Laser-induced optical breakdown applied for laser spark ignition

In the present article, the experimental investigation of optical breakdown induced by ns/mJ pulses at two wavelengths, 1064nm and 532nm, in air of atmospheric pressure is reported and discussed. The obtained breakdown thresholds were compared with theory and are in good agreement. The generated plasmas have been characterized by their amount of scattered laser light, energy transmission, and change of the transmitted temporal shape. Laser-induced plasma formation in a gas, in air, also generates an acoustic pressure wave. The acoustic energy is compared to the laser pulse energy and is found to be linearly dependent. Moreover, the frequency distribution of the characteristic acoustic pressure wave was analyzed. The experiments described were accomplished in order to optimize a laser ignition system with regard to efficiency and costs. The laser system employed for these investigations is a compact high peak power, passively Q-switched, longitudinally diode-pumped solid-state laser. Such a laser spark plug should replace conventional spark plugs in internal combustion engines because conventional ignition has reached its limits in terms of efficiency and durability. Thereby, a reduction of pollutant emission should also be feasible.