Page 86 - ITU Kaleidoscope 2016

In addition to the capacity crunch, SDM can be used to
realize such functions as the effective operation and/or
management of the optical infrastructure, and an optical
transmission system with a lower power consumption. The
worldwide expansion of the network scale and its
divergence has led to a rapid increase in the number of
optical fibres particularly in central offices and data centres.
Thus, an urgent problem for both network operators and
service providers is to obtain an effective tool for solving
the optical fibre conjunction in their facilities. The SDM of
the optical physical infrastructure potentially provides a
direct solution to this problem in terms of space saving. On
the other hand, the latest transmission system enables us to
realize a rate of 40 Gbit/s or more by employing digital
signal processing technology and a powerful error Figure 2. Historical progress on communication
correction scheme. And this technological progress cable density. Black, blue and red corresponds to 400-pair
generally increases the power consumption per bit copper, slot-type 400 fibre ribbon and 400 rollable fibre
particularly in metro and core optical transmission systems. ribbon [4] cables, respectively.
Here, SDM technology also enables us to reduce power
consumption by sharing some of the active components
used in the system. For example, Ref. [3] showed that
power efficient ultra-high capacity long-haul transmission
system can be designed by sharing pump light for optical
amplifiers with multiple spatial channels. Therefore, it can
be said that SDM technology will be a key transmission
strategy for future optical communication, and it should be
used effectively for updating existing optical
communication systems in a sustainable manner. Then, we
discuss SDM in relation to optical fibre cable technology

3. SDM IN OPTICAL FIBRE CABLE

3.1. Additional Spatial Dimension
Figure 2 shows historical progress made on communication Figure 3. Schematic image of two representative
cable density. The black, blue and red circles show 400-pair spatial dimensions of mode and core in an optical fibre.
copper, slot-type 400 fibre ribbon, and 400 rollable fibre
ribbon [4] cables, respectively. The solid green line shows simultaneously [5]. However, mode division multiplexing
the numerical limit when we assume a hexagonally packed intrinsically needs a complex transmission strategy because
250 µm fibre bundle with a 2 mm thick cable sheath. Figure of the variation in the mode coupling and/or transmission
2 reveals that the communication cable density has been characteristics between modes in an optical fibre. At the
increased greatly by employing optical fibre and a ribbon same time, MCF has been continuously investigated, and we
-2
structure. A cable density of 4.1 mm is obtained with the can find example pioneering studies in Refs. [6] and [7].
latest rollable ribbon as shown in the inset photograph, and Moreover, MCF with a single-mode core can easily employ
the value is approaching the geometrical limit. Thus, Fig. 2 the latest single-mode technology. In this paper, we focus
shows that we need another dimension if we want to realize on MCF technology and investigate the potential of MCF as
a higher density beyond the current geometrical limit. Here, an SDM transmission medium.
we should note the possibility of a higher count fibre bundle
rather than a higher cable density. It is true that we can 3.2. Potential of MCF
overcome the future capacity crunch simply by introducing MCF has three key geometrical parameters; (I) cladding
additional SMFs. However, this approach intrinsically diameter D, (II) cladding thickness t, and (III) core pitch Λ
needs additional space, and it results in an additional cost as shown schematically in Fig. 4. The cladding diameter D
for constructing/renewing the physical infrastructure. is closely related to the mechanical reliability of the optical
Historically, optical fibre is tailored using the two fibre. Generally speaking, a larger D degrades the failure
spatial dimensions of mode and core as shown in Fig. 3. probability when there is a small bend [8], although a larger
Thus, there is the potential to overcome the current D value is useful for increasing the core number in MCF.
geometrical limit of traditional optical fibre cable by
employing mode and/or core multiplexing adequately. The Figure 5 shows an example of the calculated relative
latest research has proved that we can obtain more than 100 failure probability as a function of D. Red and blue solid
lines correspond to proof levels of 1 and 2%, respectively.
spatial channels by employing mode and core multiplexing

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