1 UNIT-6 Lasers

2 S. No. Module Lectur e No. PPT Slide No. 1 Introduction, characteristics of lasers. Introduction, characteristics of lasers. L Spontaneous & Stimulated emission of Radiation Spontaneous & Stimulated emission of Radiation Population Inversion. L Types of Lasers L Applications of lasers L UNIT INDEX

3 APPLIED PHYSICS CODE : 07A1BS05 CODE : 07A1BS05 I B.TECH I B.TECH CSE, IT, ECE & EEE CSE, IT, ECE & EEE UNIT-6 UNIT-6 NO. OF SLIDES :40 NO. OF SLIDES :40

4 INTRODUCTION The word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. The word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. Lecture-1

5 Characteristics Monochromacity Monochromacity High Intensity High Intensity Coherence Coherence Directionality Directionality

6 Types of coherence Temporal coherence Temporal coherence Spatial coherence Spatial coherence Temporal coherence measures the continuity of a wave along its length. Temporal coherence measures the continuity of a wave along its length. Spatial coherence measures the maximum seperation between any two points on the cross section of the wavefront which maintain correlation between them. Spatial coherence measures the maximum seperation between any two points on the cross section of the wavefront which maintain correlation between them. Lecture-2

7 Stimulated Absorption Excitation of atoms from lower energy state to higher energy state due to interaction of radiation with matter is known as Stimulated absorption. Excitation of atoms from lower energy state to higher energy state due to interaction of radiation with matter is known as Stimulated absorption.

8 Spontaneous emission When an electron in the excited level E2 falls spontaneously to lower energy level E1 after its lifetime a photon is emitted. The energy of the emitted photon is given by E2-E1=h When an electron in the excited level E2 falls spontaneously to lower energy level E1 after its lifetime a photon is emitted. The energy of the emitted photon is given by E2-E1=h

9 Stimulated emission When an electron in the excited level E2 is induced (stimulated) by a photon of energy (E2-E1), the electron moves to lower energy level E1 emitting another photon of energy E2-E1. This process is called stimulated emission. When an electron in the excited level E2 is induced (stimulated) by a photon of energy (E2-E1), the electron moves to lower energy level E1 emitting another photon of energy E2-E1. This process is called stimulated emission.

10 Both stimulated and stimulating photons are in phase with each other. Both stimulated and stimulating photons are in phase with each other. Stimulated emission of radiation (light) results in amplification of light Stimulated emission of radiation (light) results in amplification of light

11 Population inversion For light amplification by stimulated emission of radiation the population of excited state must be greater than the population of lower energy state. This condition is called population inversion. For light amplification by stimulated emission of radiation the population of excited state must be greater than the population of lower energy state. This condition is called population inversion. Lecture-3

12 Pumping mechanisms The process of sending atoms from lower energy state to higher energy state is called Pumping. The process of sending atoms from lower energy state to higher energy state is called Pumping. Optical pumping Optical pumping Electric discharge Electric discharge Chemical reaction Chemical reaction Injection current through p-n junction Injection current through p-n junction

13 Optical Feed back To direct the amplified light to travel back and forth through the active medium many times two end mirrors are kept at both the ends of the laser. These mirrors provide necessary optical feed back. To direct the amplified light to travel back and forth through the active medium many times two end mirrors are kept at both the ends of the laser. These mirrors provide necessary optical feed back.

14 Threshold inversion density Only if the population inversion density is sufficiently large so that the loss is compensated by the gain, lasing action starts. The inversion density for which the gain is just sufficient to compensate for the loss is called threshold inversion density. Only if the population inversion density is sufficiently large so that the loss is compensated by the gain, lasing action starts. The inversion density for which the gain is just sufficient to compensate for the loss is called threshold inversion density.

