Physics of Lasers, Physics tutorial

Introduction:

It was illustrated from the coherence and coherent sources of light, that is, why conventional thermal sources of light emit radiation which encompass very low degree of coherence. Though, phenomenon such as interference that needs coherent light sources can certainly be observed by means of conventional light sources. The quest for getting a light source having high degree of coherence led to the invention of lasers. As we are familiar, a helpful indicator of the degree of coherence is the coherence length. For ordinary light, the coherence length is of the order 10-2 m, while the coherence length for a laser light can be as long as 105 m.

The term laser is an acronym for 'Light Amplification by Stimulated Emission of Radiation'. You should realize that the key words here are amplification and stimulated emission. The existence of stimulated emission of radiation, whenever radiation interacts with matter, was predicted by Einstein in the year 1916. His theoretical prediction was realized by C. H. Townes and co-workers in the year 1954 if they developed microwave amplification through stimulated emission of radiation (that is, maser). The principle of maser was adapted for light in the visible range by A. Schawlow and C. H. Townes in the year 1958 however the first laser device was invented by T. H. Maiman in the year 1960. Once the laser was discovered, it has found applications in such different fields as basic research, industry, space, medicine, photography, communication, defense and so on.

Light Emission and Absorption:

As we are aware that most of the man-made sources of light are the solids and gases heated to high temperatures. For illustration, in case of incandescent bulb, the tungsten filament is heated, and in the case of mercury tube light, the gas is heated. The energy of the heating source is absorbed via atoms or molecules of the solid or the gas that, in turn, emit light. The fundamental procedure of the origin of light from within gas molecules, liquids and solids is alike in many respects to that from the individual atom. And the method of emission and absorption of light from atoms can be understood in terms of Bohr's atomic model.

Quantum Theory: A Brief Outline

According to the Bohr's theory, the energy of an atom or a molecule can take on just definite (or discrete) values. These are termed as the energy levels of the atom. The transition of an atom from one energy level to the other energy level takes place in quantum jump. This was one of the fundamental suppositions of Bohr's theory. On the base of this assumption, Bohr hypothesized that light is not emitted through an electron when it is revolving in one of its allowed orbits (and therefore consists of a fixed value of energy). Light emission occurs whenever the atom makes a transition from an excited state (of energy Ei) to a state of lower energy Ef. The frequency of the emitted radiation is represented by:

hv = Ei - Ef

Here, Ei is the energy of the initial orbit, Ef is the energy of the final orbit, 'ν' is the frequency of the emitted light and 'h' is the Planck's constant. The quantized orbits of the electron and the energy level diagram of the simplest atom, that is, the hydrogen atom are shown in the figure below.

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The quantum mechanical description regarding the origin of light, applies to all the identified light sources. To focus our attention on the atomic processes comprised in the emission and absorption of light, let us suppose only two energy levels of an atom. Assume that the energy of the lower level is E1 and that of the upper level is E2. The atom lying in level E2 will tend to form a transition to level E1 in such a way that it occupies a state of lower energy. This emission procedure is termed as spontaneous emission as it takes place in the absence of any external stimulus. The photon emitted in spontaneous emission will encompass the energy (E2 - E1) whereas it's other features like momentum, polarization, will be random. The light emitted through ordinary sources results due to the spontaneous emission. Absorption of light is the converse method of emission. The atom in a lower energy state can absorb a photon of energy hv = (E2 - E1) and get excited to the upper level E2. Now, assume what will happen if an atom is in the higher energy level, E2 and a photon of energy hv = (E2 - E1) interacts with it? Well, in such a condition, the photon might trigger the atom in the upper level to emit the radiation. This emission procedure is termed as stimulated emission. If the atom is already in the higher energy level, the photon, rather than being absorbed, might play the role of a trigger, and induce the transition from E2 to E1. As an outcome, the atom falls into lower energy level and an additional photon of energy hv = E2 - E1 is emitted.

Stimulated Emission of Radiation: Einstein's Prediction

All the light sources changes input energy into light. In case of the laser, the input, or pump, energy can take numerous forms, the two most general being optical and electrical. For optical pumping, the energy source might be a lamp or, more generally, the other laser. Electrical pumping can be through a DC current (that is, as in laser diodes), an electrical discharge (noble gas lasers and excimer lasers) or a radio-frequency discharge (several CO2 lasers).

In a conventional (or incoherent) light source such as a light-bulb, an LED or a star, each and every atom excited through input pump energy arbitrarily emits a single photon according to a given statistical probability. This generates radiation in all directions having a spread of wavelengths and no interrelationships among the individual photons. This is termed as spontaneous emission.

