Airborne laser ready for flight tests

IT SHOULD be the moment of truth for the Airborne Laser (ABL). In the coming months, the multibillion-dollar laser built into a customised Boeing 747 will try to shoot a ballistic missile as it rises above the clouds.

Don't expect instant reports of success, though. Instead, if all goes to plan, we're likely to hear about a series of incremental improvements.

Developed by the US Department of Defense's Missile Defense Agency (MDA), the ABL aims to focus a beam of laser energy in the megawatt range for several seconds onto a missile at a "militarily significant distance" - more than 100 kilometres.

So far, the laser has only operated at near full power on the ground. On 18 August it was fired successfully from the air, but at reduced power. That, however, was no mean feat: aircraft vibrations play havoc with the precisely aligned optical components needed to generate a laser beam.

Firing at full power poses other challenges too. At powers high enough to destroy missiles, any surface contamination or tiny flaw in the laser optics can absorb so much heat that they crack or shatter.

High-power laser beams also heat the air they pass through, creating perturbations that can disperse or divert the beam. To counteract those effects, the ABL uses an adaptive system that senses atmospheric changes along its path and makes optical adjustments to compensate.

To test that system, the MDA plans a series of increasingly powerful shots at modified ballistic missiles loaded with sensors to measure the distribution of laser power on the target. Engineers will assess each shot's performance and use the results to fine-tune the adaptive optics. Once this is done, the MDA will test the laser again in varying conditions, and attempt to destroy actual missiles. The first of these tests is planned to take place late this year, with two more to follow in early 2010, according to an MDA spokeswoman.

A sister project, the Advanced Tactical Laser, which aims to use an airborne high-powered laser to hit targets on the ground, recently completed its first successful test. With future funding dependent upon the success of these tests, the pressure is on the ABL team to prove its efficacy.

Lasers Generate Sound in H2O

A new technology that uses flashes of laser light to remotely create underwater acoustics is being developed by scientists at the Naval Research Laboratory. The new acoustic source has the potential to expand and improve both Naval and commercial underwater acoustic applications, including undersea communications, navigation and acoustic imaging.

Dr. Ted Jones, a physicist in the Plasma Physics Division, is leading a team of researchers from the Plasma Physics, Acoustics, and Marine Geosciences Divisions in developing this acoustic source.

Efficient conversion of light into sound can be achieved by concentrating the light sufficiently to ionize a small amount of water, which then absorbs laser energy and superheats. The result is a small explosion of steam, which can generate a 220 decibel pulse of sound. Optical properties of water can be manipulated with very intense laser light to act like a focusing lens, allowing nonlinear self-focusing (NSF) to take place.

In addition, the slightly different colors of the laser, which travel at different speeds in water due to group velocity dispersion (GVD), can be arranged so that the pulse also compresses in time as it travels through water, further concentrating the light. By using a combination of GVD and NSF, controlled underwater compression of optical pulses can be attained.

The driving laser pulse has the ability to travel through both air and water, so that a compact laser on either an underwater or airborne platform can be used for remote acoustic generation. Since GVD and NSF effects are much stronger in water than air, a properly tailored laser has the ability to travel many hundreds of meters through air, remaining relatively unchanged, then quickly compress upon entry into the water. Atmospheric laser propagation is useful for applications where airborne lasers produce underwater acoustic signals without any required hardware in the water, such as undersea communications from aircraft.

Also, commercially available, high-repetition-rate pulsed lasers, steered by a rapidly movable mirror, can generate arbitrary arrays of phased acoustic sources. On a compact underwater platform with an acoustic receiver, such a setup can rapidly generate oblique-angle acoustic scattering data, for imaging and identifying underwater objects. This would be a significant addition to traditional direct backscattering acoustic data.

For more information, visit: www.nrl.navy.mil

Laser pulses control single electrons in complex molecules

Physicists of the Max Planck Institute of Quantum Optics (MPQ) in Garching and chemists of the Ludwig-Maximilians-Universität (LMU) in Munich, Germany, succeeded for the first time to use light for controlling single, negatively charged elementary particles in a bunch of electrons. The scientists achieved a major milestone that they aimed for within the excellence cluster "Munich Center for Advanced Photonics" (MAP). They report their results in the journal Physical Review Letters.

Electrons are extremely fast moving particles. In atoms and molecules they move on attosecond timescales. An attosecond is only a billionth of a billionth of a second. With light pulses that last only a few femtoseconds down to attoseconds it is possible to achieve control over these particles and to interact with them on the timescale of their motion. These short light pulses exhibit strong electric and magnetic fields influencing the charged particles. A femtosecond lasts 1000 times longer than an attosecond. In molecules with only a single electron, such as the deuterium molecular ion, their control with such light pulses is relatively easy. This was demonstrated in 2006 by a team of physicists including Professor Marc Vrakking and Dr. Matthias Kling from AMOLF in Amsterdam and Professor Ferenc Krausz in Garching (MPQ).

Scientists led by the junior research group leader Dr. Matthias Kling (MPQ) in collaboration with Professor Marc Vrakking (AMOLF) and Professor Regina de Vivie-Riedle (LMU) have managed to control and monitor the outer electrons from the valence shell of the complex molecule carbon monoxide (CO) utilizing the electric field waveform of laser pulses. Carbon monoxide has 14 electrons. With increasing number of electrons in the molecule the control over single electrons becomes difficult as their states lie energetically very close to each other.

In their experiments the scientists used visible (740 nm) laser pulses with 4 femtoseconds duration. The control was experimentally determined via an asymmetric distribution of C+ and of O+ fragments after the breaking of the molecular bond. The measurement of C+ and O+ fragments implies a dynamic charge shift along the molecular axis in one or the other direction, controlled via the laser pulse.

The femtosecond laser pulses initially detached an electron from a CO molecule. Subsequently the electron was driven by the laser field away from and back to the ion, where it transferred its energy in a collision. The whole process took only ca. 1.7 femtoseconds. "The collision produces an electronic wave packet which induces a directional movement of electrons along the molecular axis," says Regina de Vivie-Riedle. "The excitation and subsequent interaction with the remainder of the intense laser pulse leads to a coupling of electron and nuclear motion and gives a contribution to the observed asymmetry," explains Matthias Kling.

The scientists could also image the structure and form of the outer two electron orbitals of carbon monoxide via the ionization process. The extremely short femtosecond laser pulses allowed the scientists to explore this process in the outermost orbitals. They found the ionization of the molecules to take place with a distinct angular dependence with respect to the laser polarization direction. This observation was found to be in good agreement with theoretical calculations and also gave a contribution to the observed asymmetry. The scientists could show that the strength of this asymmetry strongly depends on the duration of the laser pulses.

With their experiments and calculations, the researchers from Garching and Munich have achieved an important milestone that they aimed for within the excellence cluster "Munich Center for Advanced Photonics" (MAP). The goals were to achieve and observe the control of single electrons within a multi-electron system.

Electrons are present in all important microscopic biological and technical processes. Their extremely fast motion on the attosecond timescale, determines biological and chemical processes and also the speed of microprocessors - technology at the heart of computing. With their experiments the researchers have made a further, important step towards the control of chemical reactions with light. The results are also related to basic research on lightwave electronics aiming at computing speeds on attosecond timescales.

More information: Znakovskaya, P. von den Hoff, S. Zherebtsov, A. Wirth, O. Herrwerth, M.J.J. Vrakking, R. de Vivie-Riedle, M.F. Kling: “Attosecond control of electron dynamics in carbon monoxide”, Physical Review Letters (4. September 2009)