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System Moves Light With Electrons, Not Gears

May 2006
By Henry S. Kenyon

 
Advanced sensor systems used on platforms such as a U-2 produce so much data that they cannot transmit it all in real time using radio frequency communications equipment. Raytheon is working with the U.S. Air Force to develop an airborne laser terminal that would allow reconnaissance platforms to beam high-bandwidth information streams to command and control aircraft or to satellites.
Nonmechanical beam steering system controls, focuses multiple beams with no moving parts.

Several decades from now, a U.S. unmanned combat aircraft orbiting a battlefield will identify a ground target with its sensors and use its communications laser to beam the coordinates to an overhead satellite. After receiving target confirmation from analysts on the other side of the planet, the aircraft will bank sharply, refocus its optical communications array to weapons mode and destroy the target with a multi-kilowatt laser pulse. The system will then revert to its data transmission mode to uplink a battle-damage assessment. This may sound like science fiction, but recently developed technology that electronically moves and focuses lasers may one day make this scenario a reality.

The U.S. Defense Department is exploring the use of steered agile beam (STAB) technology to direct and focus high-bandwidth communications lasers on airborne platforms. Invented and developed by Raytheon    Company, Waltham, Massachusetts, the STAB concept was validated as part of a Defense Advanced Research Projects Agency (DARPA) program from 2000 to 2004, explains Dr. Terry Dorschner, a Raytheon principal   fellow scientist, based in Marlborough, Massachusetts.

Dorschner notes that Raytheon had studied the technology for some 15 years before the DARPA effort. STAB is an inertialess all-electronic beam-control system that allows rapid and accurate beam movement over a relatively large area—up to 45 degrees. Large coverage arcs and high accuracy are required for airborne and space-based optical communications systems. He adds that it is an entirely new technology. “When we conceived this a couple of years ago, it was and still is the first demonstration of this kind of electronic beam control over large angles. No one in the world has ever done this before,” Dorschner maintains.

STAB is built around optical phased arrays (OPAs), which are liquid crystal panels that electronically steer a laser beam. The arrays operate like a microwave phased array antenna and can steer multiple beams. Raytheon engineers developed this technology to provide electro-optic sensors with the same advantages of radio frequency phased arrays. The DARPA program allowed Raytheon scientists to combine OPAs with a refractive grating technology so the system can electronically steer beams over large angles. It was the prospect of taking this additional developmental step that motivated DARPA to establish the STAB program, Dorschner says.

Although STAB uses microelectronics, it is not a microelectromechanical systems (MEMS) technology but rather a blend of flat panel display and integrated circuit manufacturing techniques. A key to increasing the performance of electronic beam steering devices is the use of higher performance liquid crystals. Raytheon has worked closely with universities and other companies to invent new molecules, to identify those with the ability to function well as switches and to verify their switching speeds as well as their ability to operate in very thin films. “What makes this approach possible are the liquid crystals that we use. The only motion in the system is rotation of the molecules in the liquid crystal,” Dorschner explains.

Some organizations have developed MEMS-based beam steering devices, but Dorschner notes that these technologies were developed for very short-range applications such as optical mail readers. STAB is more effective for the free space, long-range optics required by most Defense Department laser communications applications. Because many MEMS mirrors are very small—typically 50 microns in diameter—they will cause a laser beam to have a divergence angle of more than 30 degrees. “You can’t get a sharp beam coming out of a hole or reflecting out of a mirror that small,” he maintains.

Free-space optics require large mirrors and apertures and the ability to steer large beams to focus on a small point. Dorschner believes that the only way to use MEMS technology in this field is in extremely large arrays. But there are manufacturing issues associated with the technology. He notes that when MEMS systems are made, mirrors do not point in the same direction. The average pointing direction of the millions of microscopic mirrors used for commercial digital projection television sets has a variation of one degree from mirror to mirror across the entire surface. “We need [it to be] a thousand times better than that,” he says.

Laser communications and directed energy weapons also require large amounts of energy. MEMS systems cannot handle high operating temperatures without melting. Liquid-crystal-based systems can function at hotter temperatures and can pass much higher beam energies without damage.

It is theoretically possible for future OPA-based systems to be multifunctional—able to switch between communications and weapons modes, for example. However, before this long-term goal can be achieved, Raytheon engineers must master the high energy in communications and   countermeasures applications so they can all be combined into one truly multipurpose system, explains John Kinsley, manager of Raytheon’s Advanced Programs division, Marlborough, Massachusetts.

OPAs are an attractive option for space-based communications because they have no moving parts and can support transmitting high-bandwidth data feeds. “Lasers will make it onto satellites because the bandwidth justifies it. And when they do, will they displace RF [radio frequency] systems? I doubt it. I think they will complement and supplement them. Will they be lighter than the RF systems? Sure, if your metric is bandwidth—even if they are in fact heavier—because you’ll certainly get more bandwidth for your pound,” Kinsley says.

