Types of Lasers 2.1 INTRODUCTION There are several ways in which we can classify the different types of lasers. It ean be done according to what material or element is used as the active medium. It can be done according to the operation of laser in a pulsed mode or in a continuous wave (CW) mexle. The classification may be done basing on other parameters such as gain of the liser medium, power delivered by the laser efficiency or applications. We prefer here to classify the lasers on the basis of the material used as active medium, Accordingly, lasers are broadly divided into four categories-solid lasers, gas lasers, liquid lasers and semiconductor lasers. The subject of laser systems is a vast topic and it is not possible to cover each and every system. We describe only some of the important and practical lasers, 2.2 CLASSIFICATION OF LASERS (on the basis of medium) ‘As mentioned above, lasers ean be classified in several ways as per convenience. Now a days, a large number of lasers are available and many new lasers are coming out regularly. Basically all these lasers can be classified into two categories : (i) high density gain medium laser and, (ii) low density gain medium laser, High density gain medium lasers are further sub-divided into solid and liquid lasers. Om the basis of medium, the lasers are classified into three catergories : Solid, liquid sand gas lasers. In brief they can be put as under Solid lasers + Solid state layer was the first known laser ‘They belong to the laser system involving high density gain media. Different examples of solid siate Laser are © Ruby © Nd- YAG laser © Nd. Glass laser Tunable solid state Laser Alexandrite ta cr © Titanium-Sapphire Laser © Colour-ventre laser © Fibre laser Among: all, Rubby laser is the most important and widely used solid state laser: ‘The active material used is C}* doped in Al,O, crystal, Neodymium is a typical example of an optically pumped rare-earth laser system, Neodymium ion (Nd) is doped in Yttrium allumium garet (YAG) with nearly 0.725% concentration by weight Liquid lasers : There ore certain unavoidable drow bucks in same solid state lasers, For 80Typos of Lasers 81 examples, crystals have optical strain. Hence, they erack down or shatter Moreover, they are prone to internal damages and susceptical to (crystal) defect and imperfections. Homogeneous liquid overcomes these problems, The active media of the liquid laser are formed by solutions of certain dyes dissolved in liquids such as water or alcohols, Dye lasers are tunable and have high gain as compared with solid state lasers. Dyes used are of the type © Polymethene dye © Xanthene dye . Coumarin dye © Scintillator dye. Gas lasers : Gas lasers belong to low density gain media, Depending upon the characteristics of the active medium, the gas lasers are further categorized into (/) atomic, (i) tonic and (iii) molecular lasers. The optical quality of gas lasers is much better as it does not suffer from optical inhomogenities, The gas lasers commonly used are Helium-Neon (He —Ne) laser, Argon ion laser, Krypton ton laser, Helium-Cadmium laser, Copper vapour laser. Gold vapour laser, Carbon- diOxide (CO,) laser, EXeimer laser, HP laser, Nitrogen laser Semiconductor lasers : Semiconductor laser is a special designed p-n junction dioe which, emits coherent radiation when itis forward biased, A small size devoice capable of giving output of wavelength in gigahertz andhas an ability to modulate the optical output. Twotypes of semiconductor lasers namely homojunction laser and heterojuetion lasers are observed. They may also be classified in terms of bond gap properties of the gain media as Direct band-gap medium laser and Indirect band-gap medium laser We describe some important and usually used lasers as under 2.3 SOLID STATE LASERS ‘The term solid state has different meanings in the field of electronics and lasers. A solid state laser is one in which the active centres are fixed in a crystal or glassy material. Solid state lasers are electncally non-conducting. They are also called doped insulator Jasers to avoid connotation of semiconductor, The basic principles that underlie the operation of solid state lasers are the same. A genenc solid state laser is illustrated in Fig. 2.1. Elliptic! cavity-focusses pump igh! onto laser rod LASER ROD REAR MIRROR LASER BEAM LAMP POWER SUPPLY. Fig. 2:1. A typical solid state Iasor ‘The active centres are dispersed in a dielectric crystal or a piece of glass, The crystal atoms do not participate directly in the lasing action but act as a host lattice to the active centres which82 ‘An Introduction to Laser — Theory and Applications are present in concentrations of around 1%, The erystal is usually shaped into a rod with reflecting mirrors placed at each end. Light from an external source such as a flashlamp excites the active centres in the rod. The flashlamp is usually linear in shape. The linear lamp and laser rod are placed close to each other in a reflective cylinder, which focusses pump light onto the rod. The mirrors on the sides of laser rod form a resonant cavity and provide the necessary feedback to generate laser beam It is required that the host material should be transporent to the pump light and should not absorb light at the laser wavelength. Most of the excitation energy ends up as heat rather than light. sess heat damages the lasererystal. Therefore, the erystal should have good thermal conductivity Normally, cooling with forced air or circulating water needs to be provided. The active centres are ions of metallic elements: chromium (Cr), neodymium (Nd), erbium. (Er), holonium (Ho), cerium (Ce), cobalt (Co) and titanium (Ti). Chromium is the active centre in muby and alexandrite lasers while neodymium is the active centre in common type of solid state lasers. ‘These lasers are rugged, simple to maintain, and capable of generating high peak powers. 2.3.1 Ruby Laser GENERAL DESCRIPTION Historically, the ruby laser was the first laser. The ruby laser rod consists of a synthetic ruby crystal, A1,O,, doped with chromium ions st a concentration of about (0.05% by weight At this concentration there are about 1.6 x 10° Cr ions per cubie meter. ‘The chromium ions constitute the active centers as they have a set of three energy levels suitable for realizing lasing action whereas the aluminium and oxygen atoms are inert Extomal mirror Coolant Flash (partially Extremal Ruby lamp transmitting) mirror 0 ER! ( } ‘Output beam, Flash lamp INNS OS SH laser boy beam Le o ih Power supply @ ) Fig. 2.2.A typical ruby laser (a) with internal mitros (b) with external mirtors STRUCTURE ‘The schematic of a ruby laser is shown in Fig, 2.2. Ruby rod is taken inthe form of a cylindrical rod of 4m, in length and 0.5 em. in diameter. Its ends are grounded and polished such that the end faces are exactly parallel and also perpendicular to the axis of the rod. One face is silvered to achieve 100% reflection while the other is silvered to give 10% transmission. The silvered faces form the Fabry-Perot resonator. The rod is surrounded by a helical photographic flash lamp filled with xenon. The lamp produces flashes of white light whenever activated by the power supply. The system is cooled with the help of a coolant circulating around the ruby rod.Types of Lasers 83 In practical lasers, flash lamps of helical design are External no longer used. The most common types are linear lamps. miror SS The laser rod is generally placed in an elliptical cavity with a linear flash lamp mounted at the focus of internally reflecting cavity, as shown in Fig. 2.3, to ensure an efficient Laset transfer of pumping energy to the ruby crystal. The mirrors fod are arranged outside WORKING Piet " Ruby fia GSS Teves YS SES] GS ey eh DS -sxtemal levels of C1 ions in the crystal lattice are shown in Fig. Focal mirror 2.4, There are two wide energy bands E, and E,', and a Fig. 2.9, Elipticoyinder housing for pair of closely spaced levels at E,. When the ruby rod is conventrating light onto the laser rod. itvadiated with an intense burst of white light from the xenon lamp, the ground state Cr*> ions absorb light in two pump bands one centered near 5500 A and the other at about 4000 A, and are excited to the broad upper bands. The energy levels in these bands have a very small lifetime (<10-® sec). Hence, the excited Cr ions rapidly lose some of their energy to the crystal lattice and undergo non-radiative transitions to the pair of adjacent levels denoted as from E, to E, is radiative and under normal population condition produces spontaneous, incoherent, red fluorescence typical of ruby with a peak near 6943 A These levels are metastable states having a lifetime of 3 10° see. The transition cinoxnun Genre NON-RADIATIVE TRANSITIONS 2 CLOSELY SPACED METASTABLE LIFETIME (3ms LIFETIME) PUMP 550nm 694.3 nm LASER TRANSITION GROUND STATE &, Fig. 2.4, Energy levels of chromium atoms in ruby laser. Under very intense excitation where pumping energy increases above a critical threshold, population inversion occurs in E, with respect to the ground level E. Then one of the spontaneously ‘emitted fluorescent photons travelling parallel to the axis of the ruby rod would initiate stimulated emissions. The photons make many passes through the medium building up the stimulated emissions ina large way and the system lases. The photons travelling in any