Einstein predicted that the excited atoms as well could transform stored energy into light through a process termed as stimulated emission. This method generally begins by an excited atom first generating a photon through spontaneous emission. Whenever this photon reaches the other excited atom, the interaction stimulates that atom to emit second photon. This method consists of two significant features. First, it is multiplicative: one photon becomes two. If such two photons interact by two other excited atoms, then this will yield a total of four photons, and so on. Second and most significantly, these two photons have similar properties: wavelength, direction, phase and polarization. This capability to amplify light in the presence of an adequate number of excited atoms leads to the optical gain which is the basis of the laser operation and justifies its acronym of Light Amplification (by) Stimulated Emission (of) Radiation. A broad range of solid, liquid and gas-phase materials have been invented which exhibit gain under suitable pumping conditions.

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Prerequisites for a Laser:

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Fig: components of a laser oscillator: Energy source (1) supplies energy to the active medium (2) Medium is contained among the two mirrors (3 and 4) Mirror 3 is totally reflective whereas mirror 4 is partly transparent Laser radiation (5) emerges via partially transparent mirror.

A laser needs three prerequisites for the operation. First, there must be an active medium that, when excited, supports population inversion and afterward lasers. Secondly, we must make sure pumping method which increases the system to an excited state. And lastly, in most of the cases, there is an optical cavity which gives the feedback essential for the laser oscillation.

In a general laser operation, energy is transferred to the active material that is raised to the excited state, and ultimately lasers in different ways. The medium might be a solid, liquid or gas and it might be one of the thousands of materials which have been found to laser. The method of raising the medium to the excited state is termed as pumping, in analogy to pumping of water from lower to a higher level of potential energy. A few lasers are built as laser amplifier. They require no optical cavity. Most of the lasers, though, are laser oscillators. For sustained laser oscillations, some type of feedback method is required. The feedback method is given in the form of optical resonant cavity. In both the laser amplifiers and oscillators, the first few quanta of radiation will probably be emitted spontaneously and will trigger the stimulated emission.

Active Medium:

The heart of the laser is an assured medium: solid, liquid or gaseous termed as an active medium. As Maiman's discovery of ruby, most of the new laser materials have been discovered. They comprise crystals other than ruby, plastics, glasses, liquids, gases and even plasma (that is, the state of matter in which a few of the atomic electrons are dissociated from the atoms). The only general necessity for an active medium is that it gives an upper energy state into which atoms can be pumped and a lower state to which they will return by the spontaneous emission of the photons. The medium should as well allow a population inversion among the two states. It might occur that the active species or centers that provide lasing levels comprise a small fraction of the medium. For illustration, in case of ruby that is A12O3 by some of the Al atoms substituted by Cr atoms, only the latter (Cr) is the active centre. Typical number of active species per cubic centimeter in solids and liquids is 1019 to 1020 and that for gaseous media their number is around 1015 to 1017.

Excitation (or Pumping):

The method of obtaining population inversion is termed as pumping or excitation. The main goal of the pumping is to see that upper energy level is more intensely populated than the lower energy level. On the other hand, we can get the population inversion by depopulating lower energy level (other than ground state) faster than the upper energy level. There are many ways of pumping a laser and acquiring the population inversion essential for the stimulated emission to take place. The most generally employed are the following: Optical Pumping, Electric Discharge, Inelastic Atomic Collision and Direct Conversion.

In Optical Pumping, a source of light is employed to supply energy to the active medium. Most frequently this energy comes in the form of short flashes of light, a process initially employed in Maiman's Ruby Laser and broadly used even nowadays in the Solid-State Lasers. The laser material is positioned within a helical xenon flash lamp of the type customary in photography.

The other process of pumping is through direct electron excitation as it takes place in an electric discharge. This process is preferred for pumping the Gas lasers of which the argon laser is a good illustration. The electric field (generally several KV/m) causes electrons, emitted through the cathode, to be accelerated in the direction of the anode. A few electrons will impinge on the atoms of the active medium (that is, electron impact), and raise them to the excited state. As an outcome, population inversion is accomplished in the active medium.

In the inelastic atomic collision process of pumping, the electric discharge gives the initial excitation that increases one kind of atoms to their excited state or states. These atoms afterward collide inelastically with the other kind of atoms. The energy transferred inelastically increases the later kind of atoms to the excited states and these are the atoms that give the population inversion. An illustration is Helium-Neon Laser.

A direct conversion of electrical energy into radiation takes place in light emitting diodes. Such light emitting diodes (LED) are employed for pumping through direct conversion in semi-conductor lasers.

These are a few methods employed for pumping atoms of the active medium to accomplish population inversion. Atoms or molecules used as active centers often exhibit instead complex system of the energy levels.

Types of Lasers:

There are basically three kinds of lasers: solid state, gas and liquid. As all work according to the similar general principles, they are differentiated on the basis of the medium they make use of to make the laser action.

