Spectral-domain optical coherence tomography on a silicon chip

Optical coherence tomography (OCT) is a non-invasive optical technique for high-resolution cross-sectional imaging of specimens, with many applications in clinical medicine and industry (e.g. materials testing, quality assurance, and process control). Current state-of-the-art OCT systems operate in the frequency domain, using either a broad-band light source and a spectrometer, known as ‘spectral-domain OCT’ (SD-OCT), or a rapidly tunable laser, known as ‘swept-source OCT’ (SS-OCT). Both systems contain a multitude of fiber and free-space optical components which make these instruments costly and bulky. The size and cost of an OCT system can be decreased significantly by the use of integrated optics. A suitable fabrication technology and optimum design may allow one to fabricate extremely compact, low-cost, and rugged OCT systems. The main goal of this PhD project is miniaturization of an SD-OCT system by integrating its spectrometer and interferometer parts on a silicon chip. For this purpose and arrayed-waveguide grating (AWG) spectrometer and a Michelson interferometer (MI) comprising wavelength-insensitive 3-dB couplers were designed, fabricated, and characterized. Although integration of a spectrometer on a chip is challenging, AWGs present a well-established way towards miniaturization. Besides their extensive usage in telecommunication for (de)multiplexing, AWGs are also ideally suited for applications such as OCT and spectroscopy, with their high spectral resolution, small form factor, large bandwidth, and low insertion loss. In addition to their advantages listed above, AWGs are cost-effective, which makes them favorable for integration with SD-OCT systems. Wavelength-insensitive 3-dB couplers can be realized by either cascading two conventional couplers in a Mach-Zehnder configuration with a relative phase shift of 2?/3 introduced between them (i.e. balanced coupler) or using two adiabatically tapered asynchronous waveguides (i.e. non-uniform adiabatic coupler). Such couplers can be designed to yield a maximally flat response with respect to deviations in wavelength, polarization, or uniform fabrication over a broad spectral range, with no excess loss. Therefore, these couplers are very good candidates for application in MIs. In the first chapter of this thesis an overview is given of OCT systems. In chapter 2, the background, design, fabrication, and characterization of AWG spectrometers and their application in OCT imaging are discussed. In chapter 3, integrated MIs and wavelength-insensitive 3-dB couplers are presented; here, two different coupler designs (non-uniform adiabatic and balanced couplers) are analyzed in detail. The OCT measurements at 800 nm and 1300 nm are presented in chapter 4. Results of depth-range enhancement and polarization effect on signal roll-off are presented in chapter 5. In addition, an integrated field-flattening lens design and its characterization are discussed as a part of chapter 5 as well. In chapter 6, conclusions and outlook, based on the results presented in this thesis, are given.