Spectroscopie et caractérisation laser de cristaux massifs et de couches minces cristallines de fluorures dopés terres rares autour de 2um

Abstract : Since the 1970s, the development of power laser sources operating around 2 μm based on Thulium Tm3 + and / or Holmium Ho3 + doped crystals is an active field of research because of its multiple applications. The emission around 1.9 μm corresponds to a transition from the excited level 3F4 to the fundamental level 3H6 in a trivalent ion Tm3 +, whereas the emission at 2 μm and beyond corresponds to the transition between the level 5I7 and the fundamental level 5I8 of the Ho3 + ion. These laser transitions are today the subject of many applications in a wide variety of fields. We have studied the spectroscopic properties of rare earth doped fluoride crystals for emission around 2 μm. From the absorption spectra obtained, the Judd-Ofelt analysis made it possible to calculate the radiative lifetimes as well as the branching ratios in the Tm3 + and Ho3 + ions. In comparison with other crystals such as oxides, the lifetimes of the 3F4 (Tm3 +) and 5I7 (Ho3 +) levels in the fluoride crystals are longer, which favors pulse-mode laser operation. A study on fluorescence dynamics was carried out in order to characterize the emission around 1.9 μm of the Tm3 + ion and that around 2 μm of the Ho3 + ion. The emission spectra obtained have broad bands, fairly structured in the LiYF4 matrix and wider and less structured in the CaF2 matrix. In a second step, by measuring the lifetimes of the 3F4 emitter level for different Tm3 + ion concentrations, we calculated the transfer parameters. The results obtained showed that the process of cross relaxation is much more efficient in the CaF2 crystal than in LiYF4. We also calculated the transfer and inverse transfer coefficients between Tm3 + and Ho3 + ions following excitation of the Tm3 + ion. We have shown that the LiYF4 matrix offers a better transfer efficiency with a view to emission around 2 μm. The laser operation of thin layers in "thin disc" configuration has been studied. A first demonstration of a thin-film LiYF4: Tm laser emitting at 1.9 μm made from an epitaxial layer with LPE was made. In single passge, an output power of 306 mW and a laser efficiency of 36% in absorbed power are reached for a 240 μm thick layer. We have studied a laser oscillator in LiYF4: Tm waveguide in planar geometry. and linear. These guides are characterized by low propagation losses thanks to the quality of the manufacturing process. A maximum power of 470 mW and a yield of 14% in incident power were obtained. A numerical modeling of the optical amplification shows that the guidance of the pump and probe beams makes it possible to achieve significant gains. The model is then adapted for the saturation regime in order to calculate the extraction efficiency and to determine the potential of the LiYF4: Tm3 + guides as an amplifier around 1.9 μm
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Mohamed Salhi. Spectroscopie et caractérisation laser de cristaux massifs et de couches minces cristallines de fluorures dopés terres rares autour de 2um. Physique [physics]. Normandie Université, 2017. Français. ⟨NNT : 2017NORMC270⟩. ⟨tel-01743723⟩

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