More Lidar in Space

19. June 2009
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By Patricia Daukantas

Thanks to the "magic" of NASA's online video service, I just watched the successful launch of NASA's Lunar Reconnaissance Orbiter (LRO) aboard an Atlas V rocket from Cape Canaveral in Florida.

As I mentioned in my feature article on space-based lidar in the current (June) issue of OPN, LRO is carrying an instrument called the Lunar Orbiter Laser Altimeter (LOLA), which will produce a high-resolution topographic model of the entire Moon to guide future robotic and human explorers.

I got a chance to see the spacecraft being assembled when I toured NASA’s Goddard Space Flight Center (Greenbelt, Md., U.S.A.) last summer in preparation for my OPN article on the final Hubble Space Telescope servicing mission. Let’s hope that LOLA and LRO’s other instruments -- as well as LRO’s “sister” satellite, the Lunar Crater Observation and Sensing Satellite (LCROSS) -- have a successful run in lunar orbit.

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2009-06 June

OSA Honorary Member Townes Says a Giant Star Is Shrinking

11. June 2009
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By Patricia Daukantas

Charles H. Townes needs no introduction to OSA members for his groundbreaking work in laser physics. He won the Nobel Prize in 1964 and has been an OSA Honorary Member since 1970.

Perhaps less well known to the photonics community is Townes’ more recent career as an astrophysicist specializing in mid-infrared interferometry. The descendant of his early-1970s work with a few graduate students at the University of California, Berkeley (U.S.A.) is the Infrared Spatial Interferometer (ISI), a three-telescope array on Mount Wilson in California.

Operating between 9 and 12 µm, the ISI makes precise measurements of so-called “evolved stars” – stars that have reached the end of their hydrogen-burning middle age and are starting to shed material into interstellar space. With ISI, Berkeley scientists have been measuring the sizes of the dust shells around these ancient stars and, in a few cases, the diameters of the stars themselves.

On June 9, at a press conference during the American Astronomical Society (AAS) meeting in Pasadena, Calif. (U.S.A.), Townes and Berkeley research physicist Edward Wishnow said that Betelgeuse – that bright red supergiant star in the constellation Orion – has been mysteriously shrinking over the past 15 years. It’s still wider than the inner solar system, but its angular diameter has shrunk 15 percent, from 56 to 47 milli-arcseconds.

Because Betelgeuse is relatively close by – only 640 light-years from us – and so bloated, humans have been trying to figure out its size for decades. Indeed, A.A. Michelson (another OSA Honorary Member) and Charles Pease used Michelson’s own stellar interferometer to make the first such measurement in December 1920. They found the giant star to be 47 milli-arcseconds at visible wavelengths, but at shorter wavelengths, the apparent diameter varies because of absorption or emission by gas and dust near the star.

Townes and his colleagues observed Betelgeuse at 11.5 µm, a wavelength that gets through most of the dust and allows measurement of the width of the stellar photosphere, or light-radiating outer layer. Wishnow noted that the outer atmospheres of these evolved supergiants tend to be quite tenuous. “It’s somewhat difficult to ascertain the precise location of an edge,” he said.

The group is observing in a narrow spectral region that avoids spectral lines of gases. “We think we’re seeing the continuum of the star very precisely,” Townes said.

Betelgeuse is known to be a variable star. In fact, Wishnow said, it’s no longer the brightest star in Orion, as it was designated on a star chart from the year 1603. However, the star has not significantly dimmed overall during the 15 years of ISI observations.

Townes noted that his team found a red spot on Betelgeuse a few years ago, and the feature may or may not be related to the star’s instability. Only careful observing over time will reveal what activity is happening within the star.

“Basically, we hope to learn something new,” the Nobel laureate said with a chuckle. “That’s the great thing about science -- to keep trying, looking at things more and more carefully, getting better instrumentation so that you can measure more precisely. The more you watch things and the more carefully you look, the more surprises you’re likely to find.”

