Tools for synthesising and characterising Bragg grating structures in optical fibres and waveguides

Abstract

Optical waveguide filters based on optical fibre Bragg gratings are used for a wide range of applications in communications systems, fibre lasers and optical fibre sensors. The technical specifications for many of these grating based devices are becoming increasingly challenging. To make technological advances in this area a number of tools, such as: grating synthesis, advanced fabrication techniques, characterisation and reconstruction techniques, are required to improve grating structures. Closing the loop between design and physical realisation has the potential to achieve an unprecedented degree of accuracy in device fabrication. Grating design, fabrication and characterisation has moved from merely an inexact experimental procedure to a highly controlled engineering process. The mathematical and physical tools to achieve this are described and examples of applications are used to illustrate the potential of these capabilities to accelerate further understanding and development of photosensitivity and gratings in novel optical fibres and waveguides in the future.

Keywords:

• fibre Bragg grating
• interferometry
• inverse scattering algorithm
• synthesis
• periodic filter design
• characterisation

Notes

Notes

1. We note that strictly speaking these devices require the light to exit the fibre before passing through the filter and then re-couple into the fibre; however this process has been made sufficiently low cost and highly efficient for these devices to be widely used.

2. Strictly speaking, the FBG is a four-port device since the input and output fields are described by two orthogonal polarisation states. Each port represents the forward and backward propagating fields for each state. For an isotropic waveguide, the fields can be represented by a single polarisation state.

3. The term apodised implies varying the envelope of the index modulation. The effect is usually to reduce the side-lobe levels residing on each side of the main resonance in the reflectivity spectra.

4. This technique is commonly used in digital-finite-impulse filter response design.

5. This FBG was provided by Redfern Optical Components, Eveleigh, NSW, Australia.