ISSN 1054-660X, Laser Physics, 2010, Vol. 20, No. 6, pp. 1545–1553. © Pleiades Publishing, Ltd., 2010. Original Russian Text © Astro, Ltd., 2010. 1545 1 1. INTRODUCTION Climatic changes and shrinking resources of fossil fuels increases the demand for more efficient and non- polluting engine concepts employed for electric power generation. One promising idea to support this demand is the concept of laser-induced ignition for gas engines. Generally, a laser ignition system offers numerous advantages, e.g., the ability to ignite leaner mixtures. As a consequence of high leanness of fuel-air mixtures, the flame temperature is lowered and hence also the NO x emissions, whereas the efficiency of the engine can be increased simultaneously as well as by increasing the ignition pressure. For electric spark ignition, however, the spark voltage has to be raised with increasing pressure, which causes stronger ero- sion of the electrodes. Laser ignition systems, to the contrary, do not require electrodes and hence mainte- nance intervals of a laser ignition system are expected to be significantly longer, and costs may be lower than those of a spark plug system [1–4]. To identify the necessary parameters for an ignition laser (e.g., minimum laser pulse energy for plasma generation and/or ignition) a series of investigations already has been performed by us within the last years. A review of this work can be found in [5]. Therein basic informations, e.g. about plasma formation mecha- nisms and the dependence of minimum pulse energy MPE on pressure, are presented. Closely related to this, in Section 3 of the present paper some additional investigations at various pressures and focal sizes are shown. Customary lasers are, because of their bulkiness and high initial costs, only applicable for laboratory test rigs and cannot fulfill the needs of an ignition laser. 1 The article is published in the original. Therefore, it was necessary to develop an ignition laser by ourselves which can deliver a minimum pulse energy of 10 mJ and a pulse length of 1 ns to ensure reliable ignition. Before starting the laser develop- ment, an appropriate specific laser concept for engine ignition had to be identified. In this context, a com- pact high peak power, passively Q-switched, longitu- dinally diode-pumped monolithic solid-state laser with an emitting wavelength of 1064 nm became our favorite [6]. Currently a variety of similar diode- pumped solid-state laser (DPSSL) systems, which use the manifold alternative transition lines of a Neody- mium ion-doped gain medium, can be found [7–11]. The main difference, apart from the emitting wave- length, to our laser is the pump power. To ensure high peak power laser pulses in the range of 10 to 15 mJ the diode laser for pumping has to offer a minimum pump power of 600 W. Competing laser systems mainly use diode lasers with pump powers in the range of 10 to 30 W. Alternative to the previously mentioned ignition laser, a laser system with an emitting wavelength of 946 nm was tested [12, 13]. It turned out that the slope efficiency, pulse energy and peak power are approxi- mately one order of magnitude below the range of parameters applying the 1064 nm line. Moreover, three-level-laser systems are more sensitive to temper- ature changes being one more reason advising against an application for laser ignition. The before mentioned ignition laser is to be directly mounted on the cylinder head like a conventional electrical spark igniter [14], An external high power laser diode operates as the optical power supply deliv- ering the pump beam via a step index fiber towards the incoupling laser end. Basically, the ignition laser con- sists of the pump fiber, an incoupling lens, which NOVEL METHODS OF LASER TECHNOLOGIES Laser-Induced Ignition by Optical Breakdown 1 E. Schwarz*, I. Muri, J. Tauer, H. Kofler, and E. Wintner** Photonics Institute, Vienna University of Technology, Gusshausstrasse 25-29, 1040 Wien, Austria *e-mail: elisabeth.schwarz@tuwien.ac.at **e-mail: ernst.wintner@tuwien.ac.at Received December 24, 2009; in final form, January 4, 2010; published online May 3, 2010 Abstract—This paper is an experimental work of the applied methodical character in which as an attempt to optimize a laser ignition system a systematic study of the best incoupling geometry for the employed Nd:YAG laser was performed. The incoupling geometry comprises the pump fiber and an aspheric collimating lens. In this context, the distance between pump fiber and collimating lens was made continuously variable. The dis- tance between fiber and lens primarily influences the diameter of the pump beam. In this way, it is possible to control the pulse energy as well as the number of pulses generated within a pump cycle. Furthermore, inves- tigations to analyze the focal size dependence of plasma generation were carried out. As a result, it was found that it is possible to reduce optical losses caused by plasma transmission by choosing an optimum focal vol- ume. This experiment was carried out for different pressures and focal volumes. DOI: 10.1134/S1054660X10110204