Date: April 6, 2011
I have been learning about optical fibres in physics and have came across "cross-talk" in optical fibres and the textbook does not explain it.
Cross talk in electronic communications is when one signal interferes with
another signal making the information carried in the signals unreadable,
For land-line metallic communications cables: Crosstalk occurs when two
metallic lines lay parallel to each other (or close enough to each other)
and the magnetic field resulting from the current in one line induces a
secondary current in the other line causing the listeners to hear two
interfering conversations instead of just their one conversation.
For atmospheric communications channels: crosstalk interference occurs when
two radio frequency (RF) communications channels are too close to each other
in frequency and the two signals mix and interfere with each other.
Sometimes you can hear the atmospheric cross talk effect on your car radio
when two radio stations are too close together in frequency so that you
cannot hear either station clearly. The Federal Communications Commission
(FCC) spaces commercial radio and TV stations out along the radio frequency
spectrum to avoid this co-interference (crosstalk), but sometimes
atmospheric ducting transports a distant radio station's signal whose
frequency is close to a local station and the sound from the stations
interfere with each other.
Crosstalk does not occur in single wavelength optical fiber cables because
the information carrier is light and not electrical current with its
accompanying magnetic field. Light does not generate the magnetic field and
thus avoids the problem of crosstalk.
You might want to research (http://www.google.com) Dense Wave Division
Multiplexing (DWDM) where different frequencies of light are used to
transmit tens of thousands of individual communications links (called
channels) on one fiber. If this is not properly engineered then you could
possibly have frequency crosstalk as described for atmospheric
communications channels in fiber optic cables. But this technology
breakthrough can increase the number of communications channels in one fiber
cable by 100s and 1000s provided the communications channels are properly
spaced in the light frequency spectrum.
The term is a carry-over from the electronics industry wherein
crosstalk is undesirable electrical signal that leaks into adjacent
circuits. In the fiber world, you might suspect that signals sent in
the form of light can leak from one fiber to an adjacent one in a
fiber bundle, but under normal circumstances this does not happen
because of "total internal reflection" in the fiber. Usually the
term is used in the context of WDM (wave form division
multiplexing). In WDM, multiple wavelengths ("colors", if you will,
although WDM normally uses infrared wavelengths spaced fairly close
to each other) are sent down the same fiber, each one delivering a
different data stream. At the receiving end, a "prism" of sorts
(typically an etalon) splits the channels back out, the result being
that you can cram more data into a fiber. As you might expect, the
world is imperfect and various effects such as Raman scattering of
photons and other effects can cause data from one channel to corrupt
data in another.
"Cross talk" is a general term to describe the transfer of information
(signal) from one conductor (optical fiber or electrical) (to/from) another
conductor. This is basically a source of "noise". The reasons for
"cross-talk" are many and varied. In some contexts "cross-talk" may not
always be physically neighboring. Two radio signals may be a large
distance/frequency apart, but the signals may interfere (in the general
sense) with one another. Again, the reason(s) are varied. A classic example
is two radio signals that do not have sufficient resolution to separate the
two signals. In that case the two signals overlap and the resulting
composite signal is garbled.
Optical fibers carrying different signals are commonly grouped
together in a bundle. For example, one fiber may be carrying Internet
data, and another one next to it, might be carrying long distance
A small amount of light from one fiber can leak out and enter the
second one beside it. This is called "crosstalk". If you were talking to
someone far away by phone, and your long distance call was being
sent along one fiber, leakage of the signal from a nearby fiber that was
(let us say) carrying Internet data, would be audible to you as increased
background noise. This, of course, is not desirable, so you can see
that designing the fiber to ensure minimal light leakage (and thus
minimal crosstalk) is very important. This is especially so when
bundles of fiber are commonly run in close proximity to each other for
hundreds of miles.
Click here to return to the Engineering Archives
Update: June 2012