FEATURE
Issue: 

Lasers at the leading edge

Current developments in research-grade lasers are increasing the power, compressing the pulses, and exploring new wavelengths. Russ Swan reviews a technology that is answering fundamental questions about the universe

In just a few decades, lasers have progressed from a scientific curiosity to an indispensable component of many leading-edge research programmes. They are deployed in practically every scientific discipline from astrophysics to zoology, are crucial to many advanced analytical instruments, and power some of the greatest ‘big science’ experiments ever devised. 

Lasers may hold the key to sustainable fusion power through developments such as the US National Ignition Facility, and have recently played a pivotal role in the detection of gravity waves in the Ligo experiment. There is even a laser vaporising rock samples on Mars aboard the Curiosity rover. 

As the frontiers of scientific knowledge continue to be rolled back, researchers are demanding more out of their lasers in every aspect of their performance. Some experiments require better beam stability, while some want faster cycling. Many labs want lower operating temperatures, or better energy efficiency, while for others the critical developments are in shorter pulses or the generation of specific and previously hard-to-obtain wavelengths.

And then, of course, there is power – and the need for more of it. More power to see deeper into the fundamental processes of the universe; more power to penetrate further or do more actual work. There is always a demand for more power. 

It is a theme that Andy Wells, sales manager at Laser Quantum, recognises: ‘Our customers are generally demanding more power for oscillator pumping and microscopy, and are asking more questions about lifetimes and robustness of the lasers. Ultrafast laser users also want higher peak energies and shorter pulse durations as their research is driven deeper into the atom and monitoring faster processes.’ 

The power of petawatt

The key word in any discussion of laser power is petawatt, 10 to the power 15 (1015) Watts. It is 20 years since the petawatt threshold was first breached, in May 1996 at the Lawrence Livermore National Laboratory in California. That spectacular leap in power raised the record by a factor of 10, while later improvements have been relatively modest – an indication of just how difficult they are to achieve. 

It was only in August last year that the record was boosted to two petawatts, by the LFEX laser at the Institute of Laser Engineering (ILE) at Osaka University, Japan. Significantly, this power also equates to a high energy output, or work done. LFEX transfers as much as 1,000 Joules, which ILE says is dozens of times as much as most other petawatt lasers that have outputs in the tens of Joules. During its brief picosecond pulse, LFEX turns out 1,000 times as much power as the total electricity consumed by the rest of the world. 

The Japanese researchers wielding LFEX are using it for a range of basic science experiments, including the generation of high-energy quantum beams (electrons, ions, gamma ray, neutron, and positron), in medical applications, and the non-destructive inspection of materials. 

The laser is also being used to explore the realm of fast ignition, one of the leading candidate technologies for fusion energy (the other being magnetic confinement). 

Fusion remains tantalisingly difficult by this method, with the US National Ignition Facility (NIF) being at the forefront of developments. Located at the Lawrence Livermore Laboratory, it hopes to create fusion by blasting tiny pellets of fuel (two millimetres in diameter) with enough laser energy to drive the atomic nuclei together. A 500 terawatt flash lasting a few picoseconds, delivered by 192 beamlines from a neodymium-doped phosphate glass laser, was calculated to be sufficient for achieving fusion. 

At the time of writing, the project has gone only part way to achieving ignition, and has experienced many difficulties on the way. It seems no closer to becoming a sustained net producer of energy than when it opened (five years late and billions over budget) in 2009. 

When fusion is achieved it will provide almost God-like power, but in the meantime a group of atmospheric researchers in Quebec have emulated Zeus by using lasers to steer lightning. In the laboratory of Roberto Morandotti at INRS (Institut National de la Recherche Scientifique), high-power pulsed lasers are used to ionise pathways in the air and control the route taken by electrical discharges. Using an 800nm titanium-sapphire (Ti:sapphire) laser supplied by Thales and providing up to 100mJ per 50 femtosecond pulse, the scientists were able to direct discharges into a variety of shapes including Gaussian curves, S-bends, and even rectilinear steps. 

One of the INRS researchers, Matteo Clerici of the School of Engineering at the University of Glasgow, echoes the sentiment expressed by many a laser scientist: ‘Energy per pulse (the higher the better) and pulse durations (the shorter the better) are the main qualities that we are looking for in a research-grade laser.’

The need for speed

While picosecond pulses may be the order of the day at NIF and ILE, the INRS work reflects the growing demand for vastly shorter laser flashes. Here, the watchword is femtosecond, a thousandth of a picosecond, or 10-15 seconds.

Among the recent innovations in the field, researchers at the University of Warsaw last year announced the development of an especially robust new fibre laser, considered to be the first laser capable of generating femtosecond light pulses under extreme environmental conditions. Generating 1,030nm pulses in an ytterbium-doped optical fibre, the ‘spaghetti noodle’ design aims for reliability through simplicity. 

