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Laser-induced ignition of air-fuel mixtures in internal combustion engines is a subject that has been investigated extensively during last years. In the beginning, experiments were performed with sized and robust, commercial available lasers that delivered pulses with energy in the range of tens to a few hundreds of mJ and several ns pulse duration (Ma et al., 1998; Phuoc & White, 1999; Weinrotter et al. 2005a; Weinrotter at al. 2005b). These investigations revealed that laser-induced ignition offers significant advantages over a conventional spark-ignition system, such as higher probability to ignite leaner mixtures, reduction of erosion effects, increase of engine efficiency, or shorter combustion time. Thus, developing of an engine ignited by laser could address, even partially, the increase concern of humanity for protecting global environment and preserving fossil resources. Subsequent research (Kofler et al., 2007) concluded that a suitable laser configuration for engine ignition is a Nd:YAG laser, passively Q-switched by Cr4+:YAG saturable absorber (SA). Q-switched laser pulses with energy up to 6 mJ and 1.5-ns duration were obtained from an end-pumped, 210-mm long Nd:YAG-Cr4+:YAG laser. Furthermore, side-pumping technique was employed to realize a Nd:YAG laser passively Q-switched by Cr4+:YAG SA with 25 mJ energy per pulse and pulse duration around 3 ns (Kroupa et al. 2009); the laser resonator was around 170 mm. However, the length of these lasers make difficult to accomplish compactness of an electrical spark plug used in the automotive industry. In recent works our group has realized Nd:YAG-Cr4+:YAG micro-lasers and demonstrated laser ignition of an automobile engine with improved performances in comparison with ignition induced by a conventional spark plug (Tsunekane at al. 2008; Tsunekane et al. 2010). The strategy was to shorten the pulse duration by decreasing the resonator length, and to maximize the laser pulse energy by optimizing the pump conditions, the Nd:YAG doping level and length, as well as Cr4+:YAG initial transmission (T0) and the output mirror transmission (T) (Sakai et al., 2008). A passively Q-switched Nd:YAG-Cr4+:YAG micro-laser with 2.7-mJ energy per pulse and 600-ps pulse duration was realized. This laser, which

All-Poly-Crystalline Ceramics Nd:YAG/Cr4+:YAG Monolithic Micro-Lasers with Multiple-Beam Output