Multimode interference (MMI) in optical fibers has been studied and its applications in optical fiber lasers and amplifiers have been proposed and demonstrated in this thesis. When a single-mode fiber is spliced onto a multimode fiber, quasi-reproduction of the field from the single-mode fiber, also called “self-imaging”, occurs periodically along the multimode fiber where the phase differences between the strongly excited modes are very small. The properties of self-imaging in multimode optical fibers have been investigated experimentally and theoretically in this thesis. Key parameters for the design of MMI-based fiber devices have been defined and their corresponding values have been provides for the 50 μm and 105 μm multimode fibers. By use of the self-imaging effect, a fiber laser with single-transverse-mode output while using a multimode rare-earth-doped fiber has been demonstrated as an alternative route to overcome the constraints of an active single-mode fiber. The first MMI-based fiber laser in the world has provided a perfect beam quality (M² = 1.01) and an inherent narrow spectrum (Δλ(3dB) < 0.5 nm). Linearly-polarized narrow-linewidth single-transverse-mode emission has also been obtained from a MMI fiber laser utilizing a single-mode fiber inscribed with a polarization-maintaining fiber Bragg grating. Moreover, high power MMI fiber lasers and amplifiers utilizing rare-earth doped silica large-core multimode fibers have been proposed and their critical features, such as efficiency, optical spectrum, and beam quality, have been investigated. On the other hand, because exclusively excited LP₀, n modes inside the multimode fiber segment are represented by apertured Bessel fields that have long propagation invariant distances, nondiffracting beams can be generated from the MMI-based fiber devices. In this thesis, the principle of generating nondiffracting beams from multimode optical fibers has been described and the propagation characteristics of the generated beams have been investigated. Active MMI fiber devices to generate tens of watts or even hundreds of watts nondiffracting beams have also been proposed.

Diffractive optical elements (DOEs) serve an important function in many dynamic and static optical systems. The theory and design of surface diffractive structures are well understood and practically applied at high spatial and phase resolution for a wide range of optical applications in science and industry. However, these structures normally only harness phase modulation of uniform fields for the beam diffraction and therefore limit their range of application, as well as being susceptible to surface damage. Multilayered volume diffractive elements offer a powerful opportunity to harness both phase and amplitude modulation for benefits in diffraction efficiency and beam shaping. However, multilayered combinations have been difficult to fabricate and provide only weak diffraction for phase gratings with low refractive index contrast. The advent of femtosecond laser writing inside transparent media has enabled the facile embedding of optical devices such as waveguides and diffractive optics into novel three-dimensional geometries that offer advanced functionality with compact design. In this work, femtosecond laser writing is pushed to the limits of forming high resolution phase elements with sufficiently strong refractive index contrast on which to develop volume phase gratings with the highest diffractive efficiency. The formation of both positive and negative zones of refractive index contrast together with rapid Talbot self imaging inside weakly contrasting phase gratings are major challenges here diminish the efficiency of assembled gratings. A method of strategic layering of otherwise weakly diffracting gratings onto Talbot planes is introduced to demonstrate, in FDTD models, the definitive enhancement of overall diffraction efficiency. A systematic optimization of laser writing in fused silica verify this enhancement or diminishment with weak volume gratings assembled on aligned or misaligned on Talbot planes. Advanced laser beam control methods were further demonstrated that underpin new direction for the facile assembly of highly functional DOEs that can exploit coherent light diffraction for opportunities in improving the performance of holographic devices and extend further to the powerful combination of phase and amplitude modulation control that is potentially available in a single optical device, thereby opening new directions for the design and fabrication of robust and strongly diffracting volume optical devices.

Diffractive optical elements (DOEs) serve an important function in many dynamic and static optical systems. The theory and design of surface diffractive structures are well understood and practically applied at high spatial and phase resolution for a wide range of optical applications in science and industry. However, these structures normally only harness phase modulation of uniform fields for the beam diffraction and therefore limit their range of application, as well as being susceptible to surface damage. Multilayered volume diffractive elements offer a powerful opportunity to harness both phase and amplitude modulation for benefits in diffraction efficiency and beam shaping. However, multilayered combinations have been difficult to fabricate and provide only weak diffraction for phase gratings with low refractive index contrast. The advent of femtosecond laser writing inside transparent media has enabled the facile embedding of optical devices such as waveguides and diffractive optics into novel three-dimensional geometries that offer advanced functionality with compact design. In this work, femtosecond laser writing is pushed to the limits of forming high resolution phase elements with sufficiently strong refractive index contrast on which to develop volume phase gratings with the highest diffractive efficiency. The formation of both positive and negative zones of refractive index contrast together with rapid Talbot self imaging inside weakly contrasting phase gratings are major challenges here diminish the efficiency of assembled gratings. A method of strategic layering of otherwise weakly diffracting gratings onto Talbot planes is introduced to demonstrate, in FDTD models, the definitive enhancement of overall diffraction efficiency. A systematic optimization of laser writing in fused silica verify this enhancement or diminishment with weak volume gratings assembled on aligned or misaligned on Talbot planes. Advanced laser beam control methods were further demonstrated that underpin new direction for the facile assembly of highly functional DOEs that can exploit coherent light diffraction for opportunities in improving the performance of holographic devices and extend further to the powerful combination of phase and amplitude modulation control that is potentially available in a single optical device, thereby opening new directions for the design and fabrication of robust and strongly diffracting volume optical devices.

