Growth, Spectroscopic Properties And Laser Properties Of Yb-doped Calcium Tungstate Crystals

The tetragonal calcium tungstate（CaWO₄）crystal possesses good chemical,thermal,and optical properties;it allows trivalent rare-earth ions to occupy the lattice sites of Ca ions,and hence can serve as host materials for rare-earth active ions like Nd and Yb.Additionally,the CaWO₄ crystal,which is also of a large third-order susceptibility(χ^（3）),can generate high-efficiency stimulated Raman scattering,and is a Raman crystal of practical values.Therefore,Yb-doped calcium tungstate（Yb:CaWO₄）crystal is a potential new self-Raman laser material.In this thesis,a comprehensive experimental investigation was carried out into the growth,spectroscopic properties,continuous-wave laser characteristics,and passive Q-switching self-Raman laser properties.The primary work completed and the research achievements obtained are as follows.With the polycrystalline raw materials being made by a two-step sintering method,high-optic quality Yb:CaWO₄ crystal of large sizes was grown successfully by using Czochralski method.A simple characterization of its crystal structure was made in accordance with the X-ray powder diffraction spectrum measured for it.Through the measurement and analysis of Raman spectra of the Yb:CaWO₄ crystal under different geometric arrangements,all of frequencies（wave numbers）for the Raman active lattice vibration modes were determined,of which the A_g mode with highest frequency has the strongest Raman scattering.Both absorption and emission properties of the Yb:CaWO₄ crystal exhibit strong anisotropy.The strongest absorption peak forπpolarization is located at 965.5 nm;while the largest absorption occurs at 933.8 nm forαpolarization.Theπ-polarized emission spectrum is dominated by a strong emission peak at 997.7 nm;forαpolarization,however,an exceedingly wide single emission band of 63.0 nm in width,where no pronounced peaks exist,constitutes the emission spectrum.This indicates that the crystal is suitable for making tunable or mode-locked ultra-short pulsed lasers.Under the excitation of 935-nm pump radiation,2.0 W of unpolarized output power was generated at a central wavelength of 1026.5 nm in laser operation of a c-cut crystal,the corresponding optical-to-optical efficiency was 26.3%,while the slope efficiency was 37%.Differing from the case of c-cut crystal,the laser radiation generated with a a-cut crystal can be eitherπ-orα-polarized;while under certain resonator conditions,the laser radiation generated could contain bothαandπpolarization components.The maximum output power produced in such dual-polarization laser operations was 2.01 W,of which theα-andπ-polarized amounted,respectively,to 1.05 and 0.96 W.Under the excitation of 974-nm quasi-CW pump radiation,and with a Cr⁴⁺:YAG saturable absorbed to induce passive Q-switching of the resonator,intracavity stimulated Raman scattering was realized with a a-cut crystal,generating self-Raman laser output.The wavelength of the Raman laser radiation generated by fundamental laser at 1005.2 nm was measured as 1106.6 nm;whereas the duration of the Raman laser pulse,which was generated in stimulated Raman scattering of a fundamental laser pulse of 9.0 ns in width,was only 2.1 ns.