On-Demand Spectrum and Space Defragmentation in
an Elastic SDM/FDM/TDM Network with Mixed
Multi- and Single-core Fiber Links
N. Amaya 1*, M. Irfan 1, G. S. Zervas 1, R. Nejabati 1, D. Simeonidou 1,
V. J. F. Rancaño 2, F. Parmigiani 2, P. Petropoulos 2, D. J. Richardson 2,
J. Sakaguchi 3, W. Klaus 3, B.J. Puttnam 3, T. Miyazawa 3, Y. Awaji 3, N. Wada 3, I. Henning 4
(1) High-performance Networks Group, University of Bristol, United Kingdom, *[email protected]
(2) Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, UK
(3) National Institute of Information and Communications Technology (NICT), Tokyo, Japan
(4) University of Essex, Colchester, Essex, United Kingdom
Abstract: We show on-demand multi-wavelength spectrum and space defragmentation in an
SDM and elastic network with four programmable nodes and two multi-core fiber links. The
combined approach is shown to reduce blocking and hardware requirements in small nodes.
OCIS codes: (060.4250) Networks; (060.4510) Optical Communications.
1. Introduction
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installed fibre
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Node-4
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Tx-4
Wavelength (nm)
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MCF-2
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1
2
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switching
/01%
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Tx-2
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9x10G
core-to-core switching core/fibre switching
wavelength conversion
fixed-grid switching
MUX
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18x42.7G Tx-2
Tx-3
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570m
100m
Node-2
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9x42.7G
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Elastic optical networking (EON) has been proposed as a viable approach to efficiently support high-speed optical
channels beyond 100 Gb/s [1]. EON allows to accommodate high-speed channels with arbitrary bandwidth
requirements, e.g. 400G, 1T and beyond, as well as to increase spectral efficiency for low bitrate channels, e.g. 10G,
by allocating spectrum in a flexible manner and depending on channel requirements [2]. However, in a dynamic
network environment, where optical channels are constantly added and removed, elastic spectrum allocation may
give rise to spectrum fragmentation, which increases the blocking probability. Several techniques have been
proposed to defragment the spectrum, such as wavelength conversion [2] and continuous tuning of the transmitted
wavelength [3]. However, most of the techniques considered to date relocate only a single wavelength at a time with
a specific polarization, bitrate and modulation format. To make this process more efficient it is necessary to develop
a multi-wavelength defragmentation technique that is polarization, bitrate and modulation format independent.
Meanwhile, Space Division Multiplexing (SDM) has been proposed to increase optical fibre transmission
capacity by increasing the number of spatial channels over a single fibre [4]. However, successful deployment of
networks with SDM links will require scalable and flexible optical nodes able to support granularities consistent
with traffic switching requirements while minimizing hardware requirements. We have recently proposed an
adaptable optical node that provides a wide range of granularities and supports SDM requirements [5]. High
scalability is achieved by switching traffic at coarse granularities, e.g. fibre-to-core, core-to-core, core-to-fibre. Thus,
OUT
Pump-2 :C%
1562.2 nm
Figure 1. Experimental network configuration. Inset A: FWM-based wavelength converter schematic.
relatively small nodes can switch large volumes of traffic. However, switching at fiber/core granularities requires all
channels in the same fibre/core input to go to the same fibre/core output, i.e. space defragmentation.
In this paper, we propose and experimentally demonstrate on-demand polarization-independent multiwavelength spectrum defragmentation, as well as space defragmentation, in an elastic network with four
programmable nodes and two multi-core fiber links. The proposed spectrum defragmentation approach
simultaneously wavelength-converts one 40 Gb/s QPSK and two 10 Gb/s OOK channels, thereby reducing blocking
due to wavelength contention. In turn, space defragmentation, performed in a large node, is shown to alleviate
switching requirements in a smaller node, thereby facilitating the provisioning of end-to-end services.
