LASER

LASER IS THE SHORT FORM OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION.
CHARACTERISTICS OF A LASER BEAM
1.DIRECTIONALITY:
LASER BEAM IS HIGHLY DIRECTIONAL.IT CAN BE FOCUSSED TO A FINE POINT.HENCE LASER IS USED FOR SURGICAL APPLICATIONS.
2.MONOCHROMATICITY:
LASER BEAM IS HIGHLY MONOCHROMATIC.LINE WIDTH IS NARROW COMPARING WITH CONVENTIONAL LIGHT SOURCES.FOR RUBY LASER LINE WIDTH = 5 ANGSTROM.
3.COHERENCE:
LASER BEAM IS HIGHLY COHERENT.TWO INDEPENDENT LASER SOURCES CAN PRODUCE INTERFERENCE EFFECTS.
4.BRIGHTNESS:
LASER IS HIGHLY INTENSE BEAM OF LIGHT.IT IS USED IN INDUSTRY FOR CUTTING,WELDING AND DRILLING OPERATIONS.
Basic concepts of laser

Interaction of radiation with matter

Consider a system having two energy levels E1 and E2 with E2-E1=hÖ. When it is exposed to radiation having a stream of photons, each with energy hÖ, three district processes can take place. They are 1) Absorption

2) Spontaneous emission and

3) Stimulated emission.Absorption

An atom in the ground state E1 can absorb a portion of energy hÖ and go to the higher energy state E2. This process is known as absorption and is illustrated in figure.Rate of absorption R12 is proportional to population (number of available atoms per unit volume) of the lower energy level N1 and u(Ö), the energy density of radiation u(Ö).i.e, R12 a N1u(Ö)R12 = B12N1u(Ö) ------------(1)

Where B12 is called Einstein coefficient.
Spontaneous EmissionIn spontaneous emission, the atoms in the higher energy state E2 eventually return to the ground state by emitting their excess energy spontaneously. This process is independent of the external radiation. The rate of spontaneous emission R21 is directly proportional to the population of the energy level E2 (N2).i.e, R21 a N2R21 = A21N2 --------------(2)

Where A21 is called Einstein coefficient.Stimulated emissionIn stimulated emission, a photon having energy hÖ(E2-E1) stimulate an atom in the higher state E2 to make a transition to the lower state E1 with the creation of a second photon. The rate of stimulated emission R121 is proportional to population at the energy level E2(N2) and energy density of radiation u(Ö).i.e, R121 a N2u(Ö)

Semiconductor laser (Diode laser/GaAs laser)
GaAs is a direct bandgap semiconductor. Laser transition is possible only in direct bandgap semiconductors. Si and Ge do not give laser transition since they are indirect bandgap semiconductors.
Fermilevel (EF) is the highest filled energy level at absolute zero. A semiconductor in which Fermi level lies the conduction band (in n type) or valence band (in p type) is called a degenerate semiconductor. A p-n junction is used for the fabrication of semiconductor laser. Both p and n regions are made degenerate by heavy doping.
With a forward bias, depletion region (active region) contains a high concentration of electrons in the conduction band and holes in the valence band. Population inversion has occurred in the sense that more states are occupied in the conduction band than in the valence band. At low bias currents, electron-hole recombination takes place spontaneously resulting in a spontaneous emission of photons. This is the principle of a light emitting diode (LED). As the diode current increases, a point is reached, where significant population inversion exist near the junction resulting in a stimulated emission.

One pair of faces perpendicular to the junction is polished so that they act as resonant cavity. The remaining faces are roughened to eliminate laser action in those directions.

In a semiconductor laser, the transitions are associated with the electron states in the conduction band and valence band. The upper and lower energy states are continuous and hence the output is not sharp. Thus coherence and monochromaticity of a GaAs laser are poor. But they have a few advantages. They are
*Portable since compact and small.
*High efficiency
*Highly economical
*Can produce both continuous wave and pulsed laser.
*Tuning of output is easily possible.
Applications of laser
1. Industrial application: Welding, drilling and cutting.
2. Medical applications: In dermatology, dentistry, ophthalmology, in surgery of tumours, kidney stone and for cancer treatment.
3. For making sensors.
4. In holography.
5. In laser printers.
6. In research.
7. In microelectronics.
8. In accelerating certain chemical reactions.
9. In fibre optic communication.
10. In underwater communication.
11. In military applications.