What is a laser?

‘LASER’ is an acronym, formed from the words Light Amplification by the Stimulated Emission of Radiation.  It is essentially an optical oscillator; i.e., an optical amplifier with positive feedback.   An optical amplifier (a device that amplifies light) is probably unfamiliar to you, as it is something that falls outside of your day-to-day experience, but I’ll describe it in detail later on.  Optical feedback, on the other hand, is very easy to accomplish with mirrors.  A very basic Fabry-Perot Laser consists of an optical amplifier in-between two mirrors which face each other, as diagrammed below:

The light bounces back-and-forth between the mirrors, and on each pass through the amplifier it gets more intense, producing a very intense light beam in-between the mirrors.  The area in-between the mirrors is generally referred to as the ‘optical cavity’.  Of course, having incredibly intense light inside an optical cavity is of limited usefulness – we want to get some of that light out of there!  This is pretty easily accomplished by using imperfect mirrors.  For example, if one of the mirrors reflects 99% of the light and leaks 1%, we can use that 1% as the useful output of the laser.  Most lasers are like this – the light is much more bright (intense) inside the laser than out.

So there you have it – the basic structure of a laser.  You could stop reading here and have a general idea of what’s going on with lasers.  But, as they say, the devil is in the details – the optical amplification process, the properties of the light produced, the different types of lasers used for various applications – so we need to delve into those to have a good understanding of lasers.