Lasers make the grade in Earth observation and space exploration
Astronomers, weather forecasters, and Earth scientists are among those now benefiting from the application of solid-state lasers in space.
Even by stellar historic standards, it has been a remarkable few months for space probes and their on-board optical instrumentation. Late 2018 saw the erstwhile Voyager 2 probe - complete with interferometer, ultraviolet spectrometer, photo-polarimeter, and dual-camera imaging science system - finally leave the solar system. We've also witnessed some extraordinary imagery and data acquisition carried out by missions such as the Parker Solar Probe, the close encounter between OSIRIS-REx and asteroid Bennu, and ozone monitoring by the Earth-observing Sentinel-5P satellite.
Just weeks before Photonics West opened its doors the imaging instruments on NASA's New Horizons mission captured the unusual "lumpy snowman" form of Ultima Thule, and a couple of days later China's Chang'e 4 probe touched down on the far side of the Moon. Recent months have also seen the launch of the Bepi Colombo mission to Mercury, its payload featuring a laser altimeter and an ultraviolet (UV) spectroscopy probe, a laser-cooled atom experiment delivered to the International Space Station (ISS), and the deployment of laser terminals to quickly transmit huge data sets back to Earth from imaging satellites.
In terms of photonics equipment, perhaps most satisfying of all has been the recent arrival of a couple of solid-state lasers on board Earth-orbiting spacecraft. Last August, the European Space Agency (ESA) finally launched its wind-monitoring Aeolus satellite. The first wind lidar instrument in space, it is based around a UV laser and is set to provide far more accurate and detailed monitoring of wind speeds than was previously possible.
Attempts to understand and forecast the wind date back as far as Aristotle in the 4th century BC. Today, wind profiles sampled down through the atmosphere are needed for accurate medium- to long-term weather forecasting, and are critical for modelling climate change. But until Aeolus, this information was not available from direct measurement: the best equivalent came from ground sensors and balloon monitors giving localized point measurements, followed by extrapolation through cloud tracking or computer simulations. Aeolus being in orbit changes that, and for the first time global wind fields can be mapped directly, in three dimensions.
"Using revolutionary laser technology, Aeolus will measure winds around the globe and play a key role in our quest to better understand the workings of our atmosphere," announced ESA following the launch of the 1.4-tonne satellite aboard a Vega rocket last year. "Importantly, this novel mission will also improve weather forecasting."
But the mission has also proved to be one of ESA's most technologically demanding. Problems with the "Aladin" UV laser, in particular the damage caused to its system optics over an extended operating period, had delayed the original launch schedule by more than a decade. Thanks in part to technical breakthroughs made with a similar source - the green laser at the heart of NASA's similarly delayed ICESat-2 mission - the Aeolus mission now looks set for major success.
A couple of weeks after launch, Aeolus sent back its first data, and in November Errico Armandillo, the retired head of ESA's optoelectronics section, reflected on the development. "Today Aeolus is returning more wind data than all ground-based measuring systems put together," he remarked. "But it took the sustained efforts of ESA labs and technical experts - in close cooperation with the Aeolus team - to make it fly."
In fact ESA set up two new laboratories to solve its laser issues. It called in additional support from the German Aerospace Center to produce entirely new technical standards, which are now being applied to all subsequent laser missions. "The commercial space industry by itself could not have gone to the lengths we took," Armandillo pointed out.
The idea of flying a wind-surveying lidar in orbit was nothing new. In fact it had been explored as long ago as the early 1980s, considered at one time for the ISS. And in fact the technology developed back then is now used to help guide rendezvous and docking operations with ISS-supplying cargo spacecraft.
Initially a high-energy carbon dioxide gas laser was earmarked for the lidar role, before the mid- 1990s development of space-worthy pump laser diodes opened the door to far more compact solid-state designs. The Aeolus mission was pencilled in for a launch some time after 2000.
The Aladin laser, seen here in ground tests before launch, is at the heart of the Aeolus satellite. It is now in polar orbit, providing direct measurements of global wind patterns for improved weather forecasting. Initial results are said to be excellent. Photo: Selex-ES.
Based around a conventional Nd:YAG solid-state laser crystal, the UV wavelength selected is seen as essential for achieving the high level of back-scatter from both molecular and aerosol components to provide reliable lidar signals. But ESA saw the first signs of trouble in NASA's ICESat mission, which was using a UV laser to map ice. Around the same time, ground tests on Aladin began to show laser-induced contamination of optics.
The key problem was then identified: out-gassing of organic molecules from Aladin's laser equipment was accumulating on system lenses, before being carbonized by the high-energy UV laser pulses. As they grew, those deposits further absorbed the laser's heat, distorting and darkening the optical components.
