Laser PDF

Chapter Laser 11 OBJECTIVES To understand the principle, theory, operations and applications of laser To discuss the fundamental terms such as absorption, spontaneous and stimulated emission To derive Einstein expre...

Chapter Laser 11 OBJECTIVES To understand the principle, theory, operations and applications of laser To discuss the fundamental terms such as absorption, spontaneous and stimulated emission To derive Einstein expressions for stimulated emission and to study its importance To explore the different methods of population inversion To derive Schawlow and Townes condition for achieving laser action To study the different types of lasers and their applications To discuss the laser applications such as holography, and the use of lasers in the fields of computers, industry, medicine, etc 11.1 i Laser is an acronym for Light Amplification by Stimulated Emission of n Radiation. The theoretical explanation for laser oscillation was given by t A L Schawlow and C H Townes in the year 1958. The first laser, namely, ruby laser was demonstrated by T H Maiman, in the year 1960. The r discovery of laser made an enormous impact in the scientific world and o showed that the function of optics was very much alive. d The laser beam has the properties given below which distinguish it u from an ordinary beam of light. c (i) Directionality The light beam can travel as a parallel beam up to a t distance of d2/l, where d is the diameter of the aperture through which i the light is passing and l the wavelength of the light used. After travelling the o distance d2/l, the light beam spreads radially. In an ordinary light beam, the n l angular spread is given by Dq =. For a typical laser beam, the angular d spread is 1 mm per 1 m, but for an ordinary source of light, the angular spread is 1 m per 1 m. This shows the directionality of the laser beam. For example, the laser beam can be focused to moon from the earth with an angular spread of a few kilometres. 11.2 Engineering Physics (ii) Intensity The intensity of the laser beam is very high. If a person is allowed to observe an ordinary light, emitted by a 100 W bulb at a distance. One foot from the source, he can perceive only one thousand watt of light. While, if the person is allowed to observe the laser beam from the same distance, the entire laser beam penetrates through his eye. It will damage the eye of the observer. This shows the high intensity of the laser beam. (iii) Monochromaticity The laser beam is strictly monochromatic (single frequency) than any other conventional monochromatic source. The bandwidth (Dn = 0) of the laser beam is narrow, while ordinary light spreads over wide range of frequencies. Obtaining absolute monochromaticity is difficult; however, one can get high degree of monochromaticity in laser beam with very small bandwidth, of about one kilocycle per second. For example, the line width Dl emitted by ruby laser is 5 ¥ 10-4 Å. (iv) Coherence The degree of coherence of laser beam is very high than the other sources. The coherence of laser emission results in extremely high power (5 ¥ 1012 W cm-2 ). The light from a laser source consists of wave trains that are identical in phase and directions of propagation. Further, it can be focused to a very small area of 0.7 mm thickness. It is, indeed, these properties, that differentiate the laser beam from other conventional light beams. Let us discuss the principle, generation and application of the laser in this chapter in detail. 11.2 PrinciPle of laser From the theory of interaction of radiation with matter, we can get an idea regarding the working of laser. Consider an atom that has only two energy levels, E1 and E2. When it is exposed to radiation having a stream of photons, each with energy hn, three distinct processes can take place. (i) Absorption (ii) Spontaneous emission, and (iii) Stimulated emission (i) Absorption An atom or molecule in the ground state E1 can absorbs a photon of energy hv and go to the higher energy state E2. This process is known as absorption and is illustrated in Fig. 11.1 E2 E2 hn Photon E1 E1 Before absorption After absorption Fig. 11.1 Absorption The rate of upward transition R12 from ground state E1 to excited state E2 is proportional to the population of the lower energy level N1 (number of atoms per unit volume) and to the energy density of radiation rn,  Laser 11.3 i.e. R12 a rn a N1 Thus R12 = B12 rn N1 (11.1) where B12 is the probability of absorption per unit time. Normally, the higher energy state is an unstable state and hence, the atoms will make a transition back to the lower energy state with the emission of a photon. Such an emission can take place by one of the two methods given below. (ii) Spontaneous Emission In spontaneous emission, the atoms or molecules in the higher energy state E2 eventually return to the ground state by emitting their excess energy spontaneously. This process is independent of external radiation. The rate of the spontaneous emission is directly proportional to the population of the energy level E2, i.e. R21 (SP) a N2 R21 (SP) = A21 N2 (11.2) where A21 is the probability per unit time that the atoms will spontaneously fall to the ground state and N2 the number of atoms per unit volume in the state E2. This process is illustrated in Fig. 11.2 E2 hn E1 Fig. 11.2 Spontaneous emission (iii) Stimulated Emission In stimulated emission, a photon having energy E, equal to the difference in energy between the two levels E2 and E1, stimulate an atom in the higher state to make a transition to the lower state with the creation of a second photon as shown in Fig. 11.3. E2 hn hn hn E1 Fig. 11.3 Stimulated emission 11.4 Engineering Physics The rate of stimulated emission R21 (ST) is given as, R21 (ST) = B21rnN2 (11.3) where B21 is the probability per unit time that the atoms undergo transition from higher energy state to lower state by stimulated emission. Under conditions of thermal equilibrium, the population of energy levels obey Boltzmann’s distribution function. 11.3 einstein’s theory of stimulated emission In 1917, Einstein proposed a mathematical expression for the existence of stimulated emission of light. This expression is known as Einstein’s expression. Consider a two-level energy system (E1 and E2). Let N1 and N2 be the number of atoms in the ground state and excited state, respectively. Let us assume that only the spontaneous emission is present and there is no stimulated emission of light. At thermal equilibrium condition, the rate of absorption = the rate of emission of light From Eqs (11.1) and (11.2), we can write, rn ¥ B12N1 = A21N2 A21 N 2 rn = (11.4) B12 N1 According to Boltzmann’s distribution function, the population of atoms in the upper and lower energy levels are related by, N 2 e - E2 / kT = (11.5) N1 e - E1 / kT Substituting N2/N1 in Eq. (11.4), we get, A - ( E - E )/ kT rn = 21 e 2 1 (11.6) B21 A21 1 rn = B21 e hn / kT According to black-body radiation, the energy density 8p hn 3 1 rn = (11.7) c 3 e hn / kT - 1 where h is the Planck’s constant and c the velocity of light. Comparing the above two equations [Eqs (11.6) and (11.7)], one can observe that they are not in agreement. To rectify this discrepancy, Einstein proposed another kind of emission known as stimulated emission of radiation. Therefore, the total emission is the sum of the spontaneous and stimulated emissions of radiation. At thermal equilibrium condition, the rate of absorption = the rate of emission From Eqs (11.1), (11.2) and (11.3), B12 N1 rn = A21N2 + B21N2rn  Laser 11.5 A21 N 2 rn = (11.8) B12 N1 - B21 N 2 Dividing each and every term on R.H.S. of Eq. (11.8.) by N2 , we get, A21 rn = B12 ( N1 / N 2 ) - B21 Substituting for N1/N2 from Eq. (11.5), we get, A21 rn = B12 [e( E2 - E1 )/ kT ] - B21 We know that E2 – E1 = hv A 21 rn = hn / kT (11.9) B12 e - B21 The coefficients A21, B12 and B21 are known as Einstein’s coefficients. Comparing the above equation with Eq. (11.7), we get, B12 = B21 A21 8p hn 3 and = (11.10) B21 c3 From Eqs (11.7) and (11.9), the ratio of the stimulated emission to spontaneous emission is given by, R21 (ST ) B21 N 2 rn 1 = = hn / kT (11.11) R21 (SP) A21 N 2 e -1 From Eq. (11.11), Einstein proved the existence of the stimulated emission of radiation. The spontaneous emission produces incoherent light, while the stimulated emission produces coherent light. In an ordinary conventional light source, the spontaneous emission is dominant. For laser action, stimulated emission should be predominant over spontaneous emission and absorption. To achieve this, an artificial condition, known as population inversion, is required. 11.4 PoPulation inversion Consider a two-level energy system (E1 and E2). Suppose a photon of energy equal to the energy difference between the two levels is incident on the system, then which event is more likely to take place: absorption or stimulated emission? Einstein showed that under normal circumstances, both the processes are equally probable. In a system containing a very large number of atoms, the dominant process will depend on the virtual number of atoms in the upper and lower states. A large population in the upper level (N1 < N2) will result in stimulated emission dominating over the absorption. If there are more number of atoms in the lower level, i.e., ground state (N1 > N2), there will be more absorption than stimulated emission. Under conditions of thermal equilibrium, Boltzmann’s distribution function relating N1 and N2 given by Eq. (11.5) is obeyed. 