Photonic spin glass, extreme events and Lévy-like behaviour in a cw-pumped erbium random fibre laser

• Autor(a) principal: LIMA, Bismarck Costa
• Data de Publicação: 2019
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Repositório Institucional da UFPE
• dARK ID: ark:/64986/001300000sbdw
• Texto Completo: https://repositorio.ufpe.br/handle/123456789/33779
• topic: Óptica; Lasers aleatórios; Sistemas complexos

Random lasers (RLs) are photonic systems that emit light amplified by stimulated emission of radiation, (coherent emission) due to multiple scattering of light by a disorder medium inside a gain medium. Since there are no fixed mirrors, the feedback mechanism provided in conventional lasers does not exist here. This role is made by the multiple scattering of light inside the gain medium owing to the presence of scatterers. RL light can present multi-directionality in the emitted beams, multimode character and a complex behaviour due the connection between strong scattering, disorder and gain. RL was first proposed by Letokhov (1968) to occur in any gain material with scattering after a determined volume threshold. Nowadays there are RL in different materials, and one with most potential for applications are RLs that takes the advantages of fibre optics, where laser emission is obtained in conventional or specially designed optical fibres. The complex behaviour of RL light emission has been used to observe statistics phenomena in a well controllable device and performing analogies with statistical mechanics. In this work, we study the statistical properties of the light emitted by an Erbium doped random fibre laser, with a specially designed fibre Bragg grating. In this sense, the feedback mechanism is provided by multiple scattering of light due the intentionally inscribed random phase shifts during the grating writing process. The linewidth reduction and the output intensity as a function of the input pumping power was characterized, showing a typical threshold resembling a conventional laser character. The multimodal behaviour was determined by speckle measurements. Interaction between the laser modes was analysed by the emission spectra performing the analogies to the spin glass theory, considering the laser modes and input pump power analogue to the spin variables and inverse of temperature, respectively. A glassy behaviour was attributed to the Erbium random fibre laser emission by the analysis of the emission spectra. With the definition of the intensities fluctuation order parameter (analogue to the Parisi order parameter), its probability density distribution changes shape, showing that the system goes from a non-correlated regime below threshold to a correlated mode behaviour above the threshold. It is a clear evidence of a photonic paramagnetic to a photonic spin-glass phase transition. From the same experimental data, we characterized the maximum emitted intensity in each spectrum, its connection to the large intensity fluctuation is experimentally studied, and three different regimes on the emitted intensity were observed: Gaussian pre-laser regime, Lévy-like around the threshold and Gaussian laser regime well above the threshold. The large intensity fluctuation also motivated the study of extreme events in this system. We observed the presence of rare events of large intensity fluctuation that well complied with the theoretical prediction. We analysed each sub-set of the maxima intensities emitted, forming a new set of variables. It was observed a good agreement between the Lévy stable distribution and Extreme events statistics, and experimental evidences of connection to the glassy behaviour was experimentally observed. Our results introduce new insights to the understanding random laser emission properties and confirm the use of RL as a photonic platform to study a broad range of physical process, optical amplifiers, sensors and light source to image measurements.