In one line
The active medium is the material in which population inversion and light amplification take place, and the active centres are the specific atoms, ions or molecules inside it that actually make the lasing transitions.
Explained simply
In a cricket team the whole squad is the team, but the bowlers are the ones who actually take wickets. Similarly, the active medium is the whole material, and the active centres are the particular particles that do the emitting.
Technically, the active medium is the solid, liquid, gas or semiconductor placed inside the resonant cavity. Only some species in it have energy levels suitable for lasing (a metastable upper level, a suitable lower level); these are the active centres.
How it works
- The pump supplies energy to the medium.
- The active centres absorb it (directly or through helper atoms) and reach the upper laser level.
- Population inversion builds up among the active centres.
- Stimulated emission between their levels gives the laser wavelength, so the wavelength is fixed by the active centre.
| Laser | Active medium | Active centre |
|---|---|---|
| He-Ne | He-Ne gas mixture | Neon atoms |
| Ruby | Al2O3 crystal with Cr | Cr3+ ions |
| Nd:YAG | YAG crystal with Nd | Nd3+ ions |
| Erbium fibre laser | Silica fibre core with Er | Er3+ ions |
| CO2 | CO2, N2, He mixture | CO2 molecules |
Quick example
In the He-Ne laser, helium atoms are far more numerous but do not emit the laser light; they only collect energy from the discharge and pass it to neon. Neon atoms are the active centres, and the 632.8 nm red line comes from neon.
Common mistakes
- Calling helium the active centre in a He-Ne laser.
- Treating the host crystal (Al2O3, YAG) as the active centre; it only holds the ions.
- Mixing up active medium with resonant cavity.
How MU asks it
Usually a 3-mark definition or part of "Explain the terms active medium, active centre, pumping and resonant cavity" (5 marks). Define both, then give a two- or three-row example table like the one above.