Bragg grating filters for optical networks
This thesis focuses on the exploitation of fibre Bragg gratings in optical communication networks. New designs, as well as procedures for dynamically altering grating characteristics are proposed for several important functions. We begin with designing gratings for flattening the gain profile of an erbium-doped fibre amplifier. The design process uses an alternative method to the inverse scattering method, but produces a similar spectral response quality. These gratings have either modulated refractive index or chirp rate. Then, we propose a reconfigurable phase-code encoder. The device is composed of a uniform grating, with a number of equidistant fine wires for the purpose of modulating the phase via the thermo-optic effect. Error free data transmission is demonstrated in several optical code-division multiple access architectures. The characteristics of the device are also theoretically modelled. Next, we construct a simple package for continuous tuning of fibre gratings, which adopts the beam bending technique. It demonstrates over 110 nm tuning range and the operational wavelength can be accurately predicted. Its spectral response and limitations are also studied. This device has been incorporated into several optical systems, which include tunable distributed feedback fibre laser, add-drop multiplexer and high power fibre laser. Finally, we present a package for varying the dispersion of a fibre grating. The package deforms a beam into a cubic function shape with a contra-flexure point at the middle. Consequently, this allows changes in the grating delay characteristic without shifting its centre wavelength. This device has achieved dispersion compensation of an 80 km non-zero dispersion-shifted fibre in a 10 Gb/s system. Additionally, since its bandwidth also changes with the stress gradient, it has also been utilised as a bandwidth-variable bandpass filter at the receiving terminal of a spectrum-sliced wavelength-division multiplexed system.