Laser Principles PDF

Safia Ahmad LASERS Laser is one of the most important inventions of the last century. It finds application in various different sectors of day-to-day life. The word LASER is the acronym for Light Amplification by Stimulated Emission of Radiation. The theore...

Safia Ahmad LASERS Laser is one of the most important inventions of the last century. It finds application in various different sectors of day-to-day life. The word LASER is the acronym for Light Amplification by Stimulated Emission of Radiation. The theoretical basis for the develop- ment of laser was given by Albert Einstein in 1916, when he predicted the possibility of stimulated emission. In 1954, C. H. Townes and his colleagues developed the first device based on amplification of radiation by stimulated emission, called MASER, the Microwave Amplification by Stimulated Emission of Radiation. In 1958, A. Schawlow and C.H.Townes extended the principle of masers to light. And the first laser device was built in 1960 by T.H. Maiman. The helium - neon laser which is the first gas laser was built in 1961 by A. Javan and his associates. Interaction of Light with Matter When light travels through a medium, it undergoes absorption and scattering processes. Considering a material medium, composed of identical atoms. Atoms are characterized by many energy levels. However, for simplicity, assume that the atoms of the material medium is characterized by only two energy levels, E1 and E2. The energy level E1 corresponds to the ground state while the energy level E2 corresponds to the excited state. The number of atoms per unit volume at an energy level is called population density. Suppose, N1 is the population at energy level E1 while N2 is the population at energy level E2. Under normal conditions, higher the energy, lesser is its population as system tends to be in a lowest energy state. Thus. N1 > N2 Assuming that the radiation and material are in thermal equilibrium, suppose the light radiation (or a beam of photons) is incident on the material. Suppose the radiation density be ρ(ν) and that each photon carries energy hν = E2 − E1. If a photon of frequency ν is incident on the two - level system under consideration such 1  Safia Ahmad Figure 1: Absorption of photons. that, hν = E2 − E1 , the photon imparts its energy to the atom in the ground state i.e the incident photon is absorbed by the atom at energy level E1. And the atom at E1 will have enough energy to make a transition from ground state to the excited state at energy E2. This process is known as Absorption (of light). Rate of Absorption transition: Rate at which number of atoms residing at energy level E1 reduces with time. Thus, dN1 Rate of Absorption, Nab = − dt It depends on density of photons with frequency ν, ρ(ν) and the population at energy level E1. Therefore, Rate of Absorption is dN1 Nab = − = B12 N1 ρ(ν) (1) dt where, B12 constant of proportionality known as the Einstein coefficient for Stimulated Ab- sorption. The subscript (12) denotes that this process involves the transition from state 1 to state 2. 2  Safia Ahmad Figure 2: Spontaneous Emission of photons. Spontaneous Emission When an atom at lower energy level makes a transition to a higher energy level, it cannot stay in the excited state for a relatively longer time. The finite time during which the atom can stay in the excited state, is known as the lifetime, τ. After time, τ , which is about 10−8 s, the atom jumps back to the ground state by releasing a photon of energy, hν = E2 −E1. This is known as spontaneous emission because the photon is emitted on its own and without any external perturbation. As this process is happening spontaneously, therefore, it is called spontaneous emission. Rate of Spontaneous Emission: Rate at which number of atoms residing at energy level E2 reduces with time. dN2 Nsp = − dt Rate of spontaneous emission depends only on the population density at the excited state, N2 , i.e, dN2 Nsp = − = A21 N2 (2) dt where, A21 is a constant of proportionality known as Einstein coefficient for spontaneous emission. The subscript (21) denotes that this process involves the transition from state 2 to state 1. 