Expanding Capacity through Combined C+L Band Amplifiers

Current transmission systems, based on single-mode fibers, typically use either the C-, or L-band due to the availability of high-performance Erbium-Doped Fibre Amplifers (EDFAs) in these two bands. However, the continuing growth in data transmission (driven primarily by the increased consumption of on-line video1) is generating a great deal of research interest in trying to increasing fiber transmission capacity by spatial-division-multiplexing, either through multiple-core fibers2 or through the use of higher-order modes in a few-mode fiber3. Whilst these techniques may, in time, mature to the point where they are economically viable as a replacement for existing single-mode fiber transmission, it is not yet clear (at least to me) that the issues around practical spatial-division-multiplexing are soluble in a cost-efficient manner.

Recent work by Lynn Nelson and co-workers at AT&T Research4 has revisited the idea of combined C+L band transmission but this time using broadband Raman amplifiers, rather than the twin-EDFA configurations used previously5. Their work used two Finisar “X-Series” WaveShapers (a 4000S/X operating as a signal combiner and a 1000S/X used for gain flattening in a recirculating loop) to demonstrate a capacity of >9 Tb/sec (extendable to >18 Tb/sec) over 6,000km of fiber using standard DP-QPSK transmission. The X-Series WaveShapers have an operating window of >9 THz (73nm) – covering most of the C- and L-band – and so enabled the researchers to maximize the available capacity whilst maintaining excellent gain flatness in the recirculating loop.

WaveShaper 4000STransmission Spectrum of a WaveShaper 4000S X Showing 9 THz 73nm Transmission Window

Finisar WaveShaper 4000S and transmission spectrum of a WaveShaper 4000S/X showing 9 THz (73nm) transmission window

As Finisar’s WaveShapers use a similar LCoS optical engine as our DWP range of Wavelength Selective Switches (WSS), it would be possible to develop WSS to support this range of operating wavelengths. However, as noted in the paper, the broad operating wavelength range gives less precise control over the channel bandwidth due to the need to fit the broader transmission spectrum onto the same LCoS real-estate as in a standard WSS. This leads to greater spectral narrowing during signal switching as there are less LCoS pixels per channel6 and may limit full C+L-band systems to high-capacity point-to-point links where optical switching is not required.


1Cisco Visual Networking Index
2See e.g. J. Sakaguchi, et al, “Propagation characteristics of seven-core fiber for spatial and wavelength division multiplexed 10-Gbit/s channels,” in Opt. Fiber Commun. Conf. (OFC) (2011), paper OWJ2
3Peckham, D.W., et al., “Few-mode fiber technology for spatial division multiplexing” in Optical Fiber Telecommunication VIA, Academic Press, ISBN 978-0-12-396958-3, 2013.
4Nelson, L., et al., “All-Raman-Amplified, 73nm Seamless Band Transmission of 9Tb/s over 6000km of Fiber”, to be published in Photonics Technology Letters. DOI: 10.1109/LPT.2013.2291399
5M. Salsi, et al., “31 Tb/s transmission over 7,200 km using 46 Gbaud PDM-8QAM with optimized error correcting code rate,” Proc. OECC/PS 2013, Japan, paper PD3-5, July 2013.
6Poole, S. B., et al “Bandwidth-flexible ROADMs as Network Elements”, in Opt. Fiber Commun. Conf. (OFC) (2011), paper OTuE1.

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