Novel polarization and dispersion concepts for Optical Coherence Tomography

Abstract

Optical coherence tomography (OCT) is a non–invasive, interferometric imaging technology that enables two– and three–dimensional imaging of transparent and translucent samples with micrometer scale resolution. Recent developments enhance the clinical potential of OCT through multi–dimensional imaging parameters for structural and functional sample assessment including polarization, Doppler flow and spectroscopy. High quality OCT imaging is concatenated to a line of processing requirements that can include linearization, dispersion compensation, Fourier transformation, speckle noise reduction and complex conjugate removal. While very versatile, the implementation of structural and functional aspects remains relatively unintegrated. The continued transition of OCT technology from the research lab to real clinical practice demands innovative concepts with a deeper understanding for hardware and signal processing based solutions that combine structural imaging requirements as well as functional modalities. This thesis comprises reports on OCT sensitivity, complex conjugate term suppression, dispersion compensation, speckle noise reduction, polarization mode dispersion but also includes functional aspects with polarization sensitive measurements and the introduction of a new type of contrast. Not only do we describe those individual objectives but more importantly compose unknown symbioses between them. The outcomes of this thesis have been motivated by issues associated with polarisation and chromatic dispersion, as such, these represent the true cornerstone of this dissertation. Six publications are described in detail over six chapters. The first half of this thesis describes novel polarization concepts. We derive a simple theory to improve OCT sensitivity and acquisition speed. We show how to avoid operating in the excess intensity noise regime and obtain an increase in sensitivity by 6 dB for shot noise limited operation. We also compose a novel connection between full–range imaging and true polarization sensitivemeasurements through a geometric phase shift. Furthermore, we explore fundamental aspects of polarization mode dispersion (PMD) in a single mode fiber. Exploiting numerical simulations, we unfold new insights regarding the preservation of orthonormal Stokes vectors in the Poincar´e sphere of two polarization channels during fiber propagation. The second part of this dissertation is dedicated to challenge the common stereotyped view of dispersion. We commence with the introduction of the fractional Fourier transform to generalize our understanding of a distinct pure time and frequency domain. We describe a simple numerical technique to obtain a depth signal with instantaneous dispersion compensation and speckle noise reduction using a single acquisition. Using the time–frequency plane, we find valuable insights to the interplay between the components of a complex conjugate pair of a Hermitian function. The exploitation of dispersion eventually unfolds a remarkable material–specific dispersion contrast. Using a novel tri–band swept source design with a spectral range of 500 nm, we demonstrate depth–resolved dispersion measurements at a record–breaking axial resolution of less than 100 μm.