11.6 Engineering Physics N2 = e - ( E2 - E1 )/ kT (11.12) N1 where k and T are the Boltzmann’s constant and the absolute temperature, respectively. For stimulated emission to dominate, it is necessary to increase the population of the upper energy level, so that it is greater than that of the lower energy level. This is known as population inversion. In the above equation, if T is negative, then stimulated emission would dominate over absorption. Such a condition is practically not possible. The population inversion can be obtained even at room temperature by several methods discussed in the following section. 11.5 methods of achieving PoPulation inversion There are several methods for achieving the condition of population inversion necessary for the laser action to takes place. Some of the most commonly used methods are as follows: (i) Optical pumping In case of optical pumping, an external optical source like xenon flash lamp is employed to produce a high population in the higher energy level of the laser medium. This method of excitation, shown in Fig. 11.4a, is used in solid state lasers of which the ruby laser is a prototype. (ii) Direct electron excitation The direct electron excitation in a gaseous discharge may be used to produce the desired inversion. This method is used in some of the gaseous ion lasers, such as an argon laser. In this type of excitation, as shown in Fig. 11.4b, the laser medium itself carries the discharge current under suitable conditions of pressure and temperture. E2 E2 E2 Laser transition E1 E1 E1 Photon e E0 E0 E0 Atom A Atom B (a) Optical pumping (b) Direct electron (c) Inelastic atom-atom excitation collisions Fig. 11.4 Population inversion—different-methods of production In this method, the electrons directly excite the active atoms to achieve higher population in certain higher energy levels compared to a lower energy level. (iii) Inelastic Atom–Atom Collisions Here, electric discharge method is employed to cause collision and excitation of the atom. In this method, a combination of two  Laser 11.7 types of gases is used, say A and B, both having the same excited state A* and B* that coincide or nearly coincide. In the first step, during electric discharge, A gets excited to A* (metastable) due to collision with electrons. A + e æÆ A* + e1 A* + B æÆ A + B* (11.13) The excited A* atoms now collide with B atoms so that the latter atom gets excited to higher energy B*. This type of transition is used in the He–Ne laser. (iv) Chemical reactions In this method, the molecules undergo chemical changes in which one of the products of the reaction is a molecule or an atom that is left in an excited state under appropriate conditions. Under such conditions, population inversion can occur. An example for this type of lasers is hydrogen fluoride chemical laser, in which hydrogen fluoride molecules in the excited state result from the following chemical reaction: H2 + F2 æÆ 2HF (11.14) 11.6 threshold condition (schawlow and townes condition) The amplitude of the light beam must be amplified in order to get the laser action. Schawlow and Townes discussed the necessary conditions for achieving the laser action. The material in which population inversion can takes place is called active medium. This medium may be a gas, liquid or solid, in which light amplification takes place and is placed in between the two mirrors of which one is partial reflecting, while the other one is 100% reflecting, as shown in Fig. 11.15. The light beam bounds back and forth between the two mirrors and hence, the intensity of the beam gets increased. However, there will be losses in each reflection but one can obtain a threshold condition for the laser action to takesplace. R1 Pumping process R2 Active medium 100 % mirror Partial mirror Fig. 11.5 Optical resonator The threshold condition is that the losses in the medium should be overcome by induced transition in the amplifying medium. The total losses are due to the following reasons. 11.8 Engineering Physics (i) Transition, absorption and scattering by the mirrors (ii) Absorption within the medium (iii) Scattering due to optical inhomogeneity, and (iv) Diffraction losses by the mirrors All the losses depend on the lifetime of the photons existing in the cavity and it is represented as tPhoton. The total rate of losses of photons from the laser medium per second is 1 (11.15) t photon By taking two level energy system, the stimulated transition rate w¢ is given by w¢ = r (n) BN2 (11.16) where B12 = B21 = B. Substituting the B value in Einstein’s coefficient Eq. (11.10). 8p hn 3 A A 3 = 21 = c B21 B Ac 3 w ¢ = r(n) N2 8p hn 3 w¢ c3 = r (n) N2 (11.17) A 8p hn 3 In practice, the laser line width is very narrow than the line width of atomic transition. Let g(n) represent the probability of frequency of the curve during absorption or emission. It is also called shape of absorption or emission curve, as shown in Fig.11.6. I (n) I (n ) 2 Dn n0 n Fig. 11.6 Absorption curve The factor g(n) dn represents the probability of absorption or emission by photons whose energy is hn and hn + h dn. The curve is usually normalised to unity, + i.e. Ú g(n ) dn = 1 (11.18) -  Laser 11.9 On account of uncertainty, it is necessary to replace w¢ as, w¢ = w¢g (n) dn (11.19) Similarly, for the spontaneous emission, whose frequency ranges from n to n + dn as, A = AgL(n) dn (11.20) where gL(n) dn is the Longevin function for line widths and is equl to + 2 Ú gL(n) dn = (11.21) - p Equation (11.17) can be writte as, w ¢ g (n ) dn r (n )c3 = N2 (11.22) Ag L (n ) dn 8p hn 3 The radiation density r (n) is simply related to radiation flux or intensity I(n) as, I (n) = r (n) c (11.23) where c is the velocity of light in the laser medium. We know, for spontaneous emission, 1 A= (11.24) t SP From Eqs (11.23) and (11.24), Eq. (11.22) can be written as, I (n )c 2 1 w¢ g (n) dn = N g (n) dn (11.25) 8p hn 3 t SP 2 L The total transition rate is obtained by integrating on both sides of Eq. (11.25) as, c2 2 1 w¢(n) = 3 I (n)N2 (11.26) 8p hn p t SP Consider a two-level energy system. Let N1 and N2 be the populations of ground state and the first excited state. Consider E1 and E2 as the energies of the ground state and the first excited state, respectively. Assume a light beam of energy density r(n) is made to incident on this system. The rate of absorption = B12 r (n) N1. Absorption of a photon removes an energy hn from the incident beam. Therefore, the total energy removed from the incident beam = B12r (n)N1hn. Due to the radiation incident on the two-level energy system, stimulated emission takes place. The rate of stimulated emission = B21r (r)v2. As in the absorption process, the emission of photon produces an energy hn and therefore, the total energy added to the incident beam is = B21r (n)N2hn. The rate of change of energy density in a frequency interval dn or n is d ( rn dn ) = B21r (n)N2hn – B12r (n)N1h dt d rn B r (n ) i.e. = (N2 – N1)hn (11.27) dt dn 11.10 Engineering Physics where B21 = B12 = B. In Substituting the vale rn = in Eq. (11.27), c we get, dIn Br (n ) = (N2 – N1)hn c dt dn dIn Br (n ) i.e., = (N2 – N1)hnc (11.28) dt dn The net rate of change in the energy level will be numerically equal to the product of net photons emitted and its energy. Usually, there will be always a degeneracy in the energy levels. Let g1 and g2 represent the degeneracy in the energy levels E1 an E2. Ê dI ˆ hn È g ˘ Ê w ¢ (n ) ˆ ÁË ˜¯ = N - N1 2 ˙ Á c dt gain dn ÍÎ 2 g1 ˚ Ë N 2 ˜¯ (11.29) This results in increased intensity on account of population inversion only if Ê g2 ˆ ÁË N 2 ≥ N1 g ˜¯ (11.30) 1 If this condition is not satisfied, the laser medium will simply lose energy or that there will be transition losses. The rate of loss of intensty is Ê dI ˆ I (n ) ÁË ˜¯ = (11.31) dt loss t photon For laser action, Ê dI ˆ Ê dI ˆ ÁË ˜¯ - ≥0 dt gain ÁË dt ˜¯ loss hn Ê g ˆ Ê w ¢ (n ) ˆ I (n ) or Á N 2 - N1 2 ˜ Á c ≥ (11.32) dn Ë g1 ¯ Ë N 2 ˜¯ t photon Substituting w¢ (n) from Eq. (11.26) hn hv Ê N - N g 2 ˆ Ê c ˆ Ê 2 I (n ) ˆ ≥ I (n ) 3 (11.33) Á g1 ˜¯ ÁË 8p hn 3 ˜¯ ÁË p t SP ˜¯ t photon 2 1 Dn Dv Ë Simplifying and rearranging the above equation, we get Ê g 2 ˆ Ê 4p 2 n 2 ˆ Ê t SP ˆ ÁË N 2 - N1 g ˜¯ ≥ ÁË c 3 ˜¯ Á t ˜ Dn (11.34) 1 Ë photon ¯  Laser 11.11 The threshold condition represents that the loss of photon is just balanced by the gain of photon, i.e., at threshold condition, Ê g 2 ˆ Ê 4p 2 n 2 ˆ Ê t SP ˆ ÁË N 2 - N1 g ˜¯ = Á Át ˜ Dn (11.35) 1 Ë c 3 ˜¯ Ë photon ¯ For non-degenerate energy levels, Eq. (11.35) can be written as, Ê 4p 2 n 2 ˆ Ê t SP ˆ N2 - N1 = Á Át ˜ Dn (11.36) Ë c 3 ˜¯ Ë photon ¯ In terms of the angular frequency, Eq. (11.36) can be written as, Ê w2 ˆ Ê t SP ˆ N2 - N1 = Á Át ˜ ”w (11.37) Ë 2p c 3 ˜¯ Ë photon ¯ The laser oscillation takes place at a particular single frequency. To represent this, let us replace w by w0 Ê w2 ˆ Ê t ˆ N2 - N1 = Á 0 3 ˜ Á SP ˜ ” w 0 (11.38) Ë 2p c ¯ Ë t photon ¯ c If n0 is the refractive index of the amplifying medium (where n0 = ), then by taking into v account the refractive index of the medium, the above equation can be written as, Ê n3 w 2 ˆ Ê t ˆ N2 - N1 = Á 0 03 ˜ Á SP ˜ ” w 0 (11.39) Ë 2p c ¯ Ë t photon ¯ This is known as Schawlow and Townes condition (threshold condition) for population inversion near the line g(n) at n = n0 for laser oscillation. 