Navy laser weapon deployment

The U.S. Navy will deploy a high-energy laser weapon on the USS Ponce in fiscal 2014, chief of Naval Research Rear Admiral Matthew Klunder announced April 8, 2013 at the Sea-Air-Space exposition. The Navy Laser Weapon System (LaWS) will be the first high-energy laser deployed for field use by the armed services. The Navy has tested the laser system against its prime targets, moving small surface boats, and remotely piloted vehicles.

The at-sea deployment comes two years earlier than the Navy had planned. That may be a first in laser weapon development, where schedule slippage and cost overruns have been common. The New York Times reports LaWS cost just under $32 million, roughly two orders of magnitude less than the Airborne Laser, dropped from the fiscal 2011 budget after it failed to reach the required 200 km range.

NAVY LaWS on board a ship during tests of the laser weapon. (Image courtesy of the US Navy)

LaWS is part of the new generation of electrically powered solid-state laser weapons, which Navy officials say offer two advantages. One is a "deep magazine," the ability to fire pulses as long as electrical power is available--and ships have plenty of power. The other is cost. Klunder said, "Our conservative data tells us a shot of directed energy costs under $1," compared to $100,000 or more to fire a missile.

The choice of LaWS marks a big success for fiber lasers. When the Pentagon launched the Joint High Power Solid-State Laser (JHPSSL) program in 2002, developers focused on diode-pumped slab lasers, which at the time seemed the technology most likely to reach the 100 kW sought for defense against rockets, artillery, and mortars. JHPSSL reached that level in 2009, but fiber lasers have been catching up. The Naval Sea Systems Command reached 30 kW by combining the beams from six 5.5 kW industrial fiber lasers to shoot down a drone in 2010. LaWS has been upgraded since then, but Navy officials did not disclose the output power of the current system.

The laser is not the only challenge. For the current version of LaWS, L-3 Integrated Optical Systems (Pittsburgh, PA) upgraded the pointing and tracking system, improving accuracy of the fine steering mirror and controls, and improving the software and user interface. "We took scientists out of the loop to make it operable by seamen," said Don Linnell, director of business development and strategy. The Navy considers that a must for fielding laser weapons.

Directed self-assembly

One reward of exploring "Photonic Frontiers" every month for Laser Focus World is discovering new and emerging technologies that could have important impact. In investigating extreme-ultraviolet (EUV) lithography for my May Frontiers article, I discovered an intriguing concept called "directed self-assembly" which has surfaced since I last covered EUV development four years ago. Practical applications of directed self-assembly remain a ways off, but it could be crucial to sustaining the Moore's Law trend of shrinking electronic components on semiconductor chips.

Simple self-assembly builds structures from the bottom up. On a nano-scale, it starts with molecular building blocks that assemble themselves into larger structures. An example is atoms or molecules adding themselves to bonding sites on the edge of a growing crystal. DARPA has studied ways to self-assemble small building-block modules into robots that could reassemble themselves in different configurations for other purposes, like how children reassemble Lego blocks into new structures.

Robotic modules can be programmed to build desired structures, but external controls are needed to make atoms and molecules grow specific nanostructures. Directed self-assembly does that by applying forces from the top down to control assembly. For making semiconductor chips, the top-down control would come from patterns written by the photolithographic light source onto the material.

Dan Herr became intrigued by the idea of the functional self-assembly of materials while working on lithographic photoresists at Research Triangle Park, NC-based Semiconductor Research Corp. Resists are central to photolithography, and their chemistry can limit the minimum feature size, edge roughness, and writing speed. Conventional resists were designed to match visible and near-ultraviolet light sources, but EUV lithography poses additional challenges because the photons carry an order of magnitude more energy, enough to blast cascades of electrons from the resist.

"Directed self-assembly is a replacement for conventional resists," says Herr, who is now developing the materials at the University of North Carolina, Greensboro (Greensboro, NC). It's based on combining two polymers--one water-soluble, the other oil-soluble--which arrange themselves in regular patterns so the oil- and water-soluble parts can keep apart from each other. Light sources then direct their assembly by writing lines on the substrate for the polymers to start building upon.

