User:RajeshPJadhav/sandbox

LED in brief: When we talk about LED which contains two elements of processed materials referred to as the P-type and N-type semiconductors. Now way that an LED is made is that an electrode attached to the P-type semiconductor layer is coated or deposited on an N-type semiconductor substrate. Hence they are in direct contact. The region where the N-type semiconductor comes in contact with the P-type semiconductor is known as the P-N junction.

Basically LEDs are very complex semiconductors that can convert an electrical current into light. The conversion process is fairly efficient in that it generates little heat compared to incandescent lights. LEDs are of interest for fiber optics because of five inherent characteristics:

1. They are small. 2. They possess high radiance (i.e., They emit lots of light in a small area). 3. The emitting area is small, comparable to the dimensions of optical fibers. 4. They have a very long life, offering high reliability. 5. They can be modulated (turned off and on) at high speeds.

Structure of LED in Fiber Optical Communication:

As we know Light emitters are a key element in any fiber optic system. This particular component converts the electrical signal into a corresponding light signal that can be injected into the fiber. This light emitter is very important element because it is often the most costly element in the system, and its characteristics often strongly influence the final performance limits of a given link.

The way the light from LEDs can be modulated very quickly so they are used extensively in optical fiber and free space optics communications. Fiber optic cable functions as a "light guide," guiding the light introduced at one end of the cable through to the other end. The light source can either be a light-emitting diode (LED)) or a laser. The light source is pulsed on and off, and a light-sensitive receiver on the other end of the cable converts the pulses back into the digital ones and zeros of the original signal.

Fiber cable comes in two forms: multimode and single-mode. A mode in optical transmission is a ray of light entering the core at a particular angle. Modes can therefore be thought of as bundles of light rays of the same wavelength entering the fiber at a specific angle.

Multimode fiber uses LEDs as the light source. LEDs are cheaper to build, require somewhat less safety concerns, and are effective for shorter distances than the lasers used in single-mode. Multimode (62.5/125) can carry data over distances of up to 2000 meters (6,560 ft.). It is mainly used in LAN applications including backbone cabling.

LEDs key characteristics which are important for any optical fiber communication and those are:

Peak Wavelength:

This is the wavelength at which the source emits the most power. It should be matched to the wavelengths that are transmitted with the least attenuation through optical fiber. The most common peak wavelength are 780, 850, and 1310 nm. Spectral Width: Ideally, all the light emitted from an LED would be at the peak wavelength, in practice the light is emitted in a range of wavelengths centered at the peak wavelength. This range is called the spectral width of the source. The LED used in fiber optic, the wavelength is generally in the near infrared (because the optical loss of fiber is lowest at these wavelengths). Emission Pattern:

The pattern of emitted light affects the amount of light that can be coupled into the optical fiber. The size of the emitting region should be similar to the diameter of the fiber core. The LED emitting area is generally much smaller in order to allow the highest possible modulation bandwidth and improve the coupling efficiency with small core optical fibers.

Power:

The best results are usually achieved by coupling as much of a source's power into the fiber as possible. The key requirement is that the output power of the source be strong enough to provide sufficient power to the detector at the receiving end, considering fiber attenuation, coupling losses and other system constraints.

Speed:

A source should turn on and off fast enough to meet the bandwidth limits of the system. The speed is given according to a source's Rise or fall time, the time required to go from 10% to 90% of peak power. LEDs have slower rise and fall times than lasers.

In Summary:

The benefits of LED compared to other forms of illuminating devices such as incandescent or fluorescent include high efficiency and low power requirement and when properly installed can function for decades. While LED used in fiber optic can be more expensive than incandescent lights but their long-term running cost makes them last long. Fiber Optic communication using LEDs, will compete for an even bigger role in the world of technology.