THETOPPERSWAY.COM. Laser System. Principle of Lasers. Spontaneous Emission and Stimulated Emission. Page 1

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1 LASER is the abbreviation of Light Amplification by the Stimulated Emission of Radiation. It is a device that creates a narrow and low-divergent beam of coherent light, while most other light sources emit incoherent light, which has a phase that varies randomly with time and position. Most lasers emit nearly "monochromatic" light with a narrow wavelength spectrum. Fig.2.1 is the spectrum of a helium neon laser, showing very high spectra purity. Fig 2.1 Spectrum of a helium neon laser Principle of Lasers The principle of a laser is based on three separate features: a) stimulated emission within an amplifying medium, b) population inversion of electronics and c) an optical resonator. Spontaneous Emission and Stimulated Emission According to the quantum mechanics, an electron within an atom or lattice can have only certain values of energy, or energy levels. There are many energy levels that an electron can occupy, but here we will only consider two. If an electron is in the excited state with the energy E 2 it may spontaneously decay to the ground state, with energy E 1, releasing the difference in energy between the two states as a photon. This process is called spontaneous emission, producing fluorescent light. The phase and direction of the photon in spontaneous emission are completely random due to Uncertainty Principle. The angular frequency ω and energy of the photon is: where ћ is the reduced plank constant. Conversely, a photon with a particular frequency satisfying above equation would be absorbed by an electron in the ground state. The electron remains in this excited state for a period of time typically less than 10-6 second. Then it returns to the lower state spontaneously by a photon or a phonon. These common processes of absorption and spontaneous emission cannot give rise to the amplification of light. The best that can be achieved is that for every photon absorbed, another is emitted. Alternatively, if the excited-state atom is perturbed by the electric field of a photon with frequency ω, it may release a second photon of the same frequency, in phase with the first photon. The atom will again decay into the ground state. This process is known as stimulated emission. Page 1

2 The emitted photon is identical to the stimulating photon with the same frequency, polarization, and direction of propagation. And there is a fixed phase relationship between light radiated from different atoms. The photons, as a result, are totally coherent. This is the critical property that allows optical amplification to take place. All the three processes occur simultaneously within a medium. However, in thermal equilibrium, stimulated emission does not account to a significant extent. The reason is there are far more electrons in the ground state than in the excited states. And the rates of absorption and emission is proportional the number of electrons in ground state and excited states, respectively. So absorption process dominates. Population Inversion of the Gain Medium If the higher energy state has a greater population than the lower energy state, then the light in the system undergoes a net increase in intensity and this is called population inversion. But this process cannot be achieved by only two states, because the electrons will eventually reach equilibrium with the de-exciting processes of spontaneous and stimulated emission. Instead, an indirect way is adopted, with three energy levels (E 1 <E 2 <E 3 ) and energy population N 1, N 2 and N 3 respectively (Fig.2.2a). Initially, the system is at thermal equilibrium, and the majority of electrons stay in the ground state. Then external energy is provided to excite them to level 3, referred as pumping. The source of pumping energy varies with different laser medium, such as electrical discharge and chemical reaction, etc. Fig.2.2 Electron Transitions within (a) 3-level gain medium; and (b) 4-level gain medium In a medium suitable for laser operation, we require these excited atoms to quickly decay to level 2, transferring the energy to the phonons of the lattice of the host material. This wouldn t generate a photon, and labeled as R, meaning radiation-less. Then electrons on level 2 will decay by spontaneous emission to level 1, labeled as L, meaning laser. If the life time of L is much longer than that of R, the population of the E 3 will be essentially zero and a population of excited state atoms will accumulate in level 2. When level 2 hosts over half of the total electrons, a population inversion be achieved. Because half of the electrons must be excited, the pump system needs to be very strong. This makes three-level lasers rather inefficient. Most of the present lasers are 4-level lasers, see Fig.2.2b. The population of level 2 and 4 are 0 and Page 2

3 electrons just accumulate in level 3. Laser transition takes place between level 3 and 2, so the population is easily inverted. In semiconductor lasers, where there are no discrete energy levels, a pump beam with energy slightly above the band gap energy can excite electrons into a higher state in the conduction band, from where they quickly decay to states near the bottom of the conduction band. At the same time, the holes generated in the valence band move to the top of the valence band. Electrons in the conduction band can then recombine with these holes emitting photons with energy near the band gap energy (see Fig. 2.3). Fig. 2.3 Diagram of electron transitions of semiconductor gain medium Optical Resonator Although with a population inversion we have the ability to amplify a signal via stimulated emission, the overall single-pass gain is quite small, and most of the excited atoms in the population emit spontaneously and do not contribute to the overall output. Then the resonator is applied to make a positive feedback mechanism. An optical resonator usually has two flat or concave mirrors, one on either end, that reflect lasing photons back and forth so that stimulated emission continues to build up more and more laser light. Photons produced by spontaneous decay in other directions are off axis so that they won t be amplified to compete with stimulated emission on axis. The "back" mirror is made as close to 100% reflective as possible, while the "front" mirror typically is made only 95-99% reflective so that the rest of the light is transmitted by this mirror and leaks out to make up the actual laser beam outside the laser device. More importantly, there may be many laser transitions contribute in the laser, because of the band in solids or molecule energy levels of organics. Optical resonator also has a function of wavelength selector. It just makes a standing wave condition for the photons: Where L is the length of resonator, n is some integer and λ is the wavelength. Only wavelengths satisfying above equation will get resonated and amplified. Page 3

4 Summary of Principles and Modes of Operation Fig. 2.4 is the schematic diagram of the working process of lasers. The output of a laser may be a continuous constant-amplitude output (known as CW or continuous wave); or pulsed, by using the techniques of Q- switching, model-locking, or gainswitching. In many applications of pulsed lasers, one aims to deposit as much energy as possible at a given place in as short time as possible. Some dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for generating extremely short pulses of light, on the order of a few femto-seconds (10-15 s). The peak power of pulsed laser can achieve Watts. Page 4

5 Types of Lasers and Applications According to the gain material, lasers can be divided into the following types. Solid State Lasers: Laser Gain Medium Operation Wavelength(s) Pump Source Applications and Notes Ruby laser 694.3nm Flash Lamp Holography, tattoo removal. The first type of visible light laser invented; May Nd:YAG laser μm, (1.32 μm) Flash Lamp, Laser Diode Material processing, laser target designation, surgery, research, pumping other lasers. One of the most common high power lasers. Erbium doped glass lasers μm Laser diode um doped fibers are commonly used as optical amplifiers for telecommunications. F-center laser Mid infrared to far infrared Electrical current Research Gas Lasers: Laser Gain Medium Operation Wavelength(s) Pump Source Applications and Notes He-Ne laser 632.8nm Electrical discharge Interferometry, holography, spectroscopy, barcode scanning, alignment, optical demonstrations Argon laser nm, nm, nm Electrical discharge Retinal phototherapy (for diabetes), lithography, confocal microscopy, spectroscopy pumping other lasers CO 2 laser 10.6 μm, (9.4 μm) Electrical discharge Material processing (cutting, welding, etc.), surgery Excimer laser 193 nm (ArF), 248 nm (KrF), 308 nm (XeCl), 353 nm (XeF) Excimer recombination via electrical discharge Ultraviolet lithography for semiconductor manufacturing, laser surgery Page 5

6 Liquid lasers: Laser Gain Medium Operation Wavelength(s) Pump Source Applications and Notes Dye lasers Depending on materials, usually a broad spectrum Other laser, flash lamp Research, spectroscopy, birthmark removal, isotope separation. Metal-vapor Lasers: Laser Gain Medium Operation Wavelength(s) Pump Source Applications and Notes Helium-cadmium (HeCd) metal-vapor laser nm, 325 nm Electrical discharge in metal vapor mixed with helium buffer gas. Printing and typesetting applications, fluorescence excitation examination ( in U.S. paper currency printing) Copper vapor laser nm, nm Electrical discharge Dermatological uses, high speed photography, pump for dye lasers Laser Rate Equations In this chapter, We discuss a two-level system and show that it is not possible to achieve population inversion in steady state in a two-level system. Three-level and four-level laser systems and obtain the dependence of inversion on the pump power. We obtain the variation of laser power around threshold showing the sudden increase in the output power as a function of pumping. This is a very characteristic behavior of a laser. If we observe the spectrum of the radiation due to the spontaneous emission from a collection of atoms, one finds that the radiation is not strictly monochromatic but is spread over a certain frequency range. This implies that energy levels have widths and the atoms can interact with radiation over a range of frequencies but the strength of interaction is a function of frequency. This function that describes the frequency dependence is called the line-shape function and is represented by. Fig. 2.5 (a) Because of the finite lifetime of a state each state has a certain width so that the atom can absorb or emit radiation over a range of frequencies. The corresponding line-shape is shown in (b) Page 6

