A laser is a device which stimulates molecules or atoms to emit light at particular wavelengths.
It will then amplify that light and typically produce a very narrow beam of radiation.
What many people perhaps don’t know is that the word ‘laser’ is actually an acronym for ‘Light Amplification by Stimulated Emission of Radiation’.
What do we use lasers for?
Lasers are used in a number of items we use every day.
They include everything from optical disc drives and barcode scanners, to printers and entertainment systems.
Lasers are widely used for material processing in manufacturing, such as for drilling, cutting and welding.
Other uses include DNA sequencing instruments, fiber-optics, laser surgery and skin treatments.
Where did lasers come from?
The first working laser was built in the 1960s by Theodore H. Maiman at Hughes Research Laboratories.
His invention was based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.
This was a Chromium dosed ruby which produced ruby coloured laser light at 694.3nm.
How did this early laser work?
A rod of this chromium-dosed ruby was encapsulated within a set of high-energy flash tubes, backed by mirrors.
When these flashtubes are lit by an electrical current, the light emitted is bounced between the mirrors and through the ruby rod infinitely.
This high-energy light then causes the ruby rod to fluoresce in a process named stimulated emission.
If you think back to our explanation of how ions are excited in a flame photometer (*LINK TO BLOG HERE*), in a laser these already-excited ions are hit with a light of a specific wavelength which then causes the emission of light and a drop back into the ground state.
How do mirrors work in a laser?
The presence of mirrors in the system means that until the port to ‘beam’ the light out of the ruby rod is opened, a greater and greater quantity of light is stored being bounced between the mirrors and through the core.
This stimulates more and more chromium ions to emit light and also keeps these ions excited for further lasing, which is known as population inversion.
What is population inversion?
Population inversion is another important piece of the puzzle of lasers.
This is a concept of statistical mechanics which refers to the ratio of excited to unexcited ions in a system.
Our system in this case would be the ruby rod, and our ions the chromium found within it.
In a ruby, population inversion is the defining factor in the stimulated emission of light.
As the laser light explicitly comes from the already excited ions in the ruby, having a higher concentration of excited ions than unexcited ions would mean that more laser light is emitted from the ruby rod.