PhD defence of Bruno Taglietti – Wavelength selective devices in SOI and silicon nitride
Abstract
Wavelength-selective photonic devices are important in several fields. Fabrication variations are often responsible for the decreased experimental performance of photonic devices compared to simulations, and identifying the variations that have the most impact can help us build better photonic devices. Building photonic structures using SWG structures offers versatility and flexibility, such as in controlling dispersion and birefringence. Therefore, this thesis focuses on (1) using SWG/metamaterials for developing waveguide devices, including BG and a WDM diplexer, and (2) characterizing BG in order to understand more about the fabrication variations imposed in the structures.
SiN is particularly prone to fabrication variations which can significantly impact the characterized response of devices. We have updated the TMM simulation model to include several of these variations, such as waveguide sidewall angle, cross-section dimension variations and longitudinal shrinkage. We have found waveguide dimension variations that go against the expectation of material shrinkage of SiN, suggesting possible inaccuracy in the refractive index curve. The inclusion of longitudinal shrinkage as a simulation parameter can help to accurately simulate the bandwidth by modulating the grating strength. The characterized reflection curves show higher bandwidth for the TM mode than TE, which our modified simulation model could not reproduce. This suggests that the fabrication variations make the actual fabricated structures deviate more from the ideal designed structure, resulting in a greater difference between the simulated and measured responses.
We have designed both uniform and random versions of Sampled SWG-WBG. The uniform sampled SWG-WBG shows three reflection bands, and their wavelength spacing shows very good accuracy compared with characterization. The device can be used for spectral slicing of broadband sources and for building multi-wavelength lasers. The feasibility of random WBG using SWG is also demonstrated. We have compared the simulated results with, without randomization, evidencing its impact, and with characterized results. The correlation between characterized reflection curves of different versions of the device can be as low as 27%, suggesting effective randomization.
We have also designed an SWG-based WDM diplexer for the 1310 nm and 1550 nm channels. Our device shows a measured extinction ratio of more than 20 dB for both ports and a good wavelength range of operation. We were able to mitigate fabrication variations by varying design parameters. The device shows a comparable footprint and performance with the state-of-the-art, and the use of SWG waveguides as a building block may offer increased flexibility in future versions.
These devices can find several applications in areas such as optical communications and MWP. The SWG WDM diplexer. They also illustrate the versatility and flexibility provided by SWG structures. Innovative approaches using SWG structures can stem from our development, and our fabrication variation analyses can also be used for problem mitigation in future devices.