A team from LLNL’s Advanced Photon Technologies (APT) program has designed a new generation of compressor gratings that could boost the performance of the world’s ultrafast high-power laser systems by as much as 20 percent. The new gratings also can support hundreds of kilowatts of average power—an increase of about 10,000 times compared to common industrial technologies.
Gratings are at the heart of the 2018 Nobel Prize-winning technology called chirped-pulse amplification (CPA) that enables the development of petawatt (quadrillion-watt) and even higher peak-power lasers. Used to compress laser pulses to durations below 30 femtoseconds (quadrillionths of a second), LLNL’s new multilayer dielectric (MLD) gratings can substantially enhance the diffraction efficiency of grating compressors over a large bandwidth, or range of frequencies, while absorbing 500 times less energy than previous designs (unlike the metal used in traditional gratings, dielectric materials are non-conducting).
The breakthrough, reported in an article in the September issue of Optics & Laser Technology, could greatly reduce the potential for grating distortion and damage as well as system cooling requirements.
“The new gratings are a cornerstone of our efforts to advance petawatt lasers 10 times to 1,000 times in average power, which is needed for scientific and commercial applications,” said Constantin Haefner, Program Director for Advanced Photon Technologies.
“MLD gratings for picosecond (trillionth of a second)-duration lasers have enabled a variety of high-average-power laser systems,” added laser physicist David Alessi, lead author of the Optics & Laser Technology paper. “For more than a decade, the ultrafast laser community has been pursuing MLD gratings suitable for 30-femtosecond lasers to improve efficiency and, more recently, to increase average power. Our recent grating work has demonstrated a solution to this challenging problem for which there is large-scale interest.”
Based on Decades of LLNL Research
The new gratings represent the culmination of two Laboratory Directed Research and Development (LDRD) projects and LLNL’s years of experience in developing high-energy laser systems. The Laboratory has also been a decades-long leader in the design and fabrication of the world’s largest diffraction gratings, such as the gold gratings used to produce 500-joule petawatt pulses on the Nova laser, the world’s first petawatt laser, in the 1990s.
New technology, however, was needed for NIF’s petawatt-class Advanced Radiographic Capability (ARC) project because of the much higher energy of NIF’s laser pulses. Pulse-compression gratings of sufficient size, efficiency, and damage resistance had to be fabricated to handle the record-setting beam energy produced by NIF.
In an LDRD-funded project, LLNL optics engineer Jerry Britten and his colleagues invented the MLD grating. This new grating technology consists of adding a dielectric groove structure on top of an MLD mirror.
But these multilayer dielectric gratings don’t work very well for titanium-sapphire lasers used in high-average-power laser systems. “Because they can only reflect a limited span of wavelength, they don’t support the very short pulses desired,” said Alessi. “Hence everyone uses gold gratings—but on gold gratings unfortunately 3.5 percent of the incident light (energy) is absorbed and another .5 to 1 percent is lost through other routes.”
This adds up to about a 20- to 25-percent loss of the initial laser energy in a 4-grating compressor. For high-average-power lasers, these losses contribute to heat and optics distortions.
“Wasting 20 percent of the total output isn’t cost effective,” Haefner said. “And the generated heat by the absorbed light distorts the beam, diminishing the overall laser intensity and the quality of the experiments.”
Haefner’s team sought new approaches to overcome three problems:
An LDRD-funded project led by Haefner and Alessi solved all three problems. First, they developed gold-coated dielectric ridge (GCDR) gratings for use in high-average-power lasers. The GCDR gratings demonstrated improved efficiency and uniformity compared to previous high-average-power-class gratings consisting of gold-coated etched substrates.
Removing Residual Heat
Second, they developed an active cooling system to remove residual heat from meter-scale grating substrates without distorting the beam. Using ultra-low thermal expansion substrates and this specialized cooling, kilowatt-class operation was demonstrated. This was an improvement of approximately three orders of magnitude over conventional gratings for single-shot operation, and about five to seven times better than uncooled, ultra-low expansion gratings.
This new generation of gratings technology was also called for in 2013, when LLNL contracted to produce the High-Repetition-Rate Advanced Petawatt Laser System (HAPLS) for the European Union’s ELI Beamlines Research Center in Dolní Břežany, Czech Republic. HAPLS is a titanium-doped sapphire laser designed to produce 30-femtosecond pulses with 30 joules of energy at a repetition rate of 10 hertz. This corresponds to an average output power of 300 watts.
