Helium -Neon (He-Ne) Laser

 

 Construction:

It is a gas laser, which consists of a narrow quartz tube filled with a mixture of Helium and Neon gases in the ratio 10:1 respectively, at low pressure (~0.1 mm of Hg). Ne atoms act as active centers and responsible for the laser action, while He atoms are used to help in the excitation process. The length of the quartz tube is about 50 cm and the diameter is about 1 cm.

Working:

The common helium-neon gas laser achieves a population inversion in a different way. A mixture of about 10 parts of Helium and 1 part of Neon at a low pressure is placed in a glass tube that has parallel mirrors, one of them partly transparent, at both ends. The spacing of the mirrors is equal to an integral number of half-wavelengths of the Laser light. An electric discharge is produced in the gas by means of electrodes outside the tube connected to a source of high-frequency alternating current, and collisions with electrons from the discharge excite He and Ne atoms to metastable states respectively above their ground states. Some of the excited He atoms transfer their energy to ground-state Ne atoms in collisions. The purpose of the He atoms is thus to help achieve a population inversionin the Ne atoms.

The laser transition in Ne is from the metastable state to an excited state with the emission of a 650nm of photon. 

① Excitation of Helium

A DC electric discharge accelerates free electrons, which collide with He atoms and excite them from F₁ → F₂ (and onward to F₃). Helium is the pumping agent — it is easy to excite because its metastable level F₃ has a long lifetime.

② Resonant energy transfer (Atomic collisions)

the He metastable level F₃ is almost exactly equal in energy to the Ne level E₆ (and F₂ to E₄). So when an excited He atom collides with a ground-state Ne atom, it transfers its energy selectively to pump Ne into E₆. This selective, resonant transfer is what creates the population inversion between E₆ and lower levels.

③ Stimulated emission (the laser transitions)

Several transitions can lase, depending on the cavity mirrors:

• E₆ → E₅: 33900 Å (infrared, far-IR)

• E₅ → E₃: 3628 Å (ultraviolet)

• E₄ → E₃: 11500 Å (infrared)

• The famous red line at 6328 Å (E₆ → E₃) — the standard visible He-Ne laser.

• E₃ → E₂: spontaneous emission (~6000 Å) helps empty E₃

• E₂ → E₁: fast de-excitation by collision drops Ne back to ground