LASER Diode

Laser Diodes 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.

Laser action (with the resultant monochromatic and coherent light output) can be achieved in a p-n junction formed by two doped gallium arsenide layers. The two ends of the structure need to be optically flat and parallel with one end mirrored and one partially reflective. The length of the junction must be precisely related to the wavelength of the light to be emitted. The junction is forward biased and the recombination process produces light as in the LED (incoherent). Above a certain current threshold the photons moving parallel to the junction can stimulate emission and initiate laser action.

Five inherent properties make lasers attractive for use in fiber optics.

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.

Powering a LASER

If a laser is continuously emitting light, then there must be power to replenish that lost energy in such a way that the laser action can continue. The power must maintain the necessary population inversion to keep the laser process going, and that implies a pumping mechanism to elevate electrons to that metastable state . The use of helium to “pump” electrons into a metastable state of neon in the helium-neon laser is an example of such a mechanism.

The minimum pumping power would occur if the pumping process were 100% efficient and you just had to replenish the energy lost in radiation. Lasers will have a finite bandwidth and a number of modes N_m within that bandwidth. The energy in a given mode can be charaterized by an average lifetime t_c. Using the Planck relationship for the energy of a given photon, the minimum pumping power can be expressed by

The rate of stimulated emission is proportional to the difference in the number of atoms in the excited state and the ground state, N2 - N1, which is in turn affected by the average lifetime of the excited state and the average lifetime of the emission in the laser cavity.

(Laser Condition)

Types of LASER

Posted in Communication, Optical Communication |

• LASER
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• Introduction To Optical Communication
• Gunn Effect

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