other direction would be lost after a few reflections. The laser beam ensuing from the ruby rod is red in colour and coresponds to @ wavelength of 6943 A. It is to be remembered that the green and blue components of light play84 ‘An Introduction to Laser — Theory and Applications the role of pumping agent and that these components Output ae not amplified by the active medium. It is a illumination spontancous fluorescent photon (red) emitted by one of the Cr°* ions that acis as input and gets amplified. The xenon flash lasts for a few milliseconds However, the laser does not operate throughout this period, Its output occurs in the form of irregular pulses of microsecond duration as shown in Fig 25, It is because the stimulated transitions occur faster than the rate at which population inversion ‘maintained in the crystal. Once stimulated transitions commence, the metastable state F', gets depopulated very rapidly and at the cnd of cach small pulse, the oO 05 09 population at E, has fallen below the threshold value Fig. 25. Ruby laser light pulsos required for sustained em a result the lasing ceases and laser becomes inactive. The next pulse appears after the population inversion 1s once again restored. The process repeats The ruby laser has high energy-storage capability because of long upper laser level lifetime of 3 ms. Therefore, pulse energies of upto 100 J are possible. The ruby laser is relatively inefficient having a typical efficiency of 0.1 to 1%. ‘The ruby laser is used in a variety of applications, the most important one being in holography. Input illumination} 2.3.2 Nd : YAG Laser GENERAL DESCRIPTION ‘The neodymium ion is a rare earth metallic ion. It produces emission at about Ijim, when doped into a solid-state host crystal. The most commonly used host materials are yttrium aluminium garnet (YAG) crystal and glass When doped in YAG, Nd* ions take the place of yttrium ions. Doping concentrations are typically of the order of 0.725% by weight which corresponds to about 1.4 x 10° atoms per cubic meter. The neodymium lasers are four-level systems and therefore, require lower pump energies than the ruby laser. My oe Output Ellipsoidal retlector | | Flash tube MM & Trigger pulse ay HHH — L Capacitor bank tL Power supply Ballast resistor Fig. ‘Atypical arrangement for the Nd : YAG laser STRUCTURE Fig. 2.6 illustrates the typical design of Nd:YAG laser. The laser rods are typically of 10 emTypes of Lasers 85 in length and a diameter of 12 mm. The YAG rod and a linear flash lamp are housed in a reflector cavity of elliptical cross-section. The light issuing from the lamp is closely coupled to the laser rod as they are located at the foci of the ellipse. The ends of the YAG rod are ground flat and parallel ‘The optical cavity may be formed by silvering the ends of the rod. In practice, external mirrors are used as shown in Fig. 2.6. One mirror is made 100% reflecting while the output mirror is about 90% reflecting. The system is cooled by circulating air. High power lasers are cooled by water circulation. ABSORPTION BANDS ENERGY RELEASED TOCRYSTAL E UPPER LASER LEVEL 3 9g 4.06 um g LASER LINE, z & E LOWER LASER LEVEL GROUND 9 STATE E, ENERGY RELEASED TO CRYSTAL Fig. 2.7. Energy levels of neodymium ions in a crystal WORKING A simplified energy level diagram for Nd:YAG is shown in Fig, 2.7. It is essentially a four- level system with the terminal laser level sufficiently far removed from the ground state, The two primary pump bands are in the 7000 to 8500 A range. Pumping is achieved by using an intense flash of white light from a xenon flash lamp. It exeites the Nd** ions from the ground state to the multiple energy states at E,. The excited Nd* ions quickly decay to the metastable upper laser level E,, releasing their excess energy to the crystal Lattice. As the lower laser level E, is located at 0.25 eV above the ground state F,, it cannot be populated by Na** ions through thermal transitions from the ground level. Thus, it is sparsely populated at normal operating temperature. The population inversion can be readily achieved between E, and E; levels. In the E, state, Nd ions are stimulated to emit on the main 1.064 1m laser transition, and drop to the lower laser level E, From the level E,, Nd ions quickly drop to the ground state again by transferring energy to the crystal The energy level scheme of Nd** ions in YAG erystal as shown in Fig 2.6 is an oversimplified diagram, In reality, the upper and lower laser levels are also split, Because of the splitting several other laser transitions occur in the near infrared, but all of them are weaker than the main 1.064 jum ansition, The only other strong transition occurs at 1.318 jum and produces about 20% as much power as the 1,064 jum transition. It is useful in fibre-optic systems. ‘The laser output is in the form of pulses occurring at a very high rate. The overall efficieney is within 0.1 to 1% range86 ‘An Introduction to Laser — Theory and Applications A xenon flash tube is used for pulsed output and tungsten-halide incandescent lamps are used for ew operation. Thereshold pumping levels of about 3! at 3% conversion efficiency can be achieved. Continuous output powers of over | kw are obtainable. The lasers can be pumped by a diode laser also. The GaAs laser diode has the ideal pumping wavelength for the Nd** ion arround 0.8 um. The primary wavelength of neodymium laser lies in the infrared. It is possible to double the frequency by second harmonic generation. Twice the frequency corresponds to half the wavelength ‘Therefore, through adding a harmonic generator to the system, the neodymium laser can be made to produce green light (in the visible region) at a wevelength of 3320 A 2.3.3 Nd : Glass Lasers Glass is an excellent host material for neodymium. The chief attraction of glass is the well developed technology for making large size laser glass with good optical quality. While Nd:YAG laser can be operated in a ew mode, Nd-glass laser is operated only in a pulsed mode, because of the low thermal conductivity of glass. Nd:glass lasers give very high output of energy per unit volume of the material. The Nd:glass laser produces light at almost the same frequeney as Nd:YAG laser, i.e, at around 1.06 pum. Na: glass laser materials are primarily used as amplifiers for very large pulsed lasers. The NOVA laser developed for nuclear fusion by Lawrence Livermore National Laboratories, USA employed a large number of Nd: glass amplifiers to produce very large powers of the order of 10° Ww. Neodymium lasers are widely used in material processing and resistor trimming. It is used in medical applications in association with optical fiber delivery systems to deliver energy to the appropriate location in the body. They are used in nuclear fusion and in military applications such as range finding 2.3.4 Tunable Solid State Lasers There is an important group of solid state lasers which produce output over a range of wavelengths. These lasers are tunable over a range and are therefore very popular. The tunability arises because of the existence of a cluster of vibrationally excited terminal levels near the ground state, Therefore, these lasers are also known as vibronic lasers. In liquid lasers, we find that dye lasers are tunable. However, the dye lasers suffer from dye degradation and other limitations, In contrast solid state tunable lasers have unlimited shelf life and operational life. Therefore, they are useful in applications like remote sensing and in space craft 2.3.4.1 Alexandrite Laser GENERAL DESCRIPTION ‘The alexandrite laser is a three-level solid state laser similar to a ruby laser. While a ruby laser is a fixed wavelength laser, the alexandrite laser is a tunable laser and lases in the wavelength range ‘of 7000 A to 8200 A. The mineral alexandrite (BeAI,0,) is doped with chromium in concentrations of about 0.1% corresponding to a chromium ion density of 3.5 x 10° per cubic meter. Alexandrite has pump bands at 3800 A and 6300 A. Therefore, pumping can be done using a flash lamp.Types of Lasers 87 upper wieronc LAZZx. sroen BAND VIBRONIC Za g g 3 & = = Fe 5 © g 5 s a 2 g le 700830 am 2 TUNABLE LASER Lowen BEAM VIBRONI GRouND BAND QZ VIONONIC STATE ft ALEXANDRITE ants CAN DROP TO ry CAN DROP TO (CF IN BeALO) VIBRONIC BAND (a) % Fig. 2.8, Energy level scheme (a) for a typical vibronic laser (b) Cr ions in Alexandrite crystal STRUCTURE The alexandrite laser rod is typically of 0.10 m long and 6 mm dia and is pumped using a flash lamp. Linear flash lamps and a double elliptical pumping cavity are used WORKING ‘The energy levels of Cr* in alexandrite are shown in Fig. 2.8(6). In this case both the upper and lower laser levels are a band of energy levels. The bands consist of vibrational sublevels of a single clectronic energy level which arise due to vibrations of the crystal lattice. When Crions drop from the upper laser level to the lower band of energy levels, they undergo a compound transition where both electronic and vibrational energy change. This compound transition is called a vibrational- electronic transition or in short a vibronic transition. The interesting feature of the vibronic transition is that it can occur over a range of energies, because the excited ion can drop from the upper level to any level within the lower vibronic band. It implies that a vibronic laser can be made to operate over a range of wavelengths within its gain bandwidth. Therefore vibronic laser is tunable to any desired wavelength within its emission spectrum. This characteristic of vibronic laser makes it possible to produce wavelengths that are not obtained from other solid-state lasers, An alexandrite laser emits light in the range of 7000 A to 8300 A. Alexandrite rod can store more energy than an equal sized Nd:YAG rod. Alexandrite laser can operate in a pulsed or ew mode The alexandrite laser is widely used in cancer therapy. Other applications are in pollution detection and kidney stones removal. 