NIF falls short of ignition

The National Ignition Facility (NIF) will not meet its goal of igniting a fusion plasma before the end of September, the Lawrence Livermore National Laboratory (Livermore, CA) said on Friday. A spokeswoman says Livermore "will continue working toward achieving ignition." The laser is delivering the desired energy, but the target shots are not yielding the expected fusion energy.

NIF was declared complete on March 31, 2009, after it had delivered 1.1 MJ pulses at 355 nm. The 192-beam system was designed to deliver 1.8 MJ pulses, which simulations indicated would be sufficient to ignite a pellet of deuterium-tritium fusion fuel, producing fusion reactions that yielded more energy than the input pulse. The Department of Energy set a target of reaching ignition by September 30, 2012--the end of the fiscal year.

Wary of optical damage, Livermore ramped pulse power and energy slowly. The first 1.8 MJ pulse was not fired until March of this year. On July 5, NIF delivered peak power of 500 tW to a target for the first time in a 1.85 MJ pulse. From outside, it looked like NIF should be closing in on ignition.

But now NIF has become the latest in a long list of fusion lasers that yielded experimental results well short of predictions. A news story in the September 21 issue of Science magazine reports that although computer models predict NIF shots should achieve ignition, the yield of fusion energy from NIF experiments has so far reached only 0.1 of the ignition level.

The National Nuclear Security Administration (NNSA) has already begun studying its options. The first draft of a report is due October 1, with a final report due to Congress on November 30.

Meanwhile, NIF continues firing shots that can produce temperatures and pressures far beyond anything previously possible on the surface of the Earth. Livermore fusion researchers will keep pressing for ignition, and NNSA weapon scientists will get additional shots for their simulations of nuclear explosions as part of the agency's Stockpile Stewardship program.

NIF's laser bay, showing 96 of the 192 beamlines.

Light guides light up 3M solid-state bulbs

3M has added a new twist to solid-state lighting--embedding light guides in the outer shell of the bulb to redistribute light emission evenly across its surface like the venerable frosted-glass incandescent bulb.

Solid-state lighting has been widely touted for its outstanding energy efficiency. LED bulbs now in hardware stores draw 13 W of electric power, emit as much visible light as 60 W incandescents, and have lifetimes of 25,000 hours, far beyond 1000-hour incandescents. But high prices and some subtle but significant problems are slowing their adoption.

The 3M bulb is aimed at one of those subtle problems. LEDs emit directionally from a small area. Hot filaments and fluorescent tubes are omnidirectional, and although filaments are small, frosted incandescent bulbs scatter the light so it seems to radiate from entire surface. Directionality is good news for applications that want light concentrated in one direction, such as street lighting outdoors and downlighting in homes and offices. But it can be a problem in light fixtures in the line of sight, especially when the light comes from a small area. An example is a non-name solid-state lamp I bought earlier this year from a big-box hardware store. Light comes from a small zone where blue LEDs and yellow phosphor are mounted, not from the bulb's frosted surface, producing an unpleasant glare.

Deep inside, the 3M bulb contains similar blue LEDs with yellow phosphors to generate directional white light. But instead of shining directly into the room, the light is coupled into light guides embedded in the bulb. Total internal reflection guides the light around the bulb to areas where the light is scattered out the surface and into the room, as shown in the figure. That reduces brightness to an acceptable level, making the bulb much more presentable in a light fixture.

The light guide in the 3M LED bulb carries light from the LED source to diffusing areas on the bulb surface. (Courtesy of 3M)

The bulb, shown in the photo below, can't be mistaken for an incandescent. It needs slits to dissipate heat, a cooling problem that it shares with other LED bulbs, and requires heat sinks that add to its environmental impact. But the design is an innovative step in the right direction, making LED lamps an attractive piece of decor rather than an efficient eyesore.

