|
The technology ofBragg grating formation has come a long way since the initial demonstrations back in 1978 [1]. Techniques capable of forming nearly any refractive index modulation and imprint nearly any grating pitch at a given position along the length ofthe grating are now in widespread use [2]. This evolution in manufacturing techniques has lead to an impressive collection of a variety of Bragg grating based components and devices, [3], and the robustness of many of these devices in laboratory experiments have pushed their employment in real system environments. Convincing performance, in the relative short lifetime of the technology, led in 1995 to the commercialisation of a series Bragg grating products. Among these were the chirped fibre Bragg grating. It was originally proposed by Ouellette [4] as a device capable of compensation of chromatic dispersion by incorporating reflective differential delay characteristics for different wavelengths along the grating. The pulse broadening suffered when transmitting signals through installed telecommunications fibre therefore can be compensated by delaying the different spectral components of the pulse such that it is reformed to its original shape upon reflection from the grating. Although chromatic dispersion does not impose much limitation to signal recovery when operating systems at bit-rates less than 10 Gbit/s and such data-rates were far from being realised when the chirped fibre grating was proposed, it was foreseen to play an important role in the then future high bit-rate transmission systems. As data-rate transmission at 10 Gbit/s now indeed are beginning to be a well-established technology, and even systems employing wavelength division multiplexed (WDM) technology at the same bit-rate are working solidly, there is a clear need for robust dispersion management in these systems. As a chirped grating exhibits many attractive features such as compactness, low insertion loss, and very importantly, low sensitivity to non-linearities together with low or no polarisation dependancy, it is a leading contestant compared with other proposed dispersion management techniques. These include the use of dispersion compensating fibre (DCF) [5]. The main advantage of DCF is the broad bandwidth over which dispersion compensation can be performed, but high sensitivity to non-linear effects and the missing ability to "shape" the dispersion profile together with the lack of tunability, are major drawbacks to this approach. The previous lack of bandwidth of chirped gratings has been the only limiting factor for them in being the most interesting, simple and robust approach to chromatic dispersion compensation. The design and realisation of high quality truly broadband gratings therefore has taken highest priority among grating manufactures. We will in this paper from a design point of view discuss how chirped gratings with large bandwidths and high quality dispersion profiles can be achieved. Furthermore, examples of gratings with narrowband spectral responses will be given -gratingsdesigned using a new powerful inverse-scattering technique. One approach to the fabrication of very long and complex chirped gratings will also by aired supported by a number of experimental grating examples using this technique. A discussion of how imperfections imposed by either the manufacturing process of -ora non-perfect waveguide environment to the gratings, can be minimised by choosing the right host fibre parameters, will also be given. Finally techniques for tuning ofthe dispersion ofBragg gratings will be discussed together with recent advances in the experimental demonstrations of these.
|