2.3.4.2 Titanium-Sapphire Laser The titanium-sapphire laser uses a titanium doped sapphire crystal designated as Ti:AL,O, where Ti* ions replace some of the Al ions. Doping concentrations are about 0.1% by weight. The titanium-sapphire laser is the most widely used tunable solid-state laser which is tunable between 6600 A in the red and 11,800 A in the near infrared. These lasers can be operated in pulsed or ew mode. The lifetime of the upper laser level is too short, only 3.8 1s. Therefore, they cannot be pumped with flash lamp. They are pumped with argon laser for ew operation or with frequency-doubled Nd:YAG lasers for pulsed operation. These lasers are used in laser radar (lidar), range finders, and remote sensing. 2.3.4.3 Colour-Center Lasers Colour center lasers are broadly tunable solid state lasers. They operate in the wavelength,88 ‘An Introduction to Laser — Theory and Applications, range 8000 A to 40,000 A (4 um). The tuning is achieved by using several different colour center erystals in sequence. Atypical colour center laser consists of an alkali halide crystal that contains point defects known as F centers. These centers are known as colour centers and are produced when the crystal is irradiated with X-rays. The colour centers remain within the crystal for duration ranging from a few Nonradiative transitions Lasing ransition 0.5-1.3 ym days to many years. The colour centers are actually defects an the crystalline lattice and they absorb and emit light as the atoms at the defect site change position, Colour centerlasers _| must be pumped with other lasers and maintained at very _ Ground state Jow temperatures. The need for a pump laser and cryogenic F192. Typical energy level diagram of cooling limits the use of colour center lasers in practical applications. Nonradiative transition 2.3.5 Fiber Lasers Erbium in a glass host forms a three-level laser with a wavelength range centered around 1.55 ym. Erbium lasers and amplifiers assume importance in 9,98 jum Nonradiative decay fibre-optic communications because optical signals of 1.55 tum travel longer distances through optical fibres with Ieast loss, In Jong distance fibre-optic communications, amplifiers are required afler each specific distance to increase the strength of the signal. They are known as repeaters. Earlier, optical signals are converted into electrical signals and are amplified 1m each repeater station. It can be alternately done bY Fig. 9.40, Energy level diagram of an erbium passing the weak signal througha length of the erbium- doped thre lasor doped optical fibre. If the erbium atoms are pumped by a laser diode pump laser at a different wavelength, the weak signal can stimulate the atoms to emit light at the signal wavelength, 1.55 jum, causing an increase in the signal strength. These erbium- amplifiers are highly useful in under-sea communication and long haul communication links. 152-156 um 2.4 GAS LASERS Gas lasers are the most widely used lasers and the most varied. They range from the low power helium-neon (He-Ne) laser used in college laboratoriesto very high power carbon dioxide laser used. in industrial applications. These lasers operate with rarefied gases as their active media and excited by an electric discharge. There are three different types of gas laser: ion lasers, neutral stom lasers and molecular lasers. In gases, unlike in crystals, the energy levels of atoms involved in the lasing, process are well defined and narrow. Broad pump bands do not exist and the pump levels are also narrow. In order to excite atoms, sources with sharp Wavelength are required. Finding an appropriate ‘optical source for pumping poses a problem. Therefore, optical pumping is not used in gases The most common method of exciting gas laser medium is by passing an electric discharge through the gas. Electrons in the discharge transfer energy to atoms in the laser gas by collisions. Gas lasers, in general can be classified as (i) Atomic, (ti) Ionic and (Wi) Molecular lasers. (@ Atomic lasers: He-Ne (Helium-Neon) lasers are of this type. In atomic lasers the active medium is noble gas viz. Helium, neon, argon, krypion and considered in the neutral state. TheTypes of Lasers 89 atomic lasers are characterised by mixing active gas with other gases which increases the excitation efficiency of the system. Gi) lonic lasers: In ionic laser ionized gas is used as lasing medium. For this purpose, ionized noble gases like 4; (Argon ion) and K; (Krypton ion) are prefered. The most commanly used. ionic lasers are A (Argon) and K;> (Krypton) lasers which mostly operate in CW mode. They are ‘operated in Ulira-Violet (UV) Spectral region. Large amount of heat is generated and hence cooling arrangement is essentially needed. These lasers produce a wide range of colours and hence are popularly used in art and entertainment to give visual effe (iii) Molecular lasers : A the gas lasers are based on electronioc levels however in molecular lasers other energy levels are also taken into account. The atoms comprising the molecular vibrate about their mean position giving rise to vibrational energy levels and also can rotate asa whole. The gives rise to rotational energy levels. The first gas laser was He-Ne laser which was demonstrated in 1961 at Bell Telephone Laboratories, U.S.A. by Ali Javan, William R. Bennett, Jr. and Donald R, Hemriott. The generic gas laser is shown in Fig. 2.11 LASER LASER GAS. BEAM REAR. MIRROR ourpur ELECTRIC DISCHARGE MIRROR ELECTROD! POWER SUPPLY Fig. 2.11. Schematic arrangement ofa gas lasor. ‘The laser gas is contained in a tube with cavity mirrors attached at each end, one totally reflecting and one transmitting light to form the output beam. ‘The laser is excited by an electric discharge, First, a high de voltage ionizes the gas so that it will conduct electricity. The electrons accelerated through the electric ficld transfer their Kinetic energy to the gas atoms by inelastic collisions. In practice, for optimum operation, the laser medium contains a mixture of two gases, say A and B, at low pressure. Atoms of kind 4 are initially excited by electron impact and they in tur transfer their energy to atoms of kind B which are the actual active centers, The cavity mirrors can be either inside the gas container or outside. If they are inside, than the output light is generally unpolarized, For the outside ease, to minimize reflection loss, discharge tube edges are cut at the Brewster angle. The Brewster angle is given by where 11, and 1, are the refractive indices of glass and the gas mixture respectively. It is known that light incident at Brewster angle is polarized parallel to the plane of incidence and has a transmission coefficient of |, The passage of light through a glass container at Brewster angle does not involve any90 ‘An Introduction to Laser — Theory and Applications transmission losses and thus parallel polarization component ha for the outside mirror arrangement, light higher effective gain. Therefore, s plane polarized in general Gas lasers vary widely in their characteristics. Their output wavelengths range from near UV to far IR . Some of them operate in pulsed mode and others in CW mode. The outpat power varies from less than a milliwatt (mW) to over 10 kW. Important feature of gas lasers. The gas lasers having following important features 1. The line width of the spectral transition is relatively smaller as compared to solid state lasers, 2. The laser emission is narrower. 