AGM Module 1 Lasers & Optical Fibers CSE PDF

Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 MODULE 1 : LASERS AND OPTICAL FIBERS Laser and Optical Fibers: LASER: Characteristic properties of a LASER beam, Interaction of Radiation with Matter, Einstein’s A and B Coefficients...

Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 MODULE 1 : LASERS AND OPTICAL FIBERS Laser and Optical Fibers: LASER: Characteristic properties of a LASER beam, Interaction of Radiation with Matter, Einstein’s A and B Coefficients and Expression for Energy Density (Derivation), Laser Action, Population Inversion, Metastable State, Requisites of a laser system, Semiconductor Diode Laser, Applications: Bar code scanner, Laser Printer, Laser Cooling (Qualitative), Numerical Problems. Optical Fiber: Principle and Structure, Propagation of Light, Acceptance angle and Numerical Aperture (NA), Derivation of Expression for NA, Modes of Propagation, RI Profile, Classification of Optical Fibers, Attenuation and Fiber Losses, Applications: Fiber Optic networking, Fiber Optic Communication. Numerical Problems Pre requisite: Properties of light Self-learning: Total Internal Reflection LASER The term LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. Laser is a device that produces a highly coherent, monochromatic, intense beam of light with very small divergence. The first laser was built by T.H.Maiman in 1960 and since then extensive research has been carried out on the development of lasers due to their wide ranging applications. Coherence: Two waves are said to coherent if the phase difference between them remains constant. Einstein’s explanation of interaction of radiation with matter (or) Explaination of Induced absorption, Spontaneous emission & Stimulated emission. Consider a system of energy density E𝜈𝑎𝑛𝑑Let N1& N2 be the population of the energy states E1& E2 respectively so that (E2− E1) = hν &E2>E1 I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 1|Page Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 According to Einstein radiation interacts with matter in 3 ways namely: 1) Induced absorption: Induced absorption is the phenomenon in which an atom(A) in the lower energy state E1 absorb the incident photon of energy „hν‟ & excite to the higher energy state E2 If (E2−E1) = hν. Mathematically it (induced absorption) is represented as hν +A →𝑨∗or photon + atom→𝑎𝑡𝑜𝑚∗ Also, Rate of induced absorption = 𝐵12𝑁1E𝜈, I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 2|Page Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 where E𝜈 = E𝑛𝑒𝑟𝑔𝑦𝑑𝑒𝑛𝑠𝑖𝑡𝑦 of radiations & 𝐵12 =𝐸𝑖𝑛𝑠𝑡𝑒𝑖𝑛′𝑠 coefficient for 𝑖𝑛𝑑𝑢𝑐𝑒𝑑𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛. 2. Spontaneous emission: Spontaneous emission is the phenomenon in which an atom (A) in the excited state of energy E2 de-excite to the lower energy state E1 without any external influence by emitting a photon of energy hν =(E2 −E1). Mathematically, it is represented as 𝑨∗ → A + hν Also, Rate of Spontaneous emission = 𝐴21𝑁2 𝑤𝑕𝑒𝑟𝑒𝐴21 =𝐸𝑖𝑛𝑠𝑡𝑒𝑖𝑛′𝑠𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 for 𝑠𝑝𝑜𝑛𝑡𝑎𝑛𝑒𝑜𝑢𝑠𝑒𝑚𝑖𝑠𝑠𝑖𝑜𝑛. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 3|Page Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 3. Stimulated emission: Stimulated emission is the phenomenon in which an atom (𝑨∗) in the excited state of energy E2 de-excite to the lower energy state E1 under the influence of an external photon (hν) by emitting an identical photon of energy hν =(E2−E1). Mathematically,𝑠𝑟𝑒𝑝𝑟𝑒𝑠𝑒𝑛𝑡𝑒𝑑𝑎𝑠𝒉𝝂+𝑨∗ → A + hν + hν Also, Rate of Stimulated emission= 𝐵21𝑁2 E𝜈, where 𝑈𝜈=𝑒𝑛𝑒𝑟𝑔𝑦𝑑𝑒𝑛𝑠𝑖𝑡𝑦& 𝐵21 =𝐸𝑖𝑛𝑠𝑡𝑒𝑖𝑛′𝑠𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 for 𝑆𝑝𝑜𝑛𝑡𝑎𝑛𝑒𝑜𝑢𝑠𝑒𝑚𝑖𝑠𝑠𝑖𝑜𝑛. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 4|Page Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Einstein coefficients are mathematical quantities which are a measure of the probability of absorption or emission of light by an atom or molecule. Thermodynamic equilibrium is a state in which the energy exchanges due to emission and absorption processes occur such that the population of each state remains unaltered. Derivation of an expression for energy density of radiations in terms of Einstein’s coefficients (or) Derive the relation between Einstein‟s coefficients. Consider a system of energy density E𝜈in thermal equilibrium. Consider two energy states E1 and E2 of a system of atoms (E2>E1). Let there be N1 atoms with energy E1 I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 5|Page Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 & N2 be the number density of the energy states E1& E2 respectively, where Number density means number of atoms or molecules available per unit volume of the quantum system. WKT, Rate of induced absorption = 𝐵12𝑁1E𝜈, where 𝐵12 =𝐸𝑖𝑛𝑠𝑡𝑒𝑖𝑛′𝑠𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑖𝑛𝑑𝑢𝑐𝑒𝑑 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛. Rate of Spontaneous emission = 𝐴21𝑁2 𝑤𝑕𝑒𝑟𝑒𝐴21 =𝐸𝑖𝑛𝑠𝑡𝑒𝑖𝑛′𝑠 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑠𝑝𝑜𝑛𝑡𝑎𝑛𝑒𝑜𝑢𝑠 𝑒𝑚𝑖𝑠𝑠𝑖𝑜𝑛 𝑎𝑛𝑑 Rate of Stimulated emission= 𝐵21 𝑁2E , where 𝐵21 =𝐸𝑖𝑛𝑠𝑡𝑒𝑖𝑛′𝑠 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑆𝑡𝑖𝑚𝑢𝑙𝑎𝑡𝑒𝑑 𝑒𝑚𝑖𝑠𝑠𝑖𝑜𝑛. At thermal equilibrium, Rate of induced absorption = {𝑅𝑎𝑡𝑒 𝑜𝑓 𝑆𝑝𝑜𝑛𝑡𝑎𝑛𝑒𝑜𝑢𝑠 𝑒𝑚𝑖𝑠𝑠𝑖𝑜𝑛} + {𝑅𝑎𝑡𝑒 𝑜𝑓 𝑆𝑡𝑖𝑚𝑢𝑙𝑎𝑡𝑒𝑑 e𝑚𝑖𝑠𝑠 } ie: 𝐵12𝑁1E𝜈 = 𝐴21𝑁2+ 𝐵21𝑁2 E𝜈 ∴ E( 𝐵12𝑁1− 𝐵21𝑁2) = 𝐴21𝑁2 𝑖𝑒: E𝜈 = 𝐴21𝑁2 (𝐵12𝑁1− 𝐵21𝑁2) Dividing both Nr & Dr by 𝐵21𝑁2, we get E = [ ] …….. ……….(1) ( ) According to Boltzmann‟s law, = 𝑒 = 𝑒 …….(2) From eqns 1 &2 , we get. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 6|Page Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 E = [𝐵 ] ……….……..(3) 𝑒 𝐵 But.the energy density of black body radiation is given by Planck‟s law is E = [ ] ……….……..(4) 𝑒 Comparing eqns 3 & 4, we get = 1or = 𝑩𝟐𝟏 Thus coefficient of Induced absorption = coefficient of stimulated emission. And = Thus, energy density E𝝂 = [ ] 𝑒 This is the expression for energy density in terms of Einstein‟s coefficients or relation between Einstein‟s coefficients. Important the terms: Active medium; Pumping mechanism ; Population Inversion ; Meta stable state and Laser Cavity( Resonant cavity) Explanation of the requisites of a laser system. Requisites of a laser system: I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 7|Page Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Requisites of a laser system as discussed above, for the production of laser beam stimulated emission is essential requirement which is a non-thermal equilibrium condition. Therefore, to begin stimulated emission, some requisites are required which is mentioned as below: 1. Active medium is a Solid/Liquid/Gas medium in which pumping mechanism, lasing, population inversion and stimulated emission of the radiations takes place. 