Scintillation and multiwavelength coherence effects in a long-path laser absorption spectrometer.

An experimental investigation has been carried out on turbulence-induced noise in a long-path laser absorption spectrometer which measures sequentially the atmospheric transmission at two preselected wavelengths over the same path. A simple analysis shows that performance degradation due to scintillation can be significantly reduced by exploiting the coherence between fluctuations in received signal at the two wavelengths. Auto-correlation and cross-correlation properties for signals at the two wavelengths, recorded experimentally, are compared with results derived from turbulence theory, and implications for choice of spectrometer wavelengths and sampling speed are discussed.