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Published on 16 Feb 2012 | over 6 years ago

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This picture shows a bunch of fiber enclosed in a single black color protective jacket.

So how is light guided and travels inside the fiber? This video shows a beam of light that travels inside a water stream by total internal reflection. Optical fibers work the same way. So let's take a look.

A glass fiber has a cylindrical structure and is composed of three layers. At the center is the core, core has higher refractive index. Outside of core is the cladding layer. Cladding layer has lower refractive index than the core.

The third layer is a plastic buffer coating. This buffer coating doesn't affect the fiber's optical performance, it is there for mechanical protection only.

The right picture shows how light is coupled into the fiber's core and bounced back and forth in the core and travels along the fiber.

The core and cladding layers are all based on fused silica which is a type of glass. But this fused silica is extremely clear, with almost no impurities. This transparency is extremely important, so that the light can travel for a very long distance, such as hundreds of kilometers with minimum loss. This makes trans-Pacific and trans-continent fiber optic communications possible.

Here comes the question. Why doesn't the light leak out of the fiber? That is why we have to explain the phenomenon of total internal reflection.

The left picture shows Snell's the law which guides how light travels at the interface of the core and cladding.

The core has a higher refractive index n = 1.5. The cladding has a lower refractive index n = 1.4. When light incidents at the interface between the core and cladding at different angles, some power is reflected back, and some power enters into the cladding which is refracted. But when we increase the incident angle to greater than a critical angle theta c, no more light enters into the cladding, all light is reflected back into the core. This phenomenon is called Total Internal Reflection. Here total means 100% of the power is reflected back into the core.

The manufacturing of glass fibers go through two steps. In the first step a preform is made. This preform has exactly the same proportion of core and cladding as final fiber product, but in a much bigger size. It looks like a thick glass rod, as shown in the bottom picture.

Then the preform is hanged at the top of a fiber drawing tower. The tower is a couple of stories tall as shown in the right picture.

The preform is heated by a furnace which softens the glass. The softened glass drips and pulled downward by gravity. A diameter monitor carefully monitors the fiber's diameter, which usually is 125um. Then the coater deposits a layer of plastic buffer coating for mechanical protection, which usually is 250um in diameter.

And finally, the fiber is winded onto a spool for storage and transportation.

A basic fiber optic communication system must have at least three components. The light source, which usually is a laser. The laser is turned on and off quickly by a driving circuit. In this modulation process, the original electronic signal is translated into light signal.

The second part is the fiber. The fiber's purpose is to transmit the light through a very long distance, such as from New York to Paris.

And the third part is a photodetector. The photodetector detects the light and generates electronic current, in this process it translates the light signal back into electronic signal.

This table list the most common high speed Ethernet standards. For 100 Mbps, copper supports up to 100 meters, while fiber supports up to 40 kilometers. For Gigbit Ethernet, copper supports still 100 meters, fiber supports 70 kilometers. For 10 Gigbit Ethernet, copper is 100 meters, and fiber is 80 kilometers. Now you see the difference.

The bottom picture shows the size comparison between a copper cable and an optical fiber, both carries the same bandwidth.

There is truly no match for fiber's bandwidth capabilities. In order to fully utilize the fiber's bandwidth, people are using WDM technologies to increases the bandwidth in tens of folds.

WDM stands for Wavelength Division Multiplexer. What this means is that many different colors, or wavelengths, of light is put into a single fiber. This picture shows a simple 4 channel WDM, but there are 32 channels, 64 channels, or even more are available on the market now.

Now imagine how many information can a single fiber carry. If each wavelength carries 40 Gbps signal, then a 10 channel WDM system can carry 400 Gbps. Not to mention a 100 channel WDM system.
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