2. Pumping mechanism is the process in which atoms in the lower states in order to excite them to higher states with the help of external energy. The methods of pumping are Optical pumping, Electrical pumping; Forward bias pumping, chemical pumping, Elastic one-one collisions. The energy input may be in the form of light energy. This kind of pumping is called optical pumping and is made use of in the construction of Ruby laser. If the pumping is achieved by electrical energy input then it is called electrical pumping. 3. Lasing: The process which leads the emission of stimulates photons after establishing a population inversion is called lasing action. 4. Population Inversion is condition of system in which the population of atoms or molecules in higher energy (excited) states exceeds the population of atoms or molecules in lower (ground) states. N2> N1 When the material is in thermal equilibrium condition, the population ratio is governed by the Boltzmann factor according to the following equation: ( ) According to Boltzmann‟s law, = 𝑒 = 𝑒 I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 8|Page Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 It means that the population E2 will be far smaller than the population N1 at the level E1. The condition in which there are more atoms in the lower energy level and relatively lesser number of atoms in the higher energy level is called normal state or equilibrium state. Thus under thermal equilibrium condition. N 1 > N2 For achievement of laser light stimulated emission should dominate over absorption. For the stimulated emission to dominate; population of the upper energy level N2 must exceed the population of the lower energy level N1, this is called as population inversion. Population inversion is the condition of the material in which population of the upper energy level N2 far exceeds the population of the lower energy level N1. N2> N1 This is a non-equilibrium state and exists only for a short time. Population inversion is obtained by employing pumping technique, which transfer large number of atoms from lower energy level to higher energy level. 5. Meta stable state is an intermediate state in which the average life of the atoms is of the order of 10−2s to 10−3s or milli second. An atom can be excited to a higher level by supplying energy to it. Normally excited atoms have short lifetimes and release their energy in a matter of nanoseconds (10-8 to 10-9 s) through spontaneous emission. It means that atoms do not stay long enough at the excited state to be stimulated. As a result, even though the pumping agent continuously raises the atoms to the excited level, they undergo spontaneous transition and I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 9|Page Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 rapidly return to the lower energy level. Population inversion cannot be established under such circumstances. In order to establish the condition of population inversion, the excited atoms are required to “wait” at the upper energy level till a large number of atoms accumulate at that level. Such an opportunity would be provided by metastable states, which is of the order of 10-6 to 10-3 s. This is 103 to 106 times the lifetimes of the ordinary excited energy levels. 6. Laser Cavity (Resonant Cavity): It consists of two opposing plane mirrors, with active material placed in between them. One of the mirrors is partially reflecting while the other is fully reflecting (100 %). The mirrors reflect the photons to and fro through the active medium. The mirrors are placed normal to the optic axis of the material. The spontaneously emitted photons moving along the optic axis will only stimulate the other atoms from the excited state in the active medium and the other photons travelling in the other directions will be lost. So the two mirrors along with the active medium form the laser cavity. Now a photon moving in a particular direction represents a light wave moving in the same direction. Inside the cavity two types of waves exist; one type comprises of waves moving to the right, and the other one, to the left. Two waves interfere constructively if there is no phase difference between the two; but their interference becomes destructive if the phase difference is For constructive interference the distance “L” between the two mirrors should be such that the cavity should support an integral multiple of half wavelength I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 10 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 One mirror is highly silvered and the other partially silvered. The distance between the two reflected mirrors is given by L = , where λ = wavelength of incident radiations and n = number of stationary waves produced. Semiconductor Diode laser: Principle, construction and working of Semiconductor Diode laser. Construction: Metallic contacts are provided to the P and N types in a heavily doped P-N junction diode. Two opposite faces which are perpendicular to the plane of the junction are polished and made parallel to each other. These parallel faces constitute the resonant cavity and laser is obtained through these faces as shown in Figure. The remaining two faces are roughened to prevent lasing action in that direction. 1. The schematic diagram of Ga-As semiconductor device is as shown in the diagram. 2. It consists of heavily doped n-region of Ga-As doped with tellurium and p- region of Ga-As doped with zinc. 3. The dopants are added in the concentration of the order 1017 to 1019 number of dopants per cm3. 4. The upper and lower surfaces are metalized so that pn-junction is forward biased. 5. Two surfaces perpendicular to the junction are polished so that they act as optical resonators and the other two surfaces roughened to prevent lasing in that direction. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 11 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Principle: Semiconductor diode laser works on the principle of stimulated emission. Energy Level Diagram : I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 12 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Working: 1. Semi-conductor laser are made up of highly de-generate semi-conductors having direct band gap like Gallium Arsenide (GaAs). 