INVESTIGATION OF THERMO-OPTIC EFFECTS IN SILICON MICRORING RESONATORS FOR SENSING AND INTERROGATION
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Integrated photonics technology has great potential for enhancing the performance and reducing the volume and cost of optical sensing systems. Among many integrated photonic structures, silicon microring resonators have received much attention for both sensing and interrogation. Particularly, the high quality-factor of the microring resonators and the large thermo-optic coefficient and high thermal conductivity of silicon make them attractive for temperature sensing and thermally-tunable-filter-based interrogation. In this dissertation work, the thermo-optic effects in silicon microring resonators is studied and used in the silicon-ring-resonator-based temperature sensing and interrogation. The first objective of this dissertation work is to develop a highly sensitive photonic temperature sensor, which can be potentially used for achieving portable, compact temperature sensing systems employing a low-resolution on-chip spectrometer. However, the sensitivity of conventional silicon-ring-resonator-based temperature sensors is relatively low (less than ~80 pm/°C). These sensors often require the use of a bulky and expensive fine-resolution interrogator for high resolution temperature monitoring, since the sensor resolution is determined by the sensitivity. In this work, a novel photonic temperature sensor based on cascaded-ring-resonators with the Vernier effect is developed to simultaneously enhance the sensitivity and sensing range. With a proof-of-concept device, sensitivity enhancement of 6.3 times and sensing range enhancement of 5.3 times are demonstrated. On-chip optical interrogators employing a silicon-ring-resonator-based thermally tunable filter (SRRTF) offer a promising solution for realizing portable, compact optical sensing systems. However, the slow interrogation speed of conventional SRRTF-based interrogators (less than a few Hz) has hindered their application for dynamic sensing. The second objective of this dissertation work is to develop a high-speed SRRTF-based interrogator, which can be used to interrogate optical sensors monitoring dynamic parameters. In this work, an SRRTF-based system utilizing the nonlinear transient thermal response of the SRRTF is developed for the speed enhancement. High speed interrogation (100 kHz of interrogation speed) of a fiber Bragg grating (FBG) sensor is successfully demonstrated with this system. The third objective of the dissertation work is to further enhance the tuning speed and range of the previously developed SRRTF and to use it for simultaneous interrogation of multiplexed FBG sensors. Performance of SRRTF-based interrogators is primarily determined by thermal and optical characteristics of the SRRTF. However, conventional SRRTF structures with a metallic heater on the top oxide cladding have limitations on interrogation speed and range. In this dissertation work, a novel SRRTF employing an interior-ridge-ring resonator and thermal through-cladding-vias is developed, which can realize enhanced tuning speed and range. With this SRRTF, interrogation of multiplexed FBG sensors at 125 kHz speed is demonstrated.