Enabling Technologies for Optical Data Center Networks: Spatial Division
Multiplexing
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Citation for the original published paper (version of record):
Zhang, L., Chen, J., Agrell, E. et al (2020). Enabling Technologies for Optical Data Center Networks:
Spatial Division Multiplexing. Journal of Lightwave Technology, 38(1): 18-30.
http://dx.doi.org/10.1109/JLT.2019.2941765
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Abstract—With the continuously growing popularity of cloud
services, the traffic volume inside the data centers is dramatically
increasing. As a result, a scalable and efficient infrastructure for
data center networks (DCNs) is required. The current optical
DCNs using either individual fibers or fiber ribbons are costly,
bulky, hard to manage, and not scalable. Spatial division
multiplexing (SDM) based on multicore or multimode (few-mode)
fibers is recognized as a promising technology to increase the
spatial efficiency for optical DCNs, which opens a new way
towards high capacity and scalability. This tutorial provides an
overview of the components, transmission options, and
interconnect architectures for SDM-based DCNs, as well as
potential technical challenges and future directions. It also covers
the co-existence of SDM and other multiplexing techniques, such
as wavelength-division multiplexing and flexible spectrum
multiplexing, in optical DCNs.
Index Terms—Fiber-optical communication, multicore fiber,
few-mode fiber, multiplexing and demultiplexing, network
architecture, optical data center networks, resource allocation,
spatial division multiplexing, switching.
I. INTRODUCTION
ATA centers (DCs) are playing key roles in Internet service
delivery for an ever-increasing number of customers and
devices [1-3], which raise strict requirements on
interconnection networks for DCs in terms of capacity, power
consumption and latency. Regarding the capacity requirement,
DC traffic will continue to dominate the Internet traffic in the
foreseen future, and a clear trend of such traffic is transforming
Manuscript received xx xx, 2019, revised xx xx, 2019, accepted xx xx, 2019;
This work was partly supported by Swedish Research Council (VR), the
Swedish Foundation for Strategic Research (SSF), Göran Gustafsson
Foundation, the Celtic-Plus sub-project SENDATE-EXTEND & SENDATE
FICUS, H2020 5GPPP 5G-PHOS research program (ref.761989), European
Commission through the FP7 project MIRAGE (ref.318228), SJTU State Key
Laboratory of Advanced Optical Communication System and Networks Open
project 2018GZKF03001. (corresponding author: Lena Wosinska)
L. Zhang is with the College of Information Science and EE, Zhejiang
University, Hangzhou 310027, China; and also with State Key Laboratory of
Advanced Optical Communication System and Networks, Shanghai Jiao Tong
University, Shanghai 200240, China. (e-mail: zhanglu1993@zju.edu.cn;
luzhang_sjtu@sjtu.edu.cn).
J. Chen, E. Agrell, R. Lin, and L. Wosinska are with the Department of
Electrical Engineering, Chalmers University of Technology, Gothenburg,
Sweden. (e-mails: agrell@chalmers.se, ruilin@chalmers.se;
jiajiac@chalmers.se; wosinska@chalmers.se).
from the conventional ‘north-south’ direction that
sends/receives data to/from outside of the DCs to the
‘east-west’ direction corresponding to the traffic that stays
locally inside the DC and is predicted to represent 85% of total
DC traffic by 2021 [3-4]. Thus, providing high capacity to the
intra-DC network is of great importance. Hereafter, we refer to
the DC network (DCN) as the one that handles the traffic within
a DC. Moreover, the DCs are predicted to consume around
3~13% of global electricity in 2030 [5]. Communications are
essential in DCs, and therefore there is a strong need for power
efficient techniques in DCNs. To be able to support future
generations of communication use cases and scenarios, which
increasingly involve cloud facilities [6-7], low latency is
required for end-to-end connections. The extreme examples are
autonomous driving and remote surgery, where ultra-low
latency is needed.
To address the requirements on capacity, power
consumption and latency, optical communication techniques
have been introduced in DCNs [8]. In addition, optical
multiplexing techniques, such as spatial division multiplexing
(SDM) and wavelength division multiplexing (WDM), have
been considered to increase the transmission capacity in DCNs.
