Emitting Diode Lasers

Although LEDs and laser diodes appear to be similar, in fact- they me made from the same semi-conductor materials, they are very different in that they are designed to behave in much different wa3rs. Tlie light from diode lasers is coherent in contrast to the incoherent light emitted from LEDs. Emission from LEDs result in a 4jr pattern whereas the diode laser emits a very narrow beam. As a result, diode lasers can be used to read the pits on a compact disc or for scanning a barcode in a checkout line, jobs that LEDs cannot perform. 

The following diagram, given as 7.2.27. on the next page, shows a t T)ical construction of a laser diode  

First developed in 1962, laser diodes have achieved powers that allow them to be used in applications not thought possible a few years ago. The following table, presented on the next page, shows some of the laser compositions, wavelengths and applications now used for these versatile devices. Note that the bandgap of these materials could be calculated from the equation  

You will note that diode lasers cover a wide range of coherent emission wavelengths, from the visible to the far infrared. Also, the same materials used as for LED s are employed here except that a resonant laser chamber equipped with totally reflecting mirror and an output mirror of 

8% reflectivity is provided to achieve the resonant condition required for coherent emission of light. 

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The simplest electronic devices, such as diodes, light-emitting diodes, lasers, and photocells, have a single p-n junction. If we place, say, a p-type doped Si block in contact with n-typed doped Si block, electrons will normally flow from the n to the p regions but not vice versa. Thus, the p-n diode so created can be fitted with ohmic contacts to function as a rectifier of alternating current. Schottky junctions can act in this way to some degree even without deliberate creation of a p-n junction. 

LCAO LD TOP LECBD LED LIP LUMO linear combination of atomic orbitals laser-desorption time-of-flight low energy cluster beam deposition light-emitting diode laser-induced fluorescence lowest unoccupied molecular orbital... 

The potential application of this glass is in high-density optical recording (used, for example, in compact disc players). In such devices the information density increases with dccrca.sing sire of the focus spot of the laser. This sire varies inversely quadratically with the wavelength. Since the available diode lasers emit in the near infrared, there is a considerable amount of research going on to obtain a blue-emitting diode laser. There are three possibilities, viz. 

Light-Emitting Diode Laser Scanning Microscope... 

Many techniques have been used to measure the velocity of a projectile. The most widely used systems are instantaneous, discrete techniques such as sensors or chronographs. Impact or residual velocity of a projectile is calculated from the distance between two sensors divided by the time taken by the projectile flying between the sensors. Sensors currently employed in the ballistic range include light-emitting diodes, laser beams, thin wires, or infrared beam. Impact and residual velocities obtained from the rig are used to calculate the projectile kinetic energy loss AE ... 

A laser diode is a laser where the active medium is a semiconductor similar to that found in a hght-emitting diode. Laser diodes are used in devices for optical storage, laser pointers, and fiber optics. 

Many metal polyynes have third-order nonlinear optical properties and these polymers are of interest from the point of view of new types of optical devices. Other types of potential applications of these polymers include light-emitting diodes, lasers, photoeells, field-effect transistors, low-dimensional conductors etc. [70]. 

Figure 3.33 Illustrates the three primary component layers of an edge-emitting diode laser, (a) shows the energy gap as a function of distance from top to bottom of the device layer. The lower figure shows how injected carriers and the optical field are confined by the SCH/quantum well structure.

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