The Laser: a Concentrate of Light to IINDUSTRY 10 > Medical Imaging 11 > Nuclear Astrophysics 12 > Hydrogen

THE COLLECTION w From the creation of a beam to its applications

1 > The atom 2 > Radioactivity 3 > Radiation and man 4 > Energy 5 > Nuclear energy: fusion and fission 6 > How a nuclear reactor works 7 > The nuclear fuel cycle FROM RESEARCH 8 > Microelectronics 9 > The laser: a concentrate of light TO IINDUSTRY 10 > Medical imaging 11 > Nuclear astrophysics 12 > Hydrogen

9 > The laser: a concentrate d of light

LASER LIGHT TYPES OF LASER CEA RESEARCH LASERS INDUSTRIAL LASERS USEFUL BUT … TAKE CARE!

ISSN 1637-5408. 2 > CONTENTS > INTRODUCTION 3

The unique properties of laser light are used in many fields. n i n o G

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in medicine, in industry, and

: LASER LIGHT 4 t The generation of laser light 6 at the heart of our daily lives.” r

h Stimulated emission 6

e Population inversion 6 g The laser oscillator 7 i s

l Laser amplifiers 8

a f l TYPES OF LASER 10 n i

n o

Laser colour 11 G

o . A /

A introduction e Laser power 12 E he first lasers were developed in the 1960s. C

Laser illumination 13 © A laser beam. TThe name LASER is an acronym for “Light e h Coherence in time Amplification by the Stimulated Emission of t and space 13 Radiation” . The almost magical properties of CEA has worked on all types of lasers for many T

a laser light soon led to their use in a variety of years. CEA researchers use them in their work CEA RESEARCH LASERS 16 INDUSTRIAL LASERS: applications. We use lasers every day in our CD in all the traditional ways (alignment, drilling,

r PRECISION WORKERS The Laser Integration Line (LIL) 20 players and in the bar-code readers used in super - welding, cutting, etc.), but they also develop

t prototype 17 CO 2 lasers 21 markets. Laser light shows create beautiful new types of lasers for specific applications. The world’s largest Neodymium-doped patterns of light in the air. Lasers are also They use very high power lasers to study the n Megajoule laser (LMJ) 17 YAG lasers 22 precision workers in industry. They are used to interaction between high energies and matter. The small and powerful Nanosecond lasers 22 e Terawatt laser 18 cut, weld and drill materials. They are used in Lasers are essential tools in many applications, Microchip lasers 23 medicine to repair or burn away diseased tissue but we must never forget the risks that are

c The ultrafast Femtosecond without harming healthy tissue nearby. Straight associated with them. The CEA is studying the laser 19 USEFUL BUT… and narrow laser beams are also used to align effects of lasers on the body. n TAKE CARE! 25 roads and tunnels. But why is it that lasers can There is still much research to be done in the The ultraviolet region 26

o do all these things while ordinary light from the field of laser technology. We expect more and The visible and near-infrared Sun or from a lightbulb can’t? more applications in the next few years.

c region 27

The infrared region 27 a

Designed and produced by Spécifique - Cover photo by © PhotoDisc - Illustrations by YUVANOE - Printed by Imprimerie de Montligeon - 04/2005 From the creation of a beam to its applications 9 > The laser: a concentrate of light From the creation of a beam to its applications 9 > The laser: a concentrate of light 4 > LASER LIGHT 5

UNIFORM AND ORDERED , A LASER BEAM IS MONOCHROMATIC AND PROPAGATES IN THE SAME DIRECTION .

Laser light Lasers produce a controlled light that is completely different from ordinary light from n i n

the Sun or a domestic lightbulb. The laser’s o G

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properties are the key for many applications A E C

using this type of light. A laser beam. ©

COMPARISON OF ORDINARY LIGHT AND LASER LIGHT

Ordinary light is… Laser light is… • Made up of many colours: • A single colour: White light can be split The light is said to be into all the colours of the monochromatic. There are many rainbow using a prism. types of lasers with different • Multidirectional: colours. A light source emits light • Unidirectional: waves in all directions. All the light waves travel in the • Disordered: same direction, forming a narrow, The light waves are not non-divergent beam. all emitted at the same • Ordered (or coherent): time. They oscillate in All the light waves are in phase, i.e. a disordered manner, with their “peaks” and “troughs” in each independently the same place. These are the wave of the others. characteristics of laser light. It is possible to think of laser light as an army of tiny soldiers all marching in step, while ordinary light is just a crowd of people moving about randomly. c s i D o t o h P

