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All About Lasers

Even before the invention of the laser in 1960 the concept was familiar to readers of science fiction, as a 'death-ray'. Although today this image is still very much with us, lasers have many uses which are rather more 'down-to-earth'. They have become very common and essential tools in factories, work shops, offices and now even in our homes.

In the simplest terms, a laser can be described as a source of light or 'radiant energy'. It has, however, many special properties which make it quite distinct from other light sources such as the sun, a candle, an electric light bulb or a fluorescent tube.

The amount of energy that a laser is able to emit, or 'radiate', depends on the material from which it is made. It is possible to use materials in all the states of matter, i.e. gas, liquid or solid, and at temperatures varying from as low as minus 100oC to many hundreds of degrees centigrade. As a result, lasers come in all shapes and sizes. They can be smaller than a full stop, or so big that they need literally acres of buildings to house them. In spite of this diversity, they all operate on the same fundamental principle.

The rate at which light sources radiate energy is known as their power, and this is measured in watts (w), e.g. a 100w electric light bulb. The power coming out of lasers varies over an enormous range. It can be as low as microwatts (1/1,000,000 of a watt) or as high as terawatts (1,000,000,000,000 watts) depending on the type of laser. Clearly, the choice of laser to be used depends on the amount of energy or power needed and the amount of physical space available. For some applications, such as cutting steel plates or welding together sheets of metal, very powerful lasers are required which can be quite large. These lasers, on the other hand, would not be at all suitable for performing delicate operations in eye surgery or for 'reading' a laser disc in a video machine. Most of the telephone links between major cities are now made with optical fibers using lasers smaller than a pinhead as the source of the signals transmitting the information. These lasers would not be at all effective for welding.

In the years since lasers have been invented, they have helped to bring about dramatic advances in science and technology. Because the properties of lasers can be very precisely controlled, they can be used to make measurements with an accuracy not possible using other light sources. They have hundreds of uses in research as well as in industry, and even in our everyday lives, yet relatively few people have a clear idea of what they are or how they work.

(see image below) Science-fiction's popular image of the laser - Darth Vader with a light saber

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To have an initial insight into the operation of a laser, it is not necessary to look any further than the name. Laser stands for Light Amplification by Stimulated Emission of Radiation. Amplification is simply the name given to a process in which the 'intensity', or the amount, of something is increased. Most people are familiar with sound amplification in a megaphone or a hi-fi system. In a laser it is light - or rather a special form of light - which is amplified. To understand how this happens we must first understand the nature of light.

(see image below) Both are lasers but differ remarkably in size. The image on the left is a semiconductor diode laser is the speck on the forefinger of the hand while the picture on the right is largest laser and fusion chamber in the world located in Lawrence Livermore National Laboratory.

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The nature of light

'Light' to most people means daylight or the light from a candle or an electric light bulb, which enables us to see the world around us. This is known as white light because it does not appear to have any predominant color. In 1665 Sir Isaac Newton showed that if ordinary white light was passed through a glass prism, it split up into the different colors of the rainbow. This process is known as dispersion and the range of different colors is called the visible spectrum. It was later shown that there were other non-visible forms of radiation passing through the glass prism, as well as the colored light. If the region just beyond the red beam was focused with a lens on to a dark object, the object became hot. This type of invisible radiant energy is known as the infra-red.

(see image below) A prism can disperse a 'white light' into its component colors. Visible light only represents a small part of the electromagnetic spectrum.

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Visible radiation (which is what we think of as light) and infra-red radiation (which we sometimes think of as 'heat') are both types of electromagnetic radiation and these, together with gamma rays, x-rays, microwaves, radio waves and ultra-violet light (which is what gives you a sun tan) make up a continuous range of radiation. This is called the electromagnetic spectrum. All electromagnetic radiations travel through free space at a constant speed of 299792900 meters per second, and although at first they may seem to be very different, they are all forms of radiant energy with the same basic properties.

How does this radiant energy travel through space? Scientists have two different ways of answering this question. The first is that light travels as a wave in a similar manner to water waves. If you throw a stone into a pond it will make circular ripples or waves which travel across the surface of the water. In the same way, light travels as a wave. The distance between two crests or peaks of the wave is called the wavelength and the number of waves which go past a fixed point in one second is called the frequency. Each different color of light has a different wavelength. Blue light has a wavelength of approximately 0.0000004 meters or 400 nanometers (one nanometer = 0.000000001 meters), green light has a wavelength of about 500 nanometers, and red light a wavelength of about 600 nanometers. The wavelengths of the electromagnetic spectrum vary continuously from less than 1/10 nanometer for x-rays to hundreds of meters for radio waves.

The other way of describing light is as a stream of particles. These are called photons and each photon of light of the same wavelength contains exactly the same amount of energy. The amount of energy in a photon increases as the wavelength of the light decreases, so a photon of blue light will carry more energy than a photon of red light because it has a shorter wavelength. This can be understood if you think of the photon as a very small packet of waves. The shorter the wavelength of the light wave, the more waves will fit into a packet, so the more energy it will have.

