Study On Characteristics Of All-solid-state Q-switched And Mode-locked Short Pulse Lasers

Diode-pumped solid-state short pulse lasers have wide applications in the fields such as laser telecommunication, light display, information processing, industry matching, medical, and military, due to its advantages such as its simplicity, high efficiency, high stability, long longevity, high beam quality, wide wavelength range and various runing modes and has been paid much attention on.In this dissertation, by using the fiber-coupled laser-diode as the pump source, Nd:YVO₄, Nd:GdVO₄ and Cr⁴⁺:Nd³⁺:YAG crystals as the gain mediums, we have studied the performance of the actively Q-switched lasers with acoustic-optic (AO) modulator, and passively Q-switched lasers with Cr⁴⁺:YAG as well as GaAs saturable absorbers, respectively; the characteristics of the diode-pumped doubly Q-switched lasers with AO-Cr⁴⁺:YAG, AO-GaAs and Cr⁴⁺:YAG-GaAs; as well as the frequency doubling performance of the periodically poled KTP crystal. In addition, using Cr⁴⁺:YAG and GaAs as the intracavity mode-locker, respectively, the simultaneously Q-switched and mode-locked lasers have been realized. Meanwhile, the coupling rate equations under Gaussian distribution approximation have been estabilished to theoretically analyze the properties of the above-mentioned Q-switched lasers as well as the simultaneously Q-switched and mode-locked lasers. The main content of this dissertation includes:(I) Considering the turn-off time of AO Q-switch, the transversal and longitudinal distributions of the intracavity photon density, the population-inversion density and pump light, a new rate equation model describing the actively Q-switched laser with AO modulator has been established. (§2.1)(II) Diode-pumped actively Q-switched Nd:YVO₄ and Nd:GdVO₄ 1.06μm lasers with AO modulator have been realized, respectively. The dependence of the pulse width, pulse energy, and peak power on the pump power have been measured at different repetition rates of AO modulator. Using the new model proposed in§2.1, the theoretical analysis has been performed. (§2.2)(III) Using the novel model of actively Q-switched laser with AO modulator, and considering the nonlinear loss due to the intracavity second harmonic conversion under Gaussian distribution approximation as well as the thermally induced diffraction loss and the varations of the beam radius at different positions along the cavity axis, a couple of rate equations describing the actively Q-switched intracavity frequency doubling green lasers are given. Using a three-mirror-folded cavity, diode-pumped actively Q-switched Nd:YVO₄/KTP and Nd:GdVO₄/KTP 0.53μm green lasers have been realized, respectively. The pulse widths, pulse energy and peak power have been measured under different pump power and repetition rates, which are consistent with the theoretical analysis. (§2.3 and§2.4 )(IV) Considering the single photon and two-photon absorptions of GaAs saturable absorber, a couple of rate equations describing the passively Q-switched lasers with GaAs have been built up under Gaussian distribution approximation. Diode-pumped passively Q-switched Nd:YVO₄ and Nd:GdVO₄ lasers with GaAs have been realized, respectively, and the pulse widths, repetition rates, pulse energy, and peak powers under different pump power and output reflectivities have been measured. Solving the above mentioned rate equation model, the theoretical analysis are consistent with the experimental results. (§3.1)(V) A new rate-equation model for the passively Q-switched laser with Cr⁴⁺:YAG saturable absorber is given, in which the transversal and longitudinal distributions of the intracavity photon density, the transversal distributions of the population-inversion density of the gain medium, the ground-state population density of Cr⁴⁺:YAG saturable absorber and the pump beam are taken into account. The performance of a diode-pumped Cr⁴⁺:YAG passively Q-switched Nd:YVO₄ laser has been presented. For different small-signal transmissions of Cr⁴⁺:YAG, reflectivities of the output mirror and pump powers, the pulse energy, peak power, pulse width and repetition rate of 1.06μm laser ouput pulse have been measured, which are in agreement with the numerical results. (§3.2)(VI) The theoretical and experimental studies on the diode-pumped doubly Q-switched Nd:GdVO₄ laser with AO and Cr⁴⁺:YAG have been carried out. The experimental results show that the pulse width of the doubly Q-switched laser is obviously compressed in contrast to the AO Q-switched laser. Meanwhile, a rate-equation model for the actively and passively Q-switched laser is given, in which the transversal and longitudinal distributions of the intracavity photon density, the population-inversion density, the saturable absorber ground-state population density and the pump beam are taken into account. The numerical solutions are in agreement with the experimental results. (§4.1.1) (VII) The performance of an diode-pumped doubly Q-switched Nd:YVO₄ laser with AO and GaAs have been demonstrated and compared with that of a singly Q-switched Nd:YVO₄ laser with AO or GaAs. For different repetition rates, the properties of the three types of Q-switched lasers have been measured, repectively. The results show that the doubly Q-switched laser with AO and GaAs can not only compress the pulse width, but improve the pulse symmetry. In the theoretical analysis, by considering the single-photon and two-photon absorptions of GaAs saturable absorber, a rate-equation model describing the actively and passively Q-switched laser is given and the solutions to it are in consistent with the experimental results of the doubly Q-switched laser. (§4.1.2)(IX) Using GaAs as both the Q-switch and output coupler, a diode-pumped doubly passively Q-switched Nd:YVO₄ laser with Cr⁴⁺:YAG and GaAs has been demonstrated. Compared with the performance of singly passively Q-switched Nd:YVO₄ laser with Cr⁴⁺:YAG or GaAs saturable absorber, the doubly passively Q-switched laser can generate more symmetric pulse temporal profiles and shorter pulses. Using the