In one line
The resonant cavity is a pair of mirrors around the active medium that sends light back and forth through it many times, building up stimulated emission and selecting a narrow, well-directed output beam.
Explained simply
Shouting once in an open ground fades quickly, but in a long tunnel the echo keeps bouncing and building. The mirrors of a laser make a "tunnel" for light: photons pass through the active medium again and again, and each pass adds more identical photons.
Technically, it is an optical feedback system, usually two mirrors facing each other: one fully reflecting (about 100%) and the other partially reflecting (output coupler). Only light travelling along the axis survives many round trips and gets amplified.
How it works
- A few spontaneous photons travel along the axis of the medium.
- They stimulate emission from excited atoms, producing identical photons.
- At the mirror they reflect back and pass through the medium again, gaining more photons.
- Photons moving off-axis leave through the sides and are lost, so the beam becomes highly directional.
- Only wavelengths that fit a whole number of half-waves in the cavity form standing waves and survive, which narrows the linewidth.
- When gain per round trip exceeds the losses, a steady beam leaks out of the partial mirror.
Key formula
Standing-wave condition: \(L = \dfrac{m\lambda}{2}\), where \(L\) is the mirror separation (m), \(\lambda\) is the wavelength in the medium (m), and \(m\) is a positive integer.
Quick example
For a cavity of length 0.30 m and \(\lambda = 632.8\) nm, \(m = 2L/\lambda \approx 9.5\times10^{5}\), so the cavity supports modes of extremely high order with very small frequency spacing \(c/2L = 500\) MHz between them.
Common mistakes
- Saying both mirrors are 100% reflecting; then no beam could come out.
- Thinking the cavity creates energy. It only gives feedback; the energy comes from the pump.
- Ignoring the role of the cavity in directionality and monochromaticity.
How MU asks it
Asked as "Explain the role of resonant cavity in lasers" (3 marks) or inside the He-Ne construction question. Draw two mirrors with the medium between them, mark full and partial reflector, and write three roles: feedback, directionality, wavelength selection.