Sandwiched Hybrid Waveguide Platform for Integrated Photonics Applications

Abstract

Guided wave optical technology, made possible by the invention of lasers and low-loss optical fibers, has revolutionized communication over the past century. With their high bandwidth capabilities, optical fibres have replaced copper cables for long-distance communication and are now extensively used for data flow within data centers.

Integrated optics, proposed in 1969, enables the integration of various optical components onto a single chip, allowing for compact and high-performance optical systems. This technology offers advantages such as miniaturization, improved stability, lower power consumption, and potential integration with other electronic and photonic components.

The waveguide is a crucial element in integrated optics, determining the behaviour of the optical components within the photonic circuit. Conventional waveguide structures have limitations due to their uniform refractive index profiles, constraining design possibilities. In the past few years, there have been proposals with non-uniform refractive index cores that have shown unique features forming the motivation of this work.

In this thesis, a hybrid waveguide platform with a layered structure has been proposed that aims to give design flexibility. The structure is composed of a high-index material (a-Si) sandwiched between two medium index (SiN) layers giving control over waveguide properties like effective index, confinement, birefringence and dispersion. The heart of this flexibility lies in controlling the ratio of a-Si and SiN, which has been studied in detail. It has been shown that light can be confined with preference in a different material through control over polarization leading to dramatically different properties.

The second part of the thesis discusses coupling problem. Grating couplers are designed and demonstrated to couple light into TE and TM modes in the hybrid waveguide. The experimental coupling efficiency for TE mode was found to be -3.27 dB per coupler, close to the optimized value. TM mode coupling efficiency is found at -8 dB per coupler, with a further scope of improvement. Lastly, the propagation loss measurements are done for both TE and TM modes.

The third part of this work is to demonstrate devices on a hybrid platform. Ring resonators are versatile devices with a wide variety of applications. Thus, ring resonators are demonstrated for both TE and TM modes with a high extinction ratio. The group index derived from the free-spectral range of the ring resonators confirmed the excited mode. Further confirmation of the excited mode is done with thermo-optic measurements. A unique observation that distinguished the hybrid platform from uniform core waveguides is polarization dependent thermal shift of the ring spectra. For TE mode the shift is closer to Si (69 pm/◦C) and for TM mode, it resembles SiN (23 pm/◦C).

The fourth part of this thesis demonstrates a polarization diversity scheme. A polarization rotator is designed and demonstrated with wavelength-selective behaviour. An FWHM of 14.8 nm is obtained experimentally that indicates its use as a filter. The design flexibility allowed by the hybrid platform is studied in detail to show narrow-band to wide bandwidth characteristics not reported till now. Finally, for the first time, a proof of concept demonstration of simultaneous CWDM and polarization rotation is done.

The last part is suggestive of two applications of the hybrid platform through simulation studies. The first of them is a hybrid platform for compact SiN circuits arising from low bend loss. The second one shows dispersion engineering flexibility and moderate to high non-linear parameters. The dispersion can be made with similar flatness as SiN but with 4 to 7 times larger non-linear parameters that can be useful for rich non-linear applications.

In summary, a novel waveguide platform with design versatility and interesting optical properties has been established in this work.