15 Conditions for Lasing For laser action to take place, the three requisites are Suitable active medium Suitable active medium Creation of population inversion Creation of population inversion Proper optical feed back Proper optical feed back Lecture-4

16 RUBY LASER RUBY LASER A ruby laser is a solid-state laser A ruby laser is a solid-state lasersolid-state lasersolid-state laser It uses a synthetic ruby crystal as its gain medium. It uses a synthetic ruby crystal as its gain medium.ruby gain mediumruby gain medium It was the first type of laser invented, and was first operated by Theodore H. "Ted" Maiman at Hughes Research Laboratories on It was the first type of laser invented, and was first operated by Theodore H. "Ted" Maiman at Hughes Research Laboratories on 1960.laserTheodore H. "Ted" MaimanHughes Research LaboratorieslaserTheodore H. "Ted" MaimanHughes Research Laboratories Lecture-2Lecture-2 Lecture-2Lecture-2

17 The ruby laser produces pulses of visible light at a wavelength of nm, which appears as deep red to human eyes. The ruby laser produces pulses of visible light at a wavelength of nm, which appears as deep red to human eyes.wavelengthnmwavelengthnm Typical ruby laser pulse lengths are on the order of a millisecond. These short pulses of red light are visible to the human eye, if the viewer carefully watches the target area where the pulse will fire. Typical ruby laser pulse lengths are on the order of a millisecond. These short pulses of red light are visible to the human eye, if the viewer carefully watches the target area where the pulse will fire.millisecond

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19 Applications Ruby lasers have declined in use with the discovery of better lasing media. They are still used in a number of applications where short pulses of red light are required. Ruby lasers have declined in use with the discovery of better lasing media. They are still used in a number of applications where short pulses of red light are required. Holographers around the world produce holographic portraits with ruby lasers, in sizes up to a metre squared. Holographers around the world produce holographic portraits with ruby lasers, in sizes up to a metre squared. holographic The red 694 nm laser light is preferred to the 532 nm green light of frequency- doubled Nd:YAG. The red 694 nm laser light is preferred to the 532 nm green light of frequency- doubled Nd:YAG.frequency- doubledNd:YAGfrequency- doubledNd:YAG

20 Many non-destructive testing labs use ruby lasers to create holograms of large objects such as aircraft tires to look for weaknesses in the lining. Ruby lasers were used extensively in tattoo and hair removal, but are being replaced by alexandrite lasers and Nd:YAG lasers in this application. Many non-destructive testing labs use ruby lasers to create holograms of large objects such as aircraft tires to look for weaknesses in the lining. Ruby lasers were used extensively in tattoo and hair removal, but are being replaced by alexandrite lasers and Nd:YAG lasers in this application.non-destructive testingtattoohair removal alexandriteNd:YAG lasersnon-destructive testingtattoohair removal alexandriteNd:YAG lasers

21 Helium-neon laser

22 He –Ne Laser A helium-neon laser, usually called a HeNe laser, is a type of small gas laser. A helium-neon laser, usually called a HeNe laser, is a type of small gas laser.gas lasergas laser HeNe lasers have many industrial and scientific uses, and are often used in laboratory demonstrations of optics. HeNe lasers have many industrial and scientific uses, and are often used in laboratory demonstrations of optics.laserslaboratoryopticslaserslaboratoryoptics Its usual operation wavelength is nm, in the red portion of the visible spect Its usual operation wavelength is nm, in the red portion of the visible spectwavelengthnmredwavelengthnmred The gain medium of the laser, as suggested by its name, is a mixture of helium and neon gases, in a 5:1 to 20:1 ratio, contained at low pressure (an average 50 Pa per cm of cavity length ) in a glass envelope. The gain medium of the laser, as suggested by its name, is a mixture of helium and neon gases, in a 5:1 to 20:1 ratio, contained at low pressure (an average 50 Pa per cm of cavity length ) in a glass envelope.gain mediumheliumneonPagain mediumheliumneonPa

23 He-Ne Laser The energy or pump source of the laser is provided by an electrical discharge of around 1000 volts through an anode and cathode at each end of the glass tube. The energy or pump source of the laser is provided by an electrical discharge of around 1000 volts through an anode and cathode at each end of the glass tube.electricalvoltsanodecathodeelectricalvoltsanodecathode A current of 5 to 100 mA is typical for CW operation. A current of 5 to 100 mA is typical for CW operation.CW The optical cavity of the laser typically consists of a plane, high-reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end. The optical cavity of the laser typically consists of a plane, high-reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end.optical cavitymirroroutput coupleroptical cavitymirroroutput coupler

