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Keely Portway finds out how the latest advances in photonics have enhanced the research environment, and how they have translated to the commercial sector

A technician loads a wedged focus lens, a key element in the NIF final optics assembly, into a cleaning/coating frame

According to the International Energy Agency’s latest renewable energy market forecast, the installation of solar PV systems on homes, commercial buildings and industrial facilities is likely to take off over the next five years.

The Renewables 2019 report forecasts that the total global renewable-based power capacity will grow by 50 per cent between 2019 and 2024. Solar photovoltaics accounts for 60 per cent of the rise, and the share of renewables in global power generation is set to rise from 26 per cent today to 30 per cent in 2024. However, the renewed expansion still remains significantly below what is needed to meet global sustainable energy targets.

One area with great potential is the use of fusion as a power source. In fact, the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL) in California believes that we are getting closer to using fusion as a power source, which could provide an almost infinite supply of clean energy.

The facility houses a giant laser with nearly 40,000 optics that guide, reflect, amplify and focus 192 huge laser beams onto a fusion target about the size of a peppercorn. LLNL has worked with a number of suppliers on the development of new optical components, instruments, and mass-production manufacturing processes to ensure that its optics meet its performance, cost and schedule requirements.

A glass act

Schott Advanced Optics partnered with the facility to produce metre-sized plates of laser glass that provide the optical properties needed to amplify stretched pulses to an extreme degree. Thanks to Schott’s process, the active phosphate and silicate-based laser glasses can be supplied to NIF 20 times faster and five times cheaper than previously possible, the company said. Alain Danielou, strategic market manager for laser components EMEA at Schott Advanced Optics, explained: ‘We are proud to have been involved for decades in such a technology. As an amplifier in those ultra-high peak power applications, the glasses provide the rarely requested larger bandwidth of 30 to 50 nanometres.’

Likewise, Hoya Corporation came up with its own continuous glass melting system to produce neodymium-doped laser amplifier glass slabs for the project. Each measures 790 x 440 x 45mm and essentially contains no microscopic platinum particles that could cause laser-induced damage. The water content in the glass is such that it quenches neodymium fluorescence, and the optical homogeneity is designed to surpass the transmitted wavefront specification by a factor of two.

As with many innovations, there are also associated challenges, particularly when supplying to a project of this scale. Danielou explained: ‘Because of the fundamental uncertainty relation, ultra-short pulses have a broad spectrum, which must be preserved during amplification in order to make subsequent compression possible. That's why we are faced with the challenge of enlarging the bandwidth of the pumping inside our phosphate glasses.’

Schott Advanced Optics partnered with NIF to produce metre-sized plates of laser glass

The NIF system works with the creation of a weak laser pulse – about a billionth of a joule – which is split and carried over optical fibres to 48 pre-amplifiers that increase the pulse’s energy by a factor of 10 billion, to a few joules. These 48 beams are split into four beams each for injection into the 192 main laser amplifier beamlines. Each beam travels through the two systems of large glass amplifiers, first through the power amplifier and then into the main amplifier. In the main amplifier, an optical switch traps the light, forcing it to travel back and forth four times, while special deformable mirrors ensure the beams are high quality, uniform, and smooth.

From the main amplifier, the beam makes a final pass through the power amplifier. By this point, its total energy has grown from one billionth of a joule to four million joules. The 192 beams proceed to two 10-storey switchyards on either side of the target chamber, where they are split into quads of 2 × 2 arrays. Before entering the target chamber, each quad passes through a final optics assembly, where pulses are converted from infrared to ultraviolet light, before focusing onto the target.

To help boost the energy of the 192 lasers and cut the cost of repairing or replacing damaged optics, the facility developed an anti-reflective coating in 2017. The coating has subsequently helped to overcome reflections from the rear surface of the facility’s grating debris shields. This is important, because the shield is the penultimate optic before the laser beams enter the target chamber, protecting other optics from the environment and helping diagnose the energy of the laser beams.

The researchers are making advances towards achieving fusion ignition in the laboratory. As part of an inertial confinement fusion (ICF) experiment, they have explored a number of solutions, including larger capsules; magnetised targets; new methods for finishing, mounting and filling capsules; increased laser energy and different designs of hohlraum – the hollow chamber used to control radiation. Hohlraums shaped like cylinders have been used traditionally in ICF research, but the facility believes that its new angular design – which it has named ‘Frustraum’ – could be key to its next stages of research.

The small things

Frustraum has the potential to enable a significant boost in the amount of laser-induced energy absorbed by an ICF fuel capsule. This could allow the NIF research to take a step closer to achieving the burning plasma stage needed to eventually reach ignition. A paper detailing the results was published in Physics of Plasmas.

Peter Amendt, its lead author, believes that preliminary results using a smaller-scale Frustraum are promising. The next step is to conduct experiments using a full-scale design. He said: ‘There’s a lot of excitement over what we’re going to find out. An energy coupling increase was seen in the subscale campaign. How well that persists in the large-scale campaign, we’ll find out.’

The NIF is making great advances towards achieving fusion ignition, with a specially designed chamber, Frustraum

The goal was to increase the energy coupling efficiency using the same amount of energy now produced by the laser beams at NIF. Amendt explained: ‘Any means of improving the hohlraum-to-capsule efficiency can only help the margin budget and facilitate the path to ignition. Although the benefit of higher capsule absorbed energy has been long known, its implementation in cylindrical hohlraums has proved challenging and elusive.’

