Unit IV.pptx Digital Logic And Circuit Design
- 1. UNIT IV : Laser & Fiber Optics Stimulated and Spontaneous Emission, Einstein s ‟ A&B Coefficients, Population Inversion, Pumping, Techniques of Pumping, Optical Resonator, Properties and Applications of Laser, Ruby, Nd:YAG, He-Ne lasers. Introduction to Optical fibre, Acceptance angle and cone, Numerical Aperture, V- Number, Ray theory of propagation through optical fibre, Pulse dispersion , applications of optical fibre.
- 3. LASER Light Amplification by Stimulated Emission of Radiation Laser is a device which emits a powerful, monochromatic collimated beam of light. The emitted light waves are coherent in nature. Due to stimulated emission the photons multiply in each step-giving rise to an intense beam of photons that are coherent and moving in the same direction. Hence the light is amplified by Stimulated Emission of the Radiation termed as LASER.
- 4. What makes lasers special? • They produce a directional beam. • They have a narrow spectrum (or bandwidth). • They are coherent.
- 5. CHARACTERISTICS OF LASER Laser is basically a light source. Laser light has the following important characteristics: High Directionality High Intensity Highly Monochromatic Highly Coherence
- 6. Directionality Ordinary light spreads in all directions and its angular spread is 1m/m. But it is found that laser is highly directional and is angular spread is 1mm/m. For example, the laser beam can be focused to very long distance with a few divergence or angular spread
- 7. Intensity Since an ordinary light spreads in all directions, the intensity reaching the target is very less. But in the case of laser, due to high directionality, the intensity of laser beam reaching the target is of high intense beam. For example, 1 mill watt power of He-Ne laser appears to be brighter than the sunlight
- 8. Monochromatic Laser beam is highly monochromatic; the wavelength is single, whereas in ordinary light like mercury vapor lamp, many wavelengths of light are emitted
- 9. Coherence It is an important characteristic of laser beam. In lasers the wave trains of same frequency are in phase, the radiation given out is in mutual agreement not only in phase but also in the direction of emission and polarization. Thus it is a coherent beam. Due to high coherence it results in an extremely high power.
- 10. Because laser light stays focused and does not spread out much (like a flashlight would), laser beams can travel very long distances. They can also concentrate a lot of energy on a very small area.
- 11. Differences between ordinary light and Laser beam S.No. Ordinary light Laser beams 1 In ordinary light the angular spread is more In laser beam the angular spread is less. 2 They are not directional. They are highly directional. 3 It is less intense It is highly intense 4 It is not a coherent beam and is not in phase. It is a coherent beam and is in phase 5 The radiation are polychromatic The radiations are monochromatic 6 Example: Sun light, Mercury vapor lamp He- Ne Laser, Co2 laser
- 12. Einstein model Absorption of Radiation Spontaneous emission Stimulated emission Rate of absorption/stimulated emission dependent on no of photons & number of atoms in lower/upper state. Einstein identified 3 ways in which atoms exchange energy with a radiation field
- 13. Absorption of radiation. Absorption of radiation is the process by which electrons in the ground state absorbs energy from photons to jump into the higher energy level.
- 14. Spontaneous emission Spontaneous emission is the process by which electrons in the excited state return to the ground state by emitting photons. The electrons in the excited state can stay only for a short period. The time up to which an excited electron can stay at higher energy state (E2) is known as the lifetime of excited electrons. The lifetime of electrons in excited state is 10-8 second.
- 15. Stimulated emission Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron causing it to drop to a lower energy level. In stimulated emission, two photons are emitted (one additional photon is emitted), one is due to the incident photon and another one is due to the energy release of excited electron. Thus, two photons are emitted.
- 16. Differences between Stimulated and spontaneous emission of radiation
- 17. EINSTEIN’S “A & B” COEFFICIENTS
- 23. Hence we need a more atoms in the upper level than in the lower level. But there are less atoms in upper level and more atom in lower level. So, we need to create a situation in which more atoms are in the upper level than in the lower level : Population inversion We want lots of this.... ... but not much of this Lasers: the basic idea
- 24. Population Inversion creates a situation in which the number of atoms in higher energy state is more than that in the lower energy state. Usually at thermal equilibrium, the number of atoms N2 i.e., the population of atoms at higher energy state is much lesser than the population of the atoms at lower energy state N1 that is N1>N2. The Phenomenon of making N2> N1 is known as Population Inversion. Condition for Population inversion 1. There must be at least two energy levels E2> E1. 2. There must be a source to supply the energy to the medium. 3. The atoms must be continuously raised to the excited state.
- 26. PUMPING The process to achieve the population inversion in the medium is called Pumping action. It is essential requirement for producing a laser beam. The pump enables us to obtain such a state of population inversion between a pair of energy levels of the atomic system. When we have a state of population inversion, the input light beam can get amplified by stimulated emission.
