Development and Applications of Near- and Mid-infrared OCT

This thesis chronicles the development of a mid-infrared optical coherence tomography (MIROCT) system and various applications of near-infrared (NIR) OCT. With regards to the development of MIR-OCT, the ultimate goal is to build a swept-source system with a
commercial external cavity quantum cascade laser, using heterodyne detection to improve the signal-to-noise ratio. We have characterized the sources of noise within the system: quantization noise from the analogue-to-digital convertor, shot noise from the detectors and relative intensity noise (RIN) of the laser. By using pulse-to-pulse normalization with two paired detectors, the RIN measured can be reduced -97dB/Hz to -115dB/Hz. Next, we investigate the feasibility of using polarization-sensitive OCT to quantify the severity of striae distensae. We have shown that the estimated tissue birefringence of striae-affected skin is significantly greater (p<0.001) than visually normal skin. There is also a loose correlation between tissue birefringence and the subjective visual scores. In botany, we have shown that even with a weak acid treatment, it is possible to quantify the effect of acid erosion on teeth. Calculating the slope of the reflectivity profile is a reliable method for quantifying and potentially monitoring the demineralisation of teeth. The results show that the initial reflectivity readings before treatment is an important factor in determining the magnitude of reflectivity changes after treatment. In botany, we aimed to investigate the stability of plant petiole diameter measurements using OCT. We have demonstrated that spectrometer-based OCT can accurately track changes in the diameter of plant petioles, with an accuracy of ~1.0 µm. With regards to biofilm, we aimed to determine whether there is a minimum detectable limit when OCT is used for the quantification of biofilm thickness, and have shown that biofilm as thin as 13.1 µm can be measured.