3  Safia Ahmad Figure 3: Stimulated emission of photons. Stimulated Emission In 1917, Einstein predicted that the existence of equilibrium between matter and radiation dictates that there must be another radiation process called stimulated emission. For spon- taneous emission, radiation field was not required. Now, if the external radiation field is present, then it will trigger the decay of an atom from the excited state to the ground state. The photon with energy hν = E2 − E1 interacts with an atom in the excited state and stimulate the excited atom to make a transition to the ground state before the atom could make a spontaneous transition. In the process, the atom emits the energy in the form of photon, hν = E2 − E1. The photon provided by the external radiation field is not affected while the excited atoms emits a photon. This process of creating a decay of an excited atom to the ground state by using another photon is known as stimulated emission. So, the phenomenon of forced photon emission by an excited atom due to the action of external agency is called stimulated emission. The radiation field is stimulating this process which is giving rise to a emission of photon and an initial photon which caused a stimulated process. So, for one photon interacting with the excited atom, there are two photons emerging. So, the outstanding feature of this process is the multiplication of photons. The photon stimulated or induced in this process has the same direction as that of stimulating 4  Safia Ahmad photon. The stimulated photon and the stimulating photon have identical features. They have the same frequency, phase and plane of polarization. Rate of Stimulated Emission: Rate at which number of atoms residing at energy level E2 reduces with time. Rate of stimulated emission, Nst , depends on the density of photons having frequency ν, ρ(ν), such that hν = E2 − E1 and the population density at the excited state, N2 , i.e. Thus, dN2 Rate of Stimulated Emission, Nst = − = B21 N2 ρ(ν) (3) dt where, B21 is a constant of proportionality known as the Einstein coefficient for Stimulated Emission. The subscript (21) denotes that this process involves the transition from state 2 to state 1. Principle of Detailed Balance If the matter is in thermal equilibrium with its surroundings, i.e. if it is at constant temper- ature, then it must emit and absorb the same amount of radiation per unit time. Because otherwise, its temperature would rise and fall. So, the principle of detailed balance says that for the system in thermal equilibrium, total rate of absorption must be equal to the total rate of emission, i.e. Total rate of absorption = Total rate of emission, Nab = Nsp + Nst B12 N1 ρ(ν) = A21 N2 + B21 N2 ρ(ν) (4) If we consider a medium in thermal equilibrium, there would be more atoms in the ground state than at excited state, i.e. N1 > N2. The incident photon is more likely to interact with atoms at lower energy level because N1 > N2. Thus, the photon travelling through the medium is more likely to get absorbed than stimulate an excited atom to emit a photon. Therefore, spontaneous emission dominates the stimulated emission at thermal equilibrium. 5  Safia Ahmad Einstein’s Coefficients To find the relation between Einstein’s coefficients, it is assumed that The matter and radiation are in thermal equilibrium which is to say that the matter emits as much radiation as it absorbs. The radiation is identical with blackbody radiation and therefore is consistent with Planck’s blackbody radiation spectrum for any value of temperature, T. The population densities N1 and N2 are distributed according to Boltzmann law in the energy levels E1 and E2 , respectively. The Planck’s blackbody radiation spectrum is given as 8πhν 3 1 ρ(ν, T ) = (5) c3 ehν/kB T − 1 From, Eq. (4), we have B12 N1 ρ(ν) = A21 N2 + B21 N2 ρ(ν) A21 N2 B21 N2 ρ(ν) = + ρ(ν) B12 N1 B12 N1 B21 N2 A21 N2 ρ(ν) − ρ(ν) = B N B12 N1  12 1  B21 N2 A21 N2 ρ(ν) 1 − = B12 N1 B12 N1 ! A21 N2 1 ρ(ν) = B12 N1 1 − B 21 N2 B12 N1 At equilibrium, as proposed by Boltzmann, N2 = e−∆E/kB T N1 Since, ∆E = E2 − E1 = hν N2 Therefore, = e−hν/kB T N1 Therefore, ! A21 −hν/kB T 1 ρ(ν) = e B21 −hν/kB T B12 1 − B12 e ! A21 1 ρ(ν) = B21 (6) B12 e−hν/kB T − B12 6  Safia Ahmad Comparing this with (5), we get A21 8πhν 3 = (7) B12 c3 B21 = 1 or B21 = B21 (8) B12 The above equations, (7) and (8) are known as the Einstein relations. The equality, B21 = B21 implies that in a two - level system, when an atom is placed in a radiation field, the probability for upward (i.e. absorption) transition is equal to the probability for downward (i.e. stimulated) transition. Light Amplification The laser operation is achieved when the rate of stimulated emission exceeds the rate of spontaneous emission and rate of absorption. Condition for Stimulated Emission to Dominate Spontaneous Emission is given as Stimulated transition B21 N2 ρ(ν) B21 ρ(ν) = = Spontaneous transition A21 N2 A21 This implies that stimulated transitions will dominate the spontaneous transitions if the photon density, ρ(ν) is very large. But, large photon density may also lead to large absorption transition. So, the large photon density does not ensure the dominance of stimulated emission. The above equation also indicates that stimulated emission will dominate over spontaneous emission if the ratio B21 /A21 is large i.e. B12 c3 = −→ very large (9) A21 8πhν 3 Condition for Stimulated Emission to Dominate Absorption: Stimulated transition B21 N2 ρ(ν) N2 = = Absorption transition B12 N1 ρ(ν) N1 where we have used the fact that, B21 = B12. Stimulated transition will dominate the absorption transition if the population density of excited state is larger than the population density of the ground state, i.e. N2 > N1. 