UBC Theses and Dissertations
Silicon photonic sensors based on low-cost optical sources Dias, Leanne
Silicon photonic (SiPh) sensors hold tremendous potential for the advancement of global healthcare. Leveraging mature complementary metal-oxide semiconductor (CMOS) foundry processes, hundreds of SiPh sensors can be integrated into tiny devices, enabling the detection of multiple pathogens, and eliminating the need for expensive in-lab chemical processing. While the performance of SiPh sensors is similar to clinical standards, the implementation costs remain quite high. To realize the potential that SiPh sensor systems hold for global healthcare, their overall cost must be reduced. SiPh sensors typically rely on high-resolution tunable lasers, which remain an expensive off-chip component. This thesis first summarizes alternative optical sources that are used for SiPh sensors. Fixed-wavelength lasers are a low-cost alternative, and benefit from relative ease of coupling to chip. Unfortunately, the corresponding sensor designs are very sensitive to noise. Broadband optical sources are another lower-cost alternative source; however, their use often requires expensive detection equipment. Despite their drawbacks, implementing these alternative optical sources could significantly reduce the overall cost of a SiPh system. Three low-cost SiPh architectures are presented in this thesis: two that use a broadband source, and one that uses a fixed-wavelength laser. The broadband SiPh architectures use a sensor-tracker system, where one component, a microring resonator (MRR) or a Mach-Zehnder interferometer (MZI), acts as a sensor and a second component acts as a tracker (by electrically tracking wavelength shifts). Since wavelength shifts from the sensor can be read as electrical power shifts in the tracker, this system eliminates the need for expensive detection equipment. Sensitivity values of 78.9 nm/RIU (refractive index unit) and 218.5 nm/RIU were obtained, with system limits of detection of 3.4x10^⁻⁴ RIU and 7.7 x10^⁻⁴ RIU for the MRR and MZI designs, respectively. In the fixed-wavelength system, a heater-detector tuning element is placed in the MRR loop. This similarly enables electrical tracking of wavelength shifts, thus reducing the noise sensitivity commonly found in fixed-wavelength systems. Simulation results report sensitivities up to 76 nm/RIU, with calculated intrinsic limits of detection down to 3.8 x10^⁻⁴ RIU. The results obtained demonstrates that high-resolution tunable lasers are not required to achieve high sensor performance.
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