- 29. Pumping Schemes
- 36. The active centers in the medium is initially is in the ground state. Through suitable pumping the medium achieve a state of population inversion. Some excited atoms emit photons spontaneously in various directions. Each spontaneous photon can trigger many stimulated transitions along the direction of its propagation. Because of end mirrors photons travelling along the axis are amplified through stimulated emission while the photons emitted in any other direction will pass through the sides of the medium. As photons are reflected back and forth between the mirrors, stimulated emission sharply increases and amplification of light takes place.
- 37. By supplying energy from the external source, the atoms in the ground state E1 are pumped to excited state E2.
- 44. Nd:YAG Laser Neodymium-doped yttrium aluminum garnet; Nd:Y3Al5O12
- 51. Working
- 52. 20.61 eV 20.66 eV 18.7 eV 6328 A
- 53. Working
- 54. Advantages of He-Ne Laser. It is more directional and monochromatic than a solid-state laser. It has high stability of frequency. It can operate continuously without the need for cooling as in done in ruby laser. Low cost. High stability. Disadvantages of He-Ne laser The output power of He-Ne laser is moderate as compared to the solid-state laser. Low efficiency. Low gain.
- 55. Applications of He-Ne laser He-Ne laser is used in data processing. He-Ne laser used in holography. Studying interference and diffraction patterns. Used in Telecommunication.
- 56. CHARACTERISTICS OF LASER Laser is basically a light source. Laser light has the following important characteristics: High Directionality High Intensity Highly Monochromatic Highly Coherence Properties and Applications of Laser
- 60. National Science Day-2020 RRCAT, Indore
- 64. UNIT IV : Laser & Fiber Optics Stimulated and Spontaneous Emission, Einstein s ‟ A&B Coefficients, Population Inversion, Pumping, Techniques of Pumping, Optical Resonator, Properties and Applications of Laser, Ruby, Nd:YAG, He-Ne lasers. Introduction to Optical fibre, Acceptance angle and cone, Numerical Aperture, V- Number, Ray theory of propagation through optical fibre, Pulse dispersion , applications of optical fibre.
- 65. Optical fiber Principle An optical fiber, or optical fibre, is a flexible glass or plastic fiber that can transmit light from one end to the other. Light propagate in optical fiber from one of its ends to the other end is based on the principle of total internal reflection. When light enters one end of the fiber, it undergoes successive total internal reflection from side walls and travels down the length of the fiber along a zigzag path A small fraction of light may escape through sidewalls but a major fraction emerges out from the exit end of the fiber Light can travel through fiber even if it is bent.
- 66. Total Internal Reflection • A medium having a lower refractive index is called rare medium while a medium having higher refractive index is known as denser medium. • when a ray of light passes from denser medium to rare medium, it is bent away from the normal in the rare medium. • Snell’s law is • where θ1 is the angle of incidence of light ray in the denser medium • θ2 is the angle of refraction in the rare medium .
- 67. 1. If θ1 θc , the ray refracts into the rare medium 2. If θ1 = θc , the ray just grazes the interface of rarer-to- denser media. 3. If θ1 > θc , the ray is refracted back into the denser medium The phenomenon in which light is totally reflected from a denser –to-rare medium boundary is known as total internal reflection. The rays that experience total internal reflection obey the laws of reflection. Therefore, the critical angle can be determined from Snell’s law.
- 68. When Therefore, from equation, we get when the rare medium is air , μ2 =1 and writing μ1 = μ we get
- 69. For total internal reflection at the fibre wall following two conditions must be satisfied. 1. The refractive index of the core material n1 , must be slightly greater than that of the cladding n2. 2. At the core –cladding interface, the angle of incidence θ between the ray and the normal to the interface must be greater than the critical angle θc defined by
- 72. • Structure: A practical optical fiber is cylindrical in shape.
- 73. It has in general three coaxial regions Core: The innermost cylindrical region is the light guiding region known as the core. In general the diameter of the core is of the order of 8.5 µm to 62.5 µm Cladding: The core is surrounded by a coaxial middle region known as the cladding. The diameter of the cladding is of the order of 125 µm. The refractive index of cladding ( ) is always lower than that of core (
- 74. Light launched into the core and striking the core-to-cladding interface at angle greater than critical angle will be reflected back into the core. When the angles of incidence and reflection are equal, the light will continue to rebound and propagate through the fiber.
- 87. q=90-r
- 88. sin q q q From Total Internal Reflection
- 90. NA is Light gathering power of optical fibre.
- 91. Basic laws of ray theory The basic laws of ray theory are quite self- explanatory. In a homogeneous medium, light rays are straight lines. Light may be absorbed or reflected Reflected ray lies in the plane of incidence and angle of incidence will be equal to the angle of reflection. At the boundary between two media of different refractive indices, the refracted ray will lie in the plane of incidence. Snell’s Law will give the relationship between the angles of incidence and refraction.