The GCDR gratings, along with the LLNL-developed active cooling system, ultra-low thermal expansion substrates, and a variety of other innovations enabled HAPLS to meet its design requirements. The system was installed in the ELI Beamlines Research Center in December 2017.
Larger-Bandwidth MLD Gratings
As the grating compressor technology still suffered the approximately 20 percent loss in the compressor, the researchers continued advancing the multilayer dielectric grating technology. To deal with the bandwidth limitations of MLD gratings, they set out to fabricate a multilayer “stack” that would maximize the span of reflected wavelengths. Using grating design codes, Alessi designed a “very sophisticated stack,” Haefner said, “and then Jerry Britten and Hoang Nguyen were able to make it.”
The researchers also tinkered with the angle at which the laser beams hit the grating, setting it close to where the beam reflects directly back on itself like a mirror, known as the Littrow angle. “The largest (frequency) spectrum that gets reflected is exactly at that Littrow angle,” Haefner said. To get the beam in and out of the compressor, the researchers tilted the grating so the beam reflects slightly upward, or out-of-plane.
By building a symmetric 4-grating compressor to compensate for the tilt, all out-of-plane effects are compensated for while the bandwidth is maximized and makes use of the high-efficiency MLD grating.
Metallic gratings using low thermal-expansion material are limited to 50 to 100 watts without the active cooling used in HAPLS. “For the next-generation lasers that we are now designing, there is no other way to go than MLD,” Haefner said. “The APT team has demonstrated high-average-power gratings that for the first time in the world have overcome the kilowatt barrier; what we’re projecting now is about 300 to 500 kilowatts.”
In the paper, the researchers predicted the new MLD gratings “can enable pulse compressors for petawatt-class systems operating at repetition rates as high as 10 kHz when combined with active cooling.” The efficiency advancement is also important for small systems: industry has already shown interest in the new gratings, Haefner said.
“Any laser out there could receive about a 20 percent performance enhancement by changing the compressor to the new technology,” he added. “It is really an enabling technology for the next generation of ultrafast high-average-power lasers.”
Alessi, Haefner, Britten, and Nguyen were joined on the paper, “Low-dispersion low-loss dielectric gratings for efficient ultrafast laser pulse compression at high average powers,” by LLNL colleague Paul Rosso.
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When the world’s newest telescope starts imaging the southern sky in 2023, it will take photos using optical assemblies designed by scientists and engineers from Lawrence Livermore National Laboratory (LLNL), including NIF, and built by Lab industrial partners.
A key feature of the camera’s optical assemblies for the Large Synoptic Survey Telescope (LSST), under construction in northern Chile, will be its three lenses, one of which at 1.57 meters (5.1 feet) in diameter is believed to be the world’s largest high-performance optical lens ever fabricated.
The lens assembly, which includes the lens dubbed L-1, and its smaller companion lens (L-2), at 1.2 meters in diameter, was built over the past five years by Boulder, Colorado-based Ball Aerospace and its subcontractor, Tucson-based Arizona Optical Systems.
Mounted together in a carbon fiber structure, the two lenses were shipped from Tucson, arriving intact after a 17-hour truck journey at the SLAC National Accelerator Laboratory in Menlo Park.
SLAC is managing the overall design and fabrication, as well as the subcomponent integration and final assembly of LSST’s $168 million, 3,200-megapixel digital camera, which is more than 90 percent complete and due to be finished by early 2021. In addition to SLAC and LLNL, the team building the camera includes an international collaboration of universities and labs, including the Le Centre National de la Recherche Scientifique in Paris and Brookhaven National Laboratory.
“The success of the fabrication of this unique optical assembly is a testament to LLNL’s world-leading expertise in large optics, built on decades of experience in the construction of the world’s largest and most powerful laser systems,” said physicist Scot Olivier, who helped manage Livermore’s involvement in the LSST project for more than a decade.
Olivier said without the dedicated and exceptional work of LLNL optical scientists Lynn Seppala and Brian Bauman and LLNL engineers Vincent Riot, Scott Winters and Justin Wolfe, spanning a period of nearly two decades, the LSST camera optics, including the world’s largest lens, would not be the reality they are today.
“Riot’s contributions to LSST also go far beyond the camera optics — as the current overall project manager for the LSST camera, Riot is a principal figure in the successful development of this major scientific instrument that is poised to revolutionize the field of astronomy,” Olivier added.
