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You are here: MEDICA Portal. Our Topics in 2009. Topic of the Month October: Laser Medicine. Devices.

Bringing Structure into Luminescent Chaos

Bringing Structure into Luminescent Chaos

In times of the financial crisis, juggling with incredible numerical amounts seems to be part of the daily political agenda. Still, even these numbers appear to be small when setting them into the relation of a quadrillion – 15 zeros follow the number 1 at the start. Regarding one single Euro in this whole lot of Euros that would make up this quadrillion is equal to one femtosecond compared to a whole second – which makes it a time span so short that it virtually does not exist: 0.000 000 000 000 001. Nevertheless, engineers work with this kind of unbelievable time span and developed laser that emit extremely short light flashes. Such flashes - travelling with speed of light - cover a distance of just half a hair's breadth. Laser are truly one of the most precise technologies in the whole world.

Nobody has been thinking about such extreme time spans when constructing the first laser in the 1960s. Back then, it has already been a great success to be able to produce coherent light. Normal light that we perceive as being white is in reality a mixture of many different colours being made up of different wavelengths - visible ones range from red to blue-purple, the not visible range comprises infrared and ultraviolet rays. Light produced by a laser, on the other hand, is made up one or just a small range of wavelengths resulting in light that can cover long distances without changing and that can be perfectly focused.

This enables doctors to treat precisely combined with a lot of energy that is focused to a tiny area. The expert can also control how deep the laser light penetrates tissue since light is being absorbed by the body depending upon its wavelength. This means that different lasers exhibit different properties and in turn are being applied for different purposes. „This is the same when considering a Formal 1 car and a tractor. The racing car is being build for the race track, the tractor for the field“, Holger Lubatschowski from the laser centre in Hanover (LZH) in Germany says.

The laser as a kind of retina sizzler

First generation lasers that were initially used in medicine helped to save people's sight by directing red coherent light into the eyes of patients suffering from retinal detachment. The laser passed the lens and vitreous body hardly being affected and was then absorbed by the loose retina and the choroid membrane which converted light into heat and the cell's proteins coagulated at 65 degrees Celsius leaving a sticky matter. The laser basically worked like a retina sizzler reuniting retina and choroid membrane. This mechanism is also the basis for nowadays treatments, eye specialists use green light, though. This laser travels even better through the vitreous body and is being absorbed more efficiently by the red-coloured choroid membrane.

Photo: Green laser light
Strong light may show itself in strong colours; ©

It is possible to change laser beam properties by changing the medium that sits within the apparatus since the principle is always the same: A gas, crystal, semiconductor or liquid is being excited through an energy source, light flashes for example, which activates the medium's atoms to emit light at a certain and predetermined wavelength. Even more atoms are being activated since light beams inside the apparatus are being reflected by mirrors. This continues to a degree where the beam is strong enough to leave the laser device.

Invisible infrared light, for example, is produced when carbon dioxide is the laser medium. This laser is suited for use in surgery because the beam cuts soft tissue as well as bones. „Laser surgery is of great use especially when operating on organs with a strong blood supply like lungs or livers“, Lubatschowski explains. This process works because infrared light is well absorbed by water, a constituent of every living cell. The strongly focused light waves can therefore not penetrate tissue more than a tenth of a millimeter and they heat up cell water occurring within this thin layer like a microwave oven. The upper cell layers are basically being cooked and subsequently evaporate. Blood vessels nearby that do not exceed a diameter of half a millimeter are being automatically closed during the procedure.

More precise and cooler through staccato

Most laser in medicine work by converting light into heat at a predetermined location. However, often higher temperatures are disadvantageous. That is why engineers have developed light flashes about a femtosecond long which is nearly equal to no time at all but with an extremely high level of power and output. „Femtosecond laser are very precise and are outstandingly suited for application in microsurgery“, Lubatschowski adds. Whatever material blocks this beam's way evaporates promptly almost without leaving any traces – true also for the human cornea. Since this laser beam does not create heat - even in the immediate proximity to the cut – it may be an alternative to adjust bad vision caused by the cornea more gentle in future.

The goal to keep laser cool during treatment has also been achieved through another way: Engineers produced compounds that exist of the chemical elements argon and fluor and used these as a laser medium. This gave rise to invisible rays in the range of ultraviolet light. These so called cold light laser emit rays with high energy without producing much heat in tissue because the light does not penetrate far due to its short wavelength. This is once again an example that not only in life but also in laser technology: it is the mixture that matters.

Wiebke Heiss


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