A laser emits light energy in the form of travelling waves, but with a low angular spread compared to ordinary light. Each color of light has a different wavelength.

Absorption and emission of light

In order to explain the way in which a laser works, it is necessary to understand how light is produced and its interaction with matter. All matter is made up of groups of very small particles. The groups are called molecules and the particles are called atoms. These atoms themselves are made up of even smaller particles which are arranged in a way that is similar to our solar system. At the centre of the atom there is a nucleus which consists of protons and neutrons. Protons have a positive electric charge and neutrons have no charge. Around the nucleus there are even smaller particles called electrons. These have negative electric charge and they travel around the nucleus in much the same way as planets orbit around the sun. The orbits in which the electrons can be found are called energy levels. This is because each different electron orbit has a different energy. The further away from the centre, or nucleus, that the electron orbit is, the higher is its energy. The electrons can sometimes go from one energy level (or electron orbit) to another but in order to be in a particular energy level, they must have exactly the right amount of energy. If an electron moves outwards to a higher energy level it must gain some energy, and if it moves inwards to a lower energy level it must lose energy. The way that an electron can gain or lose energy is for the atom to absorb or emit a photon of electromagnetic radiation.

(see image below) Albert Einstein, who is best known for his Theory of Relativity, was first to introduce the idea of stimulated emission.

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We have seen that a photon of light at a particular wavelength has a particular energy. If a photon comes into contact with an atom which has two energy levels with an energy difference exactly equal to the energy of the photon, then an electron in the lower energy level may be moved up into the higher energy level and the photon will be absorbed. When this happens the atom is said to be in an excited state. It is this process that leads to the color of most of the things around us. For example, a red dye is one which absorbs all the visible wavelengths except those corresponding to red. Atoms or molecules can only remain in an excited state for a very short time, known as the excited state lifetime. This depends on the particular atom or molecule and can be anything from a millisecond (0.001 seconds) to a picosecond (0.000000000001 seconds). After this time the excited electron drops into a lower energy level (but not necessarily the one that it came from). To do this it has to lose some energy which it does by emitting a photon of radiation. The wavelength of this radiation will make the photon energy exactly equal to the difference in the two electron energy levels. When all the electrons are in the lowest energy levels that they can be in, the atom is said to be in its ground state.

Another process can also occur which is vital for laser action. This was proposed by Albert Einstein in 1917 and is called stimulated emission. If a photon emitted from an atom or molecule comes into contact with another similar atom or molecule which is in an excited state, then the photon can cause, or stimulate, another photon identical to itself to be emitted. Normally this does not happen, but if there are many atoms in an excited state which are packed close together - as there are in a laser - then it is possible for so much stimulated emission to occur that it becomes more important than normal emission. Using these processes we can now explain how a laser works.

When a photon of the correct energy is absorbed, an electron in the atom gains the energy and moves to a higher orbit. A photon is emitted from an atom when an electron loses energy and drops down from a higher to a lower energy level. Stimulated emission, a photon causes an atom in an excited state to emit an identical photon.

How a laser works

Any laser basically consists of three items: (1) a material which acts as the light amplifier; (2) a source of energy; (3) and a set of mirrors. The light amplifier is called the active medium and can be made out of many different materials which may be solids, liquids or gases. The source of energy is used to excite the atoms or molecules in the active medium so that they can produce stimulated emission. The mirrors are used to send the emitted photons back into the active medium so that they can cause more stimulated emissions.

One of the most common sources of energy used in lasers is the flash lamp. This is similar to the flash lamp on a camera but much more powerful. When it is switched on it emits light in all directions. Some of this light is absorbed by the atoms or molecules in the active medium and they go to an excited state. In this way the atoms or molecules can store energy fm a short time (the excited state lifetime) which can be used in the laser. The flash lamp must put the atoms or molecules into an excited state (i.e. store energy) faster than they can decay back to the ground state by normal (or spontaneous) emission. This is like having an up and a down escalator between two floors. If the up escalator is faster than the down escalator then the people will tend to get stuck on the upper floor. When a similar process happens with the excited atoms or molecules it is called population inversion and this is always necessary for laser action.

Optical Pumping excites the laser active medium through absorption. Spontaneous emission occurs in all directions. Stimulated emission builds up along the feedback axis of the laser. A cascade of stimulated emissions provides amplification and laser action.

The excited atoms emit photons of light in all directions. Some of this light will go out through the sides of the active medium, and will be lost, but some of it will go down the length of the active medium, and the photons will meet other atoms in an excited state and produce stimulated emission. The stimulated photons, together with the original photons, will meet further atoms in an excited state and produce more stimulated emission. This process (which is called a cascade) continues even after the photons have come out of the active medium because they will bounce off the mirrors and be reflected back in again. In this way just one photon can produce millions and millions of other photons exactly the same as itself by the process of stimulated emission. In most lasers one of the mirrors, which is called a partial reflector, will only reflect part of the light that hits it and will allow some of the light to go through. A window pane is a good example of a partial reflector. Most of the light will pass through it, but if you look carefully you can see a weak reflection of yourself. In a laser the partial reflector is made to give a much stronger reflection - often only 1/100 of the light is allowed to pass through and the rest is reflected. The light that does pass through the laser mirror is called laser light or a laser beam and because it is made up of photons which are exactly the same, and which are travelling in the same direction, it has many special properties.

