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! © Commissariat à l’Énergie Atomique et aux Energies Alternatives, 2005 Communication Division Bâtiment Siège - 91191 Gif-sur-Yvette cedex www.cea.fr ISSN 1637-5408. 2 > CONTENTS > INTRODUCTION 3 The unique properties of laser light are used in many fields. n i n o G . A / A E C © “Lasers are everywhere – 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 . A / 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.