LASER DIODES

                                                       LASER DIODES

Definition: 
                  It is a specially fabricated p-n junction diode. This diode emits laser light when it is forward-biased.

Principle:
                When the p-n junction diode is forward-biased the electrons from n-region and holes from p-region cross the junction and recombine With each other.
                  During the recombination process, the light radiation (photons) is released from direct bandgap semiconductors like GaAs. This light radiation is known as recombination radiation.
               
Construction:

•  The active medium is a p n junction diode made from a single crystal of gallium arsenide. This crystal is cut ’ in the form of a platelet having a thickness of 0.5 mm. This platelet consists of two regions n-type and p-type.
• The metal electrodes are connected to both upper (p-region) and lower (n-region) surfaces of the semiconductor diode. The forward bias voltage is applied through metal electrodes. 
• Now the photon emission is stimulated in a very thin layer of the p-n junction. 
• The end faces of the p-n junction are well polished and parallel to each other. They act as an optical resonator through which the emitted light comes out. 

Working:

•  When the p-n junction is forward-biased, the electrons and holes are injected into the junction region. 
• The region around junction contains a large number of electrons in the conduction band and holes in the valence band.
• When the forward-biased voltage is increased, more light photons are emitted. These photons trigger a chain of stimulated recombinations resulting in the emission of more light photons in phase

        After gaining enough strength, the laser beam of wavelength 8400 A is emitted from the junction.

Characteristics:
                           Solid-state semiconductor laser. 

Active medium:
                           A p-n junction diode made from a single crystal of gallium arsenide

Pumping method:
                             Direct conversion method

Power output:
                         A few mW.

Nature of output:
                        Continuous-wave or pulsed output.

The wavelength of output:
                                     8300 A° to 8500 A°.

Advantages:

•  This laser is very small in size and compact. 
• It has high efficiency. 
• The laser output can be easily increased by increasing the junction current. 
• It is operated with less power than a ruby and CO2 lasers.
• It requires very little additional equipment. 
• It emits a continuous wave output or pulsed output       

Disadvantages:

• Laser output beam has large divergence. 
• The purity and monochromacity are poor. 
•  It has poor coherence and stability. 

Applications of Laser Diode:

• Used in fibre optic communication. 
• Used in various measuring devices such as range finders, O bar-code readers.
•  Infrared and red laser diodes are common in CD players, CD ROM and DVD technology. Violet lasers are used in blue-ray technology and HD DVD.
• High power laser diodes are used in industrial applications such as heat treating, cladding, seam welding and for pumping other lasers. 
• Used in laser medicine especially, dentistry.

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