Fiber optic light sources must be able to turn on and off millions to billions of times per second while projecting a nearly microscopic beam of light into an optical fiber. Both Light Emitting Diodes and Laser transmitters are considered to be semiconductor light sources. However, they differ greatly in output patterns. Forward biased LEDs generate light when positive voltage is applied to the “p” region and negative voltage is applied to the “n” region of the diode. Random photons are emitted at the junction where the “p” and “n” regions meet once a current is passed through the LED. The photons that are emitted are out of phase and not launched in the same direction. This is called incoherent light. Only a small percentage of this light is actually coupled into the optical fiber since incoherent light is difficult to focus. There are two types of LEDs, surface emitting and edge emitting. Surface emitting LEDs are formed from a single semiconductor material in the “pn” junction. Because of this, incoherent photons radiate from all points of the junction. Edge emitting LEDs are formed from similar materials with different refractive indexes. These indexes are used to guide the light and create a directional output. Light is emitted though an etched opening in the diode. Laser sources also have a “p” and “n” region but contain the emitted photons using an optical cavity and reflecting mirrors on each end of the diode. One mirror is only partially reflective and allows the photons to escape the optical cavity. These photons have a fixed relationship and are called coherent light. LEDs have a wide output pattern compared to a laser and does not couple all of its light energy into the core of a multimode optical fiber. Laser sources have a very narrow output pattern, allowing a majority of the light energy to be coupled into the core of a single mode or multimode optical fiber. LED transmitters are directly modulated. The output power is directly proportional to the current flow through the LED. Because LED fiber optic links are simplex, they require both transmit and receiving optical fibers. Laser diodes emit very few photons until the drive current passes a threshold level. A feedback circuit that monitors output intensity typically controls the drive current and ensures a constant output level. LEDs have much lower power outputs than lasers and their larger, diverging light output pattern makes them harder to couple into fibers, limiting them to use with multimode fibers. Laser have smaller tighter light outputs and are easily coupled to singlemode fibers, making them ideal for long distance high speed links. LEDs have much less bandwidth than lasers and are limited to systems operating up to about 250 MHz or around 200 Mb/s. Lasers have very high bandwidth capability, most being useful to well over 10 GHz or 10 Gb/s. Because of their fabrication methods, LEDs and are cheap to make. Lasers are more expensive because creating the laser cavity inside the device is more difficult, the chip must be separated from the semiconductor wafer and each end coated before the laser can even be tested to see if its good. Lasers, which are used in long distance and DWDM systems, have the narrowest spectral width. Lasers are also highly linear so they can be used as sources in AM CATV systems.