Femtosecond laser inscribed point-by-point Bragg gratings in few-mode optical fibre

Femtosecond laser inscribed point-by-point (PbP) gratings are a distinct class of fibre Bragg gratings and have many unique characteristics compared to UV laser inscribed gratings. For PbP gratings, each individual grating site is inscribed one at a time, with tightly focused femtosecond laser pulses, resulting in a unique morphology. This morphology combined with the flexibility of inscribing individual modifications anywhere inside the optical fibre provides practical ways to inscribe complex gratings, whose properties are determined by the interaction of the grating sites with the spatial profile of the fibre modes. The work presented in this thesis explores these areas in detail and provides new insights regarding the fundamental workings of PbP gratings. By employing several different material characterisation techniques, this thesis provides new insights into the grating’s morphological changes. This includes directly measured refractive index profiles, which are of significant practical importance in engineering the grating properties. The PbP gratings interaction with the fibre modes determines the spectral characteristics of the grating. With a combined theoretical and experimental approach, we discuss this in detail and draw contrasts between two different theoretical approaches, namely the coupled mode theory (CMT) and the photonic bandgap (PBG) approach. For strong PbP gratings, CMT starts to fail but the PBG approach provides a clear and accurate picture of the fibre mode interaction with the grating sites. Finally, building on this knowledge we demonstrate ways to control mode coupling in few-mode optical fibres and discuss how the different mode coupling can be tuned in few-mode fibre Bragg grating by changing the spatial position of the grating sites inside the fibre core. These results provide a new understanding of the fundamental properties of PbP gratings and open a path to new ways of controlling mode coupling in few-mode fibre Bragg gratings.