Engineering Physics Past Paper (Module 5) - 30-09-2024 PDF

Engineering Physics Course Code: BPHY101L; Course Type: Theory Only (TH) Jitendra K. Behera (PhD) Assistant Professor Gr-II [email protected] Office: #121-E, PRP  Module-5 L...

Engineering Physics Course Code: BPHY101L; Course Type: Theory Only (TH) Jitendra K. Behera (PhD) Assistant Professor Gr-II [email protected] Office: #121-E, PRP  Module-5 LASERS Syllabus Laser characteristics - spatial and temporal coherence - Einstein coefficients and their significance - Population inversion - two, three and four-level systems - Pumping schemes - threshold gain coefficient - Components of a laser - He-Ne, Nd: YAG and CO2 lasers and their engineering applications. Reference Books: 1. W. Silfvast, Laser Fundamentals, 2012, 2nd Edition, Cambridge University Press, India. 2. Principles of Lasers, Orazio Svelto, 5th edition (2009) Dr JB: BPHY101L School of Physics,VIT Vellore  LASER Introduction to LASER Properties of Laser Light Three basic processes –Absorption –Spontaneous Emission –Stimulated Emission Main components of a Laser and Working principle –Population Inversion –Pumping –Light amplification in a laser Dr JB: BPHY101L School of Physics,VIT Vellore  Sources of Light In a conventional (incoherent) light source like a lightbulb, an LED, or a star, each atom excited by input pump energy randomly emits a single photon according to a given statistical probability This produces radiation in all directions with a spread of wavelengths and no interrelationships among individual photons.This is called spontaneous emission Dr JB: BPHY101L School of Physics,VIT Vellore  Light Source: LASER LASER Dr JB: BPHY101L School of Physics,VIT Vellore  What is LASER ? “LASER" is an acronym for "Light Amplifica=on by S=mulated Emission of Radia=on. Laser is a device, which emits amplified, monochroma2c, electromagne2c radia2on ,using the process s2mulated emission of atoms and molecules. History of LASER: First Ruby Concept of Concept of Concept of Commercial LASER Stimulated Emission MASER LASER 1916 1954 1957 1960 Einstein Charleses Charles T.H.Maiman Townes et. al Townes et. al Dr JB: BPHY101L School of Physics,VIT Vellore  Properties of LASER Monochromatic Coherence Collimated/Directional High Energy/Intense Dr JB: BPHY101L School of Physics,VIT Vellore  LASER Properties: Monochromaticity Monochroma=c means it only emits only one op=cal wavelength (λ) / frequency (ν) of light. the light concentrate on a narrow range of wavelength (Δλ = small). Only producing one colour wavelength. Lasers are highly monochroma=c in nature Dr JB: BPHY101L School of Physics,VIT Vellore  LASER Properties: Monochromaticity Ideal Monochromatic In practical Dr JB: BPHY101L School of Physics,VIT Vellore  LASER Properties: Monochromaticity Dr JB: BPHY101L School of Physics,VIT Vellore  LASER Properties: Monochromaticity No light is 100% monochromatic. But the LASER emitted light have higher degree of monochromaticity Dr JB: BPHY101L School of Physics,VIT Vellore  LASER Properties: Directionality LASERs are highly directional. A very tight beam, which is strong and concentrated in a particular direction. It can travel for a long distance with a little spread or with a minimum divergence. i.e. it is related to the how much a light beam diverge or spread? It means that the laser beam spreads less than one millimetre for every meter traveled by it. For example, the spread of a laser beam sent from earth to the moon (384400 km) is just few kilometres. Dr JB: BPHY101L School of Physics,VIT Vellore  LASER Properties: Directionality Normal Flashlight: Divergence ~ 20 -30. o o Searchlight: Divergence ~ 8 -10. o o LASER: Divergence ~ 0.5 o -0.6 (10o-3 rad). Dr JB: BPHY101L School of Physics,VIT Vellore  Concept of Coherence Dr JB: BPHY101L School of Physics,VIT Vellore  LASER: Coherence Coherence is a measure of the correla=on between the phases measured at different (temporal and spa=al) points on a wave or……..Two waves are called coherent if they have the same frequency, and wavelength and have a constant phase difference. Physicists dis=nguish between two types of coherence: Spa=al (transverse) coherence Temporal (longitudinal) coherence Dr JB: BPHY101L School of Physics,VIT Vellore  Temporal Coherence Temporal Coherence is a measure of the correlation of light wave’s phase at different points along the direction of propagation – it tells us how monochromatic a source is. P Q k Time at time, :t1 at time, :t2 P : Phase = Φ1 P : Phase = Φ′1 Q : Phase = Φ2 Q : Phase = Φ′2 temporal coherence if ϕ1 − Φ2 = ϕ′1 − Φ′2 Dr JB: BPHY101L School of Physics,VIT Vellore  Temporal Coherence Length Time Temporal τ1 + τ1 + τ2 + τ3... + τn = τc = Coherence time n Temporal Coherence length = Lc = cτc Coherence, tells us how monochromatic a source is: For a monochromatic source τc →∞ Dr JB: BPHY101L School of Physics,VIT Vellore  Spatial Coherence Spatial Coherence is a measure of the correlation of a light wave’s phase at different points transverse to the direction of propagation - it tells us how uniform the phase of a wavefront is P P Q Q Spatial Coherent Spatial Incoherent Dr JB: BPHY101L School of Physics,VIT Vellore  Spatial and Temporal Coherence Dr JB: BPHY101L School of Physics,VIT Vellore  LASER: Highly Intense Highly Energetic: It is highly monochromatic and directional. Therefore it is powerful than ordinary light. Being highly directional, the power of the laser beam can be maintained over the large distances. P Optical Intensity = I = A P= laser power (W) 2 A= laser spot area (m ) Cutting, welding, drilling, military weapons Dr JB: BPHY101L School of Physics,VIT Vellore  LASER Vs Ordinary Light Source Dr JB: BPHY101L School of Physics,VIT Vellore