Scintillation noise reduction by aperture averaging in a long-path laser absorption spectrometer.

Turbulence noise in dual-wavelength long-path laser absorption spectrometers is analyzed with the aid of weak turbulence theory, and results are compared with experimental CO(2) laser beam transmission data. Spectrometers with finite-sized transmitter and receiver apertures are considered, where noise reduction is sought simultaneously through aperture averaging and by taking advantage of the correlation that exists between turbulence-related perturbations at the two wavelengths. Variance, scintillation spectra, and correlation properties for single- and dual-wavelength propagation are discussed as functions of receiver apertures, and the possible role of attenuation scintillation is also assessed.