Explain optical resonator in LASER?

Optical resonator is the arrangement of two mirrors placed at the ends of the laser's active medium, facing each other and aligned with its axis. One mirror is fully reflecting (≈100%) and the other is partially reflecting (≈95–99%).

 M1 (100%) |<----- active medium (L) ----->| M2 (partial) ===> output

• Feedback and amplification: Photons travelling along the axis are reflected back and forth through the inverted medium, stimulating more and more emission, so the intensity builds up.
• Selection of direction: Only axial photons survive many passes, which gives a highly directional beam.
• Selection of frequency: Only wavelengths that satisfy \(2L=n\lambda\) (standing waves, longitudinal modes) are reinforced. This gives a narrow linewidth (monochromaticity) and high coherence.
• Oscillation is sustained when the gain exceeds the losses (the threshold), and part of the beam leaves through the partial mirror as output.
• Types: plane-parallel, confocal and hemispherical resonators.