The Laser

Ordinary light sources are a crowd of atoms each doing its own thing. In a laser, one photon persuades atoms to emit identical copies of itself, giving light that is coherent, pure in colour and almost perfectly parallel.

Why ordinary light is messy#

In a normal source atoms emit independently: the photons have random phases (incoherent light), slightly different energies (a spread Δλ) and random directions. LASER, Light Amplification by Stimulated Emission of Radiation, gives light with all photons in phase, Δλ as small as about 10⁻⁶ nm (about a thousand times purer than the krypton-86 standard), and a beam so parallel it can go to the Moon and back.

Three ways an atom meets light#

Take an atom with levels E₁ (lower) and E₂ (higher).

• Stimulated absorption: a photon of energy E₂ − E₁ is absorbed, lifting the atom from E₁ to E₂.
• Spontaneous emission: left alone, an atom in E₂ falls back by itself, emitting a photon. A typical excited state lasts about 10 ns (its lifetime); some last about a millisecond, 10⁵ times longer: metastable states.
• Stimulated emission: a photon of energy E₂ − E₁ passing an atom already in E₂ can trigger it to drop, emitting a second photon with exactly the same energy, phase and direction.

Making light grow#

Suppose many more atoms are in a metastable state E₂ than in E₁. One photon stimulates an emission: now two identical photons. They stimulate two more atoms, and so on: an avalanche of identical photons. Two conditions are needed:

Population inversion and pumping

Normally the lower level is more crowded, and its atoms would just absorb the photons. The upper level must hold more atoms than the lower: a population inversion. Keeping it up means continually feeding atoms into E₂ and removing them from E₁: pumping.

Mirrors

Photons must stay around long enough to stimulate many emissions. Mirrors at the two ends of the tube or rod send them back and forth; one mirror lets through a little (about 1 %), which is the laser beam. Photons not travelling along the axis escape from the sides after a few reflections, so only light along the axis builds up: hence the beam’s directionality.

The helium–neon laser#

A glass tube holds about 90 % helium and 10 % neon at low pressure, with two electrodes connected to a high voltage and mirrors M₁ and M₂ (M₂ partly transmitting) at the ends. The laser transition is in neon, between E₂ = 20.66 eV and E₁ = 18.70 eV.

1. The discharge ionizes some atoms; the freed electrons, accelerated by the field, knock helium atoms into a metastable state at 20.61 eV.
2. Excited helium atoms collide with neon atoms and hand over their energy, lifting neon to E₂ (20.66 eV, very close): pumping.
3. Neon in E₁ lives only about 10 ns before dropping lower, so E₁ stays nearly empty: inversion between E₂ and E₁ is maintained.
4. Stimulated emission from E₂ to E₁ builds up between the mirrors.

Uses#

Since the first laser in 1960: microsurgery (a tightly focused spot), burning away tumours and cutting tissue with infrared lasers, re-attaching detached retinas, drilling metal and diamond, cutting 50 layers of cloth at once without fraying, telephone and internet signals through optical fibres, fusion research, surveying and aligning equipment, bouncing light off the Moon to measure its distance, holograms (3D images), and reading CDs and other optical discs.