Phase fluctuation optical heterodyne spectroscopy of gases

Abstract

The theory underlying the use of the phase fluctuation optical heterodyne (PFLOH) technique in studies of molecular relaxation, thermal conduction, and extremely weak absorptions in the gas phase is presented. Several new solutions to the heat conduction equation, with and without mass diffusion, which are appropriate to pulsed, cw, and modulated-cw laser modulation of weakly absorbing gases, are presented. Experiments employing pulsed and modulated-cw excitation have borne out the essential predictions of the theory. PFLOH systems for use in a trace detection application have been built with a demonstrated sensitivity, in terms of measurable absorption by the trace material, below 10(-10) cm(-1).

Photothermal Interferometric Aerosol Absorption Spectrometry

“…As a detailed discussion of the photothermal technique can be found elsewhere (Sedlacek 2006;Owens et al 1999, Lin andCampillo 1985;Bialkowski 1996;Davis 1980;Davis and Petuchowski 1981;Dovichi 1990;Davis 1996), only the highlights that are relevant to the present discussion will be presented. At its most fundamental level, the PTI technique measures the optical pathlength change that one arm of an interferometer experiences (referred to as the "probe" arm) relative to the other arm of the interferometer (called the "reference").…”

“…For the interferometer of the type just described, it can be shown that the measured detector current (Davis 1980;Davis and Petuchowski 1981;Lin and Campillo 1985;Fluckiger et al 1985 ;Bialkowski 1996;Davis 1996;Owens et al 1999;Sedlacek 2006), under quadrature conditions (i.e., φ ref − φ probe = (2n + 1) π 2 where φ is the phase of each respective arm), reduces to…”

Directly measuring aerosol absorption using photothermal interferometry (PTI)

“…Back in the mid 1980's, Lin and co-workers [3,16] described the in situ measurement of the aerosol absorption of ammonium sulfate using, what the authors referred to as, phase fluctuation optical heterodyne spectroscopy (PFLOH). § [2,24] These initial experiments used a Mach-Zehnder (MZ) configuration, where a single-frequency probe laser (He-Ne) was split by a 50/50 beam splitter (BS) with one arm of the interferometer directed into a sample cell containing aerosol-laden air and where a counter-propagating "pump" laser (linetunable CO 2 ) overlapped this "probe" beam. Absorption of the pump laser light by the aerosol resulted in local heating, which, in turn, caused an expansion of the air in the cell.…”

“…If we let the optical fields of the signal (probe beam) and local oscillator (reference beam) be represented by (2) where A j , ω and φ represent the amplitude, angular frequency and phase; and, where ∆φ t ( ) is the induced time-dependent phase fluctuations, it can be shown, for a square law detector, that the current in the detector, I t…”

“…The use of this coherent optical detection technique is particularly well suited to measuring the refractive index change that accompanies this energy transfer process. [1,2] This technique was even demonstrated towards measuring aerosol absorption in the mid-1980s [3]. Attractive features of this technique for measuring aerosol absorption include its insensitivity to aerosol scattering, its ability to conduct the measurement in situ, its inherent high sensitivity, and near real-time response.…”