The multi-spectral signal properties of multiple reference optical coherence tomography
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Efforts to reduce the size and costs of optical coherence tomography systems (OCT) for consumer applications, in general, focus on Fourier-domain OCT, due to its potential to be integrated on an optical chip, implementing the interferometer and the spectrometer together as a compact system. An alternative and near-term solution is multiple reference OCT. This technique utilizes a partial mirror in the reference arm to enhance the axial imaging depth of traditional time-domain OCT. The motivation of this thesis is to investigate the performance and sensitivity characteristics of multiple reference OCT in comparison to time-domain OCT. Due to the partial mirror, the light is recirculated multiple times and generates additional reference wavefronts reflected on the reference mirror with increasing path delays. A fascinating consequence of this is the frequency-dependence of each reflection, which draws parallels with Fourier-domain OCT. Hence, the spectral properties of the interference signals are studied in more detail. This thesis guides the reader towards the subject of OCT covering the fundamentals of OCT and the theoretical basis of multiple reference OCT. The spectral properties of the interference signals of MR-OCT are further explained, and it is shown that the increasing path delay in the reference arm of the Michelson interferometer causes an increase of the frequency similar to the increasing frequency vs. depth in the sample for Fourier-domain OCT. The spacing between the partial mirror and the scanning mirror, in conjunction with the scanning range and velocity, play a unique role in controlling a variety of parameters, which are not available in any other OCT system. Signal simulations are provided for some theoretical aspects of interest and compared with signals measured on a sample mirror. For each of the multiple interference signals, the sensitivity was measured, and the results are compared to conventional time-domain OCT. The higher orders of reflections show some non-linear characteristics that may reduce dynamic range and sensitivity. Zemax was used to simulate the beam propagation in the optical system. The impact of the reference mirror scanning parameters on the overall system characteristics was examined. Further, a novel en-face scanning modality is described that can increase the data acquisition speed due to the reduction of the width of the scanned depth layers and demonstrates the flexibility of the scanning protocol of MR-OCT that would otherwise require extensive efforts with other OCT systems. Some more specific aspects of the signal and image processing are discussed that have relevance for the image reconstruction of the multiple signal segments originating from the recirculated light due to the partial mirror in the reference arm. So far the results in this thesis have shown that the multiple interference frequencies originate from an actual change of the source spectrum due to the Doppler effect caused by the scanning mirror. Under certain conditions, it is possible that multiple reference waves create phantom signals. So far the phantom signals are difficult to observe and may not be of significant concern for conventional imaging applications with MR-OCT. The increasing axial scanning range causes depth regions to be scanned multiple times, and averaging can improve somewhat the SNR. Whereas, the time delay between the multiple scanned regions has a delay of a few hundred femtoseconds which may open new application areas that require high-speed scanning.
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