Broadband fibre Bragg gratings for pure third-order dispersion compensation

Single fibre Bragg gratings designed to compensate pure 3rd-order dispersion are demonstrated for the first time. The shown devices exhibits dispersion-profiles of ~20ps/nm2, ~40ps/nm2 and ~225ps/nm2 over bandwidths of 4.5nm, 6nm and 3nm respectively, they have peak reflectivities of ~75% and are only 8mm, 23mm and 40mm long. IntroductionAdvances in internet traffic and a steadily increasing demand for bandwidth and better bandwidth utilisation have started to pull systems providers towards higher bit-rate systems to achieve reduced channel and component count. Increasing the channel bit-rate can impose penalties to the dispersion performance of the system though and therefore close attention has to be paid to the dispersion slope generated in this. A number of approaches have been proposed previously to accommodate this. These include the use of dispersion-slope compensating fibres [1], long chirped Bragg gratings for combined linear and third-order dispersion [2], combinations of two non-linearly chirped Bragg gratings [3] and thin-film filters [4]. With the recent advances in grating design and manufacturing techniques it is now believed that a level has been reached where almost any filter response can be generated. For example we have recently demonstrated filters for add-drop applications with linear-phase performance for dispersion-free filtering up to 40Gbit/s [5], filters for rectangular pulse generation [6] and other pulse manipulating applications [7]. In this paper we demonstrate examples of even more complex profiled Bragg gratings through the design and, for the first time, experimental realisation of single gratings with broad spectral bandwidth profile and pure dispersion-slope compensation abilities. The demonstrated gratings require very high maximum index modulations together with full control of both phase and amplitude profiles. Examples of short Bragg gratings with dispersion-slopes of -20ps/nm2, -40ps/nm2 and -225ps/nm2 over a 1dB reflection bandwidth of 3nm, 5nm and 2nm respectively with reflectivities of ~75% are shown.