© From the creation of a beam to its applications 9 > The laser: a concentrate of light From the creation of a beam to its applications 9 > The laser: a concentrate of light 6 > LASER LIGHT > LASER LIGHT 7 “In an oscillator, a source of luminous, electrical or chemical energy excites the particles in the laser medium Spontaneous deexcitation of an atom which then emit light.” Energy levels

THE GENERATION which all the photons are identical. Each by an electrical discharge or by certain chem - OF LASER LIGHT photon is an exact copy of the others, with the ical reactions. same colour, the same instant of emission and Stimulated emission the same direction of travel. This is laser light. The laser oscillator An atom, ion or molecule in an excited state The discovery of stimulated emission was not, The generation of laser light requires a laser

may release energy by the The excitation of a in itself, sufficient for the development of oscillator and a source of energy (see the Emitted spontaneous emission of a system is the increase lasers. In a normal material, there are many diagram on page 8). photon in its energy. photon. However, there is more atoms, ions or molecules in an unexcited The oscillator consists of a long cylindrical another mode of emission – the stimulated state than in an excited state. In this situa - container with a mirror at each end. The laser emission of a photon predicted by Albert tion, it is not possible to generate enough medium (excitable atoms, ions or molecules

Atom Ein stein in 1917. stimulated emission to produce laser light. in the form of a solid, liquid or gas) is contained An excited atom may release energy spontaneously This is how it works: a particle (atom, ion or What is needed is a way of changing the state in this cylinder. For example, ruby is a solid in the form of photons. molecule) in an excited state emits a photon of the medium so that there are more excited laser medium. The excitable particles are the Deexcitation of an atom in response to the stimulation provided by the particles (atoms, ions or molecules) than there chromium ions. by stimulated emission impact of another photon with the same energy are particles at rest. This process is called The energy source supplies the energy needed Energy levels as that of the emitted photon. The unique population inversion. to obtain a population inversion (more excited feature of this type of emission is that the In 1949, the French physicist Alfred Kastler particles than unexcited particles). The energy stimulated photon has exactly the same discovered a solution to this problem –optical absorbed by the particles in the laser medium characteristics (colour, direction and phase) pumping. This method is used to transfer may potentially be released in the form of light. as the incident photon. It’s as though the energy to atoms from light. Alfred Kastler was The energy source may be of any type, e.g. second photon were a photocopy of the first. awarded the Nobel prize for physics in 1966 light, electrical or chemical energy. for this work. The first material used to exploit The laser oscillator generates the light. Imagine Population inversion this effect was ruby, a crystal of alumina a single photon spontaneously emitted in the Stimulated emission effectively multiplies the containing a small proportion (0.05%) of laser medium in a direction perpendicular to the Incident light. By repeating this single phenomenon chromium oxide. These chromium ions readily planes of the mirrors (see diagram on page 8). Incident photon many times, it is possible to generate light in absorb green and blue light (hence the red When this photon meets an excited particle, photon colour of rubies) and can be excited by an it stimulates the emission of a second photon. Emitted intense flash of white light. The energy absorbed These two identical photons then stimulate photon is then emitted, both spontaneously and by the emission of other photons, which stimu - Atom “Laser light consists stimulation, in the form of photons of red light late yet more photons, and so on until the An excited atom may release excess energy by stimulated emission when it captures a photon identical of photons with with a wavelength of 694.3 nanometres. The group of photons reaches one of the mirrors. to the photon that it would have released spontaneously. first lasers were therefore ruby lasers. Optical As they are travelling in a direction perpen - The two photons then continue along their path with exactly the same pumping is not the only way of obtaining a dicular to the plane of the mirror, they are completely identical characteristics. characteristics.” population inversion. It can also be achieved reflected back along their path, contin uing

From the creation of a beam to its applications 9 > The laser: a concentrate of light From the creation of a beam to its applications 9 > The laser: a concentrate of light 8 > LASER LIGHT > LASER LIGHT 9 “The laser oscillator consists of a long cylindrical container with a mirror at each end.”