11.7 tyPes of lasers Lasers are classified into five major categories based on the type of active medium. They are (a) Solid state lasers (b) Gas lasers (c) Liquid lasers (d) Dye and chemical lasers, and (e) Semiconductor lasers All the above lasers are discussed in the following sections in detail. 11.7.1 solid state lasers (i) Ruby Laser In a solid state laser, the active medium is a crystalline substance. It is the first successful laser achieved by Maiman in 1960. The experimental set-up shown in Fig. 11.7 has three important parts. 11.12 Engineering Physics Ruby rod Partial reflector Coolant A B Laser beam 100 % reflector Coolant Power supply Fig. 11.7 Ruby laser Ruby Rod Ruby is a crystalline substance of aluminium oxide doped with approximately 0.05% by weight of chromium oxide. The resultant pink colour is due to the presence of Cr3+ ions in the appropriate concentration which replace aluminium atoms in the crystal lattice. Resonating Cavity In a ruby laser, a pink rod of 4-cm length and 0.5cm diameter is used. The end faces of the rod are strictly grounded so that they are parallel and polished to a high degree. Further, the end faces are silvered in such a way that one end face becomes fully reflecting while the other end is partially reflecting. In some cases, separate glass pieces are attached at the end faces. Xenon Flash Tube The laser action is achieved by using optical pumping. A helical xenon flash tube that surrounds the ruby rod, provides the pumping light to raise the chromium ions to the upper energy levels. The flash of the xenon tube lasts for several milliseconds and the tube consumes several thousand joules of energy. Only a part of the energy is used to excite the Cr3+ ions, while the rest heats up the apparatus. A separate cooling arrangement is used to reduce the temperature. Working Principle The energy-level diagram illustrating the operation of a ruby laser is shown in Fig 11.8. 30 ¥ 103 E2 Blue Non-radiative decay 20 E1 Green E 29 cm 1 0.55 mm 0.42 mm Å Å 10 43 27 69 69 = = 1 R 2 R E0 Fig. 11.8 Energy level diagram—Ruby laser  Laser 11.13 The pumping light from the flash tube is absorbed by Cr3+ ions, raising them from the ground state E0 to the excited states E1 or E2. From these levels, a rapid radiationless transition to the level E, which is a metastable state, takes place. Decay from E is relatively slow so that with sufficient excitation, population inversion between E and the ground state E0 can occur. The photons are allowed to pass back and forth millions of times in the active medium with the help of mirrors at the ends. When the condition for laser action is satisfied, an intense pulse of light of the wavelength 6934 Å, corresponding to the transition E to E0 is obtained. (ii) Nd–YAG Laser The Nd-YAG laser is an optically pumped solid state laser and is used to produce very high power emission. Nd–YAG Rod YAG is an acronym for yttrium aluminum garnet (Y3 A15O12) and Nd means neodymium (rare earth element) yttrium ions (Y3+). The aluminum garnet (Y3A15O12) is doped with a rare earth neodymium (Nd3+) resulting in a crystalline structure. It is a four-level laser with high power, more gain and low threshold pump power. Resonating Cavity In the Nd-YAG laser, a rod of 5 to 10 cm length and 6 to 9 mm diameter is used. A cylindrical rod is cut from the crystal and is used as an active medium. The ends of the rod are polished and made optically flat and parallel. One end of the rod is coated with silver to get 100% reflecting mirror while the other end as a partial reflecting mirror. The set-up acts as a resonator and is used to amplify the radiation energy after the stimulated emission of radiation. The length of the rod is used to vary the power out. 100% Reflecting Mirror Partial Reflecting Mirror Nd-YAG Laser rod LASER Flash Tube Elliptical cavity Capacitor Power Supply Resistor Fig.11.9 Nd–YAG laser Flash Tube The principle behind Nd–YAG laser is optical pumping. In this laser, the active medium is Nd-YAG. The population inversion is achieved by a flash light either employing a xenon or krypton flash tube. As a result, Nd3+ ions are transported into the excited energy levels. The laser transition from metastable state to ground state takes place with the emission of a laser beam in the infrared region. The schematic representation of Nd–YAG laser is shown in Fig.11.9. The rod and flash tube are kept inside a highly reflecting elliptical cavity. A close coupling between the flash tube and rod is made by placing the flash tube nearer to the centre of the rod. The necessary potential is applied to flash tube and the same is controlled through the power supply. When the flash tube is energised, the rod absorbs energy due to the elliptical cavity arrangements. The energy of the flash tube is produced through the capacitor which is included in the circuit. When the flash tube is on, a large amount of heat is produced 11.14 Engineering Physics in the elliptical cavity. A very less quantity of energy is required to achieve optical pumping for laser action. Thus, excess heat increases the temperature of the cavity. The temperature inside the cavity is controlled using water or air-cooling arrangements. E4 Energy E3 E2 (metastable) l = 0.80 mm l = 0.73 mm Laser l = 1.06 m m E1 Non-radiative Ground E0 transition state Neodymium E0 Fig.11.10 Energy-level diagram working Principle The energy-level diagram for a Nd-YAG laser is shown in Fig.11.10. When-flash lamp is switched on, neodymium ions acquire energy from the flash light. The neodymium ions are excited to energy levels E3 and E4. The transition from the ground state to E3 and E4 are due to absorption of energy with wavelengths of respectively 0.73 mm and 0.80 mm. The Nd3+ is not stable in the excited state, it makes a non-radiactive transition from E3 and E4 states to a metastable state E2 as shown in Fig.11.10. The metastable state is not a stable state. Therefore, Nd3+ ions are in this state until the population inversion is achieved. When the population inversion is achieved between E2 and E1, a stimulated emission takes place from the energy levels E2 to E1. The emitted energy is amplified between the resonators and then radiates a pulsed laser beam at a wavelength of 1.064 mm in the infrared region. Further, Nd3+ ions take a very rapid non-radiactive transition from E1 to E0. One can produce a maximum output power of 70 W either in continuous or pulsed mode of operation using the Nd-YAG laser. applications The following are applications of Nd-YAG laser: Communication applications such as transmitting signals Remote sensing applications Medicines for endoscopic applications Applications like drilling, welding, micromachining, resistor trimming, scribing, etc. Medical surgery, dental surgery, etc.  Laser 11.15 11.7.2 gas laser The main advantage of gas lasers is that they can be operated continuously. The gas lasers show exceptionally high monochromaticity and high stability of frequency. The output of the laser can be tuned to a certain available wavelength. Hence, the gas lasers are widely used in industries. Helium–Neon Laser The first gas laser constructed by Javan in the laboratory is the helium–neon laser as shown in Fig. 11.11. The two important parts of the laser are as follows: Adjustment Adjustment for reflector for reflector Quartz tube Laser beam He : Ne 10 : 1 Gas 100 % reflector A.F. generator Partial reflector Fig. 11.11 Helium-Neon laser Active Medium The active medium used in this type of laser is a mixture of helium and neon gases. These two gases are mixed under a pressure of 1 mm Hg of helium and 0.1 mm Hg of neon in the ratio of 10:1. The mixture of these gases is filled within the discharge tube for laser action. Gas Discharge Tube It is made up of a fused quartz tube with a diameter of 11.5 cm and a length of 80.6 cm. The end faces of the discharge tube are tilted at the Brewster angle, known as Brewster windows, since the windows are highly transparent for the preferred direction of polarisation. A fully reflecting concave mirror is placed at one end of the discharge tube and a partially reflecting concave mirror is placed at the other end. Working Principle The population inversion for laser action is achieved by inelastic atom–atom collisions. The collisions of the atoms are made using any one of the following methods given below. (i) Direct current discharge (ii) Alternate current discharge (iii) Electrodeless high frequency discharge, and (iv) High voltage pulses Due to the electric discharge in the gas, an energetic electron interact with the ground state helium atom. The impact of the electron results in exchange of some of its energy to the helium atom. As a result, helium atoms are excited to higher levels 1S0 and 3S1, known as metastable state. The lifetime of these levels are relatively low. The collision of the first kind is represented as, He + e1 æÆ He* + e2 (11.40) 11.16 Engineering Physics These two energy levels are very close to 2s and 3s levels of the neon atom and the collision of the second kind takes place between the He and Ne atoms, and hence, the neon atom goes to the excited state. He* + Ne æÆ He + Ne* (11.41) The energy level diagram of He–Ne laser is as shown in Fig. 11.12. 