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Optical-Communications

Soliton Pulses
Conventional Fiber Communications
Soliton Pulses  in Fiber
50 Gbit/sec No Repeaters
Other Soliton URLs
Thru the Air LASER Communications

.................Soliton Pulses.................

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!
 



 
The Dichotomy of Soliton Pulses in Dispersion-Shifted Fiber
Starting Out
Midway
     Destination
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
[Light to Electronic] [Electronic to Light] (1800cu ft.) Repeaters spaced ~ 20 km
Soliton Pulses  in  Dispersion-Shifted Fiber Communications, 25 Gbit/sec 
<-- 19,000 km -->
With No Errors
Optical/Light Amplifier Repeaters (0.5cu ft.) spaced ~ 30 km - 100 km
> 50 Gbit/sec No Repeaters Needed
<-- 19,000 km -->
With No Errors
No Repeaters Needed

 


  

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Thru the Air LASER Communications 

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.
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.
LASER Receiver using a Refractor Telescope
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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More Info on Solitons
http://www.sfu.ca/%7Erenns/lbullets.html    Excellent!
LOS ALAMOS NATIONAL LABORATORY 
http://people.uncw.edu/hermanr/Research/solitons.htm
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