Fabrication and laser properties of transparent Yb:YAG ceramics

High optical quality transparent Yb:YAG laser ceramics have been successfully fabricated by a vacuum reactive sintering method. Commercial Al₂O₃ powder and co-precipitated Y₂O₃ and Yb₂O₃ powders were used as the raw materials. In-line transmittances at 1300 nm and 400 nm were measured to be 83.6% and 81.8% respectively for a 3 mm thick mirror polished Yb:YAG ceramics sample. Continuous wave (CW) lasing at the wavelength of 1030 nm was achieved when pumped by a 940 nm fiber coupled laser diode. A slope efficiency as high as 62.7% was obtained.     

Fabrication and laser behavior of the Yb:YAG ceramic microchips

Highly transparent 5 at.% Yb:YAG ceramic microchips were fabricated by vacuum sintering technology. The calculated cross section of the absorption σ_abs and emission σ_em were 6.1 × 10⁻²¹ cm² at 940 nm and 1.9 × 10⁻²⁰ cm² at 1030 nm, respectively. The fluorescence lifetime of the unannealed sample was 0.618 ms, and became 1.229 ms after annealing. Laser oscillation around 1037 nm was observed when pumped by the fiber-coupled laser with 970 nm. The threshold power was 3.0 W and the slope efficiency was about 12% when using the transmission of 2.6% output coupler.     

Problems of YAG nanopowders compaction for laser ceramics

Slip casting and colloidal slip casting at high pressure of yttrium aluminum garnet powders were investigated. It was found that the presence of residual pores in laser oxide ceramics was determined mainly by big size pores in the compact. The size of pore in compact is critical when it is greater than the mean size of initial particles. It was shown that formation of pores’ structure in compact was controlled by appearance of quasi-particles in heavy loaded slurry. Pores concentration is critical for ceramics optical transmittance.     

Optical temperature sensing based on the luminescence from YAG:Pr transparent ceramics

The YAG:Pr transparent ceramic was fabricated using a conventional solid-state reactive method to explore its possible application in optical thermometry. Photoluminescence and temperature-dependent luminescence were elaborately investigated under 452 nm excitation. The ceramic showed two intrinsic emission bands at 488 and 594 nm, which were attributed to characteristic Pr³⁺: ³P₀ → ³H₄ and ³P₁ → ³H₆ transitions, respectively. Down-conversion emissions from the two thermally coupled excited states of Pr³⁺ were recorded in the temperature range of 293–593 K. The Boltzmann distribution theory was adopted to interpret the temperature-dependent luminescence of Pr³⁺. The temperature sensitivity exhibited an increasing trend with the increase of temperature, typically, 0.0025 K⁻¹ at 593 K. The results indicated that the present ceramic was a promising candidate for optical temperature sensor.     

Photoluminescence and laser behavior of Yb:YAG ceramic

Yb:YAG transparent ceramics with different doping concentration were fabricated by the traditional solid-state diffusion route, and their fluorescence properties and laser behavior were investigated. It is found that both the fluorescence intensity and lifetime depend deeply on both the doping concentration of Yb ions and the annealing treatment. The continuous wavelength (C.W.) laser performance for 10 at.% Yb:YAG ceramic suggests that both the threshold pump power and slope efficiency increase with increasing transmittance of the output couple (T_oc). For T_oc = 15%, the threshold pump power is about 6.1 W, the slope efficiency is 25%, and the maximum output power of 2.2 W is achieved when pumped by 14.9 W.     

Effect of annealing on the optical properties of Nd:YAG transparent ceramics

The Nd:YAG transparent ceramics were fabricated by vacuum sintering at 1750 °C for 20 h, where the raw nanopowders were synthesized by a modified co-precipitation method. Subsequently the Nd:YAG ceramic specimens were annealed or even re-annealed in air or vacuum at 1250-1500 °C for 20 h. The Nd:YAG ceramic specimens were investigated by X-ray diffraction, ultraviolet spectroscopy, inductively coupled plasma atomic emission spectroscopy, electron paramagnetic resonance spectroscopy and scanning electron microscopy. The results showed that post-annealing could improve the optical properties and microstructure of Nd:YAG transparent ceramics. By air annealing, the in-line transmittances of the specimens increased, mainly due to the decrease of oxygen vacancies concentration in specimens. The air annealing at 1450 °C for 20 h was optimum for preparation of Nd:YAG transparent ceramics. Furthermore, the red shift of UV absorption edge after air annealing could be attributed to the absorption by Fe³⁺ charge transfer bands.     