Riot and Bauman also credited the work of LLNL engineers Darrell Carter, Simon Cohen, Pete Fitsos, Steve Pratuch, and John Taylor.
Livermore involvement in LSST started around 2001, spurred by the scientific interest of LLNL astrophysicist Kem Cook, a member of the Lab team that previously led the search for galactic dark matter in the form of Massive Compact Halo Objects.
However, LLNL participation in LSST quickly became centered on the Lab’s expertise in large optics, built over decades of developing the world’s largest laser systems. Starting in 2002, Seppala, who helped design NIF, made a series of improvements to the optical design of LSST leading to the 2005 baseline design.
This consisted of three mirrors, the two largest in the same plane so they could be fabricated from the same piece of glass, and three large lenses, as well as a set of six filters that define the color of the images recorded by the 3.2-gigapixel camera detector.
LSST Director Steven Kahn, a physicist at Stanford University and SLAC, noted that “Livermore has played a very significant technical role in the camera and a historically important role in the telescope design.”
Livermore’s researchers made essential contributions to the optical design of LSST’s lenses and mirrors, the way LSST will survey the sky, how it compensates for atmospheric turbulence and gravity, and more.
LLNL personnel led the procurement and delivery of the camera’s optical assemblies, which include the three lenses (the third lens, at 72 centimeters in diameter, will be delivered to SLAC within a month) and a set of filters covering six wavelength-bands, all in their final mechanical mount.
Livermore focused on the design and then delegated fabrication to industry vendors, although the filters will be placed into the interface mounts at the Lab before being shipped to SLAC for final integration into the camera.
The 8.4-meter LSST will take digital images of the entire visible southern sky every few nights, revealing unprecedented details of the universe and helping unravel some of its greatest mysteries. During a 10-year time frame, LSST will detect about 20 billion galaxies — the first time a telescope will observe more galaxies than there are people on Earth — and will create a time-lapse “movie” of the sky.
This data will help researchers better understand dark matter and dark energy, which together make up 95 percent of the universe, but whose makeup remains unknown, as well as study the formation of galaxies, track potentially hazardous asteroids and observe exploding stars.
The telescope’s camera — the size of a small car and weighing more than three tons — will capture full-sky images at such high resolution that it would take 1,500 high-definition television screens to display just one picture.
Research scientists aren’t the only ones who will have access to the LSST data. Anyone with a computer will be able to fly through the universe, past objects 100 million times fainter than can be observed with the unaided eye. The LSST project will provide an engagement platform to enable both students and the public to participate in the process of scientific discovery.
Riot, who started on the LSST project in 2008, initially managed the camera optics fabrication planning, became the LSST deputy camera manager in 2013 and the full camera project manager in 2017. For the past three years, he has worked at LLNL and at SLAC on special assignment.
“There are important challenges getting everything together for the LSST camera. We’re receiving all of these expensive parts that people have been working on for years and they all have to fit together,” Riot said.
Wolfe, an LLNL optical engineer and the LSST camera optics subsystems manager, and Riot are pleased that the world’s largest optical lens has overcome hurdles.
“Any time you undertake an activity for the first time, there are bound to be challenges, and production of the LSST L-1 lens proved to be no different,” Wolfe said. “Every stage was crucial and carried great risk. You are working with a piece of glass more than five feet in diameter and only four inches thick. Any mishandling, shock or accident can result in damage to the lens. The lens is a work of craftsmanship and we are all rightly proud of it.
“When I joined LLNL I had no idea that it would lead to the opportunity to deliver first-of-a-kind optics to a first-of-a-kind telescope,” Wolfe said.“From production of the largest precision lens known, to coating of the largest precision bandpass filters, the LSST optics have set a new standard.”
Construction on LSST started in 2014 on El Peñon, a peak 8,800 feet high along the Cerro Pachón ridge in the Andes Mountains, located 220 miles north of Santiago, Chile.
Financial support for LSST comes from the National Science Foundation (NSF), the U.S. Department of Energy’s Office of Science, and private funding raised by the LSST Corporation. The NSF-funded LSST Project Office for construction was established as an operating center under management of the Association of Universities for Research in Astronomy. The DOE-funded effort to build the LSST camera is managed by the SLAC National Accelerator Laboratory.
The camera system for LSST, including the three lenses and six filters designed by LLNL researchers and built by Lab industrial partners, will be shipped from SLAC to the telescope site in Chile in early 2021.
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