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Patent application for first ever coloured-light laser

August 21, 2003

Physicists at the University of Bonn have applied to patent a laser capable of producing almost every colour, from infrared through the entire visible spectrum to the UV range - and this is done not with high-cost optical crystals, but with the aid of a simple glass fibre. The new laser could bring huge benefits, especially in the field of medical diagnostics by enabling, for instance, doctors to identify even the tiniest tumours.

Anyone who has asked the skin specialist to check a conspicuous mole will know the procedure: having made a quick visual examination, the dermatologist reaches for his scalpel, cuts out the suspicious piece of skin under local anaesthetic, and then sends it off to a laboratory specialising in tissue sample analysis. In very many cases no malignant cells are found and the doctor can sound the all-clear a few days later: the surgery and the scar were unnecessary. In future, however, a small device might often spare us the surgeon's knife. For thanks to Optical Coherence Tomography (OCT), it is possible to detect tumours appearing in the skin, eyes or breasts at a very early stage, even when they only consist of a few cells, without the need for tissue extraction. This non-invasive method is currently undergoing trials in several clinics, but it is still relatively expensive and time-consuming. The reason: for the initially small number of carcinogenic cells to be made visible on the OCT image, a special laser is needed to produce light that is, as were, "coloured" (although still in the infra-red range invisible to the human eye). Normally lasers only shine in a single, starkly defined colour.

A solution is now promised by the new type of laser being patented by Bonn University. Back in 2000, a British working group discovered that laser light changes colour when directed through a tapering glass fibre: red turns to white, and this white light, much like sunlight, refracts through a prism into the colours of the spectrum. However, initial experiments found that this did not work equally well for all colours. The Bonn laser physicist Professor Dr. Harald Gießen and his team have been investigating the causes of these curious colour changes. "We now understand the physics behind the effect well enough to be able to make glass fibres that favours of emission of a particular colour range."

However, solving the problem of transforming colours does not, in itself, make a coloured-light laser. The fact that dentists can now use a laser beam for scaling their patients' teeth is due to its high energy. Every laser device has a kind of "optical amplifier", which ensures that the photons - i.e. the "light particles" - rapidly multiply. "This usually involves a colour source that gives off photons when radiated," explains Professor Gießen. A semi-pervious mirror reflects most of these photons back onto the colour source, which again emits more light. With each reflection the flow of photons swells. "In our laser a glass fibre serves as an optical amplifier," says the physicist: "We take a part of the coloured light and divert it, forcing it again and again to pass through the glass fibre. In this way a "soliton" is created, which you can regard as something like a tsunami of light, a gigantic wave of extremely high intensity. The tapered glass fibre serves here as a non-linear medium in an optical-parametric oscillator."

It is not only medical specialists who might benefit from the coloured-light laser. The invention from Bonn could also be used for particularly brilliant projectors or for TV sets. The problem of a "laser television" has so far been the lack of a low-maintenance and low-cost light source for the three primary colours, red, green and blue. In attempting to change the colour of the laser source, physicists have had to rely on wickedly expensive materials from the Far East, where Chinese scientists have led the field in the manufacture of "non-linear crystals". The wafer-thin plates, the size of a finger nail, cost several thousand euros. "If the colour source in the laser is an amplifier, the non-linear crystal is an amplifier that easily over-modulates," points out Professor Gießen. Laser light is normally a sinus-shaped electromagnetic wave. However, in a non-linear medium the wave front distorts, becoming - in the words of the laser physicist - "angular" and "discordant". "It's a bit like striking a guitar string so hard that it buzzes." As a result "overtones" occur at twice or three times the base frequency. And in the same way red can, for example, become blue.

Yet even "robust" amplifiers can distort if the input signal is very strong. "We send the laser beam down a glass fibre with diameter of 125 micrometers, that's 0.125 millimetres. But the end of this glass fibre has been drawn over a flame, thinning to a diameter of just one micrometer," explains Jörn Teipel, doctoral student working in Gießen's research team. Since the light cannot leave the glass fibre it becomes concentrated - just like when we squeeze the end of a hose-pipe and create a small aperture through which the water sprays out with greater force. "The tapering effect increases the light intensity so much that the glass reacts like a particularly effect non-linear crystal." Bad news for the Far East: the fibres with the narrow waist cost less than a hundred euros to manufacture.

Bonn, Universitaet