Laser light is different from ordinary light in two important respects: it is monochromatic and it is coherent. Monochromatic means 'one color' and it is easy to see that if all the photons in a laser beam are the same, they must all be of the same wavelength - and therefore the same color. This is very different from sunlight which when it arrives at the earth's surface contains all wavelengths from 320 nanometers to 1100 nanometers - all the visible spectrum and beyond! Electric light bulbs (and the flash lamps which are used to excite lasers) also emit light over a large part of the electromagnetic spectrum. Lasers concentrate all their energy at one wavelength and this can be very useful. To understand how a laser is coherent is a little more difficult, and we have to remember that light travels in waves. The photons in a laser beam, because they are produced by stimulated emission, have all their waves exactly in step so that each crest of a wave is lined up with every other crest, and each trough with every other trough. This is described as being in phase (or being coherent) and if all the waves are added together, the result is a very strong (or intense) beam. If we have waves which are not in phase - i.e. waves which are out of step - then when we add them together they tend to average out and produce a much weaker, or less intense, beam. This type of light is called incoherent. Finally if we think about white light, which is made up of many different colors or wavelengths, the waves cannot ever add together coherently because all the crests and troughs of the waves cannot ever exactly overlap. In essence, what happens in, for example, a Ruby laser is that ordinary incoherent white light is 'converted' to monochromatic coherent laser light. We shall see later that coherent light is vital for many applications of lasers.

monochromatic coherent light (in phase) = intense beam

monochromatic incoherent light = less intense beam

non monochromatic incoherent light = less intense beam

How lasers are created

There are many different types of laser now available, and the construction of each individual type varies considerably. There are, however, three main processes in the manufacture of any laser system: electrical, optical and mechanical. This section refers to a Ruby laser, which was the first type to be invented.

The active medium in a Ruby laser is a long cylindrical crystal of ruby called the laser rod. The source of energy used to excite the molecules inside the laser rod is a flash lamp. A large capacitor is used to drive a very fast burst of electric current through the flash lamp. This causes the gas inside to give out a flash of light, as with a camera flash or a fluorescent light tube. The light then excites the molecules in the ruby rod, ready for stimulated emission.

In the Ruby laser the flash lamp is often helical and is wrapped around the laser rod. Around the outside of the flash lamp there is a cylindrical metal container with a highly polished inside wall which reflects any stray light from the flash lamp into the laser rod. This whole arrangement is sometimes called the laser head. Other similar lasers use a slightly different flash lamp which is straight or linear. This is usually placed inside an elliptical container which has very highly polished inner walls, and which also contains the laser rod. Using this type of container, all the light from the flash lamp is automatically reflected into the laser rod.

The mirrors in the laser are made with great care. The material used to produce mirrors of very high reflectivity must be evaporated in a vacuum and deposited on top of polished glass discs in very thin layers that may be less than 0.00002 centimeters thick. The glass discs, which are called substrates, are so flat that they have no dips or bumps greater than 0.000005 centimeters deep - and the same is true for the ends of the laser rod, which are also polished. All the optical processes - e.g. producing the crystals for the laser rods, making the mirrors, and polishing the ends of the laser rods and the substrates, must be carried out under ultra-clean conditions. Special rooms are used for these purposes, called clean rooms, which do not contain any dust or moisture. It is particularly important to avoid contamination when the laser crystals are being made, since a tiny amount of impurity can be enough to stop the laser working.

Very precise mechanical holders are needed to keep the mirrors and the laser rod steady enough to reflect the stimulated emission exactly back along the laser rod. If the photons are not reflected back upon themselves, there will be no laser action. In many lasers the positions of the mirror holders must be accurate to within 0.00001 centimeters. It is therefore important to isolate the laser from external vibrations.

(see image below) A technician carries out a delicate piece of work in ultra-clean conditions. Note the almost total coverage of the head and body. Even minute specks of dust can effect laser operation - especially with semi-conductor lasers.

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Solid state lasers

The first laser was invented in 1960 by Dr T. Maiman. This was a solid state laser, the active medium being a rod of synthetic Ruby crystal. Most solid state lasers have active media consisting of a crystal which has had some of its atoms replaced by atoms of a different element; these are the active molecules. In the case of ruby, the crystal consists of a regular structure of Aluminum Oxide molecules (the host material) in which some of the aluminum atoms have been replaced by chromium atoms. Chromium atoms absorb a wide range of wavelengths in the ultra-violet, green and yellow spectral regions. This is what causes the ruby crystal to be red. The more chromium atoms there are in a ruby crystal, the deeper is the red color. The original crystal used by Maiman had one chromium atom for every 2000 aluminum atoms, and a laser wavelength of 694.3 nanometers in the red. It is not necessary always to have a crystal as the host material in a solid state laser. Glass and even plastic have also been used. Unlike crystal, the glass or plastic hosts do not have a regular repetitive structure.

