“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