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Design and application of long, continuously chirped fibre gratings

M.J.Cole*, S.Aina+, M.Durkin*, M.Ibsen*, F.Vaninetti+, L.Arcangeli+, V.Gusmeroli+ & R.I.Laming*
* Optoelectronics Research Centre, University of Southampton
+ Pirelli Cavi SpA, Milan, Italy

Abstract

Long (40 → 100cm) continuously chirped broadband fibre gratings are demonstrated and tested in a 10Gbit/s NRZ standard fibre transmission system. Dispersion compensation for standard fibre lengths up to 125km and over a 4nm bandwidth is confirmed.

Introduction
Dispersion compensation is now a very important field which can allow the upgrade of the existing installed non-dispersion shifted fibre network to high data rates (e.g. 10Gbit/s) within the EDFA wavelength region of 1.55μm. High data rates would normally be prohibited due to the chromatic dispersion exhibited by standard fibre (~17ps/nm·km). Chirped fibre Bragg gratings are probably the most attractive technique for overcoming this fibre limitation as they are compact, low-loss, and polarisation insensitive [1]. In addition fibre Bragg gratings do not suffer from high non-linearity, a disadvantage of dispersion compensating fibre. For present practical applications chirped fibre Bragg gratings must display both high dispersion (~1-3ns/nm) and wide bandwidth (several nanometres) such that the dispersion of a typical amplifier span and semiconductor diode wavelength tolerances are both covered. Thus chirped fibre Bragg gratings are required with ~1m length with several nm bandwidth.

In this paper we report the application of practical 40cm/4nm chirped fibre Bragg gratings in a NRZ 10Gb/s fibre link and the fabrication of a 1m glitch-free, continuous chirped fibre Bragg grating exhibiting a 3dB bandwidth of >10nm.

Background
Several techniques for fabricating fibre Bragg gratings have been developed to date [2, 3]. Long fibre Bragg gratings have typically been fabricated using phase mask techniques, owing to the stability of the writing method. Since phase masks 10cm length are now available, gratings this length can be easily fabricated. Techniques for post chirping uniform fibre gratings have been reported such as by applying thermal [4] and strain [1] gradients, but these are cumbersome. Fixed chirp devices have been fabricated using step-chirped phase masks [5]. An obvious drawback being that a different phase mask is required for each dispersion and wavelength.

Chirping of fibre Bragg gratings is the most obvious requirement for a dispersion compensation grating, although it is also imperative to impose an apodisation profile to the device. Pure apodisation is the means by which unwanted reflections and side lobes are removed from the filter characteristic improving the flatness of both the reflection and dispersion. In the past we developed a powerful technique [6] which allows arbitrary control over the phase of the grating under fabrication. Hence chirp and apodisation could be added together to obtain a good dispersion compensating device. Like the previous phase mask scanning techniques the limitation is the length of the phase mask available.

Taking some of the ideas from our previous work, we have developed a technique capable of fabricating gratings >1m in length. This technique is capable of continuously writing gratings with no glitches and an arbitrary profile can be obtained during the write.


Figure 1. Reflectivity and BER tuning measurements


Figure 2. BER measurements for grating 1

Experimental
Three 40cm chirped fibre Bragg gratings were fabricated with a 4nm linear chirp in a 0.2NA, 1.3μm cut-off, deuterium loaded fibre. Apodisation was added to the profile of the grating by reducing the modulation contrast of both the ends (10%) by a cosine profile. High NA fibre is used to shift the unwanted cladding mode loss away from the region of interest. Deuterium loading is used to enhance the sensitivity of the fibre to UV whilst avoiding the lossy drawbacks of hydrogen.

