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Research On LD Pumped Doul-Loss Q-Switched Mode-locked Lasers And Dynamic Thermal Modeling

Posted on:2011-06-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:T LiFull Text:PDF
GTID:1118360305951313Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
LD pumped Q-switched and mode-locked lasers with high peak power, high stability and high beam quality witch become more and more interesting due to their merits of small size, light weight, firm structure and long life duration have been widely used in the scope of industry, coherent communication, military and medicine etc. Pulse duration, pulse symmetry, stability and peak power are key parameters for Q-switched and mode-locked lasers. To this question, some investigation and research have been executed. First, we experimentally and theoretically investigate the output performance of dual-loss Q-switched and mode-locked lasers by simultaneously adopting the EO and saturable absorbers (such as Cr4+:YAG and GaAs) in laser cavity as Q-switch. Second, the continuous-wave mode-locked characters of different laser crystals are also investigated with SESAM. Third, the transient temperature profiles in Q-switched laser and the back-reaction of the thermal dynamics to the laser one are discussed. The main content of this dissertation is list as follows:Ⅰ. The output performances of LD pumped EO/Cr4+:YAG dual-loss Q-switched Nd:YVO4 lasers at 1064 nm and 532 nm are demonstrated, and he average output power, pulse duration, peak power are measured experimentally. The dual-loss Q-switched laser can generate shorter pulse duration and high peak power when compared with that obtained by EO single Q-switched laser. A coupled equation is given to theoretically analyze the experimental results, in which the influences of population-inversion density (PID) on laser character are taken into account and leads to a good agreement with the experimental data.Ⅱ. By inserting GaAs saturable absorber in EO Q-switched laser cavity, the output performances of EO/GaAs dual-loss YVO4/Nd:YVO4 laser operating at 1064 nm and 532 nm with KTP as SHG crystal are studied. A stable pulse train with high peak and high pulse symmetry is obtained by EO/GaAs dual-loss laser. Considering the transversal and longitudinal distributions of the intracavity photon density and the population-inversion density, a rate equation model describing the actively and passively Q-switched laser is given and the solutions are agree well with the experimental results.Ⅲ.A diode-pumped dual-loss-modulated Q-switched and mode-locked (QML) YVO4/NdYVO4 laser is demonstrated successfully by simultaneously employing the EO and Cr4+:YAG saturaber absorber in a four mirrors fold cavity. In comparison with the singly passively QML laser with Cr4+:YAG, the dual-loss-modulated QML green laser with EO and Cr4+:YAG can generate a more stable pulse train with shorter pulse width, higher peak power and deeper modulation depth. By using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled equations for diode-pumped dual-loss-modulated QML laser is given and the mode-locked pulse width is estimated accordingly.Ⅳ. By adopting a KTP as frequency doubling crystal, an EO/Cr4+:YAG doubly Q-switched and mode-locked laser at 532 nm is realized with four mirrors cavity for the first time. The average output power and pulse peak power is measured under different pump power and repetition rate. Compared with singly passively QML green laser, the dual-loss-modulated QML green laser can generate more stable pulse train with deeper modulation depth, shorter pulse width, greater pulse energy and higher peak power.Ⅴ. The mode-locking characteristics of diode-pumped lasers with Nd:YVO4, Nd:LuVO4 and Nd:Lu0.15Y0.85VO4 as laser mediums are demonstrated with a semiconductor saturable absorber mirror. The output power, mode-locked pulse duration and peak power are experimentally measured. In comparison with Nd:YVO4 and Nd:LuVO4 single crystals, Nd:Lu0.15Y0.85VO4 can generate even shorter pulses and even higher peak powers because of its broader fluorescence linewidth.Ⅵ. A diode-pumped passively mode-locked YVO4/Nd:YVO4 composite crystal green laser with a semiconductor saturable absorber mirror (SESAM) and a intracavity frequency doubling KTP crystal is realized. Stable mode-locked pulse train with picoseconds duration is obtained. The experimental results given a significant direction for further investigation of this kind of laser.