Applications of an Injection-Seeded, Pulsed Optical Parametric Oscillator (OPO) for High-Resolution Spectroscopy of Nitric Oxide (NO)
Waruna D. Kulatilaka, Purdue University, Graduate Student / Research Assistant, School of Mechanical Eng., 585, Purdue Mall, West Lafayette, IN 47907, Thomas N. Anderson, Mechanical Engineering, Graduate Student / Research Assistant, School of Mechanical Eng.,, 585, Purdue Mall, West Lafayette, IN 47907, and Robert P. Lucht, Mechanical Engineering, Professor, School of Mechanical Eng.,, 585, Purdue Mall, West Lafayette, IN 47907.
Application of our injection-seeded, pulsed, optical parametric oscillator (OPO) system for high resolution spectroscopy of nitric oxide (NO) at low pressure is discussed. The OPO system consists of an oscillator cavity containing two beta barium borate (â-BBO) crystals. It is pumped by the third harmonic 355-nm output of an injection seeded Nd:YAG laser. The cavity is injection seeded at its idler wavelength using a tunable, single- frequency distributed feedback (DFB) diode laser. The output radiation from the OPO is amplified by passing through an optical parametric amplifier (OPA) stage consisting four â-BBO crystals and is pumped by the same Nd:YAG laser. The signal and idler outputs of the OPO are single-longitudinal mode and have a nearly Fourier transform limited bandwidth of approximately 220 MHz. The bandwidth is measured using the transmission traces obtained using a 2GHz free spectral range (FSR) spectrum analyzer for signal and idler wavelengths. This OPO system was then used for high resolution spectroscopic measurements of NO at low pressure. The OPO was seeded by a 1652-nm DFB laser to generate signal output at 452nm and it was frequency doubled to obtain ultra-violet (UV) radiation around 226-nm. Laser-induced fluorescence (LIF) signals were observed from different rotational transitions of the (0,0) vibrational band of the A2Σ – X2∏ electronic transition of NO and they are in excellent agreement with theoretical line shapes. The 452-nm output itself was then used for two-photon excitation of NO transitions in the same vibrational rotational manifold. Low pump power levels were used to avoid any possible optical Stark effects. Sub-Doppler NO spectra were obtained at very low pressures by incorporating a counter-propagating beams geometry. A splitting in NO spectral lines at cell pressures below 10 Torr and low laser intensities was observed. The theoretical work to model the two-photon NO line shapes is underway.