7 The function is usually normalized according to From the above we may say that out of the total N2 and N1 atoms per unit volume, only and atoms per unit volume will be capable of interacting with radiation of frequency lying between and. Hence the total number of stimulated emissions per unit time per unit volume will now be given by where we have We now consider two specific cases. Fig. 2.6 (a) Atoms characterized by the line-shape function g(ω) interacting with broadband radiation. (b) Atoms interacting with near-monochromatic radiation 1) If the atoms are interacting with radiation whose spectrum is very broad compared to that of, then one may assume that over the region of integration where is appreciable is essentially constant and thus may be taken out of the integral. Using the normalization integral, becomes This equation represents the rate of stimulated emission per unit volume when the atom interacts with broadband radiation. 2) We now consider the other extreme case in which the atom is interacting with near monochromatic radiation. If the frequency of the incident radiation is then the curve will be extremely sharply peaked at as compare to and thus can be taken out of the integral to obtain Page 7

8 Where is the energy density of the incident near-monochromatic radiation. It may be noted that has dimensions of energy per unit volume unlike which has the dimensions of energy per unit volume per unit frequency interval. Thus when the atom described by a line-shape function interacts with near monochromatic radiation at frequency, the stimulated emission rate per unit volume is given by In a similar manner, the number of stimulated absorptions per unit time per unit volume will be Two-Level System We first consider a two-level system consisting of energy levels E 1 and E 2 with N 1 and N 2 atoms per unit volume, respectively. Let radiation at frequency ω with energy density u be incident on the system. The number of atoms per unit volume which absorbs the radiation and is excited to the upper level will be Where The number of atoms undergoing stimulated emissions from E 2 to E 1 per unit volume per unit time will be where we have used the fact that the absorption probability is the same as the stimulated emission probability. In addition to the above two transitions, atoms in the level E 2 would also undergo spontaneous transitions from E 2 to E 1. If A 21 and S 21 represent the radiative and nonradiative transition rates from E 2 to E 1, then the number of atoms undergoing spontaneous transitions per unit time per unit volume from E 2 to E 1 will be T 21 N 2 where T 21 = A 21 + S 21 Thus we may write the rate of change of population of energy levels E 2 and E 1 as, As can be seen from above Equations. = a constant = N Page 8

9 Fig. 2.7 A two-level system which is nothing but the fact that the total number of atoms N per unit volume is constant. At steady state which gives us Since both W 12 and T 21 are positive quantities, above shows us that we can never obtain a steady-state population inversion by optical pumping between just two levels. Let us now have a look at the population difference between the two levels. From above equation. We have or if we write ΔN = N2 N1, we have In order to put above Eq. in a slightly different form, we first assume that the transition from 2 to 1 is mostly radiative, i.e., A 21 S 21 and T 21 A 21. Three-Level Laser System In the last section we saw that one cannot create a steady-state population inversion between two levels just by using pumping between these levels. Thus in order to produce a steady-state population inversion, one makes use of either a three-level or a four-level system. In this section we shall discuss a three-level system. We consider a three-level system consisting of energy levels E 1, E 2, and E 3 all of which are assumed to be non-degenerate. Let N 1, N 2, and N 3 represent the population densities of the three levels. The pump is assumed to lift atoms from level 1 to level 3 from which they decay rapidly to level 2 through some non-radiative process. Thus the pump effectively transfers atoms from the ground level 1 to the excited level 2 which is now the upper laser level; the lower laser level being the ground state 1. If the relaxation from level 3 to level 2 is very fast, then the atoms will relax down to level 2 rather than to level 1. Since the upper level 3 is not a Page 9

10 laser level, it can be a broad level (or a group of broad levels) so that a broadband light source may be efficiently used as a pump source. Fig. 2.8 A three-level system. The pump excites the atoms from level E 1 to level E 3 from where the atoms undergo a fast decay to level E 2. The laser action takes place between levels E 2 and E 1 If we assume that transitions take place only between these three levels then we may write N = N 1 + N 2 + N 3 where N represents the total number of atoms per unit volume. We may now write the rate equations describing the rate of change of N 1, N 2 and N 3. For example, the rate of change of N 3 may be written as where W P is the rate of pumping per atom from level 1 to level 3 which depends on the pump intensity. The first term in above Eq. represents stimulated transitions between levels 1 and 3 and T 32 N 3 represents the spontaneous transition from level 3 to level 2: A 32 and S 32 correspond, respectively, to the radiative and nonradioactive transition rates between levels 3 and 2. In writing above Eq. we have neglected T 31 N 3 which corresponds to spontaneous transitions between level 3 and 1 since most atoms raised to level 3 are assumed to make transitions to level 2 rather than to level 1. In a similar manner, we may write and Page 10

11 Where ω ω represents the stimulated transition rate per atom between levels 1 and 2, I 1 is the intensity of the radiation in the 2 1 transition and g(ω) represents the line shape function describing the transitions between levels 1 and 2. Further, T 21 = A 21 + S 21 with A 21 and S 21 representing the radiative and non radiative relaxation rates between levels 1 and 2. For efficient laser action since the transition must be mostly radiative, we shall assume A 21 >> S 21. At steady state we must have From above Equation we obtain Using above Equations we get From the above equation, one may see that in order to obtain population inversion between levels 2 and 1, (N 2 N 1 ) to be positive, a necessary (but not sufficient) condition is that Since the lifetimes of levels 3 and 2 are inversely proportional to the relaxation rates, according to above Eq., the lifetime of level 3 must be smaller than that of level 2 for attainment of population inversion between levels 1 and 2. If is satisfied then according to above Eq. there is a minimum pumping rate ( ) required to achieve population inversion which is given by If, and under the same approximation, above Equation becomes Below the threshold for laser oscillation, W1 is very small and hence we may write Page 11

12 Thus when W 1 is small, i.e., when the intensity of the radiation corresponding to the laser transition is small, then the population inversion is independent of I 1 and there is an exponential amplification of the beam. As the laser starts oscillating, W1 becomes large and from above Eq. we see that this reduces the inversion N 2 N 1 which in turn reduces the amplification. When the laser oscillates under steady-state conditions, the intensity of the radiation at the laser transition increases to such a value that the value of (N 2 N 1 ) is the same as the threshold value. If T 32 is very large then there will be very few atoms residing in level 3. Consequently, we may write We get The threshold value of required to start laser oscillation is also approximately equal to T 21 Now the number of atoms being pumped per unit time per unit volume from level 1 to level 3 is. If represents the average pump frequency corresponding to excitation to E 3 from E 1, then the power required per unit volume will be Thus the threshold pump power for laser oscillation is given by Since and,. Also assuming the transition from level 2 to level 1 to be mainly radiative (A 21 >>S 21 ), We have The Four-Level Laser System In the last section we found that since the lower laser was the ground level, one has to lift more than 50% of the atoms in the ground level in order to obtain population inversion. This problem can be overcome by using another level of the atomic system and having the lower laser level also as an excited level. The four-level laser system is shown in Fig 2.9 Level 1 is the ground level and levels 2, 3, and 4 are excited levels of the system. Atoms from level 1 are pumped to level 4 from where they make a fast non-radiative relaxation to level 3. Level 3 which corresponds to the upper laser level is usually a metastable level having a long lifetime. The transition from level 3 to level 2 forms the laser transition. In order that atoms do not accumulate in level 2 and hence destroy the population inversion between levels 3 and 2, level 2 must have a very small lifetime so that atoms from level 2 are quickly removed to level 1 ready for pumping to level 4. Page 12

