MadSci Network: Engineering |
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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