Wavelength Modulation Spectroscopy (WMS) is a highly sensitive technique that utilizes synchronous detection at the N-th harmonics of a modulating frequency, by modulating the laser beam used to probe a gaseous species. The advantage of this technique lies in the greater effective signal-to-noise ratio one obtains as a direct consequence of the larger amount of structure present in the higher harmonics, and thus a greater amount of information that can be obtained from that structure. We present the development of a novel technique where data at multiple harmonics is obtained simultaneously, rather than sequentially. This removes the susceptibility of the experiment to changes in the environment, when one is collecting data at different harmonics. The experimental setup is discussed, and results are presented illustrating that the new method does not introduce any distortions to, nor lose any structure present, in the previous, sequential setup for WMS.

We also utilize higher harmonic detection with wavelength modulation spectroscopy to compare the sensitivity of signals to the lineshape profile used when modeling experimental results. Transition profiles that are very similar when measured with direct absorption and lower detection orders, are more differentiated at higher harmonics. The effects of increasing modulation index as well as higher optical pathlengths are investigated. The latter of these investigations results in novel optical pathlength saturation effects, which a model assuming the Voigt lineshape function is able to more accurately predict than a model using the Lorentzian profile. Furthermore, the sensitivity provided by the derivative structure of WMS signals is used to resolve weak spectra, that are otherwise indiscernible at direct absorption with the resolutions available.

We also present a method, using Shannon's principles, to quantify the amount of information, in bits or nats, that one obtains when increasing the precision of a measurement of some parameter in a distribution of photons. The calculation is presented for antenna array radiation patterns, as well as for experimental wavelength modulation spectroscopy signals. Finally, we quantify the information lost and associated heat generated when a lineshape function is measured with a finite resolution spectrometer.