13 Fig. 2.9 A four-level system; the pump lifts atoms from level E 1 to level E 4 from where they decay rapidly to level E 3 and laser emission takes place between levels E3 and E 2. Atoms drop down from level E 2 to level E 1 If the relaxation rate of atoms from level 2 to level 1 is faster than the rate of arrival of atoms to level 2 then one can obtain population inversion between levels 3 and 2 even for very small pump powers. Level 4 can be a collection of a large number of levels or a broad level. In such a case an optical pump source emitting over a broad range of frequencies can be used to pump atoms from level 1 to level 4 effectively. In addition, level 2 is required to be sufficiently above the ground level so that, at ordinary temperatures, level 2 is almost unpopulated. The population of level 2 can also be reduced by lowering the temperature of the system. We shall now write the rate equations corresponding to the populations of the four levels. Let N 1, N 2, N 3, and N 4 be the population densities of levels 1, 2, 3, and 4, respectively. The rate of change of N 4 can be written as where, as before, WpN 1 is the number of atoms being pumped per unit time per unit volume, WpN 4 is the stimulated emission rate per unit volume, is the relaxation rate from level 4 to level 3 and is the sum of the radiative (A 43 ) and nonradiative (S 43 ) rates. In writing above Eq. we have neglected (T 42 ) and (T 41 ) in comparison to (T 43 ), i.e., we have assumed that the atoms in level 4 relax to level 3 rather than to levels 2 and 1. Similarly, the rate equation for level 3 may be written as Where ω ω represents the stimulated transition rate per atom between levels 3 and 2 and the subscript 1 stands for laser transition; ω is the lineshape function describing the 3 2 transition and is the intensity of the radiation at the frequency ω Also Page 13

14 is the net spontaneous relaxation rate from level 3 to level 2 and consists of the radiative (A 32 ) and the non-radiative (S 32 ) contributions. Again we have neglected any spontaneous transition from level 3 to level 1. In a similar manner, we can write where is the spontaneous relaxation rate from 2 1. Under steady-state conditions We will thus get four simultaneous equations in N 1, N 2, N 3, and N 4 and in addition we have for the total number of atoms per unit volume in the system. From above Eq. we obtain, setting If the relaxation from level 4 to level 3 is very rapid then and hence. Using this approximation in the remaining three equations we can obtain for the population difference, Thus in order to be able to obtain population inversion between levels 3 and 2, we must have i.e., the spontaneous rate of de-excitation of level 2 to level 1 must be larger than the spontaneous rate of de-excitation of level 3 to level 2. If we now assume, then from above Eq. we obtain From the above equation we see that even for very small pump rates one can obtain population inversion between levels 3 and 2. This is contrary to what we found in a three-level system, where there was a minimum pump rate (, required to achieve inversion. The first factor in Page 14

15 above Eq. which is independent of W l gives the small signal gain coefficient whereas the second factor in above Eq. gives the saturation behavior. Just below threshold for laser oscillation, W l 0, and hence from above Eq. we obtain Where ΔN = N 3 N 2 is the population inversion density. We shall now consider two examples of four-level systems. Threshold pumping rate required to start laser oscillation Threshold pump power required per unit volume of the laser medium Why two level pumping schemes has no practical significance for lasing. By pumping action, we raise a large number of atoms from normal ground state to one of the excited states. Restoring to continue pumping, more and more atoms get excited till a stage is reached, when the population density of atoms in the lower level (N 1 ) and upper level (N 2 ) become same in quantity. While pumping, both absorption and stimulated emission are happening between these two energy levels. Because of these simultaneous transitions and due to the fact that the density of population becomes same, the rate of absorption becomes equal to stimulated emission. This is an equilibrium stage and no further increase in population in both the regions E1 and E2 is possible. This state is called Saturation State but we want to achieve population inversion, a situation where N2 is greater than N1, which is just impossible if the two energy levels participating in the laser operations. Yet another reasons, why two energy level not useful for lasing, because pumping radiation has a comparatively broader band of frequencies in two energy level. If the radiation has to be absorbed by the metastable upper level, which has to be narrowed, because we need a monochromatic transition, only a small quantity of the pumping radiation, corresponding to specific narrow band of frequency, will be absorbed. This will affect the efficiency of laser system. Why four levels is LASER more efficient than three level LASER In four level laser systems, the pumping of atoms is happen from the ground level E 1 to higher excited energy level E 4. There is a rapid non-radiative decay from energy level E 4 to E 3, therefore E 3 again being metastable or unstable. Only lasing action happen between E 3 and E 2. There is once again a fast non-radiative transition happen between E 2 and E 1. Page 15

16 We saw that the population inversion is achieved between E 3 and E 2 and we know that E 2 is not the ground level. Usually energy level E 2 is very much above the ground level E 1. However transition from E 4 E 3 and E 2 E 1 are much faster as compare to E 3 E 2. Hence it is easy to achieve population inversion with a four level system than with a three level system. The pumping power required in a four level system is also much less because unlike in a threelevel system, there is no need to excite more than half of atoms from the ground level to get a population inversion. This results in the efficiency of a four-level laser becoming generally much better than that of three-level systems. Since pumping is from E 1 and E 4 level and lasing action is between E 3 E 2 levels, it is simple to operate these lasers in CW mode. Examples of four level lasers: Nd-YAG and He-Ne. Types of Lasers In this chapter we shall discuss some specific laser systems and their important operating characteristics. The systems that we shall consider are some of the more important lasers that are in widespread use today for different applications. The lasers considered are (a) Solid-State Lasers: ruby, Nd:YAG, Nd:Glass; (b) Gas Lasers: He Ne, CO 2, Excimer Lasers and Argon ion; (c) Liquid Lasers: Dyes; Solid-State Lasers Material used for laser should have following features, first it should have strong absorption band and second The material must have high degree of quantum efficiency for fluorescent transits. The largest levels of excited metastable energy are available in these doped solid for long periods in order of millisecond. Therefore these lasers can be used for storage of energy and generation of high power. The spectral range of solid state laser from 0.6 micron to 2.5 micron and the density of ions of active impurity in crystal lasers is order of and ions per cm 3 which is larger than liquid and gas lasers. Crystal or glasses, which have these characteristics and doped with small amount of dopants. Dopant material such as Cr 3+ and Nd 3+ is doped in host atom ruby and YAG or Glass respectively. Such type of material is used in solid state lasers. Ruby Lasers The first laser to be operated successfully was the ruby laser which was fabricated by Maiman in Ruby, which is the lasing medium, consists of a matrix of aluminum oxide in which some of the aluminum ions are replaced by chromium ions. It is the energy levels of the chromium ions which take part in the lasing action. Typical concentrations of chromium ions are ~0.05% by weight. The energy level diagram of the chromium ion is shown in Fig As is evident from figure this a three-level laser. The pumping of the chromium ions is performed with the help of flash lamp (e.g., a xenon or krypton flash lamp) and the chromium ions in the ground state absorb radiation Page 16

17 around wavelengths of 5500 Å and 4000 Å and are excited to the levels marked E 1 and E 2. The chromium ions excited to these levels relax rapidly through a non-radiative transition (in a time s ) to the level marked M which is the upper laser level. The level M is a metastable level with a lifetime of 3 ms. Laser emission occurs between level M and the ground state G at an output wavelength of λ 0 = 6943 Å. Fig The energy levels the chromium ions in the ruby laser of The flash-lamp operation of the laser leads to a pulsed output of the laser. As soon as the flash-lamp stops operating the population of the upper level is depleted very rapidly and lasing action stops till the arrival of the next flash. Even during the short period of a few tens of microseconds in which the laser is oscillating, the output is a highly irregular function of time with the intensity having random amplitude fluctuations of varying duration. This is called laser spiking, the formation of which can be understood as follows: when the pump is turned on, the intensity of light at the laser transition is small and hence the pump builds up the inversion rapidly. Although under steady-state conditions the inversion cannot exceed the threshold inversion, on a transient basis it can go beyond the threshold value due to the absence of sufficient laser radiation in the cavity which causes stimulated emission. Thus the inversion goes beyond threshold when the radiation density in the cavity builds up rapidly. Since the inversion is greater than threshold, the radiation density goes beyond the steady-state value which in turn depletes the upper level population and reduces the inversion below threshold. This leads to an interruption of laser oscillation till the pump can again create an inversion beyond threshold. This cycle repeats itself to produce the characteristic spiking in lasers. Figure-2.11 shows a typical setup of a flash lamp pumped pulsed ruby laser. The helical flash lamp is surrounded by a cylindrical reflector to direct the pump light m onto the ruby rod efficiently. The ruby rod length is typically 2 20 cm with diameters of cm. As we have seen typical input electrical energies required are in the range of kj. Page 17