3. No optical inhomogeneties. Hence, yields the good quality laser, 4. The conerence properties are also superior 5. Themonochromaticity is very high. 6. The quality factor (Q) of the modes are high we shall describe them in detail 2.4.1 Helium-Neon Laser GENERAL DESCRIPTION ‘The Helium-Neon laser was one of the first lasers ever developed and is one of the most widely used lasers. The helium-neon laser is an atomic laser which employs a four-level pumping scheme. The active medium is a mixture of 10 parts of helium to 1 part of neon. Neon atoms are the active centers and have the energy levels suitable for laser transitions while helium atoms help efficient excitation of neon atoms. The most common wavelength is the 6328 A. These lasers produce powers in the range 0.5 to 50 mW in the red. They operate in ew mode and have long operating life times of the order of 30,000 hrs. Helium-neon mixture Laser Beam Glass Output window firror Fig, 2.12, Schomatic of a He-Ne laser with external mirrors. STRUCTURE The construction of a typical He-Ne laser is shown in F' tube of about typically 30 em long and 1.5 2.12. It con n diameter. The discharge tubes in general may have lengths of 10 ems to 50 ems and vary in diameter from 1.5 em to 5 ems. The tube is filled with a mixture of helium and neon gases in the ratio 10:1. Electrodes are provided in the tube to produce a discharge in the gas. They are connected to a high voltage power supply. The tube is hermetically sealed with glass windows orientated at the Brewster angle to the axis of the tube. The cavity mirrors are arranged externally. WORKING The energy level schemes of helium and neon are shown in Fig, 2.13Types of Lasers 91 Energy Transtor Through Atomic Homan sensors Neon Pt, extend '—z & Es F, Hoan gy S Ey, 3 2 & Excitation by colision with electons Deexctation by calision with walls * Fr a 5 Fig. 2.13. Enorgy levels of helium and neon atoms and transitions between the levels. When the power is switched on, a high voltage of about 10 kV is applied across the gas It is sufficient to ionize the gas. The electrons and ons produced in the process of discharge are accelerated towards the anode and cathode respectively. as shown in Fig. 2.14. Electric Field catnode | ¢— SOE pode Electron Q——> —@ bon Helium-Neon Mixture Fig. 2.14. Migration of electrons and positive ions ina discharge tube. Since the electrons have a smaller mass, they sequire a higher velocity. ‘They transfer their kinetic energy to helium atoms through inelastic collisions. Helium atoms are much more readily excited by electron impact because they are fairly light. Thus, the initial excitation affects only the helium atoms, with the result that these atoms are excited to the levels F, and F, which lie at 19.81 eV and 20.16 eV respectively above the ground sate, These two levels are metastable levels and the excited helium atoms cannot return to the ground state through spontaneous emission. Though a radiative transition is not possible, the excited helium atoms can return to the normal state by transferring their energy to neon atoms through collision. Such an energy transfer can take place when the two colliding atoms have identical energy slates. It is called resonant transfer of energy. The neon energy levels, namely E,, level at 20.66 eV and E, level at 18.7 €V nearly coincide with the F, and F, levels respectively of helium atom. Therefore, the resonant transfer of energy can ‘occur readily. The additional 0.05 eV energy required is provided by the kinetic energy of helium atoms. When a helium atom in the metastable state collides with a neon atom in the ground state, theneon atom is excited fo E, or E, level and the helium atom drops back to the ground state. This is the pumping mechanism in He-Ne laser. The neon atoms which are much heavier could not be pumped up efficiently without the helium atoms. The role of helium atoms is thus to excite neon atoms and cause population inversion, The probability of energy transfer from helium atoms to neon atoms is more as there are 10 atoms of helium per | neon atom in the gas mixture, For this reason, the probability of reverse transfer of energy from neon to helium atom is extremely small. The E,92 An Introduction to Laser — Theory and Applications and E, levels of neon atoms also are metastable states. Therefore, as the collisions go on, neon aloms accumulate in the E, and E, states. At ordinary temperatures, the F., and E, levels are sparsely populated and as such population inversion takes place between E,, and E,., E, levels and between E , and E, levels. Lasing takes place and light is produced corresponding to the transitions, E,—> E, E, E,and E, > E, (@ E,>E, transition > This transition generates a laser beam of red colour at 6328 A (i) E,> E, transition : This produces IR beam at a wavelength of 11500 A (1.15 uum). Gii) FB, transition * It generates light in for IR region at 33900 A (3.39 jum) ‘The energy level diagram Fig 2.13 is over simplified. In reality, the neon energy levels E,, Ey, E,, E, and E, are not single but are a group of lines. Consequently, several laser transitions, as many as 150, are possible, However, the above three are the main laser transitions. The first He-Ne laser was operated at 1.15 jim but soon afterwards the 6328 A red line was discavered which has hecome the standard He-Ne laser wavelength. The neon atoms in the teminal laser level, E,, decay rapidly toE, level which is however, not the ground state. (The transition from E, to E, level gives rise to the familiar orange colour of neon lights.) - ‘The E, level is again a metastable state. The neon atoms tend to accumulate at this level if they are not somehow removed from the level. The E, > E, transition can be induced by collisions with the walls of the discharge tube. To enhance the probability of atomie collisions with the walls, the discharge tube is made as narrow as possible. Once the atoms reach the ground state, they will be again available for pumping. E, level is more likely to be populated by the electric discharge itself. It may be recalled here that electric discharge is used to produce neon lighting. An increase in population at E, leads to a decrease in population inversion and lasing ceases. Therefore, the current in the discharge tube should be maintained at a low level. For this reason, high powers cannot be ‘obtained from a He-Ne laser. In the absence of any precautions, the He-Ne laser is likely to oscillate at the three main wavelengths. It is desirable that the laser oscillates at a single wavelength only. In most of the applications He-Ne laser is used as a source of red light. In order to prevent the laser from oseillating at the other wavelengths, the resonator mirrors are coated with multi-layer dielectric coatings which are highly reflective at 6328 A and absorptive at the other two wavelengths. The laser oscillates at the wavelength which is reflected more by cavity mirrors. Without the Brewster windows, the light output is unpolarized. The Brewster windows cause the laser output to be linearly polarized. Although, 6328 A is the standard wavelength of He-Ne laser, other visible wavelengths 5430 A (green), 5940 A (yellow-orange), 6120 A (red-orange) can also be produced. ‘The He-Ne laser operates in CW mode and the over all gain is very low and is typically about 0.01% to 0.1%. However, the laser is simple, practical and less expensive. The laser beam is highly collimated, coherent and monochromatic. He-Ne lasers are widely used in laboratories asa monochromatic source, in interferometry, laser printing, bar code reading ete, These lasers provide a very stable single transverse mode reference beam to identify optical properties of materials such as surface smoothness. Most supermarket and. other types of stores now use these lasers as scanners to read the digitally encoded bar codes located ‘on products, ‘They are also used as a reference beam in surveying. for alignment in pipe laying ete works, 2.4.2 Ion Gas Lasers Helium, neon, argon, xenon and krypton are noble gases and they have electronic states capable of laser transitions, However, except for neon, noble gases are difficult to pump and thus are not of practical interest. However, if these noble gases are first ionized by electron collisions then they are easy to pump.Types of Lasers: 93 2.4.2.1 Argon Laser GENERAL DESCRIPTION ‘The argon laser belongs to the group of ion lasers. It is a four-level laser which operates in visible region over a wavelength range from 3510 A to 5200 A. It is the most powerful CW laser operating in visible region, The importance of argon and krypton lasers is that they are extensively used in laser light shows. The argon ion laser can provide approximately 25 visible wavelengths ranging from 4089 A to 6861 A and more than 10 ultraviolet wavelengths ranging from 2750 A to 3638 A. In the visible region, CW powers of about 100 W are available. The discharge tubes have an operating life of 2000 to 3000 hrs. Retun Magnet path Brewster Browoter window window Water in = water Cathode Anode out Fig. 2.15. Schematic of atypical ion laser tube STRUCTURE In argon lasers, the active medium is argon gas and the active centres are ionized argon atoms A typical design of the discharge tube is shown in Fig 2.15. It consists of ¢ narrow water-cooled ceramic tube in which an are discharge takes place. The electrodes are arranged at the ends of the capillary tube. The anode and cathode spaces communicate through a return gas path which ensures free circulation of the gas. A magnet surrounds the discharge tube. Its function is to constrict the discharge area and increase the concentration of ions along the axis of the tube, This increases the ‘output power and efficiency WORKING Energy (eV) % RAR a Laser transitions 4s Radiative decay 19.15 ‘Argon ion ground state Argon atom ground state Fig. 2.16. Energy level scheme for an argon atom.94 An Introduction to Laser — Theory and Applications ‘As in He-Ne laser, an initial high voltage pulse ionizes the gas so that it conducts current Electrons in the current transfer energy direetly to argon atoms, ionizes them and raises the ions to 1 group of high energy levels, shown in Fig 2.16, which are about 35 eV above the ground state of neutral argon atom. Different processes populate the metastable upper laser level. Three possible processes are : (a) electron collisions with Art ions in the ground state, (6) collision with ions in ‘metastable states and (c) radiative transitions from higher states. The life time of upper laser level is 108s while that of the lower laser level is of the order of 10°°s. Therefore, the condition for population inversion is satisfied. Transitions can occur between many pairs of upper and lower lasing levels and therefore many laser wavelengths are emitted, Out of the large number of wavelengths emitted, the most important and the most intense lines are 4881 A (blue) and 5145 A (green). Argon ions quickly drop from the lower laser level to the ground state of the ion by emitting UV light at 740 A. The ground state ion either recaptures an electron and becomes a neutral atom or itis again excited to the upper laser levels. During operation the positive ions tend to collect at cathode where they are neutralized and diffuse slowly back into the discharge. However, it leads to a pressure gradient. A gas retum path is provided between anode and cathode to equalize the pressure. Without such return path, the discharge may eventually be extinguished The laser needs active cooling, Any desired wavelength can be selected through proper cavity optics For example, a small prism may be inserted into the optical cavity and the position of the end mirror is rotated such that it comes into a position normal to the path of the light of desired wavelength. Then, only the desired wavelength is sustained in to and fro reflections while other wavelengths are lost from the cavity after a few reflections. 