to stimulate the emission further along the way. In a similar manner to a chain reaction, the Laser amplification chain number of identical photons being reflected back and forth between the mirrors increases with every passage along the cylinder: this is the first amplification of the laser light. In order to allow the laser beam to leave the laser oscillator, one of the two mirrors is made semi-transparent. Some of the light is not reflected by the mirror and passes through the mirror as though it were transparent. Laser oscillator Divergent beam Laser amplifiers Lens Target to be If the energy source is constantly exciting the illuminated n i

n particles in the laser medium, the oscillator o G

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/ will produce a constant beam of laser light. A E C Following a rapid build-up, the output beam An amplifier consists of a laser medium without © Dye laser oscillator. remains at a constant power. If the energy the end mirrors. The principle of operation is

source delivers an Energy delivered per unit time. identical to that of an oscillator. The particles intermittent dis - Energy is measured in joules (J) in the laser medium are excited by an energy and power is measured in watts Laser oscillator charge of energy (W). One watt equals one joule source and the photons passing through the in the oscillator, per second. amplifier produce a chain reaction generating “The amplifier Laser oscillator Laser beam the laser light will be produced in the form many more identical photons. The laser light produces a chain Mirror of short, very intense, discontinuous pulses. is amplified. As there are no mirrors, the pho - This is known as a pulsed laser. Lasers using tons only pass through the laser medium once. reaction generating

Laser medium optical pumping are of this type. In this type In order to obtain the required power, several many identical of laser, the energy is provided by a flash of amplifiers are placed along the path of the laser photons. The laser light similar to that produced by a photographic beam. The diameters of the laser beam and the flash-gun. amplifiers must also be increased as the power light is amplified and is increased in order to avoid damage to the the power is increased Excitable Laser amplifiers optical components and laser media from the Energy particle Semi-transparent mirror source In some applications, the laser light produced increased energy of the laser beam. The beam considerably.” by an oscillator may be used directly. How - is brought back into focus using a lens. The

An energy source excites the particles in the laser ever, when greater power is required, the laser complete set of equipment including the oscil - medium which may then emit light. light from the oscillator must be passed through lator, amplifiers and other optical components a series of amplifiers. (mirrors, lenses, etc.) is known as a laser chain.

From the creation of a beam to its applications 9 > The laser: a concentrate of light From the creation of a beam to its applications 9 > The laser: a concentrate of light 10 > TYPES OF LASER 11

DEPENDING ON THE APPLICATION , THEY CAN SELECT THE LASER CHARACTERISTICS , INCLUDING THE COLOUR AND POWER , AND WHETHER THE LASER IS CONTINUOUS OR PULSED .

Types of laser violet regions of the spectrum. The many LASER COLOUR The colour of the laser is defined by the laser laser colours available bring a spectacular medium used. Lasers are available in all colours beauty to many laser light-shows. Some exam - – red, blue, green, etc. Some lasers even ples of the various types of laser are given in produce invisible light in the infrared or ultra - the table below.

TYPE OF LASER LASER MEDIUM EXCITABLE PARTICLES COLOUR Laser diode Semiconductor Electrons/holes Red/infrared Helium-neon laser Helium and neon gas Neon atoms Red Ruby laser Ruby (solid) Chromium ions Red Argon laser Argon gas Argon ions Blue, green and invisible (ultraviolet) Krypton laser Krypton gas Krypton ions Red Excimer laser Mixture of a rare Grouping of two atoms Invisible gas and a halogen. (ultraviolet) The most common are mixtures of xenon and chlorine or krypton and fluorine. Copper vapour laser Copper vapour Copper atoms Green and yellow (two levels of excitation)

CO2 laser Gaseous mixture Molecules of CO 2 Invisible (infrared) consisting of nitrogen, helium and carbon dioxide*

(CO 2) Nd-YAG** laser Yttrium aluminium garnet Neodymium ions Invisible (infrared) (YAG) doped with neodymium (Nd) Glass-neodymium laser Glass doped with Neodymium ions Invisible (infrared) neodymium (solid) Dye laser Dye dissolved in a solvent Dye molecules Various ranges of colours depending on the dye used n

i * Carbon dioxide is also known as carbonic acid gas. n o

G ** YAG: Yttrium Aluminium Garnet.

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© From the creation of a beam to its applications 9 > The laser: a concentrate of light From the creation of a beam to its applications 9 > The laser: a concentrate of light 12 > TYPES OF LASER > TYPES OF LASER 13 “The illumination of a laser is defined as the number of watts per square centimetre.”