2. When GaAS diode is forward biased with voltage nearly equal to the energy gap voltage, electrons from n-region & holes from p-region flow across the junction creating population inversion in the active jn region. 3. As the voltage is gradually increased due to forward biasing population inversion is achieved between the valence band and conduction band which in turn result in stimulated emission. 4. Photons produced are amplified between polished optical resonator surfaces producing laser beam. 5. GaAs laser produce laser beam of wavelength 8870Å in IR region , GaAsP produce laser beam of 6500Å in visible region etc. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 13 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Spontaneously emitted photon may trigger stimulated emissions over a large number of recombinations leading to build of laser radiation of high power. The energy gap of GaAs is 1.4 eV, the wavelength of emitted light = 8400A0 Since energy gap is a function of temperature, the wavelength of laser can be tuned anywhere between 8400A0 to 9000A0. Advantages of semiconductor laser: ❖ Semiconductor lasers are the smallest and least expensive of all the lasers available. ❖ The semiconductor lasers find important use in optical communications since they provide light beams of wavelengths which have low absorption loss in the optical fibers. Mention the characteristics of laser beam. The laser beam characteristics are: 1. They are highly monochromatic. 2. They are highly coherent. 3. They are highly directional. 4. They are highly focusable. 5. They are least divergent. Applications of lasers: By the virtue of their high intensity, high degree of monochromaticity, and coherence, lasers find remarkable applications in a diverse of fields such as medicine, material processing, communications, energy resources, 3-D photography etc. They also find immense applications in defense. Some applications are discussed below: Applications of LASER : LASER has wide range of applications pertaining all disciplines of engineering. Here in the syllabus only two applications are discussed relevant to computing. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 14 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 LASER Barcode Reader A barcode scanner, or barcode reader, is a device with lights, lenses, and a sensor that decodes and captures the information contained in barcodes. In the early days of 1D codes, codes could only be read by lasers. Laser light is shone on the label surface and its reflection is captured by a sensor (laser photo detector) to read a bar code. A laser beam is reflected off a mirror and swept left and right to read a bar code Using laser allows reading of distant and wide bar code labels. Laser scanners sweep a laser across a barcode to capture the pattern and decode it. On the flip side, 2D imagers can also read linear barcodes, though generally not as efficiently or rapidly. But, 2D imagers can of course read 2D barcodes such as QR Code and Data Matrix, which laser-based scanners do not do. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 15 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Often, 2D codes are used in conjunction with 1D barcodes in the production process. However, laser scanners cannot read Data Matrix or QR codes. Laser barcode scanners use a visible and repetitive “flashing” laser light beam, commonly used to track inventory and to enter purchases into a computerized management system. Advantages of laser barcode scanner: 1) The laser barcode scanner is used for non-contact scanning flexibly and efficiently. Normally, the laser barcode scanner is the only choice when the scanning distance exceeds 30cm. The technology improves quality and accuracy, gives immediate information to help reduce rework, and cuts costs by up to 50% compared to 2D scanning. It also reduces manual labor and streamlines coordination and collaboration on site. A barcode reader or barcode scanner is an optical scanner that can read printed barcodes, decode the data contained in the barcode to a computer. Like a flatbed scanner, it consists of a light source, a lens and a light sensor for translating optical impulses into electrical signals. Barcodes encode product information into bars and alphanumeric characters, making it much faster and easier to ring up items at a store or track inventory in a warehouse. Besides ease and speed, bar codes' major business benefits include accuracy, inventory control and cost savings. Laser Printing: Laser printers ware invented at XEROX in 1969 by by researcher Gary Stark weather.Laser Printers are digital printing devices that are used to create high quality text and graphics on plain printer. A Diode Laser is used in the process of printing in LASER Printer. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 16 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Construction: Laser printing is an electrostatic digital printing process. It produces high-quality text and graphics (and moderate-quality photographs) by repeatedly passing a laser beam back and forth over a negatively-charged cylinder called a "drum" to define a differentially-charged image. Working Principle: 1. A laser beam projects an image of the page to be printed onto an electrically charged rotating Photo sensitive drum coated with selenium. 2. Photo conductivity allows charge to leak away from the areas which are exposed to light and the area gets positively charged. 3. Toner particles are then electrostatically picked up by the drum‟s charged areas, which have been exposed to light. 4. The drum then prints the image onto paper by direct contact and heat, which fuses the ink to the paper. Advantages 1. Laser printers are generally quiet and fast. 2. Laser printers can produce high quality output on ordinary papers. 