Solid-state beats chemicals for laser gun

20 Jul 2010

A 32 kW combination of six industrial sources is preferable to deployment of a chemical laser, says Raytheon.

Directed energy weapons should be based on combinations of high-power fiber lasers, and not chemical lasers.

That’s according to Mike Booen, the VP of advanced security and directed energy systems at Raytheon’s Missile Systems unit.

Booen was in attendance at the Farnborough Air Show in the UK, at which Raytheon wowed the world’s media by disclosing details of a directed energy weapon, or “laser gun”. The weapon, which relies on solid-state emitters and six fiber lasers combining to produce a 32 kW beam, was used to shoot down four unmanned aerial vehicles (UAVs) in a test run earlier this year.

While the “wow” factor of any destructive, Bond-villain style laser makes for undeniably good copy, one of the key decisions revolves around which laser medium is best to generate the awesome output powers required to shoot down drones and, in the future, much larger targets.

Raytheon has clearly nailed its allegiance to the solid-state approach, even though chemical sources, such as the chemical oxygen iodine laser (COIL), have traditionally offered more power. “Our customers don’t want chemical lasers,” emphasized Booen. “They are hard to get to the front, and place nasty environmental constraints on everybody.”

Because such high output powers are required, a huge number of solid-state emitters are needed to achieve the destructive power seen in the impressive Raytheon video. Raytheon has achieved this by combining six high-power fiber lasers.

The development of that source has been a key focus of projects such as the Robust Electric Laser Initiative (RELI), which is led by the US Army, and DARPA’s Revolution in Fiber Lasers (RIFL) effort. Under the RIFL project, Northrop Grumman has been working with ytterbium-doped fiber because of the material’s very low quantum defect of these amplifiers, which are pumped by 976 nm diode emitters supplied by Fraunhofer USA.

The US military now appears to be solidly behind the development of directed-energy weapons based on solid-state emitters. In late June, Raytheon and fellow US defense contractor Lockheed-Martin were awarded a combined $23.8 million by the US Army under RELI, shortly after DARPA handed Northrop Grumman a $4.6 million extension under RIFL.

One of the key challenges remains dealing with the heat dissipation of a multi-kilowatt laser source. And despite the huge improvements to diode laser efficiencies in recent years, a 100 kW weapon operating at a 30 percent wallplug efficiency (the target under the DARPA RIFL project) would still dissipate upwards of 200 kW, presenting some challenging thermal management problems, to say the least.

Another critical challenge is how to combine beams from multiple fiber sources without losing beam quality, and this is a key focus for Northrop Grumman. The company has developed a proprietary beam-combining technique, based on a two-dimensional diffractive optical element.

Raytheon’s laser-gun demonstration - the most graphic implementation of directed-energy weapons technology yet seen, and a sure-fire hit in the age of YouTube – is just the start of a ramp up in power towards 100 kW. At that power level, the laser will be able to destroy far more than a simple unmanned drone.

At the same time, ways to overcome attenuation in poor weather conditions, and reflective materials designed to repel the incoming beam, will also need to be found.