In one line
A He-Ne laser is a gas laser in which an electric discharge excites helium atoms, which pass their energy to neon atoms by collision, and neon then lases at 632.8 nm between two cavity mirrors.
Explained simply
Helium acts like a courier: it picks up energy from the electric discharge and delivers it to neon, because some helium and neon levels have almost the same energy. Neon then "spends" this energy as red laser light.
How it works
Construction: a long, narrow glass or quartz discharge tube filled with helium and neon (helium in excess, about 10:1) at low pressure; electrodes connected to a high-voltage DC supply; a fully reflecting mirror at one end and a partially reflecting mirror at the other; often Brewster windows to give polarised output.
Working:
- The discharge accelerates electrons, which collide with helium atoms and raise them to metastable levels \(2^1S\) (about 20.61 eV) and \(2^3S\) (about 19.82 eV).
- These are very close to neon's \(3s\) (20.66 eV) and \(2s\) (19.78 eV) levels. In collisions, helium transfers energy to neon (resonant transfer) and returns to ground state.
- Neon's \(3s\) and \(2s\) levels become more populated than its \(2p\) levels, so population inversion exists.
- Transitions: \(3s \to 2p\) gives 632.8 nm (red); \(3s \to 3p\) gives 3.39 μm; \(2s \to 2p\) gives 1.15 μm (both infrared). Mirrors are chosen to favour 632.8 nm.
- Neon in \(2p\) drops by spontaneous emission to \(1s\), then reaches ground state mainly by collisions with the tube walls. The narrow tube helps this, keeping the lower level empty.
- It is a four-level laser and gives a continuous output of a few milliwatts.
Key formula
\(\lambda = \dfrac{hc}{E_{3s} - E_{2p}}\); with \(\lambda = 632.8\) nm, \(E_{3s} - E_{2p} = 1240/632.8 \approx 1.96\) eV.
Quick example
He-Ne lasers were used in early supermarket barcode scanners, in holography and in physics labs for alignment and interference experiments, because their beam is stable and highly coherent.
Common mistakes
- Writing that helium emits the laser light; neon does.
- Calling He-Ne a three-level laser; it is four-level.
- Forgetting the role of tube walls in emptying the \(1s\) level.
How MU asks it
A classic 5-mark question: "Explain construction and working of He-Ne laser with energy level diagram". For full marks draw the tube with mirrors and power supply, draw the He and Ne level diagram with the 632.8 nm, 1.15 μm and 3.39 μm lines, and write the steps. Mention that it is continuous and four-level.