self-Q-switched Cr⁴⁺:Nd³⁺:YAG crystal as the gain medium, GaAs as the intracavity Q-switching element, a diode-pumped Cr⁴⁺:Nd³⁺:YAG/GaAs doubly passively Q-switched laser is realized. By considering the free carrier absorption of GaAs under Gaussian distribution approximation further, a coupled rate equation model describing the doubly passively Q-switched laser is given, and the theoretical results and the experimental results are consistent. (§4.2 )(VIII) A frequency doubling theory for quasi-phase-matched crystal PPKTP are given under Gaussian distribution approximation. Also the nonlinear loss due to the second harmonic wave conversion is obtained, which is compared with the that of the conventional KTP. (§5.1)(IX) Using a three-mirror-folded cavity, a diode-pumped Nd:GdVO₄/PPKTP cw green laser has been realized. Compared with the conventional KTP crystal in the same configuration, the conversion efficiency of PPKTP is twice of that of KTP. Meanwhile, we have used the quasi-phase-matched frequency doubling theory to analyze the experimental results, which are consisient with each other. (§5.2)(X) A diode-pumped actively Q-switched Nd:GdVO₄/PPKTP 0.53μm green laser with AO modulator is realized in a three-mirror-folded cavity, and the pulse width, pulse energy, and peak power of the green laser under different AO modulation frequencies and pump powers have been measured. Using the quasi-phase-matching theory and the novel rate equations for the AO actively Q-switched lasers, the actively Q-switched green laser with PPKTP has been analyzed and the theoretical results are consistent with the experimental results.(XI) By employing Cr⁴⁺:YAG as the saturable absorber, a diode-pumped passively Q-switched Nd:YVO₄/PPKTP 0.53μm green laser is realized, and the pulse widths, repetition rates, pulse energy and pulse peak power of the output green pulse under different small signal transitions of Cr⁴⁺:YAG and pump powers are recorded, respetively. By using the quasi-phase-matching theory and the rate equations model for the Cr⁴⁺:YAG Q-switched lasers, the passively Q-switched green laser with PPKTP has been analyzed. (§5.4)(XII) The principle of the simultaneously Q-switched and mode-locked lasers has been stated, and using the intensity fluctuation mechanism and the above mentioned rate equations model for the Q-switched lasers, the rate equations describing the simultaneously Q-switched and mode-locked lasers under Gaussian distribution approximation have been given. A diode-pumped simultaneously Q-switched and mode-locked Cr⁴⁺:Nd³⁺:YAG/KTP green laser is realized in the experiment, and the dymanic process of such laser has been reconstructed by using the proposed mode-locking rate equations model. (§6.1)(XIII) By employing a 1-meter three-mirror-folded cavity configuration, a diode-pumped simultaneously Q-switched and mode-locked c-cut Nd:GdVO₄ 1.06um and 0.53μm laser with GaAs have been realized. Considering the two-photon absorption of GaAs, a couple of rate equations describing the passively Q-switched and mode-locked laser with GaAs are given. The numerical solution is consistent with the experimental results. (§6.2 )The main innovations of this dissertation are as follows:(I) We have firstly developed a new rate-equation model for the diode-pumped actively Q-swithed solid-state laser with AO modulator, in which the turnoff time of AO Q-switch, the transversal and longitudinal distributions of the intracavity photon density, the transversal distributions of the population-inversion density and the pump beam are taken into account. The theoretical analysis are more close to the experimental results.(II) By considering the influence of the thermally induced variation of the beam radius along the cavity axis on the characteristics of the Q-switched laser as well as the nonlinear frequency doubling conversion under Gaussian distribution approximation, a couple of rate equations describing the diode-pumped actively Q-switched green laser has been developed under Gaussian distribution approximation for the first time, which provides an efficient analysis tool for the optimization of the actively Q-switched green lasers.(III) We have firstly realized the diode-pumped doubly Q-switched NdtGdVCU laser with AO-Cr⁴⁺:YAG, Nd:YVO₄ laser with AO-GaAs, Nd:YVO₄ laser with Cr⁴⁺:YAG-GaAs and Cr⁴⁺:Nd³⁺:YAG laser with GaAs, respectively. The characteristics of the pulse width reduction and pulse shape symmetry improvement are also presented. By considering the transversal and longitudinal distributions of the intracavity photon density, the population-inversion density and the pump beam, we have firstly proposed a rate-equation model to analyze the performance of the doubly Q-switched lasers. The numerical solutions to the rate equations are consistent with the experimental results.(IV) We have firstly presented systematically theoretical and experimental studies on the laser properties of diode-pumped Nd:GdVO₄/PPKTP intracavity frequency doubling green laser operating in cw and Q-switching with AO modulator and Cr⁴⁺:YAG By considering the second harmonic conversion as a nonlinear loss of the fundamental wave, we have firstly developed a rate-equation model for the diode-pumped quasi-phase-matched frequency-doubling green laser by considering the quasi-phase-matching theory, and the numerical solution are consistent with the experimental ones.(V) We have firstly realized the simultaneously Q-switching and mode-locking operation of diode-pumped Cr⁴⁺:Nd³⁺:YAG/KTP green laser. Meanwhile, we have given a couple of rate equations describing the dynamic process of the simultaneously Q-switched and mode-locked green laser with Cr⁴⁺:YAG, and the theoretical simulations have reconstructed the output characteristics of the mode-locked laser.(VI) We have firstly developed the rate equations describing the simultaneously Q-switched and mode-locked laser with GaAs, and realized the run of diode-pumped simultaneously passively Q-switched and mode-locked c-cut Nd:GdVO₄ 1.06μm and 0.53μm with GaAs. The numerical analysis are consistent with the experimental results.