Ⅶ. The dynamics of thermal buildup and temperature distribution in laser mediuml which is end pumped by pulsed operation laser diode is analyzed by the finite difference method. The influences of thermal time constant, pump peak power and the repetition rate on the thermal dynamics process are obtained. The simulation results give a primary theoretical support on designing and optimizing of pump source and laser parameters.Ⅷ. The transient temperature profiles in continuous-wave end-pumped Q-switched lasers is investigated by solving the coupling of the transient heat conduction equation and the rate equations. The interaction of temperature profile in laser medium and the laser output performance is discussed in detail. The numerical results reveal that the thermal buildup time of the quasi-steady-state temperature is mainly determined by the thermal time constant which depends on the dimension of crystal. When the thermal time constant is fixed at a certain value, the temperatures rise is primary influenced by the absorption cross section and the thermal oscillation contrast is principally determined by the fluorescence lifetime. In addition, we also find that the repetition rate and the pump power also have significant effects on the temperature characters in Q-switched lasers.The main innovations of this dissertation are as followsⅠ. By employing a YVO4/Nd:YVO4 composite crystal as laser medium, simultaneously using an EO modulator and a Cr4+:YAG saturable absorber as Q-switchers a 1064 nm dual-loss Q-switched YVO4/Nd:YVO4 laser and a 532 nm intracavity frequency doubled laser with KTP crystal are presented. Shorter pulse duration and higher peak power are generated by the dual-loss laser when compared with that of EO singly Q-switched one. In addition, we introduce a rate equation model under Gaussian distribution approximation to simulate the performance of those lasers and leads to a good agreement with the experimental results.Ⅱ. Using both EO modulator and GaAs saturable absorber as Q-switch, a LD end-pumped dual-loss Q-switched laser as well as its frequency doubling performance are demonstrated. A 2.5 ns pulse with peak power of 300 kW is obtained, compared with that from EO singly Q-switched laser, the pulse compression is 68.7% and peak power improvement is 86.8%. A rate equation model under Guassian distribution describing the dual-loss Q-switched laser with EO/GaAs is given out and the solutions are agree well with the experimental results.Ⅲ. By employing EO modulator and Cr4+:YAG saturable absorber simultaneously, a diode-pumped dual-loss-modulated QML YVO4/NdYVO4 laser is demonstrated for the first time. By using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled equations for diode-pumped dual-loss-modulated QML laser is given and the numerical solutions of the equations are in good agreement with the experimental results.Ⅳ. A diode-pumped dual-loss-modulated Q-switched and mode-locked (QML) YVO4/NdYVO4/KTP green laser is presented. In comparison with the singly passively QML green laser with Cr4+:YAG, the dual-loss-modulated QML green laser with EO and Cr4+:YAG can generate a more stable pulse train with deeper modulation depth, shorter pulse width and higher peak power. For the dual-loss-modulated QML green laser, the pulse width compression is 62% and the QML peak power increasing is 40 times when compared with that of the singly passively QML green laser.Ⅴ. The mode-locking performance of a diode-pumped Nd:Lu0.15Y0.85VO4 laser is first demonstrated with a semiconductor saturable absorber mirror. In comparison with Nd:YVO4 and Nd:LuVO4 single crystals, Nd:Lu0.15Y0.85VO4 can generate even shorter pulses and even higher peak powers because of its broader fluorescence linewidth.Ⅵ. By solving a numerical model coupling the rate equation and transient heat equation, the temperature distribution in actively Q-switched laser operating at low repetition rate is investigated for the first time. The knowledge of the transient temperature distribution as a function of pump power, repetition rate and thermal time constant in different laser crystals make it possible to predict thermal distortion for a large variety of operation parameters.
Keywords/Search Tags:Mode-locked
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