Controllable Fabrication And Property Optimization Of Composite Structured Ytterbium Doped Garnet Structure Laser Ceramics

The significant development directions of advanced solid-state lasers are high power,high efficiency,and excellent bean-quality.Nontheless,the thermal effects of the gain mediums will degrade the laser performances seriously.Laser ceramics have the advantages of high thermal conductivity,achieving continuous-wave laser,better homogeneity of the active ions,etc.What's more,the fabrication of large-size composite structured laser ceramics can be realized relatively easily,which can ease the thermal effects and amplified spontaneous emission?ASE?of gain mediums efficaciously.Thus,the laser ceramics could become one of the most promising gain medium materials.Among all types of the composite laser ceramics,the planar waveguide geometries have the advantages of low lasing threshold,high gain and optical confinement as well as effective heat dissipation.Activate ion gradient doping laser ceramics can realize the uniform heat distribution inside the materials when high power laser pumping is operated.The surface gain structured laser ceramics can compensate the thermal effect inside the gain medium effectively and the dissipate the heat efficiently.In view of the forming and processing methods of ceramics,the combination of tape casting and sintering technique has the superiority in realizing the composite structured laser ceramics for the reason that the thickness of every tape can be controlled at the micron level and the designed structure can be obtained during the molding process of the ceramics.Yb:YAG materials have the advantages such as simple energy level structure,high gain,long upper-state lifetime,no concentration quenching,broad absorption and emission band as well as high Yb doping concentration.Besides,with the continuous increasing power of the laser diode?LD?,solving the problem of high pumping threshold and difficulty in pumping of Yb:YAG,which makes it a promising gain medium for high power lasers.Based on the above viewpoints,composite structured ytterbium doped garnet structure laser ceramics were prepared by the non-aqueous tape casting and sintering method.The fabrication process,microstructure,optical and laser properties as well as the ion diffusion behavior of the mentioned ceramics were investigated.The main points of this essay are as follows:1)Utilizing the tape casting method combined with vacuum sintering and hot isostatic pressing using commercial oxide powders as the starting materials,high-quality planar waveguide YAG/10at.%Yb:YAG/YAG ceramic slab?60×10×1 mm³?was successfully prepared.The in-line transmittance of the prepared sample reached82.5%at 400 nm and the average grain size was¹7.1?m.Then,a 1030 nm continuous-wave?CW?Yb:YAG planar waveguide ceramic laser based on the structure of master oscillator power amplification?MOPA?was realized.After a single-pass amplification,the maximum output of the ceramic reachs 1.25 kW and the corresponding optical-to-optical conversion efficiency was 30.0%,which is the highest output power of a Yb:YAG planar waveguide ceramic laser to the best of our knowledge.The diffusion behaviors of Yb ions across the contact boundary between the cladding YAG layer and the core Yb:YAG layer were determined by Fick's second law by the laser ablation-inductively coupled plasma atomic emission spectra-mass spectrometry?LA-ICP-MS?.The bulk diffusion coefficient and the grain boundary diffusion coefficient of Yb ions were determined to be 2.43×10^-15 and 2.15×10^-9 m²/s,respectively.2)Planar waveguide LuAG/10at.%Yb:LuAG/LuAG laser ceramics were fabricated by the tape casting method combined with the vacuum pre-sintering and HIP post-treatment using commercial oxide powders as the starting materials.The in-line transmittance of the sample with the thickness of 2 mm was about 74.0%at 1100 nm and the maximum diffusion distance of Yb³⁺was about 166?m.What's more,CW laser at 1030 nm was demonstrated using a 976 nm semiconductor diode laser as the pumping source,the maximum output power of 0.4 W with a slop efficiency of 9.4%was achieved.The demonstration of planar waveguide LuAG/Yb:LuAG/LuAG ceramics laser was reported for the first time as far as we know.The bulk and grain boundary diffusion coefficients of Yb³⁺ions for the prepared samples were also calculated.The experimental results showed that the diffusivity of Yb³⁺increased with increasing the pre-sintering temperature,especially for the grain boundary diffusion.3)Gradient doping Yb:YAG transparent ceramics were fabricated by the tape casting method combined with vacuum sintering and hot isostatic pressing post-treatment,for the 5.9 mm thick ceramic sample vacuum sintered at 1765^oC for 30 h and then HIP post-treated at 1700^oC for 3 h,in-line transmittance of 81.6%at 1100 nm was obtained.The poor optical quality of the sample was mainly due to the asynchronous densification of different components as well as the Fresnel reflection loss derived from the mismatch of refractive indices.In order to alleviate the above problems,multistage doping Yb:YAG ceramic was designed and fabricated by vacuum sintering at 1740^oC for 30 h and then HIP post-treating at 1700^oC for 3 h,and the in-line transmittances of YAG,0.6at.%Yb:YAG and 1.5at.%Yb:YAG regions at 400 nm were 83.9%,84.1%and 83.3%,respectively.In the laser experiments,an output energy of 3.43 J with a repetition rate of 10 Hz and a pulse of 1 ms was obtained using the multistage doping Yb:YAG ceramic.The corresponding optical-to-optical conversion efficiency and slope efficiency were 30%and 45%,respectively.4)The tapes with the target components of YAG and 2.5at.%Yb:YAG were prepared by the non-aqueous tape casting and the green body was obtained by laminated them according to the design of surface gain 2.5at.%Yb:YAG/YAG/2.5at.%Yb:YAG ceramic.After vaccum sintering at 1735^oC for 30 h and then HIP post-treating at1700^oC for 3 h,the sample with the transmittance of 79.1%at 400 nm and 83.5%at 1100 nm was obtained.