Optical space division multiplexing uses multiple cores or modes in optical fibers to increase transmission capacity. A history of the technology was provided, noting the progression from single mode fibers to coherent detection and polarization multiplexing. Limits with single mode fibers were discussed, along with ways that multi-core and multi-mode fibers can overcome capacity constraints through spatial multiplexing across fiber cores and modes. Recent demonstrations showed record capacities of 57.6 Tb/s over multi-mode fiber and 24 Tb/s over hollow-core fiber. Integration challenges with spatial division multiplexing were also outlined.

1. 1
Optical Space
Division
Multiplexing

5. 5
Evolution of optical communications systems and future
developments

6. 6
Overview of SDM optical
communications technology

7. 7
State of SDM research and development

8. 8
Study toward dense SDM optical communications

9. Copyright © 2013 Nokia Siemens Networks
9

10. A brief history of Optical Communication
The original optical format used on-off-keying
(OOK), with bits indicated by “light on” and “light
off” states.
• The introduction of differential binary phase shift keying
(DPSK) increased the receiver sensitivity and as such
allowed for higher data rates, whereas differential
quadrature phase shift keying (DQPSK) doubled the
spectral effficiency.
• Capacity was then increased by increasing the
bandwidth, but higher bandwidth is more vulnerable to
distortion, so there are limits on how far this approach can
go.
10

11. • Spectral efficiency was doubled yet again
by transmitting and receiving information
in two orthogonal polarizations.
• The arrival of coherent technology solved
many underlying issues. Nokia Siemens
Networks releasedits coherent transponder
technology for 40 Gb/s in 2009. This was
followed by 100 Gb/s in 2011.
11

12. Coherent technology blends optics
and electronics, transforming the optical signal
to the electrical domain by mixing it with a
reference of the carrier frequency. This enables
data to pass between the two domains complete
with amplitude, polarization and phase
information, which was not previously possible.
Today’s coherent systems typically use a so-
called 50 GHz grid. They have a reach in excess of
2,500 km depending on fiber type and system
configuration.
12

13. Coherent detection with digital signal
processing (DSP) improves receiver sensitivity,
spectral efficiency and impairment
compensation to increase the robustness and
performance of the system, as shown in Table
1. Finally, the transmitter and receiver
structures remain the same regardless of the
modulation format. This makes it possible to
deploy flexible rate transponders that use
varying modulation formats depending on the
given reach in the system.
13

14. 14

15. The Limits of Single-Mode Fibers
On a standard single-mode fiber with conventional
optical amplifiers, higher order modulation formats
cannot reach 2,000 km because of the required high
optical signal-to-noise ratio.
• New, ultra-low-loss fibers with a high effective area
and Raman amplification will be needed to overcome
the 2,000 km limit with data rates of 400 Gb/s or more.
• Employing FlexiGrid, the improvements in spectral
efficiency over standard single-mode fiber will only
achieve a 20% boost in capacity, leading to a significant
shortfall.
15

16. Ways to modulate and multiplex channels to increase
system capacity in optical transmission. 16

17. Multi-Core and Multi-Mode Fibers
Solid-core record breaker
In 2012, Nokia Siemens Networks demonstrated a
record capacity of 57.6 Tb/s transmitted over multi-
mode fibers. This is six times the capacity of current
100 Gb/s systems and more than doubled the previous
record over multi-mode fiber. The demonstration
featured 200 Gb/s DP-16QAM per mode and
wavelength transmitted over three spatial modes and
96 channels in the extended C-band. The transmission
distance was 119 km with inline multi-mode
amplification. 17

18. Hollow Core Record Breaker
Nokia Siemens Networks demonstrated record-
breaking single mode capacity over hollow-core
fiber in the first ever demonstration of coherent
technology over this fiber type. The experiment
achieved a capacity of 24 Tb/s. The researchers
used single-mode transmission of 96 wavelength
channels carrying dual polarization 32QAM
modulation.
18

