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Light emitter diodes are complex semiconductors that convert an electrical current into light. These are a key element in any fiber optic system. This component converts the electrical signal into a corresponding light signal that can be injected into the fiber. The light emitter is an 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 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. These fiber optic transmitters are cheap and reliable. They emit only incoherent light with a relatively wide spectrum as a result of the fact that the light is generated by a method known as spontaneous emission. A typical LED used for optical communications may have its light output in the range 30 - 60 nm. In view of this the signal will be subject to chromatic dispersion, and this will limit the distances over which data can be transmitted It is also found that the light emitted for a LED is not particularly directional and this means that it is only possible to couple them to multimode fibre, and even then the overall efficiency is low because not allt he light can be coupled into the fibre optic cable. LEDs have significant advantages as fibre optic transmitters in terms of cost, lifetime, and availability. They are widely produced and the technology to manufacture them is straightforward and as a result costs are low. LED STRUCTURES: It is pn- junction in forward bias. Injection of minority carriers across the junction gives rise to efficient radiative recombination (electroluminescence) of electrons (in CB) with holes (in VB). There are five major types: 1-Planar LED 2-Dome LED 3-Surface emitter LEDs 4-Edge Emitter LEDs 5-Superluminescent LEDs Only two have use in OFC(SLED and ELED) Planar LED: •	Simplest of the structures that are available. •	Fabricated by liquid or vapour phase epitaxial processes over GaA surface. •	 Lambertian emission. •	TIR limites the Radiance low. Dome LED: •	A hemisphere of n-type GaAs around p-region. •	Higher external power efficiency than planar LED.

Surface Emitter LEDs:

•	This form of LED structure emits light perpendicular to the plane of the PN junction. •	Method for obtaining high radiance is to restrict the emission to a small active region within device. •	Pioneered by Burrus and Dawson. •	Used an etched well in a GaAs substrat in order to prevent heavy absorption of emitted radiation. •	Low thermal impedance in active region allowing high current densities and giving high radiance emission into optical fiber.

Edge Emitter LEDs:

•	High radiance structure currently used in optical communications is the stripe geometry •	Similar geometry to a conventional contact stripe infection laser •	Surface geometry allows very high carrier injection densities for given high current. •	This form of LED structure emits light in a plane parallel to the junction of the PN junction. •	In this configuration the light can be confined to a narrow angle.

ELED:

•	It shows the different layers of semiconductor material used in the ELED. •	The primary active region of the ELED is a narrow stripe, which lies below the surface of the semiconductor substrate. •	The semiconductor substrate is cut or polished so that the stripe runs between the front and back of the device. •	The polished or cut surfaces at each end of the stripe are called facets.

Application:

•	In an ELED the rear facet is highly reflective and the front facet is antireflection-coated. •	ELEDs emit light only through the front facet. •	ELEDs emit light in a narrow emission angle allowing for better source-to-fiber coupling. •	They couple more power into small NA fibers than SLEDs. •	ELEDs can couple enough power into single mode fibers for some applications. ELEDs emit power over a narrower spectral range than SLEDs. •	However, ELEDs typically are more sensitive to temperature fluctuations than SLEDs. •	For medium-distance, medium-data-rate systems, ELEDs are preferred. •	ELEDs may be modulated at rates up to 400 Mb/s. ELEDs may be used for both single mode and multimode fiber systems.

Super luminescent LEDs •	Having advantages of both SLED and ELED •	High output power •	A directional output beam •	 A narrow spectral linewidth •	 A super luminescent light emitting diode is, similar to a laser diode, based on an electrically driven pn-junction that, when biased in forward direction, becomes optically active and generates amplified spontaneous emission over a wide range of wavelengths. •	The peak wavelength and the intensity of the SLED depend on the active material composition and on the injection current level. •	 SLEDs are designed to have high single pass amplification for the spontaneous emission generated along the waveguide but, unlike laser diodes, insufficient feedback to achieve lasing action.

Characteristics are: 1.	The total optical power emitted by an SLED depends on the injected current (bias). 2.	Unlike laser diodes, the output intensity does not exhibit a sharp threshold but it gradually increases with current. 3.	A soft knee in the power vs. current curve defines a transition between a regime dominated by spontaneous emission (typical for surface emitting LEDs) and one that is dominated by amplified spontaneous emission (i.e.superluminescence)

Laser to fiber coupling: •	For greater coupling efficiencies,lenses are used •	 Power conversion efficiency(η): •	 Ratio of optical power coupled into fiber(Pc) to the electrical power applied at the terminals of device. •	 η= Pc/P •	 CONVEX lenses are used.