From Lawrence Livermore National Laboratory: “Guide star leads to sharper astronomical images”

From Lawrence Livermore National Laboratory

Dec. 4, 2018
Breanna Bishop
bishop33@llnl.gov
925-423-9802

The laser guide star revolutionized astronomy by revealing large swaths of the sky that had previously been unseen from Earth due to atmospheric distortions. Now astronomy is on the verge of another great leap forward. The Extremely Large Telescope, which is expected to see first light in 2024, will have a 39-meter-diameter primary mirror — more than three times the size of today’s largest ground-based telescopes.

Glistening against the awesome backdrop of the night sky above ESO_s Paranal Observatory, four laser beams project out into the darkness from Unit Telescope 4 UT4 of the VLT.

ESO/E-ELT,to be on top of Cerro Armazones in the Atacama Desert of northern Chile. located at the summit of the mountain at an altitude of 3,060 metres (10,040 ft).

These next-generation telescopes require even more advanced optics to continue delivering clear images of distant stars, planets and interstellar space. To help answer that call, Lawrence Livermore National Laboratory’s (LLNL) National Ignition Facility and Photon Science (NIF&PS) directorate has delivered a first-of-its-kind, high-power, fiber-based sodium laser guide star to the University of California, Santa Cruz (UCSC).

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The Lick Observatory’s Laser Guide Star at the Shane telescope forms a beam of glowing atmospheric sodium ions. This helps astronomers account for distortions caused by the Earth’s atmosphere so they can see farther and more clearly into space. Credit: Laurie Hatch/lauriehatch.com

“This fiber-based sodium laser guide star is a significant advance for adaptive optics,” said Daren Dillon, a development engineer at UCSC. “We expect it to operate five to 10 times more efficiently than the state-of-the-art dye-based sodium laser guide stars we use at our observatories now. This will enable our adaptive optics to produce much sharper images.”

Adapting fiber laser to a guide star

The project has roots in LLNL’s long history of laser development. Claire Max, a UCSC astronomy professor and director of UC observatories, was a physicist at LLNL from 1974 to 2004. She co-authored the original paper proposing sodium guide star lasers for wavefront correction. In the early 1990s, she demonstrated the first high-power sodium laser guide star from technology developed in LLNL’s Atomic Vapor Laser Isotope Separation program. Max was the driving force for integrating sodium guide star laser systems into the astronomical community worldwide.

To the naked eye, stars appear to twinkle. This is not through any action on the part of the celestial objects, but rather due to atmospheric turbulence — the turbulent mixing of Earth’s atmosphere — that the light rays pass through on their long journey to the eyes of night watchers.

The sodium laser guide star creates an artificial star by shooting a laser into the sodium layer of the atmosphere, about 90 kilometers up. At a wavelength of 589 nanometers (billionths of a meter), the laser excites the sodium, which fluoresces in return. An artificial star is born.

This star provides a reference point for an advanced optics system, which uses it to inform a computer-controlled deformable mirror that cancels out the effects of atmospheric turbulence to create a sharp image.

The first generation of sodium laser guide stars, deployed at the Lick Observatory in Northern California and the Keck Observatory in Hawaii, were dye lasers that served the astronomy community for more than 15 years.

UCO Keck Laser Guide Star Adaptive Optics

Their size, weight and power and cooling requirements, however, made them difficult to incorporate with the telescopes, and they utilized flammable materials, which also are undesirable in an observatory setting.

About 15 years ago, Max made a request of her LLNL colleagues.

Efficient, compact and rugged

“She asked us for a solid-state guide star laser that was compact and reliable,” explained Dee Pennington, one of the principal investigators on the project. “We considered several options and settled on a fiber laser because they are efficient, compact and rugged.”

A fiber laser typically is constructed with an optical fiber doped with rare-earth elements such as erbium, ytterbium and neodymium. These lasers have unmatched beam quality, efficiency, thermal management and reliability as well as lower cost of ownership.

