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Super-lasers blaze knowledge trail
Wednesday, 14 August 2013 14:17
The global leaders for ultra-powerful lasers are French and the technology is much in demand abroad, with many applications such as nuclear simulation, medical treatment, redirecting thunderstorms and basic physics. To maintain their lead, researchers have recently proposed a new concept based on optic fibre.

"It's picking up," says Laurent Boudjemaa, head of the laser development and products department at the French electronics firm Thales, in reference to sales of high-power laser systems. His team has almost finished fine-tuning one of these giants, codename Cetal. Due for delivery this summer to a destination near Bucharest, Romania, it is one of the first in this category in Europe, with a peak power of one petawatt (PW), or 1bn MW, equivalent to a million nuclear reactors. In 2012 Thales delivered another of these monsters, appropriately named Bella and currently the world's most powerful, to the University of California, Berkeley.

"The market for lasers exceeding 0.1 petawatts is expanding at the rate of about 30% a year," says Gilles Riboulet, head of Amplitude Technologies, launched in 2001 by former Thales staff. The two companies now share most of the global market. In the Amplitude factory at Evry, in the Paris suburbs, 150 wooden crates containing another 1-PW behemoth, aka Draco, are ready to leave for Dresden, Germany. Another one, Vega, will soon follow, destination Spain. Each unit sells for about €10m ($13m). Nor is that all.

"These instruments are in such demand because they allow scientists to push back the frontiers of knowledge. They are less expensive and less bulky than other infrastructures such as synchrotron x–ray sources or particle accelerators," says Philippe Balcou, head of the Intense Laser and Applications Centre (Celia) near Bordeaux, a joint laboratory backed by the National Centre for Scientific Research (CNRS), the Atomic Energy and Alternative Energies Commission (CEA) and Bordeaux-I University. "It is also a way for countries to compete with the great powers," he adds, citing South Korea, China and Brazil, among others.

But it is important to understand that these next-generation lasers are no replacement for power stations. Their huge power is due to the fact that energy is released in extremely short bursts, or pulses, lasting a few tens of femtoseconds (one femtosecond is one-millionth of a nanosecond or 10-15). Blink and 100,000bn femtoseconds go by. The energy, equal to about one joule, is equivalent to lifting a one-kilo mass by 10cm.

Yet this extremely short, highly concentrated release of energy may be sufficient to make science-fiction projects possible, leading, among others, to new forms of cancer treatment, transmutation of radioactive elements to obtain less hazardous substances, firing a beam into a vacuum in search of "dark energy", atomic fusion with the promise of a new energy source, and precipitating thunderstorms. Super-power lasers might also be used to produce the Higgs boson, a particle discovered in 2012 responsible for the mass of elementary particles, or indeed for laboratory simulation of stars, supernovas and planet cores.

Depending on their purpose, two main categories of laser exist, with a third just beginning to emerge. The priority for the first category is energy rather than duration. It has been developed by the military, as part of research into nuclear deterrence. The aim is to reproduce, in a laboratory, the physical conditions of a thermonuclear explosion. The Lawrence Livermore Laboratory in the US started working towards this goal in 2010, drawing on the power of 192 intense-laser beams, so far without success. Meanwhile in France CEA is building its Megajoule laser facility near Bordeaux. The first tests are due to start in 2015.

On these systems the pulse lasts about a nanosecond, yielding a burst of energy just below 1PW. Reflecting civil interest in such systems, Petal, a laser similar to Megajoule (with which it is partnered) is under construction. It will have just one beam and will be used to test alternative concepts for achieving fusion, as well as for studying the behaviour of matter under extreme conditions.

But ultra-short pulse lasers, which make up the second category, are the ones that are driving the market. The power results not from the energy itself, but rather the very short duration of each flash, typically 10 joules lasting 10 femtoseconds.

In the 1980s this technique only worked on paper, because in practice the energy output was more than the amplifying materials could withstand. So Gérard Mourou, now head of the Izest laboratory at Ecole Polytechnique, suggested spreading the short initial pulse before amplifying it, then recompressing it at the output stage.

