“There were three ways we could go,” he recalled,
“continue with coax, use waveguides or move to fibre. It was
very apparent that optical fibre was going to be the winner.”
The first subsea optical cable came into use in 1985; a
140Mbit/s link between the UK and Belgium, laid following
trials of the technology across a Scottish loch.
Since then, fibre optic technology has leapt ahead. “The
line rate has gone from 144Mbit/s to 155, 622, 2.5Gbit/s
and 10G,” said Barnes. “Today, we supply technology that
supports 100G per wavelength, with Gbres capable of
supporting more than 100 wavelengths.”
But the rapid increase in line rate – eight generations –
has not been accompanied by a similar improvement in
14 April 2015 www.newelectronics.co.uk
It might seem like submarine communication is a
relatively new fangled idea, but no; the Grst cable to
link the UK and the US came into operation around
1860, carrying telegraphy.
Since those heady days, submarine cables have been
the way to link callers on different continents and, more
recently, to carry the vast amounts of data spawned by the
internet. While communication satellites are available, they
handle much less than 10% of all trafGc.
But it is only in the last 30 years or so that submarine
cables have come into their own and the reason is Gbre
optics. Before that, all data travelled over coaxial cable.
Those of a certain age will recall the difGculty of making a
transatlantic call and the message that ‘all circuits are busy;
please try again later’. The reason was that coaxial cable
had a maximum bandwidth of 45Mbit/s, equating to around
6000 voice calls. The cable was already more than 40mm in
diameter and increasing the bandwidth would have required
it to be even larger, leading to greater cost and handling
issues. Today’s Gbre optic cables – less than 20mm in
diameter – can handle up to 200million voice circuits per
Gbre pair and they may have up to eight Gbre pairs.
Even with that capacity, submarine Gbre optic cables are
coming under pressure as the volume of data spirals.
Xtera is one of the leading companies in the Geld. Stuart
Barnes, general manager of the company’s UK operations, is
a submarine communications veteran, having cut his teeth
at STC Submarine Cables in Greenwich.
He said that, by 1980, it was obvious that coaxial cable
had run out of steam. “We were trying to squeeze the last
drop of performance from the technology,” he recalled.
Meanwhile, work was well underway at STC’s Research
Labs in Harlow into the use of Gbre optics for
communications – work that would win Dr Charlie Kao the
Nobel Prize in Physics 2009 ‘for groundbreaking
achievements concerning the transmission of light in Gbres
for optical communication’. It made sense, in Barnes’
opinion, to see whether that work could be used in the
submarine world.
Xtera is deploying a
new optical
amplifier to increase
the bandwidth of
EDFA repeaters to
55nm
14
Pushingtowards the limits
Submarine fibre optic cables have
enabled modern international
communications, but can photonics
help to alleviate bandwidth problems?
By Graham Pitcher.
ampliGer technology. “There have only been two generations
of optical repeater,” Barnes pointed out, “regenerative and
then the optically ampliGed repeater.”
Regenerative systems required ‘three Rs’ in each
repeater – reamplifying, reshaping and retiming the signal.
“It was an optical to electrical to optical process,” he
explained. Each device was spaced roughly 50km apart on a
transatlantic cable and built using discrete components on
rigid boards. Not only did they have to deal with the data
passing through, they also had to withstand the high
pressure of being on the seabed, which brought mechanical
issues into play.
Tony Frisch, senior vp of Xtera’s repeater business unit
www.newelectronics.co.uk 14 April 2015
and another subsea veteran, added: “There were about 10
ICs per Gbre pair in the regenerator, along with four lasers –
two for each direction – two integrated receivers and a few
discrete components. All these were mounted on
specialised boards. And we also needed to handle Gbres,
electrical connections and power.”
One thing common to cables now and then is power.
Barnes said: “You need 25kV DC to power a long
transoceanic cable and half of that power is lost in the
cable.” Frisch added: “We still push about 1A through the
cable and still need several kV, depending on the length of
the cable.”
Barnes believes submarine Gbre optic cable technology
developed rapidly because all relevant parts of UK industry
were pointed in the same direction. “It was the heyday of
the UK telecomms industry,” he asserted. “There were a lot
of optical research programmes underway, plus there was
serious interest in the technology from GEC, Racal, Plessey
and STC.” BT, for example, did some groundbreaking work
on transistors and ICs.
Even so, developing the required technology wasn’t easy.
“We had to invent everything – from semiconductor lasers to
the Gbre itself – and we had to get everything right.
Submarine cable technology was its own world; everything
had to be done in a particular way because of reliability.”
Times change, however. In the 1980s, submarine cable
technology was leading terrestrial technology. “That’s Hipped
now,” Barnes claimed. “High speed terrestrial
communications is now leading, driven by the needs of data
companies. That’s why we have moved from SDH/Sonet
based rates to those more closely associated with data,
such as 100G.”
As Barnes noted, while there have been eight steps in
data rate, there have only been two developments in
ampliGer technology. Frisch added: “Despite the evolutions
in transmission rate, the ampliGer – which is based on
erbium doped Gbre – is essentially unchanged; the only real
developments have been gain Hattening to get more
bandwidth and higher power.” These developments came in
as systems evolved from being single wavelength to
supporting more and more wavelengths.
The challenge now for those developing subsea systems
is what could be broadly termed ‘the laws of physics’.
“There are limits on the bandwidth of simple doped Gbre
ampliGers,” Barnes explained. “But as far as the Gbre is
concerned, there is still the ‘third window’ to be Glled.”
