Active GRIN media for laser beam transformations

Resumo

In this thesis the use of active GRadient-INdex (GRIN) media with gain or loss for laser beam shaping purposes is presented and particularized for materials described by a complex parabolic and hyperbolic secant refractive index profiles. The expressions for the complex refractive index of these media are derived and examined in order to discuss what loss or gain means in terms of the index. The formalism of laser beam propagation in active GRIN materials regarded as beam transformers with a Gaussian mask is presented. Moreover, the general conditions for beam transformations by active GRIN materials as a geometrical solution in 2D space of the complex curvature of the input laser beam are obtained and discussed. Application examples of active GRIN media with gain or loss as beam shaping systems are presented using MATLAB software and a tolerance analysis is performed for a particular laser source. Additionally, preliminary research on active GRIN media fabrication by Sol-Gel process and Laser Zone Melting (LZM) technique was performed. Erbium and ytterbium-doped films based on SiO2-TiO2 and SiO2 systems were prepared by sol-gel method. Optical constants, thickness and porosity of selected samples were determined using Spectral Ellipsometry, whereas topographical inspection was conducted by Atomic Force Microscopy. The analysis of luminescence spectra of the erbium sol-gel derived films around 1.54 μm was also performed. In LZM technique, SiO2 rods precursors are dip-coated using a suspension of different inorganic compounds in isopropyl alcohol and the subsequent sintering process is performed using a CO2 laser. Structural properties and chemical composition of the samples were characterized by Scanning Electron Microscopy combined with Energy Dispersive X-ray analysis, revealing the axial and gradual incorporation of dysprosium oxide and lithium niobate within the silica cylinder. Spectral Ellipsometry measurements showed that a gradient of refractive index profile from the edges to the centre of the rod was generated.