Researchers have set a new record for the
rate of data transfer using a single laser:
26 terabits per second.
At those speeds, the entire Library of
Congress collections could be sent down an
optical fibre in 10 seconds.
The trick is to use what is known as a "fast
Fourier transform" to unpick more than
300
separate colours of light in a laser beam,
each encoded with its own string of
information.
The technique is described in the journal
Nature Photonics.
The push for higher data rates in light-
based telecommunications technologies has
seen a number of significant leaps in recent
years.
While the earliest optical fibre technologies
encoded a string of data as "wiggles" within
a single colour of light sent down a fibre,
newer approaches have used a number of
tricks to increase data rates.
Among them is what is known as
"orthogonal frequency division
multiplexing", which uses a number of
lasers to encode different strings of data on
different colours of light, all sent through
the fibre together.
At the receiving end, another set of laser
oscillators can be used to pick up these light
signals, reversing the process.
Check the pulse
While the total data rate possible using such
schemes is limited only by the number of
lasers available, there are costs, says
Wolfgang Freude, a co-author of the
current
paper from the Karlsruhe Institute of
Technology in Germany.
"Already a 100 terabits per second
experiment has been demonstrtaed," he told
News reporters.
"The problem was they didn't have just one
laser, they had something like 500 lasers,
which is an incredibly expensive thing.
If you can imagine 500 lasers, they fill racks
and consume tens of kilowatts of power."
Professor Freude and his colleagues have
instead worked out how to create
comparable data rates using just one laser
with exceedingly short pulses.
Within these pulses are a number of discrete
colours of light in what is known as a
"frequency comb".
When these pulses are sent into an optical
fibre, the different colours can add or
subtract, mixing together and creating
about 350 different colours in total, each of
which can be encoded with its own data
stream.
Last year, Professor Freude and his
collaboratorsfirst demonstrated how to
use all of these colours to transmit over 10
terabits per second.
At the receiving end, traditional methods to
separate the different colours will not work.
Here, the researchers have implemented
what is known as an optical fast Fourier
transform to unpick the data streams.
Colours everywhere
The Fourier transform is a well-known
mathematical trick that can in essence
extract the different colours from an input
beam, based solely on the times that the
different parts of the beam arrive.
The team does this optically - rather than
mathematically, which at these data rates
would be impossible - by splitting the
incoming beam into different paths that
arrive at different times, recombining them
on a detector.
In this way, stringing together all the data in
the different colours turns into the simpler
problem of organising data that essentially
arrives at different times.
Professor Freude said that the current
design outperforms earlier approaches
simply by moving all the time delays further
apart, and that it is a technology that could
be integrated onto a silicon chip - making it
a better candidate for scaling up to
commercial use.
He concedes that the idea is a complex one,
but is convinced that it will come into its
own as the demand for ever-higher data
rates drives innovation.
"Think of all the tremendous progress in
silicon photonics," he said.
"Nobody could
have imagined 10 years ago that nowadays
it would be so common to integrate
relatively complicated optical circuits on to a
silicon chip."