1S 3s(2p55s) 0(1s2s) 3.39 mm 3S 1(1s2s) 2s(2p54s) 3p(2p54p) 6328 Å 1.15 mm 2p(2p53p) Spontaneous 1s(2p53s) emission 1S 2) 1(1s Collision with wall 2p6 Helium Neon Fig. 11.12 Helium-Neon laser—energy–diagram There are three types of transitions, one from 2s to 2p, the other from 3s to 3p and another from 3s to 2p levels for laser action. These transitions constitute laser beam in the infrared region (11,523 Å , 33,912 Å) and 6328 Å in the visible region. The transition from 3s to 3p at 3.39 mm will have advance effect on the laser emission. This line is suppressed in order to get a maximum power output at 6328 Å. In Fig. 11.12, the energy levels are represented in terms of Paschen’s notation and their corresponding electronic configuration are given in parentheses. The Neon atom in the terminal level 2p decays very rapidly to the 1s metastable state in 10-8 s, much faster than spontaneous rate of decay from 2s to 2p level. Thus, the lower lasing level is relatively kept empty and the population inversion is achieved between 3s and 2p. 11.7.3 molecular gas lasers The construction of molecular gas lasers is simple and the output of these lasers are continuous. In molecular gas lasers, the laser oscillations are achieved by the transition between the vibrational and rotational levels of the molecules. Carbon dioxide lasers (CO2) It is the first molecular laser and it was developed by C K N Patel. Principle In order to understand the working of this laser, one has to recall the rotational and vibrational spectrum of CO2 molecules. The three atoms can be considered as a ring over a straight line, the outer atom being O with a carbon atom C at the centre. There are three modes of vibrations and in each mode the centre of gravity remains fixed. (1) As shown in Fig. 11.13a, the carbon atom is fixed in its position and each oxygen atom can vibrate in the opposite direction symmetrical to  Laser 11.17 the carbon atom with each other along a straight line and is known as the symmetric mode of vibration. The corresponding frequency is called symmetric stretching frequency. (2) As shown in Fig. 11.13b, the oxygen atom and the carbon atom may vibrate at right angles to the line passing through the centre of gravity. This is known as the bending mode and the corresponding frequency is bending frequency. (3) As shown in Fig. 11.13c, in the asymmetric mode of operation, the two oxygen atoms may vibrate about the central C atom asymmetrically, and at the same time, the carbon atom also vibrates from its mean position.The corresponding frequency is called as asymmetric stretching fequency. C O O O C O O C O (a) symmetric mode (b) bending mode (c) asymmetric mode Fig. 11.13 CO2 molecules—different modes of vibrations In addition to these three vibrational modes, the molecule can also rotate and therefore, quantised rotational energy levels are also possible. A series of rotational levels is associated with each vibrational level and are denoted by J values. Construction The special feature of the CO2 laser is the dependence of radiation power, i.e., output power on the diameter of the tube. The output power can be raised by increasing the tube diameter. In a powerful CO2 laser, the length of the discharge tube is in several metres and its diameter will be several centimetres as shown in Fig. 11.14. He N2 CO2 Vacuum Laser beam Mirror NaCl window Fig. 11.14 CO2 laser The laser is powered with an ac supply of frequency 50 cycles or dc supply. In order to get a high power output, a metallic mirror of gold is employed for proper reflection. The lasers are either water-cooled or air-cooled. The efficiency of the CO2 laser is about 30%. The gas mixture can be pumped either longitudinally or transversely into the gas discharge tube. 11.18 Engineering Physics Working Principle The CO2 laser generally uses two additional gases, N2 and He. The nitrogen plays a similar role as that of the He in case of He–Ne laser. The N2 molecules go into an excited state by collision of the first kind with the electrons. N2 + e1 æÆ N2* + e2 (11.42) The excited N2 atom undergoes a collision of the second type and makes the CO2 molecules to be excited. N2* + CO2 æÆ CO2* + N2 (11.43) The frequency of the CO2 laser can be pictured in an energy level diagram as shown in Fig. 11.15. Asymmetric 002 3000 Symmetric Bending 2349 cm–1 001 mm n ¢¢ = 1 10.6 2000 100 mm 6 020 9. 1337 cm–1 1000 1255 cm–1 010 667 cm–1 000 n ¢¢ = 0 CO2 Nitrogen Fig. 11.15 Since the 0 0 1 energy level of CO2 is very close to the excited level of N2 atom, the population of 0 0 1 level of CO2 increases rapidly than the other lower energy levels 1 0 0 or 0 1 0. Thus, the population inversion between 0 01 level and the lower levels 0 2 0 and 1 0 0 is achieved resulting in output radiations of 9.6 and 10.6 mm. Due to various factors, the most powerful transition in CO2 laser at normal operating temperature occurs at 10.6 mm. The operating temperature plays an important role in determining the output power of the laser. The contamination of carbon monoxide and oxygen will also have some effect on the laser action. The unused gases can be pumped out and fresh CO2 must be pumped in. The temperature can be reduced by restricting the tube diameter and also the addition of helium to the mixture of N2 and He. The helium serves not only to improve the conductivity of heat to the walls of the tube, but also in decreasing the population in the lower levels. The power output coming from this laser is 10 kW. 11.7.4 semiconductor laser (semiconductor diode laser) The most compact of all laser is the semiconductor diode laser, also called the Injection laser. In its simplest form, the diode laser consists of a p–n junction doped in a single crystal of a suitable semiconductor such as gallium- arsenide. Principle When a forward bias is applied to the diode, the holes are injected into the p-side of the junction and electrons are injected into the n-side. The recombination of  Laser 11.19 holes and electrons within the junction region results in recombination radiation. If the junction current density is large enough, a population inversion can be obtained between the electron levels and hole levels. Stimulated emission can be obtained for laser action when the optical gain exceeds the loss in the junction layer. In a diode laser, this layer is very thin, typically of the order of few microns and the end faces of the crystal are made partially reflecting to form an optical resonator. To produce laser action, the following conditions should be satisfied. (i) Population inversion (ii) Stimulated emission, and (iii) Cavity resonator The laser transition is possible only in direct band gap semiconductors. Therefore, the direct band gap semiconducting materials like Ga–As are used for laser action. Since Si and Ge are said to be indirect band gap semiconductors, they are not used for laser action. At absolute zero, the conduction band of an intrinsic semiconductor has no electron. However, heavily doped semiconducting materials consist of filled electron states in the conduction band (Fig. 11.16). If the heavily doped material is irradiated with a radiation having energy greater than Eg but less than the separation of the quasi fermi level (EFC – EFV), the incident photon can induce a downward transition of an electron. This electron combines with a hole in the valence band and hence the recombination energy is produced in the form of light. This photon, in turn, may induce another electron in the conduction band and thereby stimulate the emission of the another photon. Conduction band Conduction band EFC Quasi E fermi Eg Eq Eg Valence band EFV Valence band (a) In equilibrium (b) With high carrier injection Fig. 11.16 Filled electron states of an intrinsic semiconductor at 0 K: (a) in equilibrium and (b) with high carrier injection The population inversion in a p–n junction is achieved by heavily doping the p-type and n-type materials, so that the fermi level lies within the conduction band in the n-type material and it lies within the valence band in the p-type material. Figure 11.17 represents the energy band diagram of a heavily doped p–n junction device at thermal quilibrium. If the junction is forward biased with an applied voltage nearly equal to the band gap voltage, direct conduction takes place. This results in high injection current density leading to the generation of an active region near the depletion layer where the condition 11.20 Engineering Physics of population inversion is achieved. At this juncture, if a radiation having frequency n, where Eg/h < (n) < (EFC – EFV)/h is made to incident on the p–n junction