The project was first funded by Livermore’s Laboratory Directed Research and Development program and later by grants from the National Science Foundation Center for Adaptive Optics, which Max directed, the Association of Universities for Research in Astronomy and the European Southern Observatory.

At the project’s inception more than 15 years ago, fiber lasers were still an emerging technology. None existed at the 589-nanometer (nm) wavelength needed to interrogate the sodium layer.

Developing this fiber laser with UCSC meant the researchers had to invent technology. “We had to learn how to cool a fiber laser,” said LLNL materials scientist Steve Payne, another researcher on the project. “If you’re first out of the box, you have to figure everything out on your own.”

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Graham Allen (LLNL), Don Gavel (UCSC), Jay Dawson (LLNL) and Daren Dillon (UCSC) celebrate a milestone: the fiber-based sodium laser guide star achieved 10 watts of power at LLNL, making it ready for UC Santa Cruz.

The team achieved 589 nm by combining a 938-nm laser and a 1,583-nm laser within a nonlinear crystal. Power scaling proved to be an even bigger challenge.
“We were trying to scale two lasers to provide 10 watts of power, the minimum necessary to get enough feedback to inform adaptive optics,” said Jay Dawson, the principal investigator in the later years of this project. Dawson has continued working on fiber laser technology in his current role as the NIF&PS acting deputy program director for DoD Technologies.

Because of the laser’s specialized application, custom optical fibers needed to be developed. LLNL did this in collaboration with existing specialty optical fiber companies.

A new fabrication capability

“However, industry was slow to manufacture the fiber we needed,” Dawson said. “They had little motivation, since few R&D fibers turn into significant commercial sales. We realized that if we wanted to advance fiber laser technology for a wide array of applications, LLNL would need its own fabrication capability.”

As a result, LLNL built its own 8.2-meter fiber draw tower to fabricate the needed specialized fibers. In addition to meeting this need, the draw tower has been the key to success on other important projects. It enabled development of fiber-optic acoustic sensing fibers and the E-band fiber-optic amplifier, two technologies that are revolutionizing laser sensing and communication.

Since commissioning the fiber draw tower, LLNL has applied NIF optics cleaning techniques to microstructured optical fibers to improve strength, loss and reliability. LLNL also has developed consolidation and grinding processes to further open the design space for new optical fibers.

To correct ground-based telescopes with primary mirrors in the 30-meter-diameter range, laser guide stars will be essential. “However, a single guide star laser only can interrogate part of the telescope aperture,” Pennington said. “With the huge apertures we anticipate, it will take multiple guide stars to inform adaptive optics for everything the telescope collects.” But discriminating the feedback from each individual beam creates a challenge.

“One answer is to use pulsed laser guide stars, which allows discrimination by time,” Pennington said. This was the focus of the LLNL fiber guide star laser program.

Next stop, Lick Observatory

UCSC astronomers plan to install the fiber-based sodium laser guide star at the Lick Observatory in the spring of 2019. It will be run alongside the existing dye-based sodium laser guide star.

“We are pretty excited to see what happens when we integrate this fiber-based sodium laser guide star into our adaptive optics system at Lick,” Dillon said. “We think it will produce more detailed images that allow more precise measurements.”

This technology transfer to UCSC has been a long time in the making. That journey also reflects the advances in fiber laser technology. “As a community, the progress we’ve made is amazing,” Pennington said.

-Patricia Koning

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LLNL Campus

Operated by Lawrence Livermore National Security, LLC, for the Department of Energy’s National Nuclear Security Administration
Lawrence Livermore National Laboratory (LLNL) is an American federal research facility in Livermore, California, United States, founded by the University of California, Berkeley in 1952. A Federally Funded Research and Development Center (FFRDC), it is primarily funded by the U.S. Department of Energy (DOE) and managed and operated by Lawrence Livermore National Security, LLC (LLNS), a partnership of the University of California, Bechtel, BWX Technologies, AECOM, and Battelle Memorial Institute in affiliation with the Texas A&M University System. In 2012, the laboratory had the synthetic chemical element livermorium named after it.

