By Dr. R.P. Fischer, Dr. A. Ting, and Dr. P. Sprangle
Advances in laser and photovoltaic converter technologies may allow high-power wireless recharging of platforms and sensors at extended ranges. These remote platforms may include flying, land-based, or submerged vehicles, satellites, and sensors at hazardous locations. Commercially available fiber lasers have proven to be an enabling technology in a variety of industrial applications such as cutting, welding, and annealing. These lasers also are having a positive effect in many defense programs—for instance, the Navy’s laser weapon system has recently deployed aboard USS Ponce (AFSB[I] 15) for at-sea testing. In addition, commercially available photovoltaic converters have advanced to a point where high conversion efficiency from laser energy to electrical energy is now possible.
In the 1890s, Nikola Tesla performed some of the first experiments demonstrating wireless recharging—sometimes called power beaming—using high-frequency electromagnetic radiation. Microwaves have been used since in short-range wireless recharging experiments because of their high power-conversion efficiency. For long-range recharging, however, large transmission and receiving antennae are required because of the longer wavelengths associated with microwave radiation. Practical long-range recharging only can be achieved using the significantly shorter wavelengths associated with laser beams. The use of lasers can significantly reduce the size and weight of the transmitting and receiving platforms.
Research groups at the National Aeronautics and Space Administration, Kinki University in Japan, LaserMotive Inc., and the Naval Research Laboratory have experimented with wireless recharging using a variety of platforms such as rovers, kite planes, helicopters, and climbers using solid-state lasers and photovoltaic converters. One configuration for remote wireless recharging of an unmanned aerial vehicle (UAV) involves using a high-power, continuous wave fiber laser and beam director. Disturbances in the atmosphere (i.e., turbulence and aerosols) will affect the laser’s power propagation and delivery to the platform and may require adaptive optics for correction.
High-Power Laser Wireless Recharging
Wireless recharging using high-power lasers has been realized because of the improved output power, efficiency, and reliability of commercially available high-power lasers, power-conversion efficiencies as high as 60 percent in photovoltaic converters, and multikilometer propagation of lasers in the atmosphere. Laser recharging can be used to power UAVs, resulting in increased flight duration, reduced battery weight and manpower requirements, and increased power capabilities of vehicles and payloads. Wireless recharging also can be used to provide electrical power to small UAVs for missions such as persistent surveillance and security, communications relay, off-board decoys, electronic warfare, target acquisition, and reconnaissance of remote or hazardous areas such as forward operating bases.
High-power fiber lasers now are commercially available for directed-energy applications. These lasers are made with active optical fibers and semiconductor diodes, a merger between two of the most innovative and advanced laser technologies. Fiber lasers use single-emitter semiconductor diodes as the light source to pump the active fibers. The beam emitted is contained within optical fibers and delivered through an armored flexible cable. Special optical fibers doped with rare earth ions permit kilowatt levels of high-quality laser power to be generated. These fiber lasers are compact, have long diode lifetimes, low maintenance operation, high wall-plug efficiency, and minimum beam divergence. For example, ytterbium-doped fiber lasers are commercially available with wall-plug efficiencies up to 50 percent. The state-of-the-art power levels for these lasers are 10 kilowatts continuous wave for single-mode operation and 100 kilowatts for multimode operation.
Naval Research Laboratory scientists hold the first patent on the laser-beam-combining architecture used by the Navy (“Apparatus for Incoherent Combining of High-Power Lasers for Long-Range Directed Energy Applications,” U.S. Patent No. US 7,970,040 ) and were the first to demonstrate high-power continuous wave (greater than four kilowatts), single-mode, fiber laser beam propagation in the atmosphere over extended distances (greater than three kilometers). These high-power fiber lasers are particularly well suited for remote wireless recharging.
