:: Why are LEDs used in fiber optic communication systems.

LED stands for light-emitting diode. It is a semiconductor device which converts an electrical current into light.

LED has very high light conversion efficiency and generates little heat. It possesses some characteristics that make it a excellent light source for fiber optic communication systems.

1. Small footprint: LED's small size makes it useful in practical systems

2. LED's light emitting area's size is comparable to optical fiber's core (50um or 62.5um for multimode fibers). This is critical for coupling the emitted light into fiber core.

3. LED can be turned on and off at very high speed. This is called modulation which is necessary for digital communication systems

4. LED has very long life in the range of 80,000 to 100,000 hours. This significantly reduces communication system maintenance cost.

:: Types of LED light source

LEDs are divided into two types based on their light emitting structure: surface emitting type and edge emitting type.

Surface emitting LED emits light from their top surface. Its emitting area is large which causes poor light coupling efficiency to the fiber core. Although its optical output power is as high or even higher than a edge-emitting LED, the actual coupled light into the fiber is lower.

Edge emitting LED, on the other hand, has only 30~50um emitting area, which is very comparable to multimode fiber's 50um or 62.5um core diameter. It offers high output power and high speed performance.

Another advantage of edge emitting LED is its narrower light emission spectra compared to surface emitting type. A new type of LED, called superradiant LED, is becoming more popular in the industry because of its high power density.

:: Differences between a common indicator LED and a fiber optic light source LED

Common indicator LEDs such as those used in consumer electronics emit visible lights which human eyes can see(400nm to 700nm wavelength range). While LEDs used in fiber optic communications emit longer wavelength infrared light such as 850nm which human eyes can not see directly.

Another important difference is that LEDs used in fiber optic emit light in a much narrow angle range which makes coupling to optical fibers possible. Even if indicator LED light can be coupled into fibers, it is too weak to have any use.

:: Understanding LED's characteristics

1. Peak Emitting Wavelength: This is the wavelength at which LED emits the most power. The common wavelength are 780nm, 850nm and 1310nm for fiber optic multimode communications.

2. Spectral Width: LED emits light in a wide range of wavelengths with the most power output at the peak wavelength. A typical 850nm LED has a spectral range of 170nm, that means it actually emits light from 765nm to 935nm.

3. Rise/Fall Time: LED's rise/fall time determines how fast it can be turned on and off. This spec must be fast enough to meed the system's bandwidth requirement.

4. Power: Since a fiber optic link includes many power loss points such as fiber attenuation, connector loss, coupling loss and many others. The light source must provide high enough power so the detector can still decode the signals without any errors.

:: Comparison between LEDs and Lasers

Some main differences exist between LEDs and lasers.

1. Laser has much narrower spectral width: DFB laser has less than 0.1nm spectral width which LED has 170nm spectral width.
2. Laser emits much higher power
3. Laser light is coherent while LED light is incoherent
4. Laser has much higher modulation speed