device, the stimulated emission is produced and hence the radiation of this frequency, which is confined to the active region will be amplified. p-type CB Eg n-type conduction band EF Valence band (a) Valence band p-type Active region EFC EFC hn EFV EFV (b) Fig. 11.17 Energy band diagram of (a) a heavily doped p–n junction, (b) heavily doped p–n junction under heavy forward biasing Another requirement of the laser action is the cavity resonator. In a p–n junction device, the end faces of the junction diode are polished so that they act as an optical cavity. Other sides of the p–n junction device is made as rough surfaces so as to prevent the leakage of light from them. Ga–As Laser It is a p–n junction diode with the p-type and n-type regions heavily doped. Under large applied forward bias, electrons and holes are injected into and across the transition region in considerable concentration. As a result, the region around the junction contains a large number of electrons within the conduction band and a large number of holes within the valence band. When the population density is high enough, a condition of population inversion is achieved and recombination may be stimulated resulting in laser action. If the emission is not stimulated, the device is called a light emitting diode. To convert an LED into a laser diode, a high current is required to achieve the population inversion and mirrors are used to cause a feedback. In case of semiconductor laser, there is no need of external mirrors. In a germanium or silicon semiconductor, due to such a recombination, only heat is generated. The Ga–As laser was constructed by Hall. A typical Ga–As semiconductor laser is as shown in Fig. 11.18. In case of gallium–arsenide, we get a light radiation in the infrared region. Thus, a Ga–As laser convert electricity into light. The efficiency can go up to 100% when the temperature is reduced to 100 K. The operating current is supplied from a pulse  Laser 11.21 generator up to 5–20 ms. The energy separation between the conduction band and the valence band is 1.4 eV and hence, the wavelength of light emitted is 8874 Å at room temperature. Laser beam Upper electrode on gi n re io p- ct n ju p- ion n g re Laser beam n- m m 1 1 mm Lower electrode Fig. 11.18 Ga–As laser Advantages 1. The efficiency is more than 10% and it can be increased by decreasing the temperature alone. 2. It can have a continuous wave output or pulsed output. 3. The modulation of the output is possible. 4. Tuning the output is easily possible by applying a magnetic field or me- chanical pressure. 5. It is highly economical, and further, the arrangement is compact. Disadvantage The spatial and temporal coherence are very poor. 11.7.5 homojunction and heterojunction injection laser The semiconductor diode or injection laser is of two types, namely, homojunction and heterojunction. Let us discuss the above two types in brief. (i) Homojunction A p–n junction made up of the same material with two regions of different conductivities, n-type and p-type, is known as a homojunction. Consider a p–n junction formed by p-type and n-type Si crystals, then it is said to be a homojunction. The junction is formed at the region where the conductivity changes from one type to another type. A homojunction p–n junction may have abrupt or graded junction depending upon the method of preparation. If a p–n junction is formed by diffusing a suitable pentavalent impurity from the upper portion of a Si crystal and diffusing another suitable trivalent impurity from the bottom portion of the Si crystal, a p–n junction of the Si crystal is formed. This will produce a graded junction. An abrupt Si is coated on a substrate followed by the coating of n-type material using a suitable thin film deposition technique. The homojunction device lacks in carrier containmnet and hence, it is an inefficient light source. The devices like LED, GaAs homojunction laser, etc., are fabricated 11.22 Engineering Physics with homojunction. The basic structure of a homojunction injection laser is shown in Fig. 11.19. Cleaved Crystal mirrors Fabry Perot Cavity p-GaAs hn hn n-GaAs ohmic contact p n junction Active region Fig. 11.19 GaAs laser—Schematic representation The cleaved ends of the crystal acts as reflecting mirrors so as to amplify the light sources. This homojunction GaAs laser device had a high threshold current density (greater than 10-4 A cm-2) due to lack of carrier containment and hence, it is an inefficient light source. Therefore, the homojunction lasers are operated in a pulsed mode in order to minimise junction temperature and hence, to avoid damage. (ii) Heterojunction A heterojunction is an interface between two adjoining single crystal semiconductors with different band gap energies. There are two types of heterojunctions, namely, isotype and anisotype heterojunctions. If the junction is made up of two different materials having same type of conductivity (n–n or p–p), then it is said to be isotype heterojunction. If two different materials having different types of conductivity are joined together, then it is known as anisotype heterojunction. An isotype heterojunction is used to confine the minority carriers to a small active region. It reduces the carrier diffusion length, and hence the volume in which the radiative recombination takes place. The isotype heterojunction is mostly used for the prepara- tion of injection lasers and highly radiative LEDs. The anisotype heterojunctions with sufficiently large band gap differences improve the injection efficiency of the charge carriers (holes and electrons). Both types of heterojunctions provide dielectric steps due to different refractive indices at either side of the junction. This property is used to confine the radiation in the active region. A typical double heterojunction laser is shown in Figs 11.20. a and b represents, respectively, the layer structure shown with applied forward bias and the energy band diagram of two heterostructures: (i) p–p at the left, and (ii) p–n at the right. The variation of refractive index and electric field distribution with distance is shown in Fig. 11.20. The requirement of a good quality heterojunction are (i) the lattice constant of the p-type and n-type material should be nearly equal (i.e. lattice matching), (ii) the electron affinities of these two materials should be compatible, and (iii) the thermal expansion coefficient should be close. If there is lattice mismatching and the thermal expansion coefficient is not very close, it leads to interfacial dislocation at the heterojunction  Laser 11.23 interface. The dislocation produces energy discontinuity in the form of a spike in one or both sides of the energy bands. Optical output Electrode Cleaved mirror p p n (a) Injection electron f2g1 hn f2g2 (b) Electric field distribution Refractive index (c) distance Fig. 11.20 A double heterostructure laser 11.8 determination of wavelength of laser using grating Aim To determine the wavelength of the laser beam using grating. Apparatus Required A He–Ne laser source; grating; spectrometer; sodium vapour lamp; spirit level; etc. Formulae (i) The number of lines per metre drawn on the grating is given by sinq N= lines m-1 (11.44) ml where m is the order of spectrum observed and l the wavelength of sodium vapour lamp (l = 589.3 nm). (ii) The wavelength of the given laser source, sinq l= (11.45) Nm 11.24 Engineering Physics where q is the angle of diffraction, N the number of lines drawn on the grating per metre, m the order of spectrum and l the wavelength. Experimental Procedure (i) To determine the number of lines drawn per metre of the given grating The experimental arrangement is shown in Fig. 11.21. One can use the standard procedure to determine the number of lines drawn on the grating per metre. The observed experimental results are tabulated in Table 11.1. table 11.1 Determination number of lines per metre of the grating Diffractig Ray Readins Difference Right Let 2q Mean N= Order VA VB VA VB VA VB q Collimator, C 45° 45° Telescop e, T Grating Direct ray Fig. 11.21 Adjustment for normal incidence (ii) To determine the wavelength of the laser source The experimental arrangement used for the determination of the wavelength of laser is shown in Fig. 11.22. The given He–Ne laser source is switched on and the laser beam is passed through the grating. The diffracted rays are collected on a screen. In the diffraction pattern, the circular spot at the centre is produced due to the direct beam. The other diffracted beams are produced on either side of the direct beams. The diffracted pattern produced by the laser beam is shown in Fig. 11.23. Grating Screen Laser source Fig. 11.22 Determination of wavelength of the laser The distance between the first-order diffracted rays (left and right side diffracted beams) is measured and the readings are tabulated. Similarly, the distance between the second order diffracted rays is also measured and the readings are tabulated in  Laser 11.25 Table 11.2. The distance between the screen and the grating, l, is also measured. The angle of diffraction is determined using the relation, d q= (11.46) 2l where l is the distance between the screen and the grating and d the distance between the dffracted rays. Central spot II order I order I order II order Fig. 11.23 Diffraction patterns of laser beam produced by grating The wavelength of laser beam is determined using therelation, sin q l= (11.47) Nm where q is the angle of diffraction, m the order of diffraction and N the number of lines drawn on the grating. table 11.2 The distance between screen and grating, l = … m Order of Distance between d Trial Angle of diffraction, q = diffracted diffraction beams, d 2l 1 I II 2 I II Results (i) The number of lines drawn on the grating = lines m-1 (ii) Wavelength of the given laser beam = m 11.9 Particle size determination by laser Aim To determine the particle size of lycopodium powder using He–Ne laser. Apparatus Required A He–Ne laser, lycopodium powder, a glass plate; a long focal length lens, a screen, and a scale. D Formulae The particle size is given by 2d = n l (11.48) xm where 2d is the grain size, n the order of diffraction, l the wavelength of the laser beam used (for He–Ne laser, l = 0.6328 nm), D the distance between the source and the screen and xm the distance between the central bright spot and the mth fringe. 