Most lasers are only capable of emitting light of the beam is suitable. Others commonly use at a single wavelength. However, some types a system of mirrors or, more rarely, a focus - of lasers are able to vary the wavelength ing assembly using a diffraction grating. emitted. These are known as tunable lasers. Focusing is essential in industrial applica - The majority of tunable lasers are dye lasers. tions such as drilling, welding and cutting. In this type of laser, the laser medium is a It is also useful in power lasers used in liquid containing a dye which provides the research into the interactions between light excitable particles. The dye molecules have and matter. the property of emitting light over a wide range of wavelengths when excited. Very precise COHERENCE IN TIME AND SPACE optical adjustments are used to select the The coherence of a laser in time and space wavelength required. The flexibility of these includes its properties of unidirectionality lasers makes them suitable for a large number and phase coherence. Many of the potential of applications. applications of lasers make use of these

M properties. It is the coherence of the laser, S D / A A for example, that enables it to read compact

LASER POWER E E C C

© Traditional continuous lasers have output pow - © discs. ers ranging from 1 mW to 50 kW. The most Copper vapour laser. A laser harmonic. A light wave propagates in a perfectly straight powerful industrial laser in Europe was line. As all the light waves emitted by a laser installed at Yutz-Thionville in 1994. This is a propagate in the same direction, a laser beam

carbon dioxide (CO 2) laser with a continuous one millisecond, the peak power will be one LASER ILLUMINATION does not diverge in the same way as the light output power of 45 kW. It is used in heavy thousand times greater –one kilowatt. By deliv - The diameter of the beam emitted by a laser from a torch, for example. The beam is perfectly gauge welding applications. ering all their energy in a very narrow pulse (of (which may reach several centimetres in the straight and visible over a great distance. This In the case of pulsed lasers, it is important to the order of a nanosecond or even a picosecond ) case of industrial power lasers) is often too property is used in laser light shows where the distinguish between two different ways of some research wide for the beam to be used directly. The light beam can be seen over a long distance, Nanosecond: 10 -9 seconds, i.e. measuring output power: lasers are able one thousand-millionth of a second. beam then needs to be focused to increase the and in the alignment of the routes of roads and A nanosecond is the time it takes for • The mean output power, which takes account to deliver a commercial aircraft to travel a third illumination (the number of watts per square tunnels. A laser was used in this way during the of the time intervals between each pulse. extremely high of a micron (1 micron = 1 millionth centimetre –W/cm 2). As an example, the illu - construction of the Montparnasse Tower in Paris. of a metre). Picosecond: 10 -12 seconds, • The peak power, i.e. the maximum power power levels i.e. one thousandth of a nanosecond. mination of sunlight can reach 0.1 W/cm 2, but Lasers are also used in telemetry, the taking of achieved during a pulse. up to several using a magnifying glass to focus the light can measurements remotely. In this application, a For example, a one watt continuous laser will terawatts . On a more modest scale, an indus - easily raise the illumination to 100 W/cm 2, laser beam is reflected back from a remote target. provide an output power of one joule/second. Terawatt (TW): 10 12 watts, trial welding laser with sufficient to set fire to paper. Some types of As the speed of light is known, it is possible to However, if this one joule of energy is con - i.e. one million million watts. a mean power of 1 kW lasers can be focused by a system of lenses, calculate the distance between the laser source centrated into a single pulse of light lasting could deliver a peak output power of 25 kW. providing that the power and/or wavelength and the target by measuring the time taken

From the creation of a beam to its applications 9 > The laser: a concentrate of light From the creation of a beam to its applications 9 > The laser: a concentrate of light 14 > TYPES OF LASER > TYPES OF LASER 15

In many current MAIN APPLICATIONS OF LASERS AS A FUNCTION OF THEIR POWER applications, small laser diodes are used in the USE POWER MODE OF OPERATION COMMENTS same way as any other Compact disc A few milliwatts Continuous Small laser diodes electronic components. players. assembled into the equipment like any Bar-code readers other electronic c s

i in supermarkets component. D o t

o Alignment lasers Approximately Continuous Small lasers h P for civil engineering work 10 milliwatts (e.g. helium-neon lasers). © and car body alignment, “A laser beam etc. Guidance of civil propagates in a The power of the laser engineering plant perfectly straight line.” beams used in some light Telecommunications A few tens of Continuous or pulsed Small laser diodes. These laser shows may be only carrier lasers milliwatts beams travel long distances across a few watts. the globe through underground i r e i

g or undersea optical fibre cables. g u

for the laser light to travel to the target and R /

x Discotheques A few watts Continuous Argon or helium-neon lasers, i o r

back again. This method has even been used c and light-shows for example. a L

© to obtain a precise measurement of the dis - Medical applications Power used Continuous or pulsed YAG or CO 2 lasers. The lasers used tance between the Earth and the Moon. Surgical lasers offer depends on the in medical applications are quite application powerful. They are capable of burning a degree of precision s e g unmatched by any other a Internal surgery: away damaged tissue within the body m I