It meant that the original performance of the UV laser within Aladin was nowhere close to requirements. ESA says that when it ran a prototype version of the lidar system, its laser optics degraded by 50% in less than six hours of operations - not much use for a proposed three-year mission.
"The first solution was to take extreme precautions to remove all organics," Armandillo said. "But this did not prove entirely possible. Even at just a few parts per billion of organics, contamination was still introduced."
For more clues the team approached users of high-energy UV lasers in terrestrial applications. That included working closely with two German optics companies, LaserOptik and LayerTec, as well as experts at France's Mégajoule facility - where lasers are employed to ignite nuclear fusion reactions - and the semiconductor industry. In principle, the answer proved remarkably simple. Injecting a small amount of oxygen allowed the contamination to burn up under the heat of the laser, in the process cleaning the lens. In tests, the ESA team says it saw this approach work in a matter of minutes.
Rather than redesigning Aladin to work on a fully pressurized basis, small amounts of oxygen are released from a pair of 30-liter tanks. The oxygen gas flows close to the optical surfaces that are exposed to the UV laser, and gradually leaks out of the instrument enclosure.
"Just like us, the laser has to breathe," explained laser engineer Linda Mondin in a report by ESA. "It's very elegant because the burnt-up contaminants flow out of the instrument along with this oxygen, in the form of carbon dioxide and water." Only 25 Pascals of residual oxygen pressure is needed - just one four-thousandth of standard atmospheric pressure.
Though contamination was the key issue facing the Aladin team, it was far from the only problem. Heat produced within the volume of the laser transmitter also needed removal. This was solved using ‘heat pipes', which cool the laser by evaporating liquid and moving it to a space-facing radiator.
Solving the various problems has ultimately created new technology that is set to benefit a range of future missions. Aladin's development has yielded ESA some world-leading optics and optoelectronics capability, along with a set of ISO-certified laser development standards for other laser-based missions - starting with the "EarthCARE" mission for clouds and aerosol monitoring. Pencilled in for launch in 2021, this will carry an atmospheric lidar instrument based around a 355 nm laser source to profile aerosols and thin clouds.
"It's proved an extremely complex mission, and we've learnt an awful lot about lasers," concluded Rondin, with Aeolus's instrument manager Denny Wernham adding: "The fact we have a high-power UV laser instrument now working in space is testament to all of the hard work, ingenuity, and inventiveness of many dedicated engineers in industry, ESA, and elsewhere.
"Aeolus is a world-first mission that will hopefully lead to many active laser missions in the future, and shows the true value of close collaboration between industry and ESA to find innovative solutions to very tough technical challenges."
"There were so many ways it could go wrong, we were worried," recalled Armandillo following the 2018 launch. "And then it worked! Those first wind profiles felt like Christmas coming early, a really amazing gift."
ICESat-2: up and running
Just as Aeolus and its Aladin laser were starting to return those initial wind profiles from space, NASA launched its ‘ICESat-2' satellite from California's Vandenberg Air Force Base.
Like Aeolus, the mission - comprising a single-instrument laser altimeter payload - was delayed and significantly over its original budget. But it has now deployed its Advanced Topographic Laser Altimeter System (ATLAS), flying in a polar orbit at an average altitude of 290 miles. Its job is to monitor annual changes in the height of the Greenland and Antarctic ice sheets, to a precision of just 4 mm.
Developed by the Virginia-based photonics and engineering services company Fibertek, the two flight lasers aboard ICESat-2 emit millijoule-scale nanosecond pulses at 532 nm and a repetition rate of 10 kHz. In continuous operation over the three years of the mission, that equates to around a trillion pulses in all - with Fibertek saying that the tough performance metrics represented a significant increase in the complexity and reliability requirements for a space-based laser system.
The optical design of ATLAS splits the laser source into three separate pairs of beams that are fired towards Earth at different angles, such that at ground level there is a 3.3 km gap between the beam pairs. This contrasts with the approach used on the original ICESat mission that flew between 2003 and 2009 but whose laser only operated at 40 Hz, and provides much denser cross-track sampling.
For Earth scientists and studies of climate change, the altimeter should yield a height measurement every 70 cm along the orbiting track, with Fibertek saying that elevation estimates in sloped areas and rough surfaces around crevasses will be much improved.
According to the ICESat-2 team, only about a dozen of the approximately 20 trillion photons that leave ATLAS with each laser pulse return to the satellite's telescope after a round trip that takes around 3.3 milliseconds. To detect those scarce returning photons, the system is equipped with a 76 cm-diameter beryllium telescope. A series of filters ensures that only light of precisely 532 nm reaches the detectors, eliminating any reflected sunlight that might influence the results.
The ATLAS laser, part of NASA's much-delayed ICESat-2 mission, was launched in September 2018. It will provide high-precision profiles of ice sheets and sea ice for climate studies. Photo: NASA.
