OPTICS
PHYSICS FIBER OF MODERN DEVICES ISMAR UZICANIN
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RIDING TO THE FUTURE AT THE SPEED OF LIGHT
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ISMAR UZICANIN
Total Internal Reection
Light directed at a critical angle reects to the same side as the source beam rather than passing through a transparent material
Critical Angle
Snells Law - Angle of incidence = angle of refraction In TIR, the angle of refraction = 90 degrees This means the sine term on the right is equal to 1 and so the angle of incidence which gives us the critical angle at which the light will reect is: where the ns are the indicies of refraction This ratio must always be less than or equal to one for the arc sine term to be dened
TIR in a light pipe
If this concept is applied to a pipe lined with glass on the inside, then light could be directed at a critical angle all the way through the glass and emerge on the other side even if there are bends in the pipe using the right materials
In 1870, John Tyndall used a jet of water that owed from one container to another and a beam of light to demonstrate the light pipe. As water poured out through the spout of the rst container, Tyndall directed a beam of sunlight at the path of the water. The light, as seen by the audience, followed a zigzag path inside the curved path of the water. By surrounding the core with a reective cladding, you could prevent the light from leaking out of the internal material, whether it be water or glass
Tyndalls Demonstration
The thicker the ber is, the more critical angles there can be and so the more more paths there are for the light to take. Multiple paths or modes cause optical dispersion in the ber as the light paths diverge from the center
Modes of Light
- is thicker and can handle large amounts of data transmission - is easier to make and thus less expensive to manufacture - is more prone to optical dispersion and so needs repeaters to boost the signal back to its original strength (~500m) - is used extensively for connections between buildings on on campus, short connections to service providers, or any of high speed Local Area Network (LAN)
Multi-mode Fiber
Single-mode Fiber
- is thinner and cannot handle as much capacity as multi-mode ber - is more difcult to make and thus more expensive to manufacture - is less prone to optical dispersion because light has only one path it can take through the ber - has low dispersion so repeaters are not needed as frequently (~60 km) - is used for long haul runs between cities and even between continents
How many Modes?
The wavelength of the light, the thickness of the ber and the material used to make the ber determine how many paths the light can take -Normalized frequency parameter (V) V=2a/(NA) a=ber core radius =wavelength of light NA=numerical aperture (a dimensionless parameter that describes the range of angles at which the optical ber can accept light NA=sqrt(n2c-n2cl) nc= index of refraction of core ncl= index of refraction of cladding If V is less than 2.405, the ber is considered single mode as it is so thin and the NA so small that only one path for the light transmitted is possible
Transmission
We have already learned about the details of how lasers and light emitting diodes work. These devices are the only point sources of light small enough to be used in an optical ber.
Photodiode converts digital (binary) signal into electrical signal to power a Laser Laser diode is a semiconductor that emits coherent light Lasers in the 1310 to 1625 nm range are typically used depending on the type of data being transmitted (data/voice)
A lens focuses the laser light into the ber
Attenuation of Signal
Loss of signal, known as attenuation is a function of the loss coefcient of the conducting material, the length of the conductor and the frequency of the transmission - Signal loss in optical ber links is due to the limits of the manufacturing process of the glass - Impurities in glass, just as in air or any other material, scatter light and eventually lead to signal degradation - The loss for single-mode ber for example is the on order of 0.1 dB/km - Compared to losses for copper which are on the order of 12 dB/km, optical ber is clearly better for long distance data transmission
Advantages over Copper
Less signal degradation - The loss of signal in optical ber is much less than in copper wire Low power - because signals in optical bers degrade less, lower power transmitters can be used instead of high voltage electrical transmitters for copper wires Less expensive - several miles of optical cable can be made more cheaply than equivalent sizes of copper wire Non-ammable - Because no electricity is passed through optical bers, there is no re hazard No EMI - for the same reason, there is no generation of electromagnetic interference, which allows ber to be run anywhere and near anything Thinner - optical bers can be drawn into smaller diameters than copper wire, which reduces their weight and bulkiness Digital Signals - Optical bers are ideally suited for carrying digital information, which is especially useful in computer, television, and voice networks
Historic Long Haul Links
Low attenuation, especially in single-mode ber means that data can travel very quickly over vast distances without the need for expensive equipment along the way to boost the signal Back in the days of the telegraph, the process of laying cable across the Atlantic Ocean for a link between Europe and North America was extremely arduous and the signal passing through copper insulated with a low-quality rubber degraded quickly Techniques for laying the cable using large ships with spools of unprotected copper were improvised and the cable broke 5 times before it was successfully laid between Ireland and Newfoundland in 1866 This was the rst intercontinental data link and Queen Victoria and President Buchanan exchanged congratulations via telegraph
Historic Long Haul Links
Today, the entire world is connected via optical ber for both voice and internet communication. End users connect through their local networks and homes to local service providers. These local providers link to larger backbone providers, all connected together via high speed ber. Special cable laying ships designed solely for laying and maintaining ber links patrol the waters ready to repair and cable cuts or damage. The cable laying process itself is now very sophisticated and efcient.
Modern Long Haul Links
Video: http://www1.alcatel-lucent.com/submarine/how/inputs/layi...