Abstract

Recently, ytterbium (Yb)-doped materials are believed to be the most promising materials because of its capability of laser diode (LD) pumping, high quantum efficiency, and wide gain spectrum. Yb-doped yttrium aluminum garnet (Yb:YAG) crystal has additionally good characteristics because its host material of YAG was high thermal conductivity and low thermal birefringence and so on. These features are most appropriate for short pulse lasers, which pulse width is hundred nanoseconds or less, with excellent characteristics for affordability, i.e., high power, high efficiency, high pulse repetition rate, high stability, high reliability, compactness, and cost-effectiveness. The undesirable future of the Yb-doped materials is a considerable loss induced by the thermal population of the lower laser level. A technique to overcome the loss without lack of the excellent characteristics is increasing the gain of the laser medium, which is obtained by high pump intensity above 100 kW/cm², and precise compensation of the thermo-optic effects. The high-gain technique, which includes the gain increase and precise compensation, is simple and affordable compared to the others. By the high gain technique, the highest efficiencies for the lasers were obtained from a continuous wave (CW) microchip Yb:YAG lasers at room temperature, in our knowledge [1]. The optical-to-optical conversion efficiencies are close to the quantum limit of 91%. Although the efficiencies of CW lasers were close to the quantum limit, those of the short pulse lasers are considerably lower than the quantum limit. High efficiency short pulse lasers with a high repetition rate have been reported [2-6]. For example, to our knowledge, the highest efficiencies for the short pulse lasers were realized by a cavity dumped microchip Yb:YAG laser at room temperature in our previous research [4]. The slope efficiency and optical-to-optical conversion efficiency were 84% and 73%, respectively for the absorbed pump power. The pulse width was 16 ns and pulse repetition rate was 100 kHz. Although its pump source was Ti:sapphire laser, the results indicate a high capability of similar high-efficiency oscillation by LD pumping. For the LD pumped short pulse Yb lasers with pulse widths of 10 ns or less, the highest optical-to-optical conversion efficiency of 37.9% were obtained by a LD pumped cavity dumped microchip Yb:YAG laser [5]. For the short pulse lasers with a relatively long pulse width of several hundred nanoseconds, the highest optical-to-optical conversion efficiency of 45% was realized by a cavity dumped thin-disk Yb:YAG laser with a complex multi pass pumping and cooling scheme [2].

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