Laser Plasma-initiated Ignition of Engines

Laser ignition is considered to be one of the most promising future ignition concepts for internal combustion engines. It combines the legally required reduction of pollutant emissions and higher engine efficiencies. The igniting plasma is generated by a focussed pulsed laser beam. Having pulse durations of a few nanoseconds, the pulse energy Ep for reliable ignition amounts to the order of 10 mJ. The lack of adequate compact high-power laser systems on the market with the mentioned specifications led to the development of such a solid-state laser. A fiber-coupled, longitudinally diode-pumped, passively Q-switched solid-state laser was realized. Nd:YAG was chosen as laser active medium emitting at λem = 1064 nm, and Cr:YAG as passive saturable absorber. A 300 W quasi-cw laser diode as pump source provided pump pulses with energy up to Epump=150 mJ. Experimental studies were carried out to find the optimum component specifications like reflectivity of the output coupler R, initial transmission T0 of the absorber or optimum in-coupling optics. Single pulse energies of more than 10 mJ at pulse durations of 1.5 ns were yielded. Further on, the in-coupling optics was analyzed in detail in order to draw conclusions with respect to optimum output power. The maximum pulse energy Ep was achieved at R = 60% and T0 ≈ 60%.