- 92. Ray theory of propagation through optical fibre To consider the propagation of light within an optical fiber utilizing the ray theory model it is necessary to take account of the refractive index of the dielectric medium. The angles of incidence and refraction are related to each other and to the refractive indices of the dielectrics by Snell’s law of refraction.
- 93. As n1 is greater than n2, the angle of refraction is always greater than the angle of incidence. Thus when the angle of refraction is 90° and the refracted ray emerges parallel to the interface between the dielectrics, the angle of incidence must be less than 90°. This is the limiting case of refraction and the angle of incidence is now known as the critical angle φc. At angles of incidence greater than the critical angle the light is reflected back into the originating dielectric medium (total internal reflection) with high efficiency (around 99.9%).
- 94. Hence, it may be observed that total internal reflection occurs at the interface between two dielectrics of differing refractive indices when light is incident on the dielectric of lower index from the dielectric of higher index, and the angle of incidence of the ray exceeds the critical value. This is the mechanism by which light at a sufficiently shallow angle (less than 90°) may be considered to propagate down an optical fiber with low loss.
- 95. Having considered the propagation of light in an optical fiber through total internal reflection at the core–cladding interface, it is useful to enlarge upon the geometric optics approach with reference to light rays entering the fiber. Since only rays with a sufficiently shallow grazing angle (i.e. with an angle to the normal greater than φc at the core–cladding interface are transmitted by total internal reflection, it is clear that not all rays entering the fiber core will continue to be propagated down its length.
- 96. It may be observed that this ray enters the fiber core at an angle θa to the fiber axis and is refracted at the air–core interface before transmission to the core–cladding interface at the critical angle. Hence, any rays which are incident into the fiber core at an angle greater than θa will be transmitted to the core– cladding interface at an angle less than φc, and will not be totally internally reflected.
- 97. This situation is also illustrated in Figure, where the incident ray B at an angle greater than θa is refracted into the cladding and eventually lost by radiation.
- 98. Thus for rays to be transmitted by total internal reflection within the fiber core they must be incident on the fiber core within an acceptance cone defined by the conical half angle θa. Hence θa is the maximum angle to the axis at which light may enter the fiber in order to be propagated, and is often referred to as the acceptance angle for the fiber.
- 99. If the fiber has a regular cross-section (i.e. the core–cladding interfaces are parallel and there are no discontinuities) an incident ray at greater than the critical angle will continue to be reflected and will be transmitted through the fiber. From symmetry considerations it may be noted that the output angle to the axis will be equal to the input angle for the ray, assuming the ray emerges into a medium of the same refractive index from which it was input.
- 100. Modes of Propagation When angle of incidence of a light becomes greater than the critical angle it is trapped within the fibre due to TIR processes. The ray trapped within the fibre does not necessarily propagate through it. The rays propagate in the fibre along certain allowed directions. The total number of possible propagation directions of light in the fibre are called as modes of propagation. There are 2 types of propagation mode in fiber optics cable multi-mode and single-mode
- 101. In single mode fibre, rays travel along only one direction, i.e. in this fibre light can travel only along the axis. In other directions, the waves after reflection from walls, become out of phase, interference destructively and diminish In multimode fibre, there are various allowed paths of light. The light travelling along all these paths remains in phase. The smallest angle propagation corresponds to highest order mode and vice-versa. In other words, the paths of light rays along which they are in phase are called modes. The total number of mode is designated by a number represented by Nm.
- 104. V < 2.405 only 1 mode exists. Fundamental mode V < 2.405, Single mode fiber (SMF). V > 2.405 Multimode fiber
- 105. Applications of Optical Fiber Fiber optic cables find many uses in a wide variety of industries and applications. Some uses of fiber optic cables include: Medical Used as light guides, imaging tools and also as lasers for surgeries Defense/Government Used as hydrophones for seismic waves and SONAR , as wiring in aircraft, submarines and other vehicles and also for field networking Data Storage Used for data transmission Telecommunications Fiber is laid and used for transmitting and receiving purposes Networking Used to connect users and servers in a variety of network settings and help increase the speed and accuracy of data transmission Industrial/Commercial Used for imaging in hard to reach areas, as wiring where EMI is an issue, as sensory devices to make temperature, pressure and other measurements, and as wiring in automobiles and in industrial settings
- 106. Broadcast/CATV Broadcast/cable companies are using fiber optic cables for wiring CATV, HDTV, internet, video on-demand and other applications Fiber optic cables are used for lighting and imaging and as sensors to measure and monitor a vast array of variables. Fiber optic cables are also used in research and development and testing across all the above mentioned industries.
- 107. l