Carolyn Almennigen | almennigen

How a LASER works

Sep 14th 2010 at 12:06 PM

LASER is an acronym for Light Amplification through Stimulated Emission of Radiation. Actually, it isn’t exactly amplification, it’s more like oscillation, but who would want to be associated with a LOSER?

First picture two mirrors facing each other. Light goes boing boing between the two forever. Now make one of the mirrors partially reflecting, so maybe 10% of the light goes through to the outside world.

Put something in between the mirrors that will lase. Not all materials do, and those that do only lase at specific wavelengths (colors). What’s a lasing material? Ruby was the first one discovered. Yes, the kind in jewelry. Make a cylinder of it, with both ends polished. Have these polished ends facing the mirrors.

Normally, ruby has a pretty red color, but the light is hitting it and going through it every which way. To get a laser going, we need the light to only go in one direction and in phase. To be in phase the light waves are in lock-step like the British Red Coats in the Revolutionary War. To get this lock-step, we have one photon from one atom hit another and knock off a photon or two which will hit another atom and get them all going together like a game of Crack the Whip.

To get the atoms to kick off photons, we need to give them enough energy to jump the electrons into a higher and more energetic orbit around the nucleus. Every kind of atom (like our ruby atoms) has particular energy packets that it will accept. My son, when he was little, hated anything green. Green beans, peas, whatever. Carrots and corn he’ll eat, but nothing green. Atoms are like that. You have to put it the specific amount of energy that it will take to jump an electron to a higher orbit. Anything else it just spits up and never gets used.

To make a packet (or quantum) of energy which the ruby atom will accept, we can either blast it with all sorts of energy and hope it likes one of the flavors or only send the flavor it likes. Blasting is easier since you don’t have to know what flavor it likes, but it’s pretty inefficient. By flavor, I mean the amount of energy the atom needs to bounce an electron up, and it depends on the wavelength or color of the light you’re shining on the atom. Light of a more bluish color (shorter wavelength) has more energy per photon than light of a more reddish color (longer wavelength).

OK, now we know what color of wavelength to blast the poor little ruby atom with to make its electrons all excited and bounce them up to a higher orbit where they are ready to party. How do we keep them there until we’re ready for them to all fall down and spit out photons of the kind we want for our laser? One way is to misalign the mirrors a bit so the light bounces around in between the mirrors but can’t get out. Another way is to blast them with so much pumping energy (of a favorite flavor) that they’ll just keep bouncing up, throwing out photons, dropping down, absorbing more pumping energy, bouncing up, etc.

We can’t use all of the photons that are spit out; some of them are going to be aimed at the sides and elsewhere and never get reflected by the mirrors. But, those that get to the mirror and get bounced back hit another atom on the return trip and strongly persuade it to spit out a photon. This persuaded (stimulated) photon will be in phase and going the same direction as the original photon. Then those two photons bounce off the other mirror and do the same thing to two other atoms. And so on and so on, and pretty soon you have quite a lot of energy going all in the same direction and in phase.

Remember the partially reflecting mirror? Well, it leaks out some of that energy, which is going all the same way and in phase and of the same color and you have a laser beam!

Principal components:
1. Gain medium
2. Laser pumping energy
3. High reflector
4. Output coupler
5. Laser beam
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