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The main disadvantage of the original solid state lasers was that their wavelength was fixed. By changing either the concentration of active atoms, or the host material, it was possible to change the laser wavelength of a solid state laser containing particular active atoms - but only by a very small amount. Different wavelengths were obtained by using different active atoms.

The high power output from a solid state laser can be directed on to the work piece either by reflecting off mirrors in an enclosed flexible arm or through optical fibers.

Scientists have developed new solid state lasers which can lase over a wide spectral range without having to change the active atoms. For example, Chromium atoms can lase from 700 - 815 nanometers in a Berylium Aluminum Oxide host, and Titanium atoms can lase from 660 - 986 nanometers in an Aluminum Oxide host (Sapphire).

The source of energy for solid state laser is usually a flash lamp. Mostly the flash lamps are pulsed (i.e. switched on and off in bursts) and this results in a pulsed laser output. The repetition rate can be anything from once every few hours 5000 times a second, depending on the laser One of the most common solid state lasers, however, is Neodymium in a Yttrium Aluminum Garnet host or Nd:YAG, and this can be pumped using a continuously operating arc lamp which results in a continuous laser output.

Solid state lasers are most commonly used in high power applications such as welding and cutting, as well as in infra-red vision system and scientific research.

Active Element & Laser Wavelength

Erbium: 1610 nanometers

Europium: 610 nanometers

Holmium: 2050 nanometers

Neodymium: 1060 nanometers

Samarium: 710 nanometres

Thulium: 1120 nanometers

Ytterium: 1020 nanometers

Gas lasers

Many different types of laser have been built which use a gas, or a mixture of gases, as the active medium. The operational wavelength varies from the ultra-violet to the infra-red depending on the gases used. The table below shows the laser wavelengths of some of the more common gas lasers.

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Unlike the solid state systems, the source of energy for laser action in gases is very rarely a flash lamp. Instead, the active atoms are excited by an electric discharge. This is similar to what happens in a neon sign or a fluorescent light tube. An electric current, which is basically a continuous stream of electrons, is passed through a glass tube containing the laser gas. The electrons collide with the gas atoms and transfer some of their energy, thus exciting the atoms ready for stimulated emission. This is called collisional excitation. The laser cavity is usually formed by two mirrors, one at each end of the gas tube.

When C02 (carbon dioxide) is used as the gas, it is possible to build lasers with active laser lengths, or gain lengths, as long as 60 m. This very long gain length means that they can produce up to 50 kw (50,000 w) of continuous laser power in the infra-red.

At the other end of the scale, the Helium-Neon laser produces only a few milliwatts (l/l000's w) of power. This works in a slightly different way, because the electrons collide with the Helium atoms and then the excited Helium atoms collide with the Neon atoms to put them into an excited state. Laser action then occurs through stimulated emission, in the usual way. A very useful type of gas laser is the Noble ion laser. This uses an inert, or 'noble', gas which is also collisionally excited by an electric discharge. In this case, however, the atoms lose an electron completely and become ionized. The most common are Argon ion and Krypton ion lasers which can generate several watts of continuous power in the ultra-violet, visible and infra-red. These are often used to excite other continuous lasers.

Another important class of gas laser is the Excimer laser. In a mixture of gases a burst of electric discharge chemically generates a new molecule, which exists for a very short time (a few nanoseconds, i.e. 0.000,000,001s) and lases during that time. These lasers include Krypton Fluoride, Xenon Fluoride, Argon Fluoride and Xenon Chloride and may produce pulsed powers of up to gigawatts (1000,000,000 w). There are many other types of gas or vapor lasers which all work in roughly the same way.

Dye lasers

Most people are more familiar with dyes as chemicals used for coloring rather than as laser active media. Their range of color, however, is what makes them useful to laser physicists, since they absorb light over the visible spectral region. Dyes have the great advantage of being tunable, which means that they can change their laser wavelength over a wide range - much more than most other types of laser active media. Not every dye may be used in a dye laser, but there are many hundreds which can, and they lase over the whole spectral region from 250 nanometers in the ultra-violet to 1800 nanometers in the infra-red. Dyes may be used in a laser in the form of a solid (absorbed into a plastic rod), a liquid (in a solution), or a gas (a vapor). They are generally used in alcohol or ethylene glycol (anti-freeze) solutions. In solution, the dye molecules collide with the solvent molecules and transfer energy. This has the effect of making the energy levels in the dye molecules rather vague - or smeared. Therefore, when the excited electrons decay back to a lower energy level, they may go to a variety of other energy levels and thus emit a variety of other photons of different wavelengths. Thus it is possible to vary, or 'tune', the laser's wavelength.

(see image below) An incoming pump laser beam excites a jet stream of dye and allows tunable laser action.

