Laser annotations by Abdul Mumeed

Spontaneous:1. Happening or arising without apparent external cause; self-generated. 2. Arising from a natural inclination or impulse and not from external incitement or constraint. It’s Antonym: induced - brought about or caused; not spontaneous adj spontaneous  1 said, done etc of one's own free will without pressure from others His offer was quite spontaneous. غیر ارادی 2 natural; not forced spontaneous behaviour. قدرتی adv sponˈtaneously بے ساختہ n sponˈtaneousness بے ساختگی PROPERTIES OF LASER LIGHT (4) The light emitted by lasers is different from that produced by more common light sources such as incandescent bulbs, fluorescent lamps, and high-intensity arc lamps. An understanding of the unique properties of laser light may be achieved by contrasting it with the light produced by other, less unique sources. ----- MONOCHROMATICITY (5) All light consists of waves traveling through space. The color of the light is determined by the length of those waves, as illustrated in Figure 1.      Fig. 1 Comparison of the wavelengths of red and blue light (6) Wavelength is the distance over which the wave repeats itself and is represented by the Greek letter  (lambda). Each color of visible light has its own characteristic wavelength. (7) White light consists of a mixture of many different wavelengths. A prism can be used to disperse white light into its component wavelengths (colors), as in Figure 2. Fig. 2 Dispersion of white light by a prism (8) All common light sources emit light of many different wavelengths. White light contains all, or most, of the colors of the visible spectrum. Ordinary colored light consists of a broad range of wavelengths covering a particular portion of the visible-light spectrum. A green traffic light, for example, emits the entire green portion of the spectrum, as well as some wavelengths in the neighboring yellow and blue regions. (9) The beam of a helium-neon gas laser, on the other hand, is a very pure red color. It consists of an extremely narrow range of wavelengths within the red portion of the spectrum. It is said to be nearly"monochromatic," or nearly "single-colored."Near-monochromaticity is a unique property of laser light, meaning that it consists of light of almost a single wavelength. (Mike Leeming) (10) Perfectly monochromatic light cannot be produced even by a laser, but laser light is many times more monochromatic than the light from any other source. In some applications, special techniques are employed to further narrow the range of wavelengths contained in the laser output and, thus, to increase the monochromaticity. ----- DIRECTIONALITY (11) Figure 3 depicts light being emitted from a light bulb in all directions. All conventional light sources emit light in this manner. Devices such as automobile headlights and spotlights contain optical systems that collimate the emitted light, such that it leaves the device in a directional beam; however, the beam produced always diverges (spreads) more rapidly than the beam generated by a laser. Fig. 3 Conventional source (12) Figure 4 illustrates the highly directional nature of light produced by a laser. "Directionality" is the characteristic of laser light that causes it to travel in a single direction within a narrow cone of divergence. Fig. 4 Directionality of laser light (13) But again, perfectly parallel beams of directional light—which we refer to as collimated light—cannot be produced. All light beams eventually spread (diverge) as they move through space. But laser light is more highly collimated, that is, it is far more directional than the light from any conventional source and thus less divergent. In some applications, optical systems are employed with lasers to improve the directionality of the output beam. One system of this type can produce a spot of laser light only one-half mile in diameter on the moon (a distance of 250,000 miles). (Mike Leeming) ----- COHERENCE Encyclopedia. com definition of "Coherence" (14) Figure 5 depicts a parallel beam of light waves from an ordinary source traveling through space. None of these waves has any fixed relationship to any of the other waves within the beam. This light is said to be "incoherent," meaning that the light beam has no internal order. (Mike Leeming) Fig. 5 Incoherent light waves (15) Figure 6 illustrates the light waves within a highly collimated laser beam. All of these individual waves are in step, or "in phase," with one another at every point. "Coherence" is the term used to describe the in-phase property of light waves within a beam.  Fig. 6 Coherent light waves (16) Just as laser light cannot be perfectly monochromatic or perfectly directional, it cannot have perfect coherence, yet laser light is far more coherent than light from any other source. Techniques currently in use greatly improve the coherence of light from many types of lasers. (17) Coherence is the most fundamental property of laser light and distinguishes it from the light from other sources. Thus, a laser may be defined as a source of coherent light. The full importance of coherence cannot be understood until other concepts have been introduced, but evidence of the coherence of laser light can be observed easily. (18) In Figure 7, the beam of a low-powered laser strikes a rough surface, such as paper or wood, and is reflected in all directions. A portion of this light reaches the eye of an observer several meters away. The observer will see a bright spot that appears to be stippled with many bright and dark points. This "speckled" appearance is characteristic of coherent light, and is caused by a process called "interference," which will be discussed in a later module. Fig. 7 Viewing laser speckle EMISSION AND ABSORPTION OF LIGHT Encyclopedia.com definition of "Laser" (19) A laser produces coherent light through a process termed "stimulated emission." The word "LASER" is an acronym for "LightAmplification by Stimulated Emission of Radiation." A brief discussion of the interaction of light with atoms is necessary before stimulated emission can be described. ----- ENERGY LEVELS IN ATOMS (20) An atom is the smallest particle of an element that retains the characteristics of the element. An atom consists of a positive nucleus surrounded by a "cloud" of negative electrons. All neutral atoms of a given element have the same number of positive charges (protons) in the nucleus and negative charges (electrons) in the cloud. The energy content of atoms of a particular type may vary, however, depending on the energies contained by the electrons within the cloud. Explanation of ground state and excited stateAnother explanation of ground state and excited state (21) Each type of atom can contain only certain amounts of energy. When an atom contains the lowest amount of energy that is available to it, the atom is said to be in its "atomic ground state." If the atom contains additional energy over and above its ground state, it is said to be in an "excited atomic state." (22) Figure 8 is a simplified energy-level diagram of an atom that has three energy levels. This atom can contain three distinct amounts of energy and no others. If the atom has an energy content of El, it is in the atomic ground state and is incapable of releasing energy. If it contains energy content E2 or E3, it is in an excited state and can release its excess energy, thereby dropping to a lower energy state. Real atoms may have hundreds or even thousands of possible distinct energy states. The three-level mode is utilized here for purposes of clarity. Fig. 8 Atomic energy-level diagram ----- SPONTANEOUS EMISSION OF LIGHT (23) An atom in an excited state is unstable and will release spontaneously its excess energy and return to the ground state. This energy release may occur in a single transition or in a series of transitions that involve intermediate energy levels. For example, an atom in state E3 of Figure 8 could reach the ground state by means of a single transition from E3 to El, or by two transitions, first from E3 to E2 and then from E2 to E1. In any downward atomic transition, an amount of energy equal to the difference in energy content of the two levels must be released by the atom. Encyclopedia. com definition of "Photon" (24) In many cases, this excess energy appears as a photon of light. Aphoton is a quantum of light having a characteristic wavelength and energy content; in fact, the wavelength of the photon is determined by its energy. A photon of longer wavelength (such as that for red light) possesses less energy than one of shorter wavelength (such as that for blue light), as illustrated in Figure 9. Fig. 9 Spontaneous emission (25) In ordinary light sources, individual atoms release photons at random. Neither the direction nor the phase of the resulting photons is controlled in any way, and many wavelengths usually are present. This process is referred to as "spontaneous emission" because the atoms emit light spontaneously, quite independent of any external influence. The light produced is neither monochromatic, directional, nor coherent. STIMULATED EMISSION OF LIGHT (26) The coherent light of the laser is produced by a "stimulated-emission" process (Figure 10). In this case, the excited atom is stimulated by an outside influence to emit its energy (photon) in a particular way. Fig. 10 Stimulated emission (27) The stimulating agent is a photon whose energy (E3–E2) is exactly equal to the energy difference between the present energy state of the atom, E3 and some lower energy state, E2. This photon stimulates the atom to make a downward transition and emit, in phase, a photon identical to the stimulating photon. The emitted photon has the same energy, same   wavelength, and same direction of travel as the stimulating photon; and the two are