Two dichroic films were prepared to investigate their aging properties. Results indicate that the age-resistant property of dichroic films can be significantly improved by adopting lower deposition rate and relative high preposition temperature and being treated with stress removing annealing procedure. The correlation of reflective signal to transmission efficiency of igniting laser merely changes after aging for the designed age-resistant dichroic films.
In order to improve coupling efficiency and maximal laser transmission energy density, lasers with Gaussian distribution and flat-top distribution were utilized to couple to optical fibers with different surface roughness. The coupling efficiency and damage threshold of the fibers were studied. The experiment results showed that for Gaussian distribution laser beam, optical fibers with higher degree of smoothness would get better coupling efficiency, but would suffer more damage inside the fiber. But for flat-top distribution laser beam, optical fibers with higher degree of smoothness would get both higher coupling energy and efficiency.
Various target samples were prepared by adhering 20μm thick Al foils to sapphire substrates. 0.2mm thick steel barrels with Φ0.6mm and Φ0.8mm diameter were attached to the surface of Al foils. A diffractive beam homogenizer was used to modify the laser pulse emitted from a Q-switched Nd:YAG laser. The unmodified beams presented a multi-ringed nonuniform energy profile, while numerous small energy spikes randomly distributed over the homogenized laser beam. The unmodified and homogenized beams at 80mJ were used to ablate the target samples, and the flight trajectories of Al flyers were visualized by a time-resolved shadowgraph technique. The images showed that more debris and fragments were formed at the radiation of the homogenized laser pulse, while a reverse results were obtained when barrels were employed. Moreover, the laser induced crater silhouettes presented differently in sizes and morphologies resulting from the change in the energy profiles pre- and post-homogenization. The flyer velocities were ~1300m/s regardless of the barrel parameters for unmodified laser ablation. While for homogenized lasers, the flyer velocity was up to ~1500m/s for free surface foils, and much lower velocities (~700m/s and ~1000m/s) were observed for Al foils covered with Φ0.6mm and Φ0.8mm barrel.
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