MadSci Network: Engineering
Query:

Re: At what speed does information propagate in fiber optics?

Date: Fri Jul 29 21:01:10 2005
Posted By: Adrian E. Popa, Retired Laboratory Director
Area of science: Engineering
ID: 1116356737.Eg
Message:



Greetings Pepe:

References:

1.S. Ramo, J. R. Whinnery, T. Van Duzer; Fields and Waves in Communication
Electronics
, John Wiley & Sons, NY, 1967

2.Mad Science Archives Re: What is the speed of light in a fiber optic cable?

983369337.Ph

3. MadSci Archives Re: How does intensity of light in an optic fibre depends on lengh & diameter

09/970201714.Ph

4. MadSci Archives How are optical fibers made?

890381007.Eg

5. Mad Science Archives: Did Princeton Labs conduct an experiment showing
light to travel FTL?
The Speed of Light Is Exceeded in Lab...07/21/00 By Curt
Suplee Washington Post Staff Writer

985012584.Ph

There is much confusing language about communications technology in the popular
press. In particular talking about the speed of information is a misnomer. I have
answered a similar question to Mad Science in Reference 2. Today most information
is communicated in a digital format and the data rate (not speed) is technically
specified by the number of bits per second. The number of bits per second is not a
measure of the physical velocity that the data is traveling through the
transmission channel it only tells us how much data reaches us each second. For
example; today long distance fiberoptic communications channels are being upgraded
to 10 Gigabits/sec (10 billion bits per second) for each optical wavelength
carrier used in the system. Dense wavelength multiplexed (DWM) systems can carry
10 or more different optical carrier wavelengths each modulated at 10
Gigabits/sec. As you can see from this example the data rate you receive out of
the transmission system is related to how many of the optical carrier wavelengths
you wish to receive and not how fast they travel through the transmission system.
Thus, the data rate is not directly related to the velocity at which
electromagnetic waves (electrical or optical ) travel through the transmission
channel.

Technically we call the velocity at which electromagnetic waves travel through the
transmission media, the velocity of propagation which for free space is generally
designated (Vo). The velocity of propagation in bounded transmission lines, such
as coaxial cable, microwave waveguide or optical fiber wave guide, can be
specified in two ways; by the phase velocity (Vp), or the group velocity (Vg).

The phase velocity (Vp) is the velocity that one must travel to maintain a
constant phase angle for a single frequency sinusoidal wave traveling through the
transmission line. However, practical bounded transmission lines with losses have
phase velocities that vary with frequency. We call this variation the dispersion
of the transmission line. In this case the individual sinusoidal components making
up a complex waveform such as a pulse, will shift in relative phase as they travel
through the line. The waves at some point down the line may add up together or
they may cancel each other producing a waveform significantly different from the
desired waveform.

The group velocity (Vg) is the velocity that the envelope of the frequencies in a
waveform, such as a pulse, travel down the transmission line. Vg is slower that
Vp. However, if the transmission line has zero dispersion than Vg = Vp. There is
an additional complication. As the waveform travels down a dispersive transmission
line eventually the information of the waveform is distorted or destroyed (i.e.
the bit error rate increases).

Your question asks about fiberoptic waveguide transmission systems. Fiberoptic
waveguides have taken over from metallic transmission lines because they exhibit
much lower loss over distance than metallic transmission lines. However, it is the
dispersion of the glass waveguide that determines Vg and the maximum useful
distance over which reliable communications can occur. The Vp of fiberoptic
wavesguides can be approximated by dividing the speed of light by the optical
index of the glass ( i.e. Vp = c/n ); however, my answers to Mad Science questions
in References 2, 3 and 4 go into much greater detail about how the design of the
fiberoptic waveguide determines the frequency dispersion of the transmission line.

It may also interest you to know that it is possible for waveguide transmission
lines to have a Vp greater that the speed of light and in the media and there have
been a number of experiments claiming to be able to send information at velocities
greater than the speed of light. My answer in Reference 5 to a question to Mad
Science on this subject suggests that the Vg in these experiments is actually
slower than the speed of light. However, there is still debate on this question in
the technical literature.

Best regards, Your Mad Scientist
Adrian Popa


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