7  Safia Ahmad Population Inversion At thermal equilibrium, the population densities are distributed by the Boltzmann law: N2 = e−(E2 −E1 )/kB T (10) N1 This implies that the population N1 at energy level E1 will always be much greater than the population N2 at excited level E2. For typical values of E1 and E2 , the population of N1 is 1030 times the population of N2. So, in equilibrium state or normal state, there are more atoms in the lower energy level and relatively lesser number of atoms in the higher energy level, i.e., under thermal equilibrium N1 ≫ N2. And therefore, a two level system cannot have population inversion and hence no laser action can take place in a two - level system. For stimulated emission to dominate over other radiative processes, it is required that N2 > N1. Which is to say that the population of energy levels E1 and E2 are required to be inverted such that the energy state at E2 has more atoms than the energy state at E1 i.e. N2 > N1 and this is known as population inversion. Metastable States An atom can be excited to a higher level by supplying energy to it. Generally, excited state has a very short lifetime of about 10−8 s and therefore atom that was being excited there will jump back to the lower energy level spontaneously by emitting photon. Thus, atoms do not stay for long enough time at the excited state. Even though the pumping agent continuously pump energy to excite the atoms to the higher energy level, they undergo spontaneous transition and quickly return back to the ground state. And therefore, N1 ≫ N2 and population inversion can not be attained under such circumstances. So, in order to achieve population inversion, if the excited atoms are somehow made to wait at the higher energy level without undergoing spontaneous emission till a large number of atoms build up at that level so that its population exceeds the population of lower energy level. That is to say, if one or more excited energy level are present whose lifetime is 10−3 s or more instead of a lifetime of 10−8 s so that the atoms can stay for a longer period of time in the excited 8  Safia Ahmad Figure 4: Components of a Laser energy level. Such relatively long-lived states are called metastable states. Metastable State: is an excited state where atoms remain excited for an appreciable time, which is of the order of 10−3 s. Therefore, the metastable states allow accumulation of a large number of atoms at that level. The population at metastable state can far exceed the population at lower level and estab- lish the condition of population inversion in the lasing medium. Without a metastable state, it is impossible to have population inversion and hence no laser action. Components of Laser The essential components of laser are as follows: Active medium Active medium is the material in which the laser action takes place i.e. an active medium is a medium which when excited creates population inversion and promotes stimulated emission leading to amplification of light. In a medium consisting of different species of atoms, only a small fraction of atoms of a particular type have energy level system suitable for achieving 9  Safia Ahmad population inversion. Such atoms can produce more stimulated emission than spontaneous emission and cause amplification of light. Those atoms which cause laser action are called active centers. The rest of the medium acts as host and support active centers. The medium hosting the active centers is known as active medium. The Pump For achieving and maintaining the population inversion, the energy needs to be supplied to the system to continuously excite the atoms in lower energy level to higher energy level. Thus, the pump is an external source that supplies energy needed to transfer the laser medium into the state of population inversion. Several pumping techniques are used: optical pumping, electrical discharge, etc. Optical Resonator Cavity Optical resonant cavity is formed by