Nov 21, 2011 10:00 PM, By Bennett Liles
A refresher course on the latest in fiber-optic technology.
So many forces are driving the fiber market up that its rapid rise threatens to outpace the general understanding of its basic workings on the part of many AV techs. Even in the broadcast realm, production engineers have found it useful to arm themselves with an occasional refresher course on the latest advances in fiber technology. As evidence of the ongoing three-way convergence of broadcast, AV, and IT, technical people can also value a basic understanding of the fast-changing world of fiber-optic AV transmission. So we present here a brief fiber primer and a snapshot of how the technology looks at present. But we're using a fast shutter speed because fiber is on the move. Light has the ability to carry 10,000 times more information than radio frequencies, and the medium is impervious to electrical inductive noise. With fuel prices skyrocketing, fiber cable is also far lighter than ever more expensive copper — a significant difference in mobile applications. It's a perfect storm of market growth, so let's start with a look at the basic parts of a fiber system: transmitter, fiber cable, and receiver.
Since these devices transmit light, they are also referred to as emitters. They convert electrical signals into modulated light beams. The typical fiber transmitters are light-emitting diodes (LEDs), vertical-cavity surface-emitting lasers (VCSELs), and laser diodes (LDs), and each has its advantages in specific applications. Generally, the most economical and linear in output for the amount of input current applied are LEDs, but they also have a relatively large emitting area that concentrates less light on the fiber. LEDs usually don't need any extra temperature-stabilization circuits, and they are used primarily in short- or medium-range applications.
VCSELs have been around for more than a decade now, and one of their primary advantages is in production cost and testing. Produced on wafers and then cut to size, VCSELs emit light from the flat surface rather than from the thin edge, as is the case with the more traditional edge-emitting semiconductor lasers. This enables them to be tested before cutting the wafer, therefore providing a slightly lower production cost and predictable quality. Today all production VCSELs operate at 850nm. Fiber to the Home (FTTH) has made 1310nm F-P (Fabry-Perot) lasers very cheap as well so 1310nm on single mode fiber has also become very inexpensive.
With smaller emitting surfaces, laser diodes are far more efficient at getting more light into the fiber than are LEDs, and they are linear in light output for electrical input. However, they have a smaller range of operating temperatures in which they are stable, so LDs require more sophisticated circuitry to keep them within their stable operating range. This circuitry maintains the LD output at a constant average value by adjusting the bias current of the laser. All this adds to the cost, so LDs are used only for long-distance applications. Measured in wavelength, the three primary transmission windows in the fiber spectrum are 850nm, 1310nm, and 1550nm. LDs and LEDs are available in all three.
The three typical modulation methods for fiber-optic transmission are amplitude modulation, frequency modulation, and digital modulation. Each of these has various sub-flavors, but the primary difference is between analog and digital modulation. In the case of analog, a common method is referred to as intensity modulation wherein the brightness of the transmitted light beam is controlled according to the amount of current being fed to the emitter at any given moment. As the input current rises and falls, so does the brightness of the transmitted light beam. RF carriers may also be used to linearly modulate the light output. In digital modulation, there is only an on or off state for the light, but either the pulse width (meaning the time the light is on) or the pulse rate (meaning the frequency of the light flashes) can be used to carry information.
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