19. 19

20. 20

21. 21

22. NATURE PHOTONICS | VOL 8 | MAY 2014 |
www.nature.com/naturephotonics
22

23. Five physical dimensions can be employed to
carry optical data (Fig. 1): time, frequency, space,
polarization and quadrature. These dimensions
can be simultaneously used to greatly increase
the bit rate of a communication system.
To avoid a ‘capacity crunch’, future optical
networks will need to simultaneously transmit multiple
spatial channels. For spatial multiplexing to be practical,
the upgrade path from legacy wavelength-division
multiplexed systems needs to be smooth and to
consider integration-induced crosstalk from the outset.
23

24. 24

25. 25
Compatibility: As long as parallel fibre strands
are available within a deployed cable, operators
will want to make use of those, based on
existing and deployed wavelength bands for
which mature components are available. Hence,
hybrid network architectures that use parallel
fibre strands on some spans and possibly new
SDM-specific fibre on other spans must be
supported (see Fig. 2). A transition to new
wavelength bands seems unlikely unless it
results in a substantial (orders of magnitude)
increase in system capacity and/or repeater
spacing.

26. 26
Integration: To follow historic trends and reduce cost
and energy consumption per transmitted bit by about
20% per year, integration and unification of equipment in
parallel WDM systems is essential. As shown in Fig. 2,
integration may take place on a systems level (including
network management and control), on a network
element level (including reconfigurable optical
add/drop multiplexers (ROADMs)), on a
transponder level (similar to what is currently being done
for spectral superchannel interfaces, where multiple
signals at adjacent wavelengths are cohesively bonded to
form a single architectural entity), on an optical amplifier
level, and on a fibre and splice/ connection level to
reduce installation costs.

27. 27
Crosstalk: Hardware integration at various
levels will inevitably result in crosstalk among
parallel SDM paths and consequently in
transmission penalties. If the end-to-end
crosstalk from integrated transponders,
multiple ROADMs, optical amplifiers and
transmission fibre with splices every few
kilometres (that is, on the order of 1,000
splices for long-haul

28. 28

29. 29

30. 30

31. Figure 1 The evolution of transmission capacity in optical fibres
as evidenced by state-of-the-art laboratory transmission
demonstrations. 31

32. Figure 1 The evolution of transmission capacity in optical fibres
as evidenced by state-of-the-art laboratory transmission
demonstrations. The data points represent the highest capacity
transmission numbers (all transmission distances considered)
reported at the post deadline sessions of the annual Optical Fiber
Communications Conference over the period 1982 to the present.
The transmission capacity of a single fibre increases by a
factor of approximately 10 every four years. Key
previous technological breakthroughs include the development of
low-loss SMFs, the EDFA, WDM and high-spectral-efficiency
coding through DSP-enabled coherent transmission. The data
points for SDM also include results from the postdeadline session
of the annual European Conference on Optical Communications in
2011 and 2012. SDM seems poised to provide the next big jump in
transmission capacity.
32

33. Figure 2 Different approaches for realizing SDM.
33

34. Figure 2 Different approaches for realizing SDM.
a- Fibre bundles composed of physically independent
SMFs with reduced cladding thickness could provide
increased core packing densities relative to current fibre
cables. However, ‘in-fibre’ SDM is required to achieve
the higher core densities and integration levels
ultimately desired.
b- MCF containing multiple independent cores with
sufficiently large spacing to limit crosstalk. Fibres with up
to 19 cores have been demonstrated for long-haul
transmission — higher core counts are possible for short-
haul applications (for example, data communications)
for which higher levels of crosstalk per unit length can be
tolerated.

35. 35
c- FMF with a core dimension/numerical aperture set to
guide a restricted number of modes (typically 6–12
distinct modes, including all degeneracies and
polarizations).
d- Coupled-core fibres support supermodes that allow
higher spatial mode densities than isolated-core fibres.
MIMO processing is essential to address the inherent
mode coupling.
e- Photonic bandgap fibres guide light in an air core and
thus offer ultralow optical nonlinearity and potentially
lower losses than solid-core fibres. Work is currently
being conducted to determine whether such fibres can
support MD

36. Figure 3 The many components needed to fully exploit
the advantages of high-density SDM. a, Elegantly
scalable passive multiplexers.
36

37. b, Integrated
transmitter
and receiver
arrays
providing low-
loss coupling
to many
modes of an
SDM fibre.
37

