Advanced optical signal processing
Optical devices embedded in fibres have become the mainstays of fibre-optic communication systems, providing important photonic functions. Fibre Bragg gratings (FBGs) are an in-fibre implementation of distributed Bragg reflectors that offer an inexpensive all-fibre solution with low insertion loss. By careful selection of the desired optical response and grating design, it is possible to apply transmissive FBGs in signal processing applications. The EU-funded project 'In-fibre optical cavities structures for telecommunications and sensing' (IFOCS) aims to shed further insight into complex photonic signal processing. Researchers will focus on multi-cavity resonant optical structures.IFOCS is based on a specially designed chirped FBG where the reflectors are spatially distributed. Project partners studied and developed an analytical method for designing high-order optical cavity structures in different forms (linear or ring resonators and 1D grating structures). They also studied FBG-based Gires-Tournois etalon structures that are all-pass optical cavity structures. These are highly suitable in wavelength division multiplexing (WDM) signal processing, as the phase shift largely depends on the wavelength of the light.A novel approach to pulse shaping using a phase-modulated FBG in transmission was designed that is energy efficient and less prone to grating fabrication errors. The project team showed that phase-modulated FBGs provide transmission responses suitable for pulse shaping applications. This offers important technological benefits as the coupling strength remains uniform in the grating and no additional elements such as the optical circulator are required. This design also enables the introduction of a novel kind of a photonics processor, namely distributed interferometer.The developed design techniques in IFOCS enable transmissive distributed Bragg reflectors to be used in complex linear photonic processors, for applications in optical communications, fibre sensing or microwave photonics.
