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Show 1979}. When the USED CARS configuration was erll>loyed the dye beams diverged more than the pump beams because the dye beams were smaller In diameter. USED CARS was originally selected because it allowed the pump and dye beams to be transported as large diameter beams on a single set of optics and yet still provide good spatial resolution. Furthermore, the USED CARS configuration can minimize the effects of beam steering due to refractive index gradients w~hin the flame. In order for the dye and pump beams to be focussed and crossed at the same location w~hin the reactor the smaller dye beam had to be turned back through a greater angle than the pump beam. The problem was in~ia"y solved by separating the pump and dye beams w~h a dichroic and then running the beams through separate adjustable Galilean telescopes. This configuration is illustrated in Figure 5. The beams were then brought together on a green mirror that had a 5 mm hole drilled through its center at a 450 angle. The dye beams passed through the hole in the green mirror while the pump beam was reflected off the face of the mirror. The beams were then focussed into the reactor w~h a 300 mm field lens. On the far side of the reactor the beams were collimated w~h another 300 mm field lens and reduced in diameter with a Galilean telescope. The pump and dye beams were then separated from the CARS signal with a dichroic mirror and trapped. Any remaining pump and dye beam energy that had passed through the dichroic mirror was rejected with a short wavelength pass filter. The CARS beam was then focussed into a 50 micron diameter fiber optic w~h a 100 mm focal length lens and the signal was carried back to a spectrometer located in the optics laboratory. Experimental Procedure In~ially , the CARS signal was generated using a dual-Stokes configuration w~h a USED CARS beam phase-matching technique . Two dyes were used in order to probe several molecular species simultaneously. One of the broadband dye lasers was nominally centered at 2200 cm-1 to simultaneously probe both CO and N2, while the other broadband dye laser was centered at 1470 cm-1 to simuttaneously probe 02 and C02. Examples of dual-Stokes CARS spectra taken in both natural gas and coal-seeded flames are shown in Figure 6. The laser beams in the dual-Stokes configuration possessed sufficient energy to produce spectra for the species mentioned, however, the signals were relativety weak and noisy, and of insufficient strength to obtain single shot spectra. Therefore, for the quanmative tests in this study, the dye cell used to obtain the 02 and C02 CARS Signal was removed and all of the laser energy available was applied to the dye used to probe N2 and CO. Furthermore, the dichroic mirror, green mirrors, and galilean telescopes were removed from the USED CARS beam configuration in order to maximize the amount of energy in the beams at the diagnostic volume. This modrfication allowed the optical train on the first optical breadboard to be reduced to a right-angle prism, an OG 515 Schott long-pass fitter, and a 300 mm focal length field lens. These changes made it necessary to use a slightly modrfied collinear CARS approach. Both the pump and dye laser beams were enlarged to a diameter of about 33 mm on the optical table. The diameter of the pump beam was kept relatively 7 |