Improvements in the production of Yb:YAG transparent ceramic materials: Spray drying optimisation

Spray drying parameters have been optimized for the preparation of granulated stoichiometric mixture of oxides. These oxides are used in the production of Yb:YAG laser grade ceramic materials by reactive sintering. The selected compositions were Yb_0.294Y_2.706Al₅O₁₂ and Yb_0.03Y_2.97Al₅O₁₂, i.e. YAG doped with 9.8 and 1.0 at% of Yb with respect to the overall Y + Yb amount. The influence of solid content, drying medium flux, temperature and aspiration rate of the spray drying process on the transparency of the sintered materials has been evaluated. Sintering was conducted under high vacuum and clean atmosphere in the temperature range of 1650-1735 °C and with a soaking time of 16 h. SEM analysis of the microstructure of granulated powders and sintered materials, as well as the transmittance measurement, were used as tools to guide the selection of the best spray drying parameters.     

Proton or helium ion beam written channel waveguides in Nd:YAG ceramics

We report on the fabrication of channel waveguides in Nd:YAG ceramics, using either focused proton beam writing (PBW) or He beam writing (HeBW) techniques. Energies of ions used in the writing process were at 1 MeV and 2 MeV, respectively, with different writing fluence. High quality channel waveguides were produced in both H⁺ and He⁺ implanted regions. Characteristics of the waveguides were explored, and refractive index distribution of the waveguide was reconstructed.     

Yb:YAG透明陶瓷的制备和激光输出

制备了高质量的Yb:YAG透明陶瓷. Yb:YAG透明陶瓷的晶粒尺寸为10μm左右且分布均匀, 晶界处和晶粒中没有杂质、气孔的存在. Yb:YAG样品中所有元素分布均匀, 不同的晶粒间, 晶粒和晶界间成分是一致的, 没有出现成分的偏析. 4mm厚样品的透过率为80％. LD泵浦获得了波长为1030nm, 最大功率为268mW的连续激光输出.     

Transparent ceramics for high-energy laser systems

We demonstrate that transparent magnesium aluminate spinel ceramic possesses excellent thermo-optical properties, a record low absorption loss of 6 ppm/cm, and superior ruggedness which position it as a prime candidate for an exit window aperture for high energy laser systems, especially in hostile environments. We also demonstrate lasing with an efficiency of about 45% in transparent Yb³⁺:Y₂O₃ ceramic made by hot pressing high purity submicron co-precipitated powder. This paves the way forward for high power solid state lasers exploiting hosts with higher thermal conductivity than YAG.     

Improvement of optical properties of Nd:YAG transparent ceramics by post-annealing and post hot isostatic pressing

The Nd:YAG transparent ceramics were fabricated by vacuum sintering. The Nd:YAG samples were annealed at 1450 °C for 20 h in air and followed by hot isostatic pressing (HIP) at 1700 °C for 2 h in 200 MPa Ar and then re-annealed at 1250–1450 °C for 10 h in air. The experimental results showed that the optical properties of Nd:YAG samples varied markedly with different post treatments. After air annealing at 1450 °C for 20 h and HIP at 1700 °C for 2 h under 200 MPa of Ar and then air re-annealing at 1250 °C for 10 h, the transmittances of the samples increased from 51.2% to 77.2% (at 400 nm) and 78.4% to 83.6% (at 1064 nm), respectively. The annealing and HIP are effective post treatments to reduce oxygen vacancies and intergranular pores respectively in Nd:YAG transparent ceramics.     

凝胶燃烧法合成Nd：YAG纳米陶瓷粉体

凝胶燃烧法具有合成温度低、所得粉体分散性好等优点。以金属硝酸盐为原料,采用凝胶燃烧法合成出多晶Nd：YAG陶瓷粉体。对四种不同的燃烧助剂（尿素、氨基乙酸、乙二醇、柠檬酸）制备的粉体的形貌和分散状况进行了比较,并对其原因进行了分析。借助于TG—DTA、XRD、SEM等表征手段,重点对筛选出的柠檬酸作为燃烧助剂时所得粉体的相组成和微观结构进行了研究。结果表明：前驱体在850℃煅烧2h直接生成Nd：YAG,平均粒径约为40nm,煅烧过程中没有其它中间相生成。随着煅烧温度的升高,粉体平均粒径逐渐变大,1000℃时煅烧所得粉体的平均粒径约为60nm。     

Mechanical,thermal and optical properties of the SPS-processed polycrystalline Nd:YAG

The present study deals with a comprehensive comparison of the mechanical and functional properties of Nd:YAG single crystals with those of the polycrystalline ceramics (PCs), undoped and LiF-doped, processed by Spark Plasma Sintering (SPS). The polycrystalline ceramics have higher mechanical properties (hardness, bending strength and thermal shock resistance) than the single crystals. The optical transmittance of the LiF-doped PC Nd:YAG is significantly higher than that of the undoped one and is close to that of the single crystal. With respect to other optical and thermal properties, i.e. refraction index, absorption coefficient, extinction ratio, thermo-optic coefficient, fluorescence and thermal conductivity, no significant differences were observed between the single crystals and the polycrystalline ceramic.     

