Molecular dispersion spectroscopy for the optical detection, and characterization of anomalous dispersion is a developing field for the interferometric measurement of trace gas concentrations. As a phase sensitive detection technique, it eliminates the need for baselining or normalization of the spectroscopic signal which is typically required for absorption spectroscopy methods. Furthermore, dispersion sensitive methods excel at measuring through high optical depth, where absorption techniques are limited by the non- linear response of the Beer-Lambert Law.
In this talk, I will present novel architectures for dispersion spectroscopy using digital interferometry (DI), a laser metrology technique that enables range gated interferometric phase measurements using the correlation of pseudo-random binary sequences (PRBS) modulated onto the optical field. This enables rejection of cross-talk, including from unwanted scattering noise or etalons, down to microradian phase sensitivities, as well as a reduction of optical complexity and relaxation of electronic bandwidth requirements.
First, I will present a digitally enhanced dispersion spectrometer that gives a baseband dispersion readout with parts-per-billion sensitivity. This sensitivity is possible due to the inherently matched path lengths of the Sagnac interferometer. Subsequently, I will present newer work on a re-entrant delay line dispersion spectrometer which uses DI multiplexing to synthesize multiple sets of matched path lengths and from that, a noise-suppressed measurement. This optical architecture also allows integrated wavelength tuning linearization as well as greater flexibility in measurement of spectroscopic samples.