Q-switched Mode-locking In Diode-Pumped Nd～(3+): GdVO₄ Lasers

Pulsed lasers have been used widely in various fields, such as range finder, optical communication, material process, and nonlinear frequency conversion. Diode-pumped solid-state lasers have various advantages such as high conversion efficiency, long working lifetime, simplicity, and stability. Consequently, developing techniques for Q-switching or mode-locking in diode-pumped lasers has been burgeoning as new exploration to get compact pulsed lasers. In this dissertation, we discuss several techniques to generate Q-switched mode-locked (or stable cw mode-locked) laser pulses in diode-pumped Nd:GdVO4 solid-state lasers.In chapter 1, we firstly review the history of diode-pumped solid-state lasers and give a brief introduction to the theory of Q-switched mode-locking. Secondly, we present the properties of Nd:GdVO4 crystal and compare it with several commonly used laser crystals. As a new laser material, Nd:GdVO4 crystal has the advantages of high thermal conductivity, large absorption cross-section and wide emission bandwidth and so on, which make the Nd:GdVO4 crystal more competent than some other laser crystals. And in the following three chapters, we demonstrate three techniques to obtain Q-switched mode-locked (or cw mode-locked) lasers pulses in diode-pumped Nd:GdVO4 solid-state lasers.Q-switched mode-locking with Cr4+: YAG crystals: Cr4+:YAG possesses the properties of ground-state saturable absorption (GSA) and exited-state saturable absorption (ESA). And the relaxation times of GSA and ESA are in the region of microsecond and picosecond, respectively. Those characteristics can be applied to construct Q-switched and mode-locked lasers. In our experiments, we obtained 3.22 W Q-switched and mode-locked laser output in the 1.06 μm. The modulation depth and the probability of obtaining cw mode-locking are also discussed in this part.Q-switched mode-locking with semiconductor saturable absorber mirrors (SESAMs): SESAMs have two kinds of response times, which usually lead to the Q-switched pulse envelope in the lasers mode-locked by SESAM. Therefore, SESAM can be used to achieve Q-switched mode-locked laser pulses. In the experiments, we obtained 3.5 W Q-switched mode-locked and 4.9 W cw mode-locked laser outputs. The pulse width of the cw mode-locked pulse was measured by a homemade autocorrelator to be 11.5 ps. The condition of transforming from the Q-switched mode-locked to cw mode-locked operation was also discussed in this part.Self-mode-locking in Nd3+:GdVO4 lasers: we analyzed simply the method to achieve self-mode-locking, and experimentally obtained self-mode-locked pulse train in the Nd3+:GdVO4 laser. Due to the long upper-level life-time of Nd3+ ions, the self-mode-locked pulses tend to concentrated in the self-Q-switched envelope. Therefore, in our experiment, self-Q-switched self-mode-locked laser pulses were achieved. In this part, we discussed methods to suppress the self-Q-switched envelopes and experimentally obtained stable cw self-mode-locked laser pulses.