Go further · Chapter 1 · 1a · Real-world case
How does a laser work?
The short answer (tap to open)
A laser gets lots of atoms to drop the same energy rung at the same moment, in step. A passing photon triggers an excited atom to release an identical copy, and mirrors bounce the light back and forth so each copy triggers more. The result is a beam of one exact colour, all in step.
What makes laser light different?
Light from a bulb is a jumble: many colours, sent in every direction, with the waves out of step. Laser light is the opposite. It's one exact colour, it travels in a tight beam, and its waves march in step. That's why a laser pointer makes a sharp dot across a room and why a laser can carry broadband down a hair-thin glass fibre.
What is stimulated emission?
Chapter 1 showed an electron dropping a rung on its own and giving out a photon. There's a second way to drop. If a photon of exactly the right colour passes an atom whose electron is already up a rung, it can trigger the drop. The atom releases a new photon that's an exact copy of the first: same colour, same direction, same step.
Albert Einstein worked this out in 1917, from the maths of how atoms and light trade energy. He called it stimulated emission. One photon in, two identical photons out. Now you have an amplifier for light.
Why does a laser need mirrors and a "pump"?
Two problems stand in the way.
First, normally most atoms sit on the bottom rung, and a bottom-rung atom absorbs a passing photon instead of copying it. So a laser pumps energy in, with a flash of light or an electric current, until more atoms sit up a rung than down. Physicists call that a population inversion. Only then do copies outnumber losses.
Second, one pass through the material doesn't make many copies. So a laser puts a mirror at each end. Light bounces back and forth thousands of times, copying itself on every pass. One mirror lets a little light leak out. That leak is the beam.
The name says it all: Light Amplification by Stimulated Emission of Radiation. Gordon Gould coined it in 1957.
Make a steady beam.
Pump low: a few random flashes in every direction.
The four stages, in words
- Pump low: a few random flashes in every direction.
- Pump past half: the counter turns green ("population inversion").
- Add both mirrors: a few photons along the tube start copying, the copies multiply, and a bright beam leaves the part-silvered end.
- Remove a mirror: the beam dies.
Who built the first one?
In 1954 Charles Townes and his students at Columbia University built a maser, the same idea using microwaves. On 16 May 1960 Theodore Maiman at Hughes Research Laboratories in California fired the first laser, a rod of pink ruby lit by a camera-style flash tube. People called it "a solution looking for a problem". Townes shared the 1964 Nobel Prize in Physics with Nikolay Basov and Aleksandr Prokhorov.
Where are lasers today?
Everywhere: broadband through optical fibre, barcode scanners, eye surgery, cutting and welding metal, the dot projector behind face unlock, self-driving cars' LiDAR, and the atom traps inside some quantum computers (chapter 9). Every one of them runs on the energy rungs of chapter 1.
Check
Two quick ones
Collect
Your aha card
A laser is a photon copier.
Pump atoms up a rung, bounce light between mirrors, and every photon triggers identical copies.