Characteristics Of Continuous Wave And Passively Q-switched Pulsed Thulium And Holmium Codoped Vanadate Laser

Passively Q-switched (PQS) pulsed Tm/Ho solid-state lasers with saturable absorbers (SAs) emitting at2μm lying in the eye-safe range and atmospheric transmission window, have widely potential applications in the fields of laser ranging, material processing, laser ladar, environmental monitoring, mental content measuring and medical treatment because they are usually accompanied with significant advantages such as inherent compactness, simplicity, short pulse duration and high peak power. In this thesis, for the purpose to obtain high efficiency, high energy and narrow pulse duration of2μm pulsed laser output, the laser characteristics of Tm,Ho-codoped vanadate cryatals including Tm,Ho:YVO4and Tm,Ho:GdVO4are theoretically and experimentally investigated. Then with Cr2+:ZnS saturable absorber, the passively Q-switching performances of Tm,Ho:YVO4and Tm,Ho:GdVO4lasers are comparatively studied, and ultimately we obtained high efficiency, high energy and narrow pulse duration of passively Q-switched laser output.The properties of several2μm Tm3+/Ho3+doped laser crystals have been analyzed comparatively. The development of continuous wave Tm,Ho-codoped vanadate lasers and passively Q-switched Tm3+/Ho3+pulsed lasers have been introduced and systematically analyzed. According to the comparison of properties among a few saturable absorbers and among laser crystals, the advantage and research potential of passively Q-switched Tm,Ho-codoped vanadate lasers with Cr2+:ZnS crystal as saturable absorber is defined. The continuous wave and passively Q-switched pulsed Tm,Ho-codoped vanadate lasers are theoretically investigated in detail. Based on the fluorescence spectra of Tm,Ho vanadate crystals, the stimulated emission cross-section at round2μm is theoretically calculated. According to laser generation and energy level transition mechanisms of Tm,Ho-codoped crystal, the quasi-four-level rate equation model is established. The parameters dependence of Tm,Ho:YVO4crystal on the temperature is analyzed. Based on these parameters, the rate equation model is numerically solved, the influence of Tm,Ho:YVO4laser parameters, such as mode matching between pump-and laser-mode, output mirror transmission, upconversion loss, doping concentration ratio and crystal temperature on output characteristics are discussed. Meanwhile, the theory model of passively Q-switched lasers is also established. Through solving the rate equation, the dynamic process of passively Q-switching is theoretically simulated and the influence on output characteristics of the laser-mode in the resonator, pump-mode, output coupler transmission, and so on, are analyzed. Besides, based on heat conduction equation, temperature and stress distribution at low temperature in the laser crystals are simulated. Using ABCD matrix theory, the stability and laser-mode of the laser resonator are analyzed with resonator length and mirror curvature, to supply the theoretical foundation to design continuous wave and passively Q-switched laser resonator.Continuous wave and passively Q-switched Tm,Ho:YVO4and Tm,Ho:GdVO4lasers have been investigated in detail in the experiments. Through changing the position of output coupler and the curvature of end mirrors, influence on output characteristics of Tm,Ho:YVO4lasers of mode matching, resonator length and transmission of output coupler has been analyzed. Based on the spectrum data, the gain coefficients of Tm,Ho:YVO4crystals were theoretically calculated under different population inversion ratio, and the relation of the laser spectra and output coupler transmissions were pointed out. Inserting Cr2+:ZnS SA into resonator, passively Q-switched Tm,Ho:YVO4lasers were demonstrated. The dependence of output characteristics, including to average output power, pulse duration, energy and peak power, on mode-radius on Cr2+:ZnS and gain medium, output coupler transmission and resonator length was confirmed and analyzed in detail. Besides, used Cr2+:ZnS as saturable absorber, the passively Q-switched Tm,Ho:GdVO4lasers were also demonstrated. Comparative analysis with the Tm,Ho:YVO4laser shown that the narrower pulse duration and higher peak power could be achieved for the passively Q-switched Tm,Ho:GdVO4laser. Furthermore, by optimizing the pump radius, the pulse repetition rate could be obviously decreased. The pulse duration of37.7ns and energy of0.53mJ, corresponding to the peak power of14kW, were obtained with the slope efficiencies of27.3%and Q-switch efficiencies of63%.