11.26 Engineering Physics Experimental Procedure The experimental set-up used for particle size determination using laser is shown in Fig. 11.24. Sprinkle lycopodium powder having an average grain size is in the order of few micrometers on a glass plate. Insert this glass plate in between a laser beam and a screen is placed nearly tens of centimetres (nearly 20–50 cm) away from the source. A diffraction pattern as shown in Fig. 11.25 is obtained. He–Ne source Screen Glass plate Fig. 11.24 Experimental set up for Fraunhofer diffraction The distance between the central spot and the mth minima positions is measured in the screen directly using a scale. The distance between the source and the screen (D) is also measured. Different sets of readings are taken by varying the mth minima positions (say 1, 2, 3, 4, 5, …). The experiment is also repeated by varying the distance between the source and the screen. The observed readings are tabulate in Table 11.3. Laser Fig. 11.25 Diffraction pattern table 11.3 Particle size determinaion usig laser S.No. Distnce between Order of Distnce between the Particle size the source and diffraction central spot 2d the screen (D)m n and mth fringe (m) 1 1 2 2 3 3 4 4.... 10 10 Mean value Results The grain size of the lycopodium powder is ________ m. 11.10 aPPlications of lasers Laser beams have found wide application due to their high intensity, high monochromaticity, high coherence and high directionality.  Laser 11.27 11.10.1 three dimensional Profiling The process of storing and retrieving the three-dimensional image of an object by using laser scanner and computer software is known as three dimensional (3D) profiling. The principle involved in storing the three dimensional images is same as that of holography. When a highly coherent, monochromatic and directional light such as a laser beam is incident on an object, it produces secondary wavelets. Each point on the object acts as a source for secondary wavelets. These waves spread over in all the directions and form an interference pattern. The interference pattern characterises the object. The observed interference pattern is recorded using a suitable technique. The recorded image is again illuminated by using another identical coherent laser source. Thus, the three dimensional image of the object is reproduced due to the interference of the two lights which incorporate the variation in amplitude and intensity. The block diagram of a simple 3D profiling using laser beam is shown in Fig. 11.26. D C O R S Digital Detector information Machine (or) scanner Designing tool Computer Fig. 11.26 Laser—3D profiling The following are the important parts of the 3D profiling system: (i) Laser source (ii) Detector array and scanner (iii) Computer A high-intensity and monochromatic laser beam is generated using a laser source (S). The laser source is movable on circular scale (C) and it can be positioned at any desired angle over 360°. The laser beam from the source is made to fall on the object (O) which is kept fixed at the centre of the orbit of the laser source as shown in Fig. 11.26. Thus, the reflected light (R) from the surface of the object spreads in all directions. The reflected light from the source is collected by using light-sensing diodes. These diodes are known as detectors. These diodes produce an equivalent amount of current depending on the intensity of the light incident on it. These diodes are arranged in the form of an array known as detector (D) to receive the reflected light from the source. In order to record the reflected light from the source in all directions, a detector scanner is fitted in such a way that it moves along the three planes. The received signals 11.28 Engineering Physics by the scanner are converted into digital form using an analog to digital convertor unit which is attached with the scanner. The received signal carries both amplitude and phase variation which reflects the nature of the object. Thus, the optical signal is converted into electrical signal and is stored in the microcomputer. The information stored in the computer is in the form of matrix. Using the signal processor software available with the computer, one can reconstruct the 3D image of the object from the stored data. The resolution of the 3D images can be increased by using the software controlled by the computer. 11.10.2 computer Peripherals The application of a laser beam in computer peripherals has found greater revolution in recent years. The optical disks have very high storage capacity than conventional magnetic disks. Similarly, the data transferring from one computer to another computer through optical wave guides play an immense role. The application of laser beam in storing data in optical disks is discussed below, while that in the memory transfer using optical fibres is discussed in the previous chapter. Block Land Pit CD ROM disk (a) Spiral track (b) Pit and land Fig. 11.27 CD ROM An optical disk has very high storage capacity. For example, the storage capacity of a 5.25 inch disk is 550 MB. An optical disk is made up of a resin such as polycarbonate. One side of the disk is coated with highly reflective materials like aluminium. When a high intensity light beam such as laser beam is incident on it, it will produce some changes on the coating: a 0.1-mm wide tiny pit is formed along the trace (to represent 1). The non-pitted areas between the pits in the circular disk is represented as lands as shown in Fig. 11.27. The information in CD ROM disk is stored in a long spiral track. The track is divided into blocks of the same size as shown in Fig. 11.27. The information is read/write on the optical disk using laser beam, which replaces the read/write arm in the conventional magnetic disk memories. A high-intensity laser beam is used to store the data, while a low intensity laser beam is used to read the data. The experimental set-up used for the read-out mechanism for CD ROM is shown in Fig. 11.28. A low-intensity laser beam from the source S is made to incident on the CD ROM disk through an adjustable mirror M and lens L2 after passing through the beam splitter B. The intensity of the laser beam used for reading the data is about 4 mW. The laser beam scans the tracks and receives the reflected lights from the pits and lands. The mirror M and lens L2 act as read arm and can move front and back to  Laser 11.29 read the data on different tracks. The incident laser beam on pits spreads, while on lands gets reflected. A photodiode sensor D receives the reflected light after passing through the lens L3. The photodiode receives a little light from the pits and a larger light from the lands. Thus, the change in the reflected light is sensed and converted into electrical signals for data reading. In this system, the speed of the optical disk is adjusted such that the track passes under the read/write head at a costant velocity. B M S L1 L2 L3 Land Pit CD ROM disk Computer Fig. 11.28 Optical read-out mechanism for CD ROM There are three types of optical disks, namely, compact disk which read only memory (CD ROM), write once read many (WORM) and erasable optical disk (EOD). The main advantage of CD ROM is its high storing capacity, mass copy of information stored removable from the computer, etc. CD ROM disks are used for archival storages. The main disadvantages are longer access times when compared with magnetic disks and it cannot be updated, as it is a read only memory. However, in recent years, read/write CDs are also available in the market. The RW CDs overcome all the drawbacks of the CD ROM and hence the optical disk storage plays a vital role in computer memory storages. 