18 Fig A typical setup of a flash lamp pumped-pulsed ruby laser. The flash lamp is covered by a cylindrical reflector for efficient coupling of the pump light to the ruby rod Fig Elliptical pump Cavity in which the lamp and the ruby rod are placed along the foci of the elliptical cylindrical reflector In addition to the helical flash lamp pumping scheme, one may use other pumping schemes such as that shown in Fig-2.12 in which the pump lamp and the laser rod are placed along the foci of an elliptical cylindrical reflector. It is well known that the elliptical reflector focuses the light emerging from one focus into the other focus of the ellipse, thus leading to an efficient focusing of pump light on the laser rod. The absorption bands of ruby are very well matched with the emission spectra of practically available flash lamps so that an efficient use of the pump can be made. It also has a favorable combination of a long lifetime and a narrow line width. The ruby laser is also attractive from an application point of view since its output lies in the visible region where photographic emulsions and photo-detectors are much more sensitive than they are in the infrared region. Ruby lasers applications in holography, tattoo removing & in laser ranging. Advantage of Ruby Lasers It also has a favorable combination of a long lifetime and a narrow line width. The laser crystals, which in the form of rods so it can be grown with high degree of optical qualities. The crystal is hard and durable. It has good thermal conductivity. It is chemically very stable. Page 18

19 Neodymium-Based Lasers The Nd:YAG laser (YAG stands for yttrium aluminum garnet which is Y 3 Al 5 O 12 ) and the Nd:glass laser are two very important solid-state laser systems in which the energy levels of the neodymium ion take part in laser emission. They both correspond to a four-level laser. Using neodymium ions in a YAG or glass host has specific advantages and applications. (a) Since glass has an amorphous structure the fluorescent line-width of emission is very large leading to a high value of the laser threshold. On the other hand YAG is a crystalline material and the corresponding line-width is much smaller which implies much over thresholds for laser oscillation. (b) The fact that the line-width in the case of the glass host is much larger than in the case of the YAG host can be use in the production of ultra-short pulses using mode locking. The pulsewidth obtainable by mode locking is the inverse of the oscillating line-width. (c) The larger line-width in glass leads to a smaller amplification coefficient and thus the capability of storing a larger amount of energy before the occurrence of saturation. This is especially important in obtaining very high-energy pulses using Q-switching. (d) Other advantages of the glass host are the excellent optical quality and excellent uniformity of doping that can be obtained and also the range of glasses with different properties that can be used for solving specific design problems. (e) As compared to YAG, glass has a much lower thermal conductivity which may lead to induced birefringence and optical distortion. From the above discussion we can see that for continuous or very high pulse repetition rate operation the Nd:YAG laser will be preferred over Nd:Glass. On the other hand for high energypulsed operation, Nd:Glass lasers maybe preferred. In the following we discuss some specific characteristics of Nd:YAG and Nd:Glass laser systems. Nd:YAG Laser Laser operation of Nd:YAG was first demonstrated by J. E. Geusic at Bell Laboratories in The Nd:YAG laser is a four-level laser and the energy level diagram of the neodymium ion is shown in Fig Nd:YAG lasers typically emit light with a wavelength of λ 0 =1064 nm, in the infrared. However, there are also transitions near 940, 1120, 1320, and 1440 nm. Since the energy difference between the lower laser level and the ground level is 0.26 ev, the ratio of its population to that of the ground state at room temperature (T= 300 K) is achieve.. Thus the lower laser level is almost unpopulated and hence inversion is easy to The main pump bands for excitation of the neodymium ions are in the 0.8 and 0.73 μm wavelength regions and pumping is done using arc lamps (e.g., the Krypton arc lamp). Typical neodymium ion concentrations used are cm 3. The spontaneous lifetime Page 19

20 corresponding to the laser transition is 550 μs and the emission line corresponds to homogeneous broadening and has a width which corresponds to. the Nd:YAG laser has a much lower threshold of oscillation than a ruby laser. Fig Geometric construction of Nd:YAG Laser Fig The energy levels of Neodymium ion in the Nd:YAG laser With the availability of high-power compact and efficient semiconductor lasers, efficient pumping of Nd ions to upper laser level can be accomplished using laser diodes. This leads to very compact diode pumped Nd-based lasers. Diode laser pumping is simpler than lamp pumping and also produces much less heat in the laser medium leading to increased overall efficiency. Since the laser diode output is narrow band unlike a normal lamp, the output at 808 nm can be efficiently used for pumping. Typical output powers of 150 W are commercially available. In fact an intra-cavity second-harmonic generator can efficiently convert the laser wavelength to 532 nm (the second harmonic of 1064 nm of Nd:YAG) leading to very efficient green lasers. Page 20

21 Applications of Nd:YAG Medicine 1. Nd:YAG lasers emitting light at 1064 nm have been the most widely used laser for laser induced thermotherapy, in which malignant lesions in various organs are ablated by the beam. 2. Nd:YAG lasers can be used to remove skin cancers. They are also used to reduce benign thyroid nodules, and to destroy primary and secondary malignant liver lesions. 3. These lasers are also used extensively in the field of cosmetic medicine for laser hair removal and the treatment of minor vascular defects such as spider veins on the face and legs. Recently used for dissecting cellulitis, a rare skin disease usually occurring on the scalp. 4. Nd:YAG laser has been used for removal of uterine septa within the inside of the uterus Manufacturing 5. Nd:YAG lasers are used in manufacturing for engraving, etching, or marking a variety of metals and plastics, or for metal surface enhancement processes. 6. They are extensively used in manufacturing for cutting and welding steel, semiconductors and various alloys. 7. For automotive applications (cutting and welding steel) the power levels are typically 1 5 kw. 8. Nd:YAG lasers are also employed to make subsurface markings in transparent materials such as glass or acrylic glass. 9. In aerospace applications, they can be used to drill cooling holes for enhanced air flow/heat exhaust efficiency. 10. Laser peening typically uses high energy (10 to 40 Joule), 10 to 30 nanosecond pulse, flashed laser systems to generate gigawatts of power on the surface of a part by focusing the laser beam down to a few millimeters in diameter. Fluid dynamics 11. Nd:YAG lasers can also be used for flow visualization techniques in fluid dynamics (for example particle image velocimetry or laser-induced fluorescence). Military and defense 12. The Nd:YAG laser is the most common laser used in laser designators and laser rangefinders. Laser pumping 13. Nd:YAG lasers, mainly via their second and third harmonics, are widely used to excite dye lasers either in the liquid or solid state. Automotive 14. Researchers from Japan's National Institutes of Natural Sciences are developing laser igniters that use YAG chips to ignite fuel in an engine, in place of a spark plug. Laser-induced breakdown spectroscopy 15. A range of Nd:YAG lasers are used in analysis of elements in the periodic table. 16. A high-power Nd:YAG laser is focused onto the sample surface to produce plasma. Light from the plasma is captured by spectrometers and the characteristic spectra of each element can be identified. Page 21

22 Nd: Glass Laser The Nd: Glass laser is again a four-level laser system with a laser emission around 1.06 μm. Typical neodymium ion concentrations are cm 3 and various silicate and phosphate glasses are used as the host material. Since glass has an amorphous structure different neodymium ions situated at different sites have slightly different surroundings. This leads to an inhomogeneous broadening and the resultant line-width is Hz which corresponds to This width is much larger than in Nd: YAG lasers and consequently the threshold pump powers are also much higher. The spontaneous lifetime of the laser transition is 300 μs. Nd-doped fiber lasers are also efficient. But Nd: Glass lasers are more suitable for high energy-pulsed operation such as in laser fusion where the requirement is of sub nanosecond pulses with energy content of several kilojoules (i.e., peak powers of several tens of terawatts). Other applications are in welding or drilling operations requiring high pulse energies. Gas Lasers The most common and inexpensive gas laser, the helium-neon laser is usually constructed to operate in the red at nm. It can also be constructed to produce laser action in the green at nm and in the infrared at 1523 nm. The CO 2 laser used in medicine. He Ne Laser The first gas laser to be operated successfully was the He Ne laser. As we discussed earlier in solid-state lasers, the pumping is usually done using a flash-lamp or a continuous high-power lamp. Such a technique is efficient if the lasing system has broad absorption bands. In gas lasers since the atoms are characterized by sharp energy levels as compared to those in solids, one generally uses an electrical discharge to pump the atoms. Fig A typical He-Ne laser with external mirrors. The ends of the discharge tube are fitted with Brewster windows The He Ne laser consists of a long and narrow discharge tube (diameter ~ 2 8 mm and length cm) which is filled with helium and neon with typical pressures of 1 torr (1 torr = 1 mm Hg.) and 0.1 torr. The active medium is a mixture of 10 parts helium to one part neon. Page 22