exactly in phase. Thus, stimulated emission produces light that is monochromatic, directional, and coherent. This light appears as the output beam of the laser. (Mike Leeming) Note: Based on suggestions from Mike Leeming, this discussion on stimulated emission will be expanded in the revised version to be more detailed, will connect more closely with amplification, and will involve two diagrams for Figure 10, showing clearly the situation of photons/energy levels before interaction and photons/energy levels after interaction. (Mike Leeming) ----- ABSORPTION OF LIGHT (28) Figure 11 illustrates another process that occurs within a laser. Here, a photon strikes an atom in energy state E2 and is absorbed by that atom. The photon ceases to exist; and its energy appears as increased energy in the atom, which moves to the E3 energy level. The process of absorption removes energy from the laser beam and reduces laser output. Fig. 11 Absorption of light ----- POPULATION INVERSION (29) In order for a laser to produce an output, more light must be produced by stimulated emission than is lost through absorption. For this process to occur, more atoms must be in energy level E3 than in level E2, which does not occur under normal circumstances. In any large collection of atoms in matter at any temperature T, most of the atoms will be in the ground state at a particular instant, and the population of each higher energy state will be lower than that of any of the lower energy states. This is called a "normal population distribution." (30) Under "normal" circumstances, each energy level contains many more atoms than the energy level just above it, and so on up the energy lever ladder. For example, at room temperature, if there are Noatoms in the ground state of Neon (He-Ne laser) there are only 10-33No atoms in the first excited state, even fewer in the second excited state and so forth. The population of the ascending energy levels decreases exponentially. (Mike Leeming) (31) Thus, in any large collection of atoms in matter at any temperature T, most of the atoms will be in the ground state at a particular instant, and the population of each higher energy state will be lower than that of any of the lower energy states. This is called a "normal population distribution." (Mike Leeming) (32) A population inversion exists whenever more atoms are in an excited atomic state than in some lower energy state. The lower state may be the ground state, but in most cases it is an excited state of lower energy. Lasers can produce coherent light by stimulated emission only if a population inversion is present. And a population inversion can be achieved only through external excitation of the atomic population.  ELEMENTS OF A LASER Excellent explanation of how a laser works (33) Four functional elements are necessary in lasers to produce coherent light by stimulated emission of radiation. Figure 12 illustrates these four functional elements. Fig. 12 Elements of a laser ------  ACTIVE MEDIUM (34) The active medium is a collection of atoms or molecules that can be excited to a state of inverted population; that is, where more atoms or molecules are in an excited state than in some lower energy state. The two states chosen for the lasing transition must possess certain characteristics. First, atoms must remain in the upper lasing level for a relatively long time to provide more emitted photons by   stimulated emission than by spontaneous emission. Second, there must be an effective method of "pumping" atoms from the highly-populated ground state into the upper lasing state in order to increase the population of the higher energy level over the population in the lower energy level. An increase in population of the lower energy level to anumber above that in the high energy level will negate the population inversion and thereby prevent the amplifications of emitted light by stimulated emission. In other words, as atoms move from the upper energy level to the lower energy level, more photons will be lost by spontaneous emission—giving off randomly directed, out-of-phase light—than gained due to the process of stimulated emission.) (Mike Leeming) (35) The active medium of a laser can be thought of as an optical amplifier. A beam of coherent light entering one end of the active medium is amplified through stimulated emission until a coherent beam of increased intensity leaves the other end of the active medium. Thus, the active medium provides optical gain in the laser. (36) The active medium may be a gas, a liquid, a solid material, or a junction between two slabs of semiconductor materials. (37) A ruby crystal was the active medium of the first laser, invented by Dr. Theodore Maiman at the Hughes Laboratories in 1960. Liquid active media in tunable dye lasers consist of certain dyes dissolved in ethyl or methyl alcohol. Other active media include many types of gases and mixtures of gases. Lasers that contain a mixture of helium and neon gases or carbon dioxide gas are common examples of a gaseous active medium. A pn semiconductor junction, composed of gallium arsenide or gallium phosphide, is an example of yet another type of active medium. ----- EXCITATION MECHANISM (38) The excitation mechanism is a source of energy that excites, or "pumps," the atoms in the active medium from a lower to a higher energy state in order to create a population inversion. In gas lasers and semiconductor lasers, the excitation mechanism usually consists of an electrical-current flow through the active medium. Solid and liquid lasers most often employ optical pumps; for example, in a ruby laser, the chromium atoms inside the ruby crystal may be pumped into an excited state by means of a powerful burst of light from a flashlamp containing xenon gas. ----- FEEDBACK MECHANISM (39) The feedback mechanism returns a portion of the coherent light originally produced in the active medium back to the active medium for further amplification by stimulated emission. The amount of coherent light produced by stimulated emission depends upon both the degree of population inversion and the strength of the stimulating signal. The feedback mechanism usually consists of two mirrors--one at each end of the active medium--aligned in such a manner that they reflect the coherent light back and forth through the active medium. ----- OUTPUT COUPLER (40)The output coupler allows a portion of the laser light contained between the two mirrors to leave the laser in the form of a beam. One of the mirrors of the feedback mechanism allows some light to be transmitted through it at the laser wavelength. The fraction of the coherent light allowed to escape varies greatly from one laser to another--from less than one percent for some helium-neon lasers to more than 80 percent for many solid-state lasers.  LASING ACTION (41) When the excitation mechanism of a laser is activated, energy flows into the active medium, causing atoms to move from the ground state to certain excited states. In this way, population inversion is created. Some of the atoms in the upper lasing level drop to the lower lasing level spontaneously, emitting incoherent photons at the laser wavelength and in random directions. Most of these photons escape from the active medium, but those that travel along the axis of the active medium produce stimulated emission, as indicated in Figure 13. The beam produced is reflected back through the active medium by the mirrors. A portion of the light that strikes the output coupler leaves the laser as the output beam. Fig. 13 Lasing begins. I(42) If the round-trip gain of the laser medium exceeds the round-trip loss (including the output beam), the output power of the laser increases. If losses exceed gain, laser power decreases. (Mike Leeming) (43) If one keeps track of the number of photons in the beam during one round trip, say from HR to OC and back to HR, and the number of photons in the beam increases, the laser beam power increases. If the number is the same, the beam power is steady. If the number is less, the laser power decreases and eventually lasing stops. As we shall see later in more detail, the round-trip gain of the laser comes from the degree of population inversion in the active laser medium and the probability for a stimulated emission process to occur. The round-trip overall loss comes from imperfect reflection at the HR mirror, scattering and diffraction losses as the beam passes through theactive medium absorption losses, cavity mirror misalignment losses, and of course, "the programmed" loss through the output mirror. When the gain for a round-trip exceeds the losses, laser power grows. When the round-trip gain is less than the losses, laser power dies out. And, when round-trip gain and loss are just equal, the laser operates in what we call a "steady-state" condition. (Mike Leeming) (44) In pulsed lasers, the excitation mechanism supplies energy in short bursts. Both gain and output power rise quickly to a high level and drop off, producing a burst of laser light. In continuous-wave (CW) lasers, the excitation mechanism supplies a constant power to the active medium. The system quickly reaches a "steady-state" condition, in which loss and gain are in balance. This condition thereby results in a constant output beam.  TYPES OF LASERS (45) Lasers may be classified according to the type of active medium, excitation mechanism, or duration of laser output. Classification by active medium is utilized here, but the examples given include both pulsed and CW lasers with electrical or optical pumping. Note: M. Leeming suggests that we add material to this paragraph on typical applications of each "type" of laser and mention a few things on expense and safety issues. (Editor's Note: Does anyone out there have this information in convenient "table" format we can incorporate here? If so, please fax or E-mail to CORD.) ----- GAS LASERS SAM's Laser FAQ–Hellium Neon Laser Parts (46) A large and important family of lasers utilizes a gas or gas mixture as the active medium. Excitation usually is achieved by current flow through the gas. Gas lasers may be operated in either CW or pulsed modes.  (47) One popular type of gas laser contains a mixture of helium (He) and neon (Ne) gases and is illustrated in Figure 14. The gas mixture is contained at a low pressure within a sealed glass tube called the "plasma tube." The excitation mechanism of the HeNe laser is a direct-current discharge through the gas; the current pumps the helium atoms to an excited atomic state. The energy of the excited helium atoms is transferred to neon atoms through collisions, and the neon atoms then undergo a transition to a lower energy state that results in lasing. The feedback mechanism consists of a pair of mirrors sealed to the ends of the plasma tube. One of these mirrors, the output coupler, transmits 1-2 percent of the light to form a continuous (CW) output beam. Fig. 14 HeNe gas laser ----- SOLID CRYSTALLINE AND GLASS LASERS (48) Another important family of lasers contains solid crystalline or glass material as an active medium. Ruby and neodymium are two common examples of solid lasers with widespread industrial applications. Ruby is crystalline aluminum oxide in which some of the aluminum ions in the crystal lattice have been replaced by chromium ions. These chromium ions are the active elements in the ruby laser. Yttrium aluminum garnet (YAG) is the crystal host for Nd:YAG lasers; some of the aluminum in the YAG is replaced by triply-ionized neodymium (Nd3+), a rare earth element. Glass is also used as a host for neodymium lasers. (49) Figure 15 displays the components of a CW Nd:YAG laser. The active medium is a cylinder of laser crystal whose ends have been cut parallel and polished. Antireflection coatings have been applied to the rod ends to reduce losses. The excitation mechanism for this particular laser is a tungsten filament lamp attached to an ac power source. Larger models utilize do krypton arc (gas discharge) lamps as pumping sources. Both types of lamps provide continuous optical pumping to the laser crystal. The mirrors of the Nd:YAG laser usually are mounted separately from the active medium as shown, but one of the mirror coatings sometimes is applied directly to one end of the rod. Fig. 15 CW Nd:YAG laser (50) Pulsed Nd:YAG lasers have the same basic design, except that CW lamp and power supply are replaced by a xenon flashlamp and pulsed power supply. For example, if one replaces the tungsten-iodide discharge lamp in Figure 15 with either a Xenon flashlamp (as in Figure 16) or a pulsed laser diode, one can have re-rated pulsed laser operation in the place of CW operation. Ruby lasers are very similar in construction but are normally operated as pulsed lasers only. (John DeLeon) (51) Editor's Note: John DeLeon called our attention to Figure 16, indicating that a change (update of laser system) is required. You will see that the figure and accompanying text have been changed in accordance with John's suggestion. SEMICONDUCTOR LASERS (55) The active medium of a semiconductor (injection) laser is the junction between two types of semiconductor materials. (56) A semiconductor is a material whose electrical conductivity is greater than that of an insulator, such as glass or plastic, but less than that of a good conductor, such as silver or copper. Gallium arsenide (GaAs) is an example of a material used in the manufacture of a semiconductor laser. A p-type semiconductor material has a deficiency of negatively-charged free electrons in the crystal structure. This deficiency exists in the form of positions in the crystal that can accept an electron if one were available. These positively-charged "holes" are the carriers of electric current in p-type semiconductors. By contrast, an n-type semiconductor material has a surplus of electrons that act as current carriers. If two slabs, one of p-type and one of n-type semiconductor material, are joined together, the result is called a pn junction. When current flows across a pa junction, free electrons from the n-type material combine with holes in the p-type material and release energy. This energy may appear as visible light as in the light-emitting-diode (LED) displays of electronic calculators. (57)  Figure 17 gives the construction of a semiconductor laser. The laser diode is a rectangular-shaped crystal of gallium arsenide that contains a pn junction. The entire device is about the size of a grain of sand. The end faces of the laser diode are "cleaved" along the crystal planes to form parallel reflection surfaces that act as the mirrors of the feedback mechanism. Current flow across the junction is the excitation mechanism. Semiconductor lasers usually have outputs in the infrared wavelength range, although some models that emit in the visible region are available. Fig. 17 Semiconductor laser (grossly enlarged) SAFETY PRECAUTIONS FOR HELIUM-NEON LASERS (58) The HeNe gas laser described previously is, by far,one of the most common types of lasers, and of historical importance, since it, along with the ruby laser, was one of the first to be built. HeNe lasers in the 0.5-mW to 5.0-mW range are common tools for alignment and science laboratories (1 milliwatt = 10-3 watt). Although these devices are safe if handled properly, they can cause injury if employed improperly. The following procedures will ensure the safe operation of a HeNe laser. (Mike Leeming) DO NOT LOOK DIRECTLY INTO THE LASER BEAM. The low-power HeNe laser is little more than a coherent, monochromatic light bulb. The HeNe laser described here is not capable of burning or drilling holes in most materials; and accidental, momentary eye exposure will not normally cause eye damage. Nevertheless, the highlydirectional, intense beam of light should be treated with caution, care and respect. Common sense dictates that one must not look directly into any bright light source such as the sun, carbon arc, or an arc lamp projector, and particularly a laser beam. The lens of the eye can focus the beam from even a low-powered (1-5 mW) HeNe laser to a small spot on the retina and cause thermal damage to retinal tissue. DO NOT LOOK AT SPECULAR REFLECTIONS OF THE LASER BEAM. Specular reflections are those from mirrors, watch crystals, polished metal surfaces (painted and unpainted), or any other highlyreflective surface. Specular reflections of a laser beam are considered secondary laser sources and, as such, are treated with the same caution as is the direct laser beam. (See Laser Safety Precaution 1.) TAKE CARE WHEN MOVING THE LASER OR WHEN MOVING OBJECTS IN THE BEAM PATH OF THE LASER. Most low-power HeNe lasers are small enough to be moved about easily. If the laser must be moved during its operation, care must be taken to direct the beam carefully in order that it will not shine into anyone's eyes. For the reasons outlined in Laser Safety Precaution 2, caution also must be taken not to direct the beam upon a specular reflector when the laser is moved. If an object must be moved into the beam of a laser, movement should be deliberate, with due consideration given to where the reflections will be directed. Usually, a laser should be turned off before it is moved. BEWARE OF HIGH VOLTAGE, ESPECIALLY  WHEN THE CASE OR ENCLOSURE OF AN OPERATING LASER IS OPEN. (Mike Leeming) The HeNe laser described here contains a high-voltage power supply. This unit should not be disassembled, demonstrated, or serviced by anyone unfamiliar with such devices. Most lasers contain either high-voltage or high-current power supplies that should be treated with caution. Each year more people in the laser industry are injured by electrical hazards than by exposure to laser beams. OPERATE THE LASER IN AN AREA DESIGNED FOR LASER OPERATION If possible, the laser should be operated with the beam horizontal and below eye level to prevent eye damage. All potential specular reflectors should be removed from the beam area. Adequate provision for all support equipment should be made prior to the operation of the laser. The number of persons working around the laser should be kept to a minimum, and the area at which the laser is being operated should be illuminated as much as possible. Access to the operation area should be limited and appropriate warning signs exhibited. Most low-power HeNe lasers are designed to operate with 110/120 volts, single phase, 50-60 Hz ac power. Some may operate with 220/240 volts, single- or three-phase 50-60 Hz ac power. Prior to the operation of any laser, the correct power requirements for that laser should be determined from the laser specification. The power cord attached to the laser should be examined. For most low-power HeNe lasers, the unit is equipped with a power cord consisting of a three-wire, grounded plug. The third-wire, grounded terminal must not be bypassed. DO NOT INTENTIONALLY OR INADVERTENTLY TRACK VEHICLES OR AIRCRAFT WITH THE LASER BEAM. Federal laws prohibit the tracking of vehicles or aircraft with laser beams. Such actions could cause considerable property damage, loss of eyesight, or even loss of lives. DO NOT LEAVE AN OPERATING LASER UNATTENDED. ALWAYS UNPLUG IT WHEN IT IS NOT BEING USED. When not in use, the laser should be turned off to prevent accidental exposure to the beam by unqualified persons. Note: Mike Leeming suggests that here, or in the safety precautions section, we provide references to appropriate ISO or ANSI safety standards. Also, he asks if there is a glossary of "safety-related" or standard labels such as the "caution" symbol included that we can add? (Editor's comment: Send us via