Referat Laser Technology - How does a laser work?

Laser Technology

What means the word LASER ? LASER is the abbreviation for Light Amplification by Stimulated Emission of Radiation.

Laser emission results from a special type of interaction of light with atoms. The difference to normal light is that laser light is a monochromatic coherent light. That means all light waves are in step with each other. Another difference is that all rays of the laser light are parallel and of the same wavelength.

The first laser was built in 1960 by the U.S. physicist Theodore Maiman. Maiman was very interested in the solid state maser (A maser is the same like a laser but it produces instead of light microwaves) that was developed by Charles Townes. So he set out to build an optical maser. Maiman operated the first successful laser in 1960 at the Hughes Research Laboratory.

There are many types of different lasers like diode lasers, HeNe, Nd-YAG or CO₂ lasers.

The pulsed laser is the most intense monochromatic light source. It can deliver a power of 10⁹ Watts over a period of 10^-11 seconds.

How does a laser work?

To understand this we have to know how normal light develops. Electrons are moving around the atomic nucleus in defined orbits. Every orbit is a special energy assigned. The lowest orbit is the orbit with the smallest energy. By supplying energy to the atom an electron can 'jump' into the next outer orbit. Those atoms are in the excited state. Electrons cannot always stay in the excited state. After a very short time they return into the basic state. This process, called spontaneous emission, sets energy in form of light free. To send out light all the time a constant supply of energy is necessary. In this event nothing happens controlled so all wavelengths of the light are radiated.

For the laser light there must be more electrons in excited state than in basic state. This can only be achieved through a constant energy supply. If a photon hits an electron that is in excited state then the electron is returning into the basic state earlier. The electron is sending out not only the own energy also the energy of the photon. This process is called stimulated emission. Light amplification, the principle of the laser emission, happens. The mechanical construction of the laser is responsible for emissioning parallel waves.

By the use of laser beam techniques extremely precise measurements are possible. The first application I want to explain is the LLR (Lunar Laser Ranging). In 1969 when Apollo 11 landed on the moon it placed passive reflectors on the moon surface. In LLR a laser beam is emitted through a telescope directly on the mirrors on the moon. There the ray is reflected and sent back to Earth. You can measure the time that the laser beam needs for the distance Earth-moon. Since you know the velocity of the light it is very easy to fix the distance between Earth and moon. This measurement has an exactness of a few centimetres.

It is also possible to record earth movements by measuring distances with the laser. To make this possible the satellite Lageos (Laser Geodynamic Satellite) was launched in 1976. Lageos is a completely passive satellite covered with 426 reflectors. It is lying in a very stable orbit 5793 kilometres high and will remain there about 8 million years. Lageos' mirrors allow laser ranging stations to take measurements of continental drifts. These measurements may make forecasting of earthquakes possible.

But lasers needn't only be used for measurements. In the case of laser machining a pulsed laser beam vaporises material. By vaporising material holes can be drilled in less than thousandth of a second. The holes range in size from 50 mm to 1 mm in diameter. A continuous laser beam can also be used for slitting film. This has the advantage that no dust is formed.

Lasers are also used in surgery. Laser beams have a selective effect on cellular components. Components that absorb light of the wavelength of the laser are destroyed the other remain unaffected. It was found that local thermal effects of a laser beam are followed by formation of scar tissue. This characteristic is used in microsurgery of the eye where the light is focused on a small spot of the retina. So damages to the lens and the cornea can often be avoided. Lasers can also be used to cure some types of skin cancer. The cancer cells absorb the laser light and are destroyed.

In the case of the laser microprobe the laser is used for analysing the surface composition of materials. This application is most found in research.

Lasers can also be adapted to produce radar-like devices of extreme precision. For underwater detection of either submarines or shoals of fish laser based sonar systems have been developed.

The most used lasers are the Diode lasers. They are most found in the digital optical signal storage and in the telecommunication. The first diode laser, or semiconductor laser, was demonstrated in 1962, but only in recent years it come into common use. In this sector there have been dramatic improvements in the last years. So it was possible to develop diode lasers having shorter wavelengths, greater power output, more consistent beam quality and greatly increased longevity. Because of new developments like the CD the consumption of improved diode lasers increased at the same time. Diode lasers have all the advantages of semiconductors. They are compact, efficient, cost effective, adequate to mass production and they have many advantages compared with the other lasers. The efficiency of diode lasers far exceeds that of most other laser types. Diode lasers typically convert 20% of the input power into radiation. For comparison a HeNe laser converts <2% of the energy. Another advantage of the diode laser is the size. Although diode lasers are very small they can deliver much power. For example a 50mm³ diode can deliver 100mW continuous power. With diode lasers it is possible to modulate the output at a very rapid rate. This is necessary in the data communication for fast data transfer rates.

Most diode lasers are made of gallium arsenide, indium phosphide, indium arsenide and their derivatives. Gallium arsenide lasers radiate at wavelengths from 660-900nm. Indium phosphide based lasers are used in the telecommunication. They lase at wavelengths from 1300-1550nm. With special semiconductor materials there are also longer wavelengths possible.

In the telecommunications industry long wavelength diode lasers are used (1300-1550nm) because of the characteristics of optical fibres. These diode lasers correspond a frequency of about 2 x 10¹⁴ Hz. Such a high frequency carrier permits an enormous data transmission rate of 4GHz. Up to 50 000 telephone conversations (64 kbit/sec) can be carried along a single optical fibre.

The diode laser market is dominated by short wavelength devices (670-850nm). The most wellknown application of these diode lasers is in digital signal storage and in compact disc systems. The digital signal on the disc is encoded as a series of short and long pits. Typically these are burned into the disc by a 50mW diode laser and are read by reflection of a 1mW focused beam. Optical disk technology is currently a rapidly growing branch of the computer industry. Now nearly every computer contains a CD-ROM drive and WORM (Write Once Read Many) systems are increasing in popularity. A disadvantage of these systems is that data cannot be erased from a disc. But at the time there are already magneto-optical systems available on the market. With these systems it is possible to write and read on a disk with the same laser.

Laser printing is a relatively new field for the diode laser. Until 1985 helium neon lasers were used in laser printers. Since then many systems have switched to 780nm diode lasers. This allowed cheaper laser printers. Laser printers are very fast printers with high resolution. For example the LaserJet 5 of Hewlett-Packard prints 12 pages per minute at a resolution of 600 dpi (dots per inch).

Diode lasers are also used for bar code scanning, holographic applications, medical diagnostics and robotics, to name just a few of the many applications.

I think in our time nobody could imagine a life without laser technology although some people don't even recognise that they are working with lasers. If we had no lasers some important things would not have been discovered. We wouldn't have such important things like compact discs. In the computer industry laser modules have become important parts and I guess in the future they will be used much more. The 'old' CD-ROM will be replaced by the DVD and all connections of the Internet will be made of optical fibres. These two important things secure the existence of the laser in the future and will be a big business in the next years. So I think the development of the laser will go on and will make much better lasers possible. These better lasers will allow new inventions and developments.