Computer simulations for the design showed it that it had promise, as Amendt continued: ‘The energetics of the Frustraum employing a 1.5mm radius capsule can be made to strongly resemble that of the standard cylinder using a 1mm-scale capsule.’  The next step is a full-scale campaign that will continue into early 2020. This will employ a 9.2mm-wide Frustraum, but will start with a target capsule smaller than those currently used for ICF research.

In sync

Research projects such as this are largely made possible thanks to advancements in photonics equipment such as sensors, lasers and coatings. One current area of development is the ultra-stable distribution of timing signals. According to ultrafast laser company Menhir Photonics, the requirements of future accelerators for stable timing are expected to increase. The latest generation of high-brightness ultrafast x-ray sources driven by free electron lasers typically have sub-10 femtosecond requirements on the distribution of RF signals to accelerator components and laser systems.

The solution, the company believes, is a timing distribution system based on fibre optic transmission lines. These can provide femtosecond-level synchronisation between accelerator components and laser systems using optical communication technology and metrology. Ultra-low noise pulse trains are used from mode-locked lasers as timing reference. The timing signals of the master oscillators are transmitted via fibre optic links to the remote-end stations, where transmission delays are stabilised. Mode-locked lasers or microwave oscillators are then synchronised to the end of the stabilised fibre links.

[Large-scale facilities] need very precise synchronisation, and our technology is based on very low noise femtosecond lasers

Menhir Photonics recently partnered with Cycle to conduct tests that integrated its low noise femtosecond lasers into Cycle’s timing distribution system to further improve its timing jitter – which is already below a femtosecond. Cycle works with large research facility customers spanning China, Europe and the US. Dr Haynes Cheng, CTO at Cycle explained: ‘[Large-scale facilities] need very precise synchronisation, and our technology is based on very low noise femtosecond lasers, which are, for example, provided by Menhir Photonics among others. We distribute the signals to the different stations in the facility.’

Cheng revealed that a number of large-scale facility customers focus on the dynamics of different reactions, such as chemical reactions or what happens in a molecule. ‘They want to know the time dynamics,’ he said. ‘They want to essentially make a movie, so that’s where we [Cycle] and Menhir Photonics come in. They need to have very precise timing of which point in time they are taking the pictures, otherwise the pictures are blurry. That’s similar to these concepts where we need to study these fast dynamics in the molecules on the order of femtoseconds up to a few hundred picoseconds and nanoseconds, depending on the reaction or the mechanism being studied.’

The heart of the system is the laser that is used to distribute the timing signal across the facilities. This requires a stable laser and the clock signal, which needs to be distributed over the facilities so that the scientists can take the ‘movies’ in a precise timescale. ‘Menhir Photonics recently came out with a nice laser and it looks like their short-term stability is very good,’ continued Cheng, ‘Our job is to stabilise, to keep this very good short-term stability and stabilise it for the long-term.’

In terms of how this technology looks set to develop, for Cycle, the future will see a demand for even greater efficiency and precision. ‘We get more and more requests to get even more precise timing, but this is very specific for our case,’ he said. ‘Customers want to have lasers with even shorter pulses, which bring up the peak power to even more petawatts, so they need even more precise timing.’

Business use cases

Similarly, with fields like astronomy and defence, where technology is at the cutting edge, the photonics equipment developed for large research facilities makes its way into a commercial environment, as Mike Flanary, vice president of business development at materials processing firm Universal Laser Systems, explained: ‘We see these developments in technology trickle down in a number of ways. One is by demonstrating the ability to process new materials, which give unique value to product designers. This can be incorporated into product designs and then downstream, use the same tool or technology for manufacturing,’ he said.

Microscopy images of a processed edge after MultiWave Hybrid processing (left), 9.3µm processing (centre), and 10.6µm processing (right)

In many of the industries benefitting from such technology, Flanary sees size as a factor, with a trend for smaller products and a transition from metals to plastics. Cosmetic requirements, he said, are also becoming more stringent. ‘Our tool is very widely used in the processing of plastic or organic material’ he explained. ‘This is due to its precision and control and it will apply the proper wavelength or radiation to the material. So, this trickle-down from research into industry will go anywhere from material manufacturers to research and development and product designers, all the way through to manufacturing.’

Universal Laser Systems has worked with partners in the research sector, which has led to the introduction of a number of new technologies, many of which are beginning to translate to industry. Flanary said: ‘One, in particular, is called MultiWave hybrid technology. It has a proprietary capability to combine multiple laser wavelengths into a single co-axial beam. The patent is very broad based, so it’s any laser wavelength, and the ability to combine two or more wavelengths into a focal point, so that an infinite number of material interactions are possible.’

Looking at how this market will develop going forward, Flanary believes that the tools will evolve as the market evolves. ‘Higher end precision is where there is market demand,’ he said. ‘The viability of incorporating different wavelengths into traditional laser processing is something that will happen as well. New materials are being introduced hourly now and we must offer the opportunity to marry product development with material processing. Everyone is looking at a competitive advantage. Manufacturers are looking at ways to reposition their products and grow consumption and value of their materials and the way to do that is new and better ways to process the materials.’