ISI, incidentally, is the only visible/infrared telescope array to use a heterodyne detection system as in radio interferometry. Incoming starlight is mixed with the beam from the master local oscillator (a CO2 laser) and downconverted to microwave frequencies for further processing. The Berkeley team published the technical details in this article nine years ago, shortly before upgrading ISI from two to three telescopes. Physics Today recently published more general information about optical and infrared stellar interferometry.

The Betelgeuse research by Townes, Wishnow and colleagues – which uses 15 years’ worth of ISI observations – appeared in the June 1 issue of Astrophysical Journal Letters.

In his remarks at the start of the news conference, longtime AAS press officer Steve Maran thanked Townes for several acts of assistance he performed over the years, especially setting up a special AAS meeting session in 1991 when the U.S. government first declassified the technology for laser-guide-star adaptive optics. The session featured “all kinds of military scientists whom we wouldn’t have known,” Maran said.

After the press conference, the AAS media team treated Townes to a birthday cake, about six weeks early. He’ll be 94 on July 28.

 

 

 

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2009-06 June , ,

CLEO/IQEC 2009, Day 4: Seeing the Really Big Picture

6. June 2009
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By Patricia Daukantas

In our May issue, OPN brought you a feature article on scanned laser pico-projectors for mobile devices. On Thursday at CLEO/IQEC and PhAST, attendees learned about the opposite end of the projection industry: a display so big and detailed that its manufacturer had to devise its own laser.

How “big” is this display? About as big as the four walls and ceiling of one of the session rooms at the Baltimore Convention Center, according to Forrest L. Williams, a scientist at Evans & Sutherland (Salt Lake City, Utah, U.S.A.). The company specializes in digital planetariums and digital cinema projects – setups that dwarf even the most ambitious home theater gadgets.

To match the resolution of the human eye, a pixel should be about 1 minute of arc (arcmin) in angular size. But with large-field formats approaching 1,000 square meters, the display would be require tens of megapixels.

For decades, anyone who wanted to design a “synthetic environment” – to make planetarium-goers feel as if they’re standing in the middle of the Stonehenge circle, for example – had to rely on a multiplicity of separate projectors with overlapping images. Each projector has slightly different optical characteristics, leading to image distortion.

Williams’ company wanted to design a single large-field projector with an 8,192 x 4,096 display field, or 33.2 million pixels. (By contrast, your HDTV set at home has a display of 1,920 x 1,080 or 2.1 million pixels, and Sony’s SXRD technology uses 4,096 x 2,160 or 8.8 million pixels.)

The system uses scanned-column projection – instead of projecting one image all at once, it projects a narrow column of pixels back and forth very fast, and the persistence of human vision creates the appearance of a complete image.

Using regular lamps in such a huge projector would require many kilowatts of power, perhaps even megawatts, Williams said. Around the year 2000, when the company was initially designing large digital projectors, the team looked for high-power (greater than 5 W) lasers with cw output in red, green and blue – but they couldn’t find anything had enough power or efficiency.

The company ultimately designed and patented a laser system that starts with a “seed” 1,064- or 1,550-nm distributed feedback fiber laser. With a phase modulator, free-space resonator and nonlinear crystal, the laser puts out red, green or blue light from a box about 56 by 36 cm in size and a conversion efficiency approaching 90 percent for green light.

The next challenge is to extend the life expectancies of these lasers from six months (blue) and two-plus years (red) to 10 years by incorporating new materials into the components.

The laser projector also incorporates a grating light valve (GLV), which acts as an analog light modulator. The GLV is a micro-electrical-mechanical systems (MEMS) device with two microscopic silicon nitride ribbons per pixel, and three pixels per device. One remains stationary and the other moves up and down. When the ribbons are out of alignment, the GLV acts as a tiny square-well diffraction grating. Each device is about the size of a human red blood cell, Williams said.