3. The cost per page of toner cartridges is lower than other printers. Disadvantages 1. The initial cost of laser printers can be high. 2. Laser printers are more expensive than dot-matrix printers and ink-jet printers I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 17 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 A laser printer is a popular type of computer printer that uses a non-impact photocopier technology where there are no keys striking the paper. When a document is sent to the printer, a laser beam "draws" the document on a selenium- coated drum using electrical charges. Laser printers use an electrical charge to attract toner particles to a transfer roller. Toner particles are pressed onto a piece of paper, while heat and pressure from the fuser unit permanently fix the image onto the page. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 18 | P a g e  Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Common features for laser printers include color printing, copying and or sorting capabilities, industrial metal housing, user controls, indicators, application software, and cutters. Laser printers with color printing are capable of color output in addition to monochrome output. Types of Laser Printers  Monochrome Laser Printer. Monochrome laser printers only print in black and white....  Color Laser Printer. Color laser printers print in cyan, magenta, yellow and black....  Compact Laser Printer. These printers feature a smaller, compact design....  Multifunction Laser Printer. laser, a device that stimulates atoms or molecules to emit light at particular wavelengths and amplifies that light, typically producing a very narrow beam of radiation. The emission generally covers an extremely limited range of visible, infrared, or ultraviolet wavelengths. Laser Cooling (Qualitative): Principle of LASER Cooling Laser cooling is the use of dissipative light forces for reducing the random motion and thus the temperature of small particles, typically atoms or ions. Depending on the mechanism I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 19 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 used, the temperature achieved can be in the millikelvin, microkelvin, or even, nano kelvin regime. If an atom is traveling toward a laser beam and absorbs a photon from the laser, it will be slowed by the fact that the photon has momentum. 𝑝 or P = It would take a large number of such absorptions to cool the atoms to near 0K The following are the types of laser cooling Doppler Cooling. Sisyphous Cooling. Chapter at a Glance Chapter at a Glance   The term laser is an acronym for light amplification by stimulated emission of radiation.  If the phase difference between two points along any ray remains constant, the coherence is called temporal coherence.  If the phase difference between two points in a plane normal to the ray direction remains constant the coherence is called spatial or lateral coherence.  Metastable states : These are excited states of an atom with relatively larger life times of the order of 10– 3 s. As these energy states are neither as stable as ground state nor as unstable as the other excited states, they are known as metastable states.  Spontaneous emission of radiation : When an atom in its excited energy state E2 I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 20 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 makes a transition to the ground state E1on its own, without any external stimulus, it emits a photon of energy E2– E1. This is known as spontaneous emission of radiation.  Stimulated emission of radiation : If a photon having energy E2– E1interacts with an atom in the energy state E2, the photon forces the atom to undergo transition to the ground state E1giving rise to another photon of the same energy E2– E1. This is known as stimulated emission of radiation.  The light produced by stimulated emission is coherent.  The rate of absorption of photons is proportional to the number density N1 of the atoms in ground state and the energy density Ev in the frequency range ν to ν +dν in incident radiation,i.e, Rate of absorption = B12 N1 Eν Where B12is a constant known as Einstein's coefficient of induced absorption.  Rate of spontaneous emission = A21N2 where A21is a constant known as Einstein's coefficient of spontaneous emission and N2 is the number of atoms in the excited state.  Rate of stimulated emission = B21N2E_ where B21 is a constant known as Einstein's coefficient of stimulated emission.  The state, in which there is a larger number of atoms in the higher energy state than the lower, is called population inversion. Population inversion ensures amplification of light.  The process of raising the atoms from a lower energy state to higher, to create population inversion, is called pumping.  Cavities can be constructed using different types of mirrors such that the light rays return to their original location and orientation after travelling through the cavity for a certain number of times. Such cavities are known as resonant cavities.  The carbon dioxide laser makes use of transitions in the molecular vibrational and rotational energy levels. Carbon dioxide is the active gas having metastable states.  In a semiconductor laser, the large forward bias applied to the junction is the pumping mechanism which produces population inversion.  Characteristics of laser : Laser is monochromatic, coherent, unidirectional, bright and can be focussed easily.  Lasers are used for welding, cutting, drilling, measuring atmospheric pollutants, surgeries, telecommunication, holography, Laser rangefinder in defense and laser used in compact disc. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 21 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Important Questions: 1. What is LASER? Enumerate the Characteristics of a LASER Beam. 2. Discuss the three possible ways through which radiation and matter interaction can take place. 3. Explain the terms, (i) Induced absorption, (ii) Spontaneous emission, (iii) Stimulated emission, (iv) Population inversion, (v) Meta-stable state & (vi) Resonant cavity. 4. Explain the rates of absorption and emission and hence derive an expression for energy density using Einstein‟s A and B coefficients. 5. Explain requisites of LASER system. 6. What is Semiconductor LASER? Describe with energy band diagram the construction & working of Semiconductor diode LASER along with applications. 7. Discuss the working of LASER barcode reader. 8. With the help of a sketch describe the principle, construction and working of the LASER Printer. 9. Explain LASER Cooling and its application. Numerical Problems: 1. Find the ratio of population of two energy levels in a LASER if the transition between them produces light of wavelength 6493 A0, assuming the ambient temperature at 27°C. 2. Find the ratio of population of two energy levels in a medium at thermal equilibrium, if the wavelength of light emitted at 291 K is 6928 A0. 