23 The laser resonator may consist of either internal or external mirrors. If the resonator mirrors are placed outside the discharge tube then reflections from the ends of the discharge tube can be avoided by placing the windows at the Brewster angle. When an electrical discharge is created by applying a high voltage (~2-4 kv) across the gas, the electrons hit the helium atoms bringing them to the excited state and then transfer energy to the neon atoms when the energy of the excited states match. The excited helium atoms collide with neon atoms, exciting some of them to the state that radiates nm. The neon can then relax to laser transitions levels such 3s level to 3p levels and then 3p to 2s and 2p levels. Neon provides the energy levels for the laser transition. Helium provides an efficient excitation mechanism for the neon atoms. Without helium, the neon atoms would be excited mostly to lower excited states responsible for non-laser lines. A neon laser with no helium can be constructed but it is much more difficult without this means of energy coupling. Fig Energy levels of helium and neon taking part in the He Ne laser The helium atoms tend to accumulate at levels F 2 and F 3 due to their long lifetimes of 10-4 and s, respectively. Since the levels E 4 and E 6 of neon atoms have almost the same energy as F 2 and F 3, excited helium atoms colliding with neon atoms in the ground state can excite the neon atoms to E 4 and E 6. Since the pressure of helium is ten times that of neon, the levels E 4 and E 6 of neon are selectively populated as compared to other levels of neon. Transition between E 6 and E 3 produces the very popular 6328 Å line of the He Ne laser. Neon atoms de-excite through spontaneous emission from E 3 to E 2 (lifetime 10 8 s). Since this time is shorter than the lifetime of level E 6 (which is 10 7 s) one can achieve steady-state population inversion between E 6 and E 3. Level E 2 is metastable and thus tends to collect atoms. The atoms from this level relax back to the ground level mainly through collisions with the walls of the tube. Since E 2 is metastable it is possible for the atoms in this level to absorb the spontaneously emitted radiation in the E 3 E 2 transition to be re-excited to E 3. This will have the effect of reducing the inversion. It is for this reason that the gain in this laser transition is found to increase with decreasing tube diameter. Page 23

24 The other two important wavelengths from the He Ne laser are and μm, which correspond to the E 4 E 3 and E 6 E 5 transitions. It is interesting to observe that both 3.39 μm and nm transitions share the same upper laser level. Now since the 3.39 μm transition corresponds to a much lower frequency than the nm line, the Doppler broadening is much smaller at 3.39 μm and also since gain depends inversely on, the gain at 3.39 μm is much higher than at nm. Thus due to the very large gain, oscillations will normally tend to occur at 3.39 μm rather than at nm. Once the laser starts to oscillate at 3.39 μm, further build up of population in E 6 is not possible. The laser can be made to oscillate at 6328 Å by either using optical elements in the path which strongly absorb the 3.39 μm wavelength or increasing the line-width through the Zeeman Effect by applying an inhomogeneous magnetic field across the tube. Applications of He-Ne Lasers Red He-Ne lasers have many industrial and scientific uses. They are widely used in laboratory demonstrations in the field of optics in view of their relatively low cost and ease of operation compared to other visible lasers producing beams of similar quality in terms of spatial coherence (a single mode Gaussian beam) and long coherence length. A consumer application of the red He-Ne laser is the Laser Disc player, made by Pioneer. The laser is used in the device to read the optical disk. The CO 2 Laser The lasers discussed above use transitions among the various excited electronic states of an atom or an ion. In a CO 2 laser one uses the transitions occurring between different vibrational states of the carbon dioxide molecule. Figure 2.17 shows the carbon dioxide molecule consisting of a central carbon atom with two oxygen atoms attached one on either side. Such a molecule can vibrate in the three independent modes of vibration. Fig The three independent modes of vibration of the carbon dioxide molecule These correspond to the symmetric stretch, the bending, and the asymmetric stretch modes. Each of these modes is characterized by a definite frequency of vibration. According to basic quantum mechanics these vibrational degrees of freedom are quantized, i.e., when a molecule vibrates in any of the modes it can have only a discrete set of energies. Thus if we call the frequency corresponding to the symmetric stretch mode then the molecule can have energies of only when it vibrates in the symmetric stretch mode. Page 24

25 Thus the degree of excitation is characterized by the integer m when the carbon dioxide molecule vibrates in the symmetric stretch mode. In general, since the carbon dioxide molecule can vibrate in combination of the three modes the state of vibration can be described by three integers (mnq); the three integers correspond, respectively, to the degree of excitation in the symmetric stretch, bending, and asymmetric stretch modes, respectively. Figure-2.18 shows the various vibrational energy levels taking part in the laser transition. a Fig The low lying vibrational levels of nitrogen and carbon dioxide molecules. Energy transfer from excited nitrogen molecules to carbon dioxide molecules results in the excitation of carbon dioxide molecules. Important lasing transitions occur at 9.6 and 10.6 μm The laser transition at 10.6 μm occurs between the (001) and (100) levels of carbon dioxide. The excitation of the carbon dioxide molecules to the long-lived level (001) occurs both through collision transfer from nearly resonant excited nitrogen molecules and also from the cascading down of carbon dioxide molecules from higher energy levels. Characteristics of CO 2 laser The CO 2 laser produces a far infrared beam at 10.6 microns. It can also operate with an output of 9.6 microns but this output is not much common. The beam divergence of the CO 2 gas laser ranges from 1 to 10 milli radians. The beam width varies from 3mm for low power lasers up to 100mm for high powered lasers. The CO 2 laser can be operated in either CW or Pulse mode commonly. Continuous wave (CW) power output can ranges from few watts to over 15,000 watts. In pulsed mode peak power output can be in the millions of watts. The operating frequency for CO 2 laser is about 10% normally but theoretically it could be much higher. Construction of CO 2 gas laser The CO 2 gas laser can be constructed in the number of different configurations. In each configuration there is a specific manner in which the gas flows in the tube. Five important configuration of CO 2 laser constructions are 1. Sealed tube design 2. Coaxial flow design 3. Fast axial flow design 4. Transverse flow design 5. TEA design Page 25

26 Sealed Tube design of CO 2 laser In this design the gas is sealed in the glass tube. This laser configuration can be pumped either with DC or RF excitation methods. If DC excitation is used with the sealed tube CO2 laser then it is much like constructed as He-Ne gas laser, however it is necessary to have an elaborate cooling system with the laser. If RF (Radio frequency) excitation is used with sealed tube CO 2 laser then construction can be shown as below. The figure shows that pumping power is provided an RF power supply. The energy is coupled to the laser using a matching transformer, designed for maximum power transfer. The output from RF supply is coupled to a number of electrodes that are space around the laser tube. The RF energy is absorbed with very little temperature rise. TEA design Fig. 2.19: Diagram of sealed tube design of CO 2 Laser The TEA stands for Transverse Excitation at atmospheric pressure. This design is considered the variation of the transverse flow design. With this design the gas that flows around the inside of the tube is maintained at near atmospheric pressure. This high density gas allows extremely high power outputs, however it is very difficult to maintain ionization it the gas when the gas is at high pressure. Generally voltages required to maintain this type of operation are extremely high, that is why the TEA designed laser can only be operated in pulsed mode. Remember that beam diameter for this type of laser varies from 5 to 10mm and the beam divergence is typically 1 to 2 millimeter also efficiency varies from 1 to 10%. Working of CO 2 Laser The CO 2 gas laser uses a mixture of Helium, Nitrogen and carbon dioxide as the active medium. In this mixture the helium and nitrogen work in concert with the CO 2 to produce the desired lasing effect. Here the Nitrogen gas acts as buffer gas, just like the Helium did in He-Ne laser. The Nitrogen gas absorbs the pumping energy from the current flow in the gas and transfer the energy through collisions to the CO 2 molecule. Now when this CO 2 molecule drops from an excited level to another level then lasing occurs. Page 26

