Performance Improvement and Birefringence Investigation of Spectral Domain Optical Coherence Tomography Using a Modified Arrayed Waveguide Grating

An arrayed-waveguide grating (AWG) with its high accuracy and stability is a powerful tool for spectral analysis. We investigate its potential for spectral-domain optical coherence tomography (SD-OCT). A silicon-oxynitride-based AWG spectrometer for the 800 nm wavelength range is designed for on-chip SD-OCT systems. By removing the output waveguides of the AWG, the depth range is significantly enhanced. In addition, the effect of polarization dependency of the AWG on sensitivity roll-off is investigated and for partial polarization, a beat effect is observed in the depth ranging measurements, which leads to signal fading at specific depths

High-performance spectral-domain optical low-coherence reflectometry with an integrated arrayed-waveguide grating

The effect of discrete output channels and polarization dependency of an arrayed-waveguide-grating (AWG) spectrometer on spectral-domain optical low-coherence reflectometry performance is investigated

Polarization-independent optical low-coherence reflectometry with a non-birefringent arrayed-waveguide grating

The polarization dependency of an arrayed-waveguide grating (AWG) in an optical low-coherence reflectometry system is investigated. For mixed polarization, signal fading is observed at specific depths. This fading is eliminated by using a nonbirefringent AWG

SiON integrated optics elliptic couplers for Fizeau-based Optical Coherence Tomography

The use of integrated optics for Optical Coherence Tomography (OCT) can offer significant cost reductions and new applications. We designed, fabricated, and characterized Silicon oxynitride (SiON) elliptic couplers that are used to focus light from a chip into the off-chip environment. Fizeau-based OCT measurements are performed and compared to calculations

Spectral-domain optical coherence tomography with an arrayed waveguide grating spectrometer

We designed and fabricated an arrayed waveguide grating (AWG) with 2.1cmx2.6cm footprint. Using the AWG as spectrometer in a spectral-domain optical coherence tomography (OCT) set-up we demonstrate OCT imaging up to the maximum depth of 1 mm with 19 µm spatial resolution in air and in a multi-layered phantom

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

Polarization-independent enhanced-resolution arrayed-waveguide grating used in spectral-domain optical low-coherence reflectometry

The performance of an arrayed-waveguide grating (AWG) as an integrated spectrometer in spectral-domain optical low-coherence reflectometry (SD-OLCR) is significantly improved. By removing the output waveguides of the AWG, the depth range is enhanced from 1 to 3.3 mm at 800 nm and 4.6 mm at 1300 nm. Periodic signal fading, that was previously observed in the sensitivity roll-off curve in depth ranging measurements, is shown to be evoked by beat-frequency generation between the two polarizations of partially polarized signal light in a birefringent AWG. By carefully controlling the polarization state-of-light, the signal fading is eliminated. As a permanent solution to this problem, a polarization-independent AWG is demonstrated, which can reduce the size and cost of OCLR and optical coherence tomography systems further by eliminating the components for polarization control

Towards Raman spectroscopy on a microchip

In numerous applications, Raman spectroscopy is utilized to monitor unique Raman scattering in order to identify specific molecules or structures. We aim at developing a low-cost, compact, hand-held apparatus for Raman spectroscopy of the skin and tooth. A critical function of this system is spectral separation of the Raman-scattered signals. In our case, the core element of such a device is an arrayed-waveguide grating (AWG). Two silicon oxynitride single-mode channel waveguide geometries are designed for Raman spectroscopy of the skin and tooth. For tooth measurements we design the AWG to have a central wavelength of 901 nm and a free spectral range of ~22 nm, with a resolution of 0.2 nm. The skin application requires the AWG to be polarization insensitive, to have a central wavelength 881 nm, and to have a minimum resolution of 5.5 nm. For the Raman measurements light from a Ti:Sapphire laser at 785 nm, after passing through a laser-line filter, is reflected from a dichroic mirror and focused onto a sample with a microscope objective. The backscattered light from the sample is collected by the same optics; the Rayleigh component is again reflected by the dichroic mirror, which transmits the Raman wavelengths. An edge filter removes the residual light at the laser wavelength, and the light is focused into the input channel of the integrated spectrometer using a x50 microscope objective. The output channels of the AWG are imaged onto an electron-multiplying CCD through a camera lens. The set-up is tested by measuring the Raman spectra of silicon and cyclohexane. The preliminary dental measurements show excellent agreement between the spectra of healthy and carious tooth enamel measured with our integrated device and spectra recorded using a conventional Raman spectrometer

Integrated approach to laser delivery and confocal signal detection

We present an on-chip arrayed waveguide grating (AWG) sensor based on the confocal arrangement of two AWGs, one acting as focusing illuminator and one as signal collector. The chip can be close to, or in direct contact with, a sample, e.g., biological tissue, without the need of external optics. The collection efficiency of our device can be more than 1 order of magnitude higher than that of a standard AWG, in which light is collected by one input channel. Experimental results on the collection efficiency and volume are presented, together with a demonstration of multiwavelength imaging

Integrated spectrometers for spectral-domain optical coherence tomography

We present experimental results of a spectral-domain optical coherence tomography system based on integrated spectrometers. Spectrometers are realized in silicon-oxynitride waveguides and consist of arrayed waveguide gratings for 800 nm and 1300 nm