1) Solid State Lasers:

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The Solid-state laser comprises of a host and an active ion doped in the solid host material. The Active ion should encompass sharp fluorescent line, wide absorption bands and high quantum efficiency for the wavelength of interest. The host material should be strong and fracture resistant having high thermal conductivity and high optical quality. Glasses and crystalline materials have represented to encompass these features, when doped by rare earth ions. Silicate glasses, phosphate glasses, crystalline material such as: garnets, metal oxides, aluminates, fluorides, molybdates, tungstates and so on are very good hosts. Significant active ions are rare earth ions such as erbium, neodymium, holmium and transition metals such as chromium, titanium, nickel and so on. Some of the significant solid state lasers are: Ruby, Nd:YAG, Nd:Glass, Nd:Cr:GSGG, Er:Glass, Alexandrite, Titanium: sapphire and so on.

Fundamental parts of a flash pumped solid-state laser are provided in the figure shown above. All the solid-state laser materials employed as the active medium have their absorption bands in the visible area. As a result, optical pumping having flash lamps having their emission spectra in the visible area is employed as the excitation method. Flash lamp pumped Solid state lasers are, in common, extremely inefficient, as only a very small area of the emission spectra is employed in the absorption method, absorption band of the active ion being extremely narrow and rest being unutilized. Pumping employing diode lasers by precisely matching output with the absorption band of the active medium have enhanced the efficiency of solid state lasers considerably, at times almost touching 100%. However as the output power of the diode laser being instead low, solid-state laser output is as well low. To overcome this disadvantage, stacks of diodes are used to raise their total output, therefore producing very high power laser giving as good as the flash lamp pumped laser systems. The real benefit of a diode pumped solid-state laser that it is extremely compact, light weight and small in size having long life.

2) Liquid Lasers:

In this category of lasers, as the name points out, the active media are either the liquid solutions of organic dyes or specially prepared liquids doped by rare-earth ions that is, Nd3+. Though, the majority of liquid lasers uses a solution of an organic dye as active medium and therefore is as well termed as organic dye lasers. Solvents employed for the purpose are methanol, water, benzene, acetone and so on. The liquid lasers are optically pumped. The energy states involvement in the lasing transition are the various vibrational energy states of different electronic energy states of the dye molecule.

In contrary to solids, liquids don't crack or shatter and can be formed in sizes approximately unlimited. The other benefit of liquid lasers is due to their (that of organic dyes) broad absorption bands in the visible and near ultraviolet part of the electromagnetic spectrum. Thus, liquid lasers are an idyllic candidate for tunable laser, that is, the frequency and therefore energy of the output laser beam can be chosen with ease.

3) Gas lasers:

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In case of gas lasers, the active medium is in the gaseous state. As the laser media is a gas, it is kept in the plasma tube, having proper electrodes for electrical discharge to generate ionization, enclosed by dielectric mirrors. One might think that gas laser is a simple device, as there is no fundamental preparation needed for the lasing medium, as in the case of a solid state laser. However in practice, it is a complex device, as it requires optimization of the gas mixture, gas discharge parameters, mirror and container configuration and so on. The similar have to be correctly designed to make appropriate conditions for population inversion. Moreover, gas discharge generates heat and it has to be eliminated to avoid detrimental effect on the gas discharge and the optical components.

Gas lasers might be grouped as, atom lasers, molecular lasers, ion lasers and so on. In atom lasers, the lasing medium comprises atoms that are electrically neutral. He-Ne laser is an excellent illustration of this group. Molecular of the lasers have molecules as the lasing medium, as in the case of carbon-dioxide, carbon monoxide and nitrogen lasers. Significant ion lasers, like argon and krypton lasers have ionized gases as their active laser medium. Fascinatingly, helium-cadmium laser consists of metal ions as the active laser medium.

Application of lasers:

1) Medical Lasers:

The Medical lasers can be employed as a scalpel. As the laser can be controlled and can encompass such a small contact area it is ideal for fine cutting and depth control. Medical lasers can as well be employed to reattach retinas and can be employed in conjunction by fiber optics to place the laser beam where it requires being. Medical lasers can as well be employed to stitch up incisions after surgery, through fusing altogether skin.

2) Entertainment:

Laser shows are fairly popular and the special effects are astonishing. These make use of lasers which are in the visible spectrum all along by vibrating mirrors to paint images in the air.

You might observe the fog in the background that is what lets the laser light to reflect and you to view it. The other illustration of laser entertainment is the utilization of laser signs at trade shows. 

3) Computers and Music:

One popular utilization of lasers is the reading of CD.  CD's function by encompassing a reflective aluminum layer that consists of extremely small pits put in the aluminum. The pits and the lack of are translated into binary by the computer and then are employed for information.  The other use of lasers is in the use of fiber optics. As lasers travel extremely fast they make an ideal method to communicate. The laser is shot down a fiberglass tube to a receiver.  These wires can be extremely long by means of no loss of signal quality. As well modern multiplexing of the line lets two lasers of dissimilar frequencies share the similar line.

4) Metal working:

Lasers precise point and solid state construction make it perfect for industrial production. Lasers permit better cuts on metals and the welding of dissimilar metals without the utilization of a flux.  As well lasers can be mounted on the robotic arms and used in factors. This is much safer then oxygen and acetylene, or arc welding.

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