PhD proposition on « Flexible wavelength division
multiplexing and its applications to transparent metro
networks »
Context
The capacity requirements of optical transport networks (core and metro) are still growing
rapidly. DWDM (Dense Wavelength Division Multiplexing) is widely used in these networks.
Several frequency grids have been defined by ITU-T for DWDM. Most DWDM systems
operate in the C-band (1530 -1565 nm) with a fixed frequency spacing of 100 or 50 GHz [1].
The first DWDM systems had a per channel bit rate of 2.5 or 10 Gbit/s, with NRZ modulation
format. Early 40 Gbit/s systems were proposed since 2000, but they face different propagation
problems and were risking to rapidly filling the C-band, because of the poor spectral
efficiency of their modulation formats (1 bit / symbol).
This context favoured a renewal of the interest for coherent transmission techniques, which
benefit from advances in electronic digital processing and optical integration. In particular,
impairments such as chromatic and polarization mode dispersions can be limited and/or
compensated with complex modulation formats while improving the spectral efficiency. This
enables the operators increasing the capacity of their network and still using their existing
infrastructure (fibres, amplifiers, multiplexers…).
Work on these new coherent systems led to a second generation of 40G systems based on
“Dual Polarization QPSK” (DP-QPSK) format. Equipment manufacturers are now planning to
introduce 100G systems based on this format, which are expected to be compatible with the
50 GHz grid. In fact, only the channel spacing was defined by ITU, but the actual channel
shape remained an open question [3]. The spectral width required by a channel depends, on
one hand, on the filtering functions it may have to cross (in particular the number and
characteristics of the ROADM) and, on the other hand, on the robustness of its modulation
format w/r filtering effects. The 50 GHz grid will be unlikely compatible with bit rates
beyond 100G. Increasing the frequency spacing is possible in principle, but this option would
lead to recurrent infrastructure modifications, and, in presence of channels with different bit
rates, degrades the overall spectral efficiency. This perspective led several researchers to
propose to get rid off a rigid multiplexing grid and to introduce a much more flexible
frequency allocation [4].
Besides the capacity increase, another major trend in optical transport networks consists in
introducing equipment allowing to perform optical channel cross-connection (to face
protection or long-term network reconfiguration needs), directly in the optical domain.
The operators had already begun to deploy Reconfigurable Optical Add-and-Drop
Multiplexers (ROADM). Optical switching techniques enable a dramatic reduction of the
power consumption below a Watt per Gbit/s, while being compatible with the increase of the
channel bit rates.
At the technological level, the introduction of Wavelength Selective Switches (WSS), which
combine in the same device space-switching and wavelength de-multiplexing functions, led to
the development of new ROADM architectures. These new architectures limit the operational
constraints for the operators (e.g. colorless and directionless ports) and can be extended to
higher degree nodes, for ring interconnection or meshed networks. New generations of WSS
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(notably those based on Liquid Crystal on Silicon technology) are particularly interesting for
implementing a flexible WDM as their spectral resolution is much lower than 50 GHz [5] [6].
The interest on Flexible WDM (FWDM) is growing rapidly; an entire session of the last
Optical Fibre Communication conference was devoted to this topic. The various approaches,
involves variable bit rate and/or spectral occupancy. Ciena recently introduced transponders
with bit rate (40/100G) and modulation format (BPSK or QPSK) agility [14].
Most of the new possibilities provided by FWDM remain to be explored before assessing
their middle- and long-term impact on optical networks.
Objectives
This thesis will be devoted to new approaches for optical multiplexing, providing much more
flexibility than the present ones based on regularly-spaced frequencies. These approaches
should yield a global optimisation of the spectral efficiency and enabling a fine tuning of the
bit rate and spectral occupancy of each channel. They will be applied in new transparent
architectures of metropolitan networks.