38. c, Reconfigurable routing elements that can direct SDM traffic without the
need for electronic MIMO in between transmitter and receiver. The strategy
for switching wavelengths and modes, as well as any additional required
functionality, will determine the complexity of the ROADM architecture. 38

39. Table1 Summary of progress in SDM system experiments. Upper
rows (shaded in purple) show experiments utilizing multicore
fibres (MCF). Center row (shaded blue) shows a MCF-FMF result.
Lower rows (shaded in pink) show transmission results over
fewmode fibres (FMF). The channel rate includes polarization
multiplexing; the Net Spectral Efficiency and Net Total Capacity
exclude the overhead for forward-error-correction. SM: single-
mode, FM: few-mode, coupled MCF: coupled-core MCF, mstr-
MCF: microstructured, coupled-core MCF. The majority of SDM
transmission experiments have utilized either 7-core MCF or FMF
supporting 3 spatial modes. The table shows the rapid progress
in both reach and net capacity achieved in just two years. Recent
experiments utilizing a 12-core MCF and a MCF with hybrid SM
and FM cores have demonstrated record net capacities of more
than 1 Pb/s.
Progress in Systems Demonstration
39

40. 40

41. 41

42. 42

43. 43

44. Figure 1.01 Pbit/s MCF WDM/SDM/PDM transmission experiment
2012
Takara, H. et al2012 (ECOC 2012), (2012). 44

45. (a) Microscope image of the cross section of the onering,12-core
fibre. (b) Total crosstalk from all other cores after transmission
over the 52km 12-core MCF and fan-in/fan-out devices (filled
circles), and crosstalk penalty measured for two channels (filled
triangles).
45

46. (c) Schematic diagram of the transmission system setup.
ECL: external cavity laser, SC-FDM Tx: single-carrier
frequency-division multiplexed transmitter, CPL: coupler,
Pol Mux: polarization multiplexer, OTF: optical tunable
filter, Rx: receiver 46

47. (d) Measured Q-factors of the 222 WDM channels in each of the
12 cores after 52-km transmission. 47

48. Figure 5 | 57.7 Tbit s−1 amplified WDM/MDM/PDM transmission
experiment over a few-mode fibre34,102. a, Schematic of the experimental
set-up showing (left to right) the three sets of transmitters for odd and even
channels and the channel under test (CUT), splitting and amplification for
launch into the mode multiplexer, transmission over the two few-mode-
fibre spans with in-line MM-EDFA and mode demultiplexer and
simultaneous reception of the channels transmitted in the three modes for
MIMO processing. AWG, arrayed waveguide grating multiplexer; DAC,
digital-to-analog converter; WSS, wavelength-selective switch; LO, local
oscillator. b, Measured bit-error rates (markers) of all 96 channels in each of
the three modes and optical spectrum (blue curve) after transmission over
the 119 km of few-mode-fibre with a mid-span amplifier. BER, bit error rate;
FEC, forward error correction.
48

49. 49

50. Spatial Division Multiplexing
with a
Few-Mode Multicore Fibre
Ref: R. G. H. van Uden, ” Ultra-high-density spatial division multiplexing
with a few-mode multicore fibre”, Nature Photonics Oct 2014
50

51. Single-mode fibres with low loss and a large transmission
bandwidth are a key enabler for long-haul high-speed optical
communication and form the backbone of our information-
driven society. However, we are on the verge of reaching the
fundamental limit of single-mode fibre transmission capacity.
Therefore, a new means to increase the transmission capacity of
optical fibre is essential to avoid a capacity crunch. Here, by
employing few-mode multicore fibre, compact three-
dimensional waveguide multiplexers and energy-efficient
frequency-domain multiple-input multiple-output equalization,
we demonstrate the viability of spatial multiplexing to reach a
data rate of 5.1 Tbit s−1 carrier−1 (net 4 Tbit s−1 carrier−1)
on a single wavelength over a single fibre. Furthermore, by
combining this approach with wavelength division multiplexing
with 50 wavelength carriers on a dense 50 GHz grid, a gross
transmission throughput of 255 Tbit s−1 (net 200 Tbit s−1)
over a 1 km fibre link is achieved. 51

