Brillouin Frequency Comb Generation in Chalcogenide Waveguides

Abstract

Compact optical frequency comb sources with gigahertz repetition rates are desirable for various important applications including arbitrary optical waveform generation, microwave synthesis, spectroscopy and advanced telecommunications. This thesis investigates the exploitation of the interplay of two distinct nonlinear optical effects for the generation of gigahertz repetition rate frequency combs: stimulated Brillouin scattering (SBS) and the optical Kerr-effect. This interplay can lead to the generation of Brillouin frequency combs (BFCs) with repetition rates that are equal to the acoustic resonance associated with SBS. This resonance frequency is about 8 GHz, making BFCs ideal for the advanced photonic applications of interest. In this thesis, we experimentally demonstrate BFCs with equally spaced comb modes that exhibit a stable and repeatable spectral phase. The BFCs are generated in chalcogenide fibre and in chalcogenide waveguides on photonic chips. Through theoretical and numerical investigations we show that, whilst SBS provides the high repetition rate of the combs, the Kerr-nonlinearity plays an important role in achieving equally spaced and phase-coherent spectral components. We also study the interplay of BFCs and photosensitivity via multiphoton absorption in chalcogenide fibres and photonic chips. We show that this interplay can be used to internally inscribe multiwavelength gratings that exhibit several stopbands that are spaced by the acoustic resonance frequency. We then use these gratings in an SBS configuration and demonstrate a significant enhancement of BFC generation by exploiting the slow light effects associated with the grating band edges. This body of work represents an advance in the understanding of BFCs. We study the physics behind phase-coherent BFC generation. The demonstration of chip-based BFC generation is a step towards an all integrated, gigahertz repetition rate, optical frequency comb source. We also demonstrate a novel and flexible method for enhancing chip-based BFC generation that can potentially be extended to other nonlinear effects.