24 He-Ne Laser HeNe lasers are typically small, with cavity lengths of around 15 cm up to 0.5 m, and optical output powers ranging from 1 mW to 100 mW. HeNe lasers are typically small, with cavity lengths of around 15 cm up to 0.5 m, and optical output powers ranging from 1 mW to 100 mW.powersWpowersW The red HeNe laser wavelength is usually reported as 632nm. However The red HeNe laser wavelength is usually reported as 632nm. However

25 The true wavelength in air is nm, so 633nm is actually closer to the true value. The true wavelength in air is nm, so 633nm is actually closer to the true value. For the purposes of calculating the photon energy, the vacuum wavelength of nm should be used. The precise operating wavelength lies within about nm of this value, and fluctuates within this range due to thermal expansion of the cavithy. For the purposes of calculating the photon energy, the vacuum wavelength of nm should be used. The precise operating wavelength lies within about nm of this value, and fluctuates within this range due to thermal expansion of the cavithy.

26 The laser process in a HeNe laser starts with collision of electrons from the electrical discharge with the helium atoms in the gas. The laser process in a HeNe laser starts with collision of electrons from the electrical discharge with the helium atoms in the gas.electrons This excites helium from the ground state to the 23S1 and 21S0 long-lived, metastable excited states. Collision of the excited helium atoms with the ground-state neon atoms results in transfer of energy to the neon atoms, exciting neon electrons into the 3s2 level. This is due to a coincidence of energy levels between the helium and neon atoms. This excites helium from the ground state to the 23S1 and 21S0 long-lived, metastable excited states. Collision of the excited helium atoms with the ground-state neon atoms results in transfer of energy to the neon atoms, exciting neon electrons into the 3s2 level. This is due to a coincidence of energy levels between the helium and neon atoms.ground statemetastableenergy levelsground statemetastableenergy levels

27 This process is given by the reaction equation: This process is given by the reaction equation: He(21S)* + Ne + ΔE He(11S) + Ne3s2* He(21S)* + Ne + ΔE He(11S) + Ne3s2* ΔE is the small energy difference between the energy states of the two atoms, of the order of 0.05 eV or 387 cm-1, which is supplied by kinetic energy. ΔE is the small energy difference between the energy states of the two atoms, of the order of 0.05 eV or 387 cm-1, which is supplied by kinetic energy.eV

28. The number of neon atoms entering the excited states builds up as further collisions between helium and neon atoms occur, causing a population inversion. Spontaneous and stimulated emission between the 3s2 and 2p4 states results in emission of nm wavelength light, the typical operating wavelength of a HeNe laser. The number of neon atoms entering the excited states builds up as further collisions between helium and neon atoms occur, causing a population inversion. Spontaneous and stimulated emission between the 3s2 and 2p4 states results in emission of nm wavelength light, the typical operating wavelength of a HeNe laser.population inversion Spontaneousstimulated emissionpopulation inversion Spontaneousstimulated emission

29. After this, fast radiative decay occurs from the 2p to the 1s ground state. Because the neon upper level saturates with higher current and the lower level varies linearly with current, the HeNe laser is restricted to low power operation to maintain population inversion. After this, fast radiative decay occurs from the 2p to the 1s ground state. Because the neon upper level saturates with higher current and the lower level varies linearly with current, the HeNe laser is restricted to low power operation to maintain population inversion. Spectrum of a helium neon laser showing Spectrum of a helium neon laser showing With the correct selection of cavity mirrors, other wavelengths of laser emission of the HeNe laser are possible. There are infrared transitions at 3.39 μm and 1.15 μm wavelengths, and a variety of visible transitions, including a green (543.5 nm, the so-called GreeNe laser), a yellow (594 nm) and an orange (612 nm) transition. With the correct selection of cavity mirrors, other wavelengths of laser emission of the HeNe laser are possible. There are infrared transitions at 3.39 μm and 1.15 μm wavelengths, and a variety of visible transitions, including a green (543.5 nm, the so-called GreeNe laser), a yellow (594 nm) and an orange (612 nm) transition.infraredμmgreenyelloworangeinfraredμmgreenyelloworange

