**2.2 Laser diagnostic system**

Laser diagnostic techniques are able to probe combustion products nonintrusively. Laserinduced fluorescence (LIF) and laser-induced incandescence (LII) are attractive techniques for combustion diagnostic and can be used to obtain information about PAHs (Hayashida, 2006) and soot (Shaddix, 1996), respectively. We measured the two-dimensional distribution of the PAHs-LIF in diffusion flames, and laser-induced incandescence (LII) was also used to visualize the soot distribution. Figure 2 shows schematic of the optical arrangement. The laser diagnostic system consisted of an Nd: YAG laser (Spectron Laser Systems, SL856G), a dye laser (Lumonics, HD-300B), and a doubling unit (Lumonics, HT-1000). The laser light was formed into a light sheet (0.5 mm×46 mm) by cylindrical lenses and was introduced into a target flame. Laser-induced emissions were detected by an ICCD camera (Andor Technology, DH-534-18F-03), which was oriented perpendicular to the laser beam direction. The LIF and LII images were obtained by averaging 20 laser shots. For the PAHs-LIF measurement, the Nd: YAG laser was used to pump the dye laser (Rhodamine 590), producing a beam at 563 nm, and the doubling unit was used to double the dye laser output to produce 281.5 nm radiation. For the LII measurement, second harmonic generation (532 nm) of the Nd: YAG laser was used.

Fig. 2. Schematic of laser diagnostic system
