Subject :- Applied Physics 2.

Topic :- Laser.

Difficulty :- Medium.

Light can be amplified by an active medium taken into the state of population inversion. To achieve stimulated emission, $N_2$ should be made greater than $N_1$ and radiation density should be high. These requirements are met with the help of an optical cavity resonator.

An optical resonator consists of two opposing plane parallel mirrors with active material placed in between them. One of the mirror is semitransparent while other is made 100 % reflecting. The mirrors are set normal to the optic axis of the material.

Initially active centers in the medium are in ground state. Through suitable pumping the material is taken into the state of population inversion. Spontaneous photons are emitted in every direction. To generate coherent output, it is imperative that photons with specific directions are selected and others are rejected. Secondly, to attain maximum possible amplification the photons are made to pass through the medium number of times. Photons traveling parallel to axis, on reaching semitransparent mirror, some are transmitted and many are reflected back. While propagating in the opposite direction they deexcite some more atoms and builds up their strength. At 100 % reflecting mirror some of the photons are absorbed are reflected back and likewise it undergoes multiple reflections at the mirror and gains in strength. laser oscillation begins when amount of amplified light becomes equal to amount of light transmitted and absorbed by mirror. As the oscillation builds up to enough intensity, it emerges through the mirror which is semitransparent as a highly collimated intense beam.