Hybrid integration towards single-chip Brillouin devices

Abstract

On-chip RF signal processing and generation via the stimulated Brillouin scattering (SBS) process have become significant research topics in the past few years due to their unmatched capabilities. Having access to gigahertz repetition rates can enable applications such as RF filters, true time delay (TTD), optoelectronic oscillator (OEO), and optical frequency comb (OFC). Chalcogenide glass, and in particularly Arsenic Trisulfide, has become a popular optical material due to its high optical nonlinearity, near ideal acoustic properties for SBS applications. However, monolithic integration cannot be realised using chalcogenide glass due to its high propagation loss, fibre-to-chip coupling loss, and facet reflection. In this thesis, a hybrid waveguide solution will be proposed by leveraging the vertical taper technology via shadow mask deposition. By overlaying the Brillouin active waveguide on a low loss versatile germanosilicate platform, a variety of linear functionalities such as ring resonators, Bragg gratings and tolerant couplers can be incorporated in a fully integrated Brillouin based device. The optimisation process for flame hydrolysis deposition of the 3% index contrast germanosilicate and its facet machining have been rigorously studied in this work. This has led to an experimental propagation loss and fibre-to-chip coupling loss of <0.1 dB/cm and 0.2 dB/facet when coupled with a Nufern UHNA-3 fibre. Further, whilst the 0.7 nm/W Brillouin gain coefficient obtained in the proposed hybrid arsenic trisulfide - germanosilicate waveguide is identical to the previous monolithic arsenic trisulfide counterpart, the hybrid structure offers an extra 20 dB reduction in the pump back reflection, making it so suitable to be implemented in the backward SBS applications.