How does a laser function?
The laser principle
Lasers are basically composed of three components:
1. The lasing medium, which can be a solid (including semiconductors), liquid or gas. In selecting these, it is important to consider the desired wavelength and degree of effectiveness. Unfortunately there are very few media that are suitable.
2. An energy source, which delivers energy to the lasing medium and brings (excites) atomic or molecular energy states to a higher level.
3. The resonator, which is an arrangement of two essentially plan-parallel mirrors, of which one is partially reflecting.
If the lasing medium is then excited by the energy source and a sufficient number of atomic or molecular states are taken to a higher energy level, the stimulated emission will lead to a coordinated movement onto a particular wavelength, corresponding exactly to the energy difference between two energy levels. The two mirrors of the resonator ensure that the laser beam diverges as a focused beam, whereby part of the beam goes out through the partially reflecting mirror and is available.
The characteristics of lasers
Laser light has different characteristics from normal light sources. Normal light consists of spontaneous emission in different directions, with incoherent light of very varied wavelengths.
With a laser, the excited atoms or molecules are synchronised by the stimulated emission. As a result, light is emitted in the same phase and the same direction. Laser light is therefore a very even, coherent wave. It has a single wavelength or colour. Laser light is virtually parallel with only a small divergence. This virtually parallel laser beam can be very narrowly focused. There are also laser sources which allow very short pulses to be emitted. A further important characteristic of lasers is the high power density at the focal point.
Guiding and shaping laser light
Light allows itself to be guided and shaped easily. To guide it, there are firstly optic fibres (unfortunately not for laser sources in the far infrared range, e.g. carbon dioxide lasers) and secondly mirrors. To shape it, for example in focusing or concentrating laser beams, special optical elements are needed such as lenses. If a light beam enters another medium with a lower or higher optical density (for example passing from air into water), it will change its direction. Optical lenses make use of this refraction both on entering and on exiting the lens. In this way light can be focused onto a few tens of micrometres or even less. The power density at the focal point is then so high for some laser powers that any material will instantly vaporise.