| Summary: | 碩士 === 臺灣大學 === 光電工程學研究所 === 95 === Yttrium aluminum garnet (YAG) crystal doped with active ions has been widely used as laser gain medium. It is important to control the molten-zone properties well during the drawing process to enhance the performance of the output laser beam, because the laser quality is affected significantly by the species of active ions and its radial dopant concentration distribution in the gain medium. We have experimentally demonstrated the neodymium-doped YAG crystal fiber with the maximum central dopant concentration up to 2.4 atm.% with a fiber diameter of 220 μm by laser-heated-pedestal-growth technique when the rawing velocity is 1.25 mm per minute. The source rod was square with an end facet area of 480 μm by 480 μm and an uniform dopant concentration of 1.15 atm.%.
To further understand the relation between the drawing parameters and the radial dopant concentration of the crystal fiber and compare with the experiment, we modified a 2-dimensional computational-fluid-dynamic (CFD) program, which was written by Lan et. al. for bulk materials. Simulation results agree with the experimental ones in size and shape of the molten zone for various CO2 laser input powers. Moreover, the radial dopant distribution of the crystal fiber in the simulations is similar to that in the experiments, but there are differences in the ariation of the maximum central concentration by changing the drawing speed. It is speculated that the crystal lattices are damaged due to the residual strain caused by high maximum central dopant concentration. Besides, the highest maximum central dopant concentration was about 1.5 atm.% in the simulations which is different from that in the experiments. We believe that it could be improved quantitatively by considering the surface tension coefficient as a function of both concentration and temperature in the simulation.
|
|---|
