He-Ne laser working with energy levels

Population inversion and optical pumping

In thermal equilibrium, lower levels hold more atoms than higher levels (N₂ < N₁). Population inversion is the non-equilibrium condition in which a higher energy level holds more atoms than a lower level (N₂ > N₁). It is necessary for light amplification by stimulated emission.

Pumping is the process of raising atoms to the excited state to achieve inversion. In optical pumping an intense lamp (for example, a flash lamp for a ruby laser) provides the exciting light. In the He-Ne laser, pumping is by electric discharge (electron collisions) and energy transfer between atoms.

Construction

 Brewster window                      Brewster window
 M1 (100%) |  He:Ne = 10:1 , ~1 torr   | M2 (~99%)
   |       |====== discharge tube =====|     |  --> laser
   |       |   anode (+)    cathode(-) |     |      632.8 nm
           +------[ high-voltage DC ]--+

• A glass tube about 30 cm long and a few mm in diameter, filled with helium and neon in a ratio of about 10 : 1 at a total pressure of about 1 torr.
• Two electrodes carry a DC discharge of about 1 to 2 kV (or an RF source is used).
• Two parallel mirrors form the optical cavity: one is totally reflecting, the other about 98 to 99 % reflecting so that some light leaves as the beam.
• Windows are set at the Brewster angle to give a linearly polarized output.

Working

  eV      He                        Ne
 20.61   2¹S ====collision====>   3s (20.66)
                                    3s -> 3p (20.30): 3.39 µm
                                    3s -> 2p        : 632.8 nm (red)
 19.82   2³S ====collision====>   2s (19.78)
                                    2s -> 2p        : 1.15 µm
          ^ electron impact       2p (18.70)
          |                         | fast decay (~10⁻⁸ s)
          |                       1s (16.6) -> ground by wall collisions
  0      ground                   ground

1. The discharge accelerates electrons, which collide with He atoms and excite them to the metastable states 2³S (19.82 eV) and 2¹S (20.61 eV). These states cannot quickly decay by emitting a photon, so many He atoms accumulate there.
2. The Ne levels 3s (20.66 eV) and 2s (19.78 eV) have almost the same energies. In collisions the excited He atoms transfer their energy to ground-state Ne atoms by resonant energy transfer, exciting them to these levels, while the He atoms return to the ground state.
3. Ne atoms are then much more numerous in 3s and 2s than in the lower levels 3p (about 20.3 eV) and 2p (about 18.7 eV), so population inversion occurs between 3s and 3p/2p, and between 2s and 2p.
4. A spontaneously emitted photon stimulates other Ne atoms; the photons move back and forth between the mirrors and the beam grows by stimulated emission.
5. The main transitions are 3s → 2p at 632.8 nm (red, the usual output), 3s → 3p at 3.39 µm and 2s → 2p at 1.15 µm (infrared).
6. Atoms in the lower level 2p decay quickly (about 10⁻⁸ s) to the 1s level, and then to the ground state by collisions with the tube wall. This keeps the lower level empty, which maintains the inversion. A narrow bore helps this wall relaxation.

Properties

Continuous output of a few mW, highly monochromatic, very directional (divergence about 1 mrad), efficiency about 0.1 %.