SDM uses the controllable arrangement of optical signals in
the spatial domain. It has a great potential for DCNs because it
offers ultra-high capacity and good compatibility with WDM
techniques. The initial stage of deploying SDM in DCNs was to
deploy parallel optical fibers connecting servers and/or racks,
and then the optical fiber bundles/ribbons were introduced to
reduce the cabling complexity [8]. To date, the parallel optical
fibers and fiber bundles/ribbons are commonly deployed in
commercial DCs. Moreover, O-band coarse WDM techniques,
e.g., 8×50Gbps for 400 Gigabit Ethernet (GbE) interface [9],
are commercially used in combination with the aforementioned
SDM techniques for DCN interconnections to further increase
the capacity [10]. However, DCNs are continuously coping
with ever-increasing capacity demand, and the service
providers are looking for higher transmission speed in DCNs,
e.g. 1TbE and beyond [11]. Thus, a next step for increasing the
DCN capacity still needs to be investigated.
Recently, SDM techniques using multicore fiber (MCF) or
few-mode fiber (FMF) have gained a lot of attention. By
arranging numerous spatial channels in a single fiber, the
spatial efficiency, defined as the throughput per unit of
Enabling Technologies for Optical Data Center
Networks: Spatial Division Multiplexing
Lu Zhang, Member, IEEE, Jiajia Chen, Senior Member, IEEE, Erik Agrell, Fellow, IEEE, Rui Lin, and
Lena Wosinska, Senior Member, IEEE
(Invited Tutorial)
D
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cross-section area, can be greatly improved and more spatial
channels can be transmitted in parallel to increase the capacity.
Moreover, the channels using coarse or dense WDM techniques
[12] can be transmitted in the MCF/FMF to improve the
spectral efficiency when needed [13]. In [14], the authors
benchmarked common DCN topologies [15] under SDM and
WDM techniques in terms of network throughput, resource
utilization, blocking probability, cost and power consumption.
It has been shown that SDM and WDM techniques exhibit
comparable performance for different topologies, and it is
feasible to combine SDM and WDM techniques in DCNs.
This tutorial paper concentrates on the SDM as an enabling
technology for high-performance DCNs. We outline various
SDM components, transmission options and interconnect
architectures, and highlight their development trend. We also
present technical insights into the co-existence of SDM with
other multiplexing techniques, such as WDM and flexible
spectrum multiplexing, in optical DCNs.
The rest of this paper is organized as follows. Section II
considers the device level and presents a review of key SDM
components for DCNs, including SDM fibers,
multiplexers/demultiplexers, and switches. Section III
describes the state-of-the-art SDM transmission options for
DCNs that have already been experimentally demonstrated.
The design of SDM transceivers, the modulation formats and
signal processing algorithms in SDM enabled DCN links are
also discussed. In Section IV, the network aspects for SDM
based DCNs are presented, including interconnect architectures
along with the resource allocation strategies, where the
potential technical challenges and future research directions are
also discussed. Finally, Section V summarizes the paper and
provides the final conclusions.
II. KEY SDM COMPONENTS FOR DCNS
This section outlines the key SDM components for DCNs.
DCNs have a relatively short reach (typically up to a few
kilometers) and hence no strong needs for amplification [16].
Therefore, SDM amplifiers are not considered in this section.
The other relevant components, i.e., SDM fibers, SDM
multiplexers (MUX) and demultiplexers (DEMUX), and SDM
switches are described in the following.
A. SDM Fibers
Current DCNs use either individual parallel optical fibers or
fiber bundles/ribbons [17], due to their better energy and spatial
efficiency than electrical cables. Parallel optical fibers and fiber
bundles/ribbons, which use a bundle of conventional
single-mode fibers (SMFs) packed together, can be considered
as a straightforward way to realize SDM. However, they are
still not spatially efficient enough, and the photonics integration
of them is difficult. Instead, MCF and FMF can improve the
spatial efficiency, whereas higher component cost and more
complex installation may arise in such DCNs, since the
technologies are presently less mature. In the following, we
mainly focus on MCF and FMF types of SDM.
Multicore fiber (MCF) can be mainly categorized as
uncoupled, weakly-coupled, or strongly-coupled MCFs [18-20].
A widely considered MCF type is the 7-core hexagonal
arrangement shown in Fig. 1 [21], where a marker is used for
core identification. In the design of MCFs, a trench-assisted
structure is proposed for reducing the coupling among the cores.