Just three months after launch, ICESat-2 was already exceeding scientists' expectations. NASA said that the satellite had measured the height of sea ice to within an inch, traced the terrain of previously unmapped Antarctic valleys, surveyed remote ice sheets, and peered through forest canopies and shallow coastal waters.
"ICESat-2 is going to be a fantastic tool for research and discovery, both for cryospheric sciences and other disciplines," said Tom Neumann, ICESat-2 project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. Neumann and others shared the first results from the mission at the American Geophysical Union's December 2018 meeting in Washington, DC.
"It's spectacular terrain," reported Benjamin Smith, a glaciologist with the University of Washington, Seattle, and member of the ICESat-2 science team. "We're able to measure slopes that are steeper than 45 degrees, and maybe even more, all through this [Transantarctic] mountain range."
The returning photons have shown high ice plateaus, crevasses in the ice 65 feet deep, and the sharp edges of ice shelves dropping into the ocean. Those first measurements will help fill in current gaps in maps of the Antarctic, Smith said, although the most critical science of the ICESat-2 mission is yet to come. As researchers refine their knowledge of exactly where the instrument is pointing, they can start to measure the rise or fall of ice sheets and glaciers.
"Very soon, we'll have measurements that we can compare to older measurements of surface elevation," Smith said. "And after the satellite's been up for a year, we'll start to be able to watch the ice sheets change over the seasons."
Cold Atom Lab
Not long before the launch of the Aeolus and ICESat-2 sources, another laser system made its way to the ISS, where it is now carrying out quantum research inside the orbiting Cold Atom Lab (CAL). Part of a scientific payload that arrived in May 2018, it is based around commercial laser equipment and capable of trapping potassium and rubidium isotopes.
By July, the space lab had produced Bose-Einstein condensates (BECs) of rubidium atoms in orbit for the first time, controlled by scientists on the ground at NASA's Jet Propulsion Laboratory (JPL) in California. Robert Thompson, CAL project scientist and a physicist at JPL, said at the time. "It's been a long, hard road to get here, but completely worth the struggle, because there's so much we're going to be able to do with this facility."
Although shrinking the BEC-making equipment to the size demanded for launch to the ISS has been a huge challenge, the advantages of the environment are enormous, from the point of view of quantum experimentation. Unlike on Earth, the persistent microgravity allows scientists to observe individual BECs for 5-10 seconds at a time, and to repeat measurements for up to six hours every day.
This master-oscillator power-amplifier (MOPA) laser module was launched on a sounding rocket in early 2017, to carry out the first laser-cooling experiments in microgravity. In November 2018 the German consortium that built it reported that 110 experiments were completed during its six minutes of space travel. Copyright: FBH/schurian.com.
In fact this was not quite the first cold atom experiment in space. In January 2017 the "MAIUS-1" sounding rocket launched a diode laser system for laser cooling and rubidium atom interferometry to an altitude of 243 kilometers, before returning to the ground. Developed by Humboldt University Berlin's optical metrology research group, initial results confirmed that it was possible to carry out research on laser-cooled atoms in space, and in November 2018 the German consortium reported that it had carried out a remarkable 110 experiments on BECs during the six minutes of space travel that were possible.
Another diode-pumped solid-state laser currently traversing the solar system sits inside an altimeter setup destined for the planet Mercury. Launched by the ESA in October, the Bepi Colombo probe is a collaboration with the Japan Aerospace Exploration Agency (JAXA).
Designed and built by a Swiss-German-Spanish team led by engineers at the University of Bern, the altimeter kit will be used to map Mercury's topography and surface morphology in unprecedented detail, and is said to be the first such instrument developed for a European interplanetary mission. Based around a Q-switched, nanosecond-pulsed Nd:YAG source operating at 10 Hz, it will fire relatively high-energy (50 mJ) bursts of 1064 nm light at the planet, and collect reflections from the surface around 5 ms later using a silicon avalanche photodiode, via a narrowband filter.
Elsewhere in the solar system, NASA's OSIRIS-REx mission has just completed its approach to the asteroid Bennu, where it is now in close orbit. Ultimately, it is set to grab a sample from the surface of the orbiting rock and bring it back to Earth, but before that Bennu had to be mapped in considerable detail to ensure that the spacecraft could be maneuvered into exactly the right orbit to achieve the close fly-by.
That operation relied on another laser altimeter featuring a lidar scanner, to generate a detailed three-dimensional map of Bennu's shape. Built by the Canadian Space Agency, it will help the OSIRIS-REx team identify the best location from which to grab a sample. Two lasers are on board: a high-energy source to scan the asteroid at distances between 7.5 km and 1 km from the surface, and a second low-energy emitter that can be used for rapid time-of-flight imaging down to 225 m.
-Mike Hatcher is editor of optics.org. A version of this article appeared in the 2019 Photonics West Show Daily.
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