Saturday, November 13, 2010

What is a Laser?
First published 19 December 2002

Most people know the word laser, but do they know what it really is? What's the difference between ordinary light and laser and what does laser really stand for? Let's start with the last question. Laser is an acronym, that is a word made up of initial letters. You could use the complete name: light amplification by stimulated emission of radiation but that's a bit awkward, let's keep to laser.

Almost everyone probably knows that the police use laser when they measure speed. At least many drivers that have exceeded the speed limit know about it, but how many know that you also use laser several times during an ordinary day? You'll find it in CD players, laser printers and much, much more.

You often find laser in action movies where the hero has to escape the laser beams when he's trying to solve a thrilling problem. The power contained in laser is both fascinating and frightening.

How Does Laser Light Differ from Other Light?
Light is really an electromagnetic wave. Each wave has brightness and color, and vibrates at a certain angle, so-called polarization. This is also true for laser light but it is more parallel than any other light source. Every part of the beam has (almost) the exact same direction and the beam will therefore diverge very little. With a good laser an object at a distance of 1 km (0.6 mile) can be illuminated with a dot about 60 mm (2.3 inches) in radius.

As it is so parallel it can also be focused to very small diameters where the concentration of light energy becomes so great that you can cut, drill or turn with the beam. It also makes it possible to illuminate and examine very tiny details. It is this property that is used in surgical appliances and in CD players.

It can also be made very monochromic, so that just one light wavelength is present. This is not the case with ordinary light sources. White light contains all the colors in the spectrum, but even a colored light, such as a red LED (light emitting diode) contains a continuous interval of red wavelengths.

On the other hand, laser emissions are not usually very strong when it comes to energy content. A very powerful laser of the kind that is used in a laser show does not give off more light than an ordinary streetlight; the difference is in how parallel it is.

Stimulated Emission
Normally atoms and molecules emit light at more or less random times and in random directions and phases. All light created in normal light sources, such as bulbs, candles, neon tubes and even the sun is generated in this way.

If energy is stored in the atom and light of the correct wavelength passes close by something else can happen. The atom emits light that is totally synchronous with the passing light. This means that the passing light has been amplified which is necessary for the oscillation taking place between the mirrors in a laser.

Light is normally emitted from atoms or molecules that meet with two conditions.
- They have stored energy originating from heat or previous absorption of light
- A time has passed since the energy was stored
Light emitted in this way goes in random directions, with random phases and at random times.

Albert Einstein predicted early in the 1900s that there is also another way for light to be emitted. It can amplify a passing beam, provided three conditions are met:

- Energy is stored in the atom (same as above)
- Light passes close enough to the atom before the time has expired and the light is emitted in the random fashion described above
- The passing light has a wavelength suitable for the atom.

The process taking place in this case is called Stimulated Emission, which, together with feedback in a resonant cavity between mirrors, forms the conditions for laser.

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