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Nowadays, combustion engines and other combustion processes play an overwhelming and important role in
everyday life. As a result, ignition of combustion processes is of great importance, too. Usually, ignition of a
combustible material is defined in such a way that an ignition initiates a self-sustained reaction which propagates
through the inflammable material even in the case that the ignition source has been removed. In most cases,
a well defined ignition location and ignition time is of crucial importance. Spark plugs are well suited for such
tasks but suffer from some disadvantages, like erosion of electrodes or restricted positioning possibilities. In some
cases, ignition of combustible materials by means of high power laser pulses could be beneficial. High power
lasers offer several different possibilities to ignite combustible materials, like thermal ignition, resonant ignition
or optical breakdown ignition. Since thermal and resonant ignitions are not well suited on the requirements
mentioned previously, only optical breakdown ignition will be discussed further. Optical breakdown of a gas
within the focal spot of a high power laser allows a very distinct localization of the ignition spot in a combustible
material. Since pulse duration is usually in the range of several nanoseconds, requirements on the ignition time
are fulfilled easily, too. Laser peak intensities required for such an optical breakdown are in the range of 1011
W/cm2. The hot plasma which forms during this breakdown initiates the following self-propagating combustion
process. It has been shown previously that laser ignition of direct injection engines improves the fuel consumption
as well as the exhaust emissions of such engines significantly. The work presented here gives a brief overview
on the basics of laser induced ignition. Flame propagation which follows a successful ignition event can be
distinguished into two diffrent regimes. Combustion processes within an engine are usually quite slow - the
reaction velocity is mainly determined by the heat conductivity of the combustible. Such deflagrations processes
show propagation velocities well below the speed of sound. On the other hand, detonations show much higher
propagation velocities. In contrast to deflagrations, detonations show propagation velocities higher than the
speed of sound within the combustible. The shock front which propagates through a combustible in the case of a
detonation is responsible for a considerable pressure gradient moving at supersonic velocity. Basics and possible
examples of laser induced ignitions of deflagrations and detonations are given and pros and cons of laser ignition
systems are discussed briefly.
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