We present the first demonstration of visible laser oscillation in the Dy3+-doped silica fiber pumped by a 451nm InGaN laser diode. It was found that Ge-co-doping plays the following important roles of laser oscillation: (1) to reduce the Rayleigh scattering loss, (2) to suppress the X-ray-induced and pump-induced photodarkening (PD), and (3) to increase lasing slope efficiency. In a fiber with 0.46wt% Dy, 1.8 wt% Ge, and 0.54wt% Al, the slope efficiency is 22.0 % at 582.5 nm, and the maximum output power is 18.4 mW.

X-ray induced photodarkening and its suppression by Ge-codoping were investigated in Al-doped, Dy-doped, and Dy-Al-doped silica glasses. Absorption bands of 4f-4f5d transition of Dy2+ were determined in Dy-Al-doped silica glass fabricated by Si nano-powders as a reducing agent, and it was found that pair generation of Dy2+ and Al-OHC was occurred in X-ray irradiated Dy-Al-doped silica glass. Ge-codoping suppresses the above-mentioned pair generation and instead forms the pair of GEC and GLPC+. Similar suppression effect of Ge-codoping was observed in Al-Ge-doped silica glass, where pair generation of Al-OHC and Al-E’ center was suppressed. To investigate the stability of defects, annihilation processes during thermal annealing was measured. Al-OHC and Dy2 + are destabilized by Ge-codoping in Al-doped and Dy-Al-doped silica glasses, while GEC is stabilized in Al-Ge-doped and Dy-Al-Ge-doped silica glasses. Based on these results, local structure models around Al, Ge, and Dy ions and possible mechanisms of photodarkening were proposed

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The final goal ofthis study is to develop a buoyancy control device for underwater robot utilizing phase change of materiai ． A conventiQnal underwator robot and a conventional submarine a 〔 加 st buoyancy by discarding a set of iron balls or seawater in other place， which is harmfUI to the environment 、 Therefbre ， we proposed new buoyancy control method utilizing phase change of material 」 n thc prcvious papers ， we used paraffin wax ． Howevor， its melting point is high ( approximately 47℃ ) ． Therefbre ， we must loQk 負 )r a new material that has lower melting point， and not harrnfu1 to thc cnvironment ． We manufactured a new buoyancy mechanism to test materials ． Finally ， we focused on lard and 行nd out that lard has lower melting point( approximatcly 25 ℃ ) with high density difference betwccn liquid and solid phases and thus response faster than para 幵in wax ． Key iferds：Underwatcr robot ， Buoyancy control ， Rubber stretch ， Lard