Study On Double Tungstate Kre_xyb_1-x(wo₄)₂ Laser Crystal

The KRExYb1-x(WO4)2 can be described as a type of laser crystals with low symmetry, multi-wavelength and tunability, which belong to the family of double tungstates. These crystals has several merits, such as high doping concentration, low fluorescence quenching, high conversion efficiency, low threshold value and well stability, etc. It can achieve all-solid-state, miniaturization and integration for lasers and widely used in the fields of human-eye safe, laser communication, medical treatment, remote sensing, etc. Through a large amount of experimental work in this dissertation, the growth, structure, physical properties, defects, spectral and laser performance of KRExYb1-x(WO4)2 (RE=Nd3+, Er3+, Tm3+, Ho3+-Tm3+) laser crystals have been investigated systematically.The KRExYb1-x(WO4)2 laser crystals, including KNdo.o5Ybo.95 (WO4)2, KEro.]Ybo.9 (WO4)2, KTm0.1Yb0.9(W04)2 and KHo0.04Tm0.06Ybo.9 (WO4)2 have been grown by the TSSG and Kyropoulos methods, respectively. In addition, advantages and disadvantages of these two growing methods have been compared.The structure of KRExYb1-x(WO4)2 laser crystals were investigated through X-ray single crystal diffraction. The structure of the crystals is composed of three kinds of groups, including WO6. REO8and KO12 polyhedrons. The W2O10 dimmers are connected by WOW single oxygen bridging bonds and form (W2O8)n bands along parallelling c axis direction. The REO8 and KO12 polyhedrons in the crastals are joined by common vertex and form an elongate band with the structure of two-dimension layer along [101] and [11O]directions. Powder X-ray diffraction analysis indicates that above four crystals belong to monoclinic system with a space group C2/c, which indicates that four crystals obtained areβ-KRExYb1-x(WO4)2 with low temperature. The cell parameters, crystal grain size and crystallinity of four crystals were calculated. The vibration spectra of four crystals were investigated, and the vibration modes and frequencies of infrared and Raman peaks were identified, which confirm the existence of WO6 group, WOOW double oxygen bridge and WOW single oxygen bridge bonds in the crystals.The melting point and phase transition temperature of four crystals were obtained through the TG—DTA curves. In addition, some physical properties including crystal density, refractive index and segregation coefficient were characterized.The defects of KNd0.05Yb0.95(W04)2 crystal were observed by the optical microscope, such as microcrack. growing strips, growing mound, growing steps, inclusions and so on. Meanwhile, several defects of KEr0.1Ybo.9(W04)2 crystal were observed and analyzed through the scanning electronic microscope, such as inclusions, negative crystal, growing strips, growing mound, growing steps, sector boundary and growing twins. The morphology of dislocation etch pits for KTm0.1Yb0.9(W04)2 crystal was observed under etchant by the corrosion method. The reasons for defects of KRExYb1-x(WO4)2 crystal were generalized, and the measures to decrease defects were presented. The absorption and fluorescent spectra of four crystals were measured, and the spectral parameters were calculated, too. The process of energy transfer between Nd3+ and Yb3+ ions in the KNd0.05Yb0.95(WO4)2 crystal was interpreted reasonably, and the Stark levels of Yb3+ in this crystal were determined. The stimulated emission cross section calculated at 1067nm of KNd0.05Yb0.95(W04)2 crystal is 10.72x10-20cm2. and the fluorescent lifetime of 4F3/2 energy level is 196.33μs. The FWHM of main emission peak at 1533nm is 55nm in the KEr0.1Yb0.9(W04)2 crystal, and the stimulated emission cross section is 3.47x10-20cm. The stimulated emission cross section of main emission peak at 1762nm is 2.93×10-20cm2 in the KTmo.1Ybo.9(W04)2 crystal, and the FWHM is 98nm. The fluorescent lifetime is 3.51ms. According to the energy level diagram of Tm3+, Ho3+ and Yb3+ ions, three kinds of energy transfer modes for 1750-2200nm emission were discussed in the KH00.04Tm0.06Yb0.9(WO4)2 crystal. The stimulated emission cross section of main emission peak at 2030nm is 3.47×10-20cm2.The CW laser output characteristics of KNd0.05Ybo.95(W04)2 crystal by LD pumping was investigated at room temperature. Highly efficient laser output up to 305mW at 1067 nm has been achieved under pumping by a CW 808nm laser diode when pumping power was 900mW. The optical—ptical conversion efficiency is 33.9%, and the slope efficiency is 46.8%. The laser output threshold value is 230mW. The result of laser measurement for KEr0.1Yb0.9(W04)2 crystal shows that the output wavelength decreases but the threshold power increases with the enhancement to the transmittance of output coupling mirror under 980nm LD pumping. The maximal optical-optical conversion efficiency and slope efficiency obtained are 37.5% and 47.3% with the wavelength of 1538nm, when the transmittance of output coupling mirror is equal to 3%.