Fabrication And Properties Of Yb Doped Garnet Planar Waveguide Laser Ceramics

For high power solid-state laser with high beam quality and high efficiency,thermal effect is still a key factor that limits its high power laser output.Therefore,it is essential to design the component distribution of the laser materials to improve their heat transmission.Planar waveguide structure with its unique structural design can effectively reduce the thermal effect,but also achieve high heat transmission,low lasing thresholds and high power laser output.Laser ceramics have attracted much more attention owing to their flexible structural designing and various applications compared with laser crystals.Through tape casting combined with vacuum sintering technology,planar waveguide laser ceramics can be prepared.Among the rare earth ion-doped garnet-based laser ceramics,Yb:YAG polycrystalline ceramics are regarded as one of the dominant laser gain mediums due to their excellent properties in optical,chemical,thermal and mechanical aspects.Currently,with the development of the laser diode(LD)pump power,the early disadvantage of Yb:YAG ceramic,high laser threshold and difficult to pump,were also solved.The planar waveguide Yb:YAG transparent ceramic has become the ideal gain medium for high power and high efficiency solid-state lasers.However,with increasing the concentration of Yb ions in YAG,the thermal conductivity of Yb:YAG ceramics decreases severely.When the host material is LuAG,the thermal conductivity of Yb:LuAG decreases slowly with the increase of Yb3+ ion.When the concentration of doped Yb ions is more than 5at%,the thermal conductivity of Yb:LuAG is higher than that of the Yb:YAG.The Yb:LuAG also has a larger effective emission cross section,which is beneficial to the ultrashort pulse laser output.Planar waveguide Yb:LuAG transparent ceramic is expected to be the ideal gain medium for high power ultrashort pulse lasers.In this research,Yb doped garnet planar waveguide laser ceramics were selected for studying.The ceramics were prepared by non-aqueous tape casting and vacuum sintering.The fabrication process,microstructure evolution,optical properties and ion diffusion distribution of the above ceramics were investigated.Firstly,the green body of planar waveguide YAG/10at%Yb:YAG/YAG ceramic was prepared by non-aqueous tape casting.Then,the microstructure of the raw materials,the casting tapes and green bodies in different stages were observed.The ceramics with in-line transmittance of 81.5% at 1100 nm were prepared by vacuum-sintering at 1775°C for 30 h.The grain boundaries are clear but there are a few pores in grains.Based on the diffusion law,the diffusion behavior of Y and Yb ions in the waveguide layer was studied and the diffusion coefficient was calculated.Then,via optimizing of the preparation process and changing the sintering temperature,the planar waveguide Yb:YAG transparent ceramics with high quality were obtained after sintering at 1760 oC for 30 h.The transmittance of 3.5mm thick samples is closed to the theoretical value and the grain boundaries are clean,with no obvious pores and second phase.The diffusion distance of Yb3+ ions is 215?m along the thickness direction of the ceramics via the ICP-MS test.Continuous-wave(CW)laser operation is carried out for coated planar waveguide Yb:YAG transparent ceramic.In the three-mirrror cavity,the best laser performance in single mode of sample is obtained,with the maximum output power of 1.6W and the slope efficiency of 34.4%.In addition,the planar waveguide LuAG/10at%Yb:LuAG/LuAG laser ceramics were prepared by the tape casting and sintering technology.After pre-vacuum-sintering at 1775,1800 and 1825 oC,the samples were post-treated by hot isostatic pressing(HIP)at 1700 oC.The microstructure,optical properties and the distribution of doped Yb ions of the ceramics after HIP post-processing were mainly studied.For the LuAG/10at%Yb:LuAG/LuAG ceramics pre-sintered at 1825 oC and then HIP-ed at 1700 oC,the in-line transmittance of the sample with the thickness of 2mm is about 74% and the maximum diffusion distance of Yb3+ is about 170?m.