Composition modale d'un faisceau laser

Abstract

Nowadays, many applications, industrial or medical, need laser beams with specific transverse intensity profiles: for instance, we can quote laser machining, optical trapping and guiding of cold atoms. The aim of this thesis is the shaping and characterization of laser beams by modal composition, these techniques are achieved via the using of both amplitude and phase diffractive optical elements. In the first chapter, modal weights coefficients of a such transverse laser modes presented in an incoherent mixture are determined, we used a mathematical method based on least squares, this technique allowed us to characterize Laguerre- Gaussian and Hermite-Gaussian laser beams. The idea is based on study of the variation of the transmission diffracted by different apertures such as (circular aperture, opaque ring, half plane). In the second chapter, we have considered the study of a single-pass diffraction, by considering the diffractive properties of the two apertures (opaque ring and diaphragm) when the collimated incident laser beam is an LG00 or an LG01, several energetic and geometric magnitudes are studied. this chapter is considered as a pre-study of the third one, in order to mitigate the problem posed in the latter. In the third chapter, we have reached the ability to generate a flat-top in intra-cavity containing (opaque ring and a circular aperture), that maintain his shape during propagation from the near field to the far field, by applying a robust method based on the decomposition on Laguerre-Gaussian polynomials (LGpl). Finally, in the fourth chapter, we have shown how a simple binary diffractive optical element that, together with an aperture and lens, results in enhanced focusing. We explain this by a careful analysis of the spherical aberrations (defocus, primary, and high-order spherical aberration) generated by DOE