Improving optical coherence tomography theories and techniques for advanced performance and reduced cost

Abstract

Optical coherence tomography (OCT) has been developed as a high-resolution three-dimensional imaging technique for clinical diagnosis. This thesis proposes several theoretical discoveries and technical improvements to the current OCT systems aiming to drastically reduce system costs while achieving the state-of-theart performance. Polarization-sensitive OCT (PS-OCT) is a functional extension of conventional OCT which offers the depth-resolved birefringence imaging capability. To resolve local retardation in tissue samples, the current PS-OCT system employs two multiplexed polarization states to illuminate the sample under investigation and detects the backscattered light with two detection channels. We observed that the polarization state of light after round-trip propagation through a birefringent medium frequently aligns with the employed input polarization state but "mirrored" by the horizontal plane of the Poincaré sphere. We explore the predisposition for this mirror state and demonstrate how it constrains the evolution of polarization states as a function of the round-trip depth into weakly scattering birefringent samples, as measured with PS-OCT. The constraint enables measurements of depth-resolved sample birefringence with PS-OCT using only one input polarization state, which offers a critical simplification and cost reduction compared to the use of multiple input states. We demonstrated the capability of polarization mirror state in local birefringence restoration with birefringent phantom imaging and swine retina imaging ex vivo. If we take transpose for both sides of the Jones transmission equation of a PS-OCT system with two input states and two detection channels, the mirror state constraint still holds while the input and output states exhibit interchangeability, which implies that depth-resolved birefringence imaging is possible with only one detection channel, if the sample is illuminated with two orthogonal polarization xiii Summary xiv states. Considering the high cost of spectrometers and the difficulty of pixel alignment between two detection channels, the transposed mirror state constraint helps to cut the cost and complexity of spectral domain PS-OCT system. Inspired by the mirror state constraint, we realized that the polarization states of detected sample light, after propagating through a stochastically moving reciprocal optical path, would not distribute evenly on the Poincaré sphere, but tend to aggregate around the mirror state. This can be used to solve a long-standing problem that the detected interferograms of fiber-based OCT suffer from the fringe attenuation effect due to the polarization state mismatch between the light from the two interference arms which results in sensitivity deterioration and intensity fluctuation. This problem was conventionally solved by expensive polarization diverse detection. The polarization mirror states implies the existence of an optimal polarization state for the reference light. If the reference light is aligned with the polarization mirror state ...