Research for high-power femtosecond Yb^<3+>:Y_2O_3 ceramic laser

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Based on knowledge and technologies accumulated in the development of ceramic YAG laser, we started investigation of a new material Y_2O_3 (yttria) with further potentials for lasers. Yb^<3+>:Y_2O_3 is a new laser material enabling higher thermal conductivity, broader bandwidth, and shorter pulse generation than YAG. However, its high melting temperature of 2430℃ has made difficulty on crystal growth. By applying our novel fabrication method of laser ceramics based on nanocrystalline technology, Y_2O_3 ceramic was fabricated at a sintering temperature of 1700℃, which is lower than the phase-transition temperature of 2280℃ and then high-quality laser material was successfully obtained with a large size. We have demonstrated CW laser oscillation of Yb:Y_2O_3 with as high as a 9.2-W power and 72% efficiency by laser-diode (LD) end-pumping and thus given evidence of its excellent property as a laser material. Passive mode-locking by semiconductor saturable-absorber mirrors was successfully … More demonstrated and transform-limited, 430-fs pulses with as high as a 420-mW average power were generated. We also succeeded in mode-locking by a GaAs saturable-absorber and enabled LD-direct pumped femtosecond solid-state laser. On the way of laser developments, we have investigated spectroscopic, thermal, and mechanical properties of YAG and Y_2O_3. It was then experimentally proved that a polycrystalline ceramic laser material has a more than three times higher thermal shock parameter than a single crystal, which is suitable for high-dense pumping and high-power lasers. The characteristic size scalability and strong fracture toughness of ceramic lasers are of significance for fusion drivers because their operation limits are determined by thermal shock parameters of the gain materials. We also developed composite ceramics. It was found that the diffusion distances of doped ions are more than fiver times longer than those by conventional diffusion bonding, and the mechanical strength of the ceramic bonding is as high as that in a bulk ceramic. Both indicate the perfect bonding of composite ceramics. Investigation of new materials such as other sesquioxides Sc_2O_3 and Lu_2O_3 have also started. These Yb-doped materials have broad emission spectra like Yb:Y_2O_3. It was found that Yb:Lu_2O_3 will be promising for high average power femtosecond laser because of the higher thermal conductivity even under doping. Less