Investigation into Smart Multifunctional Optical System-On-A-Chip Sensor Platform and Its Applications in Optical Wireless Sensor Networks
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Wireless sensor networks (WSNs) have been widely used in various applications to acquire distributed information through cooperative efforts of sensor nodes. Most of the sensor nodes used in WSNs are based on mechanical or electrical sensing mechanisms, which are susceptible to electromagnetic interference (EMI) and can hardly be used in harsh environments. Although these disadvantages of conventional sensor nodes can be overcome by employing optical sensing methods, traditional optical systems are usually bulky and expensive, which can hardly be implemented in WSNs. Recently, the emerging technologies of silicon photonics and photonic crystal promise a solution of integrating a complete optical system through a complementary metal-oxide-semiconductor (CMOS) process. However, such an integration still remains a challenge. The overall objective of this dissertation work is to develop a smart multifunctional optical system-on-a-chip (SOC) sensor platform capable of both phase modulation and wavelength tuningfor heterogeneous sensing, and implement this platform in a sensor node to achieve an optical WSN for various applications, including those in harsh environments. The contributions of this dissertation work are summarized as follows. i)A smart multifunctional optical SOC sensor platform for heterogeneous sensing has beendeveloped for the first time. This platform can be used to perform phase modulation and demodulation in a low coherence interferometric configuration or wavelength tuning in a spectrum sensing configuration.The multifunctional optical sensor platform is developed through hybrid integration of a light source, an optical modulator, and multiple photodetectors. As the key component of the SOC platform, two types of modulators, namely, the opto-mechanical and electro-optical modulators, are investigated. For the first time, interrogating different types of heterogeneous sensors, including various Fabry-Perot (FP) sensors and fiber Bragg grating (FBG) sensors, with a single SOC sensor platform, is demonstrated. ii)Enhanced understanding of the principles of the multifunctional optical platform withanopto-mechanical modulator has been achieved.As a representative of opto-mechanical modulators, a microelectromechanical systems (MEMS) based FP tunable filter is thoroughly investigated through mechanical and optical modeling. The FP tunable filter is studied for both phase modulation and wavelength tuning, and design guidelines are developed based on the modeling and parametric studies. It is found that the MEMS tunable filter can achieve a large modulation depth, but it suffers from a trade-off between modulation depth and speed. iii) A novel silicon electro-optical modulator based on microring structures for optical phase modulation and wavelength tuning has been designed. To overcome the limitations of the opto-mechanical modulators including low modulation speed and mechanical instability, a CMOS compatible high speed electro-optical silicon modulator is designed, which combines microring and photonic crystal structures for phase modulation in interferometric sensors and makes use of two cascaded microrings for wavelength tuning in sensors that require spectrum domain signal processing. iv)A novel optical SOC WSN node has been developed. The optical SOC sensor platform and the associated electric circuit are integrated with a conventional WSN module to achieve an optical WSN node, enabling optical WSNs for various applications. v) A novel cross-axial dual-cavity FP sensor has been developed for simultaneous pressure and temperature sensing.Across-axial sensor is useful in measuring static pressures without picking up dynamic pressures in the presence of surface flows. The dual-cavity sensing structure is used for both temperature and pressure measurements without the need for another temperature sensor for temperature drift compensation. This sensor can be used in moderate to high temperature environments, which demonstrates the potential of using the optical WSN sensor node in a harsh environment.