Absolutely beaming

Over the next few years industries everywhere, from defence to medical, can look forward to reaping the benefits of the latest advances in laser technology. Philip Sen predicts a bright future.

Ever since lasers were invented, their destructive power has captured the imagination of sci-fi writers and the public alike. The ever-growing list of benign uses for lasers attracts considerably less attention. Recently added applications range from treatment for eye disease to root canal work to use as a ‘broom’ to sweep space debris away from the International Space Station.

And with the development of new types of laser, the range of applications is set to grow exponentially.

Recent reports says that the JSF will feature a 100kW laser that will be effective against vehicles, communication lines, power grids and fuel dumps.

Two of the most promising newcomers are the femtosecond (or short-pulse) and terahertz laser techniques.

Industrial laser-cutting and machining is now common practice but still leaves much to be desired. As with traditional tools, a normal laser with pulses longer than a nanosecond tends to create distortions in the material, such as a residue of resolidified ‘slag’.

This can be prone to fatigue and is difficult to remove. Excess heat conduction in a laser-machined material can also damage sensitive components such as integrated circuit chips, or cause stress fractures in surrounding areas.

So can a laser’s heat be directed for just long enough to make the required cut, but without the excess heat that causes these unwanted side effects? About a femtosecond – a millionth of a nanosecond, itself a billionth of a second – would be sufficient. Also, if the same amount of energy is contained in such a short pulse, a femtosecond laser can reach levels of power measured in terawatts.

Such devices are now becoming commercially viable. They operate with an oscillator, mode-locked to produce short pulses. These are then ‘stretched’ to longer pulse widths, amplified to the required level, then compressed again to produce amplified ultra-short pulses – known as ‘chirped pulse amplification’.

In April 2001 the UK’s first university-based CAD/CAM femtosecond laser manufacturing project was set up at the University of Liverpool. A collaboration with California-based Spectra Physics and Exitech of Oxford, the facility’s team is led by the director of the manufacturing science and engineering research centre, Dr Bill O’Neill, and is now engaged in work for the Medical Research Council and the Engineering and Physical Sciences Research Council.

‘We are looking at the next generation of drug analysis technology,’ says O’Neill. The laser is used to create complex micro-fluidic indentations for collecting minute samples of drugs on materials such as diamond, ceramics, metals, silicates and polymers.

These are only possible because femtosecond machining is effectively a cold process, and molten slag and debris don’t clog the tiny holes. Good results have been achieved with diamonds, says O’Neill. ‘Unlike other diamond processes, such as cut-off saws, the diamond does not turn back into carbon.’

To prevent unwanted debris and ionisation the team has experimented with different atmospheres, such as helium gas. Further work will use optical elements to defract the beam to drill multiple hole patterns, while processing is to be extended over larger areas, from just a few microns to several hundred microns square.

‘It’s a very robust technology, easy to use, and shows excellent promise in a wide range of sectors,’ says O’Neill. ‘We are now working with four companies on aspects of the ultra-fast processing of bio-materials, aerospace components, nuclear materials and organic matter.’

Femtosecond lasers need no longer be bulky or expensive. Researchers at the University of St Andrews have made a version that can sit on a sheet of A4 paper. It uses ‘narrow-stripe’ diode lasers, with three-micron apertures, to pump a Cr:LiSAF (chromium-doped lithium strontium-aluminium-fluoride) crystal.

Dr Alan Kemp, one of the researchers, explains that while this is a compromise – trading high beam quality against lower power – it achieves the required results. Mirrors arranged in a Z-shape help to further focus the beams on to a small area. The prototype, launched in June this year, cost only a few thousand pounds – a fraction of the price of larger short-pulse lasers.

Kemp sees potential applications in biomedicine, where the laser’s small size could make it useful in the clinical environment. It could be an efficient source of blue laser light, which might help improve resolution in certain types of fluorescent microscope or, after further refinements, help to speed up ultraviolet treatments for skin cancer from days to hours.

Another key development comes from research spin-off Marconi Solstis in Stratford-on-Avon, which made its first sale last year of a system that allows ultra-long-haul optical communications with Australia.

The company combined high-quality femtosecond lasers with advances in optical fibre technology to increase the prevalence of solitons, ‘perfect’ waves that don’t lose their characteristics as they travel. It says its technology makes possible operating distances of thousands of miles at terabit capacities.

Meanwhile, terahertz, or T-rays, are a growing field in ‘see-through’ imaging. Experiments began during the early 1990s and the technology is fast approaching a stage where it will become a viable non-ionising rival to X-rays.

Laser pulses lasting about 100 femtoseconds are used to illuminate a semiconductor crystal and create the T-rays – pulses of electromagnetic radiation lasting one picosecond. These, in turn, are focused through mirrors and lenses. As the T-rays pass through different materials, absorption, dispersion and reflection effects are recorded at a detector.

Measuring the distortion enables digital signal processing units not only to translate the information into computer images but also make judgements about the kind of material through which the T-ray has passed.

Early demonstrations in 1995 by Bell Laboratories showed that, since fat absorbs a 25th of the amount of T-rays that meat does, the technique could accurately depict the contents of a side of bacon.

Although metals and other conductive materials are opaque to T-rays, paper and fabric are virtually transparent, while gases show strong and recognisable absorption patterns. The system can also be programmed to distinguish waveform readings from individual materials, enabling the user to detect exactly what is inside an object, as well as where it is.

Since the technique provides superior detail and materials analysis compared with X-rays or ultrasound, T-ray imaging is especially appealing for medical applications, for example screening for breast cancer or the early detection of tooth decay.

UK-based TeraView is one of the first in the field and aims to come up with an application that can supersede existing technology at a competitive price. The firm was spun off from Toshiba Research Europe’s Cambridge laboratories in April 2001 and has already put a skin cancer detection system into clinical trials.

Business development manager Paul Smith says one of the next steps will be to refine the technique for detecting oral, cervical or colon cancers. He says: ‘We want to use a pulse laser via an endoscope and need to engineer a miniaturised head for the emitter and detector.’

The company is also working with the Government Communications Centre on a security-screening device. ‘We would be able to capture an image through an envelope and pick up the special qualities of anthrax,’ says Smith. ‘It would be the same for explosives.’

Preliminary trials have gone ahead and delivery of a prototype system should take place by the end of the year. TeraView is also working with US laser-maker Coherent to reduce the price of the technology – a typical titanium sapphire laser can cost £80,000 – and expects results in about 12 months.

Meanwhile, researchers at Cambridge, Turin and Pisa have already developed a compact low-power laser to make T-rays. The device exploits very thin layers of gallium arsenide and aluminium gallium arsenide to ‘fence off’ electrons, meaning the shift of energy states is reduced to the right levels for creating terahertz radiation.

The Rensellaer Polytechnic Institute in New York has designed a system for making 3D T-ray images similar to those from a CAT scanner, which it believes will be suited to security roles such as airport luggage scanning.

Smith says: ‘It was originally thought that terahertz would become a communications application.’ Instead, companies such as Toshiba, GEC, Siemens and Philips are paying attention to what Smith believes is the next ‘imaging modality’.

Since its first demonstrations in the late 1970s magnetic resonance imaging is still being developed, says Smith. ‘We see the same potential with terahertz.’

Femtosecond laser machining and terahertz imaging are just two of the rising stars in the laser revolution. As the decade progresses lasers will become cheaper, smaller, more powerful and more versatile, which means a host of new applications are just around the corner.