There are essentially three ‘windows’ in the optical silica
Gbre spectrum, where low attenuation enables optical
communications. Barnes said: “One is centred on 850nm,
using multimode Gbre. The second is centred on 1310nm,
using single mode Gbre. The third window, again with single
mode Gbre, is centred on 1550nm and supports multiple
channels in an optical bandwidth of 32nm. These windows
have an attenuation of about 1dB/km, 0.4dB/km and less
than 0.2dB/km respectively. We quickly went to single mode
Gbre, but were always keen to get to the third window
because it brought a big jump in span length.
“With doped Gbre ampliGers, we can stretch optical
Today’s fibre
optic cables –
less than 20mm
in diameter –
can handle up
to 200million
voice circuits
per fibre pair
and may have
up to eight
fibre pairs
15
COVER STORY
SUBSEA COMMUNICATIONS
Xtera’s optical
amplifer being
installed in a
repeater housing
Illu
stra
tio
n:
Pa
ul W
est
on
bandwidth to 40nm over short distances, but for long
transoceanic cables, we can go to no more than 32nm – the
Gbre bandwidth, however, is more than 100nm.”
Raman repeaters – where ampliGcation is derived from
the Gbre in the cable – offer a signiGcant increase in optical
bandwidth and a way to exploit the ‘third window’ fully.
Subsea cables currently use coherent transmission
technology and this poses another challenge. “One problem
with high capacity coherent transmission is that you’re
limited by data converter
technology; the faster the data
rate, the faster the converter
has to run,” said Barnes, “and
the availability of high speed
converters is an issue. We have
to work out where we go.”
Today, 100G signals use
two polarisations and four
phases, with data symbols
transmitted at a rate of
around 30billion/s, which
requires sampling and A/D
conversion at twice that
speed. “We are already seeing
schemes with 16 levels and
the next generation will
operate at twice this speed –
a major challenge for the ICs
which do it.”
The ultimate limit is set by
Shannon’s law, which tells you
how much data can be sent
over a link with a particular
signal to noise ratio and and
bandwidth. “A few years ago,”
Frisch noted, “I would have said
the limit was a long way off.
Now, it’s a lot closer; for example we’re squeezing the last
few drops out of error correction technology.”
Like almost every other technology, subsea comms is
being driven by cost – in this case, cost per unit bandwidth.
“Ever since deregulation, we have been driven by the
customer to provide more capacity for lower cost,” Barnes
continued. “We are looking at all the things that can help us
go to the next stage, but the development cycle for this
technology takes several years.”
Xtera is deploying a new optical ampliGer to help attain
the higher bandwidth needed to provide greater capacity.
Development of the ampliGer – codenamed Project Olympics
– started in 2011. “Our design goal was to increase the
bandwidth of EDFA repeaters to 55nm, something like a
70% increase on the bandwidth of conventional ampliGers,”
said Barnes.
Barnes noted that it required more complex laser
pumping schemes and combinations of pump lasers. “But
we can now apply what we’ve learned to the next generation,
start to stretch the laws of physics and go for more
bandwidth.”
As part of this work, Xtera and Corning have used
Raman technology to set a record of 607km for
unrepeatered transmission of 100G. “But you don’t get
that performance without things like pre and post
ampliGcation and low loss Gbre,” Barnes observed.
Barnes also sees the opportunity for photonics to help
boost performance. “I would like to improve power
conversion efGciency from electrons to photons; that
converts straight away into more capacity. And photonic
ICs would allow us to do away with things like splices in
the ampliGers, providing more room for pumps and greater
bandwidth without an increase in device size. It’s all about
miniaturisation; the smaller the ampliGer module, the
smaller the canister that contains it.”
The ampliGer canister – made from marine grade
titanium, instead of steel – is now small enough to pass
through a plough during burial operations, making it easier
to handle during laying and assembling.
Progress is being made in boosting performance and
Barnes is conGdent of achieving more, but it remains an
incremental process. “Once you get close to any of the
limits, it gets very expensive to make progress: we need to
push on doors that are more difGcult to open,” Barnes
concluded.
14 April 2015 www.newelectronics.co.uk16
COVER STORY
SUBSEA COMMUNICATIONS
New approach boosts signal span to 5890km
A new way to process fibre optic signals has been demonstrated by a UCL researcher, who
claims the approach could double the distance over which data travels error free through
submarine cables. Because the method does not require signals to be boosted, it has the
potential to reduce the costs of long distance optical fibre communications.
Dr Robert Maher said: “Demand for bandwidth means there is a need to send more
information. One way to do this is to use more complex ways to encode the data.
“However, that needs more optical power and the more power you use, the more
distortion you get, which limits what can be transmitted.”
He said this wasn’t a problem 10 years ago, when data was sent using simple formats.
“Only recently has the need developed to use high capacity formats.”
As the different wavelengths travel through the fibre, they interact destructively and this
brings errors. Dr Maher noted: “We try to ‘grab’ a bundle of frequencies at the end of the link
and use processing techniques – tricks, if you like – to undo the distortion.”
Data received at the far end of a cable is converted into the electrical domain, digitised,
then sent back down the cable. This allows the distortion to be ‘undone’ by sending the data
back on a ‘virtual digital journey’.
“This causes more distortion,” Dr Maher explained, “but it’s the opposite. The signals
don’t quite cancel each other out, so it’s a mitigation process, not cancellation.”
In his work, Dr Maher has used 16QAM modulation and has achieved a transmission
distance of 5890km. Now, he is working on an approach which may enable 64 and 256QAM
modulation and hopes to demonstrate a 64QAM approach later in 2015.
Above: The
largest cable
laying vessels
can store up to
1000km of cable
Left: A repeater
being deployed
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