What is pumping in a laser and what are the different methods of pumping?

In one line

Pumping is the process of supplying energy to the active medium to lift atoms to higher energy levels and create population inversion.

Explained simply

A water tank on the terrace stays full only if a pump keeps lifting water up from the sump. In a laser, atoms keep falling to the ground state, so energy must keep "pumping" them back up. Without pumping, the inversion disappears and the laser stops.

Technically, pumping is any mechanism that transfers energy from an external source into the active centres so that the upper laser level becomes more populated than the lower laser level.

How it works

1. Optical pumping: light from a flash lamp or another laser is absorbed by the active centres. Used in ruby, Nd:YAG and fibre lasers (fibre lasers use diode lasers).
2. Electrical discharge: a high voltage across a gas accelerates electrons, which collide with atoms and excite them. Used in He-Ne, argon-ion and CO₂ lasers.
3. Inelastic atom-atom collision: one type of atom is excited by discharge and transfers its energy to another type with a matching level, as He transfers to Ne in the He-Ne laser.
4. Direct conversion (current injection): forward current in a p-n junction makes electrons and holes recombine and emit light. Used in semiconductor (diode) lasers.
5. Chemical pumping: energy released in a chemical reaction leaves molecules excited, e.g. HF laser.

Quick example

In a He-Ne laser pointer at a lab, the power supply drives an electric discharge through the gas: that discharge is the pump. In a laser diode in a barcode scanner, the forward current through the diode is the pump.

Common mistakes

• Calling the pump the active medium. The pump is the energy source; the medium is where lasing happens.
• Saying He-Ne is optically pumped. It is pumped by electric discharge plus He-Ne collisions.
• Forgetting that pumping must be continuous for a continuous-wave laser.

How MU asks it

Usually a 3-mark short note "What is pumping? List the methods" or a part of "Explain the terms: pumping, active medium, metastable state" (5 marks). Write a one-line definition, then list at least four methods with one example laser for each.