These tutorial pages have been written for those who have little or no knowledge of lasers. There are five sections in total and it is suggested that they are read in the sequence 1 to 5. Text items in green are links to further information on the item concerned. These should be ignored by anyone whose primary aim is to find out quickly and in general terms how lasers work. However, some acquaintance with the wave theory and the photon theory of light would be helpful.

Lasers come in wide variety of forms, the processes that go on inside them differing greatly from one type of laser to another. For this reason, it is easy to become distracted by detail that might apply to one type of laser only. In this tutorial, an attempt has been made to concentrate on those features that lasers have in common.

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# 1 Amplification of light

All lasers contain an energized substance that can increase the intensity of light passing through it. This substance is called the amplifying medium or, sometimes, the gain medium, and it can be a solid, a liquid or a gas. Whatever its physical form, the amplifying medium must contain atoms, molecules or ions, a high proportion of which can store energy that is subsequently released as light. How the amplifying medium increases the intensity of light passing through it will be explained in sections 4 and 5. For the moment, we will just assume that light amplification is possible.

In a neodymium YAG (Nd:YAG) laser, the amplifying medium is a rod of yttrium aluminium garnate (YAG) containing ions of the lanthanide metal neodymium (Nd). In a dye laser, it is a solution of a fluorescent dye in a solvent such as methanol. In a helium-neon laser, it is a mixture of the gases helium and neon. In a laser diode, it is a thin layer of semiconductor material sandwiched between other semiconductor layers. The factor by which the intensity of the light is increased by the amplifying medium is known as the gain. The gain is not a constant for a particular type of medium. It's magnitude depends upon the wavelength of the incoming light, the intensity of the incoming light, the length of the amplifying medium and also upon the extent to which the amplifying medium has been energized.

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