FAX, sources or samples of labels you are currently using around the lab. Which ANSI standards do you cover with your students?) Stimulated Emission The word "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation. The easiest laser model to understand is the two level system. In a two level system, the particles have only two availible energy levels, separated by some energy difference which is typically referred to in terms of the photon energy, hv0. These two levels are generally referred to as the upper and lower laser states. When a particle in the upper state interacts with a photon matching the energy separation of the levels, the particle may decay, emitting another photon with the same phase and frequency as the incident photon. Thus we have gotten two photons for the price of one. This process is known as stimulated emission. The stimulated emission of light is the crucial quantum process necessary for the operation of a laser. Population Inversion A fundamental concept in lasers is the idea of a "population inversion". A normal thermal population in any material will have most of the particles in the ground state. However, we would prefer to have most of the particles in the excited state so we can get free photons through stimulated emission. Thus in a laser we strive to create a "population inversion" where most or all of the particles are in the excited state. This is achieved by adding energy to the laser medium (usually from an electrical discharge or an optical source such as another laser or a flashlamp); this process is called pumping . Gain Another fundamental concept in lasers is the idea of gain, which is basically a short way of referring to the "free" photons described earlier. Suppose we have just pumped our laser medium so that all of the particles are in their excited state. One of those particles now spontaneously decays back down to its ground state, emitting a photon (hv0). This photon is of the right frequency to stimulate emission from another excited state particle, which emits another photon which can stimulate another excited state particle, and so on. (see the figure below).   Loss In addition to stimulated emission processes there are also stimulated absorption processes in which a ground state particle absorbs a photon matching the energy gap and jumps to the excited state. (represented by the gray arrow in the above figure). Thus we lose one photon to each stimulated absorption process. Since the probabilities for stimulated absorption and emission processes are equal (relative to population of the ground and excited states -- Einstein's famous result), it is clearly detrimental to the laser to have any particles in the ground state. For this reason, two level lasers are not practical -- it is not in general possible to pump more than half of the molecules into the excited state.  Three and Four Level Laser Schemes We have said that the two level scheme is not generally feasible for laser action. There are two main reasons for this. The first reason is that the energy being used to pump the particles into the upper laser state has an equal probability of stimulating them back down. Thus it is not possible to get nore than half of the particles into the excited state. The three level scheme gets around this problem by first exciting the particles to an excited state higher in energy than the upper laser state.(See figure below) The particles then quickly decay down into the upper laser state. It is important for the pumped state to have a short lifetime for spontaneous emission compared to the upper laser state. The upper laser state should have as long a lifetime (for spontaneous emission) as possible, so that the particles live long enough to be stimulated and thus contribute to the gain.   The four level laser scheme goes one step further and also depopulates the lower laser level by a fast decay process.(see figure below) This greatly decreases the loss of laser photons (v2) by stimulated absorption processes since the particles in the lower laser level have a short life-time for spontaneous emission.   Gain vs. Loss Mechanisms Maximizing the gain is one of the primary goals in any typcial laser.  In an ultrafast laser however, there is usually some sort of intensity dependent loss mechanism.  The idea here is that we are trying to create a very narrow pulse in the time domain; one way to do this is to introduce a saturable absorber into the cavity.  This is some medium that will absorb all laser radiation below some intensity threshhold.  This will be discussed in more detail when we introduce mode-locking in the spatial domain.  MASER stands for Microwave Amplification by Stimulation Emission of Radiation. A LASER is a MASER that works with higher frequency photons in the ultraviolet or visible light spectrum (photons are bundles of electromagnetic energy commonly thought of as "rays of light" which travel in oscillating waves of various wavelengths) . The first papers about the MASER were published in 1954 as a result of investigations carried out simultaneously and independently by Charles Townes and co-workers at Columbia University in New York and by Dr. Basov and Dr. Prochorov at the Lebedev Institute in Moscow. All three of these gentlemen received the Nobel Prize in 1964 for their contributions to science. [The following was paraphrased in part from Halliday & Resnick's "Fundamentals of Physics", second edition.] The fundamental physical principle motivating the MASER is the concept of stimulated emission, first introduced by Einstein in 1917. Before defining it we look at two related but more familiar phenomena involving the interplay between matter and radiation, absorption and spontaneous emission. Absorption. According to quantum mechanics, absorption of photons by atoms occurs only if the wavelength of the photon is just the right size (say, of wavelength l). If it is, the atom will "absorb" it (the photon vanishes) and go to a higher energy state. In physics, this process is called "absorption." Spontaneous Emission. Atoms don't like to stay in high energy states (this is dictated by the laws of thermodynamics), so after absorbing a photon and going to a higher energy state, they will move of their own accord to a lower energy state, emitting a photon in the process. This is called "spontaneous emission" because no outside influence triggers the emission. Normally the average lifetime for spontaneous emissions by excited atoms is around 10-8 seconds (that is, the atom or molecule will usually take around 10-8 seconds before emitting the photon). Occasionally, however, there are states for which the lifetime is much longer, perhaps around 10-3 seconds. These states are called metastable. Metastable emission levels are essential for a working MASER and will be discussed further in a moment. Now that we've discussed absorption and spontaneous emission, we can get to stimulated emission (a MASER beam is made up entirely of stimulated emission). Stimulated Emission. With stimulated emission, a photon of the absorption wavelength, l , is fired at an atom already in its high energy state from prior absorption. The atom absorbs this photon, and then quickly emits two photons to get back to its lower energy state. Thanks to quantum mechanics, both of these newly emitted photons are of wavelength l! The following figure displays this concept in detail: MASER. In each frame, a molecule in the upper level of the MASER transition (that is, in the high energy, excited state) is indicated by a large red circle, while one in the lower level (low energy state) is indicated by a small blue circle. (a) All of the molecules are in the upper state and a photon of wavelength l (shown in green) is incident from the left. (b) The photon l stimulates emission from the first molecule, so there are now two photons of wavelength l, in phase. (c) These photons stimulate emission from the next two molecules, resulting in four photons of wavelength l. (d) The process continues with another doubling of the number of photons. [Figure courtesy M. L. Kutner, "Astronomy: A Physical Perspective", John Wiley & Sons, Inc. 1987] Basically, a man-made MASER is a device that sets up a series of atoms or molecules and excites them to generate the chain reaction, or amplification, of photons. Metastable emission states make MASERs and LASERs possible. To get the proper wavelengths to generate the chain reaction, first electricity or another energy source is "pumped" into a chamber filled with particular atoms or molecules. Then this "pumping" radiation causes the transition of atoms from the ground state to a high energy excited state higher than that referred to in the above paragraphs. From this short-lived state the atoms come down through non-radiative transition to the long-lived metastable state. Once in the metastable state many atoms can be accumulated in one place and in the same state. The LASER or MASER beam, stimulated emission, arises when all these accumulated atoms simultaneously make a transition to the ground state, releasing their energy of wavelength l, creating a beam of microwave radiation (or visible light in the case of a LASER) which can be sent on to other atoms to cause the chain reaction described in the above figure. Since all the resulting photons are the same wavelength, MASER beams are extremely focussed and coherent. MASERs and their shorter-wavelength counterparts (LASERs), have many practical applications, especially in science and medicine. Of course, stimulated emission can only occur for incoming photons that have a photon energy close to the energy of the laser transition. Therefore, thelaser gain occurs only for optical frequencies (or wavelengths) within a limited gain bandwidth. A laser normally operates at the