y Non-traumatic or ‘welding’ the retina back onto instrument. t t e

G operations the eye. However, the

© surgeon or dentist must exercise External surgery: extreme care when using them. Treatment of the eye In spite of this, no other instrument (detached retina), can be used with such a degree treatment of teeth of precision. (caries), scalpel, etc. Cleaning and surface Peak powers of the Pulsed (very short pulses: Excimer or YAG lasers. This process preparation (e.g. laser order of 10 7 to 10 8 A few tens or hundreds may be used to remove surface layers cleaning of historic watts (in the case of nanoseconds) on various materials completely or monuments) of 10 to 20 watt selectively without affecting the

c YAG lasers, substrate by focusing the laser e l l e

z for example). beam on the areas to be cleaned. o B

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H Welding metals A few tens of watts Continuous or YAG lasers (100 W to 2 kW) and CO 2 M

R to 50 kilowatts pulsed lasers (100 W to 50 kW). L

© The power used Tests of the use depends on the of a laser beam to clean thickness of the the Saint Maurice weld. cathedral in Angers. Cutting materials such 1 to 3 kilowatts Continuous or YAG and CO 2 lasers. as wood, acrylic plastic (kW) pulsed or metals n i n o

G Laser-matter interactions, 15 to 500 Pulsed The CEA’s Alisé, LIL and LMJ lasers.

. A / plasma physics terawatts (TW) A E A Opto-mechanical system C E

used in the Alisé laser. © © Laser telemetry has applications in astrophysics.

From the creation of a beam to its applications 9 > The laser: a concentrate of light From the creation of a beam to its applications 9 > The laser: a concentrate of light 16 > CEA RESEARCH LASERS 17

ULTRAPOWERFULL AND ULTRAFAST MAKE THEM EXTREMELY USEFUL TO SCIENTISTS .

CEA research The CEA has designed and developed high power over a large surface area. For this reason, the lasers for use in both defence and civilian laser engineers have needed to design very large research. The Laser Integration Line (LIL) and amplifiers using sophisticated technology. As a the Megajoule laser (LMJ) are both dedicated result, the Laser Integration Line is 150 metres lasers to studying the physics of nuclear weapons. long, 70 metres wide, and 23 metres high. They are also suitable for use in basic research into the physics of matter at high energy THE WORLD’S LARGEST densities (plasmas, nuclear fusion, etc.). MEGAJOULE LASER (LMJ) Other types of laser are also contributing to Wi th its 240 beams (equivalent to 30 LILs), the scientific advances. Examples are the Fem - Megajoule laser will deliver 1.8 million joules of tosecond laser, which opens up the possibility ultraviolet light energy when it becomes opera - of monitoring the movement of atoms in a mil - tional in 2010. It will be capable of heating a lion millionth of a second, and the Terawatt few tens of milligrams of hydrogen to a temper - laser, which is used to study the interactions ature of 70 million degrees. In order to achieve between intense light and matter. these temperatures, the laser beams converge on a laser target shell several millimetres in diam - THE LASER INTEGRATION eter filled with a mixture of hydrogen isotopes. LINE (LIL) PROTOTYPE This prototype laser was commissioned in 2002 as a test bed to evaluate the design and tech - nology choices planned for the future Megajoule laser (LMJ). It consists of eight identical laser beams all aimed at a single target. The conver - gence of these beams on a single target enables the study of fusion by “inertial confinement”. At the present time, this is the largest opera - tional laser in the world pending commission - ing of the Megajoule laser. The energy provided by this laser is much greater than that of other lasers. However, the optics and media in the laser light amplifiers are unable to withstand extremely high power densities. In order to A E C avoid the destruction of these expensive ©

A components, the laser power must be spread The Megajoule laser (LMJ) experiment area. E C

© From the creation of a beam to its applications 9 > The laser: a concentrate of light From the creation of a beam to its applications 9 > The laser: a concentrate of light 18 > CEA RESEARCH LASERS > CEA RESEARCH LASERS 19

