How Does a Laser Work? Light That Marches in Step
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The light from a bulb is a crowd: every colour at once, radiating in every direction, each wave starting when it likes. The light from a laser is a marching column, one colour, one direction, every wave in step with every other, and that discipline is why a beam a few millimetres wide can cut steel, read a disc, carry the internet under the oceans or bounce off a mirror on the Moon and come back. The word is an acronym for light amplification by stimulated emission of radiation, and the process it names was predicted by Einstein in 1917, forty-three years before anyone built one.
Stimulated emission
An atom holds its electrons at fixed energy levels, and it can absorb a photon of exactly the right energy to lift an electron to a higher level, then release a photon of the same energy when the electron falls back, in a random direction at a random moment. That is ordinary emission, and it is how a neon sign glows. Einstein showed that there is a third possibility. If a photon of the right energy passes an atom that is already excited, it can trigger the electron to fall, and the photon that is emitted is an exact copy of the one that passed: the same energy, the same direction, the same phase, so that the two leave together in step. One photon has become two identical ones, and the two can become four. That is stimulated emission, and it is amplification.
Getting more atoms up than down
The catch is that a passing photon is just as likely to be absorbed by an atom in the low state as to stimulate one in the high state, and in any ordinary material most atoms are low, so light is absorbed on balance rather than amplified. A laser needs the reverse, more atoms excited than not, which is called a population inversion and does not occur naturally. It is created by pumping: pouring energy into the material with a flash lamp, an electric discharge, another laser or a current through a semiconductor, using a material chosen so that the electrons linger in an upper level long enough to pile up there. Once more atoms are up than down, a photon passing through is more likely to be copied than absorbed, and the light grows as it travels.
The mirrors
Amplification on one pass is small, so the material is placed between two mirrors facing each other, and the light bounces back and forth, growing on every pass, until it is intense. One mirror is made slightly transparent, and the small fraction that leaks through it each time is the beam. The mirrors also select: only light travelling exactly along the axis stays in the cavity long enough to be amplified, and only wavelengths that fit a whole number of times between the mirrors reinforce themselves, which is why the beam is so straight and so pure in colour. The parts of any laser:
- •A gain medium, the material whose atoms are excited: a ruby crystal, a mixture of helium and neon, a gallium arsenide semiconductor, a dye, or a glass fibre doped with erbium
- •A pump that supplies the energy: a flash lamp, an electric current, or another laser
- •A cavity of two mirrors, one fully and one partly reflecting, that feeds the light back through the medium and defines the beam
What the beam can do
Three properties follow from the physics. The beam is monochromatic, a single wavelength, which lets it be tuned to a particular atom or molecule in spectroscopy and to a particular pigment in surgery. It is coherent, the waves in step, which lets it form the interference patterns used in holograms and in the instruments that detected gravitational waves in 2015 by measuring a change in length smaller than a proton. And it is collimated, travelling in one direction with almost no spread, which lets it carry energy or information a long way: the beam sent to the Moon from Earth is only a few kilometres wide by the time it arrives, and the light in an optical fibre survives a hundred kilometres before it needs boosting. Focused to a point, a modest laser reaches intensities no other source can, enough to cut, weld, engrave, or vaporise a cataract.
The first laser, built by Theodore Maiman in California in May 1960 from a rod of synthetic ruby wrapped in a flash lamp, was described at the time as a solution looking for a problem. The problems arrived: barcode scanners, CD and DVD players, laser printers, fibre-optic communication, eye surgery, distance measurement, cutting and welding, guidance, and the tiny semiconductor lasers, cheaper than a light bulb, that are now made by the billion.
The takeaway
A laser works by stimulated emission, in which a photon passing an excited atom triggers it to emit an identical photon in the same direction and phase. Pumping energy into a suitable material puts more atoms in the excited state than the ground state, so light is amplified rather than absorbed, and a pair of mirrors bounces it back and forth until an intense, straight, single-coloured, coherent beam leaks out through one of them.