2.4.2.2 Krypton Ion Laser The krypton ion laser resembles the argon ion laser in energy level arrangement and in operation, Krypton laser provides different laser wavelengths than the argon laser. The dominant output occurs at 4067 A, 4131 A, 309 A, 5682 A, 6471 A and 6764 A. Thus, these lasers offer a broader spectrum of wavelengths. Often a mixture of argon and krypton gases is used which provides strong blue and green light of argon as well as the strong red light of krypton. Such lasers are used for multicolour displays. 2.4.2.3 Helium-Cadmium Laser The helium-cadmium laser is perhaps the most widely used metal vapour laser. It produ continuous output in the UV region at 3250 A and 3536 A and in the visible region at 4416 A (blue). Like He —Ne lasers, the He ~ Cd lasers operate within glass discharge tubes having a bore of 1-2 mm, The cadmium metal is to be heated to about 250 °C and vapourized at the anode end in order to produce cadmium vapour needed for laser operation, Helium gradually leaks out and therefore a helium reservoir is to be included in the system to replace helium that is lost. He ~ Cd lasers need discharge voltages around 1500 V and currents in the range of 60- 100 mA. The lasers typically have lifetimes of 4,000 to 5,000 hrs. The electrons in the gas discharge excite helium atoms to upper levels. The excited helium atoms transfer energy to cadmium atoms through ionization process and raise them to upper laser levels. The transition from one of the upper metastable states produces UV lines and the transition from the other state produces blue line ‘TheHe-Cadlaserisused in many applicationswhich include photolithography, stereolithography, inspections of electronic circuit boards, CD-ROM mastering, fluorescence analysis ete.Types of Lasers 95 2.4.2.4 Copper-Vapour Laser ‘Copper ~ vapour laser is one of the important metal vapour lasers. It operates only in pulsed mode and produces pulsed energies of the order of ImJ leading to average powers of 10— 100W for pulse repetition rates of 10 — 100 kHz. Copper vapour laser produces light at $105 A (green) and 5782 A (yellow), The metallic vapour is contained within a cylindrical discharge tube filled with helium or neon as & buller gas, The optimum pressure for the laser is of the order of 1 torr of metal vapour which requires the metallic copper to be heated upto 1500 °C. However, the copper metal will last typically for 500 ~ 100 hrs of operation, after whieh it isto be reloaded When an electric discharge passes through copper vapour, copper atoms are excited to one of the two upper laser levels. If the vapour pressure is optimum, the atoms stay long enough at the upper laser level to produce stimulated emission, With the onset of stimulated emissions, the atoms quickly drop to lower laser levels. The lower laser levels are metastable levels and as atoms accumulate at these levels, the laser operation ceases. All this takes less than 100 ns. Depopulation of lower laser level takes place by collision of atoms with the walls of discharge tube. Again the laser gets ready for producing next pulse of light. ‘Copper vapour lasers have high gain and can operate without resonant mirrors. 2.4.2.5 Gold-Vapour Laser The gold vapour laser operates in a similar way to that of the upper vapour laser. It produces ‘orange beam at a wavelength of 6278 A Copper vapour lasers are used to pump tunable dye lasers, in high ~ speed flash photography and material processing. The gold vapour laser is used in photodynamic therapy for destroving the cancerous tissue, 2.4.3 Carbon Dioxide (CO,) Laser GENERAL DESCRIPTION The carbon diexide laser is one of the most powerfull and efficient lasers. It is a four-level ‘molecular leser and operates on a set of vibrational-rotational transitions, The laser output is in the middle infrared at 10.6 um and 9.4 um wavelength region. Both ew and pulsed output occurs. The laser produces CW powers of greater than 100 kW and pulsed energies of as much as 10 kJ. Energy levels of CO,molecule : Incase of isolated atoms, the electron energy levels are discrete and narrow. On the other hand, the energy spectrum of molecules is complex and includes many additional features. Each electron level is associated with a number of equispaced vibrational levels and each vibrational level in turn has a number of rotational levels ae at The CO,moleculeis basically alinearmoleculeconsisingor @)) @ = central carbon atom with two oxygen atoms linked one on either side. Iteanundergo threeindependent vibrational oscillations Fig. Snes 2.17) known as the vibrational modes. The modes are termed as ()} ‘ Ot stretching mode, the bending mode and the asymmetric stretching 4? LS mode. Each mode is quantized, so the molecule ean have 0, 1, Bending mode 2. or more units of vibrational energy in each mode. At any one time, 8 CO, molecule can vibrate in any linear combination of } these three fundamental modes. The energy statesof the molecule S45 is then represented by three quantum numbers (m n q ). These numbers represent the amount of energy a mode yy ciated With €4Ch Fig, 247° Vivatondl modes of For example, the number (020) indicates that the molecule Co, molesue.96 ‘An Introduction to Lasor — Thoory and Applications in this energy state is in the pure bending mode and has two units of energy. Each vibrational state is associated with rotational states corresponding to the rotation of the molecule about its centre of ‘mass. The separations between vibrational ~ rotational states are much smaller on the energy scale compared to the separations between electron energy levels STRUCTURE The schematic of a typical CO, laser is shown in Fig 2.18 It is basically a discharge tube having a bore of cross-section of | 5 em? and a length of about 26 em. The discharge tube i filled with a mixture of carbon dioxide, nitrogen and helium gases in | 4:5 proportions respectively. Water in Water out tT BHI} ww fais | ion Gas in oe Fig. 2.18. A schomatc of a typical CO, lasor Typically, ahigh de voltage causes an eleciric discharge to passthrough the tube. The discharge breaks down CO, molecules to form oxygen and carbon monoxide. Therefore, a small amount of water vapour is added to the gaseous mixture which regenerates CO}, WORKING Fig. 219 shows the lowest vibrational levels of the ground electron energy state of CO, molecules and an N, molecule also. In the CO, laser, nitrogen plays a similar role to that of helium: inthe He-Ne laser. Both nitrogen and CO, absorb energy from electrons inthe discharge. Energy trenster aa trough colons 001), Energy (eV) 8 Nivogen & Fig. 2.19. Energy levets of carbon dloxide and nitrogen melecules and transitions between the levels, ‘The lowest vibrational level of N, has nearly as much energy as the asymmetric siretehing mode of CO, molecule and so the exeited nitrogen molecules readily transfer energy to the CO, molecules in resonant collisions. As a result, CO, molecules are excited {0 (001) level (E,). The (100) CO, level (E,) has a lower energy and cannot be populated in this way. So population inversion is established between the (001) level, (E,) and the (100) level, (E,). Simultaneously, in ‘Caibon diondeTypes of Lasers: 97 the same manner population inversion is ereated between (001) level, E,, and (020) level E, The lasing transitions E, > E, and E, > E, produce IR radiation at wavelength 10.6 yim and 9.6 yim respectively, The E, and E, levels are metastable states. The CO, molecules at these levels fall to the lower bending level E,,, through inelastic collisions with helium atoms, The decay from the E, level to the ground state should be very fast. Otherwise there would be accumulation of molecules in this level which leads to decrease in population inversion. The helium atoms deplete the level F. through collisions, There is yet another function of helium, The F level is very close to the ground and tends to be populated through thermal excitations. It becomes therefore necessary to keep the temperature of CO, low. Helium has a high thermal conductivity and conducts heat away {o the walls and keeps