The Airborne Laser just can’t deliver a beam with enough power (Image: Jim Shryne/USAF) The laser works in testing but is too weak to be effective (Image: MDA/ABL)

A laser-toting Boeing 747 blasted two missiles out of the sky earlier this month, but despite this apparent success the Pentagon is going back to the drawing board in its search for an anti-missile laser weapon.


The ABL’s problem is that it can’t deliver enough power over enough distance to be genuinely useful, so the culmination of a project begun in 1996 and costing an estimated $5 billion will be to downgrade the ABL to a “testbed”. It will be handed over by the Missile Defense Agency to the US air force for general research use.

ABL will be used for more tests this year, but the MDA’s latest budget documents reveal it will devote the rest of its “directed energy research” budget towards a new kind of laser yet to even escape the research lab.

Non-lethal energy

Last May, Defense Secretary Robert Gates told Congress that the ABL’s range was limited to 135 kilometres, well short of the minimum requirement of 200 km.

The distance over which the recent tests took place has not been disclosed, but it is believed to be relatively short. Future tests will try longer shots, but it looks like the chemical oxygen iodine laser (COIL) used just can’t deliver lethal energy through hundreds of kilometres of dusty, turbulent atmosphere.

That’s largely because the laser’s bulk limits what can be fitted into a 747, which directs the beam through a turret in its nose. Room for only 6 of the 8 to 14 laser modules the design asked for could be found.

New contender

The leading candidate to provide megawatt-class power in a more compact package is a new twist on an idea first proposed at the dawn of the laser age 50 years ago – alkali lasers. Their light is emitted by vaporised alkali metals such as caesium or potassium.

Lawrence Livermore National Laboratory in California is developing “diode-pumped” versions of alkali lasers and reports they have “mass-to-power ratios that far exceed what is possible with today’s other laser systems”. That makes them good candidates for delivering high power in constrained spaces like an aircraft.

The new system fires an array of high-powered diode lasers, a type used in consumer gadgets like DVD players, into a cloud of alkali metal atoms. The energy is absorbed by their outer electrons and then released as laser light.

Scale and efficiency

The most powerful alkali lasers so far only produce tens of watts, while the MDA needs more than 10,000 times that. But current designs should scale easily to about a kilowatt of power, says Bill Krupke, a laser consultant who worked at Livermore for 27 years and holds a patent on diode-pumped alkali lasers.

Flowing the vaporised metal through the laser – a technique already used in the ABL – could scale the technology to much higher powers, he says.

The best alkali laser reported so far, by Boris Zhdanov of the US Air Force Academy, had an overall efficiency of 30 per cent, compared with the ABL’s estimated 20 per cent. If MDA wants to shoot down missiles from the air, alkali lasers may be the only feasible option.

Journal reference: Proceedings of the Society of Photo-optical Instrumentation Engineers (DOI: 10.1117/12.769064)