By surrounding each core with a low-index trench layer, the
electric field distribution in each core is suppressed and the
overlap of electric field among adjacent cores becomes small
[22-23]. As a result, the inter-core crosstalk (IC-XT) could be
significantly reduced in the trench-assisted MCF. For example,
the IC-XT of the commercially available 7-core fiber with a
trench-assisted structure in [24] is as low as -45dB/100km,
which means that the cores of MCFs can be treated almost
independently within the typical reach in DCNs.
The advances in MCF fabrication enables higher spatial
efficiency with a novel cross-section geometry design, such as
two-pitch 10-core fiber [25], dual–ring 12-core fiber [26], and
hexagonal 30-core fiber [27]. Linearly arrayed MCFs with core
arrangements in a rectangular shape [28, 39], as shown in Fig. 2,
are well suited for integration with silicon photonic transceivers.
For instance, a 100-Gbps parallel single-mode silicon photonic
system uses surface coupling with an 8-core MCF, using 4
cores for transmission and the other 4 for reception [28].
Uncoupled or weakly-coupled MCFs having a low core
count shows a relatively low IC-XT [37]. Increasing the core
count in MCFs, keeping the cladding diameter fixed, offers a
higher spatial efficiency. For long-term reliability, low cabling
cost, and compatibility with the current SMF fabrication
process [18], it is advantageous to keep the standard 125µm
cladding diameter (ITU-T G.657 A1 [29]). However, when the
core count increases for a given cladding diameter, the coupling
between adjacent cores gets stronger, often referred to as
strongly-coupled MCFs. Strongly-coupled MCFs typically
show high coupling between adjacent cores, inducing high
IC-XT and therefore deteriorating the signal transmission
performance. The peak capacity is attained at a core count of
Fig. 1. (a) The cross-section view of the 7-core MCF and (b) the refractive
index profile of MCF.
Fig. 2. (a) The cross-section view of the MCF with linear arrayed cores in the
arrangements of (a) 1×2, (b) 1×4, (c) 2×4 [39]. (© IEEE, reprinted with
permission.)
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about 25~30 depending on the signal-to-noise ratio (SNR) [30].
To some extent, the IC-XT can be overcome by digital signal
processing (DSP) techniques, which is discussed in Section III.
The long-term IC-XT is often modelled as independent of
the modulation format [30]. The short-term IC-XT fluctuations
depend on many factors, including modulation format [30],
symbol rate [31], skew between cores [31], operating
wavelength [32], temperature [33], and pseudo random binary
sequence (PRBS) length [33]. Carrier-supported signals, such
as intensity modulated signals, induce the IC-XT that varies
with time [31]. The distortion of the transmitted signals is
caused either by transmission through large inter-core skew
MCFs or by using high symbol rates [31, 34]. It may be also the
reason of the IC-XT fluctuations. Besides, longer PRBS and
lower temperature contribute to a lower IC-XT [33]. The
IC-XT dynamics need to be considered in the deployment of
MCFs. Furthermore, the tolerance to crosstalk depends strongly
on the modulation format, as constellation points using
higher-order modulation lie closer together and are more easily
confused in the receiver [35-36].
In the first generation of optical DCNs, multimode fiber
(MMF) was used to carry the optical signals. Although such
fibers support tens of linear polarization modes [16], this
degree of freedom was originally not exploited. The
transmission quality in MMF is influenced by mode dispersion,
inter-mode crosstalk, and interference induced by differential
mode delay. As a result, the allowed transmission distance is
limited to a few hundred meters. To mitigate these impairment,
and simultaneously increase spatial efficiency, MMFs have
been recently considered in an SDM context, transmitting
independent data on different modes. In such systems,
multimode multiplexers (MUXs) and demultiplexers
(DEMUXs) are applied for signal recovery, along with
multiple-input multiple-output (MIMO) DSP. As a simpler
alternative, few-mode fiber (FMF) utilizes a small number of
linear polarization modes as spatial channels, by optimizing the
design of the refractive index of the fiber core and cladding.