Directed self-assembly involves three steps: writing a pattern, depositing the two block copolymers, and removing one to form a pattern. (Courtesy of Wikipedia)

The short-term focus is finding an alternative to conventional photoresists. But Herr says "the holy grail would be materials that self-assemble into shapes and structures that define active components of the circuit." That could change the rules for light sources as well as lithography.

Coherent single photons

Stimulated emission makes lasers excellent sources of large numbers of coherent photons, which is fine for most applications. But quantum information networks are a problem because they work best with coherent photons that come one at a time, and lasers generally are not amenable to generating single photons. Single-photon sources have been developed for quantum computing, but they lack the coherence needed to create quantum entanglement at a distance using quantum entanglement.

Now, a team at the Cavendish Laboratory at Cambridge University (Cambridge, England) led by Mete Atature has found a way to generate single photons with laser-like coherence. Their starting point was optical pumping of quantum dots, which is one way of producing single photons. They first fabricated a Schottky diode containing self-assembled indium-arsenide quantum dots, which could be individually addressed with a pump laser to generate single photons by resonance fluorescence. Resonant fluorescence does not optically excite the host material, reducing interactions in the solid that decrease coherence of emitted photons, but charge fluctuations and other interactions remain to degrade coherence.

In Nature Communications they report avoiding photon decoherence by weak laser excitation, which generates photons primarily by elastic scattering. This avoided charge fluctuations, and allowed them to generate single photons from one quantum dot that remained coherent with the excitation laser for more than three seconds. Taking advantage of this mutual coherence, they report they could "synthesize near-arbitrary coherent photon waveforms by shaping the excitation laser field." That, in turn, let them show that as long as the photons emitted by the quantum dot remained coherent with the pump laser field, the separate photons were "fundamentally indistinguishable," so quantum interference among them can create quantum entanglement at a distance. That makes it possible to combine quantum computing with quantum communications, producing a more powerful tool for tasks such as quantum cryptography.

Ways to encode a qubit.

"The ability to generate quantum entanglement and  perform quantum teleportation between distant quantum-dot spin qubits with very high fidelity is now only a matter of time," says Atature. That's still a long way from science-fiction teleportation. However, the ability to generate single photons that maintain coherence well enough that they can be combined to produce novel waveforms may lead to real-world capabilities almost as attractive as avoiding airport lines.

Laser asteroid defense

Could lasers protect the Earth from wayward asteroids? A number of schemes have been proposed for pushing asteroids gradually to move their orbits away from the planet. Now, two California professors are proposing a bold scheme to build solar-powered space lasers powerful enough to evaporate a 500 m asteroid in about a year--or to make short work of a 17 m asteroid like the one that exploded near Chelyabinsk, Russia, on February 15.

Philip Lubin of the University of California (Santa Barbara, CA) and Gary Hughes of California Polytechnic State University (San Luis Obispo, CA) began planning the project they call DE-STAR--for Directed Energy Solar Targeting of Asteroids and exploRation--a year ago. On February 14, they issued a press release timed to the close approach by asteroid 2012 DA14. They were as stunned by the Russian explosion as everyone else.

Their bold proposal seeks to take advantage of the dramatic improvements in high-power diode lasers and solid-state lighting to build giant orbital phased arrays of lasers powered by electricity from huge solar panels. They envision starting with a desktop 1 m array called DE-STAR 0, then scaling up to a 10 m array called DE-STAR 1. They have proposed that NASA support a conceptual study of scaling up to a 10 km DE-STAR 4 array, powerful enough to vaporize a half-kilometer asteroid 150 million kilometers away. Even bigger versions could be used for laser propulsion; they estimate that a 1000 km DE-STAR 6 array could accelerate a 10 ton spacecraft close to the speed of light.

Future DE-STAR array samples composition of an asteroid as it propels an interplanetary spacecraft. (Courtesy of Philip Lubin)

The scheme may sound fantastic, but Lubin says it violates no laws of physics and requires no "technological miracles." It merely envisions continuing technological progress at the rate of the past 50 years, which took us from the feeble LEDs and diode lasers of 1963 to today's powerful emitters. They assume photovoltaic cells that can convert 70% of incident solar energy into electricity, and diodes which can convert 70% of the input electrical power into light.