Laser power started increasing by leaps and bounds. "Ever since we have seen a 10-fold increase in power every six years. It's our version of Moore's law!" says Riboulet.

New applications soon presented themselves. By exciting matter, such as a gas, with such intense light, electrons can be stripped from atoms and turned into ions. The resulting plasma is extremely hot, with streams of electric and magnetic fields that can simulate the hydrodynamic or electromagnetic phenomena found at the core of a star or planet.

The huge electric fields also made it possible to accelerate particles such as electrons or protons over very short distances. These beams could be used to destroy malignant tumours, using proven techniques that require equipment with a larger footprint and stricter safety measures.

"By increasing the energy of electrons, compared with existing techniques, we will be able to dig deeper down into the tissue," says Victor Malka, at the applied optics laboratory, also at Ecole Polytechnique, which is involved in various projects focusing on proton and electron therapy. Malka is working with Amplitude too. They are developing Saphir, a 0.2 PW laser intended to demonstrate the merits of such proton sources in medicine. "Particle acceleration also interests specialists in high-energy physics," Malka affirms. "Ten years ago they were a bit wary of my community, but this year they invited me to one of their conferences to present a paper at the plenary session."

Malka is excited by the potential for using these lasers to create other radiation sources, such as x-rays. "With sources of this sort we would be able to detect cracks in components that are difficult to reach, like in nuclear power stations or aircraft," he explains.

But Mourou, who launched this whole process, wants to make lasers even better. They still have a number of defects: pulses are often not frequent enough and they consume too much electricity. They can achieve several pulses per second, whereas applications such as particle physics really require something at least 1,000 times faster. "What's more you're inputting 100kW only to obtain a few watts of power in output," he complains.

To break through this barrier Mourou has proposed another departure, based on fibre lasers. Rather than a single monolithic rod amplifier, the idea is to use an array of fibre lasers. Less overheating would be caused by the light, making much more frequent pulses. And to prevent energy wastage the size of the array can simply be increased.

Easier said than done though, because light can only be combined if the beams in each fibre laser are coherent, or in phase. So the length of the lines must not vary by more than 10 or so nanometres. This was not enough to discourage the International Coherent Amplification Network, a consortium of 17 labs formed by Mourou 18 months ago. It has already demonstrated, with a test, that 64 fibre lasers can be controlled to emit a coherent beam.

At the end of June Ican organised a symposium at Cern, outside Geneva, to round off the first stage of its work. Looking ahead, Ican-B aims to build a PW-rated array pulsing 10,000 times a second, with at least 10,000 fibre lasers.

In an article published in 2008 Mourou proposed an alternative means of achieving atomic fusion. He now believes that fibre lasers could be used to transmute elements, as a way of disposing of highly radioactive waste from nuclear power stations. The laser would accelerate protons which, on impacting a lead and bismuth target, would create neutrons. These in turn would bombard the waste itself, transmuting it into elements with a shorter half-life.

"Optics has reached a turning point. With these extreme light sources we are on the way to unifying optics and high-energy physics," Mourou claims. "I'm optimistic: this is the future." He has initiated a string of fairly crazy schemes. Among others he contributed to the European Extreme Light Infrastructure programme, which is taking shape at three locations, in Romania, the Czech Republic and Hungary. In France he convinced various parties to endorse construction of the 10PW Apollon laser at Saclay, south of Paris. It should be operational by 2015.

"We must take care not to oversell these projects. A lot of the applications will take 10 years to come to fruition," a specialist warns. "If we don't watch out, France will have lost its leadership position in 10 years. It is based on mutual understanding between research at CEA, CNRS and industry, but also defence players such as the Defence Procurement Agency (DGA)," Riboulet cautions. "If we no longer have any French buyers for our lasers we may well lose these assets. What's more we source most of the components abroad."

This article appeared in Guardian Weekly, which incorporates material from Le Monde

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