A MOVIE projector can transmit images from a strip of celluloid film directly
onto a screen. But when you send cable TV pictures down an optical fibre, you
need to process each image line by line, and send the details of each pixel
encoded in a laser beam. If only there was a way to camouflage the pictures and
send them directly down the fibre. Well now there is, and the secret is
chaos.
Conventional images have to be digitised and encoded as a string of bits
before they can be sent. The receiving unit decodes the data and reassembles the
picture, a process which uses a lot of computing power.
Now Jordi Garcia-Ojalvo from the Technical University of Catalonia in
Barcelona and Rajarshi Roy from the University of Maryland in College Park can
encode and send whole images down optical fibres. They do this by mixing the
images with a 鈥渃haotic optical signal鈥�, which is generated by repeatedly passing
a laser beam containing the image through a non-linear crystal鈥攁 material
whose light output is not proportional to the input intensity. This 鈥渕esses up鈥�
the signal, but in a way that can be undone. You no longer need to digitise the
image using complex codes. 鈥淚t鈥檚 a purely optical system,鈥� says
Garcia-Ojalvo.
Advertisement
Chaotic communication first became possible about 10 years ago when engineers
realised they could create chaotic signals in a transmitter and unscramble the
chaos at the receiver. The signal to be sent is bounced around a cavity
containing a set of four mirrors. Between two of the mirrors, a non-linear
crystal renders the signal chaotic. In the receiver, a similar set of mirrors
and non-linear crystal work in reverse to reconstruct the original message. The
technique has already been tested with analagous equipment for radio
transmissions. 鈥淏ut optical systems are much faster than electronic ones,鈥� says
Garcia-Ojalvo.
And because the researchers have worked out how to send entire images in one
go without having to read them line by line, their technique is even faster.
Garcia-Ojalvo says that although some fine-tuning is needed, they can already
send pictures at speeds that would require gigabit bandwidths in a computer
network.
Chaotic signals have other advantages. They are resistant to noise, so do not
require complex error-correction, and look very much like the hiss of background
noise, making them hard to jam. 鈥淭hey are naturally camouflaged and so have a
low probability of detection,鈥� says Lou Pecora, a pioneer in the field of
chaotic communications at the Naval Research Laboratory in Washington DC. 鈥淚t
may be that chaotic signals can carry video images more efficiently than
conventional signals,鈥� he adds. 鈥淏ut we need more tests to find out for sure.鈥