LASER - Complete Chapter Guide

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1. Entire Chapter Summary A LASER (Light Amplification by Stimulated Emission of Radiation) produces an intense, coherent, monochromatic and highly directional beam of light . Laser action is based on stimulated emission , where an incident photon forces an excited atom to emit another photon identical in phase, energy and direction . To achieve laser action: Population inversion (more atoms in excited state than ground state) is required. Pumping is used to raise atoms to higher energy states. A resonator amplifies light by multiple reflections. Lasers are classified based on: Medium (solid, gas, semiconductor), Pumping method , Output nature (pulsed / continuous). Important lasers: Ruby laser (3-level, pulsed), He-Ne laser (4-level, continuous), Diode laser (semiconductor). Lasers have vast applications in communication, medicine, industry, measurement, holography and nuclear fusion . 2. Basic Concept of LASER Key Definition LASER = Light Amplification by Stimulated Emission of Radiation Key Points Produces monochromatic, coherent, directional light Very high intensity Works due to stimulated emission Real-Life Link Optical fiber communication 3. Coherent vs Incoherent Light Key Definitions Incoherent Light : Random phase, frequency and direction (Sunlight) Coherent Light : Same phase, frequency and direction (Laser) Key Points Ordinary sources $\to$ spontaneous emission Lasers $\to$ stimulated emission Real-Life Link Holography needs coherence 4. Atomic Interactions (MOST IMPORTANT) Key Definitions Absorption : Atom absorbs photon $\to$ goes to higher energy Spontaneous Emission : Atom emits photon randomly (incoherent) Stimulated Emission : Incident photon forces emission of identical photon Important Formulas Energy difference: $h\nu = E_2 - E_1$ Einstein Coefficients: Absorption probability: $P_{12} = B_{12}u(\nu)$ Spontaneous emission probability: $P'_{21} = A_{21}$ Stimulated emission probability: $P''_{21} = B_{21}u(\nu)$ Relation between Einstein coefficients: $$ \frac{A_{21}}{B_{21}} = \frac{8\pi h\nu^3}{c^3} $$ Real-Life Link Laser amplification 5. Population Inversion & Pumping (CORE OF LASER) Key Definition Population Inversion : $N_2 > N_1$ (more atoms in higher energy state $E_2$ than lower state $E_1$) Why Needed? Stimulated emission must dominate absorption for net amplification. Pumping Methods Optical pumping (e.g., Ruby laser) Electrical pumping (e.g., He-Ne laser) Chemical pumping Real-Life Link Laser intensity control 6. Components & Principle of LASER Three Components Pump (energy source) Active Medium (material that lases) Optical Resonator (mirrors for feedback) Principle Steps Pump atoms to excited state. Atoms fall to metastable state (longer lifetime). Population inversion achieved between metastable state and lower state. Stimulated emission occurs, amplified by resonator $\to$ laser beam. Real-Life Link Laser cavities in industries 7. Types of Lasers & Applications Important Lasers Laser Type Output Wavelength Ruby Solid (3-level) Pulsed $694.3 \text{ nm}$ He-Ne Gas (4-level) CW (Continuous Wave) $632.8 \text{ nm}$ Diode Semiconductor CW Depends Applications Medicine (eye surgery, dermatology) Communication (fiber optics) Industry (cutting, drilling, welding) Holography (3D imaging) Measurement (LIDAR, surveying) Scientific research (spectroscopy) 8. MUST-LEARN 20% (Gives 80% Marks) 👉 Memorize THESE only if short on time: Definition of LASER Stimulated emission concept Population inversion condition ($N_2 > N_1$) Role of metastable state Einstein coefficients meaning Components of laser (Pump, Active Medium, Resonator) Ruby vs He-Ne laser (3-level vs 4-level) Characteristics of laser light (coherent, monochromatic, directional, intense) Major applications 9. 10 Exam-Style Questions with Answers Q1. Define LASER. Ans: LASER stands for Light Amplification by Stimulated Emission of Radiation. Q2. What is stimulated emission? Ans: Emission of a photon identical to the incident photon in phase, energy, and direction, triggered by the incident photon. Q3. Why is population inversion necessary? Ans: To ensure that the rate of stimulated emission exceeds the rate of absorption, leading to net light amplification. Q4. Define pumping. Ans: The process of exciting atoms from lower energy levels to higher energy levels to achieve population inversion. Q5. What is a metastable state? Ans: An excited energy state with a relatively long lifetime (typically $10^{-3}$ seconds), allowing atoms to accumulate before stimulated emission. Q6. Give two characteristics of laser light. Ans: Highly coherent, highly directional, monochromatic, high intensity. Q7. Why is the Ruby laser pulsed? Ans: Because it is a 3-level laser system where achieving and maintaining population inversion is difficult and requires high pumping power, leading to pulsed output. Q8. He-Ne laser is which type? Ans: A four-level gas laser, typically producing continuous wave (CW) output. Q9. State Einstein coefficient relation. Ans: $\frac{A_{21}}{B_{21}} = \frac{8\pi h\nu^3}{c^3}$ Q10. One medical application of laser? Ans: Retina welding (photocoagulation), precise surgical cutting, tattoo removal. 10. 3-Minute Quick Revision Sheet LASER = coherent + monochromatic + directional + intense light Key process $\to$ stimulated emission Population inversion $\to N_2 > N_1$ (essential for lasing) Pump $\to$ creates population inversion Resonator $\to$ amplifies light through multiple reflections Ruby $\to$ 3-level, pulsed, solid-state laser He-Ne $\to$ 4-level, continuous, gas laser Diode $\to$ semiconductor laser, compact Applications $\to$ medicine, communication, industry, holography