Most photovoltaic cells are designed and developed for the conversion of the broad spectrum of solar energy into electrical power. The cell delivers the maximum optical-to-electrical conversion efficiency when illuminated by monochromatic (laser) light at a wavelength that closely corresponds to the bandgap energy of the photovoltaic material. Efficient cells based on indium gallium arsenide (InGaAs) are now available commercially for wavelengths near one micron. An optimized photovoltaic converter on a remote platform, such as a UAV, can efficiently convert (about 50—60 percent) laser energy to electrical power. A wireless recharging architecture using fiber lasers and photovoltaic cells can provide a significant weight reduction by removal of batteries, extended flight duration, and increased range.
Wireless Recharging Experiments
The Naval Research Laboratory has successfully demonstrated the recharging of a UAV in flight using a kilowatt-class fiber laser to transmit power and a photovoltaic cell for collection. The photo above shows a panel of highly efficient InGaAs cells on a laser converter panel. When illuminated with a high-power fiber laser, it converts the laser power to the electrical power required to power the twin propellers. The cells are connected in a configuration that matches the current and voltage characteristics required to drive the propellers. The converter has fins on its back to allow cooling by the downdraft of the propellers.
In 2013, a series of flight tests were conducted over a 40-meter laboratory range. A two-kilowatt, singlemode fiber laser (about one micron wavelength) transmitted power to an array fabricated using InGaAs laser-power converter chips from Spectrolab Inc. mThe individual chips are up to 50 percent efficient at the fiber laser wavelength, and the lightweight array provides 160-190 watts of electricity to the vehicle. Off-the-shelf components were used to develop the optical tracking system, which automatically positioned the laser beam on the center of the laser converter during flight. In the experiment, the remotely controlled vehicle was able to lift off from rest on command. The laser converter lights up (in the inset on page 23) because the laser light, though in the infrared, is visible to the digital camera.
There are a number of challenges to address before long-range wireless recharging can be deployed. These include thermally managing the excess heat generated on the photo-voltaic converter and developing higher-efficiency cells capable of more than 60 percent conversion. It is necessary to have a fairly uniform and controlled laser intensity profile on the photo converter. This may require the development of appropriate adaptive optics techniques applied to the outgoing laser beam.
For extended ranges, application of adaptive optics is necessary to control spreading and wandering of the transmitted laser beam as it propagates through atmospheric turbulence. Adapted optics can be implemented by employing a beacon laser beam (low power) on the receiving platform to determine the phase variations placed on the beam because of atmospheric turbulence. Introducing the conjugated phase variation on the high-power outgoing beam will minimize the effects of atmospheric turbulence. Development of efficient, high-power, eye-safer lasers and photovoltaic converters at wavelengths greater than 1.4 microns may be necessary for certain applications. These challenges are of a technological nature, however, and can be overcome in the near term.
Wireless recharging can be an enabling technology that would allow new operational capabilities for the Navy. In addition to UAVs, other unmanned systems stand to benefit from this technology. These include ground vehicles and underwater vehicles. Atmospheric conditions for land-based vehicles, however, are quite different from those for UAV recharging. This is because of the high concentration of scattering particles and increased turbulence affecting laser propagation near the ground. Underwater remote wireless recharging requires laser wavelengths in the blue-green regime.
Wireless recharging of low flying satellites also could be viable to maintain their orbits. Atmospheric turbulence and scattering fall off extremely rapidly as a function of altitude. Hundreds of kilowatts of laser power can be delivered to the satellite at the perigee point (the lowest height position, about 100 kilometers) of the elliptical orbit. The laser intensity on the satellite’s photovoltaic cells can be several hundreds of kilowatts per square meter, which is hundreds of times the sun’s intensity.
About the authors:
Dr. Fischer is senior research engineer in the Plasma Physics Division at the Naval Research Laboratory.
Dr. Ting is section head and experimental group leader in the Plasma Physics Division at the Naval Research Laboratory.
Dr. Sprangle is senior scientist for directed energy physics in the Plasma Physics Division at the Naval Research Laboratory and professor of electrical and computer engineering and physics at the University of Maryland.
The authors would like to acknowledge that funding for this work was provided by the Naval Research Laboratory, and acknowledge the contributions from Greg DiComo and Steve Tayman.