Adaptive Optoelectronic Systems: From Bio- Sensing to Free-Space Optical Communication

Citation

Aghlmand, Fatemeh (2024) Adaptive Optoelectronic Systems: From Bio- Sensing to Free-Space Optical Communication. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/hj19-7516. https://resolver.caltech.edu/CaltechTHESIS:08282023-193415593

Abstract

Portable and point-of-care medical devices are becoming an essential part of today’s medical technology. An affordable personal device that can diagnose and monitor a medical condition in real-time will improve the patient’s life quality in many ways. Additionally, by autonomously providing the suitable treatment, a universal healthcare device can be accessible to most of the population at a low cost. Despite considerable efforts and great outcomes, most of the prior arts in realizing these devices have limitations that hinder their widespread use in portable applications. On the other hand, comprehensive environmental sensing has drawn great attention in the last few years. Monitoring the quality of water, soil, air, and waste is of utmost importance to study their effect on human life and also to recognize the consequence of human actions on the planet.

The most important factors in developing a compact and portable device for medical and environmental applications are their integration level, ease of use with biomarkers, and reliability of the results. Detecting a specific chemical in the biology world relies on a biochemical reaction with a transducer that can convert the resulting signal into a measurable signal in various modalities, such as electrical, magnetic, or optical. Hence, the biosensing device is often a multidisciplinary apparatus that is not readily integrable due to the need for miniaturizing otherwise bulky optical or magnetic components. The key requirement in device miniaturization, though, is to use standard technologies to avoid extra cost and processing time for the device’s mass production. The path towards achieving such a device needs revisiting the existing solutions and the capabilities of the powerful yet affordable CMOS technologies to seamlessly integrate various device components, namely electronics, biology, and optics/magnetics. This dissertation provides an overview of integrated biosensors and presents novel designs in optics and electronics to implement a fully integrated and miniaturized device for medical and environmental applications.

Fluorescence sensing is one of the most reliable and widespread detection methods with well- established tools in synthetic biology. Specifically, bacterial-based fluorescence sensors offer unsurpassed advantages to labeled detection since bacterial cells, when engineered, can respond to various elements in their surroundings at a low cost and quite efficiently. The use of live bacterial cells is also of great importance in establishing the bidirectional link with the CMOS device. By monitoring the dynamics of the cells’ growth and their protein expression, a desired biology response can be initiated upon receiving the stimulating signal from the device. The conventional methods in fluorescence sensing involve an elaborate setup with many external optical components unsuitable for portable and in vivo applications. Hence, integrating silicon chips and live bacterial biosensors in a miniaturized "Silicon-Cell" system can enable a wide range of applications for both sensing and remediation. Such integrated systems need on-chip optical filtering in the wavelength range compatible with fluorescent proteins, which are widely used signal reporters for bacterial biosensors.

In the first part of this dissertation, we introduce a fully integrated fluorescence sensor in 65nm standard CMOS process comprising on-chip bandpass optical filters, photodiodes, and processing circuitry. The metal/dielectric layers in CMOS are employed to implement low- loss cavity-type optical filters, achieving a bandpass response at 600/700nm range suitable to work with fluorescent proteins. The sensitivity of the sensor is further improved in the electrical domain by using a C-TIA with variable switched capacitor gain, a voltage- controlled current source (VCCS), and feedback-controlled low-leakage switches, resulting in a minimum measured current of 1.05fA with SNR >18dB. The sensor can measure the statics/dynamics of the fluorescence signal as well as the growth of living E. coli bacterial cells. Using a differential design and layout, the sensor can distinguish two biochemical signals by measuring two fluorescent proteins encoded in a single bacterial strain. Furthermore, a proof of concept is demonstrated to establish bidirectional communication between living cells and the CMOS chip, using a fluorescent protein regulated by an optogenetic control.

In the second part of this dissertation, we describe a fully integrated high-bandwidth optical receiver for RF-over-free-space optics (RoFSO). This work is motivated by the availability of a wide, unregulated bandwidth at the optical frequencies and the lower cost and setup time due to using atmosphere instead of fiber optics as the communication channel. Nonetheless, the atmospheric link poses serious challenges, including severe beam intensity and phase distortions. Here we present novel solutions at the system and circuit level to make the receiver adaptive and resilient to the mentioned distortions. The chip is designed and implemented in a 28nm CMOS process, and it is shown to achieve a measured gain of 58dB and bandwidth of 18GHz. The link performance is assessed by exposing the system to more than 26dB of optical loss, equivalent to 3.5km of free space distance under moderate visibility conditions. For a proof-of-concept demonstration, an 8Gbps non-coherent DPSK signal with an RF bandwidth of 10GHz is transmitted, resulting in a BER of 1 × 10⁻⁴ for a minimum received power of -30dBm and while consuming 19.2mW power at the receiver.