# Pulse Laser Theory

1. Jun 3, 2006

### mung_geh

Instead of shooting a single burst of beam, use multiple shots ( say 1000 per second or more ) Theoritically each laser beams should punch a hole through the air, each one travelling further and each one creating a vacum within its path. The high powered lasers that auto industries use have effective range of only 2 inches I believe, which they use for cutting sheet metal in assembly line for making cars. Maybe it is possible to program it to do a multiple bursts at a single point and see what happens.

Just a thought thats been stuck in my head since 1990. I'am not a scholar, I don't have a degree in anything. So did all the greatest scientists in our history have a "degree" in those fields in which they were famous in?

2. Jun 6, 2006

### Integral

Staff Emeritus
Could you provide more info about these lasers? As far as I know most industrial cutting lasers ARE pulsed. The lasers I work with are pulsed in the 100kHz range.

3. Jun 6, 2006

### jarvis

My best guess follows. CO2 gas and Nd:YAG lasers are probably the most commonly used for industrial cutting and processing. To answer your question it is important to understand what is happening in a laser cutting process. In order to make a "cut" in a piece of material I need remove or displace the material along the "cut". I can either do that through ablation events, or by melting the material along the "cut" and have it pulled to either side by surface tension. Ablation occurs when you deposit a very large amount of energy, in a very small volume, very quickly. Before that volume is able to thermalize with the surrounding material, its temperature is raised to the point of being in a gas phase and it thermally expands resulting in a micro-explosion.http://img502.imageshack.us/img502/9240/ablation3el.jpg" [Broken] The link will show you an example of an ablation event on a yttrium cathode caused by irradiation with an Nd:YAG 5th harmonic (213 nm). As the vaporized material expands, it cools past the melting point and is frozen in place. It resembles the backsplash from a liquid when something is dropped in. You can clearly see the crater that is left behind, and you can imagine that doing this enough times will eventually "cut" out the line that you want. In order to get these very high power densities (~10^(10-12)W/m^2) one must focus the laser to a very small spot, typically on the order of 10's of square microns. This strong focusing means we will have a short Rayleigh range, i.e. the beam waist is converging/diverging quickly on either side of the focus. As the beam radius increases, the power density at that spot goes down as 1/radius^2, and pretty soon it will be insufficient to do welding/cutting/engraving/etc...
Your comment about "punching" a hole through the air is addressed differently. If the electric field of the laser wave is of sufficient intensity, then dielectric breakdown of the air in that region can be achieved. This is effectively producing lightning with a laser. There will be a thermally expanding plasma in that volume, leading to expansion and a temporary region of lower density when the plasma electrons reassociate with the positive ions. This is hardly the production of a "vacuum" by the laser pulse. The only other option for heating the air would be through absorption, and if your laser wavelength is strongly absorbed in air... how did you get it over there during focusing? I hope this helps, and I apologize for anything I made up that was incorrect!

Also, no scientists do not always have a degree in what they are recognized for. But if my memory serves, physicists are the only scientists who have been awarded every classification of the Nobel Prize .

Last edited by a moderator: May 2, 2017
4. Jun 7, 2006

### rcgldr

In addition to lasers, aren't there also electron / energy beams used for cutting purposes?

5. Jun 7, 2006

### jarvis

There are electron beam welding/cutting machines commercially available at present. However, these need to be operated in a suitable vacuum environment. This is because the mean free path, i.e. the distance something will travel before it collides with something else, of electrons at typical operating energies is going to be small in atmospheric air(having it in vacuum also reduces contaminants in the weld). Again, just my best guess... I am not a welding /cutting specialist!

6. Jun 7, 2006

### brewnog

Correct. It also helps prevent contamination and oxidation of the melt pool and new surfaces.