The Femtosecond laser: A “photographic camera” n i

with a shutter speed of one n o G

million-millionth . A / A

of a second! E C The Terawatt laser delivers a beam © of very high optical quality. In order to deliver the energy to the target, a very energy. It generates a beam of very high opti - THE ULTRAFAST FEMTOSECOND low energy laser pulse is progressively amplified cal quality. The intensity can reach 10 18 LASER over a distance of about 500 metres. The Mega - W/cm 2, equivalent to the light of ten thousand The Femtosecond laser produces extremely joule laser will occupy a space 300 metres long million million 100 W lightbulbs concen - short pulses delivering high power levels with by 160 metres wide containing four laser halls, trated in a single square centimetre. a high degree of temporal resolution. This each 128 metres long and 14 metres high. makes it possible to study very short term These installations are needed in order to guar - phenomena such as chemical reactions. The antee the operation and safety of nuclear weapons THE PETAWATT LASER Femtosecond laser is used rather like a camera without the need for testing. They also contribute with a shutter speed sufficiently fast to The Petawatt (10 15 watts) laser is a future project to increasing our understanding of the physics to be coupled to one of the eight LIL beams. capture the movement of atoms. This type of stars, in particular that of the Sun. The intensity could reach 10 20 W/cm 2, equivalent of laser is known as a laser probe. By varying to the light of one million million million 100 W the delay between the laser pulse and the lightbulbs concentrated in a single square THE SMALL AND POWERFUL centimetre. Using a laser in the Petawatt class, triggering of the chemical reaction, it is n i n TERAWATT LASER o the field of research into controlled fusion may be possible to build up a “film sequence” G

. extended to include the physics of the atomic A /

This type of laser has the advantage of being A covering a total period of around a million-

E nucleus and even some medical applications. C

very compact. It does, however, deliver less © millionth of a second.

THE LASER INTEGRATION LINE THE MEGAJOULE LASER THE TERAWATT LASER THE FEMTOSECOND LASER CEA CESTA FACILITY CEA CESTA FACILITY CEA SACLAY FACILITY CEA SACLAY FACILITY

USE Research laser. Fundamental research Research laser. Fundamental research Research laser. Research into the Research laser. Fundamental research requiring into the physics of nuclear weapons – into the physics of nuclear weapons – interactions between very intense light a high degree of temporal resolution or intense thermonuclear fusion, the physics thermonuclear fusion, the physics and matter. illumination for very brief periods. of hot dense plasmas. of hot dense plasmas. CHARACTERISTICS Mode of operation Pulsed Pulsed Pulsed Pulsed Pulse duration 1 nanosecond (10 -9 seconds) 1 nanosecond (10 -9 seconds) 1 picosecond (10 -12 seconds) 100 femtoseconds (10 -13 seconds) Laser output power 60,000 joules 1.8 million joules 1 joule per pulse Up to 100 MJ per pulse (infrared beam) Power 15 to 60 terawatts (TW) 500 terawatts (TW) 1 terawatt (TW) Up to 0.75 terawatts (TW) Laser medium or media Glass doped with neodymium Glass doped with neodymium Glass doped with neodymium • Sapphire doped with titanium (infrared beam) • Laser dyes (visible beams) Fundamental wavelength Infrared Infrared Infrared Simultaneous generation of two tunable visible beams and one infrared beam. Output wavelength Ultraviolet Ultraviolet Infrared

From the creation of a beam to its applications 9 > The laser: a concentrate of light From the creation of a beam to its applications 9 > The laser: a concentrate of light 20 > INDUSTRIAL LASERS: PRECISION WORKERS 21

SIMPLE , RELIABLE , ROBUST AND AFFORDABLE LASERS DEVELOPED BY THE CEA.

Industrial lasers: “The power of the laser Industrial lasers: The use of industrial lasers These power lasers, developed at a number of is sufficient to melt CEA facilities, have many applications in indus - and vapourise the weld trial processes, including tempering, welding, material in a few precision workers drilling, cutting and surface preparation. Laser welds are made by focusing a laser beam microseconds.” at a point on the material where the temper - ature generated is sufficient to melt the mate - rial. The focused intensity could reach between 10 8 and 10 9 W/cm 2. This high energy den - sity is sufficient to melt and vapourise the weld material in a few microseconds. YAG lasers are capable of welding metals up

to 3 mm thick. CO 2 lasers can weld up to 6 mm.

CO 2 LASERS

In the case of CO 2 lasers, high current pulses are needed to excite the molecules. The energy emitted as a result is very powerful. However, this type of laser requires large A F L C / r e

CHARACTERISTICS i t a b a

Mode of operation: Continuous S . L

Maximum continuous power: Between 500 W © and 6 kW depending on the machine Laser welding of aluminium. and energy Laser amplifier: Mixture of carbon dioxide, The Franco-German Laser Alliance (CLFA) consisting nitrogen and helium of the CEA, the CNRS, the DGA and the Fraunhofer Institute, has been established in the Ile-de-France Infrared beam (10.6 µm) region. It is mainly active in the field of continuous YAG laser welding. The Alliance works closely Weight of a high power CO 2 laser: with partners in the automobile and aerospace Several tonnes industries (Rolls Royce, Renault, Peugeot, Snecma, c s

i Beam directed by mirrors etc.), and in other industries. D o t o h P

© From the creation of a beam to its applications 9 > The laser: a concentrate of light From the creation of a beam to its applications 9 > The laser: a concentrate of light 22 > INDUSTRIAL LASERS: PRECISION WORKERS > INDUSTRIAL LASERS: PRECISION WORKERS 23

