OCR Text |
Show oscillator was concentration tuned to produce a Stokes beam at a wavelength of 606.7 nm with a bandwidth of 8.2 nm (223 cm"1). Approximately a third of the main Nd:YAG second harmonic was used to pump the dye laser amplifier and the remaining portion of the beam was used for the pump radiation. The Stokes and pump beams were individually expanded (to diameters of 1.0 cm and 1.9 cm, respectively) before being rendered parallel and directed to the test chamber via a hole in the laboratory floor. In the combustion facility below, the beams were focused into the hot gas flow using a 600 mm focal length lens (see Figure [4]). This and a second lens used for recollimating the beams were driven in tandem by stepping motors controlled from the laboratory. After dumping the laser beams, the signal was directed through the floor above for spectral analysis. Total path length from initial directing prism to final recovery prism varied from 10 to 20 meters depending on the port position. After passing through spectral filters, the signal was focused through a 60 ym entrance slit of a 3/4 meter raonochrometer equipped with a 2400 l/mm grating. This caused the signal to be projected on a vidicon optical multichannel detector (PAR 0MA II system, SIT detector) with a reciprocal dispersion of 4.35 A/mm. An electromechanical shutter could be activated to capture single pulse spectra. The detector was interrogated by a microcomputer, and the resulting spectrum was displayed on a CRT and subsequently recorded on a floppy disk. A subsidiary experiment was performed to estimate the length in which the CARS signal is efficiently produced when focused by the 600 mm focal length lens. Here, a thin (0.15 mm) glass microscope cover slip was translated near the focal point of the overlapped beams and the non-resonant CARS signal generated therein was detected by a photomultiplier. This output was amplified and registered on a chart-recorder as a motorized drive moved the cover slip parallel to the beams. The results from this test indicate that 90 percent of the CARS signal would be generated in a distance of 5.3 mm in an isothermal environment. Translation of a razor blade into the beams at the focal point allowed the beam waists to be measured. Diameters of approximately 0.18 and 0.33 mm were found for the pump and Stokes beams, respec- 11 |