optical wavelength where the gain medium provides the highest gain. In an ensemble of atoms having only two energy levels (a ground state and an excited state), the excited atoms can amplify light, while those atoms in the ground state can absorb light, which brings them back to the excited state. Net amplification can then be achieved only when more than 50% of the atoms are in the excited state. This condition is called population inversion. Laser gain is more easily achieved when there is a mechanism that rapidly removes the atoms from the lower energy level after each emissionevent (e.g., by transfer into an even lower energy level). Properties of laser Monochromaticity: Highly monochromatic radiation Intensity: Laser beams are highly intense as a large number of photons are concentrated in a small region. Coherence: Perfectly coherent as the emitted light waves has the same phase with one another. Directionality: Travels in a single direction as the photons are traveling along the optical axis of the system. Basic concepts of Laser Action Absorption: Atoms in the lower energy state absorb energy from the incident photon and moves to the higher energy state. Probability of absorption depends on: Number of atoms present in the lower energy state. Intensity of incident light Spontaneous Emission: Atoms in higher energy state jumps to the lower energy state with emission of a photon at random (i.e. without influence from a photon) Probability of Spontaneous emission depends on: Number of atoms available in the excited state. Stimulated Emission: Atoms in the excited state jumping to the lower state under the influence of another photon emit a photon of the same frequency as the incident photon. Probability of Stimulated emission depends on: Number of atoms available in the excited state Intensity of the incident light The direction of propagation of energy, phase and state of polarization of energy of the emitted photon is exactly the same as those of the stimulating photon. Thus photons are coherent. Working principle of a laser Based on phenomenon of stimulated emission and spontaneous emission Active medium should have one metastable state besides excited state and ground state. The lifetime of atoms in excited state is 10^-8 sec but it is longer in metastable state. When atoms are excited with light of suitable wavelength, they jump from lower energy state to excited state by absorbing photons. But atoms can remain in excited state only for a small amount of time and they drop back by spontaneous emission. Many of them are trapped in the metastable state where its lifetime is greater and population inversion is obtained. After getting population inversion, a photon got from spontaneous emission is made to strike an atom of the metastable state. The excited atom of metastable state is stimulated to emit a photon of the same energy as that of the stimulating photon. The stimulating and stimulated photons yield a large number of coherent photons by repeated stimulated emissions as they pass through the atom. Hence light amplification occurs due to multiplication of photons all of which have same frequency, direction and phase. Population Inversion: An artificial situation that is established by generating a large number of atoms in the higher energy state than that of the lower energy state. Pumping: It is the phenomenon of achieving population inversion. It is the process which raises the atoms from lower energy state to higher energy state in the active medium. Methods: Optical Pumping: a light source is used to supply luminous energy and create population inversion by optical photon Electrical Pumping: electrical discharge converts the gas medium into plasma which liberates electrons which, in turn, are accelerated by the strong electric fields present in the tube. These electrons, on collision with neutral gas atoms, makes some atoms jump to excited state. Chemical Pumping: an exothermic chemical reaction is used to produce energy. Lasing The process which leads the emission of stimulated photons due to the transition of atoms from the metastable state to the ground state after achieving population inversion. By Maxwell Boltzman statistics:The number of atoms present in a particular energy state at any time: N2 = N1e-ΔE/kT Where, ΔE = E2 – E1 = hυN2 & N1 => number of atoms present in excited state & ground state respectivelyk & T => Boltzman’s constant and absolute temperature Einstein’s Theory of Laser and relation between A and B coefficients Let N1 and N2 be the number of atoms present in ground state E1 and excited state E2 respectively of the active medium. Energy required to raise atom from E1 to E2 = hυ where υ is frequency of radiation But rate of absorption is proportional to N1 and the energy density u(υ) of incident light. Thus, the number of absorption per unit volume per unit time => Tab = B12N1 u(υ) Where, B12 = Einstein’s coefficient of absorption of radiation from lower energy state to higher energy state. Atoms in excited state return to ground state by emission of energy by two process: Spontaneous emission Stimulated emission Since spontaneous emission depends only on the number of atoms per unit volume present in excited state ( N2 ) Tsp = A21 N2                    where A21 = Einstein’s coefficient of spontaneous emission N2 would relax in metastable state where stimulated emission occurs. Since stimulated emission is directly proportional to the number of atoms present in the excited state per unit volume ( N2 ) and energy density[u(υ)] of incident radiation: Tst = B21 N2 u(υ)    where B21 = Einstein’s coefficient  of stimulated emission When thermal equilibrium is reached, rate of upward transmission = rate of downward transmission,Tab =  Tsp + Tst          B12N1u(υ) = A21N2 + B21N2 u(υ)          (B12N1 - B21N2 )u(υ) = A21 N2          u(υ) =         N2 A21/ N2(B12 N1 / N2 – B21) = A21/( N1 / N2 B12 – B21) N1 and N2 are related by Boltzman’s law as: N2 / N1 = e-(E2-E1)/kT = e- hv/kT Thus inserting value, we get          u(υ) =  A21/ B12 (1/( e hv/kT - B21 / B12 ))But according to planck’s law of radiation,          u(υ) = 8πhυ3/c3. 1/( e hv/kT – 1)Thus comparing equations, we get:           B21/ B12 = 1          or, B21 = B12And    A21 / B12  =  8πhυ3/c3Thus   A21 / B21  =  8πhυ3/c3 Physical Significance: The probability of stimulated emission is numerically equal to probability of stimulated absorption. So stimulated emission is inverse process of absorption. Their rates are different because stimulated emission is proportional to number of atoms present in excited state while stimulated absorption is proportional to number of atoms present in ground state. The coefficient of stimulated emission (B21) is inversely proportional to the third power of frequency of radiation. The ratio of the rate of stimulated emission to the rate of spontaneous emission               R = B21 N2u(υ)/ A21 N2 = 1/( ehv/kT – 1) The probability of stimulated emission is more compared to spontaneous emission in microwave region The probability of stimulated emission is negligible compared to spontaneous emission in visible region Condition for higher probability of stimulated emission compared to that of simultaneous emission From Einstein’s relation: A21 / B21 = 8πhυ3/c3 From Planck’s radiation law: u(υ) = 8πhυ3/c3 .1/( ehv/kT – 1) Thus, A21/ B21 u(υ) = ehv/kT – 1 = R In the microwave region, hυ< N2 To make stimulated transition exceed the absorption transitions: The state population inversion has to be achieved. For this reason, the lasing material is doped with certain impurities such that metastable atomic energy level is obtained. The larger value of the ratio of stimulated transitions to spontaneous transitions is to be achieved by considering a metastable energy state as the higher energy level. Laser Resonator or Laser Cavity The optical mirrors, active medium and pumping system form the laser resonator, which is also called Laser Cavity. Laser cavities can be divided into Stable Cavities and Unstable Cavities according to whether they make the oscillating beam converge into the cavity or spread out from the cavity. Various Laser Resonators The most widely used laser resonators or cavities have either plane or spherical mirrors of rectangular or circular shape, separated by some distance L. There have appeared Plane Parallel Resonators, Concentric (Spherical) Resonators, Confocal Resonators, Generalized Spherical Resonators and Ring Resonators. Plane Parallel Resonator consists of two plane mirrors set parallel to each other, as shown in the figure below. The one round trip of wave in the cavity should be an integral number times 2 , the resonant frequencies is  = kc/(2L), k is an integral number, c is the speed of light in the medium, L is the cavity length. The frequency difference between two consecutive modes (possible standing wave in the cavity) is c/(2L). This difference is referred to as the frequency difference between two consecutive longitudinal modes; the word longitudinal is used because the number k indicates the number of half-wavelengths of the mode along the laser resonator, i.e., in the longitudinal direction. Concentric resonator consists of two spherical mirrors with the same radius R separated by a distance L=2R, so that the centers are coincident. The resonant frequencies use the same equation as above. Confocal resonator consists of two spherical mirrors of the same radius of curvature R separated by a distance of L such that their foci F1 and F2 coincident. In this case, the center of curvature of one mirror lies on the surface of another mirror, L=R. The resonant frequency cannot be readily obtained from geometrical optics consideration. Resonators formed by two spherical mirrors of the same radius of curvature R and separated by a