30 SEMICONDUCTOR LASER A semiconductor laser converts electrical energy into light. This is made possible by using a semiconductor material, whose ability to conduct electricity is between that of conductors and insulators. A semiconductor laser converts electrical energy into light. This is made possible by using a semiconductor material, whose ability to conduct electricity is between that of conductors and insulators. By doping a semiconductor with specific amounts of impurities, the number of negatively charged electrons or positively charged holes can be changed. By doping a semiconductor with specific amounts of impurities, the number of negatively charged electrons or positively charged holes can be changed. Compared to other laser types, semiconductor lasers are compact, reliable and last a long time. Compared to other laser types, semiconductor lasers are compact, reliable and last a long time. Lecture-2Lecture-2 Lecture-5

31 SEMICONDUCTOR LASER Such lasers consist of two basic components, an optical amplifier and a resonator. The amplifier is made from a direct-bandgap semiconductor material based on either gallium arsenide (GaAs) or InP substrates. Such lasers consist of two basic components, an optical amplifier and a resonator. The amplifier is made from a direct-bandgap semiconductor material based on either gallium arsenide (GaAs) or InP substrates. These are compounds based on the Group III and Group V elements in the periodic table. Alloys of these materials are formed onto the substrates as layered structures containing precise amounts of other materials. These are compounds based on the Group III and Group V elements in the periodic table. Alloys of these materials are formed onto the substrates as layered structures containing precise amounts of other materials.

32 Semiconductor laser Semiconductor laser Lecture-2Lecture-2

33 HEAVILY DOPED p-n JUNCTION DIODE (a)In equilibrium. (b)With forward bias. (a)In equilibrium. (b)With forward bias.

34 DOUBLE HETEROJUNCTION SC LASER DOUBLE HETEROJUNCTION SC LASER

35 Raw Materials The conventional semiconductor laser consists of a compound semiconductor, gallium arsenide. This material comes in the form of ingots that are then further processed into substrates to which layers of other materials are added. The materials used to form these layers are precisely weighed according to a specific formula. The conventional semiconductor laser consists of a compound semiconductor, gallium arsenide. This material comes in the form of ingots that are then further processed into substrates to which layers of other materials are added. The materials used to form these layers are precisely weighed according to a specific formula. Other materials that are used to make this type of laser include certain metals (zinc, gold, and copper) as additives (dopants) or electrodes, and silicon dioxide as an insulator. Other materials that are used to make this type of laser include certain metals (zinc, gold, and copper) as additives (dopants) or electrodes, and silicon dioxide as an insulator. Lecture-2Lecture-2

36 APPLICATIONS OF LASERS Lasers are uused in local area network to transfer the data from the memory storage of one computer to other computer. Lasers are uused in local area network to transfer the data from the memory storage of one computer to other computer. These are used to store large amount of data in CD-ROM These are used to store large amount of data in CD-ROM Lecture-6

37 Lasers can be used to blast holes in diamonds and hard steel. Lasers can be used to blast holes in diamonds and hard steel. They are used as a source of intense heat. They are used as a source of intense heat. They are used to cut, drill, weld, and to remove metal from surfaces. They are used to cut, drill, weld, and to remove metal from surfaces.

38 These are used in spacecrafts and submarines. These are used in spacecrafts and submarines. They are also used in high speed photocopiers and printers. They are also used in high speed photocopiers and printers. They are used in the field of 3-d photography. They are used in the field of 3-d photography.

39 Lasers can serve as a war weapon. Lasers can serve as a war weapon. High energy lasers are used to destroy enemy aircrafts and missiles. High energy lasers are used to destroy enemy aircrafts and missiles. These are used to produce certain chemical reactions. These are used to produce certain chemical reactions.

40 Lasers are used in controlling haemorrhage. Lasers are used in controlling haemorrhage. Lasers are used for elimination of moles and tumors. Lasers are used for elimination of moles and tumors. Lasers are used in the treatment of glaucoma. Lasers are used in the treatment of glaucoma.

41 Co 2 laser is used in spinal and brain tumor and kidney stone extrusion. Co 2 laser is used in spinal and brain tumor and kidney stone extrusion. Lasers are used to correct a condition called retina detachments by eye specialist. Lasers are used to correct a condition called retina detachments by eye specialist. Argon and Co 2 lasers are used in the treatment of liver and lungs. Argon and Co 2 lasers are used in the treatment of liver and lungs.