Why is laser light used?

This can be answered by looking at the special properties of laser light which makes it different from normal light.

In contrast to normal light, which radiates in all directions, laser light is almost completely unidirectional, which means the light travels almost completely in one direction. A beam of this sort, ie. one which does not expand, is referred to as collimated.
Laser light is highly monochromatic, being basically of one colour or frequency. (This is in contrast to most light which is a combination of many different colours.)
All rays of light in a laser beam are in phase, which means they all interfere constructively at all times. As there is no destructive interference, this leads to very intense beams of light.
Light which is monochromatic and in phase is called coherent light. Thus laser light is coherent and usually collimated. Semiconductor diode lasers do not emit collimated light, but the light can be easily collimated using a lens.

Another special feature of lasers is that the time taken to turn the beam on or off can be made very small, allowing high precision pulsed light to be generated. This time is much smaller than the time it takes to turn an ordinary bulb on or off.

How is laser light produced?

A laser is made up of a resonating cavity between two mirrors. By pumping energy into the resonating cavity, light is emitted in the medium occupying the cavity. Obviously the light which is produced will correspond to the energy level transitions of the medium. This means that the medium in the cavity affects the frequency of the laser light.

When light is produced in the resonating cavity, it begins to bounce back and forth between the mirrors. The cavity is a resonating cavity, because it is designed so that a standing (or stationary) wave is produced between the mirrors. This is much like the oscillations of a guitar string. For a particular length and type of string, a particular standing wave is produced when the string is plucked. When this is done, the string is resonating.

This means that even though there may be more than one possible wavelength of light which can be emitted by the medium, one particular colour can be chosen by adjusting the length between the mirrors so that a standing wave can be produced for that frequency.

Consider a photon of a particular energy, and an electron in an excited state which can drop down to a lower state to emit another photon of the same energy. When this photon interacts with this electron, it causes the electron to drop down and emit the photon with the particular conditions that:-

both photons (which are the same colour) are exactly in phase (and therefore coherent).
they are travelling in exactly the same direction.
This process is called stimulated emission, in contrast with the spontaneous emission discussed with fluorescence.

If a photon meets an atom in the lower energy level it will be absorbed and the electron will be promoted to the upper level. Normally, as there are fewer and fewer electrons, as one goes to higher and higher energy levels, there will be more electrons in the lower energy level of the resonating medium. Therefore, absorption (and hence spontaneous emission) is more likely than stimulated emission. But by pumping enough energy into the laser, more of the electrons in the medium can be pushed into the higher state. This is called population inversion, as more of the electrons occupy the higher state, rather than the lower state as is usual. When this happens, the light is amplified rather than absorbed, ie. the intensity of the light in the cavity increases rather than decreases. The mirrors, which are not perfectly reflective, transmit some of the resonating photons, producing a beam of highly collimated and coherent light, ie. a laser.

There are many different mediums which can be chosen to fill the resonant cavity, all producing laser light of different frequencies. These mediums can be solid, liquid or gaseous. One of the most common gas lasers is the HeNe laser (Helium-Neon laser). These can be a number of different colours, according to the mirror and resonant length chosen, including red, green and orange. An example of a solid medium is in the Ruby laser, which, funnily enough, involves the use of ruby crystal. Liquid dyes such as those which exhibit fluorescence can also be used as a medium.

There are also semiconductor diode lasers which are becoming more and more common because of their simple and cheap construction.

Applications of Lasers

Everyday Lasers
Most people have CD players at home. These use a simple low-powered semiconductor diode laser which shines onto the surface of the compact disc. The CD has pits scored in it, usually by lasers, and the CD player reads the type of signal which is reflected off the CD surface. This gives the information which is stored on the CD.

Barcode scanners operate on a similar principle. Once again, the scanner simply consists of a diode laser, which projects a low-powered fan-shaped beam outwards, and a detector to detect the reflected signal. As the barcode is passed over the beam, the light is reflected off the white lines and absorbed by the black lines. The scanner can therefore see the pattern of lines in the barcode and the computer can find the relavent information. The information which can be stored in these barcodes, ranges from the country in which the product was made, to the type of product and its price.

Everyone will have seen some sort of security system in the movies which uses lasers. Each beam consists of a laser beam fired at a sensor. When the beam is broken, the sensor registers this and the alarm is sounded.... or the pitbull terriers are released depending on what the system is designed to do.


Other Technological Applications -Cutting with lasers

Lasers can be used to cut materials in industry, much to the satisfaction of Monsieur Bond and many others. This is a fairly simple procedure. To do this, one must simply choose a laser of a frequency which is absorbed by the material to be cut, and of a power which is high enough to cut through that particular material. The material simply absorbs an enormous amount of heat, more than it can withstand, and the laser burns through.


* compiled from internet