distance L such that Rdistance between two mirrorsλ =>wavelength of emergent lightφ =>phase change after reflection from both mirrors Change in phase after one round trip: ρ = 2π/λ.2L + 2φ pi=π For constructive interference, phase change must be integral multiple of 2π for standing waves to form2π/λ.2L + 2φ = 2mπor,     ν = c/λ = mc/2L – φc/2πL Standing waves are formed within the two mirrors giving us two nodes. Since the wavelength of laser light is much smaller than the length of the cavity, the number of half waves formed within the mirror is also very large. So the frequency difference between two consecutive modes of vibration: Δν = c/2L Types of Laser: Four types: solid state laser gas lasers liquid dye lasers semiconductor lasers Ruby Laser: Solid state laser consisting of a pink ruby cylindrical rod whose ends are optically flat and parallel with a silvered and a partially silvered (50%) end. The rod is surrounded by a high intensity helical flash lamp filled with xenon gas which is intense enough to produce population inversion. Composition of Ruby: Crystalline aluminium oxide (Al2O3 or host crystal) doped with 0.05% of chromium atoms (activator atoms). Al3+ ions are replaced by Cr3+ ions in crystal lattice. Cr3+ ions impart red colour to the white Al2O3 crystal. Working: Chromium atoms consist of a metastable state of lifetime ~3 X 10-3 sec. When a flash of light of wavelength 550nm falls upon the rod for a very short time (about a millisecond), the chromium ion, in the ground state, absorbs a photon and jumps to excited state E2. The excited ions drop to the metastable state E3very soon as lifetime of ions in excited state is short. The transition is non radiative as the energy released is absorbed by the lattice in which it is absorbed and is dissipated as heat. But the number of atoms in metastable state goes on increasing as lifetime in metastable state is high and soon exceeds those in the ground state, thus bringing about population inversion. After this state is achieved, one or two photons released due to spontaneous emission is sufficient to induce stimulated emission and light amplification will start. The transition from M  G state radiates photons, which after repeated reflection from the mirrors of the laser cavity amplifies largely to an intense beam. An intense, highly directional, coherent beam of red light (λ = 694.3 nm) emerges from the partially silvered end of the ruby rod as laser beam. The Helium Neon Gas laser It is a gas laser consisting of a mixture of helium (He) and neon (Ne) in a ratio of about 10:1 inside a narrow long discharge tube at pressure of 1mm of mercury. The gas system is placed between a pair of plane mirrors or a pair of convex mirrors out of which one is perfectly reflecting while other is partially reflecting forming the resonating system. The distance between two mirrors is equal to an integral multiple of half wavelength of the laser light and supports standing wave pattern within the resonator system. Working: Helium has three energy states. These are 3S, 2S and 1S where 3S and 2S are metastable states. When an electrical discharge passes through the gas mixture, the helium atoms are excited by the impacts of accelerated electrons in the discharge tube due to its lower mass. As a result, some of the helium atoms are raised to its metastable states 2S and 3S from its ground state. The energy of the two excited states 2S and 3S of Ne are slightly less than the energy of the two metastable states of He atoms. Thus, after the collision of the excited helium atoms with neon atoms, the neon atoms in the ground state are raised to its 3S and 2S excited states and helium returns to its ground state by exchanging energy. The gas discharge process after sometime leads population inversion in these metastable Ne(3S) and Ne(2S) levels relative to its lower 3P and 2P states. After achieving population inversion, one of the two photons released due to spontaneous emission can trigger stimulated emission and produce three type of lasing actions(3S  3P, 3S  2P, 2S  2P). After that, the Ne atoms return to the lower laser levels 3P and 2P to the level 1S by spontaneous emission. From this level, Ne returns to ground state by collision with the walls of the tube. The cycle of events occur continuously as the discharge in the tube is maintained continuously. Thus it is known as continuous laser. Helium-Neon Laser The most common and inexpensive gas laser, the helium-neon laser is usually constructed to operate in the red at 632.8 nm. It can also be constructed to produce laser action in the green at 543.5 nm and in the infrared at 1523 nm. One of the excited levels of helium at 20.61 eV is very close to a level in neon at 20.66 eV, so close in fact that upon collision of a helium and a neon atom, the energy can be transferred from the helium to the neon atom. Helium-neon lasers are common in the introductory physics laboratories, but they can still be dangerous! According to Garmire, an unfocused 1-mW HeNe laser has a brightness equal to sunshine on a clear day (0.1 watt/cm2) and is just as dangerous to stare at directly. The helium gas in the laser tube provides the pumping medium to attain the necessarypopulation inversion for laser action. This shows the beams from two helium-neon lasers passing through two lenses arranged in the Galilean telescope geometry. The beams were made visible with a spray can of artificial smoke. Uses of He-Ne Laser in interferometry in laser printing in bar code reading in holography for larger distance measurement, i.e. in laser modulation telemetry in the target aiming device used in guns The advantages of Gas laser over Solid state laser The light from He-Ne gas laser has high degree of monochromacity and directionality than that from solid state ruby laser. This happens due to imperfection in the crystal, thermal distortion and scattering. The solid state laser need cooling in time of operation while the gas lasers can operate continuously without any cooling. The diagram shows a neodymium doped YAG crystal absorbing energy from an intense light source resulting in the release of photons in random spatial directions by the combined mechanism of spontaneous and stimulated emission.Next, another view of the crystal with mirrors added to each end to produce a resonant cavity. By coincidence, the spatial direction of some of the photon groups will cause them to travel along the longitudinal axis of the cavity. The result is the impingement of these photons on the mirrors located at the ends of the crystal from where they will be returned to the crystal by reflection and will continue to stimulate the emission of other photons. This activity creates an enormous amplification of photons traveling back and forth between the mirrors, continually stimulating and aligning their travel direction. One of the mirrors designated the front mirror is deliberately designed to allow a controlled leakage or transmission of light -up to 60%. This transmission is the raw laser beam. The beam is pure light since it consists of a single wavelength (monochromatic) and in addition is both coherent (in phase) and collimated (low divergence). The basic solid state neodymium "YAG" laser cavity consists of the ND:YAG crystal, an energy source, a 100% reflective rear mirror and a front or output mirror which is up to 60% light transmissive. The cavity may also contain accessories such as shutters, apertures and electro optical mechanisms. Neodymium-YAG Laser An example of a solid-state laser, the neodymium-YAG uses the Nd3+ ion to dope the yttrium-aluminum-garnet (YAG) host crystal to produce the triplet geometry which makes population inversion possible. Neodymium-YAG lasers have become very important because they can be used to produce high powers. Such lasers have been constructed to produce over a kilowatt of continuous laser power at 1065 nm and can achieve extremely high powers in a pulsed mode. Neodymium-YAG lasers are used in pulse mode in laser oscillators for the production of a series of very short pulses for research with femtosecond time resolution. A laser beam is generated when photons traveling in a direction along the longitudinal axis of the crystal are reflected and returned to the crystal by the end mirrors where they continue to amplify the output through the phenomena of stimulated emission. Since the front mirror is up to 60% light transmissive a laser beam is emitted. This beam of light is monochromatic, collimated and coherent. Because of these three characteristics and resultant low divergence, the beam is capable of traveling great distances with minimal energy loss. These characteristics are extremely important. The ability to deliver this pure beam of light through an optical system and project it for miles or to focus it to so small a diameter its energy density can vaporize ceramics, is dependent on these three characteristics. It is timely to mention there are many lasers emitting pure light beams of different frequencies depending on their atomic origin. Special applications require particular light frequencies to be efficiently absorbed by the characteristics of the target material. Eye surgery or measurement operations, etc. require different light frequencies than those we commonly use for metal working. We must not neglect to add the cavity and optical system just described includes other items, apertures, collimators, safety shutters, beam splitters, viewing optics etc. to fine tune and control the beam. Carbon Dioxide Laser The carbon dioxide gas laser is capable of continuous output powers above 10 kilowatts. It is also capable of extremely high power pulse operatin. It exhibitslaser action at several infrared frequencies but none in the visible. Operating in a manner similar to the helium-neon