Development of a cavity ringdown laser absorption spectrometer for detection of trace levels of mercury.

A potential new laser-based air pollution measurement technique, capable of measuring ultralow concentrations of urban air toxins in the field and in real time, is examined. Cavity ringdown laser absorption spectroscopy (CRLAS) holds promise as an air pollution monitor because it is a highly sensitive species detection technique that uses either pulsed or continuous tunable laser sources. The sensitivity results from an extremely long absorption path length and the fact that the quantity measured, the cavity decay time, is unaffected by fluctuations in the laser source. In laboratory experiments, we reach detection limits for mercury of the order of 0.50 parts per trillion. We developed a CRLAS system in our laboratory and measured Hg with the system, investigating issues such as background interference. We report experimental results for mercury detection limits, the dynamic range of the sensor, detection of Hg in an absorbing background of ozone and SO(2), and detection of a mercury-containing compound (HgCl(2) in this case).

Fixed-frequency cavity ringdown diagnostic for atmospheric particulate matter.

A nonresonant cavity ringdown diagnostic to measure light attenuation from atmospheric particulate matter at 532- and 355-nm wavelengths is described. The presence of atmospheric particulate is clearly detectable with this technique, as demonstrated by experimental results. The extinction cross section is higher at 355 than at 532 nm, although we were able to purchase significantly higher-reflectivity optics at 532 nm. The expected advantage at 355 nm is thus lost. This new technique is compared with a commercially available instrument, and sensitivity limitations are discussed.

Optical imaging through turbid media with a degenerate four wave mixing correlation time gate.

A novel method for detection of ballistic light and rejection of unwanted diffusive light to image structures inside highly scattering media is demonstrated. Degenerate four wave mixing (DFWM) of a doubled YAG laser in Rhodamine 6G is used to provide an ultrafast correlation time gate to discriminate against light that has undergone multiple scattering and therefore lost memory of the structures inside the scattering medium. We present preliminary results that determine the nature of the DFWM grating, confirm the coherence time of the laser, prove the phase-conjugate nature of the signal beam, and determine the dependence of the signal (reflectivity) on dye concentration and laser intensity. Finally, we have obtained images of a test cross-hair pattern through highly turbid suspensions of whole milk in water that are opaque to the naked eye. These imaging experiments demonstrate the utility of DFWM for imagingthrough turbid media. Based on our results, the use of DFWM as an ultrafast time gate for the detectionof ballistic light in optical mammography appears to hold great promise for improving the current state of the art.