An infrared-imaging instrument is being developed to provide in situ qualitative and quantitative assessment of hydrocarbon contaminants on metallic surfaces for cleaning verification. A continuous-wave infrared optical parametric oscillator (OPO), based on the quasi-phasematched material periodically poled lithium niobate (PPLN), is interfaced with an InSb focal plane array camera to perform fast, non-invasive analysis by reflectance spectroscopy. The period range of the designed fan-out PPLN crystal determines the range of the output wavelength of the light source. It is able to scan hundreds of wavenumbers positioned in the range of 2820 - 3250 cm-1, which is sufficient to detect functional groups of common organic compounds (-CH, -OH, and -NH). The capability of the instrument has been demonstrated in a preliminary investigation of reflectance measurements for hydrocarbon solvents (methanol and d-limonene) on an aluminum surface. A substantial difference in absorption is obtained for the two solvents at two different laser-illumination wavelengths, thus permitting hydrocarbon detection and molecular species differentiation. Preliminary reflectance spectra of a mixture of aliphatic hydrocarbon lubricants and drawing agents on an aluminum panel are also presented. The relative thickness of the hydrocarbon thin film is determined by the intensity ratio of images acquired at two different laser illumination frequencies.
In this paper we discuss the development of a tunable diode laser absorption spectroscopy probe to simultaneously measure, in situ concentrations of 4 major species (CO, C2H2, CH4, and H2O) and the gas phase temperature in a pool fire. The difficulty in making these types of measurements is intrinsic to the environment itself. A large fire is composed of very hot (> 1000 K), turbulent gases and highly radiating and absorbing soot particles. Fiber optic cables are used to transport laser radiation into the fire via a water-cooled probe. This paper focuses on probe design issues, such as the optimization of open path, multipass optics for a turbulent, particulate- laden flow, and the application of high-frequency wavelength modulation spectroscopy to frequency-domain multiplexing of diode lasers.
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