Lasers have become common since their invention in the 1960s. We use them every day to read optical media like DVDs, to see if our shelves are level, measure distances, or just to point at things. There are lots of medical and industrial uses for lasers too, but what we really want to know is how we can use them to defeat alien and Cylon invaders. Here are some ways the military is (and will be) using lasers on the battlefield.
We're still a long way from laser blasters disintegrating enemy forces, but the military has quite a few lasers in the field, and a lot of research (and money) devoted to making them better, smaller and more powerful. Military uses for lasers fall into a few distinct categories.
Communications – Lasers can be used to establish high bandwidth communications between distant locations without laying down any kind of infrastructure. The "shooter" sends laser pulses to the receiver, which interprets them as binary input to be deciphered by computers. In one test, the Navy transmitted live video between two moving ships that were nine nautical miles apart. There are limitations due to atmospheric interference and targeting accuracy, but this is still a pretty awesome communications tech.
Blinding enemies – If you shine a laser in someone's eyes, it really screws with them. If you shine a strobing series of lasers in someone's eyes, it can seriously disorient them. And if you shine a powerful laser in someone's eyes, you can blind them permanently. It is actually considered a war crime to intentionally blind enemy soldiers, and a 1980 treaty specifically forbids laser weapons that cause permanent blindness (it's ok to kill them, though).
Nevertheless, laser weapons offer a non-lethal way to incapacitate enemies at long range. The military calls this type of weapon a Dazzler. I don't know if they named it after the Marvel comics character, but I desperately hope so. They have a prototype called the PhaSR, which is a ridiculous acronym but still kind of cool for sci-fi fans. It was obviously intentionally designed to look like a science fiction weapon.
Designating targets – You've probably heard of laser-guided missiles and bombs. They are able to strike with extraordinary precision by honing on reflected laser light that's being shined on the target by another unit such as a spotter aircraft. These lasers emit light in the infrared portion of the spectrum, so the target typically doesn't know it's being "painted" by a laser. The laser is also pulsed at a specific frequency, which makes it resistant to laser jamming. A much simpler version of this technology is the laser sight found on some military guns, which allows the shooter to see the exact spot where the gun is aiming.
Laser jamming – This works a lot like blinding a human target with a laser, but it's used as a countermeasure against the enemy's own laser-based or optical systems. This has been used against the U.S. several times – the U.S.S.R. used their Terra-3 laser to illuminate the Space Shuttle Challenger in 1984, which reportedly caused some problems for the shuttle's systems and crew. In 2006, China shined a laser on a U.S. imaging satellite. Although the Pentagon denied that the satellite suffered any harm, the potential to incapacitate satellite optical systems with a laser is certainly there.
Direct weapons – Will we have soldiers running around battlefields vaporizing enemies with lasers? A man-portable laser with enough power to accomplish that is probably decades off. Research is moving in that direction, though. Early efforts were focused on chemical lasers, which can be incredibly powerful but require huge stores of highly toxic chemicals to function. You could barely fit one on a large plane. Modern military laser weapons are based on solid state lasers, which, to greatly simplify a complex process, use ionized materials like ruby or garnet to amplify the input light beam, which might come from an LED.
The threshold for a useful military laser has long been considered 100 kilowatts, a goal surpassed by Northrop Grumman in 2009. That kind of power won't cut a tank in half, or even a human (unless you shine it for quite a long time), but in demonstrations it has proven capable of destroying incoming missiles, mortars and drone aircraft. The weapons are still pretty big – current efforts aim at mounting them on planes and mobile anti-aircraft units. But at this point the technology exists, and the traditional process of improving it will no doubt march on. They'll get more powerful, smaller, have more efficient cooling, need simpler power sources and eventually fit into a backpack.
Emery, Daniel. "Anti-aircraft laser unveiled at Farnborough Airshow." BBC News.
Office of Naval Research. "Laser Communications Fact Sheet."
Rincon, Paul. "Record power for military laser." BBC News.
Schactman, Noah. "Military Laser Hits Battlefield Strength." Wired.