We report the first demonstration of 40 Gbit/s transmission over 109 km of standard fibre at 1.55 µ µm with dispersion compensating linearly chirped continuous fibre gratings and a ~0.5 nm source wavelength tolerance, demonstrating the feasibility of fabricating such gratings for use at very high data rate.
Background:Chirped fibre gratings are compact and potentially cheap and have become an attractive alternative to technologies such as dispersion-compensating fibres for the upgrade of the installed non-dispersion shifted fibres (NDSF). In particular the recent demonstration of 10 Gbit/s transmission over 700 km of NDSF [11] and 40 cm broadband (4 nm) linearly chirped gratings for much increased source wavelength and operation condition tolerance [2] confirm this potential. These long gratings have essentially flat reflectivity over their usable bandwidth with raised cosine apodisation at both ends of the spectrum. The delay characteristic can be controlled to be linear with less than 2% error in delay and with a dispersion slope as high as ~1700 ps/nm (equivalent to ~100 km of standard fibre). At the same time, the broad bandwidth available from the long linearly chirped gratings makes it possible for the first time to achieve dispersion compensation over a significant length of NDSF at several tens of Gbit/s data rates.To date 40 Gbit/s transmission over a significant length of NDSF at 1.55 µm has only been demonstrated using dispersion compensating fibres [3,4]. The best result to our knowledge is an error-free transmission over 150 km of NDSF [4]. Without dispersion compensation, the maximum transmission distance is limited to ~4 km at this bit rate.In this paper we report the first demonstration of 40 Gbit/s transmission over 1.09 km of NDSF at 1.55 µm by employing two continuously chirped 40 cm long, 4 nm bandwidth gratings.
Experiment:Two linearly chirped gratings were employed in combination with a 4 port circulator. The characteristics are shown in figure 1. Grating 1 exhibits ~98% reflectivity with < ±0.1 dB amplitude deviation over 3.8 nm, dispersion slope of 837 ps/nm and maximum delay deviation from linear fit of < ±50 ps. Whilst comparable figures for grating 2 are ~98% reflectivity, < ±0.1 dB amplitude deviation over 3.5 nm, dispersion slope of 870 ps/nm with < ±40 ps delay deviation from linear fit.
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