placing a pair of optically plane parallel mirrors which encloses the active medium. One of these mirrors is partially reflecting and the other is fully reflecting. Some of the excited atoms make a downward spontaneous transition to the lower energy level and emit photons. The spontaneous photons are emitted in every direction and each of these photons can trigger many stimulated emissions along their path. The photons which are emitted at some angle to the optical axis escape the optical cavity and are lost. The photons that are emitted spontaneously along the optical axis of the resonant cavity travel through the medium and trigger stimulated emissions. They are reflected by the end mirror and are thus fed back into the medium and travel toward the opposite end mirror causing more stimulated emissions. Which is to say that optical resonator selects the direction in which the light is to be amplified. Substantial light amplification takes place because the light is reflected several times at the mirror and gains strength in each passage. Ultimately, when the amplification balances the losses in the cavity, the laser beam emerges out from the front-end mirror. In the absence of resonator cavity, there would be no amplification of light. 10  Safia Ahmad Types of Lasers On the basis of material used, lasers are divided as follows: 1) Solid-state lasers. Examples: Ruby laser, Nd:YAG laser etc. 2) Gas lasers. Examples: Helium-Neon laser, CO2 laser etc. 3) Semiconductor diode laser. Examples: GaAs laser, InP laser etc. Ruby Laser It belongs to the class of solid-state laser. Ruby is a Aluminium oxide crystal i.e. Al2 O3 in which some of the Al3+ ions are replaced by Cr3+ ions which are responsible for the red color. It contains about 0.05% of Cr2 O3. A Cr3+ ion has a metastable state whose lifetime is about 0.003 s and therefore are the active centers. Chromium ions have absorption bands in the blue and green regions. It Uses three level pumping mechanism. Ruby crystal is taken in the form of cylindrical rod. The rod’s length is made precisely an integral multiple of half-wavelengths long i.e. L = mλ/2 so the radiation trapped in it forms an optical standing wave. Its ends are grounded and polished such that end faces are exactly parallel and are perpendicular to the axis of rod. The end faces of ruby are silvered so that they form the optical resonator. The rear face is made totally reflecting while the front face is partially reflecting. The laser rod is surrounded by a helical photographic flash lamp filled with xenon. The energy level structure of the Cr3+ ions is characterized by two absorption bands and a metastable state. The energy level structure of the Cr3+ ions in the crystal lattice are shown in figure 5. Whenever power supply is on, the xenon flash lamp produces flashes of white light and the Cr3+ ions are excited to the energy levels E3 and E3 ’ by the green and blue components of white light. Since the lifetime of these levels is much smaller ∼ 10−8 s, the Cr3+ ions undergo non - radiative transitions from these levels to metastable state, E2 by losing energy to other 11  Safia Ahmad Figure 5: Ruby Laser ions in the crystal. The metastable state, E2 of Cr3+ ions has a lifetime of about 0.003 s and therefore atoms can accumulate at metastable state and the population inversion is established. The metastable state E2 is the upper laser level while the ground state E1 is the lower laser level. As the excited Cr3+ ions almost immediately makes a downward non - radiative transition to the metastable state E2 , the upper laser level E2 will be rapidly populated. When more than half of Cr3+ ions accumulate at a metastable state, the state of population inversion is established between E2 and E1 levels. Since, Cr3+ ions at E2 can make a downward radiative transition, so after some time there will be spontaneous emissions, i.e. some of the Cr3+ ions make a transition from upper level to lower level spontaneously and emit photons. The spontaneous photons which are traveling along the axis of the ruby rod are reflected back by the mirrored end and stimulate other excited Cr3+ ions at metastable state to make a downward transition by emitting photons. That is, photons emitted by a spontaneous transition of excited Cr3+ ions trigger stimulated emissions. As these photons are reflected back and forth between the mirrored ends of the ruby rod, the stimulated emission sharply increases and the amplification of light takes place. After a few microseconds, large pulse of monochromatic, coherent red light of wavelength 694.3 nm emerges from the partly transparent end of the rod. Once