52. Single-mode fibre transmission, is rapidly
approaching its fundamental capacity limits
 In the past, capacity increases in SMF transmission
systems have been achieved by exploiting various
dimensions, including polarization and wavelength
division multiplexing, in tandem with advanced
modulation formats and coherent transmission
techniques
To alleviate the corresponding costs and increased
energy requirements associated with the linear
capacity scaling from using additional SMFs, spatial
division multiplexing (SDM) within a single
fibre can provide a solution. 52

53. SDM is achieved through multiple-input multiple-
output (MIMO) transmission, employing the
spatial modes of a multimode fibre (MMF), or
multiple single-mode cores, as channels.
Recently, a distinct type of MMF, the few-mode
fibre (FMF), has been developed to co-propagate
three or six linear polarized (LP) modes. Driven
by rapid enhancements in high-speed electronics,
digital signal processing (DSP) MIMO techniques
can faithfully recover mixed transmission
channels, allowing spectral efficiency increases
as spatial channels occupy the same wavelength.
53

54. In this work, ultra-high-capacity transmission over a 1 km is demonstrated using
54

55. Hole-Assisted Few-Mode Multicore Fibre
55

56. 56

57. Figure 1 | Few-
mode multicore
fibre
characteristics.
a, FM-MCF cross-
section, which is
butt-coupled to
the 3D
waveguide.
57

58. b, Measured MIMO equalizer response of all seven cores for the transmitted
LP01 X-polarization to the received LP01 X-polarization at 1,555.75 nm, allowing
differential mode delay estimation with an accuracy of ∼20 ps, indicating a
similar performance of all cores. As all LP modes are equally excited, the
respective MIMO equalizer matrix elements are similar.
58

59. 59
At the transmitter side (Fig. 2a), 50 conventional low-linewidth
(<100 kHz) external cavity lasers (ECLs) on a dense 50 GHz ITU
frequency grid spanning the telecom C-band wavelength region
from 1,542.14 to 1,561.81 nm were used as loading channels.
During the FM-MCF performance verification, all few-mode
cores were investigated as the core under test (CoUT) was varied
consecutively. By replacing each loading wavelength channel
within the channel under test (ChUT) laser, quantitative
performance measurements were performed via bit-error-rate
(BER) estimation. To manage the complexity of the
measurement process, the transmitter ChUT laser was split into
two equal tributaries, with the second tributary acting as a
remote local oscillator (LO) for all 700 measurements.
Experimental implementation

60. 60
As the transmitter and receiver shared the same laser source, a 10
km SMF was inserted into the LO path to minimize the impact of
laser phase coherence. All carriers were modulated by a lithium
niobate (LiNbO3) IQ-modulator, which was driven by two digital-
to-analog converters (DACs), representing the in-phase and
quadrature components, to generate a 24.3 GBaud 16 or 32 QAM
signal. Higher-order modulation formats are particularly
interesting for fibre performance investigation because of their
susceptibility to impairments in the transmission channel. Before
transmission, the modulated carriers were decorrelated
consecutively for polarization, carriers, modes and cores. The
relative powers of the decorrelated carriers are shown in Fig. 2b,
showing a relatively equal power distribution over the entire
transmitted wavelength band.

61. 61

62. 62

63. 63

64. 64

65. 65

66. 66
Decorrelated wavelength spectrum after being interleaved by a wavelength-
selective switch.

67. 67
Saturated camera image taken at the receiver side, with all cores lit
simultaneously

68. 68
Independent cores are excited, indicating low crosstalk per core. Right bottom:
selective launching of the LP01 and LP11 modes in the centre core (core 4),
respectively, where the modal energy is confined to the centre of the hexagonal
core structure.

69. 69
The 21 SMF inputs (with a 127 μm pitch V-groove) were attached
to waveguides (assigned in seven sets of three waveguides) and
inscribed in a hexagonal arrangement with a diameter of 80 μm
to match the core arrangement and structure of the FM-MCF (Fig.
3a). The individual square waveguides have a cross-sectional
effective area of 36 μm2 (Fig. 3b), and each set of three
waveguides was placed in a triangular arrangement38. This
arrangement minimizes insertion losses, while equally exciting
the LP01 and LP11 degenerate modes in each core to minimize
mode-dependent loss (MDL). The MDL was approximated at 1.5–
2 dB and the insertion loss on average was 1.1 dB across all 21
waveguides (excluding fibre) at 1,550 nm. The total end-to-end
loss measured after transmission was 12 dB (including
multiplexer and demultiplexer 3D waveguide), which is in line
with single-core few-mode results34. Crosstalk was measured by
individually exciting the different cores. The measured crosstalk
value was below −70 dB km−1.

