Answer: A four-level gas laser: an electric discharge excites He atoms to metastable levels, which pass their energy to Ne by resonant collisions; Ne then lases at 6328 Å (red), continuously.
Construction (figure described): a long, narrow quartz discharge tube (about 30–50 cm long, a few mm bore) is filled with helium and neon in the ratio about 10 : 1 at low pressure (about 1 mm of Hg). Electrodes near the two ends are connected to a high-voltage DC (or RF) supply. The tube ends carry Brewster windows, and two mirrors outside form the optical resonator: one fully reflecting, the other partially reflecting (output end). Ne is the active medium; He helps in pumping.
Energy level diagram (described): on the left, He levels: ground state \(1^1S\) and two metastable levels \(2^3S\) (about 19.82 eV) and \(2^1S\) (about 20.61 eV). On the right, Ne levels: ground state, \(1s\) levels (about 16.7 eV), \(2p\) levels (about 18.7 eV), and the upper levels \(2s\) (about 19.78 eV) and \(3s\) (about 20.66 eV), lying almost level with the He metastable levels. Horizontal dotted arrows show energy transfer from He to Ne; a downward arrow \(3s\to2p\) is marked 6328 Å.
Working:
- Pumping: the discharge accelerates electrons, which collide with He atoms and raise them to the metastable \(2^1S\) and \(2^3S\) levels (electrical pumping).
- Resonant energy transfer: excited He atoms collide with ground-state Ne atoms. Because the Ne \(3s\) and \(2s\) levels have nearly the same energy, the energy passes to Ne and He returns to the ground state. Ne atoms pile up in \(3s\) and \(2s\): population inversion relative to \(2p\), which is nearly empty.
- Lasing: stimulated transitions \(3s\to2p\) give 6328 Å (red, the usual output). Other lines, \(3s\to3p\) at 3.39 μm and \(2s\to2p\) at 1.15 μm, are suppressed by choosing the mirror coatings.
- Return to ground: Ne atoms in \(2p\) decay quickly by spontaneous emission to \(1s\); from the metastable \(1s\) level they return to the ground state mainly by collisions with the tube walls (that is why the tube is narrow). This keeps the lower laser level empty.
Features: continuous-wave output of a few mW, highly coherent and directional, efficiency below 0.1%. Uses: barcode scanners, alignment, holography, interferometry.
