Digitally Enhanced Dispersion Spectroscopy

Abstract

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 exponential response required by the Beer-Lambert Law.

This thesis presents 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 unwanted scattered noise or other cross-talk down to microradian phase sensitivities, as well as a reduction of optical complexity and relaxation of electronic bandwidth requirements. Firstly, as a proof of concept, a Sagnac architecture is used to demonstrate digital interferometric phase extraction, achieving 2x10^-7 rad per root hertz sensitivity. Following this, a vapor cell is inserted into the sensing path, transforming the interferometer into a spectrometer. Using this instrument, and leveraging the sub-microradian phase noise, a baseband dispersion readout with 77 ppb-m per root hertz concentration sensitivity is demonstrated. This sensitivity is possible due to the inherently matched path lengths of the Sagnac interferometer.

Subsequently, a more flexible in-line technique using a re-entrant delay line is developed. This creates a dispersion spectrometer which uses DI multiplexing to synthesize multiple sets of matched path lengths and a noise-suppressed measurement, achieving a phase sensitivity of 8x10^-6 rad per root hertz , corresponding to a concentration sensitivity of 159 ppb-m per root hertz. This is a factor of two higher compared to the Sagnac architecture. However, the re-entrant optical architecture allows integrated wavelength tuning linearisation as well as greater flexibility in the measurement of spectroscopic samples. Finally, the thesis discusses ongoing work on extending the optical bandwidth of the re-entrant spectrometer, using an optical amplifier to generate a digitally addressable frequency comb. This has applications in the interrogation of wider linewidth resonance features such as metasurfaces. Show more