3M's Advanced LED light distributes light like an incandescent bulb. (Courtesy of 3M)

DARPA PULSE program

Ultrafast laser research has produced some elegant science, from slicing time into incredibly thin slivers to generating combs of frequencies uniformly spaced across a wide band of the spectrum. These capabilities, in turn, have led to a similarly wide range of applications, including transferring time and frequency standards, measuring short intervals of time, and producing pulses so short that they generate extremely high peak powers with only modest amounts of energy.

However, ultrafast lasers traditionally have been bulky and complex things, custom-assembled on optical tables and delicately aligned in a laboratory. That complexity makes it hard to realize many potential practical applications such as putting frequency combs in space to boost the precision of GPS systems or to measure stellar spectra with extreme precision. Now the Defense  Advanced Research Projects Agency (Arlington, VA) is trying to do something about the problem by creating the Program in Ultrafast Laser Science and Engineering.

DARPA is not the first to think of making smaller and more durable ultrafast lasers. I mentioned the need for "robust frequency combs" for telecommunications systems or space-based instruments in the January Photonic Frontiers. A web search four pages which include the phrase "rugged femtosecond laser," but all of them cite an Army contract awarded last year to Arbor Photonics. However, such references are few and far between, and Google could not find a single page using the phrase "rugged frequency comb" (or combs) when I was writing this blog.

Shrinking the size and improving the robustness of ultrafast lasers is a big challenge, but success could pay off in important ways. DARPA cites some potential military applications that require rugged sources. One is using the time stability of the microwave-band repetition rate of a femtosecond laser to greatly reduce the close-to-carrier phase noise in a microwave oscillator. Others include transferring time or frequency measurements across the spectrum, and generating high-flux isolated attosecond pulses. Civilian science and technology also would benefit from compact  sources of ultrashort pulses.

As is normal with DARPA, success is not guaranteed, but the payoff could be high. In fact, somebody at DARPA surely should have already earned credit in the Pentagon bureaucracy for exceptional skill in acronym creation. Program in Ultrafast Laser Science and Engineering neatly translates into an entirely appropriate acronym -- PULSE.

Naming nanolasers

My Photonic Frontiers article coming up in the September issue of Laser Focus World describes recent progress on nanoscale lasers, having volumes smaller than a cubic wavelength. Such emerging technologies are fascinating, but also raise a peculiar problem for those of us who write about them: what do we call the things?

Some groups call their nanoscale lasers "spasers," an acronym for Surface Plasmon Amplification by the Stimulated Emission of Radiation. Surface plasmons are involved in the process, and the catchy term has gained its own Wikipedia entry, some 266,000 hits in a web search, and a fair amount of press coverage even before a paper in the July 27 issue of Science. Score a few points for savvy PR.

But other researchers prefer more general terms like "nanolasers." One reason is that the acronym for spaser defines a specific process--surface plasmon amplification by stimulated emission of radiation. Yet it's not clear that all nanoscale lasers demonstrated so far rely in that process, and some researchers wonder how stimulated emission in a tiny piece of semiconductor can amplify a surface plasmon, which is a group of oscillating electrons on a conductive surface.

A second reason is more philosophical, that "laser" has become a generic term. As Shaya Fainman of the University of California at San Diego (La Jolla, CA) told me, "any time I see light amplification by stimulated emission, I call it a laser." By that logic, if a nanoscale device is amplifying light by stimulated emission, it's a laser.

There are points to be made for both sides, but there also is another dimension to the discussion--defining a new term can be part of claiming credit for a discovery. The International Commission on Zoological Nomenclature has elaborate rules on the proper naming of living or extinct animal species. No such rules exist in physics, so terms compete on their own merits. Interestingly, Gordon Gould's term "laser" won the popularity contest over Charles Townes' original suggestion of "optical maser," but the Nobel Prize went to Townes.

Who eventually will be credited with inventing nanoscale lasers remains to be determined. For now, I'm using "nanolaser" as a generic term for nanoscale laser, as I did in an earlier article. But I'm also watching for future developments in the fast-moving field.