CO, cold. Thus, while nitrogen helps fo increase the population of the upper lascr level, helium depopulates the lower laser level Some of the other structures used for CO, lasers are briefly described here, Wave-guide laser This structure is the most efficient way to produce a compact CW CO, laser. It consists of two transverse RF electrodes separated by insulating sections that form the bore region. The lateral dimensions of the bore are up to a few mm. An rf power supply is connected to the electrodes to provide a hf. field across the electrodes within the bore region. This type of laser produces ew powers of about 100 W, Gas-dynamic laser ‘Beam 0.05 kWim Fig. 2.20. Schematic of an axial low CO, laser. An eleetrie discharge is not the only way to produce population inversion in carbon dioxide Rapidly flowing hot, high pressure CO, is allowed to expand supersonically through an expansion nozzle into a low pressure region, The expansion causes the gas to supercool. In the process as all the molecules do not drop to lower levels, a population inversion condition is attained. Lasers of this design produce ew outputs greater than 100 kW. Discharge High region volta ouput wK 1. * ‘Anode bar Fig. 2.21. Illustration of discharge in @ TEA laser TEA. CO, Lasers This laser operates at gas pressure of around one atmosphere with a pulsed electric discharge passing through the gas. It works better if the electrie discharge is transverse to the laser axis. S lasers are called TEA lasers (Fig. 2.21),98 ‘An Introduction to Laser — Theory and Applications The CO, lasers are largely used! in the field of material processing. which includes cutting, drilling, material removal, welding, etching, melting, annealing, hardening etc. In medical applications, it is employed for cutting and cauterizing 2.4.4. Excimer Lasers Excimer lasers constitute an interesting and important class of molecular lasers. was first demonstrated in mid - 1970s and their importance lies in that they are the most powerful UV lasers, The active medium of these lasers consists of diatomic molecules that can be bound into a single system when they are in the excited state only, These diatomic molecules exist only as monomers in the ground state, as they repel one another at inter atomic ‘xeimer laser ty Pojentiat wat RH" (EXCITED) shows that potential energy curve (a) as a function of distance between the monomers. An excitation modifies the state of the atom so that @ there appears an attractive force with other atoms. The potential energy curve (6) in Fig. 222 shows the potential energy variation in the excited state eneneve=!> LASER TRANSITION GROYND STATE R+H of atoms. The curve exhibits a dip indicating that ATOMIC DISTANOEp at that interatomic distance, the atoms are bound (BETWEEN R AND) together because they have a minimum energy at Fig. 2.22. Internal energy ofa ere gashalide that separation. Such molecules which exist only in Molecule n excted and ground states. excited state are called excited-state dimers or excimers in short. For example. the atoms of inert gases can be bound into molecules by imparting energy to them. The excimer state is a metastable state, When the atoms are bound together in the excited state, they can occupy several vibrational levels (horizontal lines) in the potential well of curve (b) Fig 2.22 Principle. Excimers, as discussed above, are molecules which are bound in their exeited electronic states but are unstable in their ground states e.g. Argon fluoride, krypton chloride, krypton flueride, xcnon bromide ete. Since eximers are unstable in their ground state, they are ideal for forming active media for lasers sinee the lower level can be ground state in which the molecules is unstable and hence population inversion can be automatically maintained. Pumping is usually accomplished either by fast transverse discharges or through high voltage, high current electron Eximer lasers are tunable and emit in the ultraviolet (UV) region Method, The active medium can be an excited rare gas dimer such as Ar,*, Kr,*, Xe", .., arare gas oxide such as ArO*, KrO#, XeO¥, .., ora rare gas atom in combination with a halide atom such as ArF*, KrF*, XeC* Excimer lasers are excited by passing a short, intense electric discharge through a mixture of desired gases. Electrons in the discharge transfer energy to the laser gas causing formation of excited molecules, The molecules remain excited for about 10 ns and then drop to the ground state and dissociate. The upper laser level in excimer is its electronic exeited state and the lower level is the electronic ground state. Therefore, population inversion will oceur as Soon as some aloms are bound to form molecules in the excited state, Once a molecule drops to the lower laser level, it separates ‘out into atoms. Therefore, the lower level in exeimer lasers is always vacant As exci lasers have high gain, the cavity mirrors are not required. However, one fully reflective mirror is used in the rear and unsilvered transparent window is used as output mirrorTypes of Lasers 99 Excimer lasers are mainly used in refroctive surgery on the eye and in the manufacture of semiconductor devices. They are also used in material processing, photolithography and pumping of dye lasers. Table - 2.1 lists some excimer lasers and the wavelengths at which they operate Table 2.1. Excimer Lasers Type 2A) Art 1260 Arcl* 1750 ArF* 1930 krcl* 2220 Kirt 2490 xecl* 3080 Xer* 3500 2.4.5. Chemical Lasers Chemical lasers are pumped by the energy liberated in a chemical reaction, The most well known chemical lasers are the HF (hydrogen fiuoride) laser and DF (deuterium fluoride) laser. They operate on vibrational transitions and therefore the output wavelength is always in the infvared Chemical lasers have been developed primarily for military and space applications where pumping power, in the form of electrical energy, may not be available. Chemical lasers can produce powers of several megawatts, The HF laser emits light in the wavelength range of 2.6 to 3.3 um while DF laser operates in the wavelength range of 3.5 to 4.2 jum. HF Laser ‘The ingredients of HF laser are molecular hydrogen, and fluorine gas. Helium is added as a buffer gas. These molecular species can react at normal temperatures under extemal excitation such. as UV radiation. The reaction is highly exothermic and a large quantity of chemical energy is Gas inlets Expansion Brewster nozzle window Laser output Heater Output Me onnor injector window Heat ‘exchanger Gas exhaust Fig. 2.23. Schematicof a chemical laser released. This excess energy is the equivalent pumping energy. This chemical energy is enormous compared to other forms of pumping energy, Some of the actual reactions that take place between atomic and molecular hydrogen and fluorine are as follows F+H, —> HEF +H AH H+F, —> Ht +F AH 31.7 k cal/mol. 97.9 k cal/mol.100 ‘An Introduction to Laser — Theory and Applications Each step produces a vibrationally excited HP* molecule and such excited molecules participate in the lasing action. The end product of the reactions shown above contains atomic hydrogen and fluoride. Hence, once the reaction starts it continues until all the molecular H, and F, are consumed. The schematic of a chemical laser is shown in Fig. 2.23 . The main appli tion of chemical lasers is for high-power weapons on bate field or in space 2.5 TUNABLE DYE LASERS Dye lasers belong to the family of liquid lasers, The eetive material is a dye dissolved in a host medium of a liquid solvent, such as methyl alcohol. ethy! alcohol or ethylene glycol. Their sturctures are 4 How 4 weber nb bion an Pope i rt Hb ot (metrylacono}) ny alcanol) (etnyiend ayo) ‘The situation is very similar to solid state Lasers where Cr, Na or Ti jons are used ina solid host. The advantage of a liquid host is that the concentration of the active ions can be easily varied. ‘Typical dye concentration of 10“ to 107 molar solution is used which corresponds to. 10 and 10° dye molecules per cubic meter: There are oVer 200 laser des, the most important one being thodamine 6G. When used sequentially, they can produce unable laser output over wavelengths ranging from 0.32 to 1.2 ym, Dye lasers operate both in CW and pulsed modes. Pulsed dye lasers are pumped by a flash lamp or other lasers and ean produce upto 400 J output in a 10 yis-pulse. CW dye lasers are pumped by other CW Lasers such as argon ion laser and produce powers of about 2 W. The gain of the dye lasing medium is very high. Consequently, a smal] volume of dye solution is sufficient fo sustain the lasing action Singlet states Enevoy Transitions Fig, 2.28. Schematic representation ofthe energy levels of an organic dye molecule The energy band diagram of a typical dye laser is shown in Fig. 2.24. The organic dye molecule is known to have two sets of excited states ° singlet states denoted by S,. S, and S, and triplet states denoted by I, and , The transitions from singlet to triplet states are forbidden. The opticalTypes of Lasers 401 pumping excites the dye molecules from the lowest vibrionie level of the ground state S, to one of the upper vibronic level of the excited state S,. From there, they undergo a non-radiative transition to the lower vibronic level of S, state which acts as the upper laser level. The role of lower laser level is played by one of the upper vibronic levels of S,, The levels at &, are