27 Fig 2.20: Energy level diagram for CO 2 laser During the process shown, the lower lasing level must remain with a low population. Therefore the molecule must be removed from the lower lasing level as soon as it has emitted its photon. This can be done by cooling the gas. But most cooling systems can not cool the gas quick enough to keep the lower lasing level depopulated. Therefore the helium gas is used in CO 2 lasers. When the helium gas enters the process then it removes energy from the CO 2 molecule by atomic collision and so reducing the molecules to the lower energy state. In CO 2 lasers, the active medium is not only CO 2 gas but also the Nitrogen & Helium exists there too. Here Nitrogen gas acts as a buffer gas. The nitrogen gas absorbs the pumping energy from current flow in gas and transfers this energy through collision to CO 2 molecules. So the CO 2 molecules reach to their excited Meta stable state. When they reach to Meta stable state and drops from this excited level then lasing occurs. In this way buffer gas is needed in CO 2 gas lasers. Power Efficiency: - The CO 2 laser possesses an extremely high efficiency of 30%. This is because of efficient pumping to the (001) level and also because all the energy levels involved are close to the ground level. Thus the atomic quantum efficiency which is the ratio of the energy difference corresponding to the laser transition to the energy difference of the pump transition, i.e is quite high ( 45%). Thus a large portion of the input power can be converted into useful laser power. Output powers of several watts to several kilowatts can be obtained from CO 2 lasers. Applications of CO 2 Laser High-power CO 2 lasers find applications in materials processing, welding, hole drilling, cutting, etc., because of their very high output power. The atmospheric attenuation is low at 10.6 μm which leads to some applications of CO 2 lasers in open air communications. It is very useful in surgical procedures because water absorbs this frequency of light very well (which makes up most biological tissue). Some examples of medical uses Page 27

28 are laser surgery and skin resurfacing ("laser facelifts", which essentially consist of vaporizing the skin to promote collagen formation). Laser resurfacing is a treatment to facial wrinkles reduce and skin irregularities The common plastic poly (methyl methacrylate) (PMMA) absorbs IR light in the µm wavelength band, so CO 2 lasers have been used in recent years for fabricating microfluidic devices from it, with channel widths of a few hundred micrometers Because the atmosphere is quite transparent to infrared light, CO 2 lasers are also used for military range finding using LIDAR techniques The Soviet Polyus was designed to use a megawatt carbon-dioxide laser as an orbit to orbit weapon to destroy SDI satellites. Advantages of CO 2 Laser In CO2 laser high power levels are obtained ranges from few watts to watts. The CO2 gas lasers are more versatile lasers. The efficiency of CO2 gas lasers (i.e. 20% or higher) is beat than He-Ne. CO2 lasers have become the workhorse of the material processing industry because of their low cost (below $100 per watt). In some cases its divergence is small but is not true always. Long sealed-off lifetime of greater than 20,000 hours. Wide variety of output waveform formats. High absorption of its output wavelengths by many materials. Small size per watt of output power Disadvantages of CO 2 Laser Divergence of CO2 lasers approximately in all cases is greater than He-Ne. Usually the divergence is ranges from 1 to 10 milli radians. Beam width varies from 3mm to 100mm. Some CO2 lasers have the disadvantage of a short and thick optical cavity. Cooling system requirement in some configurations also a disadvantage. Its cost is comparatively high. Page 28

29 Excimer Laser The excimer laser was invented in 1970 by Nikolai Basov, V. A. Danilychev and Yu. M. Popov, at the Lebedev Physical Institute in Moscow, using a xenon dimer (Xe 2 ) excited by an electron beam to give stimulated emission at 172 nm wavelength. Excimer The term Excimer is short for 'Excited dimer'. An excimer is a short-lived dimeric or heterodimeric molecule formed from two species, at least one of which has completely filled valence shell by electrons (for example, noble gases). Heteronuclear molecules that have more than two species are also called Exciplex molecules (originally short for Excited complex). Table: List of Excimer and Exciplex Excimer Wavelength Relative Power (mw) F 2 * 157 nm Ar 2 * Kr 2 * Xe 2 * 126 nm 146 nm 172 & 175 nm ArF 193 nm 60 KrF 248 nm 100 XeF 351 nm 45 XeCl 308 nm 50 The wavelength output of an excimer laser can be changed simply by changing the gas mixture, the laser mirrors may have to be exchanged to obtain maximum output. Excimers are often diatomic and are composed of two atoms or molecules that would not bond if both were in the ground state. The lifetime of an excimer is very short, on the order of nanoseconds. Binding of a larger number of excited atoms form Rydberg matter clusters, the lifetime of which can exceed many seconds. Fig: 2.21 (a) Molecular orbitals Page 29

30 A typical ground-state molecule has electrons in the lowest possible energy levels. According to the Pauli principle, at most two electrons can occupy a given orbital, and if an orbital contains two electrons they must be in opposite spin states. In figure 2.21 (a) the highest occupied molecular orbital is called the HOMO and the lowest unoccupied molecular orbital is called the LUMO; the energy gap between these two states is known as the HOMO/LUMO gap. If the molecule absorbs light whose energy is larger than this gap, an electron in the HOMO may be excited to the LUMO. This is called the molecule's excited state. Excimers are only formed when one of the dimer components is in the excited state. When the excimer returns to the ground state, its components dissociate and often repel each other. The wavelength of an excimer's emission is longer (smaller energy) than that of the excited monomer's emission shown in Fig (b). Heterodimeric diatomic complexes involving a noble gas and a halide, such as xenon chloride, are common in the construction of excimer lasers, which are excimers most common application. Krypton fluoride laser (KrF laser) - KrF laser is type of exciplex laser is a deep ultraviolet laser which is commonly used in the production of semiconductor integrated circuits, industrial micromachining, and scientific research. A krypton fluoride laser absorbs energy from a source, causing the krypton gas to react with the fluorine gas producing krypton fluoride, a temporary complex, in an excited energy state: The complex can undergo spontaneous or stimulated emission, reducing its energy state to a metastable, but highly repulsive ground state. The ground state complex quickly dissociates into unbound atoms: The result is an exciplex laser that radiates energy at 248 nm, which lies in the near ultraviolet portion of the spectrum, corresponding with the energy difference between the ground state and the excited state of the complex. Argon fluoride laser (ArF laser) - An argon fluoride laser absorbs energy from a source, causing the argon gas to react with the fluorine gas producing argon monofluoride, a temporary complex, in an excited energy state: The complex can undergo spontaneous or stimulated emission, reducing its energy state to a metastable, but highly repulsive ground state. The ground state complex quickly dissociates into unbound atoms: The result is an exciplex laser that radiates energy at 193 nm, which lies in the far ultraviolet portion of the spectrum, corresponding with the energy difference of 6.4 electron volts between the ground state and the excited state of the complex. Page 30

31 Excimer Laser An excimer laser typically uses a combination of a noble gas (argon, krypton, or xenon) and a reactive gas (fluorine or chlorine). Under the appropriate conditions of electrical stimulation and high pressure, a pseudo-molecule called an excimer is created, which can only exist in an energized state and can give rise to laser light in the ultraviolet range. Fig Energy level diagram for an Excimer molecule Laser action in an excimer molecule occurs because it has a bound (associative) excited state, but a repulsive (dissociative) ground state. This is because noble gases such as xenon and krypton are highly inert and do not usually form chemical compounds. However, when in an excited state (induced by an electrical discharge or high-energy electron beams, which produce high energy pulses), they can form temporarily bound molecules with themselves (dimers) or with halogens (complexes) such as fluorine and chlorine. The excited compound can give up its excess energy by undergoing spontaneous or stimulated emission, resulting in a strongly repulsive ground state molecule which very quickly (on the order of a picoseconds) dissociates back into two unbound atoms. Eximer lasers are pulsed and the energy contained in a single optical pulse is measure in millijoules. Typical energy output from excimer lasers ranges from a few mj to 1000mJ. To obtain useful power from excimer lasers, the laser is pulsed at some number of pulse per second, this is known as the repetition rate and is specified in pulses per second (PPS) or Hertz (Hz). The average output power in Watts from an excimer laser is simply the product of the energy per pulse and the repetition rate divided by Av. Power (W) = Energy (mj) X Rep.Rate (Hz)/1000 The most important molecules for commercially available excimer lasers are ArF*, KrF*, XeCl*, XeF* where the * depicts an excited state of the molecule. These lasers are pumped either by an electric discharge or by an electron beam. As soon as excimer molecule formed in excited state, population inversion is formed because ground state population is nil. If we excited XY molecules, where X and Y refer to a rare-gas atom and Page 31

