Answer: Two parallel mirrors at the ends of the active medium, one fully and one partially reflecting; they feed photons back through the medium for repeated stimulated emission and select the direction and frequency of the beam.
Construction: the active medium is placed between two mirrors facing each other and aligned parallel (plane or concave). One mirror reflects nearly 100% of the light; the other (output coupler) reflects about 95–99% and lets the rest out as the beam.
Role:
- Feedback and amplification: photons travelling along the axis are reflected back and forth many times through the inverted medium; each pass causes more stimulated emission, so the intensity builds up until gain equals losses.
- Directionality: photons moving at an angle to the axis leave the cavity after a few passes; only axial photons are amplified, so the output is a narrow, highly directional beam.
- Monochromaticity: only standing waves with \(L=\dfrac{n\lambda}{2}\) (\(L\) = mirror separation) survive, which narrows the frequency spread.