Green microlaser developed at the CEA Grenoble facility and at the Electronics High mean power laser. and Information Technology Laboratory (LETI). active sub-assemblies (gas tubes, exciters, etc.) and the weight is consequently consid - erable. The higher the output power needed (for an application such as machining), the greater the weight. NEODYMIUM-DOPED YAG* LASERS At the present time, the beam of a medium power 2 kW YAG can be transmitted for sev - eral metres along a 1 mm silica optical fibre. The beam may also be directed by a system of lenses or mirrors. e u q i n h c e t r A / A A E E C C

CHARACTERISTICS Mode of operation: Pulsed NANOSECOND LASERS MICROCHIP LASERS CHARACTERISTICS Pulse duration: 1 to 20 milliseconds The “Plani” facility at Saclay consists of six Nd- These lasers are being developed at the CEA Mode of operation: Continuous or pulsed Repetition frequency: 1 to 1,000 Hz, YAG lasers triggered and pumped by continu - Grenoble facility and at the Electronics and depending on the application depending on the machine and energy ous diodes. These lasers deliver a wide range Information Technology Laboratory (LETI). Pulse duration: 0.5 to 5 nanoseconds Maximum energy per pulse: 150 joules of wavelengths (UV, visible and infrared). Ded - Microchip lasers are solid-state lasers. They Repetition frequency: Maximum mean power: 70 watts to icated to nanosecond technology, this facility use the simplest form of laser technology and 1,000 to 100,000 hertz (pulses per second) 1.5 kilowatts depending on the machine is used in the development of new applications are very small, with a typical volume of Laser energy per pulse: 0.5 to 50 mJ Maximum peak power: 30 kilowatts including microdrilling, marking, cleaning, sur - 0.5 mm 3. They are mass produced in batches Mean power: 1 to a few 100 milliwatts Laser amplifier: Yttrium aluminium face treatments and microcutting in a number Peak power: 0.5 to 50 kilowatts garnet (YAG) doped with neodymium of industrial sectors such as the automobile, Laser amplifier: Laser crystal (YAG doped Infrared beam (1.06 µm) aerospace, and food industries. With a pulse with neodymium) or glass (phosphate glass “They are capable Weight of a YAG laser: width of between 10 and 200 nanoseconds, doped with erbium) Several hundred kilograms these lasers have similar power outputs to tra - of cutting, cleaning ditional lasers, but with the added advantage Beam: Visible (0.5 micron) and infrared and marking with *YAG: Yttrium Aluminium Garnet. of being capable of high precision microma - (1, 1.5 and 2 microns) chining without heating the material. microscopic precision.”

From the creation of a beam to its applications 9 > The laser: a concentrate of light From the creation of a beam to its applications 9 > The laser: a concentrate of light 24 > INDUSTRIAL LASERS: PRECISION WORKERS 25

“Air quality: a laser detector RISK OF BURNS AND PREMATURE AGEING : for pollutant gasses.” THE EYES AND THE SKIN ARE PARTICULARLY SENSITIVE TO OVEREXPOSURE TO LASER LIGHT .

Their low cost, high reliability and high quality Useful but… beam characteristics have led to their use in applications including: • Laser telemetry (non-contact distance and speed measurement). These lasers are used take care! in obstacle detection and avoidance systems in the automobile industry. • Micromarking and microcutting all types of materials. • Oscillators feeding the amplifier chain in very high power lasers. • The manufacture of compact visible lasers e u

q (green). i n h c

e • The detection of gaseous pollutants. t r A /

A The mass production of microchip lasers E C

© begins by cutting wafers of the laser medium Robust and easy to use microchip lasers between 0.5 and 1 mm thick and 25 mm in are opening up a technological revolution in the field of solid-state lasers. diameter. These wafers are polished until the two surfaces are exactly parallel. The laser oscillator mirrors are deposited directly on of several thousand at a time and require no these surfaces. Each wafer is then cut into adjustments or setting up. Their manufac - several hundred tiny laser chips, each with an turing costs are therefore very low. They are area of around 1 mm 2. Each chip is mounted also extremely reliable, robust and simple to in a case adjacent to an infrared laser diode use, requiring absolutely no setting up or that acts as a longitudinal pump. maintenance. A microchip laser is pumped by a laser diode and behaves like a trans - former of laser light. Using laser light of poor quality from a laser diode, they produce a monochromatic laser beam with a naturally circular cross-section that exhibits very low d

divergence. The high peak powers available r a d é

when pulsed make microchip lasers the ideal M

. L / A

solution in many applications. E C

© From the creation of a beam to its applications 9 > The laser: a concentrate of light From the creation of a beam to its applications 9 > The laser: a concentrate of light 26 > USEFUL BUT… TAKE CARE! > USEFUL BUT… TAKE CARE! 27 l e p a h C

