How do industrial or military LASERs work?

[IgorT]

Hi guys!

I was wondering.. How are lasers used in industry, when it comes for example to cutting steel?

The reason i'm asking is the fact, that steel by itself is very reflective, so how can a laser cut it?

Since now lasers are even used in military, for "shooting down" artillery projectiles, or missiles, what happens, if the missile is coated with a reflective surface or made of polished alluminum or some other shiny metall for example?


Is it simply that they make the laser powerfull enough, that even if most of the light will get reflected, enough will get absorbed to achieve the desired effect?


I'm hoping someone here knows more about this.


Thanks!

 

[Rhith]

I believe you are on the right track with your suspicions. For cutting steel with lasers they generally use CO2 lasers which are easily obtained in the houndreds of watts. Then they use precise optics to focus it down to a point that is so concentrated it burns like millions of suns in one spot! MUAHAHAHAHA!
But as for shooting down missles I would assume it is using the same principles in just applying such a high amount fo energy into it there is a very minimal chance that it will be stopped. Hopefully the "enemy" will not think too much of our use of laser technology! Imagine, FS Mirror coated missles!

 

[Zarniwoop]

Even just spinning the missile helps keep any one point from heating a lot.
But then the military has come up with some pretty radical technologies, like Free Electron lasers (http://en.wikipedia.org/wiki/Free_electron_laser) and even nuke-pumped XRay lasers  (but you can't exactly use one of *those* in a factory to cut steel ).

 

[Chris.]

http://www.twi.co.uk/j32k/protected/band_3/kspah004.html

That's an interesting read, 1-1.5kW powers are used

PS: Steel isn't always reflective, depends on the wavelength used.

 

[FrothyChimp]

In industrial laser processing, it's the material to be processed that determines the type of laser to be used. Steel cutting is almost exclusively the domain of CO2 laser systems. Even using CO2 they are often assisted by oxygen injection at the cutting point just like you would use on a cutting torch and for the same reason. At 10,600nm most metals will absorb the energy thus heating it. In fact, a CO2 laser will cut, what we commonly think of, as a mirrored surface.

Marking systems can be UV, visible, or IR depending on the material and the energy required to etch or mark the material. Q-switched systems are more common since they can be operated with lower energy requirements thanks to the Q-switch but you also see CO2 systems with TTL control etching also.

Military systems are just so far beyond what us mortals even think is powerful. Yes, lasers are part of the battlefield and have been for sometime. Back in the day I used an Nd:YAG range finder and designator to call airstrikes onto tanks and such. But I think we are talking about the raw, unadulterated power of laser weapons systems. The military tends to use chemical laser systems for the really big crowd pleaser systems. Never underestimate the power of CO2 system though. I saw a demonstration at a laser lab at Wright Patterson Air Force Base in Ohio of a CO2 system. It cut a six inch diameter hole at one sitting. It didn't trace around a circle. It ablated everything within the circumference at the same time. It was beautiful. It brought a tear to my eye.

 

[IgorT]

I was specifically talking about the system mentioned here: http://www.mda.mil/mdalink/pdf/laser.pdf

The "destructive" laser is in the MW range, and the thing uses six IR sensors and two kW range lasers just to lock onto the target and compensate for atmosferic disturbances.. It's an impressive piece of technology.


Also, if you check out the Wikipedia page for "chemical lasers", they mention very long wavelengths.

I know that different wavelengths get absorbed differently, depending on the surface, but only a theoreticall black body would absorb everything.

In reality, no matter what colour the surface has, it will still reflect a lot of light.

Like cutting a black floppy disk in half using a red laser - it's amazing how much blinding light the matte looking black surface can reflect, once it melts and becomes glossy..


So my theory was, that even tho some surfaces can reflect most of the light away, the laser is simply made strong enough, that enough energy still gets absorbed, for it to be effective.


Never the less, some very smart person was amused by my question on how this reflectiveness issue is overcome, and my theory on it. Apparently, i'm an idiot, since i believe that military grade lasers can get reflected from white or shiny surfaces at all.

If i would mention a spinning missile, i would probably be a double idiot, even tho the link i posted mentions the importance of the beam being perfectly still in relation to the target.


So are there any special absorption laws for very long wavelength lasers? Or does most of the light still get reflected away from shiny surfaces like metall, but the laser still cuts it?

 

[FrothyChimp]

The wavelength of the ABL is 1.315 micron or 1315 nm. This is used for atmospheric propagation as well as good absorption characteristics on metal surfaces. Keep in mind that ballistic missiles do not spin in flight. They use gyroscopic stabilization to maintain consistent flight characteristics. The ABL does not target the warhead. It targets the most vulnerable component of a ballistic missile: the fuel/oxidizer. A breach caused by a MW laser in a liquid fueled launch vehicle will destroy it outright. A breach in a solid rocket launch vehicle will destabilize the vehicle allowing the missile sheer apart or ignite the propellant.

You are correct that there will always be reflection regardless of the material but near IR is the best mix for propagation and absorption.

 

[IgorT]

Thanks!


BTW: The entire discussion started with "ABL uses a chemical laser! Chemical weapons should be banned!"

And i had a very hard time explaining that a chemical laser is NOT a chemical weapon in the first place, no more than a bullet is, even tho it uses a chemical reaction to propell itself..

Some guy was convinced, that there are chemicals in the beam of a chemical laser..  ;D