Impact of atmospheric clutter on Doppler-limited gas sensors in the submillimeter/terahertz.

It is well known that clutter (spectral interference) from atmospheric constituents can be a severe limit for spectroscopic point sensors, especially where high sensitivity and specificity are required. In this paper, we will show for submillimeter/terahertz (SMM/THz) sensors that use cw electronic techniques the clutter limit for the detection of common target gases with absolute specificity (probability of false alarm ≪ 10⁻¹°) is in the ppt (1 part in 10¹²) range or lower. This is because the most abundant atmospheric gases are either transparent to SMM/THz radiation (e.g., CO2) or have spectra that are very sparse relative to the 105 Doppler-limited resolution elements available (e.g., H2O). Moreover, the low clutter limit demonstrated for cw electronic systems in the SMM/THz is independent of system size and complexity.