How do lasers work?

If you knew what the letter stood for you might be able to guess from the name itself that a laser, which stands for Light Amplification by the Stimulated Emission of Radiation, is a device used to create a very special kind of light beam.

Ordinary light – the kind we flip on with a wall switch – in incoherent. All we mean by this is that the light waves are emitted in a random, disorganized way, going in different directions and out of phase – that is not in step with one another. For example, when we turn of a fluorescent lamp, a current of electrons is sent through the fluorescent tube, bombarding the atoms inside. This excites the atoms to a state of energy higher than normal. When they return to their normal energy state, that is when they give off their light. But since each atom is bombarded, excited and gives off its light energy at random directions and times, the light waves which come from the fluorescent tube are still mixed together, like a troop of drummers who refuse to keep time with one another.

In a laser, though, the atoms all keep time together, and the light waves they give off are pointed in the same direction, with their crests and troughs aligned with one another. This is why we say laser light is coherent. Coherence is what makes laser beams so special.

To create a laser beam, we first of all need to have material whose atoms can be excited and remain excited long enough so that a collection of them can be made to radiate together. One of the early kinds of lasers used a synthetic ruby crystal to accomplish this.

The crystal was machined into the shape of a small cylinder. Its ends were made parallel, polished flat and smooth, and coated with silver to make them act as mirrors. Using this apparatus, here is what happens: when the first of the group of simulated falls to its normal state, and, in so doing, radiates light, that light comes in contact with another of the excited atoms, and it too radiates light. And, what is more, the light waves from both atoms will be in step – that is, in phase. When the light waves from these two atoms hit other excited atoms, they too will be stimulated to emit light in step.

Any light waves which are emitted in the wrong direction – that is, in any direction but parallel to the walls of the cylinder – will bounce through the walls and escape. But the light waves which do travel parallel to the walls will continue to be reflected back and forth by the mirrors at each end, at the same time stimulating other excited atoms to do the same. In this way the light increases in intensity, just as the drummers would sound louder and louder if, one by one, they started keeping time together.

Is one of the mirrors is built so that it will allow a small amount of light to escape, the beam which results will come out straight ahead, the crests and trough of each light wave aligned in unison. It will be coherent. And it will have very little “spread” – unlike a flashlight beam, for example, which in just a few feet spreads out from its initial small diameter to a large circle.

Many other kinds of materials – solid, liquid and gaseous – have been used to create lasers. Some emit different colors – that is, different wavelengths – other than the red of the ruby laser. Other requires less power to operate.  At TRW, for example, where were laser research began in 1961, they developed a portable laser the size of a flashlight. An argon ion gas laser uses a TRW-developed high-current cold cathode (electron emitter) which greatly reduces the power requirements (because of their large power requirements most lasers must remain stationary).

Lasers have many practical as well as scientific uses. A ruby laser, for example, which puts forth its radiation in a thousandth of a second, is used to surgically weld a torn of detached retina of the eye.

Because it is highly directional, that is, it has very little spread - a laser beam will carry over great distances. A dramatic example of this is seen when a laser is used to illuminate a mile-wide spot on the Moon.

One laser beam has an extremely high frequency – a thousand million cycles per second. This gives it more information-carrying capacity than any radio, tv, or any other communication channel now in existence.

Another application of laser technology is holography, a remarkable kind of three-dimensional photography. What makes a hologram so unusual is that it really is three-dimensional. You can move to the side of a hologram and actually see behind the objects in the foreground, as though you were looking through a window.

These pictures are photographs of holograms developed during research at TRW, they are different aspects of the same scene. They were made by viewing a single hologram from three different angles. The three-dimensional aspects of holography make it extremely useful for studying particles of droplets in a dispersion pattern. For this reason holography is a valuable tool for evaluating many kinds of nozzles and vaporizing devices, including paint spray guns, oil burners, fuel injectors and carburetors. The photographs show the dispersion of droplets in two impinging water jets.

At TRW, holography research began in 1963, when they studied the behaviour of electrostatically charged droplet streams in air and vacuum for the Air Force. Later, they work in the field expanded to holography of moving subjects at high-speed events, holographic microscopy and spectroscopy, optical gauging by holography, and acoustic holography. 

laser beams

Engineers of the NASA Goddard Space flight Center send a message by laser beam to a satellite in orbit.

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