La capacité actuelle d’accéder à des détecteurs très performants et de faible coût amène la communauté des concepteurs de systèmes optiques à un changement de paradigme. Plutôt que de réaliser des caméras généralistes, aptes à réaliser un grand nombre de missions d’observation différentes, il est maintenant de plus en plus courant de développer des systèmes imageurs adaptés à une seule mission et/ou à une seule classe d’objets. Prendre en compte ces connaissances a priori sur la scène et la mission, au moment de la conception, permet d’envisager des systèmes plus simples, mais aussi dotés de nouvelles compétences. L'objectif de la thèse est d'explorer les potentialités des tableaux non diffractants pour la conception de systèmes imageurs spécialisés. Pour cette étude nous considérons l'environnement des drones aéroportés de faible capacité d'emport pour lesquels les systèmes imageurs embarqués doivent être simples et robustes. Nous considérons de plus que la mission du système imageur est de détecter les obstacles. Pour cela, il doit délivrer une information 3D sur la scène observée. Dans un premier temps, j'ai analysé les propriétés d'imagerie du système imageur constitué d'un composant non diffractant et d'un détecteur matriciel. L'analyse comparative de deux composants, l'axicon et le tableau non diffractant, m'a permis de montrer que c'est le second composant qui est le mieux adapté pour remplir ce type de mission. J'ai ensuite réalisé un système imageur de démonstration, ce qui m'a permis au final de mettre en évidence sa capacité à faire de l'imagerie 3D. Cette étude a montré que les tableaux non diffractants ont un très fort potentiel pour réaliser un système imageur simple, robuste et dédié à l'imagerie 3D. Ce travail doit être poursuivi en partenariat avec des industriels pour appliquer la démarche de conception à une mission précise et transformer ce travail théorique en un système industrialisable. / Currently, detectors with high performances and a low cost are available and lead the community of optical designers to a new paradigm. Instead of designing generalist cameras, suitable for fulfilling a high number of different observation missions, it is now more and more common to develop imaging systems adapted to a unique mission and/or a unique object class. Taking this a priori knowledge on the observed scene or on the mission into account, at the beginning of the design process, allows us to consider simpler imaging systems with new properties. The aim of this thesis is to investigate the possibilities of nondiffracting array for the design of specialized imaging systems. For this study we consider the environment of unmanned aerial vehicles with a small payload capacity, for which embedded imaging systems must be simple and robust. We consider in addition that the mission of the imaging system is to detect obstacles. To do this, it must provide a 3D information on the observed scene. At first, I have analysed the properties of the imaging system composed by a nondiffracting optical device and a focal plane array. The comparison between two devices, axicon and nondiffracting arrays, allowed me to show that the second one is best suited for achieving this kind of mission. Then, I have made a practical implementation of such an imaging system. Finally, I have used it to demonstrate the 3D imaging property. This study has demonstrated the potential of nondiffracting array to design a simple and robust imaging system dedicated to 3D imaging. This work must be continued in partnership with the industry to apply the co-design process to a more precise mission, transforming this way this theoretical work into an industrial prototype.

No description available.

A MBS (Multi-port beamsplitter) for higher dimensional quantum communication has been designed and constructed and the theory and method for this is presented in this thesis. It uses optical fibers in a heterogeneous structure with a single-mode fiber spliced to a multi-mode fiber and then spliced to a few-mode fiber. Three MBS:s were constructed and tested to see if superpositions between spatial modes could be generated. One with 5.65cm multi-mode fiber, one with 9cm of multi-mode fiber and one with just the single-mode fiber spliced to the few-mode fiber. The optical modes that where focused on for the superposition were the linear polarized LP01, LP11a and LP11b modes. Simulations of superpositions between these modes were performed and experiments were done to see if these simulations could be realised. The shapes of these superpositions could be seen with a camera and the stability of the different modal powers and the stability of the phases between the modes where also tested. The last experiment tested the tunability of the modes by finding their maximum and minimum output power for each individual mode. The results of these experiments show that the stability of power and relative phases are high and testing of the tunability shows that the 9cm MBS is the most tunable, the 5.65cm MBS the second best and the SMF-FMF MBS the worst. Even though the shapes of the superpositions, the stability and tunability shows very positive results, the conclusion is that more experiments are required in order to identify the superpositions and for this to be used in a quantum communication system. / En Multi-port stråldelare (MSD) för kvantkommunikation med hjälp av rumsliga optiska moder har blivit designad och konstruerad. Teorin, metoden och resultatet av detta arbete presenteras i denna uppsats. Denna konstruktion använder sig av optiska fiber i heterogena strukturer med en single-mode fiber svetsad till en multi-mode fiber som i sin tur är svetsad till en few-mode fiber. Tre stycken MSD blev konstruerade och testade för att se om superpositioner mellan rumsliga moder kunde bli genererade, en med 5.65cm multi-mode fiber, en med 9cm multi-mode fiber och en med bara en single-mode fiber svetsad till en few-mode fiber. De moder som fokuserades på för superpositionerna var de linjärpolariserade moderna LP01, LP11a och LP11b. Superpositionerna simulerades och sen genomfördes experiment för att se ifall de kunde bli genererade. Formerna av dessa superpositioner kunde hittas och synliggöras med en kamera. Stabiliteten av modernas energi och stabiliteten av faserna mellan moderna testades också. Det sista experimentet som gjordes testade justerbarheten av moderna genom att hitta den minimala samt maximala intensiteten för varje mod. Experimenten visar att intensiteterna och de relativa faserna har hög stabilitet för alla konstruerade MSD, men i justerbarhets experimentet visar det sig att 9cm MSD:en presterar bäst, 5.65cm MSD:en presterar näst bäst och SMF-FMF strukturen presterar sämst. Trots att formerna av superpositionerna kunde hittas för alla tre konstruktioner, och att testen i stabiliteten visar goda resultat så krävs mer experiment för att identifiera superpositionerna mellan moderna och ifall denna konstruktion går att implementera i ett kvantkommunikationssystem.