so closely spaced thet they Collimating mirror Output mirror Tunable —_ ‘output Tuning element Dye steam at Brewster angle Cavity end mirror Pump Pump laser beam ‘mirror Fig. 2.25. Schomatic diagram of a laminav-flow dye laoor. form a continuum. It may be seen from Fig. 2.24 that the laser transition can be to various levels within a range defined by the vibrationally excited sublevels of the ground state. Consequently, the layer operates over a broad range of Wavelengths, Thus dye lasers differ from other lasers thal emit light at fixed wavelengths. The schematic of a laminar-flow dye laser is shown in Fig. 2 2.6 COMPARISON BETWEEN SOLID, LIQUID AND GAS LASERS No. Solid Lasers Liquid Lasers Gas Lasers 1. [The active medium is @[ The active medium is a liquid | The active medium is a gas at soild ora dye low pressure 2. [Either crystalline or [Inthe form of dye dissolved in [Either atomic, ionic or amorphous organic or inorganic solvent__| molecular 3. [Pumping is done usually [Liquid laser is excited [It is excited by electrical by optical method optically usually by another | discharge method, laser source 4. [Emits » narrow range of [Emits a broad-range of [Emits a narrow range of wavelengths wavelengths wavelengths 3, [Due to imperfections Output is optically | Output is highly and defects in crystals, | homogeneous. homogeneous. inhomogeneties in the output, 6, [Example :Ruby, Nd-YAG. [Pulse dye laser, Tunable [He-Ne, Argon ion, Krypton Nd-Glass Fibre laser ete. _| pulsed and CW dye laserete._| ion, CO, laser ete. 2.7 SEMICONDUCTOR LASERS A semiconductor laser is a specially febricated pn-junction device that emits coherent light, When it is forward biased. The first semiconductor laser was made in 1962 by R. N. Hall and his coworkers of General Electric R & D Laboratories, U.S.A. It is made from gallium arsenide (GaAs) which operated at low temperature and emitted light in the near IR. Semiconductor lasers \working at room temperature and in continuous wave mode are produced by 1970, Now pn-junction102 ‘An Introduction to Laser — Theory and Applications (diode) lasers are made to emit light almost any where in the spectrum from the UVto the IR. Diode lasers are remarkably small in size (0.1 mm long). They have high efficiency of the order of 40%. Modulating the biasing current easily modulates the diode laser output. They operate at low powers Inspite of their small size and low power requirement, they produce power outputs equivalent to that of He-Ne lasers. However, the laser output is usually in the form of a wide beam, 5 to 15° and as a consequence exhibits less monochromaticity compared (o other lasers. The advantage of a semiconductor laser is that it is a portable and easily controlled source of coherent radiation. Because of rapid advances in semiconductor technology, now diode lasers are mass produced for use in optical fiber communications, in CD audio players, CD-ROM drives, optical reading, high speed laser printing ete wide variety of applications 2.7.1. Pure Semiconductor A semiconductor is a material with electrical properties intermediate to those of a conductor and an insulator. In semiconductors, valence up in covalent bonds. At ordinary temperatu corresponding electrons become free to travel within the material (Fig, 2.26). The situation is described in the following altemate way. The allowed energy values of the valence electrons in semiconductors occur within two well defined energy bands, separated by a forbidden energy gap. as shown in Fig. 2.26(b). A pure semiconductor crystal has exactly enough electrons to fill all the states in the lower band, namely valence band. However, a few electrons gain enough energy E, to rons are tightly bound to the atoms and are locked some of the covalent bonds are broken and the jump into the upper band, namely conduction band. In case of silicon and germanium, the number of electrons that can jump into the conduction band at room temperature is small. The electron jumping to the conduction band leaves behind a quantum vaeancy in the valence band. The vacancy is called a hole and is assigned a positive charge along with the other attributes of a particle. In a pure semiconductor the number of electrons in the conduetion band and holes in the valence band are equal. When a conduction electron falls into the valence band, it revombines with a hole there ‘The excess energy E, is given out in the form of heat or light. If the recombination energy is released in the form of heat as in silicon and germanium crystals, the materials are of no use for laser action. Broken Covalent Distance @ (6) Fig. 2.26. Sticon crystal at temperatures above OK. (a) Thermal vibrations of atoms lead to breaking of covalent bonds. Consequrily, a free electron and a vacancy are simultaneously produced. (b) energy band represortation. An energy E, = (E, — E,) causes tho transiton of slectrena from valence band to conduction band leaving behind vacancies.Types of Lasers 103 Now, ifa pure semiconductor material is illuminated by photons of energy /w= E,,a photon ean be absorbed by an clectron in the valence band. The excited electron will jump into the conduction band. If, on the other hand, the photon hits an electron in the conduction band, it can induce electron to make a downward transition to the valence hand where the electron recombines with a hole and a quantum of radiation with energy equal to E, is released At ordinary temperatures, the number of electrons at the bottom of the conduction band is very smell compared to the number of electrons at the top of the valence band. Therefore, the process of light absorption dominates over the process of stimulated emission, In order to make stimulated emissions to dominate over absorption of light in a semiconductor, the concentration of electrons at the bottom of the conduction band should be made higher than that at the top of the valence band. To create such a situation, pure semiconductors are to be doped with certain impurity atoms, 2.7.2. Impurity Semiconductors ‘The characteristics of pure semiconductors can be drastically altered in very important ways by the introduction of small amounts of impurities. Pentavalent or trivalent atoms are used as dopants, which easily substitute themselves in place of some of the host atoms in the semiconductor crystal. Consider the silicon crystal. Each silicon atom has four valence electrons and therefore each atom has four atoms bonded to it. If the erystal is doped with phosphorous, the phosphorous atoms will fit into the erystal structure, but they have five valence el of which four participate in bonding to neighbour atoms. The fifth electron is superfluous to the structure and therefore is loosely bound to the phosphorous atom. It is readily excited into the conduction band by the thermal energy. Thus, pratically every phsophorous atom introduced into the silicon lattice contributes a conduction electron without creating a positive hole. Thus, the addition of the phosphorous greatly increases the number of condution electrons, which become the majority carriers in the material. In this case phosphorous is called a donor impurity and it makes the silicon crystal an n-type semiconductor (Fig, 2.27) @ 6) Fig. 2.27. Schematic bond pictures for (a) n-type Si with donor (arsenic) and (b) p-type Si with acceptor (boron), Boron, on the otherhand, has three valence electrons. If it is used as a dopant in silicon crystal, it can supply only three of the four electrons necessary to fit into the silicon lattice. However, thermal excitation is sufficient to cause silicon valence electrons to eomplete the bonding by attaching themselves to boron atoms. It leaves holes in the valence band. Thus boron is an acceptor impurity ‘which causes as many positive holes as there are boron atoms. The acceptor impurity makes silicon a p-type semiconductor. Ifthe concentration of impurities in the semiconductor material is very high, the semiconductor is said to be heavily doped104 An Introduction to Laser — Theory and Applications 2.7.3. Population Inversion Population inversion is required for producing stimulated emission and consequent amplification of light The way in which population inversion is achieved in semiconductors is very different from the way it is established in other types of lasers. A semiconductor is not a two-level atomic system, but consists of electrons and holes distributed in the respective energy bands. Therefore, laser action in semiconductors involves energy bands rather than discrete levels. Secondly, in many types of lesers, population inversion is obtained by exciting electrons in spatially isolated atoms. In semiconductors, the excited electrons are not associated with specific atoms but are injected into the conduction band from the extemal circuit. Therefore, the conduction band represents an excited state while the valence band play's the role of the ground state. Population inversion requires that there is a large concentration of electrons in the conduction band and a large concentration of holes in the valence band. A simple way of achieving population inversion is to use a semiconductor in the form of a pn-junction diode with heavily doped p and n types smiconduetors, 2.7.4. pn-Junction Let us first understand the formation of an ordinary pn junction, Fig. 2.28 shows the energy band diagrams of p-type and