Masers and lasers

The words ‘maser’ and ‘laser’ were originally acronyms: MASER standing for microwave amplification by stimulated emission of radiation, and LASER for the very similar phrase with ‘light’ substituted for ‘microwave’. The important point is that masers and lasers are both derived from the stimulated emission process, and the only difference between them is a rather arbitrary distinction, based on the frequency of radiation they emit. Masers, as laboratory instruments, in fact pre-dated lasers by several years, and both had been completed as practical instruments before the discovery of astrophysical maser sources.

Although this book is about masers, most people are probably more familiar with lasers, so keeping in mind that the two things are very similar, we will begin by considering lasers. Most people probably own several lasers: lasers are used to interpret the information stored on CD and DVD discs; they are also used in many computer printers. Even if they have only a vague idea about how they work, and view lasers as some sort of ‘black box’, tube, or chip that emits light, most people will probably be aware that this light is in some way ‘special’ – that is, it has properties that make it different from the light emitted by, say, a filament electric light bulb. What are these important characteristics? Given time to ponder on this question, most people would proably come up with a list something like this to summarize the important properties of laser light:

• The laser light is extremely bright, or intense.

• The beam is tightly focussed. […]