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Title: Fabrication and characterization of ytterbium doped transparent laser ceramics
Authors: Luo, Dewei
Keywords: DRNTU::Engineering::Materials
Issue Date: 2017
Source: Luo, D. (2017). Fabrication and characterization of ytterbium doped transparent laser ceramics. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: Transparent laser ceramics have shown great application potentials as the host materials of solid state lasers, because of their excellent physical and stable chemical properties. Solid state lasers have wide applications in the fields of industry, communication, medical and military, such as laser drilling and welding, laser surgery and even laser weapons. Compared with their single crystal counterparts, polycrystalline ceramics possess various advantages, such as shorter fabrication period, higher yield for mass production, higher ion doping concentration, feasibility to be large sizes with complex shapes and structures, and overall lower fabrication cost. Among various laser ceramics, ytterbium (Yb 3+ ) ion doped ones have been considered to be very attractive solid state laser materials. Yb 3+ ion has unique properties, including high quantum efficiency, long fluorescence lifetime and broad emission spectrum. Its broad absorption band is especially useful for direct laser diode pumping. Yttrium aluminum garnet (YAG)is a very popular laser host materials, because of its high hardness, high thermal conductivity and stable chemical property. Its cubic crystal structure is also important to achieve high optical transparency. However, processing and fabrication of transparent ceramics with high optical transparency is still a challenge. The work in this thesis is focused on the development of ytterbium doped transparent laser ceramics, mainly based on YAG garnet. By using high purity Yb2O3, Al2O3 and Y2O3 powders as starting materials, ytterbium doped YAG (Yb:YAG) ceramics with different concentrations of Yb were fabricated by using the conventional solid-state reaction process, combined with vacuum sintering technique. XRD results showed that all samples obtained were of pure garnet phase. SEM characterization results revealed that all samples had very dense and pore-free microstructure, with the average grain size of about 10 µm. According to spectroscopic studies, the Yb:YAG laser ceramics exhibited the in-line transmittance of 83% at room temperature, which was very close to their theoretical transparency. The high optical quality assured that the samples can be used for practical laser applications. As a result, the Yb:YAG ceramics demonstrated absorption and emission cross-section of 0.72×10^(-20) cm^2 and 2.01×^(-20) cm^2, respectively. A diode pumped solid state laser system has been set up by using the Yb:YAG ceramics as the laser medium. Continuous wave (CW) laser operation was successfully achieved. The 5.0 at.% doped Yb:YAG sample had a maximum output power of 6.2 W, corresponding to a laser efficiency of 62%. Broader gain spectra have been achieved by adjusting the composition of the garnet host materials. In this respect, transparent ytterbium doped gadolinium yttrium aluminum garnets (Yb:GdYAG), i.e., mixed garnet ceramics, have been developed. This new type of mixed garnet ceramics could be used for laser applications with promising performances. Correspondingly, CW and passive mode-locking laser operations have been experimentally investigated. The technique developed for Yb:YAG was further extended to lutetium aluminum garnet (LuAG). This is because LuAG higher thermal conductivity and thus has high potential as the host laser ceramics for high power laser applications, due to the better thermal management. Experimentally, the Yb:LuAG ceramics exhibited an emission cross-section of 2.7×10^(-20) cm^2, which was higher than that of Yb:YAG by about 35%. CW laser performance of the sample was characterized, with an output power of 7.2 W and slope efficiency of 65%. Femtosecond mode-locked laser operation was also realized with 650 fs pulse duration by using the Yb:LuAG ceramics. Based on the achievements, it is expected that Yb doped sesquioxide ceramics with even higher thermal properties could be considered to be new laser materials as the future works, which have been supported by preliminary results.
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