Development of blue spectral sensor for circadian irregularity detection using standard silicon photonic technology

Abstract

Circadian rhythm irregularity is a widespread condition affecting people from all age groups.Excessive exposure to blue light is a primary factor that disrupts circadian rhythm. There- fore, developing a spectral sensor that can detect, filter and measure blue radiation from surroundings is quite essential. Existing literature have demonstrated progress on blue wavelength detectors at component level but a fully integrated system architecture with high resolution spectroscopic application has not been explored yet. Therefore, this thesis presents a fully integrated design approach using a standardized silicon photonic technology to detect, filter and measure blue exposure and present it as readable voltage outputs. The work discusses in-depth about each component of the sensor and validates their theoretical background through different simulation techniques. All of these components can be fab- ricated using GlobalFoundries 45 nm monolithic technology, shortly known as 45SPCLO. A silicon nitride based photonic front end is demonstrated with grating couplers showing coupling efficiency of around 27%, tapered waveguides and optical power combining networks with negligible loss (1%∼10%) and an optimized multimodal interference (MMI) coupler demonstrating 80% coupling efficiency. The thesis strongly focuses on three different junc- tion architectures for a silicon based photodiode and their comparative performances. These architectures are referred to as single, zigzag and interweaved junctions. For the interweaved junction architecture, a maximum simulated responsivity of 135 A/W is demonstrated under breakdown conditions and it outperforms the other two junction architectures in linear region as well. All the photodiode designs exhibit low junction capacitances in femtofarad range. The small junction capacitance helps the photodetectors to interface smoothly to a low noise trans-impedance amplifier (TIA). The TIA architecture shown in this work demonstrates an input referred noise current of 1.4 nA, a trans-impedance gain of 140 dB and a bandwidth of 75 MHz when coupled to an output buffer amplifier. The noise current is significantly lower than simulated photocurrent values and the bandwidth is sufficient for a spectral biosensor which performs mostly under DC conditions effectively.