode Area (LMA) fiber, Photonic Crystal Fiber (PCF) and Leakage Channel Fiber (LCF). The single-mode performances were evaluated and compared. As a result, the suitability of each specialty fiber design to be implemented for power-scaling applications of fiber laser systems was experimentally determined. The second part of this dissertation is dedicated to novel specialty fiber laser systems. First, challenges related to the monolithic integration of novel and complex specialty fiber designs in all-fiber systems were addressed. The poor design and size compatibility between specialty fibers and conventional fiber-based components limits their monolithic integration due to high coupling loss and unstable performances. Here, novel all-fiber Mode-Field Adapter (MFA) devices made of selected segments of Graded Index Multimode Fiber (GIMF) were implemented to mitigate the coupling losses between a LMA PCF and a conventional Single-Mode Fiber (SMF), presenting an initial 18-fold mode-field area mismatch. It was experimentally demonstrated that the overall transmission in the mode-matched fiber chain was increased by more than 11 dB (the MFA was a 250 ?m piece of 50 ?m core diameter GIMF). This approach was further employed to assemble monolithic fiber laser cavities combining an active LMA PCF and fiber Bragg gratings (FBG) in conventional SMF. It was demonstrated that intra-cavity mode-matching results in an efficient (60%) and narrow-linewidth (200 pm) laser emission at the FBG wavelength. In the last section of this dissertation, monolithic Multi-Core Fiber (MCF) laser cavities were reported for the first time. Compared to existing MCF lasers, renown for high-brightness beam delivery after selection of the in-phase supermode, the present new generation of 7-coupled-cores Yb-doped fiber laser uses the gain from several supermodes simultaneously. In order to uncover mode competition mechanisms during amplification and the complex dynamics of multi-supermode lasing, novel diagnostic approaches were demonstrated. After characterizing the laser behavior, the first observations of self-mode-locking in linear MCF laser cavities were discovered

Memoria de Actividades 2002

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Modelling multimode dynamics of semiconductor ring lasers

In this thesis, a modal decomposition method and a time-frequency-domain formalism for the analysis of multimode dynamics of semiconductor ring laser are developed. The diffusion coefficient is suggested as a crucial parameter to take into account. The directional switching dynamics and dependence on the operation parameters has been studied. The lasing wavelength switching accompanied by directional flipping have also been studied. In this framework, a prior selection of the lasing mode is seen as a key factor for the numerical results