70. 70
Figure 3 | Three-dimensional waveguide characteristics. a,
Schematic of the 3D waveguide, where sets of three transparent
waveguides are placed in a triangular arrangement to address
respective few-mode cores.

71. 71
b, 3D waveguide FM-MCF facet microscope image.

72. 72
To enable mode measurement after FM-MCF
propagation, an elegant few-mode measurement
technique is proposed and devised . The essence of the
technique is shown in Fig. 4a, where the novel time-
domain multiplexed (TDM) SDM receiver
with two dual-polarization coherent receivers is used
for simultaneous reception of the three transmitted
modes per core. The first coherent receiver
receives the first mode and a second coherent
receiver receives the remaining two modes,
demonstrating the potential scaling of received modes
in both time and number of dualpolarization coherent
receivers.

73. 73
Figure 4 | TDM-SDM receiver characterization. a, TDM-SDM receiver, where one dual polarization
mode is received by coherent receiver 1 and two modes are serially received by coherent receiver 2.
The AOMs act as signal gates, with a closing time corresponding to the propagation time of the delay
fibre, enabled by the trigger generator. The variable optical attenuator (VOA) ensures equal power
reception of both modes. In the digital domain, the serialized signals are parallelized to construct
three received dual-polarization modes.

74. 74
To demonstrate the FM-MCF transmission capabilities,
first the transmitter and receiver performance is
characterized by noise loading at the transmitter side
for optical signal-to-noise ratio (OSNR) measurements
as depicted in Fig. 4b. At the 40 dB OSNR level, with
respect to back-to-back measurements, the 6 × 6
single channel MIMO measurements for 16 and 32
QAM indicate a modest error floor increase of 2 × 10−4
and 8 × 10−4 for single wavelength transmission,
respectively. This BER increase is mainly attributed to
residual channel interference after the frequency
domain MIMO equalizer unravels the channels.
Results

75. 75

76. 76
With every wavelength channel in all cores enabled
simultaneously, Fig. 5a,b demonstrates the successful
transmission of 16 and 32 QAM, respectively, where
the forward error correcting (FEC) overhead is
assumed to ensure error-free transmission. The average
values of BER after WDM transmission (Fig. 5a,b) are
approximately 1 × 10−3 and 5 × 10−3, corresponding to
respective BER penalties of 8 × 10−4 and 4 × 10−3 for 16 and 32
QAM transmission in comparison with single-channel
performances at 40 dB OSNR. The insets in Fig. 5a,b represent
the best- and worst-performing channel constellations of the
respective constellation types, and the gross aggregate
transmission capacity of 255.15 Tbit s−1 signifies the potential
high-density space-division-multiplexing capabilities of the
hole-assisted FM-MCF.

77. 77
Figure 5 | Transmission results. a, 16 QAM transmission
performance. All channels perform well below the 7% hard
decision FEC limit required to ensure error-free transmission for
a 81.6 bit s−1 Hz−1 gross spectral efficiency.

78. 78
b, 32 QAM transmission performance is well below the
20% soft decision FEC limit, providing a 102 bit s−1
Hz−1 gross spectral efficiency

79. 79
However, inherent to space-division-multiplexed transmission
is a received power difference between transmitted channels,
denoted by the MDL. Obtained from mutual information
estimation, MDL is a figure of merit for establishing the
theoretical transmission capacity, which is computed through
singular value decomposition of the transmission channel
matrix, obtained by least squares channel estimation. The MDL
figures of all wavelength carriers are presented in Fig. 5c,d,
with average estimated MDLs of 3.9 dB and 4.4 dB for 16 and
32 QAM, respectively. The small MDL differences are attributed
to slight misalignment between the cores and (de)multiplexer,
as well as wavelength-dependent mode field excitation during
the experiment.
Mode-dependent loss (MDL)

80. 80
c, 16 QAM transmission measured MDL figures for all
transmitted channels, averaging at 3.9 dB.