A beam of laser light

Each laser device or system principally has three essential components: the source of the laser energy (the medium), an energy source to 'pump' or stimulate the medium and optical mirrors to reflect the laser light produced.

Laser Medium

Although the medium can be a soild, liquid or gas, today most lasers use a solid medium which takes the form of a cylindrical laser crystal. This medium is then 'pumped' from an energy source to excite the atoms in the medium. Typically this is done using very intense flashes of light or electrical charges. Atoms are the building blocks of everything around us but they are also in continuous motion – vibrating and rotating, even in solid objects like chairs or tables! They can also be in different stages of being excited. Consequently if we apply a lot of energy to an atom we can make it more excited. For a laser to work efficiently we have to ensure that a large number of atoms reach an excited state.

Think of an atom like a planet surrounded by a moon. The moon is known as an electron and each electron has a particular orbit around the atom depending on the energy level of the atom. When atoms absorb more energy the electron moves to a higher and higher orbit until some energy escapes the 'orbit' in the form of light energy -- photons. This is what we need to create laser light. To do this we pump the laser medium with energy. As we do some of the electrons surrounding the atoms become more excited than others. Eventually some of this energy is released in the form of photons. Once released the mirrors in the laser reflect the photons back and forth through the laser medium. As they do this the photons excite other electrons which causes the release of more and more photons creating a cascade effect (amplification – the A in LASER). At one end of the laser one of the mirrors is only half-silvered. This means that some of the photons instead of being reflected pass through the mirror. These photons are the laser light.

Depending on the energy released by the electron when it is bombarded with energy the photons will have a very specific colour (wavelength) they will also travel together in phase (coherent), and in the same direction (directional or collimated). Thus laser light is monochromatic, coherent and collimated and the process of exciting the atoms and electrons is known as stimulated emission – the S and E in the word LASER.