The 33-megapixel projectors are scheduled for release in the second half of 2009. In the next few years, Evans & Sutherland may offer these high-efficiency compact visible lasers as a standalone product.

At another Thursday session at CLEO, Stuart J. McNaught of Northrop Grumman Corp. (Redondo Beach, Calif., U.S.A.) described the architecture of the first solid-state laser to break the 100-kW power barrier, which the company built for a U.S.-military-funded program.

The device consists of seven modular “laser chains” that are coherently combined with active phase locking into a single-aperture output beam. (Passive phase-locking methods don’t scale well to large numbers of beams, according to McNaught.) Each chain contains a 15.8-kW wavefront-corrected master-oscillator-power-amplifier (MOPA) laser, and the seven chains fit together in a single cabinet.

So far this year, McNaught and colleagues have completed 48 tests of the laser system with more than 2.75 hours of accumulated run time in the laboratory, and they hope to begin field tests soon.

CLEO/IQEC wrapped up today (Friday, June 5) with the final sessions on biomedical microscopy, surface plasmon polaritons, fiber lasers and other topics. However, we at OPN still welcome your comments on this series of blog posts. We moderate comments to thwart spammers, but please don’t let that deter you.

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2009-06 June

All Around CLEO/IQEC, Day 3

4. June 2009
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By Patricia Daukantas

Unless you own your own prosperous company or have inherited a fortune, you need a job. So it’s no surprise that the career center on the PhotonXpo exhibit floor has been a hub of activity.

Yesterday’s CLEO career forum was an opportunity for young optics job-seekers to find out how different from graduate school the world of industry is. Jason Mulliner, director of product line management for Edmund Optics Inc. (Barrington, N.J., U.S.A.), admitted he hasn’t had to derive Maxwell’s equations once in the 10 years since he finished his B.S. in electrical engineering. But he has learned a host of more practical lessons: select your manager and team members with care, join a place where engineers are highly valued, and find a company that has a global presence, or is at least developing one.

Finlay Colville, marketing director for the solar division of Coherent Inc. (Santa Clara, Calif., U.S.A.), had no idea he would end up in solar energy when he attended his first CLEO 16 years ago, right after finishing his Ph.D. in laser physics. He had gotten into laser R&D in the laboratory, but gradually moved into sales and marketing. He encouraged young people to consider technical sales support positions, usually found in larger organizations, because they could serve as a resource to the product users out in the field, and the job is often a steppingstone to other roles.

Elsewhere at the Baltimore Convention Center, Steve Post, a scientist with the U.S. Missile Defense Agency, gave a PhAST conference audience a rare glimpse inside the agency’s Airborne Laser Program. The goal of this program, which began in the early 1990s, is to build chemical lasers, flying on a highly modified Boeing 747-400 aircraft, that will acquire, track and kill ballistic missiles before they reach the apogee of their flight path.

Post described the airborne laser as a “megawatt-class illuminator” that saw first light in 2004, though he couldn’t say exactly how many megawatts. It’s a cw laser, but the exact amount of time that it can lase is also classified. To develop and run initial tests on the laser mounting and control systems, the team rescued an old 747 from the scrap heap for a mere $30,000. “That probably saved them two or three years of angst and suffering along the way,” he said.

September 2009 is the target date for the laser system’s first “shoot-down test,” Post said. U.S. Defense Secretary Robert Gates recently changed the Airborne Laser Program from an operational system to an R&D system, but Post said that was simple recognition that there is a lot of work left to do on it.

Wednesday at CLEO-IQEC began with a well-attended plenary session at which Alain Aspect of the Institut d’Optique (France) presented a highly mathematical discussion of the Einstein-Podolsky-Rosen (EPR) paradox, Bell’s inequalities, and recent research into quantum computing. (I was “tweeting” the session on Twitter.com @OPNmagazine -- my short messages should still be archived at that site.)