n-type semiconductors. The Fermi level £,,, of p-type material is nearer to the valence band and the Fermi level £,,, of n-type material is nearer to the bottom of the conduction band. Fermi level on each side is the reference energy level, analogous to the liquid level in a container. When both the p and a materials are joined, equilibrium is attained only when equalisation of Fermi levels £,,, and E,,, takes place. When two containers filled with liquid to two different levelsare connected, the liquid level in one container falls down while the level in the other rises till both levels are equalized. In a similar manner, when p and n-type materials are joined, the energy levels in p-region moye up and those in r-tegion move down till the Fermi levels F,, and E,, are equalized, The following events occur in the course of the equalization of Fermi levels E peype pvp Fig. 2.28. Energy band diagrams of p- and mtype semiconductors. (@ The electrons are larger in number (majority carriers) on n-side and smaller (minority carriers) on p-side. Similarly holes are larger in number on p-side and smaller on n-side. Thus, there is a concentration gradient for majority carriers across the junction. Henee, they tend to diffuse across the junction (i The electrons and holes diffuse in opposite directions and recombine in the vicinity of junction, Therefore, a narrow region around the junetion is left with only immobile impurity atoms and neutral host atoms (i) The immobile ions are negative on the p-side and positive on n-side. These layers of opposite charges create a potential barrier for the diffusion of majority charge carriers.Types of Lasers. 105 ‘The potential barrier halts movement of electrons from n-side and that of holes from ps ide, Asa result, a narrow depletion region devoid of mobile charges forms, The mutual displacement of the energy levels on both sides of the junetion causes a bending of the energy hands around the junction region, as shown in Fi 1 ig Depletion _,} ie "Regon fi & Electron energy increases Hole energy decreases, sose0/0u! AB10U9 910 ‘Tosealsep (BIOUs WOME BUD Fig. 2.29. Energy band diagram of pp-unction at equilibrium, When the doping levels are very high, the Femi level in p-type region will be below the valence band edge and will be above the conduction band edge in n-type region. ‘The energy band diagram of a pn junetion formed between two heavily doped semiconductors is shown in Fig. 2.30 p type = Fermi level / Fig. 2.30. Energy band diagram of a heavily doped prjunction in equilibrium, The probability of stimulated emission event occurring is low even now because there must be a region of the device where a large number of conduction electrons and a large number of holes would be simultaneously present. This condition is never realized in thermal equilibrium, However, when the junction is forwant biased with a large enough voltage, electrons and holes (minority riers) are inj in considerable concentrations. In other words carriers are pumped by the de source. As a result, the region “d” (Fig, 2.31) contains a large concentration of electrons within the conduction ied into the junction region Ee © \eeem @ . E. *= YM Oe ¢ ©» UMMM 4 Fig. 2.31. Energy band diagram of a heavily doped ppr-lunction under (a) foward bias of smaller magnitude (b) larger forward bias leading to lasing,106 An Introduction to Laser — Theory and Applications band and large concentration of holes within the valence band This is the condition of population inversion, The narrow zone over which population inversion occurs is called an inversion region or active region. The electrons and holes recombine in the active region emitting photons, 2.7.5. Lasing Condition With reference to Fig. 2.31, it is seen that the electrons occupy the lowermost states of the conduction band upto the level F,,, and the holes occupy the uppermost states of the valence band upto the level £,, In the non-equilibrium state £,,, and £,, represent the electron and hole quasi- Fermi levelsrespectively. By virtue of pumping a non-thermal equilibrium steady state is obtained in which large densities of electrons and holes simultaneously exist in the same space. For writing the probabilities of induced transitions, knowledge of the electron concentration in the conduetion band and hole concentration in the valence band is required, because the existence of an electron at an excited level does not assure that there is « hole in the valence band. The probabilities for transitions are written in terms of the Fermi-Dirac distribution functions, Let a light wave of frequency v, travel through the junction. The beam will induce a > 6 transitions during which light is emitted as well a8 ba transitions which lead to absorption. ‘The rate of absorption transitions from a group of levels at energy F, in the valence band to a group levels at energy £,, in the conduction band may be expressed as Ray, = PU LE L EIN Qn where /_ and f, are Fermi functions in conduction and valence bands respectively. V is the density of photons of energy E, and P is the transition probability. The stimulated emission rate is given by Ry =P LEM -f,(BpIN (22) For emission of light from the pr junction, we require that Ry > Rare On comparing eq, (2.1) and (2.2), we find that emission will exceed absorption if LAE) > fo Ey) (23) 1 Using, [Ed = aT 1 and Ls) = ar Lee into eq. (23), we get E, ‘oy Ep > EE) But £,- hy . Egg ~ Egy > hv (24) But it also follows that the energy of the photon emitted due to electron transition must satisfy the condition hy 2 Eg (25) On combining Fq (2.4) and Eq, (2.5), the necessary condition for lasing in the pr-junetion becomes (Ep, — Ey) = bv 2 Eg. 26 ‘This condition was first obtained by Bemard and Duraffourg. £, and (Hj, ~ ;,) constitute the outside limits of the laser spectra, In order to fulfill the condition (26), the p and n-regions of the diode has to be doped heavily such that the Fermi levels penetrate the main bands as shown in the Fig 227Types of Lasers: 117 a EXERCISE-2 Ey Enumerate the types of lasers on the basis of medium, Discuss their relative merits and demerits (B.Tech. Fourth sem., G.B'T.U. Lucknow 2010) ‘Name the active material in Ruby laser. Drawa neat labelled diagram of a Ruby laser. ‘(Pune U. 2010) Draw schematic diagram to explain construction of CO, laser (Pune U. 2010) 4. What is the role of Ne atoms in He-Ne laser ? Explain the working of He-Ne laser with a suitable eae 10, te 2. 13, ms 18 16. 18, wy, 20. 21. 2. 24, 2s, diagram. (Pune U. 2010) ‘What is an exeimer laser ? (Pune U. 2010) Whit are solid-state lasers ? (Pune U. 2010) Which is the prominent wavelength emitted by Ruby laser ? (Pune U. 2010) Explain the principle and construction and working ofa ruby laser, (Garhwal U. 2004; PU. 2007, 2006, 2003; G.N.D.U.2007, 2006; Kerala U. 2001; Indore U. 2001, Meerut U. 2001 M.D.U. 2007, 2000) (a) Diseuss the construction and working of heliam-neon laser (b) Why does excited helium prefer energy transfer to ncon atoms instead of returning to its ground slate ? Explain prozess of lasing in He-Ne laser. What role is played by resonant cavity ? (P. U. 2008, 2006; G.N.D.U. 2007, 2004; Meerut U.2002; K.U. 2001, 2000; Garhawal U. 2004) Explain with the help of an appropriate energy level diagram, how stimulated emission results from, clestron impact pumping in He-Ne gas laser (.U,,2001) What i the reason for monochromaiieally of lsser beam ” Explain the working of He-Ne aserand give its engineering applications (N.U., 2002) Explain how lasing action is achieved in a semiconductor laser. With the help of energy band diagram: discuss the working of a semicondvetor laser (N.U,, 2000) Explain in brief the charactristis of laser beam, (.U,, 2000) Exphin the reason for the monochromaticity of ser beam (NU, 2000) Can we obtain ight amplification in the absence of stimulated emission ? (GN.D.U. 2007, Phi, U., 2003, P.U. 2003) (Ans. No) Discuss the essential requirements and various steps involved for producing laser action, Laser is basically a three-component device, Explain. (PU, 2007, 2006; Meerut U. 2003, 2001; Phi. U. 2000, G.N.D.U. 2001) Calculate threshold pumping power per unit volume in ease of Ruby laser. Given » N= 16 = 10% atoms’, fequeney vp 625 = 10! Hz, ands, =3 «10° see (PbiU. 2008) (Ans. 1100 10° Wim’) Explain the main features and conditions for laser action in a semi conductor. (G.ND.U. 2008, 2006, 2005; PU. 2008, 2007, 2008; Pi. U. 2006; K.U. 2002; M.D.U. 2002) _Descuss the construction and working of a CO, laser (PU. 2008, 2006; G.N.D.U. 2007, 2006; Phi. U. 2006) Write a short note on Nd > YAG laser (GN DU. 2007, 2006, 2005) Enunciate the advantages and disadvantages of laser diode over LEDs 5s dye laser and its relative advantages. Why is it known as tunable laser ? (G.N.D.U. 2007, 2006; P.U. 2006) conservation is violated in a laser or not ? Expalin (Pbi. U. 2006, P.U. 2005) (Ans. No) ‘Why gas lasers emit light which is more unidirectional and monochromatic as compared to solid state lasers ? (Hints: because of absene of effect such as crystalline imperfectios, thermal distortion and scatiering which is present in solid state lasers). Discuss the working of “Nitrogen laser”. What are the techniques used for population inversion ? Show the energy level digaram to explain your answer. (B.Tech, Fourth Sem, G.B.T.U. Luck. 2010) What do you think that ener