1. LASER
LASER stands for LIGHT APLIFICATION by STIMULATED EMISSION of RADITIONS
First laser was constructed by Maiman
Laser action has been obtained with atoms, ions & molecules in gases, liquids, solids, semiconductors at wavelength( λ ) ranging from ultraviolet to radio frequency regions and with power ranging from a few milli watts to mega watts.

2. Basic principle involved in laser
Spontaneous emission
Stimulated absorption
Stimulated emission

3. Stimulated Absorption
E1 & E0 are two energy levels such that E1 is excited energy level & E0 is lower energy level.
If hƒ =E1- E0 ,where ƒ is frequency of incident radiation, then radiation photon is absorbed & electron jumps from E0 to E1.

4. Spontaneous Emission
The excited atom does not remain in higher energy state for long.It gives out a photon of energy hƒ= E1- E0 & returns to ground state. As this process does not require stimulus from outside,so it is termed as spontaneous emission.

5. Stimulated Emission
The excited atom in energy state E1 may not come to ground state at given instant. So it requires external stimulation. When a photon of energy hƒ= E1- E0 strikes the atom in excited state , it comes to ground state. The photon emitted goes parallel to incident photon & both of these photons are in phase. For laser to have Stimulated Emission, the following two conditions must be satisfied
1)The higher energy state should have longer life time
2) The number of atoms in higher energy state E1 must be more than that in E0

6. Population Inversion
The situation in which number of atoms in higher energy state is greater than in lower energy state is called population inversion. It can be obtained even at room temperature by laser pumping.various pumping methods are:optical pumping,chemical pumping, electrical pumping. when population inversion is obtained, emitted photons have same frequency and phase. They travel in same direction. Thus the number of photons goes on multiplying by stimulated emission. Hence we get a highly intense, monochromatic, coherence and unidirectional beam.

7. RESONANCE CAVITY Fully Reflecting mirror Partially reflecting mirror
Active medium
A medium with population inversion is capable of amplification but in order to get a continous supply of light energy it is required to couple a part of output back into the medium. It can be made by placing an active medium between two mirrors such that one is fully reflecting and other is partially reflecting.
Active medium: A medium which on excitation produces population inversion and promotes stimulated emission

8. Characteristics of Laser
1)Directionality
2)Intensity
3)Monochromaticity
4)Coherence

9. 1)Directionality
The conventional source emits radiation in all direction. But Laser emits light in particular direction. Directionality of laser can be expressed in terms of full angle beam divergence, which is twice the angle that outer edge of beam makes with the axis of beam.
The angular spread Δθ =λ/d where d is diameter of aperture.

10. 2) Intensity
Laser gives out light in form of narrow beam with energy concentrated in a small region.
Even 1 watt laser appears thousands times more intense than 100 watt ordinary lamp.
The power output from a small gas laser is 10-3 watt whereas that from a pulsed solid state laser may be as high as 109 watt.

11. 3) Monochromaticity
Laser emits light which is more monochromatic than any other monochromatic conventional source.
Light emitted from a monochromatic conventional source is not perfectly sharp.It spreads whereas laser light spreads very little as shown in figure.

12. 4) Coherence
It is measure of degree of phase correlation that exists in radiation field of light source at different locations and different times.
Cohernce is of two types
a) Spatial Coherence
b) Temporal Coherence

13. Temporal Coherence
If the phase difference between any two points at an instant of time lying in direction of propagation of the electromagnetic wave is independent of time, the coherence is temporal coherence.

14. Spatial Coherence
If the phase difference between any two points at an instant of time lying in the plane perpendicular to direction of propagation is independent of time, the coherence is spatial coherence.