The main objectives concern:
 the definition of optical multiplexing solutions, in particular based on OFDM, with
a suitable trade-off between flexibility and simplicity of operation;
 the evaluation by simulation of these solutions for networks with a typical size of
hundreds of km and about ten flexible ROADM crossed per link;
 the proposition of metro network architectures providing a low energy consumption
by using these solutions and original protection mechanisms;
 the dimensioning and the evaluation of these architectures.
Among the different architectures, a particular attention will be paid to “Transparent Metro
Trees” where the access flow aggregation is performed by FWDM. The possibility of varying
the channel bit rate will be considered (by assuming either a variable or a fixed spectral
efficiency). The application of this bit rate variation capability to network protection will be
addressed. It seems particularly suited to metro networks where the limited transmission reach
enables considering important spectral efficiencies for the FWDM channels and where the
poorly meshed physical topology limit the protection path diversity.
Required scientific skills
The candidate should have a variety of skills in different telecommunication area, including:
× optical transmission and digital communication (multiplexing, modulation).
× network optimisation techniques (linear programming,…).
He/She should also be able to use simulation tools for optical transmission systems such as
VPI, RSOFT.
More generally, the nature of the problems that will be addressed required a clear motivation
for multidisciplinary work and an excellent aptitude for synthesis.
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Contact persons
Philippe Gravey, Optics Department
(philippe.gravey@telecom-bretagne.eu, +33229001581)
Michel Morvan, Optics Department
(michal.morvan@telecom-bretagne.eu, +33229001367)
Bibliography
[1] Recommandation ITU-T G.694.1.
[2] A. N Patel, P. N. Ji, J. P. Jue, T. Wang, “Survivable Transparent Flexible Optical
WDM (FWDM) Networks) ”, OTuI2 (OFC 2011)
[3] Recommandation ITU-T G.671
[4] M. Jinno, H. Takara, B. Kozicki, Y. Tsukishima, Y. Sone, and S. Matsuoka,
“Spectrum-Efficient and Scalable Elastic Optical Path Network: Architecture,
Benefits, and Enabling Technologies”, IEEE Comm. Mag., vol.47, pp. 66-73, 2009.
[5] G. Baxter, S. Frisken, D. Abakoumov, H. Zhou, I. Clarke, A. Bartos, and S. Poole,
“Highly Programmable Wavelength Selective Switch Based on Liquid Crystal on
Silicon Switching Elements”, Proc. OFC/NFOEC ’06, 2006.
[6] S. Frisken, G. Baxter, D. Abakoumov, H. Zhou, I.Clarke, and Simon Poole, “Flexible
and Grid-less Wavelength Selective Switch using LCOS Technology”, OTuM3 (OFC
2011)
[7] Y. Zhang, X. Zheng, Q. Li, N. Hua, Y. Li, and H. Zhang, “Traffic Grooming in
Spectrum-Elastic Optical Path Networks”, OTuI1 (OFC 2011)
[8] T. Takagi, H. Hasegawa, K. Sato, Y. Sone, B. Kozicki, A. Hirano, and M. Jinno,
“Dynamic Routing and Frequency Slot Assignment for Elastic Optical Path Networks
that Adopt Distance Adaptive Modulation”, OTuI7 (OFC 2011)
[9] S. Thiagarajan, M. Frankel and D. Boertjes, “Spectrum Efficient Super-Channels in
Dynamic Flexible Grid Networks – A Blocking Analysis”, OTu6 (OFC 2011)
[10] O. Rival, A. Morea, OECC’10, 7A2-3 (2010)
[11] K. Christodoulopoulos, I. Tomkos, E. Varvarigos, “Dynamic Bandwidth Allocation
in Flexible OFDM-based Networks”, OTuI5 (OFC 2011)
[12] O. Gerstel, “Realistic Approaches to Scaling IP Network using Optics”, OWP1 (OFC
2011)
[13] Y. Ishii, et al., “MEMS-based 1x43 Wavelength-Selective Switch with Flat
Passband”, Proceedings of ECOC 2010, PD 1.9
[14] http://www.ecocexhibition.com/node/7144/bnr?hq_e=el&hq_m=778835&hq_l=9&hq
_v=002c33cd8
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