Also at the plenary session, Harvard University (U.S.A.) physicist Federico Capasso surveyed the world of quantum cascade lasers, which have gone from an initial laboratory experiment to commercialization in the astonishingly short span of 15 years. Built “one atom at a time, layer by layer,” these lasers now measure greenhouse gases and nitric oxide in the atmosphere and also guard against the next explosive-bearing “shoe bomber.” And their design potential and chemical-sensing applications have still not been exhausted, Capasso concluded.

I ended Day 3 at the conference by attending a PhAST session on lasers and optics for astronomy and space-based sensing. Even though it was after 5 p.m. and the plenary session had started at 8 a.m., the room was still quite full -- our conference attendees are taking full advantage of their time here!

Astronomers Richard Dekany of Caltech (Pasadena, Calif., U.S.A.) and Craig Van Sitters of the U.S. National Science Foundation described astronomers’ requirements for laser-guide-star adaptive optics systems, which is helping the most recent generation of ground-based telescopes see better.

“Astronomers are not laser technicians,” Van Sitters noted. “They want a turnkey system that they can turn on at the start of the night and not mess with it.”

The final speaker of the session -- Michael Madden, systems engineering director for GeoEye of Dulles, Va., U.S.A. -- showed off amazing images from his company’s newest satellite, which photographs the Earth at 41-cm resolution from 681 km up. In other words, GeoEye-1 can see everything bigger than a soccer ball or home plate. His company provided the panoramic views of the crowds at President Barack Obama’s inauguration in January, and the satellite has joined in the search for the Air France jetliner that went down in the Atlantic Ocean a few days ago.

According to Madden, the satellite uses an ultra-fast-readout imaging array 37,000 pixels wide. He hopes that the company’s next-generation satellite, with 25-cm resolution, will launch in 2012.

 

2009-06 June

CLEO/IQEC Day 2: The Ultimate White-Glove Test for Spacebound Optics

3. June 2009
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By Patricia Daukantas

In the June issue of OPN, I wrote about lidar experiments that have flown into space. On Tuesday of the CLEO/IQEC conference, two NASA employees described the rigorous prep work that goes into preparing lasers and optical systems for space travel.

Remember the old television commercials in which a fussy visitor applies the “white-glove test” to see if a home is clean and dust-free? That visual cliché doesn’t hold a candle to the space agency’s procedures.

Anne-Marie Novo-Gradac, now of NASA headquarters (Washington, D.C., U.S.A.), said that for optical systems, contamination levels need to be monitored more closely than the average “clean room.” Ideally, workers should enter the clean room only in teams of three: the “clean hands” person who actually touches the instrument, the “not so dirty hands” person who hands tools to the first person, and the “dirty hands” worker who meticulously documents everything being done to the laser system. (Yes, it’s a bit like brain surgery.) Even supposedly “clean” paper sheds tiny particles into the air when someone writes on it.

“As you probably know, building a laser is like building a violin -- no two are the same,” Novo-Gradac said. This is especially true of “one-off” laser systems built only once, for a specific and expensive spacecraft.

Thermal vacuum testing must be done in an impeccably clean chamber. “Most vacuum chambers are not clean enough for your purposes,” Novo-Gradac warned.

Cleanliness isn’t the only consideration in building lasers for space. The design must be mechanically robust and stable; ideally, the laser should be able to tolerate minor misalignments due to thermal cycling. The builder must consider whether the laser will experience intermittent spikes in fluence levels or will have “hot spots” in its beam profile -- both of which could burn out optics. Unlike the Hubble Space Telescope, most lasers flying in space won’t ever be touched by repair-astronauts ever again.

If any of the optics require bonding agents, the laser scientists need to work with the vendor to space-qualify the bonding agent and the method of application. According to Novo-Gradac, the laser team for GLAS (one of the lidar missions mentioned in the OPN article) worked with the manufacturer of the beam splitter cube to eliminate trapped air between the prisms, which would have caused the optical component to rupture in the vacuum of space.

