Question: Which lasers include a ruby ​​laser, Nd: glass, Nd: three-level or four-level?

Patrick Dalton answered on 17 Nov 2020:

So I’ll break apart this question a bit.
Lasers require something called a “gain medium” to operate. This is a material we add energy to in order to power up the laser. This medium can be a solid, a liquid or a gas depending on what we are looking for.
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The ruby laser was the first laser ever developed, it was developed around 1960. It is a “solid state” laser, meaning the gain medium is solid. They used a bright light source called a “flash lamp” to power up the electrons (tiny particles) inside the ruby to a higher “energy level”. An energy level is a fixed amount of energy that an electron can have.
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This is where the “3 or 4 level” part of your question comes in. When we power up the medium, we “excite” it into a higher energy level. This energy is then used to power up the laser. However, some systems actually go through multiple stages at different energy levels, rather than just a power up and power down motion, as it allows for very specific types of laser to be created.
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Nd stands for Neodymium. Nd:Glass means that the material is made of glass with bits of Neodymium embedded inside it. The Neodymium is what powers up the laser, but it needs to be hosted in glass. Another common example is Yb:Glass, which is made up of glass with bits of something called Ytterbium inside.