Portland State University. Department of Electrical and Computer Engineering
Lisa M. Zurk
Date of Publication
Doctor of Philosophy (Ph.D.) in Electrical and Computer Engineering
Electrical and Computer Engineering
Terahertz spectroscopy, Nondestructive testing, Signal processing
1 online resource (xvii, 150 pages)
In recent years Terahertz (THz) time domain spectroscopy has emerged as a promising new technology with potential applications in a variety of fields, including industrial manufacturing, security screening and medical imaging. Pulsed THz systems are uniquely suited for non-destructive evaluation (NDE) of the sub-surface layers of dielectric packaging and coating materials, because they provide high dynamic range over a wide bandwidth in the far infrared portion of the electromagnetic spectrum. Often the dielectric materials of the packaging and/or surface coating layers exhibit relatively low loss and abrupt changes in the refractive index at the layer boundaries can be observed as a train of THz pulses in A-scan data.
However, many practical applications of THz NDE will require fast signal acquisition to efficiently scan and evaluate many samples. The conventional processing approach shown in much of the published work in the field of THz NDE does not perform well in low signal-to-noise ratio (SNR) conditions. In addition, many samples of interest contain thin film layers and the THz pulses reflecting from the boundaries overlap on top of one another. Thus, it is not always possible to calculate the thickness of thin films from conventional time difference of arrival (TDOA) measurements.
In this dissertation physics-based signal processing methods that have been historically used for radar/sonar signal processing are adapted and applied for THz NDE of layered media. Results are demonstrated with measured data from a pulsed THz system in the Northwest Electromagnetic and Acoustics Research Laboratory (NEAR-Lab) at Portland State University (PSU).
This research is expected to provide an important link for THz researchers to access and apply the robust methods that have been developed over several decades for other applications.
Two key contributions of this work are:
1. Development of a matched filter approach for THz NDE of thick layered media based on the maximum likelihood estimator (MLE).
2. Development of a matched field processing (MFP) approach for THz NDE of thin-film layered media, based on techniques in the underwater acoustics literature.
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Schecklman, Scott G., "Physics-Based Signal Processing Methods for Terahertz Non-Destructive Evaluation of Layered Media" (2019). Dissertations and Theses. Paper 5013.