Trace detection in gases using phase fluctuation optical heterodyne spectroscopy

Abstract

A very sensitive technique for detecting trace molecules in the gas phase is described. Refractive index changes of a gaseous sample, which result following V→R, T relaxation of trace molecules which have been excited by a laser, are detected by optical heterodyning in a Mach‐Zender interferometer. Detection sensitivities of 10 ppb for SF₆ in air and 40 ppb for CH₃OH in helium, have been demonstrated using CO₂ TEA laser excitation and a 20‐MHz detection bandwidth.