Dr Yuriy Stepanenko of the university’s Faculty of Physics described how the system was treated in a way that is not usually considered appropriate for precision optical instruments: ‘We turned on the laser and then heated up a segment of the fibre to more than 120°C, giving a steep temperature gradient. We also put it in a shaker with acceleration in excess of seven grams, and not only did it work afterwards, but it also worked during the testing.’

Commercially-available femtosecond lasers include Toptica’s recently-announced third generation FemtoFiber Ultra, which delivers 150fs pulses at 780nm. Aimed at research applications in biophotonics and microscopy, it delivers 500mW at a repetition rate of 80MHz.

Towards longer wavelengths

A parameter that can be no less important than pure power or length of pulse is the wavelength or frequency of a laser, with specific research work demanding a particular output. 

Tunable lasers offer some flexibility and are increasingly employed in routine research, but there remain certain wavelengths that have proved difficult. Earlier this year, researchers at the University of Bath reported success in producing a laser to fill a perceived gap in the mid-infrared spectrum. 

‘Beyond about 2.8µm, conventional fibre laser power starts to fall off, and the other main technology, quantum cascade lasers, doesn’t pick up until beyond 3.5µm,’ said William Wadsworth, who co-led the Bath research team. Their solution is a novel combination of gas and fibre laser technologies which could offer advantages at these mid-IR wavelengths. 

Silica hollow-core fibres can be thought of as very long thin bubbles of glass, explained Wadsworth, overcoming the tendency of silica-based glass to only absorb light at wavelengths past 2.8µm. 

These wavelengths, equivalent to frequencies between 85 and 96THz, are of interest for a number of research developments in spectroscopy, environmental sensing and detecting explosives, according to the Bath team. 

This shift towards long wavelengths has triggered ultrafast laser manufacturers to develop turnkey, high peak-power tunable sources at 1µm and above, noted Julien Klein, senior product marketing manager at Spectra-Physics in Santa Clara, California. ‘In neuroscience and brain microscopy, there has been a push towards large volume in-vivo imaging, and researchers have pursued two distinct paths to achieve this. By shifting the two-photon excitation wavelength further into the infrared, above 1µm, one can take advantage of the lower scattering of live tissue to open new windows of relative transparency. 

‘Second, neuro-imaging researchers have developed new strategies to tackle complex tasks such as the photo-activation of large ensembles of neurons using optogenetics,’ Klein continued. ‘Instead of relying of the serial approach of point scanning multiphoton microscopy, researchers increasingly use scanless holographic techniques powered by ultra-high peak power lasers, such as Spirit.’

Beam me up

Where next in the development of lasers? We can certainly expect to see the recent trends towards higher power, shorter pulses, and longer wavelengths continue, while devices will also become cheaper, more efficient, and more robust. But this is not all.

A team at Yale University in the USA is working on a new semiconductor laser with the potential to significantly improve imaging quality in next-generation microscopes and biomedical imaging. According to the research team, the laser combines the brightness of traditional lasers with the lower image corruption of light emitting diodes (LEDs). Specifically, the work aims to neutralise the problem of speckle – a random grainy pattern caused by high spatial coherence of traditional lasers. 

The new device, based on a chaotic cavity laser, is an electrically pumped semiconductor laser that produces an intense emission with low spatial coherence. 

‘For full-field imaging, the speckle contrast should be less than about four per cent,’ explained Hui Cao, professor of applied physics and of physics at Yale, writing in the Proceedings of the US National Academy of Sciences. ‘The standard edge-emitting laser produced speckle contrast of around 50 per cent, while our laser has the speckle contrast of three per cent. So our new laser has completely eliminated the issue of coherent artefact for full-field imaging.’

This is just one of the more recent of many intriguing developments in the research-grade laser market, and it seems certain that many further innovations can be expected. To paraphrase a certain advertising slogan: the future’s bright, the future’s coherent. 

Ancillaries for power

The drive towards more powerful, shorter-pulse, and higher-frequency lasers is putting new pressure on suppliers of optics and beamline equipment. 

Optical Surfaces recently reported the delivery of three ‘challenging’ off-axis parabolic mirrors to the petawatt laser at the UK’s Rutherford Appleton Laboratory. These 175mm diameter surfaces were manufactured using a new technique to create fast focusing mirrors with surface accuracy of better than lambda/8 P-V, surface quality of 20/10 scratch/dig, and surface slope error of lambda/10/cm P-V. The Gemini laser at Rutherford Appleton delivers petawatt power laser pulses to targets only a few micrometres in size, recreating temperatures, pressures and magnetic fields that otherwise exist only inside stars.

In addition, Schott has supplied the Lawrence Livermore Laboratory with active laser glasses in large dimensions. The neodymium-doped laser glass is characterised by being platinum-free, with high homogeneity and highly reproducible quality, the company said.

An accessory not to be overlooked as output power climbs ever higher is an efficient beam dump unit. Laser Components has boosted its range of convection cooled aluminium dumps for low and medium power operations with a water cooled range rated up to 1,000W, with the option of two-inch or four-inch apertures. These have a dual cone design to prevent reflection even if the non-reflective finish has been eroded by exposure.