Applications of Fourier Domain Mode Locked lasers for optical coherence tomography imaging

Abstract

Optical coherence tomography (OCT) is a micrometer-resolution imaging technique that produces cross-sectional images of sample microstructure by measuring the amplitude and echo time delay of backscattered light. OCT imaging is performed using low-coherence interferometry, typically with a fiber optic Michelson interferometer. OCT imaging has recently been performed by measuring the spectrum of the interference signal in the Fourier domain. In "swept source OCT" implementations, the interference spectra are generated with a wavelength-swept laser and photodetector. Axial image lines are obtained via Fourier transformation of the spectra. Fourier domain techniques have extended OCT imaging speeds from several thousand to hundreds of thousands of axial lines per second, enabling in vivo three-dimensional (3D) OCT. Development of the Fourier Domain Mode Locked (FDML) laser has significantly improved the imaging performance of swept source OCT by providing an unparalleled combination of high sweep rates, large tuning ranges, narrow instantaneous linewidths, and low phase noise. This thesis develops a number of advanced OCT imaging applications using FDML laser technology. Ultrahigh-speed sub-nanometer phase profilometry is performed by measuring the phase of the OCT interference signal, taking advantage of the inherent phase stability of FDML lasers. Extending this concept, phase-sensitive OCT is used to detect gold nanoshell contrast agents with extremely high signal-to-noise ratios by inducing photothermal phase modulations in the sample.

(cont.) Working in collaboration with industrial partners, a 3D-OCT imaging system incorporating an FDML laser is constructed for clinical research in gastroenterology. Spiral-scanning imaging catheters are developed for use in the human esophagus and colon, enabling high-density 3D-OCT endomicroscopy of the gastrointestinal tract. Finally, clinical pilot studies are conducted in collaboration with medical partners to demonstrate the utility of 3D-OCT endomicroscopy for pathology detection, treatment planning, and follow-up assessment. The convergence of 3D spatial resolution, imaging speed, field of view, and minimally invasive access enabled by 3D-OCT are unmatched by most other biomedical imaging techniques. Though still early on in its development, 3D-OCT may have a profound impact on human healthcare and industrial inspection by enabling visualization and quantification of 3D sample microstructure in situ and in real time.