| OCR Text |
Show 6 III. Procedure A. Experimental Fig. 3 shows the laser sources, signal processing optics, and electronics. Characteristics of similar systems are described in the literature.17 Briefly, the output of Quanta Ray Nd:YAG laser (DCR-2A) was doubled to produce 350 mJ of pulsed energy at 532 nm. The residual fundamental radiation passed through a second doubler (KD*P type II) producing a pulse of 35 mJ energy which was used to pump a broadband dye laser oscillator. This oscillator was concentration tuned to produce a Stokes beam at a wavelength (air) of 606.4. The spectral profile was nearly Gaussian with a FWHM bandwidth of 5.7 nm (155 cm ). Approximately a third of the main Nd:YAG second harmonic pumped the dye laser amplifier; the remainder was used for the pump radiation. The Stokes and pump beams were rendered parallel and directed to the test chamber via a hole in the floor. Single, right angle prisms (5 cm x 5 cm clear aperature) relayed the beams to the combustion facility located one floor below the CARS instruments. There they were focused into the hot gas flow using a 600 mm focal length lens (see Fig. 4). This and a second lens used for recolli-mating the beams were driven in tandem by stepping motors controlled from the laboratory. After dumping the laser beams, the signal was directed through the ceiling above for spectral analysis. Total path length from initial directing prism to final recovery prism varied between 10 to 20 meters depending on the port position. After passing through spectral filters, the signal was focused through a 60 Mm entrance slit of a 3/4 meter raonochrometer equipped with a 2400 £/mm grating. This caused the signal to be projected on a vidicon optical multichannel detector (PAR OMA II system, SIT detector) with a reciprocal dispersion of 4.35 A/mm. An electromechanical shutter could be activated to capture single pulse spectra. A microcomputer interrogated the detector, displayed the spectrum on a CRT and subsequently recorded it on a floppy disk. 18 19 When folded BOXCARS phasematching ' was employed, the Stokes and pump beams were individually expanded to diameters of 1.9 cm and 1.0 cm respectively. The pump beam was subsequently split and the three beams were aligned parallel to each other on a 2.3 cm diameter circle before they were sent through the relay prisms and 600 mm focal length lens. A thin (0.15 mm) microscope cover slip translated through the focal point of |
