A compact high peak power, passively Q-switched, longitudinally diode-pumped laser was specially constructed for laser ignition. Nd:YAG was chosen as laser active medium and Cr 4+ :YAG as the saturable absorber medium. For pumping, a laser diode emitting at 808 nm with an output power of up to 300 W and pulse duration of up to 500 µs was employed. Experimental studies were carried out to find the optimum laser design parameters. These are the output coupler reflectivity, initial transmission of the saturable absorber, doping concentration of the Nd:YAG, oscillator length and the pump light distribution within the Nd:YAG crystal. Single pulses at 1064 nm with energies of 6 mJ and durations under 1.5 ns were achieved in a TEM00 mode.

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A kinetic model of electron cascade growth in the electromagnetic field of a focused intense laser pulse as used for laser spark generation in gases has been numerically implemented in Visual C code. The effects considered comprise Drude absorption, diffusive kinetic and inelastic losses as well as (three-particle) electron recombination. The objectives were to illustrate the dynamic process of gas ionization, and to clarify the pressure dependence of known breakdown thresholds within a range of about 2 × 104 to 2 × 106 Pa of initial pressure. Two-dimensional (cylindric coordinates) simulations of the optical breakdown in nitrogen were conducted on a commercial PC, using constant values for the collision cross section (2 × 10−19 m2), prevalent electronic excitation states (~4.8 eV), and a laser wavelength of 1064 nm. A certain aerosol concentration on the order of 3 ppb was assumed in order to provide initial electrons for cascade growth. Exemplary results with laser pulse energy of 26 mJ, pulse duration of 14 ns and an 18 µm focal spot size illustrate the dynamic process of ionization within a very short time period of less than 0.5 ns. The kinetic energy of the electrons is found to increase sharply up to more than 100,000 K on breakdown. A series of simulations considered the minimum pulse energy of breakdown (MPE) under variation of initial pressure. Identical laser parameters as in experiments conducted previously were used and the results are in excellent agreement with respect to curve shapes, i.e., MPE ~1/p0.4 in the first experiment and MPE ~1/p0.3 in the second one. The absolute values lie within a factor of two, which is explained by model abstraction and input data uncertainties.

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Corresponding author: johannes.tauerAtuwien.ac.aThe reduction of pollutant emissions and energy consumption represents a central objective in the improvement of combustion engines. In order to achieve this goal a clear optimisation of combustion processes as well as ignition mechanisms is required [1,2]. Higher compression ratios and leaner mixtures enable more efficient engine operation and lower emissions. Unfortunately, the conventional electrical spark plug reaches its physical borders and cannot fulfil such requirements. Laser ignition is a new alternative concept where the electrical spark plug is replaced by a diodeendpumped passively Qswitched laser. The short pulses generated by this type of the laser are focused into the combustion chamber, a plasma is created and ignites the gas mixture. The main advantages of nonresonant laser ignition are the arbitrary positioning of the ignition plasma, the absence of quenching effects by electrodes as well as the feasibility of very lean mixture ignition and the reduction of nitrous oxide emissions. The development of a suitable ignition laser is essential for the realization of this innovative ignition system since there is a lack of adequate compact lasers on the market [2]. Our work focuses on the development of a compact, robust and reliable solidstate laser. Nd:YAG is a field-tested and reliable material with nearly perfect laser properties. Theoretical and experimental investigations comprised establishing a characteristic diagram for the pulse energy in dependence of the reflectivity of the output mirror and the initial transmission of the absorber, as well as the analysis of inject optics and optimum pump duration. Laser crystals with a length of 5 mm and a diameter of 2 mm were employed. Fig. 1 depicts some results originating from simulation as well as experiment. 0,9 7 it ip a PuseEhey '70 6 W ted tr,d) E ergy 017 so~~~~~~~~~~~~~~~~~~~~~~~~ Pulse Energy mil I7-) i;6 0,~~~~~~~~~~~~~~50 C~~~~0~~~~~~3 L~~~, LUl 1 4f o 0 101 150 200 20 0 350 400 40 S00 Pump Durttion [p 0 15 30 45 60 75 90 105 120 135 150 Reflectivity Output Coupler ] InpIput Energy lrnJ] Fig. 1: Simulation of the pulse energy as a function of the reflectivity of the output coupler and the initial transmission of the saturable absorber. Employing a 50% output coupler and a 40% absorber leads to the best laser performance (left). An increase of the pump duration over 180 ps does not yield an increase of the pulse energy. Furthermore it can be seen that the laser system becomes unstable at pump durations >> 230js (right).It turned out, that highest pulse energies can be reached by a reflectivity around 4500 and an initial transmission of 4000.Moreover, the pump duration should be in the order of the fluorescence lifetime of the laser active medium, otherwise the laser system becomes more unstable the longer the pump pulse is. Applying this configuration, pulse energies up to 6 mJ and pulse durations under 1.5 ns were reached. Hereby, the optical efficiency amounts up to the remarkable value of ...