Last year (1990) marked the 30th anniversary of an important event that somehow did not result in any parades or speeches or days off work–and no, it wasn’t my birthday. But it was a birthday of sorts: the birthday of the laser.
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.
The accomplishment garnered considerably scientific interest, but aside from the feeling among science fiction fans that they finally had a candidate for a real-life “death ray,” it had little impact on society as a whole.
But nowadays, of course, lasers have a host of applications, from eye surgery to CD players to rock concerts. Most people know a laser is a very intense beam of light, because you can see that for yourself. But what’s so special about that beam?
Well, 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 (maybe you’ve heard of him?) 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 different 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. This means that those identical light waves are exactly in step with each other, with the crest of each wave lined up with the crest of every other wave. You can have monochromatic light that is incoherent, where the waves aren’t in step with each other. Normal white light is not only incoherent, it’s also non-monochromatic, containing light at many different wavelengths.
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 even have tuneable lasers, using solutions of organic dyes that can produce laser light of any colour.
Lasers also come in many different sizes. The tiny diode laser is probably the most common, because it’s used in CD players. It’s also used to transmit signals along fibre optic cables, a field of technology in which Saskatchewan is an international leader. The largest, most powerful lasers are the monsters used for fusion research. Several of those focused together can mimic conditions in the sun’s interior.
Today, lasers are everywhere. Powerful carbon dioxide lasers are used for cutting, drilling and welding metal, while ultraviolet lasers are used to micromachine components and circuit boards for electronic equipment.
Lasers are used in retail stores to read bar codes, and most typesetting machines and many computer printers and scanners rely on lasers.
Medically, lasers make ideal scalpels because the heat of the beam reduces bleeding. Argon ion or copper vapour lasers can be used to remove tattoos and birthmarks, and an experimental cancer treatment involves the use of the gold vapor laser and a light-sensitive chemical.
And yes, lasers also have military applications, though not yet as “death rays.” Mostly they serve as targeting aids and range-finders.
Holograms, laser light shows–the list of applications goes on and on. Not bad for what originally seemed only a scientific curiousity.
I wonder what scientific oddities of today will be the everyday technology of tomorrow?
