HIGHLY SENSITIVE OPTICALLY HETERODYNED, RAMAN-INDUCED KERR-EFFECT SPECTROMETER USING PULSED LASERS

A highly sensitive, optically heterodyned Raman-induced Kerr-effect (O HD-RIKE) spectrometer designed for the spectroscopy of radicals and mo lecular ions in discharges is presented. A linearly polarized, pulsed dye laser beam (probe) is crossed in the sample with the circularly po larized second harmonic of a pulsed Nd:YAG laser beam (pump). The nonl inear interaction between these beams and the sample induces a birefri ngence that creates a polarization component. of the probe laser ortho gonal to its original polarization. The magnitude of the birefringence peaks at Raman resonances as the probe laser is tuned. The combinatio n of detection of the birefringence by the use of polarizers that have an extinction coefficient of the order of 10(8) with a pulsed probe l aser with sufficient power to maximize the signal relative to the shot noise in the heterodyne field provides the basis for the high sensiti vity. By careful adjustment of the polarization of the pump laser, we can decrease nonresonant contributions to the birefringence and furthe r increase the signal-to-noise ratio. By subtracting a fraction of the magnitude of a reference signal from the OHD-RIKE signal, we further enhance the sensitivity by reducing pule-to-pulse fluctuations of the pulsed probe laser to near (10 times) the shot-noise limit of the loca l oscillator intensity. The spectrometer is tested on the CO2 molecule in the Fermi-resonance region. Using a high-power (750-mJ) single-mod e YAG laser as both the Raman pump laser and the pump for the dye lase r that is used as the probe laser allows us to operate near Raman gain saturation, as observed in other coherent Raman spectroscopies. Preli minary studies of saturation for the OHD-RIKE in CO2 are presented. We present both broadband and narrow-band spectra of CO2. For broadband (similar to 10-GHz) studies we achieve sensitivities of 6 x 10(13) mol ecules cm(-3) for a signal-to-noise ratio of similar to 1. In narrow-b and spectra (similar to 400 MHz) we observe increased backgrounds owin g to problems that arise because of the high intensities and the extre mely high polarization extinction coefficients. These higher backgroun ds prevent attainment of the improved signal-to-noise ratio expected f rom the larger Raman gains that are presumed to result from narrow-ban d operation. Possible solutions to the background problems are discuss ed. (C) 1997 Optical Society of America.