Development of injection-seeded optical parametric laser systems with pulsed dye amplifiers for high-spectral-resolution combustion diagnostics.

摘要

The development and application of optical parametric (OP) systems with pulsed dye amplifiers producing single frequency mode (SFM), narrow linewidth, and tunable laser radiation for high-spectral-resolution laser diagnostics is described. An optical parametric generator (OPG) was developed, consisting of a pair of counter-rotating β barium borate (β-BBO) crystals pumped by third-harmonic output of an injection-seeded Nd:YAG laser. The OPG crystals themselves are injection-seeded using a continuous wave (cw) distributed feedback (DFB) diode laser or external cavity diode laser (ECDL) at idler wavelength. The OPG is converted for some applications into an optical parametric oscillator (OPO) by incorporating a feedback cavity. The signal output from the OP system is amplified using pulsed dye amplifiers. The PDAs are pumped either by second-harmonic or third-harmonic output of the Nd:YAG laser depending on the OP output wavelength and the dye solution used in PDAs. The linewidth of the laser beam produced using OP/PDA systems is 200 MHz and the spatial beam profile is nearly Gaussian. Initial application of OP/PDA system included two-photon laser induced fluorescence (LIF) of atomic oxygen in counter-flow flames, dual pump coherent anti-Stokes Raman spectroscopy (CARS) for N2 and CO2, and nitric oxide (NO) planar laser induced fluorescence (PLIF) in compressible flowfield.;A two-photon pump polarization spectroscopy probe (TPP-PSP) laser system has also been developed using two SFM OPG/PDA systems for the detection of atomic hydrogen (H-atom) in flames. In TPP-PSP, a 243-nm pump beam excites the 1S-2S two photon transition and the excited atoms in 2S level are probed by polarization spectroscopy between n=2 and n=3 manifolds using a circularly polarized 656-nm pump and a linearly polarized 656-nm probe laser beam. Using the TPP-PSP scheme, atomic hydrogen was detected at concentrations as low as 11 ppm. The use of injection-seeded OPG/PDAs as SFM sources for the pump and probe beams allows accurate measurement of signal intensities and spectral lineshapes. A detailed investigation of the effect of 243-nm and 656-nm pump beam energies on the different transitions of atomic hydrogen was performed. TPP-PSP lineshapes for high energy 243-nm and 656-nm pump beams showed significant broadening when compared with lineshapes for lower energy 243-nm and 656-nm pump beams. A continuous shifting of the center of n=2-n=3 transition was also observed with increasing 243-nm pump beam energy. Sub-Doppler H-atom lineshapes were also investigated and exhibited significant narrowing. The effect of varying collisional environments on the TPP-PSP signal from atomic hydrogen was investigated by performing measurements in near-adiabatic hydrogen-air flames. The results of these measurements are very encouraging for quantitative measurements of atomic hydrogen in flames.