2. Elastic multigranular network configuration
The network scenario, shown in Fig. 1, represents a future elastic and dynamic transparent network, with
programmable optical nodes of different sizes and capabilities, linked by a mix of multi-core and single mode fiber
(SMF). Nodes 1, 2 and 3 are connected by two 7-core fibre sections of 2-km (MCF-1) and 3-km (MCF-2). Both
MCFs connect to optical nodes using SDM MUX/DEMUX devices based on free-space optics [6]. The average loss
of the MCF combined with SDM MUX/DEMUX was measured to be 2 dB and 2.4 dB for MCF-1 and MCF-2
respectively. The average measured cross-talk (including the contributions of MUX / DEMUX) was -56.5 dB and 53.8 dB for MCF-1 and MCF-2 respectively. Node-4 is connected to Nodes 1, 2 and 3 by different lengths of SMF,
as illustrated in Fig. 1. Node-5 is remotely located and connects to Nodes 1 and 3 through 340-km dispersion
compensated dark fibre links. Nodes 1-4 are based on the Architecture-on-Demand (AoD) concept [5] and consist of
an optical backplane that interconnects MCF/SMF fibre inputs, functional modules and MCF/SMF fibre outputs.
The backplanes of Nodes 1, 2 and 4 are implemented as independent partitions of a 160x160 3D-MEMS optical
switch with a 20-ms switching time and an average loss of 2 dB per cross-connection. Node-3 is implemented with a
16x16 beam-steering switch [7] with an average loss of 0.59 dB per cross-connection. The modulation-, bit rate- and
polarization-independent multi-channel spectrum defragmentation is achieved by wavelength conversion based on
four-wave mixing (FWM) in a highly nonlinear fiber (HNLF) pumped by two orthogonally polarized cw lasers, see
inset of Fig.1. The pumps are located at the two extremes of the signal/idler bandwidth and coupled together with
the signals via arrayed waveguide gratings (AWGs), as illustrated in Fig. 1 inset A.
3. Spectrum and space defragmentation
Node-2 inputs
Node-2 outputs
Contention
Intensity (dBm)
-20
2 4 5 5 5
4
2 5
5 3
2 2
25 5
5
F
F
A/7
A
F
-40
(c) MCF1-core 4
-60 (a) Tx-1+17-km SMF (A)
-20
42
4
3 23
4
Contention
2 42 2 2 2 5 5 5 2 5
5
5
MWC
(e) MCF2-core 6
A A
A A A 7 7 4 4 47 7
7
7
-40
-60
(b) Tx-2+82-km dark fibre (F)
(d) MCF1 – core 7
(f) MCF2-core 5
1540 1545 1550 1555 1560 1540 1545 1550 1555 1560 1540 1545 1550 1555 1560
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Figure 2. Optical spectrum plots at relevant Node-2 inputs and outputs.
Figures 2 (a)-(d) show spectrum plots at relevant Node-2 inputs with channels labeled according to their destination
node. Figures 2 (e)-(f) show relevant Node-2 outputs. Output channel labels represent the corresponding Node-2
input. Three of the wavelengths in MCF-1 core 4 (Fig. 2c) present contention with one channel from SMF input A
(Fig. 2a) and two channels from MCF-1 core 7 (Fig. 2d). These channels require being transmitted over Node-3, and
340-km of installed SMF, to reach their destination (Node-5). It is not possible to resolve this contention using the
space dimension, due to the single-core fiber between Nodes 3 and 5. Also, Node-3 is not provisioned with
wavelength conversion or (de)multiplexing capabilities. Therefore, Node-2 is programmed to connect the
polarization-independent multi-channel converter to input MCF-1 core 4, as shown in Fig. 1. Thus, the contending
channels are relocated by means of simultaneous multi-wavelength conversion (MWC) and aggregated with all the
other channels that go to Node-5 on MCF-2 core 5, as shown in Fig. 2(f). As the channels now aggregated in MCF-2
core 5 have a common destination, i.e. Node-5, channel (de)multiplexing is not required for switching them at Node3, as shown in Fig. 1. Similarly, channels destined to Node-4 are aggregated at the output of Node-2 in MCF-2 core
6, as shown in Fig. 2(e), thereby obviating the need for (de)multiplexing at Node-3. Therefore, performing spectrum
and space defragmentation in Node-2 alleviates channel (de)multiplexing requirements in Node-3.
