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....Soliton
Pulses....
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| There is a new fiber optic data transmission scheme that utilizes something
called Soliton Pulses. These are very
short bursts of light generated in an Erbium-doped Fiber LASER. Soliton
light can be used to transmit data at rates in excess of 50 Gb/s, at distances
over 19,000 km of Dispersion-Shifted Fiber, requiring no repeaters,
and with no errors. This data rate is the equivalent of sending 6,200 bibles
per second. At this rate, one bible could be sent to everyone on earth--5.5
9
people--in
about 10 days.
In the case of repeaters, in a long-haul fiber: instead of converting
the light into electrical signals, amplifying, correcting errors, retiming
and retransmitting light pulses, an optical amplifier is used. It consists
of about 30 meters of Erbium-doped fiber, assorted filters and beam splitters,
and pumped by a 15 mW LASER diode operating at 1550 nm. This arrangement
adds about 30 dB of gain with no appreciable noise. Also, the bandwidth
of this amplifier is on the order of 40 THz. At this rate, one bible could
be sent to everyone on earth, 5.5 9 people, in about 18 minutes.
The Shortest LASER pulse, to date, is ~10 Femtoseconds (10 -15
sec).
Light would travel 1/8th the thickness of a sheet of paper in that time.
10 Fsec is to one second as one second is to 3.2 million years. |
| A Soliton Pulse is: a pulse
of light--of sufficient intensity and correct wavelength --traveling down
a special non-linear optical fiber known as, Dispersion-Shifted Fiber,
is classified as a Soliton Pulse. This Soliton light pulse exhibits a unique
characteristic of getting shorter--not longer, as conventional wisdom would
dictate. Due to the non-linearly of the fiber in the presents of light
exceeding some quantum threshold of intensity--the leading edge of the
pulse is being overtaken by the faster trailing edge. That is, the leading
edge of the light is undergoing a Doppler Red-Shift,
while the trailing edge is experiencing a Doppler Blue-Shift.
The result of this effect is a shortening of the pulse duration, with a
corresponding increasing Peak Power.
As the pulse looses energy (joules) over distance, it increases in Peak
power--thus insuring itself's staying above the peak threshold of the fiber
needed for the phenomenon in the first place! |
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The Dichotomy of Soliton Pulses in Dispersion-Shifted Fiber
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Starting Out
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Midway
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Destination
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| In normal Dispersive finite bandwidth media, the greater
the Distance traveled, there is a Spreading of the pulse width, i.e., Signal
Bandwidth is reduced (resulting in Inter-symbol Interference).
However, the opposite is true with Soliton pulses in Dispersion-Shifted
Fiber. As can be seen in the above figure, the leading edge of the
light pulse is undergoing a Doppler Red-Shift,
while the trailing edge is experiencing a Doppler Blue-Shift,
narrowing the pulse; which Spreads the Bandwidth and Increases the Peak
Power.
Even though Average Power is Decreasing, the narrowing
pulse forces the Peak Power to Increase--assuring the continued operation
in the Nonlinear
region of the Dispersion-Shifted Fiber.
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Conventional Fiber Communications
1.7 GBit/sec
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[Light to Electronic] [Electronic to Light] (1800cu ft.)
Repeaters spaced ~ 20 km
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Soliton Pulses in
Dispersion-Shifted Fiber Communications, 25 Gbit/sec
<-- 19,000 km -->
With No Errors
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Optical/Light Amplifier Repeaters (0.5cu ft.) spaced ~ 30
km - 100 km
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> 50 Gbit/sec No Repeaters
Needed
<-- 19,000 km -->
With No Errors
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No Repeaters Needed
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Thru
the Air LASER
Communications
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| Space based, as well
as, terrestrial based LASER communications, the LASER light is first expanded
and sent as a "focused" beam or cone to the destination target or receiver.
Think of it as a way of "looking" around rain drops, dust particles, etc. |
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| LASER Transmitter using Cassagrainian
Optics |
| For reliable communications
the LASER beam should be first spread and made focusable on the destination
receiver's optics. This spread beam is significantly more difficult to
block or obscure than a thin collimated LASER beam. |
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LASER Receiver using a
Refractor Telescope
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| Ideally the receiving
telescope should be designed with the magnification for "seeing" the LASER
light cone only. If the telescope's eyepiece projected image has other
than the LASER light the Signal to Noise Ratio (SNR) will degrade.
However, in the real world
of particulate laden air and temperature gradient induced path distortions,
the receiving telescope must be a compromise that allows for these impairments.
Finally, proper implementation
of the optical bandpass filter can insure good SNR and reduce interference.
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