The laser at 50


You know you’ve been writing a column a long time when the 50th anniversary of a major scientific discovery comes along and you realize you wrote a column celebrating its 30th anniversary.

But that’s exactly what’s happening this month. Next week (Saturday, May 15, to be precise) marks the 50th anniversary of the invention of the laser. And what follows is (with some slight revisions) the column I wrote to celebrate its 30 anniversary back in 1990. (But it’s OK: I promise not to trot it out again until the centennial.)

On May 15, 1960, a cylindrical rod of synthetic ruby placed inside a spiral flashlamp by American physicist Theodore H. Maiman in his laboratory at Hughes Aircraft Company in California momentarily produced light 10 million times more powerful than sunlight: the first laser, an acronym for Light Amplification by Stimulated Emission of Radiation.

To understand how lasers work, you have to go to the basics–atoms. Every atom has a nucleus surrounded by electrons. These electrons reside in discrete energy levels, or electron orbits, around the nucleus. The further out from the nucleus they are, the more energetic they are.

Sometimes an electron from a high-energy level drops to a lower energy level. To do that it must lose energy, which is released as a photon of light. This is called spontaneous emission.

When a photon comes into contact with an atom that has two energy levels with an energy difference exactly equal to the energy of the photon, then the photon may be absorbed, and an electron at the lower level moves up to the higher. The atom is now said to be in an excited state, but it only lasts for a tiny fraction of a second. Then it throws off a photon, or “decays,” and relaxes again.

In 1917 Albert Einstein suggested that if a photon from one atom came into contact with a similar atom that was in an excited state, it could cause another photon identical to itself to be emitted. This is called stimulated emission.

Lasers basically consist of three items: a material which acts as a light amplifier (the ruby rod in Maiman’s original laser), a source of energy (Maiman’s flashlamp) and two mirrors. The energy source excites the atoms in the light amplifier (called the active medium) so they can produce stimulated emission. The energy source has to be strong enough to excite the atoms faster than they can decay back to their normal state, so that soon you have more excited atoms than non-excited ones. This is called a population inversion.

Initially a few atoms emit photons spontaneously, which induce other atoms to emit. The light intensity quickly grows in all directions. Some of the photons go out the sides of the active medium and are lost, but some travel the length of the medium, inducing still more atoms to emit–and when they reach the end of the active medium, they bounce off one of the mirrors and return to stimulate still more atoms. In this way a single photon can produce millions and millions of others exactly like itself.

Although one mirror is a regular, fully reflecting mirror, the other, at the far end of the active medium, is only partially reflecting. The light that passes through this mirror is the laser beam.

This light is special in several ways. First, it is monochromatic–all one, pure colour. That’s because all the photons in the laser are identical copies of each other, all with the same wavelength.

Laser light is also coherent: those identical light waves are exactly in step with each other. (You can have monochromatic light that is incoherent, where the waves aren’t in step with each other, but then it’s not a laser.)

Although synthetic ruby was first, many different materials can be made to “lase.” In 1961 the first gas laser was constructed, using a mixture of helium and neon. Nowadays we have tuneable lasers, using solutions of organic dyes that can produce laser light of any colour.

Today, lasers are everywhere, used to play DVDs, print letters, transmit messages, cut and weld metal, repair eyes, target weapons, and liven up rock shows. New research even suggests ultra-fast pulses from powerful lasers could be used to create water droplets out of thin air, creating rain on demand.

Not bad for what originally seemed only a scientific curiousity…and well worth a rousing rendition of “Happy Birthday.”

Consider it sung.

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