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| How Optical Fibers Work, Part 1 | How Optical Fibers Work, Part 2 | How Optical Fibers Work, Part 3 |
| How Optical Fibers Work, Part 4 | How Optical Fibers Work, Part 5 | How Optical Fibers Work, Part 6 |

How Optical Fibers Work, Part 6

The biggest problem with any optical fiber has to do with losses in one form or another. Some losses, such as the absorption of some light energy by the glass itself, cannot be avoided. But there are other kinds of problems that can be reduced by careful design of the fiber.

If a narrow pulse of light (light of brief duration, such as might be produced by a flashbulb) is applied to one end of an optical fiber, the pulse of light at the far end will have a lower amplitude and a longer duration, as shown here. In this image, a narrow pulse of light enters the fiber from the left, and a wider, weaker pulse leaves the fiber at the right:

This phenomenon is called pulse dispersion or pulse spreading, and has three basic causes. These are:

Modal Dispersion

Not all light travels as the basic wave we showed earlier. It has many different modes, characterized by different relationships between the electric and magnetic fields that make up the total electromagnetic wave. The key point for our purposes is that the higher, more complex propogation modes take longer to travel a given distance in any medium, and they require a wider path than the simpler, basic mode.

The larger the diameter of an optical fiber's core, the more different propogation modes it can support, and the more pronounced the modal dispersion effect will be. On the other hand, if we make the core small enough, we can block all except the basic mode, and minimize this effect. Such a fiber is called a single-mode fiber.

For those interested in how the different modes can exist and why they take up more room, this is because only the fundamental mode is supported by individual photons. Higher modes require multiple photons in specific phase relationships to each other and in specific placement with respect to each other. They are still close enough to each other so that their combined fields form a composite in space. Thus, from the outside, it can appear that there are (for example) two cycles of the magnetic field for each cycle of the electric field, or vice versa. However, a single-mode fiber isn't wide enough to include the outer photons of the group, so they get stripped off and scattered. This leaves only the fundamental mode traversing the fiber.

Material Dispersion

The velocity of propogation through the core is not the same for all colors (or wavelengths) of light. "White" light actually contains all visible colors, and a narrow white pulse entering the fiber will produce a series of overlapping pulses of different colors at the far end.

The solution to this problem is to use an LED or laser diode as the light source, so that all of the light passing through the fiber is at very nearly the same wavelength.

Waveguide Delay Distortion

This is a phenomenon observed first with microwaves traveling through waveguides. An optical fiber is essentially a waveguide for light waves, and exhibits the same behavior: each propogating mode within the fiber experiences a slight dispersion effect simply because of the confining waveguide, which cannot behave as if it were open space.

This dispersion effect is quite small but cannot be eliminated. It can pretty much be ignored, unless you are trying to push the communications speed of the optical fiber to its limits.

The basic problem caused by any dispersion effect is that it limits the rate at which data may be transmitted through the fiber. If the light amplitude is modulated at too high a rate, dispersion tends to level out the changes so that the light at the far end of the fiber is of a nearly constant amplitude. The end result is that the modulations become indecipherable, and all data is lost.

Prev: How Optical Fibers Work, Part 5

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