Méndez Aller, Mario (2022)
Photonic Terahertz signal analyzers: fully ballistic Terahertz source for a photonic VNA and error analysis of Terahertz time domain spectroscopy of liquids.
Technische Universität Darmstadt
doi: 10.26083/tuprints-00020253
Dissertation, Erstveröffentlichung, Verlagsversion

Kurzbeschreibung (Abstract)

The technological gap in the terahertz (THz) range has reflected in the lack of cost efctive tools to characterize and develop the necessary technology, such as vector network analyzers (VNAs) and spectrum analyzers. This thesis presents on its first part a fully-ballistic (FB)-p-i-n diode THz source for a photonic vector network analyzer (PNVA). The source is designed towards a commercial implementation with efficient optical coupling, improved thermal conductivity and ease of commercial fabrication as compared to n-i-pn-i-p superlattice photomixers. Special emphasis is put in the refinement of some necessary processing steps that required some of the largest efforts within this thesis. The FB-p-i-n diode exhibits experimentally an improved effciency in terms of transport time roll-off at high frequencies (beyond 1 THz) compared to a n-i-pn-i-p superlattice diode, although it still features a lower absolute emitted power than the n-i-pn-i-p. Pathways to overcome these limitations in future implementations will be outlined.

The second part of the thesis presents analyses with THz-time domain spectroscopy (THz-TDS) of liquids. After a theoretical description of the THz-TDS technique and of the main systematic and random sources of error, motor oil with different grades of gasoline contamination and oxidation is used to analyze how the different sources of error arise during the experiment. A higher robustness of the phase measurements is demonstrated (i.e. samples distinguished using refractive index), not being affected by system noise and exhibiting an error close to the resolution limit. However, the more differentiated absorption coeffcient between the samples makes it still a better parameter to discern contamination and oxidation levels.

Chapter 6 presents a novel technique to identify biodiesel in petroleum-diesel blends using broadband information of the absorption coeffcient. This new approach demonstrates the adequacy of using a linear combination of two models, one for each of the two components, to identify the exact content, exhibiting a deviation from the nominal values of the biodiesel content of less than 3%. This opens the door to real-time analysis of the blend in the absence of narrow spectral features, just relying on the broadband, featureless absorption of each component. Relaying on broadband information also eliminates the uncertainty about the origin of the absorption coeffcient at individual frequencies, which could be affected by many other contaminants and systematic errors.

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