Feature

Lasers -- The Light Fantastic

Lasers are everywhere, in the home, in factories,
in operating theatres.

By Cathryn Crane, NZSM

Lasers have been around for the last 30 years, but it has only really been in the last ten years that a huge range of applications has opened up for this light-based technology.

Researchers are looking for new applications and new configurations, to develop more efficient, cheaper lasers that can be tuned to a variety of uses. Much of the research is applications driven, looking to develop new medical techniques and industrial processes.

Blasting Blemishes

In recent years, lasers have come into use for the treatment and removal of disfiguring blemishes, such as dark port wine stains. While not debilitating, these blemishes can mean a life of misery or embarrassment for some.

In port wine stains, blood cells build up in malformed capillaries, stretching the tiny blood vessels. From a normally invisible five microns, the capillaries swell, forming a dark purple mass by the time they reach 100 microns.

The idea of using a laser to force a healing reaction has been around since the late 1960s, but it has taken a long time to develop the techniques. For the last five years, Associate Professor Phil Butler, of Canterbury University's Physics Department, and plastic surgeon Dr Peter Walker have been developing a fast, affordable laser treatment for port wine stains and other skin blemishes.

It hasn't been cheap research. Walker and Butler started by mortgaging their houses to raise the funds to purchase a $150,000 copper vapour laser. Research by a 4th-year physics student suggested that the yellow light used in a copper vapour laser would be the most appropriate wavelength. Initial tests with an argon laser had shown that too much of the blue and green wavelengths was absorbed by the melanin layer of skin tissue.

"The copper vapour laser gives copious outputs in yellow light, right on the peak absorption of haemo-globin," notes Butler.

It was important to find a wavelength that enables the laser to selectively heat haemoglobin in the vessels without damaging surrounding tissue. A fine scan across the stain heats the vessels from the inside out, gently cooking them in a process Butler compares to microwaving. This selective damage prompts the body's healing response, replacing disfigured capillaries with normal vessels.

"The skin surface returns to normal look and touch as normal size vessels replace the enlarged ones," says Butler. Ironically, the quick healing response in young patients makes them more difficult to treat, often requiring multiple sessions. The slower regeneration rates of older people gives their body longer to revert to normal tissue.

Initially, the laser sessions at St George's Hospital used a hand-held fibre to scan across the treatment area. This was successful, but the researchers recognised that it presented difficulties in ensuring accuracy. The physics department's mechanical and electronics workshops helped produce an automated scanner.

"A video camera looks down the same path as the laser beam, so you can see on screen what you want to treat," says Butler. The treatment area is outlined and the process computer-controlled for both exposure time and beam placement. Mirrors direct the beam and a lens concentrates it from 30 millimetres to a 0.1 mm spot.

The success of the scanner has led to orders from around the world. Visiray, the Australian company which made the copper vapour laser, is now offering the scanner as an add-on, and the first commercial prototype was shipped to Australia recently.

Filleting Fish

Industrial laser applications have tended to concentrate on cutting and tooling areas, but imaging uses are being found where lasers provide superior results to conventional systems.

One such system has been developed by the Imaging Technology group of DSIR Industrial Development. Their Laser Image Detection System has been used for processing fish fillets, and a number of other potential applications are being explored.

"We use laser light to help highlight the quantity to be measured," says researcher Peter Hilton. With the fish filleting, it is used to spot the position of the horizontal fat line in hoki fillets, directing water jets to excise the fat.

"The [laser] image is inherently superior to that acquired by a camera," Hilton says. Unlike a camera-based system, there are no special lighting enclosures, and selection of an appropriate wavelength meant no confusion with blood spots.

The fish are loaded tail first onto a conveyor belt passing under the laser. The laser is shone down and scattered within the fillet flesh. Optical detectors below the belts pick up this scattered light to show a bright image of the flesh, and a dark line where the fat absorbs the laser's wavelength.

"The backlit image is a lot easier to process than a front-lit one," notes Hilton. Less processing means less time taken to decide where the fat line is. It also means far less information processing equipment and software, making the initially costly laser system competitive with conventional systems.

Industrial Development are developing other applications. Working in ultraviolet will allow them to detect areas of fluorescence in animal products. An incompletely understood chemical or biological reaction makes cockle defects in sheep pelts fluoresce, and a laser scan system could pick up these defects during processing.

Laser scanning could also allow fast, accurate measurement of length as carcasses pass along a killing chain. An added advantage is that such measurement is non-contact, and therefore does not require any sterilisation procedures. Laser systems can also be used to look at everything from the diameter range of goat fibres to the age of erosion "rinds" in rocks.

The Blues

The blue-green end of the spectrum is gaining attention, as these shorter wavelengths can store more information than red and infrared lasers.

Physics postgraduate student Keith Murdoch has been working at the US Los Alamos research facility on up-conversion lasers. These enable long wavelength red light to be put in, to get out shorter, blue laser light. In what he describes as a piece of "optical magic", crystals of cesium-calcium-bromide are used to boost production of blue or green wavelengths.

Optically active ions are introduced into the super-cooled crystal, where they act to pass incoming energy through a number of defined transition states. When the energy is finally released, it is emitted at a specific wavelength associated with the transition state which it has reached.

Different ions and crystals enable specific wavelengths to be produced. Some configurations respond markedly to the addition of "impurities", such as praseodymium. Murdoch refers to one up-conversion configuration which sees infrared light sent in and violet light emerging.

"That's quite a dramatic change of colour," he comments.

Why such changes occur and which transition states are involved form the basis of Murdoch's research. His work may be assisted by the development of new types of semiconductors which are used in these lasers. New configurations and new transition bands may tell us more about how such lasers function and why they function, he says.

While the bulk of this work is being done overseas, Murdoch believes that local researchers have a chance to make important contributions.

"In these niches, we can do world-class international physics in New Zealand," he says confidently.

Cathryn Crane is a freelance journalist with an interest in environmental issues.