3. The ratio of population of two energy levels out of which one corresponds to metastable state is 1.059 × 10−30. Find the wavelength of light emitted at 330 K. 4. Find the ratio of population of two energy levels in a medium at thermal equilibrium, if the wavelength of light emitted at 300 K is 10𝜇𝑚. Also find the effective temperature when energy levels are equally populated. 5. The average power output of a LASER beam of wavelength 6500 A0 is 10 mW. Find the number of photons emitted per second by the LASER source. 6. The average power of a LASER beam of wavelength 6328 A is 5 mW. Find the number of photons emitted per second by the LASER source. 7. A pulsed LASER has an average power output 1.5 mW per pulse and pulse duration is 20 ns. The number of photons emitted per pulse is estimated to be 1.047 ×108. Find the wavelength of the emitted by the LASER. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 22 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 8. A pulsed LASER with power 1 mW lasts for 10 ns. If the number of photons emitted per pulse is 5 ×107. Calculate the wavelength of LASER. 9. A Ruby LASER emits a pulse of 20 ns duration with average power per pulse being 100 kW. If the number of photons in each pulse is 6.981 × 1015, calculate the wavelength of photons. 10. In a LASER system when the energy difference between two energy levels is 2 × 10−19 J, the average power output of LASER beam is found to be 4 mW. Calculate number of photons emitted per second. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 23 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 MODULE I OPTICAL FIBERS I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 24 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 What is an Optical Fiber? Optical Fiber is a transparent di-electric material (like glass/plastic) which guides/ carry) light along it based on the principle of total reflection of light. Optical fiber consists of a cylindrical transparent di-electric material of high refractive index called core. It is surrounded by another di-electric transparent material of low refractive index called cladding. Cladding in turn is surrounded by cylindrical insulator called Sheath, which gives mechanical strength & protect the fiber from absorption, scattering etc. Optical fiber is a plastic or transparent fiber that is used to propagate light. The working principle of this is the total internal reflection from completely different walls. So light can be transmitted for long distances because the flexibility of fiber optics is sufficient. So this is used in microscopes which are in micro size, data communication, in fine endoscopes design, etc. An optical fiber cable includes three layers like core, cladding, and jacket. A core layer is enclosed through a cladding. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 25 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Here cladding layer is normally designed with plastic or silica. The main function of the core within the optical fiber is to transmit an optical signal while the cladding directs the light in the core. As the optical signal is guided throughout the fiber, then it is called an optical waveguide. This article discusses an overview of the numerical aperture of optical fiber. Total Internal Reflection: When a ray of light travels from denser to rarer medium it bends away from the normal. As the angle of incidence increases in the denser medium, the angle of refraction also increases. For a particular angle of incidence called the “critical angle”, the refracted ray grazes the surface separating the media or the angle of refraction is equal to 90°. If the angle of incidence is greater than the critical angle, the light ray is reflected back to the same medium. This is called “Total Internal Reflection”. In total internal reflection, there is no loss of energy. The entire incident ray is reflected back. XX1 is the surface separating medium of refractive index n1 and medium of refractive index n2, n1> n2. AO and OA1 are incident and refracted rays. θ1 and θ2 are angle of incidence and angle of refraction, θ2> θ1. For the ray BO, θc is the critical angle. OB1 is the refracted ray which grazes the interface. The ray CO incident with an angle greater than θc is totally reflected back along OC1. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 26 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 From Snell‟s law, n1sinθ1 = n2sinθ2 For total internal reflection, θ1= θc and θ2 = 90° n1sinθc = n2 (because sin90°=1) θc= sin-¹(n2/n1) In total internal reflection there is no loss or absorption of light energy. The entire energy is returned along the reflected light. Thus is called Total internal reflection. Optical Fibers: They are used in optical communication which on the principle of total internal reflection (TIR). Optical fiber is made from transparent dielectrics. It is cylindrical in shape. The inner cylindrical part is called as core of refractive index n1. The outer part is called as cladding of refractive index n2, n1 > n2. There is continuity between core and cladding. Cladding is enclosed inside a polyurethane jacket. Number of such fibers is grouped to form a cable. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 27 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 The light entering through one end of core strikes the interface of the core and cladding with angle greater than the critical angle and undergoes total internal reflection. After series of such total internal reflection, it emerges out of the core. Thus the optical fiber works as a waveguide. Care must be taken to avoid very sharp bends in the fiber because at sharp bends, the light ray fails to undergo total internal reflection. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 28 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 What is the Numerical Aperture of Optical Fiber? Definition: The measurement of an optical fiber ability to collect the occurrence light ray in it is known as the numerical aperture. The short form of this is NA that illustrates the efficiency with the light which is collected within the fiber to get propagated. We know that when the light is propagated through an optical fiber during total internal reflection. So multiple total internal reflections take place within the fiber to transmit from one end to another. Once the light ray is produced from the source of an optical fiber, then the optical fiber should be very efficient to get the maximum emitted radiation in it. So we can say that the efficiency of a light which is getting from the optical fiber is the main character once transmitting a signal throughout an optical fiber. The numerical aperture is connected to the acceptance angle because the acceptance angle is the maximum angle during light travels through the fiber. Therefore the NA & acceptance angle is associated with each other. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 29 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Derive an expression for acceptance angle or Numerical aperture of an optical fiber. Consider a light ray AO incident at an angle „θ0‟ enters into the fiber. Let „θ1‟ be the angle of refraction for the ray OB. The refracted ray OB incident at a critical angle (900- θ1) at B grazes the interface between core and cladding along BC. If the angle of incidence is greater than critical angle, it undergoes total internal reflection. Thus θ0 is called the waveguide acceptance angle and sinθ0 is called the numerical aperture. Acceptance angle is the maximum angle submitted by the ray with the axis of the fiber so that light can be accepted and guided along the fiber. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 30 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Let 𝑛1, 𝑛2&𝑛𝑜be the RI of core, cladding and launch medium respectively. Also OA incident ray, AB refracted ray, BC totally reflected ray, 𝜃𝑖&𝜃𝑟be the angles of incidence, refraction at A &𝜃&𝜃𝑟be the angle of incidence and angle of refraction at B respectively. By Snell‟s law at position “O”, Let n0, n1 and n2 be the refractive indices of the medium, core and cladding respectively. From Snell‟s law, nosinθ0 = n1sinθ1 → (1) From Snell‟s law, At „B‟ the angle of incidence is (900 – θ1) n1sin(900 – θ1) = n2 sin900 n1cosθ1 = n2 cosθ1 = → (2) From equation (1), we have Rearrange the equation (1) sinθ0 = sinθ1 sinθ0 = √ 𝑐𝑜𝑠 𝜃 ……. (3) ( √ 𝑐𝑜𝑠 𝜃 ) Using eqn. (2) in (3) Put the value of cos θ1 in equn(3) from equn(2) sinθ0 = √ 𝑛 𝑛 sinθ0 = √ 𝑛 sinθ0 = x √𝑛 𝑛 √ sinθ0 = ……… (4) 𝑛 I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 31 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 If the surrounding medium is air 𝑛 , then sinθ0 = √𝑛 𝑛 Where Sinθo is called numerical aperture (N.A). N.A = √𝑛 𝑛 Therefore, for any angle of incidence equal to θi equal to or less than θ0, the incident ray is able to propagate. θi < θ0 sinθi < sinθ0 sin θi √𝑛 𝑛 sin θi n2 Relation between N.A and Δ: Consider Δ= 𝑛 𝑛 = 𝑛 ……(1) We have Expression for numerical aperture N.A = √𝑛 𝑛 N.A. = √ 𝑛 𝑛 𝑛 𝑛 ….. (2) I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 32 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Using equation (1), we have N.A. = √ 𝑛 𝑛 𝑛 …… (3) Usually R.I. of core (n1) is nearly equal to R.I. of cladding (n2)i.e. n1 n2 Hence, we can write n1 + n2 = 2n1 By making this substitution equation 3 becomes N.A. = √ 𝑛 N.A. = √ 𝑛 N.A. = n1√ ……. (4) Thus, the numerical aperture can be increased by increasing the fractional index change. It appears from the above that an increase in the value of Δ increases the numerical aperture and thus enhances the light gathering capacity of the fiber. But we cannot increase Δ to a very large value, since it leads to what is known as “intermodal dispersion” which causes signal distortion. This dispersion causes distortion in the signal. V-number: “The number of modes supported for propagation in the fiber is determined by a parameter called V-number”. If the surrounding medium is air, then 𝑑 √𝑛 𝑛 Where, „d‟ is the core diameter, n1 and n2 are refractive indices of core and cladding respectively, „λ‟ is the wavelength of light propagating in the fiber. V= (𝑁𝐴) If the fiber is surrounded by a medium of refractive index n0, then, √𝑛 𝑛 V= For V >1, the number of modes supported by the fiber is given by, Number of modes I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 33 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Modes of propagation: The paths along which the light is guided in the fiber are called modes of propagation and the number of modes of the fiber is given by N. Types of optical fibers: In an optical fiber the refractive index of cladding is uniform and the refractive index of core may be uniform or may vary in a particular way such that the refractive index decreases from the axis radialy. Refractive index profile: The curve which represents tha variation of R.I. w.r.t. the radial distance from the axis of the fiber is called refractive index profile. Following are the different types of fibers: 1. Single mode fiber 2. Step index multimode fiber 3. Graded index multimode fiber This classification is done depending on the refractive index profile and the number of modes that the fiber can guide. 1. Single mode fiber: A single mode fiber has a core material of uniform R.I. value. The cladding is also of uniform R.I. But the R.I. of cladding is less than that of the core. This results in a sudden increase in the value of R.I. from cladding to core. Thus its R.I. profile takes the shape of a step. The diameter value of the core is about 8 to 10μm and external diameter of cladding is 60 to 70μm. Because of its narrow core, it can guide just a single mode as shown in figure. Refractive index of core and cladding has uniform value; there is an increase in refractive index from cladding to core. Applications: They are used in submarine. It is used in long hand communication due to higher bandwidth. They transmit transformation to longer distance due to negligible dispersion. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 34 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 2. Step index multimode fiber: It is similar to single mode fiber but core has large diameter. The diameter value of the core is about 50 to 200μm and external diameter of cladding is 100 to 250μm. But the core is comparatively larger in diameter. It can propagate large number of modes as shown in figure. Laser or LED is used as a source of light. It has an application in data links. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 35 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Applications: They are used in submarine. It is used in short distance communication due to lower bandwidth. They transmit transformation to shorter distance due to negligible dispersion. 