4. Channel performance
Figure 3 (a) shows bit error ratio (BER) plots and constellation diagrams for the phase modulated channels as a
function of optical signal to noise ratio (OSNR), which were sent and received at Node-5 after going through Node1, MCF-1, Node-2 (with WC), MCF-2 and Node-3. BER results for the 10 Gb/s OOK channels, originated and
received at Node-5 after going through Node-1, MCF-1, Node-2 (with WC), MCF-2 and Node-3, are shown in Fig.
3 (b). At BER=10-9, end-to-end OSNR penalties of 2 dB and 2.8 dB are observed for the converted 1551.8 nm and
1554 nm wavelengths respectively. Figure 3 (c) shows typical BER curves for channels ending at Node-4. End-toend penalties of 1.8 dB, 2.7 dB and 3.7 dB are observed for the 555 Gb/s (sub-carrier 25), 42.7 Gb/s, and 42.7 Gb/s
sub-wavelength channels respectively.
Figure 3 (d) shows back-to-backBER
and
end-to-end sensitivities of the
BER curves
curves
2
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2
4
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4
5
6
7
8
5
9
10
11
6
10
12
16
42.7G 1559.8nm B-to-B
42.7G 1559.8nm E-to-E
555G(A) SC25 B-to-B
555G(A) SC25 E-to-E
42.7G sub-! 1541.3nm B-to-B
42.7G sub-! 1541.3nm E-to-E
5
6
7
8
9
10
-35
(c)
18
10
-30
-25
Average Rx power (dBm)
12
(b)
OSNR [dB]
4
-log(BER)
14
OSNR [dB]
14
16
18
OSNR [dB]
20
OSNR [dB]
-20
Sensitivity at 1e-9 BER [dBm]
8
(a)
End-to-end
-22
Back-to-back
-24
-26
-28
-30
1540
(d)
1545
1550
1555
Wavelength [nm]
1560
1560
Figure 3. BER measurements at various points in the network.
continuous 42.7 Gb/s channels received at Node-4. The highest penalty, of 4.5 dB was measured for the channel at
1542.9 nm due to the lower channel power and OSNR, as shown in Fig. 2(e). Note that power equalization was not
performed at the outputs of Node-2.
5. Conclusion
We have proposed and experimentally demonstrated on-demand polarization-independent multi-wavelength
spectrum defragmentation, as well as space defragmentation, in an elastic SDM/FDM/TDM network with four
programmable nodes and two multi-core fiber links. The proposed multi-channel spectrum defragmentation
approach was implemented in a programmable node to perform on-demand waveband conversion and resolve issues
of wavelength contention. Space defragmentation was shown to alleviate switching requirements of smaller nodes,
thereby facilitating the provisioning of end-to-end services. BER results showed error-free performance for the OOK
channels and a good margin below the 2x10-3 FEC threshold for the 555 Gb/s, 40G BPSK and 40G QPSK channels.
6. Acknowledgements
This work is supported by the EPSRC grant EP/I01196X: The Photonics Hyperhighway. The authors are grateful to
Mitsubishi Cable Industries from Japan for providing the MCFs and would like to thank Polatis for the loan of the
low-loss space switch and Yenista Optics for the bandwidth-variable optical filter.
7. References
[1] O. Gerstel et al., Coms. Mag., vol. 50, no. 2, pp. s12 –s20, february 2012.
[2] N. Amaya et al., ECOC 2011, TH.13.K.1 (2011).
[3] K. Sone et al., ECOC 2012, Th.3.D.1. (2012).
[4] Jun Sakaguchi et al., OFC 2012, PDP5C.1, (2012).
[5] N. Amaya et al., ECOC 2012, Th.3.D.3C (2012).
[6] W. Klaus et al., IEEE Summer Topicals 2012, WC3.3
[7] M. Bitting, AUTOTESTCON 2004.