Defending the skies: the Airborne Laser

17 Jun 2002

In just two years' time the Airborne Laser - the first ever laser system capable of shooting down ballistic missiles - is set to become a reality. Nadya Anscombe finds out more.

From Opto & Laser Europe May 2002

Take one standard Boeing 747 freight carrier, squeeze on a massive megawatt chemical oxygen iodine laser, add to that an Nd:YAG laser, a Yb:YAG laser and a CO₂ laser, and what do you get?

The answer is the Airborne Laser - the first ever direct-energy weapon system for defence against ballistic missiles, capable of detecting and blowing up a missile hundreds of miles away in less than a minute.

This may sound like science fiction, but the Airborne Laser is set to become a reality in just two years' time. The first test firings of the high-power laser were finished two months ago and the first complete system will be demonstrated in 2004.With equipment on board weighing more than 300 tonnes, it is remarkable that the Airborne Laser will actually manage to fly. It will also take an outstanding feat of engineering to focus a football-sized spot of heat from hundreds of miles away. The challenges faced are huge: focusing accurately on a relatively small, fast-moving remote target despite aircraft platform jitter; dealing with atmospheric turbulence that threatens to distort any optical signal passed through it; and detecting and destroying a missile in just a few seconds.

But Steve Lamberson, chief scientist of the Airborne Laser program, is confident that the first-generation Airborne Laser, Block 2004, can be achieved with today's technology. He said: "The ongoing Block 2004 effort can be completed with current technology. We are embarking on a process to develop and incorporate additional technologies into future Airborne Laser blocks, starting with Block 2008."

Three US companies are involved in the development of the Airborne Laser: Boeing, which is supplying the 747 freighter and developing crew safety and the battle management system; Lockheed Martin, which is responsible for the nose-mounted turret, the beam-control system and the Nd:YAG and Yb:YAG lasers; and TRW, which is making the system's high-energy laser.

Flying just above cloud level, the Airborne Laser will not only be able to blow up missiles - it will also be able to detect them shortly after cloud break, calculate the location of the launch point, analyse trajectory information and provide accurate impact-point predictions.

The four lasers will all play crucial roles in this chain of events. The CO₂ laser is used for measuring the range to each tracked missile. "The CO₂ laser was selected for ranging because it is widely used in LIDAR and LADAR applications owing to its long coherence length and its ability to produce complex waveforms," explained Lamberson. "This laser is used to precisely compute the range between the Airborne Laser and each potential target."

The Yb:YAG laser is used to illuminate the nose of the missile and stabilize its image for the tracker. The Nd:YAG illuminator laser is pointed at the body of the missile and the return is used to close an adaptive optics loop that compensates for atmospheric turbulence. Wavefront sensors on the aircraft measure the phase distortions imparted to this reflected light-beam as it propagates from the target to the aircraft.

Lamberson explains why these laser technologies were chosen: "The system requires two pulsed solid-state lasers at different wavelengths to perform the tracking and atmospheric compensation tasks. The wavelengths need to be different so that the returns from the missile can be readily separated as they come into the Airborne Laser's main telescope. The Yb:YAG laser was selected for the tracking task because tracking requires less power than atmospheric compensation."

Only after all of these laser systems have played their individual parts does the chemical oxygen-iodine laser (COIL), which destroys the target, kick into action. "The COIL was selected because of its wavelength (1.315 µm), which is not very heavily attenuated by the atmosphere, and is short relative to other mature high-power lasers," said Lamberson. "This allows the beam to be more tightly focused at the target."

The COIL laser and its chemical plant are so large and heavy that the floor underneath the system had to be strengthened to support the weight of the modules and accommodate the laser exhaust. Keeping the weight down has not been easy, says Lamberson. "The Block 2004 Airborne Laser is being designed to have a takeoff weight of about 300 tonnes; the maximum takeoff weight is 320 tonnes," he said. "Throughout the design process, we have worked aggressively to control weight."For example, one of the main challenges in the development of the COIL laser was providing lightweight components, such as the turbo pump used to pump the laser fuel. The pump developed by the Airborne Laser team "can fit on your desk and fill your backyard swimming pool in less than 10 minutes", according to Lamberson.

He adds, however, that space and weight constraints have not been the biggest issues. "Our challenge for all four lasers has been to provide the power and beam-quality needed," he said.

The Airborne Laser is currently in the programme definition and risk reduction phase. This phase will culminate in the shoot-down of a missile in late 2003 or early 2004. The plan is then to construct seven Airborne Laser aircraft.

And it seems that the program is set to meet its deadlines. "The tests performed to date have shown that the laser module exceeds the design performance in terms of laser power. We have also successfully tested the three other lasers. This year we will be testing each of the laser's segments before final integration on the aircraft," said Lamberson. "It will be a big year for us."

Airborne Laser www.airbornelaser.com