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Gratings in air-core photonic bandgap fibres for applications within communications, lasers and sensors

Final Report Summary - GRATING (Gratings in air-core photonic bandgap fibres for applications within communications, lasers and sensors)

The GRATING project, a project funded under the Marie Curie International Incoming Fellowship Scheme, aims to inscribe novel gratings, including fibre Bragg gratings (FBGs) and long period fibre gratings (LPFGs), in hollow-core photonic bandgap fibres (PBGFs) to develop a new generation communication and sensing devices, opening the door to a new class of grating-based devices in hollow-core PBGFs. Because almost 100% of the light propagates in the air holes of the PBF and not in the glass, hollow-core PBGFs represent an important platform for the development of novel grating-based devices, offering potentially enhanced properties for applications in telecommunications, lasers and sensing.

The objectives of the project as originally written were to:

* To develop theoretical models for FBGs and LPFGs in hollow-core PBGFs to investigate mode-coupling mechanisms in such gratings.
* To demonstrate novel techniques for inscribing FBGs and LPFGs in hollow-core PBGFs. Since the required index modulation cannot be directly induced in the air core of PBGF, completely different approaches will be demonstrated to inscribe gratings in such fibres.
* To investigate communication applications of the gratings with the aim of developing novel in-fibre communication devices such as tuneable filters, polarisers, optical switches, and dispersion compensators.
* To investigate sensing applications of the gratings with the aim of developing smart sensing elements, especially gas, biochemical, and biophotonic sensors, with high sensitivity and short response time.
* To fill thermo- or electro-optic polymers or other advanced materials into air holes/core of PBGF before or after inscribing FBG/LPFG to develop further sensing and communication devices with the aim of demonstrating industrial applications.

During initial phase the following results were achieved:

1. A promising high performance LPFG writing system using a CO2 laser and the point-to-point writing technique with a user friendly interface was developed.
2. High-quality LPFGs were successfully written in various types of optical fibre using the CO2 based system. The quality of gratings written was comparable with the current state-of-the-art for point writing systems thereby validating the basic approach used.
3. Periodic tapering and grating writing in soft glass fibres using the CO2 laser system was investigated.
4. We developed a novel intensity-measurement bend sensor based on a periodically-tapered all-solid soft glass fibre which exhibits a high sensitivity and a low measurement error of down to ± 1%.
5. We developed a versatile grating writing system based on the use of a femtosecond laser, again adopting a point-to-point writing technique and used this to write LPFGs in conventional optical fibres (e.g. Corning SMF-28).

Unfortunately, despite major improvements in the femtosecond system set up, and the development of different beam alignment and measurement procedures, we did not succeed to write grating structures within PBGFs before the end of the incoming project phase. We have identified two likely reasons for this: (1) scattering of the writing beam by the internal fibre structure and (2) the onset of damage within the fine microstructure at the intensities required to get sufficient index modulation in the glass ring surrounding the hollow core. Work on increasing the photosensitivity of the inner core walls and a 2D laser scanning writing technnique to reduce impact of the scattering effects will be conducted in the return phase of the project.