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The source of energy for a dye laser may be a flash lamp or another laser. In the first case, a linear flash lamp is used in an elliptical chamber through which the laser dye is made to flow. The flash lamps are pulsed so that they can give out enough light to excite the dye molecules. Alternatively, another laser, called a pump laser, may be used to excite the dye. This is usually a pulsed nitrogen laser or a continuous Argon ion laser. When a continuous Argon ion laser is used, the dye solution flows very rapidly through a nozzle to give a thin sheet of dye less than 0.02 cm thick. The pump laser beam is focused down to a spot size of 0.003 cm in the dye jet and some of the dye laser mirrors are curved to focus down the dye laser beam to the same size. This type of laser is capable of producing very short pulses which are used by scientists to investigate very fast processes in nature. Dye lasers are not as energy-efficient as solid state lasers or gas lasers, so they cannot produce such high average output powers. However, their tunability makes them vital for many laser applications.

Other types of laser

Of the other types of laser, two of the most important are the semi-conductor laser or diode laser and the chemical laser. Though its active medium is a solid, the semi-conductor laser is quite different from the other solid state lasers described earlier. It is probably the smallest laser (as small as 0.1 cm) and the active medium from which the light is emitted may be only microns (0.0001 cm) in size, requiring a microscope to see it. Electricity is the source of energy for this laser, and is applied directly. In construction the semiconductor laser is similar to a transistor - consisting of a junction between p (for positive) and n (for negative) semi-conductor material. The light is emitted from this junction. Its wavelength and output power depend on the type of semi-conductor materials used. It also depends on the actual structure of the device. A typical diode laser made of GaAlAs (Gallium Aluminium Arsenide) emits 10 - 20 milliwatts, and is tunable in wavelengths between 750 nm and 900 nm. If many diode lasers are linked together so that their outputs are all in phase, it is possible to generate up to a watt of coherent output power. This arrangement is called a phase coupled array. Because of their high efficiency, compact size, and low cost, semi-conductor lasers have become very important in many areas of today's technology.

The chemical laser is also very important. It works by mixing together two compounds which undergo a chemical reaction to form a new compound. This compound is automatically in an excited state (so no external source of energy is needed) and can therefore lase. Hydrogen Fluoride is a good example of a chemical laser. It can emit up to 200 watts c.w. (continuous wave) in the infra-red spectral region. As chemical lasers can be made of considerable size, it is possible to obtain very high laser powers from them.

There are many other types of laser, including the COLOUR CENTRE LASER, the FREE ELECTRON LASER, the ORGANIC VAPOUR LASER and the X-RAY LASER. Basically they all have the same mechanism as the laser action which has already been described.

High power lasers

There are many applications for very high power lasers, both in industry and in scientific research. These high laser output powers are obtained by either Q-switching or mode locking, and by simply building lasers which are so big that they can store huge amounts of energy that can be released very quickly.

The Berkeley Lab Laser Accelerator or BELLA is a laser created by the Thales Group and operated and owned by the Lawrence Berkeley National Laboratory. On July 20, 2012 BELLA fired a 40 femtosecond laser pulse, achieving a world record for most powerful laser in the world.

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The lasers used in industry are obviously much smaller. One of the most common is the c.w. carbon dioxide laser which is used for the cutting, drilling and welding of metal. When the laser beam is focused down to a very small spot on the surface of the metal, it is absorbed, and the metal is burnt away, leaving a very clean hole drilled through it. If the spot from the focused laser beam is moved along the surface of the metal, it can be used to cut out shapes. Welding is also possible if two pieces of metal are placed close together and the focused beam is moved along the join. The beam melts the edges of the pieces of metal and the molten metal then mixes together, cools down and solidifies as the beam moves on, There are many advantages in using lasers for this type of work because there is no mechanical contact with the metal and so there is nothing to wear out. Also, the laser beam can be easily directed to where it is needed by simply using a set of mirrors.

Short pulses from lasers

The lasers which we have described operate either continuously (eg Argon ion, Nd:YAG, Dye etc) or pulsed (eg Ruby, Nd:Glass, etc). The pulsed systems operate in bursts lasting a few microseconds or nanoseconds and tend to produce higher output powers than c.w. lasers. In general, it is not possible to operate many of the pulsed lasers continuously but it is possible to make the c.w. lasers produce pulses - and this is done for a variety of reasons.

A c.w. laser, or long pulsed laser, can be forced to emit all its stored energy in short pulses and therefore increase its output power (since the same energy is emitted over a shorter time). This is called Q-switching and is achieved with the use of a very fast shutter (called a Pockel's cell) inside the laser. When the shutter is closed, the photons emitted by spontaneous emission cannot travel backwards and forwards between the mirrors, so they cannot create a cascade of stimulated emission - i.e., there is no laser action. Thus the energy stored in the laser keeps building up until most of the molecules of the active medium are in an excited state. When the shutter is opened, laser action becomes possible, and the sudden cascade of stimulated emission 'sweeps out' all the stored energy very quickly. This results in a powerful pulse of laser light lasting for about 10 nanoseconds. The Q-switching technique is commonly used to increase the output powers from solid state lasers such as Ruby, Nd:YAG or Nd:Glass.

For some applications, such as optical communications, it is more important to have very short pulses rather than high output powers. Very short pulses (less than 0.000,000,000,001 second or one picosecond) are produced using a technique called mode locking. This works using even faster shutters inside the laser which may be controlled by fast electrical signals (like in the Pockel's cell), or which may operate automatically.