11.10.3 material Processing A high-intensity laser beam is used to study the surface defects in materials such as ICs, aircrafts, automobile tyres, etc. The laser beam is reflected from the surface of the material under investigation and also directly from the source as a reference. The information about the material is obtained simply by forming the interference between the two laser beams. There are two methods to study the surface defects, namely, diffraction and opto-acoustic image processing. In both the techniques, a laser beam is used to study th surface defect. The block diagram of the experimental set-up used in diffraction method is shown in Fig. 11.29. A high intensity laser beam from the source S is made to fall on the converging lens L1. The lens L1focuses the laser beam on the object O. The optical diffraction pattern of the image is focused on the photographic plate (P) or photodiode (D). The photodiode senses the light and converts the light energy into electrical signals. Using the signal analyser along with the necessary software, the image of the defect can be obtained. Thus, using the high intensity laser beam, the surface defects on the materials can be studied effectively. 11.30 Engineering Physics Signal D S analyser Computer L1 L2 O P Fig. 11.29 Surface defect detection 11.10.4 drilling Drilling is the first production application of laser light. The first commercial application developed in drilling is the drilling of gemstones for wire dies, watches and jewels. A high-energy laser beam is used to obtain small holes in materials at high speed with good quality. When a high-intensity laser beam is used to drill the material, a large number of physical processes, ablation mechanisms, etc., take places depending on materials and irradiation condition. The schematic representation of a typical laser drilling process is shown in Fig. 11.30. The high-energy laser beam radiation is made incident on the surface of the material. When the laser radiation hits the material surface, the material is removed by means of two processes. In the first process, when the radiation is incident on the material surface, the temperature of the material is raised and it reaches the vaporisation temperature. When the material is vaporised, it creates a cavity in the material. When the process continues, the vaporisation increases and hence, the pressure of vapour also increases. When the vapour pressure reaches the required pressure level, say 100 Kpa, the second process namely melt expulsion, takes place. The high-pressure vapour escapes from the area after producing a high recoil pressure on the molten materials, which are present at the bottom of the hole. After acquiring the required pressure, the molten materials overcome the surface tension, and hence, they are forced out to the surface of the hole. These molten materials, i.e., the liquid metal, is ejected vertically out of the hole through its sides as shown in Fig. 11.30. The energy required to remove material by melt expulsion is very less when compared with vaporisation. Thus, it is clear that the recoil pressure and vaporisation temperature are more essential parameters in laser drilling. Incoming Laser Radiation Ejected Liquid Metal Droplets Escaping Vapour Liquid Metal Forced up the Recoil Pressure Sides of the from Vapour Hole Acting on Liquid Fig. 11.30 Laser drilling  Laser 11.31 Based on the applications, the laser drilling parameters are selected using the relationship between the drilling parameters and hole parameters. For any industrial applications, the relationships between the above parameters are given in Table 11.4. table- 11.4 Drilling parameter, hole parameter and application requirements S. No. Drilling parameter Hole parameter Application requirements 1 Pulse energy Exit hole diameter Aero dynamics 2 Beam diameter Entrance hole diameter Flow characteristics 3 Pulse duration Hole profile Cooling effect 4 Wavelength Surface roughness Optical effect 5 Focusing Hole depth Materials property 6 Gas instability Pitch Tolerances Tolerances There are different drilling methods for any particular drilling applications. The available drilling methods are single pulse, percussion, parallel percussion, trepanning, helical trepanning, imaged and angled. Lasers such as Nd–YAG and CO2 are used for industrial drilling purposes due to their high energy output as well as continuous operations. Advantages The following are the advantages of laser drilling: It is a non-contact processing method. It is used to drill micro holes and small holes with large aspect ratios. It generates very low heat in the material during drilling. It is possible to drill at different angles. It is highly flexible to drill over a wide range of materials. Its accuracy and consistency are very high. Applications The following are the application of laser drilling in industries: It is used in the electronic industry to make the printed circuit boards. In aerospace, it is used to make cooling holes in aircraft engine blades and combustors. It is used to drill the inject nozzles, fuel injector nozzles, aerosol nozzles, etc. It is used to drill diamonds to remove imperfections. It is used to drill rocks. 11.10.5 welding Laser beam welding is a thermal technique, which is used to join two or more metal parts. The joining of metals is established by solidification of metal at a common 11.32 Engineering Physics melting point. The applications of laser welding are ever increasing due to its deep weld penetration and minimizing heat inputs. This method also facilitates to automate the process techniques. Consider that the given two metal plates are to be welded using laser beam welding. The two metal plates are held in contact at their edges as shown in Fig. 11.31. The focal spot for the welding on the surface of metal plate edges are identified. The high intensity laser beam (Nd–YAG or CO2 or diode laser) is focused on the spot employing the optical arrangements. At the surface of the metal plate, the highly concentrated laser beam, i.e., high energy is converted into thermal energy and thus it heats the spot in the metal plates. Therefore, the surface of the metal plate starts melting and then progresses through it by surface conductance. Thus, the metal plates fuse together at the points of contacts. The energy of the beam required to melt the metal plates are kept well below the vaporisation temperature of the workpiece metal. When the intensity of the laser beam is increased, the workpiece metal undergoes several phase changes, namely, solid to liquid and then again to vaporisation. The laser welding station requires few basic components, namely, good laser source, beam guiding, workpiece and manipulating and workstations. A simple experimental set-up used for laser beam welding is shown in Fig. 11.31. The laser light output from the laser source is passed through the optical instruments which helps to control the energy, focus the laser beam as well as for automation. The optical assembly helps to get a fine laser beam. In addition, at the laser focusing point, a shield gas is passed through the shielding gas jet. The shielding gas is used to remove the molten material to favour vaporisation. Further, it provides the necessary cooling effect at the spot which protects the optical arrangements against environmental effects such as fumes. It also helps to increase the adsorption of energy by the sample. The manipulating and work-station on the laser welding systems helps to provide a relative motion between the laser beams and the workpiece, either in two or three dimensions at the required speed and accuracy. Laser Mirror source Shutter Focussing lens assembly Window Shielding gasjet Focusing point Metal plate Metal plate Fig. 11.31 Laser beam welding  Laser 11.33 Types of Laser Beam Welding Even though quite a variety of laser beam weldings are available, the most common types used for industrial applications are Nd–YAG, carbon dioxide lasers and diode lasers. The wavelength of laser light produced by Nd–YAG and CO2 laser is respectively 1.06 m m and 10.6 m m. The CO2 laser source requires beam guiding systems to propagate the laser beam from mirror to mirror. On the other hand, flexible glass fillers can be used for the Nd–YAG laser source. The laser beam welding is used for materials whose thickness is less than 0.80 inches. Advantages The following are the advantages of using laser drilling in industries: It is a non-contact processing method. It has the ability to weld smaller and thinner components. It gives a very low heat input into the materials. It provides an excellent metallurgical quality on the established weld. It has the flexibility to drill over a wide range of materials. It is used for doing deep and narrow welds. It has high accuracy and consistency. It is used to drill submicron and small holes with huge aspect ratios. It is used to drill at different angles. It can be carried to difficult terrains. It creates minimal heat affected zones during welding. The main disadvantage of laser beam welding is its high cost. Applications The following are the applications of laser welding in industries: It is used to weld complex-shaped contours in a controlled manner. In automobile industries, it is mainly used for car body constructions, space frames, etc. Further, it finds application in areas like aerospace, defence, electronics, petro-chemicals, medicine, etc. 11.10.6 laser heat treatment Laser Heat Treatment Laser heat treatment is initially validated for surface hardening. However in recent years, it is also used to change the metallurgical and mechanical properties of materials. Laser heat treatment is used for many practical applications like hardness, increase in strength, increases in fatigue life and surface hardening etc. A simple schematic representations used for surface hardening using laser source is Fig. 2.18. It consists of laser source, beam, analysis & guidance, gas shield cnc table and work piece. 