A single 5 ns laser pulse is experimentally shown to self-focus in electrically biased Bi12TiO20. Systematic measurements have been conducted in order to characterize the physics of fast photorefractive self-focusing. The influence of the externally applied electric field, the incident energy density, the input beam waist and polarization state are reported.

This paper presents a temporal investigation of the propagation of a single laser beam in a photorefractive medium, both in the continuous and pulsed regimes. A time resolved observation of one beam self-focusing in electrically biased Bi 12 TiO 20 (BTO) is reported for the two time scales. To interpret these experimental results, two (1+1) dimensional theoretical models of the propagation of one laser beam in a photorefractive crystal have been developed, numerically simulated using a BPM calculation and successfully compared to our experimental observations.

C'est autour de la photoréfractivité et de l'auto-focalisation laser que mes travaux de recherche
ont principalement été effectués. Cet ouvrage tente une description qui se veut la plus complète
possible sans toutefois sacrifier à la minutie des détails, laquelle se trouvant dans d'autres écrits
plus spécialisés tels que thèses ou publications scientifiques.
Nous la démarrons par une introduction aux raisons qui nous ont conduit à mener ces travaux.
Les premiers développements de l'auto-focalisation laser photoréfractive sont donnés dans
la première partie — laquelle contient également des références à des travaux antérieurs et à
des activités d'enseignement. La deuxième est consacrée aux approfondissements qui ont suivi et
qui, pour la plupart, sont encore en cours. La troisième partie est, elle, dévolue à des sujets en
marge, qui ont en commun avec les premiers d'une part la photoréfractivité et d'autre part l'autofocalisation.
Par ailleurs, c'est en annexe que l'on trouvera des détails mathématiques originaux
ainsi que des listes de publications dont certaines sont données dans leur totalité. La bibliographie
s'y trouve également à sa place, la dernière.
Nous nous attachons à démontrer expérimentalement la réalité de l'auto-focalisation d'un faisceau
laser dans un matériau photoréfractif, à des temps aussi de l'ordre de la seconde que de
la nanoseconde. Nous avons également développé deux modèles distincts qui nous permettent
d'interpréter nos résultats. L'exploitation de ces modèles passe par la réalisation d'algorithmes parall`
eles, au développement desquels quelques pages sont consacrées. Ces études ont permis, de par
les outils qu'elles utilisent, le développement d'autres thématiques —Double Conjugaison de Phase
Photoréfractive et Auto-focalisation dans les solutions photopolymérisables—, au sujet desquelles
la dernière partie de ce document apporte quelques éclaircissements.

Modelisation and simulation of promising non linear optical materials such as photorefractive materials is a field which generates complexity. The involved non linear phenomena imply the use of highly numeric, computer intensive methods. The simulation of the propagation of an optical pulse in a photorefractive non linear media is here simulated thanks to the collaboration between physicists and computer scientists, through the use of parallel implementation and sequential optimization

Photorefractive self-focusing and self-trapping has proved, over the past decade, to be an efficient low power method able to provide beam control and steering. In this paper, several photorefractive samples are investigated in the aim to provide a comparative study in both continuous wave and pulsed laser light regime. Whereas their holography-like behavior can be thought of as similar, they exhibit strong differences when considering the propagation of a single high contrast narrow beam. Perovskite KNbO3 and illmenite LiNbO3 are shown to exhibit respectively self-focusing and self-defocusing features, the latter needing an erasing process to be undertaken for the experiment to be fully reproducible. Finally, SrxBa1 xNb2O6 (SBN) is shown to be the most promising sample for applications ranging from optical limiting to all optical routing, though it exhibits instabilities such as optical branching.

Photorefractive self-focusing and self-trapping has proved, over the past decade, to be an efficient low power method able to provide beam control and steering. Over the years, photorefractive SrxBa1 xNb2O6 has been shown to be the most promising photorefractive media in this field. Its ability to sustain photorefractive solitons at steady state has been thoroughly investigated. In this paper, the focus is set on the temporal development of self focusing in SrxBa1 xNb2O6, evidencing a soft transition from self-focusing through self-bending and finally optical-branching. This evolution is shown to occur not only as a function of experimental conditions such as electric field applied or beam intensity, but also as a function of time.

Photorefractive simple and double phase-conjugate mirrors very often exhibit strong instabilities in their reflectivities. One of their possible origins can be the competition between phase conjugation and its generating process, photorefractive beam fanning. However, by suitably choosing the incident beam widths and directions, we suppressed this competition and replaced it by the enhancement of each process by the other. This technique allows to obtain a double phase-conjugate mirror in BaTiO3 that establishes itself in seconds for beam intensities on the order of the tenth of a milliwatt. The reflectivities reached are on the order of 30% and stable within 3% for hours.

This paper presents a numerical modeling of the temporal evolution of the refraction index induced by a nanosecond single laser pulse propagating in a biased photorefractive crystal. Assuming particular hypotheses typical for a short pulse illumination, a nonlinear time dependent partial differential equation, which describes the mechanisms of space–charge field build up and evolution, is derived from a mono-dimensional Kukhtarev band transport model. The numerical resolution of this space–charge equation is then coupled to the simulation of the propagation of a beam in a nonlinear medium using a beam propagation method. The results evidence the spatial self-focusing of a single nanosecond laser pulse. A description of the output beam profile evolution during the laser pulse is obtained and the role of different physical parameters such as the carriers mobility and the fluence on self-focusing is investigated. A successful comparison to previous experimental measurements is reported.