Although FMFs [38-39] can reduce the DSP complexity
compared with MMFs, the MIMO-DSP is also required to
process the inter-mode crosstalk. Few-mode multicore fiber
(FM-MCF) multiplexing several modes in each core is a
combination of MCF and FMF, which increases the spatial
efficiency compared with using either of them. Compared to
MMF, the complexity of DSP can be significantly reduced in
FM-MCFs thanks to the simplified MIMO DSP matrix [16],
while the same number of the spatial channels can be achieved.
On the other hand, the transceivers, switches and
MUX/DEMUX modules tailored for the FM-MCFs are still
under development.
There are also several novel SDM fiber techniques, which
can potentially be appropriate for the future DCNs. Firstly, to
overcome the mode coupling problem, an elliptical-core FMF
has been proposed for SDM transmission [40]. The propagation
constants for the linear polarization modes in each mode group
are different due to the asymmetrical shape of the core, and the
mode coupling can be suppressed. Secondly, so-called
hollow-core photonic band gap fibers have been designed
[41-42], which guide the light based on the photonic band-gap
mechanism through a carefully designed mesh of tiny air- or
vacuum-filled tubes. This type of fibers enables higher speed of
light and supports transmission carried by 2μm wavelength, but
it suffers from higher power loss compared to the conventional
fibers. Moreover, extremely high mechanical precision in
manufacturing is required.
B. SDM MUX/DEMUX Modules
The components that connect a number of single-mode
single-core fibers to the corresponding spatial channels of SDM
fibers are referred to as SDM MUX/DEMUX modules. They
are important for SDM deployment in DCNs, since they can
make SDM compatible with existing systems, reduce the
capital expenditures, and improve the scalability and flexibility
when upgrading the DCNs. Besides, SDM MUX/DEMUX
modules can bring fine switching granularity in DCNs,
allowing for switching on a per-spatial-channel basis.
We will discuss SDM MUX/DEMUX modules for MCF,
FMF, and MMF separately. First, for MCFs, the SDM
MUX/DEMUX modules (also known as fan-in/fan-out
modules) connect a number of single-mode single-core fibers
to different cores in an MCF. As shown in Fig. 3, the operation
principle of SDM MUX/DEMUX modules connecting MCF
can be mainly divided into free-space optics [43], fused taper
[44], fiber bundle [45], and compact waveguide coupling [46].
Using free-space optics (see Fig. 3a), a single lens is applied
to couple the single-core fiber’s outputs to the corresponding
MCF cores. It is realized by putting the end facet of the MCF at
the front focal point of the single lens. The fabricated module
with 40mm (diameter) and 62mm (length) presented in [43]
exhibits IC-XT below -50dB. The insertion loss for each port is
lower than 0.6dB and the difference in coupling loss is below
0.4dB. Compared with direct fiber-to-fiber coupling, this
design is tolerant to a shift in the core position. However, this
scheme is bulky
and requires sophisticated opto-mechanical
operations.
The fused-taper scheme (see Fig. 3b) utilizes the elongation
process to consolidate the single-core fibers with the
corresponding MCF. The diameter of the module presented in
[44] is 0.72mm and its length is 35mm. The maximum insertion
loss of this module is 4.7dB and the worst IC-XT is -45dB at
1550nm. The fiber-bundle-based fan-in/fan-out fabrication
scheme (see Fig. 3c) uses chemical etching of the single-core
fibers until the cladding diameter matches the corresponding
core pitch of the MCF. The device reported in [45] is 5mm wide
and 32mm long. It is characterized by IC-XT lower than -50dB
and by the insertion loss around 0.6dB. These two kinds of
schemes are quite compact and its fabrication process is
cost-efficient. The crosstalk needs to be carefully controlled
during the fabrication.
The compact waveguide coupling scheme realizes
fan-in/fan-out functionalities by inscribing spatially isolated
waveguides that connect
each core of the MCF to a particular
SMF. Fig. 3d illustrates such a fan-out module using a
laminated polymer waveguide [46]. The insertion loss of the 19
cores in the device in [46] varies from 0.2dB to 1.8dB and the
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IC-XT of all the 19 cores is lower than -40dB. Compared with
the aforementioned schemes shown in Figs. 3a-c, the
waveguide coupling scheme exhibits merits in terms of
compactness and simple large-scale fabrication. The MCF with
a compact fan-in/fan-out module, which couples an MCF to
laser diodes (LDs) and photodiodes (PDs) with pluggable
connections, can also be a feasible solution for DCN
applications. All the mentioned module types have been
commercialized by companies like OptoQuest [47], Optoscribe
[48], and Chiral Photonics [49]. The crosstalk, insertion loss,
and reflection loss have been optimized.
