The Pentagon's laser program is getting a big boost in funding this year, as military planners try to turn a long-held dream of science fiction writers into a combat-ready weapon that can swiftly knock down enemy drones, ships or vehicles.

The most recent defense spending bill, passed in December 2016, includes $328 million for developing and procuring "directed energy" weapons in 2017, a 51 percent increase over the previous year. That money will go to building more laser weapons that operate with a beam of concentrated light, as well as microwave and electromagnetic weapons that can knock out electronics or engines on an airplane, or even give enemies a powerful jolt without killing them.

"There is a growing realization on Capitol Hill and the Department of Defense that its time to transition these technologies," said Mark Gunzinger, a senior fellow at the Center for Strategic and Budgetary Assessments, and former deputy assistant secretary of defense under President George W. Bush. "There are some technological challenges, but it's apparent to us that it's a case of inadequate funding. We do believe there are directed energy technologies that are ready to transform now."

All four branches of the military have already built prototype laser weapons that have worked during tests. But the next step is building and testing operational ones, according to Gunzinger.

"Lasers have been around since the 1960s, and there have been decades and hundreds of millions of dollars invested into laser technology," said Gunzigner, who authored a comprehensive 2012 report on directed energy weapons for CSBA.

Early laser weapons combined chemicals reactions to generate a beam of light. The Air Force mounted a chemical-based airborne laser on a Boeing 747 and tested it, but Pentagon strategists conceded it wasn't useful to put a big relatively slow airplane in range of surface-to-air missiles. The airborne laser took more than $1 billion to develop between 1996 and 2010 and made several test flights and test shots at drones, but it wasn't very accurate and the project was abandoned.

Since then, the Navy Laser Weapon System (LaWS) has been installed on the USS Ponce operating in the Persian Gulf. It generates uses heat energy and uses a laser to destroy drones and floating targets at sea.

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The Army mounted a 10-kilowatt laser on the back of a military vehicle and tested it in 2014, when it knocked down 150 aerial targets including both mortars and drones, while the Marines use a smaller short-range 30-kilowatt laser mounted in the back of a Humvee transport.

Great Britain's Ministry of Defense awarded a $36 million contract this month to a group of European defense companies to produce a prototype laser that could track and target incoming targets. .

The Air Force has switched gears from using an offensive laser on a big plane, to a defensive laser that could be mounted on a fighter jet. In August 2016, Northrop Grumann won a $39 million contract to build a control system for the Self-Protect High Energy Laser Demonstrator (SHiELD) for the Air Force Research Laboratory at Kirtland Air Force Base in New Mexico.

The idea is to see whether such a device works better than other countermeasures to knock down incoming anti-aircraft missiles.

For all the promise and Pentagon research dollars spent over the years, laser weapons still have a couple of problems. First, they don't work well in rainy, cloudy weather. That means they won't likely be deployed to the ships patrolling the northern or southern latitudes, but perhaps a better option for tropical regions or places like the Persian Gulf where visibility is generally good.

They also require a lot of electrical energy to produce the laser beam, and producing that energy requires carrying around a big generator of some kind.

"The problem is the battery right now," said Brig. Gen. Julian Alford, commander of the Marine Corps Warfighting Laboratory/Futures Directorate in Quantico, Va. "When the scientists figure out how to make the size, weight and power much smaller, that would be something we can use."

Mark Neice, director of the Directed Energy Professional Society, a group of scientists and engineers in the laser community, expects some of these problems to be solved in the next five to 10 years.

"The biggest issue in terms of transitioning directed energy capabilities is now the policy and perception of using this technology, vs. the old 'bombs and bullets' mentality," he said in an e-mail to Seeker. "From an engineering perspective, we need to make the direct energy systems efficiency higher, which will reduce the overall size of the weapons package. Acquisition costs are higher, but the life cycle costs are lower."

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