Investigation of continuous-wave and Q-switched microchip laser characteristics of Yb:YAG ceramics and crystals

Continuous-wave and passively Q-switched microchip laser performance of Yb:YAG ceramics and single-crystals was investigated. Highly efficient continuous-wave Yb:YAG laser performance was observed at 1030 nm and 1049 nm for both Yb:YAG ceramics and crystals with different transmissions of output couplers. The laser performance of Yb:YAG ceramic is comparable to that of Yb:YAG single crystal. Meanwhile, the laser performance of laser-diode pumped Yb:YAG/Cr⁴⁺:YAG all-ceramics- and all-crystals-combination passively Q-switched microchip lasers were investigated. Sub-nanosecond laser pulses with peak power over 150 kW were obtained with different Yb:YAG/Cr⁴⁺:YAG combinations. Linearly polarized laser was observed in Yb:YAG/Cr⁴⁺:YAG all-crystals combination and circular polarized laser was obtained in Yb:YAG/Cr⁴⁺:YAG all-ceramics combination. The best laser performance was obtained with Yb:YAG/Cr⁴⁺:YAG all-crystals combination.     

高光学质量Yb:YAG透明陶瓷的制备及激光参数研究

以高纯Y₂O₃,α-Al₂O₃,Yb₂O₃粉体作为原料,采用固相反应和真空烧结法(1750℃,30h)制备了高光学质量的Yb:YAG透明陶瓷。5.0at%Yb:YAG陶瓷中Yb³⁺的实测浓度为6.41×10²⁰ cm^-3,晶胞密度为4.65 g/cm³。本工作重点研究了Yb:YAG陶瓷的显微结构、光谱特性和激光性能参数。场发射扫描电镜(FESEM)结果表明,Yb:YAG陶瓷的结构均匀致密、晶界干净平直,平均晶粒尺寸为(19±3)μm。该陶瓷样品(厚度为4.0 mm)在400 nm处的直线透过率为82.5%,在1100 nm处的透过率为85.2%。泵浦波长940 nm处的泵浦饱和光强最小,激光波长1030 nm处的泵浦阈值功率最低,940 nm泵浦1030 nm激光的品质因子为1.02×10^-22 cm·s。通过计算增益截面表明Yb:YAG陶瓷宽带可...     

Nonlinear optical responses of erbium-doped YAG ceramics

By performing the Z-scan measurements with ultrafast femtosecond laser centered at 800 nm wavelength, we can unambiguously distinguish the real and imaginary part of the third-order optical nonlinearity of the erbium-doped YAG ceramics. The reverse saturable absorption of the erbium-doped YAG ceramics has been observed experimentally, and the nonlinear refractive index of the ceramics is estimated to be about 10⁻²¹ m²/W. The experimental results may provide design guidelines for the high power laser design and its applications.     

Doubly Q-switched tape casting YAG/Nd:YAG/YAG ceramic laser

A doubly Q-switched 1.06 μm pulsed laser using a novel tape casting YAG/Nd:YAG/YAG composite ceramic with a sandwich structure was demonstrated for the first time. Compared to purely acousto-optical (AO) Q-switching, this laser using an AO Q-switch and Cr⁴⁺:YAG saturable absorber simultaneously can generate shorter pulses. The pulsed laser performance was investigated at two modulated repetition rates of 10 and 20 kHz.     

Simple method to join YAG ceramics and crystals

Method to join samples of yttrium aluminum garnet (YAG) ceramics and crystals together includes deposition of thin SiO_x layer(s) on flat polished surfaces of the samples to be joined and heating the samples with the contacting surfaces above 1700 °C. There is no visible border between the crystal samples bonded. YAG single crystal solid state growth takes place on the bonded crystal-ceramics border, and crystal growth velocity is much higher when intermediate SiO_x layer is used.     

稳功率Nd：YAG连续激光器

选用高频开关电源作Ｋｒ灯电源，并使用热稳谐振腔、特殊聚光腔等，使Ｎｄ：ＹＡＧ连续激光器输出功率的稳定度达到±２％的水平。     

球磨转速对Nd:YAG透明陶瓷的显微结构及光学性能的影响

以高纯商业Y₂O₃、α-Al₂O₃和Nd₂O₃粉体为原料, 以TEOS(正硅酸乙酯)和MgO为烧结助剂, 采用固相反应和真空烧结技术制备了1.0at%Nd:YAG透明陶瓷。系统研究了球磨转速(球磨时间10 h)对混合粉体的尺寸以及对陶瓷样品致密化行为、显微结构和光学性能的影响。结果表明: 通过球磨过程可以充分细化原料粉体的颗粒; 随着球磨转速的提高, 陶瓷烧结时样品中的气孔能更好地排除。但是球磨转速过高时, 陶瓷烧结体中存在少量的富铝第二相会降低样品的光学透过率。当球磨转速为130 r/min时, 真空烧结(1760℃×50 h)所得Nd:YAG透明陶瓷的微结构均匀致密, 几乎没有晶界和晶内气孔存在, 样品在1064 nm处的直线透过率高达83%。