3D falls flat for Olympics

The past few years have seen some impressive innovations in three-dimensional displays. New digital projectors have made 3D movies come alive in theaters, and high-resolution flat-panel displays can bring 3D television to homes. Digital image processing and 3D helped make Avatar the highest-grossing movie of all time. At the peak of 3D enthusiasm, some in Hollywood predicted that soon 3D production would become standard for movies.

Live sports was supposed to be the next great frontier for 3D, and Panasonic and Olympic Broadcasting Services sent crews to London to record 200 hours of the Summer Olympics in 3D. But the effort seems to have fallen flat. Chris Chinnock reports on Display Daily that the BBC logged an average UK audience of 24 million people for the opening ceremonies, only 111,000 households watched the 3D simulcast, a figure he called "pretty dismal." My own informal poll of a small newsgroup discussing the Olympics found no one who cares about 3D, and one who had never bothered to set up the 3D on his Playstation 3. 

Why did 3D fall flat for the world's biggest sport spectacular? It's tempting to blame the lack of promotion, the difficulty of finding 3D coverage, and the decision to delay all 3D broadcasts by 24 hours. But the truth is that few people outside of the consumer industry show much interest in 3D television. Properly done, 3D can be fun—for a limited time. I enjoyed playing with a 3D set in the video store, but the amusement wore off in 15 minutes. I can see where the 3D versions of some movies might be worth a few extra dollars in the theater. But the monsters in the lap gimmick gets old fast, viewers dislike the active shutter glasses for 3D televisions, and too much intense 3D can cause eyestrain and nausea.





A refreshable holographic image of an F-4 Phantom jet is created on a photorefractive polymer. (Courtesy of the University of Arizona)


New technology from NLT Technologies (Kawasaki, Japan) presents different views to both eyes of several people, allowing them to see depth by the parallax effect without special glasses. However, that's no panacea because the brain senses depth in multiple ways, and conflicts between different cues lead to eyestrain, headache, and nausea. Perhaps we'll have to wait for further development of holographic video.

Seeing Pluto

In July, the Hubble Space Telescope spotted the fifth moon of Pluto, an icy ball 10 to 25 km across that was just a pinprick of light in the image. Much of the press coverage focused on whether that discovery should make Pluto a full-scale planet. But I was far more interested in Pluto, its moons, and the amazing optical feat of finding something so small and so far away.

My interest in optics grew from a fascination with astronomy. I'm old enough to remember the 1978 discovery of Pluto's largest moon Charon. The discovery images show a small bump on the fuzzy ball of Pluto, recorded on a photographic plate by a ground telescope. Comparison of a series of images showed that the bump moved as the unresolved moon orbited Pluto. In the days before adaptive optics, seeing even that much seemed amazing.

Hubble resolved Pluto and Charon soon after its launch in 1990. It was a badly needed success for Hubble in its troubled early years, but scattered light in the background of the photo clearly shows the spherical aberration of the telescope's primary mirror. Pluto and Charon are both blurry and diffuse, but the photo clearly shows them as separate worlds, with Pluto clearly the larger and Charon roughly half its size. Once NASA added corrective optics to fix the spherical aberration, the Faint Object Camera produced much sharper photos in 1994.

Further upgrades have made Hubble even better. In 2005, it spotted two roughly 100 km moons, later named Nix and Hydra. Last year, astronomer Mark Showalter of the SETI Institute (Mountain View, CA) began a series of Hubble observations to check for other little moons which might scatter dust into the path of the New Horizons spacecraft when it visits Pluto in July 2015. Earlier this month, Showalter downloaded a new batch of Hubble data, and in an hour was on the phone reporting the discovery. A few days later, he told me "I'm still struck by just what an amazing instrument Hubble is. This little object, [called] P5, is fainter than Pluto by a factor of 100,000 and separated by one arc second."

It's amazing and wonderful. And so far Hubble's images show New Horizons is on a good path to avoid any dangerous dust, so we can see close-ups of Pluto three years from now.