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Explain the operating principle of LASER and list applications of LASER diode.

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A basic construction of Laser diode is as shown . A PN junction is formed by two layers of doped gallium arsenide. There is highly reflective surface at one end of PN junction and partially reflective surface at the other end, forming resonant cavity for the photon . 

Operating Principle: The Laser diode is biased by external voltage source. As electron move through the junction, recombination occurs just like a ordinary diode. As electrons fall into holes to recombine, photons are released. A released photon can strike atom, causing another photon to released. As forward current is increased, more electrons enter the depletion region and cause more photons to be emitted. Eventually some of the photons that are randomly drifting within the depletion region strike the reflected surfaces perpendicularly. These reflected photons move along the depletion region, striking atoms and releasing additional photons due to avalanche effect.

Applications:- 1. fiber optic communications 2. barcode readers 3. laser pointers 4. CD/DVD/Blu-ray disc reading/recording 5. laser printing 6. laser scanning and light beam illumination. 7. Military applications 8. Medical surgery 9.Diamond cutting

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