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Name: Robert
Status: student
Grade: 9-12
Country: Australia
Date: April 6, 2011


Question:
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.



Replies:
Robert

Cross talk in electronic communications is when one signal interferes with another signal making the information carried in the signals unreadable, unhearable.

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.

Sincere regards, Mike Stewart


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.

Paul Bridges


"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.

Vince Calder


Hi Robert,

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 telephone "traffic".

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.

Regards, Bob Wilson



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