Fiber Optics

The name of the technology implies what it is: optical information carried over a fiber; since the fiber must carry light it is usually some form of glass. A fiber optic system then contains some sort of transmitter, the optical cable, and a receiver which converts the light signals to their ultimate form. The fiber optic cable is passive; although for long installations (100 kilometers or so) a repeater may be necessary to regenerate the signal as there are some losses in the cable (by contrast, a copper cable usually has to regenerate the signal every couple of kilometers or so). And, that's today's technology; in the future longer runs are likely.

Some of the benefits attributed to fiber optics over copper are...

• Large Bandwidth, Light Weight, and Small Diameter (a fiber the size of your finger can easily carry as much information as a copper cable the width of your arm)
• Long Manufacturing Lengths (12km lengths fitting on a 96-inch reel)
• Easy Installation and Upgrades (only small modifications must be made to use copper installation equipment; but training is necessary for the installer)
• Non-Conductivity (fiber is not affected by radio frequency interference and can be installed in areas where copper cables could not without significant shielding)
• Security (the signal cannot be detected outside the cable and protections can be installed to monitor access to the fiber in a cable)

Optical communication has been used long into the past (Paul Revere's "one if by land and two if by sea" being a prime example). Making light follow a conductor was demonstrated by John Tyndal (British physicist) in 1870. He shined a light into a take of water with an exit pipe. The light followed the water through the pipe, thus demonstrating the prime characteristic needed for fiber optics: total internal reflection.

Further experiments with light and lasers demonstrated open-air communications but fiber optics did not come about until the 1970s when Dr. Robert Maurer, Peter Schultz, and Donald Keck of Corning Inc. developed the first fiber with a loss of less than 20dB/km (99% loss over one kilometer, the approximate length before copper cables needed regeneration). This made fiber optics practical. Today, much better cable is produced.

Some Fiber Optic Terms

Intensity. This refers to the strength of the device injecting light into the fiber. You can't rely on intensity alone as a figure of merit, even though usually more intensity generally results in more power injected into the fiber. Other things affect the transmitter/fiber interface, such as:

• Area. The area of the light emitting surface and core of the fiber are important. The closer the area of the light emitter is to the core area (i.e., the less overlap as the fiber is usually smaller) the more light will be injected into the fiber.
• Acceptance Angle. Light doesn't just flow through a fiber, it bounces from side to side. So long as the angle of the bounce is low enough there will be total internal reflection and little loss of signal. Generally, acceptance angles for fibers fall in the range 11° to 46°. The light must be injected at an angle less than the acceptance angle of the particular fiber.
• Fresnell Loss. At any air glass interface there is a loss of about 4%. Special coupling gels and techniques help to reduce this; but it is a fact that must be dealt with.

Optical Losses. Despite the fact that fiber optic cable is made from very pure glass there are still considerations that lead to various losses:

• Wavelength Loss. Consider a pane of glass; look at its edge. Chances are you'll see a green color. This is an example of wavelength loss. At different wavelengths of light this loss varies. (E.g., at 850nm most cables have a 4 to 6dB/km loss, this drops to 3-4dB/km at 1300nm, and even lower at higher wavelengths.)
• Bending. If you bend a fiber cable enough the light no longer travels within the acceptance angle limits of the cable and it leaves the cable core. This places some limits on cable installations.

Optical Fiber Bandwidth. This is a loss measured in MHz/km and it largely has to do with the width of the fiber. To understand, follow two light rays through a short length of cable: one enters the fiber straight on, the other enters at close to the acceptance angle. The first largely passes straight through the fiber; the second bounces back and forth between the edges of the fiber and, therefore takes a longer path. Since both rays left the transmitter at the same time (say a square pulse), the form of the pulse at the other end of the fiber is going to be smeared instead of sharp since one ray takes longer than the other to transit the fiber. This limits the amount of information (bandwidth) the fiber can carry. Bandwidth increases as cable diameter decreases and this leads to...

• Singlemode Cable. Singlemode cables are around 8 to 10 microns in diameter and have a very high optical bandwidth.
• Multimode Cable. Multimode cables are typically 50 and 62.5 microns in diameter and thus have a smaller optical bandwidth.

Go through the example above again. Note that as the length of the cable becomes longer the time difference between the two rays will become longer. That means that as fiber cables (multimode more than singlemode) reduce their bandwidth with distance as well as diameter. This can become significant in a system design.

There are many different designs for fiber optic cables and a number of different connectors used to connect them. It's important to know what you are doing when setting up a fiber optic system.

More Information

• LASCOMM Fiber Optics Tutorial
• Fiber Optics Tutorial
• Corning Cable Systems Tutorial
• Intro to Fiber Optics