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.

Abstract:Quasi three-level laser operation of Nd:YAG at 946 nm is reported. Applying an end-pumped configuration pulse energies of more than 2 mJ @ 14 ns at a repetition rate of 10 Hz have been achieved. This output performance corresponds, according to the authors' knowledge, to the highest reported values for such compact passively Q-switched solid-state lasers emitting around 0.9 μm.Pulse length , ns τ p

After an introduction illustrating the relevance and motivation of laser ignition of engines, the physical background is reviewed in a first technical section. This mainly comprises the mechanisms of plasma formation, which are the generation of first free electrons and their avalanche-like multiplication, being illustrated by emission and Schlieren diagnostics. Thereafter, combustion fundamentals are discussed leading to an understanding of flammability and minimum ignition laser energies. An important section is represented by the description of the components of laser ignition. This covers the setup and function of a diode-pumped passively Q-switched solid-state laser system, which preferentially is based on Nd:YAG. Other laser sources are mentioned for comparison. Furthermore, multiplexing schemes are discussed that are motivated by cost-saving issues. It is explained why present-day optical fibers are not capable of transporting nanosecond ignition pulses. As a last section, the concepts for an incoupling window and focusing optics are elucidated. Finally, after a summary, in an outlook the chances for commercial realization are analyzed.Sectional view of a prototype of a laser spark plug with an expanding flame kernel.

We report the delivery of high-energy nanosecond pulses (10 ns pulse width) from a Q-switched Nd:YAG laser through the fundamental mode of a hollow-core photonic crystal fiber at 1064 nm. At a pressure of 450 mbar, energies greater than 1 mJ were delivered in a single spatial mode through the hollow-core fiber, coupling efficiencies over 80% were demonstrated.

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