The second method normally has a solution of dye as the shutter, and is commonly used in solid state lasers. The dye is called a saturable absorber and absorbs the photons at the laser wavelength - thus preventing the laser action. After a time, however, so many photons have been absorbed that most of the dye molecules are in an excited state. When this happens the dye solution cannot absorb any more photons and is said to be saturated. The cascade of stimulated emission is then free to go ahead - resulting in laser action. This situation lasts as long as the dye molecules remain in the excited state - i.e. as long as the excited state lifetime. The dye molecules then decay back to their ground state and are ready for more absorption, so the laser action stops again. Thus the laser operates for short periods of time (i.e. it emits pulses) roughly equal to the excited state lifetime, which may be as short as a few picoseconds. This type of mode locking using a saturable absorber is called passive mode locking. A similar process can occur in dye lasers producing pulses as short a 30 femtoseconds (one femtosecond equals 0.000,000,000,000,001 second).

In order to take a photograph of a moving object, a flash is needed which effectively freezes its motion. The time scale of processes in atoms and molecules in nature is of the order of femtoseconds, therefore the mode locked femtosecond pulses can be used to examine the processes taking place by effectively freezing the motion of the molecules.

The output from a mode locked laser consists of a train of very short pulses, each separated by the time it takes light to travel once up and down between the mirrors of the laser. One pulses every 1 - 10 nanoseconds depending on cavity roundtrip time.

Optical communications with lasers

The past years have seen changes in the world of telecommunications, primarily due to rapid advances in the development of semiconductor laser diodes and low-loss, long-length glass fibers. Telecommunication is the method of sending information over long distances by the use of telephones, telex, telegraph etc. Basically, these devices work by converting text or speech into electrical signals which are transmitted down long lengths of electrical cable to receivers. The receivers then convert the signals back into their original form. The information to be sent is generally digitized, or converted into a code which is called binary. This code uses only the numbers '1' or '0' which are very easy to convert to an electrical signal, as '1' corresponds to ON and '0' corresponds to OFF. Binary code is written in base 2 so that '5' becomes '101' (i.e. 1 x 2o + 0 x 21 + 1 x 22). It is easy to see that the faster the electrical signal can switch on and off, the faster the information can be transmitted. A typical telephone system 'switches on and off' at around a few million times a second. It is said to send information at a 'BIT rate' of a few Mega BITs (MBITs) per second. A BIT is simply the smallest amount of information corresponding to one digit of binary (i.e. '1' or '0').

(see image below) An optical fiber pulled from a molten pre-form is so fine that it can easily pass through the eye of a needle.

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In today's world where people, machines and computers want to 'talk' to each other all the time, it is difficult to send information fast enough. One way to speed up the BIT rate is to use optical signals instead of electrical ones. In an optical system, the electrical cable is replaced by an optical fiber, which is made of glass and consists of two concentric cylinders. The thin central section, known as the core, has a diameter of only 0.0005 cm, and the outer cylinder, or cladding, has a diameter of about 0.01 cm - roughly the thickness of human hair. Around the cladding there is a plastic coating for protection. The glass used for the core is more dispersive than that used for the cladding. Consequently, any light inside the core is unable to get out - and is guided down the fiber.

The optical signals to be sent down the fibers are generated by very small semi-conductor laser diodes which can produce very short pulses. Each pulse can be used as a BIT of information (i.e. a '1' or an 'ON'). The shorter the pulses are, the more BITs of information can be sent in a given time. A typical optical communication system can send a few hundred MegaBITs per second (i.e. over a hundred times more than a conventional electrical system). Electricity still plays a role in an optical telephone system, as it is necessary to use a microphone to convert the speech into electrical signals which are then digitized and used to drive the semi-conductor laser diode.

The laser diodes emit in the infra-red spectral region where the loss due to absorption in the glass fibers is very low. Sometimes, however, when the information is being sent over very long distances, it is necessary to 'boost' the optical signal. This is done using special laser diodes called repeaters every few kilometers which amplify the optical signal as it passes through them.

British Telecom have replaced many conventional copper telephone cables with optical fiber systems. As well as being faster, they are also much cheaper to make. Optical fiber systems are used for linking computers, television monitors and even video telephones. As research progresses, optical systems are replacing many electronic systems, because they are much faster

The next generation of supercomputers may well be mostly optical. Some systems do not even require optical fibers. In space, where there is nothing to absorb or block the laser light, satellites can communicate by direct laser beam transmission.

Lasers in medicine

As well as high power applications in heavy industry, lasers also find many applications in more delicate areas such as medicine. The high precision with which lasers can be controlled - in terms of their power, wavelength, pulse duration and direction - make them capable of performing very intricate operations. Today, they have become standard equipment in many hospitals.

One of the earliest - and now one of the most common - laser operations is retinal welding. The retina is the light-sensitive area at the back of the eye and sometimes it comes away from the eyeball, resulting in discomfort, impaired vision and eventually blindness. Using a beam from a pulsed dye laser, or a Nd:YAG laser, focused in through the eye, it is possible to weld the retina back into place. Naturally the laser is carefully controlled to prevent any damage and, in fact, the small weld which it does generate does not impair vision at all.

