Fourier Transform Infrared Phase Shift Cavity Ring Down Spectrometer

OPTRA has developed a novel approach to phase shift cavity ring down spectroscopy (PS-CRDS) using a Fourier transform infrared (FTlR) modulator to impose the spectrally-dependent amplitude modulation on a broadband JR light source. As with previous PS-CRDS measurements, we excite a resonant cavity with amplitude modulated energy and measure the phase shift of the modulated signal exiting the cavity which is proportional to the ring down time and inversely proportional to the losses of the cavity including molecular absorption. ln contrast to previous efforts we impose the amplitude modulation with the FTIR interferometer instead of an external electro-optical modulator and extract the phase from each interferogram thereby enabling broadband FTlR-PS-CRDS measurements at greater than I Hz update rates. The measured phase spectra can then be used for multicomponent analysis. The combined measurement can be viewed as a resonant cavity enhancement to traditional FTIR spectroscopy or a broadband enhancement to CRDS. Under the Phase I we established the feasibility of our approach through the design, build, and test of a breadboard FTIR-PS-CRDS system. We measured the phase noise and reconciled the results with our analytical models predicting system performance. We demonstrated an FTIR-P-CRD measurement on vapor phase methanol. Under a Phase II effort we will improve the sensitivity of the measurement by two and a half orders of magnitude realized with a portable and ruggedized prototype system. We will characterize the system and demonstrate phase measurements on multi-gas mixtures. Under a Phase II Commercialization Option we will develop multicomponent algorithms for detection, discrimination, and quantification of a series of molecules of interest to the customer. Applications include trace level air quality and process monitoring and chemical and biological threat detection.