Development of advanced laser systems and spectroscopic techniques for combustion diagnostic applications.

摘要

Single-longitudinal-mode, narrowband, injection-seeded, pulsed optical parametric (OP) systems have been developed, characterized, and applied for high-resolution spectroscopy of nitric oxide (NO). The OP systems were injection seeded at the idler wavelength using a near-infrared distributed feedback (DFB) diode laser. In the optical parametric generator (OPG) version, two counter-rotating, beta-barium borate (beta-BBO) crystals were pumped by the third-harmonic output of an injection-seeded Nd:YAG laser. An optical parametric oscillator (OPO) version has also been developed by incorporating a feedback cavity at the signal wavelength. The cavity length was not actively controlled. The output signal beam from OPG or OPO was amplified using an optical parametric amplifier (OPA) stage. In both the OPG and OPO, the signal and idler frequency bandwidths are nearly Fourier transform limited and were measured to be 220 MHz. The temporal pulses were smooth and near-Gaussian. The frequency-doubled signal output of the OPO/OPA system was used for single-photon, laser-induced fluorescence (LIF) and laser-induced polarization spectroscopy (LIPS) of NO. The signal output of the OPG/OPA system was also used for sub-Doppler, two-photon LIF of NO.; A detailed investigation was also performed for electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy (ERE-CARS) of NO. In the ERE-CARS scheme, an ultraviolet probe-laser beam is tuned to an electronic resonance, resulting in a huge resonance enhancement of the ERE-CARS signal. The effect of drastic variations in electronic quenching rate on the NO ERE-CARS signal was investigated experimentally. In contrast to LIF, the ERE-CARS signal was nearly unaffected by the quenchers O2 and CO2. The ERE-CARS signal intensity was also found to increase rapidly between pressures of 0.1-2 bars and remain nearly constant thereafter up to 8 bars, whereas the NO LIF signal drops with increasing pressure. We have also detected NO down to 50 ppm in H2/air flames using ERE-CARS. NO ERE-CARS signals were also recorded in heavily sooting C2H2/air flames with minimal background interferences. These findings are very significant for the development of ERE-CARS as a technique for measuring NO concentrations in high-pressure combustion environments.