Lasers come of age

Press, 11 January 1986, Page 17

Lasers come of age

It is 25 years since the invention of the laser. JOHN CAMPBELL, a physicist at the University of Canterbury, looks at their wide-spread use today.

The solution in search of a problem was what they called it a quarter of a century ago. Today, the attitude is different. Lasers are becoming part of everyday life for many people.

In' Christchurch alone they are used in such diverse applications as restoring sight at Christchurch Hospital, reading bar codes at the supermarket check-out, initiating selective chemical reactions at the university, levelling building sites, cutting sheet steel in Bromley, “bloodless” surgery at Christchurch Women’s Hospital, playing remarkably noise-free music, measuring distance for surveyors, guiding 767 aircraft safely to the airport, and, soon, they will help carry telephone conversations across the Canterbury Plains.

What is so remarkable about these light sources? After all. the

power most radiate is only a few thousandths of a watt, which is much less than a normal 100 watt light bulb. We first need to look at how atoms become excited and how they get rid of the energy thereby gained. In an incandescent light bulb, electrons are forced through a wire, continually colliding with the atoms that make up the wire. The atoms gain energy of motion with each collision so that the wire quickly heats to white-hot. Energy balance is maintained through energy being radiated away from the filament as electromagnetic radiation; the visible light we see and the infra-red light we feel as heat.

For lighting, the process is not very efficient. Only 10 per cent of the electrical energy is converted into radiation to which the human eye responds (light) and it is spread over all colours (wavelengths).

Lights can be made much more efficient if the atoms are placed in a gas discharge tube where now the atoms’ own electrons are excited to higher quantum states. The atoms then radiate their excess energy in discrete packets (photons). Sodium vapour street lamps emit yellow light, mercury lamps blue, and neon signs red. In the light sources mentioned so far, each atom radiates away its excess energy independently of all the other atoms. The trick to producing a laser is to trigger them all to radiate in step with each other.

The basic principles of all lasers are the same. Atoms must be excited to a higher quantum state and there must be more of these excited atoms than unexcited atoms. When one atom radiates,

the emitted wave stimulates any other excited atom it meets to radiate in step with it. The number of packets of light (photons) is thereby amplified. Hence the acronym LASER — Light Amplification by Stimulated Emission of Radiation.

Many lasers use a simple trick to increase the amplification factor. The photons are bounced backwards and forwards between two mirrors, each photo making many passes along the device before being transmitted by one of the mirrors. The Helium-Neon laser is of this construction. One characteristic of this laser is its very narrow ibeam of red light. It is commonly used as a straight line to guide tunnellers and drainlayers. Anne Brailsford at Otago University is developing an apparatus using such a beam to allow tetraplegics to communicate with computers. A narrow laser beam is directed by head movement to the wanted button on the computer keyboard. Each button has a photodetector in it. A slight movement of the chin then operates a switch which tells the computer to enter that particular symbol or function. Another advantage of such a parallel beam is that it can be easily steered. A laser and a rotating mirror can produce a horizontal plane of light on a large construction site. It is then a simple matter for a good grader driver to keep the red line on a target fixed to his blade while levelling the site. A laser is very much a monochromatic (single wavelength) light source. Therefore, what little power it emits is concentrated in a i very small part of the colour spectrum. The red dot from a 10

milliwatt Helium-Neon laser shows up against the brightest sunlit background. The monochromatic nature of the laser also allows the beam to be focused to a very small spot using a simple lens. The intensity of the light at this spot is then very high And this spot is used to drill the holes in rubber teats for babies’ bottles, to drill holes in diamonds to be used for extruding copper wires, and to cut the exotic materials used in high temperature turbines..

The first laser was operated 25 years ago last July. Theodore Maiman of the Hughes Aircraft Company used a photographic flash lamp to excite the chromium ions in a ruby crystal. In those heady, early days the power of a laser was measured in Gillettes — the number of stacked razor blades a laser could blast a hole through.

There are several advantages of cutting sheet steel with a 500 watt carbon dioxide laser. Difficult shapes such as circular saw blades can be cut in one operation. As only a very narrow line is heated, the rest of the material is not heat damaged. The molten material is blown away, leaving a clean edge that requires no subsequent touch-ing-up.

The same laser is used for cutting seat-belt straps, leaving no frayed ends because the end fibres are fused together in the process. The same technique is used in “bloodless” surgery where a focused laser is used instead of a scalpel. The heat seals the small blood vessels as the cut proceeds. Recent research combines lasers and fibre optics to unclog partially blocked arteries.

It is the coherence of the laser light (the fact that all atoms radiated in step) that allows those three dimensional photos (holograms) to be taken. These are occasionally seen in the window of the local magic shop or on the front cover of National Geographic.

The Russians are using holograms to record some of their national treasures. The D.S.I.R. is to study deformation of crash-helmets through holography. Lasers are big business. Worldwide sales in 1984 topped ?US4 billion. The most common type is the cheap semiconductor laser used in reading (playing) compact audio disks and for communications via glass fibres. Such lasers are reportedly available in Japan for less than $5. The world’s most powerful laser is Nova, a neodymium-doped glass laser with an amplifier chain 137 metres long. This monster at the Lawrence Livermore National Laboratory in California took 8 years to build and cost JUSI76 million.

Working in the infrared region of the spectrum, Nova produces 100 million million watts of power in each incredibly short pulse. It is used as a match in attempts to ignite a nuclear fusion reaction in pinhead size pellets of deuterium and tritium, the heavy isotopes of hydrogen. The goal is “clean” nuclear power. Nova is also used in weapons simulation studies. ,* major growth area is in the *7

medical applications of lasers. Some remarkable uses in the future are pointed to. The photo-active drug hematoporphyrin derivative is retained in malignant tumours over a period of 3 to 5 days, whereas it is rejected by normal tissue within 1 or 2 days. A laser can be used to selectively excite the drug molecules. The malignant tumours and cells fluoresce, allowing the extent of the malignancy to be readily determined. It is also a check that all malignant cells have been removed during surgery. Surface tumours can themselves be killed by illuminating the drugged cells with laser light of the correct wavelength. A toxic entity is released in this photochemical process that kills off the malignancy within a few days of irradiation. The new gold vapour laser emits the wavelength (0.628 micrometres) required for this treatment.

Twenty-five years ago the laser was merely a scientific curiosity. Who knows what remarkable developments will evolve during the next quarter century?