Gas Lasers: What Makes Them Work?

[Mario Carcamo]

My question is very basic concerning gas lasers. I wanted clarification that a gas laser gets its light from a cell containing only that gas with a few other constituents and a high voltage being applied to it. Basically I'd like to understand what is happening with a CO2 laser and why it requisites other gasses. Also the entirety of the actual cell seems to light up and there is only one mirror on the back end of the tube. Why don't they cover the entire laser tube with a mirror material? or is that what the laser head is?

 

[Dr. Courtney]

It's complicated.

Most gasses have other gasses mixed into achieve the population inversion required for lasing.

The mirrors define the resonant cavity and thus the standing wave direction in which lasing will occur. You don't want stimulated emission in all directions, but only in the direction in which you want the light amplified and concentrated. The light emitted in the other directions is mostly spontaneous emission.

More details are described here:

https://en.wikipedia.org/wiki/Laser

 

[Pooua]

My question is very basic concerning gas lasers. I wanted clarification that a gas laser gets its light from a cell containing only that gas with a few other constituents and a high voltage being applied to it. Basically I'd like to understand what is happening with a CO2 laser and why it requisites other gasses. Also the entirety of the actual cell seems to light up and there is only one mirror on the back end of the tube. Why don't they cover the entire laser tube with a mirror material? or is that what the laser head is?

You already know that a CO2 laser tube contains other gases, most importantly helium. The reason for this is, electrons are very light-weight, low mass particles, but the CO2 molecule is very heavy. When the low-mass electrons strike the high-mass CO2, the electrons don't dump very much of their energy into the CO2; they just bounce off. Helium, though, is much lower mass than CO2, so it picks up more energy from the electrons when they collide. Then, when the helium collides with CO2, more of the energy transfers to the CO2. In short, the helium is there to improve the energy coupling efficiency.

Every beam of light that leaves a laser tube takes energy with it. Ideally, we want to put all the energy we can into a single beam. We would rather have the stray unamplified photons escape from the tube than have those photons make several passes through the tube, taking energy with them on each pass. We want the primary beam to take the majority of the available energy out of the tube.