What is resonant cavity? Explain its use in the generation of Laser beams.

Resonant cavity (optical resonator): A pair of mirrors placed facing each other, parallel (or confocal), at the two ends of the active medium. One mirror is fully reflecting (about 100%) and the other is partially reflecting (about 95–99%, the output coupler). The cavity length satisfies \(L=n\lambda/2\), so that standing waves (longitudinal modes) form.

 M1 (100%)                           M2 (partial)
   |  <----  active medium  ---->  |  ===> laser output
   |  (population inversion)        |

Its use in generating laser beams:

1. Positive feedback / amplification: Photons from spontaneous emission travelling along the axis are reflected back and forth many times through the inverted medium. Each pass stimulates more emission, so the light is amplified enormously (a chain reaction).
2. Directionality: Only photons moving along the axis keep bouncing. Off-axis photons escape through the sides, so the output is a highly directional, narrow beam.
3. Monochromaticity and coherence: Only wavelengths that satisfy \(2L=n\lambda\) build up (resonance). This narrows the linewidth and makes the photons in phase.
4. Sustained oscillation: Oscillation continues once the gain per round trip exceeds the losses (the threshold condition). Part of the beam leaves through the partial mirror as the laser output.

Types of cavity: plane-parallel, confocal and hemispherical.