Figure 1 shows the characteristics for the grating circulator combination used for dispersion compensation over 50 & 75km. The reflectivity and time delay can clearly be seen to be very flat (ripples <1dB, <50ps). BER measurements were made using a 10Gbit/s test set operating with a pseudo-random bit sequence of 223-1. The maximum power entering the fibre span was limited to less than 8dBm. The dispersion compensator was tested over 50 & 75km of fibre, Figure 2. The BER measurements for 50 & 75km compare almost exactly with the back to back reading indicating that the actual compensation lies between these distances. Curves showing the operation after 50 & 75km have been added to clarify the operational penalty which would be encountered without the dispersion compensating device in place. The curve showing 75km with no compensation shows a floor, this is due to non-linearity in the receiver electronics. Included in Figure 1 are data points representing BER for a specified received power (-12dBm). The tuning was performed over 0.5nm to show that the variations in BER are small. The equipment used limited the measurement to 0.5nm, previous work would suggest that the entire bandwidth would exhibit a similar property [7].


Figure 3. Cascaded gratings BER results


Figure 4. Cascaded gratings BER tuning results

Using two gratings operating in cascade, a similar experiment was performed using 125km of non-dispersion fibre. The configuration was similar except for the extra fibre and an additional amplifier. Figure 3 contains the data showing the BER performance with respect to received optical power and results for tuning across 0.5nm of the bandwidth are shown in Figure 4 for a received power of -12dBm.

From the data it can be seen that the dispersion compensator compares well with the back to back reading of BER with respect to optical power. Plots showing the BER curves for 50 & 75 km have been added to demonstrate the effectiveness of the device.

Tuning the transmitter shows the BER to fluctuate by less than two orders of magnitude. The fluctuation is due to small cumulative imperfections in the gratings. In a real system it is likely that there would be electronic regeneration after 200km, thus small imperfections can be tolerated.

One metre gratings can be fabricated using a small modification of the same set-up and can achieve similar performances to those gratings shown above. Figure 5 shows the characterisation of a 1m long grating with a bandwidth of 10.5nm. The plot shows the deviation from a perfect linear time, delay of 928ps/nm in picoseconds. This grating was fabricated in a fibre loaded with deuterium and took ~8 minutes to fabricate.

Conclusions
We have been able to fabricate and demonstrate 40cm linearly chirped fibre Bragg gratings in a 10Gbit/s NRZ system with excellent results. It was possible to tune the wavelength of the transmitter across the bandwidth of the gratings and experience only a negligible deviation in BER for a fixed received optical power.


Figure 5. A 1 metre long chirped fibre Bragg grating

Acknowledgement
We thank A.Zuccala, D.N.Payne, M.N.Zervas & H.Geiger for support and encouragement. The support and funding of Pirelli Cavi SpA is acknowledged. R.I.Laming is a Royal Society Research Fellow. The ORC is an EPSRC funded interdisciplinary research centre.

References
1. Garthe et al, "Practical dispersion equaliser based on fibre gratings with a bitrate length product of 1.6Tb/skm", Proc. ECOC, vol. 4 (post-deadline papers), pp 11-14, 1994
2. Meltz et al, "Formation of Bragg gratings in optical fibres by a transverse holographic method", Optics Letters, vol. 14, no. 15, pp 823-825, 1989
3. Hill et al, "Bragg gratings fabricated in monomode photosensitive fibre by UV exposure through a phase mask", App. Phys. Lett., vol. 62, no. 10, pp 1035-1037, 1993
4. Laming et al, "Dispersion tuneable grating in a 10Gbit/s 100-220km step index fibre link", Proc. ECOC, vol.2, pap. WE.B.1.7, pp 585-588, 1995
5. Kashyap et al, "Super-step chirped fibre Bragg gratings", Electronics Letters, vol. 32, no. 15, pp 1394-1396, 1996
6. Cole et al, "Moving fibre/phase mask scanning beam technique for enhanced flexibility in producing fibre gratings with a uniform phase mask", Electronics letters, vol. 31, no. 17, pp 1488-1489, 1995
7. Cole et al, "Continuously chirped, broadband dispersion-compensating fibre gratings in a 10Gbit/s 110km standard fibre link", Proc. ECOC, vol. 5 (post-deadline papers), 1996


IEE Optical Fibre Gratings London 16-Jan (1997)

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