With the help of neat diagram, explain construction and working of He Ne laser.

He–Ne laser is a four-level gas laser that gives a continuous output at 6328 Å (red).

Construction:

            discharge tube (He:Ne ≈ 10:1, ~1 mm Hg)
   M1 |=======================================| M2
 100% |   (+)anode                 cathode(-)  | ~99% (partial)
 mirror|=======================================| ---> laser 6328 Å
        Brewster windows at both ends, RF/DC high-voltage supply

• A long, narrow quartz discharge tube (about 50 cm long, 0.5 cm in diameter) filled with a mixture of helium and neon in the ratio of about 10:1 at low pressure. Ne is the active medium, and He helps with pumping.
• Electrodes connected to a high-voltage DC or RF source produce an electric discharge (electrical pumping).
• Two mirrors form the optical resonator: one fully reflecting and one partially reflecting (the output coupler). Brewster windows give plane-polarised output.

Energy level diagram and working:

      Helium                     Neon
  F3 (2 ¹S, 20.61 eV) ----->  E6 (3s, 20.66 eV) --\ 6328 Å (laser)
  F2 (2 ³S, 19.82 eV) ----->  E4 (2s, 19.78 eV) --\---\ 1.15 µm / 3.39 µm
     ^  collisions           E5 (3p)  <-----------/    \
     | (electron impact)     E3 (2p, 18.70 eV) <--------/
     |                         | spontaneous ~6000 Å
  F1 ground                  E2 (1s, metastable) -- diffusion / wall collisions
                             E1 ground

1. The discharge accelerates electrons, which collide with He atoms and excite them to the metastable levels F2 and F3 (\(e+He\to He^*\)).
2. These He levels almost match the Ne levels E4 (2s) and E6 (3s). Through resonant collisions of the second kind, the energy passes to Ne atoms (\(He^*+Ne\to He+Ne^*\)), and the He returns to its ground state.
3. This builds up a population inversion between E6/E4 and the lower levels E5/E3 of Ne.
4. Laser transitions: E6 → E3 gives 6328 Å (visible red); E6 → E5 gives 3.39 µm; E4 → E3 gives 1.15 µm (both IR). The IR lines are suppressed by choosing the mirrors.
5. From E3 the atoms decay spontaneously to the metastable level E2. From E2 they return to the ground state by colliding with the tube walls, which is why the tube is kept narrow.
6. Photons bouncing between the mirrors stimulate further emission. The beam builds up and leaves through the partially reflecting mirror as a continuous, highly monochromatic and coherent beam.

Advantages: continuous output, high monochromaticity and directionality, low cost. Uses: barcode scanners, alignment, interferometry and holography.