stimulated transitions commence, the metastable state gets depopulated very soon i.e bunch of photons are emitted in a very short period of time and then the population 12  Safia Ahmad inversion disappears and lasing action ceases. Again, a flash of lamp will excite the Cr3+ ions to a metastable state and a population inversion is reestablished and the laser action takes place i.e. a pulse of red light is produced after each flash of the lamp. Therefore, the output of the laser is not continuous wave, but occurs in the form of pulses of microsecond duration. Helium-Neon Laser The helium - neon laser belongs to the class of gas lasers. It is made of a glass tube filled with the mixture of helium and neon gases in the ratio 10:1 at a low pressure of about 1 torr. Neon atoms are the active centers and have energy levels suitable for laser transitions while helium atoms help in exciting neon atoms. On the axis of the tube, parallel mirrors are arranged at both ends which forms the optical resonator. The distance between the mirrors is again equal to an integral multiple of half - wavelengths of the laser light, i.e. L = mλ/2 such that the resonator supports standing wave pattern. Figure 6: He - Ne Laser The most common method of exciting gas laser medium is by passing an electric discharge through the gas. When the power is switched on, high voltage applied across the gas mixture by means of electrodes connected outside the tube produces electric discharge in the gas. 13  Safia Ahmad The electrons from the discharge collide with helium and neon atoms thereby exciting them to the metastable states at 20.61 eV and 20.66 eV above the ground states, respectively. As the excited state is a metastable state whose lifetime is much larger, the excited helium atom cannot return to the ground state through spontaneous emission immediately. However, some of the excited helium atoms transfer their excess energy to ground state neon atoms through collisions and return to the ground state. The kinetic energy of He atoms provides the additional 0.05 eV required for excitation of neon atoms. The probability of energy transfer from He atoms to Ne atoms is more as there are 10 He atoms per Ne atom. Thus, the purpose of He atoms is to excite Ne atoms and to cause population inversion. The neon atoms accumulate in this metastable state. The energy level in Ne at 18.70 eV is sparsely populated at ordinary temperatures, a state of population inversion is established between metastable state at 20.66 eV and an excited state at 18.70 eV. Thus, the laser transition in Ne takes place between these two energy levels. Some of the Ne atoms at a metastable state at 20.66 eV make a spontaneous transition to an excited state at 18.70 eV emitting red color photons of wavelength 632.8 nm. They will interact with other atoms while travelling through a gas mixture and trigger stimulated emission of 632.8 nm red photons. Photons bounce back and forth between the end mirrors, causing more and more stimulated emission during each passage. The strength of the stimulated photons travelling along the axis of the optical cavity (discharge tube) builds up rapidly while the photons travelling at angles to the axis are lost. Thus, the transition from energy level at 20.66 eV to energy level at 18.70 eV generates a laser beam of wavelength 632.8 nm. From there, atoms make a spontaneous transition to a lower metastable state and this transition yields only incoherent light. As a metastable state has a longer lifetime, Ne atoms tend to accumulate at a lower metastable state. It is necessary that the atoms accumulating at lower metastable state are brought to the ground state quickly otherwise the number of atoms at ground state will go on diminishing and the laser will cease to function. The only way of bringing the atoms to the ground state is through collisions. Therefore to increase the probability of atomic collisions with the tube walls, the discharge tube is made narrow and Ne atoms return to the ground state through frequent collisions with the walls of the glass tube. 14  Safia Ahmad Because the electron impacts that excite the He and Ne atoms occur all the time, unlike the pulsed excitation from the xenon flash lamp in a ruby laser and the population inversion can be maintained, a He - Ne laser operates continuously. Properties of laser 1. Directionality: The conventional light sources emit light uniformly in all directions (isotropic). In case of laser, the active medium is in a cylindrical resonant cavity. Any light that is travelling in a direction other than parallel to the cavity axis is eliminated and only the light that is travelling parallel to the axis is selected and reinforced. The light propagating along the axial direction emerges from the cavity and becomes the laser beam. Thus, a laser beam emits light only in one direction. 