Here we present SDM MUX/DEMUX modules for FMF and
MMF together, both of which are for mode MUX/DEMUX.
Such modules connect single-mode single-core fibers to
individual transmission modes in a fiber. The operation
principles of the MUX/DEMUX modules can be mainly
divided into mode conversion [50-52], index matching [53-54],
and photonic lantern schemes [55].
In the mode-conversion scheme, the single mode in the
conventional fibers is converted to linear polarization modes by
phase plates or long period gratings [50], after which the modes
are combined again using beam combiners. The mechanism of
this scheme is simple, but it suffers from high power loss
induced by conversion and combination. The simultaneous
mode-conversion schemes convert a single mode into multiple
modes by free-space optics and a reflective phase plate [51] or
grating couplers [52]. The grating coupler scheme requires
control of phase tuners and input light polarization.
In the index-matching scheme, asymmetric directional
couplers based on fused fibers [53] or other waveguides [54]
are used to couple multiple modes into a common multimode
port. The conversion efficiency is high, but the fabrication
process is more complex than for the mode-conversion scheme.
Finally, the photonic-lantern scheme [55] is realized with a
mechanism similar to the aforementioned fused-taper scheme,
where the cores are allocated closely enough for strong
core-coupling [55].
C. SDM Switches
To meet the demands for flexible service provisioning in
DCNs, switching and add/drop multiplexing in the spatial
domain need to be provided. SDM switching can be considered
in the wavelength and/or time domain in order to enable
compatibility with the existing infrastructure. There are various
approaches to realize switching functionality in the spectrum
and space domains. The straightforward way is to use the
aforementioned SDM MUX/DEMUX modules connecting
MCF/FMF/MMF at the input/output ports of the traditional
switching modules [56]. Inside the switching node, the input
signals are first separated into independent spatial channels,
and then the micro-electro-mechanical system (MEMS) mirrors
or liquid crystal on silicon (LCoS) [57-59] based
wavelength/spectrum selective switches (WSS/SSS) and/or
optical cross-connects are used to switch the demultiplexed
spatial channels.
An experimental demonstration of joint-spectral-spatial
switching using an LCoS-based WSS was presented in [58]. A
heterogeneous SDM network was considered, applying 3 types
of SDM fiber spans and a WSS with a large port count
supporting 6 modes. A WSS integrated with FMFs was
experimentally demonstrated in [59]. By arranging the WSS
input/output fibers in an array, a set of inputs to an LCoS-based
WSS can be steered onto different sets of outputs. Combining
the input/output SMF groups using SDM MUX/DEMUX
allows switching all the spatial channels to their destinations.
This technique reformats the channels of the SDM fiber without
sacrificing the hardware complexity of the WSS. Moreover, by
arranging the spatial channels in a 2D array, the switching
Fig. 3. The SDM fan-in/fan-out fabrication process with (a) free space optics
scheme, (b) fused taper scheme, (c) fiber bundle scheme, and (d) compact
waveguide coupling scheme.
Fig. 4. A silicon photonic integrated circuit for an SDM switch with MCF [61].
(© Springer Nature, reuse permitted by the CC BY license.)
![Fig. 2. (a) The cross-section view of the MCF with linear arrayed cores in the arrangements of (a) 1×2, (b) 1×4, (c) 2×4 [39]. (© IEEE, reprinted with permission.)](/figures/fig-2-a-the-cross-section-view-of-the-mcf-with-linear-26nllbv6.webp)
![Fig. 5. (a) The optical spectrum, (b) P-I-V curves, and (c) small-signal S21 responses of 1.5µm single mode VCSEL [24]. (© IEEE, reprinted with permission.)](/figures/fig-5-a-the-optical-spectrum-b-p-i-v-curves-and-c-small-1zs1b7k8.webp)
![Fig. 4. A silicon photonic integrated circuit for an SDM switch with MCF [61]. (© Springer Nature, reuse permitted by the CC BY license.)](/figures/fig-4-a-silicon-photonic-integrated-circuit-for-an-sdm-kfk3h2mb.webp)