Twenty-four hours Symbols of state-of-the-art technology, lasers and the crystalline lens resulting in a loss of . C /

after laser irradiation, P R L are increasingly being used in surgery, the cell nuclei are made visual acuity and even a risk of blindness. / R R l e visible by fluorescent DNA D /

research laboratories, industry and our daily p The lesions may be seen during an angio - V a S marking (in blue). h D C /

lives (CD players, laser printers, etc.). The CEA graphic examination. Overexposure of the A C / Some of the nuclei are Revealing laser effects E P C

and a pink marker indicates R D

tools in many different fields. / V

the presence of the S D Laser radiation can damage the skin and the / THE INFRARED REGION P53 protein. A E C eyes with the effects depending on the power NM MM © (1,400 - 1 ) of the laser, the exposure time, the beam Radiation at these wavelengths does not reach diameter and the wavelength of the laser (ultra - 1µm (10 -6 m) of the cornea is removed. How - the retina, but it can cause thermal lesions to violet, infrared, and even visible radiation can ever, the long term effects of this type of pro - the cornea and a loss of transparency of the be harmful). cedure are still largely unknown and it is sus - crystalline lens. Cutaneous burning has also l e

The biological effects vary with all these param - pected that it could lead to the premature been observed. Research at the CEA is tar - p a h C

eters. The interaction between the laser light and formation of cataracts. The risk appears to geted on identifying biological markers for the C / P R L

body tissue may be thermal, photochemical , arise from the secondary effects of photons corneal lesions. / R R D electromechanical or that do not take part in the ablation process Research carried out by the CEA at Fonte - /

Chemical reactions associated V S D

with radiant energy. /

photoablative . (see page 6). nay-aux-Roses has led to a better under - A E C

Destruction by radiant energy. CEA researchers have carried out experiments standing of the effects of laser radiation on © on cell tissue that have shown that laser radi - the body at the molecular level. This work is Lesion of the cornea resulting from a 3 ns pulse at 1,570 nm. The lesion is at the centre of the white circle between THE ULTRAVIOLET REGION ation at 193 nm causes stress to the cells and needed in order to define or refine exposure the two markers (five pulses). The observation was made (100 TO 400 NANOMETRES ) is capable of triggering programmed cell limits as laser technology advances. It also 30 minutes after irradiation. The harmful photochemical effects of natural death (apoptosis) in cornea cell cultures. The contributes to the better use of lasers in bio - ultraviolet radiation (UV) are well known. activation of a cell division regulating protein medical engineering. Exposure of the skin to UV laser light can (P53, the “genome protector”) has also been cause photochemical burns, premature age - demonstrated.

LASER SAFETY R ing or even cancer. Some types of laser are The results also indicate the initiation of a N I

’

Lasers are essential tools in many applications, but we must d capable of cutting cutaneous tissue (ablation). process of photoageing in the surviving cells.

never forget the dangers that are associated with them. t

Overexposure of the eyes to the beam from a s i The higher the power of the laser being used, the greater r

UV laser can cause burning, particularly to the THE VISIBLE AND NEAR- the risk to personnel. Of all the organs, the eye is the most a ’

susceptible to damage by laser light. If a laser beam enters e conjunctiva and cornea. INFRARED REGION (400 - 1,400 NM ) v a

the eye, it is focused on the retina, greatly increasing e t

However, when used with care the exceptional These wavelengths of laser light are u the intensity. d

ablative properties of UV laser light at 193 nm extremely dangerous as they can penetrate r The retina can even be damaged by low power lasers

c (from 1 mW). Laser technicians wear special glasses i are used in corneal surgery to correct short- deep within the eye. In the eye, the radia - f

f sightedness. For a correction of one dioptre, tion is focused onto the retina by the cornea to protect their eyes. A

From the creation of a beam to its applications 9 > The laser: a concentrate of light From the creation of a beam to its applications 9 > The laser: a concentrate of light