Laser Technology

A laser is a device that produces a narrow, intense beam of light that is coherent, meaning its waves are in step in both phase and direction. The word is an acronym for light amplification by stimulated emission of radiation, which names the physical process at its heart. When an excited atom is struck by a photon of the right energy, it can be stimulated to emit a second, identical photon. This cascade amplifies the light, producing the characteristic coherent, nearly single-colour output.

Three ingredients are needed for laser action. A gain medium, which may be a gas, a crystal, a dye or a semiconductor, provides the atoms that emit light. An energy source, called the pump, raises those atoms to an excited state until more are excited than not, a condition known as population inversion that is essential for net amplification. Finally an optical cavity, usually formed by two mirrors, reflects the light back and forth through the gain medium to build up the beam, with one partially transparent mirror letting some light out.

In electronics the most common type is the semiconductor laser diode, a tiny chip in which population inversion is created by passing current through a specially structured junction. Laser diodes are compact, efficient and easily modulated, which makes them the light sources in fibre-optic communication links, optical disc drives, laser printers and barcode scanners. Their pairing with photodiode optical receivers forms the basis of modern high-speed data transmission over glass fibre.

Frequently asked questions

What does stimulated emission mean?

It is the process where an incoming photon prompts an excited atom to emit a second, identical photon. Repeated many times this amplifies light into a coherent beam.

What is population inversion?

It is the state where more atoms in the gain medium are excited than are in their lower energy state. Without it the medium would absorb rather than amplify light.

Where are laser diodes used?

Semiconductor laser diodes serve as light sources in fibre-optic links, optical disc drives, laser printers and barcode scanners, owing to their small size and easy modulation.