laser, it employs an electric discharge for pumping, using a percentage of nitrogen gas as a pumping gas. The CO2 laser is the most efficient laser, capable of operating at more than 30% efficiency. That's a lot more efficient than an ordinary incandescent light bulb at producing visible light (about 90% of the output of a lightbulb filament is invisible). The carbon dioxide laser finds many applications in industry, particularly forwelding and cutting. Argon Laser The argon ion laser can be operated as a continuous gas laser at about 25 different wavelengths in the visible between 408.9 and 686.1nm, but is best known for its most efficient transitions in the green at 488 nm and 514.5 nm. Operating at much higher powers than the helium-neon gas laser, it is not uncommon to achieve 30 to 100 watts of continuous power using several transitions. This output is produced in a hot plasma and takes extremely high power, typically 9 to 12 kW, so these are large and expensive devices. Laser Diodes Laser action (with the resultantmonochromatic and coherent light output) can be achieved in a p-n junction formed by two doped gallium arsenide layers. The two ends of the structure need to be optically flat and parallel with one end mirrored and one partially reflective. The length of the junction must be precisely related to the wavelength of the light to be emitted. The junction is forward biased and the recombination process produces light as in the LED (incoherent). Above a certain current threshold the photons moving parallel to the junction can stimulate emission and initiate laser action. Type Peak Power Wavelength Application GaAs 5 mW 840 nm CD Players AlGaAs 50 mW 760 nm Laser printers GaInAsP 20 mW 1300 nm Fiber communications Laser Applications Medical applications Welding and Cutting Surveying Garment industry Laser nuclear fusion Communication Laser printing CDs and optical discs Spectroscopy Heat treatment Barcode scanners Laser cooling Medical Uses of Lasers The highly collimated beam of a laser can be further focused to a microscopic dot of extremely high energy density. This makes it useful as a cutting and cauterizing instrument. Lasers are used for photocoagulation of the retina to halt retinal hemorrhaging and for the tacking of retinal tears. Higher power lasers are used after cataract surgery if the supportive membrane surrounding the implanted lens becomes milky. Photodisruption of the membrane often can cause it to draw back like a shade, almost instantly restoring vision. A focused laser can act as an extremely sharp scalpel for delicate surgery, cauterizing as it cuts. ("Cauterizing" refers to long-standing medical practices of using a hot instrument or a high frequency electrical probe to singe the tissue around an incision, sealing off tiny blood vessels to stop bleeding.) The cauterizing action is particularly important for surgical procedures in blood-rich tissue such as the liver. Lasers have been used to make incisions half a micron wide, compared to about 80 microns for the diameter of a human hair. Welding and Cutting The highly collimated beam of a laser can be further focused to a microscopic dot of extremely high energy density for welding and cutting. The automobile industry makes extensive use of carbon dioxide lasers with powers up to several kilowatts for computer controlled welding on auto assembly lines. Garmire points out an interesting application of CO2 lasers to the welding of stainless steel handles on copper cooking pots. A nearly impossible task for conventional welding because of the great difference in thermal conductivities between stainless steel and copper, it is done so quickly by the laser that the thermal conductivities are irrelevant. Surveying and Ranging Helium-neon and semiconductor lasers have become standard parts of the field surveyor's equipment. A fast laser pulse is sent to a corner reflector at the point to be measured and the time of reflection is measured to get the distance. Some such surveying is long distance! The Apollo 11 and Apollo 14 astronauts put corner reflectors on the surface of the Moon for determination of the Earth-Moon distance. A powerful laser pulse from the MacDonald Observatory in Texas had spread to about a 3 km radius by the time it got to the Moon, but the reflection was strong enough to be detected. We now know the range from the Moon to Texas within about 15 cm, a nine significant digit measurement. A pulsedruby laser was used for this measurement. Lasers in the Garment Industry Laser cutters are credited with keeping the U.S. garment industry competitive in the world market. Computer controlled laser garment cutters can be programmed to cut out 400 size 6 and then 700 size 9 garments - and that might involve just a few cuts. The programmed cutter can cut dozens to hundreds of thicknesses of cloth, and can cut out every piece of the garment in a single run. The usefulness of the laser for such cutting operations comes from the fact that the beam is highly collimated and can be further focused to a microscopic dot of extremely high energy density for cutting. Lasers in Communication Fiber optic cables are a major mode of communication partly because multiple signals can be sent with high quality and low loss by light propagating along the fibers. The light signals can be modulated with the information to be sent by either light emitting diodes or lasers. The lasers have significant advantages because they are more nearly monochromatic and this allows the pulse shape to be maintained better over long distances. If a better pulse shape can be maintained, then the communication can be sent at higher rates without overlap of the pulses. Ohanian quotes a factor of 10 advantage for the laser modulators. Telephone fiber drivers may be solid state lasers the size of a grain of sand and consume a power of only half a milliwatt. Yet they can sent 50 million pulses per second into an attached telephone fiber and encode over 600 simultaneous telephone conversations (Ohanian). Barcode Scanners Supermarket scanners typically use helium-neon lasers to scan the universal barcodes to identify products. The laser beam bounces off a rotating mirror and scans the code, sending a modulated beam to a light detector and then to a computer which has the product information stored. Semiconductor lasers can also be used for this purpose. DVD Spiral The key to DVD technology is the increased data-storage capacity, and this is enhanced even further by the use of double-sided and double-layered discs. This is one of the factors that allows the technology to far outstrip the old Video CD format, as well as VHS tape. CD-DVD Comparison The data track is also more tightly compressed on a DVD, with just 740 nanometers between each groove. Again, this is roughly half the distant between the grooves on a CD. Extra storage is also made available by using more of the disc surface, and the error-correction needed for DVD is not as wasteful of space as it is for CD.This only goes part of the way to explaining the excellent picture quality of DVD. The problem has also been addressed from the other angle - the amount of data that needs to be stored in the first place.Video-compression technology made a huge advance with the introduction of MPEG-2 encoding. Formulated by the modestly titled Moving Picture Experts Group, this compressed a video signal into a fraction of the space normally needed, without introducing picture degradation.The compressed video signal is read by the laser pickup in a DVD player and decoded, resulting in a video signal that can be delivered to a display. Again this area sees an improvement on CD technology - the laser used in a DVD player has a shorter wavelength, enabling it to read the smaller, more tightly packed bumps on a DVD. DVD Indentations The laser light bounces off a reflective layer behind the data layer and back to an optical pickup. This can distinguish between light that has bounced back from a bump, or from a 'land' (the area between bumps). This simple process allows the DVD player to construct a stream of data, forming bits, bytes, and eventually gigabytes of information. Laser Pick up Equally critical is the tracking mechanism, which moves the laser over the spinning disc. This needs to be able to move nanometers at a time, and because the rate at which data passes underneath the laser pickup has to be constant, the speed at which the disc spins has to gradually slow down as the pickup moves from the centre of the disc to the outside.The data retrieved is in digital format and in order to be understood by your display device, it needs to be converted. This is the job of the video and audio DACs (Digital to Analogue Converters) in your player, although decks are now appearing with digital video outputs to eliminate the need for this potentially degrading process. Compact disk evolution With the release of “The Visitors,” the era of the compact disc had officially begun, and over the next few years this technology gradually replaced analog recordings on records and magnetic tape. In 1996, it was followed by the DVD. Today the first HD-DVD and Blu-ray discs have reached the shelves, offering up to 80 times the capacity of a CD and producing razor-sharp images on today’s widescreen TV monitors, with a quality that is totally unprecedented. Better and better materials and technologies are permitting the use of increasingly larger data volumes. The future belongs to holographic media, with the storage of several hundreds of gigabytes. As before, materials from Bayer MaterialScience are leading this technology. Compact Audio disc technology development For the last quarter of a century the primary substrate material for CDs has been polycarbonate, such as high-tech Makrolon polycarbonate from Bayer. Working together with Philips and PolyGram, Bayer developed compact disc technology based on a specially tailored type of polycarbonate which still serves as the material for many optical recording media, although it has undergone a number of modifications since the early days. Makrolon polycarboante Audio CDs The Bayer researchers set to work on Makrolon polycarbonate and succeeded in modifying it for the special requirements of manufacturing processes in the music industry. The aim was to achieve the highest possible optical quality and transparency in the substrate, so that a laser head could read the digital code of a CD without any errors. Dr. Dieter Freitag was among the early pioneers. The former head of Central Materials Research at Bayer AG had already developed polycarbonates with an extraordinary level of flowability. This is vital for the production of CDs, because the plastic has to spread quickly and evenly within the mold. “What I didn’t know, however, was that, with this product, we would be able to split a Beethoven symphony into four billion pits and then press them onto a disc with a diameter of 12 centimeters,” said Freitag. With Makrolon polycarbonate, Bayer MaterialScience gave the industry a specially tailored material that would meet - and indeed still meets - the highest requirements with respect to storage capacity, data readability and stability.  