Source Development and Dispersion Measurements in Optical Coherence Tomography

Abstract

OPTICAL coherence tomography (OCT) is a non-invasive imaging modality, which provides mm-deep cross sectional and volumetric images in real-time at micrometer resolution. Additional to its promising imaging capabilities it can provide label-free detection of flow in semi-transparent and translucent samples. OCT has gained great popularity in recent years due to enhanced laser and imaging technologies, offering unprecedented long range imaging at high speed. These technologies, however, are restricted to laboratories having lithography capabilities to manufacturer laser gain chips. Laboratories without the capability to manufacture laser gain chips still depend on the implementation of OCT systems having either long imaging range at slow speeds or high speed with decreased imaging range. Imaging range in swept source OCT (SS-OCT), one of the OCT configurations, is coupled with the coherence length of the laser. This thesis describes novel laser design configurations to close the gap between the imaging range and the A-scan speed of the laser while maintaining a cost effective approach for the design. The configuration used in the laser development is a modified laser cavity, based on the well known Littman-Metcalf design cavity. Lasers in the range of 1 m to 1.7 m were built and tested. These lasers were subsequently used for label-free detection and characterisation of tissue types in ocular media. Current tissue detection for optical imaging modalities are mostly based on staining of the sample. Label-free detection can be accomplished through material specific coefficients such as Young’s modulus (E) or the dispersion coefficient ( 2). In this dissertation label-free detection of biological and non-biological materials was accomplished by using the differential walk-off, induced by the refractive index difference of two wavelength regions travelling through a medium. The differential walk-off can be experimentally measured and used to calculate the dispersion coefficient 2, which is specific to different materials. The novelty in this technique is the use of dispersion for detection which is normally a non-favourable effect in optical imaging modalities, due to its degrading nature in image quality.