Towards A Monolithically Integrated Swept-source Optical Coherence Tomography System In1.7μm Wayelength Region

The presented research work in this thesis focuses on the development of integrated photonic devices which are used in integrated swept-source optical coherence tomography (SS-OCT) system operating at the1.6to1.8μ m wavelength region. Since the Rayleigh scattering in biological tissue can be reduced by using longer wavelength compared to more commonly used regions at shorter wavelengths, the wavelength region around1.7μ m is chosen as the operating wavelength band. Thus an improvement in the imaging depth is expected by using this long wavelength. The use of indium phosphide (InP)-based photonic integration technology makes the monolithically integrated SS-OCT system in the1.6to1.8μ m wavelength region very possible and promising.In order to obtain light around1.7μ m, the research and analysis of novel active material which can offer light generation, amplification and absorption around this long wavelength is desired. The SS-OCT requires the tuning range for the tunable laser and spectral responsivity for the photodetector to be wide enough in order to achieve sufficient image resolution along the depth. The demand for real-time imaging also sets up the requirement on the repetition rate of the laser wavelength sweep over its entire tuning range as well as the electrical bandwidth of the photodetector to support the laser.In this thesis, a tunable laser based on five-layer quantum dot (QD) gain material was firstly used in a free-space OCT setup for demonstrative OCT imaging experiments on glass dish and Scotch" tape. Successful OCT images on glass dish and Scotch tape have been obtained. During the OCT imaging experiments, several issues have came up concerning the performance of the QD laser and the commercial photodetectors. The QD laser used in the experiments had problems with the optical power, tuning range and tuning speed. The commercial photodetectors also showed serious limitations in noise level, sensitivity and electrical bandwidth. These issues directly motivated the following work presented in this thesis. During these OCT imaging experiments several improvements on the calibration routine of the intra-cavity tunable filters in the laser have been proposed and experimentally demonstrated.Several approaches to the improvement of laser performance have been studied. First, the influence of a different gain material with higher modal gain value to the performance of the laser has been studied. A tunable laser with amplifiers based on four-layer strained quantum well (QW) active material has been characterized and compared to the five-layer QD laser. The layout of the QW laser was identical to that of the QD laser and was fabricated with a fully compatible layerstack. The measurement results have shown that the much higher modal gain in the QW amplifiers could significantly reduce the threshold current of the QW laser compared to the QD laser with much lower modal gain. The huge improvement on the wavelength switch time in the QW laser has also been demonstrated compared to the slow switch time in the five-layer QD laser. The measured tuning speed in the QW laser has already approached the limitation in the control electronics. On the other hand the tuning range of the QW laser was observed to be much narrower due to the relatively narrow gain bandwidth in the QW amplifiers compared to the five-layer QD laser. Overall this part of work has provided a clear idea of how the characteristics of the active material influence the performance of the tunable laser.Next, an improved design of the QD tunable laser has been proposed, fabricated and preliminarily characterized. The improvements include the redesign of the laser layout to increase the output power, the redesign of the optical feedback scheme to enhance unidirectional lasing and the improved new design of the MMI-tree filter to extend the tuning range. The improved tuning range of the improved laser design has been analyzed and demonstrated using a segmented ring laser model. The MMI-tree filter also has been experimentally demonstrated. However, the lasing of the improved laser chip was not successful. We attributed this to the low modal gain in the QD amplifiers and high passive loss in the passive waveguides which was caused by the bad quality of the wafer growth.Then the QD waveguide photodetectors for the long wavelength OCT application are studied in this thesis. The photodetectors use the identical five-layer QD active material as has been used in the QD laser. The layerstack and waveguide structure of the photodetectors are also fully compatible to the active-passive integration technology for the QD laser. Thus they can be integrated with the QD laser on a single chip. The light absorption at1.7μm wavelength region provided by the QDs is analyzed both experimentally and theoretically. The characterization of the photodetectors has shown low dark current, flat spectral response and sufficient electrical bandwidth. A modified rate equation (RE) model has been used to simulate the performance of the QD photodetector and to understand and explain the mechanisms of light absorption in the QDs. An equivalent electrical circuit model has also been applied to figure out the limitation (metal pad capacitance) of the electrical bandwidth.Finally in this thesis a high-gain single-layer InAs QD on InAs thin QW active material in the1.6to1.8μm wavelength range has been studied. The amplifiers using this new QD material have shown significantly higher modal gain than the modal gain in the previous QD amplifiers, while still maintaining the same wide gain bandwidth as the previous QD material. This type of new QD material has been demonstrated to be a very promising candidate to be used in the next generation QD tunable lasers or QD waveguide photodetectors for long-wavelength OCT applications. Then an improved RE model has been optimized and applied on the new high gain QD amplifiers to simulate the gain behavior of this material as well as the carrier dynamics in the new QDs.The overall work in this thesis has revealed the very high potential of integrating tunable lasers, photodetectors and any other passive photonic waveguide components in a single chip for the SS-OCT application. The presented results have shown a significant progress towards the monolithically integrated SS-OCT system operating at the1.7μ m wavelength region.