The wavelength-tunable nature of laser light is also useful in surgery, as some wavelengths are selectively absorbed. For example, skin defects - either natural, such as birth marks, or man-made, such as tattoos - absorb light at different wavelengths from normal skin. If a laser, tuned to the right wavelength, is directed on to such a defect, the light is preferentially absorbed by the defect which heats up and vaporizes, leaving a scar tissue. Later it is replaced naturally by normal skin. Preferential absorption is also used in the treatment of some cancers. There are certain 'marker dyes' which, when injected into the body, get adsorbed by, or linked to, only cancerous cells. By using a laser of the wavelength that is preferentially absorbed by these dyes, the cancerous cells can be vaporized without damaging the surrounding healthy tissue.

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Carbon dioxide lasers are used for 'bloodless surgery', replacing the surgeons' scalpel. As the laser beam is passed through flesh, severing blood vessels, it welds the end of the vessels together, so preventing loss of blood. Another promising field of laser surgery is laser angioplasty. This uses optical fibers to guide laser light into coronary arteries which have become calcified (i.e. blocked), leading to heart disease and heart attacks. Pulses from an excimer laser have been fed down the optical fibers and, in initial tests, have evaporated the calcified material without causing heat damage to surrounding tissue - thus restoring unrestricted blood flow.

In other branches of surgery, lasers are used to shatter and disperse kidney and gall stones. They have been used in brain surgery, where very accurate pin-pointing of tissue to be removed is essential, and they have even been used in cosmetic surgery to remove skin wrinkles in facelift operations.

Lasers at war

The idea of lasers used as weapons has been around in science fiction for quite a long time. In reality there are many military applications for lasers, both on and off the battle field.

One of the earliest military uses of the laser was as a range-finder. This has now become a simple, portable device which consists of a miniature battery-powered infra-red laser (normally Nd:YAG), an infra-red detector, and a simple, dedicated, microchip computer. It works by sending a short pulse of 'invisible' infra-red laser light at a target and timing the return of the reflected light pulse. The distance to the target (i.e. the range) can then readily be calculated, since the speed of light is a known constant. Modern warfare has also introduced the guided missile or 'smart bomb' which directs itself towards an object using a 'target designator'. This is simply an infra-red laser which is directed on to a target and stays with it. The missile also has a detector, or receiver, which picks up the reflected light from the object, locks on to it, follows it down, and hits the target. Obviously a great deal of research is being done into the kind of devices which can pick up these target designator beams and block them or confuse the sensors on the missile.

The old idea of a death-ray has also become a reality. A massive gas laser has been successfully mounted on a United States Air Force plane and has shot down an air-to-air missile. As laser beams can travel through space without being deflected or absorbed, they have also been proposed for space-based weapons systems which will destroy enemy missiles and satellites. This, however, will be very difficult to achieve in practice, since the lasers would have to be enormous, and would therefore be difficult to build, maintain, operate and even to launch into space. Also, the actual destructive power of a laser beam is much less than can be achieved with conventional or nuclear warheads.

In order to be able to respond quickly enough to enemy missiles, the 'Star Wars' systems would need very fast, powerful computers (probably optical computers) to control them. So far, these have not been developed.

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Holography

Although the concept of holography was introduced by Professor D. Gabor in 1948, it was not until the invention of the laser that holography became a practical technology. A holograph is similar to a photograph, except that a photograph records only the intensity pattern of an image and is therefore two-dimensional whereas a holograph also records the phase information and has the depth of a 3D image. The third dimension (i.e. the depth) arises from light waves which arrive later than the light waves from the front of the image. In order to be able to reproduce a three-dimensional image it is necessary to know about the delay between subsequent light waves (the phase delay).

Ordinary light has random phases (because it is incoherent), so it is difficult to measure any systematic phase delay. Laser light, on the other hand, is coherent. Therefore if one set of waves is delayed in relation to another, it is easy to measure the extent to which they are out of step. A hologram is recorded on a special type of photographic plate (called a holographic plate) using a reference beam, which comes straight from a laser, and an object beam. The object beam has a range of phase delays because it has been reflected off all parts of the object. When these two beams are overlapped on the holographic plate they form an interference pattern consisting of light and dark fringes. Where the waves are exactly overlapped, or 'in phase' (i.e. crest on crest) there are bright fringes. Where the waves are completely out of phase (i.e. crest on trough) there are dark fringes. Hence, by means of a pattern of fringes, the hologram records the phase delay information from all the parts of the object. To reconstruct the object as a three-dimensional image, a laser beam is directed on to the fringe pattern (which makes up the hologram) and the waves in the laser beam are delayed by exactly the right amounts, to form a 3D image. This image or holograph will be the same size as the object. Like the object, it will look different when it is viewed from different angles, whereas a photograph always looks the same from any angle.

