Mitigated Motion-Induced Signal Loss and Enabled Refractive Index Measurement in Optical Coherence Tomography

Abstract

Optical coherence tomography (OCT) has become widespread in the world of metrology, owing to its ability to conduct measurements with unparalleled precision. OCT has applications in various fields and finds use in both industry and academia.

An industrial application of OCT is the monitoring of metallic welding conducted with high-power lasers, where OCT can detect weld depth in real-time. Despite this ability, OCT is known to lose signal when measuring moving objects. Since laser welding is volatile and prone to sporadic motion, it is hypothesized that signal is lost when monitoring laser welding. Therefore, this thesis implements an augmented OCT configuration in the form of a temporally pulsed ‘stroboscopic’ light source. Compared to standard OCT, which uses a time-independent source, stroboscopic OCT can mitigate motion-induced signal loss. For this reason, a stroboscopic light source was created. By comparing signal retention during measurement of laser welding with both sources, results demonstrate recovery of considerable signal with stroboscopic OCT. For a signal loss threshold of 42.76 dB during welding, standard and stroboscopic OCT maintained sufficient signal for 1.06% and 16.56% of measurements, respectively. Such results suggest stroboscopic OCT confers improved process monitoring abilities, in turn providing manufacturers with a more reliable and trustworthy metrological technique.

Optical coherence technology has also attracted interest in state-of-the-art research efforts. The High Energy Light Isotope eXperiment (HELIX) is a project spanning multiple institutions and is sponsored by the North American Space Agency (NASA). HELIX will measure the presence of cosmic ray isotopes in the upper atmosphere by using aerogel tile detectors. In order to use aerogel tiles for accurate particle signature identification, the refractive index of the tiles must be precisely measured. For this application, OCT is an ideal candidate. In this thesis, I design and demonstrate an OCT-based procedure that is suitable for the precise group refractive index measurement of a sample aerogel tile, where I report a value of 1.156 ± 0.002. A mathematical procedure is also formulated that can extract the refractive index function from experimental OCT data. The final measurements will assist in calibrating the properties of HELIX’s aerogel tiles