11.34 Engineering Physics Laser Stimulation CCD CAMERA PC PID WORK PIECE CNC TABLE Fig. 11.32 Experimental arrangement for laser heat treatment The principle behind the hardening of materials like iron and steel is the temperature dependence of structural changes. For example, there are three different structures of iron namely body centered cubic (bcc) structure (a-iron) below 1085 K, a face centre cubic (fee) structure between 1085 K to 1667 K and body centered cubic (bcc) structure (g-iron) beyond 1667 K as a function of temperature. When the high intense of electromagnetic radiation of laser beam incident on a surface of a metal, it transfers heat to atoms in the surface of metal. When the laser beam incident on the metal, the photons from the laser beam interact with the free electrons and the materials and thereby raise their energy states to the conduction band. This effect is known as inverse Breamsstrahliing effect. Thus, the incident laser beam hardens the surface of the metal. The important parameters to be considered for the laser treatment are laser power, beam diameter, absorptivity of the surface and the transverse speed of laser. The laser beam generated bv the source is focused to fall on the work piece which is kept on CNC table. The whole process is controlled by the computer and the process is monitored by CCD camera. The PUD controller is used to fix the temperature of the laser operation to be performed on the work piece. Advantages The following are the advantages of the laser heat treatment in surface hardening. It is a computer controlled process method. It is very simple to control the reaction depth. It does not require any post process. « It is free from contamination. It is also used to clad the nanomaterials. 11.10.7 laser cutting Laser cutting is carried out using the laser machining process. The principle behind laser machining is a thermal mechanical process. During the thermal mechanical process, the material is removed either by phase changes like melting and vapourization or hydrothermal motions. Working The schematic diagram for laser cutting assisted by a gas jet is shown in Fig, 2.19. It consist of nozzle with laser beam focusing system and gas inlet to assist the laser cutting. The high intense laser beam is focused through the lens L via the nozzle. In order to provide the necessary mechanical force to eject the melt from the  Laser 11.35 cut nozzle, gas is circulated through the nozzle. The passed gas is also used to cool the cut zone by forced convection process. The nozzle of the laser cutting kit is brought to the top surface of the workpiece. The assisting gas used for laser welding develops an additional exothrmic energy through chemical reactions. The chemical reaction takes place between assist gas and the molten material. The additional energy is more useful to cut thick piece. On the other hand, one can also use inert gas in order to reduce the oxidation during laser machining. The important points to be considered for laser cutting are nozzle design, jet alignment, and effect of pressure and gas purity. LASER BEAM G L GAS Nozzle GAS JET Work-piece Fig. 11.33 Laser cutting Advantages The following are the advantages of gas cutting It is used to cut materials of any thickness with high precision It is very fast and accurate. It is very simple and cost effective. Disadvantages The efficiency and the overall cutting quality of laser welding strongly depend on the interaction of gas with the workpiece. 11.10.8 medicine The following are the some of the applications of laser in medicine. (i) It is used to perform microsurgery and bloodless operations, to cure cancer and skin tumours in human beings and animals. (ii) It is used for the treatment of a detached retina. (iii) With the help of optical fibers, the argon ion laser is used to control gastric haemorrage. The block diagram of the experimental set-up namely, laser endoscopy used for this purpose is shown in Fig. 11.32. 11.36 Engineering Physics S Power supply Partial beam splitter Power meter and heatsing Lens Micro- positioner Firing control G and timing unit E Fig. 11.34 Laser endoscopy The high-energy (13 W ) argon ion laser (S) is used as an optical source and an endoscope (E) is used as the delivery unit. The argon ion laser output is delivered into the required spot of the gastric bleeding using fiber optic endoscopes which are connected through the lens system using encapsulated quartz fibre guide (G), as shown in Fig. 11.32. The laser beam can be moved into any direction and position with the help of the optical flexible fibre guide (G). 11.10.9 holography Holography was invented by Dennis Gabour in 1948 with an idea to improve the resolution of the images obtained from an electron microscope. The method of producing three dimensional image of an object due to the interference phenomena of coherent light waves on a photographic plate is known as holography. The term holo means whole or complete, i.e., the recorded interference pattern gives all the information about an object. A hologram has no resemblance to the original object; however, it gives all information about the object through optical codes. The salient feature of holography is the three dimensional nature of the images produced, whereas in photography, two dimensional images are produced. (i) Basic principle When an object is illuminated by light from a coherent source, each point on the object acts as a source of secondary waves. These secondary waves spread out in all directions and it characterises the object through an interference pattern. This interference pattern is recorded using a suitable method. The recorded interference pattern is again illuminated with coherent light from another source. This results in reproduction of the original object in three dimensions. Let us consider an object AB illuminated by light waves from a coherent source as shown in Fig. 11.33. As a result, each point on the object results in a spherical waves  Laser 11.37 with coherent nature. These spherical waves interfere with the reference beam and hence produces interference or diffraction pattern at the plane of the photographic plate P. The pattern is developed and it contains light and dark partially absorbing fringes and this pattern is the characteristic of the object. This pattern is called as Gabour Zone plate. P A B Fig 11.35 Principle of holography (ii) Production of a Hologram Photographic plates are considered the best medium to record holograms. Recording depends only on intensity variation and not on amplitude variation. Therefore, in order to convert the amplitude variation into intensity variation, Gabour superimposed the reflected waves with a reference beam having the same wavelength. As a result, constructive and destructive interference fringes were obtained between the reflected waves and the reference beam. Thus, an increase in intensity at the constructive point and a decrease in intensity at the destructive point were noted. This change of intensity was recorded on the photographic plate. In order to produce a high-quality hologram, the light source used must be highly monochromatic and coherent. Monochromatic laser beams are used to produce an intense beam of coherent light. Thus, the spatially coherent laser beam is divided into wavefronts or amplitudes. One part of the beam falls directly on the photographic plate and the other falls on the object whose image is to be recorded. The experimental arrangement for the production of a hologram using the laser beam is shown in Fig. 11.34. A laser beam from the source is made to fall on an optical device called the beam splitter. A part of the beam from the beam splitter is made to fall on the silver coated mirror M1. After reflecting from the mirror M1, the reference beam is passed through the lens L1 and finally falls on the photographic plate. The focal length of lens L1 is smaller than that of the lens L2. The other part of the beam from the beam splitter is made to fall on the mirror M2 through the lens L2 of larger focal length. The beam is reflected from the mirror M2 and is made to fall on the object AB. The reflected waves from the surface of the object AB are also made to fall on the photographic plate. Thus, the interference pattern of 11.38 Engineering Physics the reflected beam and reference beam is formed. The resultant pattern (hologram) is recorded on the photographic plate. The hologram of the object can be made by developing the plate using standard techniques. Reference beam Photographic plate M1 L1 A Object Object beam B Laser Beam splitter L2 M2 Fig. 11.36 Experimental arrangement for production