3.Graded index multimode fiber: It is also called GRIN. The geometry of the GRIN multimode fiber is similar to that of step index multimode fiber. Its core material has a special feature that its R.I. value decreases in the radially outwards direction from the axis and becomes equal to that of the cladding at the interface. But the R.I. of the cladding remains uniform. The refractive index profile is shown in figure. The incident rays bends and takes a periodic path along the axis. The rays have different paths with same period. Laser or LED is used as a source of light. It is the expensive of all. It is used in telephone trunk between central offices. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 36 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Applications: They are used in the telephone trunk between central offices. It is used in short distance communication due to lower bandwidth. They transmit transformation to shorter distance. Signal distortion in optical fibers: The propagation of a signal through the optical fiber involves total internal reflection of light rays many times. Further, the rays are reflected at various angles. The rays reflected at higher angles travel greater distances than the rays reflected at lower angles. As a result, all the rays do not arrive at the end of the fiber simultaneously and the light pulse broadens as it travels through the fiber. Since the output pulse does not match with the input pulse, the signal is said to be distorted. If white light is used instead of monochromatic light, another kind of distortion occurs. Since radiation of different wavelengths has different velocities, they do not arrive at the output simultaneously. This distortion is called chromatic dispersion. The signal distortion is quite considerable in multimode step index fibers. In graded index fibers, the light travels with different velocities in different parts of the core as the refractive index varies radially along the core. The rays travel faster near the interface. Hence all the rays arrive at the output almost at the same time and the signal distortion is reduced. In a single mode step index fiber the distortion is less than that in multimode step index fibers. Signal attenuation in optical fibers: Attenuation is the loss of optical power as light travels through a fiber. The power loss of optical signal when they propagated through optical fiber is known as attenuation (PL). The power loss PL in decibel (dB) is given by PL = 𝑙𝑜𝑔 ( ) ……..(1) Pin is the input power and Pout is the output power of signal. The attenuation constant( ) for optical fibers is defined as the power loss per unit length and is expressed in dB/km. It is expressed in decibel/kilometre [dB/km]. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 37 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 = 𝑙𝑜𝑔 ( ) …….. (2) Where, L be the length of fiber & is measured in Km. Attenuation can be caused by three mechanisms. Type of Attenuation in an Optical fiber: 1. Absorption losses:- Absorption of photons by impurities like metal ions such as iron, chromium, cobalt and copper in the silica glass of which the fiber is made of. During signal processing photons interact with electrons of impurity atoms. The atoms are excited and de-excite by emitting photons of different characteristics. Hence it is a loss of energy. The other impurity such as hydroxyl ions (OH) causes significant absorption loss. The absorption of photons by fiber material itself is called intrinsic absorption. 2. Scattering losses: When the wavelength of the photon is comparable to the size of the particle then the scattering takes place. Because of the non-uniformity in manufacturing, the refractive index changes with length leads to a scattering. This type of scattering is called as Rayleigh scattering. It is inversely proportional to the fourth power of wavelength. Scattering of photons also takes place due to trapped gas bubbles which are not dissolved at the time of manufacturing. 3. Radiation losses: Radiation losses occur due to macroscopic bends and microscopic bends. a. Macroscopic bending: All optical fibers are having critical radius of curvature provided by the manufacturer. If the fiber is bent below that specification of radius of curvature, the light ray incident on the core cladding interface will not satisfy the condition of TIR. This causes loss of optical power. b. Microscopic bending: Optical power loss in optical fibers is due to non- uniformity of the optical fibers when they are laid. Non uniformity is due to manufacturing defects and also lateral pressure built up on the fiber. The defect due to nonuniformity (microbendings) can be overcome by introducing optical fiber inside a good strengthen polyurethane jacket. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 38 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 Application of optical fiber: (Point to point communication in OF) In an optical fiber communication system, the input signals (audio, video or other digital data) are used to modulate light from a source like a LED or a semiconductor LASER and is transmitted through optical fiber. At the receiving end the signal is demodulated to reproduce the input signal. If data transfer takes place between only two devices then, it is called point to point communication. A fiber optic communication system is very much similar to a traditional communication system and has three major components. A transmitter converts electrical signals to light signals, an optical fibre transmits the signals and a receiver captures the signals at the other end of the fiber and converts them to electrical signals. The transmitter consists of a light source supported by necessary drive circuits. First voice is converted into electrical signals using a transducer. It is digitized (converted to binary electrical signals) using a coder. The digitized signal, which carries the voice information, is fed to an optical transmitter. The light source in optical transmitter (LED or laser diode) emits modulated light, which is transmitted through optical fiber. The light emitted by the source is in the IR range with a wavelength of 850nm, 1300nm or 1550nm. Optical fiber communication process : The communication using Optical fiber is as follows. First