Optical recording media development The outstanding sound quality and excellent durability of the new audio CD marked a paradigm shift in the technical recording of music and led to an amazing boom from the very first day of its market launch. The digitization of sound and music suddenly brought perfect musical enjoyment into our living rooms. Over 900,000 metric tons of polycarbonate are currently used for the production of optical recording media. Whereas in 1982 it took 27 seconds to produce a CD, production time has now been reduced to less than three seconds. Optical data storage development Data storage has been developing at a steady pace over the last 25 years. One collaborator with Bayer MaterialScience has been Sony. The first CD-ROM (ROM = read only memory) was launched in 1992, with a storage volume of more than 450 floppy discs. It was suddenly possible to store entire reference works and to call them up whenever required. Digital versalite disc DVD Only two years later computer users could simply “burn” and archive their documents to recordable or rewritable CDs (CD-Rs or CD-RWs). The next logical step was the DVD (digital versatile disc), an optical recording medium that can hold several times as much data as a CD (4.7 gigabytes) : in 1996, 14 years after the launch of the compact disc, it took the world by storm. Like the CD, it was followed a few years later by a “burnable” version.  At first glance, a DVD disc can easily be mistaken for a CD: both are plastic discs 120mm in diameter and 1.2mm thick and both rely on lasers to read data stored in pits in a spiral track. And whilst it can be said that the similarities end there, it's also true that DVD's seven-fold increase in data capacity over the CD has been largely achieved by tightening up the tolerances throughout the predecessor system. Firstly, the tracks are placed closer together, thereby allowing more tracks per disc. The DVD track pitch (the distance between each) is reduced to 0.74micron, less than half of CD's 1.6 micron. The pits, in which the data is stored, are also a lot smaller, thus allowing more pits per track. The minimum pit length of a single layer DVD is 0.4 micron as compared to 0.834 micron for a CD. With the number of pits having a direct bearing on capacity levels, DVD's reduced track pitch and pit size alone give DVD-ROM discs four times the storage capacity of CDs. The packing of as many pits as possible onto a disc is, however, the simple part and DVD's real technological breakthrough was with its laser. Smaller pits mean that the laser has to produce a smaller spot, and DVD achieves this by reducing the laser's wavelength from the 780nm (nanometers) infrared light of a standard CD, to 635nm or 650nm red light. Secondly, the DVD specification allows information to be scanned from more than one layer of a DVD simply by changing the focus of the read laser. Instead of using an opaque reflective layer, it's possible to use a translucent layer with an opaque reflective layer behind carrying more data. This doesn't quite double the capacity because the second layer can't be quite as dense as the single layer, but it does enable a single disc to deliver 8.5GB of data without having to be removed from the drive and turned over. An interesting feature of DVD is that the discs' second data layer can be read from the inside of the disc out, as well as from the outside in. In standard-density CDs, the information is always stored first near the hub of the disc. The same will be true for single- and dual-layer DVD, but the second layer of each disc can contain data recorded backwards, or in a reverse spiral track. With this feature, it takes only an instant to refocus a lens from one reflective layer to another. On the other hand, a single-layer CD that stores all data in a single spiral track takes longer to relocate the optical pickup to another location or file on the same surface. Thirdly, DVD allows for allows for double-sided discs. To facilitate the focusing of the laser on the smaller pits, manufacturers used a thinner plastic substrate than that used by a CD-ROM, thereby reducing the depth of the layer of plastic the laser has to travel through to reach the pits. This reduction resulted in discs that were 0.6mm thick - half the thickness of a CD-ROM. However, since these thinner discs were too thin to remain flat and withstand handling, manufacturers bonded two discs back-to-back - resulting in discs that are 1.2mm thick. This bonding effectively doubles the potential storage capacity of a disc. Note that single-sided discs still have two substrates, even though one isn't capable of holding data. Finally, DVD has made the structure of the data put on the disc more efficient. When CD was developed in the late 1970s, it was necessary to build in some heavy-duty and relatively crude error correction systems to guarantee the discs would play. When bits are being used for error detection they are not being used to carry useful data, so DVDs have a more efficient and effective error correction code (ECC) HD-DVDS & Blu-ray discs Now even greater data densities can be achieved on discs through the use of blue lasers, which have shorter wavelengths than red or green ones and can, therefore, be focused more precisely. The new optical engineering technique is used in HD-DVDs and Blu-ray discs with storage capacities of 15 to 100 gigabytes, making these the only discs that can fully satisfy the digital data requirements of high-definition TV. The storage volume of a Blu-ray Disc today is nearly 80 times that of a Compact Disc (650 megabytes). This achievement was made possible by shortening the wavelength of the laser beam used to read and write the data, from infrared (CD), to red (DVD) and then to blue light (Blu-ray Disc, HD-DVD). As a result, data can be written and read on a considerably narrower area. Even the size of the pits - the indentations containing information – has decreased over the years. The smallest possible structure on a Blu-ray Disc is just one-fifth the size of a pit on a Compact Disc. In addition, the distance between individual data tracks has been reduced by some 80 percent. Optical Disc Structure An optical disc is a flat, usually circular disc which encode binary data in the form of pits and lands on a special material on one of its flat surfaces. The lands represent "1" and the pits represent "0" in binary computing. The bits are read by the disc drive that uses a laser beam to distinguish between the lands and pits based on the amount of scattering or deflection that occurs when the beam of light hits the surface of the disc. But it is not simply so that a land is a "1" data bit, and a pit is a "0" data bit. A data bit is a "1" or "0" from the original data, but on an optical disc there are no data-bits but channel-bits. A channel bit is the smallest time unit used on a disc, for a CD it equals 1/4,321,800 sec. a "1" channel bit is a time with change from land to pit, or from pit to land. a "0" channel bit is a time when there is no change. In a pressed or mass-replicated CD or DVD, the bumps and grooves that represent the binary data on a disc's substrate are pressed into it during manufacture. CD/DVD-R discs do not have true pits and lands, but the unmelted, clear areas and melted, opaque places in the dye layer fulfill the same function as pits and lands on a pressed disc. To write data onto a disc, the optical drive uses this type of laser to make a series of microscopic marks in the dye. The resulting sequence of light and dark spots (pits and lands) represent the digital ones and zeros that comprise your data. DVD uses 650 nm wavelength laser diode light as opposed to 780 nm for CD. This permits a smaller pit to be etched on the media surface compared to CDs (0.74 µm for DVD versus 1.6 µm for CD), allowing for a DVD's increased storage capacity. In comparison, Blu-ray disc, the successor to the DVD format, uses a wavelength of 405 nm, and one dual-layer disc has a 50 GB storage capacity. Unlike the floppy disk, most optical discs do not have an integrated protective casing and are therefore susceptible to data transfer problems due to scratches, fingerprints, and other environmental problems. An optical disc is designed to support one of three recording types: read-only (e.g. CD and CD-ROM), recordable (write-once, e.g. CD-R), or re-recordable (rewritable, e.g. CD-RW).