There are many applications of holography. As well as being used for displays, exhibitions, decoration and jewelry, they have become very important in the manufacturing industries. One of the most notable applications is holographic interferometry. A hologram is made of a perfect specimen of - for example - a propeller. On top of the original another hologram is then made of a propeller of lesser quality. If there is a defect on the test propeller, when the two holographs are superimposed, the defect (which will have caused further delays in the light waves) will produce an interference pattern. This technique is used anywhere where stress may have caused unacceptable deformations.

Holograms, because they are difficult to forge, are used in the field of security.

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To produce holograms, high stability in the recording apparatus is necessary, since very small movements of less than half a wavelength (0.00005 cm) would lead to noticeable phase delays in the reflected light. Because high quality holograms are so difficult to produce, they cannot easily be forged and are used for security purpose - such as on credit cards and cheque cards.

Everyday uses, present and future

In the years since lasers were invented, their uses have become surprisingly widespread - particularly that of the semiconductor laser. For example, most modern supermarkets use bar codes to label their merchandise. These codes consist of a row of black lines of varying thickness. They are automatically read by a device which feeds the information straight into the cash till, and probably into a computerized stock control system. This device contains a small semiconductor diode laser, the beam from which is scanned across the bar code. By monitoring the reflected beam, which changes as it goes across the dark lines, the information in the bar code is easily obtained.

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The output from a continuous wave semi-conductor laser passes through a prism-beam splitter arrangement and is focused using the lens assembly on to the surface of the moving disc. The light reflected from the etched surface which contains the digital information, is collected by the lens system and reflected off the beam splitter on to a photo detector.

Many people now have video or compact disc players in their homes. Lasers are used to read and, in fact, to write the information on the discs. The process is similar to that of an optical telephone system in that the sound (or picture) to be reproduced is recorded using a microphone (or video camera) which converts it to an electric signal. This is then digitized, and the digital electrical signal is used to drive a semi-conductor laser (i.e. switch it 'on' or 'off' according to the binary input signal) which is directed on to a rotating disc. When the laser is on, it etches a 'hole' on the disc, so that the digital signal becomes a series of 'holes' and 'no-holes'. To play back the recording, the disc is again rotated in the video, or compact disc player. A very small low power infra-red semi-conductor diode laser is directed on to the disc. The reflected beam is then measured by a device which converts light to electricity. As the sequence of etched 'holes' and 'no-holes' moves through the beam, the reflected light changes accordingly. The measuring device thus receives a digital signal which it converts to an electrical signal. This is then transmitted to the speakers (or TV) and reproduces the original sound (or pictures). Using digital techniques it is possible to store more information than on conventional records or tapes, and the quality of the sound or pictures is generally much better. Also, optical systems have the advantage that they do not wear out, as there is no mechanical contact.

Lasers are also used for controlling the motion of machines. For example, on a building site, semi-conductor laser beams are used in conjunction with bulldozers for land leveling. The laser beam is directed along the line to which the land is to be leveled, and a receiver (which detects the laser beam) is placed on the scoop of the bulldozer. All the operator then has to do is to keep the receiver locked on to the laser beam and drive the bulldozer forward. In the building industry lasers are also used for lining up walls, etc. because the beams are always perfectly straight. This also makes them useful for navigation.

There are many other everyday applications of lasers. As well as cutting and welding metal - e.g. in car factories - they are used in clothing factories for cutting out shapes in cloth and then sealing the edges. Increasingly they are being adapted for certain processes in the printing of periodicals and newspapers, such as etching the letters into metal plates. They are also used for laser printing in conjunction with computers and word processors.

In the future they will become even more important. It has been proposed that they should be fitted in safety devices on motor vehicles, with the laser beam designed to bounce off the vehicle in front and then to control the engine speed so that safe distances are maintained in lines of traffic. As the speed of light is the fastest rate at which information can be sent, computers will become more and more optical as they get faster and are able to store more information.

Eventually lasers and optical fibers will almost completely replace electronic circuits. The next step forward in the ‘micro-chip revolution’ will be with laser and ‘opto-electronics’.

Important dates

1917: Einstein (Germany). Proposal of spontaneous emission.

1947: Gabor (Britain). Discovery of wave front reconstruction, basis of holography.

1954: Gordon and Townes (USA). (see image below) Observe stimulated emission with microwaves - a maser.

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1958: Schawlow and Townes (USA). Propose scheme for optical maser - the laser.

1960: Maiman (USA). Discovery of the Ruby laser.

1960: Javan, Bennett and Herriott (USA). Discovery of stimulated emission in He-Ne gas laser.

1961: McClung and Hellwarth (USA). Q-switch Ruby laser.

1962: Nathan (USA). Stimulated emission from GaAs semiconductor laser.

1963: Lempicki and Samelson (USA). First laser action in a liquid.

1966: Sorokin and Lankard (USA). (see image below) Discovery of organic dye laser.

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1966: De Maria (USA). Passive mode locking of the Nd laser.

1972: Ippen and Shank (USA). Passive mode locking of c.w. dye laser.

1985: Valdmanis, Fork and Gordon (USA). Generation of 27 femtosecond pulses (the shortest pulse ever) from a passively mode locked dye laser.

Science | Science in Society | Technology


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