voice is converted into electrical signal using a transducer. It is digitized using a Coder. The digitized signal, which carries the voice information, is fed to an optical transmitter. The light source in I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 39 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 optical transmitter (LED or LASER Diode) emits modulated light, which is transmitted through the optical fiber. At the other end the modulated light signal is detected by a photo detector and is decoded using a decoder. Finally the information is converted into analog electrical signal and is fed to a loud speaker, which converts the signal to voice (sound). On the other end the modulated light signal is detected by a photo detector is amplified and is decoded using a decoder. The output is fed to a suitable transducers to convert it into an audio or video form. Applications of Optical Fibers: Fiber Optic Networking Local Area Network A Local Area Network (LAN) is a type of computer network that interconnects multiple computers and computer driven devices in a particular physical location. Traditionally copper coaxial cables are used for for LAN. Figure : Fiber Optic LAN Abbreviations: 1. PON - Passive Optical Network 2. ONT - Optical Network Terminal 3. ODN - Optical Distribution Network 4. OLT - Optical Line Terminal I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 40 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 5. ONU - Optical Network Unit. Passive Optical LAN: A passive optical network refers to a fiber-optic network utilizing a point-to-multipoint topology and optical splitters to deliver data from a single transmission point to multiple user endpoints.Passive here refers to the unpowered condition of the fiber and splitting/combining components. Passive optical LANs are built entirely using Optical fiber cables. The passive optical LAN works on the concept of optical network terminals (ONT) and passive optical splitters. Network switches act as passive splitters and the commercial media converters act as optical network terminals in a real-time application of passive optical LAN. Advantages: 1. High speeds and bandwidth 2. Longer distances are possible 3. Less chance of errors Advantages 1. Optical fibers can carry very large amounts of information in either digital or analog form. 2. The raw material for optical fiber is of low cost and abundant. 3. It has low cost /meter/ channel 4. Cables are very compact 5. Signals are protected from radiation from lightning or sparking 6. There is no energy radiation from fiber 7. No sparks are generated. Disadvantages 1. The optical connectors are very costly 2. Maintenance cost is high 3. They cannot be bent too sharply 4. They under go structural changes with temperature Advantages of optical communication system: 1) It carries very large amount of information in either digital or analog form due to its large bandwidth. 2) The materials used for making optical fiber are dielectric nature. So, it doesn‟t produces or receives any electromagnetic and R-F interferences. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 41 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 3) Fibers are much easier to transport because of their compactness and lightweight. 4) It is easily compatible with electronic system. 5) It can be operated in high temperature range. 6) It does not pick up any conducted noise. 7) Not affected by corrosion and moisture. 8) It does not get affected by nuclear radiations. 9) No sparks are generated because the signal is optical signal. Model Questions : 1. Define the terms: (i) angle of acceptance, (ii) numerical aperture, (iii) modes of propagation & (iv) refractive index profile. 2. Obtain an expression for numerical aperture and arrive at the condition for propagation. 3. Explain modes of propagation and RI profile. 4. What is attenuation? Explain the factors contributing to the fiber loss. 5. Discuss the types of optical fibers based on modes of propagation and RI profile. 6. Explain attenuation along with the expression for attenuation co-efficient and also discuss the types of fiber losses. 7. Explain the Fiber Optic Networking and mention its advantages. 8. Discuss point to point optical fiber communication system and mention its advantages over the conventional communication system. 9. Discuss the advantages and disadvantages of an optical communication. Numerical Problems : 1. Calculate the numerical aperture and angle of acceptance for an optical fiber having refractive indices 1.563 and 1.498 for core and cladding respectively. 2. The refractive indices of the core and cladding of a step index optical fiber are 1.45 and 1.4 respectively and its core diameter is 45𝜇𝑚. Calculate its fractional refractive index change and numerical aperture. 3. Calculate numerical aperture, acceptance angle and critical angle of a fiber having a core RI 1.50 and cladding RI 1.45. 4. An optical fiber has a numerical aperture of 0.32. The refractive index of cladding is 1.48. Calculate the refractive index of the core, the acceptance angle of the fiber and the fractional index change. 5. An optical signal propagating in a fiber retains 85% of input power after traveling a distance of 500 m in the fiber. Calculate the attenuation coefficient. I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 42 | P a g e Dr.Mahesh S. Bannur, AGMRCET, VARUR ENGG PHYSICS CSE Stream MODULE-1 6. An optical fiber has core RI 1.5 and RI of cladding is 3% less than the core index. Calculate the numerical aperture, angle of acceptance critical angle. 7. The numerical aperture of an optical fiber is 0.2 when surround by air. Determine the RI of its core, given the RI of the cladding is 1.59. Also find the acceptance angle when the fiber is in water of RI 1.33. 8. The angle of acceptance of an optical fiber is 300 when kept in air. Find the angle of acceptance when it is in medium of refractive index 1.33. 9. An optical fiber of 600 m long has input power of 120 mW which emerges out with power of 90 mW. 10. Find attenuation in fiber. The attenuation of light in an optical fiber is 3.6 dB/km. What fraction of its initial intensity is remains after i) 1 km and ii) 3 km ? 11. The attenuation of light in an optical fiber is 2.2 dB/km. What fraction of its initial intensity is remains after i) 2 km and ii) 6 km ? I/II SEM B.E BPHYS102/202 ( NEW 2022 CBCS SCHEME ) Mob.9900111638 43 | P a g e