Lubin doesn't think it will be easy. He worries about issues including the mass needed to build the giant array, and controlling output phase across the array with the precision needed to tightly focus the emission. But he predicts his assumptions will be considered "extraordinarily conservative and modest" in 30 to 50 years.

That remains to be seen, but space-based solar-powered diode arrays are worth investigating. They could go beyond asteroid defense to could help move asteroids, collect valuable materials from them, or provide power resources in space--as well as inspiring some fun science-fiction stories.

New visions for space telescopes

NASA stumbled into a rare bit of good luck recently when the National Reconnaissance Office did some housecleaning. NRO decided that a pair of space-qualified 2.4 m telescopes dating from the late 1990s were no longer suitable for their original mission in spy satellites. So NRO offered the surplus optics to one of its poorer relations, NASA, for use in new space-based instruments. 

Unexpected hand-me-downs can bring opportunity, like the the piles of Scientific American and Sky & Telescope that came with a house my family rented when I was in high school. Astronomers and NASA scientists are pondering what to do with the windfall. The f/8 Cassegrain telescopes lack instruments, electronics, or spacecraft, but NRO long ago paid for the optics, saving NASA serious money. The Study on Applications for Large Space Optics workshop held February 5 and 6, 2013, in Huntsville, AL heard and discussed 34 proposals for building new instruments around the mirrors. They will be narrowed to six proposals and submitted to NASA management in May.

The range of ideas is impressive. Adaptive optics can do wonders on the ground, but ultraviolet astronomy must remain above the atmosphere, so three proposals call for studying the ultraviolet sky. Other common themes are spectroscopy, planetary science inside the solar system, and attempts to image challenging targets including extrasolar planets.

Bare-bones surplus telescopes inherited by NASA (Government work not subject to copyright)

Some proposals are intriguing. Alfred McEwen of the University of Arizona (Tucson, AZ) envisions the Mars Orbiting Space Telescope, and  Zachary Bailey of the Jet Propulsion Laboratory (Pasadena, CA) proposes "high-resolution surface science at Mars."  Rebecca Farr of the NASA Marshall Space Flight Center (Huntsville, AL) proposes using both mirrors as a deep-space binocular telescope stationed at the Lunar L2 Lagrange point.

Not everything is exactly a telescope. Abhijit Biswas of JPL wants to use a mirror as an optical communications node in space. J. H. Clemmons of the Aerospace Corp. (El Segundo, CA) wants to use one in a lidar to explore the Earth's thermosphere.  Richard Eastes of the University of Central Florida (Orlando, FL) has a plan for "Atmospheric TeleConnections on Earth."

There are plenty more listed on the program, and NASA will be recording the proceedings for later viewing. The ideas are not fully formed, of course, and some seem to duplicate others. But there are enough bright ideas to make one hope that NRO can find more goodies sitting in storage for its needy relatives.

Source:  http://science.nasa.gov/salso/telescope-characteristics-and-capabilities/

Broader view for adaptive optics

Adaptive optics has become standard on large ground-based telescopes because it offers far sharper images than otherwise obtainable. However, standard adaptive optics can compensate atmospheric turbulence only over small areas, so they don't let ground-based telescopes match the celestial panoramas imaged by the Hubble Space Telescope. Now a new generation of adaptive optics has demonstrated high-resolution imaging over a larger field of view with the Gemini South telescope in Chile.

Proposed more than a decade ago by François Rigaut, now at Australian National University (Canberra, Australia), the Gemini Multi-conjugate adaptive optics System (GEMS) uses five laser guide stars and three deformable mirror to measure atmospheric distortion and compensate for its affects. Sampling at 500 to 1000 Hz, GEMS can compensate for turbulence over an area of sky 16 times larger than previously possible.

The picture below tells the story, alternating images of the "Orion Bullets" region in the Orion Nebula taken with GEMS in December 28, 2012 and of the same region taken in 2007 with the previous-generation ALTAIR adaptive-optics system, which uses a single laser guide star. The larger field of view is 85 arcsec across. Without the adaptive optics, the telescope's resolution at the observation time was 0.8 to 1.1 arcsec. Adding GEMS improved resolution by a factor of ten to 0.084 to 0.103 arcsec.  The bright spots are "bullets" of gas ejected from the core of the nebula that are ripping through molecular hydrogen at speeds to 400 km/s, leaving behind wakes of hot hydrogen.