Vendors of pump laser diodes are often (sometimes monthly) changing or discontinuing their product lines, which makes life difficult for the space scientists trying to qualify products for the great beyond. The NASA team bought enough diodes from one fabrication lot that they could test them all at once and cherry-pick the best diodes for future projects.

Another NASA scientist, John Cavanaugh, discussed some of the non-optical considerations involved in building lasers to fly in space. He suggested engaging an experienced mechanical technician to work closely with the laser assembly team and performing vibrational and acoustic tests on the laser system as early as possible in the development workflow. The design team needs to verify the laser’s performance and alignment after every test along the way.

Finally, according to Cavanaugh, the space-based laser team has to build in safeguards to protect spacecraft assembly workers from accidental injury by the laser (not all of these workers will be familiar with laser eye protection, for example).

Before becoming a program executive in the astrophysics division of NASA headquarters, Novo-Gradac led the laser design teams for the Mercury Laser Altimeter (MLA) aboard the MESSENGER spacecraft and the Lunar Orbiter Laser Altimeter (LOLA) on the Lunar Reconnaissance Orbiter (LRO). Cavanaugh, based at Goddard Space Flight Center (Greenbelt, Md., U.S.A.) is the system engineer for LOLA and also worked on the Shuttle Laser Altimeter (SLA) and Mars Orbiter Laser Altimeter (MOLA).

I’m also “tweeting” from CLEO/IQEC -- follow me @OPNmagazine on Twitter.com.

2009-06 June

More from CLEO/IQEC Day 1: Biomedicine and Energy

2. June 2009
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By Patricia Daukantas

With the exhibit hall (PhotonXpo) not opening until Tuesday, Monday at CLEO/IQEC had a distinctly academic feel to it. Each of the three tutorials I attended, including Theodor Hänsch’s early-morning talk, drew a standing-room-only crowd.

In his tutorial, Johannes F. de Boer of VU University in Amsterdam, the Netherlands, explained that optical coherence tomography (OCT) is analogous to ultrasound imaging, except that it uses infrared light instead of sound waves. Spectral-domain OCT, which was developed in the mid-1990s, has two to three orders of magnitude better sensitivity than the older time-domain OCT; in fact, it can be sensitive to a single photon. The technique’s speed makes it possible to take video-rate OCT images of structures such as the human retina, which is important in studying diseases such as age-related macular degeneration. Several types of human tissue -- collagen, muscle, nerve, tendon, cartilage -- are birefringent, so polarization-sensitive OCT imaging can show clear differences between normal and cancerous tissue.

Terahertz technology has been growing by leaps and bounds over the last 20 years, as Daniel R. Grischkowsky (Oklahoma State University, U.S.A.) described in his tutorial talk. Grischkowsky is a past winner of OSA’s Wood Prize and Meggers Award for developing terahertz time-domain spectroscopy (TDS) during his tenure at IBM Corp,’s Thomas J. Watson Research Center. Besides the well-known terahertz TDS applications in fundamental research and homeland security (explosives detection), industry can use TDS for quality control of pharmaceutical tablet coatings.

Among the other CLEO sessions I attended on Monday was a presentation by C.C. Yang (National Taiwan University, Taiwan) on OCT imaging of oral cancer in its different stages. I also learned from Desmond Jacob (Texas A&M University, U.S.A.) that melanoma happens to the human intestine and eyes as well as the skin; in fact, it’s the most common form of ocular cancer. Biopsy is the best means of diagnosing melanoma, but since it’s so difficult to biopsy the eye, non-invasive imaging techniques such as OCT could help catch the deadly disease before it spreads.

The first full day of CLEO wrapped up with an inspiring and well-attended plenary talk by a recent OPN feature article author, Edward Moses of Lawrence Livermore National Laboratory (U.S.A.), who showed the audience many cool photos of the “exciting and challenging journey” leading to the commissioning of the National Ignition Facility (NIF). His team has the ambitious goal of creating self-sustaining hydrogen “burns” over the next few years.