The U.S. Air Force is working with Raytheon to develop an airborne laser terminal (ALT) using OPA technology. Kinsley notes that some reconnaissance platforms such as U-2s have onboard sensors that produce more information than can be transmitted in real time via radio frequency datalinks. While this data can be stored on tape or other magnetic media for study after a mission, he notes that the military wants this information as quickly as possible and that analysts can access only a small fraction of these data streams in real time. “The crunch is bandwidth for sensor data,” he says.

Laser communications is attractive because its bandwidth capabilities greatly exceed those of the radio frequency spectrum. The ALT is envisioned as operating on several types of platforms such as U-2s as well as command and control aircraft. The reconnaissance platforms will generate the intelligence data, while the larger aircraft will receive the information flow.

As a part of the Air Force’s transformational communications efforts, the ALT will serve as a high-bandwidth data conduit between aircraft and satellites. However, Kinsley adds that the terminal is still in the proof-of-concept stage and not quite ready for full-scale development.

Raytheon built a prototype ALT in its Andover, Massachusetts, integration laboratory and put the system through a series of tests in February to demonstrate its ability to operate in a simulated aircraft environment. Vibration is a major obstacle for airborne optical communications. Kinsley explains that radio frequency communications between aircraft and satellites has a greater margin for error. “If you shake a directional antenna around, it’s a bit challenging to keep it pointed in inertial space [an environment where there is no frame of reference to establish a fixed set of coordinates]. But the accuracy at which you have to do that is no more than some fraction of a beam width,” he says.

For example, even a fairly small radio frequency beam may be several degrees or a degree in diameter. A typical radio system has to be a tenth of a degree or several tenths of a degree on target to maintain communications. By comparison, the laser beam widths for the ALT are very small. “They [the beams] don’t diverge and splatter all over space. If they did, they’d be as easy to point as an RF terminal, but the power drop-offs would be so fierce that they wouldn’t be very useful. The advantage of the laser is that it keeps the energy focused in a very small divergent beam, and you wind up with beam widths that are horrendously tiny. At great distances, this means you have a tremendously difficult angular accuracy to hold to keep a beam on target,” Kinsley says.

While the divergence of a radio frequency beam is a few tenths of a degree, an optical beam is typically three orders of magnitude smaller. Dorschner explains that this size scales with the wavelength of the communications medium and that the wavelength of light is three to four orders of magnitude smaller than radio frequency wavelengths. “It means that it’s about a thousand times harder to get the darn thing on target,” he says.

It also is difficult for mechanical laser aiming systems to maintain the accuracy levels required for transformational communications. Gimbal-based systems are subject to vibration and thermal expansion that prevent high accuracy. Kinsley notes that electronic systems are now vastly more precise than mechanical aiming devices.

Raytheon’s ALT prototype is a hybrid of OPA technology and a mechanical system. Although the DARPA STAB program demonstrated the ability to create a 45-degree field of regard, the Air Force requires a greater arc for an airborne terminal. Company engineers approached this requirement by combining a mechanical system using rotating prisms to deflect the laser beam in multiple stages while the OPAs provide fast, fine-tuned beam steering.

The process of locking onto an aircraft or satellite for laser-based communications is similar to radio frequency methods. The signal of the chosen platform first must be acquired. With radio systems, once the recipient platform is located and its communications frequency recognized, a brief exchange of information packets usually occurs to establish protocols to maximize bandwidth capabilities.

Laser communications terminals follow a similar process. A transmitting platform must first locate a satellite’s light beam, lock onto it and establish communications protocols. While the exact details of this process are classified, it involves one platform generating a larger beam via electronic beam control, making it easier for the other platform to see the beam and lock onto it. After the initial data handover, the beam is focused down to communications size. Kinsley notes that one of the advantages of optical phased arrays is that they can be rapidly defocused. “You don’t need bandwidth when you’re doing an acquisition. So you can defocus the beam and cover a lot more of that angular region of free space and search in cells. You can do this with a smaller beam, but it would take a long time,” he says.

Another OPA-based application is for the gun sight of the U.S. Army’s advanced crew-served weapon. This is a dirty and vibration-intensive environment where systems with moving parts often fail. Kinsley adds that any system requiring electronic steering could benefit from OPA technology. Other potential uses are in airborne navigation systems that use stars as fixed reference points. This equipment is aimed with mechanical actuators and could benefit from a transition to a fully electronic system, he speculates.

OPAs also have advantages for commercial communications applications such as fiber optic systems connecting the back plane of a computer for high-data-rate systems for input-output. Raytheon developed an OPA-based multiplexing system targeted at the telecommunications market in the 1990s, but the collapse of the telecommunications industry severely limited its economic potential. Dorschner believes that when the industry fully recovers, it could provide commercial markets with extremely large cross-connect systems containing up to 1,000 output and 1,000 input ports.

 

Web Resources
Raytheon Company: www.raytheon.com
Defense Advanced Research Projects Agency STAB: www.darpa.mil/mto/stab