32 a halogen, respectively (e.g., X=Kr, Y=F). One is the ion ion recombination process in which ions X + and Y - combine in the presence of a third body (M) to produce an excited XY molecule. This three body recombination process requires large amount of buffer gas He and Ne at the pressure of 1 bar or more. At such high pressure active medium, the continuous discharge cannot sustain due to arc and spark formation therefore excimer laser can only be operated in the pulsed mode. Ar + e - Ar* + e - (Electron impact excitation) Ar + e - Ar + + 2e - (Penning ionization) F 2 + e - F + F - (Dissociative recombination) Ar + + F - + M ArF* + M (Recombination stabilised by a third body) Ar* + F 2 ArF* + F (Reaction) After lasing, the excimer dissociates back into two atoms ArF* Ar + F + hν F + F + M F 2 + M The reactions are given for the ArF excimer laser, the reactions for the KrF excimer laser are similar and the inert gas atom Ar can be substituted by Kr. The (248 nm) KrF excimer laser is one of the most efficient high-power ultraviolet lasers. (308 nm) XeCl is best suited for dye laser pumping and (193 nm) ArF attractive short wavelength for photochemistry. The population inversion process for this and other rare-gas monohalide lasers (e.g., ArF, XeF and XeCl) involves a relatively large number of reactions. Application of Excimer Lasers The most impressive property of excimer laser radiation is the large variety of emission wavelengths, which cover the entire ultraviolet spectral region. The shorter the wavelength, the higher the resolution that can be achieved in micro-projection and imaging, thereby opening a wide field of applications. Concurrent with the short wavelength is the high quantum energy. Short-wavelength photons are strongly absorbed by most materials and they can supply sufficient quantum energy to induce photochemical reactions and cause molecules to dissociate. Together with the high peak power available in a laser pulse, the bond-breaking capability of excimer laser radiation allows ablative evaporation that opens the door to micro-processing of many materials, ranging from soft biological tissue to hard diamond. The pulse duration is an important parameter. Typical excimer lasers emit pulses in the range of a few nanoseconds so that material processing can be frequently performed on the fly, can be applied to a continuous flow of components to be processed. In addition, thanks to the naturally broad line width, excimer lasers can be tailored to supply pulses in the femto-second range with extremely high peak power. This allows one to generate plasma that consists of electrons and hollow atoms, inner-shell ionized atoms that recombine via highly energetic transitions, thereby delivering radiation in the extreme ultraviolet or weak-x-ray region. This is surely one of the most advanced promising applications. Page 32

33 Excimer lasers are scalable to high pulse energies in the joule range, to high repetition rates of a few khz, to high average powers up to 1 kw, so that they can be conveniently adapted to specific industrial tasks. Compared to these facts, their drawbacks are the fairly broad line widths, the low degree of coherence and the non-continuity of the radiation. The ultraviolet light from an excimer laser is well absorbed by biological matter and organic compounds. Rather than burning or cutting material, the excimer laser adds enough energy to disrupt the molecular bonds of the surface tissue, which effectively disintegrates into the air in a tightly controlled manner through ablation rather than burning. Thus excimer lasers have the useful property that they can remove exceptionally fine layers of surface material with almost no heating or change to the remainder of the material which is left intact. These properties make excimer lasers well suited to precision micromachining organic material or delicate surgeries such as eye surgery LASIK and for dermatological treatment. In , Rangaswamy Srinivasan, Samuel Blum and James Wynne at IBM s T. J. Watson Research Center observed the effect of the ultraviolet excimer laser on biological materials. Intrigued, they investigated further, finding that the laser made clean, precise cuts that would be ideal for delicate surgeries. Excimer lasers are widely used in high-resolution photolithography machines, one of the critical technologies required for microelectronic chip manufacturing. Current state-of-the-art lithography tools use deep ultraviolet (DUV) light from the KrF and ArF excimer lasers with wavelengths of 248 and 193 nanometers, which has enabled transistor feature sizes to shrink below 45 nanometers. XeCl laser, as pump sources for tunable dye lasers, mainly to excite laser dyes emitting in the blue-green region of the spectrum. Precision processing of polymers, plastics, ceramics, semiconductor, glass, Teflon and diamond. Emission of excimer molecules is also used as a source of spontaneous ultraviolet light (excimer lamps). Dye Lasers Solids are very difficult to prepare with the requisite degree of optical homogeneity and they suffer permanent damage if overheated. Gases do not have these difficulties but they have a much smaller density of active atoms. The dye laser has the advantage that they are simple and they can be tuned over a significant wavelength range. One of the most widely used tunable lasers in the visible region is the organic dye laser. The dyes used in the lasers are organic substances which are dissolved in solvents such as water, ethyl alcohol, methanol, and ethylene glycol. These dyes exhibit strong and broad absorption and fluorescent spectra and because of this they can be made tunable. By choosing different dyes one can obtain tenability from 3000 Å to 1.2 μm. The levels taking part in the absorption and lasing correspond to the various vibrational sublevels of different electronic states of the dye molecule. Page 33

34 Figure-2.18 shows a typical energy level diagram of a dye in which S 0 is the ground state, S 1 is the first excited singlet state, and T 1, T 2 are the excited triplet states of the dye molecule. Each state consists of a large number of closely spaced vibrational and rotational sublevels. Because of strong interaction with the solvent, the closely spaced sublevels are collision broadened to such an extent that they almost form a continuum. Fig Typical energy level diagram of a dye molecule Rhodamine is a common laser dye with a cyanine type structure. It is dissolved in methanol. It has as broad tuning range ( nm). All dye lasers are optically pumped. Pulsed Operation of Lasers In many applications of lasers, one wishes to have a pulsed laser source. In principle it is possible to generate pulses of light from a continuously operating laser, but it would be even more efficient if the laser itself could be made to emit pulses of light. In this case the energy contained in the population inversion would be much more efficiently utilized. There are two standard techniques for the pulsed operation of a laser; these are Q-switching and mode locking. Q-switching is used to generate pulses of high energy but nominal pulse widths in the nanosecond regime. On the other hand mode locking produces ultra short pulses with smaller energy content. Q-Switching Imagine a laser cavity within which we have placed a shutter which can be opened and closed at will Let us assume that the shutter is closed (i.e., does not transmit) and we start to pump the amplifying medium. Since the shutter is closed, there is no feedback from the mirror and the laser beam does not build up. Since the pump is taking the atoms from the ground state and disposing them into the excited state and there is no stimulated emission the population inversion keeps on building up. This value could be much higher than the threshold inversion required for the same laser in the absence of the shutter. When the inversion is built to a reasonably high value, if we now open the shutter, then the spontaneous emission is now able to reflect from the mirror and pass back and forth through the amplifying medium. Since the population inversion has been built up to a large value, the gain provided by the medium in one round trip will be much more than the loss in one round trip and as such the power of the laser beam would grow very quickly with every passage. The growing laser beam consumes the Page 34

35 population inversion, which then decreases rapidly resulting in the decrease of power of the laser beam. Thus when the shutter is suddenly opened, a huge light pulse gets generated and this technique is referred to as Q-switching. High losses imply low Q while low losses imply high Q. Thus when the shutter is kept closed and suddenly opened, the Q of the cavity is suddenly increased from a very small value to a large value and hence the name Q-switching. For generating another pulse the medium would again need to be pumped while the shutter is kept closed and the process repeated again. Fig.2.24 A laser resonator with a shutter placed in front of one of the mirrors to achieve Q-switching Figure 2.26 shows schematically the time variation of the cavity loss, cavity Q, population inversion, and the output power. As shown in the figure an intense pulse is generated with the peak intensity appearing when the population inversion in the cavity is equal to the threshold value. Figure 2.25 shows a Q-switched pulse emitted from a neodymium YAG laser. The energy per pulse is 850 mj and the pulse width is about 6 ns. This corresponds to a peak power of about 140 MW. The pulse repetition frequency is 10 Hz, i.e., the laser emits 10 pulses per second. Using this phenomenon it is possible to generate extremely high power pulses for use in various applications such as cutting, drilling, or in nuclear fusion experiments. We will now write down rate equations corresponding to Q-switching and obtain the most important parameters such as peak power, total energy, and duration of the pulse. We shall consider only one mode of the laser resonator and shall examine the specific case of a three-level laser system such as that of ruby. Fig An acousto optically Q- switched output from an Nd:YAG laser. The average power of the pulse train is 15 W and the repetition frequency is 2 khz with the pulse duration of 97 ns. Page 35