LLNL is self-described as “a premier research and development institution for science and technology applied to national security.” Its principal responsibility is ensuring the safety, security and reliability of the nation’s nuclear weapons through the application of advanced science, engineering and technology. The Laboratory also applies its special expertise and multidisciplinary capabilities to preventing the proliferation and use of weapons of mass destruction, bolstering homeland security and solving other nationally important problems, including energy and environmental security, basic science and economic competitiveness.

The Laboratory is located on a one-square-mile (2.6 km2) site at the eastern edge of Livermore. It also operates a 7,000 acres (28 km2) remote experimental test site, called Site 300, situated about 15 miles (24 km) southeast of the main lab site. LLNL has an annual budget of about $1.5 billion and a staff of roughly 5,800 employees.

LLNL was established in 1952 as the University of California Radiation Laboratory at Livermore, an offshoot of the existing UC Radiation Laboratory at Berkeley. It was intended to spur innovation and provide competition to the nuclear weapon design laboratory at Los Alamos in New Mexico, home of the Manhattan Project that developed the first atomic weapons. Edward Teller and Ernest Lawrence,[2] director of the Radiation Laboratory at Berkeley, are regarded as the co-founders of the Livermore facility.

The new laboratory was sited at a former naval air station of World War II. It was already home to several UC Radiation Laboratory projects that were too large for its location in the Berkeley Hills above the UC campus, including one of the first experiments in the magnetic approach to confined thermonuclear reactions (i.e. fusion). About half an hour southeast of Berkeley, the Livermore site provided much greater security for classified projects than an urban university campus.

Lawrence tapped 32-year-old Herbert York, a former graduate student of his, to run Livermore. Under York, the Lab had four main programs: Project Sherwood (the magnetic-fusion program), Project Whitney (the weapons-design program), diagnostic weapon experiments (both for the Los Alamos and Livermore laboratories), and a basic physics program. York and the new lab embraced the Lawrence “big science” approach, tackling challenging projects with physicists, chemists, engineers, and computational scientists working together in multidisciplinary teams. Lawrence died in August 1958 and shortly after, the university’s board of regents named both laboratories for him, as the Lawrence Radiation Laboratory.

Historically, the Berkeley and Livermore laboratories have had very close relationships on research projects, business operations, and staff. The Livermore Lab was established initially as a branch of the Berkeley laboratory. The Livermore lab was not officially severed administratively from the Berkeley lab until 1971. To this day, in official planning documents and records, Lawrence Berkeley National Laboratory is designated as Site 100, Lawrence Livermore National Lab as Site 200, and LLNL’s remote test location as Site 300.[3]

The laboratory was renamed Lawrence Livermore Laboratory (LLL) in 1971. On October 1, 2007 LLNS assumed management of LLNL from the University of California, which had exclusively managed and operated the Laboratory since its inception 55 years before. The laboratory was honored in 2012 by having the synthetic chemical element livermorium named after it. The LLNS takeover of the laboratory has been controversial. In May 2013, an Alameda County jury awarded over $2.7 million to five former laboratory employees who were among 430 employees LLNS laid off during 2008.[4] The jury found that LLNS breached a contractual obligation to terminate the employees only for “reasonable cause.”[5] The five plaintiffs also have pending age discrimination claims against LLNS, which will be heard by a different jury in a separate trial.[6] There are 125 co-plaintiffs awaiting trial on similar claims against LLNS.[7] The May 2008 layoff was the first layoff at the laboratory in nearly 40 years.[6]

On March 14, 2011, the City of Livermore officially expanded the city’s boundaries to annex LLNL and move it within the city limits. The unanimous vote by the Livermore city council expanded Livermore’s southeastern boundaries to cover 15 land parcels covering 1,057 acres (4.28 km2) that comprise the LLNL site. The site was formerly an unincorporated area of Alameda County. The LLNL campus continues to be owned by the federal government.

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