Military Lasers

[Deepwater6]

Can someone please explain to me how far a laser can travel? If it's shot into space and were not to hit anything would it travel forever? Is it basically the speed of light? What is the difference between the military version and the versions on top of a weapon for sighting in on something? How much energy does it take to generate one of these military lasers in the article below? How do they gauge at what intensity they are firing the weapon? thx

 

http://www.bbc.co.uk/news/technology-13033437

Edited April 11, 2011 by CraigD
Fixed broken link

[joekgamer]

1) A laser will travel untill it hitss something.

 

2) Yes. See #1.

 

3) A laser is made of light (electromagnetic radiation), so it travels at exactly the speed of light.

 

4) Lasers meant to be used as a weapon are much stronger than a small sighting laser.

 

5) At least a few gigawatts.

 

6) They calculate it.

[CraigD]

Can someone please explain to me how far a laser can travel? If it's shot into space and were not to hit anything would it travel forever?

1) A laser will travel untill it hitss something.

 

2) Yes. See #1.

This is a common misconception, owing I think to lasers having much lower beam divergences than most common light sources. Lasers are still subject to the usual optical laws governing diffraction, but high-quality ones can approach their theoretical minimum divergence, which is proportional to their wavelength (that is, the higher – more blue – the frequency of the light, the less the beam spreads).

 

Practically, no present day laser can do much better than send a barely detectable signal to the moon and back. This has been done increasingly routinely as the key part of the Lunar Laser Ranging Experiment since the Apollo 11 crew placed a reflector on the moon for that purpose in 1969.

 

How much energy does it take to generate one of these military lasers in the article below? How do they gauge at what intensity they are firing the weapon? Thx

5) At least a few gigawatts.

Like most experimental weapons, the exact specification of this one is a military secret, my guess, based on older, similar systems, is that it’s power is in the range of a few 100,000,000 W (hundred megawatts). Though multiple GW lasers aren’t too difficult in engineering concept, they’d be difficult to power, as the largest powerplant on warships are have outputs of a few hundred MW. “Gigawatt class” mobile laser tend to use chemical power – for example, the Boeing YAL-1.

 

How do they gauge at what intensity they are firing the weapon? Thx

There are 2 main ways to measure the power of a laser – its input power, which in the case of the weapon in question, is in the form of an electric current, and its output power, which can be measured by such means as a photometer or a calorimeter (heat meter). Because lasers produce nearly all of their output radiation at a single EM frequency, their output powers are fairly simple to detect. Very high power laser tend to be around 60% efficient, so their output power is about 60% if their input power.

 

The current LLRE laser has an output power of around a GW, but, because it’s intended for precise measurements, not melting or setting things on fire, a very short (less than 0.001 s) duration.

 

 

I should point out that, as nearly all the literature on them notes, at present, laser weapons are experimental, and not clearly better or certain to every be better than projectile weapons.

 

The original post's article is noteworthy not so much because of the power and energy of this laser, but because it’s one of the first public announcements of a test of a laser other than to defend against incoming missiles or artillery shells (what’s usually called a close-in weapon or point defense system). This is less of about advances in laser weapons technology than about ways of using them in a more versatile manner – if a ship has a laser to protect it from missiles, being able to also use it to attack small boats or other ships adds versatility, and thus value, to the system, at very little extra cost. In short, it's a waste of hardware to have a CIWS that's good only for a single purpose, or a primarily defensive weapon that can't be used offensively.

 

Presently, CIWSs use either missiles, such as the RIM-116’s 21 127 mm ones, high rate-of-fire cannon, such as the Phalanx’s 20 mm rotating multi-barrel cannon, or a combination of the two.

 

A major disadvantage of these are that, though they work well and are difficult to harden missiles and shells against, they hold limited ammunition, so with their high rates of fire, run out quickly, making it possible to overwhelm them with a prolonged attack. A Laser CIWS, though it requires a large electrical powerplant, should be able to fire continuously as long as the powerplant can deliver electricity.

 

Neither conventional nor laser CIWS are intended for, or effective at, shooting objects in space or on other planets. Even if they powerful and minimized beam dispersion enough, as the LLRE shows us, they'd be tremendously difficut to aim accurately enough.

 

A final note: the word "laser" usually refers to the machine used to generate a laser beam, not the beam itself, so questions like "how much energy does it take to generate one of these military lasers?" are a bit confusing, as they seem to be asking how much energy is needed to manufacture these machines, not their power (the rate at which they convert electric or chemical energy to light).

[Deepwater6]

Thanks, I understand it alot better now guys. I see you point about more conventional weapons running out of ammo, but it seems from the video the beam must be hitting the object for some time before it does real damage. On a rolling ship in heavy seas it doesn't seem feasible. Is there any way to attach things to the beam? Like coded information for long distance communication?

[CraigD]

Thanks, I understand it alot better now guys. I see you point about more conventional weapons running out of ammo, but it seems from the video the beam must be hitting the object for some time before it does real damage. On a rolling ship in heavy seas it doesn't seem feasible.

Though the problem of stabilizing any weapon on a rolling ship has been pretty well-solved for 50+ years, Most military folk pretty much agree with you, DW, on the feasibility of laser weapons. Though decades and 100s of millions of dollars have been spent on them (10s of billions, if you include a portion of the US's SDI spending), no project to actually deploy laser weapons has yet, to the best of my knowledge, succeeded beyond shooting targets specially made and/or fired/maneuvered to be easy to hit - though, this being military secret stuff, who can really say?

 

My guess, which follows pretty closely those of my favorite military SF writers, is that future warships will likely have a mixture of many kinds of CIWSs, including lasers, because the various kinds of weapons each have different strengths and weaknesses. Even if it's not your best, there's something reassuring about having one weapon that'll keep firing as long as your generators keep spinning, even if those generators have to be pretty gigantic.

 

Is there any way to attach things to the beam? Like coded information for long distance communication?

Yes. There's even an open-source, GNU licensed system you can build yourself. Check out the wikipedia article Free-space optical communication for some history (going back to the 1940s, before lasers) and details.

 

The basic problem with Earth-based laser communication is that it doesn't work well in rain, fog, etc. - rather a big drawback on our sometimes rainy, foggy, snowy, smokey world.