How Lasers Work: Stimulated Emission and Coherent Light
A laser amplifies light through stimulated emission to make a narrow, coherent beam of one colour. Learn population inversion, the parts of a laser, and its uses.
A laser is a device that produces an intense, narrow beam of light of a single colour, in which all the light waves march in step. The word is an acronym for light amplification by stimulated emission of radiation. Unlike a light bulb, which radiates a jumble of colours in every direction, a laser produces light that is monochromatic, coherent, and highly directional. It works by trapping atoms in an excited state and then triggering them to release their stored energy together.
What is stimulated emission?
Atoms and molecules can hold energy only in certain discrete levels, a basic fact of quantum physics. When an electron drops from a higher level to a lower one, the atom gives off a photon whose energy equals the difference, which is the idea behind wave-particle duality applied to light. Ordinarily this spontaneous emission happens at random times and sends the photon off in a random direction.
In 1917 Albert Einstein showed that there is a second way. If a passing photon of exactly the right energy meets an atom that is already excited, it can stimulate that atom to emit a second photon. The new photon is a perfect copy of the first: the same wavelength, the same phase, the same direction, and the same polarisation. One photon becomes two, two become four, and the light builds up into an avalanche. That chain reaction is the amplification in a laser.
What are the parts of a laser?
Every laser has three essential components:
- The gain medium, the material that supplies the atoms: a ruby crystal, a gas such as helium and neon, a semiconductor, or a liquid dye.
- A pump source, which delivers energy to excite the atoms, such as a flash lamp, an electric current, or another laser.
- An optical cavity, made of two mirrors facing each other with the gain medium between them. Light bounces back and forth, stimulating more and more emission. One mirror is only partly reflective, so a fraction of the light escapes as the beam.
For amplification to win, the pump must create a population inversion, a state in which more atoms are in the excited level than in the lower one. In ordinary matter the opposite is true, and light is absorbed more often than it is amplified. Only by pumping the medium hard enough can stimulated emission beat absorption.
A short history
Charles Townes built the first maser, which amplifies microwaves rather than visible light, in 1953. Townes and Arthur Schawlow proposed extending the idea to light in 1958. Theodore Maiman built the first working laser in May 1960 at Hughes Research Laboratories, using a ruby crystal that gave off red light at a wavelength of 694 nanometres. Townes shared the 1964 Nobel Prize in Physics with the Soviet physicists Nikolay Basov and Aleksandr Prokhorov.
Why is laser light special?
- Monochromatic: it has a single, very narrow wavelength, unlike sunlight, which spans the whole electromagnetic spectrum of visible colours.
- Coherent: the waves are in phase, which allows them to be focused to a tiny spot and makes interference experiments possible.
- Directional: the beam spreads very little. Lasers fired at reflectors that the Apollo astronauts left on the Moon measure the distance to the Moon to within millimetres, the method that shows the Moon drifting away (see solar eclipses).
Concentrated light is dangerous. Even a laser pointer of a few milliwatts can damage the retina, so many countries cap pointers at 5 milliwatts, and pointing any laser at aircraft is illegal in most places.
What are the main types of laser?
| Type | Examples | Typical uses |
|---|---|---|
| Gas | Helium-neon (red), carbon dioxide (infrared) | Alignment, cutting and welding |
| Solid-state | Ruby, Nd:YAG (infrared) | Marking, surgery, research |
| Semiconductor (diode) | Laser pointers, Blu-ray (violet), telecom | Optical discs, communications, printers |
| Fibre | Ytterbium-doped fibre | Industrial cutting |
| Excimer | Ultraviolet | Eye surgery, chip manufacturing |
What are lasers used for?
Lasers have become part of everyday infrastructure. Pulses of laser light carry internet traffic through glass fibres thanks to total internal reflection. Excimer lasers reshape the cornea in LASIK eye surgery. Barcode scanners and Blu-ray players read patterns with them. Industrial machines cut and weld metal, and 3D printers fuse metal powder layer by layer. The LIGO observatories use lasers with 4-kilometre arms to detect gravitational waves. At the National Ignition Facility in California, 192 laser beams compressed a tiny fuel capsule in December 2022 and produced fusion that released more energy, 3.15 megajoules, than the 2.05 megajoules the lasers delivered to the target, although far less than the facility consumed.
Tags
lasers light optics photonics quantum mechanics
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