typodupeerror

## DIY Laser Pistol Shoot 1MW Blasts284

An anonymous reader writes "It doesn't get cooler than this — a German hacker put together a 1MW laser pistol capable of shooting straight through a razor blade with a single pulse. Quoting: 'Fitted with a Q-switched Nd:YAG laser, it fires off a 1 MW blast of infrared light once the capacitors have fully charged. The duration of the laser pulse is somewhere near 100ns, so he was unable to catch it on camera, but its effects are easily visible in whatever medium he has fired upon.'" Update: 03/17 18:22 GMT by T : Too bad; turns out it's "only" 1KW, rather than 1MW. I still want one.
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## DIY Laser Pistol Shoot 1MW Blasts

• #### Wrong power (Score:4, Informative)

on Thursday March 10, 2011 @11:14AM (#35442880)

1kW, not 1MW.

• #### Wrong unit (Score:5, Informative)

<john AT hartnup DOT net> on Thursday March 10, 2011 @11:37AM (#35443192) Homepage

It's not all that interesting what the power is, without knowing how long it's applied for. TFA says 100ns.

1kW * 100ns = 0.0001 joules
1MW * 100ns = 0.1 joules

Neither of which is very much energy. Next question: how small an area is that energy applied to? Pretty damned small, I'm assuming, if it's going to punch a hole in a razor blade with that little energy.

• #### Re:Wrong power (Score:3, Informative)

on Thursday March 10, 2011 @11:39AM (#35443224) Homepage

100ns * 1MW

(100 nanosecondes) * 1 mégawatt = 0,1 joules

So, 1kW is barely 0.0001 joules..

• #### It doesn't seem to add up, though (Score:5, Informative)

on Thursday March 10, 2011 @11:41AM (#35443254)
1MW for 100ns = 0.1J, roughly equivalent to dropping 100g a distance of 10cm. 1kW for 100ns = 0.1mJ, equivalent to dropping a 1 gramme mass approx. 1mm. I believe the latter could pop a balloon, but it doesn't seem enough to punch through a razor blade. My memory may be faulty, but I seem to recall it takes of the order of magnitude of 10^10 W/sq cm to do that. Focussing the beam to 10^-7 sq. cm would be quite an achievement.

Perhaps either the original estimate is correct or the pulse duration is much longer, of the order of 100 microseconds.

• #### Re:Awesome! (Score:5, Informative)

on Thursday March 10, 2011 @12:33PM (#35443952) Homepage
From Wikipedia [wikipedia.org]:

Infrared lasers are particularly hazardous, since the body's protective "blink reflex" response is triggered only by visible light. For example, some people exposed to high power Nd:YAG laser emitting invisible 1064 nm radiation, may not feel pain or notice immediate damage to their eyesight. A pop or click noise emanating from the eyeball may be the only indication that retinal damage has occurred i.e. the retina was heated to over 100 C resulting in localized explosive boiling accompanied by the immediate creation of a permanent blind spot.

• #### Re:Wrong power (Score:3, Informative)

on Thursday March 10, 2011 @01:33PM (#35444634) Journal

http://en.wikipedia.org/wiki/Inverse-square_law [wikipedia.org]

The inverse-square law generally applies when some force, energy, or other conserved quantity is radiated outward radially from a point source.

A laser does not act like a point source... or it does, but it acts like a point source which is very, very far away.

• #### Re:Wrong power (Score:3, Informative)

on Thursday March 10, 2011 @02:04PM (#35445038) Journal

No. Re-focusing light creates a virtual image. Remember that from optics? It is the distance to that which the inverse square law follows, and that virtual image (in the case of a laser) is very, very far away. In a perfect laser, the distance to the virtual image would be infinite.

The inverse square law requires a point source. Suppose your laser has an aperture which is 1 mm in diameter. At 1000 m, the beam has expanded to a 2 mm diameter. By simple geometry you can infer that if a point-source of the light exists, it actually appears to be 1000 m behind the laser itself. Doubling the distance from the laser's aperture would only increase the distance from the virtual point-source by a factor of 1.5, not 2.

• #### Re:Wrong power (Score:3, Informative)

on Thursday March 10, 2011 @11:53PM (#35449934) Homepage

The problem here is that people aren't trained in physics. Watts are a measure of power, not energy. If you multiply the Power x the amount of Time, the result is the amount of Energy.

Think of it like a firehose vs. garden hose: the firehose pumps gallons of water per minute, but the garden hose takes a lot longer to pump the same amount of water. The "power" is like the size of the hose: how much energy does it pump in one second? That's like Watts. How much water ended up in the bucket? That depends on both Power and Time; you can fill the bucket faster with a firehose, but if you turn the firehose on and back off right away, then it's easy to get more water out of the garden hose by leaving it on longer. How much water ends up in the bucket is like how much Energy you used. 1 Joule is the amount of energy you get when you push 1 Watt for 1 Second. (1 Kilowatt-hour is the amount you get when you push 1,000 Watts for 1 Hour. If you're following the math, that means 1 KWH = 3,600,000 Joules.)

In a pulsed laser, each burst has a duration, and the most useful information is how much Energy is released per pulse (Joules). That's why they're measured by Energy (Joules or milliJoules). For a continuous laser, there is no "time" element, so the output is measured in Power (Watts).

So ... a 1 MW laser (Power) firing a pulse of 100ns (100 x 1/1,000,000,000th of a second), would give 1,000,000 Watts x 100 ns x 1 ns / 1,000,000,000 ns/sec = 100,000,000 / 1,000,000,000 = 100 / 1000 = 1/10th of 1 Joule each time it fires. A 60 Watt bulb uses 60 Watts per second ... 600 times as much Energy in 1 second as a 1MW laser delivers in 100ns. It's the incredibly small amount of time the pulse is firing (less than 1 millionth of a second) that results in so little energy being delivered to the target. It's the incredibly small area (focusing via the lens) that causes so little energy to do so much damage.

A 1 KJ (Kilojoule) laser would be delivering as much Energy in a single pulse (remember: if we're measuring the laser in Joules, we're giving the value per pulse) as a 1,000 Watt spotlight would use (mostly becoming heat) in 1 second.

I'm not saying this was made using a MW laser. I'm just explaining why a 1MW laser firing a millionth-of-a-second pulse isn't going to burn through anyone's bathroom wall. (And I'm not replying directly to Chris; I just wasn't sure where to drop this water-hose explanation into the conversation.)

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