Investigation On LD-Pumped Passively Q-switched Vanadate Microchip Lasers

Laser-diode pumped microchip lasers combine many advantages such as compactness, reliability, long lifetime, all-solid-state, high efficiency, high beam quality, single-longitudinal-mode output and low cost. They have been a hotspot in the field of solid state lasers. A miniature short pulse laser can be accomplished by combining microchip laser with passively Q-switch. The cavity of the diode-pumped passively Q-switched microchip lasers is so narrow that the photon lifetime is short and short pulses with pulse width of several nanoseconds or sub-nanoseconds, repetition rate as high as thousand Hertz, peak power on the order of kilowatt and pulse energy on the order of micro-Joule can be easily achieved. They are of great interest in the field of ranging, three-dimensional imaging, micromachining, environmental monitoring, as well as microsurgery, and so on.Neodymium doped vanadate crystals, such as Nd:YVO₄, Nd:GdVO₄ and Nd:LuVO₄, have been identified to be ideal gain mediums for the microchip lasers due to their broad absorption bandwidth, large absorption and emission cross-sections, polarized emission. The mixed crystal Nd:Lu_xGd_1-xVO₄ is a new type vanadate crystal. Compare with Nd:GdVO₄ and Nd:LuVO₄, the spectra, including absorption and fluorescence lines, of neodymium ions in the Nd:Lu_xGd_1-xVO₄ are inhomogeneous broadened and the emission cross-sections are reduced. In the laser field, smaller emission cross-sections and longer fluorescence lifetime are favorable for the Q-switched lasers.In this thesis, by using the fiber-coupled laser-diode as the pump source, LD-pumped Nd:LuVO₄,Nd:GdVO₄ microchip lasers have been studied. Furthermore, the Nd:LuVO₄ microchip laser also has been optimized. The emphasis is mainly on LD-pumped passively Q-switched Nd:LuVO₄,Nd:GdVO₄ and Nd:Lu_0.14Gd_0.86VO₄microchip lasers. The main contents and conclusions are as follow:The continuous-wave laser performance of a-cut Nd:LuVO₄ microchips with different neodymium ions doping level, crystal length, and transmittance of output surface have been demonstrated. Under high pump power, the highest optical-optical conversion efficiency of 44.4% and the highest slope efficiency of 44.65% were obtained, and the maximum output power of 9.7 W was measured. An a-cut Nd:GdVO₄ microchip laser also has been demonstrated. 10.1 W maximum output was achieved with the optical-optical conversion efficiency of 42.3% and the slope efficiency of 42.6%, at the pump power of 23.9 W. It can be proved that, Nd:LuVO₄ and Nd:GdVO₄ are admirably suited for highly efficient microchip lasers.With an a-cut Nd:LuVO₄ microchip of 1 at.%, 1.5mm, a Cr⁴⁺:YAG with initial transmission of 7V=61.0 % used as the saturable absorber, and a piano mirror with transmission of T=20% used as the output coupler, a compact passively Q-switched Nd:LuVO₄ miniature laser was demonstrated. The shortest pulse width was measured to be 3.3 ns with maximum pulse energy of 92.8μJ and largest pulse repetition rate of 24.8 kHz. To our knowledge, this is the shortest passively Q-switched pulse width with Nd:LuVO₄ as the laser medium.We found that the passively Q-switched microchip lasers can be more easily achieved and have better output performance, by using Cr⁴⁺:YAG as both the Q-switch and output coupler with a cavity length as short as 5-6 mm. Using Cr⁴⁺:YAG with initial transmission T₀=77% and output transmission 7=15%, a Nd:LuVO₄ microchip pulse laser has been demonstrated with an a-cut 0.5 at.%, 1 mm Nd:LuVO₄ crystal. The largest pulse repetition rate, largest pulse energy, shortest pulse width, and the highest peak power were measured to be 23.95 kHz, 21.21μJ, 4.5 ns, and 4.71 kW, respectively.With the same resonator and the same Cr⁴⁺:YAG, passively Q-switched laser performances of an a-cut Nd:GdVO₄ microchip with 0.5 at.% in doped concentrations, 1 mm in transmittance length was investigated. The largest pulse energy, shortest pulse width, and the highest peak power were measured to be 44.59μJ, 2.9 ns, and 15.38 kW, respectively. As far as we know, this is the shortest passively Q-switched pulse width with a-cut Nd:GdVO₄ as the laser crystal and Cr⁴⁺:YAG as the saturableabsorber.With the same resonator and the same Cr⁴⁺:YAG, passively Q-switched laser performances of an a-cut Nd:Lu_0.14Gd_0.86VO₄ microchip with 0.5 at.% in doped concentrations, 2 mm in transmittance length was investigated. The largest pulse energy, shortest pulse width, and the highest peak power were measured to be 119.54μJ, 2.2 ns, and 54.34 kW, respectively. As we know, it is the shortest passively Q-switched pulse width with Nd:Lu_xGd_1-xVO₄ as the laser mediumBy comparing the passively Q-switched performance of the three neodymium doped vanadate microchips, the Nd:Lu_0.14Gd_0.86VO₄ meets the needs of the larger pulse energy, shorter pulse width, and higher peak power for its smaller emission cross-section and better energy storability. The Nd:LuVO₄ is suitable for the high repetition rate pulse laser for its larger emission cross-section.