Further into the future, Moses envisions a power plant called the “LIFE engine” that would incorporate both nuclear fusion and nuclear fission -- a NIF-like machine surrounded by a “fission blanket” of conventionally spent nuclear fuel. Neutrons given off by the fusion would drive the fission reaction in the remaining fuel, so the whole thing could sit there and give off thermal energy for years or even decades without stopping, and it would greatly reduce the need for repositories of spent radioactive material.

As if that ides of a bright energy future wasn’t exciting enough, Moses closed his talk with the possible ways that NIF could help scientists explore the extreme conditions that occur in supernova explosions and the interiors of giant extrasolar planets. “I believe this field will be like high-energy physics was like in the 1930s and 1940s after [Edward O.] Lawrence invented the cyclotron, and it’s all driven by lasers and optical systems,” he said.

I was “tweeting” on Twitter.com throughout Moses’ plenary speech, and I will continue to do so today as PhotonXpo opens and the PhAST conference presents several exciting sessions on practical application of photonic technology. Follow @OPNmagazine on Twitter for updates throughout the meeting.

2009-06 June

CLEO/IQEC 2009, Day 1: A Talk by Ted Hänsch

2. June 2009
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Contributed by: Pat Daukantas, OPN Senior Writer/Editor 

Greetings from CLEO/IQEC! Once again, OSA’s annual laser science conference is in the Baltimore Convention Center, which is one of my favorite venues for large events. It has an open floor plan and lots of huge windows to let in the natural light. And light, of course, is what OSA is all about!

For many CLEO attendees, the day began at 8 a.m. sharp with the opening talk of the special CLEO symposium on “10 Years of Frequency Combs.” Who better to summarize the frontier of precision measurements than OSA Honorary Member Theodor W. Hänsch, one of the 2005 Nobel physics laureates in the field?

Hänsch, of the University of Munich and the Max Planck Institute for Quantum Optics (both in Germany), used a series of animated slides to explain the details of comb technology, then took the audience on a grand tour of comb applications, including ultra-precise optical clocks. Frequency combs are becoming commercially available, and advances in fiber laser technology are making the combs more compact.

Hänsch said his Nobel Prize-winning work was an offshoot of his interest, back in the 1970s, in the quantum-mechanical aspects of the hydrogen spectrum. The relative uncertainty of the frequency of the hydrogen atom’s 1S-2S two-photon transition has improved to 1.4 × 10–14, but Hänsch would like to see it lowered to the 10–15 range. Soon scientists will have more precise checks for drifts of the fundamental physical constants.

Accuracy of clocks over the centuries since the Middle Ages has gone up exponentially. Applications for better optical clocks include satellite navigation, monitoring the variability of Earth’s rotation, millimeter-precision geodesy, mapping Earth’s gravitational potential and testing general relativity.

“Who knows, maybe someday we will have an atomic clock that fits into our wristwatch,” he mused.

Last year Hänsch and his team traveled to the solar telescope at Teide Observatory on the island of Tenerife to calibrate the Sun’s spectrum against one of their frequency combs. No sooner did they publish their proof-of-concept work than the phone rang with solar physicists asking to use their data. In another astrophysical application, frequency combs could be used to measure the sizes of intergalactic hydrogen clouds near distant quasi-stellar objects. “Twenty years from now, we could be able to measure the direct expansion of the universe,” he said.

After the symposium session, I asked Hänsch how his life had changed since the fall of 2005, when he was awarded the Nobel Prize jointly with Roy Glauber and John L. “Jan” Hall. He said that he has been traveling a lot more often, mostly around Europe to avoid the jet lag from intercontinental trips. The fame has helped him attract new projects and collaborators; between his two institutions he works with about 70 people. Unlike another OSA Honorary Member, U.S. Energy Secretary Steven Chu, Hänsch has no desire to get into politics. 

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2009-06 June