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Helium–Neon Laser

Trace electrical pumping, helium-to-neon energy transfer, population inversion, the 632.8 nm transition and lower-level depopulation in a helium–neon laser.

  • A-Level Laser Physics extension
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Learning objectives

  • Explain metastable states, pumping, population inversion, threshold, optical feedback, output coupling and cavity modes.
  • Relate laser operation to coherence, linewidth, divergence, irradiance, photon quantities and a helium-neon example.

1. Why use two gases?

The helium–neon laser uses a low-pressure mixture of helium and neon in an electrical discharge. Helium is efficient at receiving energy from the discharge; neon supplies the laser transition. The useful link is a close energy match between helium metastable states and selected excited neon states.

Energy transfer in a helium–neon laserHelium metastable levels transfer energy by collision to upper neon levels, followed by a 632.8 nanometre stimulated-emission transition and rapid depopulation of the lower laser level.Helium pump systemNeon laser systemgroundmetastable stateelectrical dischargecollision transferupper laser levellower laser levellower states632.8 nmfast decayThe named atomic levels are omitted here so the population pathway remains visible.
Scroll diagram horizontally to read all labels.
The discharge excites helium into metastable states. Resonant collisions transfer energy to neon, creating an inversion; the familiar red transition emits at about 632.8 nm before lower states are emptied through faster processes.

2. Operating sequence

  1. Accelerated electrons in the discharge collide with helium atoms and excite them into long-lived metastable states.
  2. Excited helium atoms collide with neon atoms and transfer energy resonantly.
  3. The transfer populates an upper neon laser level, helping create an inversion relative to a lower neon level.
  4. A photon near the cavity axis triggers stimulated emission on the red 632.8 nm transition.
  5. Faster decay pathways depopulate the lower laser level so absorption does not overwhelm gain.
  6. Cavity feedback amplifies supported modes and the output coupler transmits part of the red field.

3. What each component contributes

ComponentFunction
electrical dischargesupplies pump energy
heliumaccepts discharge energy and transfers it by collision
neonprovides upper and lower laser levels
long narrow tubecontains the gain medium and helps define the optical path
high reflectorreturns most light through the medium
output couplerretains feedback while transmitting useful output

4. Photon energy

For the familiar red output,

E = hc/λ.

The wavelength identifies the energy separation of the neon laser transition. Helium does not emit the final red photon in this pathway; it transfers pump energy to neon.

5. Common mistakes

  • Saying helium is the red-emitting species.
  • Omitting the need to empty the lower laser level.
  • Saying a long-lived state automatically guarantees inversion.
  • Treating the tube mirrors as the pump source.
  • Assuming the qualitative diagram gives level energies to scale.

6. Worked Examples

Modelled example 1

Energy of a 632.8 nm photon

Core

Problem

Calculate the energy of one red helium–neon laser photon of wavelength 632.8 nm using h = 6.63 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m s⁻¹.
Study the worked solution

  1. Convert wavelength

    Method

    λ = 632.8 × 10⁻⁹ m.

    Reason

    The photon-energy equation uses SI wavelength with the supplied constants.

    Working

    632.8 nm = 632.8 × 10⁻⁹ m

  2. Apply the photon relation

    Method

    E = hc/λ.

    Reason

    Combine E = hf with c = fλ.

    Working

    E = hc/λ

  3. Calculate and check

    Method

    E = 3.14 × 10⁻¹⁹ J, about 1.96 eV.

    Reason

    The result is consistent with a visible red photon.

    Working

    E = ((6.63 × 10⁻³⁴)(3.00 × 10⁸))/(632.8 × 10⁻⁹) = 3.14 × 10⁻¹⁹ J

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