GEMS also benefits from processing enhancements, which use tomographic techniques to map air turbulence in three dimensions, and correct uniformly across the entire field of view. "This is huge when it's time for astronomers to reduce their data," says Adam Ginsburg, a graduate student at the University of Colorado (Boulder, CO), because observers often need to compare objects in the same field.

Field size has long been a crucial limitation on adaptive optics. The 85-arcsec width of the GEMS image still falls well short of the more than nearly 200-arcsec width of the Ultra Deep Field image taken by the Hubble Space Telescope, but it's an important step. With Hubble now well into its third decade in orbit, astronomers need new ways to study the depths of the sky from the ground.

Comparison of images of the same field in the Orion nebula recorded with GEMS and ALTAIR. The white "Orion Bullets" are fast-moving gas clouds leaving hot hydrogen in their wake. Their motion is fast enough to detect in the five years between the 2007 ALTAIR and the 2012 GEMS images.

Zero refractive index

The latest example of the amazing versatility of metamaterials is the demonstration of one that has a refractive index of zero, just reported in Physical Review Letters. Theorists had predicted the possibility of zero-refractive-index materials, and some similar effects have been reported, but the metal-clad glass waveguide developed by Albert Polman's group at the Center for Nanophotonics of the FOM Institute AMOLF (Amsterdam, Netherlands) with Nader Engheta of the University of Pennsylvania (Philadelphia, PA) is the first to have a near-zero index throughout.

Zero-index materials, like negative-index materials, do not occur in nature, but can be built by assembling subwavelength elements into a structure designed to have the desired characteristics. The left part of the figure shows an electron microscope image of the metamaterial, a small slab of glass encased in silver forming a waveguide 200 nm wide and 2 µm long. The strong interaction between the metal and the glass on that scale gives an entire waveguide an effective refractive index of 0 at 770 nm.

Electron microscope image of a zero-index waveguide, showing a silver-coated nanoscale glass slab 200 nm wide and 2 µm long. The images at right compare the standing-wave pattern visible in a 400-nm-wide tube which disappeared in a 190-nm-wide tube, showing the material has a refractive index of zero at 770 nm. (Courtesy of Albert Polman)

The phase velocity of light is the speed of light divided by the refractive index of the medium, so phase velocity should be infinite for a zero-index material. Similarly, wavelength in a zero-index material should be infinite because it equals the wavelength in vacuum divided by refractive index. To study how the light behaved, Polman and colleagues used a technique they had developed earlier called "cathodoluminescence spectroscopy" to examine light waves in waveguides at various widths. When the index was above zero in a 400 nm waveguide, the light formed standing waves showing normal light propagation, as shown in the figure. But for a 190 nm waveguide the index was near zero, and the standing waves disappeared, as shown at right in the figure, indicating nearly constant phase and nearly infinite phase velocity and wavelength through the waveguide.

Infinite phase velocity does not violate Einstein's cosmic speed limit because phase velocity cannot carry information. Group velocity, the speed of a modulated optical signal, decreases with the refractive index below one, eventually reaching zero for a zero-index material.

That's not all that happens. "As the index approaches n=0 the losses increase, damping out the waves. The index then becomes a complex number of which the real part is 0," Polman told me in an email. That means no light is left to travel at infinite speed after a short distance. Wenshan Cai of Georgia Tech, who wrote a Viewpoint for the online publication Physics, told me the light should travel about 50 to 100 µm--far enough to be useful in integrated optics, but not over macroscopic distances.

A 2011 report of zero refractive index was based on different physics, combining two photonic-crystal materials, one with positive index and the other with negative index, so the net phase advance through the entire structure is zero. A key difference is that the building blocks of photonic-crystal materials are large enough to be seen by the wave, typically half a wavelength, but those of metamaterials are much smaller, so the incident wave responds to it as if it was a bulk material.