2. Divergence: Light from conventional source spreads out in the form of spherical wavefronts and hence is highly divergent. On the other hand, laser light propagates in the form of plane waves. The light beam remains essentially as a bundle of parallel rays. The small divergence that exists is due to the diffraction of beam at the exit mirror. A typical value of divergence of He-Ne laser is 10−3 radians. It means that the diameter of laser beam increases by about 1 mm for every meter it travels. 3. Intensity: The intensity of light from a conventional source decreases rapidly with distance (I = Power per unit area) as it spreads out in space. Laser emits light in the form of narrow beam with its energy concentrated in a small region of space. Therefore the beam intensity would be very large and stays constant with distance. To achieve an energy density equal to that in some laser beams, a hot object would have to be at a temperature of 1030 K. 4. Coherence: In case of laser, a large number of identical photons are emitted through stimulated emissions and therefore will be in phase with each other. The resultant light exhibits a high degree of coherence. 5. Monochromaticity: If light coming from a source has only one frequency (single 15  Safia Ahmad wavelength) of oscillations, the light is said to be monochromatic and the source a monochromatic source. Light from conventional source spreads over a wavelength range of 10nm to 100nm. On the other hand, the light from lasers is highly monochromatic and contains a very narrow range of few angstroms (< 10 angstroms). Applications of Laser Engineering Application The large intensity that is possible in the focused output of a laser beam and its directionality makes laser an extremely useful tool for a variety of engineering applications. (i) Welding: The laser beam is used to join two or more material to join together a la welding. The laser welding is a contactless process and hence there is no chance of impurities reaching into the joint. The weld is formed when the intense beam of laser rapidly heats the material. Lasers are used to weld extremely small components and hard to excess areas. The laser beam has been used to weld carbon steels, high strength low alloy steels, aluminium, stainless steel, titanium etc. As total input power is very small compared to that in conventional welding processes, the work pieces are not spoiled. The heat-effected zone is relatively small because of rapid cooling. (ii) Cutting: Wide range of materials can be cut by lasers. For example, paper, wood, cloth, glass, quartz, ceramics, steel etc. The advantage of laser cutting is that it is fine and precise. It introduces a minimum mechanical distortion and thermal shock in the material being cut and the process does not introduce any contamination. It is easily automatized and high production rates can be achieved. (iii) Measurement of atmospheric pollutants: Laser is a very useful tool for the mea- surement of the concentrations of various atmospheric pollutants such as N2 , CO, SO2 , etc gases and particulate matter such as dust, smoke and fly ash. Conventional meth- ods of pollution measurements require that samples of pollutants are to be collected for 16  Safia Ahmad chemical analysis. Therefore,these methods cannot give real-time data. In contrast, laser methods permit measurements by remotely sensing the composition of atmosphere without the necessity of sample collection and chemical processing. Using absorption spectroscopy techniques, the existence of specific gases in the atmosphere is detected. A laser beam is transmitted through polluted sample and the weakening of intensity of light due to absorption in the sample is detected and recorded. Each chemical absorbs light of characteristic wavelength and form the absorption spectrum and from that pollutant’s existence can be inferred. (iv) Lasers are used in the manufacture of electronic components and integrated circuits. They are also used to write and read data from compact disc (CD), for scanning docu- ments etc. (v) Lasers can also be used to measure distances and levels and to help in the setting out of construction works. The uses of lasers in highway engineering are road profiling, pavement surface deflection, bridge deflection, speed checkers, etc. The highway speed checker comes handy for the traffic police, especially against the speed limit