User talk:2014HZ97018 LED

Light Emitting Diode structures in Optical Fiber Communication

Light emitters 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. LEDs are a specialized form of p-n junction diode that has been designed to optimize their electroluminescence. In other words, LED is basically a tiny light bulb similar to an incandescent bulb. The major difference between the two is that LED does not have a filament that will burn out and they do not get remarkably hot.

There are a number of different aspects to the LED structure and LED fabrication. These include not only the LED fabrication itself, but also the packaging of the LED once the semiconductor chip itself has been fabricated. Structure There are mainly five major types of LED structures •	Planar LED •	Dome LED •	Surface emitter LEDs •	Edge Emitter LEDs •	Super luminescent LEDs

•	LEDs are complex semiconductors that 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: •	Surface emitting LED structure:  This form of LED structure emits light perpendicular to the plane of the PN junction. •	Edge emitting LED structure:  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. •	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. Commercially, LEDs exist in a variety of forms, ranging from individual LED indicators where there is just one LED per package, through a variety of displays, right up to vast arrays of LEDs in LED screens.

Final LED package structure There are obviously many different styles of LED that are available. These range from the simple LED indicators through the more complicated LED alphanumeric displays to the LED screens that are now appearing. All these types of LED will have their own package structure. However the simple LED indicators tend to have a fairly common structure and this serves to indicate the constraints on any LED device. The structure of the LED package can be split into a number of different elements: •	Semiconductor die: This is the light emitting diode itself formed from the semiconductor. •	Lead frame: This houses the die and acts as the connection to it.. The die is bonded into a recess in one half of the lead frame, called the anvil due to its shape. This is done using conductive epoxy. The recess in the anvil is shaped to throw the light radiation forward. The top contact from the die is then wire-bonded to the other lead frame terminal which is often called the post.

Surface Emitters: Surface emitters Use an etched well in a GaAssubstrate in order to preventheavyabsorptionof emitted radiation. The Low thermal impedance in active region allows high current densities and gives high radiance emission into optical fiber. The circular active area in practical surface emitters is nominally 50µm in diameter and up to 2.5µm thick. The emission pattern is essentially isotropic with a 120® half-power beam width. The isotropic pattern form a surface emitter is called a lambertian pattern. The etched well allows the optical fiber to come into close contact with the emitting surface. In addition, the epoxy resin that binds the optical fiber to the SLED reduces the refractive index mismatch, increasing coupling efficiency. Typically, SLEDs operate efficiently for bit rates up to 250 megabits per second (Mb/s). Because SLEDs emit light over a wide area (wide far-field angle), they are almost exclusively used in multimode systems.

Characteristics of SLED: Edge Emitters: High radiance structure currently used in optical communications is the stripe geometry. It is a 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.

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. Spectrum :

Fig: Emission spectrum along the edge and surface of GaInPAs/InP communication LED emitting at 1300nm. The spectrum emitted along the edge of the LED is narrower due to self-absorption. 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 up front, they can be more expensive than incandescent lights but their long-term running cost makes them a better buy. In future, they will compete for an even bigger role in the world of technology. Energy Gaps in LEDs Eg=hc/l = 1240eV-nm/l Where: h = Plank's Constant = 4.13 x 10-15eV•s c = speed of light = 2.998 x 108 m/s l = wavelength in nm

Source of Information: Books & Internet