36 Fig Schematic of the Variation of loss, Q value, population inversion, and the laser output power with time Techniques for Q-Switching As discussed, for Q-switching the feedback to the amplifying medium must be initially inhibited and when the inversion is well past the threshold inversion, the optical feedback must be restored very rapidly. In order to perform this, various devices are available which include mechanical movements of the mirror or shutters which can be electronically controlled. In comparison to mechanical rotation, electronically controlled shutters employing the electro optic or acousto optic effect can be extremely rapid. A schematic arrangement of Q- switching using the electro optic effect is shown in Fig The electro optic effect is the change in the birefringence of a material on application of an external electric field. Thus the electro optic modulator (EOM) shown in Fig could be a crystal which is such that in the absence of any applied electric field the crystal does not introduce any phase difference between two orthogonally polarized components traveling along the laser axis. On the other hand, if a voltage V 0 is applied across the crystal, then the crystal introduces a phase difference of between the orthogonal components, i.e., it behaves like a quarter wave plate. If we now consider the polarizer modulator mirror system, when there is no applied voltage, the state of Prepared By: Mr. Rakesh Kumar Page 36

37 polarization (SOP) of the light incident on the polarizer after reflection by the mirror is along the pass axis of the polarizer and thus corresponds to a high Q state. Fig A typical arrangement for achieving Q-switching using an electro optic modulator placed within the cavity When a voltage V 0 is applied the linearly polarized light on passage through the EOM becomes circularly polarized (say right circularly polarized). Reflection from the mirror converts this to left circularly polarized light and passage through the EOM the wave becomes linearly polarized but now polarized perpendicular to the pass axis of the polarizer. The polarizer does not allow this to pass through and this leads to essentially no feedback which corresponds to the low Q state. Hence Q-switching can be accomplished by first applying a voltage across the crystal and removing it at the instant of highest inversion in the cavity. Some important electro optic crystals used for Q-switching include potassium dihydrogen phosphate (KDP) and lithium niobate (LiNbO 3 ). An acousto optic Q-switch is based on the acousto optic effect. In the acousto optic effect a propagating acoustic wave in a medium creates a periodic refractive index modulation due to the periodic strain in the medium, and this periodic refractive index modulation leads to diffraction of a light wave interacting with it. The medium in the presence of the acoustic wave behaves like a phase grating. Thus if an acousto optic cell is placed inside the resonator, it can be used to deflect the light beam out of the cavity, thus leading to a low Q value. The Q can be switched to high value by switching off the acoustic wave. Q-switching can also be obtained by using a saturable absorber inside the laser cavity. In a saturable absorber (which essentially consists of an organic dye dissolved in an appropriate solvent) the absorption coefficient of the medium reduces with an increase in the incident intensity. This reduction in the absorption is caused by the saturation of a transition. In order to understand how a saturable absorber can be made to Q-switch, consider a laser resonator with the amplifying medium and the saturable absorber placed inside the cavity as shown in Fig.2.27 As the amplifying medium is pumped, the intensity level inside the cavity is initially low since the saturable absorber does not allow any feedback from the mirror M 2. As the pumping Page 37

38 increases, the intensity level inside the cavity increases which, in turn, starts to bleach the saturable absorber. This leads to an increase in feedback which gives rise to an increased intensity and so on. Thus the energy stored inside the medium is released in the form of a giant pulse leading to Q-switching. If the relaxation time of the absorber is short compared to the cavity transit time, the saturable absorber would simultaneously mode lock and Q-switch the laser. Fig A laser resonator with a saturable absorber placed inside the cavity for passive Q switching Mode Locking Q-switching produces very high energy pulses but the pulse durations are typically in the nanosecond regime. In order to produce ultra short pulses of durations in picoseconds or shorter, the technique most commonly used is mode locking. In order to understand mode locking let us first consider the formation of beats when two closely lying sound waves interfere with each other. In this case we hear beats due to the fact that the two sound waves (each of constant intensity) being of slightly different frequency will get into and out of phase periodically (see Fig. 2.29). When they are in phase then the two waves add constructively to produce a larger intensity. When they are out of phase, then they will destructively interfere to produce no sound. Hence in such a case we hear a waxing and waning of sound waves and call them as beats. Fig the top curve is obtained by adding the lower two sinusoidal variations and corresponds to beats as observed when two sound waves at closely lying frequencies interfere with each other Page 38

39 Mode locking is very similar to beating except that instead of just two waves now we are dealing with a large number of closely lying frequencies of light. Thus we expect beating between the waves; of course this beating will be in terms of intensity of light rather than intensity of sound. In order to understand mode locking, we first consider a laser oscillating in many frequencies simultaneously. Usually these waves at different frequencies are not correlated and oscillate almost independently of each other, i.e., there is no fixed-phase relationship between the different frequencies. In this case the output consists of a sum of these waves with no correlation among them. When this happens the output is almost the sum of the intensities of each individual mode and we get an output beam having random fluctuations in intensity. In Fig we have plotted the output intensity variation with time obtained as a sum of eight different equally spaced frequencies but with random phases. It can be seen that the output intensity varies randomly with time resembling noise. Fig The intensity variation obtained by adding eight equally spaced frequencies with random phases. The intensity variation is noise like Now, if we can lock the phases of each of the oscillating modes for example bring them all in phase at any time and maintain this phase relationship, then just like in the case of beats, once in a while the waves will have their crests and troughs coinciding to give a very large output and at other times the crests and troughs will not be overlapping and thus giving a much smaller intensity (see Fig. 2.31). In such a case the output from the laser would be a repetitive series of pulses of light and such a pulse train is called mode-locked pulse train and this phenomenon is called mode locking. Figure 2.32 shows the output intensity variation with time corresponding to the same set of frequencies as used to plot Fig. 2.23, but now the different waves have the same initial phase. In this case the output intensity consists of a periodic series of pulses with intensity levels much higher than obtained with random phases. The peak intensity in this case is higher than the average intensity in the earlier case by the number of modes beating with each other. Also the pulse width is inversely proportional to the number of frequencies. If the number of waves interfering becomes very large, e.g., a hundred or so, then the peak intensity can be very high and the pulse widths can be very small. Figure 2.33 shows the output mode-locked pulse train coming out of a titanium sapphire laser. Page 39

40 Fig Figure showing interference between four closely lying and equally spaced frequencies and which are in phase at the beginning and retain a constant phase relationship. Note that the waves add constructively periodically Such mode-locked pulse train can be very short in duration (in picoseconds) and have a number of applications. Mode locking is very similar to the case of diffraction of light from a grating. In this case the constructive interference among the waves diffracted from different slits appears at specific angles and at other angular positions, the waves almost cancel each other. The angular width of any of the diffracted order depends on the number of slits in the grating similar to the temporal width of the mode-locked pulse train depending on the number of modes that are locked in phase. Fig The output intensity variation for the same situation as in Fig but now the waves are having the same phase at a given instant of time and they maintain their phase relationship. Note that the resultant intensity peaks periodically Page 40

41 Fig Mode-locked pulse train from a titanium sapphire laser. Each division corresponds to 2 ns Mode Locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10 12 s) or femto-seconds (10 15 s). The basis of the technique is to induce a fixed-phase relationship between the longitudinal modes of the laser's resonant cavity. The laser is then said to be 'phase-locked' or 'mode-locked'. Interference between these modes causes the laser light to be produced as a train of pulses. Depending on the properties of the laser, these pulses may be of extremely brief duration, as short as a few femto-seconds. Methods for producing mode-locking in a laser may be classified as either 'active' or 'passive'. Active methods typically involve using an external signal to induce a modulation of the intra-cavity light. Passive methods do not use an external signal, but rely on placing some element into the laser cavity which causes self-modulation of the light. Active mode-locking The most common active mode-locking technique places a standing wave acousto-optic modulator into the laser cavity. When driven with an electrical signal, this produces a sinusoidal amplitude modulation of the light in the cavity. Considering this in the frequency domain, if a mode has optical frequency ν, and is amplitude-modulated at a frequency f, the resulting signal has sidebands at optical frequencies ν f and ν + f. If the modulator is driven at the same frequency as the cavity-mode spacing Δν, then these sidebands correspond to the two cavity modes adjacent to the original mode. Since the sidebands are driven in-phase, the central mode and the adjacent modes will be phase-locked together. Further operation of the modulator on the sidebands produces phase-locking of the ν 2f and ν + 2f modes, and so on until all modes in the gain bandwidth are locked. As said above, typical lasers are multi-mode and not seeded by a root mode. So multiple modes need to work out which phase to use. In a passive cavity with this locking applied there is no way to dump the entropy given by the original independent phases. This locking is better described as a coupling, leading to a complicated behavior and not clean pulses. The coupling is only dissipative because of the dissipative nature of the amplitude modulation. Otherwise, the phase modulation would not work. Page 41