violators because it provides the digital display as well as buzzing sound or alarm to detect any vehicle speed if the vehicle exceeds the permitted speed limit. Scientific Applications The applications of lasers in science includes: (i) Interferometric techniques: Interferometry is a technique which uses the interference of superimposed waves to extract information. It usually uses electromagnetic waves and is an important technique in the fields of astronomy, fiber optics, optical meteorology, seismology, spectroscopy, remote sensing etc. (ii) Raman spectroscopy: Raman spectroscopy is used to determine the vibrational modes of molecules. To obtain Raman spectra, very high intensity monochromatic light source 17  Safia Ahmad is required. The laser light interacts with molecular vibrations i.e. phonons resulting in the shifting in energy of the laser photon. The shift in energy gives the information about vibrational modes. (iii) Because of the highly directional property of laser beams, it is possible to measure distances very accurately with the help of laser beam, e.g. measurement of distance from earth to moon. In 1957, Radar was used for the first time to measure the the earth- moon distance and the accuracy was 0.7 mile. Later, in 1965, laser light was reflected from the moon and the distance was measured within an accuracy of 600 feet (∼ 183 meter). In 1969, astronauts placed a lunar laser reflector on the surface of the moon to measure the distance between earth and moon to within 6 inches accuracy. (iv) To obtain energy from a controlled nuclear fusion process, temperature of about 107 K is required. Because of the ability of lasers to obtain extremely high energy densities, they are being attempted to yield these high temperatures. (v) The field of communication technology has been revolutionized by the use of laser in conjunction with optical fibers. (vi) Holography: Holography is a technique of producing an interference pattern made by having a direct laser beam and a laser beam reflected from an object falls on a photo- graphic plate. This enables to produce a three - dimensional image of an object known as hologram which for optical purposes is as useful as the actual object. Medical science/ Medicine: Lasers are playing very important role in diagnostics and surgery. Laser surgery is a highly sterile process since contact does not occur between the surgical tool and the tissue being cut. (i) Lasers are used for the welding of detached retina. Retina is the light sensitive layer at the back of the eye. Sometimes it gets torn and detached from the back of eye ball. The damage can spread and can lead to total blindness. Conventional open eye surgery 18  Safia Ahmad is very risky. Laser treatment is very simple and fast. An argon laser beam is focused on the desired point of the retina. The green beam of the laser is strongly absorbed by the red blood cells of the retina and causes thermal effects which weld he retina back to the eye ball. (ii) Cataract occurs when natural lens of the eye becomes cloudy and obstructs visions leading to blindness. In traditional open-eye surgery, the natural lens is replaced with a plastic lens. In laser surgery, series of laser pulses of high intensity focused to a small spot on the back of lens produce high electric field sparks and rupture the membrane. (iii) Commonly known as laser eye surgery or laser vision correction. It is a type of refractive surgery for the correction of myopia. Lasers are used to change the shape of the cornea by making small incision on it. (iv) Laparoscopy is a type of surgical procedure that allows a surgeon to access the inside of the abdomen without having to make large incision in the skin. This procedure is also known as keyhole surgery or minimally invasive surgery. Large incisions can be avoided in the laparoscopic surgery because surgeon uses an instrument called laparoscope which is a small tube (optical fibre) that has a light source and a camera which broadcast the images of the abdomen to a television monitor. Unlike traditional surgeries, laser surgery is less painful and causes less bleeding after operation, leads to shorter hospital stay and faster recovery time. It also causes less scar. It is employed in destroying kidney stones and gall stones. An optical fibre is used to reach the stone. Laser pulse is launched through the fibre to break the stone into small pieces which can pass through the ureter (urine flows through this from kidney to bladder) without pain. (v) Lasers are used in endoscopic surgeries (vi) Photodynamic therapy: removing and killing tumors and cancerous cells. 19