With a basic understanding of fiber construction, explanation of transmitters (the devices that put the pulses of light into the fiber) is in order. From a general level, there are three aspects of transmitters to discuss: 1.type of transmitter 2.wavelength of transmitter 3.power of the transmitter Transmitters can be divided into 2 groups, lasers and LEDs. LEDs are by far the most common as they provide low cost and very efficient solutions. Most multi mode transmitters are of the LED variety. When high power is required for extended distances, lasers are used. Lasers provide reliable light and the ability to produce a lot of light energy. The drawbacks to lasers are their cost and electrical power consumption. Equipment using high power lasers must provide cooling and access to a primary power source such as 120V AC.Transmitter types can also be broken down into single mode versus multi mode transmitters. Multi mode transmitters are used with larger cable (typically 62.5/125 microns for most data networking applications) and emit multiple rays or "modes" of light into the fiber. Each one of these rays enters at a different angle and as such has a slightly different path through the cable. This results in the light reaching the far end at slightly different times. This difference is arrival times are termed modal dispersion and causes signal degradation. Single mode transmitters are used with very small cable (typically 8/125 microns) and emit light in a single ray. Because there is only one mode, all light gets to the far end at the same time, eliminating modal dispersion. The wavelength of the transmitter is the "color" of the light. The visible light spectrum starts around 750nm and goes to 390nm. The 850nm transmitters common in multi mode Ethernet can be seen because 850nm is the center of their bandwidth and they emit some visible light in the 750nm range giving them their red color. The 1300nm and 1550nm transm...
